U.S. patent number 10,593,979 [Application Number 14/950,204] was granted by the patent office on 2020-03-17 for membrane electrode assembly for a fuel cell, method for preparing the membrane electrode assembly, fuel cell system and vehicle.
This patent grant is currently assigned to Daimler AG, Ford Motor Company. The grantee listed for this patent is Daimler AG, Ford Motor Company. Invention is credited to Kyoung Bai, Carmen Chuy, Scott McDermid, Tran Ngo.
United States Patent |
10,593,979 |
Chuy , et al. |
March 17, 2020 |
Membrane electrode assembly for a fuel cell, method for preparing
the membrane electrode assembly, fuel cell system and vehicle
Abstract
A membrane electrode assembly for a fuel cell, with a membrane,
a catalyst layer (16) and a gas diffusion layer. The catalyst layer
(16) has a first side facing the membrane and a second side facing
the gas diffusion layer. In the catalyst layer (16) an ionomer
content increases towards the membrane. The catalyst layer (16) has
a first sublayer (22) in which catalyst particles (26) are coated
with a first ionomer (28). The catalyst layer (16) further has a
second sublayer (24) with a second ionomer (32) which is closer to
the membrane than the first sublayer (22). Pores (30) are present
at least between the coated catalyst particles (26). Further, a
method for preparing such a membrane electrode assembly, a fuel
cell system and a vehicle with a fuel cell system.
Inventors: |
Chuy; Carmen (Burnaby,
CA), McDermid; Scott (Vancouver, CA), Bai;
Kyoung (Vancouver, CA), Ngo; Tran (Vancouver,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Daimler AG
Ford Motor Company |
Stuttgart
Dearborn |
N/A
MI |
DE
US |
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Assignee: |
Daimler AG (Stuttgart,
DE)
Ford Motor Company (Dearborn, MI)
|
Family
ID: |
55967849 |
Appl.
No.: |
14/950,204 |
Filed: |
November 24, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160156054 A1 |
Jun 2, 2016 |
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Foreign Application Priority Data
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Nov 28, 2014 [CA] |
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2872682 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
8/1004 (20130101); H01M 4/861 (20130101); H01M
4/8828 (20130101); H01M 4/8663 (20130101); H01M
4/8636 (20130101); Y02E 60/521 (20130101); Y02E
60/50 (20130101); H01M 2250/20 (20130101); Y02T
90/40 (20130101); Y02T 90/32 (20130101) |
Current International
Class: |
H01M
8/10 (20160101); H01M 4/86 (20060101); H01M
4/88 (20060101); H01M 8/1004 (20160101) |
Field of
Search: |
;429/465 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004186049 |
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Jul 2004 |
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JP |
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2010251140 |
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Nov 2010 |
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JP |
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Other References
English translation of Fukuda, JP 2004-186049 A, Jul. 2004, Japan.
cited by examiner .
English translation of Nanba, JP 2010-251140 A, Nov. 2010, Japan.
cited by examiner.
|
Primary Examiner: Kelly; Cynthia H
Assistant Examiner: Alam; Rashid A
Attorney, Agent or Firm: Patent Central LLC Pendorf; Stephan
A.
Claims
The invention claimed is:
1. A membrane electrode assembly for a fuel cell, comprising a
membrane (12), a catalyst layer (16) and a gas diffusion layer
(14), wherein the catalyst layer (16) has a first side (18) facing
the membrane (12) and a second side (20) facing the gas diffusion
layer (14), wherein in the catalyst layer (16) an ionomer content
increases towards the membrane, wherein the catalyst layer (16)
comprises a first sublayer (22) in which supported catalyst
particles (26) are coated with a first ionomer (28), and a second
sublayer (24) comprising particles of a second ionomer (32) free of
catalyst, the second sublayer (24) being closer to the membrane
(12) than the first sublayer (22), the first sublayer (22) being
closer to the gas diffusion layer (12) than the second sublayer
(24), wherein pores (30) are present at least between the coated
catalyst particles (26) and also between the particles of the
second ionomer (32), wherein a porosity of the catalyst layer (16)
increases from the first side (18) towards the second side (20) of
the catalyst layer (16), and wherein the first and/or the second
ionomer is/are a mixture of an ionomer and one or more
polymers.
2. The membrane electrode assembly according to claim 1, wherein
within the first sublayer (22) an average thickness of the coating
is inferior to an average diameter of the catalyst particles
(26).
3. The membrane electrode assembly according to claim 1, wherein
within the catalyst layer (16) the coating of the coated catalyst
particles (26) avoids a contact between the second ionomer (32) and
the catalyst particles (26).
4. The membrane electrode assembly according to claim 1, wherein
the acidity and/or the molecular mass per sulfonic acid group
and/or a composition of the first ionomer (28) differs from that of
the second ionomer (32).
5. The membrane electrode assembly according to claim 1, wherein
the mixture of an ionomer and one or more polymers contains a
polymer selected from the list comprising fluoropolymers,
perfluorinated elastomers, perfluoropolyethers, polyaromatic
polymers, as well as mixtures or combinations thereof.
6. The membrane electrode assembly according to claim 1, wherein
the first sublayer (22) is obtained by mixing a powder containing
the catalyst particles (26) with the first ionomer (28), wherein
the catalyst layer (16) is obtained by application of an ink
containing the second ionomer (32) onto the at least partially
consolidated first sublayer (22).
7. The membrane electrode assembly according to claim 1, wherein
the membrane electrode assembly (10) is prepared by a method
comprising: in a first step of the preparation of the catalyst
layer (16) mixing a powder containing the catalyst particles (26)
with the first ionomer (28) to obtain the first sublayer (22), and
in a second step, applying an ink containing the second ionomer
(32) onto the at least partially consolidated first sublayer (22)
to obtain the second sublayer (24).
8. A fuel cell system, with a fuel cell stack (78) comprising a
plurality of fuel cells, wherein each fuel cell comprises a
membrane electrode assembly (10) according to claim 1, and wherein
each membrane electrode assembly (10) is arranged between a first
separator plate (92) and a second separator plate (94).
9. A vehicle with a fuel cell system (76) according to claim 8.
10. The membrane electrode assembly according to claim 1, wherein a
porosity of the cathode catalyst layer (16) increases from the
first side (18) towards the second side (20) of the catalyst layer
(16).
11. The membrane electrode assembly according to claim 5, wherein
the fluoropolymer is selected from polyvinylidene difluoride,
polytetrafluoroethylene and Teflon.RTM. AF, wherein the
perfluorinated elastomer is selected from copolymers of
hexafluoropropylene and vinylidene fluoride, and wherein the
polyaromatic polymer is selected from polyethersulfones, siloxanic
polymers, and polybenzimidazole.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a membrane electrode assembly for a fuel
cell. The membrane electrode assembly comprises a membrane, a
catalyst layer and a gas diffusion layer. The catalyst layer has a
first side facing the membrane and a second side facing the gas
diffusion layer. An ionomer content increases in the catalyst layer
towards the membrane. The invention further relates to a method for
preparing such a membrane electrode assembly, to a fuel cell system
with a fuel cell stack comprising a plurality of fuel cells, the
fuel cells each comprising such a membrane electrode assembly, and
to a vehicle.
Description of the Related Art
In a fuel cell system, fuel cells create electricity through the
electrochemical reaction that takes place when a fuel such as
hydrogen and an oxidant such as oxygen are passed across opposing
sides of the membrane electrode assembly. In a polymer electrolyte
membrane fuel cell (PEMFC) system the membrane is a polymer
electrolyte membrane (PEM) or proton exchange membrane. The
catalyst layers and the gas diffusion layers form the electrodes of
the membrane electrode assembly, namely an anode and a cathode
located on each side of the membrane.
In a fuel cell such membrane electrode assemblies are arranged
between two separator plates, wherein one separator plate comprises
channels for the distribution of the fuel and the other separator
plate comprises channels for the distribution of the oxidant. The
respective channels facing the membrane electrode assembly build a
channel structure which is called a flow field.
In a fuel cell stack a plurality of such unit cells comprising two
separator plates and the membrane electrode assembly arranged
between the separator plates are often connected in series. In such
a fuel cell stack instead of monopolar separator plates bipolar
plates can be utilized, which are electrically conductive and
contact an anode of the unit cell and a cathode of the adjacent
unit cell.
US 2008/0206616 A1 describes a catalyst coated membrane for a fuel
cell. Herein, a catalyst layer comprises smaller catalyst particles
and larger catalyst particles on a polymer electrolyte membrane.
The larger catalyst particles are proximal to a gas diffusion layer
of the membrane electrode assembly, and the smaller catalyst
particles are disposed between the larger catalyst particles and
the polymer-electrolyte-membrane. The catalyst layer also comprises
a polymer electrolyte ionomer. The ionomer concentration can vary
along the depth of the catalyst layer. The catalyst layer can have
a vertical or horizontal gradient, for example a porosity gradient,
a particle size gradient or a catalyst particle concentration
gradient. To produce the catalyst coated membrane, a first ink can
be sprayed onto a polymer electrolyte membrane substrate, wherein
the first ink composition can include catalyst particles and
polymer electrolyte ionomer particles as functional materials. A
second layer can be produced by spraying a second ink onto the
surface of the at least partially dried first layer obtained from
the first ink. The first and second inks can have different
compositions, and each composition can include different functional
materials such as a different catalyst particle composition or
different concentrations of the functional materials.
US 2011/0027696 A1 describes a fuel cell with a membrane electrode
assembly, wherein a first electrode layer of an electrode contains
a smaller amount of ionomer than a second electrode layer. Herein
the second electrode layer is closer to a polymer electrolyte
membrane of the membrane electrode assembly and the first electrode
layer is closer to a gas diffusion layer. A first gas diffusion
layer faces a first bipolar plate of a fuel cell containing the
membrane electrode assembly, and a second gas diffusion layer faces
a second bipolar plate of the fuel cell.
It is an object of the present invention to provide a membrane
electrode assembly of the initially mentioned kind, a method for
preparing such a membrane electrode assembly, a fuel cell system
and a vehicle, wherein the membrane electrode assembly provides an
improved performance to a fuel cell in which it can be
utilized.
BRIEF SUMMARY OF THE INVENTION
In the membrane electrode assembly according to the invention, the
catalyst layer comprises a first sublayer in which catalyst
particles are coated with a first ionomer and a second sublayer
comprising a second ionomer. Herein, the second sublayer is closer
to the membrane than the first sublayer. Further, pores are present
at least between the coated catalyst particles. By such a
structure, a through-plane gradient of the ionomer content can be
achieved. A high ionomer content next to the membrane provides the
catalyst layer with a good proton conduction capacity close to the
membrane. On the other hand, the pores between the coated catalyst
particles in the first sublayer lead to an elevated level of
porosity next to the gas diffusion layer and in particular next to
an (optional) microporous layer of the gas diffusion layer. Thus, a
pore network can be maintained which is capable of providing an
ample flux of a reactant within the first sublayer and also a good
removal of used reactant and/or products of the electrochemical
reaction taking place in the fuel cell. Thus, performance
improvements under a variety of test conditions can be
obtained.
By providing the first sublayer with the lower ionomer content and
the second sublayer with the higher ionomer content, the
utilization of the catalyst layer is improved. This is due to the
fact that minimal protonic and mass transport requirements are met
for a larger fraction of the catalyst layer. Both protons and
reactant are able to migrate into greater portions of the catalyst
layer thickness compared to catalyst layers not showing such a
gradient. By providing the ionomer gradient within the catalyst
layer comprising the first sublayer with the first ionomer and the
second sublayer with the second ionomer also a good interface
between the membrane and the catalyst layer is obtained.
The catalyst particles are items or units comprising a support
material such as carbon and at least one noble metal such as
platinum on the support material. The coating is a thin layer of
the first ionomer on such particles. By providing such a coating or
skin on the catalyst particles within the first sublayer, also an
in-plane gradient exists in the catalyst layer. Such a gradient
exists, for example, in a direction from the first sublayer to the
second sublayer. This is due to the fact that the coating of the
coated catalyst particles is thinner than an average size of the
particles of the second ionomer present in the second sublayer.
Performance and durability gains can be achieved as the first
ionomer type resides next to the catalyst particles, whereas the
second ionomer is present in the second sublayer of the catalyst
layer.
The membrane electrode assembly thus provides an improved
performance and an improved durability to a fuel cell in which the
membrane electrode assembly is utilized. Further, the utilization
of different ionomers within the catalyst layer provides an
advantageous material processing flexibility: it is thus possible
to combine ionomers with different chemical properties that would
otherwise be incompatible for processing in the manufacturing of
the membrane electrode assembly.
Further, the membrane electrode assembly provides an improved
structural flexibility as a variety of pore structures can be
engineered. The utilization of the first ionomer in the vicinity of
the catalyst particles also enables the placement of additives in
the vicinity of the catalyst particles. For example, additives that
avoid or reduce carbon corrosion or platinum dissolution can be
provided in the coating. This further improves the durability and
the performance of the membrane electrode assembly. As an example,
PITM additives can be utilized next to the catalyst particles, i.e.
in the coating containing the first ionomer.
Preferably, within the first sublayer an average thickness of the
coating is inferior to an average diameter of the catalyst
particles. Thus, a very thin layer or skin of the first ionomer is
present within the first sublayer of the catalyst layer. In
combination with the pores or void spaces between the coated
catalyst particles this leads to a high efficiency of the first
sublayer in catalyzing the electrochemical reactions taking place
within the catalyst layer. However, the thin coating of the first
ionomer near the catalyst material also provides the first sublayer
with good proton conductivity.
It has further proven advantageous if within the catalyst layer the
coating of the coated catalyst particles avoids a contact between
the second ionomer and the catalyst particles. Thus, the type and
the chemical properties of the first ionomer can be particularly
well adapted to the needs of the catalyst particles, whereas the
choice of the type and the properties of the second ionomer does
not need to take into account the properties of the catalyst
particles. For example, a type of ionomer which would lead to a
higher rate of crystallite dissolution within the first sublayer if
in contact with the catalyst particles does not affect the catalyst
particles if this type of ionomer is utilized for the second
sublayer isolated from the catalyst particles by the coating.
Therefore, the type of the second ionomer can be chosen in
particular to provide the second sublayer with high proton
conductivity.
Further advantageously, particles of different sizes of the second
ionomer can be present in the second sublayer, wherein smaller
particles of the second ionomer are located in at least some of the
pores present in the first sublayer. Thus, the second ionomer
penetrates into the first sublayer and there is no sharp limit
between the two sublayers. This is particularly valuable for
gradually increasing the proton conductivity in a direction from
the second side of the catalyst layer to the first side of the
catalyst layer. Thus, a particularly high proton conductivity is
present near the membrane of the membrane electrode assembly. Also
a particularly smooth change of the porosity within the catalyst
layer can be obtained. It is noted, however, that the ionomer
particle sizes may not change throughout the layer, but that their
overall distribution may vary. Moreover, it is noted that the first
side of the catalyst layer constitutes an interface between the
catalyst layer and the membrane and the second side of the catalyst
layer constitutes an interface between the catalyst layer and the
gas diffusion layer.
It has further proven advantageous if the porosity of the catalyst
layer increases from the first side towards the second side of the
catalyst layer, i.e. away from the membrane toward the surface of
the gas diffusion layer. Thus, a through-plane porosity gradient is
obtained which leads to a particularly good reactant access to the
catalyst particles within the catalyst layer. Also, products of the
electrochemical reaction taking place in the catalyst layer can be
particularly well removed from the catalyst layer.
The catalyst layer can in particular be a cathode catalyst layer.
Here, the opportunity for PEMFC (polymer electrolyte membrane fuel
cell) performance enhancement is particularly large. This is due
to, for example, rather sluggish oxygen reduction reaction kinetics
and oxygen mass transport in the cathode catalyst layer. Also, in
the cathode catalyst layer water management is of particular
relevance, as the product water is mainly present on the cathode
side of the membrane electrode assembly. Thus, larger pores in the
first sublayer which provide a good product water flux are of
particular relevance in the cathode catalyst layer.
With respect to the two different ionomers present in the catalyst
layer, in particular the acidity of the first ionomer can differ
from that of the second ionomer. Thus, a thin layer of less acidic
ionomer next to the catalyst particles provides sufficient proton
conduction, but a particularly low rate of crystallite dissolution,
i.e. of, for example, platinum dissolution. This is in particular
advantageous for an improved durability of the membrane electrode
assembly. On the other hand, a higher acidity of the first ionomer
can provide better performance of the membrane electrode assembly
in hot and dry conditions. Therefore depending on the applications
the difference in acidity of the two ionomers can be chosen
adequately.
Alternatively or additionally, a molecular mass per sulphonic acid
group of the first ionomer can differ from that of the second
ionomer. The molecular mass per sulphonic acid group is also called
the equivalent weight of the ionomer. Therefore as well by varying
the equivalent weight of the two ionomers, the properties of the
catalyst layer can be tuned to satisfy the needs of the specific
fuel cell application.
Still further, a composition of the first ionomer can differ from
that of the second ionomer. For example, perfluorosulphonic acid
(PFSA) ionomers can be utilized for the coating of the catalyst
particles, whereas hydrocarbon based ionomers can be utilized in
the second sublayer.
Moreover, the ionomer material used in this invention may not be a
pure ionomer, but it may be a mixture of an ionomer and one or more
polymers, instead. Suitable polymers are selected from the list
comprising fluoropolymers, such as polyvinylidene difluoride (PVDF)
and polytetrafluoroethylene (PTFE) and Teflon.RTM. AF,
perfluorinated elastomers--in particular copolymers of
hexafluoropropylene and vinylidene fluoride such as Tecnoflon.RTM.
of Solvay Solexis S.p.A.--, perfluoropolyethers, polyaromatic
polymers such as polyethersulfones, siloxanic polymers,
polybenzimidazole, etc., as well as mixtures or combinations
thereof. The use of ionomer/polymer mixtures instead of pure
ionomer materials provides the option to finetune the properties of
the ionomer material used, e.g. in terms of chemical properties and
concentration of the ionomer, hence the amount of ionomer present
in a catalyst layer. This is advantageous, e.g. for durability
and/or performance of a membrane electrode assembly. The
ionomer/polymer mixture can contain polymer up to 50% by weight of
the mixture. Preferably, the mixture contains polymer between 0.1
an 30% by weight of the mixture.
Further advantageously, the first sublayer can be obtained by
mixing a powder containing the catalyst particles with the first
ionomer, wherein the catalyst layer is obtained by application of
an ink containing the second ionomer onto the at least partially
consolidated first sublayer. Thus, it can particularly easily be
achieved that the second ionomer flows or penetrates into the first
sublayer and therefore a porosity gradient with increasing porosity
from the second sublayer to the first sublayer is realized.
In the method according to the invention, for preparing the
membrane electrode assembly according to the invention, in a first
step of the preparation of the catalyst layer a powder containing
the catalyst particles is mixed with the first ionomer to obtain
the first sublayer. In a second step of the preparation of the
catalyst layer, an ink containing the second ionomer is applied
onto the at least partially consolidated first sublayer to obtain
the second sublayer. Thus, a localized ionomer type in the form of
the first ionomer is provided near the catalyst surface compared to
the overall ionomer content in the bulk catalyst layer. By first
depositing a first type of ionomer near the catalyst particles
(also called first deposition) and then applying the second ionomer
as a binder (also called second deposition), ionomer and porosity
gradients throughout and within the thickness of the catalyst layer
can be obtained. This provides an improved performance to the fuel
cell utilizing the prepared membrane electrode assembly.
The fuel cell system according to the invention, which in
particular can be employed in a vehicle, includes a fuel cell stack
comprising a plurality of fuel cells. Each fuel cell comprises a
membrane electrode assembly according to the invention, and each
membrane electrode assembly is arranged between a first separator
plate and a second separator plate. Within each fuel cell the
membrane electrode assembly comprises the membrane and cathode and
anode electrodes, respectively. The cathode and anode electrodes
each comprise a catalyst layer and a gas diffusion layer.
Preferably at least the cathode catalyst layer comprises the two
sublayers.
Such a fuel cell system can include a plurality of further
components usual in particular for fuel cell systems of vehicles,
which presently do not have to be explained in detail.
The vehicle according to the invention includes a fuel cell system
according to the invention.
The features and feature combinations mentioned above in the
description as well as the features and feature combinations
mentioned below in the description of figures and/or shown in the
figures alone are usable not only in the respectively specified
combination, but also in other combinations or alone, without
departing from the scope of the invention. Thus, implementations
are also to be considered as encompassed and disclosed by the
invention, which are not explicitly shown in the figures or
explained, but arise from and can be generated by separated feature
combinations from the explained implementations.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Further advantages, features and details of the invention are
apparent from the claims, the following description of preferred
embodiments as well as based on the drawings. Therein show:
FIG. 1 schematically a part of a membrane electrode assembly
comprising a gas diffusion layer, a catalyst layer and a membrane,
wherein within the catalyst layer there is an increasing porosity
towards the gas diffusion layer and an increasing ionomer content
towards the membrane;
FIG. 2 schematically the catalyst layer according to FIG. 1,
wherein a first ionomer in the form of a coating is present in
close proximity to particles of a catalyst powder, the coated
catalyst particles forming a first sublayer within the catalyst
layer, and wherein a second ionomer forms a second sublayer within
the catalyst layer;
FIG. 3 a graph showing performance improvements due to the coating
of the catalyst particles with the first ionomer in normal
temperature operation;
FIG. 4 a graph showing performance improvements due to the coating
of the catalyst particles with the first ionomer in hot temperature
operation;
FIG. 5 a diagram showing an improvement of the effective catalyst
surface area due to the coating of the catalyst particles with the
first ionomer;
FIG. 6 a graph showing the performance improvement due to different
types and amounts of the first ionomer applied to the catalyst
particles;
FIG. 7 a scanning transmission X-ray microscopy image of a membrane
electrode assembly according to FIG. 1; and
FIG. 8 schematically a fuel cell system with a fuel cell stack
comprising the membrane electrode assemblies according to FIG.
1.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically shows a part of a membrane electrode assembly
10 for a fuel cell. The membrane electrode assembly 10 comprises a
membrane 12. The membrane 12 is a polymer electrolyte membrane
(PEM) or proton exchange membrane. The membrane electrode assembly
10 also comprises a gas diffusion layer 14. A catalyst layer 16 is
arranged between the membrane 12 and the gas diffusion layer 14.
The catalyst layer 16 has a first side 18 facing the membrane 12
and a second side 20 facing the gas diffusion layer 14. The second
side 20 can in particular be in contact with a microporous layer or
structure of the gas diffusion layer 14. The gas diffusion layer 14
has an overall high porosity which facilitates the access of
reactant gases such as hydrogen or oxygen to the catalyst layer 16.
The membrane 12 on the other hand has a high proton
conductivity.
Within the catalyst layer 16, there is an increasing porosity
towards the gas diffusion layer 14. Further, an ionomer content
increases towards the membrane 12. By these gradient structures,
i.e. the porosity gradient towards the gas diffusion layer 14 and
the ionomer gradient towards the membrane 12, both good proton
conductivity as well as ample reactant flux is provided by the
catalyst layer 16. As oxygen mass transport and product water
management are particularly important in a fuel cell comprising the
membrane electrode assembly 10, the catalyst layer 16 can in
particular be a cathode catalyst layer 16. However, the membrane
electrode assembly 10 comprises a second catalyst layer (not shown)
opposite the catalyst layer 16 shown in FIG. 1. This second
catalyst layer, which can in particular be an anode catalyst layer,
is also arranged between the membrane 12 and a further gas
diffusion layer (not shown).
In order to provide the porosity gradient and the ionomer gradient
within the catalyst layer 16, the catalyst layer 16 comprises a
first sublayer 22 and a second sublayer 24 (see FIG. 2). In the
first sublayer 22, catalyst particles 26 are coated with a first
ionomer 28. The catalyst particles 26 are separate particles of a
catalyst powder. The first ionomer 28 coating forms a thin layer or
skin around the catalyst particles 26. The catalyst particles 26
consist of noble metals such as platinum on a support such as
carbon. There are spaces or pores 30 between the coated catalyst
particles 26 which provide the first sublayer 22 with a relatively
high porosity.
As the first ionomer 28 covers each catalyst particle 26
circumferentially, the first ionomer 28 type is in very close
proximity to the catalyst powder. This allows to utilize a polymer
as first ionomer 28, which is particularly well adapted to provide
the catalyst layer 16 with improved durability. For example, an
acidity of the first ionomer 28 which forms the thin coating or
skin on the catalyst particles 26 can be lower than the acidity of
particles of the second ionomer 32 which is present in the second
sublayer 24. This improves the durability of the catalyst layer 16
as the less acidic first ionomer 28 leads to a particularly low
rate of platinum dissolution and still provides sufficient proton
conduction to the catalyst layer 16. Further, additives for example
additives to avoid or reduce carbon corrosion and/or platinum
dissolution can be easily placed at a specific location within the
catalyst layer 16, namely in close contact with the catalyst
particles 26, by providing the first ionomer 28 with such
additives.
There are also pores 34 or void spaces between the particles of the
second ionomer 32, but still the porosity of the catalyst layer 16
decreases towards the membrane 12. Also, the overall ionomer
content increases within the catalyst layer 16 in a direction from
the second side 20 to the first side 18.
To provide the catalyst layer 16 comprising the first sublayer 22
and the second sublayer 24 the catalyst particles 26 are coated in
a first step of the preparation of the catalyst layer 16. This can
be done by spray-coating or by mixing a powder containing the
catalyst particles 26 with the first ionomer 28. After formation of
the first sublayer 22, for example on a substrate 36, the second
ionomer 32 is applied to the first sublayer 22 in order to form the
second sublayer 24. During this application process smaller
particles of the second ionomer 32 can penetrate or flow into some
of the pores 30 provided within the first sublayer 22 (see FIG. 2).
Thus, a very gradual increase in porosity in a direction from the
first side 18 of the catalyst layer 16 to the second side 20 of the
catalyst layer 16 (see FIG. 1) can be achieved.
The second ionomer 32 can in particular be of a more acidic type
than first ionomer 28 in order to provide the second sublayer 24
with increased proton conductivity, in particular close to the
membrane 12. By tuning the conditions of the application process of
the second ionomer 32 onto the first sublayer 22, which can in
particular be performed by spray-coating, the ionomer gradient and
the penetration of the second ionomer 32 into the first sublayer 22
can be regulated. Also, the interface between the catalyst layer 16
and the membrane 12 can be improved.
Further, by providing the first ionomer 28 as a coating on the
catalyst particles 26, an in-plane gradient of the ionomer content
within the catalyst layer 16 is achieved. The through-plane and/or
in-plane ionomer content and/or ionomer type gradient lead to
performance and durability gains.
The catalyst layer 16 can also have a through-plane and/or an
in-plane porosity gradient. In the example shown in FIG. 1 and FIG.
2, the porosity increases towards the gas diffusion layer 14.
The performance improvement is, for example, illustrated in FIG. 3.
Herein, a cell voltage of a single fuel cell is indicated on an
ordinate 38 of a graph 40. The current density is indicated on an
abscissa 42 of the graph 40. In the graph 40 a first curve 44 shows
the polarization characteristics of a fuel cell not having the
ionomer coating on the catalyst particles 26. Other curves 46 show
the results obtained with the membrane electrode assembly 10 shown
in FIG. 1.
As can be seen from FIG. 3, higher cell voltages are obtained with
the membrane electrode assembly 10 having the described structure.
This in particular true for higher current densities. The curves 46
show the results for different quantities of ionomer loadings,
wherein the ionomer has the same equivalent weight in the different
loadings. The graph 40 in FIG. 3 shows the results in normal
temperature operating conditions of the fuel cell, for example in a
temperature range between 65.degree. C. and 70.degree. C.
A graph 48 shown in FIG. 4 also shows polarization curves, but in
this case in hot operating conditions of the fuel cell of for
example about 80.degree. C. Again, a curve 50 shows the base line
polarization, wherein other curves 52 show the performance
improvement obtained by the utilization of the membrane electrode
assembly 10 shown in FIG. 1. Again, performances above base line
levels are obtained with respect to the cell voltage for a given
current density.
In a diagram 54 shown in FIG. 5, a first pair of columns 56
visualizes the base line effective platinum surface area for wet
conditions 58 and dry conditions 60. Further pairs of columns 62
show that the addition of the ionomer spray leads to an effective
platinum dissolution surface area beyond base line levels at the
same loading. The further columns 62 show the results for different
amounts of ionomer sprays.
A further diagram 64 shown in FIG. 6 illustrates the influence of
different spraying conditions and of different ionomer equivalent
weights on the cell voltage obtained for a given current density,
for example for a current density of 1.9 A/cm.sup.2. The cell
voltage is indicated on an ordinate 66 in the diagram 64. A first
column 68 is the result obtained with an ionomer spray having the
same composition as the one which yields the results shown in a
second column 70. However, the amount of ionomer spray utilized for
the column 70 sample is higher and there is a deeper penetration of
the ionomer spray. A further column 72 shows the results obtained
with the same amount of ionomer spray as in the column 70 sample,
but with an ionomer having a lower equivalent weight. Finally, a
fourth column 74 shows the results for the base line.
FIG. 7 is a scanning transmission X-ray microscopy image of the
membrane electrode assembly 10 shown in FIG. 1. As can be seen from
this image, the membrane 12 does not have a flat, but a cragged
surface on which the catalyst layer 16 is located. Also, it is
evident that there is no sharp border between the first sublayer 22
and the second sublayer 24. However, the gradient in the ionomer
content is present, which increases from the membrane 12 towards
the gas diffusion layer 14 (not shown in FIG. 7).
FIG. 8 schematically shows a fuel cell system 76 of a vehicle. The
fuel cell system 76 comprises a fuel cell stack 78 to which fuel
such as hydrogen is provided via a supply line 80. The fuel can be
stored in a tank 82. Upon leaving the fuel cell stack 76 via an
exhaust line 84 any fuel remaining in the exhaust gas can be
recirculated to the fuel cell stack 76 via a recirculation line 86.
The fuel is provided to anode electrodes of the membrane electrode
assemblies 10. In a like manner an oxidant such as air is provided
via a supply line 88 to cathode electrodes of the membrane
electrode assemblies 10. The exhaust air leaves the fuel cell stack
76 via a further exhaust line 90.
The anode electrodes and the cathode electrodes of the membrane
electrode assemblies 10 each comprise the catalyst layer 16 and the
gas diffusion layer 14 (see FIG. 1). The electrochemical reaction
which creates electrical energy takes place when the fuel and the
oxidant are passed across opposing sides of the membrane electrode
assemblies 10.
The membrane electrode assemblies 10 are arranged between a first
separator plate in the form of an anode plate 92 and a second
separator plate in the form of a cathode plate 94. The anode plate
92 faces the anode electrode of the membrane electrode assembly 10
of a unit cell. The cathode plate 94 faces the cathode electrode of
the membrane electrode assembly 10 of this unit cell. The anode
plate 92 of a first unit cell and the cathode plate 94 of an
adjacent unit cell can form a bipolar plate assembly 96, in which
the two plates 92, 94 are joined together. The outermost electrode
assemblies 10 in the fuel cell stack 78 are not sandwiched between
two bipolar plate assemblies 96, but between one bipolar plate
assembly 96 and an end plate 98.
The plates 92, 94 joined together in the bipolar plate assembly 96
preferably form a coolant flow field (not shown), i.e. a channel
structure for a coolant fluid which removes heat generated by the
electrochemical reaction taking place in the membrane electrode
assemblies 10. Further the plates 92, 94 form reactant flow fields
for the fuel and the oxidant respectively, i.e. for the reactants
which are provided to the anode electrodes and to the cathode
electrodes of the membrane electrode assemblies 10.
The fuel cell system (76) may comprise further conventional
components, such as a humidifier, a compressors, heat exchangers,
etc. Such components are know to a person skilled in the art. Thus,
for the sake of clarity and simplicity, they are not illustrated in
FIG. 8.
* * * * *