BIPOLAR PLATE FOR FUEL CELLS

Abstract: A bipolar plate (5) for fuel cells for (a) conducting current from the anode (4) of one cell unit to the cathode (3) of the adjacent cell unit and/or (b) distributing fluid which comprises a substrate with a coating of an electrocatalytically-active material. A fuel cell assembly comprising a plurality of cell units with a bipolar plate of the present invention disposed between adjacent cell units.

Description: Bipolar Plate for Fuel Cells The present invention relates to bipolar plates for fuel cells, to fuel cells comprising such plates and particularly to so-called proton-exchange membrane fuel cells. A fuel cell is an electrochemical device in which electricity is produced without combustion of fossil fuel. In a fuel cell, a fuel, which is typically hydrogen, is oxidised at a fuel electrode, (anode) and oxygen, typically from air, is reduced at a cathode, to produce an electric current and form by-product water. An electrolyte is required which is in contact with both electrodes and which may be alkaline or acidic, liquid or solid. Heat and water are the only by-products of the electrochemical reaction in fuel cells wherein the fuel is hydrogen. Accordingly, the use of such cells in power generation offers potential environmental benefits compared with power generation from combustion of fossil fuels or by nuclear activity. In proton-exchange membrane fuel cells, hereinafter referred to for convenience as "PEM" fuel cells, the electrolyte is a solid polymer membrane which allows transport of protons from the anode to the cathode and is typically based on perfluorosulphonic acid materials. The electrolyte must be maintained in a hydrated form during operation in order to prevent loss of ionic conduction through the electrolyte. A PEM fuel cell typically comprises two electrodes, an anode and a cathode, separated by a proton-exchange membrane electrolyte. At the anode, hydrogen fuel catalytically dissociates into free electrons and protons. The free electrons are conducted in the form of usable electric current through the external circuit with which the fuel cell is in electrical contact. The protons migrate through the membrane electrolyte to the cathode where they combine with oxygen from the air and electrons from the external circuit to form water and generate heat. Individual fuel cells may be combined into assemblies, which are often referred to in the art as stacks, to provide the amount of power required. A PEM fuel cell assembly comprises a plurality of such individual cells. In a fuel cell assembly bipolar plates, also known as fluid flow field plates, play a significant role. The bipolar plate is fabricated with surface features, for example a series of corrugations or a serpentine pattern, which provide gas flow channels which ensure essentially even distribution of input gases over the electrode surfaces. The bipolar plate should have high electrical conductivity as an ohmic loss in the plate will reduce the overall assembly efficiency. Bipolar plates for fuel cells constructed from metals, referred to therein as bipolar terminal grids, have been described by Douglas et al in US 3,134,696. Bipolar plates for fuel cells constructed from carbon/polymer composites, referred to therein as bipolar current collectors-separators, have been described by Lawrence in US 4,214,969. Bipolar plates for fuel cells constructed from graphite, referred to therein as fluid flow field plates, have been described by Wilkinson et al in WO 95/16287. The disclosures in these patent specifications are incorporated herein by way of reference. We have now found that the electrical conductivity of bipolar plates for fuel cells can be increased by coating them with a coating of an electrocatalytically-active material. By "electrocatalytically-active material" we mean a material which where used as an electrode or coating therefor catalyses electrochemical reactions at high current densities at potentials close to the equilibrium potential as is more fully described by R Greef et al in "Instrumental Methods in Electrochemistry", Ellis Horwood, 1990 and by D Pletcher et al in "Industrial Electrochemistry", Chapman and Hall, 1990. According to the first aspect of the present invention there is provided a bipolar plate for fuel cells for (a) conducting current from the anode of one cell unit to the cathode of the adjacent cell unit and/or (b) distributing fluid characterised in that it comprises a substrate with a coating of an electrocatalytically-active material as hereinbefore defined. The bipolar plate according to the present invention is provided with surface features, for example an in-plane non-uniform structure, which may be regular or irregular, eg a series of corrugations or serpentine pattern, which provide gas flow channels which ensure essentially even distribution of fuel, eg input gases, over the electrode surfaces and facilitate transport of by-products, eg water, therefrom. Such surface features may be formed by techniques well known to those skilled in the art, for example, embossing or die-casting. According to a further aspect of the present invention there is provided a fuel cell comprising a) at least two bipolar plates ; b) a membrane electrode assembly disposed between the plates, which membrane electrode assembly comprises a pair of opposed electrodes with a proton-exchange membrane disposed therebetween with the proviso that where the fuel cell comprises more than two bipolar plates a membrane electrode assembly and a bipolar plate alternate throughout the cell and the membrane electrode assemblies are disposed in the fuel cell such that an anode and a cathode alternate throughout the cell; c) current-collecting means; d) means to feed gaseous hydrogen fuel to the anodes; and e) means to feed an oxygen-containing gas to the cathode. characterised in that each bipolar plates comprises a bipolar plate according to the first aspect of the present invention. The anode and cathode in the fuel cell according to the present invention may be discrete components but are preferably provided as integral parts of a single unit as is more fully described in WO 95/16287. According to a yet further aspect of the present invention there is provided a fuel cell assembly comprising a) a plurality of cell units each of which contains a proton-exchange membrane separating the cell into anolyte and catholyte chambers and provided with an anode and a cathode on opposite sides thereof; b) a bipolar plate disposed between adjacent cell units; c) current-collecting means; d) means to feed hydrogen fuel to the anolyte chambers of the cell; and e) means to feed an oxygen-containing gas to the catholyte chambers of the cell characterised in that each bipolar plates comprises a bipolar plate according to the first aspect of the present invention. In the fuel cell assembly according to the present invention the plurality of cell units may be connected in bipolar or monopolar configuration as is more fully described by Kordesch and Simaderin in "Fuel Cells and their Applications", VCH. 1996 at 49-50. the description in which is incorporated herein by way of reference. The fuel cell and the fuel cell assembly according to the present invention are connected to an external circuit via the current-collecting means. Where the fuel cell and fuel cell asembly according to the present invention are in the form of a bipolar construction the current-collecting means are preferably terminal current-collectors, more preferably comprising bipolar plates according to the present invention. Whereas both the fuel cell assembly according to the present invention and the bipolar plate according to the present invention are typically planar we do not exclude the possibility that they may be cylindrical or tubular. We do not exclude the possibility that the fuel cell, the fuel cell assembly and the bipolar plate according to the present invention may be used in liquid electrolyte fuel cells such as phosphoric acid and so-called "direct methanol" fuel cells. The electrocatalytically-active coating of which the bipolar plate according to the present invention is comprised is typically derived from a metal, metal oxide or mixtures thereof from Group 8 of the Periodic Table of Elements, namely Fe, Co. Ni, Ru, Rh, Pd, Os, Ir and Pt. Suitable electrocatalytically-active coatings comprising mixtures of platinum group metal and platinum group metal oxide are described in our EP 0,129,374. Suitable electrocatalytically-active coatings comprising mixtures of ruthenium oxide, non-noble metal oxide and noble metal or oxide thereof are described in our EP 0,479,423. Suitable electrocatalytically-active coatings comprising mixtures of cerium oxide and at least one non-noble Group 8 metal are described in our EP 0,546,714. The electrocatalytically-active coating is preferably ruthenium oxide or mixtures of ruthenium oxide with at least one of PtO, Sb 2 O 3 , Ta j O,, PdO, CeO 2 , Co 3 O 4 or preferably mixture of RuO 2 with at least one of TiO 2 , SnO 2 , IrO 2 . Where the electrocatalytically-active coating comprises a mixture of ruthenium oxide and another oxide the content of the ruthenium oxide may be in the range 0 - 100 mol %, and typically 5 - 90 mol %. The thickness of the electrocatalytically-active coating on the bipolar electrode may be in the range 0.5 - 400 gm '2 . and typically 1 - 90 gm "2 . We do not exclude the possibility that the electrocatalytically-active coating may comprise an intermediate layer between the substrate and the outer layer. As examples of such intermediate layers may be mentioned inter alia the heat-treated niobium oxide layer and the tantalum layer described in EP 0,052.986 and EP 0J 07.934 respectively. Where the coating comprises ruthenium oxide it may comprise a plurality of different layers, for example a layer of RuO TKX and a layer of RuO SnO-,. The substrate of which the bipolar electrode is comprised is typically a metal chosen from Ti, V, Cr, Mn, Fe, Co, Ni. Cu, Zr, Nb, Ag, Pt, Ta, Pb. Al. or alloys thereof, preferably titanium or stainless steel. However, we do not exclude the possibility that the substrate may be formed from a non-metallic material, for example graphite, carbon-fibre paper, carbon-fibre cloth, Ebonex (RTM). or an organic polymeric material, eg . carbon-filled polypropylene. The electrocatalytically-active coatings may be applied to the substrate by, for example, painting of a solution of precursors followed by thermal decomposition, thermal spraying, screen- printing of metal alloy. Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD), electroplating, electroless plating or spray pyrolysis. Application of a electrocatalytically-active coating comprising an outer layer of ruthenium oxide and non-noble metal oxide to a substrate by PVD is more fully described in our WO 95/05499. Application of a electrocatalytically-active coating comprising an outer layer of ruthenium oxide and non-noble metal oxide to a substrate by thermal spraying is more fully described in our WO 95/05498. Application of a electrocatalytically-active coating comprising an outer layer of (a) cerium and/or cerium oxide and at least one non-noble Group 8 metal, or (b) platinum and or platinum oxide and ruthenium and or ruthenium oxide by PVD is more fully described in our WO 96/24705. We do not exclude the possibility that different coatings may be applied to different surfaces of the bipolar plate according to the present invention. Hydrogen fuel for use in the fuel cell assembly according to the present invention may be obtained from, for example, natural gas or methanol. Oxygen for use in the fuel cell assembly according to the further aspect of the present invention may be obtained from air. It will be appreciated that in the fuel cell assembly according tothe present invention the components thereof may be provided with aligned ports, eg slots, to form a manifold to allo flow of fuel gas and oxidant gas from the means to feed such gases to the cell to the anodes and cathodes respectively. The present invention is illustrated by reference to the accompanying drawing which illustrates, by way of example only, a fuel cell assembly according to the present invention. In the drawing, ion-permeable membranes (1) and (2) have cathode electrodes (3) and (4) respectively and anode electrodes, not shown, bonded to each of their major surfaces. A bipolar plate (5) according to the present invention, provided with surface features (6), is disposed between ion-permeable membranes (1) and (2) in contact with the electrode surfaces thereof. Terminal plates (7) and (8), provided with leads (9) and (10) for delivering electric current generated in the cell assembly to an external circuit, are disposed adjacent membranes (1) and (2) respectively. In the assembly, membrane (1) is held firmly between terminal plate (7) and bipolar plate (5) so as to from an oxidant gas chamber (1 1) and a fuel gas chamber (12). In like manner, membrane (2) is held firmly between terminal plate (8) and bipolar plate (5) so as to from an oxidant gas chamber (13) and a fuel gas chamber (14). Hydrogen fuel is supplied to the anodes in the fuel gas chambers (12) and (14) via fuel gas inlet conduit (15) and by-products removed via conduit (16). Oxidant gas is supplied to cathodes (3) and (4) in the oxidant gas chambers (11) and (13) via oxidant gas inlet conduit (17) and by-products removed via conduit (18). Openings (19) and (20) located in opposite corners of membranes (1) and (2) are aligned with hydrogen gas inlet and outlet conduits (15) and (16) and with openings (21) and (22) in bipolar plate (5) to facilitate passage of hydrogen fuel gas into the fuel chambers (12) and (14) and to remove by-products therefrom. Openings, not shown, and openings (23) located in opposite corners of membranes (1) and (2) are aligned with oxidant inlet and outlet conduits (17) and (18) and with opening (24) and another not shown in bipolar plate (5) to facilitate passage of oxidant gas into the oxidant chambers (1 1) and (13) and to remove by-products therefrom. End plates (7) and (8). membranes (1 ) and (2) and bipolar plate (5) are each provided with a plurality of openings (25) through which assembly tie-rods or bolts (26) pass. In a further embodiment of the present invention, a layer of diffusion material which is electrically conducting and porous, for example a carbon-coated paper or a graphite- impregnated polymer film, is disposed in the oxidant gas chambers (11) and (13) and or in the fuel gas chambers (12) and (14). For example, the layer of diffusion material may be disposed between polar plate (5) and the adjacent electrode surfaces of membranes (1) and (2) and/or between the terminal plates (7) and (8) and the adjacent electrode surfaces of membranes (1) and (2). The present invention is further illustrated by reference to the following Examples. Examples 1 and 2 These Examples illustrate bipolar plates and fuel cells according to the present invention wherein the coating of electrocatalytically-active material comprises ruthenium oxide and titanium oxide. A coating of composition 47.5 mole % ruthenium and 52.4 mole % titanium was prepared by adding tetra-n-butyl titanate (7.47g) to a solution of ruthenium (2g), as ruthenium trichloride, in pentan-1-ol (31g). In Example 1, a portion of this solution was applied by brush to a titanium substrate which had been etched in 10% oxalic acid solution at 80-85° C for 8 hours . The coated substrate was dried at 180° C and then baked at 450° C; 12 coats were applied in this manner. Three of the coated plates were operated as bipolar plates in a PEM fuel cell and the cell voltage output thereof was determined at 1 A/cm "2 and the per-centage voltage efficiency calculated therefrom. In Example 2 the procedure of Example 1 was repeated except that the substrate was a 316L stainless steel plate which had been grit blasted. In a Comparative Test, stainless steel plates were operated as bipolar plates in a PEM fuel cell. The results are shown in the Table from which it can be seen that PEM fuel cells according to the present invention have a voltage efficiency at least 13% better than a PEM fuel cell comprising a known plate. TABLE Example 3 This example illustrates bipolar plates according to the present invention wherein the coating of electrocatalytically-active material comprises a nickel/cobalt spinel. Co(NO 3 ) 2 .6H 2 O and Ni(NO,) 2 .6H,O were dissolved in pentan-1-ol to give a total solute concentration of 0.4M and a Co:Ni ratio of 2:1. Five coats of this solution were applied by brush to an etched titanium substrate, each coat being dried for 10 mins. at 180° C. After addition of the last coat the plate was annealed at 350° C for 10 hours. The loading of coating, a mixed cobalt/nickel oxide< on the substrate was 2.5g/m 2 . Example 4-6 These Examples illustrate bipolar plates according to the present invention wherein the coating of electrocatalytically-active material comprises ruthenium oxide and tin oxide (Examples 4 and 5) and ruthenium oxide, tin oxide and iridium oxide (Example 6). In these Examples the coating was applied to a stainless steel (SS) substrate by physical vapour deposition (PVD) which is more fully described in our WO 96/24705, the disclosure in which is incorporated herein by way of reference. The SS substrates were degreased ultrasonically in Arklone (RTM), then : in Example 4 the stainless steel substrate was not subjected to any further pre-treatment; in Example 5 the stainless steel substrate was pre-treated by grit blasting and etching in 10% oxalic acid solution by making cathodic for up to 10 mins. at 4-5 volts; and in Example 6 the stainless steel substrate was pre-treated by etching in 10% oxalic acid solution by making cathodic for up to 10 mins. at 4-5 volts. The coatings were applied to the substrates using a Ru Sri source (Examples 4 and 5) or a Ru/Sn/lr source as described in our WO 96/24705. fhe loading of coating on the substrate was 35g/m 2 . Example 7 This Example illustrates a bipolar plate according to the present invention wherein the coating of electrocatalytically-active material comprises cerium oxide. A titanium substrate was etched and the coating was applied thereto by plasma spraying a cerium/nickel powder as is more fully described in our EP 0 546 714, the disclosure in which is incorporated herein by way of reference. The loading of coating on the substrate was 380g/m 2 . Example 8 This Example illustrates bipolar plates according to the present invention wherein the coating of electrocatalytically-active material comprises ruthenium and tin . An etched titanium plate and a platinum electrode were immersed in a solution of ruthenium trichloride (58 g) and stannous trichloride (205 g) in 6M hydrochloric acid (1000 ml). With the titanium plate as cathode an electric current was applied to the solution, equivalent to 0.5 kA/m 2 for 30 mins. The loading of the Ru/Sn coating on the titanium substrate was 1.5 g/n . Example 9-10 These Examples illustrate bipolar plates according to the present invention wherein the coating of electrocatalytically-active material comprises ruthenium oxide and platinum oxide. In Example 9. the substrate was a titanium plate. In Example 10. the substrate was a stainless steel plate. Five coats of a solution of RuCl, (7.4 g) and H 2 PtCl b (22.2 g) in a mixture of acetic acid (100 ml) and hydrochloric acid (900 ml) were applied by brush to the substrates. Each coat was dried at 180° C for 10 mins. and then fired at 450° C for 20 mins. After the final coat had been fired the bipolar plate was heated in air for 1 hour at 450° C. The loading of platinum on the substrates was 1.5 g/m 2 . Examples 1 1-12 These Examples illustrate bipolar plates according to the present invention wherein the substrates are non-metallic. In these Examples the substrate was Ebonex (RTM). The substrates were cleaned ultrasonically in IPA, air dried and heated at 180° C for 10 mins. In Example 11 the electrocatalytically-active coating comprised platinum oxide and iridium oxide in weight ratio 70:30 and the loading of the coating was 1.5g/m 2 . The coating was prepared by applying six coats of a solution of HJrC16, (11.9 g) and H 2 PtCl 6 (32.6 g) in pentanol (1000 ml) by brush to the substrates. Each coat was dried at 180° C for 10 mins. and then fired at 500° C for 20 mins. After the final coat had been fired the bipolar plate was heated in air for 1 hour at 450° C. In Example 12 the electrocatalytically-active coating comprised ruthenium oxide and titanium oxide in weight ratio 35:65 and the loading of the coating was 20g/m 2 . The coating was prepared as described in Examples 1-2 except that 6 coats of the solution were applied instead of 5 coats.

Claims: Claims 1. A bipolar plate for fuel cells for (a) conducting current from the anode of one cell unit to the cathode of the adjacent cell unit and/or (b) distributing fluid characterised in that it comprises a substrate with a coating of an electrocatalytically-active material as herein defined. 2. A bipolar plate for fuel cells as claimed in Claim 1 further characterised in that the substrate is metallic. 3. A bipolar plate for fuel cells as claimed in Claim 1 further characterised in that the electrocatahtically-active material is derived from one or more platinum group metals, or oxides thereof. 4. A bipolar plate for fuel cells as claimed in Claim 1 further characterised in that the electrocatalytically-active material is derived from cerium, or an oxide thereof. 5. A bipolar plate for fuel cells as claimed in Claim I further characterised in that the electrocatalytically-active material is derived from ruthenium or an oxide thereof. 6. A bipolar plate for fuel cells as claimed in Claim 5 further characterised in that the electrocatalytically-active material comprises ruthenium oxide and a non-noble metal oxide. 7. A bipolar plate for fuel cells as claimed in Claim 6 further characterised in that the electrocatalytically-active material comprises a noble metal or oxide thereof. 8. A bipolar plate for fuel cells as claimed in Claim 5 further characterised in that the electrocatalytically-active material comprises mixtures of RuO., with at least one of TiO 2 , SnO 2 . IrO,, PtO, Sb 2 O 3 . Ta^, PdO, CeO 2 . Co,O 4 . 9. A bipolar plate for fuel cells as claimed in Claim 8 wherein the electrocatalytically-active material comprises RuO2/TiO2 10. A fuel cell comprising a) at least two bipolar plates ; b) a membrane electrode assembly disposed between the plates, which membrane electrode assembly comprises a pair of opposed electrodes with a proton-exchange membrane disposed therebetween with the proviso that where the fuel cell comprises more than two bipolar plates a membrane electrode assembly and a bipolar plate alternate throughout the cell and the membrane electrode assemblies are disposed in the fuel cell such that an anode and a cathode alternate throughout the cell; c) current-collecting means; d) means to feed gaseous hydrogen fuel to the anodes; and e) means to feed an oxygen-containing gas to the cathode, characterised in that each bipolar plates comprises a bipolar plate as claimed in Claim 1. 11. A fuel cell assembly comprising a) a plurality of cell units ; and b) at least one bipolar plate as claimed in Claim 1 disposed between adjacent cell units. 12. A fuel cell assembly comprising a) a plurality of cell units wherein each unit contains an essentially gas- impermeable proton-exchange membrane separating the cell into anolyte and catholyte chambers and with an anode and a cathode on opposite sides thereof; b) a bipolar plate disposed between adjacent cell units; c) current-collecting means; d) means to feed hydrogen fuel to the anodes; and e) means to feed an oxygen-containing gas to the cathode, characterised in that each bipolar plates comprises a bipolar plate as claimed in Claim 1.

Inventors: HODGSON DAVID RONALD, MAY BARRETT, HODGSON, DAVID, RONALD, MAY, BARRETT
Applicants: HODGSON DAVID RONALD, ICI PLC, MAY BARRETT, HODGSON, DAVID, RONALD, IMPERIAL CHEMICAL INDUSTRIES PLC, MAY, BARRETT
Application year: 1999
Main classification: H01M8
IPC codes: H01M4, H01M8