Electronic Structure Engineering of Highly-Scalable Earth-Abundant Multi-Synergized Electrocatalyst for Exceptional Overall Water Splitting in Neutral Medium

Efficient neutral water splitting may represent in future a sustainable solution to unconstrained energy requirements, but yet necessitates the development of innovative avenues for achieving the currently unmet required performances. Herein, a novel paradigm based on the combination of electronic structure engineering and surface morphology tuning of earth-abundant 3D-hierarchical binder-free electrocatalysts is demonstrated, via a scalable single-step thermal transformation of nickel substrates under sulfur environment. A temporal-evolution of the resulting 3D-nanostructured substrates is performed for the intentional enhancement of non-abundant highly-catalytic Ni³⁺ and pS_n²⁻ species on the catalyst surface, concomitantly accompanied with densification of the hierarchical catalyst morphology. Remarkably, the finely engineered NiS_x catalyst synthesized via thermal-evolution for 24 h (NiS_x-24 h) exhibits an exceptionally low cell voltage of 1.59 V (lower than Pt/C-IrO₂ catalytic couple) for neutral water splitting, which represents the lowest value ever reported. The enhanced performance of NiS_x-24 h is a multi-synergized consequence of the simultaneous enrichment of oxygen and hydrogen evolution reaction catalyzing species, accompanied by an optimum electrocatalytic surface area and intrinsic high conductivity. Overall, this innovative work opens a route to engineering the active material's electronic structure/morphology, demonstrating novel Ni³⁺/pS_n²⁻-enriched NiS_x catalysts which surpass state-of-the-art materials for neutral water splitting.