Topotactically constructed nickel–iron (oxy)hydroxide with abundant in-situ produced high-valent iron species for efficient water oxidation

The low efficiency of oxygen evolution reaction (OER) is regarded as one of the major roadblocks for metal-air batteries and water electrolysis. Herein, a high-performance OER catalyst of NiFe0.2 (oxy)hydroxide (NiFe0.2-OxHy) was developed through topotactic transformation of a Prussian blue analogue in an alkaline solution, which exhibits a low overpotential of only 263 mV to reach a current density of 10 mA cm−2 and a small Tafel slope of 35 mV dec−1. Ex-situ/operando Raman spectroscopy results indicated that the phase structure of NiFe0.2-OxHy was irreversibly transformed from the type of α-Ni(OH)2 to γ-NiOOH with applying an anodic potential, while ex-situ/operando 57Fe Mössbauer spectroscopic studies evidenced the in-situ production of abundant high-valent iron species under OER conditions, which effectively promoted the OER catalysis. Our work elucidates that the amount of high-valent iron species in-situ produced in the NiFe (oxy)hydroxide has a positive correlation with its water oxidation reaction performance, which further deepens the understanding of the mechanism of NiFe-based electrocatalysts.