Title of Invention

A POROUS ELECTRODE FOR USE IN ELECTROCHEMICAL CELLS

Abstract

A porous electrode for electrochemical cells, being an anode or cathode, said electrode comprising a porous diffusion layer and catalyst layer, the porous layer being a gas supplying component prepared by a dispersion of particulate carbon, using hydrophobic polymer as binder on a substrate, heat treated at 350 , the catalyst having first and second layers, the first layer being of highly dispersed precious metal catalyst on particulate carbon at a precious metal loading of 0.125mg/cm of geometric area, the said precious metal being a metal selected from Pt group metals or alloys thereof; and the second layer being of highly dispersed catalyst (in the form of a salt of the catalyst) on particulate carbon at a precious metal loading 0.125mg’cm’ of geometric area.

Full Text

This invention relates to a porous a electrode for use in electrochemical cells. In recent years considerable attention has been directed to development of Polymer Electrolyte membrane(PEM) fuel cell for transportation applications. Further interest in such fuel cells has been intensified by the demand for nonpolluting power sources. A polymer electrolyte fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly(MEA). MEA comprises of an anode, a cathode, and a polymer electrolyte membrane. The low utilization of the high platinum loaded electrodes, and, hence, the high cost of the materials have been the major problems associated with the production of viable membrane electrode assemblies(MEA) for the PEM fuel cells. Significant increase in the effective surface area of the catalyst utilized in these electrodes will enable both improvement in the performance and reduction in the cost of the MEA. It should also be noted that increase in power density output could further reduce the capital cost per unit of power generated. An objective of the present invention is to provide a high performance, low cost MEA for PEM fuel cells, in which the porous electrodes have a relatively low platinum loading and improved platinum utilization, and, therefore, higher effective platinum surface area, by maintaining adequate rates of reactant gas transport, higher performance over prior art PEM fuel cell electrodes at practically useful current densities under practical operating conditions of temperature, pressure and gas flow rates. Accordingly, this invention provides a porous electrode suitable for use in a PEM fuel cell comprising highly dispersed precious metal catalyst on particulate carbon impregnated with proton conducting polymer(ionomer), and another component which is a gas supplying component, comprising hydrophobic polymer with particulate carbon. The finer distribution of the catalyst is achieved by treating the ionomer with salt of the catalyst and coating of the solution on the cathode of the MEA. Fuel cells in which the electrolyte is a polymer membrane are known as polymer electrolyte fuel cells, PEM fuel cells. The present invention relates to finer dispersion of electro-catalyst particles onto the cathode of the PEM fuel cell with low or modest loadings of expensive catalytic material especially suited for use in PEM fuel cell. More particularly, the invention improves fuel cell performance by dispersing the catalyst effectively at the interface of the membrane and electrode. The electrode of the present invention may form either the cathode or an anode. Such an electrode may find use in other applications, such as in liquid electrolyte fuel cells or in other applications of solid polymer electrolyte technology such as water electrolysis, electrochemical ozone generation, or electro-organic synthesis processes. This invention results in production of an electrode suitable for use in polymer electrolyte fuel cell, comprising: i) First component of the electrode is diffusion layer which is a gas supplying component, comprising of dispersal of particulate carbon and hydrophobic polymer on the suitable substrate and firing at 350°C .., —^ii.iy ui uie caiaiyst layer on the gas diffusion layer coated electrode, iii) Fabrication of MEA The gas supplying component may be prepared by dispersing particulate carbon on suitable substrate. Substrates like carbon paper, carbon cloth, and metal mesh are suitable as backing layer. It is generally preferred in this invention that the support material have low resistivity at ordinary temperatures of cell operation (e.g. from 0 to lOCC). Metals typically have resistivities less than 1xio'* ohm/cm at room temperature and are considered to be highly conductive. Solid graphite has one or two orders of magnitude greater resistivity as compared to metals, it is nevertheless an excellent conductor, as are most other economically attractive forms of carbon such as activated carbon and carbon fibers. A variety of fibrous carbon materials (carbon cloth, carbon paper, etc.) are commercially available for use as the backing material on the hydrophobic side of the Gas Diffusion Electrodes(GDE). Several types of very high surface area carbon particles, both graphitized and non-graphitized are available for use as the support material. The surface area of these available support materials can range all the way from as low as 50 m^/g to more than 1000 m^/g. A more typical range of surface area is 200-1200 m^/g. The graphitized forms of carbon tend to be relatively resistant to attack in the presence of acidic and even basic electrolytes. A slurry of vulcan XC-72(vulcan XC 72R available from Cabot Corp, Billerica, Mass, U.S.A). is dispersed in water and isopropyl alcohol by using an ultrasonicator. A measured quantity of hydrophobic polymer emulsion is added to the dispersion and the mixture is agitated in an ultrasonicator till uniform slurry is obtained. This slurry thus obtained can be applied to the substrate by brushing, screen printing or by spraying and the diffusion layer coated electrode is heat treated at 350°C for 5 hrs, temperature to provide the correct hydrophobic properties, prior to application of the active layer. Catalyst layer is coated to the electrode in two steps. The catalyst layer is called as 'Active layer'. Particulate Platinum supported carbon(vulcan XC 72R available from Cabot Corp, Billerica,. Mass, U.S.A.) with Pt to a loading of 20 wt percentage from Arora Matthey, Calcutta, India is used as catalyst. The catalyst (0.27 g) is wetted with distilled water (2 mL) and IPA/distilled water mixture in 3:1 ratio{15 mL) is added with stirring. After 15 minutes agitation, 0.2 mL of Hiflon®(Hindustan Flurocarbon Ltd. Hyderabad, India) teflon suspension containing 80 percent teflon is added with constant stirring and the mixture stirred well to obtain homogeneous ink. The ink is coated on the diffusion layer coated electrode of dimension 22cm X 20 cm by any of the above mentioned method. A high-performance GDE with better catalyst dispersion is made by this invention. Since the electrocatalyst comprises a rare or expensive metal such as a noble metal of Group VIII (e.g. platinum) or gold or silver, the presence of a substantial number of electrocatalyst particles in the active layer which are not in contact with the ionomer renders the catalyst inactive and under¬utilized which results in poor overall cell performance. In the present invention, it is preferred that part of catalyst(Group VIII metal) is present as counter ion of the ionomer in the cathode structure and deposited slowly inside the cathode during the operation of the fuel cell. This is achieved by coating the cathode with an ink comprising highly dispersed platinum onoarticulate carbon (20% weight Pt/C, 0.27 gms) and treated ionomer solution (NafioiF solution supplied by E.I, Dupont de Nemours & Co., Inc. Welmington, De 19898, USA,) until the partially catalysed layer has been thoroughly wetted with a solution. The treated ionomer component may be prepared typically by mixing tetraamineplatinum(n)chloride (4mL of 10% Nafioii® solution mixed with 0.385g of tetraamineplatinum(n)chloride) and agitating in the ultrasonicator for 15 minutes. Such a method of catalyzing can be used in fuel cell or electrolytic cell wherein the coating of the ionomer solution on the electrode or electrodes is involved and the electrodes are in contact with a liquid electrolyte or PEM electrolyte. Preferably, the coating of ionomer with catalyst is by screen printing or by brushing method. It may also be desirable to complete the preparation of the electrode by the application of a thin layer of a solubilised form of a proton- conducting polymer(ionomer solution in alcohol/ water mixture) to the fi-ont face of the electrode structure. The impregnation step must be carried out with great care so that the ion- exchange material used as the electrolyte penetrates into the catalyzable layer reasonably well but does not significantly change the gas-permeability properties of the ^ side of the electrode. Accordingly, the impregnation proceeds from the outermost surface of the catalytic face inward, partway into the cross-section of the untreated GDE, but generally not so far into the backing sheet which provides the gas-permeable regions for the reactant gas to enter from the gas side. The porous electrode, according to this invention, for electrochemical cells, being an anode or cathode, said electrode comprising a porous diffusion layer and catalyst layer, the porous layer being a gas supplying component prepared by a dispersion of particulate carbon, using hydrophobic polymer as binder on a substrate, heat treated at 350° C, the catalyst having first and second layers, the first layer being of highly dispersed precious metal catalyst on particulate carbon at a precious metal loading of 0.125mg/cm^ of geometric area, the said precious metal being a metal selected from Pt group metals or alloys thereof; and the second layer being of highly dispersed catalyst (in the form of a salt of the catalyst) on particulate carbon at a precious metal loading 0.125mg/cm^ of geometric area. The concentration of ionomer in the solution can range from 0.1 to 50 parts by weight per 100 parts by weight of solution , preferably 1-10 weight percentage, based on the weight of solution. This preferred range provides ease of impregnation and rapid drying of the impregnated electrode at ambient temperatures after impregnation. The dissolved polymer electrolyte is absorbed into the mass of finely divided support material on the surface of the catalytic face through capillary action. The electrode of the present invention is the cathode. Such cathode may find use in other applications, such as in liquid electrolyte fuel cells or in other applications of solid polymer electi-olyte technology such as water electrolysis, electrochemical ozone generation, or electro-organic synthesis processes. Using this cathode a MEA is fabricated. The anode comprised a 20 wt % Pt catalyst supported on Vulcan XC 72R carbon, at a loading of 0.25 mgPt/cm^ , which is brush coated with the soluble form of Nafiorl® -115 polymer to a loading of 0.75 mg NafifflP cm^ of the electrode dry weight. The proton conducting polymer membrane is Nafion -115 membrane produced by the E.I. Dupont de Nemours & Co., Inc. Wehnington, De 19898, USA. Cathode and anode are prepared with a Pt loading (mgPt/cm^) of 0.75 and 0.25 respectively. The cathode and anode obtained by this method is laminated along witili a membrane such that the catalyst coated surface of the anode and the cathode to the membrane at 140°C temperature and 25000 pounds force with a help of hot press. The electrodes are adherent, showed no evidence of delamination from the membrane. The membrane is cleaned prior to forming the MEA as commonly practiced in the art. MEA was incorporated in a fuel cell assembly and humidified reactants fed into the fuel cell. A low current drawn from the fuel cell such that the ion form of the catalyst is reduced to metal but essentially only on sites where support material is in contact with noble-metal sah treated ionomer. During the low current operation, electro-deposition of catalytic metal, the metal catalyst particles ( since they must be obtained from ions such as metal cations) will not form on the surface of the support material unless that support material is in contact with an ionic pathway provided by the solid deposits of polymer membrane electrolyte. Moreover, no electro-deposition of the catalytic metal will occur anywhere along an ionic pathway unless that pathway leads to or comes in contact with electrically conducting support material. Because the electro-deposition step is carried out with a low current which is controlled externally favouring small particle formation, essentially all of the electro-deposited particles of catalytic metal will be very small. Accordingly, essentially all of the electro-deposited catalytic metal will have extremely high surface area and will be provided with both an electronic pathway and an ionic pathway which is needed for the effective utilization of the deposited catalyst particle in the presence of the gaseous reactant. Virtually no noble or precious metal particles isolated or insulated from electrically conductive support material will be present; similarly, virtually no such particles isolated from polymer electrolyte will be present. Those electro-deposited catalytic metal particles which are present will generally be utilized effectively when the cell is in operation. Electrodes catalysed as above method with mixture of supported catalyst and salt of catalyst treated ionomer are found perform better than the electrodes fabricated with supported catalyst alone. 5. We Claim: 1. A porous electrode for electrochemical cells, being an anode or cathode, said electrode comprising a porous diffusion layer and catalyst layer, the porous layer being a gas supplying component prepared by a dispersion of particulate carbon, using hydrophobic polymer as binder on a substrate, heat treated at 350 , the catalyst having first and second layers, the first layer being of highly dispersed precious metal catalyst on particulate carbon at a precious metal loading of 0.125mg/cm of geometric area, the said precious metal being a metal selected from Pt group metals or alloys thereof; and the second layer being of highly dispersed catalyst (in the form of a salt of the catalyst) on particulate carbon at a precious metal loading 0.125mg’cm’ of geometric area. 2. A porous electrode for electrochemical cells, more particularly, a polymer electrolyte membrane fuel cell, as claimed in Claim 1 wherein the second layer of the said catalyst is also ionomer treated typical salt of catalyst, the Pt group of metal ion treated ionomer being impregnated on to the face adjacent to the polymer electrolyte membrane, until the said solution has wetted the face of the gas diffusion electrode and penetrated partly into the cross-section of the untreated gas diffusion electrode, thereby depositing ion- exchange polymer in contact with the substrate and said gas diffusion electrode. 3. A porous electrode as claimed in any one of the preceding Claims wherein the substrates comprise substances, such as, carbon paper, carbon cloth, metal mesh. 4. A porous electrode as claimed in any one of tile preceding Claims wherein the said substrates have low resistivity at ordinary temperature of cell operation. 5. A porous electrode as claimed in any one of the preceding Claims wherein the treated ionomer is the one exchanged with the action of the Pt group of metals. 6. A porous electrode as claimed in any one of the preceding Claims wherein the treated ionomer is a homopolymer or copolymer having anionic ionizable groups. 7. A porous electrode as claimed in any one of the preceding Claims wherein the highly dispersed precious metal catalyst is present at precious metal loading of 0.05 to 1.0 mg/cm’ of geometric area. 8. A porous electrode as claimed in any one of the preceding Claims wherein the support material comprises particles or sintered particles or fibers comprising carbon or non-noble, non- precious of fibre comprising carbon. 9. A porous electrode for electrochemical cells substantially as herein described.

Documents:

0286-mas-2001 claims duplicate.pdf

0286-mas-2001 claims.pdf

0286-mas-2001 correspondence others.pdf

0286-mas-2001 correspondence po.pdf

0286-mas-2001 description (complete) duplicate.pdf

0286-mas-2001 description (complete).pdf

0286-mas-2001 drawings.pdf

0286-mas-2001 form-1.pdf

0286-mas-2001 form-19.pdf

0286-mas-2001 form-26.pdf