Simulation Study of the Capacitance and Charging Mechanisms of Ionic Liquid Mixtures near Carbon Electrodes

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-3methylimidazolium 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.

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

“…37,38 In addition, in binary IL mixtures, interfacial enrichment of favored species has been reported in simulations and experimental studies. 39,40 The identification of chemical species at the IL/electrode interface is, however, difficult to assess directly. A very powerful method in this context is ultra-high vacuum (UHV)-based angleresolved XPS, which however can be applied only for ultrathin IL films, due to the limited escape depth of the emitted photoelectrons on the order of a few nm.…”

“… 37,38 In addition, in binary IL mixtures, interfacial enrichment of favored species has been reported in simulations and experimental studies. 39,40 …”

The effect of diameter size of single-walled carbon nanotubes on their high-temperature energy storage behaviour in ionic liquid-based electric double-layer capacitors

“…A larger space will also allow ions the freedom to reorient themselves upon the increase in potential or current densities. This ion reorientation is particularly relevant in the case with large ions, 33 and leads to charge redistribution that clears more space for the adsorption of additional ions, and maximizes charge storage in the pores of the electrode. 34,35 With such smooth accessibility, the pores in the large diameter SWCNT samples should be used more efficiently for ion storage compared to smaller SWCNTs, to the extent that the changes induced by increasing the temperature caused no practical improvement in the energy storage in larger SWCNTs at temperatures below 150 C.…”

Insights on the Behavior of Imidazolium Ionic Liquids as Electrolytes in Carbon-Based Supercapacitors: An Applied Electrochemical Approach

“…Furthermore, theoretical studies have shown the ions of an electrolyte interact specifically within the pores of the carbon electrodes. 11,[15][16][17] Unfortunately, these microscopic considerations are addressed mainly in theoretical studies that simulate conditions very different from those to which the real devices are subject.…”