Operando Unraveling of the Structural and Chemical Stability of P‑Substituted CoSe<sub>2</sub> Electrocatalysts toward Hydrogen and Oxygen Evolution Reactions in Alkaline Electrolyte

The question of whether the metal chalcogenides (phosphides) that have been acknowledged to be efficient materials for bifunctional electrocatalysts really perform as the active species or just “pre-catalysts” has been debated. Herein, a series of operando measurements, including in situ X-ray absorption spectroscopy, liquid-phase transmission electron microscopy, and in situ Raman spectroscopy, were conducted to unravel in real time the structural and chemical stability of P-substituted CoSe<sub>2</sub> electrocatalysts under both hydrogen and oxygen evolution reactions (HER and OER, respectively) in an alkaline electrolyte. It can be conclusively revealed that, in an alkaline electrolyte, the P-substituted CoSe<sub>2</sub> electrocatalyst was acting as the “pre-catalyst” rather than the real reactive species. The introduction of phosphorus is speculated to generate more vacancies or defects around Co cations in the initial CoSe<sub>2</sub> and considerably facilitates the structural transformation into the “real reactive species”, such as metallic cobalt (for HER) and cobalt oxyhydroxide (for OER).