Searching for high-performance and earth-abundant electrocatalysts for oxygen evolution reaction (OER) is of paramount significance for overall water splitting to produce hydrogen. Herein, an advanced class of CoNi selenides containing rich oxygen vacancies, with a hierarchical bundle-like and holey nanosheets as noble-metal-free catalysts were first synthesized through a facile hydrothermal method. Benefitting from abundant oxygen vacancies, bundle-like nanostructure, as well as strong synergistic effects, such CoNi selenides demonstrate a greatly enhanced surface area to supply more electrocatalytic active sites to contact with electrolyte, accompanied by largely promoted reaction kinetics, which show outstanding electrocatalytic performances for OER. Remarkably, the optimal Co1Ni0.5Se can display outstanding OER activity with the small Tafel slope of 48 mV dec(-1) and low over potential of 250 mV (at 10 mA cm(-2)), which are much superior to those of Co/Ni-based catalysts. This electrocatalyst can also maintain high activity and structure stability during long-term electrolysis of 35 h, demonstrating a desirable electrocatalyst for OER. This work elucidates the sophisticated construction of well-defined non-noble metal catalysts for the practical applications in water oxidation. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.