【作者】 王静；

【导师】 胡万彪；

【作者基本信息】 云南大学 ， 材料工程（专业学位）， 2019， 硕士

【摘要】 电极材料是超级电容器的核心,其电化学性质直接决定超级电容器性能。过渡金属氟化物BaMF4(M=Mn,Co和Ni)作为超级电容新型电极材料,因具有特殊的极化特征和层状结构,展现出极大的开发和应用前景。目前,BaMF4中M离子自身价态及其在电化学反应过程中的价态变化与电化学性质关联尚未清楚;此外,现有报道的BaMF4粒径过大(几十微米级),如何制备纳米级材料并构筑利于电化学过程电子离子输运的通道也具有极大挑战性。针对这些问题,本论文选取BaMF4家族材料中电化学性能最优异的电极材料BaCoF4,开展纳米级合成制备、微结构调控以及电化学性能和机制等研究,主要内容和结果如下:(1)系统调控水热反应的温度、时间、反应物配比和浓度、F源选取等条件因素,以CoF2、BaF2和CF3COOH同时作为F源,且Co源过量5 at%,230 ℃反应24h的最终条件,制备出纯相BaCoF4(空间群Cmc21)电极材料。所制备的BaCoF4颗粒呈现大块状层状堆积结构,尺寸为微米级。电化学测试表明BaCoF4电极材料表现出明显氧化还原和赝电容行为,典型的赝电容超级电容材料,比电容值为121 F·g-1。XPS进一步证实电化学反应的氧化还原过程呢,钴离子二价、三价之间(Co2+/Co3+)的价态变化是电化学活性和电极电荷存储的根源。(2)采用表面活性剂辅助水热方法,在纯相样品制备条件基础上添加0.4 g(0.0239mol/L)十二烷基苯磺酸钠(SDBS)调控BaCoF4的片层状堆积方式,制备出取向(晶带轴)为[3-1 0]的二维纳米片状结构,与理论上的BaCoF4离子通道[10 0]或者a轴方向较接近。由于离子通道方向上BaCoF4材料尺度减小,参与电化学反应的有效活性Co离子的数量增加,即提高了高活性位点密度,片状结构BaCoF4电极材料展现出极其优异的超级电容储能性质,比电容值提升至692.3 F·g-1,比直接制备的块体材料的值约提高了 5倍左右。因此,构筑离子通道有利于提升电极材料的综合电化学性能。(3)研究BaCoF4电极材料的热稳定性及温度微结构变化,获得了该材料稳定性的重要参数。水热制备的块体和片状结构BaCoF4均在300 ℃左右开始分解,产生BaF2。保护气氛下,BaCoF4电极材料的分解温度有所提高。分解过程中,BaCoF4颜色由粉色逐渐变为灰色至黑色,颗粒尺寸变小,导电特性提升。尽管热处理后BaCoF4的电化学性质有所提升,例如块体材料的电容值相应地提高了3倍左右,结构热稳定性仍然是该类材料作为高温电极材料需要考虑的一个问题。更多还原

【Abstract】 The electrode material is the crucial component of a supercapacitor.The electrochemical property of the electrode material directly determines the performance of the supercapacitor.As a new family of supercapacitor electrode materials,transition-metal fluoride BaMF4(M= Mn,Co and Ni)compounds,due to possessing special polarization characteristics and lamellar structures,have raised extensive attention for the promising applications.On the explorations of such material sorts,there are still several fundamental questions to be resolved.For instance,the valence state of the M cation for the as-prepared BaMF4,the valence state variations during electrochemical reaction as well as their impacts on the electrochemical behaviors,are not yet clear.On the other hand,the existing works on the BaMF4 synthesis always lead to quite large particle sizes(normally at micron scale)that are not conductive to achieve high performance for the supercapacitor because of the too less active sites.As a consequence,how to prepared the BaMF4 materials with small sizes and construct the proper channels favoring the electronic and ionic transport in order to greatly improve the active sites are also quite challenging.To address the aforementioned questions,we selected BaCoF4 that shows the best performance in BaMF4 family as the electrode material protype and imitated the studies and investigations on the nanoscale synthesis,microstructural control,electrochemical properties and mechanisms,with giving rise to the following results and conclusion:(1)Through systematical control over the hydrothermal temperatures,duration,concentrations/ratios of the starting chemicals and the F source use,pure-phase BaCoF4 with orthorhombic structure(S.G.:Cmc21)was synthesized with the final conditions:simultaneous utilizations of CoF2,BaF2 and CF3COOH as F sources with an excess of 5at%for CoF2 at 230 0C for 24 hours.The as-prepared BaCoF4 particle presents a bulk form with the size at micron-scale that consists of the multilayer lamellar structures.Electrochemical tests demonstrated the obvious REDOX and pseudo-capacitance behaviors for the present BaCoF4 electrode material with showing a specific capacitance of 121 F·g-1.The REDOX process was confirmed by XPS technique that reveals the valence state variations of the active Co cations(Co2+/Co3+)during the electrochemical reactions.To conclude,electrochemical property and charge storage performance for BaCoF4 are originated from the valence state evolutions of the active Co cations.(2)A strategy of the surfactant utilizations coupled with the aforementioned hydrothermal conditions was proposed to further thin the dimensions of the BaCoF4 electrode material.When 0.4 g(0.0239 mol/L)of the surfactant i.e.sodium dodecyl benzene sulfonate(SDBS)was used for hydrothermal reaction,BaCoF4 with nanosheet structure characterized in the[3-1 0]orientation(crystal zone axis)was synthesized.The as-synthesized BaCoF4(OR-BaCoF4)thus exhibits the feature of ionic tunnels from the crystallographic analysis because this is close to the theoretically ionic tunnel i.e.[100]or a direction of BaCoF4.As a result of the thinning along ionic tunnels that promote more active Co cations to fulfil the electrochemical reactions,the OR-BaCoF4 electrode materials exhibit the excellent energy storage properties with the specific capacitance increasing up to 692.3 F·g-1,which is almost 5 folds higher than that of the bulk BaCoF4 synthesized without adding SDBS.(3)The thermal stability and structural transformations upon high-temperature treatment were investigated with the aim to acquire the fundamental parameters for BaCoF4.Above 300 ℃,both the as-synthesized bulk BaCoF4 and OR-BaCoF4 start to decomposition,with producing BaF2.The BaCoF4 sample color changed gradually from original pink to gray and/or black,which indicates the formation of Co3+ that could enhance the electronic conductance.Heat treatment in protective atmosphere(N2)could slightly improve the thermal decomposition temperatures.Electrochemical tests demonstrated a noticeable enhancement in the charge storage,e.g.the bulk BaCoF4 after heat treatment showed an about 3-fold larger capacitance.However,if BaCoF4 is selected as a high-temperature electrode material candidate,the structural stability should be taken into consideration.更多还原