Synthesis of ZIF-67-derived CoS2@graphitic carbon/reduced graphene oxide for supercapacitor application

We report the phase-controlled synthesis of 2D reduced graphene oxide (rGO) and 3D graphitic carbon (gC) embedded CoS2@gC/rGO nanocomposite as an anode material for supercapacitor application. CoS2@gC polyhedrons sandwiched between thin layers of rGO were obtained by a sacrificial template method using the ZIF-67 metal organic framework (MOF) and graphene oxide (GO). The phase composition was altered by changing the calcination temperatures of the rGO sandwiched ZIF-67. In the presence of elemental sulfur, a single-step calcination method produced mixed phases of CoS and CoS2 that were engrained in amorphous carbon (CoSx@aC/rGO). However, the two-step calcination process yielded a phase pure CoS2 that was embedded in the graphic carbon (CoS2@gC/rGO). Phase pure CoS2 was formed as a result of the restricted diffusion of the sulfur atoms to the metallic cobalt core that was encapsulated in the graphitic carbon layers of the Co@gC/rGO composite. The CoS2@gC/rGO composite exhibited a specific capacitance of 1,188 F/g and cyclic stability of 76% and 99% coulombic efficiency. An all-solid-state asymmetric supercapacitor device was fabricated with CoS2@gC/rGO and hydrothermally reduced graphene oxide (hrGO) as the positive and negative electrodes, respectively. The device exhibited a high specific capacitance of 233 F/g at a 1.5 A/g current density and delivered high energy density of 82.88 W.h/kg at a power density of 1,199.56 W/kg. Particularly, the device conserved an energy density of 42.44 W.h/kg even at a high-power density of 7,999.9 W/kg. Additionally, it showed good cyclic stability after 10,000 cycles of repeated charging/discharging, suggesting its potential for practical supercapacitor applications.