Amorphous mesoporous nickel phosphate/reduced graphene oxide with superior performance for electrochemical capacitors.

Nickel phosphate (Ni3(PO4)2) is a promising electrode material for electrochemical capacitors, but the low intrinsic electrical conductivity and poor rate capability of Ni3(PO4)2 are the main challenges. To tackle these problems, amorphous mesoporous Ni3(PO4)2 with a pore diameter of 2-10 nm is grown on reduced graphene oxide (rGO), and a Ni3(PO4)2/rGO composite is obtained via a facile hydrothermal-calcination method in this work. The Ni3(PO4)2/rGO composite calcined at 300 °C (Ni3(PO4)2/rGO-300) possesses a uniform particle size and a high specific surface area of 198.72 m2 g-1. Benefiting from the structural characteristics, the synergistic effect of components and the high specific surface area, the Ni3(PO4)2/rGO-300 composite exhibits an extremely high specific capacitance of 1726 F g-1 at 0.5 A g-1 and an excellent rate capability of 850 F g-1 at 25 A g-1. In addition, the assembled Ni3(PO4)2/rGO-300//activated carbon asymmetric electrochemical capacitor delivers a good energy density of 57.42 W h kg-1 at a power density of 160 W kg-1. Compared with Ni3(PO4)2/rGO composites calcined at other temperatures and other nickel-phosphorus compounds reported in the literature, the Ni3(PO4)2/rGO-300 composite containing amorphous mesoporous Ni3(PO4)2 exhibits superior electrochemical performance, representing a new kind of electrode material for electrochemical capacitors.