<i>Operando</i> Identification of Active Species and Intermediates on Sulfide Interfaced by Fe₃O₄ for Ultrastable Alkaline Oxygen Evolution at Large Current Density

Abstract

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 Co9S8 and Fe3O4, that exhibits only a low potential of 350 mV and record stability of 120 h at the 500 mA cm–2 in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) and Operando Raman spectroscopy demonstrate that the initial Co9S8@Fe3O4 is reconstructed into CoOOH/CoOx@Fe3O4 and further to complete CoOOH@Fe3O4. Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe3O4 components 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@Fe3O4 exhibits 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.