Preparation And Improvement Of Licorice-based Porous Carbon Driven By Potassium Citrate,and Study Of Electrochemical Properties

With the emergence of new energy,energy storage and conversion devices,mainly batteries,fuel cells and supercapacitors,have become the main focus of development and research.Compared to conventional capacitors and rechargeable batteries,supercapacitors provide higher power density,greater temperature tolerance,and longer service life.Waste biomass-derived carbon has become the major choice for dual-layer electrode materials for its high specific surface area,rich pore size distribution and good thermal stability.But the energy density of biomass-derived carbon is low and the specific capacitance is limited.The introduction of metal compounds or heteroatomic modification can increase the active sites,enhance the conductivity and wettability of carbon-based electrode materials,and thus improve the electrochemical performance of biomass-derived carbon as an electrode material in general.Therefore,a green synthesis of waste biomass-derived porous carbon was firstly proposed,and then the electrochemical performance of the prepared licorice-based porous carbon as an electrode material was optimized by two strategies of loading nickel-manganese hydrotalcite and sulfur doping.The main results from the study were as follows:（1）Licorice-based porous carbons（CKs）were prepared by the one-pot method using potassium citrate as the activator,and it was found that potassium citrate as well as pyrolysis temperature and time had significant effects on the specific capacitance of CKs.The licorice-based porous carbon（CK-600-2h）prepared by carbonization of licorice residue at 600°C for 2 h had a high specific surface area(402.42 m²g^-1)and a high abundance of pore size distribution,and yielded a specific capacitance of 221 F g^-1 at 0.5 A g^-1.The assembled symmetric supercapacitor provided an energy density of 7.57 Wh kg^-1 at 0.5 A g^-1.（2）To address the problems of low specific capacitance and low energy density of carbon materials,a hydrothermal method was further designed for the synthesis of licorice-based porous carbon loaded nickel-manganese hydrotalcite composites.The specific capacitance of the composite（CNML）prepared by the reaction for 12 h at a Ni²⁺/Mn²⁺ratio of 4 and a hydrothermal temperature of 90°C was significantly increased(1376 F g^-1).It can maintain up to 708 F g^-1 at 10 A g^-1 and 30%of its specific capacitance after2000 cycles.The electrochemical reaction resistance was 0.752Ω（0.42 times higher than that of pure Ni-Mn hydrotalcite under the same conditions）.The mentioned results show that the CNML composite maintained the high electrical conductivity of carbon material and the high specific capacitance of Ni-Mn hydrotalcite better.（3）To address the shortage of rate performance and cyclic stability of licorice-based porous carbon-loaded hydrotalcite composites,a one-step solvothermal method was developed for the synthesis of bimetallic sulfide composites（CSNMLs）.The CSNML-4 electrode material with optimal performance obtained by adjusting the thiourea concentration exhibited a specific capacitance of 1436 F g^-1.The cyclic stability reached 60%after 2000 cycles at 10 A g^-1,which was a 30%improvement over the undoped CNML.The assembled asymmetric supercapacitor exhibited a specific capacitance of 222.84 F g^-1 at 0.5 A g^-1,a high energy density of 79.23 Wh kg^-1,and excellent cyclic stability(keep up 81%after 2000 cycles at10 A g^-1).These results illustrated that high-temperature pyrolysis of potassium citrate activated licorice-based porous carbon was an effective method for the synthesis of electrode materials.The electrochemical properties of the electrode materials were well enhanced by using metal compounds and heteroatom doping modifications.This method provided a new idea for the preparation of composite electrode materials based on biomass-derived carbon.