Key Role of Oxidising 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—and the underlying physicochemical nature they reflect—at complex solid/liquid interfaces remains a challenge. Here, we use multimodal spectroscopy to investigate polarised iridium oxide (IrOx) electrodes, a model water-oxidation catalyst, to identify potential-dependent iridium and oxygen oxidation states and their implication on the active states and reaction mechanism for water oxidation. By integrating multiple operando spectroscopic techniques (optical (UV-vis), Ir L-edge, and O K-edge X-ray absorption spectroscopy) with electrochemistry mass spectrometry and density functional theory calculations, we identify a sequential depletion of electron densities from the Ir 5d band (corresponding to Ir3+→ Ir4+→ Ir5+), followed by electron removal from the O 2p band, forming electrophilic oxygen species (O⁻1) due to enhanced Ir-O covalency and electronic states overlap at higher applied potentials. Time-resolved measurements reveal distinct lifetimes for Ir5+ and O-1 states under water oxidation conditions, showing Ir5+ remains unreactive while O-1 is consumed, at a time constant commensurate with the reaction rate, consistent with the notion that O-1 drives the oxygen evolution reaction (OER). These findings thus demonstrate the necessity of using multiple advanced operando techniques to gain unified, mechanistic understanding of the evolution of oxidation states with potential and active states for water oxidation on oxide catalysts.