Oxygen 'Sponge' Suits Redox Reactions

Figure 1. Schematic depicts strontium cobaltite "sponge" that easily sheds or absorbs oxygen atoms at relatively low temperature. Source: Oak Ridge National Laboratory.

"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," notes Ho Nyung Lee, who led the team, which included researchers from Argonne National Laboratory, Argonne, Ill., Northwestern University, Evanston, Ill., and Hokkaido University, Sapporo, Japan. Catalytic converters now rely on platinum or another noble metal to spur the redox reactions that transform undesirable compounds in exhaust gas. Use of less-costly oxide-based alternatives has been stymied because they typically require temperatures of at least 600°C to trigger the redox reactions, explain the researchers. "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," says Lee. "Granted, our material is not coming to your car tomorrow, but this discovery shows that multivalent oxides can play a pivotal role in future energy technologies."Strontium cobaltite preferential occurs in a crystalline form called brownmillerite. However, the researchers discovered that a way to synthesize the material in a more-desirable phase called perovskite. More details appear in a recent article in Nature Materials."These two phases have very distinct physical properties. One is a metal, the other is an insulator. One responds to magnetic fields, the other does not — and we can make it switch back and forth within a second at significantly reduced temperatures," notes Lee.The researchers now are exploring whether external perturbations such as strain can improve electrochemical performance, he says. Maintaining sponge performance when scaling up from the nanoscale thin films now used will pose a challenge, Lee admits.A patent is pending on the technology; one company already has expressed interest in collaborating on the development.