Proton conductors: NMR solidifies fuel cells

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  • Published: Nov 1, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Proton conductors: NMR solidifies fuel cells

Vacancies

Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopant in Sc-Doped BaZrO3 Studied by 45Sc Nuclear Magnetic Resonance

Revealing the distribution of protons and oxygen vacancies in perovskite-type proton conductors using nuclear magnetic resonance spectroscopy could open up research to develop practical intermediate-temperature solid oxide fuel cells, according to research from Japan.

Solid oxide fuel cells (SOFC) are now used as an alternative power source for homes in Japan. However, these devices are still relatively expensive, suffer from premature material degradation, take a long time to start up and have a rather high operating temperature approaching 750 Celsius. Such factors perhaps limit their more widespread use. If there were less expensive and more stable materials available that could allow an SOFC to operate in the "intermediate" temperature range of 300 to 500 Celsius they would be much more tenable in the home and be quicker to start. Such materials might also open up the possibility of using SOFCs for mobile or vehicular applications.

Proton motility

Now, a team of scientists at Tohoku University in Japan has developed a new idea to improve proton conductivity in rare-earth doped barium zirconate, BaZrO3, perovskite-type proton conductors. Rare-earth doped barium zirconate has been heralded as a breakthrough material for intermediate temperature SOFCs and other applications. However, before they become viable, there is a need to boost their proton conductivity.

Writing in the American Chemical Society's journal Chemistry of Materials, Itaru Oikawa and Hitoshi Takamura suggest a way to improve the mobility of protons in this perovskite by taking control of the oxygen vacancies as well as the protons in the material. Protons are known to be "trapped" around a rare-earth element in doped barium zirconate, which, of course, reduces its proton conductivity. This proton trapping originates in the electrostatic attraction between the negatively charged rare-earth element and the positively charged proton. However, it is possible to pair up the rare-earth element with an oxygen vacancy, which then has a net positive charge and so repels rather than traps protons.

Perfecting defects

In developing this idea, the team clarified the distribution of protons and oxygen vacancies in scandium-doped barium zirconate by combining NMR spectra with data from thermogravimetric analysis. They report that when the number of oxygen vacancies reaches about 4 mol%) in the material, the proton concentration around the zirconium ions is higher than that around the rare-earth element which indicates protons with less influence from the trapping effects of the rare-earth element.

"Because the attractive interaction between the rare-earth element and protons causes the proton trapping, introducing another defect having a positive charge - that is to say, an oxygen vacancy - appears to liberate the trapped protons," explains lead researcher Takamura. He adds that, "This idea can be applied not only to the development of ionic conductors but also other materials, such as fluorescent and catalyst materials, since the interaction of defects plays an important role in these materials." A possibility for the future is functional designer materials with controlled defects.

Related Links

Chem Mater 2015, 27, 6660-6667: "Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopant in Sc-Doped BaZrO3 Studied by 45Sc Nuclear Magnetic Resonance"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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