According to scientists at Tokyo Tech, the remarkable proton and oxide-ion (dual-ion) conductivities of hexagonal perovskite-related oxide Ba7Nb3.8Mo1.2O20.1 are promising for next-generation electrochemical devices. The novel ion-transport mechanisms they revealed should pave the way for improved dual-ion conductors, which could be critical in tomorrow’s clean energy technologies.
Clean energy technologies are the foundation of sustainable societies, and proton ceramic fuel cells (PCFCs) and solid-oxide fuel cells (SOFCs) are two of the most promising electrochemical devices for green power generation. However, these devices continue to face challenges that impede their development and adoption.
SOFCs should ideally be operated at low temperatures to avoid unwanted chemical reactions degrading their constituent materials. Unfortunately, the majority of known oxide-ion conductors, a key component of SOFCs, only have adequate ionic conductivity at high temperatures. PCFCs are not only chemically unstable in carbon dioxide atmospheres, but they also necessitate energy-intensive, high-temperature processing steps during production.
The present findings of high conductivities and unique ion migration mechanisms in Ba7Nb3.8Mo1.2O20.1 will help the development of the science and engineering of oxide-ion, proton, and dual-ion conductors.
Prof. Yashima
Fortunately, dual-ion conductors are a type of material that can solve these issues by combining the advantages of both SOFCs and PCFCs. Dual-ion conductors can achieve high total conductivity at lower temperatures by promoting the diffusion of both protons and oxide ions. This improves the performance of electrochemical devices. Although some perovskite-related dual-ion conducting materials have been reported, such as Ba7Nb4MoO20, their conductivities are insufficient for practical applications, and their underlying conducting mechanisms are unknown.
Against this backdrop, a research team led by Professor Masatomo Yashima from Tokyo Institute of Technology, Japan, decided to investigate the conductivity of materials similar to 7Nb4MoO20 but with a higher Mo fraction (that is, Ba7Nb4-xMo1+xO20+x/2). Their latest study, which was conducted in collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO), the High Energy Accelerator Research Organization (KEK), and Tohoku University, was published in Chemistry of Materials.
After screening various Ba7Nb4-xMo1+xO20+x/2 compositions, the team found that Ba7Nb3.8Mo1.2O20.1 had remarkable proton and oxide-ion conductivities. “Ba7Nb3.8Mo1.2O20.1 exhibited bulk conductivities of 11 mS/cm at 537 ℃ under wet air and 10 mS/cm at 593 ℃ under dry air. Total direct current conductivity at 400 ℃ in wet air of Ba7Nb3.8Mo1.2O20.1 was 13 times higher than that of Ba7Nb4MoO20, and the bulk conductivity in dry air at 306 ℃ is 175 times higher than that of the conventional yttria-stabilized zirconia (YSZ),” highlights Prof. Yashima.
Next, the researchers sought to shed light on the underlying mechanisms behind these high conductivity values. To this end, they conducted ab initio molecular dynamics (AIMD) simulations, neutron diffraction experiments, and neutron scattering length density analyses. These techniques enabled them to study the structure of Ba7Nb3.8Mo1.2O20.1 in greater detail and determine what makes it special as a dual-ion conductor.
Surprisingly, the researchers discovered that the high oxide-ion conductivity of Ba7Nb3.8Mo1.2O20.1 results from a unique phenomenon. It turns out that by sharing an oxygen atom on one of their corners (M = Nb or Mo cation), adjacent MO5 monomers in Ba7Nb3.8Mo1.2O20.1 can form M2O9 dimers. The breaking and reforming of these dimers produces ultrafast oxide-ion movement, similar to a long line of people relaying buckets of water (oxide ions) from one person to the next. Furthermore, AIMD simulations revealed that the observed high proton conduction was caused by efficient proton migration in the material’s hexagonal, close-packed BaO3 layers.
The findings of this study, when taken together, highlight the potential of perovskite-related dual-ion conductors and could serve as guidelines for the rational design of these materials. “The present findings of high conductivities and unique ion migration mechanisms in Ba7Nb3.8Mo1.2O20.1 will help the development of science and engineering of oxide-ion, proton, and dual-ion conductors,” says an enthusiastic Prof. Yashima.
