MULTI-LAYER FUEL CELL ELECTRODES WITH DIFFERENT LOADINGS ON THE SUPPORTED CATALYSTS

Abstract

The performance of a solid polymer membrane electrolyte fuel cell under various operating conditions can be improved via use of electrodes comprising multiple layers of supported catalyst in which the layers comprise different catalyst loadings on their respective supports. Such an electrode comprises a first component catalyst layer adjacent the membrane electrolyte and a second component catalyst layer adjacent the first component layer. The loading of the catalyst in the first component layer is greater than that in the second component layer.

Claims

1. A solid polymer electrolyte fuel cell comprising a solid polymer membrane electrolyte, an anode adjacent one side of the membrane electrolyte and comprising an anode catalyst layer, a cathode adjacent the other side of the membrane electrolyte and comprising a cathode catalyst layer, wherein at least one of the anode catalyst layer and the cathode catalyst layer comprises: a first component catalyst layer adjacent the membrane electrolyte wherein the first component catalyst layer comprises a first supported catalyst with a first catalyst loading on the support; and a second component catalyst layer adjacent the first component catalyst layer wherein the second component catalyst layer comprises a second supported catalyst with a second catalyst loading on the support; wherein the first catalyst loading is greater than the second catalyst loading.

2. The fuel cell of claim 1 wherein the cathode catalyst layer comprises: a first component catalyst layer adjacent the membrane electrolyte wherein the first component catalyst layer comprises a first supported catalyst with a first catalyst loading on the support; and a second component catalyst layer adjacent the first component catalyst layer wherein the second component catalyst layer comprises a second supported catalyst with a second catalyst loading on the support; wherein the first catalyst loading is greater than the second catalyst loading

3. The fuel cell of claim 1 wherein the first catalyst loading is greater than 50% of the weight of the first supported catalyst.

4. The fuel cell of claim 3 wherein the first catalyst loading is about 70% of the weight of the first supported catalyst.

5. The fuel cell of claim 1 wherein the second catalyst loading is less than 50% of the weight of the second supported catalyst.

6. The fuel cell of claim 5 wherein the second catalyst loading is about 30% of the weight of the second supported catalyst.

7. The fuel cell of claim 1 wherein the first catalyst loading is greater than twice that of the second catalyst loading by weight.

8. The fuel cell of claim 1 wherein the catalysts in the first and second supported catalysts comprise platinum.

9. The fuel cell of claim 1 wherein the supports in the first and second supported catalysts comprise carbon.

10. The fuel cell of claim 1 wherein the amount of catalyst in the first component layer is about the same as the amount of catalyst in the second component catalyst layer.

11. The fuel cell of claim 10 wherein the catalysts in the first and second supported catalysts comprise platinum and the amount of catalyst in the first and second component layers is about 0.25 mg Pt/cm.sup.2.

12. The fuel cell of claim 2 wherein the cathode catalyst layer is a cathode catalyst bilayer consisting of the first and second component catalyst layers.

13. The fuel cell of claim 2 wherein each of the first and second component layers comprise perfluorosulfonic acid ionomer and wherein the weight ratio of the ionomer to the supports in each of the first and second supported catalysts is about 1.

14. A method for making the fuel cell of claim 1 comprising: obtaining the first supported catalyst with the first catalyst loading on the support; obtaining the second supported catalyst with the second catalyst loading on the support wherein the first catalyst loading is greater than the second catalyst loading; and preparing the at least one of the anode catalyst layer and the cathode catalyst layer such that the first component catalyst layer adjacent the membrane electrolyte comprises the first supported catalyst, and the second component catalyst layer adjacent the first component catalyst layer comprises the second supported catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1a compares polarization plots of an inventive cell and several comparative cells operating under Normal operating conditions from the Examples.

[0016] FIG. 1b compares the cell voltages @ 1.9 A/cm.sup.2 versus number of accelerated stress test cycles of an inventive cell and several comparative cells operating under Normal operating conditions from the Examples.

[0017] FIG. 2a compares polarization plots of an inventive cell and several comparative cells operating under Hot and Dry operating conditions from the Examples.

[0018] FIG. 2b compares the cell voltages @ 1.5 A/cm.sup.2 versus number of accelerated stress test cycles of an inventive cell and several comparative cells operating under Hot and Dry operating conditions from the Examples.

[0019] FIG. 3 compares polarization plots of an inventive cell and several comparative cells operating under Cool and Dry operating conditions from the Examples.

DETAILED DESCRIPTION

[0020] 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%.

[0021] The term multiple means more than one (i.e. two or more).

[0022] The plain meaning of the term catalyst is used herein to refer to a substance which increases the rate of a chemical reaction without itself being affected. In particular herein, catalyst refers to those substances serving as an oxygen evolution catalyst and/or a hydrogen reduction catalyst

[0023] The term supported catalyst refers to a composition comprising a catalyst as defined above and a support material, in which the support material differs from the catalyst substance, and in which the catalyst has been deposited or dispersed on the support material.

[0024] With regards to supported catalysts, the term catalyst loading on the support refers to the amount of catalyst on a given amount of supported catalyst (e.g. % by weight of catalyst present on a given weight of supported catalyst). A supported catalyst as defined herein thus may have a catalyst loading on the support between 0% and 100%, but not equal to 0% or to 100%.

[0025] With regards to a catalyst layer, the total catalyst loading in the layer refers to the amount of catalyst present in a given amount of catalyst layer (e.g. % by weight of catalyst present in a given weight of catalyst layer).

[0026] In a like manner, with regards to an electrode, the total catalyst loading in the electrode refers to the amount of catalyst present in a given amount of electrode which may comprise more than one catalyst layer.

[0027] The present invention relates to solid polymer electrolyte fuel cells and stacks in which multi-layer electrodes are employed. The layers in the multi-layer electrodes comprise supported catalysts in which the catalyst loadings on the supports vary such that the catalyst loading in the layer adjacent the solid polymer membrane electrolyte is greater than the catalyst loading in the adjacent layer in the electrode. Use of such multi-layer electrodes can improve cell performance under various operating conditions. With the exception of the inventive multi-layer electrode structures, the construction of the fuel cells, and stacks thereof, can be any of the conventional constructions known to those in the art.

[0028] The multi-layer electrodes comprise two or more component catalyst layers with different compositions. In an assembled fuel cell of the invention, a first component catalyst layer in the multi-layer electrode would be located adjacent the membrane electrolyte. A second component catalyst layer would be located adjacent the first component catalyst layer on the side opposite the membrane electrolyte. Both first and second component catalyst layers in the electrode contain a suitable supported catalyst. The catalyst employed in each of the first and second component catalyst layers can be the same or different substances. Numerous options for use as catalyst in such fuel cells are known in the art. Noble metal compositions such as platinum, platinum alloys, and/or mixtures are commonly used in the art. Numerous options for use as supports in such fuel cells are also known in the art. Commonly, the supports are high surface area carbon powders (e.g. amorphous carbon powders having BET surface areas greater than about 50 m.sup.2/g, and particularly greater than about 300 m.sup.2/g).

[0029] In exemplary embodiments, platinum catalyst may be employed in both the first and second component layers. Typically, in the case of platinum catalyst, loading amounts in the range from about 20 to 70% by weight are considered to be manufacturable and hence may be considered in practice. (Loading amounts substantially greater than 70% would likely result in decreased performance because such a layer would increasingly obstruct mass transport. A pure platinum black layer for instance is harder to process into a homogenous layer. Conversely, having adequate carbon present helps processing, layer homogeneity, and ensures good electrical connectivity.) The total platinum loading in the prepared electrode may desirably be in the range from 0.1 to 0.5 mg/cm.sup.2 (e.g. about 0.25 mg/cm.sup.2). The component catalyst layers typically also include perfluorosulfonic acid ionomer and optionally other components (e.g. conductive fillers, additives to modify hydrophobicity, etc.). In an exemplary embodiment, the weight ratio of the ionomer to that of the carbon powder support can be about 1 in a layer, and the equivalent weight of the ionomer employed can be about 825. However, a wide range of weight ratios of ionomer to carbon powder support may be contemplated. And numerous options for the ionomer type and equivalent weight may be considered.

[0030] In the inventive structure, the catalyst loading on the supports in the first component catalyst layer is greater than the catalyst loading on the supports in the second component catalyst layer.

[0031] Notwithstanding this, the total catalyst loading in the first component catalyst layer as a whole may be the same as, or in principle even less than, the total catalyst loading in the second component catalyst layer. In addition, the total catalyst loading in the complete electrode may be the same as, or in principle different from, those loadings typically used in conventional fuel cell electrodes.

[0032] In exemplary embodiments, the catalyst loading on the supports in the first component catalyst layer (e.g. 70% by weight) is significantly greater than the catalyst loading on the supports in the second component catalyst layer (e.g. 30% by weight). The total catalyst loadings in each of the two component layers however can be the same. As illustrated in the Examples below, this can result in substantial performance improvements under many operating conditions. However, other loading amounts in the individual layers can be considered (e.g. the total catalyst loading in the first component layer may be substantially greater than that in the second component layer or vice versa).

[0033] Multi-layer electrodes of the invention can be prepared in various ways known to those skilled in the art. As an example, decal transfer methods may be employed. In such methods, a suitable catalyst ink is applied using screen printing or Mayer rod techniques to a transfer medium and the ink is dried to create a catalyst layer on the transfer medium. For these multiple layer embodiments, this process may simply be repeated using the desired catalyst inks in an appropriate sequence. The transfer medium with applied catalyst layers is then contacted with a desired membrane electrolyte and the multi-layer catalyst electrode is then decal transferred to the membrane by applying suitable heat and pressure. In other examples though, spray coating techniques may be used. Further, the coatings may be applied directly to the membrane electrolyte. And further still, the multi-layer electrode may be applied to a gas diffusion layer instead of the membrane.

[0034] As illustrated in the Examples below, use of the inventive multi-layer catalyst electrodes in solid polymer electrolyte fuel cells, and particularly at the cathodes, can result in significant performance improvements under a variety of operating conditions. Without being bound by theory, it is believed that the high concentration of platinum near the membrane and an increasing porosity gradient toward the gas diffusion layer provides a structure that balances proton conductivity and gas access requirements for the catalyst layer. This in turn should improve catalyst layer function and increase cell performance. Such an approach may provide benefits at both the anode and the cathode due to enhanced gas and water transport.

[0035] The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

EXAMPLES

[0036] In the following, supplies of conventional carbon supported catalyst were obtained and portions were dry milled in accordance with the method of the invention. And experimental fuel cells were made using these catalyst compositions and the performance results under various operating conditions were obtained and compared.

[0037] In these Examples, three types of commercially available carbon supported catalyst powders were employed. The carbon supports in all three were similar high surface area amorphous carbons. One type, denoted 50%, nominally comprised 50% by weight platinum catalyst highly dispersed over the carbon support surface. A second type, denoted 30%, nominally comprised 30% by weight platinum catalyst highly dispersed over the carbon support surface. A third type, denoted 70%, nominally comprised 70% by weight platinum catalyst highly dispersed over the carbon support surface.

[0038] A series of experimental fuel cells were made using these various catalyst powders in several different cathode electrode variations. The membrane electrolyte used in each cell was made of perfluorosulfonic acid ionomer that was about 15-20 micrometer thick. In all cases, catalyst layers were decal transferred onto opposite sides of the membrane to form the anode and cathode electrodes. In decal transfer, a suitable catalyst ink was first applied to a transfer medium and dried to create a catalyst layer on the transfer medium. In cases where multiple layers were involved, the process was repeated using the desired catalyst inks in an appropriate sequence. The transfer medium with applied catalyst layer was then contacted with the membrane and the catalyst layer or layers were decal transferred thereto via the application of heat and pressure. Catalyst inks for these coatings were prepared by mixing about 50 wt % of a specific carbon supported with about 20 wt % 825 EW (equivalent weight) ionomer solution, 1-propanol and distilled water resulting in a nominal ionomer to carbon weight ratio of one. The inks were jarmilled (wet) for a set period of time. The inks were then coated onto Teflon film using a Mayer rod and dried at about 60 to 80 C. for a few hours. Bilayer structures were obtained by sequential coatings of the two catalyst inks.

[0039] The anode catalyst in each cell was a conventional commercial carbon supported platinum (Pt/C) product comprising about 46% Pt by weight and the anode layer comprised about 0.05 to 0.1 mg/cm.sup.2 of Pt. Each cathode combination comprised the same total Pt loading of about 0.25 mg/cm.sup.2. Further, the weight ratio of ionomer to that of the carbon powder in the catalyst supports in each combination was about 1. However, several different catalyst layer combinations were used for the cathodes in these cells. A combination for use in a comparative cell comprised a single layer of 50% catalyst. A combination for use in another comparative cell comprised a single layer of a mix of 30% and 70% catalysts in which equal portions of the two catalysts were used by weight of Pt. Another combination for a comparative cell comprised two layers in which the first layer adjacent the membrane electrolyte solely comprised 30% catalyst and the second layer solely comprised 70% catalyst. The amounts were adjusted such that the weight of Pt in each layer was the same. Finally, a combination for an inventive cell comprised two layers in which the layers were reversed, namely the first layer adjacent the membrane electrolyte solely comprised 70% catalyst this time and the second layer solely comprised 30% catalyst. Again, the amounts were adjusted such that the weight of Pt in each layer was the same. In the following, the comparative cells comprising these different cathode catalyst layer combinations are denoted 50% single layer, 30%+70% single layer, and 30%/70% double layer respectively. The inventive cell is denoted 70%/30% double layer.

[0040] To finish fabricating individual experimental fuel cells, the preceding catalyst coated membranes (CCMs) were sandwiched between anode and cathode gas diffusion layers (GDLs) comprising commercial carbon fibre paper from Freudenberg. Assemblies comprising the appropriate CCMs and anode and cathode GDLs were then bonded together under elevated temperature and pressure and placed between appropriate cathode and anode flow field plates having straight flow field channels in order to complete the experimental fuel cell constructions.

[0041] These fuel cells were first conditioned by operating at a current density of 1.0 A/cm.sup.2, with hydrogen and air as the supplied reactants at high stoichiometries and at 100% relative humidity (RH), and at a temperature of about 70 C. overnight. Then, the performance characteristics of the fuel cells were obtained by measuring output voltage as a function of current density under a variety of operating conditions that would typically be experienced in automotive applications. In certain cases, the fuel cells were also subjected to accelerated stress testing which focused primarily on cathode catalyst layer degradation over time. This involved subjecting the fuel cell to voltage cycling between 0.1 and 1.0 volts using a square wave cycle of 2 sec and 2 sec duration respectively. After every 5000 cycles, cell performance was again evaluated by obtaining polarization curves as before. Representative results are shown in the following figures.

[0042] The operating conditions involved in this Example are summarized below and include: [0043] Normal: 68 C., 70% RH [0044] Hot and Dry: 85 C., 45% RH [0045] Cool and Dry: 40 C., 50% RH

[0046] FIG. 1a compares the polarization plots (voltage versus current density from 0 to over 2 A/cm.sup.2) of the inventive 70%/30% double layer cell to all the comparative cells operating under Normal operating conditions. The inventive 70%/30% double layer cell performed as well or better than all the comparative cells. Stress testing was then carried on all the cells under Normal operating conditions. FIG. 1b compares the cell voltages @ 1.9 A/cm.sup.2 versus number of accelerated stress test cycles under the Normal operating conditions. The inventive 70%/30% double layer cell showed significantly better results under stress testing than all the comparative cells. The comparative 30%/70% double layer cell on the other hand showed significantly worse results than all the other cells.

[0047] In a like manner to FIG. 1a, FIG. 2a compares polarization plots of all the cells, but operating under Hot and Dry operating conditions. Again, the inventive 70%/30% double layer cell performed as well as the comparative 30%+70% single layer cell, and performed significantly better than both the more conventional 50% single layer and the comparative 30%/70% double layer cells. Stress testing was then carried on all the cells under Hot and Dry operating conditions. FIG. 2b compares the cell voltages (@ 1.5 A/cm.sup.2 in this case) versus number under the Hot and Dry operating conditions. Again, the inventive 70%/30% double layer cell showed significantly better results under stress testing here than all the comparative cells.

[0048] FIG. 3 compares polarization plots of all the cells, but this time operating under Cool and Dry operating conditions. Again, the inventive 70%/30% double layer cell performed as well or better than all the comparative cells. Interestingly, the 30%+70% single layer cell performed significantly worse than all the others.

[0049] Table 1 below compares the observed output voltages of all the fuel cells @ 1.9 A/cm.sup.2 when operating under Hot and Dry operating conditions. Once again, the inventive 70%/30% double layer cell showed the best performance. The comparative 30%/70% double layer cell on the other hand showed the worst results.

TABLE-US-00001 TABLE 1 Fuel cell Cell voltage @ 1.9 A/cm.sup.2 50% single layer 0.452 30% + 70% single layer 0.484 30%/70% double layer 0.425 70%/30% double layer 0.485

[0050] The preceding Examples thus demonstrate that superior results are obtained with the inventive double layer cell under many different operating conditions, and that results at least as good as the other cells under all operating conditions.

[0051] 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.

[0052] 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. For instance, the invention is not limited just to fuel cells operating on pure hydrogen fuel but also to fuel cells operating on any hydrogen containing fuel or fuels containing hydrogen and different contaminants, such as reformate which contains CO and methanol. Such modifications are to be considered within the purview and scope of the claims appended hereto.