Preparation Of Biomass-derived Porous Carbon And Its Electrochemical Properties As Electrode Material For Supercapacitors

With the rapid development of global economy and the increasing demand for energy,exploitation of clean and efficient energy storage equipment is of great importance for sustainable development.As a new type of energy storage device,supercapacitors exhibit many advantages such as higher power density,long cycle life,wide operating temperature range,and environmental friendliness,thus have a wide application potential.Electrode is the most important part of supercapacitors,which significantly determines the electrochemical performance of supercapacitors.Among the various electrode materials,biomass-derived carbon is a promising electrode material due to the high specific surface area,low cost,mass production and environmental friendliness.In this paper,biomass was used as the precursor（sorghum）for the preparation of porous carbon materials through pre-carbonization,hydrothermal activation,and chemical activation.For improvement of the electrochemical performance,heteroatoms doping and complexing with transition metal compound were performed independently.The main research contents including follows:Part one:Preparation and electrochemical properties of sorghum-derived hierarchical porous carbon.With sorghum and KOH as precursor and activator,respectively,the effect of activation temperature on the performance of resultant porous carbons was investigated.Firstly,sorghum was pre-carbonized at 400℃and then reacted with KOH at 140℃for 4h.The resulting mixture was carbonized at different temperatures（600,700 or 800℃）to obtain a series of sorghum seeds-derived porous carbon materials（SSK-X,X=600,700 and800）.Among them,the sample with activation temperature of 700℃（SSK-700）showed the best electrochemical performance.In a three-electrode system with 6 M KOH as electrolyte,the specific capacitance of SSK-700 at 1 A g^-1is 267.6 F g^-1,which is more than 3 times higher than that of the non-activated sample（SS-700）.Furthermore,the SSK-700 based electrode showed good cycle stability,with capacitance retention rate of96.9%after 5000 charge and discharge cycles at 10 A g^-1.The symmetric supercapacitor fabricated with SSK-700 delivered an energy density of 11.7 Wh kg^-1at power density of346.7 W kg^-1.Part two:In order to improve the electrochemical performance of the biomass-derived porous carbon mentioned above,melamine was employed as the auxiliary activator and nitrogen source for the preparation of N/O co-doped porous carbon.The pre-carbonized sorghum seeds,melamine and KOH were mixed at a mass ratio of 10:5:11 and carbonized at 700℃for 2 h to obtain the N/O co-doped porous carbon（SSK-M）.The influence of melamine on the structure and properties of resultant porous carbon was investigated.The results indicate that the specific surface area of the prepared sample（SSK-M）is up to 2132m²g^-1,twice as high as that of the porous carbon without adding melamine(1074 m²g^-1).The specific capacitance of SSK-M is up to 536.7 F g^-1(0.5 A g^-1)in a three-electrode system with 6 M KOH as electrolyte.The SSK-M based symmetric supercapacitor showed a high energy density of 13.8 Wh kg^-1at power density of 298.0 W kg^-1,and a capacitance retention higher than 97%after 15,000 charge-discharge cycles,suggesting excellent cycle stability.Part three:Beyond the above study,pseudocapacitive materials such as transition metal compounds has also been investigated for the improvement of electrochemical performance of biomass-derived porous carbon.Through a one-step hydrothermal reaction between SSK-700 and KMn O₄,composite materials（SSK-700/Mn O₂）with three-dimensional needle-like network structure were prepared.The effects of hydrothermal reaction time and temperature on the electrochemical properties of the composite materials were investigated.It was found that SSK-700/Mn O₂prepared by hydrothermal reaction at 120℃for 12 h has the best performance.In a three-electrode test system using 6 M KOH as electrolyte,the specific capacitance of SSK-700/Mn O₂reaches482.7 F g^-1at 0.5 A g^-1,which nearly twice of that of SSK-700(281.3 F g^-1).After 20,000charge and discharge cycles,the retention rate of specific capacitance is 95.8%,showing excellent cycle stability.In an assembled symmetric supercapacitor,the energy density of13.7 Wh kg^-1was determined at power density of 289.1 W kg^-1.Furthermore,the flexible symmetric supercapacitor assembled with PVA/KOH gel electrolyte exhibited excellent mechanical properties,with no obvious degradation in electrochemical performance at bending angle of 90°and 180°.