Fabrication of rGO/CoS_x-rGO/rGO hybrid film via coassembly and sulfidation of 2D metal organic framework nanoflakes and graphene oxide as free-standing supercapacitor electrode

Porous cobalt sulfide (CoS_x)/reduced graphene oxide (rGO) hybrid films has been fabricated as a flexible freestanding supercapacitor electrode via the co-assembly and sulfidation of 2D metal organic framework (MOF) nanoflakes and graphene oxide (GO). Firstly, zeolitic imidazolate-67 (ZIF-67) nanocubes were added into the aqueous solution of GO to yield a mixed dispersion of ZIF-67 and GO (ZIF-GO). It was found that the morphology of ZIF-67 changed from nanocubes to 2D nanoflakes owing to the concentration change, which favored the formation of hybrid film. Secondly, the sandwich-like GO/ZIF-GO/GO hybrid film was fabricated by the successive vacuum membrane filtration of GO, ZIF-GO, and GO solutions. Finally, the hybrid film was sulfidized via a hydrothermal process using thioacetamide as the sulfur source. This process also led to the reduction of GO to rGO. The resulting rGO/CoSx-rGO/rGO hybrid film was shown to have good electrochemical performance because it combined the good pseudocapacitor property of cobalt sulfide as well as the good conductivity and electric double layer capacitor property of rGO. In addition, an all-solid-state asymmetric supercapacitor (aSC) was assembled using rGO/CoSx-rGO/rGO hybrid film and active carbon as the positive and negative electrodes, respectively. It exhibited an energy density of 10.56 Wh kg⁻¹ and a power density of 2250 W kg⁻¹. Also, it retained 92.8% of initial capacitance after 10,000 cycles. The good electrochemical performance revealed that the resulting aSC has great potential in the practical application of supercapacitors.