Characterization and control of non-stoichiometry in Pr₀.₁Ce₀.₉O₂-[d̳e̳l̳t̳a̳] thin films : correlation with SOFC electrode performance

Abstract

While the properties of functional oxide thin films often depend strongly on oxygen nonstoichiometry (6), there have been few means available for its measurement and control in a reliable and in-situ fashion. In this work, we investigate means for extracting the oxygen nonstoichiometry of dense oxide thin films as a function of temperature and oxygen partial pressure from an analysis of the chemical capacitance (Cchem) obtained by impedance spectroscopy, and the use of electrical bias as a means of systematically controlling the non-stoichiometry. We selected the PrxCeIx02-6 (PCO) solid solution system as a model system given its mixed ionicelectronic conductivity, well described defect equilibria and transport properties, and stability over with limits of temperature and oxygen partial pressure. PrCel-02-6 (PCO) thin films with x=0.01, 0.10 and 0.20 were prepared as thin film cathodes by pulsed laser deposition onto single crystalline YO. 16Zro.84 01.9 2 electrolyte substrates. The cathode reactions were examined as a function of electrode geometry, temperature, oxygen partial pressure by means of electrochemical impedance spectroscopy (EIS). A DC bias range of JE = - 100 mV to 100 mV was used to polarize the PCO films and examine the impact on area specific resistance (ASR) and film non-stoichiometry. The PCO cathodes exhibited typical mixed ionic electronic behavior including large chemical capacitance and electrode performance, as reflected in the magnitude of the ASR, found to be limited by surface oxygen exchange kinetics. With the aid of a defect equilibrium model, expressions relating chemical capacitance directly to non-stoichiometry, without need for fitting parameters, were derived. By examining the dependence of non-stoichiometry on temperature and PO2, the thermodynamic constants defining defect generation were extracted. While general agreement of these constants with bulk values derived by thermogravimetric analysis was found, confirming the suitability of using Cciie,n to measure oxygen non-stoichiometry of thin oxide films, the films were found to reduce somewhat more readily than bulk PCO. Potential sources of error observed in earlier Cchem studies on perovskite structured oxide films are also discussed. When a DC bias was applied, the non-stoichiometry of the PCO films calculated from the measured Cien, agreed well with predicted values assuming that the effective change in oxygen activity, P 0 2, eective corresponded to the value expected based on the applied Nernst potential. These results confirm the suitability of using bias across an electrochemical cell to conveniently and precisely control 6 of oxide thin films in an in-situ fashion. Of further interest was the ability to readily reach oxygen activities equivalent to PO2s as high as 280 atm. Calculated values for the surface exchange coefficient, k, were found to be comparable in magnitude to those exhibited by other popular mixed ionic electronic conductors, therefore confirming the suitability of PCO as a model mixed conducting cathode material. Interestingly, the magnitude of k was found to be largely dependent on the non-stoichiometry in the PCO films, rather than the oxygen activity in the gas phase, at all temperatures studied. This indicates the important role that defects (electronic defect and oxygen vacancy) play in the cathode reaction.