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

Abstract 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 cell-voltage of 1.45 and 1.61 V at 10 and 200 mA·cm-2 respectively with super stability for 100 h.