Preparation And Performance Improvement Of Hierarchically Porous Carbons For Supercapacitors

Activated carbons have been the most widely-used electrode materials for supercapacitors due to their good conductivity, excellent physicochemical stability, low cost, and so on. Curve and irregular micropores occupy the major part of the pores in activated carbons, in which the ion transportation rate is very slow and some of which even can not be accessible to the electrolyte ions, leading to the restriction of their effective utilization, rate capability and power density. But if the pore structure of the carbon materials can be designed properly, in which macropores can be served as ion-buffering reservoirs to minimize the diffusion distances, mesopores can promote the electrical double layer capacitance and decrease the ion-transport resistance, and micropores can provide the main sites for the formation of the electrical double layer capacitance, then the high-performance electrode materials for supercapacitors can be obtained. So in this thesis, hierarchically porous carbons with the combination of micropores, mesopores and macropores have been designed and prepared, and further optimization and modification of the hierarchically porous carbon materials have also been made. The main work in this thesis is as follows:(1) Chromium carbide-derived carbons with hierarchically porous structure and surface functionalities have been prepared through chlorination at 700 oC for 3 h and have been further treated with 16 mol L-1 HNO3. Besides, the effects of activation condition on structure and electrochemical performances of the prepared carbons have been investigated. The results show that the prepared chromium carbide-derived carbons own hierarchically porous structure with a combination of micropores, mesopores and macropores. After the nitric acid treatment, the specific surface area and the pore volume of the modified hierarchically porous chromium carbide-derived carbons are decreased, but their electrochemical performances are improved due to the surface functionalities which can enhance the wettability and pseudocapacitance. The C(Cr3C2)-M100 treated with nitric acid at 100 oC obtains the specific capacitances of 209 F g-1 and 206.5 F g-1 at the scan rate of 5 mV s-1 and the current density of 1 A g-1, respectively. Furthermore, the supercapacitor using C(Cr3C2)-M100 as the electrode material can keep the specific capacitance retention of 90 % even after 5000 cycles.(2) Hierarchically porous carbon spheres(HPCMSs) with interconnected pore networks have been prepared via a template method under the hydrothermal condition, using the furfuryl alcohol as a carbon resource, cetyltrimethyl ammonium bromide, polyacrylic acid and tetraethyl orthosilicate as templates. Besides, the effects of reactant ratio on the structure and electrochemical properties of the prepared carbons have been further investigated in detail. The results show that the HPCMS-2 sample prepared at the mole ratio of 2:1 presents the largest specific surface area of 709 m2 g-1 and the best electrochemical performances. Moreover, its specific capacitances are as high as 171 F g-1 at the scan rate of 5 mV s-1 and 221 F g-1 at the current density of 1 A g-1. The supercapacitor using HPCMS-2 as the electrode material still keeps the specific capacitance of 50 F g-1 even after 5000 cycles.(3) Three-dimensional hierarchically ordered porous carbons(3D HOPCs) have been successfully prepared through template method using silica sphere nano-array as a hard template, triblock copolymer PEO20PPO70PEO20(P123) as a soft template and sucrose as a carbon resource. Moreover, the effects of sizes of silica particle templates on structure and electrochemical properties of the 3D HOPC materials have also been studied. It can be found from the results that 3D HOPC-80 using silica spheres with the size of 80 nm as template obtains the largest specific surface area of 709 m2 g-1 and the best electrochemical performances. The specific capacitances of 3D HOPC-80 are as high as 243.5 F g-1 and 247 F g-1 at the scan rate of 5 mV s-1 and the current density of 1 A g-1, respectively. The supercapacitor using 3D HOPC-80 as electrode material holds high specific capacitance retention of 91 % after 10000 cycles.(4) Nitrogen-doped three-dimensional hierarchically porous carbon(N-3DHPC) with functionalities on the surface has been prepared by the carbonization of polyaniline covered on the three-dimensional macroporous carbon and the following KOH activation. The results show that the N-3DHPC inherits the morphology of the pristine three-dimensional macroporous carbon and processes a high specific area of 1084.0 m2 g-1. The supercapacitor using N-3DHPC as the electrode material obtains the specific capacitance of 77.1 F g-1 at the current density of 1 A g-1, and keeps high specific capacitance retention of 96 % even after 10000 cycles. Furthermore, the N-3DHPC supercapacitor obtains the energy density of 10.7 Wh kg-1 at the power density of 500 W kg-1 and still maintains the high energy density of 9.0 Wh kg-1 when the power density increases to 5000 W kg-1.