Unravelling conduction mechanisms in perovskite oxides

Researchers from the Tokyo University of Technology and Tohoku University have investigated a unique material for next-generation electrochemical devices: hexagonal perovskite-related oxide Ba₇Nb_3.8Mo_1.2O_20.1. They unveiled the material’s unique ion-transport mechanisms which could pave the way for better dual-ion conductors and a greener future.

Solid-oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) are among the most promising types of electrochemical devices for green power generation. However, they still face challenges that hinder their development. Ideally, SOFCs should operate at low temperatures to prevent unwanted chemical reactions from degrading their constituent materials. Unfortunately, most oxide-ion conductors, a key component of SOFCs, only exhibit decent ionic conductivity at elevated temperatures. PCFCs are chemically unstable under carbon dioxide atmospheres and also require energy-intensive, high-temperature processing steps during manufacturing.

Dual-ion conductors offer a solution; by facilitating the diffusion of protons and oxide ions, these conductors can achieve high total conductivity at lower temperatures, thereby improving the performance of electrochemical devices. However, the underlying conducting mechanisms behind this material remain poorly understood.

The researchers, led by Professor Masatomo Yashima from Tokyo Tech, decided to investigate the conductivity of materials similar to Ba₇Nb₄MoO₂₀, but with a higher ratio of molybdenum. Their research findings were published in Chemistry of Materials.

After screening various compositions of Ba₇Nb_4-xMo_1+xO_20+x/2, the researchers found that Ba₇Nb_3.8Mo_1.2O_20.1 exhibited proton and oxide-ion conductivities, specifically bulk conductivities of 11 mS/cm at 537°C under wet air and 10 mS/cm at 593°C under dry air.

“The total direct current conductivity at 400°C in wet air for Ba₇Nb_3.8Mo_1.2O_20.1 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-stabilised zirconia (YSZ) — a ceramic material commonly used as an electrolyte in SOFCs,” Yashima said.

To better understand the underlying mechanisms behind these high conductivity values, the researchers conducted ab initio molecular dynamics (AIMD) simulations, neutron diffraction experiments, and neutron scattering length density analyses. This enabled them to study the structure of Ba₇Nb_3.8Mo_1.2O_20.1 to determine what makes it special as a dual-ion conductor. They found that the high oxide-ion conductivity of the material originates from a unique phenomenon. Adjacent (Nb/Mo)O₅ monomers in Ba₇Nb_3.8Mo_1.2O_20.1 can form M₂O₉ dimers by sharing an oxygen atom at one of their corners. 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. AIMD simulations also revealed that the 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.

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