Use of catalytically active gas diffusion electrodes comprising a nonwoven fibrous structure in a Direct Methanol Fuel Cell

Abstract: The use of a gas diffusion electrode comprising a non-woven network of fibres, one or more catalysts components and at least one polymeric substance characterised in that the catalyst is embedded within the fibre network in a Direct Methanol Fuel Cell is disclosed.

Description: The present invention relates to the use of a gas diffusion electrode structure in a Direct Methanol Fuel Cell. Electrochemical cells invariably comprise at their fundamental level a solid or liquid electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. Gas diffusion electrodes are employed in a range of electrochemical devices, in which a gaseous reactant and/or product has to be diffused into and/or out of one of the cell electrode structures. They are designed to optimise the contact between the reactant and the electrolyte to maximise the reaction rate. Catalysts are often incorporated into gas diffusion electrode structures to increase the rates of the desired electrode reactions. Gas diffusion electrodes are employed in many different electrochemical devices, including metal-air batteries, electrochemical gas sensors, electrosynthesis of useful chemical compounds, and in particular, fuel cells. A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The hydrogen or methanol are oxidised at the anode and oxygen is reduced at the cathode. Both electrodes are of the gas diffusion type. The electrolyte has to be in contact with both electrodes and may be acidic or alkaline, liquid or solid, in nature. In proton exchange membrane fuel cells (PEMFC), the electrolyte is a solid proton-conducting polymer membrane, commonly based on perfluorosulphonic acid materials, and the combined structure formed from the membrane and the two gas diffusion electrodes is known as the membrane electrode assembly (MEA). The anode gas diffusion electrode is designed to be porous and allow the reactant hydrogen or methanol to enter the electrode from the face of the electrode exposed to the reactant fuel supply, and diffuse through the thickness of the electrode to the reaction sites which contain catalysts, usually platinum metal based, to maximise the electrochemical oxidation of hydrogen or methanol. The anode is also designed to allow electrolyte to penetrate through the face of the electrode exposed to the electrolyte and to also contact the same reaction sites. With acidic electrolyte types the product of the anode reaction are protons and these can then be efficiently transported from the anode reaction sites through the electrolyte to the cathode gas diffusion electrode. The cathode is also designed to be porous and allow oxygen or air to enter the electrode and diffuse through to the reaction sites. Catalysts are again commonly incorporated to maximise the rate of the reaction at the cathode reaction sites which combines the protons with oxygen to produce water. Product water then has to diffuse out of the electrode structure. The structure of the cathode has to be designed such that it enables the efficient removal of the product water. If water builds up in the electrode, it becomes more difficult for the reactant oxygen to diffuse to the reaction sites, and thus the performance of the fuel cell decreases. Conventionally, the gas diffusion electrodes of the PEMFC, and indeed other devices, comprise many components and are typically made up of one, two or even more layers of these components. Typically the gas diffusion electrode will comprise one or more catalyst containing layers, which are supported onto a more rigid porous substrate layer. The catalyst containing layers enhance the desired electrode reactions and comprise a catalyst, which may be formed from a high surface area catalytic metal, often one of the precious metals, particularly platinum, either unsupported, as a metal black (for example US 4927514, EP 0357077), or in a very high surface area form in which it is dispersed and supported on a high surface area electrically conducting gas porous carbon black or graphite (for example US 4447505). The catalyst component may also be a non precious metal, such as one of the transition metals. In fuel cells which employ alkaline electrolytes, the cathode gas diffusion electrode can comprise catalysts based on macrocyclic compounds of cobalt (US 4179359, EP 0 512 713). The catalyst layers may also comprise the high surface area carbon black itself, with no additional metal catalysts, in for example EP 0 026 995 where the catalyst layer for an air depolarised cathode in a chlor-alkali cell comprises carbon black materials. The catalyst layers also comprise other non-catalytic components in addition to the catalyst material, usually polymeric materials which acts as binders to hold the electrode layer together and may also perform an additional function in controlling the hydrophobic/hydrophilic nature of the final structure. In the PEMFC in particular, the catalyst layers can also comprise other polymeric materials, such as proton conducting polymers, including forms of the proton conducting electrolyte itself, which are often mixed with the catalyst components or coated onto the catalyst containing layers, from solutions of the proton conducting polymer. These catalyst layers are usually formed into suitable mixtures of the components and deposited onto a suitable porous substrate, for example conducting carbon materials such as semi graphitised papers, cloths or foams, or particularly in the case of alkaline electrolyte systems, metal meshes such as nickel or stainless steel, or in the case of sensors, various forms of porous PTFE sheet. In the acid electrolyte PEMFC the substrate is usually based on carbon paper or woven cloth materials (EP 0 026 995). These materials generally have a high bulk fibre density of greater than 0.4 g/cm 3 . The primary role of the substrate is to act as a physical support for the catalyst containing layers and to provide an electrically conducting structure in direct contact with the catalyst layer. Additionally it also enables a mechanically stable gas diffusion electrode to be produced. A major problem with conventional gas diffusion electrodes based on the carbon fibre paper substrates is the lack of flexibility due to the rigid substrate that is typically used. The conventional electrodes are consequently easily damaged on handling which leads to high reject rates during the electrode and MEA fabrication process. This obviously has an impact on cost. With conventional gas diffusion electrodes based on woven cloth substrates a problem concerns the lack of good dimensional stability, as the cloth can easily be stretched in the directions of the major planar faces (x and y directions). This can make the manufacturing of electrodes and MEAs using these substrates very difficult and therefore costly. Furthermore the complexity of the conventional gas diffusion electrode requires a number of separate components such as the substrate and the catalyst layers to be brought together which results in a lengthy manufacturing process requiring a number of steps. Again, this increases the cost per unit of these gas diffusion electrodes to higher than is currently acceptable to make applications in power generation devices, such as fuel cells, commercially viable. It is therefore an object of the present invention to provide the use of a gas diffusion electrode in a fuel cell wherein the reactant fuel is methanol (hereinafter referred to as a Direct Methanol Fuel Cell - DMFC). Accordingly, the present invention provides the use of a gas diffusion electrode comprising a non-woven network of fibres, one or more catalyst components and at least one polymeric substance characterised in that the catalyst component is embedded within the fibre network in a DMFC. Generally, the non-woven network of fibres has a density of less than 03 g/cm 3 , suitably less than 0.2 g/cm 3 and preferably less than 0.1 g/cm 3 . The fibres, within the matrix, are normally randomly orientated in the x and y direction (in-plane) producing a two dimensional isotropic structure. Additionally, random orientation in the z direction (through-plane) occurs with the inclusion of very short fibres, typically lengths of ≤ 2mm or very fine fibres, typically of diameters ≤ 1µm. It is also possible to introduce anisotropic character into the fibre matrix by using longer fibres, typically ≥50mm, in the composition, in combination with any method of laying down the layer that imparts a directional force into the mixture; such as extrusion or a paper making type processes. Fibres which are suitable for use in the present invention include carbon, glass, polymer, metal or ceramic fibres, preferably carbon, glass, metal or ceramic, typically of diameters in the range of 0.2µm to 50µm and with lengths from 0.05mm to 300mm, suitably 0.5mm to 150mm. If fibres other than carbon fibres or other electrically conducting fibres are used it is necessary that the catalyst component comprises carbon and optionally one or more other catalyst components such as a metal or a metal supported on carbon in order that the final electrode structure is electrically conducting. The catalyst component or components selected will depend on the application for which the gas diffusion electrode is being used. These may be, for example, a precious metal or a transition metal as the metal or metal oxide, either unsupported or supported in a dispersed form on a carbon support; a carbon or an organic complex, in the form of a high surface area finely divided powder or fibre, or a combination of these options. The polymeric substances act as a binder to hold the electrode layer together. Depending on the polymeric substance used it may also act to provide essential electrode structural properties, such as control of the hydrophobic/hydrophilic balance. Examples of such polymers include polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP), polyethylene, polypropylene, ethylene-propylene. The polymeric material can also act to provide the ionic conduction pathway in the electrode. Such proton conducting polymers are for example the perfluorosulphonic acid materials, produced by E.I. DuPont de Nemours, referred to as Nafion® . A first embodiment of the present invention provides the use of a gas diffusion electrode as hereinbefore described wherein the catalyst component is one or more metals or their oxides in the form of finely divided unsupported powders or as metals in a dispersed form on a carbon support in a DMFC. Suitably the one or more metals may be a precious metal (Pt, Pd, Ru, Rh, Ir, Os, Au and Ag) or a transition metal selected from groups IVB, VB, VIB, VIIB, VIII, IB or IIB of the Periodic Table in "Handbook of Chemistry and Physics", 64th Edition, CRC Press, or a combination or alloy thereof. Preferably, the one or more metals is a precious metal, particularly Pt, or an alloy thereof, A second embodiment of the invention provides the use of a gas diffusion electrode as hereinbefore described in a DMFC wherein the catalyst component is one or more carbon blacks. A major advantage of the present invention is that a free-standing, dimensionally stable and highly flexible gas diffusion electrode is obtained. The incidence of damage to the electrode on handling during manufacture is therefore minimised thus reducing the number of faulty or rejected electrodes, and consequently the cost is reduced. Furthermore, the intrinsic material cost is significantly reduced over carbon paper or woven cloths. In addition, the electrode of the invention is more amenable to high volume continuous production processes due to its high dimensional stability. A further advantage of the gas diffusion electrodes of the present invention is improved performance due to reduced mass transport losses. During operation of the PEM fuel cell (the DMFC), product water is produced at the cathode. This has to be efficiently removed from the electrode structure so that it does not build up in the structure and impede further oxygen diffusion (mass transport) to the reaction sites. With conventional electrodes this is not possible to fully achieve, and it becomes difficult to operate the cell efficiently at higher current densities, which is a desirable goal to improve power density. The presence of fibres in the catalyst layers can help to improve the efficiency of removal of the product water from the catalyst sites where it is produced into the reactant gas stream where it is transported out of the cell by the reactant gas flow. Accordingly, a further embodiment of the present invention provides a gas diffusion electrode suitable for use in electrochemical cells, said electrode comprising a current collector and a catalyst structure and characterised in that a continuous network of fibres for two-way transport is distributed in the catalyst containing layer. The present invention will now be described by way of example only which is not intended to be limiting thereof. The materials of the invention can be employed as either the anode or cathode, and indeed both anode and cathode in the electrochemical cells of the specific application. In the following examples, the electrodes are incorporated as the cathode in membrane electrode assemblies (MEAs) and evaluated in a fuel cell. The MEAs were fabricated by hot pressing the anode and cathode against each face of the solid proton conducting electrolyte membrane, as is commonly practised in the art. The anodes were of the more conventional type, currently widely employed in the PEMFC. They comprised a conventional pre-teflonated rigid conducting carbon fibre paper substrate (Toray TGP-H-090, available from Toray Industries Inc, Tokyo, Japan) to which was applied a layer of a 40 wt% platinum, 20 wt% ruthenium catalyst, supported on Cabot Vulcan XC72R (from Johnson Matthey Inc, New Jersey, USA), at an electrode platinum loading of 1.08 mg/cm 2 of electrode geometric area. EXAMPLE 1 A first particulate catalyst component was provided by dispersing 50 weight parts of a carbon black (Shawinigan black, from Chevron Chemicals, Houston, Texas, USA) in 1200 parts of demineralised water. To this was added 6 weight parts of polytetrafluoroethylene (PTFE) as a dispersion in water (ICI Fluon GP1, 64 wt% solids suspension) and the mixture stirred to entrain the PTFE particles within the carbon catalyst material. The slurry was redispersed using a high shear mixer to produce a smooth mixture. A second particulate catalyst material was provided by dispersing 100 weight parts of a 40 wt % platinum catalyst, supported on carbon black (Johnson Matthey FC-40) in 30 parts of a 9.5% dispersion of Nafion EW1100 (E I DuPont De Nemours & Co.) in water, prepared according to methods described in EPA 731,520. The particulate catalyst was dispersed using a high shear mixer to produce a smooth mixture. A pre-formed non-woven carbon fibre structure was a 17g/m 2 (≡0.07g/cm 3 ) density carbon fibre mat, supplied as Optimat 203 (from Technical Fibre Products, Kendal, Cumbria, UK). This was precoated with PTFE by soaking for 5 minutes in a solution of 120 parts by weight of GP1 PTFE emulsion in 2100 parts by weight of water then draining and allowing to dry giving a loading of 24.8% PTFE in the non-woven structure. The coated carbon fibre mat was heated to 350°C in air to sinter the PTFE. The electrode of the invention was formed by pressing the first particulate catalyst material into the non-woven carbon fibre structure using a vacuum bed to remove the water and pull the particulate catalyst material into the structure. A total fill of 3.2mg Shawinigan carbon per cm 2 carbon fibre paper geometric area was achieved. The resulting structure was heated in air to 300°C for 30 minutes. A layer of the second particulate catalyst material was then applied to one face of the filled non-woven structure to provide a platinum loading of 0.9 mg/cm 2 geometric area within the remaining carbon fibre structure and pressed at 200lb per square inch to compact the layer. The electrode formed the cathode of an MEA. with the face of the electrode comprising the platinum catalyst component bonded to the membrane electrolyte face. The membrane employed was Du Pont Nafion 117. The single cell results for air operation are shown in Figure 1 alongside an MEA containing a cathode of the more conventional type, currently employed in direct methanol PEMFC. This cathode comprised a conventional pre-teflonated rigid conducting carbon fibre paper substrate (Toray TGP-H-090, available from Toray Industries Inc, Tokyo, Japan) to which was applied a layer of the second particulate catalyst material to provide a platinum loading of 0.9 mg/cm 2 geometric area. Comparison of Example 1 with the more conventional MEA illustrates that across the current density range there is higher performance due to improved mass transport properties at operationally viable air stoichiometries. This feature of the electrode is particularly important for applications of the direct methanol PEMFC in powering vehicles, where cell operations at ever increasing current densities is a requirement to reduce the size, weight and cost of the fuel cell stack.

Claims: The use of a gas diffusion electrode comprising a flexible, free-standing catalyst layer containing a continuous, non-woven network of fibres and a polymeric substance; and optionally, a second or more layer(s) applied to the flexible, free-standing catalyst layer in a Direct Methanol Fuel Cell. The use of a gas diffusion electrode comprising a flexible, free-standing layer that comprises a mixture of (a) non-woven fibres; (b) one or more catalyst material(s); and (c) one or more polymeric substance(s); and, optionally, a second or more layer(s) applied to the flexible, free-standing layer in a Direct Methanol Fuel Cell. The use of a gas diffusion electrode according to claim 1 or claim 2, wherein the density of the non-woven network of fibres is less than 0.3g/cm 3 . The use of a gas diffusion electrode according to any preceding claim, wherein the density of the non-woven network of fibres is less than 0.1g/cm 3 . The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres have a diameter of 0.2µm to 50µm. The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres are from 0.05mm to 300mm long. The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres are bound with one or more polymeric substance(s) forming a flexible matrix. The use of a gas diffusion electrode according to any preceding claim, wherein the one or more catalyst material(s) are combined with one or more polymeric substance(s). The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres are pre-coated with one or more polymeric substance(s). The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres are selected from the group consisting of carbon, glass, polymer, metal and ceramic fibres. The use of a gas diffusion electrode according to any preceding claim, wherein the non-woven fibres are carbon. The use of a gas diffusion electrode according to any preceding claim, wherein the catalyst material(s) is one or more metals or their metal oxide, in the form of finely divided unsupported powders or as metals in a dispersed form on a carbon support. The use of a gas diffusion electrode according to any preceding claim, wherein the catalyst material(s) is one or more precious metals or a transition metal or combination or alloy thereof. The use of a gas diffusion electrode according to any preceding claim, wherein the catalyst material(s) is or includes carbon black. The use of a gas diffusion electrode according to any preceding claim, wherein the electrode comprises a second catalyst layer applied to the flexible free-standing catalyst layer. The use of a gas diffusion electrode according to any preceding claim, wherein the electrode comprises a second catalyst layer applied to the flexible, free-standing catalyst layer, which second layer comprises one or more metals or their oxides in the form of an unsupported finely divided powder or one or more metals in dispersed form on a carbon support. The use of a gas diffusion electrode according to any preceding claim, wherein the electrode comprises a second catalyst layer applied to the flexible, free-standing catalyst layer, which second layer comprises one or more precious metals or a transition metal or combination or alloy thereof. The use of a gas diffusion electrode according to any preceding claim, wherein the electrode comprises a second or more layer(s) applied to the flexible, free-standing layer, wherein at least the second layer comprises a particulate metal supported on carbon black in a polymer dispersion.

Inventors: DENTON JAN, GASCOYNE JOHN MALCOLM, POTTER ROBERT JOHN, DENTON, JAN, GASCOYNE, JOHN MALCOLM, POTTER, ROBERT JOHN
Applicants: JOHNSON MATTHEY PLC, JOHNSON MATTHEY PUBLIC LIMITED COMPANY
Application year: 1997
Main classification: H01M4
IPC codes: H01M8, H01M4, C25B11