GB2190399A - Multi-metal electrode - Google Patents

Abstract

An electrode consists of two or more metals sputtered onto an electrode carrier so as to provide a variety of surface sites for different steps in the electrocatalytic process. Binary or more-component alloys of metal can be sputtered from a sputtering target comprising an alloy of, or a mixture of powders of, the metals. Alternatively targets of two or more metals can be moved into sputtering position in turn thereby producing successive layers of different metals. The metals may be sputtered onto porous or non-porous members of materials such as PTFE to make fuel-cell electrodes or metallised-membrane sensor electrodes.

Description

SPECIFICATION Multi-metal electrode This invention relates to an electrode comprising two or more metals and to a method of making it.

Electrodes are solid electronic conductors, and in use are in contact with liquid ionic conductors, termed electrolytes. At the interface between electrodes and electrolytes, electron transfer processes take place. Whether these processes are needed for sensing the presence and concentration of a material (as in a sensor), or for power generation (as in a battery or fuel cell), it is likely that the electrocatalytic properties of the electrode will play an important part in the processes.

An electrode of high electrocatalytic activity is likely to have on its surface a variety of sites.

A "site" is a portion of the surface of which one of various possible processes occurs. For instance there will be sites that are suitable for the adsorption of one or other component of the electrocatalytic process. There are likely to be other sites which components, or complexes or activated complexes of the components, need to diffuse for reaction with each other or with the surface or for electron transfer. One example is the oxidation of carbon monoxide on the surface of a platinum electrode in dilute sulphuric acid. It is likely that for high electrocatalytic activity the electrode needs to have sites of non-oxidised platinum for the adsorption of the carbon monoxide.It is also likely that the carbon monoxide needs to diffuse to a portion of the surface on which there are particular types of oxidised platinum species, by which the carbon monoxide can be oxidised. The oxidised species may then need to diffuse to yet a third type of site in order to leave the surface to make room for further adsorption or reaction.

One way in which catalysts are endowed with a heterogeneiety of sites is by making them of a combination of several components. Thus, for instance, binary alloys of platinum-group metals are known for fuel-cell electrodes. By "metal" we also include any electronic conductor.

According to the invention a method of making an electrode comprises sputtering two or more metals onto an electrode carrier.

Binary or more-component alloys of metals could be sputtered onto porous or non-porous membranes of materials such as PTFE to make fuel-cell electrodes or metallised-membrane sensor electrodes. This could be achieved by having sputtering targets made from the appropriate alloys. It could also be achieved by upwards sputtering of a mixture of powders of the appropriate metals. Thus, in the invention, preferably the sputtering target is of an alloy of, or a mixture of powders of, the said two or more metals.

However, if an apparatus is available in which the targets of two or more metals can be moved into sputtering position in turn, then an electrode can be constructed, in which successive layers consist of different metals. The relative thicknesses and order of the different layers can easily be varied. This allows the heterogeneiety of the catalytic surface to be varied even more than in simple homogeneous alloys. Thus, preferably there is a plurality of sputtering targets, each of one only, or some, of the said or of the same metals in different proportions, and optionally the various targets are sputtered sequentially.

It a sputtering substrate is kept cool, for instance with cooling water, as in the fabrication of metallised-membrane electrodes, then the resulting sputtered layer is more porous. The hotter the layer is allowed to be during the sputtering process and afterwards, the less porous it will be, and the more intercalated the different layers will be. In any case, it is likely that some sintering and interdiffusion of the metals constituting the layered electrode will take place even at ambient temperatures. It is however preferred for such interdiffusion to stop short of total homogeneity, and the preferred electrode has differentiated layers of metals (including alloys) rather than being of one uniform alloy.

Claims (6)

1. A method of making an electrode comprising sputtering two or more metals onto an electrode carrier.

2. A method according to Claim 1, wherein the sputtering target is of an alloy of, or a mixture of powders of, the said two or more metals.

3. A method according to Claim 1, wherein there is a plurality of sputtering targets, each of one only, or some, of the said metals, or of the same metals in different proportions.

4. A method according to Claim 3, wherein the various targets are sputtered sequentially.

5. An electrode made by a method according to any preceding claim.

6. An electrode according to Claim 5, comprising differentiated layers of metals.