S-doped micro/mesoporous carbon-graphene composites as efficient supercapacitors in alkaline media

Composites of polymer (styrene-sulfonate sodium salt) derived carbon with highly oxidized graphite oxide were synthesized and tested as supercapacitors. The materials were characterized using adsorption of nitrogen, SEM/EDX, elemental analysis, thermal analysis-mass spectroscopy and potentiometric titration. The electrochemical performance was evaluated using cyclic voltammetry, galvanostatic charge-discharge techniques and impedance spectroscopy in 6 M KOH. Addition of the graphene phase increases DC conductivity, volume of small micropores, and sizes of mesopores. These changes enhance the capacitance. Sulfur species located in small micropores affect the charge of the carbon surface and decrease its affinity to adsorb water. This results in a specific electrosorption of electrolyte ions and thus in highly efficient space utilization. Sulfones and sulfoxides located in the larger pores (mesopores) contribute to a pseudocapacitive effect. On the materials tested similar to 110 F g(-1) was measured in spite of the small surface area (about 600 m(2) g(-1)). This leads to a high volumetric capacitance of up to 65 F cm(-3) (without a special densification).