Direct numerical simulations of the modified Poisson-Nernst-Planck equations for the charging dynamics of cylindrical electrolyte-filled pores

Abstract

Understanding how electrolyte-filled porous electrodes respond to an applied potential is important to many electrochemical technologies. Here, we consider a model supercapacitor of two blocking cylindrical pores on either side of a cylindrical electrolyte reservoir. A stepwise potential difference 2Φ between the pores drives ionic fluxes in the setup, which we study through the modified Poisson-Nernst-Planck equations, solved with finite elements. We focus our discussion on the dominant timescales with which the pores charge and how these timescales depend on three dimensionless numbers. Next to the dimensionless applied potential Φ, we consider the ratio R/Rb of the pore's resistance R to the bulk reservoir resistance Rb and the ratio rp/λ of the pore radius rp to the Debye length λ. We compare our data to theoretical predictions by Aslyamov and Janssen (Φ), Posey and Morozumi (R/Rb), and Henrique, Zuk, and Gupta (rp/λ). Through our numerical approach, we delineate the validity of these theories and the assumptions on which they were based.