Scientists at the U.S. Department of Energy's Oak Ridge National Laboratory have developed a new oxygen "sponge" that can easily absorb or shed oxygen atoms at low temperatures. Materials with these novel characteristics would be useful in devices such as rechargeable batteries, sensors, gas converters, and fuel cells. Materials containing atoms that can switch back and forth between multiple oxidation states are technologically important but very rare in nature, said ORNL's Ho Nyung Lee, who led the international research team. "Typically, most elements have a stable oxidation state, and they want to stay there," he said. "So far, there aren't many known materials in which atoms are easily convertible between different valence states. We've found a chemical substance that can reversibly change between phases at rather low temperatures without deteriorating, which is a very intriguing phenomenon." Many energy-storage and sensor devices rely on this valence-switching trick, known as a reduction-oxidation, or "redox" reaction. For instance, catalytic gas converters use platinum-based metals to transform harmful emissions such as carbon monoxide into nontoxic gases by adding oxygen. Less expensive oxide-based alternatives to platinum usually require very high temperatures—at least 600 to 700°C—to trigger the redox reactions, making such materials impractical in conventional applications. "We show that our multivalent oxygen sponges can undergo such a redox process at as low as 200°C, which is comparable to the working temperature of noble metal catalysts," Lee said. The team's material consists of strontium cobaltite, which is known to occur in a preferred crystalline form called brownmillerite. Through an epitaxial stabilization process, the ORNL-led team discovered a new recipe to synthesize the material in a more desirable phase known as perovskite.