GB2484886A

GB2484886A - Membrane

Info

Publication number: GB2484886A
Application number: GB1011904.8A
Authority: GB
Inventors: Michael Ian Petch
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2010-07-15
Filing date: 2010-07-15
Publication date: 2012-05-02
Also published as: GB201011904D0

Abstract

A membrane for use in a proton exchange membrane fuel cell at a temperature in excess of 100Â°C comprises a membrane formed from a polymer capable of being doped with one or more acid groups or which is pre-complexed with one or more acid groups, and a porous particulate material is disclosed. The polymer may be a polyazole. The porous particulate material may be 20-80 % by weight of the polymer and may be a stable, non-electrically conducting inorganic material.

Description

MEMBRANE

The present invention relates to a proton-conducting polymer electrolyte membrane, suitable for use in fuel cells, and in particular a polyelectrolyte membrane formed from a polyazole, wherein the membrane further comprises a porous particulate material.

It is known that membranes comprising polymers which are capable of complexing with acid groups or being doped with acid groups can be used in proton exchange membrane (PEM) fuel cells and can provide a viable alternative to PEM fuel cells comprising a perfluorosulfonic acid polymer, for example Nafion®, for use at temperatures in excess of 100°C. For example, membranes formed from polybenzimidazole and doped with phosphoric acid are well known (US 5,525,436).

However, it has been found that such membranes are susceptible to compression when used in the fuel cell, which can lead to membrane thinning and eventually shorting of the fuel cell electrodes. This leads to failure of the fuel cell and therefore a much shorter lifetime than is required for practical applications.

Tt is therefore an object of the invention to provide a membrane which is resistant to compression and which overcomes the problems of shorting of the electrodes.

Accordingly, the present invention provides a membrane, suitably for use in a proton exchange membrane fuel cell at a temperature in excess of 100°C, said membrane being formed from a polymer capable of being doped with one or more acid groups or which is pre-complexed with one or more acid groups, and a porous particulate material.

Pre-complexing of the polymer with the acid groups may be carried out by a polycondensation reaction of the monomer and acid components, such as described in U52006/0079392.

I

In one particular embodiment, the polymer is based on a polyazole structure, for example: polybenzimidazole; polyimidazole; polybenzthiazole; polybenzoxazole; polyoxadiazole; polythiadiazole; polythiazole; preferably the polymer is a polybenzimidazole. A number of polymers comprising a polybenzimidazole structure are known, for example: poly(2,5-benzimidazole) (AB-PBI); poly(2,2 -m-phenylene-5,5 - bibenzimidazole) (m-PBI); poly-[(l -(4,4 -diphenylether)-S-oxybenzimidazole)- benzimidazole] (PBI-OO); poly[( 1-93,3 -phthalide-p-phenylene)-5-oxybenzimidazoleO-benzimidazolel (PBT-OPh). Most preferably, the polymer is in-PBI or AB-PBI.

The acid groups with which the polymer is capable of being doped or which are pre-complexed to the polymer are suitably sulphuric acid groups, polyphosphonic acid groups or phosphoric acid groups, preferably phosphoric acid groups. Suitably, the polymer is capable of being doped by, or is pre-complexed to at least two, preferably at least three acid groups (molecules) per repeating polymer unit. Suitably, there are no more than six, preferably no more than five acid molecules per repeating unit, although much higher levels of doping or pre-complexing of up to 50 mols acid per repeat polymer unit are possible and are within the scope of the invention.

The membrane has a thickness suitably in the region of 50-200 microns, more suitably 70-150 microns, and preferably 70-100 microns.

The porous particulate material is present in a suitable amount such that compression of the membrane leading to shortage of the electrodes is prevented. Suitably, it is present in an amount of from 20-80 % by weight of the polymer component of the membrane, more suitably in an amount of from 30 -70% by weight and preferably from -70% by weight.

The particle size of the porous particulate material is dependent upon the thickness of the membrane. The porous particulate material has a maximum dimension that is smaller than the thickness of the membrane.

The particulate material is preferably porous such that acid groups can be held within the pores of the particulate material, thus not reducing the proton conductivity of the membrane. The porous particulate material is suitably a stable, non-electrically conducting, inorganic material. In one preferred embodiment of the invention, the particulate material is silicon carbide.

In one embodiment, the porous particulate material is added to the polymer solution prior to casting of the membrane.

Membranes of the present invention are suitable for use in proton exchange membrane fuel cells, in particular those that operate at temperatures in excess of 100°C and preferably in excess of 140°C. If the membrane of the invention has been pre-complexed with acid groups, then no further processing is required before it is combined with anode and cathode catalyst layers and anode and cathode gas diffusion layers to form a membrane electrode assembly (MEA), However, if the membrane has not been pre-complexed with acid groups, doping of the membrane is required. This is carried out according to methods known in the art, for example by immersing the membrane in a suitable acid, for example sulphuric acid or phosphoric acid. The doped membrane may then be combined with the various layers as described above to form the MEA.

Alternatively the anode and cathode catalyst layers may be pre-doped with acid groups, and the acid is transferred and doped into the membrane during the assembly process to form the MEA and in the conditioning of the MEA in the fuel cell. Still alternatively, a combination of doping the membrane and the anode and/or cathode may be used.

Accordingly, a further aspect of the invention provides an MEA comprising a membrane of the invention.

The anode and cathode catalyst layers each comprise an electrocatalyst, which may be a finely divided unsupported metal powder, or may be a supported catalyst wherein small metal particles are dispersed on electrically conducting particulate carbon supports. The electrocatalyst metal is suitably selected from (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium), (ii) gold or silver, (iii) a base metal, or an alloy or mixture comprising one or more of these metals or their oxides.

The preferred electrocatalyst metal is platinum, which may be alloyed with other precious metals or base metals. If the electrocatalyst is a supported catalyst, the loading of metal particles on the carbon support material is suitably in the range lO-9Owt%, preferably 15- 75wt% of the weight of resulting electrocatalyst. Suitably the carbon support used for the cathode catalyst is a carbon black that has been treated at high temperature to impart a high level of corrosion resistance, or is produced as a highly corrosion resistant carbon black in its production process.

The anode and cathode gas diffusion layers are suitably based on conventional gas diffusion substrates. Typical substrates include non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the 112315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SQL Technologies GmbH, Germany or AvCarb® series from Ballard Power Systems Inc, or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into a MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of fuel cell and the operating conditions that will be used. The substrate can be made more wettable by incorporation of materials such as carbon blacks via impregnation from liquid suspensions, or can be made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the melting point of the polymer. A microporous layer may also be applied to the gas diffusion substrate on the face that will contact the electrocatalyst layer. The microporous layer typically comprises a mixture of a carbon black, which is suitably a corrosion resistant material, and a polymer such as polytetrafluoroethylene (PTFE).

Claims

CLAIMS1. A membrane, suitably for use in a proton exchange membrane fuel cell at a temperature in excess of 100°C, said membrane being formed from a polymer capable of being doped with one or more acid groups or which is pre-complexed with one or more acid groups, and a porous particulate material.
2. A membrane according to claim 1, wherein the polymer is a polyazole.
3. A membrane according to claim 1 or claim 2, wherein the porous particulate material is present in an amount of from 20-80 % by weight of the polymer.
4. A membrane according to any preceding claim, wherein the porous particulate material is a stable, non-electrically conducting, inorganic material.
5. A membrane according to claim 4, wherein the porous particulate material is silicon carbide.
6. A membrane electrode assembly comprising a membrane according to any one of claims ito 5.