Capacitance of the Double Layer Formed at the Metal/Ionic-Conductor Interface: How Large Can It Be?

TL;DR

This paper proposes a new theory explaining exceptionally large capacitance at metal/ionic-conductor interfaces by considering discrete ion binding to image charges, surpassing mean-field predictions.

Contribution

It introduces an alternative ionic double layer model accounting for ion-image charge binding, explaining large capacitance beyond mean-field limitations.

Findings

Capacitance can reach extremely high values due to ion-image charge binding.

Large voltages cause ion depletion and collapse of capacitance to mean-field levels.

The model aligns with observed small double layer widths of about 0.3 Å.

Abstract

The capacitance of the double layer formed at a metal/ionic-conductor interface can be remarkably large, so that the apparent width of the double layer is as small as 0.3 \AA. Mean-field theories fail to explain such large capacitance. We propose an alternate theory of the ionic double layer which allows for the binding of discrete ions to their image charges in the metal. We show that at small voltages the capacitance of the double layer is limited only by the weak dipole-dipole repulsion between bound ions, and is therefore very large. At large voltages the depletion of bound ions from one of the capacitor electrodes triggers a collapse of the capacitance to the mean-field value.