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

Abstract

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.