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 $\tau =4L^2/\left({\pi }^{2}D\right)$. Our results imply that strongly correlated, strongly confined electrolytes—such as ionic liquids in the surface force balance apparatus—move slower than predicted so far.

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• Capacitance
• Charge
• Charge dynamics
• Electrochemical properties
• Electrochemistry
• Electrokinetic flows
• Electrostatic double-layer capacitors
• Ionic transport
• Nonequilibrium & irreversible thermodynamics
• Ionic fluids
• Ions
• Liquid-solid interfaces
• Nonequilibrium systems