Fabrication Of Carbon And Cabon-based Composites And Their Application In Electrochemical Energy Storage

Portable energy storage devices provide convenience for people's life.Compared with the traditional energy storage devices-battery,supercapacitor show higher power density and cycling life.However,the energy density of supercapacitor determined by the electrode materials used is low which limit the application in many fields.Given the high surface area,excellent electroconductivity and tunable porosity,carbon materials were widely used in energy storage devices.One of the effective methods to improve the performance of supercapacitor is to develop porous carbon materials with appropriate porous structure.Also,the energy density can be enhanced via fabrication hybrid energy storage devices such as Li-ion capacitor.This thesis developed a general method(leavening method)to tune the porous structure of biomass-derived materials in the second chapter.Taking cellulose as the original materials,hierarchically porous carbon materials with macro-,meso-,and micropores can be prepared through“leavening method”.To verify the general applicability of the leavening method,the tertiary mixture of cellulose,lignin,and hemicellulose with different mass ratios was studied.When the amount of cellulose and hemicellulose is higher than 50%,the carbon materials derived from mixture via“leavening method”contain macro-,meso-,and micropores.As the cellulose and hemicellulose is most crude biomass is higher than 50%,the“leavening method”pyrolysis products originate from crude biomass also contain hierarchically porous structure which confirm the general applicability of“leavening method”.The product derived from cellulose via“leavening method"(Ccel-LE)feature high specific surface area(1893 m2/g),was processed as the electrode for supercapacitor in 6 M KOH tested in two-electrode system.The specific capacitance is 253 F/g at 0.1 A/g and 115 F/g at 10 A/g,which indicated Ccel-LE have a good rateability.The energy density of the device is 8.8 W h/kg when the power density is 200 W/kg.The energy density can be further improved by using 1 M Emim BF4/AN as the electrolytes in which the voltage window was expanded to 2.0 V.In this case,the energy density is 14.7 W h/kg when the power density is 265 W/kg.Carbon materials is ideal materials for electronic double layer capacitor whose energy density is usually lower than 10 W h/kg.Fabrication asymmetric device is one of effective approaches to improve energy density of supercapacitor.The third chapter of this research aimed to boost the performance of Li-ion supercapacitor by design carbon coated Nb2O5 loaded in activated carbon cloth.Nb2O5 was firstly loaded in the actived carbon cloth which was then coated by coordination compound formed by mixing FeCl3 and tannin.The Nb2O5 mass loading of the the electrode is about 7.7 mg/cm2 because of the hydrophilicity of active carbon cloth.The carbon coated T-Nb2O5(C/T-Nb2O5@ACC)can be obtained after heat treatment at 700 ?.The research here introduce Fe into the carbon layer in order to enhance its graphitic degree during thermal treatment process.An appropriate conductivity and defects can enhance the performance of Nb2O5.The specific capacitance of C/T-Nb2O5@ACC is 2.0 mA h/cm2(250 mAh/g)at the current density of 0.1 mA/cm2 with mass loading of 7.7 mg/cm2.When the current density increase to 10 mA/cm2,the specific capacitance is 0.78 mA h/cm2(102 mA h/g)giving rise to 40%.The sample in absence of carbon coating cannot afford capacity when the current density is 5 mA h/g.These results demonstrate the the carbon coating strategy boost the performance of Nb2O5.Then,the Li-ion capacitor was assembled by using C/T-Nb2O5@ACC as the anode and high temperature treated carbon cloth(TCC)containing plentiful micropores as the cathode.The device assembled here give energy density of 26.9 W h/kg(3.0 mW h/cm3)at 77 W/kg(8.6 mW/cm3),and when the power density increased to 1512 W/kg(169 mW/cm3),the energy density is 10.5 W h/kg(1.17 mW h/cm3).Moreover,a robust stability of 75%was achieved after 10,000 charge-discharge test.To overcome the cumbersome electrode processing of powder sample,commercial carbon cloth was used as the support to load active materials as free-standing electrode in third chapter.However,the volume energy density is low because the support is inert and cannot provide capacitance.In the fourth chapter of this research,FeCl3 preinsertion strategy was used to expand the lattice of graphite whose lattice distance increase from 0.32 nm to 0.90 nm.The Li-ion diffusion kinetics can be enhanced through the preinsertion approach.Also,the three-dimension structure and the free-standing prosperities helped to reduce the tortuosity and improve the rate ability of the electrode materials.The FeCl3 insertion carbon cloth(CCGic)was served as the anode and Li foil was used as the counter electrode in order to fabricate half-cell.The surface capacity is 3.0 mAh/cm2 at 0.5 mA/cm2 and when current density increase 10 mA/cm2,the surface capacity is 0.5 mA h/cm2 which is 25 times of original carbon cloth.Then,Li-ion capacity was assembled by using CCGic as the anode and TCC as the cathode.This devices achieve 5.2 mW h/cm3 at 6 mW/cm3 and still maintain 3.89 mW h/cm3 at 120 mW/cm3 which is better than the devices based on Nb2O5.The capacitance retention is 87%after 10,000 charge-discharge test indicate a good stability of this device.This thesis aimed to enhance the performance of energy storage devices through porous structure optimization,fabrication of carbon-based composites,graphite lattice modification as well as preparation of three-dimensional framework,which can offer an appropriate alternative for energy storage devices with high performance.