Two-dimensional (2D) metal organic frameworks (MOFs) are emerging as low-cost oxygen evolution reaction (OER) electrocatalysts,however,suffering aggregation and poor operation stability.Herein,ultrafine Fe3O4 nanoparticles (diameter:6 ± 2 nm) are homogeneously immobilized on 2D Ni based MOFs (Ni-BDC,thickness:5 ± 1 nm) to improve the OER stability.Electronic structure modulation for enhanced catalytic activity is studied via adjusting the amount of Fe3O4 nanoparticles on Ni-BDC.The optimal Fe3O4/Ni-BDC achieves the best OER performance with an overpotential of 295 mV at 10 mA cm-2,a Tafel slope of 47.8 mV dec-1 and a considerable catalytic durability of more than 40 h (less than 5 h for Ni-BDC alone).DFT calculations confirm that the active sites for Fe3O4/Ni-BDC are mainly contributed by Fe species with a higher oxidation state,and the potential-determining step (PDS) is the formation of the adsorbed O* species,which are facilitated in the composite.