Quantifying Chemical Expansion of Non-Stoichiometric Oxide Thin Films: Challenges and Opportunities

Non-stoichiometric oxides used for the electrodes and electrolytes of solid oxide fuel cells (SOFCs) typically exhibit chemical expansion behavior due to the large defect concentrations required for high ionic conductivity or gas reactivity. Robust SOFC design requires knowledge of how chemical expansion contributes to mechanical strains at interfaces, as such deformation promotes dislocation mobility, delamination, and fracture. Direct measurement of chemical expansion via X-ray diffraction or dilatometry typically requires long time scales (minutes to hours) to allow equilibration of gas atmospheres or bulk samples, or to allow adequate signal detection in the absence of synchrotron access. Here, we describe a new approach to directly and rapidly quantify dynamic chemical expansion of non-stoichiometric oxides in situ at elevated temperatures up to 650^ºC, and demonstrate this method for the mixed-ionic-electronic conducting Pr_0.10Ce_0.90O_2-δ (PCO) as a model SOFC electrode.  The activity of oxygen is modulated via sinusoidal electrical bias signal, while amplitude and phase lag of film expansion are detected with second-scale temporal resolution and sub-nanometer displacement resolution. These dynamic chemical expansion measurements are coupled with defect models of PCO to advance understanding of defect concentration and mobility under oscillating electrical potential. Extensions of this approach to estimate activation energies, lateral expansion differences, and breathing modes of multilayered electrodes are discussed. This approach provides facile access to dynamic chemical expansion under in operando conditions of SOFC electrodes, facilitating improved design of materials that withstand large chemomechanical changes during SOFC start-up, shut-down, and redox cycling.