Preparation Of Biomass-derived Carbon Materials/MnO₂ Composite And Its Application In Supercapacitors

Supercapacitor is one of the most promising energy storage devices with high power density,highly reversible charge storage process and long cycle life.It has shown great potential in hybrid vehicles,portable electronic devices and other functional fields.In recent years,three-dimensional porous carbon materials have become one of the most interested electrode materials for high-performance supercapacitors due to their low ion diffusion impedance,unique pore structure and topological structure.As sustainable,cheap and environmentally friendly natural resources with hierarchical structure,biomass can be easily transformed into three-dimensional carbon materials,which hold great potential in the future high performance supercapacitors.However,due to the uncontrolled microstrure and limited electrochemical properties,the specific capacity and energy density of three-dimensional porous carbon materials derived from biomass are not competitive.In this thesis,two methods were employed to control the pore structure of porous carbon derived from corncob and bamboo.Three-dimensional aerogel was prepared by using biomass carbon as template through directed self-assembly with other functional materials.In order to improve the capacitive performance of biomass derived three-dimensional carbon materials,Mn O₂ was introduced by electropolymerization.The effects of deposition methods and deposition parameters on the growth and morphology of Mn O₂ were investigated.The biomass derived carbon materials/Mn O₂ composites with excellent electrochemical properties were obtained and further applied in supercapacitors.Specific research contents of this thesis are as follows:1.Two corncob-derived carbon electrode materials mainly composed by micropores?activated carbon,AC?and mesopores/macropores?corncob carbon,CC?were prepared and studied after anodic electrodeposition of Mn O₂.The capacity of Mn O₂/activated carbon composite?Mn O₂@AC?electrode did not increase obviously after Mn O₂ electrodeposition,while that of Mn O₂/corncob carbon composite?Mn O₂@CC?electrode increased up to 9 times reaching 4475 m F cm^-2.An asymmetric all-solid-state supercapacitor?ASC?was fabricated using AC as anode,Mn O₂@CC as cathode and polyvinyl alcohol?PVA?/Li Cl gel as electrolyte.An ultrahigh specific capacitance of 3455.6 m F cm^-2 at 1 m A cm^-2,a maximum energy density of 1.56 m W h cm^-2 and a long lifetime of 10000 cycles can be achieved.This work provides insights in understanding the function of Mn O₂ in biomass-derived electrode materials,and a green path to prepare ASC from waste biomass with excellent electrochemical performance.2.A three-dimensional composite aerogel with synergistic effects of corncob-derived AC,cellulose nanofiber?CNF?and graphene oxide?GO?was prepared by self-assembling of graphene with the other components.The composite aerogel has good conductivity,high specific surface area and mechanical strength.AC is used as the conductive substrate to enhance the specific surface area and specific capacitance of GO hybrid aerogels.CNF acts as an intensifier,on the one hand,it can improve the mechanical strength of the composite aerogels.On the other hand,its hydrophilicity can increase the wettability of GO in aqueous solution.Light weight and ultra-thin AC/GO/CNF aerogels can be obtained by changing the amount of CNF and molding in different moulds.After Mn O₂ electrodeposition,the AC/GO/CNF@Mn O₂composite electrode shows an ultrahigh specific capacitance of 4800 m F/cm².Finally,a sandwich-like ASC was prepared by using AC/GO/CNF as anode,AC/GO/CNF@Mn O₂ as cathode and PVA/Na₂SO₄ gel as electrolyte.The device exhibited a high energy density of 0.365m W h cm²,power density of 18000 m W cm^-2and long cycle stability.The AC/GO/CNF@Mn O₂composite demonstrates great potential as a power source in high-performance supercapacitors.3.A ternary nanocomposite was prepared by co-electrodepositing of Mn O₂ and polypyrrole?PPY?on biomass derived bamboo carbon.This method overcomes the shortcomings of traditional biomass derived carbon,Mn O₂ and PPY composite structure,which fully utilizes the synergistic and complementary effects of Mn O₂ and PPY.Mn O₂ nanoparticles was inserted into PPY chain,which can improve the conductivity of bamboo carbon/PPY/Mn O₂composite.At the same time,Mn O₂ particles provide rigid support and conductive path through crosslinking with PPY chains,thus the rate performance of composites can be improved?capacitance value remains 48.2%?.In addition,due to the dual capacitance contribution of PPY and Mn O₂,the bamboo carbon/PPY/Mn O₂ electrode has a high specific capacitance of 2436m F/cm².When it was further assembled into the ASCs with AC/GO/CNF anode,this device exhibited a high energy density of 0.426 m W h cm^-2,high power density of 16000 m W cm^-2and long cycle stability,which shows great potential in high-performance supercapacitors.