Electro-chemical polarization around metallic particles - Part 2: The role of diffuse surface charge

Matthias Bücker, Sabine Undorf, Adrián Flores Orozco, Andreas Kemna

Research output: Contribution to journalArticlepeer-review

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, electro-migration and diffusion processes in bulk electrolyte and diffuse layer are modelled by the Poisson-Nernst-Planck system of partial differential equations. To include the effect of a fixed diffuse charge, we consider a constant electric ζ-potential at the surface of the particle, which leads to the build-up of a static diffuse layer. Furthermore, a minor fraction of electro-active 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 out-side the two hemispheres of the sphere. The other two are volume-diffusion processes; (i) one driven by the reaction currents through the particle surface and (ii) the other by the unequal electro-migration 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 ζ-potential at the particle surface, and thus the static diffuse layer, only moderately change the relaxation of the field-induced diffuse layer: With increasing magnitude of the ζ-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 towards lower frequencies. The volume-diffusion process due to the reaction currents remains practically unaffected by the static diffuse layer.
Original languageEnglish
JournalGeophysics
DOIs
Publication statusPublished - 12 Nov 2018

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