Effect of side chain modifications in imidazolium ionic liquids on the properties of the electrical double layer at a molybdenum disulfide electrode

Abstract

Knowledge of how the molecular structures of ionic liquids (ILs) affect their properties at electrified interfaces is key towards the rational design of ILs for electric applications. Polarizable molecular dynamics simulations were performed to investigate the structural, electrical and dynamic properties of electric double layers (EDLs) formed by imidazolium dicyanamide ([ImX1][DCA]) at the interface with molybdenum disulfide electrode. The effect of side chain of imidazolium on the properties of EDLs was analyzed by using) and 1-(2-hydroxyethyl)-3-methylimidazolium ([ImO1]) as cations. Using [Im21] as reference, we find that the introduction of octyl or benzyl groups significantly alters the interfacial structures near the cathode because of the reorientation of cations. For [Im81], the positive charge on the cathode induces pronounced polar and non-polar domain separation. In contrast, the hydroxyl group has minor effect on the interfacial structures. [ImB1] is shown to deliver slightly larger capacitance than other ILs even though it has larger molecular volume than [Im21]. This is attributed to the limiting factor for capacitance being the strong association between counter-ions, instead of the free space available to ions at the interface. For [Im81], the charging mechanism is mainly the exchange between anions and octyl tails, while for the other ILs, the mechanism is mainly the exchange of counter-ions. Analysis on the charging process shows that the charging speed does not correlate strongly with macroscopic bulk dynamics like viscosity. Instead, it is dominated by local displacement and reorientation of ions.

Polarizable molecular dynamics simulations on the conductivity of pure 1-methylimidazolium acetate systems

“…Integration was done using the velocity Verlet algorithm with the temperature-grouped Dual–Nosé–Hoover thermostat as described in ref. 40 and 41, which implements an additional temperature group for the center-of-mass translations. The non-Drude particles were kept at 300 K and the Drudes at 1 K. The collision frequency was set to 10 ps −1 for the atoms and to 200 ps −1 for the Drude particles.…”

Microstructural probing of phosphonium-based ionic liquids on a gold electrode using colloid probe AFM

“…2 For example, the processes of energy storage and release are closely related to the EDL structure in IL-based supercapacitors, which is composed of an inner tightly bound ion layer and an outer loose layer with alternating cations and anions. [3][4][5][6][7] The formation of either a multilayer alternating cation and an anion structure or a distinctive dense counterion monolayer in ILs across the longitudinal direction to the surface has been confirmed by both experimental 4,[6][7][8][9][10][11] and theoretical [12][13][14][15][16] studies and suggested to play a crucial role in the capacitive performance at IL-electrode interfaces. 16,17 The capacitance of ILs at the electrode surface is reported to be determined by, and reflects, the microstructural changes within the innermost region of the EDL.…”

“…[3][4][5][6][7] The formation of either a multilayer alternating cation and an anion structure or a distinctive dense counterion monolayer in ILs across the longitudinal direction to the surface has been confirmed by both experimental 4,[6][7][8][9][10][11] and theoretical [12][13][14][15][16] studies and suggested to play a crucial role in the capacitive performance at IL-electrode interfaces. 16,17 The capacitance of ILs at the electrode surface is reported to be determined by, and reflects, the microstructural changes within the innermost region of the EDL. 14 Generally, most cations and anions prefer to form 'neutral aggregates', i.e., the cation-anion alternating structure in the EDL because the ion distribution is mainly dominated by coulombic interactions.…”

Microscopic Simulations of Electrochemical Double-Layer Capacitors

“…The purple points are the image charges. Reproduced with permission from ref . Copyright 2021 AIP Publishing.…”