Al-doped Co9S8 encapsulated by nitrogen-doped graphene for solid-state asymmetric supercapacitors

Abstract Nowadays, the operation of cobalt sulfide is continuously realized for electrode fabrication in the energy storage field. However, such material agonized from the depressed conductivity, collapsed volume by cycling, poor retention rate, and passive nanostructure design hampering the wide application. As a solution, Al doping was exploited as an effective approach for improving the conductivity of cobalt sulfide. Herein, a novel type of core/sell architecture was successfully designed from aluminum-doped cobalt sulfide encapsulated by nitrogen-doped graphene (Al-doped Co9S8@NG) through the solvothermal/sulfuration of ZIF67 structure subsequent by wrapping with Ppy layer and calcination in argon gas at various temperatures. The developed core-shell material was utilized as a promising cathode material for solid-state asymmetrical supercapacitor device (AsScs). Interestingly, integrating the morphology, and composition merits could upgrade the Al-doped Co9S8@NG600 electrode with highly enhanced supercapacitive features. The electrode was attained a supreme specific capacity of about 736 C/g at an applied current density of 1 A/g, ultra-long cycle stability of 92% after performing 10000 cycles, and a remarkable retention rate (∼71%). Further, the capacitive and diffusion-controlled participations for the electrode were analyzed using standard numerical packages in Python. Motivally, Al-doped Co9S8@NG core/shell as a positive electrode was assembled with a negative electrode synthesized from activated PANI-derived carbon nanorods (ACNRs) in a solid-state AsSCs device. The device could give outstanding values from specific capacitance, energy density, and power density of 134 F/g, 53.3 Wh/kg, and 0.954 kW/kg, respectively with a considerable cycle life stability of 93% after consuming 10000 cycles. Further, when two AsSCs devices were linked in series, multicolor LED could be lightened for 25 s approving their availability for making modern portable electronics. Such impressive findings can open the gate for utilizing MOFs in designing new metal sulfide-graphene composites as efficient energy storage electrodes for industrial uses.