(353c) Iridate Perovskites As Highly Active Electrocatalysts for Water Oxidation in Acidic Conditions

Advancements in capture efficiency coupled with decrease in cost of electricity from renewable sources provides a unique opportunity for electrochemical conversion processes to make a significant impact on our current energy landscape and chemical industry infrastructure. Electrochemical reduction reactions such as hydrogen evolution and CO₂ reduction are promising routes for production of storable, high purity, carbon-free or carbon-neutral fuels, as well as commodity chemicals. Many such electrocatalytic conversion processes are conducted in aqueous environments, and all of them require a source of protons and electrons to drive the reaction. The oxygen evolution reaction (OER) takes advantage of using water as an abundant resource and provides the necessary protons and electrons to drive these fuel-producing electroreductive reactions. Additionally, development of OER catalysts that are stable and active in acidic conditions could have a direct impact on improvement of devices for polymer electrolyte membrane-based water electrolyzers for fuel production.

We present investigation of a class of catalysts that has demonstrated remarkable activity and stability in acidic electrolyte. Some significantly outperform rutile IrO₂ and RuO₂, the only other OER catalysts to have reasonable stability and activity in acidic electrolyte. Previous work has shown that a highly active catalyst surface is formed via leaching of Sr from surface layers of crystalline SrIrO₃ thin films upon exposure to acidic electrolyte and electrochemical potential cycling. These experimental results are supported by DFT calculations which reveal that the SrIrO₃ perovskite is thermodynamically unstable relative to aqueous Sr²⁺ and propose possible Sr-deficient overlayer structures that may form with electrochemical testing. Through a combination of lattice matching and ab initio molecular dynamics, anatase IrO₂ and IrO₃ overlayers are suggested as highly active motifs that may be present at the restructured catalyst surface. We have developed an expanded series of iridate perovskite catalysts that take advantage of cation leaching from surface layers to access unique surface structures supporting a range of catalytic activity for the OER. Catalyst activity and stability are assessed by alternating cyclic voltammetry (CV) and chronopotentiometry. X-ray photoemission spectroscopy (XPS) of the catalyst surface and x-ray diffraction (XRD) of the catalyst bulk structure before and after electrochemical testing are used to identify motifs that may be responsible for improvements in catalytic activity.