Mesoporous, hierarchical core/shell structured ZnCo₂O₄/MnO₂ nanocone forests for high-performance supercapacitors

ZnCo₂O₄/MnO₂ nanocone forests with a mesoporous, hierarchical core-shell structure and a large surface area were hydrothermally grown on 3D nickel foam. The supercapacitor electrodes prepared from the unique structure exhibits exceptional specific capacitances of 2339 and 1526Fg^-1 at current densities of 1 and 10Ag^-1, respectively, and long-term capacity retention of ~95.9% after 3000 cycles at 2Ag^-1 and 94.5% after 8000 cycles at 10Ag^-1. These values are proven to be the highest when the capacitances are compared between the current study and similar core/shell-structured metal oxide electrodes taken from the literature. Many synergistic effects are identified to be responsible for the observations: namely, highly conductive 3D Ni foam substrate that totally eliminate binders and conductive additives; high crystalline quality of the ZnCo₂O₄ core which is directly grown on the conductive current collector, allowing fast electron transport; and the mesoporous MnO₂ shell with a huge surface area for fast ion diffusion and intimate electrode/electrolyte contact. In addition, the nanostructured core and shell have redox reactions with anions and cations from the electrolyte, respectively, both of which contribute much to electrochemical charge storage.