Publication date: 10th April 2024
Finding the optimal mixed conducting materials for use in Solid Oxide Cells (SOCs) or catalytic applications requires detailed knowledge of the electrochemical charge transport properties and oxygen exchange kinetics. These properties are governed by the defect chemistry of the material and cation segregation phenomena, which both are very closely linked to the oxygen stoichiometry under various temperature, atmosphere and ageing conditions.
Here, we present two new, fast and precise cell types for coulometric titration and quasi in-situ XPS measurements to investigate the bulk defect chemistry and link it to the surface chemistry of the mixed conducting perovskite Sr1-xTi0.6Fe0.4O3-δ with x ranging from 0 to 0.07. For powder samples, we use a miniature titration cell with dedicated pumping and sensing electrodes. Thin film samples deposited by Pulsed Laser Deposition (PLD) on YSZ were used in sealed oxide ion battery [1] type cells, on which oxide ions are directly exchanged by pumping through the electrolyte. The big advantage of coulometry over thermogravimetry is the much broader pO2 range that is accessible electrochemically, compared to gas mixtures. By running multiple oxidation and reduction cycles within and beyond the thermodynamic stability limit, we can differentiate between reversible oxygen stoichiometry changes and irreversible metal exsolution reactions, with corresponding phase transitions. This enables us to investigate differences in oxygen stoichiometry, thermochemical stability and kinetics of metal exsolution between materials on a fundamental level. For the thin film samples, these coulometric techniques were combined with simultaneous Electrochemical Impedance Spectroscopy (EIS) and in-situ X-ray Spectroscopy (XPS), thus quantifying surface stoichiometry, and linking transition metal oxidation states on the surface with bulk oxygen vacancy concentration determined from coulometry.
In summary, we could show that coulometric techniques and in-situ XPS are valuable techniques to characterise the bulk defect chemistry and surface reducibility of various oxide materials.
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