Rechargeable batteries and fuel cells are seen as the two contenders to serve as a power source for the next generation of environmentally friendly vehicles. A significant barrier to achieving greater efficiency in the latter is the slow rate of oxygen production from the cathode, which limits the power output of the device. Now an unexpected find by MIT researchers regarding the behavior of incredibly thin sheets of material could lead to major improvements .

Fuel cells

In fuel cells, a fuel such as hydrogen or methanol reacts in the presence of a catalyst, releasing its energy chemically rather than being burned. As a result, they can produce electricity from fuel without releasing greenhouse gases or other pollutants, and so are considered a promising alternative approach for generating electricity – not only for environmentally friendly vehicles but also for stationary power systems.

In present fuel cells, the rate of oxygen production is the limiting factor in the power output of the device. There are two major kinds of fuel cells in development: solid-oxide fuel cells (SOFCs) and proton-exchange membrane fuel cells (PEMFCs). The MIT research addresses potential improvements in the cathode in SOFCs, which could find application in large-scale systems such as electric power plants.

Through thick and thin

In many cases, thin layers of a material — which may be just a few molecules in thickness — exhibit properties different from solid blocks of the same material. But even though this is a known phenomenon, the nature of the difference the MIT team found in the behavior of thin films of a mineral called perovskite - in this case, deposited as a thin layer on the surface of a crystal of zirconia - “was very much unexpected,” says Yang Shao-Horn, associate professor of mechanical engineering and materials science and engineering at MIT, who led the research.

Thin-film material offers potential insights

By creating the kind of high-purity thin films of material used in the MIT study - in this case, as thin as 20 nanometers, or billionths of a meter - it is possible to study the details of how the surface of the material reacts in much greater detail than has been possible in research with bulk materials. This research shows that unique thin-film characteristics can enhance catalytic activity.

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.

Possible advantages

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 degrees Celsius or higher, the new approach might lead to materials that could work at 500 degrees Celsius, as was the case in these tests.

Still early days

This work is just the first step, however. 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.

The MIT research team's study was supported by the NSF, the U.S. Department of Energy, Oak Ridge National Laboratory and the King Abdullah University of Science and Technology. The results are published in the German journal Angewandte Chemie.