Systematic investigation of spinel-type ferrite MFe2O4 (M = Mn, Fe, Co, Ni, Zn): in-situ and ex-situ X-ray approaches to oxygen evolution reaction

Metal oxide of spinel family has performed great potential to replace RuO2 or IrO2 toward oxygen evolution reaction (OER), but the fundamental mechanism of spinel oxides is still far from a complete understanding especially for the role of metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface condition (mor-phology and defects), it’s a great challenge to have a fair assessment of electrocatalytic performance of spinel system. Herein, we demonstrated a series of MFe2O4 (M = Mn, Fe, Co, Ni, Zn) family with a well-controlled morphology to achieve comprehensive study of electrocatalytic activity toward OER, in which most of the divalent metals (M2+) of MFe2O4 were located at octahedral sites and well-defined single-crystal nanocubes were synthesized to minimize the influence caused by crystallographic sites/morphology of variables for each samples. The effects of M2+ ion in MFe2O4 can be realized through investigating the OER behaviors of MFe2O4 and that of γ-Fe2O3 as a reference to have in-depth understanding of the roles of divalent/trivalent metal ions. In-situ X-ray diffraction and ex-situ X-ray absorption spec-troscopies were employed to provide direct evidence that the superior OER activities of CoFe2O4 and NiFe2O4 samples could be attributed to a remarkable phase transformation forward metal oxyhydroxide, while the rest of compounds remain unchanged during oxygen evolution reaction.