【作者】 曲群婷；

【导师】 吴宇平；

【作者基本信息】 复旦大学 ， 物理化学， 2010， 博士

【摘要】 按照储能机理的不同,超级电容器可以分为以下三种：双电层电容器、法拉第赝电容器和混合型超级电容器。双电层电容器主要是通过在电极和电解质之间的界面上所形成的双电层电容来储能,该类电容器具有很高的功率密度和极好的循环性能。法拉第赝电容器,主要是通过在电极的表面或近表面发生快速可逆的化学吸附／脱附或氧化还原反应来储能,该反应的特点是有法拉第电流产生,其理论比电容和能量密度比双电层电容器高出10-100倍。混合型超级电容器的两个电极分别采用不同的储能机理,其中一个电极选用赝电容类或二次电池类电极材料,另一电极选用双电层电容类碳材料。因此混合型电容器可同时具有法拉第赝电容器或二次电池的高能量密度和双电层电容器的高功率密度。与有机电解液相比,水系电解液具有更高的离子电导率,因此其功率性能远远优于有机电解液。就能量密度而言,可从提高体系的工作电压U和放电比电容C两方面来提高能量密度E。有机电解液的工作电压通常能达3V。水系电解液的理论工作电压为1.23V,通过提高H2和O2析出的过电位可将工作电压提高到2V。对于放电比电容C,电容器类电极材料在水系电解液中的电容值通常高于有机电解液。综合考虑到电压和比电容值,水系电解液的能量密度则可能超过有机电解液。此外,与有机电解液相比,水系电解液价格更加便宜,安全性更好,环境污染小,且操作方便。总体看来,水系电解液更适合用作超级电容器的电解液。由于强酸性和强碱性水溶液对环境仍有较大的污染,而且基于这两类水系电解液的电容器仍存在价格昂贵、物质在电极沉积、爬碱、电解液消耗、电极稳定性差而影响能量密度和循环寿命等问题。因此本工作着重研究基于中性水溶液的混合型超级电容器。本工作以提高混合型电容器的功率密度、能量密度和循环性能为根本目的,一方面从法拉第反应的电极材料入手,制备具有高比表面积、结构稳定的纳米材料,提高材料的能量、功率密度和长期稳定性,另一方面对不同种类中性水溶液对电极材料电化学性能的影响进行研究,从而优选出最佳电解液,并对电极材料的反应机理进行分析研究。(1)通过对比活性炭在0.5mol/L Li2SO4、Na2SO4和K2SO4水溶液中的电化学性能,发现其倍率性能在K2SO4中最好,而在Li2SO4中最差。交流阻抗结果显示活性炭电极在三种电解液中的等效串联电阻以Li2SO4的顺序递减,说明水合离子在电解液本体和活性炭电极孔内部的迁移速度是按照Li+<Na+<K+的顺序逐渐增加,因此活性炭电极在K2S04中的倍率性能最好。此外,最初的电化学循环能够有效活化电极,使得离子在活性炭孔内的动力学扩散更容易进行。与Li盐相比,K盐和Na盐在自然界中存在更丰富,且更廉价易得,因此基于K+和Na+的水系电解液比Li+更适合在电容器中应用。(2)通过溶液共沉淀法制备了比表面积高和结晶度非常低的δ-MnO2纳米棒,对该材料在Li2SO4、Na2SO4和K2SO4水溶液中的电化学性能进行对比。在低扫速下,由于Li+半径最小,Li+在MnO2固相中可逆的嵌／脱反应产生了额外的容量,使得MnO2在Li2SO4水溶液中的比电容最高(达201F／g)。在高扫速下,MnO2电极在K2SO4水溶液中的比电容最高,这可归因于K+的水合离子半径最小和离子电导率最高。交流阻抗结果显示MnO2电极在K2SO4水溶液中的电化学电阻最小。所组装成的AC/K2SO4/MnO2混合电容器可在0-1.SV进行可逆的充放电循环。在2kW/kg的功率密度下,能量密度有17Wh/kg,高于对称型电容器AC/K2SO4/AC和文献中报道的混合电容器AC/Li2SO4/LiMn2O4。同时显示出极好的循环性能,在没有除氧的条件下,23000次循环后电容损失小于6%。(3)为进一步研究δ-MnO2的电化学反应机理,通过高温固相法制备了同属6晶型并具有较高结晶度的KxMnO2·nH2O纳米材料,并对比了该材料在Li2SO4、Na2SO4和K2S04电解液中的电化学行为。结果显示电极材料在三种电解液中分别发生Li+、Na+和K+的嵌／脱反应,而且KxMnO2·nH2O电极的倍率性能和循环性能都是在K2SO4中最好,Li2SO4中最差,Na2SO4中居中。其倍率性能顺序与该材料在三种电解液中的电化学电阻大小相关。其循环性能顺序与该材料在三种电解液中充放电结束时晶格参数的变化剧烈程度有关。AC/K2SO4/KxMnO2·nH2O混合电容器的能量密度稍微低于前面工作中的AC/K2SO4/MnO2纳米棒混合电容器,这是由于MnO2纳米棒的颗粒尺寸明显小于KxMnO2·nH2O颗粒,从而具有更多的表面活性位置发生反应。(4)由于基于MnO2材料的混合电容器的能量密度与电池相比,仍然较低,而具有更高放电容量的尖晶石LiMn2O4材料则可能提高电容器的能量密度,同时为了提高LiMn2O4的倍率和循环性能,本工作制备了由纳米晶粒围成的有序大孔结构的LiMn2O4并将其用于水系电解液中。该材料在0.5mol/L Li2SO4水溶液中的可逆容量达118mAh/g。AC/Li2SO4/大孔LiMn2O4混合电容器的能量密度达42Wh/kg,并显示出极好的高倍率性能。在10000循环后放电容量损失小于7%,这可归功于大孔LiMn2O4材料的结晶度高、电极极化小、结构稳定度好、Mn溶解量低和有序多孔结构等因素。(5)通过水热法制备了结晶度较高的V2O5·0.6H20纳米带,对其作为混合型电容器正极材料的性能进行了探索和初步研究。发现该材料在K2S04水溶液中的电化学反应是基于K+在层间结构中可逆的嵌／脱反应。AC/K2SO4/V2O5·0.6H2O混合电容器能在0-1.8V间进行可逆的充放电循环,能量密度达29Wh/kg,并显示出非常好的倍率性能,但是该混合电容器的循环性能较差,应进一步对V2O5·0.6H20材料进行改性提高其循环性能。更多还原

【Abstract】 According to charge storage mechanisms, supercapacitors can be divided into three types, i.e., electrochemical double layer capacitors (EDLCs), faradic pseudo-capacitors, and hybrid supercapacitors. EDLCs store energy through the charge separation at the electrode/electrolyte interface, and they show very high power density and excellent cycling performance. Pseudocapacitors characterized by the presence of faradic current, store energy by using the fast chemical absorption/desorption or redox reactions on the electrode surface, and their theoretical specific capacitance and energy is 10-100 times higher than that of EDLC. The two electrodes used in hybrid supercapacitors store energy in different manners. One of them uses pseudocapacitors type or secondary battery type electrode materials, and the other electrode uses EDLCs type carbon materials.The power capability of aqueous electrolyte is much higher than that of organic electrolyte due to their higher ionic conductivity. In the case of energy density, it can be improved by increasing the working voltage (U) and specific capacitance (C). The working voltage of organic electrolyte can be up to 3V. The theoretical working voltage of aqueous electrolyte is 1.23V, which can be widened to 2V by increasing the overpotentials of H2 and O2 evolution. As for the specific capacitance, the capacitor-type electrode materials usually shows higher capacitance values in aqueous electrolyte than in organic electrolyte. Based on the above consideration, the energy density of aqueous electrolyte may exceed organic electrolyte. Moreover, aqueous electrolytes are cheaper, safer, and more environmentally friendly than organic electrolytes. Therefore, aqueous electrolyte seems to be more suitable as electrolyte for hybrid supercapacitors.Since the strong acidic and alkaline aqueous electrolytes pose environmental pollution, and the capacitors based on them still have the problems of high-price, precipitation of materials on the electrode, electrolyte consumption, poor electrode stability, low energy density, and poor cycling life, etc., here we focus on the investigation of neutral aqueous electrolytes based hybrid supercapacitors.The aim of this dissertation is to improve the power density, energy density and cycling performances of hybrid supercapacitors. On the one hand, nano-scaled faradic electrode materials with high specific surface area and structural stability were prepared to improve the power density, energy density and cycling performances of hybrid supercapacitors. On the other hand, the influence of various neutral aqueous electrolytes on the electrochemical performances of electrode materials was investigated, from which the optimal electrolyte was selected and the reaction mechanism of electrode material was analyzed.(1) The electrochemical performances of activated carbon in 0.5 mol/L Li2SO4, Na2SO4, and K2SO4 aqueous electrolytes were investigated. The equivalent series resistance (ESR) obtained from Nyquist plots decreases in the order of Li2SO4> Na2SO4>K2SO4, signifying that the migration speeds of the hydrated ions in the bulk electrolyte and within the inner pores of AC electrode increase in the order of Li+< Na+<K+. As a result, the rate behaviors of AC in the three electrolytes improve in the order of Li2SO4<Na2SO4<K2SO4. The initial electrochemical cycles can activate the electrode and make the dynamic dispersion of ions within the pores much easier compared with that prior to cycling. In addition, as the electrolyte salts based on Na+ and K+ are more easily available and cheaper than those of Li+, the aqueous electrolytes based on K+ and Na+ salts are more suitable as electrolytes for capacitor application.(2) 8-MnCO2 nanorods with high surface areas and very low crystallinity were prepared from solution precipitation method and their electrochemical performance was investigated in Li2SO4, Na2SO4, and K2SO4 aqueous electrolytes. CV results show that at the slow scan rates, MnO2 shows the largest capacitance (201 F/g) in Li2SO4 electrolyte since the reversible intercalation/deintercalation of Li+ in the solid phase produces an additional capacitance besides the capacitance based on the absorption/desorption reaction. At the fast scan rates, MnO2 shows the largest capacitance in K2SO4 electrolyte due to the smallest hydration radius and highest ionic conductivity of K+. The assembled AC/K2SO4/MnO2 hybrid supercapacitor could be cycled reversibly between 0 and 1.8 V with energy density of 17Wh/kg at 2kW/kg, much higher than those of AC/K2SO4/AC supercapacitor and AC/Li2S04/LiMn204 hybrid supercapacitor. Moreover, this supercapacitor exhibits excellent cycling behavior with no more than 6% capacitance loss after 23,000 cycles at 10C rate even the dissolved oxygen is not removed.(3) In order to investigate the electrochemical reaction mechanism ofδ-MnnO2 more clearly, KxMnO2·nH2O with high crystallinity was prepared by solid-state method and its electrochemical behaviors in Li2SO4, Na2SO4 and K2SO4 aqueous solutions were compared. Results show that the electrode reaction of the KxMnO2·nH2O electrodes in the above three electrolyte involves the intercalation/deintercalation of Li+, Na+, and K+, respectively. Both the rate behavior and cycling performance of KxMnO2·nH2O electrodes in the three electrolyte are the best in K2SO4 solution and worst in Li2SO4 solution. The sequence of rate behavior is related to the electrochemical impedance of KxMnO2·nH2O electrodes in the three electrolytes. The sequence of cycling performance is related to the degree of the structural change of the electrode material at the end of charge and discharge. The energy density of AC/K2SO4/KxMnO2·nH2O hybrid supercapacitor is a little lower than the former work based on AC/K2SO4/MnO2 nanorods. This is because the particle size of MnO2 nanorods is much smaller than that of KxMnO2·nH2, thus possessing more active sites for electrode reaction.(4) As the energy density of the hybrid supercapacitors based on nano MnO2 materials at the low power density is still lower than that of current batteries, and spinel LiMn2O4 can discharge more capacity, here we prepared macroporous LiMn2O4 materials and use them in aqueous electrolyte. The discharge capacity of porous LiMn2O4 in Li2SO4 solution can be up to 118mAh/g. The assembled AC/Li2SO4/porous LiMn2O4 hybrid supercapacitor shows a high energy density of 42Wh/kg, and exhibits excellent rate behaviors. This supercapacitor also show excellent cycling performance with no more than 7% capacity loss after 10000 cycles, which can be ascribed to the high crystallinity, good structural stability, porous structure and nanograins of the prepared LiMn2O4.(5) V2O5-0.6H2O nanoribbons with high crystallinity were prepared and their electrochemical behaviors as cathode materials for the hybrid supercapacitors were investigated primarily. Results show that K+ ions can intercalate/deintercalate reversibly in the V2O5·0.6H2O interlayer space. AC/K2SO4/V2O5·0.6H2O hybrid supercapacitor was successfully assembled, which can be cycled reversibly in the voltage region of 0-1.8V and presents an energy density of 29Wh/kg. Although this supercapacitor shows very good rate behavior, its cycling behaviors is not good. Further works on the modification of V2O5·0.6H2O materials are needed to improve its cycling performances.更多还原