Novel core–shell structure composite NiCo2O4–Vo@ZIF with amorphous ZIF shell and oxygen-vacancy-rich core for asymmetric supercapacitors

Supercapacitors are highly attractive devices for the storage of clean renewable energy. The combination of oxygen-vacancy-rich metal oxides and amorphous ZIF is synergistic and complementary for the formation of stable electrode materials for supercapacitors. In this study, NiCo2O4–Vo@ZIF with a novel core–shell structure was formed by first treating NiCo2O4 with NaBH4, followed by solvothermally forming an amorphous ZIF shell on the exterior of the NiCo2O4. Appropriate amounts of NaBH4 improved the external morphology of NiCo2O4 and introduced oxygen–vacancies, which made more active sites. Furthermore, the use of ZIF, exhibited advantages such as improving the stability of NiCo2O4–Vo core, increasing the specific surface area and catching ions like a fishing net. Therefore, the resulting core–shell structure exhibited excellent electrochemical properties. Finally, the fabricated NiCo2O4–Vo@ZIF was further assembly with activated carbon to constitute an asymmetric supercapacitor, which achieved 62.9 Wh/kg energy density at 850 W/kg power density and maintained 91.91 % of the capacity after 5000 cycles. Therefore, the developed NiCo2O4–Vo@ZIF structure shows a considerable great future as the energy storage devices.