Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory

TL;DR

This paper uses classical density functional theory (DFT) to analyze the structure of electric double layers in capacitors, comparing in- and out-of-plane properties with simulations, and identifies limitations of DFT at high concentrations and potentials.

Contribution

It provides the first DFT-based calculation of the in-plane structure of electric double layers and evaluates DFT's performance against molecular dynamics simulations.

Findings

DFT accurately predicts charge layering perpendicular to electrodes.

DFT performs poorly in predicting in-plane structure compared to simulations.

Limitations of DFT are identified at high concentrations and potentials.

Abstract

The ongoing scientific interest in the properties and structure of electric double layers (EDLs) stems from their pivotal role in (super)capacitive energy storage, energy harvesting, and water treatment technologies. Classical density functional theory (DFT) is a promising framework for the study of the in- and out-of-plane structural properties of double layers. Supported by molecular dynamics simulations, we demonstrate the adequate performance of DFT for analyzing charge layering in the EDL perpendicular to the electrodes. We discuss charge storage and capacitance of the EDL and the impact of screening due to dielectric solvents. We further calculate, for the first time, the in-plane structure of the EDL within the framework of DFT. While our out-of-plane results already hint at structural in-plane transitions inside the EDL, which have been observed recently in simulations and experiments, our DFT approach performs poorly in predicting in-plane structure in comparison to simulations. However, our findings isolate fundamental issues in the theoretical description of the EDL within the primitive model and point towards limitations in the performance of DFT in describing the out-of-plane structure of the EDL at high concentrations and potentials.