ANODE CATALYST LAYER FOR USE IN A PROTON EXCHANGE MEMBRANE FUEL CELL

Abstract

A catalyst layer including: (i) a first catalytic material, wherein the first catalytic material facilitates a hydrogen oxidation reaction suitably selected from platinum group metals, gold, silver, base metals or an oxide thereof; and (ii) a second catalytic material, wherein the second catalytic material facilitates an oxygen evolution reaction, wherein the second catalytic material includes 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, wherein the transition metal is preferably selected from the group IVB, VB and VIB; and the first catalytic material is supported on the second catalytic material. The catalyst can be used in fuel cells, supported on electrodes or polymeric membranes for increasing tolerance to cell voltage reversal.

Claims

1. A method for increasing the tolerance to incidences of cell reversal at the anode of a proton exchange membrane fuel cell, said method comprising providing an anode catalyst layer comprising a first catalytic material which facilitates a hydrogen oxidation reaction and a second catalytic material which facilitates an oxygen evolution reaction, wherein the second catalytic material comprises iridium or iridium oxide and one or more metals M or an oxide thereof, wherein M is selected from the group consisting of Group IVB, VB and VIB metals and Sn, and wherein the first catalytic material is supported on the second catalytic material.

2. The method according to claim 1, wherein all the first catalytic material is supported on the second catalytic materials.

3. The method according to claim 1, wherein up to 90% of the first catalytic material is not supported on the second catalytic material.

4. The method according to claim 1, wherein M is selected from the group consisting of Ti, Zr, Hf, Nb, Ta and Sn.

5. The method according to claim 1, wherein M is selected from the group consisting of Ti, Ta and Sn.

6. The method according to claim 1, wherein the second catalytic material has a surface area of at least 10 m.sup.2/g.

7. The method according to claim 1, wherein the second catalytic material is a particulate, an aerogel, acicular or fibrous.

8. A method for improving the tolerance to incidences of cell reversal at the anode of a proton exchange membrane fuel cell, said method comprising providing an anode catalyst layer comprising Pt supported on IrTa oxide.

Description

[0018] Accordingly, the present invention provides a catalyst layer comprising: [0019] (i) a first catalytic material, wherein the first catalytic material facilitates a hydrogen oxidation reaction; and [0020] (ii) a second catalytic material, wherein the second catalytic material facilitates an oxygen evolution reaction, and wherein the second catalytic material 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,

[0021] characterised in that the first catalytic material is supported on the second catalytic material.

[0022] In one embodiment, all the first catalytic material is supported on the second catalytic material. In a second embodiment, some of the first catalytic material (for example up to 90%, suitably up to 70%, more suitably up to 50%, preferably up to 25% and more preferably up to 5%) is not supported on the second catalytic material and exists as discrete unsupported particles.

[0023] 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. In a preferred embodiment, M is not Ru.

[0024] 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).

[0025] The atomic ratio of iridium to (total) metal M in the second catalytic material is from 20:80 to 99:1, suitably 30:70 to 99:1 and preferably 60:40 to 99:1.

[0026] Suitably, the second catalytic material has a surface area of at least 10 m.sup.2/g, more suitably at least 15 m.sup.2/g, and preferably at least 30 m.sup.2/g.

[0027] The second catalytic material may be of any form suitable for forming a porous catalyst layer, for example a particulate, an aerogel (foam-like), acicular, fibrous etc. If fibrous, the fibres are suitably less than 500 nm in length, preferably less than 200 nm and may be made by a variety of process, including electrospinning

[0028] The first catalytic material comprises a metal (the primary metal), which is suitably selected from [0029] (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium), or [0030] (ii) gold or silver, or [0031] (iii) a base metal

[0032] or an oxide thereof.

[0033] 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. Suitably, the weight ratio of the primary metal of the first catalytic material to the second catalytic material is 1:99 to 70:30, preferably 5:95 to 40:60.

[0034] The first catalytic material may be supported on the second catalytic material by adding the second catalytic material (suitably in solid form) to an acidic solution of a precursor of the first catalytic material, with rapid stirring. Stirring is continued for several days, after which the resulting slurry was collected by filtration, washed and air-dried at elevated temperature.

[0035] The catalyst layer of the invention has utility in a number of applications, but particularly as the catalyst layer at the anode of a fuel cell, in particular a proton exchange membrane fuel cell. In one preferred embodiment, the catalyst layer is used at the anode of a proton exchange membrane fuel cell which is subject to incidences of cell reversal during practical real-life operation. The catalyst layer may also show improved tolerance to anode performance degradation caused by the presence of low levels of carbon monoxide impurities in the hydrogen fuel supply, particularly when compared to a low surface area HOR electrocatalyst, such as unsupported platinum (e.g. platinum black).

[0036] In one embodiment of the invention, the catalyst layer comprises a third catalytic material wherein the third catalytic material may be the same or different to the first catalytic material and comprises a metal (primary metal) as defined hereinbefore for the first catalytic material. Suitably, the third catalytic material is unsupported. Suitably, the third catalytic material is the same as the first catalytic material. The third catalytic material may account for 0-50% of the total of the first catalytic material and the third catalytic material.

[0037] Suitably, the total loading of the primary metal of the first (and if present third) catalytic material in the catalyst layer is less than 0.5 mg/cm.sup.2, and is preferably from 0.01 mg/cm.sup.2 to 0.4 mg/cm.sup.2, most preferably 0.02 mg/cm.sup.2 to 0.2 mg/cm.sup.2. The loading will depend on the use of the catalyst layer and suitable loadings will be known to those skilled in the art.

[0038] 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 Nafion and ionomers made from hydrocarbon polymers.

[0039] The catalyst layer of the invention has utility in PEM fuel cells. Accordingly, a further aspect of the invention provides an electrode, suitably an anode, comprising a gas diffusion layer (GDL) and a catalyst layer according to the invention.

[0040] 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 suitably based on conventional non-woven carbon fibre gas diffusion substrates such as rigid sheet carbon fibre papers (e.g. the TGP-H series of carbon fibre papers available from Toray Industries Inc., Japan) or roll-good carbon fibre papers (e.g. the H2315 based series available from Freudenberg FCCT KG, Germany; the Sigracet series available from SGL Technologies GmbH, Germany; the AvCarb series available from Ballard Material Products, United States of America; or the NOS series available from CeTech Co., Ltd. Taiwan), or on woven carbon fibre cloth substrates (e.g. the SCCG series of carbon cloths available from the SAATI Group, S.p.A., Italy; or the WOS series available from CeTech Co., Ltd, Taiwan), For many PEMFC and DMFC applications the non-woven carbon fibre paper, or woven carbon fibre cloth substrates are typically modified with a hydrophobic polymer treatment and/or application of a microporous layer comprising particulate material either embedded within the substrate or coated onto the planar faces, or a combination of both to form the gas diffusion layer. 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 m 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.

[0041] 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.

[0042] 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 FuMA-Tech GmbH as the fumapem P, E or K series of products, JSR Corporation, Toyobo Corporation, and others. The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength. For example, the membrane may comprise an expanded PTFE substrate. 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 Celtee-P membrane which will operate in the range 120 C. to 180 C.

[0043] 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.

[0044] 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: [0045] (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; [0046] (ii) a catalysed membrane coated on one side only by a catalyst layer may be sandwiched between (a) a gas diffusion layer and an electrode, the gas diffusion layer contacting the side of the membrane coated with the catalyst layer, or (b) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention; [0047] (iii) a catalysed membrane coated on both sides with a catalyst layer may be sandwiched between (a) two gas diffusion layers, (b) a gas diffusion layer and an electrode or (c) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention.

[0048] 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.

[0049] 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 fuelled 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.sup. 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.

[0050] 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.

[0051] The invention will now be further described by way of example only.

[0052] Preparation of IrTa mixed oxide catalyst 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.

[0053] Preparation of Pt/IrTa Oxide Catalyst

[0054] Hexachloroplatinic acid (H.sub.2PtCl.sub.6) solution containing 2.0 g of Pt was diluted to 500 ml with water. Formic acid (60 ml) was added to the Pt solution and stirred. To the resulting solution, IrTa Oxide (18.0 g) was added with rapid stirring. Stirring was continued for 6 days. The slurry was collected by filtration, washed copiously with water and dried in air at 105 C. The product was ground in a mortar and pestle.

[0055] Yield: 19.4 g

[0056] Metal assay (wt %): Pt=9.21%, Ir=46.3%, Ta=18.5%

[0057] CO metal area=13.6 m.sup.2/g-Pt

[0058] XRD characterised as indicating a Pt and IrTa oxide phase; Pt crystallite size 5.4 nm, IrTa oxide phase crystallite size 7.0 nm, lattice parameters a =4.584 , c=3.175 .