Spinel-structured hollow nanospheres prepared by a soft-template solvothermal method for high-performance supercapacitors

Multi-element transition-metal oxides have attracted much attention in the field of energy storage due to their excellent specific capacitance and multiplicity. Herein, Ni_0.5Mn_0.5Co₂O₄ hollow electrode materials were prepared by a novel solvothermal method using polyvinylpyrrolidone (PVP) as a soft template, followed by a subsequent annealing treatment. The materials were characterized by XRD, XPS, SEM, TEM, and BET, and their supercapacitor properties were tested on an electrochemical workstation. The unique hollow structure of Ni_0.5Mn_0.5Co₂O₄ can act as a “charge reservoir” for exposing more active sites and increases its specific surface area to 74.67 m²·g⁻¹. In the three-electrode system, the electrode material of Ni_0.5Mn_0.5Co₂O₄ exhibits a high specific capacitance (2952.5 F·g⁻¹ at 1 A·g⁻¹ and 540 F·g⁻¹ at 15 A·g⁻¹). Meanwhile, the electrode material still possesses 80.8% of the original capacitance after 3000 cycles at a current density of 15 A·g⁻¹, demonstrating good cycling stability. Finally, the electrode material was assembled with activated carbon to form an asymmetric supercapacitor (ASC). The electrochemical performance of the ASC device tested in the two-electrode system exhibited an energy density of 118.2 Wh·kg⁻¹ when the power density was 1428 W·kg⁻¹. In addition, the ASC device could light up a small light bulb. The redox reaction kinetics was controlled by both diffusion and capacitive behaviors. With increasing scanning rate, the capacitive contribution gradually dominated the kinetics. The electrode material of Ni_0.5Mn_0.5Co₂O₄ offers possibilities for actual applications in the future.