Synthesis Of Hierarchical Porous Carbons And Their Electrochemical Behavior

The supercapacitors and lithium ion batteries with high energy density and power density have been put into industrial productions and applications, in the field of energy storge and power supply. The controllable synthesis of electrode material with high energy density, power density, long cycle stability and safety and the study on energy storage mechanism as well are being a hot scientific research topic.Porous carbon electrode materials with the advantages of unique chemical stability, good electronic conductivity, large specific surface area and relatively low production costs, have already became the excellent candidates for the use in the electrochemical energy storage. In order to improve the electrochemical capacity, the cycle stability and achieve a high energy density and a power density, it is highly required simultaneously to maintain a high specific surface area and enlarge the pore diameter of porous carbon electrode, thus facilitating the diffusion of electrolyte into the electrode porous chanles. In addition, surface modification, such as doping with other heteroatoms, is considered as another effective method to improve the surface wettability and increase the electronic conductivity, ensuring a high energy density. In this thesis, based on solution chemistry route, we have synthesized a series of porous carbon materials, and investigated the relationship between the structural characteristics and electrochemical properites.(1) Hierarchically porous carbons with variable pore sizes at multi-length-scale, a nitrogen and boron co-doped and local graphitized framework, and high mechanical strength were synthesized through the selfassembly of poly(benzoxazine-co-resol) with ionic liquid C16mimBF4and a carbonization process. In this synthesis, the ionic liquid acts both as a structure directing agent and a heteroatom precursor. The obtained porous carbons have a specific surface area lower than376m2g-1and thus a high skeleton density. With such heteroatom doped skeleton structures and fully interconnected macropores, mesopores and micropores, the hierarchically porous carbon shows outstanding electrochemical performance, e.g. a superior high gravimetric capacitance (Cg) of247F g-1, an interfacial capacitance(Cs) of66mF cm-2(calculated based on the discharge curve with a constant current density of0.5A g-1), whilst a high volumetric capacitance (Cv) of101F cm-3compared to those reported in the literature. Cycling stability tests indicate that the carbon exhibits a capacitance retention of96.2%after4000charge-discharge cycles, strongly reflecting an excellent long-term cyclability of the electrode. Due to its unique skeleton structure and high conductivity, such hierarchically porous carbon shows promise as an electrode material for supercapacitors.(2) P-containing ionic liquid acted both as structure directing agent and co-solvent, porous carbons with continuous structure combined N, P co-doping have been synthesized.The carbon materials present the specific surface area of492～605m2g-1, pore volume of0.26～0.39cm3g-1, N and P contentance is0.7-0.72%and0.76～1.2%, respectively. The designed hierarchical porous carbon shows a good electrochemical performance, e.g. a high gravimetric capacitance (Cg) of209F g-1, interfacial capacitance (Cs) of35μF cm-2, and a high capacitance maintance of87%even at high current density of30A g-1.(3) Highly uniform foam-like carbon monolith with mesoporous structure has been synthesized using an ultrasound-assisted air bubbling strategy in self-assembled F127and poly-(benzoxazine-co-resol)-based aqueous emulsions. The air bubbling strategy is easy and green as the use of additional inorganic foaming agent or solid templates is not required. Furthermore, it has been found that an ultrasound treatment initiates the rearrangement of surfactant F127and induces the generation of a defective mesostructure. The synthesized foam-like carbon monolith has been evaluated as an electrode material for supercapacitors. It can retain87%of the initial specific capacitance (151F g-1, at the current density of0.5A g-1) even at the high current density of10A g-1when used as the electrode for supercapacitor.(4) In order to create abundant porosity, TEOS was introduced into the poly-(benzoxazine-co-resol) reaction system. The alkalinity of the reaction system can catalyzd the hydrolysis of TEOS, and then the homogeneous carbon/silica composites were obtained throught a sol-gel process. After the removal of silica by NaOH leaching, the porous carbon monolith with a dual mesoporous structure was obtained. The specific surface area can be reach876～1110m2g-1and the pore volume is in range of0.88～1.16cm3g-1. Used as the lithium battery electrode, such porous carbon presents a high reversible capacitie of660mAh g-1at the current density of100mA g-1. Even at the high current density of3000mA g-1, the reversible capacity can reach215mA h g-1with a stable long-term cycly stability.