U.S. patent application number 14/174605 was filed with the patent office on 2014-06-05 for fuel cell comprising at least two stacked printed circuit boards with a plurality of interconnected fuel cell units.
This patent application is currently assigned to UCL Business PLC. The applicant listed for this patent is Imperial Innovations Limited, UCL Business PLC. Invention is credited to Daniel John Leslie BRETT, Anthony Robert John KUCERNAK.
Application Number | 20140154604 14/174605 |
Document ID | / |
Family ID | 43904464 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140154604 |
Kind Code |
A1 |
BRETT; Daniel John Leslie ;
et al. |
June 5, 2014 |
Fuel Cell Comprising at Least Two Stacked Printed Circuit Boards
with a Plurality of Interconnected Fuel Cell Units
Abstract
A fuel cell comprising at least two stacked fuel cell boards
(22) which each comprise a membrane of substantially gas impervious
electrolyte material and at least two electrode pairs wherein the
anode and cathode of each said electrode pair are arranged on
respective faces of said membrane. An electrode of each pair of
electrodes is connected to an electrode of an adjacent pair of
electrodes by a through-membrane connection (13) or by an external
connection on a Printed Circuit Board, comprising an electrically
conductive region of said electrolyte material. A method for
forming the through-membrane electrical connections in the
electrolyte membrane is also disclosed.
Inventors: |
BRETT; Daniel John Leslie;
(Gerrards Cross, GB) ; KUCERNAK; Anthony Robert John;
(London, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UCL Business PLC
Imperial Innovations Limited |
London
London |
|
GB
GB |
|
|
Assignee: |
UCL Business PLC
London
GB
Imperial Innovations Limited
London
GB
|
Family ID: |
43904464 |
Appl. No.: |
14/174605 |
Filed: |
February 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14022465 |
Sep 10, 2013 |
|
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14174605 |
|
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Current U.S.
Class: |
429/437 ;
429/434; 429/457; 429/465 |
Current CPC
Class: |
H01M 8/0269 20130101;
H01M 8/1007 20160201; H01M 8/2465 20130101; H01M 8/0263 20130101;
H01M 8/2483 20160201; H01M 8/0228 20130101; H01M 8/04082 20130101;
Y02E 60/50 20130101; H01M 8/04029 20130101; H01M 8/1004 20130101;
H01M 8/241 20130101; H01M 8/0297 20130101 |
Class at
Publication: |
429/437 ;
429/465; 429/434; 429/457 |
International
Class: |
H01M 8/24 20060101
H01M008/24; H01M 8/04 20060101 H01M008/04; H01M 8/10 20060101
H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2011 |
GB |
1103590.4 |
Claims
1-24. (canceled)
25. A fuel cell comprising at least two fuel cell boards, each
individually switchable by providing at least one switch on each of
said fuel cell boards.
26-68. (canceled)
69. A fuel cell as claimed in claim 25, wherein when the fuel cell
boards in the fuel cell are stacked, adjacent fuel cell boards in
the stack are separated by an insulating spacer.
70. A fuel cell as claimed in claim 69, wherein the spacer has
integrated coolant conduits.
71. A fuel cell as claimed in claim 69, wherein the spacer is
shaped to assist the distribution of reactants.
72. A fuel cell as claimed in claim 69, further comprising water
supply or extraction conduits.
73. A fuel cell as claimed in claim 25, wherein each fuel cell
board comprises a plurality of anodes and a plurality of cathodes
and wherein on each face of each fuel cell board, the sequence of
anodes and cathodes alternates such that each anode, except the
anode on edge of the fuel cell board, lies between two cathodes,
and each cathode, except the cathode on edge of the fuel cell
board, lies between two anodes, and the cathodes of two adjacent
fuel cell boards lie substantially opposite each other.
74. A fuel cell as claimed in claim 73, wherein a first reactant is
delivered to a pair of anodes on a first and second adjacent fuel
cell boards by means of a single first reactant distribution
channel on one reactant distribution layer, and a second reactant
is delivered to a pair of cathodes on said first and second
adjacent fuel cell boards by means of a single second reactant
distribution channel on said reactant distribution layer.
75. A fuel cell as claimed in claim 73, wherein each face of each
fuel cell board carries either only anodes or only cathodes, and
the fuel cell boards in the fuel cell are arranged so that the
anodes of two adjacent fuel cell boards lie substantially opposite
each other, and the cathodes of two adjacent fuel cell boards lie
substantially opposite each other.
76. A method of controlling the power output profile of a fuel cell
having at least two fuel cell boards comprising selectively
switching one or more fuel cell board.
77. A method as claimed in claim 76, comprising the step of
switching fuel cell boards according to a duty cycle.
78. A method as claimed in claim 76, further comprising depositing
a corrosion resistant layer onto the surface of a current
collection and/or distribution layer of a fuel cell.
79. The method of claim 78, wherein the corrosion resistant layer
is formed by applying an ink comprising a conducting polymer or a
conductive metal oxide, or carbon conductive ink, to the surface of
a current collection and/or distribution layer.
80. The method of claim 79, wherein the conducting polymer is
selected from polyaniline, polypyrrole, polythiophene and
poly(3,4-ethylenedioxythiophene) (PEDOT).
81. The method of claim 79, wherein the conductive metal oxide is
selected from Ti4O7; Ti0.9Nb0.1O2; or tin oxide doped with
antimony, fluorine, or indium.
82. The method of claim 79, wherein the conductive polymer has a
perfluorinated substituent, especially where that perfluorinated
substituent is a perfluorinated aliphatic chain, or contains
perfluorinated aliphatic chains.
83. The method of claim 79, wherein the monomers forming the
conductive polymer comprise a perfluorinated alkyl chain.
84. The method of claim 79, wherein the surface of a current
collection and/or distribution layer undergoes direct
derivatisation of its surface, utilising a silane or diazo reagent
to form a corrosion resistant layer.
85. The method of claim 84, wherein the silane or diazo reagent
comprises perfluoro properties.
86. The method of claim 85 wherein the silane or diazo reagent
comprises a pefluorinated benzene, or an aryl ring further
comprising a perfluorinated alkyl chain.
87. The method of claim 77, wherein a combination of corrosion
resistant layers is applied to the surface of a current collection
and/or distribution layer, the method comprising the step of
applying an ink to the surface of a current collection and/or
distribution layer, and/or electrodepositing a conductive polymer
onto the surface of a current collection and/or distribution layer,
and/or direct derivatisation of the surface of a current collection
and/or distribution layer, with a silane or diazo reagent.
Description
TECHNICAL FIELD
[0001] This invention relates to fuel cells, for example solid
polymer electrolyte fuel cells. This invention also relates to the
use of Printed Circuit Board (PCB) technology in the manufacture of
fuel cells of the present invention, the use of corrosion-resistant
coatings, and the use of a Shape Memory Alloy (SMA) in low profile
valves in fuel cells.
BACKGROUND ART
[0002] A fuel cell is an electrochemical device which generates
electrical energy and heat from an oxidant (e.g. pure oxygen or
air) and a fuel (e.g. hydrogen or a hydrogen-containing mixture, or
a hydrocarbon or hydrocarbon derivative). Fuel cell technology
finds application in portable, mobile and stationary applications,
such as power stations, vehicles and laptop computers.
[0003] Typically, a cell comprises two electrodes, an anode and a
cathode, which are separated by an electrolyte membrane that allows
ions (e.g. hydrogen ions) but not free electrons to pass through
from one electrode to the other. A catalyst layer on the electrodes
accelerates a reaction with the fuel (on the anode electrode) and
oxidant (on the cathode electrode) to create or consume the ions
and electrons. The electrons freed at the anode form an electrical
current, which is used to perform work and then flows to the
cathode where the electrons are consumed.
[0004] A single pair of electrodes separated by an electrolyte
membrane is called a Membrane Electrode Assembly (MEA). A fuel cell
MEA operating under a moderate load produces an output voltage of
about 0.7 V, which is too low for many practical considerations.
Conventionally, in order to increase this voltage, MEAs are
assembled into a stack as shown in FIG. 1. Each MEA 1 has a layer
of "electrolyte membrane" 1a, which is an ion-permeable membrane
sandwiched between two electrolyte layers, and an anode 2 and a
cathode 3 on opposite faces of the electrolyte membrane. Adjacent
MEAs can be separated by an electrically conducting bipolar
separator plate 4 and hydrogen fuel 5 and oxygen gas 6 flow through
the channels provided on opposed faces of the bipolar plate. End
plates 9 are connected to an external circuit via an electrical
connection 7, 8. The number of these MEAs in a stack in a fuel cell
determines the total voltage, and the surface area of each membrane
electrode determines the total current.
[0005] A problem with current fuel cell geometry is that when fuel
cells are stacked in this manner, the electrical current flows
perpendicular to the face of an MEA. Hence, this stacking requires
separator plates to conduct the current from the positive electrode
of one cell to the negative electrode of the next. Furthermore,
failure of an MEA, for example due to pinhole formation through the
membrane electrolyte or short-circuiting of electrodes across the
membrane, results in the entire stack needing to be shut down. Yet
further, if a single MEA is not performing as well as the others,
current will be driven through it, which results in its rapid
degradation. If one MEA is destroyed then the whole fuel cell stack
becomes unusable.
[0006] In the known arrangement the bipolar plates are commonly
made of graphite carbon or stainless steel and must be electrically
conductive, gas impervious and incorporate the flow-field channels
for the distribution of reactants, and possibly also of coolant,
across the faces of the MEA in their surface. Thus, the material
composition of the bipolar plates is constrained and the plates are
complex and expensive to manufacture. Furthermore, maintaining the
correct water content in the electrolyte membrane is essential to
optimising its performance. The membrane requires a certain level
of moisture to operate and conduct the ionic current efficiently so
that the cell current does not drop. Water produced by the cell is
removed by the flow of gas along the cathode, or wicked away.
Accordingly, corrosion of metallic bipolar plates in the humid
environment of the cell is a common problem, limiting the materials
from which the bipolar plate can be made. Overheating of the fuel
cell stack is a further problem and cooling is necessary. This is
usually achieved by the provision of further plates comprising
channels to circulate coolant water through the middle of the
stack, which is cumbersome and impractical for many applications.
Further still, the electrical output of a stack is modulated and
regulated by using monolithic power electronics. This is expensive
and point failure of these power electronics also leads to failure
of the whole fuel cell system.
[0007] One known alternative is described in "New SPFC-Technology
with Plastics", by K. Ledjeff and R. Nolte, Proceedings of the
First International Symposium on New Materials for Fuel Cell
Systems, 1995, p 128-134. The authors describe a banded structure
of a single electrolyte membrane to generate a high output voltage,
without employing stack technology. The electrodes and bipolar
plates of a standard construction fuel cell are mounted in a
co-planar configuration, with each fuel cell MEA present as a band
joined to a second adjacent MEA band. A problem with this approach
is that the membrane structure requires careful assembly and
sealing of the adjacent bands to avoid mixing of oxidant and
fuel.
[0008] The invention is set out in the claims. The construction of
a stacked fuel cell is greatly simplified, as plastic or other
non-electrically conductive spacers may be used between adjacent
MEAs as opposed to complicated and expensive bipolar plates.
Because of the layout of the fuel cell of the present invention,
there is low risk of fuel mixing with oxygen. Individual MEAs or
fuel cell boards in the fuel cell stack may also be incorporated
with an independent switch which provides additional control of the
fuel cell.
[0009] In the following discussion the invention is described by
reference to three embodiments. The description of these
embodiments is not to be understood to limit the scope of the
present invention and is merely exemplifying the invention.
BRIEF DESCRIPTION OF THE FIGURES
[0010] Specific embodiments of the invention will now be described
by reference to the accompanying Figures in which:
[0011] FIG. 1 shows a schematic side view of a stacked fuel cell of
the known type.
[0012] FIG. 2 (a) shows a cross-sectional side view of a fuel cell
board according to the first embodiment of the present
invention.
[0013] FIG. 2 (b) shows a perspective view of a fuel cell board
according to the first embodiment of the present invention.
[0014] FIG. 3 shows a sectional side view of a fuel cell comprising
a stack of fuel cell boards according to the first embodiment of
the present invention.
[0015] FIG. 4 shows a perspective view of a fuel cell system
according to the first embodiment of the present invention.
[0016] FIG. 5 (a) shows a cross-sectional side view of a spacer
with surface channels.
[0017] FIG. 5 (b) shows a perspective view of a spacer with surface
channels.
[0018] FIG. 5 (c) shows a plastic spacer viewed from above or
below, having an internal conduit provided for a coolant.
[0019] FIG. 6 shows a partially exploded perspective view to show
the internal structure of a section of the fuel cell stack of the
first embodiment.
[0020] FIG. 7 shows a perspective, partial cut-away view of a fuel
cell stack of the second embodiment of the present invention.
[0021] FIG. 8 shows an exploded view of a section of the fuel cell
detailing the MEA layout of the second embodiment of the present
invention.
[0022] FIG. 9 shows an exploded view of the individual layers of
the PCB board and MEAs of the second embodiment of the present
invention.
[0023] FIGS. 10 (a) and 10 (b) show a plan view of a current
collection and distribution layer of the second embodiment of the
present invention.
[0024] FIG. 11 shows a plan view of a reactant distribution layer
of the second embodiment of the present invention.
[0025] FIG. 12 shows an exploded view of the individual layers of
the PCB board with reactant flow of the second embodiment of the
present invention.
[0026] FIGS. 13 (a) and 13 (b) show a cross sectional view of the
third embodiment of the present invention.
[0027] FIG. 13 (c) shows a plan view of the individual layers of a
fuel cell board of the third embodiment of the present
invention.
[0028] FIG. 14 shows the lower half of a reactant channel and
illustrates the use of a Shape Memory Alloy (SMA) as a low profile
valve.
[0029] A first embodiment of the invention can be understood
referring to FIGS. 2 (a) and 2 (b) in which electrode pairs are
arranged in a series along either side of a single layer of polymer
electrolyte 10, such as a Nafion.TM. membrane, to form a membrane
or sheet, termed here a fuel cell board 22. Anodes 11, separated by
gaps 15, are situated on one face of this membrane and cathodes 12,
separated by gaps 15, are situated on the other face of this
membrane. The anode and cathode respectively of two adjacent
electrode pairs may partially overlap. Through-membrane electronic
connections 13 connect the electrodes across the membrane in the
overlapping region and are produced by a homogeneous chemical
deposition process described in more detail below. A catalyst layer
14 adjacent to the electrodes encourages the reactions at the
electrodes. A fuel 16, such as hydrogen gas, flows along the face
of the fuel cell board 22 supporting the anodes; an oxidant 17,
such as oxygen gas or air, flows along the face of the fuel cell
board supporting the cathodes. One electrode at the edge of the
upper face and one electrode at another edge of the lower face of a
fuel cell board are connected to an external circuit via an
electrical connection 18, 19. In this series arrangement, the
surface area of an electrode pair determines the size of the
current for a fuel cell board 22, but the voltage builds up
corresponding to the number of electrode pairs on that fuel cell
board 22.
[0030] In the preferred embodiment, the thickness of the
electrolyte membrane layer 10 is between 1-200 .mu.m, and
preferably between 5-100 .mu.m. The electrode bands 11, 12 are 1
mm-5 cm in width, preferably 2 mm-1 cm in width. The gaps 15
between the electrode bands are between 0.1 mm-1.5 cm wide,
preferably between 0.2 mm and 1 cm wide. The width of the
through-membrane electronic connectors 13 is 1 .mu.m-2 mm, and
preferably 10 .mu.m-1 mm.
[0031] FIGS. 3 and 4 show a fuel cell, wherein multiple fuel cell
boards 22 are stacked between two endplates 21 in order to provide
increased current. Electrically insulating spacers 20 are
integrated into the stack between each of the fuel cell boards each
comprising an spacer composed of electrically insulating material,
which can incorporate a reactant distribution system as well as
optionally containing channels or conduits associated with the
cooling system and optionally containing a distribution system for
delivering or removing water. The spacers 20 may be arranged in any
suitable layout, as well as possibly containing channels or
conduits associated with the provision of water required for the
hydration of the membranes or the extraction of product water, as
discussed in more detail below. The size of a single cell (that is,
the surface area of a pair of electrodes) determines the size of
the current for a fuel cell board. The number of these single cells
on a fuel cell board determines the voltage produced. The number of
fuel cell boards in a stack determines the size of the total
current of the fuel cell stack. Manifolds of any appropriate form
(not shown in FIGS. 3 and 4) supply and collect the reactants
respectively at the inlets and outlets of the cell; and may
incorporate further inlets and outlets for the coolant and the
hydration/product water.
[0032] As there is no requirement for electrical contact between
the fuel cell boards in the present invention, the bipolar plate
used in current fuel cell designs can be replaced by a simple
plastic spacer which is easily manufactured and hence cheap.
Similarly, as it is no longer necessary to stack the anode of one
fuel cell board next to the cathode of the next, adjacent fuel cell
boards may have their anode sides (or cathode sides) facing each
other in which case porous spacers 20 can be provided as there is
no risk of the gases mixing. This results in a significant increase
in packing density of the fuel cell as one gas flow can serve two
fuel cell boards, reducing the number of seals, and increasing the
power density. Repeat distances (i.e. the thickness of the combined
fuel cell board and feed region) may be as low as about 0.2 mm.
Hence a far higher volumetric power density of the fuel cell system
is achievable. Further, the channels in the surface are easily
formed for gas circulation and internal conduits are easily
integrated into the spacer, allowing coolant and/or water to
circulate and providing a cooling and water distribution system for
the stack. Channels provided for the reactant gases and channels or
conduits associated with the cooling and/or water distribution
system are separate as discussed in more detail below.
[0033] The end cathodes 12 and anodes 11 on each fuel cell board
are connected to respective first and second output lines 23, 24
via electrical connections 18, 19. The connection between each fuel
cell board in the stack and the second output line 24 can be
controlled by a switch mechanism 25 such as a field-effect
transistor (FET) switch providing power handling and control
directly at the cell. Each of these switches can be controlled by
individual control lines 26. For this reason, failure of a fuel
cell board is not serious, as each individual fuel cell board can
be "switched off" or its loading may be reduced, allowing power
handling and control at the cell using cheap low-current
switches.
[0034] FIGS. 5 (a) to (c) show in more detail an insulating spacer
20 with channels 27 etched into its surface for the reactants to
flow through and one possible layout of an internal cooling channel
28 (shown by dotted lines), which allows a cooling gas or liquid to
circulate between the layers of the fuel cell boards. For most
efficient cooling, these channels may be provided in the spacers
between every layer, or between multiple layers such as every
2.sup.nd, 3.sup.rd or 4.sup.th fuel cell board layer in the fuel
cell stack.
[0035] Alternatively, as shown in FIG. 7, cooling may be provided
by means of a fan 46 situated at one or more edges of the fuel cell
stack, blowing air through the stack in channels. This leads to
effective cooling being achieved, as the single channel necessary
for reactants allows faster flow of gas through the layers of the
stack than a bipolar plate or a further separate plate used for a
cooling system. Further, a larger volume of gas may be used, which
also improves cooling. Coolant is pumped through the cooling
channels to maintain the stack at a desired operating
temperature.
[0036] FIG. 6 shows a partially exploded cross sectional view of
the internal structure of a section of the fuel cell stack. Banded
MEAs 30 are arranged with the anodes of (in this illustration)
vertically adjacent MEAs facing each other and the cathodes of
vertically adjacent MEAs also facing each other. Oxygen or air A is
delivered to the cathodes, while fuel B, such as hydrogen, is
delivered to the anodes. The porous, electrically non-conductive
spacer 31 having gas distribution channels forms a gas distribution
layer using ribs 32 of a flexible electrically non-conductive
material. The ribs are positioned along the gaps 33 between the
bands of electrodes. Each fuel cell board is sealed from the
atmosphere with a seal 34 and is attached to an external circuit,
or bus bar 35, for example via an FET switch 36.
[0037] The porous, electrically non-conductive spacer 31 may be
made of a rigid plastics material. Preferably the ribs 32 and seals
34 are made of polydimethylsiloxane (PDMS), or a similar flexible
material that provides a tight seal.
[0038] In order to fabricate the fuel cell stack, multiple bands of
electrodes are, for example, screen printed onto both the upper and
lower face of a membrane, the anodes and cathodes being in the form
of bands running in the same direction as, and connected in pairs
by, through-membrane connections. The through-membrane electronic
connections are created in bands along the electrolyte membrane, as
described below. Each membrane is coated with a plurality of
electrodes in this way forms a fuel cell board. When complete,
these fuel cell boards are stacked alternately with insulating
spacers formed of moulded plastics.
[0039] In order to deposit through-membrane electrical conductors,
it is necessary to chemically deposit a metal or other electrically
conductive material within the membrane. This material must be
chemically stable within the membrane under fuel cell operating
conditions, and may typically be a precious metal (e.g. Pt, Au, Ru,
Ir, Rh, Pd) or an oxide of a precious metal. The general approach
involves placing the membrane in a mask and exposing it to a
precursor to the electrically conductive material. The precursor is
allowed to diffuse into the membrane, following which the membrane
is exposed to a reactant species which reacts with the precursor so
as to cause the deposition of particles within the membrane
material. This reactant species may be an acid or base, it may be
an oxidant or reductant, or it may destabilize the complex
containing the precursor. This second step `fixes` the position of
the through membrane conductor. The fixing step may also be
electrochemical--two electrodes may be placed on either side of the
membrane, and a current passed so as to induce reduction or
oxidation of the precursor at the point at which the
through-membrane electrical conductors are to be placed. The
particles produced in the "fixing step" may or may not be
electrically conducting. A post-treatment involving a further
chemical species or treatment step may optionally be used to render
those particles deposited in the membrane conductive or to improve
the conductivity of the particles deposited (for instance, a
chemical reductant may be applied to reduce a non-conducting
precursor particle to a conducting state). The precursor
introduction-fixing-post-treatment process may be repeated more
than once to improve the conductivity of the through membrane
connection.
[0040] A preferred method used to deposit conductive bands in the
membrane will now be described. The membrane is placed into a mask
before immersing it in a solution or allowing a solution to flow
over it. The solution contains a metal complex, for example a gold-
or platinum-complex (e.g. Pt(NH.sub.3).sub.4Cl.sub.2 or
AuCl.sub.2), in a high concentration. The metal ions/complex
diffuse(s) into the membrane, in the regions where it is not
protected by the mask. The metal-complex solution is then replaced
by a solution containing a strong reductant in a high concentration
(e.g. NaBH.sub.4, or HCOOH). The reducing agent reduces the metal
ions to their metallic state, leaving metal particles deposited in
the membrane. These metal particles aggregate and are able to
conduct electrical current.
[0041] Impregnation of the precious metal complex or salt into the
membrane is dependent upon the concentration of the ions in the
solution next to the membrane material, the diffusion coefficient
of the molecule in the membrane material and the thickness of the
membrane material. The temperature of the impregnation affects the
diffusion coefficient of the ion which increases with increasing
temperature.
[0042] The characteristic diffusion time for a distance L is:
.tau.=L.sup.2/D, where D is the diffusion coefficient of the
molecule in the ion exchange polymer. Therefore, assuming a
diffusion coefficient of 10.sup.-6 cm.sup.2 s.sup.1, a membrane
thickness of 200 .mu.m, and allowing the metal complex solution to
diffuse in from both sides of the membrane, yields a characteristic
time of 100 seconds. The membrane would normally be left for a
multiple of the characteristic time in order to allow to system to
reach equilibrium, therefore it would take approximately 250
seconds for the metal ions to impregnate the membrane material.
[0043] There is significant benefit in using a thinner membrane, as
halving the membrane thickness quarters the characteristic time
.tau.. Using the thin Nafion.TM. membranes which are common in
modern fuel cell systems gives a characteristic time of around 20
seconds. Diffusion of the subsequent reactant requires an
equivalent length of time to that required for the precious metal
precursor, resulting in the cycle of impregnation, reduction and
washing taking between 2 and 3 minutes. This process may be
repeated several times in order to produce improved
through-membrane contact.
[0044] To avoid the creation of very diffuse through-membrane
connections, the metal complex may alternatively be reduced with an
electron beam or a visible or UV light beam directed onto the
electrolyte membrane, in the location where the through-membrane
connection is required, resulting in a very well defined and
precisely located through-membrane connection. This method has the
added benefit that a mask is not necessary, as one could immerse
the entire electrolyte membrane in the metal-complex solution, and
selectively create the conducting through-connections.
[0045] Another alternative is to allow a composition containing a
reducing agent, which is activated by visible or UV light, and the
metal complex to diffuse into the electrolyte membrane. Thus when
visible or UV light is shone onto the membrane, the metal ions are
reduced. Precise, well-defined through-membrane connections can be
formed either by placing the membrane into a mask before directing
visible or UV light onto it, or without using a mask, by beam
geometry.
[0046] Because electronic connectors are integrated into the
membrane without compromising the integrity of the membrane, there
is low risk of fuel mixing with oxygen.
[0047] In a second embodiment of the present invention, a fuel cell
stack 40 is provided. A perspective, partial cut-away view is shown
in FIG. 7 and is described in more detail in reference to FIG. 8.
FIG. 8 shows an exploded view of a section of the fuel cell
detailing the MEA layout. Each fuel cell board 41 of the fuel cell
stack comprises a plurality of polymer electrolyte membranes 50a,
50b, 50c, etc., each of these membranes supporting an anode 51 and
a cathode 52. Alternatively, each fuel cell board may comprise one
single polymer electrolyte membranes, which supports a plurality of
anodes and cathodes. This does not alter the mode of operation
described below.
[0048] For clarity, the construction of the fuel cell boards and
the fuel cell stack is described herein in terms of `horizontal`
and `vertical` planes, in accordance with the embodiments
illustrated in the Figures. However, these terms are used for
clarity only, and are not limiting on the scope of the invention.
It will be clear to the reader that the fuel cell boards can be
arranged in any plane, not just the horizontal plane. Further, the
term `directly opposite` is not limited to the electrodes being in
register.
[0049] The anode lies on one face of the polymer electrolyte and
lies directly opposite a cathode on the opposite face of the same
electrolyte membrane layer. Together, the anode, cathode and
electrolyte layer form an MEA 59. In one (horizontal) plane, anodes
on one face of a polymer electrolyte layer 50a are adjacent to
cathodes on the same face of the adjacent electrolyte layer 50b.
That is, in a horizontal plane, the sequence of anode and cathode
positioning in adjacent MEAs alternates for each MEA. If one single
polymer electrolyte membrane is used, the sequence of anodes and
cathodes along each face alternates, with the respective cathodes
and anodes on the opposite face also alternating.
[0050] Thus, each fuel cell board comprises an alternating sequence
of anodes and cathodes on its two opposing faces. Anodes and
cathodes on each face of the electrolyte membrane in a horizontal
plane are separated by gaps 55. In the corresponding vertical
plane, anodes on one MEA 54a face anodes of the adjacent MEA 54b,
and cathodes on one MEA 54b face cathodes of the adjacent MEA 54c.
Thus when a plurality of fuel cell boards are stacked together,
pairs of adjacent anodes and pairs of adjacent cathodes alternate
in the plane vertical to the singly alternating anodes and
cathodes.
[0051] As shown in FIG. 9, between each planar layer of MEAs 59, is
a PCB board 60 that is made up of three individual layers. This PCB
board comprises a first current collection and distribution layer
61, a reactant distribution layer 62, and a second current
collection and distribution layer 63. These three layers are bonded
together to form a single PCB board 60. By virtue of the current
collection and distribution layer, current does not cross the
electrolyte layer, but moves laterally from anode to cathode
parallel to the horizontal plane of electrodes. Consequently, no
through-membrane connections are necessary for the current to
flow.
[0052] The individual layers of the PCB boards, i.e. the first and
second current collection and distribution layers and the reactant
distribution layers, are adhered together into a solid structure
using an epoxy-containing glass fibre composite. The PCBs may be
fabricated from pre-impregnated composite fibres, such that they
contain an amount of the material used to bond the individual
layers together and to bond the MEAs to the PCBs, or a
pre-impregnated composite fibre mask may be applied to the PCBs.
The MEAs may be laser bonded onto a PCB, thereby creating a fuel
cell board 41. To create the fuel cell stack, a plurality of boards
are laminated together. The gaps between the electrodes, and the
sealing achieved in these gaps by the epoxy resin, prevent the fuel
and oxidant gas flows from mixing, and prevent the fuel coming into
contact with the cathode and the oxidant coming into contact with
the anode, as described in more detail below. As this lamination
step results in a solid structure, with good contact between the
individual layers, the usually necessary heavy end boards become
redundant. Accordingly, a monolithic, light, and completely sealed
structure is produced. A simple PCB can be used as the end
board.
[0053] When assembled into a fuel cell stack, an anode of a first
MEA on a first fuel cell board lies vertically directly opposite an
anode of a second MEA on a second fuel cell board, wherein the
first and second fuel cell board are horizontally adjacent in the
fuel cell stack. Similarly, a cathode of a first MEA, on a first
fuel cell board, lies vertically directly opposite a cathode of a
second MEA on a second fuel cell board,
[0054] wherein the first and second fuel cell board are
horizontally adjacent in the fuel cell stack.
[0055] The structure of a current collection and distribution layer
61 is shown in more detail in FIG. 9. FIG. 9 shows a section of the
layer 61, corresponding to two adjacent MEAs 59. Referring to FIG.
9, the layer 61 consists of a frame 65, with panels of electrical
distribution tracks 66 that link the anode 59a of one MEA to the
cathode 59b of the vertically adjacent MEA. By virtue of the
electrical distribution tracks of the current collection and
distribution layer, the MEAs on each individual fuel cell board are
connected in electrical series. As indicated by the arrows in FIG.
9, the anode 59a and cathode 59b are located on the underside of
the MEA shown. The upper side of the MEA carries the corresponding
cathode 59c and anode 59d, respectively.
[0056] FIGS. 10a and 10b show a plan view of a current collection
and distribution layer 61 for a planar layer of 16 MEAs. In FIG.
10b (and at the left hand side end of the layer shown in FIG. 10a),
the plan view of a first and second current collection and
distribution layer bonded together is shown. That is, the tracks 66
of the first and second current collection and distribution layer
are visible. FIG. 10a (with the exception of the left hand side end
of the layer) shows a plan view of only one current collection and
distribution layer. Only a single layer of tracks is visible.
[0057] Holes 67 may be provided at regular intervals in the frame
65 to form part of an integral cooling system. These holes may form
vertical cooling channels 42, as shown in FIG. 7. One or more fans
46 may be included to force the air into the cooling system.
[0058] The current collection and distribution layer 61 also
includes holes 68 forming part of vertical reactant channels 43
(shown in FIG. 7), at the two opposed edges of the current
distribution layer, adjacent to the narrow ends of the electrode
bands.
[0059] These vertical channels 42, 43 are formed when the fuel cell
boards 41 are stacked. In the embodiment shown in FIG. 7, the
vertical fuel inlet and outlet channels run close to one edge 44 of
the layer, and the vertical oxidant inlet and outlet channels run
close to the opposite edge 45 of the layer. In the embodiment shown
in FIGS. 7-12, there are two vertical fuel and oxidant channels per
electrode. These vertical reactant channels connect with the
channels provided in the reactant distribution layer.
[0060] The structure of a reactant distribution layer 62 is shown
in detail in FIGS. 9 and 11, where a section of this layer 62,
corresponding to two adjacent MEA 59 electrode pairs, is shown.
This section is repeated, such that the number of reactant
distribution channels corresponds to the number of MEAs or
electrode pairs in the plane. The reactant distribution layer 62
comprises a frame 70, with two channels 71, 72, each forming a
planar reactant distribution loop, and holes located close to two
opposed edges of the frame, at the narrow ends of the distribution
loops, forming vertical reactant channels.
[0061] The holes along one edge of the frame 70 are sequentially
inlet 73a, 74a and outlet 73b, 74b channels for a reactant, for
example, a fuel, such as hydrogen and the holes along the opposite
edge of the frame are sequentially inlet 75a, 76a and outlet 75b,
76b channels for a reactant, for example, an oxidant, such as
oxygen or air. This is shown in FIGS. 9 and 12. The holes 73, 74,
75, 76 on the reactant distribution layer 62 line up with the holes
68 in the current collection and distribution layer 61 to create
the vertical reactant distribution channels 43 shown in FIG. 7.
[0062] In the reactant distribution layer shown in FIG. 12, a
reactant, for example a fuel, flows through the vertical channels
80 on one edge of the frame 70. When the location of the vertical
channel corresponds to the location between two anodes 59a, 59a',
the vertical channel is connected to the reactant distribution loop
72 of the reactant distribution layer 62. Accordingly, the fuel
will flow from the vertical inlet channel 73a, into the entrance 81
of the distribution loop 72 of the reactant distribution layer, and
along the faces of the anodes 59a, 59a' in the planes directly
above and below the reactant distribution layer. The exit 82 of the
distribution loop connects with the vertical fuel outlet channel
73b. The vertical channels 76a, 76b at the opposite edge of the
frame, for example carrying the oxidant, are holes in the frame 70
that are not connected to the distribution loop 72.
[0063] In the planar adjacent MEA, the electrodes facing the
distribution channel 71 are both cathodes 59b, 59b'. Accordingly,
the layout of the distribution channel 71 associated with these
electrodes is reversed: In the reactant distribution layer shown in
FIGS. 9, 11 and 12, the oxidant flows through the vertical channels
90 on the right hand edge of the frame. When the location of the
vertical channel corresponds to the location in the reactant
distribution layer 62 between two cathodes 59b, 59b', the vertical
channel is connected to the reactant distribution loop 71 of the
reactant distribution layer 62. Accordingly, the oxidant flows from
the vertical inlet channel 75a, into the entrance 91 of the
distribution loop of the reactant distribution layer, and along the
faces of the cathodes in the planes directly above and below the
reactant distribution layer. The exit 92 of the distribution loop
is adjacent to the entrance 91 of the distribution loop, and is
connected to the vertical fuel outlet channel 75b that is adjacent
to the vertical inlet channel 75a. The vertical channels 74a, 74b
at the opposite edge of the frame, carrying the fuel, are not
connected to the distribution loop 71.
[0064] In the adjacent fuel cell boards of the fuel cell stack, the
sequence of anodes and cathodes is reversed. The reactant
distribution channel layout thus alternates in sequence both in the
horizontal and vertical plane, in order to supply vertically
positioned pairs of anodes and cathodes with fuel and oxidant,
respectively.
[0065] As in the first embodiment of the invention, the polymer
electrolyte layer may be any electrolyte membrane, which allows
ions (e.g. hydrogen ions) but not free electrons to pass through
from one electrode to the other, for example, a sheet of Nafion.TM.
membrane. The same preferred dimensions as described above for the
first embodiment apply.
[0066] The reactant gas flows are kept separated by virtue of seals
in the gaps between the electrodes of adjacent MEAs. These seals
are achieved by impregnating the PCB boards with epoxy compounds,
which are activated to create a tight seal upon lamination and
boding with the MEA. If appropriate, further sealing may be
incorporated by using seals made of PDMS, for example at the outer
edges of a fuel cell board.
[0067] The vertical channels are connected to one or more reactant
manifolds along the two opposed edges of the stack, which supplies
and collects the reactants.
[0068] The frame of the reactant distribution layer may also
comprise a series of holes 100 at regular intervals in the frame to
form part of an integral cooling system, as described below.
[0069] The holes 67, 100 provided for the cooling system may be
positioned between every electrode in a horizontal plane, or
between every second, third, fourth, fifth, sixth, seventh, eight,
ninth or tenth electrode, depending on the cooling required. The
holes need not be at regular intervals between electrodes, but can
be at any suitable interval.
[0070] In a third embodiment of the present invention, each fuel
cell board 41 of the fuel cell stack comprises one or a plurality
of polymer electrolyte membranes arranged in a plane. In a first
alternative embodiment a) for each fuel cell board a series of
individual membranes is aligned in a plane, with an anode on one
face of the membrane and a cathode on the opposite side of the
membrane. In a second alternative embodiment b) a single membrane
is provided for each fuel cell board, with a series of anodes on
one face of the membrane and a series of cathodes on the opposite
side of the membrane. In both embodiments a) and b), all the anodes
are positioned on one face of the one or more polymer electrolyte
membranes in a horizontal plane, and all the cathodes are
positioned on the opposite face of the one or more polymer
electrolyte membranes in a horizontal plane. As in the second
embodiment, anodes and cathodes lie directly opposite each
other.
[0071] As in the first embodiment, in the third embodiment the fuel
cell boards are stacked so that the anode-side faces of two
adjacent fuel cell boards face each other, and the cathode-side
faces of two adjacent fuel cell boards face each other. In this
manner, the fuel can be delivered to all the anodes on two adjacent
fuel cell boards, and the oxidant can be delivered to all the
cathodes on two adjacent fuel cell boards, in a simple manner.
[0072] As in the second embodiment, in the third embodiment the
MEAs are bonded onto a PCB board made of three separate layers, to
form a fuel cell board. In the fuel cell stack, each planar series
of MEAs is located between two PCB boards. Reactant delivery to the
electrodes is achieved by the reactant distribution layer of the
PCB board, as described for the second embodiment. Thus, the
electrically insulating spacer of the first embodiment (by
reference to FIGS. 3 and 4, each spacer 20) is replaced by a PCB
layer.
[0073] As for the second embodiment, in the third embodiment, heavy
end plates are not necessary, as the fuel cell boards are laminated
together into a solid structure. A simple PCB can be used instead
of the heavy end board.
[0074] In embodiment a) of the third embodiment, in which each fuel
cell board comprises a series of individual membranes, the
electrical connections between anodes and cathodes are made either
through the gaps between the membranes, as holes can easily be
machined into the PCB boards, or by externally connecting the
current collection layer of the PCB board of the anode side to the
current distribution layer of the PCB board of the cathode
side.
[0075] In embodiment b) of the third embodiment, in which a single
membrane is provided for each fuel cell board, the electrical
connections between anodes and cathodes are made either by
through-membrane connections, as described for embodiment 1, or by
externally connecting the current collection layer of the PCB board
of the anode side to the current distribution layer of the PCB
board of the cathode side.
[0076] The PCB reactant distribution layer is described in more
detail with reference to the second embodiment of this invention.
However, due to the different layout of anodes and cathodes in the
third embodiment, the current collection and distribution layers
are not required when the electrical connection is made via
through-membrane connections or when electrical connections are
made through the PCB board. When the electrical connection is made
by externally connecting the PCB boards of the anode side to the
PCB boards of the cathode side, the PCB boards facing the anodes
will have a first and a second current collection layer, separated
by a reactant distribution layer and the PCB boards facing the
cathodes will have a first and a second current distribution layer,
separated by a reactant distribution layer. The connections are
made between the anode of a first MEA and the cathode of a second
MEA, between the anode of a second MEA and the cathode of a third
MEA, and so forth, depending on the number of electrode pairs on
the fuel cell board.
[0077] FIG. 13 (a) shows two layers of a fuel cell stack,
comprising an MEA 110, PCB material 111, an electrically conducting
layer 112 that conducts the current from the anode of a first cell
to the cathode of the next cell, and reactant distribution channels
113 to carry the reactants to the electrodes.
[0078] FIG. 13 (b) shows an alternative layout.
[0079] FIG. 13 (c) shows an extract of the fuel cell structure,
with one set of fuel cell electrodes sandwiched between two sets of
three separate boards. The individual subsections are laminated
together. Four fuel cell electrodes 120 are configured in
electrical series. Adjacent to the cathode side is a lower cathode
contact board 121 and an upper anode contact board 122. The contact
boards have slots cut into them so that reactant from the inlet
channel can flow to the outlet channel. The boards carry conducting
bars 123, 124 that carry the current laterally. In FIG. 13 (c) the
cathode board is shown with the conductor layer facing up, whereas
in reality the conductors would face towards the electrodes 120.
Adjacent to the lower cathode contact board 121 is an inner air
distribution layer 125. This layer has sets of inlet and outlet
channels 127 for the air (oxygen), and holes for the hydrogen inlet
and outlet channels 128. Each channel flows to the corresponding
reactant plenum. Adjacent to the upper anode contact board 122 is
an inner hydrogen distribution layer 126, comprising hydrogen inlet
and outlet channels 129 and air inlet and outlet channels 130.
Adjacent to the inner air distribution layer 125 is an upper
cathode contact board 131 and Adjacent to the inner hydrogen
distribution layer 126 is a lower anode contact layer 132. The
contacts on the lower side of the lower cathode contact board 121
are designed to connect to the connectors of the upper anode
contact board 122, placing the MEAs 120 in electrical series.
[0080] As in the first embodiment of the invention, the polymer
electrolyte layer may be any electrolyte membrane that allows ions
(e.g. hydrogen ions) but not free electrons to pass through from
one electrode to the other, for example, a sheet of Nafion.TM.
membrane, and the same preferred dimensions apply, with the proviso
that the membrane may be provided in individual sections, rather
than in one complete piece.
[0081] PCB technology (or other similar technology) is used to
manufacture elements of the fuel cell stack of the second and third
embodiment. This enables the elements to be manufactured in large
quantities and at low cost. For example, multiple flow field boards
can be manufactured at the same time, by using thin laminate boards
which are stacked and then simultaneously routed. Individually
routed boards are then stacked and laminated together.
[0082] PCBs for the present invention are produced in the known
way. Insulating layers may be made of dielectric substrates such as
FR-1, FR-2, FR-3, FR-4, FR-5, FR-6, CEM-1, CEM-2, CEM-3, CEM-4,
CEM-5, polytetrafluoroethylene, and G-10, which are laminated
together with an epoxy resin prepreg. In order to yield conductive
areas, a thin layer of copper is either applied to the whole
insulating substrate and etched away using a mask to retain the
desired conductive pattern, or applied by electroplating.
[0083] Each individual layer of the PCB board is 30 .mu.m-2 mm
thick, preferably 50 .mu.m-1 mm thick, more preferably 0.1 mm-0.8
mm thick, most preferably about 0.4 mm thick. Each PCB board is
thus 90 .mu.m-6 mm thick, preferably 150 .mu.m-3 mm thick, more
preferably 0.3 mm-2.4 cm thick, most preferably about 1.2 cm thick.
The electrode layer is 0.1 mm-1 mm thick, preferably 0.3-0.6 mm
thick, more preferably 0.4 mm thick. Thus the cell pitch is
preferably 1.6 mm thick (1.2+0.4 mm), thus allowing 16 cells per
inch.
[0084] For the first, second and third embodiment of the present
invention, the electrode bands are 1 mm-5 cm in width, preferably 5
mm-15 mm in width, more preferably about 1 cm in width. The size of
the gaps between the electrode bands is dependent on whether they
accommodate cooling channels. The gaps between the electrode bands
with a cooling channel are between 1 mm-1.5 cm wide, preferably
between 2 mm and 1.2 cm wide, more preferably between 5 mm and 1 cm
wide. The gaps between the electrode bands without a cooling
channel are between 0.5 mm-1 cm wide, preferably between 2 mm and 8
mm wide, more preferably between 3 mm and 6 mm wide.
[0085] For the first, second and third embodiment of the present
invention, a catalyst layer is preferably provided on the
electrodes. This layer may be made of suitable catalytic material
for the reactions of interest, as is commonly understood by a
researcher skilled in the art of producing fuel cells. For example,
the catalyst layer may be composed of platinum nanoparticles
deposited on carbon and bound with an proton conducting polymer
(e.g. Nafion.TM.), as described in "PEM Fuel Cell Electrocatalysts
and Catalyst Layers Fundamentals and Applications", Jiujun Zhang
(Ed.), 1st Edition., 2008, XXII, 1137 p. 489 illus.,
Springer-Verlag London, ISBN: 978-1-84800-935-6.
[0086] A gas diffusion layer may be included adjacent to the
catalyst layer. The gas diffusion layer may be fabricated or
deposited in any appropriate manner as will be familiar to the
skilled reader. For example, the gas diffusion layer in a typical
fuel cell is composed of carbon in one of a number of forms mixed
with a number of binders and additives to modify the wetting
characteristics of the layers. Typically the gas diffusion layer
adjacent to the catalyst layer is composed of a microporous layer
of carbon powder bound with PolyTetraFluoroEthylene (PTFE) (this
layer has very small pores). Adjacent to this microporous layer is
a further backing layer, typically composed of carbon
fibers--either woven into a cloth, or bound together in some form
of non-woven material, such as a paper. This layer has pores of a
larger size. The combination of these two layers provides a
gradation in pore size in moving from the gas-channel to the
catalyst layer. Sometimes rather than two discrete layers, the
microporous layer and the porous backing layer interpenetrate.
[0087] Typically the thickness of the gas diffusion layer is around
100-1000 .mu.m. In commonly used modem fuel cells, the choice of
carbon as the major constituent of the gas diffusion layer is
dictated by the further constraint that electrical current must be
conducted from the catalyst layer to the ribs of the bipolar plate.
Most other materials that might be used are either not sufficiently
corrosion resistant (many other metals), or are too expensive
(gold, platinum etc.)
[0088] In the design of the current invention, the gas diffusion
layer may be composed of the same materials i.e. carbon powder
and/or fibers bound together with a suitable binder and treated
with a suitable chemical to modify its hydrophilicity. However,
because transport to electrons in a direction normal to the surface
of the electrode is not required, the gas diffusion layer may
alternatively be composed of non electrically conducting materials
which nonetheless have suitable properties. The exact nature and
electrical conducting properties are dictated by the electrical
conductivity requirements of the surface layer--because current
must be conducted along the surface of the MEA, requiring a high
enough value of the electrical conductivity so that ohmic loss
(surface current.cndot.layer resistance) is small (i.e. <10-20
mV).
[0089] Examples of material out of which the gas diffusion layer
can be composed are porous forms of the following: inorganic oxides
(Al.sub.2O.sub.3, SiO.sub.2); plastics: (ptfe, poly ethylene, poly
sulfones, etc); other inorganic materials: nitrides, carbides,
phosphates, sulphates etc. In some cases, e.g. for thick catalyst
layers, the catalyst layer may provide sufficient electrical
conduction itself--in this case the gas diffusion layer does not
need to be electrically conducting and may even be omitted. In
other cases it may be necessary to have some of the current carried
through the gas diffusion layer, in which case it would need to be
composed (at least partially) from carbon or other electrically
conducting material. In all cases, the gas diffusion layer can be
made to be quite thin, for example, less than 100 .mu.m, more
preferably less than 25 .mu.m. A thinner gas diffusion layer
enhances transport of reactants to and from the catalyst layer.
[0090] Under typical operating conditions, it is necessary to
provide a cooling mechanism to prevent the fuel cell form
overheating. Cooling plates are used in some known fuel cells and
can be incorporated into the fuel cell stacks of the present
invention. However, the thickness of these plates limits the
density of the fuel cell. Cooling of the fuel cell stacks of the
first, second and third embodiment of the invention may be effected
by inclusion of one or more of cooling plates, cooling channels
and/or heat pipes. Further, the PCB boards of the second and third
embodiments may further incorporate a coolant channel.
[0091] In the second and third embodiments of the present
invention, integral cooling of the fuel cell stack is simple due to
PCB technology (holes). Air or water cooling, or heat pipes can be
used. For example, fans for air cooling may be provided, pushing
air into a plenum, which forces air into, for example, vertical
channels running through the fuel cell stack. These cooling
channels are the result of horizontally stacking the boards with
holes in their frames, as described above. Heat pipes may be used
in combination with, or in place of, the cooling channels.
[0092] Heat pipes are described in detail in "An introduction to
heat pipes: modeling, testing, and applications", G. P. Peterson,
Wiley, New York, 1994, ISBN 047130512X. A heat pipe is a sealed
hollow tube made of a material with high thermal conductivity at
both ends, filled with a fraction of a percent by volume of a
working fluid (or coolant), forming a closed-loop capillary
recirculation system. Heat pipes operate when there is a
temperature difference between the two ends of the pipe, by
employing evaporative cooling to transfer thermal energy from one
end of the pipe to the other by the evaporation and condensation of
a working fluid or coolant. Alternatively, flat, planar heat pipes
may be used.
[0093] Each fuel cell board of the first, second and third
embodiment of the present invention is connected via its first
cathode and its last anode to an external circuit. These electrical
connections are fabricated by known means. The switching mechanism,
such as an FET switch, and possibly a voltage measuring apparatus
of any appropriate known type, are incorporated into the fuel cell
board. The voltage measuring apparatus allows quick and simple
monitoring of the voltage output of each fuel cell board when it is
either connected or disconnected from the stack bus-bar. This
voltage measurement is then used to adjust the load pattern of the
fuel cell boards, as described below. The switch is located between
the electrical connection of an electrode and the stack bus bar and
the voltage measuring apparatus is connected to the first cathode
and the last anode of each fuel cell board. Individual control
lines are used to operate each of the switches.
[0094] In operation, the fuel cell is enclosed in a housing and is
sealed from the atmosphere. Reactants are fed into the fuel cell
channels through sealed connections. Seals may, for example, be
made of PDMS. In particular, fuel (e.g. H.sub.2) and oxidant (e.g.
O.sub.2) are fed into appropriate channels of the fuel cell stack,
with fuel being supplied to the anodes and oxidant to the cathodes.
The electrical current thus formed can be taken directly or the
output of the fuel cell board can be modulated utilising the
aforementioned switch. Modulating the output of the fuel cell board
may result in a significantly improved performance when the cell is
operating on reformate or methanol (in a Direct Methanol Fuel Cell)
or other fuels which result in poisoning of the fuel cell over
time.
[0095] A constant power output of the stack may be achieved in a
variety of ways. For example, all fuel cell boards may be loaded at
all times. Alternatively, the fuel cell boards may be divided into
groups and these groups may be "switched on" in turn in a
synchronous manner (i.e. switching occurs at a defined time for all
fuel cell boards). The fuel cell boards may also be switched in an
asynchronous or quasi-asynchronous manner--i.e. each fuel cell
board is connected and disconnected to the load for a defined
period and frequency individually specified for each fuel cell
board. By switching the fuel cell boards so that they are only
connected to the load for a proportion of the time according to a
duty cycle, the output power of the stack can be continuously
modified. For example, if over a given sample period of time only
50% of the fuel cell boards are connected to the load, then the
output power of the fuel cell stack will be similarly reduced. The
manner in which this 50% is achieved may be brought about in a
multitude of ways--for example half of the fuel cell boards may be
disconnected from the load and half connected for the entire
period; alternatively all fuel cell boards may be connected to the
load, but each connected for only half of the sample period.
Alternatively again, half of the fuel cell boards may be connected
to the load for one quarter of the sample period, and the other
half for three quarters of the sample period etc. The choice of the
specific scheme or duty cycle used may depend on the performance of
individual fuel cell boards, the need to avoid localized heating or
`hot spots`, the need to avoid flooding of cathode sites with
product water, the need to prevent dehydration of the membrane, or
the need to counteract poisoning of the electrodes. It will be
noted that the duty cycle may be predetermined or may be controlled
in real time based on monitored performance of the fuel cell, for
example in a closed feedback loop with the voltage measuring
apparatus described above. Part-time use of fuel cell boards may
also improve efficiency as one can achieve optimum load conditions
and power conversion for each individual fuel cell board rather
than for the fuel cell stack which is a limitation of current
designs. By including additional switching and filtering components
on the fuel cell boards, a smoothly varying output, for example a
sinusoidal wave, may be obtained, in addition to simple
"changeovers" or steps from one potential to another.
[0096] In the case of failure of a fuel cell board, the failed fuel
cell board may be switched out altogether. This may also be
performed in advance of the failure on the basis of diagnostics or
predictive models.
[0097] Each fuel cell board can carry its own electronic circuitry,
with each module feeding the power into an electrical bus. That is,
each board contains its own power electronics and controller. The
latter monitors the performance of the fuel cell electrodes, local
humidity and temperature. It can also control the shape memory
alloy (SMA) valves to throttle the flow of reactant to the
electrodes on that board, as described in more detail below. Thus
the power electronics can be put directly onto each horizontal
board. In this manner, the status of each electrode can be
monitored for degradation. This information gives feedback to
enable the electronics to be modulated so a particular board of
electrodes can be used less, thereby slowing the degradation
process, or by completely shutting down a board of electrodes. This
control enables the protection of underperforming boards, thereby
increasing the longevity of the entire fuel cell stack.
[0098] A further benefit of having electronics directly on each
board is that they can each be configured to convey power in a
different way. One example is to use the fuel cell to work
multiphase electric motors and provide current to different coils
at different times, thereby increasing the efficiency, increasing
control of performance and increasing control at low torque. Each
of the horizontal boards comprising the series of electrodes would
drive one phase.
[0099] If, for example, an electrode is underperforming or has
become faulty, it is not only possible to switch out the affected
board using the individual electronics, but it is also possible to
stop the fuel or oxidant supply to specific electrodes. In such a
case, it is preferable to particularly stop the flow of hydrogen
through cells that have been shut off. Stopping the gas flow is
difficult, due to the low profile gas inlet. However, it has been
found that the gas flow through the inlet may be blocked by using a
Shape Memory Alloy (SMA) as a low profile valve. A Shape Memory
Alloy is a metal alloy that deforms in response to the application
of temperature (e.g. heat) or an electromagnetic field (e.g.
application of current), and returns to its original shape after
the application of e.g. heat or current ceases. This is described
in reference to FIG. 14, which shows three views of the lower half
of a reactant channel (the top half is not shown for clarity). FIG.
14(a) shows the reactant-inlet valve open. The arrow indicates
where reactant flow occurs. The SMA may be either a foil which
distorts on heating to close off the channel, as shown in FIG.
14(b), or it may be in the form of a wire attached to a flap, where
the wire changes shape upon heating, thereby drawing an object, for
example a flap, which closes the channel, as shown in FIG. 14(c).
SMAs that may be used in the present invention include
nickel-titanium, copper-aluminium-nickel, copper-zinc-aluminium,
and iron-manganese-silicon alloys. Preferably the SMA used is a
nickel-titanium alloy.
[0100] The fuel cell of the present invention comprises at least
two fuel cell boards, each fuel cell board may comprise either
[0101] a) one membrane of substantially gas impervious electrolyte
material supporting at least two anodes on a first face of the
membrane and two cathodes on a second face of the membrane; or
[0102] b) at least two membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane.
[0103] Preferably, the fuel cell comprises at least three fuel cell
boards. More preferably the fuel cell comprises between four and
fifty fuel cell boards, for example, eight, ten, sixteen or twenty
fuel cell boards.
[0104] Preferably each fuel cell board comprises either
[0105] a) one membrane of substantially gas impervious electrolyte
material supporting at least three anodes on a first face of the
membrane and three cathodes on a second face of the membrane;
or
[0106] b) at least three membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane.
[0107] More preferably each fuel cell board comprises either
[0108] a) one membrane of substantially gas impervious electrolyte
material supporting at least three anodes on a first face of the
membrane and three cathodes on a second face of the membrane; for
example, at least four anodes on a first face of the membrane and
four cathodes on a second face of the membrane, for example, at
least ten anodes on a first face of the membrane and ten cathodes
on a second face of the membrane, for example, at least sixteen
anodes on a first face of the membrane and sixteen cathodes on a
second face of the membrane, for example, at least twenty anodes on
a first face of the membrane and twenty cathodes on a second face
of the membrane; or
[0109] b) at least three membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane; for
example, at least four membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane, for
example, at least ten membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane, for
example, at least sixteen membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane, for
example, at least twenty membranes of substantially gas impervious
electrolyte material, each supporting one anode on a first face of
the membrane and one cathode on a second face of the membrane.
[0110] When the current is conducted by means of a current
collection and distribution layer, the surface area of an electrode
on a membrane, or on the at least two membranes, should
substantially correspond to the dimensions of a reactant
distribution channel on a reactant distribution layer, and to the
size of a panel of electrical distribution tracks on the current
collection and distribution layer, or to the size of a panel on the
current collection layer and the current distribution layer,
depending on the specific embodiment. Preferably, the surface area
of an electrode will be such that as much of its surface area as
possible is in contact with a reactant distribution channel.
[0111] The flexible characteristics of the present invention make
it suitable for use in applications which require a high degree of
resilience and fault tolerance, along with a high power density.
For instance it may be used within a stationary power supply system
which is required to work for long periods without manual
intervention, or for applications which require operation under
degraded conditions due to partial damage.
[0112] Because of the ability to monitor the fuel cell for
degradation and switch out a whole plate containing a faulty
electrode, electrodes can be built to a lower tolerance than
possible for known fuel cells. This in turn lowers the cost of
manufacturing the fuel cells of the present invention compared to
known fuel cells.
[0113] Thus all embodiments of the present invention have the
advantages of easily separating the fuel and air flows, resulting
in a simpler, cheaper system, and the ability to monitor the
operating status (e.g. degradation) of each element of the fuel
cell stack and to independently control elements of the fuel cell
stack.
[0114] It will be appreciated that aspects of the invention can be
interchanged or juxtaposed as appropriate. The fuel used is not
restricted to hydrogen, but may be any suitable fuel. For example,
the new geometry fuel cell stack described herein is also
applicable to methanol used in Direct Methanol fuel cells. The
electrodes deposited on the membrane need not be straight bands,
but may be any appropriate shape e.g. they may be curved, or their
width may vary with position. Similarly, any of the cooling or gas
diffusion channels need not be straight, and may for example be of
a serpentine design. Also, the first embodiment of the invention is
not limited to having only one cooling or gas diffusion channel in
each spacer.
[0115] The membrane material is not limited to Nafion.TM., and any
suitable ion-exchange electrolyte, such as a fluoropolymer sulfonic
acid membrane may be employed.
[0116] In the first embodiment, the spacer between the stacked
sheets may be plastic, but any suitable electrically insulating
material may be used. Channels in the surface on either face of the
spacer need not line up or be of equal dimensions.
[0117] The switch mechanism for controlling the electronics of the
fuel cell board is not restricted to FET-type switches; any
suitable switch, such as a junction transistor, may be used.
[0118] Whereas gold or platinum metal complex containing solutions
are preferred for the fabrication of the through-membrane
electronic connectors, any suitable precious-metal or suitable
compound may be used. The catalyst is preferably Pt or a Pt-alloy
and is optionally supported onto carbon, but is not limited to
these examples.
[0119] A further aspect of the present invention relates to a
corrosion resistant coating that may be applied in the first,
second and third embodiments of the invention. This coating is used
to passivate the copper surface of the PCB and stop the copper
layer from corroding. This is important because the conditions in a
fuel cell are very acidic and oxidizing, under which conditions
copper corrodes. If the copper is oxidized from Cu.sup.0 to
Cu.sup.2+, this leads to the disintegration of the copper surface
of the PCB, and the consequently, the fuel cell.
[0120] Accordingly, a coating is needed that prevents H.sub.2O from
reaching the copper surface of the PCB, but still allows good
electrical conduction. The copper surface of the PCB may for
example be the current collection and/or distribution layer or
`plate`. The coating must be stable and must not undergo any
electrochemical or chemical degradation within the fuel cell
environment. At the same time the material must be hard-wearing, as
it must not become detached from the copper surface. It is
important that the coating does not have any cracks or holes in it
that would allow water ingress.
[0121] Three different approaches to achieve these effects are
detailed below. A combination of two or all three approaches is
also considered herein, in order to achieve the best
performance.
[0122] In a first approach, the copper layer is coated with an ink
or paint composed of electrically conductive particles. These
particles must be passive and must not undergo significant
oxidative or acidic corrosion.
[0123] Examples of suitable particles include carbon (in the form
of graphite particles) or electrically conductive titanium
suboxide, for example, Ti.sub.4O.sub.7, obtainable commercially as
Ebonex.RTM.. Other materials may also be considered, for instance
Ti.sub.0.9Nb.sub.0.1O.sub.2, which may be more resistant to
corrosion, doped tin oxide (doped with antimony, fluorine, or
indium to improve conductivity), or an ink composed of electrically
conductive polymer particles. Examples of suitable polymers include
polyaniline, polypyrrole, polythiophene and
poly(3,4-ethylenedioxythiophene) (PEDOT).
[0124] Carbon inks for coating copper tracks for PCBs are known and
commercially available.
[0125] An ink is obtained by mixing the conductive compound, for
example graphite, or the titanium, or tin compounds described
above, with a solvent and a polymeric binder to form a slurry, or
ink. The concentration of particles used in the ink is typically a
volume fraction of greater than 20 vol %, for example, between 20
vol % and 70 vol %, for example, between 20 vol % and 60 vol %.
This volume fraction is selected in order to enable good
particle-particle contact, for conductivity, and also to contain
sufficient binder to create a stable coating layer. The particles
make up the conductive phase of the coating layer when the ink is
coated onto a substrate. The polymeric binder may be any suitable
polymer, for example, polyethylene, or an epoxide polymer, such as
a thermosetting epoxy polymer.
[0126] Inks containing conductive polymers are used in the
electronics industry for displays, although they have not
previously been used in fuel cell current collection plates. The
term `current collection plate` is used to encompass a range of
components in the fuel cell which may also be called bipolar plate,
monopolar or unipolar current collector, gas diffusion layer, gas
transport layer etc.
[0127] The other materials exemplified above are not known in the
field of inks or passivating coating layers for PCBs and fuel cell
current collection and/or distribution layers or plates.
[0128] A second approach involves direct electrodeposition of an
electrically conducting polymer onto the copper surface of the PCB.
There are many possible polymers which can be considered including
polyaniline, polypyrrole, polythiophene and
poly(3,4-ethylenedioxythiophene) (PEDOT).
[0129] Stabilisation of the polymer to electrochemical oxidation
and reduction and improvement in electrical conductivity may be
achieved by incorporating a polymeric anionically charged polymer
such as poly(styrenesulfonate) or Nafion.RTM..
[0130] The use of such electrically conducting polymers has been
considered within fuel cells before, predominantly within the
catalyst layer. The use of these materials as corrosion protection
layers in general has been disclosed in the literature, but there
has been no disclosure regarding the use of these materials as
corrosion protection layers in fuel cells.
[0131] The Applicant has examined the use of fluorinated polymer
derivatives as an electrically conductive corrosion resistant layer
and discovered that fluorinating the polymer by adding a pendant
pefluoro alkyl chain increases the surface energy of the resulting
coating and makes it water repellent. This reduces the likelihood
of corrosion. Higher surface energies also lead to better wear
resistance due to decreased friction.
[0132] In a preferred embodiment, the conducting polymer is chosen
such that it has a high fluorine content, for example, wherein the
polymer comprises a perfluorinated alkyl chain.
[0133] PEDOT has previously been considered as a passivating layer
because of its stability and high electrical conductivity. The
Applicant has examined fluorinated PEDOT as a passivating layer
directly electrodeposited onto a fuel cell current collection
and/or distribution layer or plate.
[0134] A method of producing the EDOT monomer and its
polymerisation are described in Benedetto et al., Electrochimica
Acta, Vol 53, 11, 20 Apr. 2008, 3779-3788. Examples of EDOT
monomers are shown below:
##STR00001##
[0135] A third approach involves the direct attachment of an
organic group to the copper surface to reduce water adsorption and
increase the surface energy of the surface. A number of different
approaches are possible including the reaction of a silyl-chloride
with a hydrophobic backbone, a silyl alkoxide with a hydrophobic
backbone, or a perfluorinated alkoxy silane, with the surface.
Example materials are shown in Table 1 below. An example of this
approach, used for organic electronics purposes, is given in Lee,
Relationship between the chemical nature of silanes and device
performance of polymer light emitting diodes, Thin Solid Films, Vol
515, 4, 5 Dec. 2006, 2705-2708.
TABLE-US-00001 TABLE 1 ##STR00002## ##STR00003## ##STR00004##
##STR00005## ##STR00006##
[0136] This approach has not been previously disclosed for use with
fuel cell current collection and/or distribution layers or
plates.
[0137] For example, one possibility in this approach is to use the
formation of a diazo compound from a functionalised aniline
followed by reaction of the diazo compound with the substrate or
electrochemical reduction to lead to direct electrochemical
deposition of the material.
[0138] A Scheme showing the production of diazo derivative followed
by derivatisation of a surface either by direct reduction due to
surface hydrogen (1) or electrochemical reduction (2), or direct
electrochemical deposition (3) is outlined below:
##STR00007##
[0139] Other approaches for producing the diazo salt are possible,
and there are a range of possible reaction conditions.
[0140] A chemical process has been disclosed for fuel cell catalyst
layers in which the penta fluoro aryl group is bonded to a carbon
surface by Xu et al., Electrochem. Solid-State Lett., Vol 8, 10,
pA492-A494, (2005). However, there is no disclosure on the
application of this process to fuel cell current collection and/or
distribution layers or plates.
[0141] Examples of perfluoro aniline derivatives for use with the
diazo approach are shown in Table 2 below. Other examples include
aniline with the same pendant chains as those shown for the EDOT
monomers (1)-(3), above.
TABLE-US-00002 TABLE 2 ##STR00008## (2) ##STR00009## (3)
##STR00010## (4) ##STR00011## (5) ##STR00012## ##STR00013## (6)
##STR00014## (7) ##STR00015## (8)
[0142] In this approach, it is also possible to use a chemical
process to coat the copper surface with perfluoro derivatised aryl
groups on silicon. Whilst this process has been described in
Stewart et al., J. Am. Chem. Soc. 126 (1): 370, the application of
this approach to fuel cell current collector and/or distribution
layers or plates has not been described.
[0143] An electrochemical process described in Lyskawa and
Belanger, Chem. Mater., 2006, 18 (20), 4755-4763, has been used to
derivatise both metal and carbon, but has not been applied to fuel
cell current collector and/or distribution layers or plates.
[0144] There is no disclosure of the deposition of fluorinated
materials using the electrochemical approach, as is described by
the present invention.
[0145] The electrochemical approach devised by the Applicant to
deposit the perfluoro group on a surface is different from normal
approaches, because of the instability of the intermediate azo
compound.
[0146] The following are examples of use that are intended to
illustrate the invention, and not intended to limit the scope of
the claims: [0147] (a) A conductive carbon ink is screen printed
onto the copper electrode surface and dried in an oven at
120.degree. C. Examples of suitable inks are [0148] 1. Carbon
conductive ink XZ302-1 manufactured by Sun Chemical Corporation.
[0149] 2. Carbon conductive ink SD 2843HAL manufactured by Peters
Lackwerke GmbH. [0150] (b) Synthesis of compound (I) of the EDOT
monomers was undertaken as described in Benedetto et al. A 10 mM
solution of (1) in acetonitrile containing 0.1 M Bu.sub.4N PF.sub.6
was electropolymerised onto the copper surface by sweeping the
electrochemical potential to a value of 1.1V. [0151] (c)
(Pentafluorophenyl)triethoxysilane was dissolved in 0.5 mol
dm.sup.-3HCl to a concentration of 10 mmol dm.sup.-3. NaNO.sub.3
was added to this solution in an ice bath to a concentration of 20
mmol dm.sup.-3. A copper surface was placed in this solution and
held at a potential of -0.2 V vs. a saturated calomel electrode for
five minutes. [0152] (d) A copper surface was cleaned and coated
with tridecafluoro-1,1,2,2-tetrahydrooctyl-1-siloxane. The silane
solution was prepared by dissolving
1H,1H,2H,2H-Perfluorooctyltriethoxysilane in 100 ml of
butanol/H.sub.2O (95/5) solution containing a few drops of acetic
acid at a concentration of 0.2 wt. %. The solution was stirred for
60 minutes to hydrolyze silane materials and then the cleaned
copper surface was dip coated with the hydrolysed silane solution.
After dip coating, the silane treated copper substrates were washed
with toluene several times to get a layer of silane. The silane
coated substrate was dried at 160.degree. C. for 5 minutes to
remove residual solvents after toluene washing.
[0153] The following examples relate to combinations of coating
techniques, to achieve more than one corrosion resistant coating
layer: [0154] (e) The method of example (a) is followed by the
method of example (b), in which electropolymerisation occurs on the
surface produced in (a), rather than on the "copper surface"
referred to in example (b). [0155] (f) The method of example (a) is
followed by the method of example (c), in which
(pentafluorophenyl)triethoxysilane is coated onto the surface
produced in (a), rather than on the "copper surface" referred to in
example (c). [0156] (g) The method of example (a) is followed by
the method of example (d), in which
tridecafluoro-1,1,2,2-tetrahydrooctyl-1-siloxane is coated onto the
surface produced in (a), rather than on the "copper surface"
referred to in example (d). [0157] (h) The method of example (b) is
followed by the method of example (c), in which
(pentafluorophenyl)triethoxysilane is coated onto the surface
produced in (b), rather than on the "copper surface" referred to in
example (c).
* * * * *