Porous hollow carbon spheres: facile fabrication and excellent supercapacitive properties

A facile method has been developed for the fabrication of hierarchically porous hollow carbon spheres (HCS) by using K2CO3 impregnated starch as the raw material. The shell of HCS is highly graphitized and abundant in oxygen-containing surface groups. The presence of K2CO3 activates the carbon matrix with numerous hierarchical pores during heat treatment. A slow heating rate is the key to fabricate hollow carbon spheres. The by-product of water possibly serves as the template and promotes the formation of HCS. The carbonization duration plays an important role. Extending the heating time enhances both the porosity and the graphitization degree of the shell. The HCS-10 exhibits a large specific surface area of 517.5 m2 g−1 and pore volume of 0.265 cm3 g-1. The unique hollow spherical morphology and suitable porous structure bestow HCS-10 excellent supercapacitive performances. Derived from the CV curves, a symmetrical supercapacitor of HCS-10 exhibits a maximum specific capacitance of 317 F g-1 at a slow scan rate of 5 mV s-1 and a high rate capacitance of 222.2 F g-1 at a fast scan rate of 300 mV s-1 with the potential window of 1.0 V by using 6 M KOH solution as the electrolyte. Galvanostatic charge-discharge displays a high capacitance of 265.4 F g-1 at 1 A g-1 and a high rate capacitance of 137.9 F g-1 even at a high current density of 100 A g-1. The high capacitance retention of 99.6 % over 25000 charge/discharge cycles further confirms the cycling stability and rate performance with varied current densities of 1, 2, 5, 10 and 1 A g-1. Also, the presence of oxygen-containing groups broadens the working voltage up to 1.5 V. Notably, the highest power density of 100 kW kg-1 is achieved at current density of 100 A g−1 and working voltage of 1.0 V. Cycling tests deliver a compatible data at 1.4 V and 10 A g-1, a high capacitance of 286 F g-1 with a high energy density of 77.88 Wh kg-1 and a large power density of 14 kW kg-1. The excellent supercapacitive performances of HCS-10 will possibly fit the growing demands for the energy storage devices.