| Super capacitor is an efficient energy storage device, which is easy to use, comparedwith other energy storage devices. It possess high specific capacity, high power density,long cycle life, environment friendly and many other advantages, which has a broadprospects for development and has become the hot spot of scientists from all over the world.Studies have shown that the supercapacitor electrode material is the key factors affecting itsperformance, and electrolyte properties also affect the nature of supercapacitors. A coal tarpitch (CTP), which is the byproduct of coking enterprises processing process, is used assupercapacitor precursor because it is cheap with high carbon yield. This work explores theeffects of pore structures of carbon materials from all kinds methods on electrodeperformance.The research using tannic acid as electrolyte additives is expected to greatlydecrease the cost of the preparation of electrode materials and improve specific capacitance.In the research process, all kinds of test methods such as scanning electron microscopy,transmission electron microscopy, nitrogen adsorption/deadsorption, cyclic voltammograms,constant current charge, discharge and electrochemical impedance have been used. Themain results were as follows:(1) Porous carbon materials were prepared from coal tar pitch by zinc oxide whisker ashard template and chemical vapor deposition method. The results showed that the porouscarbon materials with rich microporous structure, and the degree of graphitization wasreduced. The pore size is3.4nm and BET surface area is531.0m2g-1. Cyclicvoltammograms test showed its capacitance is mainly given by electric double layer.Constant current charge and discharge curve possessed good symmetry. The specificcapacitance is140.3F g-1in the current density of1A g-1. Electrochemical impedanceshowed contact resistance was1.07Ω and excellent capacitance characteristic in lowfrequency area.(2) Pitch-based carbon electrode materials with high capacitance by using nano-sizedoxides as templates and combination with KOH activation method. The preparationconditions of electrode materials were optimized with orthogonal array design (OAD). The results showed the kinds of template agent have a significant impact on the iodineadsorption value. The synthetic carbon material possessed the highest iodine adsorptionvalue under the optimum conditions, which Al2O3was used as a template, activationtemperature of700°C, CTP/template mass ratio of1:1, respectively. The main factorsinfluencing the specific capacitance of the materials are the surface area and mesoporousvolume ratio. The carbon materials have the greatest iodine adsorption value of2238mg·g-1and the greatest surface area of2071.6mg·g–1when micropororous and mesoporous volumeratio is0.94and the pore size is2.25nm. At the same time, specific capacitance alsoreached a maximum231F·g–1at current density of1A·g-1. Constant current charge anddischarge curve possess good symmetry. This suggests that material has goodelectrochemical capacitance characteristics. The material has good cycle stability becausespecific capacitance remains93.2%after cycle500times. Ac impedance shows contactresistance is1.1Ω. So the carbon materials of Al-1-1-700is very suitable for supercapacitor areas.(3) Pitch-based carbon materials were prepared by using nano-sized calcium carbonateas templates, which produced many pores using itself placeholder and pyrolysis. Theiriodine adsorption increased significantly in compared to the carbon materials withoutadding calcium carbonate. The carbon materials with the mass ratio of2:1CTP and calciumcarbonate (CTP-Ca-2-1) have the maximum iodine adsorption value of1728mg g-1. Furtherresearch shows that the BET surface area of CTP-Ca-2-1is1336m2g1with pore sizebeing mainly between0.5to3nm. The material is rich in micropores and mesoporestructure with mesopore volume fraction of64.65%. Electrochemical properties showed thespecific capacitance of the carbon materials with calcium carbonate template have highervalue than without template. The specific capacitance of CTP-Ca-2-1has the largest valueof209F g-1at the current density of0.4A g-1. Ac impedance curves suggest the contactresistance of CTP-Ca-2-1is1.2Ω, indicating its good electrical conductivity. Cycle life testshows that specific capacitance remains92.54%after1000cycling times, which showed anexcellent cycle stability.(4) Carbon materials were prepared by the pyrolysis of coal tar pitch and lignin. Theporosity and oxygen content of the materials were improved when lignin pyrolyzed. The results showed that oxygen content of the carbon materials has been greatly improved. Thelignin pyrolysis are slowly weightless in wide range. This phenomenon is conducive todevelopment and extension of channel. The results also shows that the carbon materialsfrom the mass ratio of lignin and coal pitch is1:2(CAC-1-2) has the largest iodineadsorption value1915mg g-1and specific capacitance189F g-1. Electrochemical analysisshowed that CV curves of CAC-1-2is close to rectangular and constant current charge anddischarge curve has good symmetry, which shows that the material has goodelectrochemical capacitance characteristics.The specific capacitance has the maximumvalue of189F·g–1at the current density of0.5A·g–1. The specific capacitance can remain95.8%after500cycle times, which suggest CAC-1-2has the good cycle stability. However,the carbon materials from high lignin content, have poor performance of ac impedance.CAC-1-2is ideal super capacitor materials.(5) Coal tar pitches (CTP) are used as precursor of electrode material because of theirlow cost and high carbon yield. In this study, CTP-based activated carbon materials (CACs)with high capacitance for supercapacitors were prepared by doping elemental N withco-carbonization of CTP and melamine. The electrochemical performance and texturalcharacterization of CACs were investigated by constructing a single electrode capacitor in a6M KOH electrolyte, N2adsorption, scanning electron microscopy, and XPS. Results showthat CACs with mass proportion of melamine and CTP being1:5possess a surface area of2573m2g1, with mesopore volume accounting for56percent of the total volume. Afterco-carbonization, N content on the surface of the CACs increased by134%compared tothat of undoped activated carbons. Their capacitance reached up to228F g-1at a currentdensity of1A g-1. The high electrochemical performance of the CACs is attributed to itsadditional pseudo-capacitance and high surface area. The specific capacitance stillpossessed94.2%retention after1000cycles, indicating good cycle stability.(6) Manganese oxide, which was prepared by pyrolysis at550°C, was doped intoactivation carbon materials from coal tar pitch (CAC). Electrochemical properties showedthat the composites materials doped with different proportion of manganese oxide cangreatly increase specific capacitance of the composite materials. The composites materialfrom the mass ratio of CAC and manganese oxide being4:1(CAC-Mn-4-1) has the highest specific capacitance value, which increased by60.1%and68.6%compared with CAC at acurrent density of0.5and1A g-1, respectively. It illustrates faradaic pseudocapacitancefrom manganese oxide greatly increased the specific capacitance of the composite material.The specific capacitance value of CAC-Mn-4-1is243.5F g-1at a current density of0.5Ag-1. Ac impedance test showed that C/Mn composites possess better capacitor characteristicsthan CAC in low frequency regions. Cycle life test shows that the capacitance remains91.5%after1000times, which showed CAC-Mn-4-1has good cycle stability. Energydensity of the symmetry capacitor assembled with CAC-Mn-4-1has a good retention ratiowith the increasement of power density. Its energy density is greater than11.5Wh Kg-1,superior to the general carbon materials. Therefore, CAC-Mn-4-1is suitable for applicationin the field of super capacitor.(7) Tannic acid as electrolyte additive was first studied using commercial activatedcarbon as working electrode. The results showed that tannins can obviously improve thespecific capacitance of the capacitor. The specific capacitance of the capacitor increased by22.4%at the current density of0.5A g-1when the tannic acid concentration is5%than0%.Research showed that the increasement of specific capacitance is mainly due to oxidationreduction reaction from the structure of the adjacent phenolic hydroxyl/quinone. Cycle testshowed the capacitance remained93.5%at tannins-5%-KOH electrolyte after1000cycletimes. Electrochemical impedance showed that tannins addition is not conducive toresistance and capacitive properties, which is possible that because tannic acid reduced theconductive ability of electrolyte. The contact resistance increased only by5%after1000times, indicating that1000times have little impact on ac impedance properties of thecapacitor. Tannins as the electrolyte additives is more advantageous to reduce the cost dueto its cheap price. |
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