【作者】 苏凌浩；

【导师】 张校刚；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2009， 博士

【摘要】 鉴于目前超级电容器研究中炭基材料的比电容较小和钌基材料的价格昂贵等缺陷,寻找其它成本低廉、性能良好的替代电极材料已经势在必行。双金属氢氧化物由于具有独特的片层结构,有望成为下一代超级电容器的电极材料。这类化合物具有丰富的片层,能够提供大的表面积,可以提高双电层电容;同时,层板中的过渡金属元素可以提供大量的电化学活性位,即,产生法拉弟赝电容。利用双金属氢氧化物作为超级电容器的电极材料,可以将双电层电容和法拉弟赝电容两种电荷储存机理结合在一起,大大改善超级电容器的性能,因而具有巨大的发展潜力。本文以钴铝双金属氢氧化物（Co-Al LDH）作为具体的研究对象,采用直接沉淀法或均匀沉淀法制备Co-Al LDH层状材料,通过FE-SEM、XRD、FT-IR等技术对Co-Al LDH层状材料的形貌和结构进行表征,并从结构改性、提高材料的导电性、筛选合适的电解质、加入氧化还原电对以及构筑纳米片薄膜等几个方面详细研究了Co-Al LDH层状材料的赝电容行为,对双金属氢氧化物作为超级电容器的电极材料的可行性进行了系统的分析和评价。首先,采用均匀沉淀法制备了Co-Al LDH纳米片材料,并研究了其在KOH碱性溶液中的电化学电容行为。这种Co-Al LDH材料呈现规则的六边形片状形貌,厚度约为几十个纳米;在2 A g^-1的大电流密度下具有赝电容活性,其质量比电容为192 F g^-1。同时,从探索超级电容器的储荷机理入手,研究了向KOH溶液中添加氧化还原电对(Fe（CN）₆^3-/Fe（CN）₆^4-)对Co-Al LDH电极的赝电容行为的影响。由于Fe（CN）₆^3-/Fe（CN）₆^4-电对的良好的电化学可逆性能,能够接受或提供Co-Al LDH电极在充/放电过程中顺利完成化合态Co（II）/Co（III）之间的转化所需的电子,即,起到“穿梭电荷”的作用。交流阻抗和Tafel测试表明,加入氧化还原电对Fe（CN）₆^3-/Fe（CN）₆^4-之后,Co-Al LDH电极的传荷电阻降低,交换电流密度增加。其次,通过对Co-Al LDH进行离子交换、剥片和电泳沉积,研究了Co-Al LDH纳米片薄膜的赝电容行为。同块体材料不同,Co-Al LDH纳米片薄膜表现出极其优异的赝电容行为:在5¹000 mV s^-1的扫描速度范围内,都呈现出类似矩形的CV曲线;充/放电曲线为直线对称的I-V响应。究其原因,在于经剥离得到的Co-Al LDH纳米单片,脱离了原来块体材料中层间阴离子和其它片层的束缚,得以与电解质更充分地接触;同时,电化学活性位（Co元素）也最大限度地暴露在外,提高了活性材料的利用率。再次,针对超级电容器的电极材料在长期充/放电循环过程中由于结构发生变化而引起电容性能衰减的缺点,研究了对Co-Al LDH进行结构改性对其赝电容行为的影响。具体做法是,先利用直接沉淀法制备苯甲酸根占据层间空隙的Co-Al LDH,继之在惰性气氛下焙烧。利用LDH的结构记忆效应,将焙烧所得的产物在浓KOH溶液中浸泡后又复原为Co-AlLDH。由于复原所得的Co-Al LDH层间嵌有炭材料,在充/放电循环过程中对其结构有稳固作用。即使在2 A g^-1的大工作电流下,循环1000周后,电容保持率仍为100 %。另外,研究了不同电解质对Co-Al LDH电极的赝电容行为的影响。发现在LiOH、NaOH和KOH三种电解质的水溶液中,Co-Al LDH的电容行为有很大的差别,其根本原因在于电解质中阳离子的不同。对于LiOH和NaOH电解质而言,除了发生钴元素的化合态Co（II）/Co（III）之间的转化外,由于Li+、Na+离子的半径较小,还可以同时发生Li+或Na+离子与OH-离子作为离子对共同嵌入/脱出Co-Al LDH的晶格的电极过程;对于KOH来说,由于K+离子的半径较大,不能嵌入Co-Al LDH的晶格,只发生化合态Co（II）/Co（III）之间的转化过程。最后,研究了提高电极材料的导电性对Co-Al LDH的赝电容行为的影响。通过向Co-Al LDH纳米片材料中物理掺加多壁碳纳米管,改善了电极的导电性,由附着在Co-Al LDH纳米片表面的多壁碳纳米管构成良好的导电网络,不仅减小传荷电阻,而且使电极的比电容值从192.0 F g^-1增大到342.4 F g^-1,循环寿命也大大改善。在此基础上,从容量匹配的角度出发,对复合电极材料中多壁碳纳米管和Co-Al LDH纳米片的配比进行了合理优化。经优化后制备的“自复合”电极的赝电容行为有了很大改善,比较接近于炭材料和水合二氧化钌的特性,即,矩形的CV曲线和近乎直线的I-V响应。总之,通过上述一系列的研究,系统获取了各种因素对Co-Al LDH电极的赝电容行为的影响作用,表明这类具有特殊层状结构的双金属氢氧化物是一种很有希望的超级电容器的电极替代材料。更多还原

【Abstract】 Owing to the drawbacks in the supercapacitors nowadays that the specific capacitance of carbon-based materials is relatively low and that RuO2 is expensive, it is vital to develop new alternate electrode materials with low cost and good properties. Layered double hydroxides （LDHs） as a family of compounds with unique lamellar structure are promising for next-generation supercapacitors in that electrical double layered capacitance and faradaic pseudocapacitance can be simultaneously acquired because of their abundant slabs and electrochemically active sites if they are used as active electrode materials.In this thesis, Co-Al layered double hydroxide （Co-Al LDH）, which is prepared through the method of direct coprecipitation or homogeneous coprecipitation and characterizated by SEM, XRD, and FTIR techniques, is employed as active electrode material for supercapacitor. The effects on its pseudocapacitive performances from such influencing factors as structural modification, electrical conductivity, electrolyte, redox couple and nanosheet film are investigated and discussed in detail. Meanwhile, the feasibility of LDHs as active electrode materials for supercapacitors is evaluated and a systematic study is carried out.First, Co-Al LDH nanosheets with regular hexagonal morphology are synthesized by homogeneous coprecipitation via the gradual hydrolysis of urea, whose specific capacitance is 192 F g^-1 in 1 M KOH solution. Meanwhile, the capacitive properties of Co-Al LDH is examined by adding hexacyanoferrate （II） and （III） solely or jointly into 1 M KOH aqueous solution so as to understand the charge-storage mechanism for a pseudocapacitor. Owing to the high reversibility, Fe（CN）₆^3-/Fe（CN）₆^4- ion pair act as electron relay at the electrode/electrolyte interface during charge and discharge by coupling in the redox transition of Co（II）/Co（III） in the Co-Al LDH electrode. Electrochemical impedance spectra and Tafel curves provide direct evidences with decreased charge-transfer resistance and increased exchange current density in the alkaline solution containing hexacyanoferrate ions, respectively.Second, the pseudocapacitive performances of film electrodes consisting of Co-Al LDH nanosheets are detected. The nanosheets are obtained after Co-Al LDH’s consecutive treatments of ion-exchange with chloride, nitrate ions, and delamination in formamide. Thin films on ITO glass are fabricated through a method of electrophoretic deposition using Co-Al LDH nanosheets as building blocks. Compared to bulky Co-Al LDH material, Co-Al LDH nanosheet film exhibits excellent electrochemically capacitive properties in alkaline KOH solution: rectangle-like CV curves in a rather wide range of scan rate from 5 to 1000 mV s^-1 and almost symmetric straight charge/discharge curves, which benefits from the unique two-dimensional morphology of the nanosheets and the utmost exposure of redox active sites.Third, it is investigated to modify Co-Al LDH’s structure so as to resolve the deformation and collapse of the structure of electrode material during charge/discharge circling at large current. A solid Co-Al LDH is synthesized through a three-step procedure including direct coprecipitation, heat treatment, and reconstruction. After sintering the Co-Al LDH containing benzoate at 600 oC for 3 h in argon gas flow, Co-Al double oxides are obtained. When immersed in aqueous 6 M KOH solution in air, the double oxides restack to Co-Al layered double hydroxides again with more regular crystal than before. The restacked Co-Al LDH reveals good capacitance retention of 100 % after cycling 1000 times of charge/discharge at a big current of 2 A g^-1, which results from the stability of carbon, created during the pyrolysis of benzoate and inserted in resulting double oxides, on Co-Al LDH’s structure.Four, the influence from electrolyte on the pseudocapacitive performances of Co-Al LDH is evaluated. The electrochemically capacitive behavior of Co-Al LDH is quite different in LiOH, NaOH, and KOH aqueous solutions with a result of the distinct cations. It is found that Co-Al LDH undergoes two independent electrode processes in LiOH aqueous solution, involving the simultaneous intercalation of an ion-pair, i.e. lithium cation and hydroxyl group, which is different from the mechanisms in NaOH and KOH aqueous solutions. The reason is thought to be the selective intercalation into Co-Al LDH for alkali metal ions due to their respective ionic radius. Only Li+ and Na+ cations are suitable for intercalating in the remaining vacancies of （OH）6 octahedra. Compared with lithium and sodium cations, potassium ion has too large radius to fill into the [Co（OH）6] or [Al（OH）6] octahedral vacancies.At last, the effect of electrode’s electrical conductivity on the pseudocapacitive performances of Co-Al layered double hydroxide is studied by mixing multi-wall carbon nanotubes （MWCNTs） with Co-Al LDH. It is found that either specific capacitance (342.4 F·g^-1) or long-term performance of all composite electrodes at high current density is superior to pure LDH electrode (192 F·g^-1) due to the network of MWCNTs attached to the surface of LDH, which betters interconnection of Co-Al LDH particles and decreases contract resistance. On this basis, MWCNTs and Co-Al LDH, as two electroactive materials, are integrated in one electrode to form a new-concept self-hybrid composite electrode in the light of the thought that different materials should match correctly according to their respective charge-storage mechanisms. Cyclic voltammetry and chronopotentiometric techniques are employed to evaluate the electrochemical behaviors of the symmetric self-hybrid supercapacitor in 1 M KOH solution, giving the results that the CV curves approach rectangle shapes, and that charge/discharge curves are basically symmetrical.In a word, on the basis of above analysis and researches, a conclusion can be drawn out that LDHs with special layered architecture are hopeful in supercapacitors as alternate electrode material.更多还原