(Invited) Impact of Size Scale on Electro-Chemo-Mechanical Coupling Properties in MIECs: Bulk and Thin Film (Pr,Ce)O2-δ

Many oxides exhibit advantageous properties in energy conversion and storage applications, such as solid oxide fuel cells (SOFCs), heterogeneous catalysis, photoelectrochemical cells, and batteries, due in part to their high temperature stability, corrosion resistance, and dopant cation solubility, respectively. In particular, mixed ionic and electronic conducting oxides have been demonstrated to aid in increasing reaction kinetics at electrodes in SOFCs due to the simultaneous presence of electrons and oxygen vacancies. With changes in oxygen partial pressure and temperature during normal operation operation, the defect concentration and hence transport characteristics of MIEC oxides inevitably change. In addition, defects are often coupled to a significant lattice dilation, known as chemical expansion, which under some circumstances leads to mechanical failure. Furthermore, as length scales are reduced, the energetics for electron and vacancy formation have been shown to be modified. In this presentation, recent work examining the the interplay of electro-chemo-mechanical properties of bulk ceramic to thin film size scales will be discussed. This work was facilitated by using the model material system, (Pr,Ce)O_2-δ (PCO), which, due to the ease of reduction of Pr from 4+ to 3+ valence, exhibits significant MIEC behavior at relatively high oxygen pressures (e.g. air). Impedance spectroscopy was used to study both the oxygen exchange resistance and oxygen content (via chemical capacitance) of cells consisting of PCO thin films deposited on yttria stabilized zirconia substrates. When compared to defect formation energies derived from a detailed thermogravimetric and conductivity model for bulk PCO, films were found to exhibit enhanced reducibility. This phenomenon was further verified using an in situ optical transmission measurement of films, coupled with chemical capacitance, which correlated the amount of absorbing Pr⁴⁺ centers to oxygen vacancy concentration. Additionally, oxygen exchange probed by optical transmission relaxation measurements on “bare” films exhibited slower kinetics as compared to corresponding electrochemical measurements. The role of metal electrodes as well as impurities in modifying oxygen exchange rate will be discussed, as well as new means to improve oxygen exchange of the aged “bare” film surface. Lastly, thermo-chemical expansion measurements of bulk and thin film PCO samples will be presented with discussion related to the atomistic role of individual point defects and defect association.