One-Step Electrodeposition of Co9S8 on Ni Foam for Efficient Alkaline Overall Water Splitting

Abstract Developing binder-free, earth-abundant bifunctional electrocatalysts for alkaline overall water splitting is crucial for scaling alkaline water electrolysis beyond noble-metal electrodes. In this work, nonacobalt octasulfide (Co9S8) is directly grown on nickel foam through a binder-free, one-step galvanostatic electrodeposition method. The influence of deposition time on the catalytic behavior was systematically investigated. Structural and surface analyses (XRD, HRTEM/SAED, XPS) confirm Co9S8 as the dominant phase across all deposition times, while systematic electrochemical characterization reveals a clear structure–activity relationship; an intermediate deposition (20 min, CSN-20) produces an interconnected porous architecture that maximizes electrochemically accessible surface area (Cdl = 75 mF cm–2) and minimizes charge-transfer resistance. The optimized electrode (CSN-20, deposited at 100 mA cm–2 for 20 min) exhibits superior bifunctional activity, requiring overpotentials of 121 mV for HER and 267.4 mV for OER in 1.0 M KOH, indicating fast reaction kinetics. CSN-20 sustains this bifunctional activity for 120 h of continuous operation at 50 mA cm–2 under HER, OER, and full two-electrode conditions, with posttest XRD/XPS confirming that the bulk Co9S8 phase is retained while a surface oxy(hydroxide) layer forms during OER, consistent with a self-optimizing active surface rather than catalyst degradation. A two-electrode electrolyzer assembled with the optimized electrode (CSN-20||CSN-20) requires 1.66 V to deliver 10 mA cm–2. This study establishes a facile, ecofriendly, and scalable route for developing efficient alkaline water splitting electrodes, without heteroatom doping, heterostructuring, or postsynthetic treatment.