Outstanding electrochemical performance of nickel-cobalt phosphate sheets with abundant mesopores for use in aqueous asymmetric supercapacitors and Ni–Zn batteries

Abstract

Transition metal phosphate materials (TMPs), especially bimetallic phosphates, have been regarded as an attractive electrode material for energy storage applications. However, phosphate materials with high capacitance and long cycle stability have scarcely been reported in the literature, due to their poor conductivity and small surface area. In this work, novel mesoporous nickel-cobalt phosphate sheets supported on conductive nickel foam (NF) have been successfully prepared by a facile and straightforward hydrothermal reaction. The resulting material shows a particularly attractive electrochemical performance, with a fairly high specific capacitance of 3722 F g−1 (7.44 F cm−2) at a current density of 1 A g−1. Even at current density values of 25 A g−1, the specific capacitance remains sufficiently high, that is, 1900 F g−1 (3.8 F cm−2). The significantly improved capacitance of nickel-cobalt phosphate material arises from the effective combination of the synergistic effect between two metal species, abundant mesopores and the high specific area of sheets, contributing to faster electrochemical kinetics, shortening ion transport channels and providing plentiful active sites for reaction. Furthermore, an asymmetric supercapacitor device using nickel-cobalt phosphate as cathode electrode and activated carbon (AC) as anode electrode has been assembled. This device exhibits a prominent energy density of 57.22 Wh kg−1 at a power density of 400 W kg−1 and excellent cycling performance at a large current density of 30 A g−1 (about 98% capacitance retention for 30000 cycles). Surprisingly, nickel-cobalt phosphate sheets firstly applied in an aqueous Ni–Zn battery deliver a high capacity of 195.5 mAh g−1 at 1 A g−1 and good rate performance (104.6 mAh g−1 at 10 A g−1). Such experimental findings suggest that mesoporous nickel-cobalt phosphate sheets are candidate materials to be employed in aqueous supercapacitors and Ni–Zn batteries.