Role of electrolyte pH on water oxidation for iridium oxides

Understanding the effect of non-covalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy and surface-enhanced infrared absorption spectroscopy to probe the effect of non-covalent interactions on OER activity of IrOx in acidic and alkaline electrolyte. Our results suggest the active species for OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species is higher in alkaline electrolyte. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilise oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolyte; the larger repulsive interaction between them results in significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.