Novel ultrafast solid oxide-ion conductors for low-temperature fuel cells

Novel Sillén oxyhalides exhibit high oxide-ion conductivity at low temperatures, chemical stability, and negligible chemical expansion. One of the materials achieves 0.01 S cm⁻¹ bulk conductivity at 233 °C—within the “Norby gap.” The discovery reveals an unexpected vacancy-mediated mechanism for oxide-ion transport despite excess oxygen, introducing a new materials-design concept for fast oxide-ion conductors and low-temperature solid oxide fuel cells. The present findings may open a new research field, “Oxide Ionics.”

Solid-oxide fuel cells (SOFCs), which convert chemical energy directly into electrical power, are promising energy-conversion technologies because they can operate on various fuels, including hydrogen, natural gas, and biogas. When hydrogen is used as the fuel, water is the main product of the electrochemical reaction. However, conventional SOFCs generally operate at high temperatures, creating challenges related to material degradation, manufacturing costs, and compatibility between cell components.

Commercially available SOFC electrolytes, such as yttria-stabilized zirconia (YSZ), typically require high operating temperatures of around 700°C–1,000 °C to maintain high ionic conductivity. However, such high temperatures can accelerate material degradation and increase costs. Considerable efforts have therefore been devoted to developing electrolyte materials that can operate at lower temperatures of 200 °C–500 °C. Yet, materials that simultaneously combine sufficient oxide-ion conductivity, chemical stability, and negligible chemical expansion are very rare in this temperature range. This gap between 200 and 500 °C, called the “Norby gap,” represents an important challenge not only for SOFCs, but also for various chemical and energy-conversion processes.

In a breakthrough, a research team led by Professor Masatomo Yashima of the Department of Chemistry, School of Science at Institute of Science Tokyo (Science Tokyo) in Japan, has discovered a novel Sillén oxybromide, Bi_1.9Te_0.1LaO_4.05Br, that exhibits exceptional bulk oxide-ion conductivity within the Norby gap. “This oxide-ion conductor not only demonstrates significantly higher ion conductivity than leading proton conductors but also exhibits negligible chemical expansion and high chemical stability. We believe these findings could lay the foundation for a new research field, 'Oxide Ionics,'" says Yashima.

The team included graduate students Yusuke Kanenori and Hidetoshi Udagawa, Assistant Professors Kei Saito, Yuichi Sakuda, and Kotaro Fujii from Science Tokyo; Professor Tatsumi Ishihara and Dr. Maksymilian Kluczny from Kyushu University, Japan; Professor Stephen Skinner and Dr. Mudasir A. Yatoo from Imperial College London, UK; as well as Professor Kazuhiro Mori from the High Energy Accelerator Research Organization (KEK) and J-PARC, Japan. Their study will be published in Nature Communications on October 14, 2026.

In a previous work, the present researchers reported high oxide-ion conduction in a related Sillén oxychloride, Bi_1.9Te_0.1LaO_4.05Cl, but its conductivity was insufficient to achieve high oxide-ion conductivity within the Norby gap. In the present work, they replaced the chloride ion Cl^– of Bi_1.9Te_0.1LaO_4.05Cl with a larger and more polarizable bromide ion, Br^–, and synthesized the Bi_1.9Te_0.1LaO_4.05Br sample and investigated its structural and transport properties.

Notably, Bi_1.9Te_0.1LaO_4.05Br exhibits a high bulk oxide-ion conductivity of 0.01 S cm^–1 at 233 °C within the Norby gap, far surpassing that of representative electrolytes such as YSZ and lanthanum strontium gallium magnesium oxide (LSGM), which require temperatures of 644 and 534 °C, respectively, to achieve the same conductivity. Its oxide-ion conductivity also exceeds even the proton conductivity reported for several representative proton-conducting ceramics. Moreover, it exhibits high chemical stability in various atmospheres, including dry and wet oxidizing, reducing, and CO₂ atmospheres, at temperatures up to 300 °C. It also exhibits negligible chemical expansion.

Their crystal structure analysis shows a Sillén-type structure, with highly conductive triple fluorite-like slabs sandwiched by Br layers, providing a distinctive structural environment for fast oxide-ion transport. This sandwich structure provides a new material-design principle for high oxide-ion conduction.

The most surprising finding concerns how the oxide ions move. Remarkably, despite the excess oxygen of 0.05 in Bi_1.9Te_0.1LaO_4.05Br, both oxygen vacancies and interstitial oxide ions exist at the lattice oxygen sites and interstitial oxygen sites, respectively. Through diffraction studies, first-principles calculations and ab initio molecular dynamics simulations, the team uncovered that the oxide-ion transport in this material proceeds through a vacancy-mediated mechanism. Namely, oxide ions migrate from a lattice site to a neighboring vacant lattice site through vacancies despite the excess oxygen atoms and interstitial oxide ions. This finding challenges the conventional expectation that excess-oxygen oxide-ion conductors should predominantly transport oxide ions through an interstitialcy diffusion mechanism.

“Our findings demonstrate that strategically engineered oxygen vacancies enable ultrafast oxide-ion transport even in materials containing excess oxygen atoms,” remarks Yashima. “This overturns a widely accepted picture of how oxide ions should move in excess-oxygen materials and opens a new direction for designing fast oxide-ion conductors.”

The team also synthesized other Sillén oxybromides and iodides that demonstrated high electrical conductivity, suggesting that this family of oxyhalides can offer a promising platform for fast oxide-ion conductors.

The researchers note that the total conductivity of the present material is lower than its bulk conductivity because of grain-boundary resistance. Further optimization of the material's microstructure, including grain growth and densification, will therefore be important for translating its exceptional bulk conductivity into practical electrolyte performance.

Looking ahead, Yashima says, “This discovery establishes a new foundation for designing novel oxide-ion conductors based on vacancy-mediated transport in materials with excess oxygen atoms. By sandwiching highly oxide-ion conductive triple fluorite-like slabs between Br layers, we may be able to develop a broader family of novel oxide-ion conductors for low-temperature SOFCs, solid oxide electrolysis cells, and other technologies.”

Authors:

Yusuke Kanenori¹, Hidetoshi Udagawa¹, Kei Saito¹, Yuichi Sakuda¹, Kotaro Fujii¹, Maksymilian Kluczny², Mudasir A. Yatoo³, Kazuhiro Mori^4,5,6, Tatsumi Ishihara^2,7,8, Stephen J. Skinner^3,7, and Masatomo Yashima^1,8

Title:

Vacancy-mediated ultrafast oxide-ion conduction in excess-oxygen Sillén oxybromides

Journal:

Nature Communications

DOI:

10.1038/s41467-026-77848-8

Affiliations:

¹Department of Chemistry, Institute of Science Tokyo, Japan
²Department of Automotive Science, Kyushu University, Japan
³Department of Materials, Imperial College London, UK
⁴Institute of Materials Structure Science/J-PARC Center, High Energy Accelerator Research Organization (KEK), Japan
⁵Graduate Institute for Advanced Studies, The Graduate University for Advanced Studies (SOKENDAI), Japan
⁶Graduate School of Science and Engineering, Ibaraki University, Japan
⁷International Institute for Carbon Neutral Energy Research, Kyushu University, Japan
⁸Center for Energy System Design (CESD), Kyushu University, Japan

• Breakthrough proton-conducting ceramic material for clean energy | Science Tokyo News
• Boosting fuel cell efficiency with water vapor | Science Tokyo News
• Breakthrough in High-Performance Oxide-Ion Conductors Using Rubidium | Science Tokyo News
• Solving the Problems of Proton-Conducting Perovskites for Next-Generation Fuel Cells | Former Tokyo Tech
• Discovering Exceptional Oxide Ion Conductivity at Lower Temperatures | Former Tokyo Tech
• Toward Sustainable Energy Applications with Breakthrough in Proton Conductors | Former Tokyo Tech
• Shedding Light on Unique Conduction Mechanisms in a New Type of Perovskite Oxide | Former Tokyo Tech
• Ba2LuAlO5: A New Proton Conductor for Next-Generation Fuel Cells | Former Tokyo Tech
• Discovering Hidden Order in Disordered Crystals | Former Tokyo Tech
• Novel Oxychloride Shows High Stability and Oxide-Ion Conduction through Interstitial Oxygen Site | Former Tokyo Tech
• Elucidating the Mechanism of High Proton Conduction to Develop Clean Energy Materials | Former Tokyo Tech
• Fueling the Future with New Perovskite-related Oxide-ion Conductors | Former Tokyo Tech
• New Ba7Nb4MoO20-Based Materials with High Oxygen-Ion Conductivity Could Open Sustainable Future | Former Tokyo Tech
• New High Proton Conductors with Inherently Oxygen Deficient Layers Open Sustainable Future | Former Tokyo Tech
• Getting through the bottleneck—A new class of layered perovskite with high oxygen-ion conductivity | Former Tokyo Tech
• Apatite-Type Materials without Interstitial Oxygens Show High Oxide-Ion Conductivity by Overbonding | Former Tokyo Tech
• Discovery of a new structure family of oxide-ion conductors “SrYbInO4” | Former Tokyo Tech
• Discovery of a new crystal structure family of oxide-ion conductors: NdBaInO4 | Former Tokyo Tech

• Masatomo Yashima | Science Tokyo Research Information DB - Science and engineering fields
• Kei Saito | Science Tokyo Research Information DB - Science and engineering fields
• The Yashima Research Group
• Department of Chemistry, School of Science
• School of Science
• Materials-Positive Society Visionary Initiative