【作者】 李英芝；

【导师】 张清华；

【作者基本信息】 东华大学 ， 材料学， 2012， 博士

【摘要】 超级电容器是一种新型的储能器件,既有传统电容器的高功率、长寿命、免维修、无污染等优点,又具有电池高能量密度的优点。双电层电容器有高功率、长寿命的优点,但是能量密度偏低；法拉第电容器比双电层电容器拥有更高的能量密度,但是由于涉及氧化还原反应,使其循环寿命较低。双电层电容器和法拉第电容器的结合有利于提高材料的电化学特性。聚苯胺以其合成方便、形貌可控等优点成为超级电容器电极材料研究的重要材料之一。本文通过原位聚合方法,将导电PANI分别负载在海藻酸钠、磺化倍半硅氧烷和纳米碳材料等载体中,得到不同形态结构的纳米复合材料,分析了材料的合成条件与形态结构的关系,并对该电极材料的电化学性质做了详细表征。海藻酸钠(SA)是一种富含羟基的线性生物大分子,通过pH值控制SA在水中的降解,可得到理想的生物高分子模板。利用模板诱导法制备了不同结构的PANI/SA复合纳米纤维,其中大面积网状结构的PANI/SA纳米复合纤维具有较高的比表面积(65.5m2g-1)和较好的孔径分布。网状结构中大量的孔隙为电解液离子迅速扩散提供了便捷通道；连续网状的PANI纳米纤维为电荷的迁移提供连续的通道；50-100nm的纤细直径使PANI纤维具有较大的比表面积,为氧化还原反应提供了较大的反应接触面积。优化的PANI/SA电极材料具有1612F g-1的高比电容和大功率特性,1000次循环后,仍保持了74%的初始电容。八苯基倍半硅氧烷(POSS)是一种含Si-O-Si笼型内核、八个顶点上带有苯基的纳米材料。通过苯基磺化得到带磺酸基的倍半硅氧烷(SOPS),磺酸基的存在大大提高了SOPS在水中的分散性。以SOPS作为掺杂酸,利用原位聚合制备得到PANI/SOPS复合材料。电化学测试表明,SOPS的引入明显提高了PANI/SOPS复合材料的比电容和循环寿命。在1A g-1的电流密度下,SOPS含量为2%的PANI/SOPS具有最大的比电容,达到1810Fg-1; SOPS含量为25%的PANI/SOPS电极在3000次循环后仍能保持82%的初始比电容。可能是SOPS上的磺酸基与PANI分子链有很好的作用力,使SOPS能以掺杂酸的角色与PANI达到分子／纳米水平的杂化。通常电极充放电过程中,离子在电极材料中脱出/嵌入会引起PANI基体的膨胀/收缩,Si-O-Si笼型内核的存在增强了PANI/SOPS复合材料微观结构的强度,避免了微观结构的塌陷或破坏。PANI/SOPS的纳米棒组装成有序的菊花形结构,使电解液离子的扩散路径更短,有利于提高电极材料的响应性,从而明显提高了电极材料的功率特性。石墨烯(graphene, G)是一种二维结构的新型碳系材料,具有极好的导电性、机械性、大比表面积等众多优点。用PANI包覆石墨烯制备二元复合电极材料,既具有双电层电容特性(石墨烯提供大比表面积),又具有赝电容特性(PANI可发生氧化还原反应)。石墨烯优良的机械性能将增强复合材料结构的稳定性,其优异的电导特性将改善复合材料的导电性。通过改进传统的石墨烯氧化还原制备方法,能够得到分散性能良好的还原石墨烯(r-G),避免了石墨烯的自团聚。用r-G与苯胺单体原位聚合,成功制备了聚苯胺/石墨烯(PANI/r-G)复合材料。电化学测试表明石墨烯的加入提高了PANI作为电极材料的利用率,当石墨烯的含量为6%时,PANI/r-G的比电容达到846F g-1；1000次循环后,仍保持74%初始比电容,比纯PANI电极稳定性有明显提高。电化学剥离法制备石墨烯比氧化还原法相对较为简便。我们在Na2SO4电解液中,用电化学剥离高纯碳棒制备石墨烯(ec-G),得到了品质更好的石墨烯。用ec-G与苯胺单体原位聚合,制备PANI/ec-G的复合材料。当ec-G的含量为4%时,在1A g-1电流密度下,PANI/ec-G的比电容达到895F g-1。当ec-G的含量为6%时,1000次循环后,PANI/ec-G保留了79%的初始比电容。ec-G在复合物中形成了导电网络有利于电子迁移；同时对PANI基体有机械支撑作用,提高了电极材料结构的稳定性。因此ec-G的引入有效地提高了PANI/ec-G复合材料电化学性能。膨胀石墨(EG)是石墨经热或微波处理,体积急剧膨胀的一种碳材料。相比天然石墨,EG石墨片层具有较大的间距,有利于超声剥离方法得到高品质的石墨烯。SEM、TEM、AFM、Raman光谱分析发现,EG超声剥离制备的石墨烯尺寸较大、结构较完整。将不同超声时间的EG石墨片层与苯胺单体原位聚合,制备不同剥离程度的石墨片层与聚苯胺的复合物(PANI/e-EG)。当超声时间较短时,PANI能很好的包覆EG石墨片层(PANI在EG石墨片层上形成有序的凸点),得到PANI插层的PANI/e-EG复合材料；当超声时间较长时,EG被超声剥离成多层石墨烯,PANI不能很好的包覆石墨烯。电化学测试表明,随着超声时间的延长,PANI/e-EG复合电极的电化学性能下降。在1Ag-1电流密度下,未强超声剥离的膨胀石墨片层与PANI的复合材料PANI/e-EGO比电容为1257F g-1；1000次循环后,保持了84%的初始比电容。分析认为,比石墨烯相比,EG为复合材料提供更好的骨架支撑、电子通道,所以其电化学性能更优良。随后我们对上述实验现象做了进一步研究。精细地合成了PANI纳米棒在EG界面上垂直生长的PANI/EG复合材料,电化学测试发现EG三维骨架结构大大提高了PANI的电极利用率,使其比电容、功率密度和循环寿命都有很大提高。在1A g-1电流密度下,含有10%EG的复合材料PANI/EG10%具有1665F g-1的比电容；2000次循环后,保持了87%的初始比电容。分析认为,有序的纳米棒阵列缩短了电解液离子在界面之间的扩散路径,大比表面积增加了电解液／活性物质的接触面积；EG的骨架结构为电荷迁移提供了有效途径,增强了复合材料微观结构的稳定性；复合材料的多孔结构为电解液迁移提供了良好的通道。更多还原

【Abstract】 Supercapacitors, also called ultracapacitors, are promising energy storage devices which bridge the gap between batteries and conventional capacitors. They can provide higher energy density than conventional capacitors and much higher power density than batteries. Supercapacitors exhibit a promising set of features such as high power density, fast charge/discharge rate, sustainable cycling life, and safe operation. Based on the charge storage mechanism, supercapacitors can be divided into two categories. One is electrochemical double layer capacitor (EDLC), which stores charges electrostatically via reversible ion absorption at the electrode/electrolyte interface. EDLCs can provide ultrahigh power and excellent cycle life due to fast and nondegradation process between electrode active materials and electrolyte. However, the energy density of EDLC is limited by the finite electrical charge separation at the interface of electrode materials and electrolyte, and the availability of surface area. The other is the pseudocapacitor or redox supercapacitor, which stores energy by fast and reversible redox reactions. Because of the Faradaic process underpinning the energy stored in a pseudocapacitor, the pseudocapacitor often suffers from lack of stability during cycling. In this work, we want to fabricate electrode materials to combine these two charge storage mechanisms (Faradaic and non Faradaic), their synergic effects to improve device characteristics.Polyaniline (PANI), as a kind of widely studied electronically conducting polymers, is a promising candidate for electrode materials due to excellent capacities for energy storage, easy synthesis, non-toxic and low cost. Compared with metallic oxide, PANI possesses better conductivity. Its electrical properties can be modified by the oxidation state of the main chain and degree of protonation. And most importantly, the structures of PANI can be controlled by template-induced or self-assembly in different experimental conditions. Therefore, we want to design various structures of PANI composites, characterize their electrochemical properties, and reveal the relationship among composition, structure and performances.Sodium Alginate (SA) consists of a linear block co-polymer of1,4-linked (3-D-mannuronic (M) and R-L-guluronic acid (G), bearing abundant carboxyl and hydroxyl groups. We synthesized various templates of SA by controlling pH values of aqueous solution, and then fabricated the PANI/SA composites by in situ polymerization. The electrochemical characterization reveals that the electrode (PANI/SA) with large scale net-work structure exhibits a high electrochemical performance. PANI/SA nanofibers possess thin diameters (50-100nm), large specific surface area (65.5m2g-1) and an appropriate pore size distribution. The nanostructure of electrode materials generates high electrode/electrolyte contact area and short path lengths for electronic transport and electrolyte ion. The specific capacitance of an optimum electrode is up to1612F g-1and it maintains74%after1000cycles. The approach is simple and can be easily extended to fabricate nanostructural composites for supercapacitor electrode materials.Polyhedral oligomeric silsesquioxane (POSS),(RSiO1.5)n, has a cubic silica core with a diameter of0.53nm and a spherical radius of1-3nm including the functionalized organic arms. POSS is often used as an additive in nanoreinforced organic-inorganic hybrid materials. In this work, POSS was functionalized by direct sulfonation, and the porous and ordered hierarchical nanostructure of polyaniline/sulfonated polyhedral oligosilsesquioxane (PANI/SOPS) was subsequently fabricated by in situ polymerization. The morphologies of the PANI/SOPS nanocomposites can be controlled by adjusting the concentration of SOPS. Comparing with the pure PANI, the PANI/SOPS electrode exhibits a higher specific capacitance of1810F g-1, faster reflect of oxidation/reduction on high current changes and better cyclic stability. The specific capacitance maintains82%after3000cycles. The excellent performance may be attributed to the nano-architecture of electrode materials and the support of the SOPS nanoparticles.Synergetic interaction between PANI and SOPS significantly improves the porosity and the stability of electrode, yielding excellent electrochemical property. This result suggests that the construction of porous and ordered hierarchical nanostructure is a novel and effective way for improving the electrochemical properties of conducting polymersGraphene has attracted much research attention due to its two-dimensional and unique physical properties, such as high electronic transport properties, excellent mechanical strength, and elasticity and superior thermal conductivity. We designed and prepared a series of polyaniline/graphene (PANI/G) composites. A modified chemical exfoliation method was used to produce graphene. In chemical reduction stage, the DMF/H2O (9:1) mixed solution was used as reaction solution instead of conventional aqueous solution. We obtained individual disperse graphene, subsequently fabricated PANI/G composites via in situ polymerization. The electrochemical characterization demonstrated that the PANI/G electrode with6%graphene content exhibits relatively high specific capacitance (846F g-1) and good long-term cycling stability. After1000cycles, the discharge capacitance retention of the PANI/G electrode is74%while that of the pure PANI is only43%. Graphene improves the stability and conductivity of electrode materials structures, so the PANI was effectively utilized.The high quality and large-area graphene were obtained by the electrochemical exfoliation of graphite in Na2SO4electrolyte. Aniline monomer was adsorbed on the nanosheets to prepare PANI/G composites via in situ polymerization. The electrochemical characterization demonstrated that PANI is effectively utilized with the assistance of graphene conductive skeletons in the electrode. The composites electrodes achieve a good specific capacitance of895F g-1at1A g-1and long-term life. It may be due to the net-work of graphene in composites, which not only support a skeleton for PANI matrix but also supply paths for electronic transport.Expanded graphite (EG) is a kind of modified graphite with a hierarchical porous nature and good conductivity. In this work, EG was exfoliated by sonication with a cylindrical tip. The exfoliation degree of EG can be controlled by ultrasonic time. The EG flakes were coated by PANI using in-situ polymerization. The electrochemical characterization illustrated that the long ultrasonic time of EG flakes had a negative effect on the electrochemical properties of PANI/EG electrodes. SEM images revealed that the EG three-dimensional (3D) structure aggravated under long time sonication, and graphene sheets intercalated into random PANI nanorods. While the ordered PANI nanorods grew onto EG flakes with no or short time sonication. The EG supplied more effectively mechanical skeleton for PANI and transport paths for ions and electron than graphene, therefore the carbon supports of EG for PANI composite electrodes is more effective.The above experiment results are very interesting, arising our future attention. An oriented array of polyaniline (PANI) nanorods grown on expanded graphite (EG) nanosheets were fabricated by in situ polymerization. The morphologies of PANI/EG nanocomposites can be controlled by changing the ratio of EG to aniline monomer. The PANI/EG nanocomposites exhibited high specific capacitance, high rate capability and significant cyclic stability. An excellent specific capacitance as high as1665F g-1was observed in the PANI/EG electrode with10%EG content. The composite electrode material also exhibited significant rate capability with specific energy of113.8Wh kg-1and specific power of560kW kg-1at the current density of8A g-1, respectively, and good long-term cycling stability. In composites, EG serves as excellently3D conductive skeletons to support a highly electrolytic accessible surface area of redox active PANI and supply a direct path for electrons. Such3D nanoarchitecture composite is very promising for the next generation of high performance electrochemical supercapacitors.更多还原