Sulfur modification of worm-like exfoliated graphite with crumpled graphene sheets derived from mesocarbon microbeads for electrochemical supercapacitors

Abstract

Chemical exfoliation and unzipping of graphitic mesocarbon microbeads tend to form bone-like aggregates of graphene oxide (GO) sheets (marked as O-EMCMB). Heat treatment of O-EMCMB leads to the formation of partially reduced GO fragments (marked as HT-EMCMB) with hydrophobic sheets. Sulfur doping of O-EMCMB through hydrothermal process with sodium sulfide results in the formation of sulfur-doped and reduced GO with worm-like shape (marked as S-EMCMB). S-EMCMB has more crumpled sheets and hydrophilic surface than HT-EMCMB. Cyclic voltammetry reveals that S-EMCMB electrode gains more pseudocapacitive charge-storage capacity than the O-EMCMB and HT-EMCMB electrodes in 6 M KOH solution. S-EMCMB electrode exhibits high rate performance, cycling stability, and coulombic efficiency during galvanostatic charge and discharge (GCD) processes, its specific capacitance attains 314 F g−1, far greater than the O-EMCMB (67 F g−1) and HT-EMCMB (222 F g−1) under a GCD current of 1 A g−1. The enhanced charge-storage properties of S-EMCMB can be ascribed to its high hydrophilicity and electrical conductivity after incorporation of sulfur-related functional groups into the crumpled graphene sheets. This unique worm-like configuration inhibits graphene sheets from restacking and enables rapid transport of ions and electrons through hydrophilic pores for facilitating the redox kinetics at the electrolyte-electrode interfaces.