WO2019030557A1

WO2019030557A1 - Free-standing oer anode catalyst layers for fuel cells

Info

Publication number: WO2019030557A1
Application number: PCT/IB2018/000705
Authority: WO
Inventors: Yuquan ZOU; Hao Zhang; Scott Mcdermid; Sumit Kundu; Dorina Manolescu; Yunsong Yang
Original assignee: Daimler Ag; Ford Motor Company
Priority date: 2017-08-11
Filing date: 2018-06-15
Publication date: 2019-02-14

Abstract

Free-standing oxygen evolution reaction (OER) anode catalyst layers for a solid polymer electrolyte fuel cell and methods for making them are disclosed. The anode catalyst layers comprise a porous polymer sheet (e.g. expanded PTFE) and a mixture comprising an OER catalyst and a hydrocarbon ionomer which is impregnated in the pores of the porous polymer sheet. Use of such a separate OER anode catalyst layer in a catalyst coated membrane assembly of a fuel cell provides better voltage reversal tolerance than admixtures and use of the hydrocarbon ionomer can slow down dissolution of the OER catalyst. Further, the preparation methods are suitable for mass production.

Description

FREE-STANDING OER ANODE CATALYST LAYERS FOR FUEL CELLS

BACKGROUND Field of the Invention

This invention relates to bilayer anode catalyst layer designs for providing voltage reversal tolerance in solid polymer electrolyte membrane fuel cell stacks. In particular, the invention relates to discrete anode catalyst layers comprising OER catalyst for such designs.

Description of the related art

Fuel cells electrochemically convert a fuel reactant (e.g. hydrogen) and an oxidant reactant (e.g. oxygen or air) to generate electric power. Solid polymer electrolyte fuel cells are a type of fuel cell which employs a proton conducting, solid polymer membrane electrolyte (e.g. perfluorinated sulfonic acid ionomer) between cathode and anode electrodes. Gas diffusion layers are typically employed adjacent each of the cathode and the anode electrodes to improve the distribution of gases to and from the electrodes. In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided adjacent the gas diffusion layers to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary byproduct in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1 V, a plurality of cells is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.

To simplify the manufacture of solid polymer electrolyte fuel cells and stacks, components known as catalyst boated membranes (CCMs) are usually prepared in which an anode and a cathode are bonded in layer form to opposite sides of a membrane electrolyte layer. Each of the anode and cathode comprise appropriate catalysts. Thus, a CCM is a bonded, layered assembly comprising an anode catalyst layer, a membrane electrolyte layer, and a cathode catalyst layer.

The components in a CCM are all thin and relatively fragile. Further, the electrolyte membrane is typically not dimensionally stable and can swell when in contact with solvents used in typical inks or coatings. Thus it can be challenging to find suitable techniques for mass production of CCMs. Among the many known methods for preparing CCMs, decal transfer methods are probably the most commonly used. However, techniques have been developed which employ reinforcement layers to assist both in CCM production as well as to improve the CCM's mechanical properties. For instance, WO2013/064640 discloses an "integral" approach to first coat the cathode layer onto a supporting substrate, followed by electrolyte membrane coating, in which an expanded polytetrafluoroethylene (ePTFE) substrate pre-impregnated with ionomer dispersion is introduced and then adhered to the cathode layer. Finally, the anode layer is coated onto the membrane ionomer layer to form the CCM. In this approach, only one ePTFE sheet is used for mechanical reinforcement of the CCM. In other approaches, more than one reinforcement layer may be employed in CCM fabrication. For instance, US20130202986 discloses a fuel cell construction comprising a reinforced electrode assembly comprising first and second porous reinforcement layers.

The incorporation of one or more reinforcement layers in a CCM advantageously provides improved mechanical strength and in-plane hydration stability (i.e. the dimensional stability of the CCM in the planar directions as a function of hydration state). This is important with regards to long-term durability of commercial fuel cell stacks. For instance, US5547551 or EP1998393 discloses the incorporation of a reinforcement layer (e.g. ePTFE) in the middle of electrolyte membrane. The swelling of the ionomer layer can be constrained by such a reinforcement layer.

A problem associated with large series stacks of fuel cells is that, if for some reason a cell (or cells) in the series stack is not capable of delivering the same current being delivered by the other cells in the stack, that cell or cells may undergo voltage reversal. Depending on the severity and duration of the voltage reversal, the cell may be irreversibly damaged and there may be an associated loss in cell and stack performance. Thus, it can be very important in practical applications for the cells in large series stacks to either be protected against voltage reversal or alternatively to have a high tolerance to voltage reversal.

A voltage reversal condition can arise for instance due to a fuel starvation condition existing on the fuel cell anode (i.e. where the anode receives insufficient fuel for intended operation). A fuel starvation condition can happen during start up from below freezing temperatures as a result of ice blockages in the anode, or during operation at normal operating temperatures as a result of anode "flooding" (where liquid water blocks passageways in the anode). It is well recognized that such conditions can lead to cell voltage reversal due to the associated rise of anode potential, and further can lead to corrosion of the carbon supports which are typically used to support the anode catalyst (typically platinum). As a consequence of this corrosion, a loss in effective platinum surface area occurs at the anode and cell function is degraded. Therefore, a voltage reversal tolerant anode is an important design requirement for the anodes in commercial fuel^* cell stacks.

There are several ways to improve fuel cell anodes for purposes of voltage reversal tolerance. For example, increasing the Pt catalyst loading used and using catalysts that are more resistant to oxidative corrosion can help. Further, in one well studied approach, a suitable secondary catalyst material is incorporated into the anode to facilitate water hydrolysis (also known as the oxygen evolution reaction or OER). By promoting water hydrolysis over the reactions causing corrosion of the anode catalyst supports, voltage reversal tolerance can be significantly improved. This approach is described for instance in US6517962 and US6936370 in which the incorporated secondary catalyst materials for promoting OER included ruthenium, iridium, and/or their oxides.

In the art, different ways have been disclosed for introducing such a secondary OER catalyst into the anode. The OER catalyst may be mixed directly with the primary anode catalyst (e.g. carbon supported Pt catalyst). Alternatively, the secondary OER catalyst may be incorporated in a distinct, separate layer (e.g. a bilayer design as disclosed in US2013/002289). The former method is straightforward and more cost-effective compared to the latter. However, the observed activity of the OER catalyst using the latter method, and hence the reversal tolerance of fuel cells in which the OER catalyst appears in a separate layer, is significantly improved compared to that using the former admixed method. Thus it is apparent that the structure of the anode layer as a whole can have a significant impact on the effectiveness and function of any incorporated OER catalyst.

As another improvement, it has been reported that the use of hydrocarbon ionomer in anodes comprising OER catalyst is beneficial because the OER catalyst is stabilized by the presence of hydrocarbon type of ionomer. As disclosed in US2013/0157169, the use of hydrocarbon ionomer can significantly slow down the dissolution and migration of the catalyst metal when compared to that observed using the more traditional polyfluorosulfonic acid ionomer (PFSA).

There remains a need in the art to improve the preparation process of CCMs for mass production purposes. Further, there is a need to improve the reactivity of OER catalyst incorporated into the anode catalyst layers for purposes of voltage reversal tolerance in order to lower the required loading of OER catalyst and reduce cost. Further still, there is a need to improve on the stability or durability of the catalyst layers during fuel cell operation. The present invention fulfills these and other needs. SUMMARY

Free-standing oxygen evolution reaction (OER) anode catalyst layers for a solid polymer electrolyte fuel cell and methods for making them have been developed. The anode layers comprise a porous polymer sheet (e.g. expanded polytetrafluoroethylene) and a mixture comprising an OER catalyst and a hydrocarbon ionomer which is impregnated in the pores of the porous polymer sheet. Use of such a separate OER anode catalyst layer in a catalyst coated membrane assembly of a fuel cell provides better voltage reversal tolerance than admixtures and use of the hydrocarbon ionomer can slow down dissolution of the OER catalyst. Further, the preparation methods are suitable for mass production.

In the present invention, a suitable porous polymer sheet can be greater than 85% porous and can have an average pore size of about 600 nm. A suitable OER catalyst comprises iridium oxide. The average particle size of the OER catalyst can be about 200 nm and the loading of the OER catalyst can be from about 20 to 50 micrograms/cm² and particularly from about 30 to 35 micrograms/cm². A suitable hydrocarbon ionomer comprises a suitable sulfonated polyphenylene (e.g. as for instance disclosed in CA2933122). And in the mixture, additional ionomers may be included, such as perfluorosulfonic acid ionomer or another type of hydrocarbon ionomer. In the OER anode catalyst layer of the invention, a suitable weight ratio of the OER catalyst to the hydrocarbon ionomer is about 5: 1. An OER anode catalyst layer of the invention may be employed in a catalyst coated membrane assembly (CCM) for a solid polymer electrolyte fuel cell. Such a CCM comprises a solid polymer electrolyte membrane comprising a proton-conducting membrane ionomer, and a cathode and a bilayer anode bonded to opposite sides of the solid polymer electrolyte membrane. The cathode comprises a cathode oxygen reduction reaction (ORR) catalyst. The bilayer anode comprises a hydrogen oxidation reaction (HOR) anode catalyst layer adjacent the electrolyte membrane and an OER anode catalyst layer of the invention located away from the electrolyte membrane. The HOR catalyst used can comprise platinum supported on carbon particles.

The OER anode catalyst layer of the invention can be made using simple methods suitable for mass production purposes. In one exemplary method, an OER anode catalyst layer can be made via the following steps:

preparing a dispersion comprising the OER catalyst, the hydrocarbon ionomer, and a carrier solvent, immersing the porous polymer sheet in the dispersion whereby the dispersion impregnates the pores of the porous polymer sheet, and

drying the impregnated porous polymer sheet, thereby removing the carrier solvent.

In an alternative exemplary method, an OER anode catalyst layer can be made via the following steps: preparing an ink comprising the OER catalyst, the hydrocarbon ionomer, and a solvent, coating the ink onto the porous polymer sheet whereby the dispersion impregnates the pores of the porous polymer sheet, and

drying the impregnated porous polymer sheet, thereby removing the solvent. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic illustration of a solid polymer fuel cell comprising a bilayer anode which includes an OER anode catalyst layer of the invention. Figure 2a shows a schematic illustration of a first method for preparing a free-standing OER anode catalyst layer of the invention.

Figure 2b shows a schematic illustration of a second method for preparing a free-standing OER anode catalyst layer of the invention.

Figure 3 compares the polarization curves (voltage versus current density) of a comparative fuel cell to a fuel cell of the invention.

DETAILED DESCRIPTION

Herein, in a quantitative context, the term "about" should be construed as being in the range up to plus 10% and down to minus 10%.

The acronym HOR refers to "hydrogen oxidation reaction" and a HOR catalyst is specifically selected for oxidizing fuel at the anode in a solid polymer electrolyte fuel cell. A suitable HOR catalyst is platinum, or more preferably carbon supported platinum. However, various other precious or non-precious metals, alloys, or mixtures may be considered. The acronym OER refers to "oxygen evolution reaction" and an OER catalyst is specifically selected to hydrolyze water at the anode in a solid polymer electrolyte fuel cell. Further, an OER catalyst is defined herein to be a catalyst other than the HOR catalyst (i.e. is materially different from the HOR catalyst used in the anode) and is selected to be more effective at water hydrolysis than the HOR catalyst. A suitable OER catalyst is iridium oxide. However, various other oxides, compositions, alloys, or mixtures may be considered.

The acronym ORR refers to "oxygen reduction reaction" and an ORR catalyst is specifically selected for reducing oxidant at the cathode in a solid polymer electrolyte fuel cell. A suitable ORR catalyst is platinum, or more preferably carbon supported platinum. However, various other precious or non-precious metals, alloys, or mixtures may be considered.

A schematic illustration of an exemplary solid polymer fuel cell of the invention is shown in Figure 1. A single such fuel cell is shown and it comprises a CCM of the invention, which in turn comprises a bilayer anode that includes an OER anode layer of the invention.

In Figure 1, fuel cell 1 comprises CCM 2 which is located between a pair of gas diffusion layers (GDLs), namely anode GDL 10 and cathode GDL 11. In turn, this assembly is located between a pair of flow field plates, namely fuel flow field plate 12 and oxidant flow field plate 13. CCM 2 comprises solid polymer electrolyte membrane 8, cathode catalyst layer 9, and bilayer anode 3. Cathode catalyst layer 9 and bilayer anode 3 are bonded to opposite sides of solid polymer electrolyte membrane 8.

Bilayer anode 3 comprises HOR anode catalyst layer 4 adjacent electrolyte membrane 8 and OER anode catalyst layer 5 located away from electrolyte membrane 8 (and adjacent anode GDL 10 in the exemplary embodiment of Figure 1). OER anode catalyst layer 5 comprises porous polymer sheet 6 (e.g. expanded PTFE) which has been impregnated with mixture 7 comprising particulate OER catalyst (e.g. Ir02) and hydrocarbon ionomer (e.g. a suitable sulfonated Diels-Alder polyphenylene as described in CA2933122 which describes the making of specific polyphenylenes via a Diels-Alder reaction). [For illustrative purposes, only a single fuel cell is shown in Figure 1. In a more typical fuel cell stack, the fuel and oxidant flow field plates (12, 13) would typically be mated together to form a unitary bipolar plate. The bipolar plate would also typically incorporate coolant flow fields between the mated plates to allow for coolant flow therein and thus for efficient heat removal from the stack during operation.] The porous polymer sheet in OER anode catalyst layer 5 provides mechanical support for mixture 7 and allows for it to be made separate from CCM 2 and even as a separate component for bilayer anode 3. A suitable reinforcing porous polymer sheet material for use in the present invention is expanded PTFE (ePTFE). However, other porous expanded polymer sheet such as porous polypropylene (PP), porous polyvinylidene fluoride (PVDF), porous polyethersulfone (PES), and the like can be readily employed instead. Further, other types of porous polymer sheet may be considered (i.e. porous polymer sheet other than expanded polymer sheet). For instance, porous electrospun sheet and porous sheet made from nano- fibres may be considered. For expanded polymer sheet, representative sheet porosities range from about 60 to 90%, thicknesses from about 4 to 8 micrometers thick, and average pore sizes from about 0.2 to 1 micrometers although other porosities, thicknesses, and average pore sizes may be considered. As illustrated in the following Examples, suitable OER anode catalyst layers can be made using ePTFE that is 89% porous, 5 micrometers thick and has an average pore size of 600 nm.

The OER catalyst used in OER anode catalyst layer 5 can be any suitable secondary catalyst material for promoting water hydrolysis at the anode (e.g. as described in the aforementioned US6517962 and US6936370) and thereby providing improved tolerance to voltage reversal. Iridium and its oxides or mixed oxides comprising iridium can be particularly desirable.

The hydrocarbon ionomer used in OER anode catalyst layer 5 can be any suitable such ionomer, e.g. sulfonated Diels-Alder polyphenylene, or combination of such ionomers. In addition to the hydrocarbon ionomer, other ionomer types (e.g. PFSA ionomer) may also be included in anode catalyst layer 5. The ionomers employed in the OER anode catalyst layer serve as a binder as well as for providing ionic access to the OER catalyst. Preferably then a sufficient amount is employed for these purposes. However it is also important to allow for adequate flow of gases through the OER anode catalyst layer. As illustrated in the following Examples, it is possible to achieve all these requirements using an appropriate weight ratio of OER catalyst to hydrocarbon ionomer in mixture 7 (e.g. about 5: 1).

As mentioned, the use of a porous polymer sheet in the OER anode catalyst layer serves to reinforce it and also to reinforce the bilayer anode and CCM in which it is incorporated. Several characteristics are improved as a result (e.g. handling characteristics, hydration stability). Again, as disclosed in US2013/002289, the use of a separate layer for a given amount of OER catalyst in the anode can provide for superior reversal tolerance compared to using an admixture of HOR and OER catalysts in the anode. And as disclosed in US2013/0157169, the use of hydrocarbon ionomer in the OER anode catalyst layer can significantly slow down the dissolution and migration of the catalyst metal when compared to that observed using the more traditional PFSA ionomer.

OER anode catalyst layer 5 may be fabricated in at least two simple ways that are suitable for purposes of mass production. Figure 2 shows schematic illustrations of two methods for preparing a free-standing OER anode catalyst layer of the invention. The method of Figure 2a involves uptake of a dispersion into porous polymer sheet 6 while the method of Figure 2b involves coating a dispersion onto porous polymer sheet 6.

In both methods shown in Figure 2, OER catalyst dispersion 15 is prepared in step 21 which comprises a dispersion of OER catalyst 7 in a solution comprising hydrocarbon ionomer dissolved in a suitable carrier solvent (e.g. an alcohol and water). Along with the advantages that the hydrocarbon ionomer provides in the OER product anode catalyst layer, the hydrocarbon ionomer also usefully stabilizes the dispersion for manufacturing purposes. As illustrated in the Examples below, care must be taken not to use an excessive amount of ionomer since too much ionomer in the product catalyst layer can adversely affect gas permeability therethrough.

In Figure 2a, a continuous web of porous polymer sheet 6 is immersed in a bath of OER catalyst dispersion 15 in immersing step 22a whereby the pores of porous polymer sheet 6 are fully impregnated with dispersion 15. The web is then continuously dried at step 23 to remove the carrier solvent to produce a continuous web of OER anode catalyst layer which can be subsequently cut as desired into free-standing OER anode catalyst layers 5.

In the alternative method of Figure 2b, dispersion 15 now serves as a catalyst coating ink. Here, a continuous web of porous polymer sheet 6 is directed against backer roll 30 while coating head 31 applies a continuous coating of dispersion 15 thereto in coating step 22b. As before, the web is then continuously dried at step 23 to remove the dispersion solvent to produce a continuous web of OER anode catalyst layer which can be subsequently cut as desired into free-standing OER anode catalyst layers 5.

The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

EXAMPLES Several free-standing OE anode catalyst layer samples were prepared according to the method generally shown in Figure 2b. The porous polymer sheet employed was an expanded PTFE sheet with thickness of about 5 micrometers, porosity of 89%, and an average pore size of 600 nm. Two similar dispersions were prepared which comprised iridium oxide (IrC ) particulate catalyst as the OER catalyst, carrier solvent mixtures of isopropyl alcohol and water (in a 3: 1 weight ratio respectively), but differing amounts of sulfonated Diels-Alder polyphenylene hydrocarbon ionomer. The IrC>2 catalyst had an average particle size of about 200 nm. The weight concentration of catalyst in the dispersions was about 5%. The weight concentrations of the hydrocarbon ionomer in the dispersions was either about 1% or 0.5%. (Thus, the weight ratio of OER catalyst to hydrocarbon ionomer in the former dispersion would be about 5: 1.) The ion exchange capacity (IEC) of the sulfonated Diels-Alder polyphenylene hydrocarbon ionomer was between about 2.2 and 2.4 meq/g.

A Mayer bar coating method was used to apply the dispersions to expanded PTFE samples mounted on backing substrates. The dispersions readily penetrated the pores of the e-PTFE and thereafter the solvent was removed by drying. The loading of applied OER catalyst in the dried samples was determined to be about 32 micrograms/cm². (During the preparation process, it was noted that the 1% hydrocarbon ionomer dispersion remained stable for 2 weeks. Similar dispersions but without any hydrocarbon ionomer added were clearly unstable after about 6 hours.) The produced anode catalyst layer samples were easily peeled off the backing substrate, were self-supporting, and could be handled manually without any damage or failure.

SEM images of the prepared samples suggested that the coatings were uniformly applied through the complete thickness of the e-PTFE. Further, the particulate catalyst appeared to be uniformly distributed on the fibre structure of the e-PTFE. It appeared that there was sufficient volume within the pores to accommodate additional coating if desired.

Through-plane gas permeability values were measured for representative samples from each type of dispersion and were compared to values obtained from two different but conventional anode gas diffusion layers and the supplied e-PTFE prior to coating. The measurements were made using a standard Gurley permeability test procedure (in which a fixed volume of air is pushed through the samples under standard conditions and the time taken to do so is indicative of relative gas permeability). The as-supplied e-PTFE had a Gurley time of about 2 seconds. The two conventional anode GDLs had Gurley times of about 85 and 94 seconds. The representative samples prepared in accordance with the invention had Gurley times of about 64 and 102 seconds for the dispersions made with 0.5% and 1.0% hydrocarbon ionomer concentrations respectively. These results show that the gas permeability for these representative samples are in a range that is acceptable for use in fuel cell applications (i.e. similar to conventional GDLs). The gas permeabilities were substantially lower than that of the e-PTFE sheet per se. And, given the large difference observed in permeability between the representative samples, it is clear that the hydrocarbon ionomer concentration can have a significant effect on product permeability.

Experimental fuel cells were then prepared to roughly compare the performance and voltage reversal tolerance characteristics of cells made with OER anode catalyst layers of the invention to the characteristics of conventional fuel cells comprising OER anode catalyst as an admixture in the anode. The former cells comprised anode catalyst layers as prepared above using a 1% hydrocarbon ionomer concentration and are denoted as Inventive fuel cells in the following. The latter cells are denoted as Comparative fuel cells in the following. The anodes in both types of fuel cells employed equal amounts of Pt catalyst and ΓΓ<¾ catalyst as HOR and OER catalysts respectively. Further, each type of fuel cell used the same catalyst loadings. However, the Inventive fuel cells comprised a bilayer anode structure as shown in Figure 1. The Comparative fuel cells instead comprised a single anode layer comprising a simple admixture of the HOR and OER catalyst. Otherwise, the construction of the two types of cells was similar and conventional.

Figure 3 compares the polarization curves (voltage versus current density) of a Comparative fuel cell to an Inventive fuel cell under conventional operating conditions for automotive applications (e.g. using hydrogen and air as the supplied reactants at 50% RH, and at a temperature of 70°C). As is evident from Figure 3, the performance of the Inventive fuel cell was slightly inferior to that of the Conventional fuel cell but is roughly the same. In a similar performance comparison under conditions similar to automotive conditions in a hot environment, similar results were obtained. Namely, the performance of the Inventive fuel cell was somewhat inferior to that of the Conventional fuel cell but still similar enough.

The voltage reversal characteristics of cells were determined using testing that simulates an extended reversal event occurring in a cell in a stack that is undergoing fuel starvation. Here, the cells are first operated normally at a current density of 1 A/cm². Then the current is turned off, the reactant supply to the anode is switched from hydrogen to nitrogen instead, and 0.2 A/cm² is forced through the cells, thereby subjecting it to voltage reversal conditions. Typically, the cell voltage would roughly plateau at a value between 0 and about -3 volts for a variable amount of time and then drop off suddenly to a value much less than -5 V, at which point testing ended. The length of time to this sudden drop off point is representative of the cell's ability to tolerate voltage reversal. In this testing, an Inventive fuel cell exhibited a reversal time of 360 minutes while the Comparative fuel cell exhibited a reversal time of 850 minutes. While the reversal tolerance of the Inventive fuel cell was thus somewhat inferior compared to that of the Conventional fuel cell, it should be. noted that a similar fuel cell without any added OER anode catalyst only exhibits a reversal time of a mere 10 minutes.

The preceding Examples demonstrated that mechanically acceptable free-standing OER anode catalyst layers of the invention can easily be made by techniques amenable to mass production. While the results obtained to date in experimental fuel cells are somewhat inferior to state-of-the-art fuel cells, this may be a result of increases in interfacial resistances between the OER anode catalyst layer and the adjacent layers in the experimental fuel cells tested (e.g. between layer 5 and either or both of layers 4 and 10 in Figure 1). Improvements in these interfacial resistances may thus be expected to improve these results such that they are no longer inferior.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A free-standing oxygen evolution reaction (OER) anode catalyst layer for a solid polymer electrolyte fuel cell comprising:

a porous polymer sheet; and

a mixture comprising an OER catalyst and a hydrocarbon ionomer impregnated in the pores of the porous polymer sheet.

2. The OER anode catalyst layer of claim 1 wherein the porous polymer sheet is expanded polytetrafluoroethylene.

3. The OER anode catalyst layer of claim 1 wherein the porous polymer sheet is greater than 85% porous.

4. The OER anode catalyst layer of claim 1 wherein the average pore size of the porous polymer sheet is about 600 nm.

5. The OER anode catalyst layer of claim 1 wherein the OER catalyst comprises iridium oxide.

6. The OER anode catalyst layer of claim 1 wherein the average particle size of the OER catalyst is about 200 nm.

7. The OER anode catalyst layer of claim 1 wherein the loading of the OER catalyst is from about 20 to 50 micrograms/cm².

8. The OER anode catalyst layer of claim 7 wherein the loading of the OER catalyst is from about 30 to 35 micrograms/cm².

9. The OER anode catalyst layer of claim 1 wherein the hydrocarbon ionomer comprises sulfonated polyphenylene.

10. The OER anode catalyst layer of claim 9 wherein the mixture additionally comprises perfluorosulfonic acid ionomer or another type of hydrocarbon ionomer.

11. The OER anode catalyst layer of claim 1 wherein the weight ratio of the OER catalyst to the hydrocarbon ionomer is about 5: 1.

12. A catalyst coated membrane assembly for a solid polymer electrolyte fuel cell, the catalyst coated membrane assembly comprising a solid polymer electrolyte membrane comprising a proton-conducting membrane ionomer, and a cathode and a bilayer anode bonded to opposite sides of the solid polymer electrolyte membrane, the cathode comprising a cathode oxygen reduction reaction (ORR) catalyst, and the bilayer anode comprising a hydrogen oxidation reaction (HOR) anode catalyst layer adjacent the electrolyte membrane and an OER anode catalyst layer away from the electrolyte membrane, characterized in that the OER anode catalyst layer is the OER anode catalyst layer of claim 1.

13. The catalyst coated membrane assembly of claim 12 wherein the HOR catalyst comprises platinum supported on carbon particles.

14. A solid polymer electrolyte fuel cell comprising the catalyst coated membrane assembly of claim 12.

15. A method of making the OER anode catalyst layer of claim 1 comprising:

preparing a dispersion comprising the OER catalyst, the hydrocarbon ionomer, and a carrier solvent;

immersing the porous polymer sheet in the dispersion whereby the dispersion impregnates the pores of the porous polymer sheet; and

16. A method of making the OER anode catalyst layer of claim 1 comprising: - ·

preparing an ink comprising the OER catalyst, the hydrocarbon ionomer, and a solvent;

coating the ink onto the porous polymer sheet whereby the dispersion impregnates the pores of the porous polymer sheet; and

drying the impregnated porous polymer sheet, thereby removing the solvent.