Neutrons lead the search for oxygen ion conductors

Scientists at the Institut Laue-Langevin (ILL) in collaboration with the University of Montpellier-2 are one step closer to understanding how certain materials display high oxygen mobility at room-temperature

Ionic conductors are everywhere. Without them, we’d never have developed the high energy density batteries we now depend on. Electrochromatic windows would be an impossibility, and gas sensors far less sensitive than those we have today. But there is one particular class of these materials whose behavior has remained elusively unexplained, until now.

Oxygen ion conductors have attracted considerable interest due to their potential technological application in solid oxide fuel cells. The challenge, however, is that most materials display this behavior only at temperatures above 500 °C, limiting their use and resulting in stability problems over time. In addition, these results led to the conclusion that a thermally-activated diffusion process was behind the behavior. But, when the same effect was observed in a small number of materials at room temperature, an alternative explanation was needed.

Using a unique combination of facilities, an international team of researchers recently investigated one such material – Sr₂ScGaO₅ – with a so-called Brownmillerite-type structure. Published in the Journal of Physical Chemistry C [DOI: 10.1021/acs.jpcc.5b02173], their results showed that there is a close correlation between materials displaying moderate or ambient temperature mobility and the presence of high structural disorder of oxygen ions, suggesting that this may be the source of the effect. The team used neutron and x-ray diffraction, alongside NMR characterization, to carry out the experimental work. Density Functional Theory calculations were used to carry out the accompanying modelling.

Previous works from the Institut Laue-Langevin (ILL) on similar materials suggested that lattice vibrations may trigger oxygen mobility at ambient temperatures, and this paper further strengthens that claim. According to ILL instrument scientist, Andrea Piovano, “What we’ve found is that the movement of oxygen ions actually dynamically changes the crystal structure, even at low temperatures – so-called dynamical disordering. We’re now on the hunt for a global explanation for this effect.”

Developing an understanding of oxygen ion mobility across a range of temperatures will be key to the development of more efficient solid oxide fuel cells, and may also help in the development of future sensors and catalysts.

Corallini, S. et al, “One-Dimensional Oxygen Diffusion Mechanism in Sr₂ScGaO₅ Electrolyte Explored by Neutron and Synchrotron Diffraction,¹⁷O NMR, and Density Functional Theory Calculations” J. Phys. Chem. C, 2015, 119, 11447−11458