New Ba7Nb4MoO20-Based Materials with High Oxygen-Ion Conductivity Could Open Sustainable Future

Scientists at Tokyo Institute of Technology (Tokyo Tech), Imperial and High Energy Accelerator Research Organization (KEK) Institute of Materials Structure Science, discover new Ba₇Nb₄MoO₂₀-based materials with high oxygen-ion (oxide-ion O^2-) conductivities—"the hexagonal perovskite-related oxides"—and shed light on the underlying mechanisms responsible for their conductivity. Their findings lead the way to uncovering other similar materials, furthering research on developing low-cost and scalable renewable energy technologies.

Over the past few years, fuel cells have become a focal point of research in eco-friendly technology because of their superior abilities to store and produce renewable energy and clean fuel. A typical type of fuel cell gaining ground is the oxide-ion-conducting fuel cell, which is primarily made of materials through which oxide ions (oxygen ions: O^2-), can easily move. New materials with higher conductivity at low and intermediate temperatures, provide a number of advantages over commonly used fuel cells based on yttria-stabilized zirconia (YSZ) electrolytes, such as higher power generation efficiency, longer lifetimes, and lower costs.

However, only a limited number of such materials are known and their application to developing fuel cells has largely remained at the laboratory scale. To truly achieve a sustainable energy economy, new oxide-ion conductors with high conductivity need to be discovered that can allow low-cost and efficient scaling up of these technologies.

Scientists from Tokyo Tech, Imperial and KEK set out to address this need, and in a recent study, identified a new oxide-ion-conducting material that may be a representative of an entire family of oxide-ion conductors.

The material in question has the chemical formula Ba₇Nb_3.9Mo_1.1O_20.05 and is classified as a "hexagonal perovskite-related oxide." Prof Masatomo Yashima, who led the study, explains: "Ba₇Nb_3.9Mo_1.1O_20.05 shows a wide stability range and predominantly oxide-ion conduction in the oxygen partial pressure range from 2 × 10^-26 to 1 atm. Surprisingly, bulk conductivity of Ba₇Nb_3.9Mo_1.1O_20.05, 5.8 × 10^-4 S/cm, is remarkably high at 310 ^oC, and higher than bismuth oxide- and zirconia-based materials (Fig. 1)."* Prof Stephen Skinner comments that the fast oxide ion transport was unambiguously confirmed using the ¹⁸O tracer diffusion technique at Imperial.

Prof Yashima and his team note that the crystal structure of Ba₇Nb_3.9Mo_1.1O_20.05 contains oxygen-deficient layers (c' in Fig. 2a and Fig. 2c) and that its high oxide-ion conductivity is attributable to the oxide-ion migration on the c' layers. In fact, they succeed in experimental visualization of O1−O5 oxide-ion diffusion pathways by the neutron-diffraction measurements at a high temperature 800 ^oC with SuperHRPD diffractometer of Prof Takashi Kamiyama's group at KEK/J-PARC (Figs. 2b,d). Prof Yashima says that the oxide ions migrate via interstitialcy diffusion mechanism through interstitial octahedral O5 and lattice tetrahedral O1 oxygen sites and that the (tetrahedral)–(octahedral) diffusion pathways on the c' layer in Ba₇Nb_3.9Mo_1.1O_20.05 is the same as those in another hexagonal perovskite-related oxide Ba₃MoNbO_8.5–δ. Therefore, Prof Yashima and his team claim that "The common feature of the diffusion mechanism would be a guide for design of oxide-ion conductors with the hexagonal perovskite related structures and that the present finding of high oxide-ion conductivities in rare-earth-free Ba₇Nb_3.9Mo_1.1O_20.05 suggests the ability of various hexagonal perovskite related oxides as superior oxide-ion conductors".*

(a) Refined crystal structure and (b) corresponding yellow isosurface of maximum-entropy method neutron scattering length densities (MEM NSLDs) at 0.36 fm/ų of Ba₇Nb_3.9Mo_1.1O_20.05 at 800 ^oC. Refined crystal structure (c) and corresponding MEM NSLD distribution (d) on the ab plane at z = 0 of Ba₇Nb_3.9Mo_1.1O_20.05 at 800 ^oC. In panel d, the contour lines from 0 to 2 fm/ų by the step of 0.2 fm/ų. Arrows in panel c denote the directions of oxide-ion O1-to-O5 migration.

Reference

Authors :	Masatomo Yashima1, Takafumi Tsujiguchi1, Yuichi Sakuda1, Yuta Yasui1, Yu Zhou2, Kotaro Fujii1, Shuki Torii3,4, Takashi Kamiyama3,4, Stephen J. Skinner2
Title of original paper :	High Oxide-Ion Conductivity through the Interstitial Oxygen Site in Ba7Nb4MoO20-Based Hexagonal Perovskite Related Oxides
Journal :	Nature Communications
DOI :	10.1038/s41467-020-20859-w
Affiliations :	1 Department of Chemistry, School of Science, Tokyo Institute of Technology 2 Department of Materials, Imperial College London 3 Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) 4 Japan Proton Accelerator Research Complex (J-PARC)

• Press release "New Ba₇Nb₄MoO₂₀-Based Materials with High Oxygen-Ion Conductivity Could Open Sustainable Future"
• New High Proton Conductors with Inherently Oxygen Deficient Layers Open Sustainable Future | Tokyo Tech News
• Apatite-Type Materials without Interstitial Oxygens Show High Oxide-Ion Conductivity by Overbonding | Tokyo Tech News
• Discovery of a new structure family of oxide-ion conductors "SrYbInO4" | Tokyo Tech News
• Discovery of a new crystal structure family of oxide-ion conductors: NdBaInO4 | Tokyo Tech News
• The Yashima Research Group
• Researcher Profile | Tokyo Tech STAR Search - Masatomo Yashima
• Department of Chemistry, School of Science
• High Energy Accelerator Research Organization (KEK)
• Department of Materials, Imperial College London
• Latest Research News