Operando Identification of Active Species and Intermediates on Sulfide Interfaced by Fe₃O₄for Ultrastable Alkaline Oxygen Evolution at Large Current Density

Transition-metal sulfides are investigated as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media; however, the real active species remain elusive and the development of oxyhydroxides reconstructed from sulfides delivering stable large current density at low applied potentials is a great challenge. Here, we report a synergistic hybrid catalyst, composed of nanoscale heterostructures of Co₉S₈and Fe₃O₄, that exhibits only a low potential of 350 mV and record stability of 120 h at the 500 mA cm^-2in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) and Operando Raman spectroscopy demonstrate that the initial Co₉S₈@Fe₃O₄is reconstructed into CoOOH/CoO_x@Fe₃O₄and further to complete CoOOH@Fe₃O₄. Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe₃O₄components prevent the aggregation of CoOOH. Operando infrared spectroscopy indicates cobalt superoxide species (*OOH) as the active intermediates during the OER process. Density functional theory calculations demonstrate the formation of *OOH as the rate-determining step of OER and CoOOH@Fe₃O₄exhibits a lower energy barrier for OER. Our results provide an in-depth understanding of the dynamic surface structure evolutions of sulfide electrocatalysts for alkaline OER and insights into the design of excellent nanocatalysts for stable large current density.