Simulation study of the capacitance and charging mechanisms of ionic liquid mixtures near carbon electrodes

Published

January 4, 2019

Author(s)

Alta Y. Fang, Alexander Y. Smolyanitsky

Abstract

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-ethyl-3-methylimidazolium (EMIM$^+$) and different fractions of bis(trifluoromethylsulfonyl)imide (TFSI$^-$) and tetrafluoroborate (BF$_4^-$), in conjunction with planar graphene sheets as electrodes. We demonstrate that ion-electrode van der Waals attractions 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 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 mechanism 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 counter-ion adsorption and improves capacitance at the negative electrode.

Citation

Journal of Physical Chemistry C

Volume

123

Issue

3

Pub Type

Journals

Download Paper

https://doi.org/10.1021/acs.jpcc.8b10334

Keywords

ionic liquid mixture, supercapacitor, molecular dynamics

Citation

Fang, A. and Smolyanitsky, A. (2019), Simulation study of the capacitance and charging mechanisms of ionic liquid mixtures near carbon electrodes, Journal of Physical Chemistry C, [online], https://doi.org/10.1021/acs.jpcc.8b10334 (Accessed April 18, 2024)