Vertically-aligned cobalt nickel phosphide nanowires (Co-Ni-P NWs) are synthesized on Ni foam by phosphorizing cobalt carbonate hydroxide precursor NWs in red phosphorous vapor at an elevated temperature. The as-fabricated self-supported integrated electrode (Ni@Co-Ni-P) exhibits outstanding electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline solution, delivering a cathodic current density of 100 mA cm(-2) at a small overpotential of 137 mV and a Tafel slope of 65.1 mV dec(-1). Furthermore, the electrode shows remarkable catalytic performance towards the oxygen evolution reaction (OER), affording an anodic current density of 90.2 mA cm(-2) at an overpotential of 350 mV, superior to many other transition metal based OER catalysts. Given the well-defined bifunctionality, an alkaline electrolyzer is assembled using two symmetrical Ni@Co-Ni-P as the cathode and anode, respectively, which demonstrates outstanding catalytic performance for sustained water splitting at varying current densities from 10 to 240 mA cm(-2). Significantly, the Ni@Co-Ni-P electrolyzer is able to operate for 3175 h (ca. 132 days) without degradation at an industry-relevant current density of 100 mA cm(-2), leading to exceptionally high H-2 production rate of 311 mmol h(-1) catalyst cm(-2) with an energy efficiency of 76% at ca. 1.9 V. (C) 2016 Elsevier B.V. All rights reserved.