New material could enable future energy technologies

Materials containing atoms that can switch back and forth between multiple oxidation states are technologically important but very rare in nature, said Oak Ridge National Laboratory’s (ORNL) Ho Nyung Lee, who led the international research team that published its findings in Nature Materials.

‘Typically, most elements have a stable oxidation state, and they want to stay there,’ Lee said in a statement. ‘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.’

According to ORNL, a Tennessee-based laboratory that is part of the US Department of Energy, many energy storage and sensor devices rely on this valence-switching, a process known as a reduction-oxidation (redox) reaction. Catalytic gas converters use platinum-based metals to transform carbon monoxide into non-toxic gases by adding oxygen. Less expensive oxide-based alternatives to platinum usually require very high temperatures - at least 600 to 700 degrees Celsius - 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 degrees Celsius, which is comparable to the working temperature of noble metal catalysts,’ Lee said. ‘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.’

The team’s material consists of strontium cobaltite, which is known to occur in a preferred crystalline form called brownmillerite. Through an epitaxial stabilisation process, the ORNL-led team discovered a new way to synthesise the material in a more desirable phase known as perovskite. The researchers have filed an invention disclosure on their findings.

‘These two phases have very distinct physical properties,’ Lee said. ‘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.’

The study, ‘Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge,’ involved ORNL’s Hyoungjeen Jeen, Woo Seok Choi, Matthew Chisholm, Michael Biegalski and Dongwon Shin; Argonne National Laboratory’s Chad Folkman, I-Cheng Tung, Dillon Fong and John Freeland; and Hokkaido University’s Hiromichi Ohta.