Researchers from MIT and Oak Ridge National Laboratory, led by MIT professor Yang Shao-Horn, found that La0.8Sr0.2CoO3-δ (LSC) epitaxial thin films deposited on the surface of a crystal of zirconia exhibit better oxygen reduction kinetics than bulk LSC. The finding could lead to more powerful solid oxide fuel cells in the future—the new research suggests that output could be increased by up to a hundredfold by using thin films of certain perovskite compounds.
A paper on the research was published online 22 June in the journal Angewandte Chemie International Edition.
In many cases, thin layers of a material exhibit properties different from solid blocks of the same material. Even though this is a known phenomenon, the nature of the difference the team found in the behavior of the LSC thin films “was very much unexpected,” says Dr. Shao-Horn.
In current fuel cells, the rate of oxygen reduction (that is, oxygen atoms combining with hydrogen) is the limiting factor in the power output of the device. Many teams are pursuing ways of improving the efficiency and reducing the costs of two major kinds of fuel cells: solid-oxide fuel cells (SOFCs) and proton-exchange membrane fuel cells (PEMFCs). The work on think films addresses potential improvements in the cathode in SOFCs, which could find application in large-scale systems such as electric power plants.
Previous research had found the opposite, that thin films of some perovskite materials were a hundred times less reactive than the bulk material, Shao-Horn says.
“To our knowledge, this is the first time these thin films have been shown to exhibit” the increased activity, Shao-Horn says. The team is continuing research to verify their hypothesis about the reasons for the increased activity, and to explore a family of materials that may exhibit similar properties. “We are working on determining why” the activity level is so high, Shao-Horn says, suggesting that the increased reactivity of the material may result from a stretching of the surface. This may change the content of oxygen vacancies or the electronic structure of the material, possibilities that are being examined in Shao-Horn’s group.
While many fuel cells use electrodes made from precious metals such as platinum, the electrodes in this experiment are made from relatively abundant materials such as cobalt, lanthanum and strontium, Shao-Horn says, so they should be relatively inexpensive to produce. In addition, this material works at much lower temperatures than existing SOFC electrodes, which could be an advantage because “at lower temperatures, material degradation can be much reduced,” she says. Whereas current cells work at temperatures of 800 °C or higher, the new approach might lead to materials that could work at 500 °C, as was the case in these tests.
Shao-Horn stresses that this is the beginning of a new fundamental research area, and could lead to exploration of a whole family of possible compounds in search of one with an optimal combination of high catalytic activity and high stability. This highly reactive material could find a home in places other than fuel cells: for instance, in high-temperature sensors and in membranes used to separate oxygen from nitrogen and other gases, she says.
Gerardo Jose la O’, Sung-Jin Ahn, Ethan Crumlin, Yuki Orikasa, Michael D. Biegalski, Hans M. Christen, Yang Shao-Horn (2010) Catalytic Activity Enhancement for Oxygen Reduction on Epitaxial Perovskite Thin Films for Solid-Oxide Fuel Cells.
Angewandte Chemie International Edition Early View doi: 10.1002/anie.20100192