Proton diffusion mechanisms in the double perovskite cathode material GdBaCo₂O_5.5: A molecular dynamics study

GdBaCo₂O_5+x compounds have demonstrated to be very efficient cathode materials not only in solid oxide fuel cells but also in proton conducting fuel cells. In this last case, the excellent properties could be due to the presence of mixed electron-proton conduction. We study here the diffusion of the proton in this material using molecular dynamics simulations. Two different diffusion mechanisms are observed. The predominant mechanism is the standard proton transfer between two neighbouring oxygen atoms combined with the rotation of H around its first neighbour oxygen atom. The second mechanism consists in the migration of the OH group where both oxygen and hydrogen atoms diffuse together. Strong spatial correlations between successive proton jumps are evidenced. This is likely related to the presence of oxygen vacancies and to the concerted diffusion of hydrogen and oxygen atoms.