Researchers at the Tokyo Institute of Technology (Tokyo Tech), in collaboration with Tohoku University, Australian Nuclear Science and Technology Organization (ANSTO) and the High Energy Accelerator Research Organization (KEK), recently investigated a promising material for next-generation electrochemical devices: hexagonal perovskite-related oxide Ba7Nb3.8Mo1.2O20.1. The team unveiled the material's unique ion-transport mechanisms, that could pave the way for better dual-ion conductors.
Clean energy technologies are the cornerstone of sustainable societies, and solid-oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) are among the most promising types of electrochemical devices for green power generation. These devices, however, still face challenges that hinder their development and adoption.
Ideally, SOFCs should operate at low temperatures to prevent unwanted chemical reactions from degrading their constituent materials. Unfortunately, most known oxide-ion conductors, a key component of SOFCs, only exhibit adequate ionic conductivity at elevated temperatures. As for PCFCs, not only are they chemically unstable under carbon dioxide atmospheres, but they also require energy-intensive, high-temperature processing steps during manufacturing. Dual-ion conductors, however, offer a solution to these problems. By facilitating the diffusion of both protons and oxide ions, these conductors can achieve high total conductivity at lower temperatures, thereby improving the performance of electrochemical devices. Still, the underlying conducting mechanisms behind this material remain poorly understood.
Against this backdrop, a research team, led by Professor Masatomo Yashima from Tokyo Tech, set out to investigate the conductivity of materials similar to Ba7Nb4MoO20 but with a higher ration of molybdenum.
After screening various compositions of Ba7Nb4-xMo1+xO20+x/2, the team found Ba7Nb3.8Mo1.2O20.1 to have remarkable proton and oxide-ion conductivities. "We discovered that 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", explained Yashima. "The total direct current conductivity at 400 °C in wet air for Ba7Nb3.8Mo1.2O20.1 was 13 times higher than that of Ba7Nb4MoO20, and the bulk conductivity in dry air at 306 °C is 175 times higher than conventional yttria-stabilized zirconia (YSZ) - a ceramic material commonly used as an electrolyte in SOFCs."
To illuminate the underlying mechanisms behind these high conductivity values, the scientists then conducted ab initio molecular dynamics (AIMD) simulations, neutron diffraction experiments, and neutron scattering length density analyses. This 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.
They discovered that the high oxide-ion conductivity of the material originates from a unique phenomenon. Adjacent (Nb/Mo)O5 monomers in Ba7Nb3.8Mo1.2O20.1 can form M2O9 dimers by sharing an oxygen atom at one of their corners (M = Nb/Mo cation). Much like a long line of people relaying a bucket of water can accelerate its transportation, the breaking and reforming of these dimers gives rise to ultrafast oxide-ion movement. Furthermore, the AIMD simulations revealed that the observed high proton conduction arose from efficient proton migration in the hexagonal, tightly-packed BaO3 layers in the material.
The results of this study highlight the potential of perovskite-related dual-ion conductors and could serve as guidelines for the rational design of these materials. "Uncovering the high conductivities and unique ion migration mechanisms in Ba7Nb3.8Mo1.2O20.1 will help push the engineering of oxide-ion, proton, and dual-ion conductors, leading us to even better conducting materials for next-generation energy technologies," concludes Yashima.