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Multifold operando spectroscopy reveals an Fe4+-containing active site at the surface of cobalt iron oxyhydroxides for the oxygen evolution reaction.

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The hetero-atomic interaction has been the subject of many investigations, due to their heterogeneity, the individual roles of the atoms are still difficult to realize. Herein, an electrocatalyst with a hetero-atomic pair confined on a tungsten phosphide (WP) substrate so that the Fe 3 + -site of the pair is distal to the surface is shown to deliver an extremely low overpotential of 192 mV at 10 mA cm À 2 and one of the highest oxygen production turnover frequencies (TOF) of 2.1 s À 1 at 300 mV under alkaline environment for the oxygen evolution reaction (OER). Operando characterization shows the Lewis acidic Fe 3 + site boosts a large population of Co 4 + /3 + and the deprotonation of coordinated water, allowing simultaneously enhanced electrontransfer as well as the proton-transfer. A significant contribution from the WP substrate modulates the order of hydroxide transfer in the pre-equilibrium step (PES) and rate-determining-step (RDS), leading to a remarkable OER performance.

The actual chemical environment of metal phosphides or chalcogenides during hydrogen evolution reaction (HER) has been debated. In this work, pyrite-phase NiP2, Se-doped NiP2, and NiSe2 were investigated by means of in situ and ex situ characterizations to dynamically study the transformation during electrocatalysis. We found that doping with selenide would generate more vacancies to facilitate the activation on water molecule, in which hydroxide poisoning on active site would be suppressed by the presence of selenide. Furthermore, the radial distance of Ni–Ni pair can serve as a descriptor to identify the mechanism of HER. For NiP2 and Se-doped NiP2, the radial distance of the metal (Ni–Ni) pair during HER is identical to the as-prepared one, implying that the HER active site is on a single metal site with a Heyrovsky step. A drastic bond contraction of Ni–Ni pair on NiSe2 indicates dual-site activation correlated with a chemical desorption of Tafel step. The role of anion effects and metal sites are comprehensively investigated to further gain the fundamental insights from the point of view on improving HER electrocatalytic performance.