U.S. patent application number 14/194247 was filed with the patent office on 2014-08-14 for fuel cell stack and fuel cell system.
This patent application is currently assigned to Sharp Kabushiki Kaisha. The applicant listed for this patent is Sharp Kabushiki Kaisha. Invention is credited to Toshiyuki FUJITA, Hironori KAMBARA, Masashi MURAOKA, Tomohisa YOSHIE.
Application Number | 20140227621 14/194247 |
Document ID | / |
Family ID | 40304377 |
Filed Date | 2014-08-14 |
United States Patent
Application |
20140227621 |
Kind Code |
A1 |
FUJITA; Toshiyuki ; et
al. |
August 14, 2014 |
FUEL CELL STACK AND FUEL CELL SYSTEM
Abstract
A fuel cell stack formed by stacking two or more fuel cell
layers each constituted of one or more unit cell and a fuel cell
system including the same are provided. Any two fuel cell layers
adjacent to each other each have one or more gap region. At least a
part of the gap region in one fuel cell layer of any two fuel cell
layers adjacent to each other is in contact with a unit cell
constituting the other fuel cell layer. The gap region in one fuel
cell layer and the gap region in the other fuel cell layer
communicate with each other. The fuel cell stack is excellent in
fuel or oxidizing agent supply performance and it realizes high
power density.
Inventors: |
FUJITA; Toshiyuki; (Osaka,
JP) ; KAMBARA; Hironori; (Osaka, JP) ;
MURAOKA; Masashi; (Osaka, JP) ; YOSHIE; Tomohisa;
(Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sharp Kabushiki Kaisha |
Osaka |
|
JP |
|
|
Assignee: |
Sharp Kabushiki Kaisha
Osaka
JP
|
Family ID: |
40304377 |
Appl. No.: |
14/194247 |
Filed: |
February 28, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12671878 |
Feb 2, 2010 |
8741500 |
|
|
PCT/JP2008/063644 |
Jul 30, 2008 |
|
|
|
14194247 |
|
|
|
|
Current U.S.
Class: |
429/456 |
Current CPC
Class: |
H01M 8/1011 20130101;
H01M 2008/1095 20130101; H01M 8/241 20130101; Y02E 60/50 20130101;
H01M 8/0258 20130101; Y02E 60/523 20130101 |
Class at
Publication: |
429/456 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2007 |
JP |
2007-202146 |
Mar 28, 2008 |
JP |
2008-087571 |
May 13, 2008 |
JP |
2008-126377 |
Claims
1-30. (canceled)
31. A fuel cell stack, comprising: two or more stacked fuel cell
layers each constituted of one or more unit cell and having a gap
region in the layer; and a current collection unit, said unit cell
including an anode catalyst layer, an electrolyte membrane, a
cathode catalyst layer, and a fuel supply unit for supplying a fuel
to said anode catalyst layer, and each said unit cell can be
supplied with an oxidizing agent gas in a plurality of directions
from outside of the fuel cell stack.
32. The fuel cell stack according to claim 31, comprising two or
more stacked fuel cell layers in which two or more unit cells are
arranged such that the gap region is provided between adjacent unit
cells.
33. The fuel cell stack according to claim 32, wherein said unit
cell is in a shape of an elongated strip.
34. A fuel cell system, comprising: the fuel cell stack according
to claim 31; and an auxiliary equipment for promoting flow of the
oxidizing agent gas into the fuel cell stack.
Description
CROSS REFERENCE
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/671,878, filed Feb. 2, 2010, allowed, which is the U.S.
national phase of International Application No. PCT/JP2008/063644
filed 30 Jul. 2008, which designated the U.S. and claims priority
to JP Application No. 2007-202146 filed 2 Aug. 2007; JP Application
No. 2008-087571 filed 28 Mar. 2008; and JP Application No.
2008-126377 filed 13 May 2008, the entire contents of each of which
are hereby incorporated by reference in this application.
TECHNICAL FIELD
[0002] The present invention relates to a fuel cell stack and a
fuel cell system including the fuel cell stack.
BACKGROUND ART
[0003] Expectations for a fuel cell as a power supply for portable
electronic devices supporting the information-oriented society have
recently been growing from a point of view of high power generation
efficiency and high energy density as a stand-alone power
generation apparatus. The fuel cell generates electric power
through such reaction as electrochemical oxidization of a reducing
agent (such as hydrogen, methanol, ethanol, hydrazine, formalin,
and formic acid) at an anode electrode and electrochemical
reduction of an oxidizing agent (such as oxygen in air) at a
cathode electrode.
[0004] The fuel cell, however, suffers a problem of low power
density per volume. In seeking for a fuel cell having a smaller
size and lighter weight, a fuel cell achieving high power density
has been desired.
[0005] In general, except for a high-temperature fuel cell such as
a molten carbonate cell, such conventional fuel cells as a polymer
electrolyte fuel cell, a solid oxide fuel cell, a direct methanol
fuel cell, and an alkaline fuel cell are based on a unit cell as a
constituent unit, that has a planar structure obtained by stacking
an anode separator in which an anode flow channel for supplying a
reducing agent is formed, an anode current collector for collecting
electrons from an anode catalyst layer, an anode gas diffusion
layer, an anode catalyst layer, an electrolyte membrane, a cathode
catalyst layer, a cathode gas diffusion layer, a cathode current
collector for feeding electrons to the cathode catalyst layer, and
a cathode separator in which a cathode flow channel for supplying
an oxidizing agent is formed in this order. Each unit cell produces
a high current with a low voltage.
[0006] In addition, it is also general to provide the anode
separator and the cathode separator not only with a role to supply
the reducing agent and the oxidizing agent separately to the anode
catalyst layer and the cathode catalyst layer respectively but also
with a role as the anode current collector and the cathode current
collector respectively, by employing an electrically conductive
material as a material for these separators.
[0007] Normally, as individual unit cells are low in voltage, a
fuel cell is formed as a fuel cell stack in which a plurality of
unit cells are stacked to be able to output a high voltage. Here,
the plurality of unit cells are stacked such that the anode
electrode of a unit cell is electrically in contact with the
cathode electrode of a unit cell adjacent thereto.
[0008] In such a layered fuel cell stack, intimate electrical
contact between the layers should be maintained. If contact
resistance becomes high, internal resistance of the fuel cell
becomes high and overall power generation efficiency is lowered. In
addition, the fuel cell stack usually includes a sealing material
for sealing each of the reducing agent and the oxidizing agent in
each separator, and each layer should be pressed with quite strong
force in order to ensure sealing performance and electrical
conductivity. Accordingly, fastening members for pressing each
layer such as a presser, a bolt and a nut have been required, which
resulted in a large and heavy fuel cell stack and low power
density.
[0009] Further, as each separator requires a flow channel for
uniformly supplying the reducing agent or the oxidizing agent to
each unit cell all over a plane of the catalyst layer, the
separator is large in thickness, which caused lower power density.
If the flow channel in the separator is narrowed to make the
thickness of the separator smaller, pressure loss becomes greater.
Then, it is inevitable to use large auxiliary equipment such as a
pump or a fan for supplying the reducing agent and the oxidizing
agent, and in addition, power consumption by the auxiliary
equipment also increases. Consequently, power density of a fuel
cell system as a whole is lowered.
[0010] In order to solve such problems, improvement in power
density of a fuel cell by increasing density in a power generation
area, that is, a power generation area included in a unit volume of
the fuel cell, has been aimed. For example, WO03/067693 (Patent
Document 1) discloses a fuel cell layer, in which functions of a
gas diffusion layer, a catalyst layer and an electrolyte layer are
integrated into a single substrate, and proposes a fuel cell
constituted of a smaller number of parts than in a conventional
fuel cell or fuel cell stack having a layered planar structure.
[0011] More specifically, the fuel cell layer described in Patent
Document 1 is connected to an external load with a fuel plenum, an
oxidizing agent plenum, and a porous substrate communicating with
the fuel plenum and the oxidizing agent plenum. The fuel cell layer
also has a porous substrate and numerous fuel cells formed using
the porous substrate. Each fuel cell has a distinct channel, a
first catalyst layer disposed on the first channel wall, a second
catalyst layer disposed on the second channel wall, an anode formed
from the first catalyst layer and an cathode formed from the second
catalyst layer, and an electrolyte disposed in the distinct channel
to prevent transfer of fuel to the cathode and to prevent transfer
of oxidizing agent to the anode. The fuel cell also has a first
coating disposed on at least a portion of the porous substrate to
prevent fuel from entering a portion of the porous substrate, a
second coating disposed on at least a portion of the porous
substrate to prevent oxidizing agent from entering a portion of the
porous substrate, a first sealant barrier disposed on the first
side, a second sealant barrier disposed on the second side, a third
sealant barrier disposed between the fuel cells, a positive
electrical connection disposed on the first side, and a negative
electrical connection disposed on the second side.
[0012] When a plurality of microscopically dimensioned fuel cells
are formed within a single substrate, higher overall power
densities can be achieved. In addition, as the multiple fuel cells
within the single substrate can be formed in parallel, fuel cells
of high capacity can be constructed. The combination of fuel cells
within a single substrate minimizes the reliance on externally
applied seals and clamps. A number of variations of the fuel cell
layer are envisioned. Examples of the variations include having the
fuel and oxidizing agent plenums dead-ended, having the fuel cell
layer enclosing a volume, having the porous substrate in a
non-planar, or alternately planar, configuration and having the
fuel cell layer enclose a volume in a cylindrical shape.
[0013] The fuel cell layer described in Patent Document 1, however,
requires separation between the oxidizing agent and the reducing
agent for each of a surface and a rear surface of the fuel cell
layer. In addition, in any fuel cell stack structure described in
Patent Document 1, the fuel and the oxidizing agent should be
supplied to the fuel cell layer in an in-plane direction. In a case
where three or more fuel cell layers are stacked in a direction of
layer thickness, the fuel plenum or the oxidizing agent plenum
should be provided in order to supply the fuel and the oxidizing
agent, which requires a prescribed interval (gap) between the fuel
cell layers. Further, as the fuel plenum or the oxidizing agent
plenum provided between the fuel cell layers should be sealed, a
fastening member is required for ensuring sealing performance of a
sealing material. Moreover, as supply and exhaust of the fuel and
the oxidizing agent to the fuel cell layer in the in-plane
direction is restricted, it is difficult to supply the fuel and the
oxidizing agent through natural convection. In particular, as
supply of air representing the oxidizing agent is difficult,
auxiliary equipment requiring motive power such as a fan or a pump
is required.
[0014] Namely, a fuel cell structure described in Patent Document 1
has suffered a problem of low power density when a stack structure
in which fuel cell layers are stacked in a direction of layer
thickness responds to demands for high output power on an equipment
side. Meanwhile, when a stack structure in which fuel cell layers
are stacked in an in-plane direction of the fuel cell layer
responds to demands for high output power on an equipment side,
arrangement of fuel cells on the equipment side and design of a
mechanism thereof are much restricted, because the fuel cell layers
require a large area in the in-plane direction.
[0015] Japanese Patent Laying-Open No. 5-41239 (Patent Document 2)
discloses a high-temperature fuel cell module in which at least two
stacks formed by stacking a plurality of cells constituted of an
anode, a solid electrolyte, a cathode, and a ceramic separator form
a part or the entirety of a manifold. According to such a
configuration, effective use of an electrode area can be made and
cost reduction can be achieved because the manifold can be formed
with a simplified structure.
[0016] A fuel cell stack described in Patent Document 2, however,
cannot three-dimensionally supply the fuel or the oxidizing agent
into the fuel cell stack, as in the case of Patent Document 1
above. Therefore, such auxiliary equipment as a pump or a fan for
supplying the fuel or the oxidizing agent at quite a flow rate is
required, which results in a large-sized fuel cell system and
increase in power consumption by the auxiliary equipment and hence
low power density
Patent Document 1: WO03/067693
Patent Document 2: Japanese Patent Laying-Open No. 5-41239
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0017] The present invention was made to solve the above-described
problems, and an object of the present invention is to provide a
fuel cell stack highly integrated in three-dimension, that is
excellent in fuel or oxidizing agent supply performance and capable
of supplying the fuel or the oxidizing agent without using
auxiliary equipment or with low power consumption by the auxiliary
equipment and achieving high power density, as well as to provide a
fuel cell system including the same.
Means for Solving the Problems
[0018] The present invention provides a fuel cell stack including
two or more stacked fuel cell layers each constituted of one or
more unit cell, any two fuel cell layers adjacent to each other
each having one or more gap region, at least a part of the gap
region in one fuel cell layer of any two fuel cell layers adjacent
to each other being in contact with the unit cell constituting the
other fuel cell layer, and the gap region in one fuel cell layer
communicating with the gap region in the other fuel cell layer.
[0019] The gap region in one fuel cell layer and the gap region in
the other fuel cell layer are preferably formed in different
portions, when viewed in a direction of stack of the fuel cell
layers. In addition, all gap regions in the fuel cell stack
preferably communicate with one another.
[0020] The fuel cell layer preferably includes a portion having a
shape elongated in a longitudinal direction within the fuel cell
layer.
[0021] Any two fuel cell layers adjacent to each other described
above are stacked such that the unit cell constituting one fuel
cell layer preferably intersects with, and further preferably is
orthogonal to, the unit cell constituting the other fuel cell
layer.
[0022] In the fuel cell stack according to the present invention,
preferably, at least one fuel cell layer of the fuel cell layers in
the fuel cell stack has a plurality of unit cells, and the
plurality of unit cells each have a shape elongated in a
longitudinal direction within the fuel cell layer.
[0023] The fuel cell stack according to the present invention may
have a spacer layer between any two fuel cell layers adjacent to
each other described above, or at least one fuel cell layer may be
replaced with a spacer layer.
[0024] The spacer layer is preferably electrically conductive, and
is preferably formed of a porous body.
[0025] In addition, the fuel cell stack according to the present
invention may have current collectors at respective opposing ends
thereof in a direction of stack of the fuel cell layers.
[0026] In the fuel cell stack according to the present invention
above, the unit cell is preferably a direct methanol fuel cell.
[0027] In addition, the present invention provides a fuel cell
system including the fuel cell stack described in any part above
and auxiliary equipment for promoting flow of air into the fuel
cell stack, and provides a fuel cell system including the Fuel cell
stack described in any part above, a switch circuit electrically
connected to the current collectors, and a control circuit for
controlling the switch circuit.
[0028] Moreover, the present invention provides a fuel cell stack
including at least one fuel cell layer constituted of one or more
unit cell and at least one spacer layer constituted of one or more
spacer as stacked, at least one fuel cell layer and/or at least one
spacer layer having a gap region in the layer.
[0029] The fuel cell layer and the spacer layer are preferably
alternately stacked.
[0030] Such a fuel cell stack according to the present invention
preferably includes at least one fuel cell layer in which two or
more unit cells are arranged such that a gap region is provided
therebetween and/or at least one spacer layer in which two or more
spacers are arranged such that a gap region is provided
therebetween.
[0031] In addition, preferably, such a fuel cell stack according to
the present invention at least includes a fuel cell layer in which
two or more unit cells are arranged such that a gap region is
provided therebetween and a spacer layer stacked on the fuel cell
layer, in which two or more spacers are arranged such that a gap
region is provided therebetween, the gap region in the fuel cell
layer communicating with the gap region in the spacer layer.
[0032] The spacer constituting the spacer layer described above is
preferably arranged to intersect with the unit cell constituting
the fuel cell layer.
[0033] Preferably, such a fuel cell stack according to the present
invention at least includes the fuel cell layer (A), the spacer
layer, and the fuel cell layer (B) in this order, and two or more
unit cells (b) in the fuel cell layer (B) are arranged in a region
directly on or directly under the unit cells (a) in the fuel cell
layer (A), respectively. Alternatively, preferably, such a fuel
cell stack according to the present invention at least includes the
fuel cell layer (A), the spacer layer, and the fuel cell layer (B)
in this order, and two or more unit cells (b) in the fuel cell
layer (B) are arranged directly above or directly under the gap
regions in the fuel cell layer (A), respectively.
[0034] In addition, preferably, the fuel cell stack according to
the present invention at least includes the fuel cell layer (A),
the spacer layer, and the fuel cell layer (B) in this order, and
the spacers arranged at respective opposing ends among two or more
spacers constituting the spacer layer are arranged in contact with
end portions in a longitudinal direction of the unit cells (a) in
the fuel cell layer (A) and the unit cells (b) in the fuel cell
layer (B).
[0035] Preferably, the two or more unit cells constituting the fuel
cell layer are arranged substantially in parallel such that a gap
region is provided therebetween, and the two or more spacers
constituting the spacer layer are arranged substantially in
parallel such that a gap region is provided therebetween.
[0036] At least any of the unit cell and the spacer constituting
the fuel cell stack is preferably in a shape of an elongated
strip.
[0037] The spacer layer described above may be constituted of two
or more spacers including a spacer formed of a porous body and a
spacer formed of a non-porous body. In this case, at least spacers
arranged at respective opposing ends of the spacer layer among the
two or more spacers are preferably formed of a non-porous body. In
addition, the spacer layer preferably includes at least one spacer
having a hydrophilic surface.
[0038] In such a fuel cell stack according to the present
invention, the unit cell is preferably a direct methanol fuel cell
or a polymer electrolyte fuel cell.
Effects of the Invention
[0039] According to the fuel cell stack having such a structure
that the unit cells are three-dimensionally stacked and the fuel
cell system to which this fuel cell stack is applied in the present
invention, the fuel or the oxidizing agent can be supplied without
using auxiliary equipment or with low power consumption by the
auxiliary equipment and high power density can be achieved. In
addition, the present invention can provide the fuel cell stack and
the fuel cell system achieving a smaller size, lower cost and
higher power density, free from fastening members such as a
presser, a bolt and a nut.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic diagram showing a preferred example of
a fuel cell stack according to the present invention.
[0041] FIG. 2 is a schematic cross-sectional view showing a
preferred example of a unit cell employed in the present
invention.
[0042] FIG. 3 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present
invention,
[0043] FIG. 4 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention
[0044] FIG. 5 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0045] FIG. 6 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0046] FIG. 7 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention
[0047] FIG. 8 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0048] FIG. 9 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0049] FIG. 10 is an enlarged cross-sectional view of the unit cell
in FIG. 9.
[0050] FIG. 11 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present
invention.
[0051] FIG. 12 is a perspective view showing an example of the fuel
cell stack including a fan inside.
[0052] FIG. 13 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present
invention.
[0053] FIG. 14 is an enlarged detail view of a region A in FIG.
13(a).
[0054] FIG. 15 is a partially enlarged schematic cross-sectional
view of another preferred example of the fuel cell stack according
to the present invention.
[0055] FIG. 16 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present
invention.
[0056] FIG. 17 is a top view showing another preferred example of
the fuel cell stack according to the present invention.
[0057] FIG. 18 is a top view showing another preferred example of
the fuel cell stack according to the present invention.
[0058] FIG. 19 is a top view showing another preferred example of
the fuel cell stack according to the present invention.
[0059] FIG. 20 is a cross-sectional view of a unit cell forming an
uppermost fuel cell layer in the fuel cell stack shown in FIG.
19.
[0060] FIG. 21 is a top view showing another preferred example of
the fuel cell stack according to the present invention.
[0061] FIG. 22 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0062] FIG. 23 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0063] FIG. 24 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0064] FIG. 25 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0065] FIG. 26 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0066] FIG. 27 is a cross-sectional view showing another preferred
example of the unit cell employed in the fuel cell stack according
to the present invention.
[0067] FIG. 28 is a cross-sectional view schematically showing an
example of preferred bonding and integration of fuel cell layers in
the present invention.
[0068] FIG. 29 is a top view schematically showing an example of a
method of supplying a fuel to the fuel cell stack according to the
present invention.
[0069] FIG. 30 is a top view schematically showing another example
of a method of supplying a fuel to the fuel cell stack according to
the present invention.
[0070] FIG. 31 is a top view schematically showing another example
of a method of supplying a fuel to the fuel cell stack according to
the present invention.
[0071] FIG. 32 is a schematic diagram showing a preferred example
of a fuel cell system according to the present invention.
[0072] FIG. 33 is a schematic diagram showing a manner of
controlling the fuel cell system according to the present
invention.
[0073] FIG. 34 is a schematic diagram for illustrating a method of
extracting a current.
[0074] FIG. 35 is a schematic diagram showing a manner of
controlling the fuel cell system according to the present
invention.
[0075] FIG. 36 is a schematic diagram showing a part of a
configuration of a unit cell in Example 1.
[0076] FIG. 37 is a schematic diagram showing a configuration of a
fuel cell layer in a fuel cell stack in Example 1.
[0077] FIG. 38 is a schematic diagram showing a configuration of a
fuel cell layer in a fuel cell stack in Example 2.
[0078] FIG. 39 is a diagram showing a first layer and a second
layer in the fuel cell stack in Example 2.
[0079] FIG. 40 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0080] FIG. 41 is a schematic diagram showing an example of a basic
configuration of a fuel cell stack including a spacer layer.
[0081] FIG. 42 is a schematic diagram showing a preferred example
of the fuel cell stack having the spacer layer.
[0082] FIG. 43 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer.
[0083] FIG. 44 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer.
[0084] FIG. 45 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer.
[0085] FIG. 46 is a perspective view showing another preferred
example of the fuel cell stack according to the present
invention.
[0086] FIG. 47 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer, FIG. 47(a)
being a top view thereof and FIG. 47(b) being a side view.
[0087] FIG. 48 is a top view and a cross-sectional view showing a
shape of a flow channel substrate fabricated in Example 3.
[0088] FIG. 49 is a top view and an enlarged view showing a shape
of an anode current collector fabricated in Example 3.
[0089] FIG. 50 is a top view and an enlarged view showing a shape
of a cathode current collector fabricated in Example 3.
[0090] FIG. 51 is a perspective view schematically showing
diffusion bonding of the flow channel substrate, the anode current
collector, the cathode current collector, and a spacer in Example
3.
[0091] FIG. 52 is a perspective view schematically showing joint of
an MEA to the anode current collector in Example 3.
[0092] FIG. 53 is a perspective view and a cross-sectional view of
a two-dimensional stack, in which an adhesive is applied to an MEA
end portion obtained in Example 3.
[0093] FIG. 54 is a cross-sectional view showing joint between
two-dimensional stacks in Example 3.
[0094] FIG. 55 is a cross-sectional view showing a structure of a
fuel cell stack in which a plurality of two-dimensional stacks are
integrated
[0095] FIG. 56 is a schematic perspective view showing the fuel
cell stack including a manifold obtained in Example 3.
[0096] FIG. 57 is a perspective view schematically showing
diffusion bonding of a flow channel substrate, an anode current
collector and a cathode current collector in Example 4.
[0097] FIG. 58 is a cross-sectional view of a two-dimensional
stack, in which an adhesive is applied to an MEA end portion
obtained in Example 4.
[0098] FIG. 59 is a cross-sectional view showing joint between
two-dimensional stacks in Example 5.
[0099] FIG. 60 is a cross-sectional view showing the fuel cell
stack obtained in Example 5.
[0100] FIG. 61 is a perspective view schematically showing
diffusion bonding of a flow channel substrate, an anode current
collector, a cathode current collector, and a spacer in Example
6.
[0101] FIG. 62 is a cross-sectional view showing the fuel cell
stack obtained in Example 6.
[0102] FIG. 63 is a graph showing a current-voltage characteristic
and a characteristic of current-power density on average of samples
No. 1 and No. 2 and a two-layered stack obtained by stacking
samples No. 1 and No. 2.
[0103] FIG. 64 is a graph showing a current-voltage characteristic
and a current-power density characteristic of samples No. 1, No. 2
and No. 3 and a three-layered stack obtained by stacking samples
No. 1, No. 2 and No. 3.
[0104] FIG. 65 is a perspective view showing a structure of a fuel
cell stack fabricated in Comparative Example 1.
[0105] FIG. 66 is a graph showing a current-voltage characteristic
and a current-power density characteristic of the fuel cell stack
in Comparative Example 1.
[0106] FIG. 67 is a graph showing results of continuous power
generation tests for a two-layered stack fabricated in Example 7, a
two-layered stack fabricated in Example 9, and a two-layered stack
fabricated in Comparative Example 1 (stacks having cell intervals d
of 2 mm and 3 mm, respectively).
[0107] FIG. 68 is a graph and a table showing power generation
characteristics of a sample No. 4 (one-layered stack), a
two-layered stack, and a three-layered stack fabricated in Example
10.
[0108] FIG. 69 is a graph showing continuous power generation
characteristics of a fuel cell stack fabricated in Example 11, at
an output current density of 100 mA/cm.sup.2 and at temperatures of
25.degree. C. and 40.degree. C.
[0109] FIG. 70 is a schematic diagram showing an example of an
orientation of an anode electrode and a cathode electrode of the
unit cell in the fuel cell stack according to the present
invention.
[0110] FIG. 71 is a schematic diagram showing another example of an
orientation of an anode electrode and a cathode electrode of the
unit cell in the fuel cell stack according to the present
invention.
[0111] FIG. 72 is a schematic diagram showing an example of an
orientation of an anode electrode and a cathode electrode of the
unit cell in the fuel cell stack including a spacer layer according
to the present invention.
DESCRIPTION OF THE REFERENCE SIGNS
[0112] 100, 300 fuel cell stack; 101, 301, 501, 701, 801, 901 first
fuel cell layer; 102, 302, 502, 702, 802, 902 second fuel cell
layer, 103, 303, 401, 601, 703, 803, 903, 1101, 1601, 1701, 1901,
4101, 4201, 4301, 4401, 4501, 4701 unit cell; 104a, 104b gap
region; 201, 403, 507, 604, 706, 806, 907, 1403, 1503, 2001, 2201,
2301, 2401, 2501, 2601, 2701, 2801, 4001, 4305, 4601 electrolyte
membrane; 202, 402, 508, 603, 705, 805, 906, 1402, 1502, 2002,
2202, 2302, 2402, 2502, 2602, 2702, 2802 anode catalyst layer; 203,
404, 506, 605, 707, 807, 908, 1404, 1504, 2003, 2203, 2303, 2403,
2503, 2603, 2703, 2803 cathode catalyst layer; 204, 509, 602, 704,
804, 905, 1401, 1501, 2004, 2204, 2304, 2404, 2504, 2604, 2704,
2804 anode conductive porous layer; 205, 505, 606, 708, 808, 909,
1405, 1505, 1506a, 1506b, 2005, 2205, 2305, 2405, 2505, 2605, 2705,
2805 cathode conductive porous layer; 206, 910, 1406, 2006, 2306,
2406, 2506, 2606, 2706, 2806 fuel flow channel; 503 first unit
cell; 904 separator; 1702 branch portion; 2008 cathode current
collector; 2009 anode current collector; 208, 2206, 2407, 2507,
2607, 2809 hydrophilic layer; 210, 2207 gas-liquid separation
layer, 207, 1407, 2007, 2208, 2307, 2408, 2508, 2608, 2708, 2810
through hole; 209, 2308, 2409, 2509, 2707, 2807, 2808 fuel
permeability control layer; 2410 bracket-shaped flow channel, 2709
reinforcement member; 2811 bonding layer; 809 insulating layer;
4002, 4602 substrate, 4102, 4202, 4302, 4402, 4502, 4702 fuel cell
layer; 4103, 4203, 4303, 4403, 4503a, 4503b, 4603 spacer; 4104,
4204, 4304, 4404, 4504, 4704 spacer layer; 4703a spacer formed of
porous body; and 4703b spacer formed of non-porous body.
BEST MODES FOR CARRYING OUT THE INVENTION
[0113] <<Fuel Cell Stack>>
[0114] A fuel cell stack according to the present invention will be
described hereinafter in detail with reference to embodiments. The
embodiments shown below are all directed to a direct methanol fuel
cell (hereinafter also referred to as DMFC) generating electric
power by directly extracting protons from methanol, in which a
methanol aqueous solution is adopted as the fuel while air
(specifically, oxygen in the air) is adopted as the oxidizing
agent. The DMFC is advantageous in that (1) it does not require a
reformer and (2) a fuel container can be reduced in size as
compared with a canister of a high-pressure gas represented by
hydrogen, because it employs liquid methanol higher in volume
energy density than a gaseous fuel. Therefore, the DMFC is suitably
applicable as an alternative to a power supply for small-sized
equipment, in particular, a secondary battery for portable
equipment. In addition, the DMFC is also advantageous in that, as
the fuel is liquid, a narrow curved space portion that has been a
dead space in the conventional fuel cell system can be used as a
fuel space and the DMFC is less likely to be restricted in terms of
design. The DMFC is preferably applicable to portable, small-sized
electronic devices also owing to such advantages.
[0115] In general, in the DMFC, the following reaction occurs at an
anode electrode and a cathode electrode. Carbon dioxide is
generated as a gas on the anode electrode side and water is
generated on the cathode electrode side.
Anode electrode:
CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.-
Cathode electrode: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
Embodiment 1
[0116] FIG. 1 is a schematic diagram showing a preferred example of
a fuel cell stack according to the present invention, FIG. 1(a)
being a perspective view thereof, FIG. 1(b) being a top view and
FIG. 1(c) being a side view. A fuel cell stack 100 in FIG. 1 is
formed by stacking a first fuel cell layer 101 and a second fuel
cell layer 102, and each layer is constituted of five fuel cell
unit cells (hereinafter simply referred to as a unit cell) 103. In
both of these fuel cell layers, unit cell 103 is arranged at a
distance from an adjacent unit cell, and first fuel cell layer 101
has a gap region 104a and second fuel cell layer 102 has a gap
region 104b. In the present embodiment, gap region 104a and gap
region 104b each have a parallelepiped shape.
[0117] Here, the "unit cell" is herein defined as one unit
constituting the fuel cell stack and as a structure including an
MEA (Membrane Electrode Assembly) and other constituent members
added as necessary to the MBA for providing a power generation
function or for other purposes. Other constituent members are not
particularly limited, and they include, for example, a fuel flow
channel for supplying the fuel, an air flow channel for supplying
air, a separator, an anode current collector, a cathode current
collector, and the like. In addition, the "MEA" is defined as an
assembly including at least an electrolyte membrane and an anode
catalyst layer and a cathode catalyst layer sandwiching the
electrolyte membrane, and it herein encompasses also an example in
which an anode conductive porous layer is provided on the anode
catalyst layer and a cathode conductive porous layer is provided on
the cathode catalyst layer. Moreover, the "separator" herein refers
to a constituent member of a unit cell playing a role for
separation of the fuel and the oxidizing agent (such as air).
[0118] In the present embodiment, each fuel cell layer has a shape
elongated in a longitudinal direction within the fuel cell layer,
and each unit cell 103 constituting the fuel cell layer also has a
shape elongated in the longitudinal direction (the unit cell having
such a shape may hereinafter be referred to as a unit cell in a
"shape of an elongated strip"), more specifically, a parallelepiped
shape. Unit cells 103 in an identical layer are identical in their
direction of elongation. Namely, unit cells 103 within the
identical layer are arranged in parallel at regular intervals.
Second fuel cell layer 102 is stacked on first fuel cell layer 101
such that the longitudinal direction (direction of elongation) of
the unit cells constituting the second fuel cell layer is
orthogonal to the longitudinal direction (direction of elongation)
of the unit cells constituting first fuel cell layer 101. According
to such a configuration, a part of an upper side of gap region 104a
in first fuel cell layer 101 comes in contact with the unit cell
constituting second fuel cell layer 102. When the unit cell has a
longitudinal direction, a plurality of unit cells can be stacked
over the gap region in a stable manner and hence a stack structure
of the fuel cell layers can readily be constructed. In addition, by
employing a unit cell in a shape of a planar elongated strip, a
contact area between the unit cells as stacked is larger than in
employing a unit cell in a shape of an elongated strip having
surface irregularities or in a cylindrical shape. Therefore,
physical strength of the obtained fuel cell stack is improved.
Moreover, as the contact area between the unit cells is greater,
electrical contact resistance can be low in stacking with
electrically serial connection. Further, by employing the planar
unit cell, a surface area per volume can be larger than a columnar
unit cell. As electrically serial connection along a direction of
stack can be made while suppressing electrical contact resistance
to be low, the amount of a current that flows in the in-plane
direction of the fuel cell layer can be lowered. Accordingly, a
thickness of a current collector can be decreased or a current
collector can be eliminated, and high integration of the fuel cell
stack can be achieved. Consequently, smaller size, lighter weight
and lower cost of the fuel cell stack can be achieved.
[0119] In addition, in the present embodiment, as stacking is
carried out such that the longitudinal direction (direction of
elongation) of the unit cells constituting second fuel cell layer
102 is orthogonal to the longitudinal direction (direction of
elongation) of the unit cells constituting first fuel cell layer
101, gap region 104a and gap region 104b each having a
parallelepiped shape are similarly aligned to be orthogonal to each
other when viewed in a direction of stack of the fuel cell layers
and they are formed in different portions except for an
intersecting region (a region where gap region 104a and gap region
104b intersect with each other) (see FIG. 1(b)).
[0120] By aligning the unit cells to be orthogonal to each other,
an area of the intersecting region of the unit cells is small and a
distance of diffusion of air to the inside of the intersecting
region is short, so that supply of air to the intersecting region
is satisfactory. By making shorter a length of the unit cell in a
direction of a short side as well, the area of the intersecting
region can be made further smaller. By making shorter the length in
the direction of a short side, a distance of convection of oxygen
in the in-plane direction of the unit cell on the cathode side is
short, so that convection supply of oxygen can be less likely to be
disturbed even when air is naturally supplied. Consequently, air
can more efficiently be supplied toward the cathode of the unit
cell within the fuel cell stack in which the fuel cell layers are
three-dimensionally stacked, without the use of auxiliary equipment
for supplying the air.
[0121] In fuel cell stack 100 in FIG. 1, air serving as the
oxidizing agent can be supplied either by opening at least a part
of a surface of the fuel cell stack to atmosphere or by using
auxiliary equipment such as an air pump or a fan. Here, the
supplied air passes through gap regions 104a and 104b provided
between the unit cells and spreads through the inside of fuel cell
stack 100, and it is supplied to the cathode catalyst layer of each
unit cell. Here, the unit cells constituting second fuel cell layer
102 are arranged over a part of the upper side of gap region 104a
in first fuel cell layer 101. Then, an area of contact between the
air present in the gap region and each unit cell increases. Thus,
air supply to the cathode catalyst layer of the unit cell is
significantly improved.
[0122] In fuel cell stack 100, the gap region is
three-dimensionally formed and in addition, all gap regions (four
gap regions 104a and four gap regions 104b) communicate with one
another. Since such a configuration achieves higher porosity of the
fuel cell stack and thus better air permeability, convection of the
air is improved. Namely, according to such a configuration, the air
introduced in the fuel cell stack can be supplied through the
communicating gap regions into fuel cell layer 100 through natural
convection or diffusion. In addition, natural convection of the air
within the fuel cell stack is also satisfactory. The air within the
fuel cell stack warmed by heat originating from power generation is
emitted to the outside through the communicating gap regions as a
result of convection and the air is efficiently taken in through a
side surface or a lower surface of the fuel cell stack. Therefore,
an air convection rate is improved and such auxiliary equipment as
an air pump or a fan for air supply is not necessarily required,
which enables size reduction of a fuel cell system including the
fuel cell stack. In addition, as air convection resistance
decreases, power generation characteristics of the fuel cell can be
improved. Alternatively, even in the case of using auxiliary
equipment such as the air pump or the fan, wind power necessary for
supplying the air to the inside of the fuel cell stack can be
reduced, which enables lower power consumption or a smaller size of
the auxiliary equipment.
[0123] Further, according to such a configuration, a degree of
freedom in a manner of use of the fuel cell stack can significantly
be improved. For example, in whichever direction the fuel cell
stack may be inclined for use, warmed air can be exhausted from an
upper portion and air can be taken in from the side and the bottom,
so that convection of the air can satisfactorily be maintained. In
addition, even when the fuel cell stack is mounted on equipment and
for example the upper surface and the lower surface of the fuel
cell stack are covered, the air can be exhausted or taken in from
the side surface. In particular, as a polymer electrolyte fuel cell
(PEFC) or a direct methanol fuel cell (DMFC) can operate at a low
temperature around room temperature, for example, such a package as
a heat insulating structure necessary for a fuel cell operating at
a high temperature such as a solid oxide fuel cell (SOFC) is not
required. Therefore, in the case of a polymer electrolyte fuel cell
(PEFC) or a direct methanol fuel cell (DMFC) in particular, as the
air in atmosphere can be taken into the fuel cell stack
three-dimensionally from all directions, the need for auxiliary
equipment for supplying the air can be obviated. Therefore, the
fuel cell stack according to the present invention is particularly
preferably implemented by a fuel cell capable of operating at a low
temperature around a room temperature such as a polymer electrolyte
fuel cell (PEFC) or a direct methanol fuel cell (DMFC). In
addition, better air permeability in the fuel cell stack and a
greater surface area of the fuel cell stack improve also heat
radiation property of the fuel cell stack, and temperature increase
can be suppressed even when the fuel cell stack is highly
integrated. Therefore, according to the fuel cell stack of the
present invention, deterioration of power generation
characteristics of the fuel cell stack due to heat can be
suppressed and influence on the equipment by the heat originating
from power generation can be less even when the fuel cell stack is
mounted on the equipment or the like.
[0124] In fuel cell stack 100, unit cells 103 are equal in height,
that is, the fuel cell layers are equal in thickness. As the unit
cells in the fuel cell layer are thus identical in height, contact
between upper and lower unit cells is satisfactory and poor contact
can be lessened even when a unit cell is stacked over a plurality
of unit cells.
[0125] In addition, in fuel cell stack 100, the unit cells are
arranged in a single fuel cell layer at regular intervals (that is,
a width of the gap region is equal). Here, regular intervals refer
to such a condition that an error of all gap intervals is within a
range of .+-.0.25 mm. A plurality of gap regions are equal in
cross-sectional area in a surface perpendicular to the direction of
stack of the fuel cell layers. As described above, as unit cells
103 in the same layer are arranged in parallel, the plurality of
gap regions 104a are identical in area and unit cells 103 are also
equal in height. Therefore, the plurality of gap regions 104a has
the same space volume. This is also the case with gap region 104b.
In fuel cell stack 100, a current flows electrically serially in a
direction of stack, through portions where the unit cells intersect
and come in contact with each other. On the other hand, a current
in a portion where the unit cells do not intersect with each other
once flows in the in-plane direction of the fuel cell layer to the
closest portion where the unit cells intersect and come in contact
with each other. Thereafter, when the current reaches the portion
where the unit cells intersect and come in contact with each other,
it flows in the direction of stack. As the gap regions are arranged
at regular intervals, a distance of flow through the fuel cell
layer in the in-plane direction becomes uniform, and hence local
heat generation due to resistance loss does not take place. In
addition, local overvoltage is less likely. As deterioration in
output characteristics of the fuel cell due to local heat and
deterioration in output characteristics due to deterioration of a
catalyst metal as a result of application of a potential to such an
extent as elution of a catalyst metal due to overvoltage thus do
not occur, the fuel cell stack can have longer life.
[0126] The interval between the unit cells arranged in each fuel
cell layer is not particularly limited so long as the interval is
sufficient for the air to diffuse or pass as a result of natural
convection, however, the interval is set preferably in a range from
0.001 cm to 1 cm and more preferably in a range from 0.05 cm to 0.2
cm.
[0127] In fuel cell stack 100, unit cells 103 are all identical in
shape. Therefore, the fuel cell stack in the present embodiment
includes, as second fuel cell layer 102, a fuel cell layer
identical in shape to first fuel cell layer 101, and it can be said
that second fuel cell layer 102 is stacked as rotated by 90.degree.
with respect to first fuel cell layer 101. Here, the fuel cell
layers identical in shape mean that error of an outer dimension of
all unit cells is within a range of .+-.0.25 mm and arrangement and
the shape of the unit cells coincide by inversion or rotation. As
the unit cells are all identical in shape, variation in
characteristics due to the difference in shape of the unit cell can
be suppressed. In addition, as the unit cells can be manufactured
in a singe manufacturing process, manufacturing cost can be
reduced. The unit cells constituting one fuel cell layer intersect
with the unit cells constituting the other fuel cell layer by
stacking two fuel cell layers at different angles as in the present
embodiment, so that three-dimensionally communicating gap regions
can readily be formed.
[0128] It is noted herein that a fuel cell stack having such a
structure that a plurality of unit cells in a shape of an elongated
strip as shown in FIG. 1 are stacked to form a number sign shape in
multiple layers may be referred to as a "number-sign-shaped fuel
cell stack."
[0129] Here, the fuel cell stack in the present embodiment may be
modified, for example, as follows. Initially, the plurality of unit
cells 103 constituting first fuel cell layer 101 do not necessarily
have to be identical in shape (in a length in the longitudinal
direction, a width or a height). This is also the case with second
fuel cell layer 102. So long as at least two unit cells are
identical in shape, an effect the same as described above can be
obtained in that region. In addition, the number of unit cells
within one fuel cell layer is not particularly limited. At least
one fuel cell layer should only have two or more unit cells and
thus at least one gap region. If there are a plurality of gap
regions, a width thereof (a distance between the unit cells) may be
different. A distance between the unit cells within first fuel cell
layer 101 may be different from a distance between the unit cells
in second fuel cell layer 102. Moreover, it is not necessary to
arrange the plurality of unit cells in parallel within one fuel
cell layer.
[0130] First fuel cell layer 101 and second fuel cell layer 102 do
not necessarily have to be identical in shape. In addition, an
angle between the longitudinal direction (direction of elongation)
of the unit cells constituting the second fuel cell layer and the
longitudinal direction (direction of elongation) of the unit cells
constituting first fuel cell layer 101 does not have to be set to
90.degree., and any angle may be set. In an example where two fuel
cell layers identical in shape are stacked to obtain a fuel cell
stack as in the present embodiment as well, an angle of the upper
fuel cell layer with respect to the lower fuel cell layer is not
particularly limited.
[0131] In addition, the fuel used for the fuel cell stack according
to the present invention is not limited to the methanol aqueous
solution, and other fuels containing hydrogen atoms in their
molecular structure may be employed. The fuel refers to a reducing
agent supplied to the anode catalyst layer. Specifically, examples
of the reducing agent include: gaseous fuels such as hydrogen, DME
(dimethyl ether), methane, butane, and ammonia; alcohols such as
methanol and ethanol, acetals such as dimethoxymethane, and
carboxylic acids such as formic acid; esters such as methyl
formate, hydrazine, or liquid fuels such as sulfurous acid,
bisulfite, thiosulfate, dithionite, hypophosphorous acid, and
phosphorous acid; and a substance obtained by dissolving a solid
fuel such as ascorbic acid in water. The fuel is not limited a fuel
consisting of one type, and a mixture of two or more types may be
employed. A methanol aqueous solution is preferably employed from a
point of view of low fuel cost, high energy density per volume,
high power generation efficiency (low overvoltage), and the like.
Examples of the oxidizing agent used for the fuel cell stack
according to the present invention include oxygen, hydrogen
peroxide and nitric acid, however, oxygen in air is more preferably
used from a point of view of cost for the oxidizing agent and the
like.
[0132] An internal structure of unit cells 103 constituting first
fuel cell layer 101 and second fuel cell layer 102 will now be
described. FIG. 2 is a schematic cross-sectional view showing a
preferred examples of the unit cell employed in the present
invention. It is noted that other preferred examples of the unit
cell will be described later. The unit cell shown in FIG. 2 is
constituted of an electrolyte membrane 201, an anode catalyst layer
202 arranged on one surface of electrolyte membrane 201, a cathode
catalyst layer 203 arranged on the other surface of electrolyte
membrane 201, a hydrophilic layer 208 arranged in contact with a
surface of anode catalyst layer 202 opposite to its surface opposed
to electrolyte membrane 201, an anode conductive porous layer 204
arranged in contact with a surface of hydrophilic layer 208
opposite to its surface opposed to anode catalyst layer 202, a
cathode conductive porous layer 205 arranged in contact with a
surface of cathode catalyst layer 203 opposite to its surface
opposed to electrolyte membrane 201, a fuel flow channel 206 which
is a space for fuel transportation formed within anode conductive
porous layer 204, a fuel permeability control layer 209 covering
one surface of fuel flow channel 206, and a gas-liquid separation
layer 210 in contact with a surface of anode conductive porous
layer 204 opposite to its surface in contact with hydrophilic layer
208. Fuel permeability control layer 209 is arranged in contact
with hydrophilic layer 208. In addition, anode conductive porous
layer 204 is formed by forming in an anode conductive plate, a
number of through holes 207 penetrating in a direction of layer
thickness thereof.
[0133] The methanol aqueous solution used as a fuel is supplied to
the entire surface of a power generation portion of the unit cell
through fuel flow channel 206, supplied to hydrophilic layer 208
through fuel permeability control layer 209, and thereafter held in
hydrophilic layer 208 and uniformly supplied to anode catalyst
layer 202. Hydrophilic layer 208 is preferably an electrically
conductive layer having a hydrophilic surface and formed of a
porous body. Thus, generated carbon dioxide passes through a hole
within hydrophilic layer 208 and then successively through through
hole 207 in anode conductive porous layer 204 and gas-liquid
separation layer 210, and it is exhausted to the outside. As carbon
dioxide is quickly exhausted through through hole 207 in the
vicinity of anode catalyst layer 202, peel-off of the electrolyte
membrane or the catalyst layer or change in shape thereof due to
increased pressure of carbon dioxide in the unit cell does not
occur and supply of the fuel is not disturbed either.
[0134] When the fuel cell stack structure according to the present
invention is applied, carbon dioxide exhausted through through hole
207 in anode conductive porous layer 204 is exhausted either into
cathode conductive porous layer 205 of an adjacent unit cell or
into the gap region. Carbon dioxide exhausted into cathode
conductive porous layer 205 diffuses in the in-plane direction,
reaches the gap region, and it is exhausted through the gap region
to the outside of the fuel cell stack as air diffuses. Thus, carbon
dioxide quickly exhausted to the outside of the unit cell can
quickly be exhausted through the gap region in the fuel cell stack
to the outside of the fuel cell stack as air diffuses.
[0135] As carbon dioxide can be exhausted through through hole 207
provided in anode conductive porous layer 204 and penetrating in
the direction of thickness, it is not necessary to form a carbon
dioxide exhaust path in the anode conductive plate in the unit cell
shown in FIG. 2. In order to reliably and quickly exhaust carbon
dioxide, a cross-sectional area of an exhaust flow channel should
be secured. In the present embodiment, however, it is not necessary
to do so and the anode conductive plate can be made thinner.
Therefore, the unit cell can be made thinner and the fuel cell
stack obtained by stacking the unit cells can achieve higher power
density.
[0136] In addition, as the methanol aqueous solution is supplied
through fuel permeability control layer 209, entry of carbon
dioxide into fuel flow channel 206, which disturbs supply of the
methanol aqueous solution, does not occur. Therefore, as it is not
necessary to use auxiliary equipment such as a pump to cause the
methanol aqueous solution to flow through fuel flow channel 206 at
a prescribed flow rate or higher in order to exhaust carbon
dioxide, the need for the pump can be obviated. Moreover,
conventionally, in order to reduce pressure loss in supply with a
pump, a cross-sectional area of the fuel flow channel has had to be
great. By providing fuel permeability control layer 209, however,
the need for the pump can be obviated, and therefore such a
conventional problem can be solved. Further, as the cross-sectional
area of fuel flow channel 206 can be made smaller, the methanol
aqueous solution can be supplied by capillarity to all corners of
the power generation portion. The unit cell can thus be made
thinner and the need for the auxiliary equipment can be obviated,
so that higher power density of the fuel cell can be achieved.
[0137] Gas-liquid separation layer 210 is preferably made of a
porous body including a number of pores having such a pore diameter
as allowing passage of a gas but not allowing passage of a liquid
up to a prescribed pressure. Thus, such a structure that carbon
dioxide can be exhausted but the methanol aqueous solution does not
leak to the outside can be obtained. It is noted that through hole
207 may be filled with the gas-liquid separation layer.
[0138] Hydrophilic layer 208 not only plays a role of holding the
fuel and uniformly supplying the fuel to anode catalyst layer 202,
but also plays a role of preventing lowering in output from the
fuel cell as oxygen introduced from through hole 207 reaches anode
catalyst layer 202, because the pores in the hydrophilic layer are
filled with a liquid (methanol aqueous solution).
[0139] The air is supplied from atmosphere through cathode
conductive porous layer 205 to cathode catalyst layer 203. More
specifically, the air supplied to the fuel cell stack is supplied
through the gap region provided between the unit cells to the
inside of the fuel cell stack and supplied through holes in cathode
conductive porous layer 205 to cathode catalyst layer 203.
[0140] According to the structure of the unit cell as described
above, a thin unit cell having a thickness not greater than about 1
mm can be fabricated. In the fuel cell stack according to the
present embodiment, in order to minimize use of auxiliary
equipment, preferably, a length of a short side of the unit cell is
set to 1 to 3 mm, a thickness thereof is set to 0.5 to 1 mm, a
length of a long side thereof is set to 30 to 100 mm, a gap
interval (a width of the gap region) is set to 0.2 to 2 mm, and the
number of stacked fuel cell layers is set to 4 to 8 Thus, a fuel
cell stack achieving high power density and small size as described
above can be provided.
[0141] Anode catalyst layer 202 contains a catalyst promoting
oxidation of the fuel. As a result of oxidation reaction of the
fuel on the catalyst, protons and electrons are generated. In
addition, cathode catalyst layer 203 contains a catalyst promoting
reduction of the oxidizing agent. The oxidizing agent takes in
protons and electrons on the catalyst and reduction reaction
occurs.
[0142] For example, a layer including a catalyst-supporting carrier
and an electrolyte is used as anode catalyst layer 202 and cathode
catalyst layer 203 described above. Here, an anode catalyst in
anode catalyst layer 202 has a function, for example, to accelerate
a reaction speed at which protons and electrons are generated from
methanol and water, the electrolyte has a function to transmit the
generated protons to electrolyte membrane 201, and the anode
carrier has a function to conduct the generated electrons to anode
conductive porous layer 204. On the other hand, a cathode catalyst
in cathode catalyst layer 203 has a function to accelerate a
reaction speed at which water is generated from oxygen, protons and
electrons, the electrolyte has a function to conduct protons from
electrolyte membrane 201 to the vicinity of the cathode catalyst,
and the cathode carrier has a function to conduct the electrons
from cathode conductive porous layer 205 to the cathode catalyst.
Though the anode carrier and the cathode carrier have a function to
conduct electrons, the catalyst also has electron conductivity.
Accordingly, it is not necessarily required to provide a carrier.
In that case, supply and reception of electrons to/from anode
conductive porous layer 204 or cathode conductive porous layer 205
is carried out by the anode catalyst and the cathode catalyst,
respectively.
[0143] Examples of the anode catalyst and the cathode catalyst
include a noble metal such as Pt, Ru, Au, Ag, Rh, Pd, Os, and Ir, a
base metal such as Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W, and Zr, an
oxide, a carbide and a carbonitride of these nobles metal and base
metal, and carbon. A single material or combination of two or more
types of these materials can be used for the catalyst. The anode
catalyst and the cathode catalyst are not necessarily limited to a
catalyst of the same type, and different substances may be used
therefor.
[0144] An electrolyte used for anode catalyst layer 202 and cathode
catalyst layer 203 is not particularly limited so long as the
electrolyte has proton conductivity and electrically insulating
property, however, a solid or a gel not dissolved by methanol is
preferred. Specifically, an organic polymer having strong acid
group such as sulfonic acid group and phosphoric acid group or weak
acid group such as carboxyl group is preferred. Examples of such
organic polymers include sulfonic acid group containing
perfluorocarbon (NAFION.RTM. manufactured by Du Pont), carboxyl
group containing perfluorocarbon (Flemion.RTM. manufactured by
Asahi Kasei Corporation), a polystyrene sulfonic acid copolymer, a
polyvinyl sulfonic acid copolymer, an ionic liquid (ordinary
temperature molten salt), sulfonated imide,
2-acrylamide-2-methylpropane sulfonic acid (AMPS), and the like. In
addition, in an example where a carrier provided with proton
conductivity which will be described later is used, the carrier
conducts protons and the electrolyte is not necessarily
required.
[0145] The carrier used in anode catalyst layer 202 and cathode
catalyst layer 203 is preferably made of a carbon-based material
having high electrical conductivity, and examples of such materials
include acetylene black, Ketjen Black.RTM., amorphous carbon,
carbon nanotube, carbon nanohorn, and the like. The examples
include not only the carbon-based materials but also a noble metal
such as Pt, Ru, Au, Ag, Rh, Pd, Os, and Ir, a base metal such as
Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W, and Zr, and an oxide, a
carbide, a nitride, and a carbonitride of these noble metal and
base metal. A single material or combination of two or more types
of these materials can be used for the carrier. In addition, a
material provided with proton conductivity, specifically, sulfated
zirconia, zirconium phosphate and the like, may be used for the
carrier.
[0146] A thickness of each of anode catalyst layer 202 and cathode
catalyst layer 203 is preferably set to 0.5 mm or smaller, in order
to lower resistance in proton conduction and resistance in electron
conduction and in order to reduce diffusion resistance of the fuel
(such as methanol) or the oxidizing agent (such as oxygen). In
addition, as a catalyst sufficient for improving output as a cell
should be carried, the thickness is preferably set to 0.1 .mu.m or
greater.
[0147] Electrolyte membrane 201 is not particularly limited so long
as a material having proton conductivity and electrically
insulating property is employed, however, electrolyte membrane 201
is preferably formed as appropriate of a polymer membrane, an
inorganic membrane or a composite membrane that has conventionally
been known.
[0148] Examples of polymer membranes include a
perfluorosulfonic-acid-based electrolyte membrane (NAFION
(manufactured by Du Pont), a Dow membrane (manufactured by The Dow
Chemical Company), ACIPLEX.RTM. (manufactured by Asahi Kasei
Corporation), and Flemion (manufactured by Asahi Glass Co., Ltd.))
and the like, as well as a hydrocarbon-based electrolyte membrane
composed, for example, of polystyrene sulfonic acid, sulfonated
polyether ether ketone and the like. The inorganic membrane is
composed, for example, of phosphoric acid glass, cesium hydrogen
sulfate, polytungstophosphoric acid, ammonium polyphosphate, and
the like. In addition, examples of composite membranes include a
Gore-Select.RTM. membrane (GORE-SELECT.RTM. manufactured by W. L.
Gore & Associates).
[0149] When the temperature of the fuel cell stack attains to a
temperature around 100.degree. C. or higher, sulfonated polyimide,
2-acrylamide-2-methylpropane sulfonic acid (AMPS), sulfonated
polybenzimidazole, phosphonated polybenzimidazole, cesium hydrogen
sulfate, ammonium polyphosphate, an ionic liquid (ordinary
temperature molten salt), or the like, that has high ion
conductivity even though water content is low, is used as a
material for the electrolyte membrane and such a material is
preferably used in a form of a membrane. The electrolyte membrane
preferably has proton conductivity of 10.sup.-5 S/cm or higher, and
use of a polymer electrolyte membrane having proton conductivity of
10.sup.-3 S/cm or higher, such as a perfluorosulfonic acid polymer
or a hydrocarbon-based polymer, is further preferred.
[0150] Anode conductive porous layer 204 has a function to supply
and receive electrons to/from anode catalyst layer 202. A carbon
material, an electrically conductive polymer, a noble metal such as
Au, Pt and Pd, a metal such as Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu,
Zn, and Su, Si, a nitride, a carbide, a carbonitride, or the like
thereof, an alloy such as stainless, Cu--Cr, Ni--Cr and Ti--Pt, and
the like are preferably used as a material for the anode conductive
plate forming anode conductive porous layer 204, and at least one
element selected from the group consisting of Pt, Ti, Au, Ag, Cu,
Ni, and W is further preferably contained. In addition, when a
metal poor in corrosion resistance under acid atmosphere such as
Cu, Ag and Zn is used, a noble metal such as Au, Pt and Pd and a
metal material having corrosion resistance, an electrically
conductive polymer, an electrically conductive nitride, an
electrically conductive carbide, an electrically conductive
carbonitride, an electrically conductive oxide, and the like may be
used as a surface coating. Thus, the fuel cell can have longer
life. By containing the element above, specific resistance of anode
conductive porous layer 204 becomes lower. Therefore, voltage
lowering due to the resistance of anode conductive porous layer 204
can be lessened and higher power generation characteristics can be
obtained.
[0151] As described above, anode conductive porous layer 204 shown
in FIG. 2 is formed by forming in the anode conductive plate,
through holes 207 penetrating in a direction of layer thickness.
Thus, efficiency in exhausting carbon dioxide generated in anode
catalyst layer 202 can be improved. Here, "penetrating" means
passage from one surface to the opposite surface. A plurality of
through holes 207 more preferably communicate with one another
Examples of the anode conductive plate (the anode conductive porous
layer) having holes penetrating in the direction of layer thickness
include a porous metal layer obtained by providing a plurality of
holes in a plate or a foil, a porous metal layer like a mesh or an
expanded metal, and the like. In addition, examples of materials
having a plurality of through holes communicating with one another
include a foam metal, a metal web, a sintered metal, carbon paper,
and a carbon cloth.
[0152] Cathode conductive porous layer 205 has a function to supply
and receive electrons to/from cathode catalyst layer 203, and has
holes communicating between the outside of the unit cell and
cathode catalyst layer 203. In general, during power generation by
the unit cell, cathode conductive porous layer 205 is maintained at
a potential higher than anode conductive porous layer 204.
Accordingly, a material for cathode conductive porous layer 205
preferably has corrosion resistance as high as or higher than anode
conductive porous layer 204.
[0153] Cathode conductive porous layer 205 may be made of a
material similar to that for anode conductive porous layer 204. In
particular, however, for example, a carbon material, an
electrically conductive polymer, a noble metal such as Au, Pt and
Pd, a metal such as Ti, Ta, W, Nb, and Cr, a nitride, a carbide or
the like thereof, an alloy such as stainless, Cu--Cr, Ni--Cr and
Ti--Pt, and the like are preferably used. In addition, when a metal
poor in corrosion resistance under acid atmosphere such as Cu, Ag,
Zn, and Ni is used, a noble metal and a metal material having
corrosion resistance, an electrically conductive polymer, an
electrically conductive oxide, an electrically conductive nitride,
an electrically conductive carbide or the like, and an electrically
conductive carbonitride may be used as a surface coating.
[0154] A structure of cathode conductive porous layer 205 is not
particularly limited so long as it has communicating holes allowing
supply of oxygen in the air around the fuel cell stack to cathode
catalyst layer 203. In order to supply oxygen to the cathode
catalyst layer located at a portion of contact between the stacked
fuel cell layers, however, holes communicating in a direction of
stack and a direction perpendicular to the direction of stack are
preferably provided in cathode conductive porous layer 205.
Examples of such materials include a foam metal, a metal web, a
sintered metal, carbon paper, a carbon cloth, and the like. The
direct methanol fuel cell stack according to the present invention
preferably has such a structure that, in a region where adjacent
fuel cell layers are stacked and overlie each other, air enters
also in a direction of a cross-section in the direction of layer
thickness of cathode conductive porous layer 205 and reaches
cathode catalyst layer 203.
[0155] Cathode conductive porous layer 205 has porosity preferably
of 30% or higher for lowering diffusion resistance of oxygen and
preferably of 95% or lower for lowering electrical resistance, and
more preferably has porosity from 50% to 85%. Cathode conductive
porous layer 205 has a thickness preferably not smaller than 10
.mu.m for lowering diffusion resistance of oxygen in a direction
perpendicular to the direction of stack of cathode conductive
porous layer 205 and preferably not greater than 1 mm for lowering
diffusion resistance of oxygen in the direction of stack of cathode
conductive porous layer 205, and more preferably a thickness from
100 .mu.m to 500 .mu.m.
[0156] As shown in FIG. 2, the unit cell used in the present
invention preferably includes not only the cathode conductive
porous layer, the cathode catalyst layer, the electrolyte membrane,
the anode catalyst layer, and the anode conductive porous layer,
but also gas-liquid separation layer 210 provided in order to
prevent fuel leakage and to exhaust a reaction product generated at
the anode and fuel permeability control layer 209 provided in order
to restrict a permeate flux of methanol to the catalyst layer. The
gas-liquid separation layer and the fuel permeability control layer
will be described in detail.
[0157] The gas-liquid separation layer is provided in order to
suppress leakage of methanol through the through holes in the anode
conductive porous layer to the outside of the unit cell, and it is
mainly a layer having impermeability of liquids such as water or a
methanol aqueous solution but having gas permeability. In addition,
according to the present invention, the gas-liquid separation layer
is preferably provided also with electrical conductivity.
Specifically, a mixture of a material having a property to separate
between a gas and a liquid and an electrically conductive material
may be employed. Examples of such materials include a porous layer
composed of a mixture of a fluorine-based polymer represented by
PTFE (polytetrafluoroethylene) and PVDF (polyvinylidenfluoride) and
acetylene black, Ketjen Black, amorphous carbon, carbon nanotube,
carbon nanohorn, and the like (hereinafter, the gas-liquid
separation layer provided with electrical conductivity will simply
be referred to as the gas-liquid separation layer). In order to
prevent leakage of methanol, the gas-liquid separation layer is
preferably provided on the surface of the anode conductive porous
layer.
[0158] The fuel permeability control layer suitably provided for
the fuel flow channel preferably has fuel diffusion resistance in
the direction of thickness thereof and has a function to suppress a
permeate flux of the fuel, and it is more preferably made of a gas
impermeable material. The shape of the fuel permeability control
layer is not particularly limited so long as the functions
described previously are achieved, and it may have a fuel permeable
function by providing small holes penetrating in the direction of
thickness thereof. In an example where the methanol aqueous
solution is adopted as the fuel, the fuel permeability control
layer is preferably formed of a polymer membrane, an inorganic
membrane or a composite membrane. Examples of polymer membranes
include silicon rubber, a perfluorosulfonic-acid-based electrolyte
membrane (NAFION (manufactured by Du Pont), a Dow membrane
(manufactured by The Dow Chemical Company), ACIPLEX (manufactured
by Asahi Kasei Corporation), and Flemion (manufactured by Asahi
Glass Co., Ltd.)), as well as a hydrocarbon-based electrolyte
membrane composed, for example, of sulfonated polyimide,
polystyrene sulfonic acid, sulfonated polyether ether ketone, and
the like. The inorganic membrane is composed, for example, of
porous glass, porous zirconia, porous alumina, and the like.
Examples of the composite membrane include a Gore-Select membrane
(manufactured by W. L. Gore & Associates). In addition,
examples of the fuel permeability control layer having penetrating
small holes include a fuel permeability control layer fabricated
with a photosensitive resin. In an example where a photosensitive
resin polymerized or cured by ultraviolet rays or X-rays, an
ultraviolet-ray or X-ray non-transmissive mask can be used to
provide a through hole in a columnar shape or a through hole like a
slit in a direction of layer thickness.
[0159] In the fuel cell structure described in Patent Document 1
above, carbon dioxide may be stored in the porous substrate or the
fuel plenum, supply of the methanol aqueous solution to the
catalyst layer may be disturbed, and output of the fuel cell may be
lowered due to diffusion resistance of the methanol aqueous
solution. Accordingly, the methanol aqueous solution should flow
within the fuel plenum at quite a flow rate, and hence auxiliary
equipment such as a pump consuming a large amount of electric power
is required. As the fuel is supplied through the fuel permeability
control layer, carbon dioxide is not stored in the fuel flow
channel (or the fuel plenum), and the fuel can be supplied in a
stable manner. Thus, it is not necessary to use auxiliary equipment
such as a pump consuming a large amount of electric power or use
thereof can be minimized, and thus power density of the fuel cell
system can be enhanced. In addition, by supplying the fuel with its
permeate flux being controlled, the need for a sealing material for
sealing, a fastening member for securing sealing property, and a
separator for preventing the fuel from reaching the cathode
catalyst layer can be obviated, and a smaller size, lower cost and
higher output of the fuel cell stack can be achieved.
[0160] Though not specifically shown in FIG. 2, by forming
electrolyte membrane 201 to be wider in the in-plane direction than
anode catalyst layer 202 and cathode catalyst layer 203,
preferably, electrolyte membrane 201 and anode conductive porous
layer 204 come in contact with each other and electrolyte membrane
201 and cathode conductive porous layer 205 come in contact with
each other at end portions in the cross-section of the unit cell,
and bonding thereof is more preferred. This is preferred in order
to prevent the fuel from going around the end portion of
electrolyte membrane 201 to reach cathode catalyst layer 203 from
anode catalyst layer 202. Such a structure is realized, for
example, by forming the electrolyte membrane by applying a solution
of the electrolyte membrane to the end portions of anode conductive
porous layer 204 and cathode conductive porous layer 205 across a
prescribed width to a thickness equal to or greater than that of
each of anode catalyst layer 202 and cathode catalyst layer 203,
the thicknesses being substantially identical to each other, and
then by bonding these conductive porous layers through thermal
welding. Instead of forming the electrolyte membrane from the
solution of the electrolyte membrane, the electrolyte membrane may
be adhered to and formed on the anode conductive porous layer and
the cathode conductive porous layer in advance. Alternatively,
instead of forming the electrolyte membrane on the conductive
porous layer, the electrolyte membrane may be formed at opposing
ends of electrolyte membrane 201 formed to be wider where the
catalyst layer is not stacked, such that the electrolyte membrane
covers end surfaces of the catalyst layer and has a thickness as
large as the catalyst layer. By doing so as well, such a structure
that the conductive porous layer comes in contact with both of the
catalyst layer and the electrolyte membrane can be realized. In
addition, a layer formed between electrolyte membrane 201 and anode
conductive porous layer 204 and/or cathode conductive porous layer
205 to cover the end surfaces of the catalyst layer does not
necessarily have to be an electrolyte membrane, and the layer may
be formed with an adhesive. Examples of such adhesives include a
polyolefin-based adhesive, an epoxy-based adhesive and the like.
When anode conductive porous layer 204 and cathode conductive
porous layer 205 are porous in the cross-section of the end
portion, the cross-section of the end portion of anode conductive
porous layer 204, hydrophilic layer 208, anode catalyst layer 202,
and electrolyte membrane 201 and a boundary between adjacent layers
are preferably sealed with a sealant made of an adhesive or the
like in order to prevent the fuel from leaking from the
cross-section of the end portion of anode conductive porous layer
204 and reaching cathode catalyst layer 203 through cathode
conductive porous layer 205, and in addition to the above, more
preferably, the cross-section of the end portion of cathode
catalyst layer 203 and cathode conductive porous layer 205 is also
sealed with a sealant.
[0161] In the unit cell structured as described above, on the anode
side, referring to FIG. 2, the fuel reaches anode catalyst layer
202 through fuel flow channel 206, oxidation reaction of the fuel
occurs in anode catalyst layer 202, and protons and electrons are
generated. The generated electrons flow to anode conductive porous
layer 204 and the protons move through electrolyte membrane 201 to
cathode catalyst layer 203. On the other hand, on the cathode side,
the air supplied into the fuel cell stack through the fuel cell
layer or the gap region in the unit cell is supplied to cathode
catalyst layer 203 through cathode conductive porous layer 205 of
the unit cell. In cathode catalyst layer 203, reduction reaction of
oxygen with protons and electrons supplied by cathode conductive
porous layer 205 occurs and water is generated.
Embodiment 2
[0162] FIG. 3 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present invention,
FIG. 3(a) being a perspective view thereof, FIG. 3(b) being a top
view, and FIG. 3(c) being a side view. A fuel cell stack 300 in
FIG. 3 is formed by alternately stacking a first fuel cell layer
301 and a second fuel cell layer 302, and the total number of
stacks is eight. Each layer is constituted of five unit cells 303,
and fuel cell stack 300 has forty unit cells as a whole. This
embodiment is different from Embodiment 1 above in the number of
stacked fuel cell layers. By increasing the number of stacks, a
higher voltage can be output.
[0163] In fuel cell stack 300, unit cells 303 are equal in height,
that is, the fuel cell layers are equal in thickness. As the unit
cells in the fuel cell layer are thus identical in height, contact
between upper and lower unit cells is satisfactory and poor contact
can be lessened even when a unit cell is stacked over a plurality
of unit cells. In addition, the eight fuel cell layers in total are
stacked in a direction perpendicular to a direction along a plane
of the fuel cell layer. Thus, in a case of serial electrical
wiring, an electrical conduction path can be short and resistance
loss can be lessened. In addition, as the fuel cell layers are
stacked in the direction perpendicular to the direction along the
plane of the fuel cell layer, a dead space for wiring or the like
can be reduced and a space volume occupied by the fuel cell stack
can be decreased, so that a fuel cell stack achieving a smaller
size and higher power density can be obtained.
[0164] Here, in fuel cell stack 300, all fuel cell layers are
identical in shape, however, they do not necessarily have to be
identical in shape. In addition, unit cells 303 constituting fuel
cell layer 300 may all be identical in shape or some of them may be
different in shape. So long as at least two of the fuel cell layers
constituting fuel cell stack 300 are identical in shape,
manufacturing cost can be reduced and construction of the stack
structure can be facilitated in those fuel cell layers. Moreover,
so long as at least two unit cells in a single fuel cell layer are
identical in shape, variation in characteristics due to difference
in shape of the unit cell described above can be suppressed in
those unit cells.
[0165] The structure of the fuel cell stack, an effect thereof, and
possible modifications are otherwise the same as in Embodiment 1
above. In the present embodiment, though the fuel cell layers are
stacked such that an overlying gap region portion is in a
quadrangular shape when viewed in the direction of stack, the fuel
cell layers may naturally be stacked such that the overlying gap
region portion is in a polygonal shape, such as triangular,
pentagonal or hexagonal. The internal structure of each unit cell
can be the same as in Embodiment 1 above.
[0166] Here, in the fuel cell stack, for example as shown in FIG.
3, formed by stacking a plurality of fuel cell layers in each of
which unit cells in the same parallelepiped shape are arranged at
regular intervals such that the unit cells constituting the fuel
cell layer are orthogonal to the unit cells constituting the fuel
cell layer adjacent to the former fuel cell layer, referring to
FIG. 3, a width X1 of the unit cell is preferably set to 1 mm or
greater from a point of view of ease in fabrication of the unit
cell. In addition, taking into consideration the fact that a
convection rate of oxygen is greater as an area of intersection of
the unit cells is smaller and the fact that generated water is
quickly exhausted as vapor, width X1 of the unit cell is preferably
as narrow as possible, specifically, it is preferably 5 mm or
smaller. A width X2 of the gap region between the unit cells with
respect to width X1 of the unit cell (interval between the unit
cells) (X2/X1) is preferably not greater than 1 from a point of
view that a greater ratio of an MEA occupied in the fuel cell layer
leads to higher power density of the fuel cell stack, and
preferably it is not smaller than 0.2 from a point of view of ease
in air intake into the fuel cell stack. Provided that X1 and X2
satisfy the range above, a thickness X3 of the cathode conductive
porous layer included in the unit cell (a total thickness when the
cathode conductive porous layer is adjacent to the spacer in the
fuel cell stack (see FIGS. 61 and 62 which will be described
later)) is preferably not smaller than 0.2 mm from a point of view
of ease in air intake and convection in the fuel cell stack and
preferably not greater than 2 mm from a point of view of
improvement in power density of the fuel cell stack.
Embodiment 3
[0167] FIG. 4 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
Though the fuel cell stack in the present embodiment is similar to
that in Embodiment 2 above in the shape of each fuel cell layer and
the shape of each unit cell, it is characterized by an internal
structure of the unit cell. Specifically, a unit cell 401 is formed
by stacking an anode catalyst layer 402, an electrolyte membrane
403 and a cathode catalyst layer 404 in this order. According to
such a configuration, as a conductive porous layer and a separator
are not required, the number of members is decreased and lower cost
and higher power density can be achieved.
[0168] In addition, as in the embodiment above, each fuel cell
layer in the fuel cell stack in the present embodiment is
constituted of a plurality of unit cells 401 arranged at intervals,
and the unit cells in a single fuel cell layer are aligned such
that the unit cells are orthogonal to unit cells in the adjacent
fuel cell layer. Thus, a ratio of an area of the anode catalyst
layer and the cathode catalyst layer facing the gap region
increases and the fuel and the oxidizing agent can efficiently be
supplied. An interval between the unit cells arranged within each
fuel cell layer is the same as in Embodiment 1 above.
[0169] A highly selective catalyst, which is low in reactivity in
oxidation reaction of methanol but high in reactivity in reduction
reaction of oxygen and protons, is preferably used for cathode
catalyst layer 404. Thus, even when methanol reaches the cathode
catalyst, a cathode potential can be high and high power-generation
efficiency can be achieved. Examples of the highly selective
catalysts above include a composite catalyst obtained by having
platinum particles carry heteropoly acid
(H.sub.3PW.sub.12O.sub.40), a catalyst of a cobalt porphyrin
complex, a platinum catalyst to which chloro-substituted cobalt
bisdicarbollide complex is adsorbed, an alloy catalyst of Ru
(ruthenium) and Se (selenium), an alloy catalyst of Pt, Co and the
like, a metal carbide such as tungsten carbide, and the like.
[0170] A catalyst or a catalyst layer structure, with which
methanol is mainly oxidized but reduction reaction of oxygen is
poor, is preferably used for anode catalyst layer 402, and anode
catalyst layer 402 is preferably composed of a material promoting
methanol oxidation reaction at the anode. Examples of a highly
selective catalyst layer structure of anode catalyst layer 402
include such a structure that a carrier of a highly electrically
conductive carbon-based material (such as acetylene black, Ketjen
Black, amorphous carbon, carbon nanotube, and carbon nanohorn)
carrying fine metal particles of Pt or an alloy of Pt and Ru that
are catalyst particles and a proton-conductive polymer binder such
as NAFTON are contained and a surface of the carbon-based material
is modified with hydrophilic functional group such as carboxylic
acid group or hydroxyl group. Owing to the hydrophilic functional
group, anode catalyst layer 402 is infiltrated with the methanol
aqueous solution and a coating film of the methanol aqueous
solution is formed. Thus, an amount of oxygen that reaches anode
catalyst particles is significantly smaller than an amount of the
methanol aqueous solution present in anode catalyst layer 402.
Consequently, influence by oxygen on reaction in anode catalyst
layer 402 can be suppressed and lowering in output characteristics
of the fuel cell due to the influence by oxygen can mostly be
avoided.
[0171] In the present embodiment, preferably, the methanol aqueous
solution and oxygen are mixed and the mixture is sprayed or
supplied with a pump. As pressure loss is lessened by the gap
regions formed to three-dimensionally communicate with one another
in the fuel cell stack, the fuel mixture can be supplied to all
corners of the fuel cell stack while reducing power consumption by
auxiliary equipment such as a pump.
Embodiment 4
[0172] FIG. 5 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment is formed by
alternately stacking a first fuel cell layer 501 and a second fuel
cell layer 502. First fuel cell layer 501 includes five first unit
cells 503 arranged at a distance from one another, and has a gap
region between these first unit cells 503. Each first unit cell 503
has an internal structure the same as the structure shown in FIG.
2. Specifically, each first unit cell 503 has an anode conductive
porous layer 509, an anode catalyst layer 508, an electrolyte
membrane 507, a cathode catalyst layer 506, and a cathode
conductive porous layer 505 in this order. Second fuel cell layer
502 is formed of a single second unit cell and it has no gap
region. An internal structure of the second unit cell is the same
as that of first unit cell 503. An interval between the unit cells
arranged within first fuel cell layer 501 is the same as in
Embodiment 1 above.
[0173] In such a structure as well, the fuel or the oxidizing agent
can be taken into the fuel cell stack through the gap region in
first fuel cell layer 501, without the use of auxiliary equipment.
Even when the auxiliary equipment is used, pressure loss in
supplying the fuel or the oxidizing agent to the inside can be
lessened and it can be supplied with low power being consumed.
First fuel cell layer 501 having the gap region plays a role as a
spacer between second fuel cell layer 502 having no gap region and
another second fuel cell layer 502 adjacent thereto having no gap
region, so that power generation in a spacer portion can be
realized and electrically serial wiring is facilitated.
Embodiment 5
[0174] FIG. 6 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment has a structure
formed by stacking five fuel cell layers identical in shape each
constituted of five unit cells 601. Each unit cell 601 has an
internal structure the same as the structure shown in FIG. 2 and
all unit cells are identical in shape. Specifically, each unit cell
601 has an anode conductive porous layer 602, an anode catalyst
layer 603, an electrolyte membrane 604, a cathode catalyst layer
605, and a cathode conductive porous layer 606 in this order. In
the present embodiment, on the fuel cell layer having five unit
cells 601 and the gap regions, the fuel cell layers identical in
shape are stacked such that the positions of the unit cells
coincide and the positions of the gap regions coincide. Therefore,
the structure is such that four large gap regions in total, with
individual gap regions being coupled to one another, are provided.
An interval between the unit cells arranged within each fuel cell
layer is the same as in Embodiment 1 above.
[0175] In such a structure as well, the fuel or the oxidizing agent
can be taken into the fuel cell stack through the gap region in the
fuel cell layer, without the use of auxiliary equipment. Even when
the auxiliary equipment is used, pressure loss in supplying the
fuel or the oxidizing agent to the inside can be lessened and it
can be supplied with low power being consumed
Embodiment 6
[0176] FIG. 7 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment is formed by
alternately stacking a first fuel cell layer 701 constituted of
eight unit cells 703 arranged at a distance from one another and a
second fuel cell layer 702 constituted of seven unit cells 703
arranged at a distance from one another. The structure of unit cell
703 is the same as in FIG. 2. Specifically, each unit cell 703 has
an anode conductive porous layer 704, an anode catalyst layer 705,
an electrolyte membrane 706, a cathode catalyst layer 707, and a
cathode conductive porous layer 708 in this order. First fuel cell
layer 701 and second fuel cell layer 702 are structured such that a
gap region is provided between the unit cells and the gap region
lies between two adjacent fuel cell layers. The fuel cell layers
are stacked in series, and the adjacent fuel cell layers form an
electron conduction path, as the anode conductive porous layer and
the cathode conductive porous layer come in electrical contact with
each other. An interval between the unit cells arranged within each
fuel cell layer is the same as in Embodiment 1 above.
[0177] In such a structure as well, the fuel or the oxidizing agent
can be taken into the fuel cell stack through the gap region in the
fuel cell layer, without the use of auxiliary equipment. Even when
the auxiliary equipment is used, pressure loss in supplying the
fuel or the oxidizing agent to the inside can be lessened and it
can be supplied with low power being consumed.
Embodiment 7
[0178] FIG. 8 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment is formed by
alternately stacking a first fuel cell layer 801 constituted of
five unit cells 803 arranged at a distance from one another and a
second fuel cell layer 802 constituted of two unit cells 803
arranged at a distance from each other, and a gap region is
provided between the unit cells. An internal structure of unit cell
803 is similar to that in FIG. 2. Specifically, each unit cell 803
has an anode conductive porous layer 804, an anode catalyst layer
805, an electrolyte membrane 806, a cathode catalyst layer 807, and
a cathode conductive porous layer 808 in this order. An interval
between the unit cells arranged within each fuel cell layer is the
same as in Embodiment 1 above.
[0179] Here, a direction of stack of the members above constituting
each unit cell 803 is perpendicular to the direction of stack of
the fuel cell layers. In addition, each unit cell 803 has an
insulating layer 809 on each of upper and lower surfaces of the
fuel cell layer in the direction of stack. Thus, the fuel cell
layers are electrically insulated from one another, so that a
current in each unit cell is drawn toward an end portion through
anode conductive porous layer 804 and cathode conductive porous
layer 808, collected there, and extracted to the outside. According
to such a configuration, as a surface of anode conductive porous
layer 804 and a surface of cathode conductive porous layer 808 can
occupy a larger area of a unit cell surface facing the gap region
between the unit cells, supply of the fuel or the oxidizing agent
can be improved and convection resistance can be lowered. Power
generation efficiency can thus be improved.
[0180] Further, the fuel or the oxidizing agent can be taken into
the fuel cell stack through the gap region in the fuel cell layer
without the use of auxiliary equipment. Even when the auxiliary
equipment is used, pressure loss in supplying the fuel or the
oxidizing agent to the inside can be lessened and it can be
supplied with low power being consumed.
Embodiment 8
[0181] FIG. 9 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment is formed by
alternately stacking a first fuel cell layer 901 constituted of
three unit cells 903 arranged at a distance from one another and a
second fuel cell layer 902 constituted of five unit cells 903
arranged at a distance from one another. Each unit cell 903 has a
separator 904, an anode conductive porous layer 905, an anode
catalyst layer 906, an electrolyte membrane 907, a cathode catalyst
layer 908, and a cathode conductive porous layer 909 in this order.
A direction of stack of the members above constituting each unit
cell 903 is in parallel to the direction of stack of the fuel cell
layers. Each fuel cell layer is structured such that a gap region
is provided between the unit cells and a part of the gap region
lies between two adjacent fuel cell layers. The fuel cell layers
are stacked in series, and the adjacent fuel cell layers form an
electron conduction path, as cathode conductive porous layer 909
and separator 904 come in electrical contact with each other. An
interval between the unit cells arranged within each fuel cell
layer is the same as in Embodiment 1 above
[0182] FIG. 10 is an enlarged cross-sectional view of unit cell 903
in FIG. 9. As shown in FIG. 10, unit cell 903 has separator 904 in
which a fuel flow channel 910 is formed, on a surface of anode
conductive porous layer 905 opposite to a surface in contact with
anode catalyst layer 906.
[0183] Here, separator 904 is preferably formed of an electrically
conductive material. In addition, in order to avoid exhaust of the
fuel through separator 904 to the outside of unit cell 903,
separator 904 preferably has no opening hole. Moreover, in order to
avoid exhaust of the fuel to the outside of unit cell 903,
separator 904 and electrolyte membrane 907 are preferably bonded to
each other with a sealant or an adhesive at left and right end
portions in FIG. 10, with anode catalyst layer 906 and anode
conductive porous layer 905 being interposed.
[0184] The fuel reaches anode conductive porous layer 905 through
fuel flow channel 910 by means of auxiliary equipment such as a
pump and it is supplied to anode catalyst layer 906 through anode
conductive porous layer 905. Therefore, generated carbon dioxide is
exhausted to the outside through fuel flow channel 910 together
with unreacted fuel, by means of the auxiliary equipment such as a
pump.
[0185] In the present invention, in a case of a structure where a
fuel supply flow channel is completely separate from an oxidizing
agent supply flow channel, a substance supplied through the gap
region in the fuel cell layer does not necessarily have to be air
serving as the oxidizing agent, and the fuel may be supplied
through the gap region. In this case, the anode and the cathode are
reversed, so that the oxidizing agent can be supplied through a
flow channel provided in the cathode conductive porous layer. For
example, in the embodiment shown in FIG. 10, the oxidizing agent
may be separated by separator 904 and supplied through fuel flow
channel 910 formed within separator 904, while the reducing agent
may be supplied through the gap region in the fuel cell layer.
[0186] According to the structure of the fuel cell stack in the
present embodiment, the air can be taken into the fuel cell stack
through the gap region in the fuel cell layer without the use of
auxiliary equipment. Even when the auxiliary equipment is used,
pressure loss in supplying the fuel or the oxidizing agent to the
inside can be lessened and it can be supplied with low power being
consumed.
Embodiment 9
[0187] FIG. 11 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present invention,
FIG. 11(a) being a top view thereof and FIG. 11(b) being a side
view. In the present embodiment, in each fuel cell layer, unit
cells 1101 are arranged such that a gap region provided between
unit cells 1101 is wider in a central portion in the fuel cell
stack. Specifically, in each fuel cell layer, unit cells 1101 are
arranged such that a width of a gap region located around the
center of the fuel cell layer is greater than a width of a gap
region located proximate to an outer peripheral portion.
Consequently, a space volume of the gap region located in the
central portion in the fuel cell stack is greater than that of the
gap region located at an end portion of the fuel cell stack. Thus,
the gap region is greater toward the central portion of the fuel
cell layer in at least one direction within the surface of the fuel
cell layer, so that the gap region along a vertical line in the
center when viewed from a side surface of the fuel cell stack and
the gap region along a line extending from a central gap to a
direction of depth when viewed from the top of the fuel cell stack
are wider. In natural air supply, oxygen concentration around the
central portion of the fuel cell stack is lower than around an
outer periphery of the stack. According to the fuel cell stack
structure in the present embodiment, however, as shown in FIG.
11(b), pressure loss in sending air toward the central portion of
the fuel cell stack in a direction from a lower surface to an upper
surface of the fuel cell stack (in a direction from the rear to the
front in the plane in FIG. 11(a)) into the fuel cell stack, for
example, by using a fan or a blower, is lessened. Therefore, the
air is readily supplied to the central portion of the fuel cell
stack and oxygen concentration around the central portion of the
fuel cell stack can be raised. Consequently, power generation
characteristics of the fuel cell stack can be improved. It is noted
that an internal structure of each unit cell 1101 can be the same
as the structure in FIG. 2.
[0188] In the present embodiment, unit cell 1101 is preferably
arranged in such an orientation that a surface thereof on the
cathode side, with the electrolyte membrane serving as the
reference, faces the flow of the air, that is, such an orientation
that the cathode catalyst layer side and the anode catalyst layer
side are upstream and downstream of the flow of the air,
respectively, with the electrolyte membrane serving as the
reference. Thus, the air readily reaches the cathode catalyst layer
through the cathode conductive porous layer and diffusion
resistance of oxygen can be lowered, which brings about improvement
in power generation efficiency and higher power density of the fuel
cell stack. In addition, by arranging the unit cells in such an
orientation, difference in oxygen partial pressure between the
inside and the outside of the fuel cell stack can be made smaller
and difference in a power generation amount per power generation
area (power generation variation) can be reduced. As a portion
where a power generation amount locally increases can thus be
decreased, load due to local heat or load due to overvoltage of the
catalyst can be suppressed, and lowering in output characteristics
can be suppressed. Moreover, power generation by the fuel cell
stack warms the air inside and upward convection of the warmed air
is caused. Here, as the space along the vertical line in the
central portion is large, an effect of convection can be enhanced.
Further, as heat inside the fuel cell stack can efficiently be
radiated, excessive temperature increase can be prevented.
[0189] In the present embodiment, whichever surface of the fuel
cell stack may face upward, the gap region along the vertical line
in the center is great and hence heat convection of the air can
promote supply of the air into the fuel cell stack. Therefore, the
air can satisfactorily be taken into the fuel cell stack without
the use of auxiliary equipment such as a pump or a fan.
[0190] In using a fan or a blower, preferably, it is fixed and
arranged such that a surface shown in FIG. 11(a) faces up and the
flow of the air is caused in the direction the same as the flow of
heat convection by means of the fan or the blower. As an amount of
the air supplied by the fan or the blower can thus be decreased,
the fuel cell can generate electric power with lower power
consumption. In addition, when the air is supplied by using the
fan, a fan 1201 is preferably provided within the fuel cell stack
as shown in FIG. 12. Alternatively, when a fuel cell system has two
or more fuel cell stacks, the fan is preferably provided between
these fuel cell stacks. Thus, the inside air of which oxygen
partial pressure has been lowered can forcibly be exhausted to the
outside and fresh air is taken in, so that the air in the fuel cell
stack can efficiently be changed.
[0191] In the case of the fuel cell stack in the present
embodiment, the entire power generation area itself is as large as
in the fuel cell stack in FIG. 3 described previously. Meanwhile,
power generation characteristics and power density of the fuel cell
stack can be improved by making higher power generation area
density (fuel cell density) of the fuel cell in the outer
peripheral portion of the fuel cell stack where air is readily
supplied while lowering power generation area density (fuel cell
density) in the central portion of the fuel cell layer and reducing
pressure loss in air supply into the fuel cell stack. According to
the present embodiment, such improvement in the power generation
characteristics and power density of the fuel cell stack can be
achieved with a simplified method of designing as appropriate the
gap regions in the fuel cell stack. In the present embodiment,
preferably, the width of the gap region (the distance between the
unit cells) is set to 0.1 mm to 2 mm in the case of a small gap and
set to approximately 2 mm to 5 mm in the case of a large gap.
[0192] Here, any of the fuel cell stacks according to Embodiments 1
to 9 described above is preferably arranged such that the air
readily escapes toward the upper surface with respect to a ground
surface when the air in the fuel cell stack is warmed by heat
generated during power generation and heat convection occurs. In
addition, with respect to the flow of the air due to heat
convection, preferably, the cathode catalyst layer side and the
anode catalyst layer side are located upstream and downstream of
the flow of the air respectively, with the electrolyte membrane
serving as the reference. Thus, as the air readily reaches the
cathode catalyst layer through the cathode conductive porous layer,
reduction in diffusion resistance of the oxygen can be achieved,
which brings about improvement in power generation efficiency and
higher power density of the fuel cell stack.
[0193] In addition, in any of the fuel cell stacks according to
Embodiments 1 to 9 described above, when the flow of the air is
caused by such auxiliary equipment as a fan, a blower or an air
pump to supply the air to the fuel cell stack, the flow of the air
caused by the auxiliary equipment is preferably caused in the
direction the same as the direction of heat convection. The flow of
the air inside the fuel cell stack can thus be improved without
preventing the flow by heat convection. As described previously,
with respect to such flow of the air-, preferably, the cathode
catalyst layer side and the anode catalyst layer side are located
upstream and downstream of the flow of the air respectively, with
the electrolyte membrane serving as the reference
Embodiment 10
[0194] FIG. 13 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present invention,
FIG. 13(a) being a side view thereof and FIG. 13(b) being a top
view. As shown in FIG. 13(a), the fuel cell stack according to the
present embodiment is configured such that a central fuel cell
layer 1301 has a largest thickness and a fuel cell layer has a
smaller thickness toward a direction of an end of the fuel cell
stack (in a vertical direction in FIG. 13(a)). In FIG. 13(a), a gap
region in the fuel cell stack is wide along a central line, while a
gap region along a line closer to an upper or lower end of the fuel
cell stack is narrow. The gap regions are equal in area of a
cross-section perpendicular to the direction of stack of the fuel
cell layers (see FIG. 13(b)), however, the fuel cell layer has a
thickness (a thickness of the unit cell) greater toward the center.
Therefore, a space volume of the gap region is greater toward the
center. An interval between the unit cells arranged within each
fuel cell layer is the same as in Embodiment 1 above. According to
the configuration in the present embodiment as well, an effect the
same as in the fuel cell stack in FIG. 11 can be obtained.
[0195] The fuel cell stack in the present embodiment can be
installed, for example, such that the air flows from a direction
shown with an arrow in FIG. 13 by means of a fan or a blower and it
is supplied to the inside of the fuel cell stack. Thus, pressure
loss in the fuel cell stack is lessened, air can be supplied to the
inside of the fuel cell stack with low power consumption by the fan
or the blower, and power density of the overall fuel cell system
can be improved.
[0196] FIG. 14 is an enlarged detail view of a region A shown in
FIG. 13(a). An internal structure of each unit cell is the same as
the structure shown in FIG. 2. In the present embodiment, as shown
in FIG. 14, a thickness of the fuel cell layer is varied by
adjusting a thickness of a cathode conductive porous layer 1405. It
is noted that the thickness of the fuel cell layer can be adjusted
also by varying a thickness of each member, without limited to
cathode conductive porous layer 1405.
[0197] In addition, in a portion where the fuel cell layers
intersect and come in contact with each other, air is supplied from
a cross-section of cathode conductive porous layer 1405 and the air
reaches a cathode catalyst layer 1404. Therefore, a greater
thickness of cathode conductive porous layer 1405 increases an
amount of supply of oxygen and thus output current density can be
high, which is preferred. The power generation characteristics can
thus be improved. Namely, by increasing the thickness of cathode
conductive porous layer 1405 in the central portion of the fuel
cell stack, not only the air can readily be taken into the fuel
cell stack but also the power generation characteristics in the
portion where the fuel cell layers intersect and come in contact
with each other can be improved. In addition, air is supplied well
around an outer surface of the fuel cell stack than around the
central portion, and hence a partial pressure of oxygen in the air
is also high. Accordingly, relatively good power generation
characteristics are obtained. Therefore, the thickness of the
cathode conductive porous layer in the fuel cell layer in the
central portion of the fuel cell stack is made larger to improve
passage of the air. Meanwhile, by making smaller the thickness of
the cathode conductive porous layer in the fuel cell layer close to
the outer surface of the fuel cell stack as well, power density of
the fuel cell stack as a whole is improved.
Embodiment 11
[0198] FIG. 15 is a partially enlarged schematic cross-sectional
view of another preferred example of the fuel cell stack according
to the present invention. An overall configuration of the fuel cell
stack in the present embodiment can be as shown, for example, in
FIG. 3. An interval between the unit cells arranged within each
fuel cell layer is the same as in Embodiment 1 above. In the
present embodiment, a cathode conductive porous layer 1506a located
at a portion of contact between the stacked, adjacent fuel cell
layers has a larger thickness than a cathode conductive porous
layer 1506b in a portion other than the portion of contact. A
single cathode conductive porous layer having a large thickness
only in the portion of contact may be adopted as the cathode
conductive porous layer, or the thickness may be ensured by
layering two or more cathode conductive porous layers only at the
portion of contact. At that contact portion, the air diffuses
within the cathode conductive porous layer in a direction along the
plane perpendicular to the direction of layer thickness of the
cathode conductive porous layer, and oxygen is supplied to a
cathode catalyst layer 1504. Accordingly, as the cathode conductive
porous layer at the contact portion is greater in thickness, an
amount of oxygen that can be introduced in the cathode conductive
porous layer increases and a larger amount of air can be supplied
to cathode catalyst layer 1504. Therefore, a limit amount of
current that can be extracted increases and an amount of oxygen in
the vicinity of cathode catalyst layer 1504 increases, so that
diffusion resistance of oxygen can be lowered and power generation
efficiency can be improved.
[0199] In addition, by making smaller the thickness of the cathode
conductive porous layer located in a portion other than the portion
of contact, the gap region within the fuel cell stack becomes
greater and a passageway for the air is wider. Accordingly, air is
readily introduced in the fuel cell stack and convection of air in
the fuel cell stack is satisfactory. Moreover, as a distance
traveled by oxygen within the cathode conductive porous layer to
reach the cathode catalyst layer becomes shorter, diffusion
resistance of oxygen can be lowered and power generation
characteristics can be improved. So long as the thickness of the
fuel cell layer in the portion other than the portion of contact is
decreased, convection of the air within the fuel cell stack is
satisfactory and hence a layer to be made thinner is not limited to
the cathode conductive porous layer. From a point of view of
improvement in power generation efficiency through lowering in
convection resistance of oxygen, it is preferred to increase the
thickness of the cathode conductive porous layer at the portion of
contact and to decrease the thickness of the cathode conductive
porous layer in a portion other than the portion of contact. Power
density of the fuel cell stack can thus be improved.
Embodiment 12
[0200] FIG. 16 is a schematic diagram showing another preferred
example of the fuel cell stack according to the present invention,
FIG. 16(a) being a top view and FIG. 16(b) being a side view. In
the present embodiment, as shown in FIG. 16(a), among a plurality
of unit cells constituting each fuel cell layer, a unit cell having
a width smaller toward a center of the fuel cell layer is adopted
as a unit cell 1601 located in a central portion of the fuel cell
stack. The "width" herein refers to a width in a direction
perpendicular to a longitudinal direction of the unit cell. Thus, a
cross-sectional area of the gap region in the central portion of
the fuel cell layer (a cross-sectional area in a direction
perpendicular to the direction of stack) and a space volume are
greater than those in a peripheral portion thereof. It is noted
that each unit cell is configured as in FIG. 2. The fuel cell
layers are identical in shape, and the fuel cell layers are stacked
such that the unit cells constituting one fuel cell layer of the
adjacent fuel cell layers are orthogonal to the unit cells
constituting the other fuel cell layer. An interval between the
unit cells arranged within each fuel cell layer is the same as in
Embodiment 1 above.
[0201] According to such a configuration, for example as shown in
FIG. 16(b), when the fuel cell stack is installed such that the air
flows from the lower surface of the fuel cell stack toward the
upper surface (the surface shown in FIG. 16(a)), the gap region is
greater toward the central portion in a plane perpendicular to the
direction of flow of the air. Accordingly, pressure loss of the air
that passes through the central portion of the fuel cell stack is
reduced, the air is readily supplied to the central portion of the
fuel cell stack, and power generation characteristics of the fuel
cell stack can be improved. In addition, as the air readily flows
to the central portion of the fuel cell stack, pressure of the air
supplied by such auxiliary equipment as a fan or a blower can be
lowered. Therefore, power consumption by the auxiliary equipment
can be reduced and power density of the fuel cell system as a whole
including the fuel cell stack and the auxiliary equipment can be
increased. Further, an effect the same as in the embodiment shown
in FIG. 11 is obtained. Though FIG. 16 shows an example where a
width of the unit cell continuously decreases toward the central
portion, the width of the unit cell may be decreased in a stepped
manner (in a stepwise fashion).
Embodiment 13
[0202] FIG. 17 is a top view showing another preferred example of
the fuel cell stack according to the present invention. In the
present embodiment, each fuel cell layer is formed of a single
comb-shaped unit cell 1701 having five comb teeth. Specifically,
each fuel cell layer 1701 is formed of a single unit cell including
a branch portion 1702 having a shape elongated in a longitudinal
direction within the fuel cell layer. Branch portions 1702 are
identical in a direction of elongation and arranged in parallel to
one another. Here, the comb shape refers to such a shape that
elongated comb teeth portions are all joined together at one end.
In the present embodiment, a gap region refers to a space between
the comb teeth. The fuel cell layers are identical in shape and
stacked as alternately rotated by 90.degree.. Each unit cell can
have an internal structure the same as in FIG. 2. An interval
between the comb teeth is the same as in Embodiment 1 above.
[0203] According to such a structure, at least two surfaces of side
surfaces of the fuel cell stack as well as an upper surface and a
lower surface thereof have a gap for air intake. Therefore,
convection of air caused by heat is likely to occur and the air can
satisfactorily be taken into the fuel cell stack without using such
auxiliary equipment as a pump or a fan. In addition, even when the
auxiliary equipment such as a pump or a fan is used, the air can be
supplied with low power consumption.
[0204] Here, as shown in FIG. 17, the fuel cell layers are
preferably stacked such that the gap region is formed also in a
root portion of the comb-shaped fuel cell layer when viewed in the
direction of stack of the fuel cell layers. Thus, the air can be
taken into the fuel cell stack also from the surface where the root
portion of the comb is located. In addition, in mounting the fuel
cell stack in the present embodiment on equipment, if two surfaces
of the fuel cell stack are covered with wall surfaces of the
equipment, the fuel cell stack is preferably arranged such that the
root portion of the comb-shaped unit cell (a comb teeth coupling
portion) is located on a wall surface side of the equipment.
[0205] In the fuel cell stack in the present embodiment, the fuel
cell layer is formed of a single unit cell as described above. Such
a configuration is preferred in that time and effort for aligning a
plurality of unit cells can be eliminated, positioning of the gap
region is facilitated, lowering in yield due to operational error
such as dropping can also be suppressed, and manufacturing cost can
be reduced. Further, a power generation area can be increased at a
connecting portion at an outer peripheral portion (the root portion
of the comb-shaped unit cell).
[0206] Though not shown, a width (a width in a direction
perpendicular to the longitudinal direction of the branch portion)
of the branch portion (the comb tooth portion) when viewed from the
upper surface (the surface shown in FIG. 17) preferably decreases
toward the center of the fuel cell layer within the surface of the
fuel cell layer. Thus, an effect the same as in the fuel cell stack
in FIG. 16 can further be obtained.
[0207] A structure shown in FIGS. 18(a) to (c) can be illustrated
as other structures having portions elongated in a longitudinal
direction and a portion where the elongated portions are connected
together. FIG. 18(a) shows an antenna shape, FIG. 18(b) shows a
gear wheel shape, and FIG. 18(c) shows a serpentine shape. The
serpentine shape is preferred, for example, in that the fuel at the
anode electrode is readily fed to all corners and circulated by
means of such auxiliary equipment as a pump and a volume of a flow
channel in a portion other than the fuel cell can be decreased.
Embodiment 14
[0208] The fuel cell stack according to the present embodiment will
be described with reference to FIGS. 19 and 20. FIG. 19 is a top
view of the fuel cell stack according to the present embodiment,
and FIG. 20 is a cross-sectional view of a unit cell forming an
uppermost fuel cell layer in the fuel cell stack according to the
present embodiment. As shown in FIG. 19, unit cells 1901 forming
each fuel cell layer are characterized in that they are arranged in
a fan shape within the fuel cell layer and gap regions are provided
such that one ends in a direction of length of unit cells 1901
concentrate at one point. By arranging the unit cells as shown in
FIG. 19, when the fuel cell layers are stacked, the gap region in a
central portion when viewed in the direction of stack can be
great.
[0209] In the present embodiment, the unit cell forming the
uppermost fuel cell layer preferably has an internal structure in
which a cathode current collector 2008 corresponding to an electron
supply electrode is arranged on a cathode conductive porous layer
2005 and an anode current collector 2009 corresponding to an
electron extraction electrode is arranged on an anode conductive
porous layer 2004, as shown in FIG. 20. Cathode current collector
2008 is in electrical contact with cathode conductive porous layer
2005, and cathode current collector 2008 is preferably identical in
shape to the unit cell when viewed from the upper surface, so as
not to deteriorate diffusion of air as it lies over the gap region
in the fuel cell stack.
[0210] In a fuel cell stack as in the present invention in which
unit cells are stacked in the number sign shape and fuel cell
layers are arranged electrically in series, an area where a current
flows in the in-plane direction is small, except for opposing ends
of the fuel cell stack, and hence the unit cells in the fuel cell
layer constituting an inner portion of the fuel cell stack do not
necessarily require an anode current collector and a cathode
current collector. For example, when a cathode electrode of the
unit cell constituting an uppermost fuel cell layer in the fuel
cell stack (that is, on the cathode catalyst layer side with the
electrolyte membrane serving as the reference) is located at an
uppermost portion of the fuel cell layer and an anode electrode of
the unit cell constituting a lowermost fuel cell layer in the fuel
cell stack (that is, on the anode catalyst layer side with the
electrolyte membrane serving as the reference) is located at a
lowermost portion of the fuel cell layer, the configuration may be
such that only the cathode current collector is provided on the
uppermost layer of the fuel cell stack while it is electrically
connected to the cathode catalyst layer, only the anode current
collector is provided on the lowermost layer of the fuel cell stack
while it is electrically connected to the anode catalyst layer, and
the unit cells in the fuel cell layers constituting the inner
portion of the fuel cell stack are not provided with anode current
collectors and cathode current collectors. According to such a
configuration, decrease in the number of parts of the fuel cell
stack, reduction in thickness, cost reduction, and the like can be
achieved. The anode current collector and the cathode current
collector will be described hereinafter.
[0211] The anode current collector basically corresponds to an
electron extraction electrode of the fuel cell stack as described
above, and it has a function to supply and receive electrons
to/from the anode conductive porous layer. In the anode current
collector, as a current flows in a direction of its length
(in-plane direction), a distance for electron conduction is longer
than in the anode conductive porous layer. Therefore, a material
having good electrical conductivity is preferably used. The need
for the anode current collector depends on electrical conductivity
of the anode conductive porous layer. When the anode conductive
porous layer is made, for example, of a carbon material, an
electrically conductive polymer or the like and electrical
conductivity is relatively low, electrical conductivity can be
improved by providing the anode current collector in the fuel cell
stack. In contrast, when the anode conductive porous layer is made,
for example, of a metal or the like and electrical conductivity is
relatively high, it is not particularly necessary to provide an
anode current collector. Meanwhile, when electrical conductivity of
the anode conductive porous layer is relatively low, an anode
current collector may be provided for each unit cell for the
purpose of assisting electron conduction in the anode conductive
porous layer and to lower electron conduction resistance, and in
this case, the anode current collector is added to the
configuration of the unit cell.
[0212] For suppression of voltage lowering, a noble metal such as
Au, Pt and Pd, a metal such as Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu,
Zn, and Su, Si, a nitride, a carbide or the like thereof, an alloy
such as stainless, Cu--Cr, Ni--Cr and Ti--Pt, and the like, that
are low in electron conduction resistance, are preferably used as a
material for the anode current collector, and at least one element
selected from the group consisting of Pt, Ti, Au, Ag, Cu, Ni, and W
is further preferably contained. In addition, when a metal poor in
corrosion resistance under acid atmosphere such as Cu, Ag and Zn is
used, a noble metal such as Au, Pt and Pd and a metal material
having corrosion resistance, an electrically conductive polymer, an
electrically conductive nitride, an electrically conductive
carbide, an electrically conductive oxide, and the like may be used
as a surface coating.
[0213] The shape of the anode current collector is not particularly
limited, however, a penetrating or communicating hole in the anode
conductive porous layer preferably communicates with a hole
provided in the anode current collector in that efficiency in
exhausting the exhaust gas generated in the anode catalyst layer is
excellent. The anode current collector is preferably formed of a
porous metal layer having a plurality of holes penetrating in a
direction of layer thickness, and such a shape can preferably be
illustrated by a shape having a plurality of holes in a plate or a
foil, or a mesh or an expanded metal. As the holes in the anode
conductive porous layer and the holes in the anode current
collector communicate with one another, carbon dioxide can
efficiently be exhausted and partial pressure of carbon dioxide
within the anode catalyst layer and the anode conductive porous
layer can be lowered. Thus, peel-off at an interface between the
anode catalyst layer and the anode conductive porous layer and an
interface between the anode current collector and the anode
conductive porous layer can be suppressed.
[0214] The cathode current collector corresponds to an electron
supply electrode of the fuel cell stack as described above, and it
has a function to supply and receive electrons to/from the cathode
conductive porous layer. In the cathode current collector, as a
current flows in a direction of its length (in-plane direction), a
distance for electron conduction is longer than in the cathode
conductive porous layer. Therefore, a material having good
electrical conductivity is preferably used. The need for the
cathode current collector depends on electrical conductivity of the
cathode conductive porous layer. When the cathode conductive porous
layer is made, for example, of a carbon material, an electrically
conductive polymer or the like and electrical conductivity is
relatively low, electrical conductivity can be improved by
providing the cathode current collector in the fuel cell stack. In
contrast, when the cathode conductive porous layer is made, for
example, of a metal or the like and electrical conductivity is
relatively high, it is not particularly necessary to provide a
cathode current collector. Meanwhile, when electrical conductivity
of the cathode conductive porous layer is relatively low, a cathode
current collector may be provided for each unit cell for the
purpose of assisting electron conduction in the cathode conductive
porous layer and to lower electron conduction resistance, and in
this case, the cathode current collector is added to the
configuration of the unit cell.
[0215] A material for the cathode current collector may be the same
as that for the anode current collector, however, for example, a
noble metal such as Au, Pt and Pd, a metal such as Ti, Ta, W, Nb,
and Cr, a nitride, a carbide or the like thereof, an alloy such as
stainless, Cu--Cr, Ni--Cr and Ti--Pt, and the like are preferably
used. In addition, when a metal poor in corrosion resistance under
acid atmosphere such as Cu, Ag, Zn, and Ni is used, a noble metal
and a metal material having corrosion resistance, an electrically
conductive polymer, an electrically conductive oxide, an
electrically conductive nitride, an electrically conductive
carbide, and the like may be used as a surface coating.
[0216] The shape of the cathode current collector is not
particularly limited so long as it has a communicating hole
allowing supply of oxygen in the air outside the cathode current
collector to the cathode conductive porous layer or to the cathode
catalyst layer, however, for example, a foam metal, a metal web, a
sintered metal, a shape having a plurality of holes in a plate or a
foil, or a metal mesh or an expanded metal may be used. The cathode
current collector has porosity preferably of 30% or higher for
lowering diffusion resistance of oxygen and preferably of 95% or
lower for lowering electrical resistance, and more preferably has
porosity from 50% to 85%.
[0217] In the present embodiment, the anode current collector and
the cathode current collector are arranged to be in electrical
contact with respective opposing ends (a lower end and an upper
end) of the uppermost fuel cell layer in the direction of stack.
Therefore, it is not necessary to provide a current collector for
each unit cell and the number of members can be decreased. In
addition, at the opposing ends of the uppermost fuel cell layer in
the direction of stack, the current flows in a direction of length
of the unit cell and is collected. Accordingly, by providing a
current collector having high electrical conductivity, resistance
in electrical conduction can be lowered. In addition, in the
present embodiment, one ends of the unit cells in a direction of
length are arranged to concentrate at one point and the cathode
current collector and the anode current collector form a positive
terminal and a negative terminal, respectively. An arrow shown in
FIG. 19 shows a direction of flow of a current in the cathode
current collector. As the ends of the current collector concentrate
at one point, wire routing is easy and air intake is not disturbed
by reduction in size of the fuel cell stack or wire routing. When a
switch circuit which will be described later is provided, the
current collector is preferably insulated from a current collector
provided in another unit cell.
[0218] It is noted that unit cells other than the unit cells
constituting the uppermost fuel cell layer, for example, all unit
cells constituting the fuel cell stack, may be provided with the
anode current collectors and the cathode current collectors,
without limited to the above. According to such a configuration, a
resistance value in a portion where a current flows through the
fuel cell layer in the in-plane direction can be lowered, which can
result in improvement in power generation efficiency.
[0219] The fuel cell stack in the present embodiment can be
modified to a structure as shown in FIG. 21 by modifying a shape of
the unit cells so long as air supply is not disturbed. The fuel
cell stack in FIG. 21 is constituted of unit cells in a trapezoidal
(sectorial) shape and has an increased power generation area as
compared with the fuel cell stack in FIG. 19. According to such a
configuration, the fuel cell stack having high power density is
provided.
Embodiment 15
[0220] FIG. 40 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack according to the present embodiment has such a
structure that the "number-sign-shaped fuel cell stack" similar to
that shown in FIG. 3 is stacked on a fuel cell layer formed of a
single, relatively large unit cell in a shape of a flat plate
having no gap region. In other words, the fuel cell stack in the
present embodiment can be said to have such a structure that the
lowermost fuel cell layer in the number-sign-shaped fuel cell stack
as shown in FIG. 3 (alternatively, may also be referred to as the
uppermost layer, if the fuel cell stack in the present embodiment
shown in FIG. 40 is inverted) is implemented as a fuel cell layer
formed of a single, relatively large unit cell in a shape of a flat
plate having no gap region. Namely, the difference from the fuel
cell stack shown in FIG. 5 resides in that the fuel cell layer
formed of a single unit cell is arranged at the lower end or the
upper end of the fuel cell stack. An internal structure of the unit
cell is not particularly restricted, and a structure may be formed,
for example, by stacking an anode conductive porous layer, an anode
catalyst layer, an electrolyte membrane, a cathode catalyst layer,
and a cathode conductive porous layer in this order. More
specifically, in addition to the structure shown in FIG. 2, a unit
cell structure which will be described later or the like may be
adopted. For example, a length and a width of the lowermost (or the
uppermost) unit cell may be equal to a length in a longitudinal
direction of the unit cell constituting the number-sign-shaped fuel
cell stack, however, they are not limited thereto and they may be
longer or shorter than that. In addition, the fuel cell stack
stacked on the fuel cell layer having no gap region and formed of a
single unit cell is not limited to the stack having the structure
shown in FIG. 3, and a fuel cell stack in any embodiment described
above may be employed.
[0221] In mounting the fuel cell stack on equipment or the like, in
many cases, the fuel cell stack is arranged on a substrate 4002
within the equipment. In the fuel cell stack in the present
embodiment, preferably, in arranging the fuel cell stack on
substrate 4002, as shown in FIG. 40, the fuel cell stack is
arranged such that the fuel cell layer side formed of a single
large unit cell is located on substrate 4002 side. As air cannot
basically be taken into the fuel cell stack from substrate 4002
side, air intake efficiency in such arrangement is equal to that in
a case where any surface of the fuel cell stack not having a fuel
cell layer formed of a single large unit cell is arranged on the
substrate side. Meanwhile, use of the fuel cell layer formed of a
single large unit cell can increase a power generation area.
Increase in the power generation area per volume can improve volume
power density.
[0222] In addition, by forming such a structure that the
number-sign-shaped fuel cell stack is stacked on the fuel cell
layer formed of a single large unit cell above, a temperature in
the fuel cell stack increases as a result of power generation by
the large unit cell and air in the fuel cell stack is warmed. Then,
an ascending air current is produced and flow of the air can be
improved through a three-dimensional air passageway (the gap
region) provided in the number-sign-shaped fuel cell stack portion
and power generation efficiency can also be enhanced in accordance
with temperature increase in the fuel cell stack. In particular,
when an area for installing the fuel cell stack in the equipment is
limited, by employing such a fuel cell stack excellent in air
supply and having improved volume power density as the fuel cell
stack according to the present invention, a thickness of the fuel
cell can be suppressed and prescribed output can be supplied, which
is extremely effective. Moreover, according to the present
invention, as the fuel cell stacks can three-dimensionally be
stacked and can be made compact, a degree of freedom in design or
the like on the equipment side on which the fuel cell stack is to
be mounted can advantageously be improved. For example, in small
portable equipment such as an electronic dictionary containing a
one-segment broadcast tuner and requiring high power consumption or
in an example where a fuel cell is mounted on a back of a liquid
crystal display, the fuel cell stack in the present embodiment is
used and arranged on the substrate with arrangement as described
above, so that the fuel cell stack can be mounted without extending
off the small equipment or the area of the back of the liquid
crystal display. Therefore, a degree of freedom in design or the
like such as a size of such equipment on which the fuel cell stack
is to be mounted can be improved.
[0223] The fuel cell layer formed of a single unit cell above is
preferably arranged such that its anode side (the anode catalyst
layer side with an electrolyte membrane 4001 serving as the
reference, in FIG. 40, the anode side being denoted as "AN";
similarly, the cathode catalyst layer side being referred to as the
cathode side with electrolyte membrane 4001 serving as the
reference; in FIG. 40, the cathode side being denoted as "CA"; to
be understood similarly hereinafter) is located on the substrate
side. Thus, as it is not necessary to provide an air passageway
between the substrate and the fuel cell stack, the anode side of
the fuel cell layer can be brought in contact with the substrate
and volume power density can further be improved.
[0224] Here, the embodiments of the present invention described
above all have such a structure that adjacent fuel cell layers are
in intimate contact, however, the present invention is not
necessarily limited as such. For example, a configuration may also
be such that a spacer layer is arranged between adjacent fuel cell
layers or one or more fuel cell layer is replaced with a spacer
layer. Here, the spacer layer refers to a layer playing a role for
creating a space in the fuel cell stack and formed of one
constituent member or two or more constituent members. In addition,
a member forming the spacer layer is herein referred to as a
"spacer". A fuel cell stack according to the present invention
provided with a spacer layer will be described hereinafter in
detail with reference to embodiments.
Embodiment 16
[0225] FIG. 41 is a schematic diagram showing an example of a basic
configuration of a fuel cell stack including a spacer layer, FIG.
41(a) being a perspective view thereof and FIG. 41(b) being a side
view thereof. The fuel cell stack shown in FIG. 41 includes a fuel
cell layer 4102 constituted of five unit cells 4101 arranged
substantially in parallel at a distance from one another and a
spacer layer 4104 stacked thereon. Spacer layer 4104 is constituted
of five spacers 4103 arranged substantially in parallel at a
distance from one another, and each spacer 4103 is arranged to
intersect with unit cells 4101. In FIG. 41, each spacer 4103 is
stacked to be orthogonal to unit cells 4101. Such arrangement
implements the fuel cell stack in which a gap region in fuel cell
layer 4102 three-dimensionally communicates with a gap region in
spacer layer 4104. Therefore, the fuel cell including such a spacer
layer can also achieve an effect the same as in the fuel cell stack
in the embodiments above. In addition, in the present embodiment,
each spacer 4103 is made of an electrically conductive material,
and thus each unit cell 4101 is electrically connected with spacer
4103 being interposed.
[0226] Here, in the fuel cell stack shown in FIG. 41, each unit
cell 4101 has a parallelepiped shape and spacer 4103 also similarly
has a parallelepiped shape. The shape of the unit cell and the
spacer is not limited as such. For example, as shown in the
embodiments above, the unit cell may adopt various shapes such as a
shape of an elongated strip other than the parallelepiped shape.
The spacer may be identical to or different from the unit cell in
shape. In addition, the number of unit cells constituting the fuel
cell layer and the number of spacers constituting the spacer layer
are not particularly limited, so long as the number is one or more.
It is noted that at least one of the fuel cell layer and the spacer
layer preferably has a gap region. Moreover, a width of the gap
region (a distance between the unit cells or between the spacers)
in the fuel cell layer and the spacer layer may be the same or
different. Further, it is not necessary to arrange the unit cells
and the spacers in parallel or substantially in parallel.
Furthermore, an angle of intersection of the spacer and the unit
cell is not limited to 90.degree., and various angles may be
adopted. Other possible modifications of the present embodiment are
the same as in Embodiment 1 and the like above. Though the spacer
layer is stacked on the fuel cell layer in FIG. 41, the fuel cell
layer may be stacked on the spacer layer.
[0227] In addition, the spacer layer may include at least two
spacers that are not arranged at a distance from each other or
arranged at a distance from each other only in part. For example,
the spacer layer can include at least two spacers arranged such
that at least a part of a side surface of a spacer comes in contact
with at least a part of a side surface of another spacer. Moreover,
the spacer layer does not have to have a gap region. For example,
the spacer layer may have no gap region and may be formed of a
single spacer, like second fuel cell layer 502 shown in FIG. 5.
[0228] In order to improve porosity in the fuel cell stack and to
achieve good air permeability, the spacer is preferably made of a
porous body. Particularly when the spacer layer does not have a gap
region, from a point of view of facilitating movement of a gas
within the fuel cell stack, the spacer forming the spacer layer is
preferably made of a porous body. It is noted that various
modifications described above are also suitably applied to the
embodiments below.
Embodiment 17
[0229] FIG. 42 is a schematic diagram showing a preferred example
of the fuel cell stack having the spacer layer, FIG. 42(a) being a
perspective view thereof, FIG. 42(b) being a top view and FIG.
42(c) being a side view. As illustrated, the present embodiment has
such a structure that a fuel cell layer 4202 constituted of five
unit cells 4201 arranged substantially in parallel at a distance
from one another and a spacer layer 4204 constituted of five
spacers 4203 arranged substantially in parallel at a distance from
one another are alternately stacked. Such a structure is the same
as a structure in which a plurality of basic structures shown in
FIG. 41 are stacked (the fuel cell stack in FIG. 42 is obtained by
stacking three basic structures in FIG. 41), and it can also be
defined as a structure in which some fuel cell layers in the fuel
cell stack in FIG. 3 were replaced with the spacer layers. In the
present embodiment, the spacer layer is identical in shape to the
fuel cell layer. Thus, the fuel or the air is readily taken into
the fuel cell stack and a stable stack can be obtained. In
addition, the spacer layer preferably conducts electrons. Thus,
serial wiring in the direction of stack is facilitated and an
electrical conduction path is shortest, so that electrical
resistance can be suppressed. Possible modifications of the present
embodiment are the same as in Embodiment 16 and the like above. It
is noted that a lowermost layer and an uppermost layer in the fuel
cell stack in the present embodiment may be formed by the fuel cell
layer or by the spacer layer. In addition, as shown in FIG. 42, the
unit cell and the spacer are preferably in a shape of a planar
elongated strip. As a contact area as stacked is thus larger than
that of a shape of an elongated strip having surface irregularities
or a cylindrical shape, physical strength of the obtained fuel cell
stack is improved. Moreover, as the contact area between the unit
cell and the spacer is large, electrical contact resistance can be
low in stacking with electrically serial connection. For example,
when a unit cell in a cylindrical shape having an outer
circumferential surface as a cathode surface is employed, serial
wiring connection in the direction of stack cannot be made.
Electrically serial wiring connection in the direction of stack
while suppressing electrical contact resistance is enabled, so that
a value of a current that flows in the in-plane direction of the
fuel cell layer can be lowered. Accordingly, a thickness of a
current collector can be decreased or the current collector can be
eliminated, and high integration of the fuel cell stack can be
achieved. Consequently, smaller size, lighter weight and lower cost
of the fuel cell stack can be achieved.
[0230] In addition, in the fuel cell stack shown in FIG. 42,
stacking is carried out such that the longitudinal direction
(direction of elongation) of the unit cells constituting the fuel
cell layer is orthogonal to the longitudinal direction (direction
of elongation) of the spacer. Accordingly, an area of a region
where the unit cell and the spacer intersect with each other is
small. As a distance of diffusion of air to the inside of the
intersecting region is thus short, air supply in the intersecting
region is satisfactory. Moreover, by making shorter a length of the
unit cell in a direction of a short side as well, the area of the
intersecting region can be made further smaller. By making shorter
a length in a direction of a short side, a distance of diffusion of
oxygen in the in-plane direction of the unit cell on the cathode
side is short, so that diffusion supply of oxygen can be less
likely to be disturbed even when air is naturally supplied.
Consequently, air can more efficiently be supplied toward the
cathode of the unit cell within the fuel cell stack in which the
fuel cell layers are three-dimensionally stacked, without the use
of auxiliary equipment for supplying the air.
[0231] Here, in the fuel cell stack, as shown for example in FIG.
42, where the fuel cell layer obtained by arranging a plurality of
parallelepiped unit cells identical in shape in parallel at regular
intervals and the spacer layer obtained by similarly arranging a
plurality of parallelepiped spacers identical in shape in parallel
at regular intervals are alternately stacked such that the unit
cells constituting the fuel cell layer are orthogonal to the
spacers constituting the spacer layer adjacent to that fuel cell
layer and where each unit cell constituting the fuel cell layer is
arranged directly under (or directly on) each unit cell
constituting the fuel cell layer adjacent to that fuel cell layer,
referring to FIG. 42, the unit cell preferably has a width W1 not
smaller than 1 mm from a point of view of ease in fabrication of
the unit cell. In addition, in view of the fact that a distance of
convection of oxygen is preferably shorter taking into account
concerns about supply disturbance caused by a natural convection
rate of oxygen, the fact that an area of a region where the spacer
and the unit cell intersect with each other is preferably small,
and the fact that a distance of convection of vapor is preferably
shorter taking into account the fact that quick exhaust of
generated water as vapor is preferred, width W1 of the unit cell is
preferably as small as possible, and specifically, it is preferably
5 mm or smaller. A width W2 of a gap region between the unit cells
(interval between the unit cells) with respect to width W1 of the
unit cell (W2/W1) is preferably not greater than 1 from a point of
view that a greater ratio of an MEA occupied in the fuel cell layer
leads to higher power density of the fuel cell stack, and
preferably it is not smaller than 0.2 from a point of view of ease
in air intake into the fuel cell stack. Provided that W1 and W2
satisfy the range above, a thickness W3 of the spacer is preferably
not smaller than 0.2 mm from a point of view of ease in air intake
and convection in the fuel cell stack and preferably not greater
than 2 mm from a point of view of improvement in power density of
the fuel cell stack.
Embodiment 18
[0232] FIG. 43 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer, FIG. 43(a)
being a perspective view thereof and FIG. 43(b) being a side view.
The fuel cell stack shown in FIG. 43 has such a structure that a
fuel cell layer 4302 constituted of five unit cells 4301 arranged
substantially in parallel at a distance from one another and a
spacer layer 4304 constituted of five spacers 4303 arranged
substantially in parallel at a distance from one another are
alternately stacked. Difference from the fuel cell stack in FIG. 42
resides in that, attention being paid to two adjacent fuel cell
layers with the spacer layer being interposed in the fuel cell
stack in FIG. 42, each unit cell constituting one fuel cell layer
is arranged directly under (or directly on) the unit cell
constituting the other fuel cell layer, while in the fuel cell
stack in FIG. 43, each unit cell constituting one fuel cell layer
is arranged in a region where a unit cell constituting the other
fuel cell layer is absent, that is, directly under (or directly
above) the gap region in the fuel cell layer. The unit cells in the
fuel cell stack in FIG. 43 thus configured are preferably arranged
such that a surface on the cathode side, with an electrolyte
membrane 4305 of unit cell 4301 serving as the reference, faces a
direction of gravity, as shown in FIG. 43(b).
[0233] As the unit cells in the fuel cell stack are arranged in an
orientation as shown in FIG. 43, air within the fuel cell stack is
warmed by heat originated from power generation and the warmed air
moves in a direction (upward direction in FIG. 43(b)) opposite to
the orientation of gravity. Then, ambient air is introduced from
below the fuel cell stack into the fuel cell stack and an ascending
air current is produced. As the fuel cell stack in FIG. 43 is
arranged such that the cathode side of the unit cell faces the
orientation of this ascending air current, a cathode surface of the
unit cell is located on a perpendicular extension line of the gap
region in the fuel cell layer. Therefore, the air that has passed
through the gap region once impinges on a cathode surface of the
unit cell and then diffuses to left and right, and then moves
upward (see an arrow in FIG. 43(b)). Thus, even under such a
passive condition as not using auxiliary equipment such as a fan or
a blower, air can effectively be supplied to the cathode surface of
the unit cell and the air is readily diffused to all corners in a
cathode catalyst. As thus convection resistance of the air can be
lowered and oxygen concentration in a cathode catalyst portion can
be raised, power generation efficiency of the unit cell can be
enhanced. The fuel cell stack achieving high power density is thus
realized. It is noted that possible modifications of the present
embodiment are the same as in Embodiment 16 and the like above. In
addition, in the fuel cell stack in the present embodiment, the
lowermost layer and the uppermost layer may be formed by the fuel
cell layer or the spacer layer.
Embodiment 19
[0234] FIG. 44 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer, FIG. 44(a)
being a perspective view thereof, FIG. 44(b) being a top view and
FIG. 44(c) being a side view. The fuel cell stack shown in FIG. 44
has such a structure that a spacer layer 4404 constituted of five
spacers 4403 arranged substantially in parallel at a distance from
one another and a fuel cell layer 4402 constituted of five unit
cells 4401 arranged substantially in parallel at a distance from
one another are alternately stacked. Here, in the fuel cell stack
in the present embodiment, two spacers located at opposing ends
among spacers 4403 constituting spacer layer 4404 are arranged
directly under and directly on opposing end portions of fuel cell
layer 4402 adjacent to the spacer layer. Namely, the two spacers
located at the opposing ends are arranged to be in contact with end
portions in a longitudinal direction of unit cells 4401
constituting fuel cell layer 4402 adjacent to the spacer layer. In
the example shown in FIG. 44, a position of a sidewall surface on
an outer side of each of the two spacers located on the opposing
ends coincides with a position of a sidewall surface on an outer
side of the unit cell. According to such a configuration, as
physical strength of the fuel cell stack can be improved, the
structure of the fuel cell stack can further be stabilized.
[0235] The fuel cell stack in the present embodiment can be
modified as follows. For example, an end surface of the fuel cell
layer (the sidewall surface on the outer side of the unit cell)
does not necessarily have to coincide with an end surface of the
spacer layer (the sidewall surface on the outer side of each of the
spacers located on the opposing ends), and it may slightly project
outward from the end surface of the spacer layer (the sidewall
surface on the outer side of each of the spacers located on the
opposing ends), or may be located slightly inward. In such a case,
in order to further stabilize the structure of the fuel cell stack,
difference in position between these end surfaces is preferably not
greater than 1 mm. In addition, in the fuel cell stack in the
present embodiment, the lowermost layer and the uppermost layer may
be formed by the fuel cell layer or the spacer layer.
[0236] As in Embodiments 16 to 19 above, the spacer layer may be
constituted of one or more spacer. By forming the spacer layer with
two or more spacers and arranging the spacers at a distance from
each other, a gap region can be formed between the spacers. Here,
the spacer layer is preferably constituted of spacers identical in
outer dimension to a unit cell included in any fuel cell layer in
the fuel cell stack according to the present invention. In
addition, preferably, the spacer layer is identical in shape to any
fuel cell layer, and any fuel cell layer and spacer layer in the
fuel cell stack according to the present invention have gap regions
in a similar shape. Here, being identical in shape means that error
of the outer dimension of all unit cells and spacers is within a
range of .+-.0.25 mm and arrangement and the shape of the unit
cells in the fuel cell layer coincide with those of the spacers in
the spacer layer by inversion or rotation. Thus, such a structure
that the fuel or the air is readily taken into the fuel cell stack
can be obtained, and consequently, the need for auxiliary equipment
can be obviated. Even when the auxiliary equipment is necessary,
power consumption by the auxiliary equipment can be lowered.
Embodiment 20
[0237] FIG. 45 is a schematic diagram showing another preferred
example of the fuel cell stack having the spacer layer, FIG. 45(a)
being a top view and FIG. 45(b) being a side view. The fuel cell
stack shown in FIG. 45 has such a structure that a spacer layer
4504 constituted of a plurality of spacers and a fuel cell layer
4502 constituted of five unit cells 4501 arranged substantially in
parallel at a distance from one another are alternately stacked, as
in the embodiments above. It is noted in the fuel cell stack in the
present embodiment that the spacers constituting spacer layer 4504
are arranged only in a portion sandwiched between two adjacent fuel
cell layers 4502 and they are not arranged in a gap region portion
in the fuel cell layer. Namely, referring to FIG. 45(a), when the
fuel cell stack is viewed from above, a spacer 4503a located in a
central portion of the fuel cell stack (a dotted line region in
FIG. 45(a)) is hidden under unit cell 4501 (formed only in a region
directly under unit cell 4501), and no spacer is present in the gap
region between the unit cells. According to such a configuration,
convection of the air or the fuel into the fuel cell stack is
satisfactory and shortage of oxygen or the fuel in the fuel cell
stack is less likely.
[0238] In the present embodiment, as shown in FIG. 45(a), spacers
located at opposing ends of the spacer layer among the spacers
constituting spacer layer 4504 are denoted as spacers 4503b in a
shape of an elongated strip such as a parallelepiped shape (without
limited to this shape), and these spacers may be arranged to
intersect with a plurality of unit cells constituting adjacent fuel
cell layer 4502 and to be located directly under and directly on
the opposing end portions of unit cell 4501 constituting adjacent
fuel cell layer 4502 or the vicinity thereof. Physical strength of
the fuel cell stack is thus increased. Preferably, as shown in FIG.
45(a), the spacer in a central portion of the fuel cell stack is
not arranged in the gap region in the adjacent fuel cell layer but
the spacer is arranged at opposing ends of the spacer layer across
the plurality of unit cells and the gap regions. As concentration
of oxygen or the fuel is low in a portion around a central portion
of the fuel cell stack, a path for the air or the fuel in the fuel
cell stack is preferably wide in the central portion of the fuel
cell stack, and in order to effectively improve strength of the
fuel cell stack, the spacer is preferably arranged on an outer side
of the fuel cell stack, across the plurality of unit cells and gap
regions. All spacers including also the spacers located at opposing
ends of the spacer layer may naturally be arranged only in the
portion in contact with the two adjacent fuel cell layers.
Embodiment 21
[0239] FIG. 46 is a perspective view showing another preferred
example of the fuel cell stack according to the present invention.
The fuel cell stack in the present embodiment has such a structure
that a fuel cell stack similar to that shown in FIG. 44 (a fuel
cell stack having a spacer layer constituted of spacers 4603) is
stacked on a fuel cell layer formed of a single, relatively large
unit cell in a shape of a flat plate having no gap region. Such a
configuration in which the fuel cell stack is constructed on the
fuel cell layer formed of a single relatively large unit cell is
the same as in Embodiment 15 above, and the same effect can be
obtained. Details and possible modifications of the present
embodiment are also the same as in Embodiment 15 above. For
example, the fuel cell stack stacked on a fuel cell layer formed of
a single unit cell having no gap region is not limited to a stack
having a similar structure shown in FIG. 44, and the fuel cell
stacks in Embodiments 16 to 20 above may be adopted. In addition,
in arranging the fuel cell stack in the present embodiment on a
substrate 4602, as shown in FIG. 46, the fuel cell stack is
preferably arranged such that the fuel cell layer side formed of a
single large unit cell is located on substrate 4602 side. Moreover,
the fuel cell layer formed of a single unit cell above is
preferably arranged such that its anode side (the anode catalyst
layer side with an electrolyte membrane 4601 serving as the
reference) is located on the substrate side. As thus it is not
necessary to provide an air passageway between the substrate and
the fuel cell stack, the anode side of the fuel cell layer can be
brought into contact with the substrate and volume power density
can further be improved.
[0240] Here, the spacers used in Embodiments 16 to 21 above may be
made of a porous body. By being made of a porous body, the fuel or
the air passes through the porous body and the fuel or the air can
be supplied to the catalyst layer through the spacer layer also in
a region where the spacer layer and the fuel cell layer are
stacked. Accordingly, diffusion resistance of the fuel or oxygen is
lowered and more output current can be extracted. Preferred
examples of materials for the spacer layer include polymers
excellent in acid resistance and chemical resistance such as
polyimide, PVDF, PTFE, PEEK.RTM., and the like, as well as
electrically conductive polymers and the like such as polyaniline
and polythiophene. In addition, from a point of view of acid
resistance and chemical resistance, metal oxides such as titanium
oxide, silica and zirconia oxide are preferred. Moreover, from a
point of view of acid resistance, chemical resistance and electron
conductivity, preferred examples include a noble metal such as Au
and Pt, a metal forming passivity on a surface of
corrosion-resistant stainless and titanium, carbon, and the like.
The spacer is preferably formed of one or more of the materials
above Examples of the shape of the spacer include a mesh shape, a
non-woven fabric shape, a foam, a sintered body, a mixture of two
or more of them, and the like.
[0241] When a spacer made of a porous body of which surface is
hydrophilic is employed as the spacer, water produced and condensed
in the fuel cell stack can be absorbed and then exhausted to the
outside of the fuel cell stack. Accordingly, characteristics of the
fuel cell stack can be maintained well for a long time. Examples of
porous bodies of which surface is hydrophilic include non-woven
fabric made of a polymer such as cotton and polyester, a metal
oxide such as titanium oxide and silica, a metal porous body of
which surface alone is coated with a metal oxide, a metal-polymer
composite in which a hydrophilic polymer is applied only to a
surface of a metal porous body for modification, and the like. From
a point of view of reduced length of an electrical path, more
preferably, the porous body is electrically conductive. A porous
body of which surface is hydrophilic may naturally be used together
with a porous body of which surface is water-repellent, a
non-porous body, or the like.
[0242] In addition, the spacer may be made of a non-porous body. By
being made of a non-porous body, physical strength is
advantageously improved as compared with the case of being made of
the porous body. When the spacer is electrically conductive and
also plays a role as an electrical wire, not only physical strength
but also lowering in electrical resistance can be achieved.
[0243] Further, use of the spacer made of a non-porous body and the
spacer made of a porous body as combined is also preferred. In this
case, for example as shown in FIG. 47, as spacers constituting a
spacer layer 4704, more preferably, spacers 4703a made of a porous
body are arranged in high proportion in a central portion of the
fuel cell stack, while spacers 4703b made of a non-porous body are
arranged in high proportion on an outer side of the fuel cell stack
(in the vicinity of opposing ends of the spacer layer).
Concentration of oxygen or the fuel is lower around the central
portion of the fuel cell stack than around an outer peripheral
portion, due to consumption thereof during power generation,
however, use of the spacer made of a porous body can achieve good
diffusion of oxygen or the fuel. Meanwhile, as concentration of
oxygen or the fuel is higher on the outer side of the fuel cell
stack than in the central portion, the non-porous spacer is used to
achieve increased physical strength and lower electrical
resistance. Arrangement of the spacer made of a non-porous body and
the spacer made of a porous body is not limited to that shown in
FIG. 47, and arrangement can be designed as appropriate, for
example, by alternately arranging these spacers, from a point of
view of electrical resistance, supply of a reactant, physical
strength, and the like.
[0244] The number of unit cells in the fuel cell layer in each
embodiment (Embodiments 1 to 21) described above is by way of
example and the number thereof is not necessarily limited thereto.
The number of unit cells within the fuel cell layer can be set as
appropriate, depending on demands for an output voltage, a size or
the like of the fuel cell stack. In addition, in each embodiment
described above, such a structure that a first fuel cell layer and
a second fuel cell layer are alternately stacked has been described
by way of example, however, the structure is not necessarily
limited thereto. For example, a portion where the first fuel cell
layer is stacked on the first fuel cell layer or a portion where
the second fuel cell layer is stacked on the second fuel cell layer
may be present within the fuel cell stack. So long as a region
where the first fuel cell layer and the second fuel cell layer are
adjacently stacked is present in at least a part in the fuel cell
stack, a gap region is formed at least in that region and an effect
of the present invention can be obtained.
[0245] In the fuel cell stack according to the present invention,
when electrical wires in the fuel cell stack are connected in
series, adjacent fuel cell layers are preferably stacked such that
the anode side of the unit cell constituting one fuel cell layer
(the anode electrode) is opposed to the cathode side of the unit
cell constituting the other fuel cell layer (the cathode
electrode), as shown in FIG. 70. On the other hand, for example as
shown in FIG. 71, a structure may naturally be such that a stacking
pattern of the anode electrode and the cathode electrode is
reversed at the substantial center in the direction of stack in the
fuel cell stack, without limited as above. Here, the cathode
electrodes are located on respective opposing outer surfaces of the
fuel cell stack (a lower surface and an upper surface), and
therefore, an area of the cathode electrodes exposed to atmosphere
can be increased. When the anode electrode and the cathode
electrode are arranged in accordance with the configuration shown
in FIG. 71, a portion around the central portion of the fuel cell
stack where concentration of oxygen tends to be low serves as the
anode electrode, which is preferred in that an average value of
concentration of oxygen at the surface of the cathode electrode is
high, in regard to all cathode electrodes constituting the fuel
cell stack. As the concentration of oxygen at the cathode electrode
is high, power generation efficiency is improved and power density
per volume of the fuel cell stack can be improved. In addition, in
exhausting produced water as well, vapor is more readily exhausted
to the outside (for example, to the atmosphere) as the cathode side
is closer to the outer surface. Accordingly, the produced water is
less likely to stay in the fuel cell stack and the produced water
can be less likely to clog an air supply path. Thus, the fuel cell
stack in which the unit cells are further highly integrated can be
fabricated. Power density of the fuel cell stack can thus be
improved in the example shown in FIG. 71, extraction from the cell
is achieved by 4-series and 2-parallel wired connection, in which
the anode electrodes facing with each other in the central portion
serve as a negative terminal and two surfaces of the cathode
electrodes of the fuel cell layer on the respective outer surfaces
are combined as one to serve as a positive terminal. In addition,
as shown in FIG. 72, the fuel cell stack having the spacer layer
between the fuel cell layers can also have a similar structure, and
the similar effect can be obtained.
[0246] The fuel cell stack according to the present invention as
above has a gap region serving as an air passageway in the inside
thereof, so that a gas can readily enter and exit from the inside
of the fuel cell stack. In the fuel cell stack in which unit cells
in a shape of an elongated strip such as a parallelepiped shape are
stacked in a number sign shape or in the fuel cell stack in which
the fuel cell layer and the spacer layer are alternately stacked in
a number sign shape, all air passageways three-dimensionally
communicate with one another, so that a gas can further readily
enter and exit from the inside of the fuel cell stack. Each unit
cell produces water from the cathode electrode side through power
generation reaction. In an example where the fuel cell stack
including highly integrated unit cells and having improved volume
power density is fabricated, the produced water condenses in the
fuel cell stack and clogs the air passageway, which has resulted in
failure to maintain continuous output. According to the present
invention, however, produced water evaporates by heat generated as
loss in power generation by the fuel cell stack and it is
satisfactorily exhausted to the outside of the fuel cell stack.
Thus, increase in a vapor pressure in the fuel cell stack and
condensation is prevented, and thus output electric power can
continuously be supplied in a stable manner. As the unit cells are
highly integrated and stacked, a temperature of the fuel cell stack
is also readily raised and evaporation of water is promoted.
Namely, according to the fuel cell stack of the present invention,
by highly integrating the unit cells, synergistically, produced
water is satisfactorily exhausted as vapor to the outside of the
fuel cell stack without condensed water clogging the air
passageway, oxygen can be taken into the fuel cell stack in a
stable manner, and output electric power can continuously be
supplied in a stable manner. The communicating space (the gap
region) in the fuel cell stack has three-dimensional opening which
opens to the outside of the fuel cell stack. Therefore, however the
orientation of the fuel cell may be varied while being carried as
in being mounted on portable equipment or the like, any opening
faces upward and vapor can satisfactorily be exhausted to the
outside of the fuel cell stack on an ascending air current
originating from heat generation by the fuel cell stack.
[0247] <Structure of Unit Cell>
[0248] The unit cell used in the fuel cell stack according to the
present invention will now be described in detail. Some preferred
examples have already been described. The unit cell used in the
present invention is configured to have at least an electrolyte
membrane and an anode catalyst layer and a cathode catalyst layer
sandwiching the electrolyte membrane. This configuration
corresponds to a minimal configuration of the unit cell. As the
number of members can be reduced by fabricating the fuel cell stack
with this minimal configuration, cost for the members can be
lowered. In addition, as a thickness of the unit cell can be made
smaller, output per volume of the fuel cell stack can be improved.
Moreover, such an effect as simplification of a production process
is achieved, because such an operation process as thermocompression
bonding of a conductive porous layer can be eliminated. In this
structure, since separation of methanol or air supplied to the
anode catalyst layer or the cathode catalyst layer is difficult, a
catalyst highly selective to oxygen or methanol is more preferably
used for at least one of the anode catalyst layer and the cathode
catalyst layer.
[0249] For separation of methanol and air and more efficient
current collection, the unit cell having the minimal configuration
described above is preferably provided with a separator having
electron conductivity. In an example where an anode conductive
porous layer and a cathode conductive porous layer are provided, if
the anode conductive porous layer is provided with a hydrophilic
layer, air introduced in the anode catalyst layer can be suppressed
because diffusion resistance of air is high in the hydrophilic
layer filled with the methanol aqueous solution, whereby the need
for the separator can be obviated. In addition, by providing highly
electrically conductive anode conductive porous layer and cathode
conductive porous layer, electrical resistance between the unit
cells or between the fuel cell layers can be suppressed and higher
output of the fuel cell stack can be achieved. The unit cell shown
in FIG. 2 described already also has such a configuration. In the
unit cell shown in FIG. 2, the methanol aqueous solution serving as
the fuel diffuses in the hydrophilic layer from the fuel flow
channel formed within the anode conductive porous layer through the
fuel permeability control layer and it is supplied to the anode
catalyst layer.
[0250] The unit cell used in the present invention is preferably
formed by stacking the cathode conductive porous layer, the cathode
catalyst layer, the electrolyte membrane, the anode catalyst layer,
and the anode conductive porous layer described above in an
integrated manner. As a result of integration, without an external
pressure, intimate contact between adjacent members constituting
the unit cell is secured and electrons or a substance can
satisfactorily be transported between the adjacent members. Thus, a
member for fastening a constituent member of the unit cell is not
necessary, which allows size reduction of the unit cell. Here,
integration in the present invention refers to such a state that
members in the unit cell are not removed from one another without
external pressure, and specifically to a state joined by chemical
bonding, an anchoring effect, adhesive force, or the like.
[0251] The unit cell used in the present invention is provided with
a fuel flow channel for supplying the methanol aqueous solution
serving as the fuel, and a fuel flow channel forming member is
preferably used for forming the fuel flow channel. The fuel flow
channel is a space for transporting a fuel communicating from a
cartridge holding the fuel, and one or more flow channel for
transportation from the cartridge is preferably provided for each
unit cell. As the fuel flow channel should only establish
communication and have a function for fuel transportation, this
space may be made of a porous material such as a foam metal, a
metal web, a sintered metal, carbon paper, and a carbon cloth. In
order to supply the fuel in an amount necessary and sufficient for
the fuel cell stack, the fuel flow channel forming member is
preferably made of such a material and made in such a shape as
lowering resistance in diffusion of the fuel toward downstream of a
direction of flow of the fuel. For example, in terms of a shape, an
extra fine tube capable of sucking up the fuel by capillarity is
preferred, and in terms of a material, an inner surface of the flow
channel is preferably chemically modified with polar functional
group (hydrophilic) such as OH group or COOH group. As a method of
making a surface hydrophilic, a reform process using a plasma
surface treatment system (PS-601S/PS-1200A manufactured by Kasuga
Electric, Inc.) or the like, or an ashing process used in a
semiconductor process is suitably employed. In addition, the fuel
flow channel preferably has the fuel permeability control layer at
least in a part thereof or the fuel flow channel forming member
itself is preferably made of a material for the fuel permeability
control layer. As thermocompression bonding is generally used in
integration of unit cell constituent members, the fuel flow channel
forming member is more preferably made of a material having high
heat resistance, that is not thermally decomposed at a temperature
around 130 to 180.degree. C. Though a direction or a length of the
fuel flow channel forming member or the formed fuel flow channel in
the direction along the plane of the anode conductive porous layer
is not limited, elongation in a direction perpendicular to a
direction of layer thickness of the unit cell is preferred. Such
arrangement as parallel elongation in the direction of length of
the anode conductive porous layer or arrangement in a serpentine
shape can be exemplified.
[0252] When the fuel is supplied without using a pump representing
external motive power, one end portion of the fuel flow channel
(hereinafter referred to as a fuel flow channel terminal end
portion) downstream of a direction of flow of the fuel in the
formed fuel flow channel is open. This is because, in supplying the
fuel by capillarity, air in the fuel flow channel should be
exhausted to the outside. In order to suppress flow of the fuel out
of the fuel flow channel terminal end portion, an opening has a
diameter preferably in a range from 1 .mu.m to 500 .mu.m. Here, air
may be introduced in the fuel cartridge, or the pressure in the
fuel flow channel may be lowered when supply of the methanol
aqueous solution is stopped, which leads, for example, to
introduction of air. Accordingly, the diameter of the opening is
preferably decreased by using a porous material capable of
exhausting air in the fuel flow channel while suppressing flow-out
of the fuel. Such a porous material is preferably the same as a
material for the gas-liquid separation layer, and a specific
example includes a porous layer made of a mixture of fluorine-based
polymer represented by PTFE (polytetrafluoroethylene) or PVDF
(polyvinylidenfluoride) and a carbon material.
[0253] It is noted that other unit cell constituent members may
also attain a function for fuel transportation, which is a function
of the fuel flow channel forming member. Thus, the need for the
fuel flow channel forming member can be obviated, cost can be
reduced, and the number of fabrication steps can be decreased.
Specifically, the anode conductive porous layer can be provided
with a fuel transportation function, which means that the fuel flow
channel is formed in the anode conductive porous layer. In this
case, the anode conductive porous layer preferably has a function
to transport the fuel in a direction perpendicular to the direction
of layer thickness of the anode conductive porous layer in the
inside thereof or along its surface. Means for providing a space
communicating in a direction perpendicular to the direction of
layer thickness in the anode conductive porous layer can be
exemplified as means for providing such a transportation function.
More specifically, examples of such means include a metal plate in
which a flow channel is formed, a foam metal, a metal web, a
sintered metal, carbon paper, a carbon cloth, and the like.
[0254] The structure of the unit cell preferably used in the
present invention and fuel supply to the fuel cell stack will now
be described with reference to the drawings. In the present
invention, fuel is supplied preferably by forming a fuel flow
channel with a fuel flow channel forming member or by providing the
anode conductive porous layer with a function as a fuel flow
channel forming member. Here, at least a part of the fuel flow
channel is preferably formed within the anode catalyst layer or the
anode conductive porous layer, from a point of view of a smaller
thickness of the unit cell.
[0255] Fuel supply to the fuel cell stack in the present invention
can be achieved by using the unit cell in which the anode
conductive porous layer is provided with a function as the fuel
flow channel, as shown in FIGS. 22 and 23.
[0256] In the unit cell in FIG. 22, an anode conductive porous
layer 2204 also serves as the fuel flow channel. In the inside of
the unit cell, the fuel is transported in a direction perpendicular
to a direction of layer thickness of anode conductive porous layer
2204 and the transported fuel permeates toward a surface of contact
with an anode catalyst layer 2202 for supply. Regarding
transportation of the fuel in anode conductive porous layer 2204,
quick diffusion in a direction perpendicular to the direction of
layer thickness can achieve uniform fuel supply to anode catalyst
layer 2202, and therefore, a coefficient of diffusion in the
direction perpendicular to the direction of layer thickness is
preferably greater than a coefficient of diffusion in the direction
of layer thickness. In the present structure, as anode conductive
porous layer 2204 also serves as the fuel flow channel, it is not
necessary to separately provide a fuel flow channel, which is
advantageous in that the number of members necessary for
configuring the unit cell is decreased and fabrication is
facilitated. In addition, anode conductive porous layer 2204
required in the present structure preferably has prescribed
strength for a thermocompression bonding process in integration of
the unit cell, and it is preferably made of a material of which
dimension or structure is not varied by 30% or more even pressing
under a pressure of 0.1 t/cm.sup.2. In addition, in order to attain
a function as the fuel flow channel, a material that is not
dissolved and does not contract and expand depending on a fuel,
water and a temperature for use is preferred. Examples of materials
satisfying such conditions include a foam metal, a metal web and a
sintered metal.
[0257] In order to improve a fuel holding function of anode
conductive porous layer 2204, a hydrophilic layer 2206 is provided
between anode conductive porous layer 2204 and anode catalyst layer
2202. Hydrophilic layer 2206 not only holds a fuel aqueous solution
but also prevents oxygen in the air from reaching anode catalyst
layer 2202. Though the methanol aqueous solution is supplied to
anode catalyst layer 2202 through hydrophilic layer 2206, air that
passes through anode conductive porous layer 2204 cannot pass
through, because hydrophilic layer 2206 is filled with the methanol
aqueous solution and there are not sufficient holes for air
diffusion. Therefore, the air is not supplied to anode catalyst
layer 2202 and methanol oxidation reaction at anode catalyst layer
2202 is not impeded. Thus, the need for a separator for preventing
the air from reaching anode catalyst layer 2202 can be obviated,
and the fuel cell stack achieving lower cost, a smaller thickness
and higher power density can be realized. A gas such as carbon
dioxide, which is a by-product generated as a result of power
generation reaction, is gaseous at room temperature, and its volume
is very large as compared with methanol and water representing a
reactant. Accordingly, the pressure in anode catalyst layer 2202 is
increased. As a result of pressure increase, carbon dioxide is
either exhausted in a direction along the plane perpendicular to
the direction of layer thickness of the unit cell member (a lateral
direction in the drawing) or exhausted to atmosphere through a
through hole 2208 provided in anode conductive porous layer 2204
after passing through hydrophilic layer 2206. As carbon dioxide is
satisfactorily exhausted from anode conductive porous layer 2204
through hydrophilic layer 2206, it is not necessary to exhaust
carbon dioxide generated in the flow channel together with the fuel
as in the conventional example, and the fuel can be supplied in a
stable manner without the use of auxiliary equipment such as a
pump.
[0258] Hydrophilic layer 2206 preferably connects anode catalyst
layer 2202 and anode conductive porous layer 2204 to each other
with low electrical resistance, and hydrophilicity is preferably
improved by chemically modifying a surface of catalyst carrying
carbon of anode catalyst layer 2202 or anode conductive porous
layer 2204 with polar functional group (hydrophilic group) such as
OH or COOH, in terms of better wettability therebetween and
improvement in adhesiveness. A preferred example is composed of
conductive particles in which a surface of carbon particles such as
acetylene black and Ketjen Black is chemically modified with polar
functional group (hydrophilic group) such as OH group or COOH
group, and a polymer having hydrophilic functional group
functioning as a binder, such as Nafion 117, Nafion 112, Flemion,
or sulfonated polyimide. Here, as shown in FIG. 22, instead of
forming the hydrophilic layer by subjecting only the surface of
anode conductive porous layer 2204 to hydrophilic treatment, the
inside of the porous layer may be subjected to hydrophilic
treatment to replace the hydrophilic layer.
[0259] Further, as shown in FIG. 22, in order to prevent leakage of
the methanol aqueous solution to atmosphere, a gas-liquid
separation layer 2207 is preferably formed on a surface of anode
conductive porous layer 2204 opposed to hydrophilic layer 2206. In
the fuel cell stack according to the present invention, the anode
conductive porous layer in the unit cell should be in electrical
contact with the cathode conductive porous layer or the current
collector in the adjacent unit cell. Therefore, electrical
conductivity is preferably improved by further adding an
electrically conductive material to gas-liquid separation layer
2207.
[0260] In the unit cell shown in FIG. 23, a fuel flow channel 2306
is formed in a surface of an anode conductive porous layer 2304 on
the side of an anode catalyst layer 2302. The fuel is transported
through fuel flow channel 2306 and supplied to anode catalyst layer
2302 through a fuel permeability control layer 2308 provided to
cover fuel flow channel 2306. Examples of a method for forming fuel
flow channel 2306 include melt extrusion, transfer molding,
compression molding, and the like, if anode conductive porous layer
2304 is made of an electrically conductive polymer or the like. If
anode conductive porous layer 2304 is made of a metal, a
metal-containing material such as a nitride, a carbide, a
carbonitride, or the like thereof, or the like, examples include
press forming and etching process. If anode conductive porous layer
2304 is made of carbon, examples include compression molding,
injection molding and the like.
[0261] Anode conductive porous layer 2304 including fuel flow
channel 2306 preferably has prescribed strength for a
thermocompression bonding process in integration of the unit cell,
and it is preferably made of a material having less surface strain,
of which dimension or structure is not varied by 10% or more even
pressing under a pressure of 0.1 t/cm.sup.2. In consideration of
electrical conductivity or ease in molding, a plate-shaped
electrically conductive substrate having a fuel flow channel and a
through hole for exhausting CO.sub.2 and made of a metal or a
nitride, a carbide, a carbonitride, or the like thereof is more
preferably used as anode conductive porous layer 2304. Here, the
fuel flow channel is not limited so long as it is a communicating
space as described above. Accordingly, a flow channel space formed
in the surface of the anode conductive porous layer may be
configured such that it is buried with a porous body material such
as a foam metal, a metal web and a sintered metal and the
communicating space in the porous material serves as the fuel flow
channel. As the fuel flow channel is formed of a porous material,
strength of a member required in the thermocompression bonding
process can be improved and the space of the fuel flow channel can
be prevented from collapsing. In addition, the porous body material
described above may be a part of the anode conductive porous layer,
and in this case as well, the fuel flow channel can be prevented
from collapsing.
[0262] In the unit cell shown in FIG. 23, in order to suppress
passage of methanol, preferably, fuel permeability control layer
2308 is provided for a part of fuel flow channel 2306 and it is
arranged such that electrical contact between anode conductive
porous layer 2304 and anode catalyst layer 2302 is maintained with
the hydrophilic layer (not shown) being interposed. The role or the
material of the hydrophilic layer is as described above. Further,
in addition to the hydrophilic layer described above, a surface of
a fuel flow channel forming portion of the anode conductive porous
layer serving as an inner wall of the fuel flow channel and a
surface of the fuel permeability control layer on the side forming
an inner surface of the fuel flow channel are preferably subjected
to hydrophilic treatment for lowering diffusion resistance of
methanol or the methanol aqueous solution. Examples of such
hydrophilic treatment include a method of chemically modifying the
surfaces with polar functional group (hydrophilic group) such as OH
group or COOH group. By applying the fuel flow channel formed by
the surface subjected to hydrophilic treatment, diffusion
resistance of the methanol aqueous solution is low and the methanol
aqueous solution can quickly be transported in a direction of the
flow channel. In addition, though the methanol aqueous solution is
supplied to anode catalyst layer 2302 through fuel permeability
control layer 2308, carbon dioxide does not stay in fuel flow
channel 2306 and does not disturb diffusion of the methanol aqueous
solution, because fuel permeability control layer 2308 does not
allow passage of carbon dioxide. Therefore, supply at a constant
flow rate by using a liquid delivery pump or the like is not
necessary, and the methanol aqueous solution can be supplied
without using auxiliary equipment. Here, the fuel is supplied
through fuel flow channel 2306 by capillarity.
[0263] In the unit cell shown in FIG. 23, in order to prevent
leakage of the methanol aqueous solution to atmosphere, it is not
necessary to separately provide a gas-liquid separation layer as in
the unit cell shown in FIG. 22. If it is difficult to hold the
fuel, however, a gas-liquid separation layer is preferably further
provided between anode conductive porous layer 2304 and the
hydrophilic layer. Formation of the gas-liquid separation layer is
the same as in the unit cell shown in FIG. 22.
[0264] In addition, the fuel may be supplied to the fuel cell stack
according to the present invention by using a unit cell in which a
fuel flow channel is separately arranged in a portion adjacent to
the anode catalyst layer, as shown in FIGS. 24 to 27.
[0265] In the unit cell shown in FIG. 24, a bracket-shaped flow
channel 2410 is formed on a hydrophilic layer 2407 formed on a
surface of an anode conductive porous layer 2404, and a fuel
permeability control layer 2409 is provided to cover bracket-shaped
flow channel 2410 so that the fuel is supplied through this layer.
Naturally, bracket-shaped flow channel 2410 may be formed on the
surface of anode conductive porous layer 2404. In the configuration
illustrated in FIG. 24, bracket-shaped flow channel 2410 and fuel
permeability control layer 2409 serve as flow channel constituent
members to thereby form the fuel flow channel. In the unit cell,
atmosphere is acidic because protons are generated as a result of
reaction at the anode catalyst and they are conducted from the
anode to the cathode. Therefore, bracket-shaped flow channel 2410
is preferably made of a highly acid-resistant material. In
addition, for a thermocompression bonding process in integration of
the unit cell, preferably, bracket-shaped flow channel 2410 has
prescribed strength and it is made of a material of which dimension
or structure is not varied by 10% or more even pressing under a
pressure of 0.1 t/cm.sup.2. Moreover, bracket-shaped flow channel
2410 is preferably made of a material that is not dissolved and
does not contract and expand depending on a fuel, water and a
temperature for use. As a material satisfying such conditions,
polyimide, PTFE (polytetrafluoroethylene), PVDF
(polyvinylidenfluoride), polycarbonate, polyethylene,
polypropylene, a polyacrylic resin, a polyolefin-based polymer, an
epoxy-based resist resin, a noble metal such as Au, Pt and Pd, a
metal such as C, Ti, Ta, W, Nb, Ni, Al, Cr, Ag, Cu, Zn, and Su, Si,
a nitride, a carbide or the like thereof, an alloy such as
stainless, Ti--Pt, and the like are preferably used. In addition,
when a metal poor in corrosion resistance under acid atmosphere
such as Cu, Ag and Zn is used, coating with a noble metal such as
Au, Ag and Pt is preferred. Further, the inner wall of
bracket-shaped flow channel 2410 is preferably subjected to
hydrophilic treatment in order to lower diffusion resistance of the
methanol aqueous solution. For example, a method of forming a
surface of the inner wall by chemically modifying the same with
polar functional group (hydrophilic group) such as OH group or COOH
group, a reform process using a plasma surface treatment system
(PS-601S/PS-1200A manufactured by Kasuga Electric, Inc.) or the
like, an ashing process used in a semiconductor process, and the
like are available.
[0266] Examples of a method of forming bracket-shaped flow channel
2410 include melt extrusion, transfer molding, compression molding,
and the like if it is made of a fluorine-based resin or the like,
examples thereof include press forming and etching process if it is
made of a metal or the like, and examples thereof include
compression molding, injection molding, and the like if it is made
of carbon.
[0267] Bracket-shaped flow channel 2410 does not necessarily have
to be in contact with hydrophilic layer 2407 formed on the surface
of anode conductive porous layer 2404 or with anode conductive
porous layer 2404, and it may be arranged to be embedded in anode
catalyst layer 2402. In addition, bracket-shaped flow channel 2410
does not necessarily have to be independent of each other, and
bracket-shaped flow channels 2410 may be connected to each other.
If bracket-shaped flow channels 2410 are connected to each other,
such arrangement as preventing electrical contact between anode
catalyst layer 2402 and anode conductive porous layer 2404 or as
blocking a proton conduction path within anode catalyst layer 2402
is not preferred, and it is necessary, for example, to provide a
hole in a connection portion.
[0268] The methanol aqueous solution passes through fuel
permeability control layer 2409 and it is supplied to anode
catalyst layer 2402. Air that passes through anode conductive
porous layer 2404 cannot pass through, because there are not
sufficient holes for air diffusion due to presence of provided
hydrophilic layer 2407, as in the unit cell in FIG. 22. Anode
catalyst layer 2402 comes in electrical contact with anode
conductive porous layer 2404 with hydrophilic layer 2407 being
interposed, at low contact resistance. In addition, presence of
fuel permeability control layer 2409 avoids introduction of carbon
dioxide generated in anode catalyst layer 2402 into the fuel. As at
least a part of the fuel flow channel is thus formed by fuel
permeability control layer 2409, a direction of passage of the fuel
can be determined and introduction of carbon dioxide into the fuel
flow channel can be prevented. In addition to the fact that carbon
dioxide does not disturb supply of the methanol aqueous solution,
the methanol aqueous solution is quickly transported in the
direction of flow channel by hydrophilicity of the inner wall of
the fuel flow channel or by capillarity and it can be supplied to
the catalyst layer in a stable manner. Supply at a constant flow
rate by using a liquid delivery pump or the like is thus not
necessary, and the methanol aqueous solution can be supplied
without using auxiliary equipment. It is not necessary to
separately provide a gas-liquid separation layer as in the unit
cell shown in FIG. 22 in order to prevent leakage of the methanol
aqueous solution to atmosphere, however, if it is difficult to hold
the fuel, a gas-liquid separation layer is preferably further
provided between anode conductive porous layer 2404 and hydrophilic
layer 2407. Formation of the gas-liquid separation layer is the
same as in the unit cell shown in FIG. 22.
[0269] In the unit cell shown in FIG. 25, a tubular fuel flow
channel 2506 is formed by a hollow fuel permeability control layer
2509. The fuel passes through this layer from an inner wall side to
an outer wall side of the tube and the fuel is supplied to an anode
catalyst layer 2502. As tubular fuel flow channel 2506 is formed by
fuel permeability control layer 2509, methanol can uniformly be
supplied in all directions. In addition, as fuel flow channel 2506
also functions as fuel permeability control layer 2509, the number
of necessary members can be decreased and thus cost can be reduced.
Here, "tubular" does not necessarily mean that fuel flow channel
2506 has an annular cross-section as shown in the drawing and it
can have various shapes such as an oval shape.
[0270] In addition, for a thermocompression bonding process in
integration of the unit cell, the fuel permeability control layer
forming the fuel flow channel preferably has prescribed strength
and it is preferably made of a material of which dimension or
structure is not varied by 10% or more even pressing under a
pressure of 0.1 t/cm.sup.2. It is noted, however, that a material
of which dimension or structure is varied by 10% or more may be
used provided that methanol can be transported in the direction of
flow channel along the inner wall of the tube in a deformed state.
Here, the unit cell is preferably integrated in consideration of
volume change, for example, by providing a fuel flow channel
forming member or a space formed by the fuel flow channel forming
member in advance with a space that expands by methanol. In
addition, fuel permeability control layer 2509 forming fuel flow
channel 2506 is preferably made of a material that is not dissolved
and does not contract and expand depending on a fuel, water and a
temperature for use. Examples of a material for fuel permeability
control layer 2509 forming fuel flow channel 2506 include a
microtube made of silicon (hereinafter referred to as a silicon
tube), a hollow fiber membrane made of poly-4-methyl-1-pentene,
polyimide, polyamide, or polyethylene, and a semipermeable membrane
tube made of a porous membrane of regenerated cellulose
(cellophane), acetylcellulose, polyacrylonitrile, Teflon.RTM., or
polysulfone.
[0271] In order to lower diffusion resistance of methanol or the
methanol aqueous solution, the inner wall of the fuel flow channel
is preferably subjected to hydrophilic treatment. For example, a
method of chemically modifying a surface of the inner wall of fuel
permeability control layer 2509 with polar functional group
(hydrophilic group) such as OH group or COOH group, a reform
process using a plasma surface treatment system (PS-601S/PS-1200A
manufactured by Kasuga Electric, Inc.) or the like, an ashing
process used in a semiconductor process, and the like are
available. According to the fuel flow channel subjected to
hydrophilic treatment, diffusion resistance of the methanol aqueous
solution is low and methanol can quickly be transported in the
direction of flow channel. In addition, though the methanol aqueous
solution is supplied to anode catalyst layer 2502 through fuel
permeability control layer 2509, carbon dioxide does not stay in
fuel flow channel 2506 and does not disturb diffusion of methanol,
because fuel permeability control layer 2509 does not allow passage
of carbon dioxide. Therefore, supply at a constant flow rate by
using a liquid delivery pump or the like is not necessary but
supply can be achieved by capillarity. Therefore, the methanol
aqueous solution can be supplied without using auxiliary equipment.
A hydrophilic layer 2507 may be provided between anode catalyst
layer 2502 and anode conductive porous layer 2504. A gas-liquid
separation layer may be provided between anode conductive porous
layer 2504 and hydrophilic layer 2507, if necessary.
[0272] In the unit cell shown in FIG. 26, a fuel flow channel 2606
is formed on a hydrophilic layer 2607 formed on a surface of an
anode conductive porous layer 2604. The fuel passes from an inner
wall side to an outer wall side of fuel flow channel 2606 and it is
supplied to an anode catalyst layer 2602. Naturally, fuel flow
channel 2606 may be formed on the surface of anode conductive
porous layer 2604. For a thermocompression bonding process, fuel
flow channel 2606 preferably has prescribed strength and it is
preferably made of a material of which dimension or structure is
not varied by 10% or more even pressing under a pressure of 0.1
t/cm.sup.2. In addition, fuel flow channel 2606 is preferably made
of a highly acid-resistant material such as an acid-resistant
photosensitive resin used for a photoresist, such as polyimide.
PTFE (polytetrafluoroethylene), PVDF (polyvinylidenfluoride),
polycarbonate, polyethylene, polypropylene, a polyacrylic resin, a
polyolefin-based polymer, and a polyepoxy-based resin, a poly-metal
oxide, and the like. In order to form a fine fuel flow channel, an
acid-resistant photosensitive resin used for a photoresist is more
preferably used. An exemplary procedure for forming the fuel flow
channel is a method of forming a lateral wall of the flow channel
by hot-laminating a dry film of a photosensitive resin to a surface
of the anode conductive porous layer, thereafter arranging an
ultraviolet-ray or X-ray non-transmissive mask having a plurality
of through holes like slits, and irradiating the mask with
ultraviolet rays or X-rays to polymerize and cure the resin in a
slit portion. As the resin is not polymerized and cured in a
portion that was not irradiated due to presence of the mask, it can
readily be removed with a developer. Thereafter, complete curing is
carried out. The formed lateral wall of the flow channel extends
perpendicularly to a surface of the anode conductive porous layer.
In order to achieve a uniform depth of the flow channel, tolerance
of a thickness of the cured photosensitive resin is preferably
within a range of 5%. In addition, a dry film of a photosensitive
resin is arranged on the formed lateral wall of the flow channel in
parallel to the anode conductive porous layer with hot-laminating
and an upper wall of the flow channel is formed by using a mask
similarly provided with slits, to thereby form the fuel flow
channel. In order to facilitate adjustment of a fuel supply amount,
a hole or a slit for passage of the methanol aqueous solution is
preferably provided in the upper wall of the fuel flow channel. The
hole or the slit is preferably sufficiently small in order to avoid
introduction of carbon dioxide generated in the anode catalyst
layer into the flow channel through the hole or the slit. In
addition, the inner wall of the flow channel described above
(constituted of the lateral wall and the upper wall) fabricated
from the photosensitive resin preferably has improved
hydrophilicity through chemical modification of its surface with
polar functional group (hydrophilic group) such as OH or COOH, a
reform process using a plasma surface treatment system (PS-601
S/PS-1200A manufactured by Kasuga Electric, Inc.) or the like, an
ashing process used in a semiconductor process, and the like. Thus,
carbon dioxide does not stay in fuel flow channel 2606 and does not
disturb diffusion of methanol. Therefore, supply at a constant flow
rate by using a liquid delivery pump or the like is not necessary,
and the methanol aqueous solution can be supplied without using
auxiliary equipment. Hydrophilic layer 2607 may be provided between
anode catalyst layer 2602 and anode conductive porous layer 2604. A
gas-liquid separation layer may be provided between anode
conductive porous layer 2604 and hydrophilic layer 2607, if
necessary.
[0273] The unit cell shown in FIG. 27 is characterized in that, as
in the unit cell in FIG. 25, a tubular fuel flow channel 2706 is
formed by a fuel permeability control layer 2707, the fuel is
supplied through this layer from an inner side to an outer surface
side of the tube, and in addition, a reinforcement member 2709 is
provided to be embedded in an anode catalyst layer 2702.
Reinforcement member 2709 is arranged to suppress change in a
thickness or a shape of anode catalyst layer 2702 and fuel flow
channel 2706 during thermocompression bonding generally used in
integration of unit cell members, with an expectation to facilitate
arrangement of the tubular flow channel and to suppress
displacement of the fuel flow channel during thermocompression
bonding or the like. A material for reinforcement member 2709 is
preferably an acid-resistant material that is not dissolved and
does not contract and expand depending on a fuel, water and a
temperature for use. In addition, as thermocompression bonding is
employed, the material preferably has prescribed strength and
preferably its dimension or structure is not varied by 10% or more
even pressing under a pressure of 0.1 t/cm.sup.2. Examples of
materials satisfying such conditions include an acid-resistant
photosensitive resin used for a photoresist, such as polyimide,
PTFE (polytetrafluoroethylene), PVDF (polyvinylidenfluoide),
polycarbonate, polyethylene, polypropylene, a polyacrylic resin, a
polyolefin-based polymer, and a polyepoxy-based resin, a poly-metal
oxide, and the like. In order for the reinforcement members to
extend at very short intervals, an acid-resistant photosensitive
resin is more preferably used. In addition, in order to facilitate
control of a fuel supply speed or to achieve a uniform thickness of
the catalyst layer, tolerance of a thickness of the reinforcement
member is preferably within a range of 5%. Further, a hydrophilic
layer may be provided between anode catalyst layer 2702 and an
anode conductive porous layer 2704. A gas-liquid separation layer
may be provided between anode conductive porous layer 2704 and the
hydrophilic layer, if necessary.
[0274] In addition, in order to obtain a desired fuel supply flux
or to maintain a shape of the unit cell member, construction based
on combination of the fuel flow channels shown in FIGS. 22 to 27 is
also possible. For example, the fuel flow channel shown in FIG. 24
and the fuel flow channel shown in FIG. 25 may be combined and
arranged.
[0275] The present invention may include a
current-collector-integrated MEA (Membrane Electrode Assembly) or a
current-collector-integrated unit cell having such a structure that
a cathode conductive porous layer, a cathode current collector, a
cathode catalyst layer, an electrolyte layer, an anode catalyst
layer, an anode current collector, and an anode conductive porous
layer are stacked in this order. By integrating the current
collector, a satisfactorily low contact resistance between the
current collector and the catalyst layer can be maintained without
pressing with the use of such a fastening member as a bolt or a
nut, and electrical resistance in the in-plane direction of the
catalyst layer can be lowered. In such a
current-collector-integrated unit cell, for example, a current
collector having a flat-plate-shaped structure including a
plurality of opening portions penetrating in a direction of
thickness, such as a mesh shape or a punching metal shape of a
noble metal such as Au or a corrosion-resistant metal is preferably
used as the current collector, and at least one of the conductive
porous layer and the catalyst layer preferably enters the opening
portion. In addition, the cathode conductive porous layer and the
anode conductive porous layer are preferably electrically
conductive. The current-collector-integrated MEA and the
current-collector-integrated unit cell can be fabricated by
fabricating two stack structures (for the anode side and the
cathode side) each obtained by fixing the current collector on the
conductive porous layer and applying a catalyst thereon to form a
catalyst layer, and thereafter by hot pressing the resultant
structures with an electrolyte membrane being sandwiched
therebetween.
[0276] Preferably, the fuel cell stack according to the present
invention is configured such that the fuel cell layers configured
to have at least one unit cell described above are stacked. FIG. 28
is a cross-sectional view schematically showing an example of
preferred bonding and integration of the fuel cell layers in the
present invention. As illustrated, a direction of stacking a
cathode conductive porous layer 2805, a cathode catalyst layer
2803, an electrolyte membrane 2801, an anode catalyst layer 2802,
and an anode conductive porous layer 2804 constituting the unit
cell is preferably the same as a direction of stack of the fuel
cell layers. For example, when the direction of stack of the layers
constituting the unit cell is perpendicular to the direction of
stack of the fuel cell layers, the anode conductive porous layers
come in contact with each other or the cathode conductive porous
layers come in contact with each other between the adjacent fuel
cell layers, and therefore, an insulating layer for insulation
therebetween should be provided and the number of required members
increases. In addition, in arrangement in a perpendicular
direction, an area of a portion of contact between the fuel cell
layers decreases and electrical resistance in current collection
may increase. Accordingly, the direction of layer thickness of the
unit cell is preferably the same as the direction of layer
thickness of the fuel cell layer. In FIG. 28, cathode catalyst
layer 2803 is arranged above anode catalyst layer 2802 in each fuel
cell layer, however, arrangement is not limited thereto. Stacking
may be such that anode catalyst layer 2802 is located above cathode
catalyst layer 2803 and the direction of stack of the layers
constituting the unit cell is the same as the direction of stack of
the fuel cell layers.
[0277] As shown in FIG. 28, the fuel cell layers are preferably
stacked in series in the direction of stack with a bonding layer
2811 which will be described later being interposed. The serially
stacked structure refers to such a stack structure that the cathode
conductive porous layer in the fuel cell layer is electrically
connected to the anode conductive porous layer in the fuel cell
layer stacked on the former fuel cell layer. As the serially
connected structure does not require another wiring, a
manufacturing process can be simplified.
[0278] Bonding between the fuel cell layers will be described with
reference to FIG. 28. Bonding between the fuel cell layers is
preferably carried out by using an electrically conductive
adhesive, with electrically conductive bonding layer 2811 made of
the electrically conductive adhesive being interposed. In addition,
the electrically conductive adhesive is more preferably a mixture
of a thermosetting polymeric adhesive and highly electrically
conductive powders Examples of the thermosetting polymeric adhesive
include a polyolefin-based polymer such as 1152B of Three Bond Co.,
Ltd., as well as an epoxy resin, a phenolic resin, a melamine
resin, and a urea resin. The thermosetting polymeric adhesive
undergoes heated polymerization during thermocompression bonding
under pressure and thereafter it is cooled to room temperature
still under pressure, whereby the fuel cell layers are
satisfactorily joined to each other. Therefore, by using the
thermosetting polymeric adhesive, a fastening member can be
eliminated. In an example using a two-component epoxy resin such as
Quick 5 manufactured by Konishi Co., Ltd., it is cured at a low
temperature from room temperature to about 100.degree. C.
Therefore, bonding is more preferably carried out in a slightly
pressurized state at room temperature.
[0279] For example, powders of corrosion-resistant materials
including a carbon-based material such as acetylene black, Ketjen
Black, amorphous carbon, carbon nanotube, and carbon nanohorn, a
noble metal such as Au, Pt and Pd, a metal such as Ti, Ta, W, Nb,
Ni, Al, Cr, Ag, Cu, Zn, and Su, Si, a nitride, a carbide, a
carbonitride, or the like thereof, an alloy such as stainless,
Cu--Cr, Ni--Cr and Ti--Pt are preferably used as the highly
electrically conductive powders. The highly electrically conductive
powders not only connect the fuel cell layers to each other with
low electron conduction resistance, but also allows carbon dioxide
generated from anode catalyst layer 2802 to escape through a gap
formed between powders toward cathode conductive porous layer 2805,
so that carbon dioxide can be exhausted to the gap region in the
fuel cell stack through cathode conductive porous layer 2805. In
the case of an electrically conductive adhesive having an
insufficient gap between powders, electrically conductive bonding
layer 2811 should be applied such that a prescribed gap is created
between the fuel cell layers, rather than applying the same on the
entire surfaces of the opposing fuel cell layers. Namely, for
example, the electrically conductive adhesive is applied only to a
portion around an outer side of a portion where the first fuel cell
layer and the second fuel cell layer are opposed to each other for
bonding, in order not to clog through holes 2810 provided in anode
conductive porous layer 2804 in the fuel cell in the second layer.
As the gap region is thus formed between the fuel cell layers,
carbon dioxide can reliably be exhausted through through hole 2810
and cathode conductive porous layer 2805.
[0280] In addition, the hole in anode conductive porous layer 2804
in one of adjacent fuel cell layers and the hole in cathode
conductive porous layer 2805 in the other fuel cell layer
preferably communicate with each other. A part of water or vapor
produced at cathode catalyst layer 2803 passes through that gap and
further through anode conductive porous layer 2804 to reach anode
catalyst layer 2802 or a hydrophilic layer 2809, so that it can be
reused as a reactant at the anode. The water produced on the
cathode side can thus be reused on the anode side.
[0281] As shown in FIG. 28, in the fuel cell stack according to the
present invention, cathode conductive porous layer 2805 in the fuel
cell layer and anode conductive porous layer 2804 in the fuel cell
layer stacked over the same and adjacent thereto are preferably
integrated. Cathode conductive porous layer 2805 and anode
conductive porous layer 2804 are bonded by bonding layer 2811 and
integrated to each other. Bonding layer 2811 is electrically
conductive and porous, so that good electrical contact between the
stacked fuel cell layers can be ensured without external pressure.
As a presser, a bolt, a nut, and the like, that are members for
fastening the stacked fuel cell layers to each other, are thus not
required, a smaller size and lower cost of the fuel cell stack can
be achieved. Details will be described in a section of a
manufacturing method. In addition, as described above, water
produced at cathode catalyst layer 2803 evaporates and passes
through cathode conductive porous layer 2805, bonding layer 2811
and anode conductive porous layer 2804, it is cooled by hydrophilic
layer 2809 as a result of temperature difference, and a part
thereof is trapped. This water can be used for reaction at anode
catalyst layer 2802. As the water produced on the cathode side can
thus be reused, methanol concentration in the methanol aqueous
solution in the fuel cartridge can be raised, and power generation
for a long time can be achieved with a small fuel cartridge.
[0282] <Fuel Supply>
[0283] In the fuel cell stack according to the present invention,
as shown in FIG. 29, a fuel such as a methanol aqueous solution can
be supplied from a cylinder holding the fuel through a fuel flow
channel to each unit cell. The cylinder holding the fuel is
preferably arranged in at least any one of gaps in four corners of
the fuel cell stack formed by stacking the unit cells.
[0284] Fuel supply to the fuel cell stack shown in FIGS. 19 and 21
will be described. As shown in FIGS. 30 and 31, radially arranged
unit cells constituting one fuel cell layer are connected to a
first fuel cartridge 3001, and radially arranged unit cells
constituting a fuel cell layer adjacent thereto are connected to a
second fuel cartridge 3002. First fuel cartridge 3001 and second
fuel cartridge 3002 are each installed in a region where one ends
of the unit cells concentrate. As the fuel cartridge is arranged at
where the fuel cells concentrate, routing of the fuel flow channel
is facilitated and manufacturing cost is reduced. In addition, as
the fuel is directly supplied from the fuel cartridge to each unit
cell, the fuel is uniformly supplied to the unit cells. Therefore,
power generation characteristics uniform among the unit cells are
readily obtained and power generation characteristics of the fuel
cell stack can be improved.
[0285] <Method of Manufacturing Fuel Cell Stack>
[0286] So long as the fuel cell stack according to the present
invention is structured as described above, a manufacturing method
thereof is not particularly restricted. Preferably, however, the
method includes at least one of (1) a first step of forming an
opening in a current collector or forming a conductive porous
layer, (2) a second step of forming a catalyst layer, (3) a third
step of integrating constituent members of a unit cell, (4) a
fourth step of forming a fuel cell layer, and (5) a fifth step of
stacking and integrating the current collector and the fuel cell
layer, and more preferably the method includes all these steps.
[0287] (1) First Step
[0288] In an example where a metal plate or a metal foil is
employed as a current collector or the like, a method of forming a
plurality of openings in a plane by a punching method, an etching
method, a laser method, drilling, and the like can be adopted as a
method of forming an opening in a current collector or a method of
forming a conductive porous layer.
[0289] In the punching method, openings can be formed by
fabricating a mold for forming a prescribed opening pattern,
pressing the mold against the current collector or the like, and
performing punching. According to such a manufacturing method, a
plurality of openings can be formed at once without the use of a
special apparatus and inexpensive working can be realized. In
addition, as this is machining free from heat, openings can be
formed without restriction on a material or quality of the material
for a selected current collector or the like.
[0290] In the etching method, openings are formed by using a
photoetching process used in a printed wiring technique or the
like. Openings can be formed by applying or laminating a
photosensitive resist to any one surface of a metal plate or a
metal foil to form a photosensitive layer, forming a resist pattern
for the metal plate or the metal foil such that the pattern remains
after exposure, and performing development and etching. According
to such a manufacturing method, a plurality of openings can be
formed at once and processing of fine opening patterns can be
achieved.
[0291] In the laser method, such laser as excimer laser, carbon
dioxide laser or xenon laser is used for forming openings. Here,
focused laser can be used to form a prescribed opening pattern,
while moving a current collector or the like on an X-Y stage for
each opening.
[0292] (2) Second Step
[0293] As a method of forming a catalyst layer, for example, a
technique to uniformly apply a paste obtained by dispersing
catalyst particles, electrically conductive particles and an
electrolyte in an organic solvent with a bar-coating method, a
screen printing method, a spray coating method, or the like and to
remove the organic solvent in the paste to form the catalyst layer
can be adopted. According to such a manufacturing method, a
catalyst layer having a large number of pores can be formed and an
effective surface area of catalyst particles can be increased. The
catalyst layer means the anode catalyst layer and the cathode
catalyst layer.
[0294] In addition, the catalyst layer can be formed directly on
the conductive porous layer. As the catalyst layer is thus
integrally formed in advance on the conductive porous layer, an
interface having good adhesiveness can be obtained.
[0295] Moreover, the catalyst layer can be formed also by forming a
catalyst layer on a base material such as a plastic film and
thereafter transferring the catalyst layer to the conductive porous
layer. According to such a manufacturing method, a catalyst layer
can separately be formed in advance and dispersibility of catalyst
particles and electrically conductive particles can be improved.
Therefore, even an organic solvent with which an insulating layer
is inevitably formed on a surface of a conductive porous layer can
be used as a solvent for the paste.
[0296] Further, in the step of forming an anode catalyst layer on
the anode conductive porous layer, a method of forming the anode
catalyst layer by arranging a fuel flow channel forming member on a
surface of the anode conductive porous layer on which the anode
catalyst layer is to be formed and forming the anode catalyst layer
from above the fuel flow channel forming member such that the fuel
flow channel forming member is embedded in the anode catalyst layer
can be adopted. As the fuel flow channel forming member is thus
embedded in the anode catalyst layer, an interface where good
adhesiveness between the fuel flow channel forming member and the
anode catalyst layer is achieved can be obtained. Furthermore, in
the step of forming an anode catalyst layer on the anode conductive
porous layer, a method of forming the anode catalyst layer by
forming the anode catalyst layer in advance on the anode conductive
porous layer, then arranging the fuel flow channel forming member
on the anode catalyst layer formed in advance, and forming the
anode catalyst layer from above the fuel flow channel forming
member such that the fuel flow channel forming member is embedded
in the anode catalyst layer can also be adopted. Thus, good
adhesiveness can be obtained at an interface between the anode
catalyst layer and the anode conductive porous layer and an
interface between the fuel flow channel forming member and the
anode catalyst layer.
[0297] (3) Third Step
[0298] As a method of integrating constituent members of the unit
cell, for example, a method of integrally forming the unit cell
through thermocompression bonding can be adopted. For example, the
step of arranging the anode conductive porous layer and the cathode
conductive porous layer on each of which the catalyst layer has
been formed in the second step above, such that the anode catalyst
layer and the cathode catalyst layer are opposed to each other with
the electrolyte membrane being interposed and performing
thermocompression bonding at a temperature exceeding a softening
temperature or a glass transition temperature of an electrolyte in
the electrolyte membrane or the catalyst layer by using a hot
pressing apparatus can be adopted. As the members are thus joined
by chemical bonding, an anchoring effect, adhesive force, or the
like, electron conduction resistance or ionic conduction resistance
at an interface between the members can be lowered. Through this
step, for example, the cathode conductive porous layer, the cathode
catalyst layer, the electrolyte membrane, the anode catalyst layer
in which the fuel flow channel forming member has been embedded,
and the anode conductive porous layer are stacked in the direction
of layer thickness of the unit cell in this order and
integrated.
[0299] (4) Fourth Step
[0300] An example of a method of forming the fuel cell layer
includes a method of preparing a plurality of unit cells fabricated
in the third step above and arranging the unit cells at prescribed
intervals on a plane such that the anode conductive porous layer
and the cathode conductive porous layer face the same direction.
Thus, the fuel cell layer including a plurality of
two-dimensionally arranged unit cells can be formed. In addition,
the fuel cell layer can also be formed by fabricating a first fuel
flow channel fixed by arranging first fuel flow channel forming
members in advance at prescribed intervals and connecting the first
fuel flow channel forming members to second fuel flow channel
forming members such that flow channel spaces communicate with one
another and by fabricating the unit cell on a plane by using the
first fuel flow channel such that the anode conductive porous layer
and the cathode conductive porous layer face the same direction in
accordance with the third step above.
[0301] (5) Fifth Step
[0302] An example of a method of stacking and integrating the
current collector and the fuel cell layer includes the following
method. Initially, using the fuel cell layers obtained in the
fourth step above, the fuel cell layers are stacked such that they
intersect with each other at such a prescribed angle that the anode
conductive porous layer and the cathode conductive porous layer are
opposed to each other and come in contact with each other. Here, an
electrically conductive adhesive is applied to a contact portion in
the anode conductive porous layer or the cathode conductive porous
layer. Then, by subjecting a stack structure obtained by stacking
the current collectors on the anode conductive porous layer and the
cathode conductive porous layer located at opposing ends of the
stack structure to thermocompression bonding, the fuel cell stack
in which the fuel cell layers are integrated can be fabricated. As
the electrically conductive adhesive is applied, the fuel cell
layers are connected to each other with low electron conduction
resistance. Accordingly, higher output of the fuel cell stack can
be achieved and a fastening member can be eliminated as a result of
integration. Preferably, the electrically conductive adhesive is a
mixture of a thermosetting polymeric adhesive and highly
electrically conductive powders. The thermosetting polymeric
adhesive described above is cured by heat during thermocompression
bonding and achieves good bonding between the fuel cell layers.
Therefore, a fastening member is not necessary in the fuel cell
stack. The highly electrically conductive powders connect the fuel
cell layers to each other at low electron conduction resistance,
and in addition some of carbon dioxide generated in the anode
catalyst layer can efficiently be exhausted through a gap formed
between the powders. In integration of the stack structure through
thermocompression bonding, it is also possible to manufacture the
integrated fuel cell stack by carrying out thermocompression
bonding once, after all fuel cell layers and current collectors
constituting the fuel cell stack are stacked. On the other hand,
the integrated fuel cell stack may be manufactured by dividing all
fuel cell layers constituting the fuel cell stack into a plurality
of sets, integrating the fuel cell layers in each set through
thermocompression bonding, stacking the integrated sets and the
current collectors, and carrying out thermocompression bonding
again.
[0303] <<Fuel Cell System>>
[0304] A fuel cell system according to the present invention will
be described. FIG. 32 is a schematic diagram showing a preferred
example of the fuel cell system according to the present invention,
FIG. 32(a) being a perspective view thereof, FIG. 32(b) being a top
view, and FIG. 32(c) being a side view. FIGS. 32(b) and 32(c) show
only the mounted fuel cell stack. The fuel cell system shown in
FIG. 32(a) includes the fuel cell stack as in FIG. 3, switches A,
B, C, D, E, a, b, c, d, and e, and a control circuit. The control
circuit sends a signal to the switches and controls opening and
closing thereof. The switch is implemented, for example, by a
semiconductor device such as a bipolar transistor and a MOS
transistor or a mechanical switch such as an electromagnetic
relay.
[0305] In addition, as shown in FIGS. 32(b) and 32(c), the fuel
cell stack includes current collectors only at opposing ends
thereof, the current collectors are provided in a plurality of unit
cells in the fuel cell layers at opposing ends respectively, and
the current collectors included in the plurality of unit cells are
electrically isolated from each other. The anode conductive porous
layer and the cathode conductive porous layer in each unit cell
preferably have high electrical resistance, as well as resistivity
preferably in a range from 0.01 to 1 .OMEGA.cm. Thus, as the fuel
cell layer not having a current collector has the conductive porous
layer having a high resistance value, a current that flows through
the fuel cell layer in a lateral direction is less. Though a method
of controlling a switch will be described later, an area or a
portion where a current flows in a vertical direction can be
controlled by using the switch. The switch is preferably joined to
each current collector through a lead. An FPC (Flexible Printed
Circuit) is preferably used as a lead in terms of a smaller
thickness, lighter weight and ease of installation of wires.
[0306] FIG. 33 is a schematic diagram showing a manner of
controlling the fuel cell system according to the present
invention. In FIG. 33(a), all switches A, B, C, D, E, a, b, c, d,
and e are in the ON state. In a region S, a current flows in a
vertical direction. Namely, portions where a longitudinally
extending unit cell and a laterally extending unit cell intersect
with each other are stacked in a direction of stack with no gap, so
that a current can flow in the direction of stack.
[0307] In FIG. 33(b), switches a, b, c, d, e, and A are in the ON
state, and switches B, C, D, and E are in the OFF state. Therefore,
a current can be extracted from a column of switch A, however, a
current cannot flow from a power generation portion of other unit
cells because of high resistance, and output can hardly be
obtained. By thus controlling the switches, in this example, 25
regions from which a current is to be extracted can be controlled
with a small number of, that is, ten, switches. Lower cost,
simplification of wiring, and a smaller size of the fuel cell
system can be achieved by decreasing the number of switch
members.
[0308] In the fuel cell system according to the present invention,
control is preferably carried out such that a current is
successively extracted from a power generation region of a fuel
cell to which the fuel is supplied. As shown in FIGS. 34(a) to (d),
a case where a methanol flow channel is formed from a methanol
cartridge will be described by way of example. When methanol is
supplied from the methanol cartridge, from a state where all
switches are turned off, initially, switches a and E are turned on.
As shown in FIG. 34(b), a current is mainly extracted from a power
generation region of the intersecting unit cell closest to the
methanol cartridge. As the output current is extracted, the
temperature increases, methanol is warmed, and a diffusion rate
within the methanol flow channel is increased. Then, in FIG. 34(c),
switches a, b, D, and E are turned on and other switches are turned
off. In addition, as shown in FIG. 34(d), switches a, b, c, C, E,
and D are turned on, and so on. Thus, a current is extracted
successively from a power generation region of the fuel cell closer
to the cartridge toward a power generation region thereof farther
therefrom, so that methanol is quickly supplied to a power
generation region of the fuel cell farthest from the cartridge and
methanol can spread all over the fuel cell. The fuel cell can thus
soon be started up. In addition, electric power from a portion
enabled to generate electric power can be supplied to auxiliary
equipment such as a pump or a fan or a portion where application
load is low, to thereby not only speed up start-up of the fuel cell
but also to speed up launching of the application. Moreover, for
example by actuating a fan, launching while improving power
generation characteristics at the time of start-up can also be
carried out, so that generated electric power at the time of
start-up can effectively be utilized.
[0309] In addition, the fuel cell system according to the present
invention includes an inclination sensor, and the switches are
preferably controlled to increase an amount of power generation by
the fuel cell located below, for producing an ascending air
current. The switches are controlled by means of the inclination
sensor such that a power generation region at a position closest to
a ground surface is responsible for highest output. An operation of
the switches will be described specifically in the fuel cell stack
shown in FIG. 35. In a steady state, switches e, A, B, C, D, and E
are turned on and switches a, b, c, and d intermittently repeat
switch ON and OFF. Thus, a largest amount of output electric power
is extracted from power generation regions of the unit cells
located lowest, and the temperature of those unit cells is highest
due to heat generation caused by internal resistance. As an
ascending air current thus occurs, warmed air diffuses in the fuel
cell stack and the air can satisfactorily be taken into the fuel
cell stack. Thus, by using the inclination sensor to detect a power
generation region of the unit cell closest to the ground and
causing that unit cell to generate electric power to attain the
highest temperature under switching control, air convection can be
improved and the need for a fan or a pump can be obviated or power
consumption can be reduced.
[0310] The present invention will be described hereinafter in
further detail with reference to examples, however, the present
invention is not limited thereto.
Example 1
[0311] A Nafion 117 membrane (manufactured by Du Pont) having a
size of 6 mm.times.48 mm and a thickness of approximately 175 .mu.m
was employed as the electrolyte membrane. A catalyst paste was
prepared in accordance with the following procedure. An anode
catalyst paste was prepared by placing catalyst carrying carbon
particles consisting of Pt--Ru particles of which Pt carrying
amount was 32.5 wt % and Ru carrying amount was 16.9 wt % and
carbon particles (TEC66E50 manufactured by Tanaka Kikinzoku Group),
20 wt % Nafion alcohol solution (manufactured by Sigma-Aldrich
Co.), ion exchanged water, isopropanol, and zirconia beads at a
prescribed ratio in a container made of PTFE, mixing these with the
use of a stirrer and deaerator at 50 rpm for 50 minutes, and
removing the zirconia beads. In addition, a cathode catalyst paste
was prepared as in the case of the anode catalyst paste, by using
catalyst carrying carbon particles consisting of Pt particles of
which Pt carrying amount was 46.8 wt % and carbon particles (TEC
10E50E manufactured by Tanaka Kikinzoku Group).
[0312] Two types of tubes of a silicon tube having an outer
diameter of 0.3 mm .phi. (an inner diameter of 0.2 mm .phi.) and a
length of 48.5 mm (KN3344391 manufactured by Tech-Jam) and a PTFE
tube for manifold having an outer diameter of 2 mm .phi. (an inner
diameter of 1 mm .phi.) and a length of 42.5 mm (KN3344350
manufactured by Tech-Jam) were used as fuel flow channel forming
members constituting the fuel flow channel, and the tubes were
coupled to each other so that fifteen silicon tubes in total are
branched to extend from the PTFE tube as in FIG. 36. Coupling was
achieved by providing a hole having a diameter of 0.5 mm in the
PTFE tube, inserting the silicon tube therein, and sealing the same
with an epoxy-resin-based adhesive having good chemical resistance.
The silicon tubes were connected such that the directions of their
length were perpendicular to a direction of length of the PTFE
tube, and an interval between the silicon tubes was as narrow as 2
mm, and an interval between sets of three silicon tubes was as wide
as 5 mm. A wire made of Ti and having a diameter of 0.15 mm .phi.
was inserted in each silicon tube.
[0313] Then, in order to form the hydrophilic layer, a paste
consisting of hydrophilic carbon and Nafion was prepared by placing
an 20 wt % alcohol solution of Nafion (manufactured by
Sigma-Aldrich Co.) containing the hydrophilic carbon (Aqua-Black
001, manufactured by Tokai Carbon Co., Ltd.), ion exchanged water,
isopropanol, and zirconia beads at a prescribed ratio in a
container made of PTFE, mixing these with the use of a stirrer and
deaerator at 50 rpm for 50 minutes, and removing the zirconia
beads.
[0314] Carbon paper GDL31BC (manufactured by SGL Carbon Japan Co.,
Ltd.) having one surface subjected to water-repellent treatment
with a layer composed of a fluorine-based resin and carbon
particles was made to have a size of 6 mm.times.48 mm, which was
employed as the anode conductive porous layer. The paste above
consisting of hydrophilic carbon and Nafion was applied with a
bar-coater onto a water-repellent surface of the conductive porous
layer, followed by drying, to form the hydrophilic layer having a
thickness of 40 .mu.m. The silicon tubes were rested on this
hydrophilic layer and arranged such that the silicon tubes were in
parallel to a direction of a long side of the conductive porous
layer and a tip end of the silicon tube and one end in the
direction of the long side of the conductive porous layer coincide
with each other. Here, as in FIG. 36, three silicon tubes per one
conductive porous layer were disposed as aligned such that
centerlines in a direction of length of the silicon tube were
located at positions at distances of 1 mm, 3 mm and 5 mm
respectively, with respect to a direction of a short side of the
conductive porous layer, and the silicon tubes were aligned on five
conductive porous layers in total in accordance with the similar
procedure.
[0315] Then, in order to fix the aforementioned silicon tubes onto
the conductive porous layer, an end portion of the silicon tube in
the direction of length was tentatively secured to the conductive
porous layer with a cellophane tape. While the silicon tube was
located on the conductive porous layer, the silicon tube and the
conductive porous layer were installed on a screen printing plate
such that the conductive porous layer and an opening portion of
6.times.48 mm in the screen printing plate were in parallel to a
direction along plane and the anode catalyst paste described above
was applied. The screen printing plate was held in a thermostatic
chamber at 60.degree. C. each time the anode catalyst paste was
applied, so as to dry away the solvent contained in the anode
catalyst paste. By repeating this process several times, the anode
catalyst layer having a thickness of approximately 0.4 mm from the
surface of the conductive porous layer was formed. Through the
processes above, the silicon tubes tentatively secured to the
conductive porous layer were fixed in such a state as embedded in
the catalyst layer as shown in FIG. 25. A stack structure of the
conductive porous layer, a set of three methanol supply paths, and
the anode catalyst layer formed in accordance with the procedure
above is referred to as an "anode unit". Five anode units were
fabricated for fifteen silicon tubes in total.
[0316] Instead of the silicon tube above, a hollow fiber or a
dialysis tube may be employed. In the case of a hollow fiber, a
hollow fiber having an outer diameter of 0.22 mm (material:
poly-4-methyl-1-pentene manufactured by DIC Corporation) can be
used, a length thereof is in conformity with that of the silicon
tube, and a procedure for producing the anode unit is also the
same. In the case of a dialysis tube, a dialysis tube having a
two-dimensional width of approximately 4 mm (material: regenerated
cellulose) and a length of 48.5 mm was employed instead of the
silicon tube and coupled to a connection port provided in the PTFE
tube. In using the dialysis tube as well, a procedure for producing
the anode unit is the same as in the case of the silicon tube. As
the two-dimensional width of the dialysis tube is shorter than a
length of a short side of the conductive porous layer, the anode
catalyst layer and the conductive porous layer have a sufficient
contact surface, however, by further decreasing the two-dimensional
width, the contact surface can also be increased. As carbon dioxide
representing a product cannot be exhausted from a portion of
contact between the dialysis tube and the conductive porous layer,
exhaust of the product is facilitated by increasing the contact
surface.
[0317] Carbon paper GDL31BC (manufactured by SGL Carbon Japan Co.,
Ltd.) having one surface subjected to water-repellent treatment
with a layer composed of a fluorine-based resin and carbon
particles was made to have a size of 6 mm.times.48 mm, which was
employed as the cathode conductive porous layer. The cathode
catalyst paste was applied to a water-repellent surface of the
conductive porous layer by using a screen printing plate, that was
held in a thermostatic chamber at 60.degree. C. for ten minutes to
dry away the solvent contained in the cathode catalyst paste, to
thereby form the cathode catalyst layer (the stack structure of the
conductive porous layer and the cathode catalyst layer is
hereinafter referred to as a "cathode unit"). Five cathode units
were fabricated in accordance with the similar procedure.
[0318] A titanium plate having a thickness of 0.1 mm and a size of
6 mm short side.times.52 mm long side was used as the current
collector. By drilling, a pattern of openings at a pitch of 0.550
mm each having a diameter of 0.45 mm was formed in a region of 6
mm.times.48 mm.
[0319] An olefin-based resin 1152B manufactured by Three Bond Co.,
Ltd., which is a thermosetting polymeric adhesive, was employed as
the electrically conductive adhesive used for forming the bonding
layer, and XC72 (Vulcan) was employed as highly electrically
conductive powders. Here, 1152B was pasty, and 1152B and XC72 were
placed in a mortar at a ratio of 1.7:1 and they were sufficiently
mixed with each other. Conditions for curing 1152B were set to
100.degree. C. and 30 minutes and the bonding layer was formed by
curing.
[0320] Further stacking was performed in the order of the anode
unit (the conductive porous layer being located below), the
electrolyte membrane and the cathode unit (the cathode catalyst
layer being located below) from below, such that the anode catalyst
layer and the cathode catalyst layer overlapped with each other
with the electrolyte membrane being interposed. As five anode units
were formed for the methanol supply paths described above, five
stack structures were fabricated by further stacking in accordance
with the similar procedure. A Teflon spacer having a size of 25
mm.times.50 mm and a thickness of 1.0 mm was employed. The stack
structure was installed in the center of a stainless plate having a
size of 100 mm.times.100 mm and a thickness of 3 mm. One Teflon
spacer was arranged on the stainless plate for each side thereof,
at a distance of 1 cm from each side on an outer perimeter of the
stainless plate in parallel thereto. The stack structure and the
spacer were sandwiched between stainless plates each having a size
of 100 mm.times.100 mm and a thickness of 3 mm. Thermocompression
bonding was performed for two minutes at 130.degree. C. and at 10
kgf/cm.sup.2 in a direction of thickness of the stainless plate, to
integrate the stack structure and to form the unit cell.
Specifically, interfaces of the stacked members are joined to each
other through an anchoring effect, adhesive force of Nafion
contained in the catalyst layer, or the like. Thereafter, the wire
above was slowly pulled out of the silicon tube. As the tip end of
the silicon tube is open and such a state allows methanol sucked up
by capillarity to flow out, a paste consisting of an NMP
(2-methylpyrrolidone) solution of PVDF containing PTFE particles
was applied to a side surface, from which the tip end of the
silicon tube of the unit cell projects. The organic solvent
contained in the paste was removed by drying and this process was
repeated until the tip end of the silicon tube was closed by a
porous body. As a result of this process to close the tip end of
the silicon tube with the porous body, methanol is not exhausted
but air introduced in the tube is exhausted through the porous
body. The fuel cell layer constituted of a group of five unit cells
fabricated in accordance with the procedure described above was
adopted as a "first layer".
[0321] In accordance with the procedure the same as that for the
first layer, the fuel cell layers to serve as a second layer to a
fifth layer were fabricated. As shown in FIG. 37, further stacking
was performed such that a direction of a long side of the unit
cells forming the second layer is perpendicular to a direction of a
long side of the unit cells forming the first layer, and the
electrically conductive adhesive described above was applied to a
surface of the cathode conductive porous layer in the first layer
that is to serve as a contact portion. Regarding the third layer
and so on, further stacking was similarly performed such that a
direction of a long side of the unit cells is perpendicular to that
of a layer located directly below, and the electrically conductive
adhesive was similarly applied to each contact portion. Five layers
in total, that is, 25 unit cells, were formed.
[0322] The current collector was arranged on the lowermost surface
of the unit cells forming the first layer and the uppermost surface
of the unit cells forming the fifth layer such that one end of the
current collector and one ends of the unit cells coincide with one
another. Here, the electrically conductive adhesive was applied to
a contact surface. A Teflon spacer having a size of 25 mm.times.50
mm and a thickness of 4.5 mm was employed. The unit cell group and
the current collector were disposed on a stainless plate having a
size of 100 mm.times.100 mm and a thickness of 3 mm, and one spacer
was arranged on the stainless plate at a distance of 1 cm from each
side of an outer perimeter of the stainless plate in parallel
thereto. The unit cell group and the current collector were
sandwiched between stainless plates each having a size of 100
mm.times.100 mm and a thickness of 3 mm. Thermocompression bonding
was performed for ten minutes at 130.degree. C. and at 0.1
kgf/cm.sup.2 in a direction of thickness of the stainless plate, to
integrate the fuel cell layer and the current collector and to
fabricate the fuel cell stack. An interface of each layer was
joined through an anchoring effect, adhesive force of Nafion, a
thermosetting resin, or the like, and the layers were in contact
with each other with low contact resistance.
[0323] Four corners in a direction along a plane of the integrated
fuel cell stack above were empty spaces, and a cylinder made of
resin was suitably provided in that space as shown in FIG. 29. The
cylinder is connected to the methanol cartridge and it holds
sufficient methanol. Five PTFE tubes in total extending from the
integrated unit cells and PTFE joints provided in the cylinder
(made of abitakku, for an outer diameter of 2 mm .phi..times.an
inner diameter of 1 mm .phi.) were connected to one another so that
methanol is supplied to each unit cell. A sufficient supply
pressure was applied from the methanol cartridge to the cylinder,
so that a liquid was supplied into the silicon tubes serving as the
fuel flow channels.
Example 2
[0324] Nafion 117 membrane (manufactured by Du Pont) in a shape of
a trapezoid having an upper base of 6 mm, a lower base of 12 mm and
a height of 48 mm and a thickness of approximately 175 .mu.m was
employed as the electrolyte membrane. A catalyst paste was
fabricated as in Example 1.
[0325] A silicon tube having an outer diameter of 0.3 mm .phi. (an
inner diameter of 0.2 mm .phi.) and a length of 48.5 mm (KN3344391
manufactured by Tech-Jam) was employed as the fuel flow channel
forming member.
[0326] Carbon paper GDL31BC (manufactured by SGL Carbon Japan Co.,
Ltd.) having one surface subjected to water-repellent treatment
with a layer composed of a fluorine-based resin and carbon
particles was made to have an upper base of 6 mm, a lower base of
12 mm and a height of 48 mm, which was employed as the anode
conductive porous layer. The silicon tubes above were rested on the
water-repellent surface of the conductive porous layer and disposed
such that the tip end of the silicon tube and the lower base of the
trapezoidal conductive porous layer coincide with each other. Here,
three silicon tubes per one conductive porous layer were disposed
as aligned such that centerlines in a direction of length of the
silicon tube are located at positions at distances of 1 mm, 3 mm
and 5 mm with respect to the upper base of the conductive porous
layer and at distances of 2 mm, 6 mm and 10 mm with respect to the
lower base thereof, respectively.
[0327] In order to fix the aforementioned silicon tubes onto the
conductive porous layer, an end portion of the silicon tube was
tentatively secured to the conductive porous layer with a
cellophane tape. While the silicon tube was located on the
conductive porous layer, the silicon tube and the conductive porous
layer were installed on a screen printing plate such that the
conductive porous layer and a trapezoidal opening having an upper
base of 6 mm, a lower base of 12 mm and a height of 48 mm in the
screen printing plate were in parallel to a direction along plane,
and the anode catalyst paste described above was applied. The
procedure for forming the anode catalyst layer is the same as in
Example 1. Through the processes above, the silicon tubes
tentatively secured to the conductive porous layer were fixed in
such a state as embedded in the catalyst layer as shown in FIG.
25.
[0328] A process for filling the tip end of the silicon tube with
the porous body is in conformity to that in Example 1. The
conductive porous layer, a set of three methanol supply paths, and
the anode catalyst layer formed in accordance with the procedure
above were stacked as the anode unit, and six anode units were
fabricated with the same procedure.
[0329] A Teflon spacer having a size of 78 mm.times.78 mm and a
thickness of 1.0 mm was worked into a shape as shown in FIG. 38.
Specifically, six hollowed portions in total were provided in a fan
shape such that the electrolyte membrane and the cathode unit are
fitted therein and centerlines in a direction of height of the
trapezoidal members are located at intervals of 18 degrees, and the
hollowed portions were symmetrical around a diagonal line of the
Teflon spacer. In addition, as an upper base portion of the
trapezoidal hollowed portion is also hollowed, the silicon tube
serving as the fuel flow channel can be fitted without being in
contact with the Teflon spacer even when the anode unit is fitted
in the trapezoidal hollowed portion described previously. Four such
Teflon spacers were prepared.
[0330] A trapezoidal titanium plate having a thickness of 0.1 mm,
an upper base of 5.5 mm, a lower base of 12 mm, and a height of 52
mm was used as the current collector. By drilling, a pattern of
openings at a pitch of 0.550 mm each having a diameter of 0.45 mm
was formed in a trapezoidal region having an upper base of 6 mm, a
lower base of 12 mm, and a height of 48 mm. It is noted that the
lower base of the current collector coincides with the lower base
of a region where the opening pattern is formed.
[0331] Further stacking was performed in the order of the anode
unit (the conductive porous layer being located below), the
electrolyte membrane and the cathode unit (the cathode catalyst
layer being located below) from below, such that the anode catalyst
layer and the cathode catalyst layer overlapped with each other
with the electrolyte membrane being interposed. This stack
structure was fitted in the Teflon spacer above and the upper bases
of the trapezoidal stack structures were aligned. As shown in FIG.
38, six stack structures were aligned, they were sandwiched between
the stainless plates in accordance with the procedure as in Example
1, and thermocompression bonding was performed for two minutes at
130.degree. C. and at 10 kgf/cm.sup.2 in a direction of thickness
thereof, to integrate the stack structure. A group of six unit
cells fabricated in accordance with this procedure serves as the
first fuel cell layer (hereinafter referred to as the first layer).
This first layer and the spacer were not separated from each other
and they were held in the fitted state.
[0332] The fuel cell layers to serve as a second layer to a fourth
layer were fabricated in accordance with the procedure the same as
that for the first layer, and the layers were not separated from
the Teflon spacer but held together, as in the case of the first
layer.
[0333] As shown in FIG. 39, the first layer and the second layer
were arranged such that they are opposite on a diagonal line of the
spacer and the sides of the Teflon spacers coincide with one
another, and the electrically conductive adhesive was applied to a
contact portion (a surface of the cathode conductive porous layer
in the first layer). The integrated structure obtained by layering
the first layer and the second layer was sandwiched between
stainless plates each having a thickness of 3 mm, and
thermocompression bonding was performed for ten minutes at
130.degree. C. and at 10 kgf/cm.sup.2 in a direction of thickness
of the stainless plate for integration. After integration,
sufficient cooling was performed and thereafter the Teflon spacer
in the second layer was removed in a direction of thickness of the
fuel cell. The third layer and the fourth layer were also
integrated in accordance with the procedure described above, and
the Teflon spacer in the third layer was removed. Here, the spacers
in the first layer and the fourth layer are not removed.
[0334] The integrated structures were layered such that
orientations of respective Teflon spacers in the first layer with
the second layer lying above and in the fourth layer with the third
layer lying below are opposite on a diagonal line, and the second
layer and the third layer were sandwiched. In addition, the
electrically conductive adhesive was applied to a contact portion
(a surface of the cathode conductive porous layer in the second
layer). The first layer, the second layer, the third layer, and the
fourth layer were layered in this order from below, and the first
layer and the third layer were in parallel to each other and the
second layer and the fourth layer were in parallel to each other,
in the direction along the plane. The Teflon spacer having a size
of 100 mm.times.100 mm and a thickness of 4 mm has a through hole
of 78 mm.times.78 mm, and the fuel cell layer and the current
collector above were disposed in this through hole, that was
sandwiched between stainless plates each having a size of 100
mm.times.100 mm and a thickness of 3 mm. Thermocompression bonding
was performed for ten minutes at 130.degree. C. and at 10
kgf/cm.sup.2 in a direction of thickness of the stainless plate, to
integrate the fuel cell layer. The contact surfaces of the second
layer and the third layer were in electrical contact with each
other, with low contact resistance. Here, the Teflon spacers in the
first layer and the fourth layer were removed in the direction of
thickness of the fuel cell layer.
[0335] The current collector was arranged such that the upper base
of the trapezoidal unit cell and the upper base of the current
collector lie over the lowermost surface of the unit cells forming
the first layer and the uppermost surface of the unit cells forming
the fourth layer. Here, the electrically conductive adhesive was
applied to the contact portion. The Teflon spacer having a size of
100 mm.times.100 mm and a thickness of 4 mm has a through hole of
78 mm.times.78 mm, and the current collector described above was
disposed in this through hole, which was sandwiched between
stainless plates each having a size of 100 mm.times.100 mm and a
thickness of 3 mm. Thermocompression bonding was performed for ten
minutes at 130.degree. C. and at 10 kgf/cm.sup.2 in a direction of
thickness of the stainless plate, to integrate the unit cell group
and the current collector above and to fabricate the fuel cell
stack. The unit cell group and the current collector were in
electrical contact with each other, with low contact
resistance.
[0336] Two corners in a direction along the plane of the fuel cell
stack described above were empty spaces, and cylinders made of
resin could suitably be provided in those spaces as shown in FIG.
31. The cylinder was connected to the methanol cartridge and it
holds sufficient methanol. The silicon tubes were connected to a
connection port provided in the cylinder such that 36 silicon tubes
in total (for example, 18 silicon tubes from the first layer and 18
silicon tubes from the third layer) extend from the fuel cell stack
for one cylinder, so that methanol is supplied to each unit cell.
Similarly, 36 silicon tubes in total (for example, 18 silicon tubes
from the second layer and 18 silicon tubes from the fourth layer)
were also connected to another cylinder.
Example 3
[0337] A fuel cell stack was fabricated with the following
method.
[0338] (1) Fabrication of MEA (Membrane Electrode Assembly)
[0339] Initially, a catalyst paste was prepared in accordance with
the following procedure. An anode catalyst paste was prepared by
placing catalyst carrying carbon particles consisting of Pt--Ru
particles of which Pt carrying amount was 32.5 wt % and Ru carrying
amount was 16.9 wt % and carbon particles (TEC66E50 manufactured by
Tanaka Kikinzoku Group), 20 wt % Nafion alcohol solution
(manufactured by Sigma-Aldrich Co.), ion exchanged water,
isopropanol, and zirconia beads at a prescribed ratio in a
container made of PTFE, mixing these with the use of a stirrer and
deaerator at 50 rpm for 50 minutes, and removing zirconia beads. In
addition, a cathode catalyst paste was prepared as in the case of
the anode catalyst paste, by using catalyst carrying carbon
particles consisting of Pt particles of which Pt carrying amount
was 46.8 wt % and carbon particles (TEC 10E50E manufactured by
Tanaka Kikinzoku Group).
[0340] Then, carbon paper GDL25BC (manufactured by SGL Carbon Japan
Co., Ltd.) having one surface subjected to water-repellent
treatment with a layer composed of a fluorine-based resin and
carbon particles was made to have a size of 1.8 mm.times.23 mm,
which was employed as the anode conductive porous layer. The anode
catalyst paste described above was applied to a water-repellent
surface of the anode conductive porous layer by using screen
printing, that was held in a thermostatic chamber at 60.degree. C.
each time the anode catalyst paste was applied, so as to dry away
the solvent contained in the anode catalyst paste. By repeating
this process several times, the anode catalyst layer having a
thickness of approximately 0.4 mm was formed on the anode
conductive porous layer. A stack structure of this anode conductive
porous layer and the anode catalyst layer is referred to as an
"anode unit". Similarly, GDL25BC of 1.8 mm.times.23 mm was employed
as a cathode conductive porous layer, and the cathode catalyst
layer was formed on this water-repellent surface to a thickness of
0.1 mm. A stack structure of this cathode conductive porous layer
and the cathode catalyst layer is hereinafter referred to as a
"cathode unit".
[0341] Thereafter, a Nafion 117 membrane (manufactured by Du Pont)
having a size of 100 mm.times.100 mm and a thickness of
approximately 175 .mu.m was employed as the electrolyte membrane,
and the anode unit, the electrolyte membrane and the cathode unit
were successively stacked in this order from below such that the
anode catalyst layer and the cathode catalyst layer overlapped with
each other with the electrolyte membrane being interposed and the
anode catalyst layer and the cathode catalyst layer come in contact
with the electrolyte membrane. Here, the cathode unit and the anode
unit were arranged such that they were arranged in three rows and
ten columns at regular intervals in the electrolyte membrane of 100
mm.times.100 mm. Thereafter, a titanium plate having a thickness of
0.45 mm and a size of 100 mm in a longitudinal direction and 100 mm
in a lateral direction was arranged on each of four sides of the
stack structure, at a distance by 1 cm from the stack structure.
This titanium plate is provided in order for the stack structure to
have a thickness greater than 0.45 mm when it is pressed. Then,
this stack structure and the titanium plate were placed on a
stainless plate having a thickness of 1 mm and a stainless plate
having a thickness of 1 mm was further placed thereon, so that
these stainless plates sandwich the stack structure and the
titanium plate and thermocompression bonding was performed for two
minutes at 130.degree. C. and at 10 kgf/cm.sup.2 in a direction of
thickness thereof, to integrate the stack structure. By cutting out
the stack structure to a size of the conductive porous layer (1.8
mm.times.23 mm), an MEA in a shape of an elongated strip having a
width of 1.8 mm and a length of 23 mm was obtained.
[0342] (2) Fabrication of Flow Channel Substrate and Anode Current
Collector and Cathode Current Collector
[0343] As shown in FIG. 48, a flow channel in a serpentine shape
was formed by etching an SUS substrate having a width of 11 mm, a
length in a longest portion of 36 mm, and a thickness of 0.2 mm.
FIG. 48(a) is a top view of the obtained flow channel substrate,
and FIG. 48(b) is a cross-sectional view along the line A-A' shown
in FIG. 48(a). The unit of a numeric value shown in FIG. 48 is mm.
As shown in FIG. 48(b), the flow channel has a width of 1 mm and a
depth of 0.1 mm. As shown in FIG. 48(a), the flow channel substrate
has two projections (a width of 2 mm.times.a length of 3 mm) at its
lower end. The projection serves as a manifold insertion port. In
addition, this flow channel substrate has three through holes like
a slit each having a width of 1 mm.times.a length of 25 mm (see
FIG. 48(a)). These through holes and three through holes like a
slit formed in the anode current collector which will be described
later serve as an air path in the fuel cell layer and the fuel cell
stack obtained thereby. In the flow channel substrate shown in FIG.
48, a wall formed between the through hole like a slit and the flow
channel adjacent thereto has a width of 0.5 mm.
[0344] In addition, the anode current collector in a shape as shown
in FIG. 49 and the cathode current collector in a shape as shown in
FIG. 50 were fabricated with etching. The anode current collector
in FIG. 49 is made of a substrate in a shape of a flat plate made
of stainless (SUS316L) having a width of 11 mm, a length of 36 mm
and a thickness of 0.1 mm. As shown in FIG. 49(a), three through
holes like a slit are formed in this anode current collector. FIG.
49(b) is an enlarged view of a region A in the anode current
collector shown in FIG. 49(a). As shown in FIG. 49(b), in region A
of the anode current collector, a plurality of openings each in a
hexagonal shape of which side is 0.25 mm long are provided in a
region having a width of 0.5 mm from the centerline in the
longitudinal direction of region A toward opposing ends, with a
line width of 0.1 mm being left. The anode current collector
provided in the lowermost fuel cell layer in the fuel cell stack is
provided with an output extraction terminal. It is noted that the
unit of a numeric value shown in FIG. 49 is mm.
[0345] Meanwhile, the cathode current collector shown in FIG. 50 is
made of a substrate in a shape of a flat plate made of stainless
(SUS316L) having a width of 2 mm, a length of 25 mm and a thickness
of 0.1 mm. In the present manufacturing example, four such
substrates were used. FIG. 50(b) is an enlarged view of the cathode
current collector shown in FIG. 50(a). As shown in FIG. 50(b), a
plurality of hexagonal through holes are provided in the cathode
current collector (a hole diameter of 0.6 mm and a percentage open
area of 70%). Four cathode current collectors provided in the
uppermost fuel cell layer in the fuel cell stack are provided with
an output extraction terminal, as shown in FIG. 50(c). A composite
cathode current collector shown in FIG. 50(c) is formed by
integrating four cathode current collectors by means of the output
extraction terminal.
[0346] Here, the flow channel substrate and the anode current
collector above were worked such that four MEAs in a shape of an
elongated strip can be arranged with prescribed gap regions being
formed in parallel in a single substrate. As will be described
later, though the anode current collector and the MEAs each in a
shape of an elongated strip are arranged on this flow channel
substrate, here, each MEA is prevented from two-dimensionally
coming apart. In addition, by providing a prescribed gap in a
single flow channel substrate or anode current collector and
forming unit cells thereon, a fuel cell stack including a fuel cell
layer in which a plurality of unit cells are two-dimensionally
arranged with a prescribed gap and having excellent mechanical
strength can be fabricated. Though the MEA formed on the flow
channel substrate should naturally be prevented from peeling off
from the flow channel substrate, a method therefor will be
described later.
[0347] (3) Diffusion Bonding of Flow Channel Substrate, Anode
Current Collector, Cathode Current Collector, and Spacer
[0348] Then, the flow channel substrate, the anode current
collector and the cathode current collector described above and six
spacers (each in a parallelepiped shape having a width of 0.5 mm, a
length of 20 mm and a thickness of 0.5 mm) made of stainless
(SUS316L) which is a non-porous body were prepared, that were
joined together in the order of the cathode current collector, the
spacer, the flow channel substrate, and the anode current collector
from below as shown in FIG. 51, to thereby obtain a joint structure
(a total thickness of 0.8 mm). Joint was achieved by hot-pressing
diffusion bonding. It is noted that six spacers were arranged at
intervals of 2 mm. In addition, in diffusion bonding, the surface
of the anode current collector made of SUS316L on the flow channel
substrate side was plated with gold to a thickness of 1 .mu.m.
Plating with gold can prevent stainless from corrosion and can also
improve joint property in diffusion bonding. In the obtained joint
structure, the anode current collector was stacked on the flow
channel substrate, and a plurality of through holes in region A of
the anode current collector communicate with the flow channel in
the flow channel substrate. The MEA in a shape of an elongated
strip was arranged on this region A. Diffusion bonding is
preferably used for joint of the flow channel substrate, the anode
current collector, the cathode current collector, the spacer, and
the like as above, however, a joint method such as laser welding
may be employed.
[0349] (4) Joint of MEA
[0350] Then, as shown in FIG. 52, four MEAs each in a shape of an
elongated strip (0.3 mm thick) obtained as above were joined to
region A of the anode current collector of the joint structure
above. Joint was carried out as follows. Initially, an NMP
(2-methylpyrrolidone) solution of PVDF (5 wt %) and XC72 (Vulcan)
which is carbon particles were mixed and kneaded such that the
carbon particles accounted for 7 wt % with respect to PVDF, to
thereby prepare an electrically conductive ink. Thereafter, this
electrically conductive ink was applied to the surface of the anode
conductive porous layer (GDL25BC) of the MEA to a thickness of 10
.mu.m by using a bar-coater, followed by drying. Then, the MEA was
arranged on region A of the anode current collector. Thereafter, a
titanium plate having a thickness of 0.7 mm and a size of 10 mm in
a longitudinal direction and 100 mm in a lateral direction was
arranged on each of four sides of the stack structure, at a
distance by 1 cm therefrom. This titanium plate was provided in
order for the stack structure to have a thickness greater than 0.7
mm when it is pressed. Thereafter, this stack structure and the
titanium plate were placed on a stainless plate having a thickness
of 1 mm and a stainless plate having a thickness of 1 mm was
further placed thereon, so that these stainless plates sandwich the
stack structure and the titanium plate, and thermocompression
bonding was performed for two minutes at 130.degree. C. and at 5
kgf/cm.sup.2 in a direction of thickness thereof, to flatten the
MEA and to fix the MEA to the anode current collector. By thus
applying the electrically conductive ink consisting of PVDF and
carbon particles to the MEA, PVDF is softened during
thermocompression bonding and introduced in the through holes in
the anode current collector, so that the MEA can be fixed through
an anchoring effect. In addition, as a result of improvement in
adhesiveness owing to the anchoring effect, electrical contact
resistance between the anode current collector and the MEA can be
lowered. Though GDL25BC (manufactured by SGL Carbon Japan Co.,
Ltd.) forming the conductive porous layer of the MEA does not have
one surface subjected to water-repellent treatment with a layer
composed of a fluorine-based resin and carbon particles, a
conductive porous layer having opposing surfaces subjected to
water-repellent treatment may be employed. A stack structure thus
obtained, in which a plurality of MEAs (four in the present
example) were joined to the anode current collector, is hereinafter
referred to as a two-dimensional stack, because a plurality of MEAs
were arranged on a plane.
[0351] (5) Application of Adhesive to MEA End Portion
[0352] Then, as shown in FIG. 53, the adhesive was applied to the
end portions of the MEA. FIG. 53 shows a two-dimensional stack to
which the adhesive was applied to the end portions of the MEA, and
FIG. 53(b) is a cross-sectional view along the line B-B' in FIG.
53(a). A one-component thermosetting epoxy resin that is cured at
100.degree. C. in 30 minutes, in which epoxy resin and phenol-based
resin were mixed, was employed as the adhesive. By thus applying
the adhesive to the end portions of the MEA and sealing the end
portions, leakage of the fuel can be sealed and adhesiveness to the
anode current collector can be improved.
[0353] Here, though the adhesive is not limited to those mentioned
above, the epoxy resin is preferably used as the adhesive from a
point of view of resistance to acid, resistance to methanol
(chemical resistance), or the like. In addition, an adhesive
exhibiting a thermosetting characteristic at a relatively low
temperature not higher than approximately 100.degree. C. is
preferred from a point of view of prevention of deterioration due
to solubility and decomposition of the MEA and aggregation of a
catalyst. Moreover, a reaction start temperature of an adhesive to
be used is preferably 60.degree. C. or higher so that the adhesive
is not cured during storage at room temperature.
[0354] (6) Joint Between Two-Dimensional Stacks (Construction of
Fuel Cell Stack)
[0355] Then, as shown in FIG. 54, the two-dimensional stacks in
which the adhesive was applied to a plurality of (four in the
present example) MEA end portions were subjected to
thermocompression bonding for 30 minutes at such conditions as
100.degree. C. and 0.1 kgf/cm through hot pressing for joint to
each other, to thereby obtain the fuel cell stack. Joint was
achieved by stacking a necessary number of two-dimensional stacks
and thereafter subjecting the stack structures to hot pressing. As
a result of thermocompression bonding, the adhesive applied to the
end portions of MEA was cured, and hence firm joint between the
two-dimensional stacks can be achieved. FIG. 55 is a
cross-sectional view showing a structure of the fuel cell stack in
which a plurality of two-dimensional stacks were integrated.
According to such a joint method, a single hot pressing process can
fabricate the fuel cell stack in which a necessary number of
two-dimensional stacks were joined, a time for manufacturing can be
shortened, and manufacturing cost can be reduced. In addition, the
fuel cell stack fabricated with the method as above achieves
improved reliability of strength, because the anode current
collector, the cathode current collector and the MEAs were firmly
fixed by the adhesive. The fuel cell stack having many MEA end
portions as in the present invention includes many bonded portions
as a result of bonding above. As compared with the fuel cell stack
obtained by stacking a single large fuel cell, strength of the fuel
cell stack can significantly be enhanced particularly in its
central portion. In the fuel cell stack obtained by stacking a
single large fuel cell, only an outer peripheral portion of that
fuel cell layer can be bonded or fastened with a bolt, a nut or the
like, and a central portion thereof is not pressed. Therefore,
peel-off of the MEA or poor electrical contact between the MEA and
the current collector is likely.
[0356] (7) Installation of Manifold
[0357] Then, as shown in FIG. 56, a manifold for supplying the fuel
to each layer in the fuel cell stack obtained as above was
inserted. After insertion of the manifold, a gap between an inlet
and an outlet of the flow channel substrate and the manifold was
sealed with a sealant. A manifold made of an acrylic resin was
employed as the manifold. In addition, a two-component type epoxy
resin that is cured at room temperature was used as a sealant for
sealing. In the fuel cell stack in the present example, as the fuel
flow channel is in a serpentine shape, manifold connection portions
can concentrate at one location and the manifold can be reduced in
size. Ease in insertion and sealing of the manifold is thus
improved and reduction in cost can be achieved by improved yield
and reduction in production time. In addition, as the manifold is
reduced in size, a volume of the manifold, which disturbs air
intake into the fuel cell stack, can be decreased, and hence air
can satisfactorily be taken into the fuel cell stack.
[0358] (8) Insulating Coating of Flow Channel Substrate
[0359] In the fuel cell stack obtained as above and including the
manifold as shown in FIG. 56, a distance between the flow channel
substrate and the anode current collector in the adjacent fuel cell
layer is short. Accordingly, depending on external stress,
electrical short-circuit may occur and output from the fuel cell
stack may become poor. Therefore, in order to avoid such a
disadvantage, as shown in FIG. 56, a surface of the flow channel
substrate where the flow channel is not formed, a surface of a
portion of the anode current collector where the MEA is not
arranged, or surfaces of both of them is (are) preferably coated
with an insulating coating or provided with an insulating spacer.
It is more preferable to provide an insulating spacer, in terms of
improvement in strength of the fuel cell stack. In addition, it is
further preferable to arrange the insulating spacer at an outer
peripheral end portion of the fuel cell stack, in terms of
improvement in strength.
Example 4
[0360] The cathode current collector, the flow channel substrate,
and the anode current collector used in Example 3 were used, and
these were joined together in the order of stack of the cathode
current collector, the flow channel substrate and the anode current
collector from below as shown in FIG. 57, to obtain a joint
structure without including a spacer. Joint was achieved as in
Example 3, and hot-pressing diffusion bonding was performed. Here,
in the present example, joint was made by arranging four cathode
current collectors to be orthogonal to the flow channel substrate.
Thereafter, as in Example 3, a two-dimensional stack in which the
adhesive was applied to the end portions of the MEA was obtained.
FIG. 58 shows a cross-sectional structure of the obtained
two-dimensional stack. From then on, the fuel cell stack including
a manifold was fabricated as in Example 3 except for stacking and
joining adjacent two-dimensional stacks such that they intersect
with each other at an angle of 900.
Example 5
[0361] In the present example, initially, a two-dimensional stack
in which the adhesive was applied was obtained as in Example 4,
except that the adhesive was applied to the entire end portions of
the anode conductive porous layer, the anode catalyst layer, the
electrolyte membrane, and the cathode catalyst layer in the MEA and
a part of the end portion of the cathode conductive porous layer
(the end portion closer to the cathode catalyst layer). Thereafter,
as shown in FIG. 59, the electrically conductive adhesive
containing a thermosetting resin as in Example 3 was applied onto
the cathode conductive porous layer in one two-dimensional stack,
hot pressing under the conditions the same as in Example 3 was
carried out to thermally cure the adhesive, and the two-dimensional
stacks were joined to each other at the cathode conductive porous
layer of the two-dimensional stack and the cathode current
collector in the two-dimensional stack adjacent thereto, such that
they intersect with each other at an angle of 90.degree. to thereby
fabricate the fuel cell stack. As the two-dimensional stacks can
thus readily be joined together by hot pressing, a thermosetting
adhesive is preferably used. FIG. 60 shows a cross-sectional
structure of the obtained fuel cell stack. By thus not coating at
least a part of the end portion of the cathode conductive porous
layer with the adhesive, air can effectively flow into the cathode
conductive porous layer from a direction perpendicular to the
direction of stack of the MEA (in-plane direction).
Example 6
[0362] In the present example, initially, the flow channel
substrate, the anode current collector and the cathode current
collector used in Example 3 and four spacers (each in a
parallelepiped shape having a width of 2 mm, a length of 25 mm and
a thickness of 0.5 mm) made of stainless (SUS316L) which is a
porous body were prepared, that were joined together in the order
of stack of the spacers, the cathode current collector, the flow
channel substrate, and the anode current collector from below as
shown in FIG. 61, to thereby obtain a joint structure. Joint was
achieved as in Example 3, and hot-pressing diffusion bonding was
performed. Thereafter, after the MEAs were joined as in Example 3,
the adhesive was applied to the end portions of the MEAs, to
thereby obtain the two-dimensional stack in which the adhesive was
applied to the MEA end portions. Thereafter, hot pressing under the
conditions the same as in Example 3 was carried out to thermally
cure the adhesive, and the two-dimensional stacks were thus joined
to each other at the spacers in the two-dimensional stack and the
anode current collector in the two-dimensional stack adjacent
thereto, such that they intersect with each other at an angle of
90.degree. to thereby fabricate the fuel cell stack. As the
two-dimensional stacks can thus readily be joined together by hot
pressing, a thermosetting adhesive is preferably used. FIG. 62
shows a cross-sectional structure of the obtained fuel cell stack.
The spacers used in the present example have an outer dimension
(except for thickness) the same as that of the cathode current
collector and the cathode conductive porous layer, and it is
stacked on the cathode current collector in a manner substantially
coinciding with the same. Such a configuration is preferred in
terms of improvement in strength owing to increase in a joint area
between the spacer and the conductive porous layer of the MEA,
lowering in electrical resistance, and air permeability within the
fuel cell stack.
[0363] Joint between the constituent members through diffusion
bonding as above is preferred, for example, from the following
aspects.
[0364] (a) As atoms in metal members are joined, an interface
resistance value is as low as in a bulk metal and bonding strength
is high.
[0365] (b) An acid-resistant SUS member highly resistant to an acid
solution and an alcohol solution can be used as a constituent
member and long-term reliability of joint is also high.
[0366] (c) As a process of the current collector is fine, etching
is preferred. Here, as a process can totally be performed from the
process of the current collector until structuring of the fuel cell
stack and the joint process, design and registration for joint at a
fine portion (stacking of a plate material having a width of 100
.mu.m can also be carried out) can be performed.
[0367] (d) As registration of a fine portion can be achieved, joint
strength can be made higher and interface resistance can be lowered
even when a width of a margin for bonding between the flow channel
substrate and the current collector is small. Thus, a proportion of
the margin for bonding can be less and a proportion of a width of
the flow channel can be greater. Then, an effectively working power
generation area of the MEA to which fuel is supplied becomes
greater and power generation characteristics are improved. For
example, a width of the margin for bonding can also be reduced from
0.5 mm to approximately 0.3 mm or smaller. In this case, assuming
that an MEA having a width of 2 mm is used, a ratio of the flow
channel can be increased from 50% to 70% or higher.
[0368] (e) As compared with an example where the electrically
conductive adhesive is used for joint, leakage of the fuel through
a gap between the flow channel substrate and the current collector
is less likely (the electrically conductive adhesive is more likely
to cause leakage when a gap is created due to peel-off or
dissolution).
[0369] (f) Without concerns about liquid leakage from between the
flow channel substrate and the current collector at the end
portion, by providing the fuel permeability control layer between
the flow channel and the catalyst layer, seeping of the fuel from a
cross-section of the end portion of the fuel cell (a gap in an
unsealed portion) does not occur without complete sealing of the
end portion of the MEA for leaving no gap (sealing of a
cross-section at the end portion of the MEA and a cross-section at
the end portion of the anode conductive porous layer), electric
power can satisfactorily be generated, and carbon dioxide can be
exhausted from the end portion of the MEA.
[0370] (g) As a part in which all members for one layer in the fuel
cell stack have been joined can be fabricated, the step of stacking
the fuel cell stack includes only the step of joining the MEA and
the part in which all members for one layer in the fuel cell stack
have been joined through hot pressing, whereby manufacturing of the
fuel cell stack can be simplified.
Example 7
[0371] Eight MEAs each in a shape of an elongated strip (a
thickness of 0.45 mm, a width of 1.8 mm, and a length of 23 mm)
were fabricated with a method the same as in Example 3. A flow
channel substrate obtained by increasing the number of flow
channels in a longitudinal direction in the flow channel substrate
from four in FIG. 48 to eight and the anode current collector
obtained by increasing the number of regions A in the anode current
collector from four in FIG. 49 to eight were bonded to each other
through diffusion bonding, and thereafter eight MEAs were arranged
on the anode current collector. The used anode current collector
has a width of region A of 2 mm and a through hole like a slit
having a width of 1 mm, as in FIG. 49. Eight MEAs were arranged on
eight regions A respectively such that a pitch between the MEAs in
a shape of an elongated strip (a distance from a left end of an MEA
to a left end of an adjacent MEA) was set to 3 mm. Then, eight
cathode current collectors increased in the number of cathode
current collectors from four in FIG. 50(c) were made to lie along
the surface of GDL25BC on the cathode side (the cathode conductive
porous layer) and subjected to thermocompression bonding for two
minutes under such conditions as 100.degree. C. and 0.1
kgf/cm.sup.2 through hot pressing, to tentatively bond the anode
current collector, the MEAs, and the cathode current collectors to
one another. Then, an epoxy-based adhesive was applied to the end
portions of the MEAs and the end portions of the current
collectors, to bond the MEAs and the current collectors to one
another and to seal the end portions of the MEAs. One fuel cell
layer was thus fabricated. Two such fuel cell layers were
fabricated, that are referred to as samples No. 1 and No. 2,
respectively. Thereafter, titanium non-woven fabric (porosity of
60%, a thickness of 0.5 mm, a width of 1.5 mm, a length of 30 mm,
and linear 20 .mu.m) of Bekinit K.K., which is a titanium porous
body, and a non-porous body (a parallelepiped having a length of 30
mm, a thickness of 0.5 mm and a width of 0.5 mm) made of SUS316L
were used as the spacers, and these were alternately arranged at a
pitch of 4 mm (a distance from a left end of a spacer to a left end
of an adjacent spacer), to thereby form the spacer layer.
Thereafter, two fuel cell layers of samples No. 1 and No. 2 above
sandwiched this spacer layer, the manifold was inserted as shown in
FIG. 63(b), and an extraction terminal of the anode current
collector and an extraction terminal of the cathode current
collector were joined to form serial wiring in order to further
lower electrical resistance, to thereby form the fuel cell stack
(hereinafter also referred to as a two-layered stack). It is noted
that each spacer and the fuel cell layer were bonded with a
two-component type electrically conductive adhesive (Dotite SH-3A
manufactured by Fujikura Kasei Co., Ltd.). FIG. 63(b) shows a
schematic perspective view of this two-layered stack.
[0372] FIG. 63(a) shows a current-voltage characteristic and a
characteristic of MEA average current-power density of sample No.
1, sample No. 2 and the two-layered stack above. Measurement was
conducted by supplying a methanol aqueous solution of 3 mol/L as
the fuel with a pump and passively supplying air without using
auxiliary equipment for power generation. As shown in FIG. 63(a),
maximum average power densities of samples No. 1 and No. 2 and the
two-layered stack were 38.5 mW/cm.sup.2, 37.4 mW/cm.sup.2 and 42.1
mW/cm.sup.2, respectively. The maximum value of the average power
density improved in the two-layered stack, because two fuel cell
layers were stacked and the temperature readily increased so that
catalyst activity improvement or the like was achieved and hence
power generation efficiency improved.
[0373] Here, the maximum volume power density (output per fuel cell
stack volume, unit: W/L) of the two-layered stack was calculated as
follows. Initially, a region surrounded by a bold frame shown in
FIG. 63(b) is regarded as a substantial volume of the fuel cell
stack (hereinafter also referred to as a stack volume, excluding a
region where a manifold was installed). Then, this stack volume is
calculated as 23 mm.times.23 mm.times.1.9 mm=1 cm.sup.3, because
the fuel cell layer has a thickness of approximately 0.7 mm, the
spacer has a thickness of 0.5 mm, and the two-layered stack
obtained by stacking the former has a thickness of 1.9 mm as found
in measurement. On the other hand, the total area of the MEAs in
the bold frame above is calculated as 2.3 cm.times.0.18
cm.times.8.times.2 layers=6.624 cm.sup.1. Therefore, the output
from the two-layered stack is calculated as 42.1
mW/cm.sup.2.times.6.624 cm.sup.2=279 mW. Therefore, the maximum
volume power density of the two-layered stack can be calculated as
279 mW/1 cm.sup.3=279 W/L.
Example 8
[0374] Another fuel cell layer the same as in Example 7 was
fabricated and used as sample No. 3. Then, as in Example 7, three
fuel cell layers of samples No. 1 to No. 3 were used to fabricate
the fuel cell stack constituted of three fuel cell layers and two
spacer layers arranged between the fuel cell layers (hereinafter
also referred to as a three-layered stack). FIG. 64(b) shows a
schematic perspective view of this three-layered stack. FIG. 64(a)
shows a current-voltage characteristic and a characteristic of MEA
average current-power density of sample No. 1, sample No. 2 and
sample No. 3 and the three-layered stack. The measurement
conditions were the same as in Example 7. As shown in FIG. 64(a),
maximum average power densities of samples No. 1, No. 2 and No. 3,
and the three-layered stack were 38.5 mW/cm.sup.2, 37.4
mW/cm.sup.2, 39.3 mW/cm.sup.2, and 41.1 mW/cm.sup.2, respectively.
The maximum value of the average power density improved in the
three-layered stack, because three fuel cell layers were stacked
and the temperature readily increased so that catalyst activity
improvement or the like was achieved and hence power generation
efficiency improved.
[0375] Here, the maximum volume power density of the three-layered
stack was calculated as follows. Initially, a region surrounded by
a bold frame shown in FIG. 64(b) is regarded as a substantial
volume of the fuel cell stack (stack volume), as in the two-layered
stack in Example 7 Then, this stack volume is calculated as 23
mm.times.23 mm.times.3.0 mm=1.587 cm.sup.3, because the fuel cell
layer has a thickness of approximately 0.7 mm, the spacer has a
thickness of 0.5 mm, and the three-layered stack obtained by
stacking the former has a thickness of 3.0 mm as found in
measurement. On the other hand, the total area of the MEAs in the
bold frame above is calculated as 2.3 cm.times.0.18
cm.times.8.times.3 layers=9.936 cm.sup.2. Therefore, the output
from the three-layered stack is calculated as 41.1
mW/cm.sup.2.times.9.936 cm.sup.2=408 mW. Therefore, the maximum
volume power density of the three-layered stack can be calculated
as 408 mW/1.587 cm.sup.3=257 W/L
Comparative Example 1
[0376] FIG. 65 is a perspective view showing a structure of a
conventional fuel cell stack fabricated in the present Comparative
Example. A fabrication procedure is as follows. Initially, as in
Example 3, the anode catalyst layer and the cathode catalyst layer
were formed on GDL25BC (conductive porous layer) of 23 mm.times.23
mm. Under the hot pressing conditions the same as in Example 3, an
MEA in which GDL25BC, the anode catalyst layer, the electrolyte
membrane (Nafion 17) of 3 cm.times.3 cm, the cathode catalyst
layer, and GDL25BC were stacked in this order was fabricated. The
electrolyte membrane that extends off the conductive porous layer
was cut away and the MEA having an outer dimension of 23
mm.times.23 mm was prepared.
[0377] It is noted that a member having such an outer dimension
that an output extraction terminal as in FIG. 49 is provided in a
rectangle of 25.times.40 mm and obtained by plating SUS316L having
holes having a diameter of 0.6 mm in a flat plate of 0.1 mm
thickness in a hexagonal close packing pattern at a pitch of 0.7 mm
and having an opening in a portion having an area of 23 mm.times.23
mm with Au to a thickness of 1 m was employed as the anode current
collector.
[0378] In addition, a member having such an outer dimension that an
output extraction terminal as in FIG. 50(c) is provided in a
rectangle of 23 mm.times.23 mm and obtained by plating SUS316L
having holes having a diameter of 0.6 mm in a flat plate of 0.1 mm
thickness in a hexagonal close packing pattern at a pitch of 0.7 mm
and having an opening in a portion having an area of 23 mm.times.23
mm with Au to a thickness of 1 .mu.m was employed as the cathode
current collector.
[0379] Thereafter, the fabricated MEA was sandwiched such that it
coincides with the opening portion in the anode current collector
and the cathode current collector. The flow channel substrate
having an outer dimension as in FIG. 48 (an SUS substrate having a
thickness of 0.2 mm and a width 25 mm.times.a length at a longest
portion 40 mm as well as insertion ports for two manifolds at end
portions as in the flow channel substrate in FIG. 48), in which the
flow channel is formed directly under where the MEA of 23
mm.times.23 mm is arranged such that the fuel flows, was made to
coincide with the anode current collector, and an outer perimeter
was sealed with an adhesive. Quick 5, which is an epoxy adhesive,
was used as the adhesive.
[0380] Finally, as shown in FIG. 65, the manifold was inserted and
the output extraction terminals of the anode current collector and
the cathode current collector were connected to form serial
wiring.
[0381] FIG. 66(a) shows a current-voltage characteristic and a
characteristic of MEA average current-power density in the example
where the two fuel cells thus fabricated were stacked (that is, the
fuel cell stack having the structure shown in FIG. 65). Measurement
was conducted by supplying a methanol aqueous solution of 3 mol/L
as the fuel with a pump and passively supplying air without using
auxiliary equipment for power generation. This fuel cell stack is a
stack in which two fuel cells (cells) each having an outer
dimension of 2.23.times.2.23 cm and a surface area of 5 cm.sup.2
were stacked in series. When an interval d between the cells is
narrowed, air is less likely to be supplied and a value of maximum
average power density has lowered due to shortage of air.
Consequently, as shown in the table in FIG. 66, when two cells were
three-dimensionally stacked, maximum volume power density was 167
W/L when cell interval d was set to 1 mm, it was 159 W/L when the
interval was set to 2 mm, it was 153 W/L when the interval was set
to 3 mm, and it was 140 W/L when the interval was set to 4 mm. It
is noted that maximum average power density was calculated,
regarding a region surrounded by a bold frame shown in FIG. 65 as a
stack volume, as in Examples 7 and 8.
[0382] Thus, according to the present invention, the maximum volume
power density of the fuel cell stack could be increased to
approximately 1.5 times or higher than the conventional fuel cell
stack. In addition, volume power density could be improved without
the use of such auxiliary equipment as a fan for supplying air. The
total output power of the two-layered stack in Example 7 according
to the present invention was 279 mW and the total output power of
the three-layered stack in Example 8 was 408 mW. In an example
where the fuel cell stack has a two-dimensional outer dimension of
23 mm.times.23 mm, the conventional fuel cell including a single
fuel cell layer can output only approximately 217 mW even with the
output of 41.1 mW/cm.sup.2, which is the maximum value of the
average power density of sample No. 3. On the other hand, according
to the three-dimensionally highly-integrated fuel cell stack of the
present invention, with the increase in thickness by 1.2 mm in the
two-layered stack and increase in thickness by 2.3 mm in the
three-layered stack, output could be increased to approximately
1.29 times in the case of the two-layered stack and to
approximately 1.88 times in the case of the three-layered stack, as
compared with the conventional fuel cell. Thus, in a case where an
area for installing a fuel cell in equipment is limited, a fuel
cell stack from which a large amount of output can be extracted
with slight three-dimensional increase in thickness can be
fabricated.
[0383] If output from equipment, which is a load of the fuel cell
stack, is large, an area required for the MEA is great. However,
two-dimensional arrangement does not allow arrangement in an area
equal to or greater than a mountable area of the equipment.
Therefore, such a case as requiring stack on the MEA often occurs.
In this case, if an area of the MEA is large, that is, an area of
the fuel cell layer is large, as in the conventional fuel cell, a
distance to the fuel cell layer to be stacked thereon becomes
greater in terms of intake of oxygen. In the conventional example,
in the case of a fuel cell layer having a square MEA of 5 cm.sup.2,
for three-dimensional stack while extracting power density of the
MEA equivalent to that in the case of one layer, an interval
between the fuel cell layers should be set to approximately 3 to 4
mm. If an area of the fuel cell layer is greater than the above,
the interval between the fuel cell layers should be set to 4 mm or
greater or auxiliary equipment such as a fan is required. According
to the present invention, by increasing the number of unit cells in
a shape of an elongated strip from eight to eight or more or by
increasing a length of the unit cell in a shape of an elongated
strip, stacking can be carried out while maintaining constant the
interval between the fuel cell layers (a thickness of the spacer).
Namely, even though output of the load is increased, the fuel cell
stack can be fabricated without lowering volume power density. In
addition, according to the present invention, the fuel cell layers
can be brought closer to one another and highly integrated.
Therefore, influence by a temperature owing to power generation in
the adjacent fuel cell layer is great, and power generation
efficiency and maximum average power density can be improved. In
addition, temperature increase raises a saturated vapor pressure
and an amount of condensed water produced in the fuel cell stack is
decreased. Moreover, according to the fuel cell stack in the
present invention, as an air passageway is three-dimensionally
formed in the fuel cell stack, produced water that has turned to
vapor is satisfactorily exhausted through the passageway to the
outside of the fuel cell stack and thus electric power can be
supplied in a stable manner
Example 9
[0384] The fuel cell stack was fabricated by alternately arranging
cotton non-woven fabric instead of titanium non-woven fabric that
had served as the spacer in the fuel cell stack in Example 7 and a
spacer made of SUS316L. The cotton non-woven fabric has a width of
1.5 mm and a thickness of 0.5 mm, and it is longer than an outer
dimension of the fuel cell stack by 2 cm. Therefore, a portion that
has extended off was folded back to the back of the fuel cell
stack. The configuration in Example 9 is otherwise the same as in
the fuel cell stack in Example 7.
[0385] FIG. 67 shows results of continuous power generation tests
(a voltage-time characteristic evaluation test at an output current
of 100 mA/cm.sup.2) for a two-layered stack fabricated in Example
7, a two-layered stack fabricated in the present example, and the
two-layered stacks fabricated in Comparative Example 1 (stacks
having cell intervals d of 2 mm and 3 mm, respectively). The
continuous power generation test was conducted under such
conditions as 40.degree. C. an output current of 100 mA/cm.sup.2,
and being open to atmosphere. In the case of the two-layered stack
in Comparative Example 1, continuous output for 20 minutes was
unsuccessful even with cell interval d being set to 2 mm, due to
air supply being blocked by produced water. When cell interval d
was set to 3 mm in the two-layered stack in Comparative Example 1,
continuous power generation characteristics substantially
equivalent to those in Example 7 were exhibited. On the other hand,
the two-layered stack according to the present example including as
the spacer, cotton non-woven fabric, which is a hydrophilic porous
body, exhibited excellent continuous power generation
characteristics, because condensed produced water was absorbed and
exhausted to the back side of the fuel cell stack and the water
evaporated. Thus, it can be found that, by using a spacer made of a
hydrophilic porous body, water produced from the unit cell can
evaporate to the outside and excellent continuous power generation
characteristics can be maintained. An amount of produced water
includes an amount of water produced as a result of combustion of
methanol with oxygen at the cathode as methanol moves from the
anode electrode through the electrolyte membrane to the cathode
side or an amount of transferred water included in the fuel. The
continuous power generation characteristics can further be improved
by using an electrolyte membrane in which a crossover amount of
methanol or water is reduced, and thus a fuel cell stack achieving
further higher output can be obtained.
[0386] The fuel cell stack according to the present invention has
such a structure as achieving high output and facilitating exhaust
of produced water as vapor, as compared with a conventional
structure, and it can be concluded that such a structure is
advantageous for higher integration. In particular, it is extremely
advantageous in that higher integration can be achieved without the
use of auxiliary equipment for supplying air.
Example 10
[0387] Five MEAs each in a shape of an elongated strip (a thickness
of 0.45 mm, a width of 2.35 mm, and a length of 23 mm) were
fabricated with a method the same as in Example 3. Then, the flow
channel substrate and the anode current collector were bonded to
each other through diffusion bonding, and thereafter the five MEAs
were arranged on the anode current collector. Specifically, the
five MEAs were arranged on regions A at five locations respectively
such that a pitch between the MEAs in a shape of an elongated strip
was set to 3.5 mm. The flow channel substrate used here has a
structure the same as that of the flow channel substrate in FIG.
48. Namely, in the flow channel substrate, a through hole like a
slit has a width of 1 mm, a wall formed between the through holes
like a slit and a flow channel adjacent thereto has a width of 0.5
mm, the flow channel has a width of 1.5 mm and a depth of 0.1 mm,
the flow channel substrate has a thickness of 0.2 mm, and the
number of flow channels in a longitudinal direction was increased
from four to five (therefore, four through holes like a slit were
provided and projections of 2.5.times.3 mm serving as manifold
insertion ports were located on a diagonal line).
[0388] In addition, the anode current collector used here has an
outer dimension the same as that of the flow channel substrate in
the present example. More specifically, the anode current collector
has a structure the same as that of the anode current collector in
FIG. 49, in which the number of regions A was increased from four
to five (therefore, four through holes like a slit were provided),
the through hole like a slit has a width of 1 mm and a length of 25
mm, region A has a width of 2.5 mm, and an opening in a hexagonal
shape of which side is 0.25 mm long is provided in a region having
a width of 0.75 mm from the centerline in the longitudinal
direction of region A toward opposing ends, with a line width of
0.1 mm being left.
[0389] Thereafter, the composite cathode current collector in which
the number of cathode current collectors in the composite cathode
current collector in FIG. 50(c) was increased from four to five,
one cathode current collector has a width of 2.5 mm, and a distance
between the cathode current collectors is set to 1 mm was made to
lie along the surface of GDL25BC on the cathode side (the cathode
conductive porous layer) and subjected to thermocompression bonding
for two minutes at such conditions as 100.degree. C. and 0.1
kgf/cm.sup.2 through hot pressing, to tentatively bond the anode
current collector, the MEAs, and the cathode current collectors to
one another. A two-component type epoxy adhesive was applied to the
end portions of the MEAs and the end portions of the current
collectors to bond the MEAs and the current collectors to one
another and to seal the end portions. One fuel cell layer was thus
fabricated. Three such fuel cell layers were fabricated, that are
denoted as samples No. 4, No. 5 and No. 6, respectively.
Thereafter, the spacer layer the same as in Example 7 was
fabricated, with which a two-layered stack and a three-layered
stack were fabricated. The two-layered stack is a fuel cell stack
having such a structure that the fuel cell layer of sample No. 4,
the spacer layer and the fuel cell layer of sample No. 5 were
stacked in this order. Meanwhile, the three-layered stack is a fuel
cell stack having such a structure that the fuel cell layer of
sample No. 4, the spacer layer, the fuel cell layer of sample No.
5, the spacer layer, and the fuel cell layer of sample No. 6 were
stacked in this order
[0390] FIG. 68 shows power generation characteristics of sample No.
4 (one-layered stack), a two-layered stack, and a three-layered
stack. FIG. 68(a) is a graph showing a current-voltage
characteristic and a characteristic of MEA average current-power
density of sample No. 4 (one-layered stack), the two-layered stack
and the three-layered stack FIG. 68(b) is a table summarizing power
generation characteristics and shapes of these stacks. Measurement
was conducted by supplying a methanol aqueous solution of 3 mol/L
as the fuel with a pump and passively supplying air without using
auxiliary equipment for power generation, as in Example 7. As shown
in FIG. 68, maximum average power densities of sample No. 4, the
two-layered stack and the three-layered stack were 39.5
mW/cm.sup.2, 43.7 mW/cm and 42.2 mW/cm.sup.2, respectively. It is
noted that maximum average power densities of samples No. 5 and No.
6 were 41.2 mW/cm.sup.2 and 41.5 mW/cm.sup.2, respectively. Even
when a width of the MEA was increased to 2.35 mm, lowering in the
average power density of the MEA was not observed in the range of
the output current density in the present example. By stacking two
or three layers, the temperature of the fuel cell and the
temperature of air in the fuel cell stack increase, so that
catalyst activity improvement or the like was achieved and hence
power generation efficiency improved. Thus, the average power
density of the MEA improved.
[0391] In Examples 7 and 8, the MEA had a width of 1.8 mm, and a
width of the through hole like a slit in the anode current
collector and the flow channel substrate or a distance between the
cathode current collectors, which is a width of an air path, was
set to 1 mm. On the other hand, in the present example, the width
of the MEA was increased to 2.35 mm and a width of the air path was
set to 1 mm. As a ratio of the MEA to a gap in a plane is higher,
two-dimensionally higher integration can be achieved. In the
present example, as shown in FIG. 68(b), the two-layered stack
having a thickness of 0.19 cm exhibited a maximum value of volume
power density of 328 W/L and the three-layered stack having a
thickness of 0.3 cm exhibited a maximum value of volume power
density of 300 W/L. In the present example, the maximum value of
the volume power density was significantly improved as compared
with Examples 7 and 8. Thus, according to the present invention, by
optimizing a shape (such as a width) of the MEA, a shape (such as a
width) of an air path (a gap region), or the like, power density
can significantly be improved. Such optimization can be achieved by
optimizing arrangement of the fuel cells (unit cells) within the
fuel cell stack.
[0392] If the fuel cell stack is preferably arranged in a
three-dimensional space having a height in mounting the fuel cell
stack on equipment, importance is preferably placed on air supply
and a ratio of the gap region within the fuel cell stack is made
higher. If the fuel cell stack is preferably arranged in a space
having a small thickness as if being two-dimensional, a ratio of
the MEA is preferably made higher to improve two-dimensional
integration. The fuel cell stack according to the present example
can also achieve an effect the same as that of the fuel cell stacks
in Examples 7 and 8. According to the fuel cell stack in the
present example, the maximum value of volume power density could be
increased to approximately two times as high as that of the
conventional fuel cell stack in Comparative Example 1.
Example 11
[0393] Three fuel cell layers having the same structure as the fuel
cell layer fabricated in Example 10 (samples No. 4, No. 5 and No.
6), cotton non-woven fabric which is a hydrophilic porous body to
serve as a spacer, and a titanium porous body were used to
fabricate the fuel cell stack having three fuel cell layers.
Specifically, six spacers made of cotton non-woven fabric having a
thickness of 0.4 mm and a width of 1.5 mm were arranged at
intervals of 2 mm, and titanium non-woven fabric (porosity of 60%,
a thickness of 0.4 mm, a width of 1.5 mm, a length of 20 mm, and
linear 20 .mu.m) of Bekinit K.K., which is a titanium porous body,
was arranged at opposing end portions in a longitudinal direction
of each MEA such that the entire fuel cell stack is electrically
serially wired, followed by bonding by a two-component,
electrically conductive adhesive (Dotite SH-3A manufactured by
Fujikura Kasei Co., Ltd.) to form the spacer layer. The fuel cell
stack was thus fabricated by stacking the fuel cell layer, the
spacer layer, the fuel cell layer, the spacer layer, and the fuel
cell layer in this order. Here, a length of the cotton spacer that
was used was longer than an outer dimension of the fuel cell stack
by 2 cm from the end portion. Therefore, as in Example 9, a portion
that has extended off was folded back to the back of the fuel cell
stack. In addition, in order to further lower electrical
resistance, an extraction terminal of the anode current collector
and an extraction terminal of the cathode current collector were
joined to form serial wiring.
[0394] FIG. 69 shows continuous power generation characteristics of
the obtained fuel cell stack at an output current density of 100
mA/cm.sup.2 and at temperatures of 25.degree. C. and 40.degree. C.
The left ordinate represents an output voltage (V) of the fuel cell
stack, and the right ordinate represents volume power density (W/L)
of the fuel cell stack. As shown in FIG. 69, in the continuous
power generation test for 60 minutes, under any temperature
condition, lowering in the output voltage was as low as
approximately 10%. In addition, at the temperature of 25.degree. C.
as well, continuous power generation for one hour could be achieved
while maintaining high volume power density not lower than 220 W/L.
In general, when a highly-integrated stack is placed in equipment,
an ambient temperature may be raised as the stack is surrounded by
a housing. In the fuel cell stack in the present example, under the
condition that the ambient temperature was set to 40.degree. C.,
power generation efficiency improved as compared with the condition
that the ambient temperature was set to 25.degree. C., and high
volume power density not lower than 250 W/L was exhibited (see FIG.
69). Thus, the fuel cell stack in the present example can achieve
continuous power generation while maintaining high volume power
density.
[0395] It should be understood that the embodiments and the
examples disclosed herein are illustrative and non-restrictive in
every respect. The scope of the present invention is defined by the
terms of the claims, rather than the description above, and is
intended to include any modifications within the scope and meaning
equivalent to the terms of the claims.
* * * * *