Elastoplastic Properties of (Pr, Ce)O2-δ Thin Films

Solid oxide fuel cells (SOFCs) rely on long term, consistent operation of non-stoichiometric oxide components under elevated temperatures and oxygen partial pressure gradients.  Materials that exhibit the desired ion transport and reactivity properties generally also exhibit electro-chemo-mechanical coupling that can affect mechanical stability in operando.  The importance of chemomechanical coupling such as chemical expansion of SOFC electrodes is increasingly relevant to the goal of reduced operating temperatures enabled by thin film electrodes. Here, we explore this coupling experimentally and computationally, for the model SOFC cathode material, (Pr, Ce)O_2-δ. We quantified the Young’s elastic modulus E of (Pr, Ce)O_2-δ thin films at temperatures up to 600^ºC and oxygen partial pressures pO₂ below 10^-3 atm via environmentally controlled nanoindentation. The observed decrease in E with increased temperature or decreased pO₂ correlated with the changes in oxygen vacancy concentration and lattice parameter expected due to chemical expansion. We compared these results with those we calculated for several compositions of bulk (Pr, Ce)O_2-δ via density functional theory, and found that the experimentally observed reduction in mechanical stiffness of reduced (Pr, Ce)O_2-δ thin films exceeded that predicted computationally for bulk counterparts. These results demonstrate that the in operando mechanical properties of non-stoichiometric oxides used in thin film SOFCs can differ significantly from those expected from characterization of bulk forms or at standard temperature and pressure. Thus, this chemomechanical coupling must be considered when designing mechanically robust SOFCs with thin film electrode components.