Electrochemical performance enhancement of flexible graphene supercapacitor electrodes by carbon dots modification and NiCo2S4 electrodeposition

Abstract In this work, flexible graphene supercapacitor electrodes were prepared from biomass-derived carbon dots/graphene composite film (CDGF) and hydrogel (CDGH) with/without an electrodeposited NiCo2S4 layer. Results showed that the dispersive CDs with abundant heteroatoms effectively inhibited the aggregation of graphene sheets and increased the number of active sites. By combining CDs modification and NiCo2S4 electrodeposition, the electrochemical performance of the prepared ternary composite electrodes is significantly enhanced. In particular, the CDGF-NiCo2S4 electrode has a specific capacitance of up to 1348 F g−1 at a current density of 0.5 A g−1. The resultant CDGF-NiCo2S4 flexible symmetric supercapacitor using 1.0 M H2SO4 as aqueous electrolyte shows a large specific capacitance (313 F g−1 at 0.5 A g−1) and a high energy density of 85.1 Wh kg−1 at a power density of 353 W kg−1. By contrast, the CDGH-NiCo2S4 flexible solid-state supercapacitor using polyvinyl alcohol (PVA)/H2SO4 as gel electrolyte has improved cycling stability (83.1% capacitance retention after 10000 charging/discharging cycles) and a better mechanical flexibility (96.8% capacitance retention after 1000 bending cycles). This work demonstrates the importance of CDs and metal sulfides in modifying graphene-based composite electrodes for flexible supercapacitor.