Multishelled Nickel–Cobalt Oxide Hollow Microspheres with Optimized Compositions and Shell Porosity for High-Performance Pseudocapacitors

Nickel–cobalt oxides/hydroxides have been considered as promising electrode materials for a high-performance supercapacitor. However, their energy density and cycle stability are still very poor at high current density. Moreover, there are few reports on the fabrication of mixed transition-metal oxides with multishelled hollow structures. Here, we demonstrate a new and flexible strategy for the preparation of hollow Ni–Co–O microspheres with optimized Ni/Co ratios, controlled shell porosity, shell numbers, and shell thickness. Owing to its high effective electrode area and electron transfer number (n3/2A), mesoporous shells, and fast electron/ion transfer, the triple-shelled Ni–Co1.5–O electrode exhibits an ultrahigh capacitance (1884 F/g at 3A/g) and rate capability (77.7%, 3–30A/g). Moreover, the assembled sandwiched Ni–Co1.5–O//RGO@Fe3O4 asymmetric supercapacitor (ACS) retains 79.4% of its initial capacitance after 10 000 cycles and shows a high energy density of 41.5 W h kg–1 at 505 W kg–1. Importantly, the ACS device delivers a high energy density of 22.8 W h kg–1 even at 7600 W kg–1, which is superior to most of the reported asymmetric capacitors. This study has provided a facile and general approach to fabricate Ni/Co mixed transition-metal oxides for energy storage.