碳电极微孔内离子液体进出孔迁移的计算研究

Based on its porous structure and high specific surface area, porous carbon is a suitable candidate for high-performance electric double layer capacitor with ionic liquids. The complexity of the porous structure, however, makes it difficult to distinguish the influence of specific pore size and shape on double layer capacitance. In particular, micropore owns a similar size to ionic liquids, which could either form compact double layer efficiently within the micropore, or get excluded and leave the pore empty. Therefore, we study the system of microporous carbon electrode immersed in ionic liquids electrolytes. Micropore is formed by structurally optimizing the constituent sp2 carbons in hexagonal geometry. Both ab-initio and classical molecular dynamic simulations are adopted to investigate the ion density distribution with varying electrode potential, pore size, pore geometry and initial configuration of ions inside micropore. We will summarize the links between pore structure, ion migration and electric double layer capacitance. As far as power density is concerned, the ion conductivity is computed while the possible correlation between cation and anion diffusion is also explored. In addition, the density functional theory based on statistical mechanics will be employed to study the free energy profiles along the ion migration path into and out of micropore. The reasons behind the ion migration will be revealed from energy’s perspective. At the same time, electrochemical measurements are to be conducted on porous carbon electrodes. Impedance spectrum data will be used to firstly derivate the equivalent circuit at approximate pore size distribution, and secondly compare with the equivalent resistance and capacitance obtained from simulation results.

多孔碳材料具有高比表面积，浸入离子液体中形成双电层电容，可望实现较高的储能密度。然而多孔碳中的孔结构复杂多样，难以区分特定孔尺寸和形态对电容的影响。尤其是和离子尺寸接近的微孔，既可能吸附离子在孔内形成紧密的双电层，又可能因离子难以迁入造成孔的空置。因此本项目以碳电极微孔/离子液体电容系统为对象，通过sp2杂化碳原子的优化构成一系列微孔结构。使用从头算、分子动力学模拟等方法，在不同微孔结构、尺寸、初始构象的条件下，统计微孔内离子分布如何随时间、随电势变化，归纳微孔结构-离子进出孔迁移-电容性能三者之间的联系。功率性能方面，计算孔道内离子传输的电导率，探索阴、阳离子扩散的相关性。结合统计力学的密度泛函理论，计算自由能沿迁移路径的变化，揭示离子迁入、迁出微孔的原因。对多孔碳电极进行电化学性能测试，基于交流阻抗谱图推算出多孔碳整体的等效电路，依据孔径分布近似换算成单个微孔的等效电容、阻抗，与模拟结果比较。

本项目中以不同类型（凹面、凸面）与不同几何结构（球体、平板、圆珠）为基础结构构筑碳电极模型，通过分子动力学模拟实现了电化学测试，同时与多孔碳电极实验的循环伏安法、交流阻抗测试对比。通过比较不同微孔结构配合多种离子液体电解液，总结了电极微孔结构与电容性能之间的一般规律。利用经典密度泛函理论计算自由能表达式中的各能量项，通过不断改变狭缝孔尺寸，计算了孔内离子分布与两表面之间作用力的变化规律，同时测试了改变电解液浓度等的影响，总结了两平板电极之间长程作用力衰减的机理。在动力学性质模拟中，总结电极表面电荷的极化与附近吸附离子运动的关联，为提高超级电容器的储能密度、快速充放电能力提供了理论计算依据。基于交流阻抗谱图推算出不同几何结构的碳电极模型的整体等效电路，分析了该体系电容、电阻性能在不同温度和电极构型条件下的变化规律，圆满完成了项目研究目标。