Effects of Surface Transition and Adsorption on Ionic Liquid Capacitors

Abstract

Room temperature Ionic liquids (RTILs) are synthetic electrolytes with superior electrochemical stability to conventional aqueous based electrolytes, allowing significantly enlarged electrochemical window for application as capacitors. In this study, we propose a variant of an existing RTIL model for solvent-free RTILs, accounting for both ion-ion correlations and nonelectrostatic interactions. Using this model, we explore the phenomenon of spontaneous surface charge separation in RTIL capacitors and find that this transition is a common feature for realistic choices of the model parameters in most RTILs. In addition, we investigate the effects of asymmetric preferential ion adsorption on this charge separation transition and find that proximity of the transition in this case can result in greatly enhanced energy storage. Our work suggests that differential chemical treatment of electrodes can be a simple and useful means for optimizing the energy storage in RTIL capacitors.

Stability of ionic liquid modeled by composite Coulomb-Yukawa potentials

“…(11)]. Despite these differences, the ability of the model from Chao and Wang [27] to account for experimentally observed hysteresis effects [34,35] applies analogously to our present model. In summary, we present a comprehensive mean-field description of an ionic liquid with ion-specific, composite Coulomb and Yukawa interactions on a densely packed lattice.…”

“…Consider a solvent-free ionic liquid that consists of monovalent anions and cations of comparable size with a composite Coulomb-Yukawa interaction potential as function of ion-ion distance r: u aa /k B T = l B /r + ae −κr /r for an anion-anion pair, u ac /k B T = −l B /r + be −κr /r for an anion-cation pair, and u cc /k B T = l B /r + ce −κr /r for a cation-cation pair, where k B T denotes the thermal energy unit (Boltzmann constant times absolute temperature) and l B the Bjerrum length. The three interaction strengths of the Yukawa potential contributions, a, b, c, introduce ion specificity and, as has been suggested previously [22,[27][28][29][30][31], account for steric effects and/or ionion correlations. We express them conveniently by the matrix A h = {{a, b}, {b, c}}.…”

“…Limmer [25] has implemented the short-ranged attraction by a phenomenological Landau-Ginzburg approach and Downing et al [26] through a Bragg-Williams free energy. Chao and Wang [27] generalize the BSK model and show that it too predicts a phase transition near an electrode. However, like BSK, they neglect the penetration of the Yukawa field into the electrode, leading to an incomplete boundary condition.…”

“…Chao and Wang [27] employ two potentials, Coulomb and Yukawa, to model the composite pair interaction u aa = u cc = −u ac = k B T l B (1 − we −κr )/r, where w is a dimensionless parameter. This generalizes the BSK model to capture the more general case a = c = −b = wl B and thus α = √ wλ.…”

Image-charge effects on ion adsorption near aqueous interfaces

“…The ICI induced by the graphitic surfaces promotes microstates with charge separated ion distributions across the carbon electrode, where the local electroneutrality can be violated due to nanoconfinement while global neutrality is still maintained across the system. Furthermore, the enhanced charge fluctuation in the longitudinal direction induced by ICI under CW boundary is also applicable to nonaqueous electrolytes and pure ionic systems; for example, this enhanced charge fluctuation is likely to play a role in the recently proposed spontaneous charge separation in solvent-free ionic liquid capacitors predicted by field-theoretic models (72)(73)(74).…”