Preparation And Capacitance Characteristics Of Carbon-based Electrode Materials Derived From Camellia Oleifera

Forest-based bioenergy is gaining global popularity due to its multiple benefits and increased global energy needs.However,the high-value utilization of Camellia oleifera still faces great challenges.After carbonization and activation,the doped carbon materials derived from Camellia oleifera display hierarchical porous structures that facilitate ion transport and shorten diffusion pathways for enhanced charge storage in supercapacitor（SCs）.However,there are faces a daunting challenges to construct three-dimensional porous electrode materials with high specific capacitance and fast ion electron transfer by a low cost,environmentally friendly,and simple method.Therefore,in this paper,porous carbon composites were prepared based on the precursor from Camellia oleifera.The structures and chemical composition of electrode materials have been reasonable constructed and designed to enhance the rapid transfer of ions and electrons in the electrochemical systems.Therefore,a series novel multi-scale carbon matrix composite with unique surface morphology,high conductivity and high specific surface area has been developed.The main works of this dissertation are devided into the following chapters:1.Camellia oleifera is usually rich in lignocellulose and other valuable compounds.Therefore,the camellia tissue（such as stem and root）with 3D structure was considered to be an ideal precursor for synthesis of binder-free carbon materials.In this chapter,porous carbon materials were developed from the camellia plant by direct carbonization and the electrochemical test were carried out.It was found that the Camellia oleifera stem is an ideal candidate for SCs electrode material due to the N-6 and porous structure.The micropore ratio and the content of heteroatom doping are of great significance to the electrochemical properties of carbon electrode materials.Therefore,the free-binder CW-800 electrode shows a higher specific capacitance of 130 F g^-1 and rate performance(0.1-50 A g^-1,58.5%).More important,the capacitance retention was 92.5%of specific capacitance was maintained at a current density of 20 A g^-1 after 10000 charge/discharge cycles in 3 M KOH electrolyte.2.Hierarchical porous carbon derived from wood（NCW）with adjustable pore structure and N doping was certainly prepared by one-step of NH₄Cl activation,doping and carbonization.NH₄Cl is considered as an effective N dopant to produce N-6 with superb OH^-penetration/absorption.The prepared NCW-2 electrode with interconnected three-dimensional（3D）hierarchical porous structure shows higher specific surface area(981 m² g^-1)and micropore ratio（70.8%）,providing an effective interface area for charge storage.Therefore,the specific capacitance of NCW-2 in the three-electrode system at a low current density of 0.1 A g^-1 was 350 F g^-1,and a superior capacitance retention rate of 94.8%after 10000 cycles of 20 A g^-1 can be realized.3.NCW@Fe_xO_y composite negative electrode was produced by combination of electrodeposition and post heat-treatment to construct a multi-scale composite electrode for effectively improving the electrochemical performance.The results show that the phase structure and interface chemical bond in the composites can be controlled by heat treatment temperature.NCW@Fe₂O₃ with 3D porous structure and the C-O-Fe chemical bond between NCWs and Fe₂O₃ to alleviate the transport constrains and thus hugely promote the kinetics of ion adsorption and electron transport.Therefore,a maximum specific capacity of 603 F g^-1 in the three-electrode system was obtained,much higher than that of NCW.The specific capacitance remains 405 F g^-1 even at 50 A g^-1and capacitance retention of 85.5%after 10000cycles at 10 A g^-1 can easily realized.4.Controllable vertical growth of Ni Co₂O₄ nanosheets positive electrode with oxygen vacancy on the surface of wood-derived porous carbon was realized via electrodeposition technology and followed by a heat-treatment process.The composition with oxygen vacancies exposed more active sites for electrochemical reation and enhanced conductivity.Therefore,NCW@Ni Co₂O₄ electrode presented a high specific capacitance of 2038 F g^-1 at 1 A g^-1,and the capacity retention was87.2%after 10000 cycles at 10 A g^-1.5.For the practical application of supercapacitors,symmetrical devices and asymmetrical devices are assembled by using different electrodes.Benefiting from the appropriate pore size distribution and the optimal number of N/O-based functional groups of NCW,the symmetrical supercapacitor based on NCW electrode shows high energy density(22.32 Wh kg^-1,1.24 m Wh cm^-2 and 3.11 m Wh cm^-3),and superior capacitance retention of 94.7%was obtained after 30000 cycles.Moreover,due to the synergy of multifunctional composites,the asymmetric supercapacitors assembled from NCW and NCW@Fe₂O₃//@Ni Co₂O₄ shows a wider voltage of 1.5 V and a higher energy density of 83.9 Wh Kg^-1(area energy density of 3.26 m Wh cm^-2 and volume energy density of 10.19 m Wh cm^-3).