Study On Fabrication And Properties Of High Performance Porous Nano-Carbon Aerogels

Carbon nanomaterials-based aerogels possess three-dimensional(3D)connected conductive networks and good mechanical properties,thus could serve as self-supporting electrodes,with broad application prospects in new energy storage systems such as supercapacitors and rechargeable batteries.However,as electrodes carbon nanomaterials show low specific capacitance and rate capability due to their unoptimized microstructures.To this end,researchers usually enhance the performance by methods such as compositing carbon structures with pseudo-capacitive materials or optimizing the microstructures of porous carbon electrodes.Due to the introduction of pseudo-capacitive materials,there are still problems,such as the decreases of cycling stability,and difficult to precisely adjust the microstructure of the porous carbon on sub-nanometer scale.In terms of the above issues,this thesis developed porous nano-carbon aerogels with advanced microstructures to serve as self-supporting electrodes of supercapacitors and potassium ion batteries.Excellent electrochemical properties were obtained through a systematic and detail study with corresponding theoretical simulation and calculation.High specific capacitance and significantly improved cycling stability were achieved through designing and fabricating carbon nanotube(CNT)/conducting polymer hybrid sponges with core-shell and double-sheathed core-shell structures.A high performance porous-carbon nanotube(PCN)structure was developed which showed ultrahigh rate capability and ultrafast charging properties as supercapacitor electrodes,and ultrastable performance in K ion batteries.The mechanism of K ion storage in sub-nano pores was revealed by theoretical calculations.Based on a 3D porous CNT sponge with even deposited and thickness-controlled polyaniline(PANI)coating,that is a core-shell CNT/PANI sponge,high specific capacitance(746 F/g)and good rate capability were obtained.Meanwhile,PANI shell could enhance mechanical properties and conductivity of the CNT sponge,and the hybrid sponges not only achieved high volumetric capacitance(632 F/cm~3)after densification,but also served as sensitive and stable strain sensors and reinforced epoxy-based nanocomposites with high conductivity and mechanical properties.To inhibit the weak cycling stability of conducting polymers,a hybrid sponge with double sheathed core-shell structures was prepared by depositing different polymers on CNTs.Two double-sheathed structures,CNT/PANI/polypyrrole(PPy)and CNT/PPy/PANI,were obtained which possess high specific capacitance and stable cycling performance.Mechanism study showed synergistic effect between coated PANI and PPy layers during charging-discharging process,which could improve the cycling stability.Electrical double-layer(EDL)capacitor performances mainly depend on specific surface area and pore structures which can provide ion adsorption sites,so designing porous carbon with controlled micro-structures is a prerequisite to construct high performance electrode materials.This thesis proposed a pore-creation method combining dilute nitric acid treatment and air-etching process to prepare 1D PCN structures consisting of tube-walls with evenly and densely distributed pores and highly centralized pore-size,from the PPy nanotube aerogel precursors.This unique micro-structure obtained specific capacitance of 193 F/g(1 A/g)as a supercapacitor electrode,and enabled ultra-fast charge and discharge under high rate of 1000 A/g,showing high specific capacitance(120 F/g)and high power density(265 kW/kg),and the ultrafast charging properties are very stable,possessing 88%retention even after 100 thousand cycling tests.Mechanism study revealed favorable kinetics for this high rate performance,which is high contribution from EDL capacitance(85%).Carbon materials possess high theoretical capacity and energy density as PIB anodes,but the process of K ion insertion into carbon layers is relatively sluggish and causes large volume expansion,which are not good for rate capability and cycling performance.This thesis for the first time studied the effect of sub-nano pores on K ion storage mechanism.Through optimizing the micro-structures of PCN and precisely adjusting their pore-size and pore volume,and then systematic electrochemical tests,PCN with high sub-nano pore volume could achieve capacity of 253.7 mAh/g(50 mA/g)and excellent rate capability.Meanwhile,the electrode exhibited very stable long cycling performance under a wide rate range(from 70 mA/g to 5 A/g).Mechanism analysis showed that the excellent rate performance was associated with a dominating surface capacitive-controlled process.The microstructures of PCN were simulated by first principles calculations.The results showed that sub-nano porous structures(at 0.8 nm)possessed very small diffusion energy and low variation of lattice constants,indicating their ultrafast charging properties and stable long cycling performance.The revealed effect of sub-nano pores on K ion storage could offer a potential route for designing and developing high performance PIB anode materials.