Preparation And Electrochemical Properties Of Functional Carbon Materials

Electrode material is one of the most important factors affecting the energy storage of supercapacitors.Carbon materials is widely used as electrode material for supercapacitors.Electrochemical double layer capacitors based on carbon electrode materials have been difficult to break the bottleneck of their low specific capacitance and energy density.There are two main methods to improve the capacitance of carbon materials:（i）carbon materials as the modified components or carriers to composite with other active materials;（ii）the chemical modification of the carbon material itself.This paper mainly focuses on the design and preparation of functional carbon materials and their electrochemical properties.On the one hand,electroactive polyaniline was used to modify carbon film with interfacial affinity.The specific capacitance of the carbon film material was increased by introducing electroactive polyaniline.On the other hand,heteroatom was used to modified carbon fiber material.The performance of carbon fiber capacitance was improved by the introduction of pseudocapacitance from containing oxygen,nitrogen and sulfur functional group.The main research ideas and contents of this paper are as follows:（1）The synthesis and electrochemical performance study of PANI@FC@NFThe electroactive crack-free carbon film material was designed and synthesized.Through hydrothermal carbonization and thermal carbonization of glucose,complete carbon film was coated on the surface of nickel foam（NF）substrate to prepare full carbon film（crack-free）coated nickel foam（FC@NF）.Then the electrochemical active polyaniline layer was deposited on the surface of FC@NF to prepare the FC@NF electrode modified by polyaniline（PANI@FC@NF）.The electrochemical test of FC@NF and PANI@FC@NF was carried out in 0.5 M H₂SO₄ electrolyte.At the current density of 1 m A cm^-2,the specific capacitance increases from 28.7 m F cm^-2 of FC@NF to 610 m F cm^-2 of PANI@FC@NF.At the current density of 2.0 m A cm^-2,the capacitance retention of PANI@FC@NF was 77.63%after 1000cycles.Based on polyvinyl alcohol-H2SO4 gel electrolyte and PANI@FC@NF electrode material,a solid symmetrical supercapacitor was constructed.At a power density of 0.9 m W cm^-2,the energy density of PANI@FC@NF supercapacitor was 41μWh cm^-2.At current density of 5 m A cm^-2,after 1000 cycles,the capacitance retention of PANI@FC@NF supercapacitor was 85.6%.The results show that the PANI@FC@NF electrode achieves high cycling stability in an acidic electrolyte based on the interfacial protection of a crack-free carbon film.Based on the interfacial affinity of the crack-free carbon film to combine polyaniline,the PANI@FC@NF electrode achieved high capacitance performance.（2）The preparation and electrochemical performance study of activated carbon fiberCarbon fiber materials with electroactive oxygen-containing functional groups on the edge of carbon skeleton were designed and synthesized.Activated carbon fiber(TE_ROTCF)electrode materials were prepared by combining electrochemical activation with thermal activation.At the current density of 1 m A cm^-2,the specific capacitance of TE_ROTCF was1426 m F cm^-2,while that of non-activated carbon fiber was only 709 m F cm^-2.When the current density increased from 1.0 m A cm^-2 to 10 m A cm^-2,the capacitance retention of activated carbon fibers was 66.62%,while that of non-activated carbon fibers was only0.85%.The carbonyl group was introduced at the edge of the carbon skeleton can perform a reversible Faraday reaction and provide additional pseudocapacitance,so TEROTCF shows high capacitance performance.Thermal activation improves graphitization and electron conductivity of carbon fiber,so TE_ROTCF exhibits good rate capability.Based on polyvinyl alcohol-KOH gel electrolyte and TE_ROTCF electrode material,a solid symmetrical supercapacitor was constructed.At a power density of 125.0μW cm^-2,the energy density of PANI@FC@NF supercapacitor was 26.0μWh cm^-2.The results showed that the activated carbon fiber(TE_ROTCF)containing carbonyl group shows excellent electrochemical capacitance and rate capability.（3）The preparation and electrochemical performance study of S/N co-doped activated carbon fiber electrodeIn order to further improve the capacitance of carbon fiber materials,S/N co-doped activated carbon fiber were designed and synthesized.Hydroxyl-rich carbon fiber（CF-OH）was prepared by electrochemical activation.S/N-ACF was prepared by the thermal treatment of thiourea bonded hydroxyl-rich carbon fiber（CF-OH）.As a contrast,the sulphur/nitrogen co-doped carbon fiber（S/N-CF）is prepared by the thermal treatment of thiourea adsorbed carbon fiber.S/N-ACF shows higher amount of S/N doping（4.56 at.%N and 3.16 at.%S）than S/N-CF（1.25 at.%N and 0.61 at.%S）.CF-OH can bond the decomposition products of thiourea（CS₂,H₂NCN and NH₃）through hydrogen bond of-NH···O and-S···OH during thermal treatment process,thus causing high doping degree of S/N-ACF.Accordingly,the specific capacitance increases from 1196 m F cm^-2 for S/N-CF to 2704 m F cm^-2 for S/N-ACF at 1 m A cm^-2.The rate capability increases from 54.48%for S/N-CF to 64.71%for S/N-ACF at a current density increase from 1 to 40 m A cm^-2.High content of nitrogen and sulfur functional groups were introduced into the carbon skeleton edge of activated carbon fiber to perform reversible Faraday pseudoreaction and provide pseudocapacitance.S/N-ACF shows excellent capacitance performance.Its carbon skeleton contains more heterocyclic nitrogen and heterocyclic sulfur,which is conducive to providing more delocalized electrons and enhancing the electronic conductivity,thus S/N-ACF shows excellent rate capability.Based on polyvinyl alcohol-H₂SO₄ gel electrolyte and S/N-ACF electrode material,a solid symmetrical supercapacitor was constructed.At a power density of 350μW cm^-2,the energy density of S/N-ACF supercapacitor was 184.7μW h cm^-2.The results suggest that S/N-ACF with high S/N co-doping degree showed better capacitance and rate capability.