Understanding and designing versatile electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte is of great interest and importance towards overall water splitting. Here we investigated the crystal-plane-dependent electrocatalytic activity of NiCo2O4 crystals by combining experimental studies and theoretical calculations for the first time. It is revealed that NiCo2O4 nanosheet exposing {1 1 0} crystal planes shows higher catalytic activity for both HER and OER than that of NiCo2O4 octahedron exposing {1 1 1} crystal planes and NiCo2O4 truncated octahedron exposing {1 1 1} and {1 0 0} crystal planes. Furthermore, we grew the nanosheet with {1 1 0} planes on nickel foam (NF), which could guarantee fast electron and ion transport, rapid release of evolved bubbles and good structural stability, leading to the improved electrochemical performance. Acting as both anode and cathode electrocatalysts with a two-electrode electrolyzer in alkaline medium, NiCo2O4 nanosheet array produced a small cell voltage of 1.59 V to drive a current density of 10 mA cm(-2). The research presented here is of both fundamental and instructional significance because it unveils that selectively exposing specific crystal planes is very effective to receive promising catalysts for overall water splitting. (C) 2017 Elsevier Inc. All rights reserved.