Unifying the Alkaline Hydrogen Evolution/oxidation Reactions Kinetics by Identifying the 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<sub>ad</sub>)-water-alkali metal cation (AM<sup>+</sup>) adducts, on the basis of the observations that enriching the OH<sub>ad</sub> abundance via surface Ni benefits the HER/HOR; increasing the AM<sup>+</sup> concentration only promotes the HER while varying the identity of AM<sup>+</sup> affects both HER/HOR. The presence of OH<sub>ad</sub>-(H<sub>2</sub>O)<sub>x</sub>-AM<sup>+</sup> in the double layer region facilitates the OH<sub>ad</sub> removal into the bulk forming OH<sup>-</sup>-(H<sub>2</sub>O)<sub>x</sub>-AM<sup>+</sup> <i>as per</i> the hard-soft acid-base (HSAB) theory, thereby selectively promoting the HER. It can be detrimental to the HOR <i>as per</i> the bifunctional mechanism as the AM<sup>+</sup> destabilizes the OH<sub>ad</sub>, which is further supported by the CO oxidation results. This new notion may be important for alkaline electrochemistry.