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

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. proportional to exp(- (psi) over tilde) where (psi) over tilde 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)(psi) over tilde](1/(1-q)) = exp(q) (-(psi) over tilde) 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. (C) 2021 Elsevier B.V. All rights reserved.