Dynamic density functional theory for the charging of electric double layer capacitors

Abstract

We consider the charging of a model capacitor comprised of two planar electrodes and an electrolyte. Upon switching on a voltage difference, electric double layers build up in this setup, which we characterize with a classical dynamic density functional theory (DDFT) that accounts for electrostatic correlations and for molecular excluded volume of finite-sized ions and solvent molecules. Our DDFT predicts the electrode charge Q( t) to form exponentially with two timescales: at early times, the system relaxes on the RC time, namely, λ D L/[ D(2 + σ/ λ D)], with λ D being the Debye length, L being the electrode separation, σ being the ion diameter, and D being the ionic diffusivity. Contrasting an earlier DDFT study, this early-time response does not depend on the applied potential. At late times, the capacitor relaxes with a relaxation time proportional to the diffusion time L2/ D.