基于双阳离子型离子液体超级电容器中界面现象的研究

As an advanced class of electrical energy storage devices, supercapacitors, also called electrical double layer capacitors (EDLCs), are being of importance to energy storage in the green energy community, especially for storing electricity generated from the solar or wind. To address the main issue of supercapacitor development, owing to their exceptionally wide electrochemical window, ionic liquids (ILs) have received a great deal of attention in recent years, emerging as good candidates for electrolytes used in supercapacitor. However, to date, most studies on IL-based supercapacitor focused on monocationic ILs (MILs), i.e., with monovalent cations, whereas the work on supercapacitor based on the newly-developed dicationic ILs (DILs, i.e., each cation carries two unit charges) is quite elusive, due to their later and more difficult synthesis, and efforts are required in this area to develop new-type supercapacitor. Thus, on the basis of my previous investigations of MIL supercapacitor, the research proposed in this project will aim at addressing key issues in supercapacitor with DILs as the electrolyte. Since supercapacitors store electrical energy via ion electrosorption directly in the electrical double layers (EDLs) at the electrolyte/electrode interface, EDLs, generally in thickness of few nanometers, play a dominant role in the underlying energy storage mechanism and the resulting device performance. In this project, the molecular dynamics simulation will be taken as the major approach to investigating the interfacial phenomena (e.g., the microstructure and capacitive property of EDLs) in DIL-based supercapacitors with different-designed electrode surfaces, compared with experimental observations. The results, obtained in molecular level, would be of great significance to fundamental understanding of the energy storage mechanism in supercapacitor, and, in the help of comparison with the observations from counterpart MILs, could render useful guidance and new ideas to develop high-performance IL-based supercapacitor with enhanced energy density.

离子液体作为超级电容器的电解质正受到越来越多的重视。然而，迄今有关离子液体超级电容器的工作大部分都集中于单阳离子型（即单价阳离子）离子液体为电解质的研究领域，而对于新近开发的双阳离子型（即双价阳离子）离子液体，相关研究还很少。鉴于此，申请者拟在前期研究单阳离子型离子液体超级电容器的基础上，研究双阳离子型离子液体在超级电容器中的重难点问题。具体方法是，以单阳离子型离子液体为比较对象，以分子层面的计算模拟为主要手段，结合实验观测结果，深入探究双阳离子型离子液体在不同电极表面的微观结构和电容特性，揭示与之相关的储能机理，从而给开发具有高性能的超级电容器提供科学依据和设计思路。本项目的开展将由简到繁、逐步深入：先明了非受限状态下双阳离子型离子液体与单阳离子型的异同，然后揭示非孔隙受限下该离子液体形成的双电层的结构和电容特性，再研究不同孔隙内双电层的储能机制，同时研究添加有机溶剂的影响机理。

超级电容器因其具有功率密度高、循环寿命长、稳定性优、环境友好等优点，在储能领域正受到越来越多的关注；而离子液体作为超级电容器的新型电解质可以进一步提高其电容性能和工作温度，是目前国内外的研究热点。本项目结合工程热物理和材料科学的知识，研究了基于双阳离子型和单阳离子型离子液体超级电容器中的双电层储能固液界面。主要研究内容包括：平板型电极表面离子液体双电层的研究、非平板型电极表面离子液体双电层的研究、混合离子液体用作电解质的研究以及把离子液体拓展到碳捕捉上的初步研究。研究成果主要包括：（1）获得了数十种离子液体的力场参数，并附以实验结果进行校验，开发了特定的、可对离子液体进行自动建模和基本物性分析的分子动力学软件包；（2）研究了平板型电极上离子液体双电层的微观结构和电容性能，发现采用非对称性双阳离子型离子液体可以提高超级电容器的能量密度，表明通过改变离子液体的离子形态是提高超级电容器电容性能的有效途径之一；（3）利用模拟和实验对比发现原子力显微镜的探针在测量过程中对离子具有选择性：探针倾向于探测到体积或质量较大的阳离子或阴离子；（4）研究了离子液体在具有缺陷碳电极表面上的微观表征，模拟结果和利用高分辨率的原子力显微镜所观察到的实验现象较为一致，并发现表面缺陷大小会影响离子分布的震荡特性；（5）研究了离子液体混合物在超级电容器中储能机理，揭示了带电界面上离子液体中极少量的水的微观分布及其随电极表面上施加电压的变化规律，并阐述了该规律的形成和作用机理；这一工作被来自于四个国家、六个大学的研究者们评价为“第一个关于含水离子液体在带电界面的计算研究”；（6）拓展了双阳离子型离子液体在碳捕捉中的研究，发现双阳离子型离子液体相比对应的单阳离子型离子液体具有更高的CO2/N2选择比。依托该项目，共发表SCI期刊论文15篇、做国际国内邀请报告13次。由于本项目中期完成情况较好，被选做基金委项目进展口头汇报。