U.S. patent application number 15/002734 was filed with the patent office on 2016-05-19 for catalyst layer, membrane electrode assembly, and electrochemical cell.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. The applicant listed for this patent is KABUSHIKI KAISHA TOSHIBA. Invention is credited to Yoshihiro Akasaka, Taishi Fukazawa, Shigeru Matake, Wu Mei, Katsuyuki Naito.
Application Number | 20160141631 15/002734 |
Document ID | / |
Family ID | 47911621 |
Filed Date | 2016-05-19 |
United States Patent
Application |
20160141631 |
Kind Code |
A1 |
Mei; Wu ; et al. |
May 19, 2016 |
CATALYST LAYER, MEMBRANE ELECTRODE ASSEMBLY, AND ELECTROCHEMICAL
CELL
Abstract
A method of manufacturing a membrane electrode assembly,
including: forming a catalyst layer precursor containing a mixture
of a catalyst material and a pore-forming material on a substrate
having a flatness of 60% or more; removing the pore-forming
material from the catalyst layer precursor on the substrate,
thereby forming a catalyst layer containing the catalyst material
and having a porosity of 20 to 90% by volume; transferring the
catalyst layer from the substrate to a gas diffusion layer, to
provide an electrode; and bonding the catalyst layer of the
electrode to an electrolyte membrane, to provide a membrane
electrode assembly.
Inventors: |
Mei; Wu; (Yokohama-shi,
JP) ; Matake; Shigeru; (Yokohama-shi, JP) ;
Fukazawa; Taishi; (Tokyo, JP) ; Akasaka;
Yoshihiro; (Kawasaki-shi, JP) ; Naito; Katsuyuki;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Tokyo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
47911621 |
Appl. No.: |
15/002734 |
Filed: |
January 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13628649 |
Sep 27, 2012 |
9276269 |
|
|
15002734 |
|
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|
Current U.S.
Class: |
429/482 ;
429/523; 429/524 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 4/8605 20130101; H01M 2008/1095 20130101; H01M 4/92 20130101;
H01M 8/1004 20130101 |
International
Class: |
H01M 4/92 20060101
H01M004/92; H01M 4/86 20060101 H01M004/86; H01M 8/1004 20060101
H01M008/1004 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2011 |
JP |
2011-213299 |
Claims
1-6. (canceled)
7: A method of manufacturing a membrane electrode assembly, the
method comprising: forming a catalyst layer precursor comprising a
mixture of a catalyst material and a pore-forming material on a
substrate having a flatness of 60% or more; removing the
pore-forming material from the catalyst layer precursor on the
substrate, thereby forming a catalyst layer comprising the catalyst
material and having a porosity of 20 to 90% by volume; transferring
the catalyst layer from the substrate to a gas diffusion layer, to
provide an electrode; and bonding the catalyst layer of the
electrode to an electrolyte membrane, to provide a membrane
electrode assembly.
8: The method of claim 7, further comprising: bonding a second
catalyst layer of a second electrode to the electrolyte
membrane.
9: The method of claim 7, wherein the substrate has a flatness of
70% or more.
10: The method of claim 7, wherein the catalyst layer has a single
layer structure satisfies a relation:
R.sub.1.gtoreq.R.sub.0.times.1.2, wherein: R.sub.1 is an alignment
ratio of the catalyst layer; and R.sub.0 is an alignment ratio of
the catalyst material in powder form having a random crystalline
plane distribution, wherein each of the alignment ratios is
calculated from a X-ray diffraction spectrum having a diffraction
angle 2.theta. range from 10 to 90 degree measured using
Cu-K.alpha.-rays, and is defined as a ratio of a diffraction peak
area contributed by the most closely packed crystalline planes of a
material to a total area of all diffraction peaks of the same
material at the 20 range from 10 to 90 degree.
11: The method of claim 7, wherein a ratio of pores having a pore
diameter ranging from 5 to 100 nm to all pores in the catalyst
layer is 50% by vol or more.
12: The method of claim 10, wherein a spacing of the most closely
packed crystalline planes is in a range from 95 to 98% of a
corresponding spacing of the catalyst material in powder form with
random crystalline plane distribution.
13: The method of claim 7, wherein the catalyst layer comprises 30
at % or more of platinum or iridium.
14: The method of claim 7, wherein the catalyst layer comprises an
alloy having a composition of formula (I):
Pt.sub.yRu.sub.zT.sub.1-y-z (I), wherein: 0.2.ltoreq.y.ltoreq.0.8;
0.ltoreq.z.ltoreq.0.8; and T is at least one element selected from
the group consisting of W, Hf, Si, Mo, Ta, Ti, Zr, Ni, Co, Nb, V,
Sn, Al, and Cr.
15: The method of claim 7, wherein the catalyst layer comprises an
alloy having a composition of formula (II): Pt.sub.uM.sub.1-u (II),
wherein: 0<u.ltoreq.0.9; and M is at least one element selected
from the group consisting of Co, Ni, Fe, Mn, Ta, W, Hf, Si, Mo, Ti,
Zr, Nb, V, Cr, Al, and Sn.
16: The method of claim 7, wherein a thickness of the catalyst
layer is from 20 nm to 10 .mu.m.
17: The method of claim 10, wherein the catalyst material comprises
Pt, and each of the alignment ratios is calculated with the
following equation: R=an area of (111)/[an area of(111)+an area of
(200)+an area of (311)] wherein the peaks (111), (200), and (311)
are presented in the range from 10 to 90 degrees of the diffraction
angle 2.theta..
18: The method of claim 7, wherein the catalyst layer precursor
comprising the mixture of the catalyst material and the
pore-forming material is sputtered on the substrate.
19: A method of manufacturing an electrochemical cell, the method
comprising: sandwiching a membrane electrode assembly obtained by
the method of claim 8 with a first separator and a second
separator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2011-213299, filed
Sep. 28, 2011, the entire contents of which are incorporated herein
by reference.
FIELD
[0002] Embodiments described herein relate generally to a catalyst
layer, a membrane electrode assembly, and an electrochemical
cell.
BACKGROUND
[0003] Studies of electrochemical cells have been enthusiastically
made in recent years. For example, a fuel cell among
electrochemical cells includes a system configured to
electrochemically react with the fuel such as hydrogen with an
oxidizer such as oxygen to generate electric power. Among these
fuel cells, a polymer electrolyte membrane fuel cell (PEFC) is
operable at a lower temperature than other fuel cells and produces
water as a reaction product so that it is clean to environment and
has been therefore put to practical use as power sources for
household stationary use and for vehicles.
[0004] In a catalyst layer contained in each electrode of PEFC, a
carbon-supported catalyst obtained by supporting a catalyst
material on a carbon black support is generally used.
[0005] When PEFC is used, for example, as a power source for
vehicles, the carbon support contained in the catalyst layer is
corroded by start and stop operations, and also, the catalyst
supported on the carbon support is itself also dissolved. It is
reported that this promotes the deterioration of the catalyst layer
and the membrane electrode assembly (MEA) including the catalyst
layer. For this, it is desired to improve the durability of the
catalyst layer.
[0006] In light of this, the adoption of a carbonless catalyst
layer formed by sputtering or vapor deposition of catalyst material
is examined. For example, there is a catalyst layer obtained by
sputtering a whisker substrate with platinum. There is also a
catalyst layer including a void layer obtained by stacking a
catalyst material layer and a pore-forming material layer
alternately on each other and then, by dissolving/removing the
pore-forming material layer. The deterioration of the catalyst
support caused by corrosion can be avoided by the use of the
carbonless catalyst layer like this. However, the resistance of
these catalysts to dissolution is still insufficient and therefore
needs to be further improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a cross-sectional view of a membrane electrode
assembly according to an embodiment;
[0008] FIG. 2 is a view showing an image of a catalyst layer
according to an embodiment observed by a scanning type electron
microscope;
[0009] FIG. 3 is a view showing an image of a catalyst layer
according to an embodiment observed by a scanning type electron
microscope;
[0010] FIG. 4 is a view schematically showing a method of measuring
X-ray diffraction used for a catalyst layer according to an
embodiment;
[0011] FIG. 5 is a view showing XRD spectrums of a catalyst layer
according to an embodiment and a powdered catalyst material;
[0012] FIG. 6 is a view schematically showing an example of an
electrochemical cell according to an embodiment; and
[0013] FIG. 7 is a view showing the outline of a dissolution
resistance test.
DETAILED DESCRIPTION
[0014] In general, according to one embodiment, there is provided a
catalyst layer containing a catalyst material. The catalyst layer
satisfying requirements below:
[0015] a porosity of 20 to 90% by vol; and
[0016] a relation R.sub.1.gtoreq.R.sub.0.times.1.2.
[0017] In the above inequality, R.sub.1 represents an alignment
ratio (R) of the catalyst layer and R.sub.0 represents an alignment
ratio of the catalyst material in powder form in which crystalline
planes of the catalyst material orient randomly. Each of the
alignment ratios is calculated from a X-ray diffraction spectrum
having a diffraction angle 2.theta. range from 10 to 90 degree
measured using Cu-K.alpha.-rays, and is defined as a ratio of the
diffraction peak area contributed by the most closely packed
crystalline planes of a material to the total area of all
diffraction peaks of the same material at the 28 range from 10 to
90 degree.
[0018] Embodiments of the present invention are explained below in
reference to the drawings. In each of the drawings, portions which
exhibit the same or similar functions are indicated by the same
reference numerals and any overlapping descriptions are
avoided.
[0019] FIG. 1 is a cross-sectional view of a membrane electrode
assembly according to the embodiment.
[0020] The membrane electrode assembly (hereinafter also referred
to as MEA) 1 is provided with a first electrode 2, a second
electrode 3, and an electrolyte membrane 4. The electrolyte
membrane 4 is inserted between the first electrode 2 and the second
electrode 3. On one surface of the electrolyte membrane 4, a first
catalyst layer 5 and a first gas diffusion layer 6 are stacked in
this order to form a first electrode 2. Further, on another surface
of the electrolyte membrane 4, a second catalyst layer 7 and a
second gas diffusion layer 8 are stacked in this order to form a
second electrode 3.
[0021] First, the catalyst layer according to the embodiment will
be explained. Although it is enough that at least one of the first
electrode 2 and second electrode 3 includes the catalyst layer
having a structure explained below, the catalyst layer 5 and
catalyst layer 7 both preferably have the following structure. When
the first and second catalyst layers 5 and 7 both have the
following structure, the catalyst materials contained in the first
and second catalyst layers 5 and 7 may be the same or
different.
[0022] Hereinafter, as the term indicating one of the first and
second catalyst layers 5 and 7 or both of the first and second
catalyst layers 5 and 7, the term "catalyst layer 5, 7" is also
used.
[0023] The catalyst layer 5, 7 according to the embodiment contains
a catalyst material. The catalyst material contains a metal or two
or more elements selected from the group consisting of precious
metals such as Pt, Ru, Rh, Os, Ir, Pd, and Au. Such a catalyst
material is superior in catalyst activity, conductivity, and
stability.
[0024] According to other example, the catalyst material may be a
complex oxide or a mixture oxide containing oxides of two or more
metals selected from the above group.
[0025] When the catalyst layer 5, 7 is used in hydrogen oxidation
reaction or hydrogen generation reaction, the catalyst layer 5, 7
contains, for example, Pt.
[0026] When the catalyst layer 5, 7 is used in oxidation reaction
of a reformed hydrogen gas containing CO or alcohol such as
methanol and ethanol, the catalyst layer 5, 7 contains an alloy
having a composition represented by the formula
Pt.sub.yRu.sub.zT.sub.1-y-z. Here, 0.2.ltoreq.y.ltoreq.0.8 and
0.ltoreq.z.ltoreq.0.8, and the element T is at least one element
selected from the group consisting of W, Hf, Si, Mo, Ta, Ti, Zr,
Ni, Co, Nb, V, Sn, Al, and Cr. In this case, the alloy contains 20
to 80 at % of Pt, 0 to 80 at % of Ru, and 0 to 80 at % of element
T.
[0027] When the catalyst layer 5, 7 is used in oxygen reduction
reaction, the catalyst layer 5, 7 contains an alloy having a
composition represented by, for example, the formula
Pt.sub.uM.sub.1-u. Here, u is defined by the following equation:
0<u.ltoreq.0.9, the element M is at least one element selected
from the group consisting of Co, Ni, Fe, Mn, Ta, W, Hf, Si, Mo, Ti,
Zr, Nb, V, Cr, Al, and Sn. In this case, the alloy contains greater
than 0 at % to 90 at % or less of Pt, and 10 at % or more to less
than 100 at % of the element M.
[0028] When the catalyst layer 5, 7 is used in oxygen generation
reaction, the catalyst layer 5, 7 contains an oxide of at least one
metal selected from the group consisting of Ir, Ru, Pt, Au, Rh, and
Os, or a complex oxide of these metal oxides and an oxide of Ta or
Sn.
[0029] The catalyst layer 5, 7 preferably contains platinum or
iridium in a ratio of 30 at % or more (i.e. 30 to 100 at %). Such a
composition enables the compatibility between the activity and
durability of a catalyst material.
[0030] FIGS. 2 and 3 each shows a view showing an image of a
catalyst layer according to the embodiment observed at a
magnification of 200,000 by a scanning electron microscope (SEM).
As shown in FIGS. 2 and 3, each of the catalyst layer 5, 7 has a
porous structure including pores although a structure of the
catalyst layer depends on a process of fabricating and a
composition of the catalyst layer. A structure of the catalyst
layer shown in FIG. 2 exhibits large holes while a structure of the
catalyst layer shown in FIG. 3 has a structure in which the
catalyst materials are connected each other compactly as one.
[0031] The catalyst layer 5, 7 has a porosity of 20 to 90% by vol.
If the porosity is designed to be excessively small, there is the
case where the supply of fuel, removal of reaction products, and
transfer of protons do not work smoothly. If the porosity is
designed to be excessively large, the catalyst layers 5, 7 are
easily broken and there is therefore the case where only
insufficient resistance to dissolution is obtained. The catalyst
layers 5, 7 preferably have a porosity of 40 to 90% by vol. Such a
large porosity is advantageous in increasing the specific surface
areas of the catalyst layers 5, 7 to thereby obtain high catalyst
activity.
[0032] In one example, the ratio of pores having a pore diameter
ranging from 5 to 100 nm to all pores is 50% by vol or more (i.e.
50 to 100% by vol). When this ratio is high, mass transfer in the
catalyst layers 5, 7 is promoted. The volume and ratio of pores in
the catalyst layers 5, 7 can be controlled by, for example, the
particle diameter and amount of pore-forming material which will be
explained later and sputtering conditions.
[0033] The thickness of the catalyst layers 5, 7 is, for example,
20 nm to 10 .mu.m.
[0034] The catalyst layer 5, 7 according to the embodiment may be
stacked prior to use.
[0035] When the catalyst layers are stacked, a void layer or a
fiber layer may be inserted between the catalyst layers. In this
case, the average thickness of each catalyst layer is preferably 20
to 200 nm. If the average thickness is too low, this causes
increase in production cost and it is difficult to obtain long-term
dissolution resistance. If the average thickness is too high on the
other hand, there is the possibility of deterioration in the
characteristics of an electrochemical cell because the amount of
fuel to be supplied per specific surface of the catalyst layer is
reduced.
[0036] The void layer is a vacant spacing between the catalysts
layers. Further, the fiber layer is made of carbon fibers, carbon
nanofibers, or carbon nanotubes. The fiber layer is formed such
that it has a porosity of 50% or more.
[0037] The average thickness of the void layer or fiber layer is
preferably 10 to 500 nm. When the average thickness is too low,
there is the possibility of insufficient fuel supply and
unsatisfactory removal of products obtained by electrode reaction.
If the average thickness is too high on the other hand, the
characteristic improvement obtained by the introduction of the void
layer or fiber layer is reduced, and also, there is the possibility
of increased production cost.
[0038] Moreover, there is the possibility that the layer containing
the catalyst is dissolved when pores are formed.
[0039] When the catalyst layers are stacked as mentioned above,
mass transfer can be more promoted than in the case of disposing
only one thick catalyst layer.
[0040] The catalyst layer 5, 7 satisfy a relation
R.sub.1.gtoreq.R.sub.0.times.1.2. In the above inequality, R.sub.1
represents a alignment ratio of the catalyst layer and R.sub.0
represents a alignment ratio of the catalyst material in powder
form in which crystalline planes of the catalyst material orient
randomly. Here, the catalyst material in powder form means the
catalyst material constituting the catalyst layer 5, 7.
(hereinafter also referred to as "powdered catalyst material).
[0041] More specifically, the alignment ratio is calculated from a
X-ray diffraction spectrum having a diffraction angle 2.theta.
range from 10 to 90 degrees measured using Cu-K.alpha.-rays, and is
defined as a ratio of the diffraction peak area contributed by the
most closely packed crystalline planes of a material to the total
area of its all diffraction peaks at the 28 range from 10 to 90
degrees.
[0042] The Cu-K.alpha.-rays are X-rays having a wavelength of
0.15418 nm and a spectrum obtained by measuring X-ray diffraction
using these X-rays is also hereinafter called "XRD spectrum".
[0043] The ratio R.sub.1 is preferably 1.3 times or more of the
ratio R.sub.0.
[0044] Although the reason of the high durability of the catalyst
layer 5, 7 according to the embodiment has not been clarified
completely, it is inferred that, because the ratio R.sub.1 is
sufficiently larger than the ratio R.sub.0 as mentioned above,
there is a good alignment of different crystalline grains and a
high percentage of the closely packed crystalline planes of the
catalyst material in the surfaces facing another electrode in the
catalyst layer 5, 7 according to the embodiment. The good alignment
improves surface homogeneity, which, along with the closely packed
crystalline planes in the surface helps to suppress catalyst
dissolution.
[0045] The following explanations are furnished as to a method of
measuring XRD spectrums of the catalyst layer and powdered catalyst
material.
[0046] FIG. 4 is a view schematically showing a method of measuring
X-ray diffraction used for a catalyst layer according to the
embodiment.
[0047] In the measurement of the XRD spectrum of the catalyst
layer, a plate sample obtained by forming the catalyst layer 5, 7
on a substrate 40 is used. X-rays are applied to the sample as
shown in FIG. 4 to conduct X-ray diffraction measurement, thereby
obtaining XRD spectrums.
[0048] On the other hand, the XRD spectrum of the powdered catalyst
material can be obtained in the following manner. First, the
catalyst material with the same composition is prepared by
arc-melting method or sintering method or the like and ground to
obtain a powder sample having an average particle diameter of 50
.mu.m or less. Also, the powder sample may be obtained by using
carbon black as a support and by sputtering the catalyst on the
surface of stirring carbon black. The powder material produced in
the above methods has a random distribution of crystalline grain
and crystalline planes. Then, X-ray diffraction of the sample is
measured to obtain an XRD spectrum. In the case where the ASTM card
of the catalyst material is present and the material described in
the ASTM card is a powder, the XRD spectrum described in the ASTM
card may be used as the XRD spectrum of the powdered catalyst
material.
[0049] When the catalyst layer is formed from two or more types of
catalyst materials, it is only necessary that the ratio R.sub.1 of
at least one type of catalyst material is 1.2 times or more of the
ratio R.sub.0.
[0050] FIG. 5 is a view showing XRD spectrums of a catalyst layer
according to an embodiment and a powdered catalyst material. FIG. 5
is an XRD spectrum when using a catalyst material containing Pt,
wherein a main peak derived from Pt is present in a range of a
diffraction angle 2.theta. ranging from 35 to 90 degrees in the XRD
spectrum in FIG. 5 showing the range of a diffraction angle
2.theta. ranging from 10 to 90 degrees.
[0051] Pt exhibits a face centered cubic (fcc) structure. For the
powdered catalyst material and the catalyst layer according to the
embodiment the peak between 36 to 44 degrees in FIG. 5 is regarded
as (111) peak, the most closely packed crystalline planes of Pt.
When main peaks (111), (200) and (311) are presented in the range
from 10 to 90 degrees of a diffraction angle 2.theta., the
alignment ratio (R) is calculated using the following equation.
R=an area of (111)/[an area of (111)+an area of (200)+an area of
(311)]
In an example shown in FIG. 5, the ratio R.sub.1 obtained for the
spectrum S1 is about 1 because few peaks other than (111) are
found. On the other hand, the ratio R.sub.0 obtained for the
spectrum S0 is less than 0.5 because large peaks (200) and (311)
are found other than (111) and then R.sub.0 is calculated from the
integrated intensities of each peak using the above equation.
Accordingly the ratio R.sub.1 is about two or more times of the
ratio R.sub.0.
[0052] It is preferable that the spacing of the most closely packed
planes in the catalyst layer 5, 7 is, for example, in a range from
95 to 98% of the one in its corresponding powdered catalyst
material. It is considered that the shorter spacing means a
stronger atomic bonding energy, and thus improve the resistance to
the catalyst dissolution. When the spacing is in the above range,
the durability can be improved with maintaining the activity of the
catalyst. Moreover, the oxygen reduction activity of platinum can
be improved by slightly reducing the lattice spacing. This reason
is considered to be that the adhesion of reactant to and desorption
of reaction products from the surface of the catalyst are well
balanced.
[0053] The catalyst layer 5, 7 is manufactured by forming a layer
including a catalyst material and a pore-forming material as a
catalyst layer precursor and by removing the pore-forming material
from this catalyst layer precursor. More specifically, the catalyst
layer 5, 7 is manufactured in the following procedures.
[0054] First, a catalyst layer precursor including a mixture of a
catalyst material and a pore-forming material is formed on a
substrate by sputtering or vapor deposition. Specifically, the
catalyst material and pore-forming material are simultaneously
formed on the substrate by sputtering or vapor deposition. Or, a
target obtained by mixing the catalyst material with the
pore-forming material is used to carry out sputtering or vapor
deposition.
[0055] As the pore-forming material, one having higher solubility
in a washing solution used in acid washing or alkali washing which
will be explained later than the catalyst material is used.
Although the pore-forming material is a metal or metal oxide, a
metal is preferable because it generally has a high forming rate
and can be removed in a short time and generally has a good
operability and a low fabrication cost. As the metal to be used as
the pore-forming material, at least one metal selected from the
group consisting of Mn, Fe, Co, Ni, Zn, Sn, Al, and Cu is
preferable. Particles of ceramics such as oxides or nitrides may
also be used. In the following explanations, the pore-forming
material is regarded as a pore-forming metal.
[0056] The structure and stability of an oxide can be controlled by
introducing oxygen into the atmosphere in the sputtering or vapor
deposition. At this time, the partial pressure of the oxygen in the
atmosphere is preferably designed to be less than 20%. There is the
case where not all the pore-forming metal can be removed in the
process of removing the pore-forming metal from the catalyst layer
precursor which will be explained later. For this, the ratio
occupied by the pore-forming metal in the catalyst layer precursor
may be designed to be higher than the target porosity.
[0057] As the substrate, one having a flatness of 60% or more and
preferably 70% or more is used. Here, a reference plane is defined
as the plane parallel to the surface of a substrate viewed
macroscopically. And the flatness is defined as the ratio of the
area of the orthographic projection on the reference plane
contributed from a flat region with an angle of 10 degrees or less
to the reference plane of the substrate surface to the area of the
orthographic projection on the reference plane of the whole
substrate surface. The flatness of the substrate on which the
catalyst film is formed affects the growth and orientation of metal
or alloy crystals. When the flatness is too low, it is difficult to
raise the ratio of R.sub.1 to R.sub.0. For example, a carbon sheet
or Teflon (trademark) sheet is used as the substrate.
[0058] In succession, the pore-forming metal is dissolved, for
example, by washing using an acid or alkali solution, and/or by the
electrolytic method to remove the metal from the catalyst layer
precursor. A catalyst layer having pores is obtained in this
manner.
[0059] When an acid solution is used as the washing solution, the
catalyst layer precursor can be dipped in, for example, nitric
acid, hydrochloric acid, sulfuric acid, or mixture solution of
these acids for about 5 min to 50 hr. At this time, the acid
solution is heated to a temperature of about 50 to 100.degree. C.
Further, bias voltage may be applied to promote the dissolution of
the pore-forming metal. Moreover, heat treatment may be performed
after these treatments.
[0060] In order to suppress the dissolution of the catalyst
material during the dissolution of the pore-forming metal, a
process of fixing the catalyst layer precursor to the substrate may
be performed in advance. For example, the catalyst layer precursor
may be impregnated with a polymer solution such as Nafion
(manufactured by Du Pont), then dried before conducting the
pore-forming metal removal.
[0061] A polymer solution such as Nafion (manufactured by Du Pont)
may be added to the obtained catalyst layer by spraying or
impregnation method to improve the proton conductivity of the
catalyst layer and the adhesion to other members.
[0062] As mentioned above, a part of the pore-forming metal may be
left in the resulting catalyst layer 5, 7. The residual
pore-forming metal forms a stable oxide and is considered to
contribute, for example, to the restriction on the growth of the
catalyst material, maintenance of the structure of the catalyst
layer, and promotion of proton conductivity.
[0063] Next, materials other than the catalyst layer will be
explained with reference to FIG. 1.
[0064] The electrolyte membrane 4 contains, for example, an
electrolyte having proton conductivity. This electrolyte membrane 4
has the function of conducting protons derived from the fuel
supplied to the first electrode 2 to the second electrode 3. As the
electrolyte having proton conductivity, fluororesins having a
sulfonic acid group (for example, Nafion (manufactured by Du Pont),
Flemion (manufactured by Asahi Glass Co., Ltd.), and Aciplex
(manufactured by Asahi Kasei Corporation) or inorganic materials
such as tungstic acid or phosphorous-tungstic acid may be used. The
thickness of the electrolyte membrane 4 may be properly determined
in consideration of the characteristics of the obtained MEA 1. The
thickness of an electrolyte membrane 4 is preferably 5 to 300 .mu.m
and more preferably 10 to 150 .mu.m from the viewpoint of strength,
dissolution resistance, and output characteristics of MEA 1.
[0065] When MEA 1 is used in a fuel cell, the first electrode 2 and
second electrode 3 are an anode and a cathode respectively and
hydrogen is supplied to the anode and oxygen/Air is supplied to the
cathode. The first electrode 2 and second electrode 3 may
optionally contain a first gas diffusion layer 6 and a second gas
diffusion layer 8 respectively. Hereinafter, as the term indicating
one of the first and a second gas diffusion layer 6 and 8 or both
of the first and second gas diffusion layers 6 and 8, the term "gas
diffusion layer 6, 8" is also used.
[0066] The gas diffusion layer 6, 8 preferably contains a water
repellent. The water repellent promotes the water-repellency of the
gas diffusion layer 6, 8 to thereby prevent the occurrence of the
so-called flooding phenomenon that the water created by power
generation is not discharged from the inside of the catalyst layer
5, 7 to thereby causes clogging by water. Examples of the
water-repellent include fluorine type polymer materials such as a
polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),
polyhexafluoropropylene, and
tetrafluoroethylene-hexafluoropropylene copolymer (FEP). The
water-repellent agent may be introduced into the gas diffusion
layer 6, 8 after the catalyst layer 5, 7 is formed.
[0067] According to another embodiment, there is provided a
membrane electrode assembly including: a first electrode and a
second electrode, at least one of which containing the catalyst
layer described above; and an electrolyte membrane inserted between
the first electrode and the second electrode.
[0068] MEA according to the embodiment is manufactured by the
following procedures.
[0069] First, the catalyst layer 5, 7 according to the embodiment
are transferred to the gas diffusion layer 6, 8 or to the
electrolyte membrane 4. These layers and electrolyte membrane are
stacked as shown in FIG. 1 and are bonded by heating and pressing
to obtain MEA 1.
[0070] The above materials are generally bonded by using a hot
press machine. A press temperature is lower than a glass transition
temperature of the polymer electrolyte to be used as the binder in
the electrodes 2 and 3 and electrolyte membrane 4 and is generally
100 to 400.degree. C. A press pressure is generally 5 to 200
kg/cm.sup.2 though depending on the hardness of the electrodes 2
and 3.
[0071] when the catalyst layer 5, 7 is formed on the gas diffusion
layer 6, 8 or on the electrolyte membrane 4 directly, there is the
possibility that the catalyst layer having a larger ratio R.sub.1
as mentioned above cannot be obtained.
[0072] In MEA 1 according to the embodiment, the catalyst layer 5,
7 has high resistance to dissolution.
[0073] According to another embodiment, there is provided an
electrochemical cell including: the membrane electrode assembly
described above; and a first separator and a second separator which
sandwich the membrane electrode assembly therebetween.
[0074] FIG. 6 is a view schematically showing an example of an
electrochemical cell according to the embodiment. The
electrochemical cell shown here is assumed to be a fuel cell to
explain.
[0075] A fuel cell 100 has a structure in which the membrane
electrode assembly 1 shown in FIG. 1 is sandwiched by a first
separator (specifically, an anode separator) 31 and a second
separator (specifically, a cathode separator) 32. Here, the first
electrode 2 is an anode and the second electrode 3 is a
cathode.
[0076] The anode separator 31 and the cathode separator 32 are each
provided with a passage 20 used to supply air or fuel to MEA 1. A
seal 30 is disposed on each side of the catalyst layers 5 and 7 and
on each side of the gas diffusion layers 6 and 8 to prevent fuel
leakage and oxidizer leakage from the membrane electrode assembly
1.
[0077] A fuel cell 100 is manufactured by stacking single cells
including this MEA 1 and two separators 31 and 32 and by connecting
these single cells in series. A higher electromotive force can be
obtained by using a plurality of MEAs 1. No particular limitation
is imposed on the shape of the fuel cell 100 and an appropriate
shape is properly selected corresponding to the desired
characteristics of a fuel cell such as voltage. Here, the fuel cell
100 is assumed to have a stack structure as shown in FIG. 6 to
explain. However, the fuel cell 100 may have a plane configuration.
Further, the number of single cells to be combined is not limited
to 3 as shown in FIG. 6.
[0078] As the fuel, for example, hydrogen, reformed gas, an aqueous
solution containing at least one selected from the group consisting
of methanol, ethanol, and formic acid may be used.
[0079] Although the above explanations are furnished as to a fuel
cell as one example of the electrochemical cell of the embodiment,
the above electrochemical cell can be an electrolyte cell in
another example. For example, the electrolyte cell contains an
oxygen evolution catalyst electrode in place of the anode as the
first electrode 2.
[0080] According to a further example, the electrochemical cell
according to the embodiment is a micro electro mechanical systems
(MEMS) type electrochemical cell.
[0081] The electrochemical cell according to the embodiment
contains MEA including the catalyst layer having high dissolution
resistance and catalyst activity and is therefore superior in
long-term stability.
EXAMPLE
[0082] Examples and comparative examples will be explained
below.
[0083] <Fabrication of a Catalyst Layer>
Examples 1 to 8
[0084] A layer containing a metal or alloy and a pore-forming
material as shown in Table 1 was formed on a carbon sheet (G347B,
manufactured by Tokai Carbon Co., Ltd.) having a flatness of 70% or
more by sputtering to obtain a catalyst layer precursor. In each of
example, a sputtering parameter for a metal or alloy, and
pore-forming metal were so adjusted that a catalyst loading amount
was 0.1 mg/cm.sup.2 and a porosity is a value shown in Table 1.
[0085] In succession, the obtained catalyst layer precursor was
subjected two times to acid treatment in which the precursor was
dipped at 80.degree. C. in an aqueous 10 wt % sulfuric acid
solution for 30 min, and then, the precursor was washed with pure
water, followed by drying to obtain a catalyst layer.
Comparative Example 1
[0086] A monolayer catalyst layer containing Pt was formed on a
whisker substrate (organic pigment: Pigment Red 149, average
diameter: about 50 nm) by sputtering. At this time, the catalyst
layer was formed such that a loading amount was 0.10
mg/cm.sup.2.
Comparative Example 2
[0087] A carbon paper Toray 060 (manufactured by Toray Industries,
Ltd.) provided with a carbon layer 5 to 50 .mu.m in thickness was
used as a substrate. Layers each containing an alloy having a
composition represented by Pt.sub.0.25Mn.sub.0.75 and layers
containing Mn (pore-forming material) were alternately formed on
the substrate to make a stacked structure. Here, 5 alloy layers and
4 pore-forming layers (thickness of pore-forming layer: 50 nm) were
formed. At this time, the process was performed such that the total
loading amount of the catalyst was 0.10 mg/cm.sup.2. In succession,
the processes for dissolution of the pore-forming metal and washing
and drying of the catalyst layer were performed in the same manner
as in Examples 1 to 8 to obtain a catalyst layer having a stacked
structure.
Comparative Examples 3 and 4
[0088] Using a carbon sheet used in Examples 1 to 8, a layer
containing Pt was formed on the carbon sheet by sputtering to
obtain a catalyst layer precursor. At this time, amounts of the
metal and pore-forming metal, and a sputtering parameters were so
adjusted that a catalyst loading amount was 0.1 mg/cm.sup.2 and a
porosity is a value shown in Table 1. In succession, processes for
dissolution of the pore-forming metal and washing and drying of the
catalyst layer were performed in the same manner as in Examples 1
to 8 to obtain a catalyst layer.
[0089] The catalyst layers obtained in Examples and Comparative
Examples were evaluated in the following procedures.
[0090] <Fabrication of Electrodes>
[0091] Electrodes were fabricated in the following procedures by
using the catalyst layers obtained in the Examples and Comparative
Examples above.
[0092] Each catalyst layer obtained in Examples 1 to 8 and
Comparative Examples 1, 3, and 4 was thermal compression-bonded and
transferred to a carbon paper Toray 060 (manufactured by Toray
Industries, Ltd.) provided with a carbon layer 5 to 50 .mu.m in
thickness on the surface thereof at 150.degree. C. under a pressure
of 20 kg/cm.sup.2 for 5 min to obtain a electrode. Since the
catalyst layer of Comparative Example 2 used a carbon paper
provided with a carbon layer as the substrate, the substrate and
the catalyst layer formed thereon were used as it was as the
electrode.
Comparative Example 5
Pt Standard Electrode
[0093] 2 g of a commercially available particulate Pt catalyst
(Model number: TEC10E50E-HT, manufactured by Tanaka Kikinzoku Kogyo
K.K.), 5 g of pure water, 5 g of a 20% Nafion (manufactured by Du
Pont) solution, and 20 g of 2-ethoxyethanol were sufficiently
stirred to disperse, thereby preparing a slurry. The obtained
slurry was applied to a carbon paper (manufactured by Toray
Industries, Ltd., 350 .mu.m) processed by water-repellent treatment
and dried to obtain a Pt standard electrode having a Pt catalyst
loading density of 0.1 mg/cm.sup.2.
[0094] <Fabrication of MEA>
[0095] A square specimen of 3.2 cm.times.3.2 cm having an area of
about 10 cm.sup.2 which was cut from each electrode fabricated
above was used as a cathode. The Pt standard electrode fabricated
in Comparative Example 5 was used as an anode.
[0096] Nafion 112 (manufactured by Du Pont) was sandwiched between
these electrodes, which were bonded with each other at 125.degree.
C. under a pressure of 30 kg/cm.sup.2 for 10 min by thermal
compression bonding to obtain a MEA.
[0097] <Fabrication of a Single Cell of a Fuel Cell>
[0098] MEA obtained above was sandwiched between a first separator
and a second separator which each includes a passage, to obtain a
single cell of a polymer electrolyte fuel cell.
[0099] The fabricated catalyst layer, MEA, and single cell were
evaluated for the following articles.
[0100] 1. Porosity, Pore Diameter and Pore Diameter
Distribution
[0101] First, each catalyst layer obtained in Examples 1 to 8 and
Comparative Examples 1 to 4 was cut. When the catalyst layer had,
for example, a rectangular form, the center of the short side of
the rectangle was cut in parallel to the long side. Further, the Pt
standard electrode of Comparative Example 5 was also cut in the
same manner. The position of the center of the section was observed
by SEM. It was confirmed that each catalyst layer obtained in
Examples 1 to 8 and Comparative Examples 3 and 4 had a single layer
structure including pores. It was also confirmed that the catalyst
layer of Comparative Example 1 had a dense single layer structure
and the catalyst layer of Comparative Example 2 had a stacked
structure in which catalyst layers including pores and void layers
were alternately laminated. The electrode of Comparative Example 5
was confirmed to have a single layer structure including pores.
[0102] Further, the catalyst layer or the standard electrode were
cut at the upper part, center part, and lower part along the
direction of the thickness and further, each part was observed at 3
positions by SEM. SEM images observed at a magnification of 200,000
in a total of 9 visual fields were obtained and the catalyst
material was distinguished from pores based on the contrast to
calculate the area occupied by the pores in each visual field. The
volume of the pores was calculated based on this area. The ratio of
the volume occupied by the pores in the entire catalyst layer was
calculated in each visual field to determine an average of these
ratios in 9 visual fields as the porosity of the sample.
[0103] Further, in each visual field, the volume of pores having a
pore diameter of 5 to 100 nm and the volume of all pores were
calculated based on their area results. Then, from these values,
the ratio of the pores having a pore diameter of 5 to 100 nm to all
pores was calculated. An average of the ratios obtained in 9 visual
fields was determined as the pore diameter distribution ratio (vol
%).
[0104] In this case, with regard to Comparative Example 2 having a
stacked structure, the void layer present between the catalysts
layers was excluded from the aforementioned pores.
[0105] 2. R.sub.1/R.sub.0 and Crystalline Planes Spacing
[0106] XRD spectrums of the catalyst layers obtained in Examples 1
to 8 and Comparative Examples 1 to 4 were measured by an X-ray
diffraction analyzer using Cu-K.alpha. rays (wavelength: 0.15418
nm). The measurement was made in a diffraction angle 2.theta. range
from 10 to 90 degrees. An XRD spectrum of the Pt standard electrode
obtained in Comparative Example 5 was also measured in the same
manner. Because this Pt standard electrode is constituted of
platinum nano-particle powders and therefore, the distribution of
crystalline planes is random. For this, with regard to the Pt
standard electrode, the XRD spectrum of a powder sample obtained by
grinding the Pt standard electrode was measured. In this case, the
obtained spectrum was similar to the XRD spectrum of the ASTM card
of Pt.
[0107] With regard to each XRD spectrum, the peaks derived from the
catalyst material in catalyst layers were identified to find the
alignment ratio R.sub.1 using the approach explained before.
[0108] On the other hand, powder samples with the same composition
as the catalyst material obtained in Examples 1 to 8 and
Comparative Examples 1 to 4 were prepared. XRD spectra of these
powder samples were likewise measured and the peaks derived from
the catalyst material were identified to find the alignment ratio
R.sub.0 using the approach explained before.
[0109] Then, the ratio of R.sub.1 to R.sub.0 (that is, ratio
R.sub.1/R.sub.0) was found in each of Examples and Comparative
Examples. According to the need, a simple method was used in which
a peak area calculated from peak strength and half-value width was
used in place of the above integrated intensity.
[0110] Further, as to the above catalyst material, the spacing of
its most closely packed crystalline planes was also obtained form
its XRD spectrum and thus the ratio of its spacing to that obtained
from its ASTM card was calculated.
[0111] As to Examples 6 to 8, catalyst powders with the same
composition as those used in Examples 6 to 8 were prepared by
arc-melting method and then, ground into powder having an average
particle diameter of 50 .mu.m or less to produce a powder sample
having a random crystalline plane distribution. This powder sample
was measured by X-ray diffraction to obtain an XRD spectrum, which
was used as the XRD spectrum of each of Examples 6 to 8.
[0112] 3. Single Cell Voltage
[0113] In the obtained single cell, hydrogen was supplied as the
fuel to the anode at a flow rate of 20 ml/min and air was supplied
to the cathode at a flow rate of 50 ml/min and the single cell was
made to discharge at a current density of 1 A/cm.sup.2 to measure
cell voltage (V) after 50 hr. At this time, the single cell was
kept at 50.degree. C. 4. Dissolution resistance The catalyst
dissolution resistance was evaluated according to the load response
durability protocol shown in FIG. 7. The single cell was kept at
70.degree. C. while supplying hydrogen to the anode at a flow rate
of 40 ml/min and nitrogen to the cathode at a flow rate of 40
ml/min. In this condition, a cycle involving (1) a step of
maintaining a voltage of 0.6 V for 5 sec and in succession, (2) a
step of maintaining a voltage of 0.9 V for 5 sec was repeated
30,000 times. Thereafter, cell voltage was measured and compared
with that measured after 50 hr to calculate the deterioration rate.
A sample having a deterioration rate less than 10% was rated as
"dissolution resistance A", a sample having a deterioration rate of
10 to 25% was rated as "dissolution resistance B", and a sample
having a deterioration rate exceeding 25% was rated as "dissolution
resistance C".
[0114] The results obtained for the above evaluation are shown in
Table 1.
TABLE-US-00001 TABLE 1 Pore diameter Pore-forming distribution
Catalyst material material Porosity (%)* R.sub.1/R.sub.0 Example 1
Pt Al 20 50.0 1.6 Example 2 Pt Al 35 60.0 1.4 Example 3 Pt Al 65
70.0 1.3 Example 4 Pt Fe 75 80.0 1.2 Example 5 Pt Fe 90 80.0 1.2
Example 6 Pt0.8 Ni0.1 Ta0.1 Mn 75 70.0 1.3 Example 7 Pt0.7 Co0.3 Mn
40 55.0 1.6 Example 8 Pt0.8 W0.1 Mo0.1 Fe 65 60.0 1.4 Comparative
Example 1 Pt (wisker substrate) -- -- No pore 1.1 Comparative
Example 2 Pt0.25 Mn0.75 (stacked) Mn 40 50.0 1.1 Comparative
Example 3 Pt Al 5 40.0 1.6 Comparative Example 4 Pt Al 95 70.0 1.0
Comparative Example 5 Pt particles -- 70 30.0 1.0 Lattice plane
interval (%) Cell voltage Dissolution resistance Example 1 98.0
0.65 A Example 2 97.0 0.67 A Example 3 96.5 0.65 B Example 4 96.5
0.66 B Example 5 96.5 0.65 B Example 6 96.5 0.68 A Example 7 97.0
0.66 B Example 8 96.5 0.67 B Comparative Example 1 100.0 0.62 C
Comparative Example 2 96.5 0.64 C Comparative Example 3 99.5 0.63 C
Comparative Example 4 96.5 0.65 C Comparative Example 5 100.0 0.63
C *a ratio of pores having a pore diameter ranging from 5 to 100
nm
[0115] The catalyst layers of Examples 1 to 8 each had a ratio
R.sub.1/R.sub.0of 1.2 or more. Further, the spacing of the most
closely packed crystalline planes in the catalyst layer was in the
range from 96.5 to 98.0% of that of the corresponding powder
sample. The cell voltage of the single cell containing each of
these catalyst layers was high and had better dissolution
resistance. Further, in each of the catalyst layers of Examples 1
to 8, a ratio of pores having a pore diameter ranging from 5 to 100
nm to all pores was 50% by vol or more.
[0116] As is clear from Table 1, Examples 1 to 8 each had a higher
cell voltage and dissolution resistance than each of Comparative
Examples 1 to 5 when used for single cells of a fuel cell.
[0117] According to the above embodiments or examples, a catalyst
layer which has sufficient dissolution resistance and high catalyst
activity, and a membrane electrode assembly and an electrochemical
cell provided with the catalyst layer can be provided.
[0118] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *