Preparation of hierarchical porous carbon from corncob hydrochar by KCl enhancing K2CO3 activation for electrode material of supercapacitor

[Display omitted] •Hierarchical porous carbon can be obtained from hydrochar by KCl-K2CO3 activation.•KCl enhancement effect was best when mass ratio of K2CO3 to KCl was 1:1.•Volume ratio of mesopore and macropore to micropore was 1:2.2 for HCPC-1-1-800-IMP.•SBET and pore volume of HCPC-1-1-800-IMP was 1914 m2 g−1 and 0.96 cm3 g−1 respectively.•Energy density of HCPC-1-1-800-IMP was as high as 22.7 Wh kg−1 at 750 W kg−1. Hydrothermal carbonization (HTC) is a green and efficient technology for biomass valorization, and hydrochar (HC) from HTC was good precursor of carbon materials. In this paper, hierarchical porous carbon derived from corncob hydrochar (HCPC) was prepared by KCl molten salt enhancing K2CO3 activation. The surface morphology, carbon structure and pore structure evolution of HCPC and its electrochemical performance for supercapacitor was investigated. The results showed that when the activation temperature was 800 °C and the mass ratio of HC:KCl was 1:1, KCl obviously improved pore structure and induced rich amorphous carbon for HCPC in the K2CO3 activation process, and during this process, the KCl enhancement effect could be further excited by impregnation as premixing method. Specifically, the specific surface area and the total pore volume of HCPC-1-1-800-IMP was 1914 m2/g and 0.96 cm3/g respectively, and the volume ratio of mesopore and macropore to micropore was about 1:2.2. Furthermore, the result also indicated that HCPC-1-1-800-IMP had the specific capacitance of 255.4 F/g at 0.5 A/g in 6 M KOH. Moreover, when HCPC-1-1-800-IMP was used as electrode material in the symmetric supercapacitor, it exhibited a good specific capacitance of 72.7 F/g at 1 A/g and a high energy density of 22.7 Wh/kg at 750W/kg. After 5,000 cycles at 5 A/g, the coulomb efficiency remained close to 100 % and the specific capacitance retention rate was 93.6 %. The results of this work may provide a theoretical support for high-value utilization of biomass, as well as for clean and efficient preparation of hierarchical porous carbon.