Thermal synthesis of electron deficient oxygen species on crystalline IrO2

Abstract

Water splitting is a promising technology in the path towards complete renewable energy within the hydrogen economy but overcoming the sluggishness of the oxygen evolution reaction (OER) is a major challenge. Iridium-based oxides remain the most attractive materials for the OER under acidic conditions since they offer the combination of activity and stability. Gaining knowledge about how these materials have such an ability is of great interest to develop improved electrocatalysts for the OER. Among the different iridium-based oxides the materials with high concentrations of electron deficient oxygen (O^I−) have been shown to have higher OER activity, however, they also have high dissolution rates, seemingly due to the presence or formation of Ir^III species. In contrast, rutile-type IrO₂, which does not contain Ir^III species, has high dissolution resistance but the OER activity remains comparatively low as only low coverages of O^I− species are formed under OER. The apparent link between O^I− and Ir^III species that comes from these observations has yet to be proven. In this work, using ab initio thermodynamics and in situ X-ray photoelectron and absorption spectroscopy we show that the same electrophilic O^I− species that appear on Ir-based oxides under OER can be formed on Ir^IV+δ by mild thermal oxidation of rutile-type IrO₂, without the presence Ir^III species.