Dynamic reconstruction in nickel sulfide-based precatalysts for water electrolysis

Nickel sulfides have emerged as a versatile class of precatalysts for alkaline water electrolysis, wherein the as-synthesized phase serves as a precursor that transforms into the catalytically relevant state under operating conditions. This behavior has shifted attention from a conventional catalyst-centered view toward a reconstruction-centered perspective, in which structural evolution becomes a central determinant of both catalytic activity and long-term stability. In this review, nickel sulfides are examined from this reconstruction-centered perspective. The crystal chemistry of representative Ni-S phases is first outlined as the structural foundation for their distinct thermodynamic stability, electronic properties, and transformation propensities. Reconstruction under oxygen-evolution conditions is then analyzed, with emphasis on sulfur oxidation and leaching, Ni(OH)2/NiOOH shell formation, sulfur-leaching kinetics, dopant-mediated effects, and electrolyte-coupled interactions that collectively determine whether the transformation remains surface-limited or proceeds into bulk conversion. Reconstruction under hydrogen evolution conditions is subsequently explored, highlighting phase convergence, partial surface Ni-O formation, and stabilization strategies involving electron-acceptor incorporation, interfacial confinement, and sulfur-vacancy tuning. Recent progress in translating Ni-S precatalysts into self-supported electrodes, high-current-density alkaline devices, and anion exchange membrane electrolyzers is further evaluated, underscoring that reconstruction also governs practical durability and catalyst-layer behavior. Based on these mechanistic and device-level insights, this review identifies key design principles that should guide the future development of Ni-S precatalysts, including control of reconstruction depth, tuning of sulfur-vacancy energetics, intentional heterointerface engineering, management of electrolyte-derived species, and balancing activation with long-term stability. Overall, nickel sulfides are positioned as promising precatalyst platforms whose performance is governed not by structural invariance but by the extent to which reconstruction can be directed toward a stable and catalytically optimal operating-state interface.