Developing highly active and inexpensive electrocatalysts for oxygen evolution reaction (OER) is critical to large-scale applications of electrochemical water splitting. In the present work, a novel porous NiO/NiCo2O4 heterostructure is constructed by two-stage calcination of nickel-cobalt bimetallic hydroxide precursors prepared using a microwave-assisted hydrothermal method, in which abundant interfaces are constituted. The NiO phase stabilized in NiCo2O4 matrix is the nanometer scale (ca. 13 nm). The porous nanoscale NiO/NiCo2O4 heterostructure shows a 10 mA cm(-2) current density under the overpotential of 264 mV which outperforms the noble catalyst RuO2. It is demonstrated that the highly active Ni3+ species generated by the oxidation of nanometer scale NiO surface are responsible for enhanced OER. Additionally, DFT calculations certify that the heterostructure interfaces promote the chemisorption of OH intermediates. This strategy of increasing the intrinsic activity and improving the chemisorption abilities for oxygen-containing intermediates by constructing nanoscale heterostructure electrocatalysts provides a feasible method to accelerate the reaction rate of OER. (C) 2019 Elsevier Inc. All rights reserved.