Operando 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 Co 9 S 8 and Fe 3 O 4, 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 Co 9 S 8 @Fe 3 O 4 is reconstructed into CoOOH/CoO x @Fe 3 O 4 and further to complete CoOOH@Fe 3 O 4 . Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe 3 O 4 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@Fe 3 O 4 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.