To design water-splitting catalysts for energy conversion and storage with better metal atom utilisation and activity[1-2], it is necessary to understand how the photo- and electro-catalytic performance of catalyst materials relates to their chemical configuration at the atomic level. Herein, we use operando time-resolved spectroelectrochemistry to investigate the kinetics of multi-reduced and multi-oxidized localised intermediates. Mono-metallic and bi-metallic molecular catalysts were investigated to rationalize more complex multi-metallic systems. First, earth-abundant molecular cobalt catalysts for H2- and CO2-reduction were studied on a photoelectrode based on mesoporous TiO2 . The accumulation of charge in the catalyst, necessary for catalysis, was observed in hundreds of microseconds and was critically quenched by valence band holes under microseconds. It was then shown that regulating the applied potential, the excitation light intensity and the photoelectrode surface coverage was an effective approach to prevent recombination while simultaneously favouring charge accumulation. Second, the electrochemical activity of molecular and inorganic iridium-based water-oxidation catalysts were compared per iridium state . Three multi-electron oxidation states involving Ir3+, Ir4+ and Ir4.x+ were identified in both catalysts. The generation of Ir4.x+-based states was found to be the potential determining step for catalytic water-oxidation, whilst H2O2 oxidation was observed to be driven by the generation of Ir4+ states. The TOFs over potential indicate a first order reaction mechanism for H2O2 oxidation by Ir4+ states in IrOx and for water oxidation by the molecular catalyst, and a potential-dependent mechanism for water oxidation in IrOx involving the co-operative interaction of multiple Ir4.x+ states. This insight into the intrinsic reaction kinetics based on localised states from our spectroelectrochemical data offers a promising alternative to more widely applied Tafel and Butler-Volmer analyses for disordered metal oxide electrocatalysts such as IrOx, which is of interest to enhance the performance and cost effectiveness of electrolysers and to develop earth-abundant catalysts.
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 Bozal-Ginesta, C., Mesa, C.A., Eisenschmidt, A., Francàs, L., Shankar, R., Antón-García, D., Warnan, J., Willkomm, J., Reynal, A., Petit, C., Reisner, E., Durrant, J.R., Chemical Science, 2021, 12, 946-959, DOI: 10.1039/d0sc04344c
 Bozal-Ginesta, C., Rao, R.R., Mesa, C.A., Liu, X., Hillman, S., Stephens, I., Durrant, J.R., Operando spectroelectrochemical analysis of active state kinetics in water-oxidation IrOx electrocatalysts, submitted