Highly efficient non-noble materials for water splitting are essential for renewable energy application. Herein, cobalt phosphide nanoparticles with high P/Co ratio are synthesized and display overpotentials of 348 mV (or 343 mV) for oxygen evolution reaction (OER) with surface oxidized active species formed and 200 mV (or 184 mV) for hydrogen evolution reaction (HER) in 1 M NaOH (or 1 M KOH) at 10 mA cm(-2). The improved OER could be ascribed to the smaller particle size for increased active site number and more importantly the improved intrinsic activity probably assisted by more phosphates formed during electrocatalytic reaction; while the improved HER performance is mainly attributed to the increased active sites in the form of Co-P bonds. In a two-electrode configuration, the catalysts exhibit excellent overall water splitting performance, with 10 mA cm(-2) at 1.58 V and a long-time reaction stability without attenuation during 10 h operation. (C) 2019 Elsevier Inc. All rights reserved.