Demystifying the active roles of NiFe-based oxides/(oxy)hydroxides for electrochemical water splitting under alkaline conditions

[Display omitted] •The water splitting technology and its need for current energy demand are introduced.•The mechanism of water splitting on NiFe-based catalysts is discussed in depth.•The necessity of LDH phase formation under OER conditions are emphasized.•The role of Fe3+ in improving the water splitting performance of NiFe-based catalysts and the advantages over other trivalent cations are clarified. NiFe-based compounds have received increasing attention due to their low cost, availability, as well as excellent H2 evolution reaction (HER) and the O2 evolution reaction (OER) catalytic properties. Although it is agreed that the implementation of high activity, the co-existence of Ni and Fe is essential, the identification of active sites and the mechanism of water splitting in NiFe-based compounds have long been controversial. This review focuses on the catalytic centers of NiFe-based oxides/(oxy)hydroxides and related mechanism in alkaline water oxidation process from the perspective of thermodynamics and kinetics. Furthermore, NiFe layered double hydroxides (NiFe LDHs) are introduced separately, emphasizing the necessity of LDH phase formation during water splitting. The review highlights the roles of Fe3+ and the third transition metal Co in NiFe-based catalysts. Finally, a brief perspective on NiFe-based catalysts is proposed. This review will be a good complement to the existing literatures and will contribute to a better study of NiFe-based catalysts.