Reaction Kinetics of Gas–Solid Exchange Using Gas Phase Isotopic Oxygen Exchange

The oxygen transport kinetics of heterogeneous gas–solid exchange has been investigated on the basis of a two-step reaction mechanism, linking surface catalysis to solid-state self-diffusion, via gas phase isotopic oxygen exchange on the mixed ionic electronic conductor (MIEC) La_0.6Sr_0.4Co_0.2Fe_0.8O_3–<i>x</i> (LSCF) and electronic conductor (La_0.8Sr_0.2)_0.95MnO_3±<i>x</i> (LSM). The catalytic activity of LSCF is higher than that of LSM toward the elementary step of oxygen dissociation, likely caused by a higher vacancy concentration. The apparent activation energy for surface exchange of LSCF is lower than values obtained from bulk characterization techniques. The diffusion coefficient (<i>D</i>) for LSM at different temperatures shows a huge deviation from literature values, and an alternate exchange mechanism has been proposed. The fast transport pathway is attributed to the substoichiometry of LSM in the near surface region. These results have significant implications for the improvement of the oxygen reduction reaction for the design of higher-performance materials and the importance and limitations of isotope exchange experimental design