Defect-Engineered Co9S8/NiS@NF Nanoarchitectures: Uniting Heterojunctions and Sulfur Vacancies for Superior Overall Water Electrolysis

The development of durable, high-performance electrocatalysts capable of sustaining industrial-level current densities is critical for advancing alkaline water electrolysis. In this work, we present a hierarchical Co₉S₈/NiS heterojunction directly grown on nickel foam via hydrothermal and controlled sulfurization process. The self‐supported electrode delivers low overpotentials of 260 mV (OER) and 40 mV (HER) at 10 mA cm^-2 in 1 M KOH. Long‐term stability tests of OER and HER respectively confirm 95% and 96% current retention at 100 mA cm^-2 over 30 h. When employed in a two-electrode configuration, the Co₉S₈/NiS@NF electrode pair achieves 10 mA cm^-2 overall water splitting at just 1.38 V, outperforming most reported transition‐metal sulfide systems. Comprehensive structural and spectroscopic analyses reveals that intimate electronic coupling at the Co₉S₈/NiS interface, together with abundant sulfur vacancies, synergistically optimizes charge distribution and lowers kinetic barriers. Moreover, the integrated nano-flower and nano-needle morphology of self-supporting integrated electrode provides an expanded electrochemically active surface area and efficient mass-transport channels. This dual‐modulation strategy—combining heterojunction engineering with defect chemistry—provides a scalable path for designing non‐precious‐metal electrocatalysts that meet the demands of practical hydrogen production.