Nonstoichiometry in Oxide Thin Films Operating under Anodic Conditions: A Chemical Capacitance Study of the Praseodymium-Cerium Oxide System

PrxCe1xO2-delta can potentially serve as both the cathode and anode in solid oxide fuel cells (SOFCs), given that it exhibits significant mixed ionic-electronic conductivity (MIEC) under both cathodic and anodic conditions. While MIEC is strongly dependent on oxygen nonstoichiometry (delta), there have been few ways to extract this information reliably and in situ, particularly for thin films. In this work, this is achieved by analysis of chemical capacitance values extracted from impedance spectroscopy data obtained on electrochemical cells of the form Pr0.1Ce0.9O2-delta/Y0.16Zr0.84O1.92/Pr0.1Ce0.9O2-delta operating at temperatures in the range of 450700 degrees C and over the oxygen pressure (pO(2)) range of 10(33)10(14) atm. In combination with our prior investigation, the oxygen nonstoichiometry over a very wide range of pO(2) values is derived. We found that delta in thin films differs from that of the bulk (delta(film) > delta(bulk)). This follows from a decreased enthalpy of reduction of 4.12 +/- 0.25 eV for the film, compared to 4.52 +/- 0.37 eV for the bulk.