With the increasing demand for dissolving pulp, large quantities of hemicelluloses were generated and abandoned. These hemicelluloses are very promising biomass resources for preparing carbon spheres. However, the pore structures of the carbon spheres obtained from biomass are usually poor, which extensively limits their utilization. Herein, the carbon microspheres derived from hemicelluloses were prepared using hydrothermal carbonization and further activated with different activators (KOH, K2CO3, Na2CO3, and ZnCl2) to improve their electrochemical performance as supercapacitors. After activation, the specific surface areas of these carbon spheres were improved significantly, which were in the order of ZnCl2 > K2CO3 > KOH > Na2CO3. The carbon spheres with high surface area of 2025 m2/g and remarkable pore volume of 1.07 cm3/g were achieved, as the carbon spheres were activated by ZnCl2. The supercapacitor electrode fabricated from the ZnCl2-activated carbon spheres demonstrated high specific capacitance of 218 F/g at 0.2 A/g in 6 M KOH in a three-electrode system. A symmetric supercapacitor was assembled in 2 M Li2SO4 electrolyte, and the carbon spheres activated by ZnCl2 showed excellent electrochemical performance with high specific capacitance (137 F/g at 0.5 A/g), energy densities (15.4 Wh/kg), and good cyclic stability (95% capacitance retention over 2000 cycles).
Keywords: carbon microspheres; hemicelluloses; hydrothermal carbonization; supercapacitor.