Professor of Ceramics and Electronic Materials
Director, Crystal Physics and Electroceramics Laboratory (CPEL)
MIT Department of Materials Science and Engineering
Many energy related materials rely on transport of ions into and out of electrodes and membranes, for example, Li ions in batteries, O ions in solid oxide fuel cells (SOFCs) and steam electrolyzers, and H ions in hydrogen storage devices. Significant mechanical stresses often accompany these changes in chemical composition, also referred to as chemical expansion. For example, Pr0.1Ce0.9O2-o, a candidate solid oxide fuel cell (SOFC) cathode material, exhibits a >200% increase in its effective thermal expansion coefficient due to oxygen loss upon heating in air. Under the large oxygen partial pressure (pO2) gradient typical of SOFC operation, chemical expansion can result in cracking of electrolyte membranes, and, as a consequence, models have been developed to predict safe operating conditions.
This chemo-mechanical coupling between oxygen stoichiometry and expansion is defined, analogous to thermal expansion, by a chemical coefficient of expansion, which experimentally has been observed to depend on material composition and structure. The atomic origins of the chemical expansion in fluorite and perovskite structured oxides are explored by atomic level computational methods and validated by experimental data including lattice dilation, defect generation and carrier localization. The implications of chemical expansion, including discussion of models developed to predict its impact on SOFCs as well as secondary effects, namely reduction in elastic modulus, as well as a case study of chemical expansion in Pr0.1Ce0.9O2-o, correlating oxygen non-stoichiometry with expansion is presented.