Fuel cells: Bringing order to defects and making way for oxygen to move

Materials that allow easy movement of oxygen ions are essential for solid oxide fuel cells. The material discovered in this research could enable more efficient solid oxide fuel cells to operate at much lower temperatures than current technology (~800°C).

Metal oxides are important materials found in many energy technologies such as fuel cells, superconductors, and thermoelectric systems. Most metal oxides contain oxygen deficiencies or vacancies as point defects which are often uniformly distributed in the material. A challenging goal is controlled generation and positioning of these oxygen deficiencies to create novel structures and functional properties. Researchers at the Pacific Northwest National Laboratory have accomplished just that in their discovery of a new, non-stoichiometric metal oxide SrCrO_2.8. While attempting to prepare thin films of stoichiometric SrCrO_3.0, they found that a non-stoichiometric form with the composition SrCrO_2.8 is formed instead.

This new material contains ordered arrays of SrO₂ planes interleaved between layers of tetrahedrally coordinated Cr⁺⁴ ions and separated by ~1nm. Moreover, when mildly heated in air, it reversibly transforms from a semiconducting form with rhombohedral (diamond-like) structure to a metallic form with a so-called cubic perovskite structure. The ordered oxygen vacancies in the rhombohedral form allow oxygen ions to diffuse through the material quickly and easily at low temperatures (~250°C). This property is exceedingly important for solid oxide fuel cell technology which currently requires very high operating temperatures (~800°C). First-principle calculations provided insights into the formation and stability of the non-stoichiometric SrCrO_2.8 and how oxygen ions could move so easily through the material. The aggregation of oxygen vacancy defects into ordered arrays is a property of interest for not only more efficient solid oxide fuel cells but also would be useful for other applications such as thermoelectrics.

DOE Office of Science, Basic Energy Sciences program. The research included staff and use of the facilities supported by the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility supported by the Biologic and Environmental Sciences program. Computational facilities were supported by Pacific Northwest National Laboratory.