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• Fe-leaching induces surface reconstruction of Ni-Fe alloy/NC during OER. • Fe-leaching during OER benefits the exposure of more active sites. • The high valent Ni, pyridinic N and metal-N species are the exact active centers. • The fully activated catalyst exhibits excellent OER activity. Understanding the dynamic transformation process of the oxygen evolution reaction (OER) catalysts is of importance for their rational design. Here we first report that Fe-leaching could induce the electrochemical activation of strongly coupled Ni-Fe alloy on N-doped carbon composites to boost the OER performance. It is demonstrated that the leaching of unstable FeOOH mixed layer from the surface of FeNi 3 alloy is responsible for the surface reconstruction and exposure of more active sites. Furthermore, the N-doped carbon, FeNi 3 alloy, together with the accompanying generation of Fe-doped Ni(OH) 2 , synergistically promote the OER activity after fully activation. It is also verified that the high valent Ni species, pyridinic N and the metal-N species are the exact active centers and the fully activated catalyst Fe 0.2 Ni 0.8 /NC-600-a is among the best non-noble OER catalysts to date, which exhibits superior OER performance with a low overpotential (η 20 = 290 mV) and small Tafel slope (76 mV dec−1). This study is helpful to understand the fundamentals of the Ni-Fe alloy catalysts and provides guidance to design advanced catalysts for water oxidation through in-situ electrochemical activation. [ABSTRACT FROM AUTHOR]