独特形貌氧化锰纳米电极材料的可控制备及其电容性质研究

【作者】 朱刚；

【导师】 刘宗怀；

【作者基本信息】 陕西师范大学 ， 无机化学， 2013， 博士

【摘要】 氧化锰材料由于资源丰富、价格低廉、环境友好、理论比电容高等优点,被认为是最具发展潜力的超级电容器电极材料之一。本论文采用水热法可控制备了一系列不同晶相和形貌的氧化锰纳米材料,探讨了水热制备条件对氧化锰纳米电极材料形貌和晶相的影响,系统研究了氧化锰纳米电极材料结构、比表面积、形貌及结晶性等参数对其电容性质的影响。论文的主要内容如下：(1)以高锰酸钾为锰源、有机胺/铵和无机铵为还原剂,发展了制备大比表面积介孔Birnessite型层状氧化锰纳米材料的简易方法。以尿素为反应还原剂,通过低温水热反应制备了片层状氧化锰。制备材料的BET比表面积为230m2/g,反应体系中的尿素对于氧化锰粒子的生成和氧化锰结构中介孔的形成起着至关重要的作用,且水热反应温度对制备材料的晶相和形貌有显著影响。以硫酸钱为反应还原剂时,制备得到了花状微球形氧化锰,花状微球由薄的纳米片组装而成,比表面积为280m2/g,平均孔径为3.8nm。制备形貌规则、粒径大小分布均匀层状氧化锰的最优条件是：高锰酸钾与硫酸铵物质的量之比为1：0.5,水热反应温度为90℃,反应时间为24小时,该方法是一种制备介孔结构层状氧化锰纳米材料的绿色策略。当CTAOH为反应还原剂原料时,成功制备了分级结构层状氧化锰纳米材料,其比表面积为105m2/g,平均孔径为7.8nm。反应中CTAOH既是还原剂,又是表面活性剂,CTAOH的引入对于氧化锰分级结构的形成起着非常重要的作用。水热反应温度对制备材料的晶相和形貌有显著影响,随着反应温度的升高,产物由花球状氧化锰转变为具有棒状形貌碱式氧化锰。(2)采用两步法制备了纳米带状高结晶性Birnessite型氧化锰材料。以高锰酸钾和乙醇反应所得碱式氧化锰为前驱物,在140℃的水热温度和10mol/L KOH溶液中反应72小时,制备得到了比表面积为53m2/g的带状Birnessite型层状氧化锰纳米材料。水热反应温度、反应时间和碱式氧化锰的加入量均会影响制备产物的晶相。在高浓度碱液中,碱式氧化锰经历了溶解-重结晶过程,最终转变为层状氧化锰纳米带。(3)发展了一维隧道型线状α-MnO2和棒状β-MnO2纳米材料制备新方法。以锰粉为锰源、K2S2O8为氧化剂,采用水热合成技术可控制备了一维隧道型线状α-MnO2和棒(?)β-MnO2纳米材料。该实验方法简单,无催化剂和模板剂添加,且产物纯度高。研究结果表明,水热反应温度、反应时间和体系中K+离子浓度均是影响产物晶相和形貌的重要因素。K+离子是α-MnO2的模板剂,且高的K+离子浓度可以保持在高温处理时α-MnO2晶相不发生变化。通过改变实验条件,可以实现α-MnO2纳米线向β-MnO2纳米棒的转变。温度和酸度共同促使α-MnO2转变为β-MnO2,两者缺一不可,高温和高的H+离子浓度有利于β-MnO2的生成。(4)采用循环伏安、恒流充放电和交流阻抗技术,系统研究了制备的不同形貌层状和隧道型氧化锰纳米材料的电容性质。通过比较和分析制备的氧化锰电极材料的电化学测试结果,发现影响氧化锰电容性质的因素是多方面的,如材料的晶相、结晶性、比表面积和形貌等。通常,层状氧化锰和具有较大孔道尺寸的隧道型氧化锰的比电容较高,而具有较小孔道尺寸的隧道型氧化锰由于电解质离子难以进入隧道而具有很低的比电容。低结晶性氧化锰材料由于晶格松弛,离子嵌入／脱出反应容易进行,因而比高结晶性材料电容性质优异。大比表面积氧化锰材料往往可以提供更多的氧化还原反应活性位点而具有更高的比电容。氧化锰材料的形貌与其比表面积相关,而且会影响离子嵌入／脱出的路径,因而也是决定材料电容性质的重要因素之一。分级结构氧化锰材料在进行能量存储时具有很大优势,因为其兼具纳米尺寸构筑单元和亚微米尺寸堆积结构的优点。(5)分级结构层状氧化锰纳米电极材料具有优异的电容性质。在不同形貌层状和隧道型氧化锰纳米材料中,分级结构层状氧化锰纳米电极材料具有优异的电容性质。优良电容性质归因于材料低的结晶度、高的比表面积和独特的分级结构。当扫描速度为5mV/s时,比电容为347F/g。循环寿命测试结果表明,在扫描速度为20mV/s的条件下,10000次循环后的比电容衰减仅为2.5%。同时,由该分级结构层状氧化锰电极和石墨烯电极组成的不对称超级电容器显示了高的能量密度和功率密度,功率密度为400W/kg时相应的能量密度为20.9Wh/kg。分级结构层状氧化锰纳米电极材料是组装性能优良超级电容器的候选电极材料。更多还原

【Abstract】 Manganese oxides with different crystal structures and morphologies are considered as one of the most potential materials for supercapacitors due to their abundant resources, relatively low cost, environmentally friendly nature and high theoretical specific capacitance. In this paper, a series of manganese oxide nanomaterials with different crystal structures and morphologies have been prepared by the hydrothermal treatment method, and the hydrothermal treatment conditions are discussed systematically. On the basis of the preparation of manganese oxide nanomaterials with different crystal structures and morphologies, the effects of crystal structure, specific surface area, morphology and crystallinity on the capacitive properties of manganese oxide nanomaterials have also been investigated. The research contents are as follows:(1) KMnO4as manganese source and organic amine/ammonium and inorganic ammonium as reducing agent, a simple preparation method of manganese oxide nanomaterials with large surface area and layered structure is developed.Layered manganese oxide with plate-like morphology has been hydrothermally prepared at90℃for24h in a reaction system of KMnO4and urea. The BET specific surface area of the layered manganese oxide with plate-like morphology is230m2/g. Research results indicate that urea plays a crucial role for the formation of the manganese oxide nanomaterial with large surface area and layered structure. And hydrothermal temperature has great effects on the crystal phase and morphology of the prepared materials. When (NH4)2SO4is used as reductant, birnessite-type manganese oxide with flower-like microsphere morphology and large specific surface area has been prepared by hydrothermal treating a mixture solution of KMnO4and (NH4)2SO4at90℃for24h. Results indicate that the birnessite-type manganese oxide shows novel flower-like microsphere morphology and a specific surface area of280m2/g, and the flower-like microsphere consists of the thin nano-platelets. On the basis of the optimizing experiments, the optimization preparation conditions of the manganese oxide nanomaterial with large surface area and layered structure is a molar ratio of KMnO4to (NH4)2SO4=1:0.5. This preparation approach is a green strategy for synthesizing layered manganese oxide nanomaterials because no organic solvent or surfactant is added in the reaction system. When cetyltrimethylammonium hydroxide (CTAOH) is used as reductant, hierarchical manganese oxide nanomaterial has been simply prepared via a hydrothermal treatment technology in a mixed solution of KMnO4and cetyltrimethylammonium hydroxide (CTAOH) at90℃for12h. The obtained material has a specific surface area of105m2/g and the average pore size is7.8nm. CTAOH serves as both a reductant and a surfactant, and it plays a very important role in the formation of the hierarchical structure. Hydrothermal temperature has an obvious influence for the crystalline and morphology of the obtained materials. In company with the hydrothermal treatment temperature to180℃, the layered birnessite structure has completely transformed into purity y-MnOOH with rod-like morphology.(2) Birnessite manganese oxide material with belt-like morphology and high crystallinity has been prepared through two-step hydrothermal reaction.Firstly, MnOOH is prepared by reacting KMnO4and ethanol at140℃for24h. Then, the obtained MnOOH is hydrothermally treated at140℃for72h after it is well dispersed in10mol/L KOH aqueous solution. The specific surface area of the belt-like manganese oxide nanomaterial is53m2/g. Hydrothermal treatment temperature, reaction time and the amout of MnOOH all can affect the crystalline phases of the obtained materials. A transformation process has been observed. MnOOH is dissolved and followed by recrystallizing, and finally transformed into the birnessite manganese oxide material with belt-like morphology in a strong basic solution.(3) A new preparation method of both α-MnO2nanowires and β-MnO2nanorods has been developed.Manganese powder is used as manganese source and K2S2O8is used as oxidant. a-MnO2nanowires and β-MnO2nanorods are controllably prepared with a hydrothermal method at150℃and180℃for24h, separately. The method is simple for no catalysis or template reagent is added, and the products have high purity. Research results show that hydrothermal temperature, reaction time and the concentration of K+ions in the reaction system affect the crystal phase and moiphology of the products. K+ions serve as both template and structure stabilizer for a-MnO2. The crystal phase of a-MnO2maintains unchanged in aqueous solution with high K+ions concentration when treated under higher hydrothermal temperature. Under certain conditions, α-MnO2nanowires can transform into β-MnO7nanorods. Higher hydrothermal temperature and H+ions concentration favor the the formation of β-MnO2. (4) The electrochemical properties of the prepared manganese oxide nanomaterials with different crystallinities and morphologies have been systematically investigated.Research results indicate that the capacitive properties of the prepared manganese oxide nanomaterials with different crystallinities and morphologies connects with their crystal phase, crystallinity, specific surface area and morphology. Generally, the specific capacitance of layered manganese oxides and tunnel manganese oxides with larger cavity sizes is higher than that of tunnel manganese oxides with smaller cavity sizes. The capacitive behavior of manganese oxide nanomaterials with low crystallinity is superior to than those with high crystallinity due to their loose lattice. Manganese oxide nanomaterials possessing high specific surface area show high specific capacitance because they can provide more redox electroactive sites. Morphology of manganese oxide materials is related to their specific surface area and also affects the transport/diffusion path lengths for ions and electrons. Hierarchical structure manganese oxide materials have found to be one of the best systems for energy storage, because it offers both advantages of nanosized building blocks and submicrometer-sized structures.(5) Manganese oxide nanomaterial with hierarchical structure has good capacitance.The perfect electrochemical property of hierarchical manganese oxide nanomaterial is ascribed to the unique hierarchical structure, poor crystalline and relatively high specific surface area. Manganese oxide nanomaterial with hierarchical structure exhibits not only high specific capacitance of347F/g, but also excellent cycle stability (97.5%capacitance retention after10000cycles at a scan rate of20mV/s). An asymmetric supercapacitor based on the obtained manganese oxide as a positive electrode and graphene as a negative electrode is assembled. The assembled asymmetrical supercapacitor gives a high energy density of20.9Wh/kg at a power density of400W/kg. The hierarchical manganese oxide nanomaterial is a very promising electrode material for assembling supercapators with good capacitive performance.更多还原