Chemical Epitaxy of Iridium Oxide on Tin Oxide Enhances Stability of Supported OER Catalyst

Significantly reducing the iridium content in oxygen evolution reaction (OER) catalysts while maintaining high electrocatalytic activity and stability is a key priority in the development of large-scale proton exchange membrane (PEM) electrolyzers. In practical catalysts, this is usually achieved by depositing thin layers of iridium oxide on a dimensionally stable metal oxide support material that reduces the volumetric packing density of iridium in the electrode assembly. By comparing two support materials with different structure types, it is shown that the chemical nature of the metal oxide support can have a strong influence on the crystallization of the iridium oxide phase and the direction of crystal growth. Epitaxial growth of crystalline IrO₂ is achieved on the isostructural support material SnO₂, both of which have a rutile structure with very similar lattice constants. Crystallization of amorphous IrO_x on an SnO₂ substrate results in interconnected, ultrasmall IrO₂ crystallites that grow along the surface and are firmly anchored to the substrate. Thereby, the IrO₂ phase enables excellent conductivity and remarkable stability of the catalyst at higher overpotentials and current densities at a very low Ir content of only 14 at%. The chemical epitaxy described here opens new horizons for the optimization of conductivity, activity and stability of electrocatalysts and the development of other epitaxial materials systems.