Park Suehyun, McDaniel Jesse G
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Chem Phys. 2020 Feb 21;152(7):074709. doi: 10.1063/1.5144260.
Ionic liquids are widely used as electrolytes in electronic devices in which they are subject to nanoconfinement within nanopores or nanofilms. Because the intrinsic width of an electrical double layer is on the order of several nanometers, nanoconfinement is expected to fundamentally alter the double layer properties. Furthermore, in confined systems, a large portion of the ions are interfacial, e.g., at the electrode interface, leading to significant deviations of electrostatic screening and ion dynamics as compared to bulk properties. In this work, we systematically investigate the interference between electrical double layers for nanoconfined ionic liquids and the resulting influence on the structure, dynamics, and screening behavior. We perform molecular dynamics simulations for the ionic liquids [BMIm][BF ] and [BMIm][PF ] confined between two flat electrodes at systematic separation distances between 1.5 nm and 4.5 nm for both conducting and insulating boundary conditions. We find that while ion dynamics is expectedly slower than in the bulk (by ∼2 orders of magnitude), there is an unexpected non-linear trend with the confinement length that leads to a local maximum in dynamic rates at ∼3.5-4.5 nm confinement. We show that this nonlinear trend is due to the ion correlation that arises from the interference between opposite double layers. We further evaluate confinement effects on the ion structure and capacitance and investigate the influence of electronic polarization of the ionic liquid on the resulting properties. This systematic evaluation of the connection between electrostatic screening and structure and dynamics of ionic liquids in confined systems is important for the fundamental understanding of electrochemical supercapacitors.
离子液体在电子设备中被广泛用作电解质，在这些设备中它们会受到纳米孔或纳米膜内的纳米限域作用。由于双电层的固有宽度在几纳米的量级，纳米限域作用预计会从根本上改变双电层的性质。此外，在受限体系中，很大一部分离子是界面性的，例如在电极界面处，这导致与本体性质相比，静电屏蔽和离子动力学有显著偏差。在这项工作中，我们系统地研究了纳米限域离子液体的双电层之间的相互干扰以及由此对结构、动力学和屏蔽行为产生的影响。我们对离子液体[BMIm][BF ]和[BMIm][PF ]进行分子动力学模拟，在1.5纳米至4.5纳米的系统分离距离下，对导电和绝缘边界条件下的两个平面电极之间进行模拟。我们发现，虽然离子动力学预计比本体中慢（约2个数量级），但存在一个意想不到的与限域长度的非线性趋势，导致在约3.5 - 4.5纳米限域时动力学速率出现局部最大值。我们表明这种非线性趋势是由于相反双电层之间的干扰产生的离子相关性。我们进一步评估限域对离子结构和电容的影响，并研究离子液体的电子极化对所得性质的影响。这种对受限体系中离子液体的静电屏蔽与结构和动力学之间联系的系统评估，对于从根本上理解电化学超级电容器非常重要。
