Assembling amorphous (Fe-Ni)Cox-OH/Ni3S2 nanohybrids with S-vacancy and interfacial effects as an ultra-highly efficient electrocatalyst: Inner investigation of mechanism for alkaline water-to-hydrogen/oxygen conversion

A hierarchical nanocluster hybrids of trimetallic (Fe-Ni)Cox−OH nanofilms decorated amorphous Ni3S2 with S-vacancy supported on nickel foam (NF) was proposed via rapid two-step electrodeposited pathway for ultra-highly efficient alkaline water-to-hydrogen/oxygen conversion. [Display omitted] •Hierarchical (Fe-Ni)Cox−OH/Ni3S2 nanocluster hybrids with S-vacancy supported on nickel foam (NF) are synthesized via rapid two-step electrodeposited pathway.•DFT calculations and experimental results confirm amorphous S-vacant Ni3S2 plays a crucial role for HER.•In situ newly formed ultrathin NiOOH nanosheets on near-surface Ni3S2 layer coupling with (Fe-Ni)Cox-OH are identified as intrinsic OER active species.•Amorphous (Fe-Ni)Cox-OH/Ni3S2 nanohybrids represent the best bifunctional electrocatalysts hitherto, achieving cell-voltage of 1.61 V at 200 mA cm−2, along with super stability. Rational design of earth-abundant electrocatalysts with ultra-high activity and durability for the electrochemical water-to-energy conversion systems is still a significant challenge. Herein, a hierarchical nanocluster hybrids of trimetallic (Fe-Ni)Cox−OH nanofilms decorated amorphous Ni3S2 with S-vacancy supported on nickel foam (NF) was proposed via rapid two-step electrodeposited pathway. By effectively breaking long-range-order of Ni3S2 to form S-vacant amorphous phase, combining with the strong electronic interactions between the (Fe-Ni)Cox−OH and Ni3S2, it resulted in favorable water adsorption ability, free-energy of H* adsorption (ΔGH*), and fast mass/charge transfer for hydrogen evolution process. The (Fe-Ni)Cox−OH/Ni3S2 nanohybrids displayed ultralow overpotential of 91 and 145 mV at 100 and 1400 mA·cm−2 respectively with admirable durability for 100 h at 200 mA·cm−2 in 1 M KOH, even exceeding Pt plate. The amorphous S-vacant Ni3S2 plays a crucial role for hydrogen evolution: compared with both original and defective Ni3S2, the free energy of disordering Ni3S2 for *H−OH and H* intermediates more tend to thermodynamical neutrality according to density functional theory (DFT) calculations. For water-to-oxygen evolution (η100 = 280 mV), in situ newly formed ultrathin NiOOH nanosheets on near-surface Ni3S2 layer coupling with (Fe-Ni)Cox−OH nanofilms can be identified as intrinsic OER active species; and the host metallic Ni3S2 phase provides wonderful conductivity. Impressively, the bifunctional (Fe-Ni)Cox−OH/Ni3S2 nanohybrid represents the best electrocatalyst so far, achieving