The role of individual nickel surface species in the hydrogen evolution reaction on nickel in alkaline electrolytes

The role of Ni surface species in the hydrogen evolution reaction (HER) under alkaline conditions remains a key challenge in the development of cost-effective and efficient catalysts for sustainable hydrogen production. Here, we systematically investigate well-defined Ni surfaces through carefully designed experiments to unravel the contributions of specific Ni species (NiO, Ni(OH)2, and NiHx) to HER kinetics. Using detailed electrochemical and surface characterization, combined with density functional theory (DFT) calculations, we demonstrate that NiO, contrary to a body of literature, acts as a passive spectator, blocking active sites without altering intrinsic activity; NiHx inhibits HER by modifying the electronic structure of Ni; and Ni(OH)2 promotes HER by facilitating water dissociation and creating new, highly active sites in its vicinity on metallic Ni. Our findings establish a unified framework for understanding the intrinsic activity of Ni-based catalysts, addressing discrepancies in the literature and providing insights into the design of next-generation alkaline water electrolyzers.