Nanoscale Design And Application Of Electrode Materials And Energy Density Enhancement And Their Applications

The development of sustainable clean energy and related energy storage materials is one of the key directions of the future energy field.In a variety of energy storage systems,supercapacitors can operate with a fast charge-discharge rate,high power density,and excellent cycle stability.According to the different energy storage mechanisms,supercapacitors can be divided into two categories:electrostatic adsorption-based double-layer capacitance（represented by activated carbon）and Faraday reaction-based pseudo-capacitance（represented by ruthenium dioxide）.However,the low energy density of supercapacitors limits their market use compared to lithium-ion batteries.China’s 13th Five-Year Plan suggests that the energy density and power density of supercapacitors should reach 50 Wh kg^-1 and 5,000 W kg^-1 respectively,and the required cost should be less than 1.2 yuan/Wh.At present,although the mainstream electrode material activated carbon with cost advantage has a long service life in the market,the energy density in supercapacitors is generally only 3-5 Wh kg^-1,far lower than this technical index.Design criteria for high-performance supercapacitors include high specific capacitance（energy stored per unit mass,volume,and specific surface area）,good rate capability（specific capacitance maintained at large scan rates or current densities）,and cycle stability,while the cost and toxicity of the active material should also be taken into account.Therefore,the development of key electrode materials with low cost,simple synthesis process,especially with large specific capacitance,high power density,and good stability has important academic research value in the energy storage field and has broad prospects and strategic significance in the field of national new energy and sustainable development.This paper embarks from the energy storage formula of the supercapacitor:E=0.5 CV²,with development of high energy-power density supercapacitor electrode materials as the goal,the synthetic methodologies including microwave strengthen,metal-templated method,soluble salt-templated,and dual-templated strategies are developed for the design of electrode materials with improved specific capacitance.The energy density was improved by assembling asymmetric supercapacitors and using high-voltage electrolytes to widen the working voltage window.The practical application of the supercapacitor（such as the application at low temperature）is preliminarily explored.To conclude,the following significant conclusions are obtained in this thesis:1)Microwave-enhanced preparation ofα-phase three-dimensional Ni Co dihydroxide（Ni Co DH）microspheres as cathode materials for high energy density supercapacitors:The energy density was improved by assembling high-capacitance Ni Co DH cathode material with activated carbon into an asymmetric capacitor.The growth process of Ni Co DH under microwave irradiation was investigated by a time-evolution experiment.The results showed that with the increase of microwave heating time,Ni Co DH changed from 3D flower clusters of 8 min to microspheres of 30 min.Electrochemical tests show that the specific capacitance of Ni Co DH electrode is 1120 F g^-1 at 1 A g^-1 and 996 F g^-1 at 10 A g^-1,and the specific capacitance retention rate is 88.9%.At a current density of 10 A g^-1,the specific capacitance reached 122.5%of the initial value after 2000 cycles.The results show that the good stability may be attributed to the synergistic effect of Ni and Co elements,the ion exchange phenomenon between the CNO^-anion and OH^-electrolyte ion,and the continuous and compact three-dimensional structure of Ni Co DH.In addition,asymmetric capacitors are assembled to improve the energy density.The operating voltage range of the asymmetric supercapacitor with Ni Co DH as the positive electrode and coal-derived activated carbon as the negative electrode is 0-1.6V.When the power density is 400 W kg^-1,the energy density is 42.5Wh kg^-1.2)Preparation of graphitized mesoporous onion-like carbons（OLCs）by iron-based template for supercapacitor application:Using a combination of catalytic graphitization and hard template,a novel catalytic graphitization process using biomass gelatin as a carbon source to prepare mesoporous onion carbon with controllable shell for supercapacitor electrode material was reported.Due to the significant cross-linking between gelatin and iron nitrate,the composite aerogels formed iron carbide（Fe₃C）nanoparticles（NPs）in 800℃argon calcination and embedded in the three-dimensional interconnected macroporous carbon framework.Fe₃C NPs can be used as both template and graphitization catalysts to form OLCs with a mesoporous size of about 8 nm after acid washing.Owing to abundant exposed adsorption-desorption sites and rapid ion transport path of the hollow structure,enhanced electron transfer of the graphitized shell,and pseudocapacitance contributed from high oxygen doping,the optimized sample OLC-3 has a specific capacitance of 251.2 F g^-1 at 0.5 A g^-1,and a specific capacitance of 84.1 F g^-1 at a high current density of 50 A g^-1.3)Three-dimensional cross-coupled macro-mesoporous carbon prepared by soluble Na NO₃ salt template method for high-energy-power density supercapacitors:The specific surface area,pore structure,and other properties of porous carbon were further designed and optimized by using template method.By virtue of the solubility and high-temperature decomposability of Na NO₃,the highly dispersed Na NO₃nanocrystals can be used as macroporous hard templates and mesoporous activators in gelatin aerogels when annealing above 600 ^oC.The carbon materials prepared by this method have the following characteristics:i)Highly developed three-dimensional interconnected macro-mesoporous skeleton;ii)Ultrahigh SSA approaching 3000 m² g^-1 with mesopore domination;iii)Narrow mesopore within 2-4 nm,and macropores within 50-150 nm;iv)N‐self doping without the addition of any other nitrogen source;v)Low oxygen content or high C/O ratio.The optimized sample C-0.75-900 has excellent compatibility with the high-voltage EMIBF₄ ionic liquid electrolyte,achieving the goal of high energy-power density at 4 V operating voltage.A high energy density of 92 Wh kg^-1 was obtained at 1 k W kg^-1,and still remained at 39Wh kg^-1 even when the power density increased up to 200 k W kg^-1.4)Synergy of mesoporous carbon electrodes with binary EMIBF₄-GBL anti-freezing electrolyte for high-energy-power density supercapacitors at low temperature:The practical application of mesoporous electrode materials was explored.Nitrogen-doped mesoporous hierarchical porous carbon（NMHC）electrode material was prepared by a dual-salt-template strategy.NMHC has a unique"sandwich"type meso-macro-mesoporous nanostructure,with an ultrahigh specific surface area of 2637.4 m² g^-1 and a pore volume of 1.679 cm³ g^-1.GBL is introduced into EMIBF₄ to form EMIBF₄-GBL binary electrolyte with anti-freezing properties,which maintains the ionic conductivity of 2.3 S cm^-1 at-50°C.The anti-freezing mechanism of binary EMIBF₄-GBL electrolyte was investigated by Raman,¹H NMR,and DFT.It is found that the C=O bond in GBL strongly interacts with the 2H site on the imidazole ring of EMI⁺,which breaks the interionic hydrogen bond between EMI⁺and BF₄^-at the 4H site and weakens the hydrogen bond at the5H site.Because NMHC has a larger specific surface area,a higher proportion of accessible mesopores,NMHC has a good synergistic effect with EMIBF₄-GBL binary electrolyte,and NMHC shows faster electron transfer and ion diffusion kinetics than commercial YP50 microporous activated carbon.At-50°C,a two-electrode coin capacitor composed of NMHC shows an energy density of 61 W h kg^-1 at a power density of 925 W kg^-1,and retains an energy density of 31 W h kg^-1 at a power density of 9250 W kg^-1.