Surface Memory in Nickel HER Catalysts: Hydroxide Formation at Open-Circuit Potential

Abstract

Grid-coupled electrolyzers must keep up with the variable nature of renewable power, placing nickel-based alkaline HER catalysts under rapidly changing potentials. Yet the surface chemistry of nickel-based electrodes during transitions between biased operation and open-circuit potential (OCP) remains poorly resolved. Here we resolve the morphological and chemical evolution of Ni electrodes during cycling between reductive bias and OCP. We combine in situ X-ray photoelectron spectroscopy with isotopically labeled NanoSIMS, complemented by microscopy methods, to map morphological and chemical evolution under controlled cycling. We find that reductive bias generates a transient surface that undergoes rapid hydroxylation upon return to OCP; the Ni(OH)2 observed at OCP therefore reflects the preceding electrochemical history rather than serving as a direct degradation product or mechanistic intermediate. This “memory effect” links surface speciation to prior bias conditions and identifies OCP transitions as a critical regime for hydroxide growth and irreversible surface transformations. More broadly, the results underscore the need to account for non-steady-state operation when designing and benchmarking alkaline electrocatalysts for grid-coupled water electrolysis.