The DFT calculations revealed that the defects not only create more active sites to enable the reactions, but also alter the electronic structures of existing five-coordinated Ru atoms to promote the intrinsic caThe DFT calculations indicated that the high-index surface containing highly under-coordinated Ru (CN = 3) sites can effectively reduce the OER overpotential. Simultaneously, the Cu-dopant can modulate the electronic structures to enhance the OER activity and induce the formation of unsaturated Ru sites on the surface.talytic activity.