Triple conducting oxides as air electrode in protonic ceramic cells: A study towards structure, transport properties and electrochemical performance of selected triple ionic-electronic conducting oxides

Protonic ceramic cells, which include air electrodes as essential components, are key devices in the global energy transition process. Triple ionic-electronic conducting (H+, O2-, e-) oxides are considered promising candidates for use as air electrode, owing to their high electrochemical activity towards the oxygen reduction reaction and water dissociation reaction when operated at intermediate to low temperatures (700 – 500 °C). This thesis focuses on the investigation of the structure, transport properties and electrochemical performance of two material systems: iron-doped barium zirconates BaZr1-xFexO3-δ and barium-doped lanthanum cobaltites La1-xBaxCoO3 δ, both of which adopt a perovskite structure and both are reported to exhibit excellent electrochemical performance as air electrode. Chapter 2, 3 and 4 investigate the structure and transport properties of BaZr1-xFexO3-δ, La1-xCaxFeO3 δ and La1-xBaxCoO3 δ under dry conditions, their thermodynamic quantities and oxygen ion conductivities are reported. Chapter 5 and 6 focus on the electrochemical performance of La1-xBaxCoO3 δ and BaZr1-xFexO3-δ. A cathodic reaction model based on bulk oxygen ion conduction is proposed. Evaluation of the electrode polarization resistances indicates in the investigated series La0.4Ba0.6CoO3-δ and BaZr0.5Fe0.5O3-δ are the best-performing electrodes. In Chapter 7, the oxygen ion and proton transport properties of La5.4WO11.1-δ are investigated by means of electrical conductivity relaxation measurements. An apparent two-fold relaxation behavior is observed at low currents even under dry conditions, suggesting use of electrodes with poor reversibility in conductivity and conductivity relaxation measurements may lead to erroneous results and interpretations.