Oxygen Ionic Transport in Brownmillerite-Type Ca2Fe2O5-δ and Calcium Ferrite-Based Composite Membranes

Oxygen ionic transport in mixed-conducting Ca2Fe2O5-δ brownmillerite was analyzed in light of potential applications in the composite materials for oxygen separation membranes and solid oxide fuel cell cathodes. The lattice defect formation and oxygen diffusion mechanisms were assessed by the computer simulations employing molecular dynamics and static lattice modeling. The most energetically favorable oxygen-vacancy location is in the octahedral layers of the brownmillerite structure, which provide a maximum contribution to the ionic migration in comparison with the structural blocks comprising iron-oxygen tetrahedra. The activation energies for the vacancy and interstitial diffusion in the tetrahedral layers, and also between the octahedral and tetrahedral sheets, are several times higher. The calculated values were found comparable to the experimental activation energy for ionic conduction in Ca2Fe2O5-δ, 147 kJ/mol, determined by the steady-state oxygen permeation measurements. The dense membranes of model composite Ca2Fe2O5-δ - Ce0.9Gd0.1O2-δ with equal weight fractions of the components (CGCF5) were sintered and characterized. No critical interdiffusion of the composite constituents, leading to their decomposition, was found by X-ray diffraction and electron microscopic analyses. The electrical conductivity of this composite, with an activation energy of 37 kJ/mol, is intermediate between two parent compounds and is dominantly p-type electronic as for Ca2Fe2O5-δ. Since the ion- and electron-conducting phases are well percolated in the composite ceramics, the oxygen permeation fluxes through CGCF5 are considerably higher than those of both constituents.