Green synthesis of iron-doped cobalt sulfide <i>via</i> synergistic electronic and structural engineering in ethaline deep eutectic solvent for efficient oxygen evolution reaction

The development of high-efficiency, earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is essential for scalable green hydrogen production, yet challenges persist in balancing activity, stability, and cost. Herein, we present a sustainable approach to synthesize Fe-doped cobalt sulfide (Co-S-30Fe) nanoparticles using an ethaline deep eutectic solvent-mediated strategy, which enables precise control over Fe incorporation to optimize both structural and electronic properties. The engineered Co-S-30Fe/NF electrode exhibited exceptional OER performance in alkaline media, requiring an overpotential of only 278 mV at 100 mA cm^-2, with a Tafel slope of 44.6 mV dec^-1 and outstanding operational stability. Spectroscopic analyses revealed that Fe³⁺ doping induces three synergistic effects: (1) coexistence of dynamically active Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺ redox couples, (2) substantial oxygen vacancy generation, and (3) ethaline-directed self-assembly of monodisperse nanospheres (∼96 nm) with 31.6% higher electrochemical surface area. This synergy of electronic reconstruction, defect engineering, and morphology control significantly enhances charge transfer kinetics (67% reduction in charge-transfer resistance) and intrinsic catalytic activity (4.4-fold increase in turnover frequency) compared to undoped Co-S. Critically, <i>in situ</i> electrochemical reorganization during the OER induced a surface transformation into oxygen-rich Co(Fe)-O/OH species, addressing the activity-stability trade-off. When integrated into a Co-S-30Fe/NF‖Pt/C/NF electrolyzer, the system achieved overall water splitting at low cell voltages of 1.53 V and 1.75 V (10 and 100 mA cm^-2, respectively) while maintaining stable operation for 100 h at 10 mA cm^-2.