【作者】 汪形艳；

【导师】 王先友；

【作者基本信息】 湘潭大学 ， 材料学， 2011， 博士

【摘要】 超级电容器又称电化学电容器,它结合了传统电容器高比功率和二次电池高比能量的优点,在电动汽车、航空航天、移动通信、国防和消费性电子产品等领域有着广阔的应用前景。电极材料是决定超级电容器性能的主要因素,因此电极材料的研究一直是该领域的热点方向。本文采用反相微乳法及常温常压干燥技术、氧化-活化技术制备出高性能碳微球（CMB）,提出常温常压干燥制备碳微球的新方法,取代传统的超临界干燥法,解决了制备工艺复杂、成本高、性能难以控制的技术难题。利用扫描电镜（SEM）、X-射线衍射（XRD）、透射电镜（TEM）、氮气吸脱附等温线及循环伏安测试（CV）等探讨了制备条件对材料形貌、结构和性能的影响,发现当搅拌速度为480 rpm,V_s/V_h为0.01时,800℃炭化得到的碳微球具有良好的球形和部分石墨化的微观结构,孔径集中在中孔范围。采用6 M硝酸对碳微球进行活化处理,用傅立叶红外光谱（FT-IR）、氮气吸脱附等温线、循环伏安、恒流充放电及交流阻抗测试等比较了活化前后碳微球的物理化学性能。结果表明:硝酸活化使得碳微球比表面积、孔径略有增大,同时表面含氧和含氮官能团增加,从而改善了电极材料的润湿性能,有利于电解液离子的吸附和双电层的形成。在6 M KOH溶液中,当电流密度为1 A g^-1时,单电极比电容从196.8 F g^-1增加到246 F g^-1,能量密度为8.5 Wh kg^-1。而且活化后的碳微球具有良好的倍率性能,大电流放电时,比电容衰减较小。采用金属盐溶液浸渍和热分解新方法,在碳微球表面分别包覆Co₃O₄、Mn₂O₃和NiO,制备了Co₃O₄/CMB、Mn₂O₃/CMB和NiO/CMB三种超级电容器电极材料。发现包覆后的过渡金属氧化物/CMB复合材料不但具有高的双电层电容,而且具有高的氧化还原准电容。纳米级的金属氧化物均匀地分散在碳微球表面,明显地改善了碳微球的电容性能。当Co₃O₄掺杂量为10%且电流密度为1 A g^-1时,10%-Co₃O₄/CMB单电极比电容高达350.2 F g^-1。此外,10%-Co₃O₄/CMB复合材料表现了良好的倍率性能和循环稳定性;Mn₂O₃是另一种能提供高氧化还原准电容的电极材料,当Mn₂O₃/CMB复合材料中Mn₂O₃的包覆量为10%且电流密度为1 A g^–1时,10%-Mn₂O₃/CMB单电极比电容为333.8 F g^–1;包覆在碳微球表面的NiO是纳米晶须状,大量纳米晶须状NiO包覆在CMB表面,使NiO/CMB成为海胆状的核壳结构,其中15%-NiO/CMB复合材料单电极比电容最高可达356.2 F g^–1,同时具有优良的循环稳定性。采用共沉淀法制备了二元金属氧化物Co_xNi_1-xO,通过X-射线能谱分析（EDX）和循环伏安等确定了最佳的Co:Ni比为2:3,相应的氧化物组成为Co_0.70Ni_0.30O,是一种具有球形多孔的纳米片层结构电极材料,孔径在中孔范围。以Co_0.70Ni_0.30O为正极,活化碳微球为负极组装成Co_0.70Ni_0.30O/KOH/ACMB非对称型电容器,工作电压可达到1.6 V,同时具有双电层电容和法拉第准电容。当1 A g^-1放电时,能量密度达27.5 Wh kg^–1,5 A g^-1放电时,能量密度仍然保持在24.4 Wh kg^–1,能量密度是活化碳微球对称型电容器的三倍以上,而且具有较好的循环稳定性。以活化碳微球为电极活性物质,以1 M Et₄NBF₄的乙腈溶液为电解液,组装成有机扣式电容器。有机电容器的工作电压明显提高,可达到3.0 V。1 A g^-1放电时,比能量达37.4 Wh kg^–1,远远高于活化碳微球在KOH中的比能量,即使在5 A g^-1大电流放电,比能量仍然保持在29.4 Wh kg^–1。有机电容器表现出较小的漏电流与良好的电压保持率。循环5000次后容量保持率在95%以上。同时,有机电容器具有较宽的工作温度范围,在0⁷0℃之间表现出良好的电容性能。系统研究了活化碳微球阻抗特性变化规律,为该材料的研究和应用提供了理论依据。交流阻抗测试表明活化碳微球电极具有典型的多孔电极特征,欧姆电阻Rs较小,约为2.0Ω。当温度T = 0⁵5℃时,扩散阻抗随温度增加略有降低,电容器表现为典型的双电层电容行为。当T≥65℃时,在相同电位下,电荷转移电阻R_ct随温度升高而增加;在相同温度下,R_ct随电位的增加而增加,表明存在法拉第氧化还原副反应。并且发现低频区Warburg阻抗的相位角|δ|在45°⁹0°之间,说明碳微球电极过程主要受吸附和半无限扩散混合动力学过程控制。电容器具有较小的时间常数,0℃时,τR最大为3.5 s,在25⁶5℃温度区间,τR为1.0 s左右,T > 65℃时,τR增大。电容器在低频时,接近纯的电容行为,在高频时,接近于纯的电阻行为。通过低频阻抗谱数据计算电容器比电容最大为28.4 F g^–1,电容器可达到的能量密度和功率密度分别为35.5 Wh kg^–1和112.5 kW kg^–1。更多还原

【Abstract】 Supercapacitor （also called electrochemical capacitor） combines the advantages of both conventional capacitors and rechargeable batteries. It has not only much more energy density than conventional capacitors, but also much higher power density than rechargeable batteries. Therefore, supercapacitors exhibit broad prospects in power source applications such as electric vehicles, spatial, military, mobile telecommunication, and consumer electronics. Electrode material is a key component of the supercapacitor and it determines the performance of supercapacitor. The research and development of electrode materials have become a hot topic in both academic and industrial field.In this study, carbon microbeads （CMB） were prepared by inverse emulsion polymerization and ambient drying technique. Compared with supercritical drying technique, new method for preparing CMB under ambient pressure drying conditions resolved the problems, such as complex process, expensive cost and performance was difficult to control. The influence of preparation conditions on the morphology and performance was investigated by scanning electron microscopy （SEM）, X-ray diffraction （XRD）, transmission electron microscopy （TEM）, N2 sorption isotherm and cyclic voltammetry （CV）. The results showed that the optimal stirring speed and V_s/V_h ratio were 480 rpm and 0.01, respectively. The CMB prepared at 800℃is a typical mesoporous carbon material with partially graphitized structure. In order to improve the surface structure and electrochemical performances, CMB was activated in 6 M HNO3, and the physicochemical and electrochemical performances of activated CMB （ACMB） were characteristized by Fourier transform infrared （FT-IR） spectrum, CV, galvanostatic charge-discharge, and electrochemical impedance spectrum （EIS）. It was found that the activation caused a slight enhancement in specific surface area and pore size. Besides, the oxygen and nitrogen functional groups on the surface of CMB were detected, which could improve the hydrophilicity of CMB and the capability to form electric double-layer. Thus ACMB has better electrochemical properties. The specific capacitance of electrode increases from 196.8 to 246 F g^-1 at a current density of 1 A g^-1 in 6 M KOH and its energy density is 8.5 Wh kg^-1. Furthermore, ACMB shows good rate capability, the specific capacitance has tiny decrease even at large current density.Co₃O₄/CMB, Mn₂O₃/CMB, and NiO/CMB composites for supercapacitor electrode materials were synthesized by in situ coating method and their supercapacitive behaviors were investigated. It was found that both double-layer capacitance and Faradic pseudo-capacitance existed in the metal oxide/CMB composites. The metal oxide/CMB composites showed better electrochemical performance than pure CMB since nano-sized metal oxides homogenously encapsulated on the surface of CMB. The optimum content of Co₃O₄ in Co₃O₄/CMB composite is 10 wt.%. The specific capacitance of the 10%-Co₃O₄/CMB electrode is up to 350.2 F g^-1 at a current density of 1 A g^-1. Moreover, 10%-Co₃O₄/CMB composite supercapacitor exhibits good high-rate capability and excellent cycle life. Mn₂O₃ is another electrode material which can provide high redox pseudo-capacitance. When 10 wt.% Mn₂O₃ is coated on the surface of CMB , the specific capacitance of electrode is up to 333.8 F g^-1 at a current density of 1 A g^-1. Nanowhisker-like NiO was coated on the surface of CMB and the appearance of NiO/CMB composites become sea urchin-like morphology with core-shell structure. The 15%-NiO/CMB composite exhibits the best capacitive properties, the specific capacitance of electrode is up to 356.2 F g^-1. Besides, the symmetric supercapacitor using 15%-NiO/CMB composite as the electrode active material shows stable cycling performance.Co_xNi_1-x oxides were successfully synthesized by chemical coprecipitation method. The results of EDX and CV showed that Co_0.70Ni_0.30 oxide was obtained when the optimum molar ratio of the Co: Ni was 2:3. The Co_0.70Ni_0.30 oxide shows the spherical morphology with mesoporous structure, which is consisted of many interleaving thin nano?akes. An asymmetric supercapacitor (Co_0.70Ni_0.30 oxide/KOH/ACMB) was fabricated with Co_0.70Ni_0.30 oxide as the positive electrode and ACMB as the negative electrode. The asymmetric supercapacitor represents both double-layer capacitance and Faraday pseudo-capacitance, and its work voltage is extended to 1.6 V. The specific energy is 27.5 Wh kg^-1 at a current density of 1 A g^-1 and still keeps 24.4 Wh kg^-1 even at a current density of 5 A g^-1. The specific energy is three times higher than that of a symmetric ACMB capacitor. The hybrid supercapacitor also demonstrates good cycling performance.A symmetrical button cell supercapacitor was assembled with ACMB as the active material and 1 M Et₄NBF₄/AN as electrolyte. The work voltage of organic capacitor is increased up to 3.0 V. The specific energy is 37.4 Wh kg^-1 at a current density of 1 A g^-1 and still keeps 29.4 Wh kg^-1 even at a current density of 5 A g^-1, which is much more than that of in 6 M KOH. Organic capacitor exhibits a low leakage current and good voltage retention rate. The capacitance retention of supercapacitor remained 95% after 5000 cycles. Besides, Organic capacitor showed good capacitive behavior in the range of temperature from 0 to 70℃.Regularity of electrochemical impedance spectra for ACMB was studied in systems, AC impedance measurements show that the ACMB electrode has typical characteristic of porous electrode and the ohmic resistance （Rs） is about 2.0Ω. The capacitor shows typical double-layer capacitive behavior and the diffusion resistance decreases with the increase of T from 0 to 55℃. When the T≥65℃, at fixed voltage, charge-transfer resistance (R_ct) increases with temperature; the increase in R_ct with voltage at fixed T clearly indicates the occurrence of faradaic processes. The phase angle |δ| values are between 45°⁹0°, indicating the mixed kinetic processes of adsorption-limited process and semi-infinite diffusion-like limited step. The time constant is 3.5 s at 0℃, andτ_R keeps about 1.0 s at the range of 25⁶5℃. When T > 65℃,τ_R begins to increase. The supercapacitor balances between the ideal capacitor and ideal resistive behavior: resistive at high frequencies and capacitive at low frequencies. The maximum capacitance of supercapacitor calculated from low-frequency impedance data is 28.4 F g^-1. The energy density and power density of capacitor can be achieved 35.5 Wh kg^-1 and 112.5 kW kg^-1, respectively.更多还原