3D urchin-shaped Ni3(VO4)2 hollow nanospheres for high-performance asymmetric supercapacitor applications

Abstract

3D urchin-shaped Ni₃(VO₄)₂ hollow nanospheres were synthesized by a facile, template free, hydrothermal method. The size of the urchin-shaped Ni₃(VO₄)₂ hollow nanospheres was ∼500 nm and they were composed of ∼10 nm thick sheet-like building block units. The morphological evolution was sensitive to alkaline media and ∼50 nm nanoparticles were formed when liquid ammonia was replaced by sodium hydroxide. The formation of [Ni(NH₃)₆]²⁺ complex ions (hexaamminenickel(II) ions) and subsequent slow release of nickel ions to the growing crystal seem to have resulted in the formation of hollow urchin-shaped nanostructures. The electrochemical supercapacitor properties of these two nanostructures were investigated and it was found that the urchin-shaped nanospheres exhibited better performance than the nanoparticles in all respects. The as-fabricated porous urchin-shaped Ni₃(VO₄)₂ nanosphere electrode exhibited a specific capacity of 402.8 C g⁻¹ at 1 A g⁻¹ with enhanced rate capability and an excellent capacity retention of 88% after 1000 cycles. An asymmetric supercapacitor was fabricated using Ni₃(VO₄)₂ nanospheres as the cathode and activated carbon (AC) as the anode and the electrochemical properties were studied at various scan rates in the potential range of 0.0–1.6 V. The as-fabricated asymmetric supercapacitor (Ni₃(VO₄)₂//AC) achieved a high specific capacity (114 C g⁻¹), energy density (25.3 W h kg⁻¹) and power density (240 W kg⁻¹). Moreover, this asymmetric supercapacitor displayed an excellent life cycle with 92% specific capacity retention after 1000 consecutive charge–discharge cycles. The impressive electrochemical performance of the Ni₃(VO₄)₂ nanospheres, owing to their large surface area, pore volume and 3D structure, makes them a promising candidate for the future high energy storage systems.