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

TLDR

Despite the fundamental and practical significance of the hydrogen evolution and oxidation reactions (HER/HOR), their kinetics in base remain unclear. The presence of OHad-(H₂O)ₓ-AM⁺ in the double‑layer facilitates OHad removal into the bulk, forming OH⁻-(H₂O)ₓ-AM⁺ per hard–soft acid–base theory, selectively promoting HER while destabilizing OHad and potentially hindering HOR via a bifunctional mechanism, as supported by CO oxidation results. Alkaline HER/HOR kinetics can be unified by the catalytic roles of adsorbed hydroxyl–water–alkali metal cation adducts; enriching OHad via surface Ni benefits both reactions, increasing AM⁺ concentration promotes HER only, and varying AM⁺ identity affects both HER and HOR, underscoring the importance of this concept for alkaline electrochemistry.

Abstract

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 OHad 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)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.