U.S. patent number 10,714,762 [Application Number 15/522,030] was granted by the patent office on 2020-07-14 for electrode catalyst layer for fuel cell, and fuel cell membrane electrode assembly and fuel cell using the catalyst layer.
This patent grant is currently assigned to NISSAN MOTOR CO., LTD.. The grantee listed for this patent is DAIMLER AG, FORD MOTOR COMPANY, NISSAN MOTOR CO., LTD.. Invention is credited to Tetsuya Mashio, Shinichi Takahashi.
United States Patent |
10,714,762 |
Takahashi , et al. |
July 14, 2020 |
Electrode catalyst layer for fuel cell, and fuel cell membrane
electrode assembly and fuel cell using the catalyst layer
Abstract
Provided is a catalyst layer for fuel cell which has a high
catalytic activity and enables maintaining the high catalytic
activity. Disclosed is an electrode catalyst layer for fuel cell
including a catalyst containing a catalyst carrier having carbon as
a main component and a catalytic metal supported on the catalyst
carrier, and a polymer electrolyte having a sulfonic acid group
(--SO.sub.3H) as an ion exchange group, in which the catalyst has
the R' (D'/G intensity ratio) of 0.6 or less, which is the ratio of
D' band peak intensity (D' intensity) measured in the vicinity of
1620 cm.sup.-1 relative to G band peak intensity (G intensity)
measured in the vicinity of 1580 cm.sup.-1 by Raman spectroscopy,
and has BET specific surface area of 900 m.sup.2/g catalyst carrier
or more, and mole number of a sulfonic acid group in the polymer
electrolyte relative to weight of the catalyst carrier is 0.7
mmol/g or more and 1.0 mmol/g or less.
Inventors: |
Takahashi; Shinichi (Kanagawa,
JP), Mashio; Tetsuya (Kanagawa, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO., LTD.
DAIMLER AG
FORD MOTOR COMPANY |
Yokohama-shi, Kanagawa
Stuttgart
Dearborn |
N/A
N/A
MI |
JP
DE
US |
|
|
Assignee: |
NISSAN MOTOR CO., LTD.
(Yokohama-shi, JP)
|
Family
ID: |
55857221 |
Appl.
No.: |
15/522,030 |
Filed: |
October 8, 2015 |
PCT
Filed: |
October 08, 2015 |
PCT No.: |
PCT/JP2015/078615 |
371(c)(1),(2),(4) Date: |
April 26, 2017 |
PCT
Pub. No.: |
WO2016/067879 |
PCT
Pub. Date: |
May 06, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170338496 A1 |
Nov 23, 2017 |
|
Foreign Application Priority Data
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|
|
|
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Oct 29, 2014 [JP] |
|
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2014-220577 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/8652 (20130101); H01M 4/92 (20130101); H01M
8/1041 (20130101); H01M 8/1004 (20130101); H01M
4/8825 (20130101); H01M 4/926 (20130101); H01M
8/1032 (20130101); H01M 4/8605 (20130101); H01M
2008/1095 (20130101); H01M 2300/0082 (20130101) |
Current International
Class: |
H01M
4/92 (20060101); H01M 8/1032 (20160101); H01M
8/1004 (20160101); H01M 8/1018 (20160101); H01M
4/88 (20060101); H01M 8/1041 (20160101); H01M
4/86 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2005-026174 |
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Jan 2005 |
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JP |
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2008-77974 |
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Apr 2008 |
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JP |
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2009-123474 |
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Jun 2009 |
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JP |
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2009-199935 |
|
Sep 2009 |
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JP |
|
2009-242233 |
|
Oct 2009 |
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JP |
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2010-129397 |
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Jun 2010 |
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JP |
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2011-159634 |
|
Aug 2011 |
|
JP |
|
WO-2013027627 |
|
Feb 2013 |
|
WO |
|
WO 2015/045852 |
|
Apr 2015 |
|
WO |
|
Primary Examiner: Slifka; Sarah A.
Assistant Examiner: Thomas; Brent C
Attorney, Agent or Firm: Foley & Lardner LLP
Claims
The invention claimed is:
1. A membrane electrode assembly for fuel cell comprising a solid
polymer electrolyte membrane, a cathode catalyst layer arranged on
one side of the electrolyte membrane, and an anode catalyst layer
arranged on the other side of the electrolyte membrane, wherein at
least one of the cathode catalyst layer or the anode catalyst layer
is a catalyst layer comprising a catalyst containing a catalyst
carrier having carbon as a main component and a catalytic metal
supported on the catalyst carrier, and a polymer electrolyte having
a sulfonic acid group (--SO.sub.3H) as an ion exchange group,
wherein the catalyst has the R' (D'/G intensity ratio) of 0.35 or
more and of 0.6 or less, which is the ratio of D' band peak
intensity (D' intensity) measured in the vicinity of 1620 cm.sup.-1
relative to G band peak intensity (G intensity) measured in the
vicinity of 1580 cm.sup.-1 by Raman spectroscopy, and has BET
specific surface area of 900 m.sup.2/g catalyst carrier or more,
and mole number of a sulfonic acid group in the polymer electrolyte
relative to weight of the catalyst carrier is 0.7 mmol/g or more
and 1.0 mmol/g or less, wherein the weight ratio of the polymer
electrolyte relative to the catalyst carrier is 0.5 or more and 1.0
or less, wherein a portion of the catalyst is coated with the
polymer electrolyte, wherein the catalyst carrier is obtained by
heat treatment in which the heat treatment temperature is over
1300.degree. C. and 1880.degree. C. or less, and the atmosphere is
an inert atmosphere.
2. The membrane electrode assembly for fuel cell according to claim
1, wherein the equivalent weight (EW) of the polymer electrolyte is
500 g/eq. or more and 1200 g/eq. or less.
3. The membrane electrode assembly for fuel cell according to claim
1, wherein the catalytic metal is platinum or comprises platinum
and a metal component other than platinum.
4. The membrane electrode assembly for fuel cell according to claim
3, wherein the platinum content per unit area of the catalyst layer
is 0.2 mg/cm' or less.
5. The membrane electrode assembly for fuel cell according to claim
3, wherein the platinum content relative to the catalyst is 20% by
weight or more and 60% by weight or less.
6. A fuel cell comprising the membrane electrode assembly for fuel
cell set forth in claim 1.
7. The membrane electrode assembly for fuel cell according to claim
1, wherein the catalyst has pores with a radius of 1 nm or more and
the catalytic metal is supported inside the pores with a radius of
1 nm or more.
8. The membrane electrode assembly for fuel cell according to claim
1, wherein the catalyst comprises mesopores with a radius of 1 nm
or more, and a portion of openings of the mesopores is not coated
with the polymer electrolyte.
9. The membrane electrode assembly for fuel cell according to claim
1, wherein the mole number of the sulfonic acid group in the
polymer electrolyte relative to the weight of the catalyst carrier
is given by: [mmol/g]=(AB).times.1000, wherein A=the weight ratio
of the polymer electrolyte relative to the catalyst carrier (I/C
ratio), and B=EW, wherein equivalent weight=weight of the polymer
electrolyte per mole of the sulfonic acid group in the polymer
electrolyte [g]/[mol] ([g]/[eq]).
10. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst comprises mesopores with a radius of
1 nm or more, the catalytic metal is supported inside the mesopores
with a radius of 1 nm or more, and the catalytic metal supported
inside the mesopores with a radius of 1 nm or more is not contacted
with the polymer electrolyte.
11. The membrane electrode assembly for fuel cell according to
claim 1, wherein the mole number of the sulfonic acid group in the
polymer electrolyte relative to the weight of the catalyst carrier
is 0.8 to 0.9 mmol/g.
12. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst has an R (D/G intensity ratio) of 1.7
or more, which is the ratio of D band peak intensity (D intensity)
measured in the vicinity of 1360 cm.sup.-1 relative to G band peak
intensity (G intensity) measured in the vicinity of 1580 cm.sup.-1
by Raman spectroscopy.
13. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst carrier has BET specific surface area
of 1200 m.sup.2/g or more.
14. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst comprises micropores with a radius of
less than 1 nm and mesopores with a radius of 1 nm or more, the
catalytic metal supported inside the mesopores more than inside the
micropores.
15. The membrane electrode assembly for fuel cell according to
claim 14, wherein a pore volume of mesopores with a radius of 1 nm
or more is greater than a pore volume of micropores with a radius
of less than 1 nm.
16. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst carrier is obtained by a heat
treatment of a carbon material, the carbon material comprising
pores with a radius of less than 1 nm and pores with a radius of 1
nm or more, wherein a pore volume of the pores with a radius of
less than 1 nm is 0.3 cc/g carrier or more.
17. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst carrier is obtained by a heat
treatment of a carbon material comprising pores with a radius of
less than 1 nm and pores with a radius of 1 nm or more, wherein a
mode radius of pore distribution of the pores with a radius of less
than 1 nm is 0.3 nm or more and less than 1 nm.
18. The membrane electrode assembly for fuel cell according to
claim 1, wherein the catalyst carrier is obtained by a heat
treatment of a carbon material comprising pores with a radius of 1
nm or more, wherein a mode radius of pore distribution of the pores
with a radius of 1 nm or more is 1 nm or more and less than 5 nm,
wherein a pore volume of the pores with a radius of 1 nm or more
and less than 5 nm is 0.4 cc/g carrier or more.
Description
TECHNICAL FIELD
The present invention relates to an electrode catalyst layer for
fuel cell, and a membrane electrode assembly for fuel cell and a
fuel cell using the catalyst layer.
BACKGROUND ART
A polymer electrolyte fuel cell (PEFC) using a proton-conductive
solid polymer membrane operates at lower temperature compared to
other types of a fuel cell such as a solid oxide fuel cell or a
molten carbonate fuel cell, for example. For such reasons, the
polymer electrolyte fuel cell is expected to be used as a
stationary power supply or a power source for a moving object such
as an automobile, and actual application thereof has been already
started.
For the polymer electrolyte fuel cell, an expensive metal catalyst
represented by Pt (platinum) or Pt alloy is generally used.
Furthermore, as a carrier for supporting the metal catalyst,
graphitized carbon is used from the viewpoint of water repellency
and corrosion resistance. It is described in JP 2005-26174 A to use
a carrier in which average lattice plant spacing of a [002] plane,
that is, d002, is 0.338 to 0.355 nm, specific surface area is 80 to
250 m.sup.2/g, and volume density is 0.30 to 0.45 g/ml. In JP
2005-26174 A, it is described that the durability of a cell is
improved by using the graphitized carbon.
SUMMARY OF INVENTION
However, although the catalyst using the carrier described in JP
2005-26174 A has high durability, it has a problem that, as an
electrolyte is in contact with catalytic metal particles and a
transport path of the reaction gas (in particular, O.sub.2) to a
catalytic metal is blocked, the catalytic activity is lowered.
One object of the present invention is to provide an electrode
catalyst layer which has high durability and an excellent gas
transportability.
Another object of the present invention is to provide an electrode
catalyst layer which has an excellent catalytic activity.
Still another object of the present invention is to provide a
membrane electrode assembly and a fuel cell which have an excellent
power generation performance.
MEANS FOR SOLVING PROBLEMS
The inventors of the present invention conducted intensive studies
to solve the problems mentioned above. As a result, it was found
that the problems can be solved when a catalyst with specific
surface area and specific D'/G intensity ratio is used and an
electrode catalyst layer in which mole number of sulfonic acid
group in a polymer electrolyte relative to weight of a catalyst
carrier is within a specific range is used, and the present
invention is completed accordingly.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic cross-sectional view illustrating the basic
configuration of a polymer electrolyte fuel cell according to an
embodiment of the present invention. In FIG. 1, 1 denotes a polymer
electrolyte fuel cell (PEFC), 2 denotes a solid polymer electrolyte
membrane, 3 denotes a catalyst layer, 3a denotes an anode catalyst
layer, 3c denotes cathode catalyst layer, 4a denotes an anode gas
diffusion layer, 4c denotes a cathode gas diffusion layer, 5
denotes a separator, 5a denotes an anode separator, 5c denotes a
cathode separator, 6a denotes an anode gas passage, 6c denotes a
cathode gas passage, 7 denotes a refrigerant passage, and 10
denotes a membrane electrode assembly (MEA).
FIG. 2 is a schematic explanatory cross-sectional view illustrating
the shape and structure of catalysts (a) and (c) according to an
embodiment of the present invention. In FIG. 2, 20 denotes a
catalyst, 22 denotes a catalytic metal, 23 denotes a carrier
(catalyst carrier), 24 denotes a mesopore, and 25 denotes a
micropore.
FIG. 3 is a schematic explanatory cross-sectional view illustrating
the shape and structure of a catalyst (b) according to an
embodiment of the present invention. In FIG. 3, 20' denotes a
catalyst, 22' denotes a catalytic metal, 23' denotes carrier
(catalyst carrier) and 24' denotes a mesopore.
FIG. 4 is a graph illustrating the result of evaluation of platinum
coating on carriers B and C which was prepared in Reference
Examples 2 and 3 and carrier F which was prepared in Reference
Example 6 of Experiment 1.
DESCRIPTION OF EMBODIMENTS
One embodiment of the present invention is an electrode catalyst
layer for a fuel cell including a catalyst containing catalyst
carrier having carbon as a main component (in the present
specification, it is also simply referred to as a "catalyst
carrier" or a "carrier") and a catalytic metal supported on the
catalyst carrier, and a polymer electrolyte having a sulfonic acid
group (--SO.sub.3H) as an ion exchange group (it is also simply
referred to as an "electrolyte" in the specification), in which the
catalyst has the R' (D'/G intensity ratio) of 0.6 or less, which is
the ratio of D' band peak intensity (DI intensity) measured in the
vicinity of 1620 cm.sup.-1 relative to G band peak intensity (G
intensity) measured in the vicinity of 1580 cm.sup.-1 by Raman
spectroscopy, and has BET specific surface area of 900 m.sup.2/g
catalyst carrier or more, and mole number of a sulfonic acid group
in the polymer electrolyte relative to weight of the catalyst
carrier is 0.7 mmol/g or more and 1.0 mmol/g or less (in the
present specification, the fuel cell electrode catalyst layer is
also referred to as an "electrode catalyst layer" or a "catalyst
layer").
Namely, the catalyst contained in a catalyst layer for a fuel cell
of this embodiment satisfies the following constitutions (I) and
(II): (I) BET specific surface area is at least 900 m.sup.2/g
catalyst carrier; and (II) R' (D'/G intensity ratio), which is the
ratio of D' band peak intensity (D' intensity) measured in the
vicinity of 1620 cm.sup.-1 relative to G band peak intensity (G
intensity) measured in the vicinity of 1580 cm.sup.-1 by Raman
spectroscopy, is 0.6 or less.
Meanwhile, as described herein, the G band measured in the vicinity
of 1580 cm.sup.-1 by Raman spectroscopy is also simply referred to
as "G band." As described herein, the D' band measured in the
vicinity of 1620 cm.sup.-1 by Raman spectroscopy is also simply
referred to as "D' band." Furthermore, each peak intensity of the G
band and D' band is also referred to as "G intensity" and "D'
intensity", respectively. Furthermore, the ratio of D' intensity
relative to G intensity is also simply referred to as "R' value" or
"D'/G intensity ratio."
Herein, the G band is a peak derived from graphite which is
measured in the vicinity of 1580 cm.sup.-1 (vibration inside
hexagonal lattice of a carbon atom) by Raman scattering analysis.
Furthermore, D' band is observed in the vicinity of 1520 cm.sup.-1
as a shoulder of G band by Raman scattering analysis. The D' band
is derived from a disorder or a defect of a graphite structure, and
it is present when the crystal size of graphite is small or many
edges are present on a graphene sheet. Unlike the center part of a
graphene molecule (6-membered ring), the electron state at edge
(end part) of a graphene molecule easily becomes a start point of
carbon corrosion. In other words, small R' value means that the
edge amount is small in carbon (graphene), which is a start point
of electrochemical corrosion as present in a graphite structure.
Accordingly, the durability can be improved by the above (II) so
that a decrease in catalytic activity can be effectively suppressed
and prevented.
Meanwhile, G band, D' band, and D band which is described below,
and their peak intensity are well known in the related field. For
example, reference can be made to R. Vidano and D. B Fischbach, J.
Am. Ceram. Soc. 61 (1978) 13-17 or G. Katagiri, H. Ishida and A.
Ishitani, Carbon 26 (1988) 565-571.
The carrier described in JP 2005-26174 A is obtained by
graphitization of carbon particles with heat treatment at 2000 to
3000.degree. C. (paragraph [0016]). The carrier described in JP
2005-26174 A can have a carrier with improved durability according
to a graphitization treatment. However, as the specific surface
area of the carrier is as small as 250 m.sup.2/g or less, it is
impossible to have electric double layer capacity at sufficient
level when a catalytic metal is supported thereon.
Meanwhile, the catalyst used in this embodiment satisfies the above
(I). With the above (I), carbon powder has sufficient specific
surface area, and thus it has high electric double layer capacity.
As such, according to this catalyst, the dispersibility of the
catalyst is improved so that an area for electrochemical reaction
can be increased. In other words, the power generation performance
can be improved.
Meanwhile, when R' value is lowered by graphitization of carbon for
the purpose of improving durability, the catalyst carrier turns out
to have a hydrophobic property. As such, if a polymer electrolyte
having a hydrophobic structure in a main chain, for example,
fluorine-based polymer electrolyte or the like, is used, the
polymer electrolyte can easily adsorb onto a carrier on which the
catalytic metal is supported. Since the electrolyte can more easily
adsorb onto a surface of a catalytic metal compared to gas such as
oxygen, when such carrier is used, a surface of the catalyst or an
opening (entrance) of a pore is coated at high ratio by an
electrolyte. As a result, the gas transportability within a
catalyst layer is lowered, and thus a decrease in catalytic
activity and a decrease in power generation performance are
yielded. In order to obtain a sufficient power generation
performance, an expensive metal such as platinum needs to be used
in a large amount, and it leads to high production cost of a fuel
cell.
Based on the above findings, the inventors of the present invention
realized that, when a catalyst satisfying the above constitutions
(I) and (II) is used, the gas transportability can be improved as
the catalyst layer satisfies the following constitution (III);
(III) Mole number of a sulfonic acid group in the polymer
electrolyte relative to weight of the catalyst carrier is 0.7
mmol/g or more and 1.0 mmol/g or less.
Since a sulfonic acid group has a strong interaction with a
catalytic metal, the sulfonic acid group adsorbed onto the
catalytic metal also increases as the amount of sulfonic acid group
increases. As such, the polymer electrolyte main chain can also
easily reach the catalyst. In addition, as the adsorption of a main
chain with a hydrophobic structure in a polymer electrolyte onto a
carrier with a hydrophobic property increases, the catalyst metal
is coated with an electrolyte at high rate. Meanwhile, it is
believed that as the mole number of a sulfonic acid group in the
polymer electrolyte relative to weight of the catalyst carrier is
1.0 mmol/g or less, a contact between the catalytic metal and
electrolyte is suppressed, and thus part of the reaction gas (in
particular, O.sub.2) can be directly supplied without mediated by
an electrolyte to improve the gas transportability. Inventors of
the present invention found that, even when the catalytic metal is
not in contact with an electrolyte, the catalyst can be effectively
utilized according to forming of a three-phase interface with
water. For such reasons, as part of the catalyst is coated with an
electrolyte (only part of the electrolyte is in contact with a
metal catalyst), sites not requiring pass-through of an electrolyte
increase, and thus the gas transportability can be improved.
Accordingly, the reaction gas (in particular, O.sub.2) can be
transported more rapidly and also more efficiently to a catalytic
metal so that the catalyst can exhibit a high catalytic activity,
that is, the catalytic reaction can be promoted. This effect can be
also effectively exhibited under conditions with a high load. Thus,
a membrane electrode assembly and a fuel cell having the catalyst
layer of the present invention exhibit a high current and voltage
(iV) property (voltage reduction at high current density is
suppressed), and they have an excellent power generation
performance.
Furthermore, when the BET specific surface area of the catalyst is
900 m.sup.2/g or more, the catalyst carrier has many pores, in
particular, mesopores that are described below. As the entrance of
the pores is clogged by a polymer electrolyte, the gas
transportability into the pores is impaired. Since the catalytic
metal is supported in the pores, when the gas transportability into
the pores is impaired, the catalytic activity is lowered.
Meanwhile, as the mole number of a sulfonic acid group in the
polymer electrolyte relative to weight of the catalyst carrier is
set at 1.0 mmol/g or less, part of the catalyst is coated with a
polymer electrolyte so that clogging of pore entrance by a polymer
electrolyte can be suppressed and efficient transport of gas into
pores can be achieved.
Meanwhile, when the mole number of a sulfonic acid group in the
polymer electrolyte relative to weight of the catalyst carrier is
small, adsorption of a polymer electrolyte onto a catalytic metal
decreases, and thus it is easier for the polymer electrolyte to be
present as aggregates in a catalyst layer. It is believed that, as
a result, the gas transportability in a catalyst layer is lowered.
As the mole number of a sulfonic acid group in the polymer
electrolyte relative to weight of the catalyst carrier is set at
0.7 mmol/g or more, the gas transportability (in particular, oxygen
transportability) can be improved.
Meanwhile, the above mentioned mechanism for exhibiting the effect
of the present invention is just an assumption, and the present
invention is not limited to such assumption.
According to the above embodiment, a transport path of gas is
ensured by controlling the amount of a catalytic metal in a
suitable range to which reaction gas can reach without passing
through an electrolyte. As such, the electrode catalyst layer can
have an improved gas transportability. Furthermore, as the catalyst
has high specific surface area, the catalytic activity is
excellent. Furthermore, according to the present embodiment, as the
catalyst has low D'/G intensity ratio, the electrode catalyst layer
has high durability so that a high catalytic activity is
maintained.
The catalyst layer for fuel cell of this embodiment can therefore
exhibit a high catalytic activity, and also can maintain the
activity. In addition, a membrane electrode assembly and a fuel
cell having this catalyst layer have excellent power generation
performance and durability. As such, another embodiment of the
present invention is a membrane electrode assembly for a fuel cell
which includes the aforementioned electrode catalyst layer for a
fuel cell. Still another embodiment of the present invention is a
fuel cell which includes the membrane electrode assembly for fuel
cell.
Hereinbelow, one embodiment of the catalyst of the present
invention and one embodiment of a catalyst layer, a membrane
electrode assembly (MEA), and a fuel cell using the catalyst are
described in detail with suitable reference to the drawings.
However, the present invention is not limited to the following
embodiments. Meanwhile, each drawing is exaggerated for the
convenience of description, and the dimensional ratio of each
constitutional element can be different from actual ratios.
Furthermore, when descriptions of the embodiment of the present
invention are given in view of the drawings, the same elements are
given with the same symbols for describing the drawings, and
overlapped descriptions are omitted.
Furthermore, as described herein, "X to Y" for representing a range
means "X or more and Y or less." Furthermore, unless specifically
described otherwise, operations and measurements of physical
properties or the like are performed at room temperature (20 to
25.degree. C.)/relative humidity of 40 to 50%.
[Fuel Cell]
A fuel cell has a membrane electrode assembly (MEA) and a pair of
separator having an anode-side separator having a fuel gas passage
for flowing fuel and a cathode-side separator having an oxidant gas
passage for flowing an oxidant. The fuel cell according to this
embodiment has excellent durability and it can exhibit very high
power generation performance.
FIG. 1 is a schematic cross-sectional view illustrating the basic
configuration of a polymer electrolyte fuel cell (PEFC) 1 according
to an embodiment of the present invention. PEFC 1 has a solid
polymer electrolyte membrane 2, and a pair of catalyst layers
(anode catalyst layer 3a and cathode catalyst layer 3c) to sandwich
the solid polymer electrolyte membrane 2. A laminated body
constituted by the solid polymer electrolyte membrane 2 and the
catalyst layers (3a and 3c) is sandwiched by a pair of gas
diffusion layers (GDL) (anode gas diffusion layer 4a and cathode
gas diffusion layer 4c). Thus, the solid polymer electrolyte
membrane 2, the pair of the catalyst layers (3a and 3c) and the
pair of the gas diffusion layers (4a and 4c) are stacked to
constitute a membrane electrode assembly (MEA) 10.
In the PEFC 1, the MEA 10 is further sandwiched by a pair of
separators (anode separator 5a and cathode separator 5c). In FIG.
1, the separators (5a and 5c) are shown as being located on both
ends of the illustrated MEA 10. However, in a fuel cell stack in
which a plurality of MEAs is stacked up, the separators are also
generally used as the separators for the adjacent PEFC (not shown).
In other words, the MEAs form a stack by sequentially laminated via
the separators in a fuel cell stack. In other words, a fuel cell
stack is constituted in such a manner that the MEAs are
sequentially stacked via a separator to form a stack. Meanwhile, in
an actual fuel cell stack, gas seal members are provided between
the separator (5a and 5c) and the solid polymer electrolyte
membrane 2, or between the PEFC 1 and the adjacent other PEFC.
However, they are not illustrated in FIG. 1.
The separators (5a and 5c) are obtained by, for example, applying a
press forming process to thin plates with a thickness of 0.5 mm or
less, forming a corrugating shape as shown in FIG. 1. The convex
areas of the separators (5a and 5c) seen from the MEA side are in
contact with the MEA 10. Therefore, an electrical connection with
the MEA 10 is surely obtained. Furthermore, the concave areas as
viewed from the MEA of the separator (5a and 5c) (spaces between
the separator and the MEA derived from the concave-convex shape of
the separator) function as a gas passage through which gas flows at
the time of the operation of the PEFC 1. Specifically, a fuel gas
(for example, hydrogen or the like) flows in gas passage 6a of the
anode separator 5a, and an oxidant gas (for example, air or the
like) flows in gas passages 6c of the cathode separator 5c.
Meanwhile, the concave areas as viewed from the opposite side of
the MEA of the separator (5a and 5c) become a refrigerant passage 7
through which a refrigerant (for example, water) flows to cool the
PEFC at the time of the operation of the PEFC 1. Furthermore, the
separator is generally provided with a manifold (not shown). The
manifold functions as a connection means for connecting each cell
when constituting a stack. By having such a constitution,
mechanical strength of the fuel cell stack can be obtained.
Meanwhile, according to the embodiment illustrated in FIG. 1, the
separator (5a and 5c) is formed to have a concave-convex shape.
However, the separator is not limited to have such concave-convex
shape, and as long as it can exhibit the function as a gas passage
and a refrigerant passage, it can have any shape such as flat shape
or a partial concave-convex shape.
The fuel cell having MEA of the present invention as described
above exhibits an excellent power generation performance and
excellent durability. Herein, a type of the fuel cell is not
particularly limited. Although descriptions are given above by
having a polymer electrolyte fuel cell as an example, other
examples include an alkali fuel cell, a direct methanol fuel cell,
and a micro fuel cell. Among them, as having a small size and high
density and high output, a polymer electrolyte fuel cell (PEFC) can
be preferably mentioned. Furthermore, the aforementioned fuel cell
is also useful as a stationary power supply in addition to a power
source for a moving object such as an automobile which has limited
loading space. It is particularly preferably used as a power source
for a moving object such as an automobile where high output voltage
is required after stopping operation for a relatively long
time.
A type of fuel gas used at the time of the operation of the fuel
cell is not particularly limited. Examples of the fuel gas include
hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,
secondary butanol, tertiary butanol, dimethyl ether, diethyl ether,
ethylene glycol and diethylene glycol. Particularly, hydrogen and
methanol are preferably used in terms of having a high output
property.
Furthermore, the use for which the fuel cell can be applied is not
particularly limited, but it is suitably applied to a motor
vehicle. The electrolyte membrane-electrode assembly of the present
invention has an excellent power generation performance and
excellent durability and it allows obtainment of a cell with small
size. For such reasons, the fuel cell of the present invention is
particularly advantageous when it is applied to a motor vehicle
from the viewpoint of installing it on a vehicle. Accordingly, the
present invention provides a motor vehicle having the fuel cell of
the present invention.
Hereinbelow, the members constituting the fuel cell of the present
invention are briefly described, but the technical scope of the
present invention is not limited to the following embodiments.
[Electrode Catalyst Layer (Catalyst Layer)]
The electrode catalyst layer (catalyst layer) of the present
invention may be either a cathode catalyst layer or an anode
catalyst layer, but is preferably a cathode catalyst layer. As
described above, in the catalyst layer of the present invention, a
catalyst can be effectively used by forming three-phase interfaces
with water even when the catalyst and the electrolyte are not in
contact with each other, because water is formed in the cathode
catalyst layer.
As described herein, the catalyst layer essentially contains a
catalyst, in which a catalytic metal is supported on the catalyst
carrier, and an electrolyte.
In the catalyst layer, the mole number of sulfonic acid group in a
polymer electrolyte relative to weight of a catalyst carrier is 0.7
mmol/g or more and 1.0 mmol/g or less, preferably 0.75 to 0.95
mmol/g, and more preferably 0.8 to 0.9 mmol/g from the viewpoint of
improving the gas transportability.
The mole number of sulfonic acid group in a polymer electrolyte
relative to weight of a catalyst carrier is obtained as described
below. Mole number of sulfonic acid group in a polymer electrolyte
relative to weight of a catalyst carrier [mmol/g]=(A/B).times.1000
[Math. 1]
In the formula, A=weight ratio of a polymer electrolyte relative to
a catalyst carrier (I/C ratio), B=EW (equivalent weight weight of a
polymer electrolyte per mole of a sulfonic acid group in the
polymer electrolyte) [g]/[mol] ([g]/[eq]).
When a polymer electrolyte with two different kinds of EW (ion
exchange group is a sulfonic acid group) is used, a mole number
obtained by dividing I/C ratio of each polymer electrolyte by EW of
each polymer electrolyte is used.
Specifically, it can be obtained as described below. Hereinbelow,
descriptions are given for a case in which two kinds of polymer
electrolytes (the polymer electrolyte 1 and the polymer electrolyte
2) are used. Mole number of sulfonic acid group in a polymer
electrolyte relative to weight of a catalyst carrier
[mmol/g]={(A1/B1)+(A2/B2)}.times.1000 [Math. 2]
In the formula, A1=weight ratio of a polymer electrolyte relative
to the catalyst carrier in the polymer electrolyte 1 (I/C ratio),
B1=EW (weight ratio of a polymer electrolyte relative to the amount
of sulfonic acid group in the polymer electrolyte 1 [g]/[mol],
A2=weight ratio of a polymer electrolyte relative to the catalyst
carrier in the polymer electrolyte 2 (I/C ratio), B2=EW (weight
ratio of a polymer electrolyte relative to the amount of sulfonic
acid group in the polymer electrolyte 2 [g]/[mol].
Hereinbelow, the same shall apply when plural polymer electrolytes
are used.
The mole number of sulfonic acid group in a polymer electrolyte
relative to weight of a catalyst carrier can be controlled by
suitably adjusting the EW of a polymer electrolyte and I/C
ratio.
(Polymer Electrolyte)
The polymer electrolyte is not particularly limited as long as it
has a sulfonic acid group as an ion exchange group, and the
conventionally known knowledge can be properly referred to. The
polymer electrolyte is roughly divided into a fluorine-based
polymer electrolyte and a hydrocarbon-based polymer electrolyte,
depending on the kind of ion exchange resin that is a constituent
material. Among them, a fluorine-based polymer electrolyte is
preferred as a polymer electrolyte. The catalyst has high
hydrophobicity at conditions in which R' value is 0.6 or less.
However, by having the constitution of (III) above, it becomes
difficult for the electrolyte to adsorb onto a catalyst even when
the fluorine-based polymer electrolyte with high hydrophobicity is
used, and thus it is more likely that the effect of the present
invention is obtained at high level. Furthermore, from the
viewpoint of having excellent heat resistance, chemical stability,
durability, and mechanical strength, the fluorine-based polymer
electrolyte is preferable.
Examples of the ion exchange resin that constitutes a
fluorine-based polymer electrolyte include perfluorocarbon sulfonic
acid based polymers such as Nafion (registered trademark,
manufactured by Du Pont), Aciplex (registered trademark,
manufactured by Asahi Kasei Corporation Ltd.), and Flemion
(registered trademark, manufactured by Asahi Glass Co., LTD.),
trifluorostyrene sulfonic acid based polymers, ethylene
tetrafluoroethylene-g-styrene sulfonic acid based polymers,
polyvinylidene fluoride-perfluorocarbon sulfonic acid based
polymers, and the like. Among them, a fluorine-based polymer
electrolyte consisting of a perfluorocarbon sulfonic acid based
polymer is used.
The hydrocarbon-based electrolyte specifically includes sulfonated
polyether sulfon (S-PES), sulfonated polyaryl ether ketone,
sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole
alkyl, sulfonated polystyrene, sulfonated polyether ether ketone
(SPEEK), sulfonated polyphenylene (S-PPP), and the like.
The above-mentioned ion exchange resins may be used singly in only
one kind or in combinations of two or more kinds. Also, the
above-mentioned materials are not exclusive, and other materials
can be used as well.
The conductivity of protons is important in the polymer electrolyte
which serves to transfer protons. Here, in the case where EW of the
polymer electrolyte is too large, ion conductivity of the whole
catalyst layer deteriorates. Accordingly, the catalyst layer of
this embodiment preferably contains the polymer electrolyte with
small EW. On the other hand, in the case where EW is too small, the
hydrophilicity is so high that smooth movement of water becomes
difficult. From this point of view, equivalent weight (EW) of a
polymer electrolyte is preferably 500 g/eq. or more and 1200 g/eq.
or less, and more preferably 700 g/eq or more and 1100 g/eq or
less. Meanwhile, EW (Equivalent Weight) represents the equivalent
weight of an exchange group with proton conductivity. The
equivalent weight is dry weight of an ion exchange membrane per
equivalent of the ion exchange group, and it is represented by a
unit of "g/eq". When a commercially available polymer electrolyte
is used, literature values described in a catalogue of the product
and so on is used as EW. Furthermore, when EW of a polymer
electrolyte is unclear, it can be obtained by base neutralization
titration which uses sodium hydroxide.
Furthermore, the weight ratio of a polymer electrolyte relative to
a catalyst carrier (I/C ratio) is, considering the ion conductivity
and proton conductivity, preferably 0.5 or more and 1.0 or less,
and more preferably 0.6 or more and 0.9 or less.
In addition, the catalyst layer contains two or more kinds of
polymer electrolytes with different EW in the power generation
surface, and it is preferable to use a polymer electrolyte with a
lowest EW among polymer electrolytes in a region in which a
relative humidity of gas in a passage is 90% or less. By adopting
such material arrangement, the resistance value becomes small,
irrespective of the current density region, and cell performance
can be improved. EW of the polymer electrolyte used in a region in
which a relative humidity of gas in a passage is 90% or less, that
is, the polymer electrolyte with a lowest EW is preferably 900 g/eq
or less. Accordingly, the above-mentioned effects are more secured
and remarkable.
Furthermore, it is desirable to use the polymer electrolyte with a
lowest EW in a region with a temperature higher than the average
temperature of the inlet and outlet of cooling water. Accordingly,
the resistance value becomes small, irrespective of the current
density region, and cell performance can be further improved.
Furthermore, it is desirable to use the polymer electrolyte with a
lowest EW in a region within the range of 3/5 from at least one of
gas supply ports of fuel gas and oxidant gas, with respect to the
passage length, from the viewpoint of reducing the resistance value
of fuel cell system.
(Catalyst)
(II) R' (D'/G intensity ratio) which is a ratio of D' intensity
relative to G intensity of the catalyst is 0.6 or less.
According to the above (II), the amount of the edge of carbon
(graphene) which becomes a start point of electrochemical corrosion
in the graphite structure of the catalyst carrier can be kept at
sufficiently low level. Accordingly, the durability can be improved
and a reduction in the catalytic activity when supported with a
catalytic metal can be effectively suppressed and prevented. From
the viewpoint of further improvement of durability, the R' value
(D'/G intensity ratio) of the catalyst is preferably 0 to 0.6, and
more preferably 0 to 0.51.
In addition to above, the catalyst preferably has (II') R (D/G
intensity ratio) of 1.7 or more, which is a ratio of D intensity
relative to G intensity. Meanwhile, the D band measured in the
vicinity of 1360 cm.sup.-1 by Raman spectroscopy is also herein
simply referred to as "D band". Furthermore, peak intensity of D
band is also referred to as "D intensity." Furthermore, the ratio
of the D intensity relative to the G intensity is simply referred
to as "R value" or "D/G intensity ratio." Herein, D band is
observed in the vicinity of 1360 cm.sup.-1 by Raman scattering
analysis, and it results from a disorder or a defect in a graphite
structure. It appears when orientation property of graphene
molecule is high or graphitization level is high. In other words, a
high R value means low graphitization level of carbon powder
(carrier). For such reasons, when R value is 1.7 or more, electric
double layer capacity per surface area of carbon powder becomes
larger, and thus the catalytic activity can be more effectively
improved. Considering a further improvement of the electric double
layer capacity (catalytic activity), the R value (D/G intensity
ratio) of a catalyst is preferably more than 1.75 and 2.5 or less,
and more preferably 1.8 to 2.4.
Meanwhile, in the present specification, the R' value is obtained
by measuring Raman spectrum of a catalyst (or a catalyst carrier
precursor which will be described later) by using a micro Raman
spectrometer and calculating the relative intensity ratio between
the peak intensity in the vicinity of 1620 cm.sup.-1 (D' intensity)
referred to as a D' band and the peak intensity in the vicinity of
1580 cm.sup.-1 (G intensity) referred to as a G band, that is, the
peak area ratio of (D' intensity/G intensity). Similarly, the R
value is obtained by measuring Raman spectrum of a catalyst (or a
catalyst carrier precursor which will be described later) by using
a micro Raman spectrometer and calculating the relative intensity
ratio between the peak intensity in the vicinity of 1360 cm.sup.-1
(D intensity) referred to as a D band and the peak intensity in the
vicinity of 1580 cm.sup.-1 (G intensity) referred to as a G band,
that is, the peak area ratio of (D intensity/G intensity). As for
the peak area, the area obtained by Raman spectrometric measurement
which is described below is used.
(Raman Spectrometry Measurement)
The Raman spectrum was measured using a microlaser Raman SENTERRA
(manufactured by Bruker Optics K.K.) as a measuring apparatus, at
room temperature (25.degree. C.), exposure of 30
seconds.times.integration of 4 times, in the following conditions.
Meanwhile, the peaks of G band, D' band and D band can be
determined by peak fitting based on Gaussian distribution.
[Formula 1]
<Measurement Conditions>
Excitation wavelength: SHG of Nd: YAG, 532 nm
Laser output: 3 mW
Spot size: .about.1 .mu.m
Detector: CCD
The catalyst has (I) BET specific surface area of at least 900
m.sup.2/g catalyst carrier. It is more preferably at least 1000
m.sup.2/g catalyst carrier, more preferably 1000 to 3000 m.sup.2/g
catalyst carrier, and particularly preferably 1100 to 1800
m.sup.2/g catalyst carrier. With the specific surface area
described above, the catalyst has sufficient specific surface area,
and thus large electric double layer capacity can be achieved.
In the case of the specific surface area as described above,
sufficient mesopores and micropores that are described below can be
secured, thus while securing micropores (lower gas transport
resistance) sufficient for gas transport, more catalytic metal can
be stored (carried) in the mesopores. Also, the electrolyte and the
catalytic metal in the catalyst layer can be physically separated
(contact between the electrolyte and the catalytic metal can be
more effectively suppressed and prevented). Therefore, the activity
of the catalytic metal can be more effectively utilized. In
addition, as the micropores function as a transport path of gas, a
three-phase interface with water is more significantly formed so
that and the catalytic reaction can be more effectively
promoted.
According to this embodiment, part of the catalyst is coated with
an electrolyte so that clogging by the electrolyte of an entrance
of a mesopore in which a catalytic metal is supported can be
suppressed, and thus efficient transport of gas to the catalytic
metal can be achieved.
Meanwhile, the "BET specific surface area (m.sup.2/g catalyst
carrier)" in the present specification is measured by the nitrogen
adsorption method. In detail, about 0.04 to 0.07 g of sample
(carbon powder, catalyst powder) is accurately weighed, and sealed
in a test tube. The test tube is preliminarily dried in a vacuum
dryer at 90.degree. C. for several hours to obtain a measurement
sample. An electronic balance (AW220) manufactured by Shimadzu
Corporation is used for weighing. Meanwhile, as for the coated
sheet, about 0.03 to 0.04 g of the net weight of a coating layer in
which the weight of Teflon (registered trademark) (substrate) of
the same area is deducted from the total weight of the coated sheet
is used as a sample weight. Next, the BET specific surface area is
measured at the following measurement conditions. A BET plot is
obtained from a relative pressure (P/P.sub.0) range of about 0.00
to 0.45, in the adsorption side of the adsorption and desorption
isotherms, thereby calculating the BET specific surface area from
the slope and intercept thereof.
[Formula 2]
<Measurement Conditions>
Measurement instrument: High accuracy all--automated gas adsorption
instrument manufactured by BEL Japan, Inc. BELSORP 36
Adsorption gas: N.sub.2
Dead volume measurement gas: He
Adsorption temperature: 77 K (temperature of liquid nitrogen)
Pre-measurement treatment: vacuum dry at 90.degree. C. for several
hours (set on the measurement stage after purging with He)
Measurement mode: isothermal adsorption process and desorption
process
Measurement relative pressure P/P.sub.0: about 0 to 0.99
Setting time for equilibration: 180 seconds for every relative
pressure
The catalyst preferably satisfies one of the following (a) to (c):
(a) the catalyst has pores with a radius of less than 1 nm and
pores with a radius of 1 nm or more, a pore volume in the pores
with a radius of less than 1 nm is 0.3 cc/g carrier or more, and
the catalytic metal is supported inside the pores with a radius of
1 nm or more; (b) the catalyst has pores with a radius of 1 nm or
more and less than 5 nm, a pore volume of the pores is 0.8 cc/g
carrier or more, and the catalytic metal has a specific surface
area of 60 m.sup.2/g carrier or less; and (c) the catalyst has
pores with a radius of less than 1 nm and pores with a radius of 1
nm or more, a mode radius of pore distribution in the pores with a
radius of less than 1 nm is 0.3 nm or more and less than 1 nm, and
the catalytic metal is supported inside the pores with a radius of
1 nm or more. Meanwhile, in the present specification, the catalyst
satisfying the above (a) is also referred to as the "catalyst (a)",
the catalyst satisfying the above (b) is also referred to as the
"catalyst (b)", and the catalyst satisfying the above (c) is also
referred to as the "catalyst (c)".
Instead of the above, or in addition to the above, the catalyst
preferably satisfies the following (d): (d) the catalyst has a mode
radius of pores with a radius of 1 nm or more of pore distribution
of 1 nm or more and less than 5 nm, the catalytic metal is
supported inside the pores with a radius of 1 nm or more, the mode
radius is the same or less than the average particle radius of the
catalytic metal, and a pore volume in the pores with a radius of 1
nm or more and less than 5 nm is 0.4 cc/g carrier or more.
Meanwhile, in the present specification, the catalyst satisfying
the above (d) is also referred to as the "catalyst (d)".
Hereinbelow, the catalyst (a) to (d) are described in detail as
preferred modes.
(Catalysts (a) and (c))
The catalyst (a) contains a catalyst carrier and a catalytic metal
supported on the catalyst carrier and satisfies the following
constitutions (a-1) to (a-3): (a-1) the catalyst has pores with a
radius of less than 1 nm (primary pores) and pores with a radius of
1 nm or more (primary pores); (a-2) a pore volume of the pores with
a radius of less than 1 nm is 0.3 cc/g carrier or more; and (a-3)
the catalytic metal is supported inside the pores with a radius of
1 nm or more.
In addition, the catalyst (c) contains a catalyst carrier and a
catalytic metal supported on the catalyst carrier and satisfies the
following constitutions (a-1), (c-1) and (a-3): (a-1) the catalyst
has pores with a radius of less than 1 nm and pores with a radius
of 1 nm or more; (c-1) a mode radius of pore distribution in the
pores with a radius of less than 1 nm have is 0.3 nm or more and
less than 1 nm; and (a-3) the catalytic metal is supported inside
the pores with a radius of 1 nm or more.
Meanwhile, in the present specification, the pores with a radius of
less than 1 nm are also referred to as a "micropore." In addition,
in the present specification, the pores with a radius 1 nm or more
are also referred to as a "mesopore."
As described above, the inventors of the present invention have
found that, even when a catalytic metal does not contact an
electrolyte, the catalytic metal can be effectively used by forming
three-phase interfaces with water. Therefore, in the catalysts (a)
and (c), by adopting a constitution that the (a-3) the catalytic
metal is supported inside the mesopores in which the electrolyte
cannot enter, and thus the catalytic activity can be improved.
Meanwhile, when the catalytic metal is supported inside the
mesopores in which the electrolyte cannot enter, the transport
distance of gas such as oxygen is increased, and gas
transportability is lowered, thus a sufficient catalytic activity
cannot be elicited, and catalytic performance is deteriorated under
high load conditions. On the other hand, if the (a-2) the pore
volume of micropores in which the electrolyte and the catalytic
metal may not or cannot enter at all is sufficiently secured, or
the (c-1) the mode radius of the micropores is set large, the
transport path of gas can be sufficiently secured. Therefore, gas
such as oxygen can be efficiently transported to the catalytic
metal in the mesopores, namely, gas transport resistance can be
reduced. According to this constitution, gas (for example, oxygen)
passes through micropores (gas transportability is improved), and
gas can be efficiently contacted with the catalytic metal.
Therefore, when the catalysts (a) and (c) are used in the catalyst
layer, micropores are present in large volume, thus a reaction gas
can be transported to the surface of the catalytic metal present in
the mesopores via the micropores (path), and gas transport
resistance can be further reduced. Therefore, the catalyst layer
containing the catalysts (a) and (c) can exhibit higher catalytic
activity, namely, the catalytic reaction can be further promoted.
Therefore, the membrane electrode assembly and the fuel cell having
the catalyst layer containing the catalysts (a) and (c) can further
increase power generation performance.
FIG. 2 is a schematic explanatory cross-sectional view illustrating
the shape and structure of the catalysts (a) and (c). As
illustrated in FIG. 2, the catalysts (a) and (c) illustrated by 20
consist of a catalytic metal 22 and a catalyst carrier 23. Also, a
catalyst 20 has pores 25 with a radius of less than 1 nm
(micropores) and pores 24 with a radius of 1 nm or more
(mesopores). The catalytic metal 22 is supported inside the
mesopores 24. Also, it is enough that at least a part of the
catalytic metal 22 is supported inside the mesopores 24, and a part
may be supported on the surface of the catalyst carrier 23.
However, it is preferable that substantially all of the catalytic
metal 22 is supported inside the mesopores 24, from the viewpoint
of preventing the contact between the electrolyte and the catalytic
metal in the catalyst layer. The phrase "substantially all of the
catalytic metal" is not particularly limited so long as it is the
amount that can sufficiently improve the catalytic activity. The
phrase "substantially all of the catalytic metal" is present in an
amount of preferably 50% by weight or more (upper limit: 100% by
weight) and more preferably 80% by weight or more (upper limit:
100% by weight), in the whole catalytic metal.
The fact that "the catalytic metal is supported inside the
mesopores" in the present specification can be confirmed by
reduction in the volume of mesopores before and after supporting
the catalytic metal on the catalyst carrier. In detail, the
catalyst carrier (hereinbelow, also simply referred to as a
"carrier") has micropores and mesopores, and each pore has a
certain volume, but when the catalytic metal is supported in these
pores, the volume of each pore is reduced. Therefore, when the
difference between the volume of mesopores of the catalyst
(carrier) before supporting the catalytic metal and the volume of
mesopores of the catalyst (carrier) after supporting the catalytic
metal [=(volume before supporting)-(volume after supporting)]
exceeds 0, it means that "the catalytic metal is supported inside
the mesopores". Similarly, when the difference between the volume
of micropores of the catalyst (carrier) before supporting the
catalytic metal and the volume of micropores of the catalyst
(carrier) after supporting the catalytic metal [=(volume before
supporting)-(volume after supporting)] exceeds 0, it means that
"the catalytic metal is supported inside the micropores".
Preferably, the catalytic metal is supported in the mesopores more
than in the micropores (that is, reduction value of the volume of
mesopores between before and after supporting>reduction value of
the volume of micropores between before and after supporting). It
is because gas transport resistance is reduced, and thus a path for
gas transport can be sufficiently secured. The reduction value of
the pore volume of mesopores between before and after supporting
the catalytic metal is preferably 0.02 cc/g or more, and more
preferably 0.02 to 0.4 cc/g, in consideration of the reduction in
gas transport resistance, securing of the path for gas transport,
and the like.
In addition, it is preferable that the pore volume of pores with a
radius of less than 1 nm (micropores) (of the catalyst after
supporting the catalytic metal) is 0.3 cc/g carrier or more, and/or
the mode radius (modal radius) of pore distribution of micropores
(of the catalyst after supporting the catalytic metal) is 0.3 nm or
more and less than 1 nm. More preferably, the pore volume of
micropores is 0.3 cc/g carrier or more, and the mode radius of pore
distribution of micropore is 0.3 nm or more and less than 1 nm.
When the pore volume and/or mode radius of micropores is within the
above range, micropores sufficient for gas transport can be
secured, and gas transport resistance is small. Therefore, a
sufficient amount of gas can be transported to the surface of the
catalytic metal present in the mesopores via the micropores (path),
thus a high catalytic activity can be exhibited, namely, the
catalytic reaction can be promoted. Also, electrolyte (ionomer) and
liquid (for example, water) cannot enter the micropores, only gas
is selectively passed (gas transport resistance can be reduced).
The pore volume of micropores is more preferably 0.3 to 2 cc/g
carrier, and particularly preferably 0.4 to 1.5 cc/g carrier, in
consideration of the effect of improving gas transportability. In
addition, the mode radius of pore distribution of micropores is
more preferably 0.4 to 1 nm, and particularly preferably 0.4 to 0.8
nm. The pore volume of pores with a radius of less than 1 nm is
herein also simply referred to as "the pore volume of micropores".
Similarly, the mode radius of pore distribution of micropores is
herein also simply referred to as "the mode radius of
micropores".
The pore volume of the pores with a radius of 1 nm or more and less
than 5 nm (mesopores) (of the catalyst after supporting the
catalytic metal) is not particularly limited, but is preferably 0.4
cc/g carrier or more, more preferably 0.4 to 3 cc/g carrier, and
particularly preferably 0.4 to 1.5 cc/g carrier. When the pore
volume is within the above range, more catalytic metal can be
stored (supported) in the mesopores, and the electrolyte and the
catalytic metal in the catalyst layer can be physically separated
(contact between the catalytic metal and the electrolyte can be
more effectively suppressed and prevented). Therefore, the activity
of the catalytic metal can be more effectively utilized. Also, by
the presence of many mesopores, the catalytic reaction can be more
effectively promoted. In addition, the micropores act as a
transport path of gas, and three-phase interfaces are more
remarkably formed by water, thus the catalytic activity can be
further improved. The pore volume of pores with a radius of 1 nm or
more and less than 5 nm is herein also simply referred to as "the
pore volume of mesopores".
The mode radius (modal radius) of pore distribution of pores with a
radius of 1 nm or more (mesopores) (of the catalyst after
supporting the catalytic metal) is not particularly limited, but is
preferably 1 to 5 nm, more preferably 1 to 4 nm, and particularly
preferably 1 to 3 nm. In the case of the mode radius of pore
distribution of mesopores described above, a more sufficient amount
of the catalytic metal can be stored (supported) in the mesopores,
and the electrolyte and the catalytic metal in the catalyst layer
can be physically separated (contact between the catalytic metal
and the electrolyte can be more effectively suppressed and
prevented). Therefore, the activity of the catalytic metal can be
more effectively utilized. Also, by the presence of large-volume
mesopores, the catalytic reaction can be more effectively promoted.
In addition, the micropores act as a transport path of gas, and
three-phase interfaces are more remarkably formed by water, thus
the catalytic activity can be further improved. The mode radius of
pore distribution of mesopores is herein also simply referred to as
"the mode radius of mesopores".
The "radius of pores of micropores (nm)" in the present
specification refers to a radius of pores measured by the nitrogen
adsorption method (MP method). Also, the "mode radius of pore
distribution of micropores (nm)" herein refers to a pore radius at
a point taking a peak value (maximum frequency) in the differential
pore distribution curve that is obtained by the nitrogen adsorption
method (MP method). The lower limit of the pore radius of
micropores is the lower limit that can be measured by the nitrogen
adsorption method, that is, 0.42 nm or more. Similarly, the "radius
of pores of mesopores (nm)" refers to a radius of pores measured by
the nitrogen adsorption method (DH method). Also, the "mode radius
of pore distribution of mesopores (nm)" refers to a pore radius at
a point taking a peak value (maximum frequency) in the differential
pore distribution curve that is obtained by the nitrogen adsorption
method (DH method). Herein, the upper limit of the pore radius of
mesopores is not particularly limited, but is 5 nm or less.
The "pore volume of micropores" in the present specification refers
to a total volume of micropores with a radius of less than 1 nm
present in the catalyst, and expressed as a volume per 1 g of the
carrier (cc/g carrier). The "pore volume of micropores (cc/g
carrier)" is calculated as a downside area (integrated value) under
the differential pore distribution curve obtained by the nitrogen
adsorption method (MP method). Similarly, the "pore volume of
mesopores" refers to a total volume of mesopores with a radius of 1
nm or more and less than 5 nm present in the catalyst, and
expressed as a volume per 1 g of the carrier (cc/g carrier). The
"pore volume of mesopores (cc/g carrier)" is calculated as a
downside area (integrated value) under the differential pore
distribution curve obtained by the nitrogen adsorption method (DH
method).
The "differential pore distribution" in the present specification
refers to a distribution curve obtained by plotting a pore size on
the horizontal axis and a pore volume corresponding to the pore
size in the catalyst on the vertical axis. That is to say, in the
case of regarding the pore volume of the catalyst obtained by the
nitrogen adsorption method (MP method in the case of micropores; DH
method in the case of mesopores) as V and the pore diameter as D, a
value (dV/d (log D)) obtained by dividing that differential pore
volume dV by logarithmic difference of the pore diameter d (log D)
is determined. Moreover, the differential pore distribution curve
is obtained by plotting this dV/d (log D) to the average pore
diameter of each section. The differential pore volume dV indicates
the increment of the pore volume between measuring points.
The method for measuring the radius of micropores and pore volume
by the nitrogen adsorption method (MP method) is not particularly
limited, and for example, the method described in known documents
such as "Science of Adsorption" (second edition, written jointly by
Seiichi Kondo, Tatsuo Ishikawa and Ikuo Abe, MARUZEN Co., Ltd.),
"Fuel Cell Characterization Methods" (edited by Yoshio Takasu,
Masaru Yoshitake, Tatsumi Ishihara, Kagaku-Dojin Publishing Co.,
Inc.), and R. Sh. Mikhail, S. Brunauer, E. E. Bodor J. Colloid
Interface Sci., 26, 45 (1968) can be used. The radius of micropores
and pore volume by the nitrogen adsorption method (MP method) are a
value herein measured by the method described in R. Sh. Mikhail, S.
Brunauer, E. E. Bodor J. Colloid Interface Sci., 26, 45 (1968).
The method for measuring the radius of mesopores and pore volume by
the nitrogen adsorption method (DH method) is not also particularly
limited, and for example, the method described in known documents
such as "Science of Adsorption" (second edition, written jointly by
Seiichi Kondo, Tatsuo Ishikawa and Ikuo Abe, MARUZEN Co., Ltd.),
"Fuel Cell Characterization Methods" (edited by Yoshio Takasu,
Masaru Yoshitake, Tatsumi Ishihara, Kagaku-Dojin Publishing Co.,
Inc.), and D. Dollion, G. R. Heal: J. Appl. Chem., 14, 109 (1964)
can be used. The radius of mesopores and pore volume by the
nitrogen adsorption method (DH method) are a value herein measured
by the method described in D. Dollion, G. R. Heal: J. Appl. Chem.,
14, 109 (1964).
The method for producing the catalyst having specific pore
distribution as described above is not particularly limited, but it
is usually important that the pore distribution (micropores and
mesopores in some cases) of the carrier is set to the pore
distribution described above. Specifically, as the method for
producing a carrier having micropores and mesopores, and a pore
volume of micropores of 0.3 cc/g carrier or more, the methods
described in publications such as JP 2010-208887 A (specification
of US 2011/318,254, the same applies hereafter) and WO 2009/75264 A
(specification of US 2011/058,308, the same applies hereafter) are
preferably used. Furthermore, as a method for producing a carrier
having micropores and mesopores, and having micropores with a mode
radius of pore distribution of 0.3 nm or more and less than 1 nm,
the methods described in publications such as JP 2010-208887 A and
WO 2009/75264 A are preferably used.
(Catalyst (b))
The catalyst (b) contains a catalyst carrier and a catalytic metal
supported on the catalyst carrier and satisfies the following
constitutions (b-1) to (b-3): (b-1) the catalyst has pores with a
radius of 1 nm or more and less than 5 nm; (b-2) a pore volume in
the pores with a radius of 1 nm or more and less than 5 nm is 0.8
cc/g carrier or more; and (b-3) a specific surface area of the
catalytic metal is 60 m.sup.2/g carrier or less.
According to the catalyst having the constitutions of the (b-1) to
(b-3) described above, filling of the pores of the catalyst with
water is suppressed, and then pores contributing to transport of a
reaction gas is sufficiently secured. As a result, a catalyst
excellent in gas transportability can be provided. In detail, the
volume of mesopores effective for gas transport is sufficiently
secured, and further, the specific surface area of the catalytic
metal is reduced, and thus the amount of the water maintained in
the mesopores in which the catalytic metal is supported can be
sufficiently reduced. Therefore, filling of the inside of the
mesopores with water is suppressed, thus gas such as oxygen can be
more efficiently transported to the catalytic metal in the
mesopores. In other words, the gas transport resistance in the
catalyst layer can be further reduced. As a result, the catalytic
reaction is promoted, and the catalyst (b) of this embodiment can
exhibit higher catalytic activity. Therefore, a membrane electrode
assembly and a fuel cell having a catalyst layer using the catalyst
(b) of this embodiment are excellent in power generation
performance.
FIG. 3 is a schematic explanatory cross-sectional view illustrating
the shape and structure of the catalysts (b) according to an
embodiment of the present invention. As illustrated in FIG. 3, the
catalyst 20' of the present invention consists of a catalytic metal
22' and a catalyst carrier 23'. The catalyst 20' has pores 24' with
a radius of 1 nm or more and less than 5 nm (mesopores). The
catalytic metal 22' is mainly supported inside the mesopores 24'.
Also, it is enough that at least a part of the catalytic metal 22'
is supported inside the mesopores 24', and a part may be supported
on the surface of the catalyst carrier 23'. However, it is
preferable that substantially all the catalytic metal 22' is
supported inside the mesopores 24', from the viewpoint of
preventing the contact between the electrolyte (electrolyte
polymer, ionomer) and the catalytic metal in the catalyst layer.
When the catalytic metal contacts the electrolyte, the area ratio
activity of the surface of the catalytic metal is reduced. On the
other hand, according to the above constitution, it is possible to
make the electrolyte not to enter the mesopores 24' of the catalyst
carrier 23', and thus the catalytic metal 22' and the electrolyte
can be physically separated. Moreover, three-phase interfaces can
be formed with water, and consequently the catalytic activity is
improved. The phrase "substantially all the catalytic metal" is not
particularly limited so long as it is the amount that can
sufficiently improve the catalytic activity. The "substantially all
the catalytic metal" is present in an amount of preferably 50% by
weight or more (upper limit: 100% by weight) and more preferably
80% by weight or more (upper limit: 100% by weight), in the whole
catalytic metal.
The pore volume of pores with a radius of 1 nm or more and less
than 5 nm (mesopores) in the catalyst (b) is 0.8 cc/g carrier or
more. The pore volume of mesopores is preferably 0.8 to 3 cc/g
carrier, and particularly preferably 0.8 to 2 cc/g carrier. In a
case where the pore volume is within the range described above,
pores contributing to transport of a reaction gas is much secured,
thus transport resistance of the reaction gas can be reduced.
Therefore, the reaction gas can be rapidly transported to the
surface of the catalytic metal stored in the mesopores, thus the
catalytic metal is effectively utilized. Furthermore, in a case
where the volume of mesopores is within the range described above,
the catalytic metal can be stored (supported) in the mesopores, and
the electrolyte and the catalytic metal in the catalyst layer can
be physically separated (contact between the electrolyte and the
catalytic metal can be more effectively suppressed and prevented).
As described above, in the embodiment in which the contact between
the catalytic metal in the mesopores and the electrolyte is
suppressed, the activity of the catalyst can be more effectively
utilized, as compared with the case where the amount of the
catalytic metal supported on the surface of the carrier is
high.
In addition, in the catalyst (b), the catalytic metal (catalyst
component) has a specific surface area of 60 m.sup.2/g carrier or
less. The catalytic metal has a specific surface area of preferably
5 to 60 m.sup.2/g carrier, more preferably 5 to 30 m.sup.2/g
carrier, and particularly preferably 10 to 25 m.sup.2/g carrier.
The surface of the catalytic metal is hydrophilic, and water
generated by catalytic reaction is likely to adsorb, thus water is
likely to be maintained in the mesopores in which the catalytic
metal is stored. When water is maintained in the mesopores, gas
transport path becomes narrow, and the diffusion rate of the
reaction gas in water is low, thus gas transportability is reduced.
On the other hand, when the specific surface area of the catalytic
metal is set relatively small as the above range, the amount of
water adsorbed to the surface of the catalytic metal can be
reduced. As a result, water is hard to be maintained in the
mesopores, and the water content in the catalyst and also in the
catalytic layer can be reduced. Therefore, transport resistance of
the reaction gas can be reduced, and the catalytic metal is
effectively utilized. The "specific surface area of the catalytic
metal" in the present invention can be measured by the method
described in, for example, Journal of Electroanalytical Chemistry
693 (2013) 34 to 41, or the like. The "specific surface area of the
catalytic metal" herein adopts the value measured by the following
method.
(Method for Measuring Specific Surface Area of Catalytic Metal)
With regard to the cathode catalyst layer, electrochemical
effective surface area (ECA) is measured by cyclic voltammetry.
Hydrogen gas humidified so as to be saturated at a measurement
temperature is flowed into the opposed anode, and this anode is
used as a reference electrode and a counter electrode. Nitrogen gas
similarly humidified is flowed into the cathode, and valves of
entrance and exit of the cathode are closed immediately before
starting measurement, and nitrogen gas is sealed. Measurement is
performed in this state, in the following conditions, using an
electrochemical measuring system (manufactured by HOKUTO DENKO
CORP., model: HZ-5000).
[Formula 3]
Electrolyte solution: 1M sulfuric acid (manufactured by Wako Pure
Chemical Industries Ltd., for measurement of harmful metal)
Scanning rate: 50 mV/s
Number of cycles: 3 cycles
Lower limit voltage value: 0.02 V
Upper limit voltage value: 0.9 V
The method for producing the catalyst having specific pore volume
as described above is not particularly limited, but it is usually
important that the mesopore volume of the carrier is set to the
pore distribution described above. Specifically, as the method for
producing a carrier having mesopores, and a mesopore volume of 0.8
cc/g carrier or more, the methods described in publications such as
JP 2010-208887 A (specification of US 2011/318,254, the same
applies hereafter) and WO 2009/075264 A (specification of US
2011/058,308 A, the same applies hereafter) are preferably
used.
It is preferable that, in the catalysts (a) and (c), at least apart
of the catalytic metal is supported inside the mesopores, and in
the catalyst (b), at least a part of the catalytic metal is
supported inside the mesopores. Here, the size of the catalytic
metal supported in the mesopores when the catalytic metal is
supported in the mesopores is preferably larger than the size of
the mesopores (Embodiment (i)). According to the constitution, the
distance between the catalytic metal and the inner wall of the pore
of the carrier is reduced, and the space in which water can be
present is reduced, namely, the amount of water adsorbed to the
surface of the catalytic metal is reduced. Also, water is subjected
to interaction of the inner wall of the pore, and thus a reaction
of forming a metal oxide becomes slow, and a metal oxide is hard to
be formed. As a result, deactivation of the surface of the
catalytic metal can be further suppressed. Therefore, the catalyst
of this Embodiment (i) can exhibit higher catalytic activity,
namely, the catalytic reaction can be further promoted.
In Embodiment (i), the mode radius (modal radius) of pore
distribution of mesopores (of the catalyst after supporting the
catalytic metal) is preferably 1 nm or more and 5 nm or less, more
preferably 1 nm or more and 4 nm or less, further preferably 1 nm
or more and 3 nm or less, and particularly preferably 1 nm or more
and 2 nm or less. With the mode radius of pore distribution as
described above, the sufficient amount of the catalytic metal can
be stored (supported) in the mesopores, and the electrolyte and the
catalytic metal in the catalyst layer can be physically separated
(contact between the catalytic metal and the electrolyte can be
more effectively suppressed and prevented). Therefore, the activity
of the catalytic metal can be more effectively utilized.
Furthermore, due to the presence of pores with large volume
(mesopores), the activity and effect of the present invention can
be more significantly exhibited so that the catalyst reaction can
be more effectively promoted.
In Embodiment (i), the average particle size of the catalytic metal
(catalytic metal particles) (of the catalyst after supporting the
catalytic metal) is preferably 2 nm or more and 7 nm or less, and
more preferably 3 nm or more and 5 nm or less. When the average
particle size of the catalytic metal is twice or more of the mode
radius of pore distribution as described above (when the mode
radius is half or less of the average particle size of the
catalytic metal), the distance between the catalytic metal and the
inner wall of the pore of the carrier is reduced, and the space in
which water can be present is reduced, namely, the amount of water
adsorbed to the surface of the catalytic metal is reduced. Also,
water is subjected to interaction of the inner wall, and thus a
reaction of forming a metal oxide becomes slow, and a metal oxide
is hard to be formed. As a result, deactivation of the surface of
the catalytic metal can be suppressed, and high catalytic activity
can be exhibited. Namely, the catalytic reaction can be promoted.
Also, the catalytic metal is relatively firmly supported in the
pores (mesopores), and the contact with the electrolyte in the
catalyst layer is more effectively suppressed and prevented.
Moreover, elution due to potential change is prevented, and
temporal performance deterioration can be also suppressed.
Therefore, catalytic activity can be further improved, namely, the
catalytic reaction can be more efficiently promoted.
(Catalyst (d))
The catalyst (d) contains a catalyst carrier and a catalytic metal
supported on the catalyst carrier and satisfies the following
constitutions (d-1) to (d-4): (d-1) the catalyst has pores with a
radius of 1 nm or more having a mode radius of pore distribution of
1 nm or more and less than 5 nm; (d-2) the catalytic metal is
supported inside the pores with a radius of 1 nm or more; (d-3) the
mode radius is half or less of the average particle size of the
catalytic metal; and (d-4) a pore volume in the pores with a radius
of 1 nm or more and less than 5 nm is 0.4 cc/g carrier or more.
Meanwhile, as described herein, "half of the average particle size
(1/2 times of the average particle size)" is also referred to as
"average particle radius."
According to the catalyst having the constitutions of the (d-1) to
(d-4) described above, by taking a constitution that the catalytic
metal is supported inside the pores (mesopores) in which the
electrolyte cannot enter, the catalytic metal inside the pores
forms three-phase interfaces with water, and the catalyst can be
effectively utilized. As a result, the activity of the catalyst can
be improved. In detail, particularly, the (d-3) the mode radius of
the pores is set to half or less of the average particle size of
the catalytic metal, and thus the distance between the catalytic
metal and the inner wall of the pore of the carrier is reduced, and
the space in which water can be present is reduced, namely, the
amount of water adsorbed to the surface of the catalytic metal is
reduced. Also, water is subjected to interaction of the inner wall
of the pore, and thus a reaction of forming a metal oxide becomes
slow, and a metal oxide is hard to be formed. As a result,
deactivation of the surface of the catalytic metal can be further
suppressed. Thus, the catalyst (d) of this embodiment can exhibit
high catalytic activity, namely, the catalytic action can be
promoted. Therefore, a membrane electrode assembly and a fuel cell
having a catalyst layer using the catalyst (d) of this embodiment
are excellent in power generation performance.
The catalyst (d) according to an embodiment of the present
invention contains a catalytic metal and a carrier. The catalyst
also has pores (mesopores). Herein, the catalytic metal is
supported inside the mesopores. Also, it is sufficient that at
least a part of the catalytic metal is supported inside the
mesopores, and a part may be supported in the surface of the
carrier. However, it is preferable that substantially all the
catalytic metal is supported inside the mesopores, from the
viewpoint of preventing the contact between the electrolyte and the
catalytic metal in the catalyst layer. The "substantially all the
catalytic metal" is not particularly limited so long as it is the
amount that can sufficiently improve the catalytic activity. The
"substantially all the catalytic metal" is present in an amount of
preferably 50% by weight or more (upper limit: 100% by weight) and
more preferably 80% by weight or more (upper limit: 100% by
weight), in the whole catalytic metal.
The pore volume of the mesopores of the catalyst (d) is 0.4 cc/g
carrier or more, preferably 0.45 to 3 cc/g carrier, and more
preferably 0.5 to 1.5 cc/g carrier. When the pore volume is in the
above range, more catalytic metal can be stored (supported) in the
mesopores, and the electrolyte and the catalytic metal in the
catalyst layer can be physically separated (contact between the
catalytic metal and the electrolyte can be more effectively
suppressed and prevented). Therefore, the activity of the catalytic
metal can be more effectively utilized. In addition, by the
presence of many mesopores, the catalytic reaction can be more
effectively promoted.
The mode radius (modal radius) of pore distribution of the pores of
the catalyst (d) is 1 nm or more and less than 5 nm, preferably 1
nm or more and 4 nm or less, more preferably 1 nm or more and 3 nm
or less, and further preferably 1 nm or more and 2 nm or less. In
the case of the mode radius of pore distribution as described
above, a sufficient amount of the catalytic metal can be stored
(supported) in the mesopores, and the electrolyte and the catalytic
metal in the catalyst layer can be physically separated (contact
between the catalytic metal and the electrolyte can be more
effectively suppressed and prevented). Therefore, the activity of
the catalytic metal can be more effectively utilized. In addition,
by the presence of pores with large volume (mesopores), the action
and effect according to the present invention are more remarkably
exhibited, and the catalytic reaction can be more effectively
promoted.
The method for producing the catalyst having specific pore
distribution as described above is not particularly limited, but it
is usually important that the mesopore volume of the carrier and so
on is set to the pore distribution described above. As the method
for producing those carriers, the methods described in publications
such as JP 2010-208887 A and WO 2009/075264 A are preferably
used.
(Catalyst Carrier)
The catalyst carrier contains carbon as a main component. The
phrase "contain(s) carbon as a main component" herein is a concept
containing both "consist(s) only of carbon" and "consist(s)
substantially of carbon", and an element other than carbon may be
contained. The phrase "consist(s) substantially of carbon" refers
to that 80% by weight or more of a whole, and preferably 95% by
weight or more of a whole (upper limit: less than 100% by weight)
consists of carbon.
The catalyst carrier is not particularly limited, but is preferably
carbon powder. Furthermore, the R' value is substantially the same
also by the catalyst supporting process set forth below, thus in
order that a catalyst may satisfy the condition of the above (II),
it is preferable that the catalyst carrier also satisfies the above
(II):
(II) The R' (D'/G intensity ratio) which is the ratio of D'
intensity relative to G intensity is 0.6 or less.
According to the above (II), the amount of the edge of carbon
(graphene) which becomes a start point of electrochemical corrosion
in the graphite structure can be kept at sufficiently low level.
Therefore, by using such carbon powder in the catalyst, the
durability can be improved and a reduction in the catalytic
activity when supported with a catalytic metal can be effectively
suppressed and prevented. From the viewpoint of further improvement
of durability, the R' value (D'/G intensity ratio) of carbon powder
is preferably 0 to 0.6, and more preferably 0 to 0.51.
In addition to above, because the R value is substantially the same
also by the catalyst supporting process set forth below, thus in
order that a catalyst may satisfy the condition of the above (II'),
it is preferable that the catalyst carrier preferably has (II') the
R (D/G intensity ratio) which is a ratio of D intensity relative to
G intensity of 1.7 or more. Since such catalyst carrier has low
graphitization level, electric double layer capacity per surface
area of carbon powder increases, and thus the catalytic activity
can be more effectively improved. Considering a further improvement
of the electric double layer capacity (catalytic activity), the R
value (D/G intensity ratio) of a catalyst carrier is preferably
more than 1.75 and 2.5 or less, and more preferably 1.8 to 2.4.
The BET specific surface area of the catalyst carrier can be a
specific surface area which is sufficient to carry the catalyst
component in a highly dispersed manner. The BET specific surface
area of the carrier is substantially equivalent to the BET specific
surface area of the catalyst. Thus the BET specific surface area of
the carrier is preferably 900 m.sup.2/g or more, more preferably
1000 m.sup.2/g or more, and particularly preferably 1100 m.sup.2/g
or more. Also, the upper limit of the BET specific surface area of
the carrier is not particularly limited, but is preferably 3000
m.sup.2/g or less, and more preferably 1800 m.sup.2/g or less. In
the case of the specific surface area as described above,
sufficient mesopores and also, in some cases, sufficient micropores
can be secured, thus further more catalytic metal can be stored
(supported) in the mesopores with better dispersibility. Also,
mesopores and also micropores in some cases sufficient for gas
transport can be secured, thus gas transport resistance can be
further reduced. In addition, the electrolyte and the catalytic
metal in the catalyst layer can be physically separated (contact
between the catalytic metal and the electrolyte can be more
effectively suppressed and prevented). Therefore, the activity of
the catalytic metal can be more effectively utilized. Moreover,
local flux in the vicinity of the catalytic metal particles becomes
small, thus a reaction gas is rapidly transported, and the
catalytic metal is effectively utilized. Also, by the presence of
many pores (mesopores) and micropores in some cases, the action and
effect according to the present invention are further remarkably
exhibited, and the catalytic reaction can be more effectively
promoted. Also, the balance between dispersibility of the catalyst
component on the catalyst carrier and effective utilization rate of
the catalyst component can be properly controlled. In addition, the
micropores act as a transport path of gas, and three-phase
interfaces are more remarkably formed by water, thus catalytic
activity can be further improved.
Furthermore, size of the catalyst carrier is not particularly
limited. From the viewpoint of easy supporting, catalyst use rate,
and controlling the thickness of an electrode catalyst layer within
a suitable range, the average particle size (diameter) of the
catalyst carrier is preferably 5 to 2000 nm, more preferably 10 to
200 nm, and particularly preferably 20 to 100 nm. As for the value
of the "average particle size of a catalyst carrier", unless
specifically described otherwise, a value which is measured by use
of an observational means such as a scanning electron microscope
(SEM) and a transmission electron microscope (TEM), and is
calculated as an average value of particle size of the particles
observed in several to several tens of visual fields is used.
Similarly, the "particle size (diameter)" means, among the lengths
of a line going through a center of a particle connecting any two
points on a particle contour, the longest length.
When the catalyst satisfies the requirement of any of the above
constitutions (a) to (d), t is preferable that the catalyst carrier
also satisfies the same requirement of the constitutions (a) to
(d).
It is preferable that the catalyst carrier satisfies at least one
of the following constitutions (1) to (3). (1) (a-1) it has pores
with a radius of less than 1 nm (primary pore) and pores with a
radius of 1 nm or more (primary pore); and (a-2) a pore volume in
the pores with a radius of less than 1 nm is 0.3 cc/g carrier or
more. (2) (a-1) it has pores with a radius of less than 1 nm and
pores with a radius of 1 nm or more; and (c-1) a mode radius of
pore distribution in the pores with a radius of less than 1 nm is
0.3 nm or more and less than 1 nm. (3) (d-1) the mode radius of
pore distribution in the pores with a radius of 1 nm or more is 1
nm or more and less than 5 nm; and (d-4) a pore volume in the pores
with a radius of 1 nm or more and less than 5 nm is 0.4 cc/g
carrier or more. Furthermore, in (3), it is preferable that (b-2) a
pore volume in the pores with a radius of 1 nm or more and less
than 5 nm is 0.8 cc/g carrier or more. More preferable range of the
pore volume of micropores in (a-2), mode radius of pore
distribution of micropores in (c-1), mode radius of pore
distribution of pores with a radius of 1 nm or more in (d-1), pore
volume of the pores with a radius of 1 nm or more and less than 5
nm in (d-4) and the like are the same as those described in the
sections of the catalysts (a) to (d).
(Catalytic Metal)
The catalytic metal constituting the catalyst has a function of a
catalytic action of an electrochemical reaction. The catalytic
metal used in the anode catalyst layer is not particularly limited
so long as it provides a catalytic action for the oxidation
reaction of hydrogen, and a known catalyst can be similarly used.
In addition, the catalytic metal used in the cathode catalyst layer
is not also particularly limited so long as it provides a catalytic
action for the reduction reaction of oxygen, and a known catalyst
can be similarly used. Specifically, the catalytic metal can be
selected from metals such as platinum, ruthenium, iridium, rhodium,
palladium, osmium, tungsten, lead, iron, copper, silver, chromium,
cobalt, nickel, manganese, vanadium, molybdenum, gallium, and
aluminum, as well as their alloys.
Of these, those that contain at least platinum are preferably used,
in order to improve catalytic activity, anti-toxicity against
carbon monoxide and the like, heat resistance, and the like.
Namely, the catalytic metal is preferably platinum or contains
platinum and a metal component other than platinum, and is more
preferably platinum or a platinum-containing alloy. Such catalytic
metal can exhibit high activity. When the catalytic metal is
platinum, in particular, platinum with small particle size can be
dispersed on a surface of carbon powder (carrier), and thus the
platinum surface are per weight can be maintained even when the use
amount of platinum is lowered. Furthermore, when the catalytic
metal contains platinum and a metal component other than platinum,
use amount of expensive platinum can be lowered, and thus it is
preferable from the economical point of view. The alloy
compositions should preferably contain 30 to 90 atom % of platinum,
although it depends to the type of metal to be alloyed, and the
content of the metal to be alloyed with platinum should be 10 to 70
atom %. Alloy is a collective name of a combination of a metal
element combined with one or more kinds of metal elements or
non-metallic elements, such combination having metallic
characteristics. The structure of an alloy can be an eutectic alloy
which is a mixture of crystals of different component elements, a
solid solution which is formed by completely molten component
elements, a compound where the component elements are an
intermetallic compound or a compound forming a compound of a metal
with a non-metal, or the like, and may be any of them in the
present application. In this case, the catalytic metal used in the
anode catalyst layer and the catalytic metal used in the cathode
catalyst layer may be appropriately selected from the above. Unless
otherwise noted herein, the descriptions for catalytic metals for
the anode catalyst layer and for the cathode catalyst layer have
the same definitions for both. However, the catalytic metals for
the anode catalyst layer and for the cathode catalyst layer need
not be the same, and may be appropriately selected so as to provide
the desired action described above.
The shape and size of the catalytic metal (catalyst component) are
not particularly limited, and any shape and size similar to those
of a known catalyst components can be adopted. For example, those
having granular, scaly, or layered shape can be used, and granular
shape is preferred.
The average particle size (diameter) of the catalytic metal
(catalytic metal particles) is not particularly limited. However,
it is preferably 3 nm or more, more preferably more than 3 nm and
30 nm or less, and particularly preferably more than 3 nm and 10 nm
or less. If an average particle size of the catalytic metal is 3 nm
or more, the catalytic metal is relatively firmly supported in the
carbon powder (for example, inside the mesopores of carbon powder),
and the contact with the electrolyte in the catalyst layer is more
effectively suppressed and prevented. In addition, when the
catalyst carrier has the micropores, the micropores are remained
without being blocked by the catalytic metal, and transport path of
gas is more favorably secured, and gas transport resistance can be
further reduced. Moreover, elution due to potential change is
prevented, and temporal performance deterioration can be also
suppressed. Therefore, catalytic activity can be further improved,
namely, the catalytic reaction can be more efficiently promoted. On
the other hand, if an average particle size of the catalytic metal
particles is 30 nm or less, the catalytic metal can be supported on
the catalyst carrier (for example, inside the mesopores of carbon
powder) by a simple method, and the electrolyte coating of the
catalytic metal can be reduced. In the case of using the catalyst
(a) and/or (c) as a catalyst, the average particle size of the
catalytic metal (catalytic metal particles) is preferably 3 nm or
more, more preferably more than 3 nm and 30 nm or less, and
particularly preferably more than 3 nm and 10 nm or less. In
addition, in the case of using the catalyst (b) as a catalyst, the
average particle size of the catalytic metal (catalytic metal
particles) is preferably more than 3 nm, more preferably more than
3 nm to 30 nm, and particularly preferably more than 3 nm to 10 nm.
If an average particle size of the catalytic metal is more than 3
nm, the specific surface area of the catalytic metal can be made
small. As a result, as described above, the amount of water
adsorbed to the surface of the catalytic metal can be reduced, and
mesopores contributing to transport of a reaction gas can be
secured in a large amount. Therefore, transport resistance of the
reaction gas can be reduced. Moreover, elution due to potential
change is prevented, and temporal performance deterioration can be
also suppressed. Therefore, catalytic activity can be further
improved, namely, the catalytic reaction can be more efficiently
promoted. On the other hand, if an average particle size of the
catalytic metal particles is 30 nm or less, the catalytic metal can
be supported inside the mesopores of the carrier by a simple
method, and the electrolyte coating of the catalytic metal can be
reduced. Furthermore, in the case of using the catalyst (d) as a
catalyst, the average particle size of the catalytic metal is twice
or more of the mode radius of pore distribution of mesopores (the
mode radius is half or less of the average particle size of the
catalytic metal). Here, the average particle size of the catalytic
metal (catalytic metal particles) is preferably 2 nm or more and 7
nm or less, and more preferably 3 nm or more and 5 nm or less. When
the average particle size of the catalytic metal is twice or more
of the mode radius of pore distribution as described above, the
distance between the catalytic metal and the inner wall of the pore
of the carrier is reduced, and the space in which water can be
present is reduced, namely, the amount of water adsorbed to the
surface of the catalytic metal is reduced. Also, water is subjected
to interaction of the inner wall, and thus a reaction of forming a
metal oxide becomes slow, and a metal oxide is hard to be formed.
As a result, deactivation of the surface of the catalytic metal can
be suppressed, and high catalytic activity can be exhibited.
Namely, the catalytic reaction can be promoted. Also, the catalytic
metal is relatively firmly supported in the pores (mesopores), and
the contact with the electrolyte in the catalyst layer is more
effectively suppressed and prevented. Moreover, elution due to
potential change is prevented, and temporal performance
deterioration can be also suppressed. Therefore, catalytic activity
can be further improved, namely, the catalytic reaction can be more
efficiently promoted.
Meanwhile, the "average particle size of the catalytic metal
particles" or the "average particle radius of the catalytic metal
particles" in the present invention can be determined as the
crystallite radius obtained from the half-band width of the
diffraction peak of the catalytic metal component in the X-ray
diffraction, or an average value of the particle diameter of the
catalytic metal particles examined by using a transmission-type
electron microscope (TEM). The "average particle size of the
catalytic metal particles" or the "average particle radius of the
catalytic metal" herein is a crystallite radius obtained from the
half-band width of the diffraction peak of the catalytic metal
component in the X-ray diffraction.
The content of the catalytic metal per unit catalyst coated area
(mg/cm.sup.2) is not particularly limited so long as sufficient
dispersion degree of the catalyst on the carrier and power
generation performance are obtained, and is, for example, 1
mg/cm.sup.2 or less. However, in the case where the catalyst
contains platinum or a platinum-containing alloy, the platinum
content per unit catalyst coated area is preferably 0.2 mg/cm.sup.2
or less. In the catalyst layer of this embodiment, the electrolyte
coating of the catalytic metal is suppressed so that the activity
per catalyst weight can be enhanced. Accordingly, it is possible to
lower the use amount of an expensive catalyst. The lower limit
value is not particularly limited so long as power generation
performance is obtained, and it is 0.01 mg/cm.sup.2 or more, for
example.
In the present specification, the induction coupled plasma emission
spectrometry (ICP) is used for measuring (confirming) the
"catalytic metal (platinum) content per unit catalyst coated area
(mg/cm.sup.2)". The method for obtaining desired "catalytic metal
(platinum) content per unit catalyst coated area (mg/cm.sup.2)" can
be also easily performed by a person skilled in the art, and the
adjustment of the composition (catalyst concentration) and coating
amount of slurry allows the control of the amount.
In addition, for a case in which the catalytic metal contains
platinum, the platinum content relative to the catalyst is
preferably 20% by weight or more and 60% by weight or less, and
more preferably 20% by weight or more and less than 50% by weight
or less. The supported amount in the above-mentioned range is
preferable by reason of allowing sufficient dispersion degree of
the catalyst components on the carrier, the improvement in power
generation performance, the economic advantages, and the catalytic
activity per unit weight. Here, the "catalyst supporting rate" in
the present invention is a value obtained by measuring the weights
of the carrier before supporting the catalytic metal and the
catalyst after supporting the catalytic metal.
The catalyst layer may contain an additive such as a
water-repellent agent such as polytetrafluoroethylene,
polyhexafluoropropylene or tetrafluoroethylene-hexafluoropropylene
copolymer, a dispersing agent such as a surfactant, a thickener
such as glycerin, ethylene glycol (EG), polyvinyl alcohol (PVA) or
propylene glycol (PG), and a pore-forming material, as
necessary.
The thickness of the catalyst layer (dry film thickness) is
preferably 0.05 to 30 .mu.m, more preferably 1 to 20 .mu.m, and
further preferably 2 to 15 .mu.m. Meanwhile, the above thickness is
applied to both the cathode catalyst layer and the anode catalyst
layer. However, the thicknesses of the cathode catalyst layer and
the anode catalyst layer may be the same or different from each
other.
(Method for Producing Catalyst Layer)
The method for producing the catalyst layer of the present
invention is not particularly limited, and for example, the known
methods such as the method described in JP 2010-21060 A are
applied, or properly modified and applied. Preferable embodiments
will be described below.
First, a catalyst is prepared. The catalyst can be obtained by
supporting a catalytic metal on a catalyst carrier.
The catalyst carrier is preferably obtained by a heat treatment of
a carbon material. According to a heat treatment, a catalyst
carrier having R' value (D'/G intensity ratio) of 0.6 or less can
be obtained.
The BET specific surface area of a carbon material is not
particularly limited, but is preferably 900 m.sup.2/g or more, more
preferably 1000 to 3000 m.sup.2/g, even more preferably 1100 to
1800 m.sup.2/g, and particularly preferably 1200 to 1800 m.sup.2/g
in order for the BET specific surface area of the catalyst carrier
to satisfy the above condition (I). In the case of the specific
surface area as described above, a sufficient gas transportability
(lower gas transport resistance) and performance (supporting a
sufficient amount of the catalytic metal) can be achieved. In
addition, by using a carrier with large BET specific surface area
in particular, more efficient supporting (storing) of the catalytic
metal inside a carrier (in particular, mesopore) can be
achieved.
It is preferable that the carbon material satisfies at least one of
the following constitutions (1) to (3). (1) (a-1) it has pores with
a radius of less than 1 nm (primary pore) and pores with a radius
of 1 nm or more (primary pore); and (a-2) a pore volume of the
pores with a radius of less than 1 nm is 0.3 cc/g carrier or more.
(2) (a-1) it has pores with a radius of less than 1 nm and pores
with a radius of 1 nm or more; and (c-1) the mode radius of pore
distribution of the pores with a radius of less than 1 nm is 0.3 nm
or more and less than 1 nm. (3) (d-1) the mode radius of pore
distribution of the pores with a radius of 1 nm or more is 1 nm or
more and less than 5 nm; and (d-4) a pore volume of the pores with
a radius of 1 nm or more and less than 5 nm is 0.4 cc/g carrier or
more. Furthermore, in (3), it is preferable that (b-2) a pore
volume of the pores with a radius of 1 nm or more and less than 5
nm is 0.8 cc/g carrier or more. More preferred range of the pore
volume of micropores in (a-2), mode radius of pore distribution of
micropores in (c-1), mode radius of pore distribution of pores with
a radius of 1 nm or more in (d-1), pore volume of the pores with a
radius of 1 nm or more and less than 5 nm in (d-4) and the like are
the same as those described in the sections of the catalysts (a) to
(d).
The carbon material is produced by a method described in the
publications such as JP 2010-208887 A or WO 2009/75264 A. The
conditions of the heat treatment for obtaining a carbon material
having desired pores are different depending on the material, and
are properly determined so as to obtain a desired porous structure.
Generally, the high heating temperature brings a tendency for a
mode size of the pore distribution to shift toward the direction of
a large pore diameter. Therefore, such heat treatment conditions
may be determined in accordance with the material while confirming
the porous structure and it can be easily determined by a person
skilled in the art.
The material of the carbon material is not particularly limited, as
long as it contains carbon as a main component. However, a material
allowing easy formation of a catalyst carrier satisfying the
aforementioned R' value or the aforementioned BET specific surface
area is preferable. In addition, the material that can form pores
having pore volume or mode size (primary pores) inside the carrier
and has sufficient specific surface area and sufficient electron
conductivity for supporting the catalyst component inside the
mesopores in a dispersion state is preferable. Specifically,
examples include carbon powder made of carbon black (such as ketjen
black, oil furnace black, channel black, lamp black, thermal black
and acetylene black), and activated carbon. The phrase "the main
component is carbon" indicates that carbon atoms are contained as
the main component, and is a concept including both "consisting
only of carbon atoms" and "consisting substantially of carbon
atoms", and elements except carbon atoms may be contained. The
phrase "consisting substantially of carbon atoms" indicates that
the mixing of approximately 2 to 3% by weight or less of impurities
is allowable.
Furthermore, the average particle size (average secondary particle
size) of the carbon material is not particularly limited, but it is
preferably 20 to 100 nm. From the viewpoint of easy supporting,
catalyst use rate, or the like, the average particle size (average
primary particle size) of the carbon material is 1 to 10 nm, and
preferably 2 to 5 nm. When it is within this range, mechanical
strength is maintained even when the aforementioned pore structure
is formed on the carrier, and the catalyst layer can be controlled
within a suitable range. As for the value of the "average particle
size of a carbon material", unless specifically described
otherwise, a value which is measured by use of an observational
means such as a scanning electron microscope (SEM) and a
transmission electron microscope (TEM), and is calculated as an
average value of particle size of the particles observed in several
to several tens of visual fields is used. Similarly, the "particle
size (diameter)" means, among the lengths of a line going through a
center of a particle connecting any two points on a particle
contour, the longest length.
The conditions for a heat treatment of a carbon material are not
particularly limited, but the treatment is performed so as to
obtain a catalyst carrier satisfying the above constitution (II)
(R' value (D'/G intensity ratio) is 0.6 or less) and the
constitution (I) (BET specific surface area is 900 m.sup.2/g
carrier or more). Specifically, the heat treatment temperature is
preferably 1300.degree. C. or more and 1880.degree. C. or less,
more preferably 1380 to 1880.degree. C., and even more preferably
1400 to 1860.degree. C. For the heat treatment, the temperature
increase rate is preferably 100 to 1000.degree. C./hour and is
particularly preferably 300 to 800.degree. C./hour. The heat
treatment temperature (retention time at a predetermined heat
treatment temperature) is preferably 1 to 10 minutes, and
particularly preferably 2 to 8 minutes. The heat treatment can be
performed in an air atmosphere, or an inert atmosphere such as
argon gas or a nitrogen gas. In such conditions, carbon powder for
satisfying the above specific surface area as the constitution (I)
and the R' value as the constitution (II), or the R value of the
constitution (I), (II), and (II') is conveniently obtained.
Meanwhile, when the heat treatment conditions are less than the
above lower limit (heat treatment conditions are too mild), there
is a possibility that the edge amount of carbon (graphene) is not
sufficiently lowered. On the other hands, when the heat treatment
conditions are more than the above upper limit (heat treatment
conditions are too severe), there is a possibility that
graphitization proceeds too much, and the BET specific surface area
of carbon (graphene) becomes too small.
The resultant obtained by a heat treatment of the carbon material
corresponds to a catalyst carrier.
Subsequently, a catalytic metal is supported on the catalyst
carrier.
The method for supporting the catalytic metal on the catalyst
carrier is not particularly limited. Preferably, the method
includes (i) a step of depositing a catalytic metal on the surface
of the catalyst carrier (deposition step) and (ii) a step of
performing a heat treatment, after the deposition step, to increase
the particle size of the catalytic metal (heat treatment step). The
above method increases the particle size of the catalytic metal by
subjecting the catalytic metal to a heat treatment after
deposition. Therefore, a catalytic metal with a large particle size
can be supported inside the pores (especially mesopores) of the
catalyst carrier.
A preferred embodiment of the method for producing the catalyst
will be described below, but the present invention is not limited
to the following embodiment.
(i) Deposition Step
In this step, the catalyst metal is deposited on the surface of the
catalytic carrier. This step is a known method, and for example, a
method of immersing a catalyst carrier in a precursor solution of
the catalytic metal, followed by reducing is preferably used.
The precursor of the catalytic metal is not particularly limited,
and properly selected depending on the kind of the catalytic metal
to be used. Specifically, chlorides, nitrates, sulfates, chlorides,
acetates and amine compounds of the catalytic metal such as
above-mentioned platinum and the like can be exemplified. More
specifically, chlorides such as platinum chloride
(hexachloroplatinate hexahydrate), palladium chloride, rhodium
chloride, ruthenium chloride and cobalt chloride, nitrates such as
palladium nitrate, rhodium nitrate and iridium nitrate, sulfates
such as palladium sulfate and rhodium sulfate, acetates such as
rhodium acetate, amine compounds such as dinitrodiamine platinum
nitric acid and dinitrodiamine palladium and the like are
preferably exemplified. Also, the solvent used to prepare the
precursor solution of the catalytic metal is not particularly
limited so long as it can dissolve the precursor of the catalytic
metal, and it is properly selected depending on the kind of the
precursor of the catalytic metal to be used. Specific examples
include water, acids, alkalis, organic solvents and the like. The
concentration of the precursor of the catalytic metal in the
precursor solution of the catalytic metal is not particularly
limited, and is preferably 0.1 to 50% by weight and more preferably
0.5 to 20% by weight, when converted in terms of metal.
The reducing agent includes hydrogen, hydrazine, sodium
thiosulfate, citric acid, sodium citrate, L-ascorbic acid, sodium
borohydride, formaldehyde, methanol, ethanol, ethylene, carbon
monoxide, and the like. A gaseous substance at normal temperature
such as hydrogen can also be supplied by bubbling. The amount of
the reducing agent is not particularly limited so long as it is the
amount that can reduce the precursor of the catalytic metal to a
catalytic metal, and the known amount is similarly applicable.
The deposition conditions are not particularly limited so long as
the catalytic metal can be deposited on a catalyst carrier. For
example, the deposition temperature is preferably a temperature
around the boiling point of the solvent, and more preferably a room
temperature to 100.degree. C. Also, the deposition time is
preferably 1 to 10 hours and more preferably 2 to 8 hours. The
precipitation step may be performed while stirring and mixing, as
necessary.
Accordingly, the precursor of the catalytic metal is reduced to a
catalytic metal, and the catalytic metal is deposited (supported)
on the catalyst carrier.
(ii) Heat Treatment Step
In this step, after the (i) deposition step, a heat treatment is
performed to increase the particle size of the catalytic metal.
The heat treatment condition is not particularly limited so long as
it is the condition that can increase the particle size of the
catalytic metal. For example, the heat treatment temperature is
preferably 300 to 1200.degree. C., more preferably 500 to
1150.degree. C., and particularly preferably 700 to 1000.degree. C.
Also, the heat treatment time is preferably 0.02 to 3 hours, more
preferably 0.1 to 2 hours, and particularly preferably 0.2 to 1.5
hours. The heat treatment step may be performed in a hydrogen
atmosphere.
Accordingly, the particle size of the catalytic metal can be
increased in the catalyst carrier (especially, in the mesopores of
the catalyst carrier). Therefore, the catalytic metal particles are
hard to desorb (from the catalyst carrier) to the outside of the
system. Therefore, the catalyst can be more effectively
utilized.
Subsequently, a catalyst ink containing the catalyst obtained
above, a polymer electrolyte and a solvent is prepared. The solvent
is not particularly limited, and the normal solvent used in forming
a catalyst layer can be similarly used. Specific examples include
water, cyclohexanol, lower alcohols with a carbon number of 1 to 4,
propylene glycol, benzene, toluene, xylene and the like. Other than
these, acetic acid butyl alcohol, dimethyl ether, ethylene glycol
and the like may be used as a solvent. These solvents may be used
singly in one kind or in mixed liquid of two or more kinds.
Among them, a water-alcohol mixed solvent with a high content ratio
of water is preferably used as the solvent. It is preferable to use
a mixed solvent with a high content ratio of water as a dispersion
medium, because it can prevent electrolyte from coating the
entrance of mesopores. Here, a mixed weight ratio of water and
alcohol (water/alcohol) is preferably 55/45 to 95/5, and more
preferably 60/40 or more and less than 91/9.
Water is not particularly limited, and tap water, pure water,
ion-exchange water, distilled water and the like can be used. Also,
alcohol is not particularly limited. Specific examples include
methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,
2-methyl-1-propanol, 2-butanol, 2-methyl-2-propanol, cyclohexanol,
and the like. Among them, methanol, ethanol, 1-propanol,
2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol and
2-methyl-2-propanol are preferable. By using such high affinity
lower alcohol, extremely uneven distribution of the electrolyte can
be prevented. Furthermore, among the above alcohols, alcohol with
boiling point of lower than 100.degree. C. is preferably used.
Examples of the alcohol with boiling point of lower than
100.degree. C. include alcohol selected from a group consisting of
methanol (boiling point: 65.degree. C.), ethanol (boiling point:
78.degree. C.), 1-propanol (boiling point: 97.degree. C.),
2-propanol (boiling point: 82.degree. C.), and 2-methyl-2-propanol
(boiling point: 83.degree. C.). The alcohol can be used singly in
only one kind or in a mixture of two or more kinds.
As described above, the polymer electrolyte is roughly classified
into a fluorine-based polymer electrolyte and a hydrocarbon-based
polymer electrolyte, depending on the kind of ion exchange resin
which is a constituent material. Among them, the electrolyte is
preferably a fluorine-based polymer electrolyte. By using a
hydrophobic fluorine-based polymer electrolyte as described above,
the electrolyte is further likely to agglomerate with increasing
water content in the solvent.
The amount of the solvent constituting the catalyst ink is not
particularly limited so long as it is an amount such as to allow
the electrolyte to be completely dissolved. Specifically, the
concentration of the solid matter containing the catalyst powder,
the polymer electrolyte and the like is preferably 1 to 50% by
weight and more preferably about 5 to 30% by weight in the
electrode catalyst ink.
Meanwhile, in the case of using additives such as water-repellent
agent, dispersing agent, thickener and pore-forming material, these
additives may be added to the catalyst ink. In this case, the
addition amount of the additives is not particularly limited so
long as it is an amount such as not to disturb the above effect of
the present invention. For example, the addition amount of each of
the additives is preferably 5 to 20% by weight, with respect to the
whole weight of the electrode catalyst ink.
Next, the catalyst ink is applied on the surface of a substrate. An
application method on the substrate is not particularly limited and
known methods can be used. Specifically, the application can be
performed using a known method such as spray (spray coating)
method, Gulliver printing method, die coater method, screen
printing method, and doctor blade method.
On this occasion, a solid polymer electrolyte membrane (an
electrolyte layer) or a gas diffusion substrate (a gas diffusion
layer) can be used as the substrate onto which the catalyst ink is
applied. In such a case, after forming the catalyst layer on the
surface of a solid polymer electrolyte membrane (an electrolyte
membrane) or a gas diffusion substrate (a gas diffusion layer), an
obtained laminated body may be directly used for producing a
membrane electrode assembly. Alternatively, the catalyst layer may
be obtained by forming the catalyst layer on the substrate which is
a peelable substrate such as polytetrafluoroethylene (PTFE) [Teflon
(registered trademark)] sheet, and then peeling the catalyst layer
portion off the substrate.
Lastly, a coated layer (membrane) of the catalyst ink is dried
under the room atmosphere or an inert gas atmosphere at room
temperature to 150.degree. C. for 1 to 60 minutes. Thus, the
catalyst layer is formed.
[Membrane Electrode Assembly/Fuel Cell]
According to further another embodiment of the present invention, a
membrane electrode assembly for fuel cell containing the above
electrode catalyst layer for fuel cell is provided. Namely, a fuel
cell membrane electrode assembly having a solid polymer electrolyte
membrane 2, a cathode catalyst layer arranged on one side of the
electrolyte membrane, an anode catalyst layer arranged on the other
side of the electrolyte membrane, and a pair of gas diffusion
layers (4a and 4c) which sandwich the electrolyte membrane 2, the
anode catalyst layer 3a and the cathode catalyst layer 3c is
provided. Then, in this membrane electrode assembly, at least one
of the cathode catalyst layer and the anode catalyst layer is the
catalyst layer of the embodiment described above.
However, in consideration of the necessity for the improvement in
proton conductivity and the improvement in the transport property
(the gas diffusion property) of reactant gas (especially O.sub.2),
at least the cathode catalyst layer is preferably the catalyst
layer of the embodiment described above. However, the catalyst
layer according to the above-mentioned embodiment is not
particularly limited; for example, the catalyst layer may be used
as the anode catalyst layer, or as both the cathode catalyst layer
and the anode catalyst layer.
According to further another embodiment of the present invention, a
fuel cell having the membrane electrode assembly of the
above-mentioned embodiment is provided. Namely, an embodiment of
the present invention is a fuel cell having a pair of an anode
separator and a cathode separator which sandwich the membrane
electrode assembly of the above-mentioned embodiment.
The constituents of the PEFC 1 using the catalyst layer according
to the above-mentioned embodiment will be described below with
reference to FIG. 1. However, the characteristics of the present
invention lie in the catalyst layer. Therefore, the specific
constitutions of members except the catalyst layer constituting the
fuel cell may be properly modified with reference to the
conventionally known knowledge.
(Electrolyte Membrane)
The electrolyte membrane consists of, for example, a solid polymer
electrolyte membrane 2 such as can be seen in the constitution
illustrated in FIG. 1. This solid polymer electrolyte membrane 2
has the function of allowing the protons generated in an anode
catalyst layer 3a to be selectively transmitted to a cathode
catalyst layer 3c along the membrane thickness direction during the
operation of a PEFC 1. Also, the solid polymer electrolyte membrane
2 serves as a partition wall to prevent the fuel gas supplied to
the anode side from mixing with the oxidant gas supplied to the
cathode side.
An electrolyte material composing the solid polymer electrolyte
membrane 2 is not particularly limited, and can be properly
referred to the conventionally known knowledge. For example, the
fluorine-based polymer electrolyte and the hydrocarbon-based
polymer electrolyte, which are described as the polymer electrolyte
in the above, may be used. On this occasion, it is not necessary to
use the same as the polymer electrolyte used for the catalyst
layer.
The thickness of the electrolyte membrane may be properly
determined in consideration of the characteristics of the obtained
fuel cell, and is not particularly limited. The thickness of the
electrolyte membrane is ordinarily approximately 5 to 300 .mu.m.
The balance between the strength during the manufacturing process
of the membrane, the durability during usage, and output
performance during use can be properly controlled, when the
thickness of the electrolyte membrane is within such a range.
(Gas Diffusion Layer)
The gas diffusion layers (the anode gas diffusion layer 4a and the
cathode gas diffusion layer 4c) have the function of promoting the
diffusion of the gas (the fuel gas or the oxidant gas) supplied
through the gas passages (6a and 6c) of the separator to the
catalyst layers (3a and 3c), as well as the function as the
electronic conduction path.
A material composing a substrate of the gas diffusion layers (4a
and 4c) is not particularly limited, and can be properly referred
to the conventionally known knowledge. Examples thereof include
sheet-like materials with conductivity and porosity, such as
fabrics made of carbon, paper-like paper-making material, felt and
unwoven fabric. The thickness of the substrate may be properly
determined in consideration of the characteristics of the obtained
gas diffusion layer, and it may be approximately 30 to 500 .mu.m.
When the thickness of the substrate is a value within such a range,
the balance between the mechanical strength and the diffusivity of
gas, water and the like can be properly controlled.
The gas diffusion layer preferably contains water-repellent agent
with the aim of enhancing water repellency to prevent a flooding
phenomenon and the like. Examples of the water-repellent agents
include, but not particularly limited to, fluorine-based polymer
materials such as polytetrafluoroethylene (PTFE),
polyfluorovinylidene (PVdF), polyhexafluoropropylene and
tetrafluoroethylene-hexafluoropropylene copolymer (FEP), as well as
polypropylene and polyethylene.
Also, in order to further improve water repellency, the gas
diffusion layer may be such as to have a carbon particle layer
comprising an aggregate of carbon particles containing the
water-repellent agent (a microporous layer; MPL, not shown in the
drawings) on the catalyst layer side of the substrate.
The carbon particles contained in the carbon particle layer are not
particularly limited, and conventionally known materials such as
carbon black, graphite and expanded graphite may be properly
adopted. Among them, carbon black such as oil furnace black,
channel black, lamp black, thermal black and acetylene black may be
preferably used by reason of having excellent electron conductivity
and large specific surface area. The average particle size of the
carbon particles is preferably approximately 10 to 100 nm. Thus,
high drainage by capillary force is obtained, and the contact with
the catalyst layer also can be improved.
Examples of the water-repellent agent used for the carbon particle
layer include the same as the above-mentioned water-repellent
agent. Above all, the fluorine-based polymer materials may be
preferably used by reason of being excellent in water repellency
and corrosion resistance during the electrode reaction.
The mixing ratio between the carbon particles and the
water-repellent agent in the carbon particle layer should be
approximately 90:10 to 40:60 at weight ratio (carbon
particles:water-repellent agent) in consideration of the balance
between the water repellency and the electron conductivity.
Incidentally, also the thickness of the carbon particle layer is
not particularly limited and may be properly determined in
consideration of the water repellency of the obtained gas diffusion
layer.
(Method for Producing Membrane Electrode Assembly)
The method for producing the membrane electrode assembly is not
particularly limited, and a conventionally known method can be
used. For example, it is possible to use the method of transferring
by means of a hot press or coating the catalyst layer on the solid
polymer electrolyte membrane, drying it, and joining the gas
diffusion layer to it, or the method of preparing two gas diffusion
electrodes (GDE) by p