Chemical Strain and Oxidation-Reduction Kinetics of Epitaxial Thin Films of Mixed Ionic-Electronic Conducting Oxides Determined by X-Ray Diffraction

The study of the chemical strain kinetics induced by oxygen surface exchange in transition metal oxides is of increasing relevance for understanding the fundamental mechanisms for oxygen reduction and evolution reactions (ORR/OER) of oxide catalysts as well as for novel cathode materials for intermediate-temperature solid oxide fuel cell (SOFC) technology.

In most transition metal oxide materials the variation in the oxygen stoichiometry produced by oxygen exchange with the atmosphere is often accompanied by subtle cell volume changes, the so-called chemical strain. Generally, the incorporation of charged defects (oxygen vacancies or interstitials) is expected to cause a cell expansion from the equilibrium intrinsic cell because of variations in the radii of the transition metal ions as well as Coulomb repulsion between those defects. Any slight variation in the oxygen content would proportionally result in a measurable volume increase.  The ability to dynamically follow the cell volume variations allows establishing a direct correlation with oxidation/reduction kinetics.

In this talk we will describe recent development of a new methodology to analyze chemical strain kinetics in epitaxial thin films after redox cycling by using X-ray diffraction time-resolved measurements in conventional lab diffractometer [1], as depicted in Fig 1(a). With this technique we have been able to measure average cell parameter changes as small as 0.1 ppm in a time scale of a few seconds in a wide variety of SOFC cathode materials like La₂NiO_4+δ, Ba_0.5Sr_0.5Co_0.8Fe_0.3O_3-δ, La_0.8Sr_0.2CoO_3-δ, and GdBaCo₂O_5+δ, as well as in epitaxial bilayers. Some of the results of the oxidation kinetics for the different cathode materials are depicted in the Arrhenius plot in Figure 1(b).

We will analyze the suitability of those materials for their application as cathode materials in intermediate-temperature SOFC.  We will also describe the usefulness of this methodology as complementary to other well-established techniques such as Electric Conductivity Relaxation (ECR) and ¹⁸O Isotopic Exchange Depth Profiling (IEDP) experiments. 

[1] R. Moreno, P. García, J. Zapata, J. Roqueta, J. Chaigneau and J. Santiso, Chem. Mater. 25 (2013) 3640-3647