Operando capturing of surface self-reconstruction of Ni3S2/FeNi2S4 hybrid nanosheet array for overall water splitting

Earth-abundant catalysts for both Oxygen evolution reaction (OER) and Hydrogen evolution reaction (HER) are highly required for electrochemical water splitting. Transition metal sulfides have been considered as efficient catalysts for both OER and HER, but their real active species in operation condition are still controversial. Herein, self-supported Ni3S2/FeNi2S4 nanosheets (Ni-Fe-S) are synthesized and in-situ Raman measurement is conducted to monitor the phase transformation under realistic conditions. Combined with a series of ex-situ technologies, it can be concluded that, Ni-Fe-S nanosheets function as pre-catalysts which undergo dynamically structural reconstruction during electrolysis. The active species are determined to be γ-NiOOH for OER and metallic Ni0 for HER in 1 M KOH media, respectively. As-converted products only need overpotentials of 50 mV for HER and 201 mV for OER to achieve the current density of 10 mA cm-2, respectively. A two-electrode electrolyzer can afford a current density of 10 mA cm-2 at a low cell voltage of 1.55 V. This work provides new perspectives to recognize the catalytic evolution process of nickel-based sulfides for water splitting and sheds light on the rational design of bifunctional materials in energy devices.