The development of active, stable, and more affordable electrocatalysts for acidic oxygen evolution reaction (OER) is of great importance for the practical application of electrolyzers and the advancement of renewable energy conversion technologies. Currently, IrO2 is the only catalyst with high stability and activity, but a high cost. Further optimization of the catalyst is limited by the lack of understanding of catalytic behaviors at the acid-IrO2 interface. Here, in strong interaction with the experiment, we develop an explicit model based on grand-canonical density function theory (GC-DFT) calculations to describe acidic OER over IrO2. Compared to the explicit models reported previously, hydronium cations (H3O+) are introduced at the electrochemical interface in the current model. As a result, a variation in stable IrO2 surface configuration under the OER operating condition from previously proposed complete *O-coverage to a mixture coverage of *OH and *O is revealed, which is well supported by in situ Raman measurements. Moreover, the accuracy of predicted overpotential is increased in comparison with the experimentally measured. More importantly, an alteration of the potential limiting step from previously identified *O → *OOH to *OH → *O is observed, which opens new opportunities to advance the IrO2-based catalysts for acidic OER. In addition, direct seawater electrolysis is also investigated as an extension of pure water-based electrolysis, which relies on freshwater, a resource expected to become increasingly scarce. We present in situ characterizations and first principle theoretical calculations that reveal the dynamic surface phase transition through the competitive adsorption and the oxygen selectivity change across various pH and potential conditions. Notably, the selectivity dominating reaction steps are identified by the sensitivity analysis. Our results highlight the importance of combining in situ techniques and theoretical calculations for the interfacial reaction mechanism study, opening new opportunities for optimizing IrO2-based catalysts in water and seawater electrolysis.
