Understanding two voltammetric features of water reduction and water oxidation in mild pH solutions

Electrification of many processes requires the use of aqueous solutions under mild pH conditions where the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) can become competing reactions. The HER and OER under mild pH conditions show peculiar voltammetric behaviours, specifically two reductive or oxidative features, that are not observed in strongly acidic and basic solutions. These behaviours cannot be fully explained by thermodynamic considerations only and are particularly complex owing to the involvement of multiple water species (H₃O⁺, H₂O and OH^–) and the conversion between these species via water autodissociation and acid–base neutralization reactions. This Analysis provides a systematic and conceptual explanation of the effect of pH, potential, stirring and buffer on the thermodynamics and kinetics of the HER and OER, providing fundamental and yet essential insights into comprehending HER and OER behaviours under mild pH conditions, and their implications for other aqueous reactions more broadly.