Exploring the relationship between high-valent iron content and current intensity of NiFe 0.2 -O x H y during oxygen evolution reaction by operando 57 Fe Mössbauer spectroscopy. 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 NiFe 0.2 (oxy)hydroxide (NiFe 0.2 -O x H y ) 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 NiFe 0.2 -O x H y was irreversibly transformed from the type of α -Ni(OH) 2 to γ -NiOOH with applying an anodic potential, while ex-situ / operando 57 Fe 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.