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

Transition-metal sulfides are investigated as promis-ing electrocatalysts for oxygen evolution reaction (OER) in alkalinemedia; however, the real active species remain elusive and thedevelopment of oxyhydroxides reconstructed from sulfidesdelivering stable large current density at low applied potentials isa great challenge. Here, we report a synergistic hybrid catalyst,composed of nanoscale heterostructures of Co9S8and Fe3O4, thatexhibits only a low potential of 350 mV and record stability of 120h at the 500 mA cm-2in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) andOperandoRaman spec-troscopy demonstrate that the initial Co9S8@Fe3O4is recon-structed into CoOOH/CoOx@Fe3O4and further to completeCoOOH@Fe3O4.OperandoXAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH actsas active species and Fe3O4components prevent the aggregation of CoOOH.Operandoinfrared spectroscopy indicates cobaltsuperoxide species (*OOH) as the active intermediates during the OER process. Density functional theory calculations demonstratethe formation of*OOH as the rate-determining step of OER and CoOOH@Fe3O4exhibits a lower energy barrier for OER. Ourresults provide an in-depth understanding of the dynamic surface structure evolutions of sulfide electrocatalysts for alkaline OER andinsights into the design of excellent nanocatalysts for stable large current density

Automated summary

This study reports a synergistic hybrid catalyst composed of nanoscale heterostructures of Co9S8 and Fe3O4, which 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. The reconstructions of Co9S8@Fe3O4 into CoOOH/CoOx@Fe3O4 and then to complete CoOOH@Fe3O4 are revealed through voltage-dependent soft X-ray absorption spectroscopy and Operando Raman spectroscopy. The completely reconstructed CoOOH acts as the active species and Fe3O4 components prevent the aggregation of CoOOH.