Exploration of low-cost transition metal based electrocatalysts for water oxidation and understanding their structure-activity relationship are greatly desired for clean and sustainable chemical fuel production. Herein, a core-shell (CS) NiFeCr metal/metal hydroxide catalyst was fabricated on 3D Cu nanoarray substrate by a simple electrodeposition-activation method. Synergistic promotion effect between electronic structure modulation and nano-structure regulation was presented on CS-NiFeCr OER catalyst: the 3D nano-architecture facilitates the mass transport process, the in-situ formed interface metal/metal hydroxide heterojunction accelerates the electron transfer, and the electronic structure modulation by Cr incorporation improves the reaction kinetics. Benefiting from the synergy between structural and electronic modulation, the catalyst shows superior activity towards water oxidation under alkaline conditions: overpotential of 200 mV at 10 mA/cm2 current density and Tafel slope of 28 mV/dec. I would like to use DFT to calculate binding energy, dissociation energy, and other mechanistic properties.