Electrochemical polarization around metallic particles — Part 2: The role of diffuse surface charge

Abstract

Numerical simulations are carried out to study the induced-polarization response of a charged metallic sphere, which immersed in electrolyte solution is covered by a static diffuse layer. The metallic sphere itself is assumed to be perfectly conductive; electromigration and diffusion processes in bulk electrolyte and diffuse layer are modeled by the Poisson-Nernst-Planck system of partial differential equations. To include the effect of a fixed diffuse charge, we consider a constant electric [Formula: see text]-potential at the surface of the particle, which leads to the buildup of a static diffuse layer. Furthermore, a minor fraction of electroactive cations engages in oxidation-reduction reactions at the particle surface, which allows charges to be transferred across the solid-liquid interface. Upon excitation by a low-frequency electric field, we observe the coupling of three polarization processes in the composite material consisting of metallic particle and surrounding electrolyte. The first is related to the dynamic charging of field-induced diffuse layers immediately outside the two hemispheres of the sphere. The other two are volume-diffusion processes: (1) one driven by the reaction currents through the particle surface and (2) the other driven by the unequal electromigration transport of anions and cations through the static diffuse layer. Diffuse-layer relaxation and volume diffusion due to reaction currents can also be observed around uncharged metallic particles and clearly dominate the macroscopic polarization response. The [Formula: see text]-surface potential at the particle surface, and thus the static diffuse layer, only moderately change the relaxation of the field-induced diffuse layer: With the increasing magnitude of the [Formula: see text]-potential, we observe an increase of the low-frequency electrical conductivity of the particle in suspension, a reduction of its polarization magnitude, and a shift of its characteristic frequency toward lower frequencies. The volume-diffusion process due to the reaction currents remains practically unaffected by the static diffuse layer.