Key role of oxidizing species driving water oxidation revealed by time-resolved optical and X-ray spectroscopies

Abstract Oxidation states underpin the understanding of active states, reaction mechanisms and catalytic performance of electrocatalysts. However, determining them at complex solid–liquid interfaces is challenging. Here we use multimodal spectroscopy to investigate polarized iridium oxide (IrO _x ) electrodes, a model water oxidation catalyst, to identify potential-dependent iridium and oxygen oxidation states. By integrating multiple operando spectroscopies (optical (ultraviolet–visible), Ir L-edge and O K-edge X-ray absorption spectroscopy) with electrochemistry mass spectrometry and density functional theory calculations, we identify the sequential depletion of electron densities from the Ir5 d band (corresponding to Ir ³⁺ →Ir ⁴⁺ →Ir ⁵⁺ ), followed by electron removal from the O2 p band, forming electrophilic oxygen species (O ⁻¹ ) due to enhanced Ir–O covalency and electronic state overlap. Time-resolved measurements reveal distinct lifetimes for Ir ⁵⁺ and O ⁻¹ states under water oxidation conditions, Ir ⁵⁺ remains unreactive whereas O ⁻¹ is consumed at a time constant commensurate with the reaction rate, indicating that O ⁻¹ drives the oxygen evolution reaction. These findings demonstrate the necessity of using multiple operando techniques to gain a unified understanding of the evolution of oxidation states and active sites with potential for water oxidation on oxide catalysts.