Abstract
There is a growing interest in the properties of ionic liquids (ILs) and IL-solvent mixtures at metallic interfaces, particularly due to their applications in energy storage. The main focus so far has been on electrical double layers with ILs far from phase transitions. However, systems in the vicinity of their phase transformations are known to exhibit some remarkable features, such as wetting transitions and capillary condensation. Herein, we develop a mean-field model suitable for the IL-solvent mixtures close to demixing, and combine it with the Carnahan-Starling (CS) and lattice-gas expressions for the excluded volume interactions. This model is then solved analytically, using perturbation expansion, and numerically. We demonstrate that, besides the well-known camel and bell-shaped capacitances, there is a bird-shaped capacitance, having three peaks as a function of voltage, which emerge due to the proximity to demixing. In addition, we find that the camel-shaped capacitance, which is a signature of dilute electrolytes, can appear at high IL densities for ionophobic electrodes. We also discuss the differences and implications arising from the CS and lattice-gas expressions for excluded volume interactions in the context of our model.
Highlights
Ionic liquids (ILs) and ionic liquid (IL)–solvent mixtures have become the focus of research in electrochemistry due to their unique properties, such as exceptional electrochemical and thermal stability, and low vapour pressure
Treating solvent as a continuum, but describing ILs as amenable to phase separate into the ion-rich and ion-dilute phases, we showed that the capacitance and stored energy become sensitive functions of temperature in the vicinity of demixing
These results suggest that the asymmetry in ion sizes may reduce the capacitance calculated in this work, and will bring asymmetry in the capacitancevoltage dependence, but the qualitative behaviour due to proximity to demixing shall be captured already by a model featuring the same sizes of cations and anions
Summary
Ionic liquids (ILs) and IL–solvent mixtures have become the focus of research in electrochemistry due to their unique properties, such as exceptional electrochemical and thermal stability, and low vapour pressure. Gongadze and Iglic [48] proposed an improved mean-field model of EDLs that accounts for such ion-size asymmetry, and found that it leads to a pronounced decrease of the capacitance and to shape asymmetry of the capacitance-voltage curves (with respect to the potential of zero charge), which seems to be consistent with the experimental observations [51, 52] These results suggest that the asymmetry in ion sizes may reduce the capacitance calculated in this work, and will bring asymmetry in the capacitancevoltage dependence, but the qualitative behaviour due to proximity to demixing shall be captured already by a model featuring the same sizes of cations and anions. We briefly describe the bulk system, i.e., the system in the absence of an electrode
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