Impedance response of ionic liquids in long slit pores

TL;DR

This paper develops a simplified, semi-analytical model to study the impedance response of ionic liquids in slit pores, incorporating ion layering and transport, validated by numerical simulations, and explores how pore size and electrostatic effects influence dynamic properties.

Contribution

It introduces a reduced-order model combining density functional theory and impedance analysis, enabling efficient qualitative comparisons and insights into ionic liquid behavior in nanopores.

Findings

Impedance response depends on pore width and potential difference.

Electrostatic effects beyond mean-field alter response time dependence.

Peaks in response time occur at specific pore widths.

Abstract

We study the dynamics of ionic liquids in a thin slit pore geometry. Beginning with the classical and dynamic density functional theories for systems of charged hard spheres, an asymptotic procedure leads to a simplified model which incorporates both the accurate resolution of the ion layering (perpendicular to the slit pore wall) and the ion transport in the pore length. This reduced-order model enables qualitative comparisons between different ionic liquids and electrode pore sizes at low numerical expense. We derive semi-analytical expressions for the impedance response of the reduced-order model involving numerically computable sensitivities, and obtain effective finite-space Warburg elements valid in the high and low frequency limits. Additionally, we perform time-dependent numerical simulations to recover the impedance response as a cross-validation step. We investigate the dependence of the impedance response on system parameters and the choice of density functional theory used. The inclusion of electrostatic effects beyond mean-field qualitatively changes the dependence of the characteristic response time on the pore width. We observe peaks in the response time as a function of pore width, with height and location depending on the potential difference imposed. We discuss how the calculated dynamic properties can be used together with equilibrium results to optimise ionic liquid supercapacitors for a given application.