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 (OHad)-water-alkali metal cation (AM(+)) adducts, on the basis of the observations that enriching the H-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 OHad-(H2O) (H2O)(x)-AM(+) in the double-layer region facilitates the OHad 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 OHad, which is further supported by the CO oxidation results. This new notion may be important for alkaline electrochemistry.