Hydrothermally Synthesized Sulfur-Doped Graphite as Supercapacitor Electrode Materials

Sulfur-doped graphite (S-DG) was synthesized using a sustainable method. Low-cost cellulose fiber and carbon disulfide (CS2) were used as precursors for carbon and sulfur (S) sources, respectively, and a multistep chemical and thermochemical synthesis process was employed to obtain S-DG. Advanced microscopic and spectroscopic techniques and porosimetry analyses were performed to probe the surface morphology, surface area, microstructures, and chemical bonding states of S-DG. Interestingly, X-ray photoelectron spectroscopy revealed that the S loads were 3.81 and 7.02 atom % when reactions were carried out in aqueous and dimethyl sulfoxide (DMSO) media, respectively. The specific surface areas were calculated as 522.77 and 693.27 m2 g–1 for the two materials, respectively. To our best knowledge, the 7.02 atom % S load is the highest S doping reported so far in a graphite material where laboratory chemicals have been used as precursors. Both materials, i.e., S-DGWater and S-DGDMSO, were tested as supercapacitor electrodes in a portable electrochemical cell with a three-electrode system to check their potential and stability for the fabrication of renewable energy storage devices. Cyclic voltammetry and galvanostatic charging–discharging techniques were performed within a potential window of 1 V in the presence of 1 M Na2SO4 as an electrolyte. The maximum specific capacitances (CSp) of S-DGWater and S-DGDMSO were obtained as 155.61 and 261.43 F g–1 (at a scan rate of 10 mV s–1), respectively. The result concluded that the extent of S doping appeared to be the key factor for control of the peak current. Importantly, our investigation suggests that the S load in graphite facilitates a diffusion-driven storage mechanism and a higher amount of S may enhance surface-controlled storage as well. Both materials show excellent cyclic stability with >98% retention of the initial specific capacitance over 10000 cycles of charging–discharging.