Understanding the properties of oxide ion conductors for efficient energy devices
The crystal structure of bismuth vanadate is made up of two types of sub-lattice: the Bi-O sub-lattice (depicted in yellow) and the V-O sub-lattice (depicted in brown).
23 July 2019
Given the ever-increasing need for cleaner, more efficient energy sources, materials scientists are constantly searching for new ways to understand and exploit the properties of functional materials to create more environmentally-friendly electrical devices. A team of scientists from the Institut Laue-Langevin (ILL) and Durham University, UK, have used neutron scattering techniques to characterise the material properties of oxide ion conductors, solids with highly mobile oxide ions, which can be used for several energy and environmental applications, such as oxygen sensors and pumps or solid oxide fuel cells (SOFCs).
The study, which was recently published in the Journal of the American Chemical Society, is part of an on-going collaboration between Professor Ivana Evans at Durham University, and Dr Andrea Piovano and Professor Mark Johnson at the ILL.
“Our ultimate goal is to understand the properties and exploitable behaviour of oxide ion conductors at the atomic level,” says Professor Ivana Evans. “This means understanding their structure and dynamics, and how they evolve with temperature. By examining the relationship between the atomic-level and macroscopic properties, we can apply this knowledge to develop new and improved materials.”
One of the most promising applications of oxide ion conductors is as electrolytes in solid oxide fuel cells (SOFCs), devices for efficient electricity generation. Oxide ion conductors have several properties which make them suitable for use in fuel cells. The solid electrolytes in SOFCs allow the movement of ions through the device without the need for a liquid or soft membrane separating the electrodes as in traditional fuel cells – this results in higher current densities and reduced risk of current leakage. SOFCs allow efficient energy conversion from chemical energy to electricity without creating environmentally-noxious by-products. They can also work with a variety of fuels and are less affected by possible impurities present in them. “To be able to harness and optimise these properties, we need to understand how the ion conduction mechanisms operate at the microscopic level,” says Dr Andrea Piovano.
The main disadvantage of oxide ion conductors is that, for those currently being used for SOFCs, very high temperatures of more than 750 – 800 oC are required to achieve the necessary level of ion conductivity. In order to develop SOFCs which could be used for domestic or transport applications, it is crucial to develop materials that show high oxygen ion conduction at lower temperatures. One oxide ion conductor which is known to have remarkable oxide ion conductivity at relatively low temperatures is a form of bismuth vanadate, a material with the chemical formula Bi0.913V0.087O1.587.
The team analysed bismuth vanadate to investigate the mechanisms contributing to its oxide ion dynamics. They used quasielastic neutron scattering to directly observe oxide ion dynamics. This technique measures small energy exchanges between the neutrons and the sample during random motion processes in the material and, crucially, it can be used to distinguish between long-range diffusion and localised dynamics. Data were collected on the IN16B backscattering spectrometer at ILL, enabling detection of the longest timescale oxygen ions diffusion process of any fluorite-type solid electrolyte to date. Furthermore, it is only the second case to date of observing oxide ion dynamics on a nanosecond timescale by quasielastic neutron scattering in any material.
The experiments revealed that two main mechanisms contribute to the favourable oxide ion dynamics of bismuth vanadate. Firstly, the slower of the two mechanisms, is the long-range diffusive movement of oxide ions through the Bi-O sub-lattice, one of two types of sub-lattice that make up the material’s crystal structure (depicted in yellow in the figure above). The second, faster process involves the motion of oxygen atoms within the V-O sub-lattice, the other sub-lattice within the crystal (depicted as brown polyhedra in the figure above). This does not contribute to conduction directly, instead, it facilitates the movement of oxygen atoms into and out of the V-O sub-lattice, creating vacancies which further facilitate conduction via the first process.
These insights can be used to inform ways of chemically modifying bismuth vanadate, as well as other similar oxide ion conductors, to improve its properties further. For example, doping – introducing impurities into a material to modify conductivity – can be used to generate more vacancies in the Bi-O sub-lattice, improving conductivity.
Further studies could investigate other material properties, such as long-term stability, processability and compatibility with the electrode components of a fuel cell. “The next step is to build on the recently published results and continue to use advanced experimental and computational methods to develop and characterise new solid electrolytes with high oxide ion conductivities at low temperatures,” says Professor Ivana Evans.
Re.: Insight into Design of Improved Oxide Ion Conductors: Dynamics and Conduction Mechanisms in the Bi0.913V0.087O1.587 Solid Electrolyte. J. Am. Chem. Soc.2019141259989-9997,
Instruments : Neutron scattering data were collected on time-of-flight spectrometer IN6 and on the backscattering spectrometer IN16B
About ILL – Institut Laue-Langevin is an international research centre based in Grenoble, France. It has led the world in neutron-scattering science and technology for almost 50 years, since experiments began in 1972. ILL operates one of the most intense neutron sources in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are constantly upgraded. Each year 1,200 researchers from over 40 countries visit ILL to conduct research into condensed matter physics, (green) chemistry, biology, nuclear physics, and materials science. The UK, along with France and Germany is an associate and major funder of the ILL. www.ill.eu