Tokyo Tech scientists have revealed the remarkable dual-ion conductivities of Ba7Nb3.8Mo1.2O20.1, an oxide related to hexagonal perovskite. This discovery holds significant promise for advancing electrochemical devices crucial in the realm of clean energy technologies. Solid-oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) are promising for green power generation, yet they face challenges such as high operating temperatures and energy-intensive manufacturing processes.
SOFCs ideally require low temperatures to prevent unwanted chemical reactions, but many oxide-ion conductors, vital for SOFCs, only exhibit good ionic conductivity at elevated temperatures. PCFCs, on the other hand, face chemical instability in carbon dioxide atmospheres and necessitate high-temperature processing during manufacturing.
The solution lies in dual-ion conductors, which can support the diffusion of both protons and oxide ions. This enables high total conductivity at lower temperatures, enhancing electrochemical device performance. While some perovskite-related dual-ion conductors have been reported, their conductivities often fall short for practical applications.
In response, a research team led by Professor Masatomo Yashima from Tokyo Institute of Technology delved into materials similar to Ba7Nb4MoO20 but with a higher Mo fraction (Ba7Nb4-xMo1+xO20+x/2). Their collaborative study, involving the Australian Nuclear Science and Technology Organization (ANSTO), the High Energy Accelerator Research Organization (KEK), and Tohoku University, was published in Chemistry of Materials.
After screening various compositions, Ba7Nb3.8Mo1.2O20.1 emerged with exceptional proton and oxide-ion conductivities. Professor Yashima notes, “Ba7Nb3.8Mo1.2O20.1 exhibited bulk conductivities of 11 mS/cm at 537°C under wet air and 10 mS/cm at 593°C under dry air.”
The researchers then delved into the material's underlying mechanisms through ab initio molecular dynamics (AIMD) simulations, neutron diffraction experiments, and neutron scattering length density analyses. They discovered that the high oxide-ion conductivity results from the formation of M2O9 dimers, akin to a relay of buckets passing oxide ions.
Moreover, efficient proton migration in the hexagonal close-packed BaO3 layers contributes to high proton conduction. These findings not only highlight Ba7Nb3.8Mo1.2O20.1's potential but also serve as guidelines for designing perovskite-related dual-ion conductors in science and engineering. Professor Yashima emphasizes, “The present findings will help the development of science and engineering of oxide-ion, proton, and dual-ion conductors.”