Charging dynamics of electric double layer capacitors including beyond-mean-field electrostatic correlations

Abstract

Electric double layer (EDL) formation underlies the functioning of supercapacitors and several other electrochemical technologies. Here, we study how the EDL formation near two flat blocking electrodes separated by $2L$ is affected by beyond-mean-field Coulombic interactions, which can be substantial for electrolytes of high salt concentration or with multivalent ions. Our model combines the Nernst-Planck and Bazant-Storey-Kornyshev (BSK) equations; the latter is a modified Poisson equation with a correlation length $\ell_c$. In response to a voltage step, the system charges exponentially with a characteristic timescale $\tau$ that depends nonmonotonically on $\ell_c$. For small $\ell_c$, $\tau$ is given by the BSK capacitance times a dilute electrolyte's resistance, in line with [Zhao, Phys. Rev. E 84, 051504 (2011)]; here, $\tau$ decreases with increasing $\ell_c$. Increasing the correlation length beyond $\ell_c\approx L^{2/3}\lambda_D^{1/3}$, with $\lambda_D$ the Debye length, $\tau$ reaches a minimum, rises as $\tau\propto \lambda_D\ell_c/D$, and plateaus at $\tau=4L^2/(\pi^2 D)$. Our results imply that strongly correlated, strongly confined electrolytes - ionic liquids in the surface force balance apparatus, say - move slower than predicted so far.