Unifying the Hydrogen Evolution and Oxidation Reactions Kinetics in Base by Identifying the Catalytic Roles of Hydroxyl-Water-Cation Adducts

Despite the fundamental and practical significance of the hydrogen evolution and oxidation reactions (HER/HOR), their kinetics in base remain unclear. Herein, we show that the alkaline HER/HOR kinetics can be unified by the catalytic roles of the adsorbed hydroxyl (OH_ad)-water-alkali metal cation (AM⁺) adducts, on the basis of the observations that enriching the OH_ad abundance via surface Ni benefits the HER/HOR; increasing the AM⁺ concentration only promotes the HER, while varying the identity of AM⁺ affects both HER/HOR. The presence of OH_ad-(H₂O) _x-AM⁺ in the double-layer region facilitates the OH_ad removal into the bulk, forming OH^--(H₂O) _x-AM⁺ as per the hard-soft acid-base theory, thereby selectively promoting the HER. It can be detrimental to the HOR as per the bifunctional mechanism, as the AM⁺ destabilizes the OH_ad, which is further supported by the CO oxidation results. This new notion may be important for alkaline electrochemistry.