Developing a multistep surface reaction mechanism to model the impact of H2 and CO on the performance and defect chemistry of La0.9Ca0.1FeO3−δ mixed-conductors

This study examines the impact of hydrogen (H2) and carbon monoxide (CO) on the oxygen permeation and defect chemistry of La0.9Ca0.1FeO3−δ (LCF) membranes. By conducting two separate experiments with hydrogen-argon (Ar) and carbon monoxide-argon mixtures, we show that the oxygen flux (JO2) increases by more than one order of magnitude when either fuel is introduced at the fuel side as compared to non-reactive experiments. Our measurements reveal that H2 oxidation is faster compared to CO oxidation, hence higher fluxes can be achieved with the former. Using X-Ray Diffraction (XRD), we verify that the aforementioned performance rise is achieved without sacrificing the stability of the material. We show that JO2 increases due to surface reactions of fuel with lattice oxygen ions and hence, two 4-step surface reaction mechanisms are proposed for either fuel. The mechanisms are thermodynamically consistent and coupled with detailed diffusion of charged species within the material by utilizing a Poisson-Nernst-Planck (PNP) model in the dilute limit. The model demonstrates that surface reactions impose a limit on the rise of the oxygen flux, while charged species diffusion within the material is fast. For LCF, reduction of Fe+3 to Fe+2 during fuel oxidation is faster compared to Fe+4 reduction to Fe+3 and this behavior significantly impacts the kinetic rates.