Concentration Fluctuations and Capacitive Response in Dense Ionic Solutions

TL;DR

This study uses molecular dynamics simulations to explore how ion concentration affects the electrochemical capacitance of dense ionic solutions, revealing a non-monotonic relationship influenced by ion motion and solvation fluctuations.

Contribution

It demonstrates the non-monotonic behavior of capacitance and correlation length in dense ionic solutions, linking microscopic fluctuations to macroscopic electrochemical response.

Findings

Capacitance first increases then decreases with ion concentration.

Correlation length exhibits non-monotonic behavior with ion concentration.

Charge fluctuations are significantly affected by ion-solvent interactions.

Abstract

We use molecular dynamics simulations in a constant potential ensemble to study the effects of solution composition on the electrochemical response of a double layer capacitor. We find that the capacitance first increases with ion concentration following its expected ideal solution behavior, but decreases upon approaching a pure ionic liquid in agreement with recent experimental observations. The non-monotonic behavior of the capacitance as a function of ion concentration results from the competition between the independent motion of solvated ions in the dilute regime and solvation fluctuations in the concentrated regime. Mirroring the capacitance, we find that the characteristic decay length of charge density correlations away from the electrode is also non-monotonic. The correlation length first decreases with ion concentration as a result of better electrostatic screening but increases with ion concentration as a result of enhanced steric interactions. When charge fluctuations induced by correlated ion-solvent fluctuations are large relative to those induced by the pure ionic liquid, such capacitive behavior is expected to be generic.