Development of New Mixed-Metal Ruthenium and Iridium Oxides as Electrocatalysts for Oxygen Evolution

We review recent research into oxides of platinum-group metals (PGMs), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction. These are used in membrane electrode assemblies in devices such as electrolysers, for water splitting to generate hydrogen as fuel, and in fuel cells where they provide a buffer against carbon corrosion. In these situations, proton-exchange membrane layers are used, and highly acid-resilient electrocatalyst materials are required. The range of structure types investigated includes perovskites, pyrochlores and hexagonal perovskite-like phases, where the PGM is partnered by base metals in complex chemical compositions. The role of chemical synthesis in the discovery of new oxide compositions is emphasised, particularly to yield powders for processing into membrane electrode assemblies. We highlight emerging work that shows how leaching of the base metals from the multinary compositions occurs during operation to yield active PGM-oxide phases, and how attempts to correlate stability with crystal structure have been made. Implications of these discoveries for the balance of activity and stability needed for effective electrocatalysis in real devices are discussed.