Design And Synthesis Of High Performance Capacitive Carbon Materials And Their Electrochemical Performances

As a new style of eco-friendly energy storage devices,supercapacitors capable of being fully charged or discharged within seconds and running millions of cycles with minimal performance loss have drawn great attention for applications that require high power delivery or fast energy harvesting.However,they store charges predominantly at the interface between solid electrode and liquid electrolyte,which makes the low energy density.So how to improve the energy density of supercapacitors while maintaining high power density is still a key scientific problem in this field.Electrode materials are the key components in supercapacitors,their properties have an important influence on the electrochemical performance.Among the electrode materials in supercapacitors,cabon materials with high specific surface area,abundant pore structure,controlled surface chemisty and good conductivity,are regarded as one of the most widely used commercial electrode materials.Improving the specific capacitance is an effective approach to improve the energy density of supercapacitors.To realize the efficient energy storage of carbon materials,this thesis focuses on designing the structure of the porous carbon materials by developing new pore forming strategies and working on new approaches towards developing nanocomposites,and further the relationship between the microstructures and the electrochemical properties is studied.The thesis elucidates the influence of structure and surface chemistry of the carbons on their electrochemical performance and provides a basis for fabrication of supercapacitors with high energy density,high power density and excellent cycling performance.Specifically,the thesis includes the following parts:(1)By taking advantage of the low boiling point and volatile properties of metal zinc,a complex derived from L-glutamic acid and zinc chloride was employed to synthesize highly microporous carbons via facile pyrolysis.L-glutamic acid,a new carbon precursor with nitrogen functionality,coordinated with zinc chloride resulting in a homogeneous distribution of Zn2+ on a molecular level.During pyrolysis to 910 ?,the evaporation of in situ formed zinc species created an abundance of micropores together with the inert gases.The obtained carbons exhibited high specific surface area(SBET:1203 m2 g-1)and a rich nitrogen content(4.52 wt%).In excess of 89%of the pore volume consisted of micropores with pore size ranging from 0.5 to 1.2 nm.These carbons have been shown to be suitable for use as supercapacitor electrodes,and have been tested in 6 M KOH where a capacitance of 217 F g-1 was achieved at a current density of 0.5 A g-1.A long cycling life of 30000 cycles was achieved at a current density of 1 A g-1,with only a 9%loss in capacity.(2)A new strategy to prepare high performance of capacitive carbons with high surface area,good electrical conductivity,high tap density as well as outstanding electrolyte accessibility to the intra-pore space by using amino acid copper complex as carbon precursors was proposed.The in situ formed CuCl acted as porogens for generation of ultramicropores and supermicropores during carbonization,depending on the ratio of CuCl2 to L-glutamic acid.When the amount of CuCl2 was exactly controlled for coordination of Cu2+ with COO-and NH2-of L-glutamic acid,the formation of atomic COO-Cu and Cu-N ensured the molecular level distribution of Cu2+ and kept nitrogen active sites stable.The evaporation of CuCl from inside out triggered the formation of carbonaceous materials with specific surface area of 860 m2 g-1 and open ultramicropores concentrated at around 0.52 nm even at a temperature as low as 350 ?.When the amount of CuCl2 exceeded the theoretical coordinated proportions,the molten salt effect of the aggregated CuCl particles led to the generation of supermicropores of 0.72 nm and 1.12 nm with surface area up to 2051 m2 g-1 while maintaining 3.22 wt%nitrogen content and a pack density of 0.35 g cm-3 after pyrolysis at 900 ? and removal of Cu species residue with hydrochloric acid.Moreover,the in situ produced metallic Cu features of catalytic conversion of amorphous carbon to sp2-hybridized carbon,which facilates improve the conductivity of the carbon materials.Besides,the extraction of CuCl2 from the filtrate by simple distillation made it possible to achieve cyclic utilization of copper salt.Such capacitive carbons contributed a specific capacitance of 273 F g-1 at 0.5 A g-1 and a charge transfer of 0.21 ? with a cycle life over 20000 cycles with perfect capacitance retention as a supercapacitor electrode.(3)The carbon materials were created by using the unique carbon precursor synthesized through coordination of L-glutamic acid with boric acid,which enabled the incorporation of nitrogen and boron to the carbon matrix,along with the creation of defective sites in the carbon frameworks.The materials were applied as electrodes materials exhibiting pseudocapacitive behaviors obviously.Boron doping shows catalytic effect on oxygen chemisorption on carbon surface,the content of oxygen in the material was as high as 13 at%;nitrogen functional groups are considered to not only generate pseudocapacitance in the mechanisms of acting as electron donor to attract protons or enhancing charge density of space charge layer,but also strengthen oxidation/reduction of quinone.During pyrolysis the in-situ formed B2O3 were acted as molten salt templates to form micropores and mesopores,which depended on the ratio of L-glutamic acid to H3BO3.When the amount of H3BO3 was exactly controlled for coordination with L-glutamic acid,the boron species were highly dispersed in the complex,thus the materials were dominated by micropores after washing with hot water;when the amount of H3BO3 exceeded the theoretical coordinated proportions,the molten salt effect of the aggregated B2O3 particles led to the generation of mesopores.The optimized material delivered a remarkable capacitance(675 F g-1 at 0.5 A g-1)and extraordinary rate performance(61%capacity retention at 100 A g-1)in 1 M H2SO4 electrolyte as results of affording efficient redox reactions and the high electric double-layer capacitance.