【作者】 韩燕；

【导师】 刘双喜；

【作者基本信息】 南开大学 ， 材料物理与化学， 2013， 博士

【摘要】 超级电容器作为新型储能元件,由于循环寿命长,可逆性良好,能量密度和功率密度高等的优点,一经问世便受到广泛关注。其中,电极材料作为超级电容器的重要组成部分,很大程度上决定了超级电容器的性能。研究表明,具有大比表面积,高电导率,适当孔径分布和规则孔道结构,化学性质稳定的多孔炭材料能够成为理想的电极材料。本文采用水蒸汽活化废轮胎热解炭黑,模板-水热法,模板-溶剂蒸发法制备出具有不同孔道结构的碳质多孔材料,并考察了其在水系电解液(6MKOH)中的电化学性能。论文主要研究内容与结果如下：(1)采用水蒸汽活化处理纯化后的热解炭黑制备活性炭。随着活化温度,活化时间的增加,活性炭的产率逐渐减小,比表面积和孔体积逐渐增加,微孔体积逐渐减小,说明活化过程中炭与水蒸汽发生氧化反应,使孔径不断扩大。综合考虑活化过程中炭材料的产率及所得炭材料的孔道结构特点,选取AC-800-4和AC-850-2炭材料进行进一步的电化学性能研究。研究结果表明,所制备的活性炭材料具有良好的电化学可逆性,其中AC-800-4的比电容较高(110Fg-1),等效串联电阻较小(0.34Ω),但电荷转移电阻和频率的响应时间增加。原因可能是水蒸汽活化处理过程中,AC-800-4炭材料具有较窄的孔道结构和较高的官能团含量。其中,窄的孔道结构不利于电解液在材料内部的扩散传输,使频率的响应时间增加；材料表面含氧官能团增加,一方面,改善材料表面润湿性,使电化学反应过程中等效串联电阻降低,另一方面,其氧化还原反应产生赝电容,使整体比电容提高,但同时使电荷转移电阻增加。采用浓硝酸对AC-800-4进行表面改性处理,改性后活性炭的形貌、孔道结构和石墨微晶结构基本保持不变,含氧官能团含量增加,电化学性能明显提高,比电容从110Fg-1提高到140Fg-1,且循环性能良好。这主要是由于引入的含氧官能团可以增加材料表面的极性,改善材料表面润湿性,增加电极材料与电解液的有效接触面积；同时在充放电过程中发生氧化还原反应,形成赝电容,从而增加电极材料的比电容。(2)分别以葡萄糖和酚醛树脂为碳源,Si02溶胶为模板,采用水热方法制备具有不同结构特征和表面化学组成的碳空心球。以葡萄糖为碳源合成的碳空心球g-CHS形状规则,大小均一,表面光滑,分散性良好,具有介孔/大孔核及微孔壳构成的多级孔道结构,且材料表面连有一定的含氧官能团。以酚醛树脂为碳源合成的碳空心球p-CHS表面粗糙,壁厚不均匀,团聚现象严重。球壁内部丰富的微孔,空心球核和空心球团聚形成介孔和大孔结构,构成材料的多级孔道结构。与g-CHS材料相比,p-CHS材料的微孔含量增加,石墨化程度降低,表面官能团含量增加。电化学性能研究结果显示：g-CHS电极材料显示了较为优越的电化学性能,具有较高的质量比电容和面积比电容(266F g-1和40.4μFcm-2),且循环性能稳定,倍率性能良好。这主要是由于,与g-CHS相比,p-CHS材料因具有较高的微孔率,不利于电解液离子在电极材料中的扩散与传输,使电解液与电极表面的接触电阻增加,材料的有效接触面积降低,材料的比电容减小,尤其在大电流密度下的充放电性能变差；较高的官能团含量,使p-CHS电极材料呈现混合电容特征,一方面由于赝电容的存在,材料的比电容有所提高,另一方面电荷转移电阻增加,循环过程中官能团易发生不可逆反应,导致容量衰减。(3)以葡萄糖为碳源,SiO2溶胶为模板,采用更为简单的溶剂蒸发方法制备蜂窝状的多孔炭材料g-HHPC。所得g-HHPC材料由相互连接的中大孔孔道及较短的微孔孔道构成,具有部分石墨化结构,表面连有一定的含氧官能团。对比具有相似比表面积,孔体积及石墨化程度的碳空心球g-CHS,g-HHPC电极材料具有较高的质量比电容(272Fg-1)和面积比电容(41.5μF cm-2),良好的倍率性能,电极的响应时间短,说明g-HHPC材料内部相互连接的介孔/大孔结构,使电解液极易进入孔道,有利于电解液离子在材料内部的快速传输,较短的微孔孔道结构使电解液在材料内部的扩散距离减小,有效接触面积增加。但g-HHPC电极材料的电容衰减率相对较大(22%),说明含氧稍高的表面官能团,虽然有利于改善材料的表面润湿性,提高材料的比电容,但循环过程中不稳定易发生不可逆的还原反应造成比电容的衰减。总之,电极材料的电化学性能是材料孔道结构和表面化学组成等几个方面综合作用的结果。材料的孔道结构中,大孔和介孔有利于电解质离子的扩散传输,微孔有利于提高电解液离子与电极的有效接触面积。材料的表面含氧官能团,一方面改善材料的润湿性,提高电极表面对电解液离子的吸附,降低材料的等效串联电阻,另一方面,引入的含氧官能团可能与水分子结合,空间位阻增加,使材料的孔径减小,阻碍电解液离子在孔道内的传输,同时由于发生氧化还原反应,使电极等效串联电阻及电荷转移电阻增加。更多还原

【Abstract】 As new energy storge devices, supercapacitors have received extensive attention because of their unique including long cycle life, superior reversibility, and high energy and power density. In general, the electrode material, as an important part of the supercapacitor, is crucial to ensure a good performance of the supereapacitor. Among the exploration of supercapacitor electrode materials, porous carbon materials are considered as the main candidate for supercapacitors in terms of large surface area, high conductivity, suitable pore size distribution and regular structure, long-term cycle stablity, as well as electrochemical reproducibility.Based on the consideration, a series of carbon materials with different surface areas and pore structures were prepared by steam activation from the industrial pyrolytic char, tenplate-solvothermal method and tenplate-solvent evaporation method. And the electrochemical performances of the carbons as supercapacitor electrode materials were investigated in6M KOH electrolyte. The main results are shown as follows:(1) Activated carbons (ACs) derived from the industrial pyrolytic tire char were prepared by steam activation and first evaluated as potential electrode materials for supercapacitor. With the activation temperature and time increasing, the surface areas and pore volumes of ACs increase gradually, while the micropore volume decreases. This suggests a continuous widening of pores featured in the development of mesopores and macropore at the expense of micropores and surface area. Base on the yield and textural properties, the ACs obtained at800℃for4h (AC-800-4) and850℃for2h (AC-850-2) are selected to study the electrochemical performances as supercapacitor electrode material. The results demonstrate that the obtained ACs display good eletrochemical properties. Compared to the AC-850-2electrode, the AC-800-4electrode has a higher capacitance (110F g-1), lower equivalent series resistance (0.34Ω), but processes a larger charge transfer resistance and time constant, which could be attributed to less pore size and higher content of oxygen-containing functional groups.The AC-800-4was further modified by concentrated nitric acid (labeled as m-AC) and then used as electrode material for supercapacitors. It is found that the morphology, porous texture and localized graphitic structure for the AC-800-4and m-AC have little difference, but the oxygen content increases and the functional groups change after acid treatment. The electrochemical results demonstrate that the m-AC electrode displays higher specific capacitance (140F g-1), better stability and cycling performance, which can be attributed to the increase of surface oxygen-containing functional groups introduced by acid treatment. These groups can improve the wettability of the electrode to enhance the utilization efficiency of the surface area and bring stable pseudocapacitance, leading to a notable improvement of the specific capacitance and the cycle performance.(2) Carbon hollow-spheres with different structure and surface chemical properties were prepared by hydrothermal treatment directly using colloidal silica as template, glucose and phenolic resol as carbon source (marked as g-CHS and p-CHS), respectively.The carbon hollow-spheres using glucose as carbon source (g-CHS) have regular shape, uniform size, smooth surface and homogeneous dispersion. In the meantime, the as-prepared sample possesses a hierarchical porous structure with micropore shell and meso/macropore core and moderate oxygen functional groups. While, the carbon hollow-spheres using phenolic resol as carbon source (p-CHS) have rough surface, nonuniform wall thickness and serious partical agglomeration. Compared with the g-CHS, the p-CHS have more micropore structure, lower degree of graphitization and higher oxygen content.On the comparison study for the electrochemical performances as supercapacitor electrode materials, it shows that the g-CHS electrodes possess higher specific capacitance (about266F g-1at1A g-1in KOH aqueous electrolyte), longer cyclic life and more excellent rate capability, which can be attributed to the higher ratio of mesopores/macropores, facilitating fast ion diffusion and offering a good charge accommodation. Furthermore, the p-CHS show the characteristics of the hybrid capacitor due to the high oxygen content, which enhance the specific capacitance but negatively influence the cycle stability.(3) Honeycomb hierarchical porous carbon was prepared by an easy solvent evaporation method directly using colloidal silica as template, glucose as carbon source (marked as g-HHPC). The as-prepared sample possesses three-dimensionally honeycomb-like sturcture with interconnected meso/macropore and abundant micropore, low degree of graphitization and moderate oxygen functional groups.Compared with the g-CHS with similar specific surface area, pore volume and degree of graphitization, the obtained HHPC as an electrode material exhibits better capacitive performances with higher specific capacitances (about292F g-1and44.5μF cm-2at1A g-1in KOH aqueous electrolyte), better electrochemical stability and rate capability, and less time constant, which can be mainly attributed to the unique three-dimensional hierarchical porous structure, not only promoting the mass transfer/diffusion of ions into the pores effectively but also offering a good environment for charge accumulation. While, the higher capacitance loss of22%could be due to the slow reduction of functional groups during cycling in a non-reversible way.In summary, the electrochemical properties of the electrode materials depend upon several parameters, for instance, the porous structure and the surface chemical properties. And the effects of these parameters are interlinked and revealed in combination. For the porous structure, mesopores and macropores can promote the mass transfer/diffusion of ions into the pores, while micropores can yield a higher surface area for charge accumulation. For the surface oxygen-containing functional groups, on the one hand, these groups can improve the wettability of the electrode to enhance the utilization efficiency of the surface area and bring pseudocapacitance, leading to a notable improvement of the capacitive performance. On the other hand, the introduced functional groups may result in pore obstruction and increase the charge transfer resistance due to the redox reaction of the functional groups.更多还原