【作者】 刘颖；

【导师】 闫鹏勋；

【作者基本信息】 兰州大学 ， 材料物理与化学， 2014， 博士

【摘要】 能源是二十一世纪最重要的研究课题之一。伴随着全球经济的快速发展,以及化石燃料枯竭、环境污染等问题日益严峻,迫切要求人们开发能取代化石燃料的可再生清洁能源及其相关的高效能源转化和存储技术。在许多应用领域,化学电源和电化学电容器(Electrochemical Capacitors,简写为ECs,也称超级电容器)是电化学能源转化和存储最为有效、实用的技术。近年来,电化学电容器因具有电容量大、功率密度高、充放电速度快、循环寿命长、工作温度范围宽、环境友好以及安全可靠等优点而备受人们的关注。电化学电容器的储电性能强烈依赖于电极中采用的活性物质。根据储能机理的不同,电化学电容器电极材料可分为碳材料、过渡金属氧化物和导电高分子三类。在它们之中,层状结构水钠锰矿(birnessite-type manganese dioxide,简写为Bir-MnO2)因其较高的理论容量和离子渗透率,以及环境友好、资源丰富等优点,被视为下一代高性能电化学电容器的理想电极材料之一。然而,Bir-MnO2电极材料存在电导率低、比表面积小和易溶解等不足,这极大地限制了它在电化学电容器领域的广泛应用。目前,纳米化和复合材料是提高Bir-MnO2电化学性能的主要途径。本论文研究以提高Bir-MnO2电极材料的电容性能为目标,从合理设计电极材料微观结构的思路出发,设计并制备出了几种基于Bir-MnO2的电极材料。通过系统研究这几种电极材料的储能特性,阐明了它们各自“成分-结构-性能”之间的关系。论文的主要研究内容和实验结果如下：1.改进传统水热法制备Bir-MnO2纳米材料的缺点(高温、还原剂、表面活性剂或模板等),采用一种简单的低温水热法制备出Bir-MnO2纳米片结构。SEM和TEM图像显示,所制备的Bir-MnO2纳米片长度为几百纳米,厚度在5～10nm之间。将其用于电化学电容器的电极,充放电电流密度0.1A/g时的比容量达到169F/g；且当电流密度增加到5A/g时,比容量仍为96F/g,具有较好的倍率特性。同时,Bir-MnO2电极材料也表现出良好的循环稳定性,在1A/g下充放电循环1000次后的容量保持率为94%。2.氧化石墨烯(GO)的表面上具有大量的含氧官能团,比如羧基、羟基、羰基和环氧基等。这些官能团不仅使得GO能在水中形成稳定悬浮液,还能吸附其他离子或分子而合成复合材料。借助这种优势,本文采用一种简单的水热法,通过控制反应物浓度、反应温度和反应时间等参数,在GO基底上垂直生长了Bir-MnO2纳米片阵列,从而制备出一种新型电极材料-层状二氧化锰／氧化石墨烯(Bir-MnO2/GO)纳米复合材料。实验结果表明,通过改变反应时间可制备出了不同MnO2含量的Bir-MnO2/GO复合材料,且反应时间12h时,所制备的电极材料电化学性能最好。在1mol/L Na2SO4电解液中,Bir-MnO2/GO复合材料的比容量达到213F/g(充放电电流密度为0.1A/g),1A/g下充放电1000次后的循环寿命达到98.1%。另一方面,电化学阻抗谱分析结果进一步显示,Bir-MnO2/GO电极的等效串联电阻比Bir-MnO2电极低,意味着电荷更易在Bir-MnO2/GO电极材料中嵌入／脱嵌,从而表现出优于单一组分Bir-MnO2和GO电极的电化学性能。3.GO的导电性较差,将其作Bir-MnO2纳米结构的载体时,电极材料的储电性能提升空间有限。因此,采用比表面积大、导电性良好的还原氧化石墨烯(rGO)作为生长Bir-MnO2纳米片阵列的基底,将能制备出更高性能的电化学电容器电极材料。基于这种分析,我们采用水热法合成了一种新型的层状二氧化锰／石墨烯(Bir-MnO2/rGO)纳米复合材料,并通过控制反应时间探讨了该材料的形成机理。实验结果表明,Bir-MnO2/rGO复合材料的特殊结构提供了良好的导电性、快速的电子和离子传输速率,以及较高的MnO2利用率,使其在1mol/L Na2SO4电解液中表现出了优于Bir-MnO2和rGO的比容量(充放电电流密度0.2A/g时的比容量达到315F/g)、倍率特性(电流密度增加到6A/g时的比容量为193F/g)和循环寿命(3A/g下充放电循环2000次后的容量保持率依旧为87%)。4.与Bir-MnO2的理论比容量值(1370F/g)相比,Bir-MnO2/rGO复合电极材料的比容量还比较低,仍具有较大的提升空间。另一方面,石墨烯复合材料的研究近年来正在逐渐兴起。但目前的研究主要集中在二元复合材料,对基于石墨烯、过渡金属氧化物和导电聚合物等三类材料复合形成的多元复合材料的研究还比较少。因此,为了进一步提高Bir-MnO2/rGO复合材料的电容性能,我们以多元复合材料为研究对象,首次提出了一种微观结构独特且尚未见报道的多元复合电极材料设计方案,即在石墨烯上垂直生长Bir-MnO2纳米片阵列,并在Bir-MnO2/rGO纳米结构外面包覆导电聚合物聚3,4-乙烯二氧噻吩-聚苯乙烯磺酸(poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate),简写为PEDOT:PSS)。同时,对所设计的Bir-MnO2/rGO/PEDOT:PSS多元复合材料作为电化学电容器电极材料的可行性进行了初步探究。实验结果表明,30wt%PEDOT:PSS含量的Bir-MnO2/rGO/PEDOT:PSS复合材料表现出最好的电容特性。在电流密度为0.1A/g时,其比容量达到249F/g。更多还原

【Abstract】 Energy is one of the most important research topics in this century. With the rapid development of global economy, the depletion of fossil fuels, as well as the increasing environmental pollution, there is an urgent need to seek renewable and clean energy sources that can substitute fossil fuels, and to develop efficient technologies associated with energy conversion and storage. In many application areas, the most effective and practical technologies for electrochemical energy conversion and storage are batteries and electrochemical supercapacitors (or ultracapacitors). In recent years, electrochemical capacitors (ECs) have attracted significant attention, mainly due to their high power density, rapid charging-discharging rates, long lifecycle, wide operating temperature range, environmental compatibility, and good operational safety.The properties of ECs intimately depend on the active materials used in electrodes. According to different energy storage mechanisms, the electrode materials for ECs can be categorized into three types:carbon materials, transition metal oxides, and conducting polymers. Amongst them, birnessite-type manganese dioxide (Bir-MnO2) is believed to be a promising electroactive material for the next generation of supercapacitors, due to its high theoretical specific capacitance, high ion permeability, environmental compatibility, low cost, and abundance in nature. However, the birnessite MnO2electrode materials are suffered from some disadvantages, such as poor electronic conductivity, low specific surface, and partial dissolution, which greatly limit its wide application in ECs. Lately, preparing nanostructures or composite materials is the major method to improve the electrochemical properties of Bir-MnO2.In order to meet the demand of high-performance electrode materials for ECs, this dissertation designed and prepared several Bir-MnO2based electrode materials with the idea of rationally designing the microstructure of materials. By systematical studie on the energy storage characteristics of these materials, the relationships between the component, structure and performance were illustrated. The concrete research contents and results are summarized as follows:1. By overcoming the disadvantages of traditional hydrothermal methods for birnessite-type MnO2nanostructures, such as high temperature, reductants, surfactants and/or templates, Bir-MnO2nanosheets were synthesized by a low-temperature hydrothermal method. Nanosized Bir-MnO2sheets with lateral size of a few hundred nm, and thickness of5-10nm were observed from SEM and TEM images. To assess the as-synthesized MnO2nanosheets for use in supercapacitors, the electrode exhibits a high specific capacitance of169F g-1at a current density of0.1A g-1, good rate capability with a capacitance of96F g-1even at a high current density of5A g-1, as well as excellent cycle stability with capacitance retention of94%at1A g-1after1,000cycles.2. There are a large number of the oxygen-containing functionalities on the surface of GO, such as epoxide, hydroxyl, car-bonyl and carboxyl groups, et al. It not only makes GO form stable suspension in water, but also can easily adsorb ions or monomers to fabricate composites. Depending on this inherent advantages of GO, we synthesized an advanced birnessite-type manganese dioxide/graphene oxide (Bir-MnO2/GO) hybrid via a simple hydrothermal methods. In this hybrid, MnO2nanosheets arrays were vertically grow on graphene oxide flakes by adjusting the concentrations of reactants, the reaction temperature and time in hydrothermal procedure. The experimental results revealed that Bir-MnO2/GO hybrids with different Bir-MnO2content could be prepared by changing the reaction time, and the hybrid with a reaction time of12h showed the best electrochemical performances. In an electrolyte of1M Na2SO4, it exhibited a high specific capacitance (213F g-1at current density of0.1A g-1), a good rate capability (even80F g-1at10Ag-1) and a long cycle life with the capacitance retention ratio of98.1%at1A g-1after1,000cycles. Furthermore, the EIS measurements demonstrated the electrochemical resistance of MnO2nanosheet which directly grows on GO is reduced, indicating easier access for intercalation/deintercalation of charges in hybrid. Thus, the Bir-MnO2/GO hybrid displayed enhanced energy storage performances compared with the GO and Bir-MnO2.3. The conductivity of GO is too bad to effectively enhance the capacitive performances of Bir-MnO2nanosheets. In order to synthesis high-performance electrode materials, it would be much better to use the reduced graphene oxide (rGO) as the substrate for Bir-MnO2nanosheets arrays. Based on this analysis, we used a facile hydrothermal method to fabricate the novel birnessite-type manganese dioxide/reduced graphene oxide (Bir-MnO2/rGO) hybrid, in which Bir-MnO2nanosheets arrays were uniform and vertically grow on rGO flakes. The formation mechanism of the hybrid is also discussed based on a series of time-dependent experiments. The results revealed that the unique structure of the hybrid provided good electronic conductivity, fast electron and ion transport, and high utilization of MnO2, which made the Bir-MnO2/rGO electrode exhibit much higher specific capacitance (315F g-1at a current density of0.2A g-1) and better rate capability (even193F g-1at6A g-1) compared with both the rGO and Bir-MnO2electrodes. Moreover, the capacitance of the electrode is still87%retained after2000cycles at a charging rate of3A g-14. One hand, the specific capacitances of the as-synthesized Bir-MnO2/rGO hybrid are far below the theoretical value of Bir-MnO2(1370F g-1). Thus, there is still room for the improvement of electrochemical capacitance performances. On the other hand, the MnO2/rGO composites have attracted considerable interest, and plenty of such composites have been reported in recent years. However, to date, researches on this field are mainly focuse on graphene-based binary composites, ternary composites based on graphene, transition metal oxides, and conducting polymers have seldom been reported for use as electrochemical capacitors. Hence, we took ternary composites as the object of the further reseach for Bir-MnO2/rGO hybrid, and designed a new ternary composite with unique structure. In this composite, Bir-MnO2nanosheets arrays were vertically grow on rGO flakes, and the Bir-MnO2/rGO nanostructure was wrapped by poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS). Here, the practicability of the as-designed ternary composites using as an electroactive material for electrochemical capacitors was preliminarily explored. The experimental results revealed that the Bir-MnO2/rGO/PEDOT:PSS composite containing30wt%PEDOT:PSS exhibited the best electrochemical performances. A specific capacitance of249F g-1could be obtained at a current density of0.1A g-1.更多还原