The advancements of sustainable energy storage and conversion systems will benefit from the exploring of cost-effective and high-performance electrode materials with multifunctions. In this study, a novel three-dimensional (3D) electrode, consisting of hybridized Ni(PO3)(2)-MnPO4 nanosheet array on self-supported conductive nickel foam (Ni(PO3)(2)-MnPO4/NF), is established to demonstrate the promising applications of an class of transition metal phosphates (TMPOs) for electrochemical water splitting and supercapacitors. By means of the Ni(PO3)(2)-MnPO4/NF electrode with rationally designed nanosheet array structure and abundant defects, we demonstrate the good catalytic activity of TMPOs for hydrogen evolution reaction (125 mV at 10 mA cm(-2)). It is also revealed as a robust oxygen-evolving catalyst, yielding a current density of 10 mA cm(-2) at ultralow overpotential of 116 mV (corresponding to the electrode potential of 1.346 V). When tested as bifunctional electrocatalysts to drive overall water splitting, the applied potential difference is as low as 1.41 V for the whole cell to achieve the current density of 10 mA cm(-2). Moreover, Ni(PO3)(2)-MnPO4/NF could serve as excellent supercapacitor electrode with superior capacitance behavior (areal specific capacitance of 14.48 F cm(-2) at 5 mA cm(-2)) and good stability. In an aqueous asymmetric supercapacitor, a high energy density of 45.6 Wh kg(-1) is obtained at a power density of 1.1 kW kg(-1). (C) 2019 Elsevier Ltd. All rights reserved.