▎ 摘　　要

Oxygen evolution reaction (OER) is regarded as a limit-efficiency process in electrochemical water splitting generally, which needs to develop the effective and low-cost non-noble metal electrocatalysts. Oxygen vacancies have been verified to be beneficial to enhance the electrocatalytic performance of catalysts. Herein, we report the facile synthesis of reduced CoFe2O4/graphene (r-CFO/rGO) composite with rich oxygen vacancies by a citric acid assisted sol- gel method, heat treatment process and the sodium borohydride (NaBH4) reduction. The introduction of graphene and freezing dry technique prevents the restacking of GO and the aggregation of CFO nanoparticles (NPs) and increases the electronic conductivity of the catalyst. Fast heating rate and low anneal temperature favors to obtain low crystallinity and lattice defects for CFO. NaBH4 reduction treatment further creates the rich oxygen vacancies and electrocatalytic active sites. The obtained r-CFO/rGO with high specific surface area (108 m(2) g(-1)), low crystallinity and rich oxygen vacancies demonstrates a superior electrocatalytic activity with the smaller Tafel slope (68 mV decb(-1)), lower overpotential (300 mV) at the current density of 10 mA cm(-2), and higher durability compared with the commercial RuO2 catalyst. This green, low-cost method can be extended to fabricate similar composites with rich defects for wide applications. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.