WO2008007136A1

WO2008007136A1 - Membrane electrode assembly for direct methanol fuel cell

Info

Publication number: WO2008007136A1
Application number: PCT/GB2007/050384
Authority: WO
Inventors: Michael Ian Petch
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2006-07-14
Filing date: 2007-07-06
Publication date: 2008-01-17
Anticipated expiration: 2009-01-14
Also published as: TW200826342A; GB0613951D0

Abstract

A membrane electrode assembly, suitable for use in a direct methanol fuel cell, is disclosed. The membrane electrode assembly comprises an anode gas diffusion layer (5), an anode catalyst layer (4) comprising an anode electrocatalyst, a proton exchange membrane (3), a cathode electrocatalyst layer (2) comprising a cathode electrocatalyst and a cathode gas diffusion layer (1). A methanol oxidation catalyst is incorporated within the cathode gas diffusion layer and/or is present in an additional catalyst layer (6) between the cathode electrocatalyst layer and the cathode gas diffusion layer.

Description

MEMBRANE ELECTRODE ASSEMBLY FOR DIRECT METHANOL FUEL

CELL

The present invention relates to a membrane electrode assembly suitable for use in a direct methanol fuel cell.

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. Proton-conducting membranes are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.

The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymeric membrane. On either side of the membrane there is an electro catalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. .Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer, must allow products to be removed from the electrocatalyst layer, and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.

Direct methanol fuel cells are a promising alternative power source for portable devices such as mobile telephones and laptop computers. Methanol is a readily available fuel that is easy to store and transport and has a high energy density. Methanol or a mixture of methanol and water is supplied to the anode, and an oxidant, usually air or oxygen, is supplied to the cathode. The anode and cathode reactions are shown in the following equations:

Anode: CH₃OH + H₂O → CO₂ + 6H⁺ + 6e Cathode: 3/2O₂ + 6H⁺ + 6e → 3H₂O

Methanol crossover is a phenomenon wherein methanol passes from the anode, through the membrane, to the cathode, instead of reacting at the anode. It is generally viewed as a problem because it decreases fuel utilisation efficiency and leads to a loss in fuel cell performance. In WO 03/003494, methanol crossover is used beneficially to adjust the temperature of the fuel cell. Methanol crossover is facilitated, and the methanol is combusted at the cathode, thereby heating the fuel cell. It is suggested that a cathode electrocatalyst adapted for promoting methanol combustion may be employed in the cathode.

The present inventors have sought to provide an MEA that is suitable for use in a direct methanol fuel cell, and that is adapted to take advantage of methanol crossover. Accordingly, the present invention provides a membrane electrode assembly, suitable for use in a direct methanol fuel cell, comprising an anode gas diffusion layer, an anode catalyst layer comprising an anode electrocatalyst, a proton exchange membrane, a cathode electrocatalyst layer comprising a cathode electrocatalyst and a cathode gas diffusion layer, characterised in that a methanol oxidation catalyst is either incorporated within the cathode gas diffusion layer and/or is present in an additional catalyst layer between the cathode electrocatalyst layer and the cathode gas diffusion layer.

The methanol oxidation catalyst promotes combustion of methanol that has passed through the membrane and the cathode electrocatalyst layer. In the system of WO 03/003494, the methanol is combusted at the cathode electrocatalyst layer. By contrast, when the MEA of the present invention is used in a direct methanol fuel cell, methanol combustion may occur at the cathode electrocatalyst layer, but will additionally occur at the methanol oxidation catalyst which is either within the cathode gas diffusion layer or within a separate catalyst layer. The advantage of using a separate methanol oxidation catalyst, is that this catalyst can be optimised for methanol combustion whilst the cathode electrocatalyst can be optimised for oxygen reduction. The MEA of the present invention will help to eliminate methanol emissions from a direct methanol fuel cell, or may improve the water management within the fuel cell as the heat of combustion evaporates water in the cathode gas diffusion layer.

Suitable methanol oxidation catalysts include the platinum group metals

(platinum, palladium, rhodium, ruthenium, indium and osmium), which are optionally supported on metal oxide or carbon support materials. A preferred methanol oxidation catalyst is platinum. Suitably a platinum group metal methanol oxidation catalyst is present at between O.OOlg/m² and lg/m², preferably between 0.01 and O.lg/m², based on the weight of the platinum group metal.

In a first embodiment of the invention, the methanol oxidation catalyst is incorporated within the gas diffusion layer and in a preferred embodiment is present throughout the thickness of the gas diffusion layer. In an alternative embodiment, the methanol oxidation catalyst is incorporated only within a limited region of the gas diffusion layer, e.g. a region adjacent to one of the faces of the gas diffusion layer.

In a second embodiment of the invention, the methanol oxidation catalyst is present in an additional catalyst layer between the cathode electrocatalyst layer and the cathode gas diffusion layer. Suitably the additional catalyst layer does not contain ion- conducting material so that the methanol oxidation catalyst promotes the gas phase combustion of methanol rather than an electrochemical oxidation of methanol. Preferably the additional catalyst layer comprises a methanol oxidation catalyst and a hydrophobic polymer, and more preferably consists of a methanol oxidation catalyst and a hydrophobic polymer such as polytetrafluoroethylene.

The anode and cathode gas diffusion layers are suitably conventional gas diffusion layers based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan or Ul 05 or Ul 07 paper available from Mitsubishi Rayon, Japan), woven carbon cloths (e.g. the MK series of carbon cloths available from Mitsubishi Chemicals, Japan) or non-woven carbon fibre webs (e.g. ELAT series of non-woven substrates available from E-TEK Inc, USA; H2315 series available from Freudenberg FCCT KG, Germany; or Sigracet® series available from SGL Technologies GmbH, Germany). The carbon paper, cloth or web is typically modified with a particulate material either embedded within the layer or coated onto the planar faces, or a combination of both. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE).

Proton exchange membranes that are suitable for use in direct methanol PEM fuel cells are well known to the skilled person. The membrane may be based on a perfluorinated sulphonic acid material such as Nafion® (DuPont), Flemion® (Asahi Glass) and Aciplex® (Asahi Kasei). Alternatively, the membrane may be based on a sulphonated hydrocarbon membrane such as those available from Polyfuel. The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength. For example, the membrane may comprise a proton-conducting membrane and a matrix of silica fibres, as described in EP 875 524 or the membrane may comprise an expanded PTFE substrate. The thickness of the membrane is suitably from 25 μm to 200μm.

Electrocatalysts that are suitable for use at the anode and cathode electrocatalyst layers are well known to the skilled person. The electrocatalyst may be a finely divided metal powder (metal black), 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 anode electrocatalyst metal is platinum, which may be alloyed with other precious metals such as ruthenium, or base metals such as molybdenum or tungsten. The preferred cathode electrocatalyst metal is platinum. If the electrocatalyst is a supported catalyst, the loading of metal particles on the carbon support material is suitably in the range 10-100wt%, preferably 15-75wt%. The electrocatalyst layers suitably comprise other components, such as ion-conducting polymer, which is included to improve the ionic conductivity within the layer. The anode and cathode electrocatalyst layers suitably comprise proton- conducting polymer, which is included to improve the proton conductivity within the layer.

The methanol oxidation catalyst may be incorporated into the cathode gas diffusion layer by methods that are known to the person skilled in the art. For example, the gas diffusion layer may be impregnated with metal salts that are subsequently reduced and fired. Alternatively, the gas diffusion layer may be impregnated with a preformed catalyst comprising a metal on a support material. The gas diffusion layers, catalyst layers and proton exchange membrane may be combined to form an MEA by processes known to the skilled person, including lamination processes.

An MEA wherein an additional catalyst layer comprising a methanol oxidation catalyst is present between the cathode electrocatalyst layer and the cathode gas diffusion layer may be prepared by methods that are known to the person skilled in the art. For example, a catalyst ink consisting of a methanol oxidation catalyst, a hydrophobic polymer (e.g. polytetrafluoroethylene) and a solvent may be printed onto the surface of the cathode gas diffusion layer and subsequently combined with a membrane coated with the cathode electrocatalyst layer.

The MEA may further comprise components that seal and/or reinforce the edge regions of the MEA. The MEA may be incorporated into a fuel cell stack using conventional methods. The invention provides a direct methanol fuel cell comprising an MEA according to the invention. The invention further provides a portable device (e.g. a portable power supply, a laptop computer or a mobile telephone) incorporating a direct methanol fuel cell according to the invention.

The present invention further provides a method of operating a direct methanol fuel cell according to the invention, comprising steps of (a) supplying methanol and optionally water to the anode; and

(b) supplying air or oxygen to the cathode.

For a more complete understanding of the invention, reference is made to a schematic drawing wherein: Fig. 1 is a schematic diagram showing an MEA according to a first embodiment of the invention.

Fig. 2 is a schematic diagram showing an MEA according to a second embodiment of the invention.

Figure 1 shows an MEA consisting of a cathode gas diffusion layer (1), a cathode electrocatalyst layer (2), a proton-conducting membrane (3), an anode electrocatalyst layer (4) and an anode gas diffusion layer (5). The cathode gas diffusion layer (1) contains a methanol oxidation catalyst.

Figure 2 shows an MEA consisting of a cathode gas diffusion layer (1), a cathode electrocatalyst layer (2), a proton-conducting membrane (3), an anode electrocatalyst layer (4), an anode gas diffusion layer (5) and an additional catalyst layer (6) between the anode cathode gas diffusion layer (1) and the cathode electrocatalyst layer (2). The additional catalyst layer (6) contains a methanol oxidation catalyst.

Claims

1. A membrane electrode assembly, suitable for use in a direct methanol fuel cell, comprising an anode gas diffusion layer, an anode catalyst layer comprising an anode electrocatalyst, a proton exchange membrane, a cathode electrocatalyst layer comprising a cathode electrocatalyst and a cathode gas diffusion layer, characterised in that a methanol oxidation catalyst is either incorporated within the cathode gas diffusion layer and/or is present in an additional catalyst layer between the cathode electrocatalyst layer and the cathode gas diffusion layer.

2. A membrane electrode assembly according to claim 1, wherein the methanol oxidation catalyst is incorporated within the cathode gas diffusion layer.

3. A membrane electrode assembly according to claim 2, wherein the methanol oxidation catalyst is present throughout the thickness of the cathode gas diffusion layer.

4. A membrane electrode assembly according to claim 2, wherein the methanol oxidation catalyst is incorporated only within a limited region of the cathode gas diffusion layer.

5. A membrane electrode assembly according to claim 1, wherein the methanol oxidation catalyst is present in an additional catalyst layer between the cathode electrocatalyst layer and the cathode gas diffusion layer.

6. A membrane electrode assembly according to claim 5, wherein the additional catalyst layer comprises a methanol oxidation catalyst and a hydrophobic polymer.

7. A direct methanol fuel cell comprising a membrane electrode assembly according to any preceding claim.

8. A portable device comprising a fuel cell according to claim 7.

9. A method of operating a direct methanol fuel cell according to claim 7, comprising steps of (a) supplying fuel and optionally water to the anode; and

(b) supplying air or oxygen to the cathode.