A two-step surface exchange mechanism and detailed defect transport to model oxygen permeation through the La0.9Ca0.1FeO3−δmixed-conductor

Abstract

A framework to model materials experiencing defect mechanism similar to that of La1−xCaxFeO3−δ (LCF) membranes is considered. Using the La0.9Ca0.1FeO3−δ mixed-conductor, we propose a model that incorporates a two-step thermodynamically consistent surface exchange mechanism and charged species transport within the material using the Planck–Nernst–Poisson model. The surface reaction mechanism has been proposed under equilibrium; the current study expands the theory to finite rate kinetics and shows significant kinetic limitations for oxygen incorporation. Existing models describing the diffusion of oxygen vacancies may not be applicable at low oxygen partial pressures. Evidence shows that for the La1−xCaxFeO3−δ membrane, simultaneous presence of multiple Fe states should be considered. As a result, the surface chemistry and charged species transport models developed in this study conserve the Fe sites and the electroneutrality condition both at the feed and sweep sides of the membrane. The role of a strict electroneutrality condition within the material is also examined. Using inert argon experimental data, we demonstrate the fidelity of the model. Predictions reveal that no crossover in the concentration of FeFe• and FeFe′ takes place within our operating conditions; thus, electronic conductivity within the material is of p-type.