Three-dimensional Co3O4/CoS hierarchical nanoneedle arrays electrode grown on nickel foam for high-performance asymmetric capacitors

Asymmetric capacitors have attracted attention owing to their advantage of high power density, safety and low cost, while the limited energy density and cyclic stability impact the applications in high-energy storage field. In this work, the nano-Co3O4/CoS core-shell structures are successfully designed on nickel foam via a facile hydrothermal method for Co3O4 nanoneedle arrays and electrodeposition for ultrathin CoS nanosheets. The energy density of the asymmetric capacitor using a nano-Co3O4/CoS electrode can reach 22 Wh kg-1 at 1 A g-1, which is several times larger than the single Co3O4 electrode. The results show that the hierarchical porous structure can accelerate electron and ion transport, reduce the charge transfer resistance, and thus improve the capacitive performance of hybrid electrodes. Moreover, the modified asymmetric capacitor shows good cycling stability with 90.1% energy density retention over 5000 cycles at 5 A g-1.