Preparation of Plant-based Hierarchical Porous Carbon and Research on the Electrochemical Performances of Supercapacitor

Author:Guo Zuo Zuo

Supervisor:yang ru


Degree Year:2018





Supercapacitors have attracted increasingly growing attention as efficient energy storage systems because of their significantly longer cycling life and higher power density than most batteries.Compared with both batteries and fuel cells,the major challenge of supercapacitors is their low energy density.Generally,the performance of supercapacitors primarily depends on electrode materials.Porous carbon materials are good candidates for supercapacitor electrode materials because of their sustainable precursors,tunable morphologies for specific ions,simple preparation processes and especially low costs.In addition,with the increasing scarcity of fossil fuels,the synthesis of porous carbons derived from biomass waste for energy application has been attracting more and more attention.In this thesis,we prepared porous carbon materialsspecial morphology from various biomass by different methods.The effect of preparation conditions on the pore structure,the surface chemical state and conductivity of porous carbon materials,and the physical-chemical properties correlating to their electrochemical performance have been researched emphatically.This thesis mainly contains the following content:In the third chapter,porous carbons with high nitrogen,large specific surface area(SSA)and oxygen content,fairly good electrical conductivity were successfully prepared through the pyrolysis of the hemp stems involving KOH under NH3 flow.In this method,NH3 can not only effectively enhance the activation property of KOH,but also introduce nitrogen into carbon framework even at high temperature.The performance of the sample prepared by dual-activation of KOH and NH3 is much better than the samples solely activated by KOH or NH3.The dual-activated sample exhibits high SSA(1949 m2 g-1),hierarchical porosity,high electrical conductivity(3.5 S cm-1)and high nitrogen content(4.4 wt.%).The obtained sample shows high specific capacitance of 352 F g-1 at 0.1 A g-1 and excellent rate capability(67%capacitance retention ratio even at 30 A g-1)in 6 M KOH electrolyte.Moreover,the as-assembled symmetric supercapacitor displays a high energy density of 99.5 Wh kg-1 in an ionic liquids(EMIM TFSI)electrolyte system,still maintaining a value of 27.7 Wh kg-1 even at high power density 21660 W kg-1.More interestingly,this supercapacitor can light up a red light-emitting-diode more than 8 minutes only charged 4.4 seconds.In the fourth part,soybean root,possessing abundant honey-like epidermal and ground tissue,was utilized to synthesize 3-Dimensional(3D)hierarchical porous carbons through a facile chemical activation method and employed for high-energy supercapacitor electrode materials.The as-prepared sample with high SSA(2143 m2 g-1),high graphitization degree and 3D hierarchical porosity,exhibits remarkable electrochemical performance,such as a high specific capacitance of 276 F g-1,outstanding rate capability and excellent cycling stability(98%retention after 10000 cycles at 5 A g-1)in 6 M KOH electrolyte at a two-electrode system.In addition,the as-assembled symmetric supercapacitor delivers a superior energy density of 9.6 Wh kg-1 in 6 M KOH.In order to improve the energy density,we measured the non-aqueous symmetrical supercapacitors in a neat ionic liquid of EMIM BF4 with a wide potential window of 3.5 V.The energy density enhanced to 100.5 Wh kg-1 in ionic liquid system.This result is higher than that of many reported biomass based porous carbons for supercapacitors.In addition,a value of 40.7 Wh kg-1 is still maintained even at ultrahigh power density 63000 W kg-1.These energy densities are comparable to that of the Ni metal hydride battery,but the supercapacitor can be charged or discharged in seconds or minutes.In the fifth chapter,porous carbons with ultrahigh SSA were obtained by carbonization and KOH-activation of available biomass tremella.Influences of KOH/C mass ratio on the surface chemistry and pore structure were systematically studied.The as-prepared sample possesses ultrahigh BET SSA of 3760 m2 g-1 and high content of surface oxygen.So,the sample shows a very high specific capacitance of 284 F g-1 at 1 A g-1 in 6 M KOH and 240 F g-1 at 0.5 A g-1 in 1 M Na2SO4,superior cycle stability(99%and 94%capacitance retention after 10000 cycles),and good rate performance in a symmetric two-electrode supercapacitors.To further improve the operating voltage,supercapacitors were assembled in neat EMIM BF4(3 V).The energy density of this device can reach 65.6 W h kg-1,which is higher than most previously reported carbon materials.Moreover,the energy density of 28 Wh kg-1 is still maintained even at an ultrahigh power density 19700 W kg-1.In the last chapter,we successfully prepared 3D hierarchical porous carbon using lignin as the precursor by a facile template-free method.In this method,the hydrothermal carbonization can produce plentiful surface holes,and following KOH activation step generates abundant cross-link macropores in the holes and forms incalculable micropores and small mesopores in the walls and surfaces of holes.The obtained samples exhibit unique 3D hierarchical porous structure,high SSA(1660 m2 g-1)and high electrical conductivity(5.4 S cm-1).Consequently,the sample exhibits high specific capacitance(420 F g-1),superior rate capability and outstanding durability in a three-electrode system in 6 M KOH.In addition,the assembled symmetric supercapacitor delivers a superior energy density of 46.8 Wh kg-1 in ionic liquid system.Moreover,a value of 22.9 Wh kg-1 is still maintained even at ultrahigh power density 25400 W kg-1.These results suggest that biomass-derived porous carbons are the ideal electrode materials for high-performance supercapacitors owing to its low-cost,environmentally friendly,and renewable source material.