Conduction pathways and mixed ionic-electronic conductivity below 500 °C in CaxY3-xFe5O12-δ materials

Mixed ionic-electronic conducting cathode material plays an important role in the operation of efficient low-temperature solid oxide fuel cells (LT-SOFCs). In the present study, we address the issue of achieving ionic conduction pathways and high mixed ionic-electronic conductivity below 500 degrees C in cathode materials for LT-SOFC technology. We report high mixed ionic-electronic conductivity in the low temperature range (similar to 10(1)-10(3) S cm(-1) at 350-500 degrees C) in the calcium-substituted yttrium iron garnets CaxY3-xFe5O12-delta (x = 0.15). Oxide ion conduction is governed by an oxygen vacancy concentration mechanism presented here by soft bond valence sum distribution analysis of experimental neutron diffraction data up to 450 degrees C. The continuous minimum energy conduction pathways for oxide ions in the (100) plane have been visualized by soft bond valence sum analysis. The electronic conduction in present garnets occurs due to hopping of holes from Fe4+ to Fe3+ chemical states confirmed by Mossbauer spectroscopy. Importantly, the studied materials are found to be thermodynamically stable at least up to high temperatures of 800 degrees C, which supports well to frequent thermal cycles in SOFC devices. Another important parameter, viz. the thermal expansion coefficient, is found to be comparable with that for the conventionally used anode, electrolyte, and interconnect materials. On the basis of this study, we conclude that the present cathode materials would be useful for an efficient LT-SOFC technology.