High Performance Reversible Solid Oxide Cells with Porous La0.6Sr0.4FeO3-Δ Oxygen Electrodes

Abstract

As a mixed electronic and ionic conductor (MIEC), (La,Sr)FeO3-δ has been extensively studied to replace manganite based materials as oxygen electrode of intermediate temperature (600-800oC) reversible solid oxide cells (RSOCs). However, compared to Sr doped LaCoO3 material, the electrochemical performance of cobalt-free (La,Sr)FeO3-δ needs to be improved to meet the requirements of commercial applications. Contrary to the conventional solid-state reaction and sol-gel method, molten salt synthesis (MSS) is a simple, versatile, and environmental-friendly approach to synthesize highly active perovskite catalysts with controllable compositions and morphologies. Especially, the corrosion effect of molten salts and the salt-removal process in MSS process have potential to manufacture particles with open porous topographic structure, resulting in large specific surface area. Therefore, it’s promising to use the MSS method to enhance the activity of cobalt-free (La,Sr)FeO3-δ. In this study, the MSS method was employed to synthesize perovskite La0.6Sr0.4FeO3-δ (LSF) at 850oC as the oxygen electrode of a button solid oxide cell. In the material preparation procedure, moderate chloride composite was added in the reactants, forming a liquid environment to accelerate the reaction. At 800 °C, the button cell exhibits a peak power density of 1.73 Wcm-2 using pure H2 as fuel and reaches an electrolysis current density of 1.50 Acm-2 under 1.3 V in 50% CO2/50% H2, both of which are much better than the reference cell with a La0.6Sr0.4FeO3-δ electrode prepared by sol-gel method. The button cell also demonstrates a stable performance under a constant current density of 0.5 Acm-2 in SOFC mode for 200 hours. We also discussed the mechanism for the ultra-high electrochemical performance of La0.6Sr0.4FeO3-δ by charactering the difference between LSF powders synthesized by MSS method and sol-gel method. The results show that LSF using MSS powders possess a great plenty of pores in the grains. Meanwhile, the valance and the coordinator number of Fe atoms are reduced owing to the poor solubility of oxygen and the corrosion in the molten chlorides, leading to more oxygen vacancies. Therefore, we concluded that the combining effect of the microstructure and the enhanced amount of oxygen vacancies make the porous LSF electrode exhibits attractive electrochemical performance in both SOFC mode and SOEC mode. Figure 1