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The performance of several negative ͑fuel͒ and positive ͑air͒ electrode compositions for use in reversible solid oxide fuel cells capable of operating both as a fuel cell and as an electrolyzer was investigated in half-cell and full-cell tests. Negative electrode compositions studied were a nickel/zirconia cermet ͑Ni/YSZ͒ and lanthanum-substituted strontium titanate/ceria composite, whereas positive electrode compositions examined included mixed ion-and electron-conducting lanthanum strontium ferrite ͑LSF͒, lanthanum strontium copper ferrite ͑LSCuF͒, lanthanum strontium cobalt ferrite ͑LSCoF͒, and lanthanum strontium manganite ͑LSM͒. While titanate/ceria and Ni/YSZ electrodes performed similarly in the fuel cell mode in half-cell tests, losses associated with electrolysis were lower for the titanate/ceria electrode. Positive electrodes gave generally higher losses in the electrolysis mode when compared to the fuel cell mode. This behavior was most apparent for mixed-conducting LSCuF and LSCoF electrodes, and discernible but smaller for LSM; observations were consistent with expected trends in the interfacial oxygen vacancy concentration under anodic and cathodic polarization. Full-cell tests conducted for cells with a thin electrolyte ͑7 m YSZ͒ similarly showed higher polarization losses in the electrolysis than fuel cell direction.

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

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