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

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 Ir5d band (corresponding to Ir³⁺→Ir⁴⁺→Ir⁵⁺), followed by electron removal from the O2p band, forming electrophilic oxygen species (O^-1) due to enhanced Ir-O covalency and electronic state overlap. Time-resolved measurements reveal distinct lifetimes for Ir⁵⁺ and O^-1 states under water oxidation conditions, Ir⁵⁺ 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.