Use of an anode catalyst layer

Abstract

A method of operating a fuel cell having an anode, a cathode and a polymer electrolyte membrane disposed between the anode and the cathode, includes feeding the anode with an impure hydrogen stream having low levels of carbon monoxide up to 5 ppm, and wherein the anode includes an anode catalyst layer including a carbon monoxide tolerant catalyst material, wherein the catalyst material includes: (i) a binary alloy of PtX, wherein X is a metal selected from the group consisting of rhodium and osmium, and wherein the atomic percentage of platinum in the alloy is from 45 to 80 atomic % and the atomic percentage of X in the alloy is from 20 to 55 atomic %; and (ii) a support material on which the PtX alloy is dispersed; wherein the total loading of platinum group metals (PGM) in the anode catalyst layer is from 0.01 to 0.2 mgPGM/cm.sup.2.

Claims

1. A method of operating a fuel cell comprising an anode, a cathode and a polymer electrolyte membrane disposed between the anode and the cathode, wherein said method comprises feeding the anode with an impure hydrogen stream comprising carbon monoxide in an amount of between 2 ppm to 5 ppm, and wherein the anode comprises an anode catalyst layer comprising a carbon monoxide tolerant catalyst material, wherein the catalyst material comprises: (i) a binary alloy of PtX, wherein X is rhodium, and wherein the atomic percentage of platinum in the alloy is from 45 to 80 atomic % and the atomic percentage of X in the alloy is from 20 to 55 atomic %; and (ii) a support material on which the PtX alloy is dispersed; wherein the total loading of platinum group metals (PGM) in the anode catalyst layer is from 0.01 to 0.2 mgPGM/cm.sup.2, wherein the fuel cell produces stable voltages when the anode is fed an impure catalyst stream comprising 5 ppm of carbon monoxide.

2. The method according to claim 1, wherein the atomic percentage of Pt in the binary alloy is from 50 to 75 atomic % and the atomic percentage of X is from 25 to 50 atomic %.

3. The method according to claim 1, wherein the amount of platinum in the supported catalyst is 10-50 wt % of the total weight of the binary alloy plus support material.

4. The method according to claim 1, wherein the anode further comprises a second catalyst.

5. The method according to claim 4, wherein the second catalyst is an oxygen evolution catalyst.

6. The method according to claim 2, wherein the atomic percentage of Pt in the binary alloy is from 50 to 75 atomic % and the atomic percentage of X is from 25 to 50 atomic %.

7. The method according to claim 2, wherein the amount of platinum in the supported catalyst is 10-50 wt % of the total weight of the binary alloy plus support material.

8. The method according to claim 2, wherein the anode further comprises a second catalyst.

9. The method according to claim 3, wherein the anode further comprises a second catalyst.

10. The method according to claim 8, wherein the second catalyst is an oxygen evolution catalyst.

11. The method according to claim 9, wherein the second catalyst is an oxygen evolution catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the anode half-cell polarisation data in a hydrogen stream comprising 2 ppm carbon monoxide.

DETAILED DESCRIPTION OF THE INVENTION

(2) Preferred and/or option features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise.

(3) The invention provides the use of an anode catalyst layer in a proton exchange membrane fuel cell. During operation of the fuel cell, a hydrogen stream comprising up to 5 ppm carbon monoxide is fed to the anode. The anode catalyst layer comprises a binary alloy of PtX, wherein X is rhodium or osmium.

(4) By the term platinum group metals is meant metals selected from the group ruthenium, rhodium, palladium, osmium, iridium and platinum; in the context of the present invention the metals platinum, rhodium and osmium are specifically intended.

(5) The atomic percentage (atomic %) of platinum in the binary alloy is from 45 to 80 atomic % and of X is from 20 to 55 atomic % (i.e, a ratio of from 0.8:1 to 4:1); suitably the atomic percentage of platinum is from 50 to 75 atomic % and of X is from 25 to 50 atomic % (i.e. a ratio of from 1:1 to 3:1).

(6) In one embodiment, the atomic percentage of platinum is 50 atomic % and of X is 50 atomic % (i.e, a ratio of 1:1).

(7) In a second embodiment, the atomic percentage of platinum is 66.6 atomic % and of X is 33.3 atomic % (i.e. a ratio of 2:1).

(8) In a third embodiment, the atomic percentage of platinum is 75 atomic % and of X is 25 atomic % (i.e. a ratio of 3:1).

(9) In the present context, atomic % means atomic percentage, i.e. the percentage based on atoms or moles of the total of platinum and metal X; any additional non-metallic components (e.g. carbon) are not taken into consideration. The atomic percentages given are the nominal atomic percentages that are targeted; in practice when the alloy is made the actual assay may typically be subject to a deviation of 5% from the nominal atomic percentages.

(10) By the term alloy we mean that there is at least some interaction and incorporation of the metal X into the platinum lattice, but the incorporation is not necessarily uniform throughout the whole alloy particle. The atomic percentage of the metal X in the alloy may be determined by standard procedures known to those skilled in the art; for example by wet chemical analysis digestion of the sample followed by inductively coupled plasma (ICP) emission spectroscopy.

(11) The catalyst material of the invention is a supported catalyst (i.e. the binary alloy is dispersed on a support material). Suitably the amount of the binary alloy is 5-50 wt %, suitably 10-40 wt %, based on the weight of platinum versus the total weight of the supported catalyst (i.e. the binary alloy plus the support material). In a supported catalyst according to the present invention the PtX alloy is suitably dispersed on a conductive high surface area support material, for example a conductive carbon, such as an oil furnace black, extra-conductive black, acetylene black or heat-treated or graphitised versions thereof, or carbon nanofibres or nanotubes. It may also be possible to use a non-conducting support material, such as inorganic metal oxide particles if the catalyst is deposited sufficiently well over the surface to provide the required electronic conductivity or if further additives are included to provide the necessary conductivity. The catalyst of the invention preferably consists essentially of the PtX alloy dispersed on a conductive carbon material. Exemplary carbons include Akzo Nobel Ketjen EC300J (or heat treated or graphitised versions thereof), Cabot Vulcan XC72R (or heat treated or graphitised versions thereof) and Denka Acetylene Black.

(12) To form the anode catalyst layer, the catalyst material is applied directly to one side of a substrate using a range of well-established techniques. The substrate may be a porous gas diffusion layer (to form an anode electrode) or a proton exchange membrane (to form a catalyst coated membrane). Alternatively, the anode catalyst layer may be applied to a decal transfer substrate, the anode catalyst layer subsequently being transferred from the decal transfer substrate to a gas diffusion layer or proton exchange membrane by techniques known to those skilled in the art. The decal 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.

(13) The catalyst material is formulated into an ink, comprising an aqueous and/or organic solvent, and a solution form of a proton conducting polymer (e.g. as disclosed in EP 0 731 520) and deposited onto either the substrate or decal transfer substrate using well known techniques, such as spraying, printing and doctor blade methods. The catalyst material is applied to the substrate or decal transfer substrate to provide an anode catalyst layer having a PGM loading of 0.01 to 0.2 mgPGM/cm.sup.2, suitably 0.01 to 0.15 mgPGM/cm.sup.2 and preferably 0.01 to 0.1 mgPGM/cm.sup.2.

(14) Typical gas diffusion layers, suitable for use as the substrate onto which the anode catalyst layer is applied, are fabricated from gas diffusion 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 gas diffusion layer. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). Suitably the gas diffusion layers 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 gas diffusion layer that contacts the catalyst material. The formed anode electrode (gas diffusion layer plus anode catalyst layer) is subsequently combined with a proton exchange membrane and a cathode electrode to form a membrane electrode assembly (MEA) by methods known to those skilled in the art.

(15) The proton exchange 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 ionomer such as Nafion (DuPont), Flemion (Asahi Glass), Aciplex (Asahi Kasei) and Aquivion (Solvay Plastics); 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 one or more expanded PTFE layers.

(16) Typical membranes onto which the anode catalyst layer is applied to form a catalyst coated membrane include the proton exchange membranes listed above. The formed catalyst coated membrane is subsequently combined with an anode gas diffusion layer and a cathode gas diffusion layer by methods known to those skilled in the art to form a membrane electrode assembly, Suitably gas diffusion layers included those listed hereinbefore.

(17) The anode catalyst layer may further comprise additional components, for example a second catalyst, such as an oxygen evolution catalyst. Examples of such catalysts are known to those in the art.

(18) Further aspects of the invention provide the use of an anode electrode, a catalyst coated membrane or a membrane electrode assembly in a proton exchange membrane fuel cell, wherein the anode electrode, catalyst coated membrane or membrane electrode assembly comprise an anode catalyst layer comprising a carbon monoxide tolerant catalyst material, wherein the catalyst material comprises: (i) a binary alloy of PtX, wherein X is a metal selected from the group consisting of rhodium and osmium, and wherein the atomic percentage of platinum in the alloy is from 45 to 80 atomic % and the atomic percentage of X in the alloy is from 20 to 55 atomic %; and (ii) a support material on which the PtX alloy is dispersed;
wherein the total loading of platinum group metals (PGM) in the anode catalyst layer is from 0.01 to 0.2 mgPGM/cm.sup.2; and
wherein during operation of the fuel cell a hydrogen stream comprising up to 5 ppm carbon monoxide is fed to the anode catalyst layer.

(19) The invention will now be described in more detail with reference to the following examples, which are illustrative and not limiting of the invention.

General Method for the Preparation of the Catalyst Material for Examples 1 to 3

(20) Carbon black was dispersed in water using a shear mixer and transferred to a reaction vessel. Solid NaHCO.sub.3 or 1M NaOH was added. Subsequently, a mixed solution of the rhodium salt or the osmium salt and H.sub.2PtCl.sub.6 was added. When deposition of the metals was complete the catalyst was recovered by filtration and washed on the filter bed with demineralised water until free of soluble ions. The material was dried and then annealed at high temperature in an inert atmosphere.

Example 1: Pt.SUB.75.Rh.SUB.25

(21) Ketjen EC300J 5 g

(22) Pt 4.25, 21.7 mmol as H.sub.2PtCl.sub.6 (16.88 g, 25.18% Pt)

(23) Rh 0.75 g, 7.3 mmol as RhCl.sub.3 (1.8 g, 41.44% Rh)

(24) NaHCO.sub.3 (14 g, 0.167 mol) 10% excess

(25) Annealing temperature: 500 C. for 30 minutes

Example 2: Pt.SUB.50.Rh.SUB.50

(26) H.sub.2PtCl.sub.6 solution (24.88% Pt)=65.83 g (16.38 g, 0.0840 mmol Pt)

(27) RhCl.sub.3 (42.40% Rh)=20.39 g (8.64 g, 84 mmol Rh)

(28) KetjenEC300J (9.8% moisture)=27.70 g (24.99 g dry)

(29) NaHCO.sub.3=69.82 g (0.8312 mol)

(30) Annealing temperature: 500 C. for 30 minutes

Example 3: Pt.SUB.75.Os.SUB.25

(31) H.sub.2PtCl.sub.6=6.0 g (1.5 g, 0.0769 mol Pt)

(32) Na.sub.2OsCl.sub.6=1.18 g (0.50 g, 0.0263 mol Os)

(33) 1M NaOH=As required

(34) Ketjen EC 300J (2.8% moisture)=4.1 g

(35) Annealing temperature: 500 C. for 2 hours.

Comparative Example 1

(36) The Pt catalyst used was HiSPEC 2000 available from Johnson Matthey Fuel Cells Limited.

Comparative Example 2

(37) The PtRu catalyst used was HiSPEC 10000 available from Johnson Matthey Fuel Cells Limited.

Preparation of Catalyst Coated Membrane (CCM)

(38) A dispersion comprising 0.5 g of Example 1 and sufficient aqueous Nafion solution (11% solids) to target a 120% dry Nafion weight with respect to the catalyst support material weight was shear-mixed to form an ink. The process was repeated using Example 2 and Comparative Examples 1 and 2.

(39) Using the inks obtained above, anode catalyst layers with platinum loadings as detailed in Table 1 were formed on one side of a 17 m reinforced PFSA membrane. A cathode catalyst layer comprising HiSPEC 9100 (commercially available from JMFCL) and 120% Nafion (with respect to the carbon support material weight) and having a platinum loading of 0.4 mgPt/cm.sup.2 was formed on the other side of the membrane to produce a (CCM).

MEA Testing

(40) The CCMs were combined with anode and cathode gas diffusion layers without hot bonding to form the MEA. MEA testing was conducted at 80 C., 7.2 psig with fully humidified gas reactants. Anode polarisation curves, obtained by feeding H.sub.2 to the cathode electrode, were obtained using (i) pure H.sub.2, (ii) 5 ppm carbon monoxide in H.sub.2, and (iii) 2 ppm carbon monoxide in H.sub.2 supplied to the anode electrode.

(41) The electrochemical areas of each of the anode electrocatalyst were measured using CO stripping voltammetry using conventional methods.

(42) Table 1 provides details of the catalyst materials, loading of the catalyst in the anode, the electrochemical area and performance (mV) at a current of 1 A/cm.sup.2 for half cells operating on pure H.sub.2, 2 ppm carbon monoxide in H.sub.2 and 5 ppm carbon monoxide in H.sub.2. The half-cell polarisation curve for 2 ppm carbon monoxide in H.sub.2 over the complete current range tested is given in FIG. 1.

(43) From Table 1, it can be seen that while there is relatively little difference in the performance of the catalysts of the invention and the comparative examples when operating on H.sub.2, the catalysts of the invention show improved half-cell performance (i.e. lower anode overpotential or voltage) when compared to the Comparative Examples when carbon monoxide is introduced into the H.sub.2 stream. It is also seen that although there is a variation in the catalyst surface area between the Examples (see Electrochemical Area); there is no direct correlation between the catalyst surface area on the tolerance to carbon monoxide. Thus, a clear benefit from the alloy catalysts is observed.

(44) FIG. 1 gives the anode polarisation curves of the Examples and Comparative Examples when operating under H.sub.2+2 ppm carbon monoxide. Significant performance losses (higher anode voltages) are observed for both of the Comparative Examples (Pt and PtRu) owing to the adverse impact of carbon monoxide on the electrocatalyst. In contrast, all Examples of the invention show a much lower half-cell voltage (overpotential) when compared to the Comparative Examples, indicating that the carbon monoxide present in the hydrogen fuel stream is having less of a poisoning effect on these electrocatalysts.

(45) TABLE-US-00001 TABLE 1 Wt % Performance @ 1A/cm.sup.2 loading Loading mV, half cell of Pt on of PGM Electrochemical 2 ppm 5 ppm Catalyst catalyst in anode Area (m.sup.2/g carbon carbon Example No Material support (mgPGM/cm.sup.2) PGM H.sub.2 monoxide monoxide Comparative Pt 10 0.1 70.6 75 353 .sup.a Example 1 Comparative PtRu 40 0.08 82.8 70 148 .sup.a Example 2 Example 1 Pt.sub.3Rh 40 0.093 115.4 60 105 260 Example 2 PtRh 30 0.094 76.8 81 85 119 .sup.a It was not possible to obtain a value for Pt and PtRu when operating on 5 ppm carbon monoxide in H.sub.2 as the degree of poisoning by carbon monoxide was such that highly unstable, i.e. non-equilibrium, voltages were observed.