By using a gastight electrochemical cell with flowing air as the reference environment, we were able to achieve an oxygen partial pressure (P02) as low as 1016 atm inside the cell. The conductivity of SrFeCo05O, has been studied as a function of P02 and temperature. It was found that in the high-P0, range, interstitial oxygen ions (Or') and electron holes (Fe,= h') are the dominant charge carriers, while in the low-P02 range, oxygen vacancies (V) and electrons (e') are dominant. At 800°C in air; total conductivity and ionic conductivity of SrFeCo05O, are 17 and 7 5 cm', respectively, and the ionic transference number is 0.4. A semiconductor-metal-semiconductor transition is found in this system in a reducedoxygen environment. Defect dynamics in this system can be understood by means of the trivalence-to-divalence transition of Fe ions when P02 is reduced. A defect model has been proposed. By using the conductivity results, we were able to estimate oxygen permeation through a ceramic membrane made of SrFeCo55O. The oxygen permeability we calculated is consistent with that measured at the conversion reactor. To confirm the ionic transference number measured by electron-blocking method, electromotive force measurement was carried out and obtained consistent results.
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