GB2471837A - A membrane electrode assembly with a iridium coated membrane - Google Patents

Abstract

A membrane electrode assembly (MEA) comprising a membrane coated with a layer of iridium. The coating may be applied by forming an ink from iridium powder and applying the ink to the anodic side of the membrane using an airbrush. The MEA with the iridium coated membrane had improved performance when compared with an MEA with an iridium oxide coated member.

Description

In-situ catalyst conversion

Field of the Invention:

This invention relates to the in-situ conversion of metallic catalyst to a metallic oxide catalyst in electrochemical cells or stacks of cells.

Background to the Invention:

Iridium oxide is widely regarded as one of the anode catalysts of choice in a proton exchange membrane electrolyser. Iridium is often cited as one of the most corrosion resistant element it also offers a low oxygen evolution activation overvoltage in its oxide form when used in a finely divided high surface area arrangement. The same can be said for ruthenium oxide, both ruthenium and iridium are used for illustrative purposes in this application and are not intended to limit the general scope of in-situ oxidation of metal catalysts or combinations of metals to metal oxides to improve catalytic activity.

The use and application of metal oxides is known but the process for the conversion may involve the use of high temperatures, (typically 500c in the case of Ir) which may limit the materials to which the oxide can be applied and the processes which can be used to achieve the application.

Summary of the Invention

It is proposed that oxidation of iridium (or other suitable metals) in situ offers performance and cost benefits, and most importantly allows the conversion process to be carried out in the presence of materials of low thermal stability e.g materials which might melt, evaporate or burn during conventional processing. Cost reductions also come about by reducing the number of steps in production of a membrane electrode assembly (MEA). This is applicable to both metal powders used in a catalytic ink formulation and metals deposited onto the electrode meshes. In some cases the metal state may be cheaper and easier to purchase than the oxide state, for example, unlike iridium, iridium oxide has limited commercial availability. Fine iridium oxide powder is typically produced via the Adams fusion or colloid method using an iridium salt precursor and requiring several steps. It can also be produced via thermal or electrochemical oxidation of iridium powder.

21r + 3H20 -1r203 + 6H + 6e E0= O.926-O.O59pH (1) Jr + 2H20 -IrE)2 + 4H + 4e E0= O.926-O.O59pH (2) 1r203 + H20 -21r02 + 2H +2e E0= O.926-O.O59pH (3) The oxidation of such metals in situ results in a partial oxidation of the total quantity of metal present to offer a further gain in terms of efficiency as the electrical resistance through the catalyst layer is reduced compared to one with a fully prior-oxidised layer.

For iridium it is envisaged that only the surface layer iridium will oxidise as the environmental conditions (pH, oxygen/water presence and applied potential) enable the above reactions to occur. A passivating layer will form protecting unexposed iridium from further oxidation. Any iridium present which is isolated from any of these prerequisite conditions will remain unchanged. Therefore the iridium oxide formed will have good contact with the membrane at critical points where electrolysis is favourable, without converting all the iridium (better conductivity) to iridium oxide.

Example of the Invention Ink formulations were made using: 1. commercially produced Iridium powder 2. commercially produced Iridium powder, which was oxidised to Iridium oxide in-house in a furnace at 500°C for 5 hours prior to ink manufacture.

The powders were analysed by energy dispersive X ray using a scanning electron microscope to confirm the composition of each powder prior to use in an ink. Analysis showed that the furnace treatment oxidised the majority of the Iridium powder.

The two powders were then made into inks, with all other components of the inks the same in both cases. Each ink was applied to anodic side of a membrane using an airbrush to give a similar loading and the membrane electrode assembly tested within a cell with all other variables unchanged. Current density-voltage characteristics of the MEA are shown in figure 1.

Results showed that the MEA made with iridium powder had improved performance compared to that made with Iridium oxide powder. To confirm that iridium inked MEA5 were not just showing a transient improvement due to incomplete oxidation, one such membrane was swept across the voltage range multiple times. Little change in potential was observed suggesting that a steady state had been achieved. Results of multiple sweeps are shown in figure 2. The IV curves show that this is consistent with the formation of iridium oxide at least as a surface coating on the original iridium metal particles used in the ink.

Further analysis of the iridium inked membrane showed evidence of iridium oxidation, with weight and atomic weights being close to the ratio achieved for furnace iridium powder, and quite different from the original iridium powder used in the ink formulation, as shown in table 1.

Table 1: Iridium analysis, pre and post oxidation via furnace treatment and in-situ.

Weight% Atomic% IrM IrM Iridium powder 92.23 45.21 Iridium powder after furnace oxidation to iridium oxide powder 75.95 26.27 Iridium ink MEA after operation in an electrolyser 83.50 25.56

Claims (2)

1. CLAIMS1 A membrane electrode assembly, wherein the membrane is coated with a layer of iridium or another similar metal.
2. 2. The use of iridium or other similar metals as a coating for a membrane electrode assembly, wherein the metal can be converted into its oxide form, in situ.