High-performance spinel NiMn2O4 microspheres self-assembled with nanosheets by microwave-assisted synthesis for supercapacitors

Abstract

Spinel NiMn₂O₄ microspheres self-assembled with nanosheets directly grown on a 3D nickel foam were successfully prepared by a facile microwave-assisted hydrothermal process. The structural and morphology evolution behavior and the charge storage mechanism of the as-prepared samples were thoroughly characterized and assessed. It was found that the formation of the product occurred in a stepwise manner, and the steps involved were nucleation, regular growth, and a self-assembly process (from nanosheets to small microspheres and further aggregation to large microspheres), resulting in a flower-like microsphere structure. Furthermore, a study on the energy storage mechanism of the prepared spinel NiMn₂O₄ indicated that the diffusion-controlled process dominated charge storage at low scan rates, while the capacitive process was dominant as the scan rate increased. As a result, the as-prepared NiMn₂O₄ showed high specific capacitance up to 768.9 F g⁻¹ at 1 A g⁻¹ when measured by a charge–discharge test and excellent cycling stability (85.8% capacity retention and near 100% Coulombic efficiency after 6000 cycles at 5 A g⁻¹), which indicated that the fabricated products show a great potential in commercial supercapacitor applications.