Operando Spectroscopic Monitoring of Metal Chalcogenides for Overall Water Splitting: New Views of Active Species and Sites

Metal‐based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X‐ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure–activity relationships of low‐crystalline cobalt sulfide (L−CoS) catalysts toward overall water splitting. The operando results for L−CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S−Co−Co−S moieties as the true catalytically active species. In comparison, during the acidic HER, L−CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co−S−Co configurations. Further operando characterizations highlight the crucial formation of high‐valent Co4+ species in L−CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co‐LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity. Combined operando XAS and electrochemical characterizations unravel a divergent structural optimization of cobalt sulfide catalysts toward overall water splitting in different media. Our results highlight the crucial influence of in situ formed active species on the catalytic performance, and we show that these transient coordination environments are not evident from ex situ characterizations.