Synergistic sulfur engineering in Amorphous Co–S nanoparticles via ChCl–EG DES–mediated synthesis for efficient overall water splitting

This study presents a green strategy for synthesizing amorphous cobalt sulfide (Co–S) bifunctional electrocatalysts via a choline chloride–ethylene glycol deep eutectic solvent (DES) under ambient conditions, addressing ionic coordination dynamics and defect engineering for enhanced solid–state ionic/electronic transport in energy conversion. By modulating the equilibrium between Co²⁺ and S₂O₃^2- ions in Ethaline, we fabricated monodisperse Co–150S nanoparticles (~ 78 nm) with tailored sulfur content (S/Co = 1.9), an amorphous architecture, and abundant oxygen vacancies. These structural features synergistically optimized ionic diffusion pathways and electronic conductivity, achieving exceptional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities in alkaline media. The Co–150S/NF electrode demonstrated a volcano-like sulfur-dependent activity profile, achieving the highest electrochemically active surface area (ECSA, 3.7 cm²) and ultralow overpotentials of − 105 mV (HER) and 277 mV (OER) at 10 mA cm⁻², comparable to benchmark Pt/C and RuO₂ catalysts. Post-electrolysis characterization revealed dynamic structural reorganization during HER and OER operations, involving over 80 at% sulfur depletion and the formation of metastable Co-rich phases that maintained catalytic functionality. In overall water splitting, the system required only 1.62 V to drive 10 mA cm⁻² with minimal activity decay (1.5 mV h⁻¹ over 26 h). Mechanistic investigations revealed that sulfur incorporation initiates a multiscale optimization process: (i) DES–mediated ionic confinement prevents particle aggregation, promoting uniform nanosphere formation; (ii) modulation of the electronic structure through nonstoichiometric Co–S coordination; and (iii) defect engineering through the enrichment of oxygen vacancies. This study provides insights into ionic coordination mechanisms in non–aqueous solvents and defect–mediated ion transport in amorphous solids, suggesting a potential strategy for developing electrocatalysts applicable to related energy storage technologies.