Wang Zhenxing, Yang Yang, Olmsted David L, Asta Mark, Laird Brian B
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA.
J Chem Phys. 2014 Nov 14;141(18):184102. doi: 10.1063/1.4899176.
A major challenge in the molecular simulation of electric double layer capacitors (EDLCs) is the choice of an appropriate model for the electrode. Typically, in such simulations the electrode surface is modeled using a uniform fixed charge on each of the electrode atoms, which ignores the electrode response to local charge fluctuations in the electrolyte solution. In this work, we evaluate and compare this Fixed Charge Method (FCM) with the more realistic Constant Potential Method (CPM), [S. K. Reed et al., J. Chem. Phys. 126, 084704 (2007)], in which the electrode charges fluctuate in order to maintain constant electric potential in each electrode. For this comparison, we utilize a simplified LiClO4-acetonitrile/graphite EDLC. At low potential difference (ΔΨ ⩽ 2 V), the two methods yield essentially identical results for ion and solvent density profiles; however, significant differences appear at higher ΔΨ. At ΔΨ ⩾ 4 V, the CPM ion density profiles show significant enhancement (over FCM) of "inner-sphere adsorbed" Li(+) ions very close to the electrode surface. The ability of the CPM electrode to respond to local charge fluctuations in the electrolyte is seen to significantly lower the energy (and barrier) for the approach of Li(+) ions to the electrode surface.
双电层电容器（EDLCs）分子模拟中的一个主要挑战是选择合适的电极模型。通常，在这类模拟中，电极表面是通过在每个电极原子上使用均匀的固定电荷来建模的，这忽略了电极对电解质溶液中局部电荷波动的响应。在这项工作中，我们评估并比较了这种固定电荷方法（FCM）与更实际的恒电位方法（CPM），[S.K.里德等人，《化学物理杂志》126，084704（2007）]，在CPM中，电极电荷会波动以维持每个电极中的恒定电势。为了进行这种比较，我们使用了一种简化的LiClO4 - 乙腈/石墨双电层电容器。在低电位差（ΔΨ⩽2 V）时，两种方法对于离子和溶剂密度分布产生的结果基本相同；然而，在较高的ΔΨ时会出现显著差异。在ΔΨ⩾4 V时，CPM离子密度分布显示出非常靠近电极表面的“内球吸附”Li(+)离子（相对于FCM）有显著增强。CPM电极对电解质中局部电荷波动的响应能力被发现显著降低了Li(+)离子接近电极表面的能量（和势垒）。
