Praseodymium-Cerium Oxide (Pr x Ce 1-x O 2−δ ; PCO), a potential three way catalyst oxygen storage material and solid oxide fuel cell (SOFC) cathode, exhibits surprisingly high levels of oxygen nonstoichiometry, even under oxidizing (e.g. air) conditions, resulting in mixed ionic electronic conductivity (MIEC). In this study we examine the redox kinetics of dense PCO thin films using impedance spectroscopy, for x00.01, 0.10 and 0.20, over the temperature range of 550 to 670°C, and the oxygen partial pressure range of 10 −4 to 1 atm O 2 . The electrode impedance was observed to be independent of electrode thickness and inversely proportional to electrode area, pointing to surface exchange rather than bulk diffusion limited kinetics. The large electrode capacitance (10 −2 F) was found to be consistent with an expected large electrochemically induced change in stoichiometry for x00.1 and x00.2 PCO. The PCO films showed surprisingly rapid oxygen exchange kinetics, comparable to other high performance SOFC cathode materials, from which values for the surface exchange coefficient, k q , were calculated. This study confirms the suitability of PCO as a model MIEC cathode material compatible with both zirconia and ceria based solid oxide electrolytes.

While the properties of functional oxide thin fi lms often depend strongly on their oxygen stoichiometry, there have been few ways to extract this information reliably and in situ. In this work, the derivation of the oxygen nonstoichiometry of dense Pr 0.1 Ce 0.9 O 2 − δ thin fi lms from an analysis of chemical capacitance obtained by impedance spectroscopy is described. Measurements are performed on electrochemical cells of the form Pr 0.1 Ce 0.9 O 2 − δ / Y 0.16 Zr 0.84 O 1.92 /Pr 0.1 Ce 0.9 O 2 − δ over the temperature range of 450 to 800 ° C and oxygen partial pressure range of 10 − 5 to 1 atm O 2 . With the aid of a defect equilibria model, approximations relate chemical capacitance directly to non-stoichiometry, without need for fi tting parameters. The calculated non-stoichiometry allows extraction of the thermodynamic constants defi ning defect generation. General agreement of these constants with bulk values derived by thermogravimetric analysis is found, thereby confi rming the suitability of this technique for measuring oxygen non-stoichiometry of thin oxide fi lms. Potential sources of error observed in earlier chemical capacitance studies on perovskite structured oxide fi lms are also discussed.

Electrospun polyaniline (PAni) fibers doped with different levels of (+)‐camphor‐10‐sulfonic acid (HCSA) are fabricated and evaluated as chemiresistive gas sensors. The experimental results, based on both sensitivity and response time, show that doped PAni fibers are excellent ammonia sensors and that undoped PAni fibers are excellent nitrogen dioxide sensors. The fibers exhibit changes in measured resistances up to 60‐fold for ammonia sensing, and more than five orders of magnitude for nitrogen dioxide sensing, with characteristic response times on the order of one minute in both cases. A time‐dependent reaction‐diffusion model is used to extract physical parameters from fitting experimental sensor data. The model is then used to illustrate the selection of optimal material design parameters for gas sensing by nanofibers.

Simultaneous in situ optical absorption and electrochemical impedance spectroscopy measurements were performed, for the first time, at elevated temperature on a metal oxide thin film exhibiting oxygen nonstoichiometry, utilizing Pr0.1Ce0.9O2−δ (10PCO) as a model system. Chemical capacitance measurements, capable of providing explicit values of δ, were used to determine the optically absorbing center (Pr4+) concentration and thereby the optical extinction coefficient for Pr4+. The absorption coefficient was found to exhibit a linear dependence on Pr4+ concentration, validating the use of optical absorption to examine defect concentration trends and allowing derivation of the extinction coefficient εPr4+ = 5.01 ± 0.14 × 10–18 cm2. Values of Pr4+ concentration derived from the chemical capacitance and corresponding trends in optical absorption were found to be self-consistent, validating the thin film defect model for 10PCO, thereby confirming that the oxygen reduction enthalpy in thin film 10PCO is lower than that in the bulk. The non-contact optical absorption technique thus provides an additional in situ method for investigating the defect equilibria of thin films and is expected to aid in confirming whether and under what conditions the defect thermodynamics of films differ from that of their bulk counterparts.

In this paper, we focus on the effect of processing‐dependent lattice strain on oxygen ion conductivity in ceria based solid electrolyte thin films. This is of importance for technological applications, such as micro‐SOFCs, microbatteries, and resistive RAM memories. The oxygen ion conductivity can be significantly modified by control of lattice strain, to an extent comparable to the effect of doping bulk ceria with cations of different diameters. The interplay of dopant radii, lattice strain, microstrain, anion‐cation near order and oxygen ion transport is analyzed experimentally and interpreted with computational results. Key findings include that films annealed at 600 °C exhibit lattice parameters close to those of their bulk counterparts. With increasing anneal temperature, however, the films exhibited substantial compaction with lattice parameters decreasing by as much as nearly 2% (viz, Δd600–1100 °C: –1.7% (Sc+3) > –1.5% (Gd+3) > –1.2% (La+3)) for the annealing temperature range of 600–1100 °C. Remarkably 2/3rd of the lattice parameter change obtained in bulk ceria upon changing the acceptor diameter from the smaller Sc to larger La, can be reproduced by post annealing a film with fixed dopant diameter. While the impact of lattice compaction on defect association/ordering cannot be entirely excluded, DFT computation revealed that the main effect appears to result in an increase in migration energy and consequent drop in ionic conductivity. As a consequence, it is clear that annealing procedures should be held to a minimum to maintain the optimum level of oxygen ion conductivity for energy‐related applications. Results reveal also the importance to understand the role of electro‐chemo‐mechanical coupling that is active in thin film materials.

The electrical conductivity (σ) of Ce 0.8 Zr 0.2 O 2−δ was measured and modeled using a defect chemical-based model including contributions from singly and doubly ionized oxygen vacancies, acceptor impurities, and small polaron electrons. By analyzing the pO 2 dependence of σ in terms of the defect model, a transition between an impurity dominated regime at high pO 2 and lower temperature to one controlled by the simultaneous generation of electrons and doubly ionized oxygen vacancies at low pO 2 and higher temperature is identified. At even lower pO 2 , or equivalently larger deviations from stoichiometry, evidence is presented for a further transition to singly ionized oxygen vacancies accompanying electron generation. Temperature induced conductivity relaxation measurements are successfully applied in deconvoluting electron generation and migration contributions to the activation energy. Key parameters are extracted including the enthalpy of reduction H r of 2.87 ± 0.06 eV and the electron hopping or migration energy of 0.354 ± 0.005 eV. Both the activated electron mobility and the broad maximum in conductivity observed under the most reducing conditions support the small polaron model for electron transport in Ce 0.8 Zr 0.2 O 2−δ . Consistent with earlier findings, Zr, though isovalent with Ce, markedly enhances the reducibility, and thereby the oxygen storage capability of ceria− zirconia solid solutions.

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