Nature Catalysis, Published online: 07 September 2026; doi:10.1038/s41929-026-01605-9 Understanding the role of nickel surface species in the hydrogen evolution reaction under alkaline conditions is crucial for developing precious-metal-free water electrolysers. Now, by systematically studying well-defined Ni surfaces, the authors dissect the contribution of NiO, NiHx and Ni(OH)2 species to the hydrogen evolution reaction.