Modeling and Experiments of Binary Electrolytes in the Presence of Diffusion, Migration, and Electro-Osmotic Flow

Abstract

Combined diffusion, migration, and advection of ions in a binary electrolyte plays a role in various applications, including water electrolysis, electrodeposition, deionization, and electrophoresis. Here we analyze a dilute binary electrolyte with arbitrary ion valencies in a porous or nonporous medium using the one-dimensional Nernst-Planck equations. We examine how advection influences the limiting current, diffusion potential, and overall potential, deriving broadly useful analytical expressions. We provide experimental results for the electro-osmotic flow through a submicroporous separator in an alkaline water electrolysis setup. The time evolution of the potential is followed from the initial timescale of double-layer charging, followed by the diffusional timescale, to the time at which a limiting current is reached. For the longer timescales, a quasisteady model is shown to predict the time evolution of the advection-modified potential drop reasonably well. Additional interesting features arising due to electro-osmotic drag and unsteady diffusion are observed and explained.