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 Co 9 S 8 /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 Co 9 S 8 /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 Co 9 S 8 /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.