Bulk and Surface Properties of New Materials for Solid Oxide Fuel Cell Electrodes
Theoretical Chemistry: Molecules, Materials, Surfaces
Final Report Abstract
We studied the structure, electronic properties, thermodynamic stability and reactivity of two promising classes of materials for electrodes in solid oxide fuel cells (SOFCs) using density-functional theory (DFT) calculations and ab initio molecular dynamics (MD) simulations. The two classes are layered cobalt oxide based YBaCo4 O7+δ compounds for cathodes and liquid Sn for anodes. For YBaCo4 O7+δ we investigated the changes in crystal structure and the couplings of the magnetic moments upon oxygen uptake and predicted the local atomic re-arrangement in the oxygen-rich compound with δ=1. We analyzed in detail how the oxidation state and the magnetic moments of Co ions are modified at surfaces. In contrast to the bulk, all Co ions at a surface carry a magnetic moment and the couplings can change from anti- to ferri- to fully ferromagnetic between surface layers depending on the specific surface structure. In our calculation on the reactivity of the cobalt oxide surfaces we observed that it can be mandatory to include an electron transfer from defect state in the bulk to the surface by band bending to fill holes in the valence band, which otherwise would exist due to the polar nature of a surface termination. Otherwise, rather wrong results on adsorption energies, activation barriers for adsorption and vibrational signatures of adsorbates are obtained. Such a modification of the surface electronic structures of polar crystal terminations has not yet been discussed in the literature. Finally, from our ab initio MD simulations we propose that the chemical reactions at the liquid Sn anode take place at the solid/gas interface by direct interaction of dissolved O atoms with fuel molecules from the gas, without the need to fragment and dissolve the fuel molecules in the molten tin.
