Catalyst Layer For Use In A Fuel Cell

Abstract

A catalyst layer includes (i) an electrocatalyst, and (ii) a water electrolysis catalyst, iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from transition metals and/or Sn, with the exception of ruthenium. Such a catalyst layer has utility in fuel cells that experience high electrochemical potentials.

Claims

1. A catalyzed membrane comprising a solid polymeric membrane, a first catalyst layer, and a second catalyst layer, wherein at least one of the first catalyst layer or the second catalyst layer comprises: (i) an electrocatalyst, and (ii) a water electrolysis catalyst, wherein the water electrolysis catalyst comprises a single oxide structure comprising iridium and tantalum, wherein said catalyzed membrane is configured for use in a proton exchange membrane fuel cell.

2. The catalyzed membrane according to claim 1, wherein the water electrolysis catalyst is unsupported.

3. The catalyzed membrane according to claim 1, wherein the electrocatalyst comprises a metal which is selected from the group consisting of: (i) platinum group metals. (ii) gold or silver, (iii) abase metal, and an oxide thereof.

4. The catalyzed membrane according to claim 1, wherein the electrocatalyst is supported on an inert support.

5. The catalyzed membrane according to claim 4, wherein the inert support is non-carbonaceous.

6. The catalyzed membrane according to claim 1, wherein the electrocatalyst is unsupported.

7. The catalyzed membrane according to claim 6, wherein the electrocatalyst is unsupported platinum.

8. An electrode comprising a gas diffusion layer and a catalyst layer, wherein: the gas diffusion layer comprises a gas diffusion substrate based on carbon paper, woven carbon cloths or non-woven carbon fiber webs; and the catalyst layer comprises: (i) an electrocatalyst, and (ii) a water electrolysis catalyst, wherein the water electrolysis catalyst comprises a single oxide structure comprising iridium and tantalum.

9. A membrane electrode assembly comprising the electrode of claim 8.

10. A membrane electrode assembly comprising the catalyzed membrane of claim 1.

11. A fuel cell comprising the electrode of claim 8.

12. A fuel cell comprising the catalyzed membrane of claim 1.

13. The catalyzed membrane according to claim 1, wherein the weight ratio of water electrolysis catalyst to electrocatalyst is from 10:1 to 1:10.

14. The catalyzed membrane according to claim 1, wherein the weight ratio of water electrolysis catalyst to electrocatalyst is from 0.75:1 to 5:1.

15. The electrode of claim 8, wherein the water electrolysis catalyst is unsupported.

16. The electrode of claim 8, wherein the electrocatalyst comprises a metal which is selected from the group consisting of: (i) platinum group metals, (ii) gold or silver, (iii) a base metal, and an oxide thereof.

17. The electrode of claim 8, wherein the electrocatalyst is supported on an inert support.

18. The electrode of claim 17, wherein the inert support is non-carbonaceous.

19. The electrode according to claim 8, wherein the electrocatalyst is unsupported.

20. The electrode according to claim 8, wherein the electrocatalyst is unsupported platinum.

21. The electrode according to claim 8, wherein the weight ratio of water electrolysis catalyst to electrocatalyst is from 10:1 to 1:10.

22. The electrode according to claim 8, wherein the weight ratio of water electrolysis catalyst to electrocatalyst is from 0.75:1 to 5:1.

23. A catalyzed membrane comprising: (i) an anode catalyst layer comprising: an electrocatalyst comprising platinum; and a water electrolysis catalyst comprising a single oxide structure comprising iridium and tantalum; (ii) a perfluorosulphonic acid membrane; and (iii) a cathode catalyst layer comprising platinum.

24. The catalyzed membrane according to claim 23, wherein the electrocatalyst comprises carbon-supported platinum.

25. The catalyzed membrane according to claim 23 wherein the cathode catalyst layer comprises carbon-supported platinum.

26. The catalyzed membrane according to claim 23 wherein the water electrolysis catalyst is unsupported.

27. A membrane electrode assembly comprising: (i) a first gas diffusion layer comprising a first waterproofed carbon paper coated with a first hydrophobic microporous layer; (ii) the catalyzed membrane according to claim 23; and (iii) a second gas diffusion layer comprising a second waterproofed carbon paper coated with a second hydrophobic microporous layer.

28. The membrane electrode assembly according to claim 27, wherein the electrocatalyst comprises carbon-supported platinum.

29. The membrane electrode assembly according to claim 27, wherein the cathode catalyst layer comprises carbon-supported platinum.

30. A catalyzed membrane comprising: (i) an anode catalyst layer comprising platinum; (ii) a perfluorosulphonic acid membrane; and (iii) a cathode catalyst layer comprising: an electrocatalyst comprising platinum; and a water electrolysis catalyst comprising a single oxide structure comprising iridium and tantalum.

31. The catalyzed membrane according to claim 30, wherein the anode catalyst layer comprises carbon-supported platinum.

32. The catalyzed membrane according to claim 30, wherein the electrocatalyst comprises carbon-supported platinum.

33. The catalyzed membrane according to claim 30, wherein the water electrolysis catalyst is unsupported.

34. A membrane electrode assembly comprising: (i) a first gas diffusion layer comprising a first waterproofed carbon paper coated with a first hydrophobic microporous layer; (ii) the catalyzed membrane according to claim 30; and (iii) a second gas diffusion layer comprising a second waterproofed carbon paper coated with a second hydrophobic microporous layer.

35. The membrane electrode assembly according to claim 34, wherein the anode catalyst layer comprises carbon supported platinum.

36. The membrane electrode assembly according to claim 34, wherein the electrocatalyst comprises carbon-supported platinum.

37. The catalyzed membrane of claim 1, wherein the water electrolysis catalyst is prepared by: (a) combining a suspension of IrCl.sub.3 in water with a solution of TaCl.sub.5 in concentrated hydrochloric acid to form a precursor solution; (b) spray drying the solution to form a spray dried powder; and (c) calcining the spray dried powder in air to obtain the water electrolysis catalyst.

38. The electrode of claim 8, wherein the water electrolysis catalyst is prepared by: (a) combining a suspension of IrCl.sub.3 in water with a solution of TaCl.sub.5 in concentrated hydrochloric acid to form a precursor solution; (b) spray drying the solution to form a spray dried powder; and (c) calcining the spray dried powder in air to obtain the water electrolysis catalyst.

39. The catalyzed membrane of claim 23, wherein the water electrolysis catalyst is prepared by: (a) combining a suspension of IrCl.sub.3 in water with a solution of TaCl.sub.5 in concentrated hydrochloric acid to form a precursor solution; (b) spray drying the solution to form a spray dried powder; and (c) calcining the spray dried powder in air to obtain the water electrolysis catalyst.

40. The catalyzed membrane of claim 30, wherein the water electrolysis catalyst is prepared by: (a) combining a suspension of IrCl.sub.3 in water with a solution of TaCl.sub.5 in concentrated hydrochloric acid to form a precursor solution; (b) spray drying the solution to form a spray dried powder; and (c) calcining the spray dried powder in air to obtain the water electrolysis catalyst.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0023] FIG. 1 provides the results of an assessment of cell reversal tolerance among conventional membrane electrode assemblies and embodiments according to the present disclosure.

[0024] FIG. 2 illustrates the results of an evaluation of the impact of prolonged cell reversal conditions on fuel cell performance for a membrane electrode assembly according to the present disclosure.

[0025] FIG. 3 depicts the results of a determination of the loss of cell voltage with multiple simulated start-up and shut-down cycles for comparative example 4 and example 4 according to the present disclosure.

DETAILED DESCRIPTION

[0026] It is therefore an object of the present invention to provide a catalyst layer comprising alternative water electrolysis catalysts, which have comparable activity to state of the art water electrolysis catalysts for the oxygen evolution reaction, but which demonstrates good performance and durability when incorporated in a MEA and operated under practical real-life fuel cell operating conditions.

[0027] Accordingly, the present invention provides a catalyst layer comprising: [0028] (i) an electrocatalyst; and [0029] (ii) a water electrolysis catalyst, wherein the water electrolysis catalyst comprises iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from the group consisting of transition metals and Sn, with the exception of ruthenium.

[0030] Suitably, M is selected from the group consisting of group IVB, VB and VIB metals and Sn; more suitably selected from the group consisting of Ti, Zr, Hf, Nb, Ta and Sn; preferably selected from the group consisting of Ti, Ta and Sn.

[0031] The iridium or oxide thereof and the one or more metals (M) or oxide thereof may either exist as mixed metals or oxides or as partly or wholly alloyed materials or as a combination of the two or more. The extent of any alloying can be shown by x-ray diffraction (XRD).

[0032] The atomic ratio of iridium to (total) metal M in the water electrolysis catalyst is from 20:80 to 99:1, suitably 30:70 to 99:1 and preferably 60:40 to 99:1.

[0033] The electrocatalyst comprises a metal (the primary metal), which is suitably selected from [0034] (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium), or [0035] (ii) gold or silver, or [0036] (iii) a base metal [0037] or an oxide thereof.

[0038] The primary metal may be alloyed or mixed with one or more other precious metals, or base metals or an oxide of a precious metal or base metal. The metal, alloy or mixture of metals may be unsupported or supported on a suitable inert support. In one embodiment, if the electrocatalyst is supported, the support is non-carbonaceous. Examples of such a support include titania, niobia, tantala, tungsten carbide, hafnium oxide or tungsten oxide. Such oxides and carbides may also be doped with other metals to increase their electrical conductivity, for example niobium doped titania. In one preferred embodiment, the electrocatalyst is unsupported platinum.

[0039] The electrocatalyst and water electrolysis catalyst may be present in the catalyst layer either as separate layers or as a mixed layer or as a combination of the two. If present as separate layers, the layers are suitably arranged such that the water electrolysis layer is next to the membrane and therefore supplied with water diffusing back to the anode from the cathode. In a preferred embodiment, the electrocatalyst and the water electrolysis catalyst are present in the catalyst layer as a mixed layer.

[0040] Suitably, the ratio of the water electrolysis catalyst to electrocatalyst in the catalyst layer is from 10:1 to 1:10 with the electrocatalyst. The actual ratio will depend on whether the catalyst layer is on the anode or cathode. In the case of an anode catalyst layer, the ratio is suitably from 0.05:1 to 10:1; preferably, from 0.75:1 to 5:1. In the case of a cathode catalyst layer, the ratio is suitably from 1:1 to 1:10; preferably from 0.5:1 to 1:5.

[0041] Suitably, the loading of the primary metal of the electrocatalyst in the catalyst layer is less than 0.4 mg/cm.sup.2, and is preferably from 0.01 mg/cm.sup.2 to 0.35 mg/cm.sup.2, most preferably 0.02 mg/cm.sup.2 to 0.25 mg/cm.sup.2.

[0042] The catalyst layer may comprise additional components, such as an ionomer, suitably a proton conducting ionomer. Examples of suitable proton conducting ionomers will be known to those skilled in the art, but include perfluorosulphonic acid ionomers, such as Nation® and ionomers made from hydrocarbon polymers.

[0043] The catalyst layer of the invention has utility in PEM fuel cells. Accordingly, a further aspect of the invention provides an electrode comprising a gas diffusion layer (GDL) and a catalyst layer according to the invention.

[0044] In one embodiment, the electrode is an anode, wherein the water electrolysis catalyst comprises iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from the group consisting of transition metals and Sn, with the exception of ruthenium. Suitably, M is selected from the group consisting of group IVB, VB and VIB metals and Sn; more suitably selected from the group consisting of Ti, Zr, Hf, Nb, Ta and Sn; preferably selected from the group consisting of Ti, Ta and Sn.

[0045] In a further embodiment, the electrode is a cathode wherein the water electrolysis catalyst comprises iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from the group consisting of transition metals and Sn, with the exception of ruthenium. Suitably, M is selected from the group consisting of group IVB, VB and VIB metals and Sn; more suitably selected from the group consisting of Ti, Zr, Hf, Nb, Ta and Sn; preferably selected from the group consisting of Ti, Ta and Sn.

[0046] The catalyst layer can be deposited onto a GDL using well known techniques, such as those disclosed in EP 0 731 520. The catalyst layer components may be formulated into an ink, comprising an aqueous and/or organic solvent, optional polymeric binders and optional proton-conducting polymer. The ink may be deposited onto an electronically conducting GDL using techniques such as spraying, printing and doctor blade methods. Typical GDLs are fabricated from substrates based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan or U105 or U107 paper available from Mitsubishi Rayon, Japan), woven carbon cloths (e.g. the MK series of carbon cloths available from Mitsubishi Chemicals, Japan) or non-woven carbon fibre webs (e.g. AvCarb series available from Ballard Power Systems Inc, Canada; H2315 series available from Freudenberg FCCT KG, Germany; or Sigracet® series available from SGL Technologies GmbH, Germany). The carbon paper, cloth or web is typically modified with a particulate material either embedded within the layer or coated onto the planar faces, or a combination of both to produce the final GDL. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). Suitably the GDLs are between 100 and 400 μm thick. Preferably there is a layer of particulate material such as carbon black and PTFE on the face of the GDL that contacts the catalyst layer.

[0047] In PEM fuel cells, the electrolyte is a proton conducting membrane. The catalyst layer of the invention may be deposited onto one or both faces of the proton conducting membrane to form a catalysed membrane. In a further aspect the present invention provides a catalysed membrane comprising a proton conducting membrane and a catalyst layer of the invention. The catalyst layer can be deposited onto the membrane using well-known techniques. The catalyst layer components may be formulated into an ink and deposited onto the membrane either directly or indirectly via a transfer substrate.

[0048] The membrane may be any membrane suitable for use in a PEM fuel cell, for example the membrane may be based on a perfluorinated sulphonic acid material such as Nafion® (DuPont), Flemion® (Asahi Glass) and Aciplex® (Asahi Kasei); these membranes may be used unmodified, or may be modified to improve the high temperature performance, for example by incorporating an additive. Alternatively, the membrane may be based on a sulphonated hydrocarbon membrane such as those available from Polyfuel, JSR Corporation, FuMA-Tech GmbH and others. The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength, such as expanded PTFE or a non-woven PTFE fibre network. Alternatively, the membrane may be based on polybenzimidazole doped with phosphoric acid and include membranes from developers such as BASF Fuel Cell GmbH, for example the Celtec®-P membrane which will operate in the range 120° C. to 180° C. and other newer developmental membrane such as the Celtec®-V membrane.

[0049] In a further embodiment of the invention, the substrate onto which the catalyst of the invention is applied is a transfer substrate. Accordingly, a further aspect of the present invention provides a catalysed transfer substrate comprising a catalyst layer of the invention. The transfer substrate may be any suitable transfer substrate known to those skilled in the art but is preferably a polymeric material such as polytetrafluoroethylene (PTFE), polyimide, polyvinylidene difluoride (PVDF), or polypropylene (especially biaxially-oriented polypropylene, BOPP) or a polymer-coated paper such as polyurethane coated paper. The transfer substrate could also be a silicone release paper or a metal foil such as aluminium foil. The catalyst layer of the invention may then be transferred to a GDL or membrane by techniques known to those skilled in the art.

[0050] A yet further aspect of the invention provides a membrane electrode assembly comprising a catalyst layer, electrode or catalysed membrane according to the invention. The MEA may be made up in a number of ways including, but not limited to:

[0051] (i) a proton conducting membrane may be sandwiched between two electrodes (one anode and one cathode), at least one of which is an electrode according to the present invention;

[0052] (ii) a catalysed membrane coated on one side only by a catalyst layer may be sandwiched between (i) a gas diffusion layer and an electrode, the gas diffusion layer contacting the side of the membrane coated with the catalyst layer, or (ii) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention;

[0053] (iii) a catalysed membrane coated on both sides with a catalyst layer may be sandwiched between (i) two gas diffusion layers, (ii) a gas diffusion layer and an electrode or (iii) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention.

[0054] The MEA may further comprise components that seal and/or reinforce the edge regions of the MEA for example as described in WO2005/020356. The MEA is assembled by conventional methods known to those skilled in the art.

[0055] Electrochemical devices in which the catalyst layer, electrode, catalysed membrane and MEA of the invention may be used include fuel cells, in particular proton exchange membrane (PEM) fuel cells. The PEM fuel cell could be operating on hydrogen or a hydrogen-rich fuel at the anode or could be fueled with a hydrocarbon fuel such as methanol. The catalyst layer, electrode, catalysed membrane and MEA of the invention may also be used in fuel cells in which the membranes use charge carriers other than protons, for example OH— conducting membranes such as those available from Solvay Solexis S.p.A., FuMA-Tech GmbH. The catalyst layer and electrode of the invention may also be used in other low temperature fuel cells that employ liquid ion conducting electrolytes, such as aqueous acids and alkaline solutions or concentrated phosphoric acid. Other electrochemical devices in which the catalyst layer, electrode, catalysed membrane and MEA of the invention may be used are as the anode electrode of regenerative fuel cells where the hydrogen oxidation and oxygen evolution reactions are both performed, and as the anode of an electrolyser where oxygen evolution is performed by the water electrolysis catalyst and contaminant hydrogen is recombined with oxygen by the electrocatalyst.

[0056] Accordingly, a further aspect of the invention provides a fuel cell, preferably a proton exchange membrane fuel cell, comprising a catalyst layer, an electrode, a catalysed membrane or an MEA of the invention.

[0057] The invention will now be further described by way of example only.

[0058] Preparation of Water Electrolysis Catalysts

[0059] IrTa Mixed Oxide Catalyst

[0060] IrCl.sub.3 (76.28 g, 0.21 mol Ir) was suspended in water (500 ml) and stirred overnight. TaCl.sub.5 (32.24 g, 0.090 mol Ta) was added to concentrated hydrochloric acid (200 ml) with stirring to give a slightly milky solution. The Ta solution was stirred into the IrCl.sub.3 solution and kept until ready to use. The solution was spray dried and calcined in air to yield a 70 at % Ir 30 at % Ta mixed oxide catalyst.

[0061] IrSn Mixed Oxide Catalyst

[0062] An IrSn mixed oxide water electrolysis catalyst was prepared in an analogous manner to the IrTa mixed oxide water electrolysis catalyst described above. A 70 at % Ir 30 at % Sn mixed oxide catalyst was obtained

[0063] IrTi Mixed Oxide Catalyst

[0064] High surface area TiO.sub.2 (3.0 g) was stirred in water (500 ml) and IrCl.sub.3 (92.2 g) added. The suspension was warmed to 75° C. and 1M NaOH was added dropwise until the pH remained stable at 7. The suspension was cooled, and the catalyst product was collected by filtration and washed with water. The material was calcined in air to yield a 87 at % Ir 13 at % Ti mixed oxide catalyst.

[0065] Anodes catalyst layers were made as listed in Table 1, by screen printing the appropriate ink onto a decal transfer substrate to give the required loading. The catalyst inks were made according to the techniques described in EP 0 731 520. Where the ink contained both electrocatalyst and water electrolysis catalyst, an ink containing the electrocatalyst was first made, and the water electrolysis catalyst was subsequently added.

TABLE-US-00001 TABLE 1 Ratio of water Water electrolysis Electrolysis catalyst:electro- Example No. Electrocatalyst Catalyst catalyst Comparative 1 Pt/carbon (0.225 mg Pt cm.sup.−2) Comparative 2 Pt/carbon RuO.sub.2/IrO.sub.2 1:1.25 (0.2 mg Pt cm.sup.−2) (90:10 at % Ru:Ir, 0.16 mgcm.sup.−2) Comparative 3 Pt black IrO.sub.2 1:1.25 (0.36 mg Pt cm.sup.−2) (0.29 mgcm.sup.−2) Comparative 4 Pt/heat-treated carbon (0.384 mg Pt cm.sup.−2) Example 1 Pt black IrO.sub.2/Ta.sub.2O.sub.5 1:1.25 (0.33 mg Pt cm.sup.−2) (70:30 at % Ir:Ta, 0.26 mgcm.sup.−2) Example 2 Pt black IrSn 1:1.25 (0.38 mg Pt cm.sup.−2) (70:30 at % Ir:Sn, 0.30 mg cm.sup.−2) Example 3 Pt black IrO.sub.2/TiO.sub.2 1:1.25 (0.38 mg Pt cm.sup.−2) (87:13 at %, 0.30 mgcm.sup.−2) Example 4 Pt/heat-treated IrO.sub.2/Ta.sub.2O.sub.5 1:10 carbon (70:30 at % Ir:Ta, (0.44 mg Pt cm.sup.−2) at0.044 mg cm.sup.−2

[0066] An MEA was produced by combining the anode with a conventional, carbon supported cathode catalyst layer of ˜0.4 mg Pt cm.sup.−2 and a perfluorinated sulfonic acid membrane by the well known decal transfer method to produce a catalyst coated membrane (CCM). The CCM was assembled between two sheets of waterproofed carbon paper coated with a hydrophobic microporous layer to form the complete membrane electrode assembly. This was then tested in a 1 cm.sup.2 active area fuel cell under simulated starvation conditions at 80° C. The cell was first operated with humidified hydrogen and air flowing over the anode and cathode respectively. A current of 500 mA cm.sup.−2 was applied for 5 minutes to allow the MEA to reach a constant condition. The current was then dropped to 200 mA cm.sup.−2 and the hydrogen supply switched to nitrogen. The current drawn from the cell was then kept constant until either 90 minutes passed or the cell voltage dropped below −2.5 V. The results are shown in FIG. 1. The results indicate that the catalyst layers of the invention (Examples 1, 2 and 3) perform better than Comparative Example 1 and comparably to Comparative Example 2 and 3.

[0067] A similar MEA was prepared from Example 1 and then tested in a 242 cm.sup.2 active area fuel cell under simulated starvation conditions at 80° C. The cell was first operated with humidified hydrogen and air flowing over the anode and cathode respectively. A current of 500 mA cm.sup.−2 was applied for 5 minutes to allow the MEA to reach a constant condition. The current was then dropped to 200 mA cm.sup.−2 and the hydrogen supply switched to nitrogen. The current drawn from the cell was then kept constant until either 90 minutes passed or the cell voltage dropped below −2.5 V. Polarisation curves were measured using air, 21% oxygen in helium (Helox) and pure oxygen both before and after the reversal testing to see if any damage had occurred. The results are shown in FIG. 2. The performance of the MEA after 90 minutes of reversal matches the initial performance very closely across all conditions indicating the stability of the electrode.

[0068] MEAs were also prepared with catalyst layers according to Comparative 4 and Example 4 on the cathode and a standard catalyst layer on the anode. The resistance of the MEA to simulated start-stop conditions was tested by mounting the MEA in a single cell with an active area of 242 cm.sup.2 and, after conditioning, subjecting the MEA to the following sequence: (i) holding the current at a relatively high current density for 15 minutes with hydrogen on the anode and air on the cathode; (ii) reducing the load and holding for 30 seconds; (iii) stopping the supply of hydrogen to the anode, removing the load and purging the anode and cathode with air; (iv) reintroducing the hydrogen to the anode and holding at a very low current density for 10 seconds; (v) applying a similar load to that in step (ii) and maintaining for 30 seconds; (vi) increasing the load to a medium current density and holding for 5 minutes.

[0069] A single cycle consists of steps (ii) to (vi); step (i) was performed initially and then after every ten cycles. The performance loss during step (vi) was monitored as a function of the number of cycles.

[0070] The loss of cell voltage for Comparative Example 4 and Example 4 are shown in FIG. 3. It takes many more cycles for Example 4 to reach the same loss of cell voltage as Comparative Example 4 because of the protective action of the IrTa water electrolysis catalyst.