On the Gouy-Chapman-Stern model of the electrical double-layer structure with a generalized Boltzmann factor

TL;DR

This paper extends the classical Gouy-Chapman-Stern model of the electrical double layer by incorporating a generalized Boltzmann factor using Tsallis nonextensive statistics, allowing for more accurate modeling of ion distributions and capacitance in various electrolytes.

Contribution

It introduces a generalized ion concentration distribution based on Tsallis statistics into the GCS model, providing a new analytical framework for EDL behavior.

Findings

The model can reproduce both inverse bell-shaped and bell-shaped capacitance curves.

Analytical expressions for differential capacitance are derived.

The approach captures deviations from classical Boltzmann behavior in EDLs.

Abstract

The classical treatment of the electrical double-layer (EDL) structure at a planar metal/electrolyte junction via the Gouy-Chapman-Stern (GCS) approach is based on the Poisson equation relating the electrostatic potential to the net mean charge density. The ions concentration in the diffuse layer are assumed to follow the Boltzmann distribution law, i.e. $\propto \exp(-\tilde{\psi})$ where $\tilde{\psi}$ is the dimensionless electrostatic potential. However, even in stationary equilibrium in which variables are averaged over a large number of elementary stochastic events, deviations from the mean-value are expected. In this study we evaluate the behavior of the EDL by assuming some small perturbations superposed on top of its Boltzmann distribution of ion concentrations using the Tsallis nonextensive statistics framework. With this we assume the ion concentrations to be proportional to $ [ 1- (1-q) \tilde{\psi}]^{{1}/{(1-q)}} = \exp_q({- \tilde{\psi}})$ with $q$ being a real parameter that characterizes the system's statistics. We derive analytical expression and provide computational results for the overall differential capacitance of the EDL structure, which, depending on the values of the parameter $q$ can show both the traditional inverse bell-shaped curves for aqueous solutions and bell curves observed with ionic liquids.