High-Temperature pO2 Stability of Metal Oxides Determined by Amperometric Oxygen Titration

Abstract

For any mixed ionic-electronic conducting (MIEC) oxide an assessment of its basic material properties should include chemical stability as a function of oxygen partial pressure pO2. However, the stability range is not always easily experimentally accessible, but decomposition is linked to oxygen exchange between MIEC sample and surrounding atmosphere. A zirconia-based “oxygen pump” setup is introduced, which enables precise pO2 control of the atmosphere surrounding the MIEC sample. This is facilitated continuously over a pO2 range between 10−20…1 bar at temperatures between 700°C and 1000°C. By measuring the electric current necessary for pumping oxygen into or out of the sample chamber, the corresponding amount of oxygen is determined with a high precision. Furthermore, the time-scale of the continuous pO2 change during this so-called amperometric titration can easily be adjusted to the oxygen exchange/decomposition kinetics of the analyzed sample. In this way, the oxygen uptake or release of an MIEC oxide is determined as a function of pO2, temperature, and time. The amperometric titration setup presented here is discussed in detail and validated using a defect-chemically well-characterized SrTiO3 single crystal and a CuO powder sample. Furthermore, advantages as well as pitfalls are discussed. The subsequent assessment of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) powder sample yields valuable information on oxygen stoichiometry changes during continuous pO2 changes and, finally at low-pO2 values, irreversible demixing phenomena. The stability limits of this MIEC material are thus determined within a temperature range of 700 to 900°C.