Gas diffusion substrates

Details Gas diffusion substrates Gas diffusion substrates

2001-09-14

2003-03-11

Bell, Bruce F. (Department: 1741)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S047000, C429S047000, C429S006000, C429S006000, C204S283000

Reexamination Certificate

active

06531240

ABSTRACT:

The present invention relates to a novel porous gas diffusion substrate and a porous gas diffusion electrode which may have application in electrochemical devices, for use for example in a fuel cell, and a process for the manufacture of the substrate and electrode.
BACKGROUND OF THE INVENTION
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. Electrocatalysts 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. Conventionally, gas diffusion electrodes 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 electrocatalyst containing layers, which are supported onto a more rigid porous substrate layer.
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 a membrane electrode assembly (MEA). Alternatively, the MEA may be formed from two porous gas diffusion substrates and a solid proton-conducting polymer membrane catalysed on both sides; or the MEA may be formed from one gas diffusion electrode and one gas diffusion substrate and a solid proton-conducting polymer catalysed on the side facing the gas diffusion substrate. The anode gas diffusion electrode or substrate is designed to be porous and allow the reactant hydrogen or methanol to enter from the face of the electrode or substrate exposed to the reactant fuel supply, and diffuse through the thickness of the electrode or substrate to the reaction sites which contain electrocatalysts, 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 or substrate 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 or substrate. The cathode is also designed to be porous and allow oxygen or air to enter the electrode or substrate and diffuse through to the reaction sites. Electrocatalysts 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 cathode 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 cathode, it becomes more difficult for the reactant oxygen to diffuse to the reaction sites, and thus the performance of the fuel cell decreases. In the case of methanol fuelled PEMFCs, additional water is present due to the water contained in the methanol, which can be transported through the membrane from the anode to the cathode side. The increased quantity of water at the cathode requires removal. However, it is also the case with proton conducting membrane electrolytes, that if too much water is removed from the cathode structure, the membrane can dry out resulting in a significant decrease in the performance of the fuel cell.
Traditionally, the gas porous substrates used in PEMFC are based on high density materials such as rigid carbon fibre paper (i.e. Toray TGP-H-60 or TGP-H-90 from Toray Europe Ltd., 7 Old Park Lane, London, W1Y 4AD) or woven carbon cloths, such as Zoltek PWB-3 (Zoltek Corporation, 3101 McKelvey Road, St. Louis, Mo. 63044). Substrates such as these are usually modified with a particulate material embedded within the fibre network and often coated on to the surface. Typically these particulate materials comprise a carbon black and polymer mix (the polymer most frequently used being polytetrafluoroethylene, PTFE). The coating, or embedding, is carried out in order to improve the water management properties, to provide a continuous surface on which to apply the catalyst layer and to improve the electrical conductivity.
More recently, electrode structures based on a porous substrate comprising a non-woven network of carbon fibres (carbon fibre structures such as Optimat 203, from Technical Fibre Products, Kendal, Cumbria, UK) with a particulate material embedded within the fibre network as disclosed in EP-A-0 791 974 have shown comparable performances to structures based on carbon fibre paper or cloth. Electrodes based on non-woven materials of this type give a physically strong, dimensionally stable and handleable structure at a cost compatible with auto-motive power applications.
However, although these electrodes perform reasonably well under most conditions, they lack the flexibility to modify the water management characteristics, particularly when operating at high current densities. Control of this property is highly important to ensure the optimum functioning of the PEMFC which may operate under a range of conditions of, for example, temperature, pressure, reactant gas flow rates and reactant gas level of humidification.
Recent attempts have been made to improve the performance of gas diffusion electrodes by minimising all transport resistances occurring in the electrochemical reaction, by making appropriate changes in the electrode structure. For example, a method proposed is to include a pore-former to the catalyst mixture prior to being applied to the electrode substrate, and subsequent treatment of the electrode to remove the pore-forming material to leave pores. Such methods are proposed in Fuel Cells Bulletin, No 6, March 1999, page 6 and in EP-A-0797265. However, the introduction of a pore-former results in the need for an additional processing step, ie the subsequent removal of the pore-former, and therefore presents a disadvantage in terms of the manufacturing and total cost. Furthermore, complete removal of the pore-former would be required since the pore-former material itself may be deleterious to the performance of the electrode; it would be difficult to ensure complete removal and some form of analysis would be required to determine the extent to which the pore-former had been removed, thus further adding to the cost.
It is an object of the present invention to provide a porous substrate, suitable for use for example in a gas diffusion electrode, which retains good gas diffusion properties, but also has greatly improved water management properties.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a gas diffusion substrate comprising a porous structure and a filler material, characterised in that the filler material comprises (i) a base filler material which is electrically conducting and carbonaceous, and (ii) one or more

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