Two-dimensional electrochemical model for mixed conductors: a study of ceria

TL;DR

This study develops a 2D electrochemical model for ceria-based mixed conductors, revealing how in-plane drift-diffusion and cross-plane currents influence electrode resistance under various conditions.

Contribution

The paper introduces a 2D finite-element model that distinguishes between in-plane and cross-plane electronic currents in ceria electrochemical cells, enhancing understanding of rate-limiting processes.

Findings

In-plane drift-diffusion current can be rate-limiting under certain conditions.

Surface reaction rate constant significantly affects electrode polarization.

Model fits experimental resistance data by separating surface and bulk contributions.

Abstract

A two-dimensional small bias model has been developed for a patterned metal current collector $|$ mixed oxygen ion and electronic conductor (MIEC) $|$ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\text{Pt} | \text{Sm}_{0.15}\text{Ce}_{0.85}\text{O}_{1.925} | \text{Pt}$ symmetric cell in a $\text{H}_2-\text{H}_2\text{O}-\text{Ar}$ (reducing) atmosphere from 500 to $650^o C$. Using a two-dimensional finite-element model, we show that two types of electronic current exist within the cell: an in-plane drift-diffusion current that flows between the gas $|$ ceria chemical reaction site and the metal current collector, and a cross-plane current that flows between the two metal electrodes on the opposite side of the cell. By fitting the surface reaction constant $\tilde k_f^0$ to experimental electrode resistance values while fixing material properties such as bulk ionic and electronic equilibrium defect concentrations and mobilities, we are able to separate the electrode polarization into the surface reaction component and the in-plane electron drift-diffusion component. We show that for mixed conductors with a low electronic conductivity (a function of oxygen partial pressure) or a high surface reaction rate constant, the in-plane electron drift-diffusion resistance can become rate-limiting in the electrode reaction.