Tailoring microstructure and conductivity of porous hollow carbon spheres to enhance their performance as electrode materials for supercapacitors

Porous carbon materials are widely used in electrical double-layered capacitors for energy storage applications due to their abundant pores, high specific surface area and excellent electrochemical stability although their intrinsic electrical conductivity is generally low. Traditional high-temperature treatment was always adopted to boost the electrical conductivity of porous carbon materials by enhancing their graphitization degree, while this would always be accompanied by severe damage to pore structure and corresponding electrochemical performance. To solve the dilemma between boosting electrical conductivity and retaining pore structure, in this research, we proposed a low-temperature catalytic graphitization for hollow carbon spheres (HCSs) prepared by modified Stober method accompanied with a special pore structure tailoring strategy. By altering the timing of adding carbon source, followed by secondary pore-forming and catalytic graphitization, HCSs with thin and porous shell walls along with enhanced conductivity were synthesized. Compared with the untreated HCSs or high-temperature graphitized HCSs, HCSs fabricated by the aforementioned combined strategies exhibited enhanced pore volume, specific surface area (2160.1 m(2).g(-1), 165.3 % of the untreated group) and conductivity (16.44+0.05 S.cm(-2), 221.5% of the untreated group), demonstrating much better electrochemical performance as the mass specific capacitance reached 256.7 F.g(-1) and a volume specific capacitance reached 102.7 F.cm(-3) at a current density of 1 A.g(-1). Moreover, due to the largely promoted conductivity, the capacitance with the catalytic graphitized HCSs as the electrode material could be further boosted up to 273.4 F.g(-1) at 1 A.g(-1) by exempting the generally indispensable conductive agents and a high capacitance retention of 100.7 % was achieved after 5000 cycles at a relatively high current density of 10 A.g(-1). This could be attributed to local graphitization brought by dispersed catalyst particles instead of entire atom rearrangement during high-temperature graphitization which would result in severe pore vanishing. The synergy of low-temperature catalytic graphitization and secondary pore-forming might be a novel strategy in electrode material fabrication for advanced supercapacitors or other energy storage devices.