U.S. patent application number 10/731168 was filed with the patent office on 2004-06-17 for method of fabricating a membrane-electrode assembly.
Invention is credited to Fischer, Andreas, Thate, Sven, Wessel, Helge.
Application Number | 20040112754 10/731168 |
Document ID | / |
Family ID | 32336169 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040112754 |
Kind Code |
A1 |
Thate, Sven ; et
al. |
June 17, 2004 |
Method of fabricating a membrane-electrode assembly
Abstract
The invention relates to a method of fabricating a
membrane-electrode assembly (MEA), particularly for PEM fuel cells,
wherein the MEA comprises a polymer-electrolyte membrane (PEM) with
reaction layers applied to both sides and possibly with gas
distribution layers, and at least one of the reaction layers
includes at least one catalytic component and an electron
conductor, the method comprising the following procedural steps: A)
The introduction of ions of the at least one catalytic component
into the polymer-electrolyte membrane and/or into an ionomer
introduced into the reaction layers, B) the application of the
electron conductor to both sides of the polymer-electrolyte
membrane, C) the electrochemical deposition of the ions of the
catalytic component from the polymer-electrolyte membrane and/or
from the ionomer, introduced into the reaction layers, on the
electron conductor onto at least one side of the
polymer-electrolyte membrane.
Inventors: |
Thate, Sven; (Neuleiningen,
DE) ; Fischer, Andreas; (Mannheim, DE) ;
Wessel, Helge; (Mannheim, DE) |
Correspondence
Address: |
Herbert B. Keil
KEIL & WEINKAUF
1350 Connecticut Ave., N.W.
Washington
DC
20036
US
|
Family ID: |
32336169 |
Appl. No.: |
10/731168 |
Filed: |
December 10, 2003 |
Current U.S.
Class: |
205/102 ;
205/164 |
Current CPC
Class: |
H01M 2008/1095 20130101;
H01M 4/8853 20130101; H01M 8/1004 20130101; H01M 4/8605 20130101;
H01M 4/8817 20130101; C25D 7/00 20130101; H01M 4/881 20130101; Y02E
60/50 20130101; Y02P 70/50 20151101 |
Class at
Publication: |
205/102 ;
205/164 |
International
Class: |
C25D 005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 2002 |
DE |
10257643.2 |
Claims
We claim:
1. A method of fabricating a membrane-electrode assembly (MEA),
particularly for PEM fuel cells, wherein the MEA comprises a
polymer-electrolyte membrane (PEM) with reaction layers applied to
both sides and possibly with gas distribution layers, and at least
one of the reaction layers includes at least one catalytic
component and an electron conductor, the method comprising the
following procedural steps: A) The introduction of ions of the at
least one catalytic component into the polymer-electrolyte membrane
and/or into an ionomer introduced into the reaction layers, B) the
application of the electron conductor to both sides of the
polymer-electrolyte membrane, C) the electrochemical deposition of
the ions of the catalytic component from the polymer-electrolyte
membrane and/or from the ionomer, introduced into the reaction
layers, on the electron conductor onto at least one side of the
polymer-electrolyte membrane.
2. The method as claimed in claim 1, wherein the electrochemical
deposition of the ions of the catalytic component in step C) is
carried out under fuel cell conditions.
3. The method as claimed in claim 2, wherein a variation of
operating conditions is effected during the deposition under fuel
cell conditions.
4. The method as claimed in claim 1, wherein the electrochemical
deposition of the ions of the catalytic component in step C) is
carried out under electrolytic conditions.
5. The method as claimed in claim 4, wherein the electrolytic
conditions comprise the application of a constant or time-variant
DC voltage or an AC voltage.
6. The method as claimed in claim 1, wherein in step C) at least
one element from the 3.sup.rd to 14.sup.th group of the periodic
table of the elements is deposited as the catalytic component onto
the electron conductor on at least one side of the
polymer-electrolyte membrane.
7. The method as claimed in claim 1, wherein in step C) at least
one of the elements Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn,
Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic component on
the cathode-side electron conductor.
8. The method as claimed in claim 1, wherein in step C) at least
one of the elements Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn,
Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic component on
the anode-side electron conductor.
9. The method as claimed in claim 1, wherein the electron conductor
comprises carbon in the form of a bonded fiber web, fibers or
powder.
10. The method as claimed in claim 1, wherein the electron
conductor applied in step B) comprises at least one catalytic
component from the group consisting of Pt, Co, Fe, Cr, Mn, Cu, V,
Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W.
11. The method as claimed in claim 1, wherein in step B), together
with the electron conductor, an ion conductor is applied to at
least one side of the polymer-electrolyte membrane.
12. The method as claimed in claim 1, wherein the catalytic
component in step A) is introduced into the polymer-electrolyte
membrane in an amount of from 0.000005 to 0.05 mmol/cm.sup.2.
Description
[0001] The invention relates to a method of fabricating
membrane-electrode assemblies, particularly for PEM fuel cells,
which comprise catalytically active electrodes.
[0002] Fuel cells are energy converters that convert chemical
energy into electrical energy. In a fuel cell, the electrolytic
principle is inverted. Nowadays, various types of fuel cells are
known which generally differ from one another with respect to their
operating temperature. The design of the cells, however, is in
principle the same for all types. They generally comprise two
electrodes, an anode and a cathode, at which the reactions proceed,
and an electrolyte between the two electrodes. In the case of a
polymer-electrolyte membrane fuel cell (PEM fuel cell) the
electrolyte used is a polymer membrane which conducts ions
(especially H.sup.+ ions). The electrolyte has three functions. It
establishes the ionic contact, prevents electrical contact, and
additionally ensures that the gases fed to the electrodes are kept
separate. As a rule, the electrodes are supplied with gases which
are reacted as part of a redox reaction. The electrodes have the
functions of feeding in the gases (e.g. hydrogen or methanol and
oxygen), of removing reaction products such as water or CO.sub.2,
of catalytically reacting the starting materials and of drawing off
or supplying electrons. The conversion of chemical into electrical
energy takes place at the three-phase boundary of catalytically
active sites (e.g. platinum), ion conductors (e.g. ion exchange
polymers), electron conductors (e.g. graphite) and gases (e.g.
H.sub.2 and O.sub.2). Crucially, the catalysts should have as large
an active surface area as possible.
[0003] In the prior art, the fabrication of PEM fuel cells usually
involves mixing a catalyst supported on carbon black with a
solution or suspension of an ion conductor (ionomer) and applying
the mixture to the ion-conducting membrane. This has the drawback
of not ensuring that the catalyst present on the electron conductor
will indeed have been made completely accessible by the ion
conductor and thus be able to be active.
[0004] Moreover, high (cost-intensive) catalytic loading is often
required, owing to nonoptimal catalyst utilization. This is due to
the fact that a large proportion of the catalysts applied over the
entire area of the membrane remains virtually unutilized, since a
proportion of the catalysts is not made accessible by the ionomer
and the ion conduction in the membrane usually does not take place
over its entire area, but e.g. via ion channels resulting from
phase separation, or via pores of a non-conductive polymer which
are filled with an ion conductor. Catalyst atoms located at the
ends of such channels are able to take a particularly effective
part in the electrochemical reaction. In addition, in
fabricate-reinforced or heterogeneous membranes regions will be
formed which take no part in ion transport and consequently shield
a proportion of the catalysts present in the electrode. That
fraction of the catalyst which is consequently not utilized
contributes to the excessive costs of such a fuel cell.
[0005] The attempt to systematically locate the catalysts solely in
the electrochemical reactive zones is disclosed by U.S. Pat. No.
5,084,144. With this method of enhancing the electrocatalytic
activity of a gas diffusion electrode, a catalytic metal is
electrolytically deposited on the gas diffusion electrode from an
electrolytic fluid. The catalytic metal is therefore not deposited
systematically solely at the ends of the ion channels of the
membrane, but over the entire area. A further drawback of this
method is that expensive electrolytic fluids containing noble
metals are required, which are laborious and expensive to work up.
Moreover, the utilization of the noble-metal catalyst dissolved in
the electrolyte is low. Furthermore, the electrolyte fluid usually
contains not just the desired catalytic ions, but also other ions
which are deposited on the gas diffusion electrode and which
represent contaminants of the deposited catalyst layer.
[0006] A method of electrolytically depositing metals on a solid
electrolyte is disclosed by DE-A 28 21 271, wherein the solid
electrolyte, in the dried state, treated in a solution containing
the metal as a salt, is inserted into an electrolytic cell and is
subjected to an electrolytic process. In so doing, the cell is kept
at a constant current density over a predetermined period. This
results in a continuous surface cutting of this electrolyte, which
means that catalyst atoms are present not only at those sites which
are catalytically active under fuel cell conditions, but on the
entire solid electrolyte surface.
[0007] It is therefore an object of the present invention to
provide a method by which optimal catalytic loading is achieved on
the electrodes of a PEM fuel cell.
[0008] This object is achieved by a method of fabricating a
membrane-electrode assembly (MEA), particularly for PEM fuel cells,
wherein the MEA comprises a polymer-electrolyte membrane (PEM) with
reaction layers applied to both sides and possibly with gas
distribution layers, and at least one of the reaction layers
includes at least one catalytic component and an electron
conductor, the method comprising the following procedural
steps:
[0009] A) The introduction of ions of the at least one catalytic
component into the polymer-electrolyte membrane and/or into an
ionomer introduced into the reaction layers,
[0010] B) the application of the electron conductor to both sides
of the polymer-electrolyte membrane,
[0011] C) the electrochemical deposition of the ions of the
catalytic component from the polymer-electrolyte membrane and/or
from the ionomer, introduced into the reaction layers, on the
electron conductor onto at least one side of the
polymer-electrolyte membrane.
[0012] The polymer-electrolyte membrane in the context of the
present invention is to be understood as meaning either a polymer
membrane serving as the electrolyte or a polymer membrane whose
pores are filled with a substance serving as an electrolyte (e.g.
with an ionomer, acid).
[0013] The at least one membrane-electrode assembly (MEA), composed
of the components electrode/membrane/electrode arranged like a
sandwich, represents the central element of the PEM fuel cell. A
PEM fuel cell usually comprises a stack-like arrangement of a
multiplicity of membrane-electrode assemblies. Each electrode
usually comprises a reaction layer and, in the case of fuel cells
that run on gases, a gas distribution layer.
[0014] The gas distribution layer can serve as a mechanical support
for the electrode and ensures that the respective gas is properly
distributed across the reaction layer and that the electrons are
collected. A gas distribution layer is required, in particular, for
fuel cells operating with hydrogen on the one hand and oxygen or
air on the other hand.
[0015] The reaction layer is where the electrochemical reaction
proper takes place during fuel cell operation. At least one of the
reaction layers contains at least one catalytic component which
catalytically supports e.g. the reaction of oxidation of hydrogen
or of reduction of oxygen. Alternatively, however, the reaction
layers may contain a plurality of catalytic substances having
different functions. In addition, the reaction layer may contain a
functionalized polymer (ionomer) or a nonfunctionalized
polymer.
[0016] Further, an electron conductor in the reaction layers
serves, inter alia, to conduct the electric current which flows
during the fuel cell reaction, and as a support material for
catalytic substances.
[0017] To implement the method according to the invention, ions of
a catalytic component are first of all introduced into the
polymer-electrolyte membrane. In the same way, the ions can
additionally be introduced into the ionomer which may, if required,
have been incorporated into the reaction layer.
[0018] The polymer-electrolyte membrane consists of
cation-conductive polymer materials which hereinafter are referred
to as ionomer. Customarily, a tetrafluorethylenefluorvinyl ether
copolymer having acid functions, especially sulfuric acid groups,
is used. Such a material is commercially available, for example,
under the trade name Nafion.RTM. by E. I. du Pont. Examples of
ionomer materials that can be used in the present invention are the
following polymer materials or mixtures thereof:
[0019] Nafion.RTM. (Dupont; USA)
[0020] perfluorinated and/or partially fluorinated polymers such as
"Dow experimental membrane" (Dow Chemicals, USA),
[0021] Aciplex-S.RTM. (Asashi Chemicals, Japan)
[0022] Raipore R-1010 (Pall Rai Manufacturing Co., USA),
[0023] Flemion (Asahi Glass, Japan)
[0024] Raymion.RTM. (Chlorine Engineering Corp., Japan).
[0025] Alternatively, however, other, especially fluorine-free,
ionomer materials can be used, e.g. sulfonated phenol formaldehyde
resins (linear or crosslinked); sulfonated polystyrene (linear or
crosslinked); sulfonated poly-2,6-diphenyl-1,4-phenylene oxides,
sulfonated polyarylethersulfones, sulfonated polyarylethersulfones,
sulfonated polyaryletherketones, phosphonated
poly-2-6-dimethyl-1,4-phenyl oxides, sulfonated polyetherketones,
sulfonated polyetheretherketones, arylketones or
polybenzimidazoles.
[0026] In addition, those polymer materials are used which include
the following components (or mixtures thereof):
[0027] Polybenzimidazol-phosphoric acid, sulfonated polyphenylenes,
sulfonated polyphenylene sulfide and polymeric sulfonic acids of
the type polymer-SO.sub.3X (X=HH.sub.4.sup.+, NH.sub.3R.sup.+,
NH.sub.2R.sub.2.sup.+, NHR.sub.3.sup.+, NR.sub.4.sup.+).
[0028] In addition to the above-listed polymer materials, the ion
exchange materials used may include further inorganic and/or
organic components (e.g. silicates, minerals, clays, silicones)
which have a positive effect on the properties of the ionic
exchange material (e.g. conductivity).
[0029] Also possible is the use of porous non-conductive polymers
which require their conductivity by the pores being filled with
e.g. an ionomer (for example Goreselect, Gore, USA) or an acid (for
example H.sub.3PO.sub.4, H.sub.2SO.sub.4, methanesulfonic acid, . .
. ).
[0030] The introduction of the ions of a catalytic substance into
the polymer-electrolyte membrane is effected by a technique known
in the prior art. Preferably, the catalytic substance is present
ionically in a solution with which the polymer-electrolyte membrane
is impregnated. In the process, ion exchange causes the ions of the
catalytic substance to bind to the membrane, e.g. in Nafion.RTM. to
bind to ionic SO.sub.3H groups.
[0031] In the case of acid-filled membranes, it is possible to mix
the acid with the catalytic substance for the purpose of
introducing the ions of the catalytic substance into the
membrane.
[0032] In one embodiment of the present invention, the diffusion of
the ions of the catalytic substance into the polymer-electrolyte
membrane is promoted by an external electric field being
applied.
[0033] The next step B) in the method according to the invention is
the application of the electron conductor to both sides of the
polymer-electrolyte membrane. This purpose can be served by a
technique known in the prior art, for example a dry or wet spray
technique with the aid of which the electron conductor present as a
powder or possibly dissolved in an ionomer solution is sprayed
directly onto the membrane or onto a support, followed by optional
hot compression bonding to the membrane. Further options of
applying include e.g. screen printing or sintering followed by
optional hot compression-bonding to the membrane. Also conceivable
is the introduction of ions of the catalytic component into an
ionomer incorporated into the electron conductor layer.
[0034] Prior to the next step C) of the method according to the
invention, the at least one membrane-electrode assembly is largely
finished and installed in an apparatus which allows an electric
current to be impressed or reactants (e.g. H.sub.2/O.sub.2) to be
fed in while at the same time an electric current is tapped off.
Also conceivable would be a continuous procedure, in which step C)
of the method according to the invention is carried out, and the at
least one membrane-electrode unit as processed is then installed in
a PEM fuel cell.
[0035] In the membrane-electrode assembly installed, prior to step
C), in a PEM fuel cell or in another apparatus suitable for
performing the electrochemical deposition, the catalytic substance
introduced into the membrane in step A) is present in the form of
ions bound within the membrane (for example to its negatively
charged sulfone groups). These are deposited electrochemically, in
step C) of the method according to the invention, from the
polymer-electrolyte membrane onto the electron conductor on at
least one side of the polymer-electrolyte membrane. The
electrochemical deposition is to be understood, in this context, as
the deposition of the catalytic components while chemical energy is
converted into electrical energy or vice versa, the mechanism being
ion migration within the membrane and an electrode reaction taking
place.
[0036] An advantage of the method according to the invention is
that as a result of the electrochemical deposition of the catalytic
component from the membrane, said catalytic component can be
deposited only where the electrochemically active three-phase
boundary is also present. The catalytic component is therefore
deposited specifically onto the electron conductor in those
locations where the ion channels of the membrane terminate. As a
result, a continuous layer of the catalytic component is not
formed, the catalyst instead being deposited only at those plates
where it is optimally utilized. The result is an effective
reduction in catalytic loading without a decrease in the fuel cell
performance. For example it is possible to operate fuel cells
having a Pt loading of the electron conductor of less than 1
mg/cm.sup.2. The reduction in the catalytic loading advantageously
means a cost reduction for MEA fabrication, since the catalytic
components used are often metals. Since the catalytic component in
the present invention is present in finely dispersed form as ions
in the interior of the polymer-electrolyte membrane, another point
is that, in contrast to the deposition of catalyst ions from a
solution, with the method according to the invention no impurities
in the form of other undesirable ions are deposited on the electron
conductor, but only the catalyst ions present in the membrane. A
further advantage of the method according to the invention is the
small number of procedural steps for fabricating a
polymer-electrolyte membrane fuel cell. This too has a positive
effect on costs. These possible cost reductions make the use of PEM
fuel cells more attractive for wired commercial use, for example in
fuel cell vehicles or in stationary fuel cell systems for domestic
power supplies.
[0037] In a preferred embodiment of the present invention, the
electrochemical deposition of the ions of the catalytic component
in step C) of the method according to the invention is effected by
operating an apparatus which allows an electrical current to be
tapped off and fuel cell reactants to be fed in under fuel cell
conditions, e.g. by operating a PEM fuel cell on the fuel
conditions. By varying the operating conditions during the
deposition (load cycle, current, voltage, gas composition,
temperature, pressure etc.) it is possible for the result of the
deposition (dispersity, particle size) to be systematically
controlled.
[0038] If the electrochemical deposition is carried out e.g. under
H.sub.2/O.sub.2, air fuel cell conditions, gas distribution layers
are required which must be applied to the reaction layers or to the
respective electron conductor before step C) of the method
according to the invention is carried out. The gas distribution
layers (for example bonded carbon fiber web (E-Tek carbon cloth) or
carbon paper (e.g. Toray carbon paper (Electrochem. Inc.),
Spectracorp carbon paper (Spectracorp), Sicracet Gas Diffusion
Media (SGL Carbon))) are applied, prior to the electrochemical
deposition (step C)) by laying on, rolling, hot pressing or other
techniques known to those skilled in the art. Then the anode of the
apparatus is fed with e.g. hydrogen and the cathode is fed with
e.g. oxygen. At the anode, which preferably already includes a
catalyst which lowers the activation energy for this reaction,
H.sup.+ ions and electrons are produced by oxidation of the
hydrogen. The H.sup.+ ions, together with the ions of the catalytic
component introduced into the membrane in step A) migrate through
the membrane to the cathode which preferably already includes a
small amount of the catalytic component and at which the reduction
of oxygen to water and the deposition of the catalyst cations takes
place. The electrons required for the reduction flow through an
external electric circuit from the anode to the cathode. Thus the
catalyst cations are advantageously, without an additional
procedural step, deposited at precisely those locations on the
electron conductor so as to be firmly attached thereto, where they
are optimally utilized for the fuel cell.
[0039] Alternatively, electrochemical deposition of the at least
one catalytic component onto the electron conductor in a liquid
milieu is possible, for example in a directoxidation fuel cell such
as a direct-methanol fuel cell.
[0040] In a further preferred embodiment of the present invention,
the electrochemical deposition of the ions of the catalytic
component in step C) of the method according to the invention is
effected by operating an apparatus which allows an electric current
to be impressed for the electrolytic deposition of the catalytic
component. This is done, for example, by operating the apparatus
(e.g. a PEM fuel cell) under electrolytic conditions. Once the
membrane-electrode assembly prior to step C) of the method
according to the invention has been largely finished and been
installed in the apparatus (e.g. a PEM fuel cell), the ions present
in the membrane are deposited electrolytically on the electron
conductor. The catalyst ions thus deposited are located precisely
as targeted at the end of ion-conducting regions of the membrane
and are thus wholly active. The electrolysis can be fed out by a DC
voltage being applied to the electrodes of the apparatus (e.g. fuel
cell). As a result, the metal ions which are present in uniform
dispersion in the polymer-electrolyte membrane, are deposited
cathodically on the electron conductor. Depending on which electron
conductor (on the anode or cathode side of the fuel cell) the
catalytic component is to be deposited on, the polarity of the
electrodes during the electrolysis is chosen accordingly.
[0041] In a preferred embodiment of the present invention, the
electrolysis for depositing the catalytic component is carried out
by applying a time-variant, e.g. pulsed DC voltage or a
time-variant DC current to the electrodes of the fuel cell, with
the resultant advantage that control of the particle size of the
deposited particles and of the surface morphology of the catalyst
(e.g. the noble metal) becomes possible.
[0042] In a further preferred embodiment of the present invention,
the electrolysis for depositing the catalytic component from the
membrane onto the respective electron conductor is carried out by
applying an AC voltage (alternatively a DC voltage whose polarity
is periodically reversed) or a DC voltage with an AC voltage
superimposed thereon to the electrode in the fuel cell. With this
procedure, the catalytic component can continue to be deposited
alternately on the two electrodes. This has the advantage that the
catalyst ions are deposited uniformly and in very finely dispersed
form on both electrodes.
[0043] In a preferred embodiment of the present invention, in step
C) at least one element from the 3.sup.rd to 14.sup.th group of the
periodic table of the elements (PTE), equally preferably from the
8.sup.th to 14.sup.th group of the PTE is deposited as the
catalytic component onto the electron conductor on at least one
side of the polymer-electrolyte membrane. These electrocatalysts
promote the fuel cell reaction (oxidation of hydrogen or reduction
of oxygen) catalytically. As a result of the method according to
the invention, these catalytically active components are applied in
highly dispersed form to the surface of the electron conductor
serving as a support. The abovementioned catalytically active
components are introduced into the polymer-electrolyte membrane, in
step A) of the method according to the invention, in a
concentration of preferably 0.000005 to 0.05 mmol/cm.sup.2.
[0044] In a preferred embodiment of the present invention, in step
C) at least one of the elements Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd,
Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic
component on the cathode-side electron conductor in the fuel cell.
While this is done, a further catalytic substance required for
lowering the activation energy for the fuel cell reaction may
already be present on the electron conductor. For example, in step
C) of the method according to the invention, copper can be
deposited as a second catalytically active substance on an electron
conductor which already supports platinum as the first
catalytically active substance. The abovementioned catalytically
active components Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn,
Zn, Au, Ag, Rh, Ir or W to be deposited on the cathode side of the
fuel cell are introduced, in step A) of the method according to the
invention, into the polymer-electrolyte membrane in an
amount/concentration of preferably 0.000005 to 0.05
mmol/cm.sup.2.
[0045] It has been found that there are problems, under fuel cell
operating conditions, with the cathodic reduction of oxygen:
[0046] At cathodic electrode material of the fuel cell as described
in the prior art, highly reactive peroxidic oxygen species (e.g.
HO., HOO.) are formed which cause irreversible damage to the
proton-permeable membrane and the ionomer of the electrode. It was
found that additives which have deperoxidation-active properties
and are specifically introduced into or at the electrode material
produce a sustainable increase in the lifetime or service life and
economic efficiency of fuel cells. Here, the term
deperoxidation-active is to be understood as the property of
preventing the formation of peroxides and the retrospective
decomposition of peroxides as already formed. Peroxides in this
context are all compounds of type R--O--O--R and the corresponding
free radicals (RO. or ROO.), where R is preferably H. For example,
HOO. is a peroxidic radical corresponding to H.sub.2O.sub.2
(hydrogen peroxide). As a result of disposing suitable
deperoxidation-active compounds and/or elements in or on the fuel
cell electrodes, rapid breakdown of the peroxides or suppression of
formation of peroxides is achieved under fuel cell conditions.
Irreversible damage to the ionic exchange membranes by reactive
peroxides is no longer observed. This is surprising, since
according to the principle of microreversibility those substances
which decompose peroxides are also able to form peroxides. For
example, platinum under fuel cell conditions acts as peroxide
former owing to the permanent O.sub.2 supply. Under different
conditions it is used for the decomposition of peroxide. Only by
introducing further deperoxidation-active additives is it possible
to decompose the peroxides formed on the platinum in the fuel cell
or to inhibit their formation. The active components to be
mentioned for such elements or compounds acting as
deperoxidation-active additives are primarily the metals Co, Fe,
Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W. The
said metals are therefore deposited as catalytic components,
preferably by means of the method according to the invention, on
the cathode-side electron conductor. They are introduced, in step
A) of the method according to the invention, into the
polymer-electrolyte membrane in an amount/concentration of
preferably 0.000005 to 0.05 mmol/cm.sup.2.
[0047] Another important object of fuel cell fabrication is the
reduction of cathode-side overpotentials. Progress in this respect
is achieved by catalysts comprising a plurality of active
components. In a preferred embodiment of the present invention, a
plurality of catalytic components are therefore deposited on the
electron conductor on the cathode side in the fuel cell. In so
doing it is possible, on the one hand for a plurality of catalytic
components, in step A) of the method according to the invention, to
be introduced into the polymer-electrolyte membrane (PEM), which
are then, in step C), jointly deposited on the electron conductor,
and/or, on the other hand, where components that are already
catalytic to be applied together with the electron conductor onto
the PEM in step B), followed by the deposition thereonto, in step
C) of additional catalytic components.
[0048] In a preferred embodiment of the present invention, in step
C) at least one of the elements Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni,
Mo, Sn, Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic
component on the anode-side electron conductor in the fuel
cell.
[0049] In the course of "cold combustion" of methanol in the fuel
cell, a byproduct formed in small amounts is carbon monoxide (CO).
This results in an increase in the CO concentration on the anode
side of the fuel cell, and a catalytic component (e.g. platinum)
which serves to promote the anode reaction catalytically, is
consequently loaded with CO. In H.sub.2O.sub.2 fuel cells, for
which the hydrogen is supplied by hydrocarbon reformation, CO is
likewise introduced into the fuel cell. In both cases, the free
surface area of the catalytic component for H.sub.2 adsorption and
oxidation is therefore reduced, resulting in "CO poisoning" of the
fuel cell. Via oxidation of the CO on the CO-loaded catalyst by
H.sub.2O it is possible to effect a "desorption" of the CO
molecules. Base cocatalysts, e.g. ruthenium, allow H.sub.2O to be
absorbed at lower anode potentials, thus contributing to an
increase in the CO tolerance on the anode side of the fuel cell.
According to the invention, such catalytic components can be
deposited precisely on target on the electron conductor on the
anode side, for example ruthenium on a Pt-C electron conductor,
thereby reducing the risk of CO poisoning of the fuel cell. The
abovementioned components Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo,
Sn, Zn, Au, Ag, Rh, Pt, Ir or W to be deposited on the anode side
of the fuel cell are introduced, in step A) of the method according
to the invention, into the polymer-electrolyte membrane in an
amount/concentration of preferably 0.000005 to 0.5
mmol/cm.sup.2.
[0050] In the method according to the invention, the electron
conductor used preferably contains at least one metallic element in
the form of bonded fiber web, fibers or powder. Also conceivable is
the use of electron-conducting polymers as electron conductors.
Particular preference is given to the use of finely dispersed C
blacks or graphite powders as electron conductors. In the fuel
cell, the carbon black or the graphite, by means of the large
surface area of their particles, serve as electrically conductive
gas-porous supports for at least one catalytic component. By means
of the method according to the invention, said catalytic component
can be applied to the electron conductor which has previously been
bonded to a polymer-electrolyte membrane.
[0051] In a preferred embodiment of the present invention, the
electron conductor applied in step B) of the method according to
the invention comprises at least one catalytic component from the
group consisting of Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn,
Zn, Au, Ag, Rh, Ir or W. Here, the electron conductor, when it is
bonded in step B) to the membrane, already serves as a support for
at least one catalytic component (e.g. platinum), and at least one
further catalytic component (e.g. Ru or Cu) or the catalytic
component already present (e.g. additional Pt) is deposited, in
step C) of the method according to the invention, on said
catalyst-containing electron conductor. Thus it is possible, in the
method according to the invention, for e.g. a catalytic component
which enhances CO tolerance to be deposited on the anode side onto
an electron conductor on which a catalytically active component
which catalytically promotes the fuel cell reaction is already
present. On the cathode side, for example, a deperoxidation-active
component can be deposited on an electron conductor/catalyst
combination.
[0052] In a preferred embodiment of the present invention, in step
B), together with the electron conductor, an ion conductor (e.g. an
ionomer solution or suspension) is applied to at least one side of
the polymer-electrolyte membrane. The joint application of ionomer
and electron conductor advantageously results in the electron
conductor being made accessible to a high degree by means of
ionomer, which means a large 3-phase interface area.
* * * * *