Simulation Study of the Capacitance and Charging Mechanisms of Ionic Liquid Mixtures near Carbon Electrodes
Simulation Study of the Capacitance and Charging Mechanisms of Ionic Liquid Mixtures near Carbon Electrodes
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DOI：
10.1021/acs.jpcc.8b10334
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发表时间：
2019-01-24
影响因子：
3.7
通讯作者：
Smolyanitsky, Alex
中科院分区：
文献类型：
--
作者：
Fang, Alta;Smolyanitsky, Alex
双电层电容器的性能受到电解质选择的强烈影响，并且由离子液体混合物组成的电解质已经显示出实现高能量密度的前景。在这里，我们进行全原子分子动力学模拟的离子液体含有1-ethy 1 -3-methylimidazolium和不同分数的双（三氟甲基磺酰基）酰亚胺和四氟硼酸盐，结合平面石墨烯片作为电极。我们表明，相对离子电极货车的范德华相互作用中起着重要的作用，在第一界面层附近的不带电电极吸附的离子的人口。在带电电极附近，我们发现离子液体混合物通常表现出介于两种纯离子液体之间的积分电容。我们表征了携带各种表面电荷的电极附近的累积离子密度，揭示了不同离子液体的不同充电机制，这与离子的相对尺寸有关。最后，在离子液体混合物中，我们确定了一种有效的离子交换现象，其中电极的充电导致两种类型的阴离子在第一界面层中的密度的不同趋势，这增强了对阴离子的吸附，提高了负电极的电容。
The performance of electric double-layer capacitors is strongly influenced by the choice of electrolyte, and electrolytes comprised of ionic liquid mixtures have shown promise for enabling high energy densities. Here we perform all-atom molecular dynamics simulations of ionic liquids containing 1-ethy1-3-methylimidazolium and different fractions of bis(trifluoromethylsulfonyl)imide and tetrafluoroborate, in conjunction with planar graphene sheets as electrodes. We demonstrate that relative ion electrode van der Waals interactions play an important role in the population of ions adsorbed in the first interfacial layer near uncharged electrodes. Near charged electrodes, we find that the ionic liquid mixtures generally exhibit integral capacitances intermediate between the two pure ionic liquids. We characterize cumulative ion densities near electrodes carrying various surface charges, revealing different charging mechanisms for different ionic liquids, which we relate to the relative sizes of the ions. Finally, in the ionic liquid mixtures we identify an effective ion exchanging phenomenon wherein charging of the electrodes leads to different trends in the densities of the two types of anions in the first interfacial layer, which enhances counterion adsorption and improves capacitance at the negative electrode.