Operando Spectral and Electrochemical Investigation into the Heterophase Stimulated Active Species Transformation in Transition-Metal Sulfides for Efficient Electrocatalytic Oxygen Evolution

In-depth understanding of electrocatalytically active species transformation during oxygen evolution reaction (OER) is highly guidable to design effective electrocatalysts for water splitting. Herein, cobalt sulfide (CoS_x) nanovesicles decorated with iron sulfide (FeS_x) heterophases were successfully synthesized by using a metal-organic framework precursor through a solvothermal method. The obtained CoS_x/FeS_x exhibited much elevated OER activity as compared to pristine CoS_x and even most previously reported cobalt-based electrocatalysts, with the overpotential of 304 mV at 10 mA cm^-2 in 1 M KOH. As revealed by operando Raman and X-ray absorption spectroscopy measurements, the electrocatalytically active species transformation in CoS_x nanovesicles could be stimulated by the incorporated FeS_x heterophases due to the increased average valence state of cobalt and lowered coordination of cobalt sites. As a result, the electrocatalytically active cobalt oxyhydroxides could be more easily formed on the catalyst surface during OER and thus contributed to the highly improved performance. This real-time spectral and electrochemical insight into the electrocatalytically active species transformation may guide the rational design of highly efficient electrocatalysts for water splitting from the viewpoint of electronic structure-electrocatalytic activity relationship.