Rubidium could be the next key player in oxide-ion conductors. Researchers at Institute of Science Tokyo have discovered a rare rubidium (Rb)-containing oxide-ion conductor, Rb₅BiMo₄O₁₆, with exceptionally high conductivity. Identified through computational screening and experiments, its superior performance stems from low activation energy and structural features like large free volume and tetrahedral motion. Its stability under various conditions offers a promising direction for solid oxide fuel cells and clean energy technologies.
Oxide-ion conductors enable oxide ions (O²⁻) to transport in solid oxide fuel cells (SOFCs), which can run on diverse fuels beyond hydrogen, including natural gas, and biogas, and even certain liquid hydrocarbons. This flexibility makes them particularly valuable during the transition to a hydrogen economy. While SOFCs hold transformative potential from an energy sustainability perspective, their widespread adoption is still challenged by their cost, durability, and operating temperature range. Overcoming these hurdles requires the development of better oxide-ion conductors, and researchers in the world are constantly trying out new materials with different chemical compositions. Could rubidium (Rb) be the key to high-performance oxide-ion conductors?
A research team from Institute of Science Tokyo (Science Tokyo), Japan, led by Professor Masatomo Yashima at the Department of Chemistry, School of Science, set out to answer this question. They explored the untapped potential of Rb as the next major advancement in next major advance in oxide-ion conductor technology through a systematic and comprehensive approach. Their findings were published online in Chemistry of Materials on February 2, 2025.
Since Rb+ is one of the largest cations (second only to the cesium ion), crystalline Rb-containing oxides are expected to have a larger lattice and free volumes, potentially leading to lower activation energy for oxide-ion conductivity. Based on this idea, the researchers first performed a computational screening of 475 Rb-containing oxides using bond-valence-based energy calculations. They found that palmierite-type oxide materials, which have a crystal structure similar to the naturally occurring mineral palmierite, exhibited a relatively low energy barrier for oxide-ion migration.
Considering that several bismuth (Bi)-containing materials and molybdenum (Mo)-containing oxides exhibited high oxide-ion conductivity in previous studies, the team selected Rb5BiMo4O16 as a promising candidate. To validate their selection, they conducted a series of experiments, including material synthesis, conductivity measurements, chemical and electrical stability tests, and detailed compositional and crystal structure analyses. They also performed theoretical calculations and ab initio molecular dynamics simulations to explore the underlying mechanisms behind the measured properties.
The results were highly promising. As Yashima remarks, "Surprisingly, Rb5BiMo4O16 exhibited a high oxide-ion conductivity of 0.14 mS/cm at 300 °C, which is 29 times higher than that of yttria-stabilized zirconia at 300 °C and comparable to the leading oxide-ion conductors with similar tetrahedral moieties." Several factors were identified by the research team to explain this exceptional oxide-ion conductivity. First, the large Rb atoms facilitate a low activation energy for oxide ion conductivity. This oxide-ion conductivity is further enhanced by the rotation and arrangement of the MoO₄ tetrahedra within the crystal lattice. In addition, the anisotropic large thermal vibration of oxygen atoms in the material also contributes to oxide-ion conductivity. Finally, the presence of large Bi cations with a lone pair of electrons also plays an important role in lowering the activation energy for oxide-ion migration.
Another remarkable aspect of Rb5BiMo4O16 is its stability at high temperatures under various conditions, including CO2 flow, wet air flow, wet 5% hydrogen in nitrogen flow, and its stability at about 21 °C in water. "The discovery of Rb-containing oxides with both high conductivity and high stability may open a new avenue for the development of oxide-ion conductors," comments Yashima. "We expect that these advances will lead to new applications and markets for Rb, as well as contribute to lowering the operating temperature and reducing the cost of solid oxide fuel cells."
Further research in this field could pave the way for better oxide-ion conductors in sustainability-focused energy applications, as well as in devices like oxygen membranes, gas sensors, and catalysts.
About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to create value for and with society."
About KEK
KEK was established to promote various types of research as a center of excellence for the overall development of Japan's accelerator science (including particle and nuclear research using high-energy accelerators, research on the structure and function of materials, including living organisms, research aimed at improving accelerator performance, and related fundamental technologies). As an Inter-University Research Institute Corporation, KEK provides research opportunities to researchers both domestically and internationally. With its Tsukuba and Tokai campuses as centers of excellence, KEK actively participates in international collaborative experiments and developments. Additionally, KEK is responsible for the School of High Energy Accelerator Science at the Graduate University for Advanced Studies (SOKENDAI).
About J-PARC
The J–PARC stands for Japan Proton Accelerator Research Complex and is an ***accelerator-based research facility with intense proton beams at Tokai-mura, Ibaraki, Japan***. J-PARC has three proton accelerators and three research facilities with using MW-class high power proton beams, which generate neutrons, muons, mesons and neutrinos ***for experiments***, to underlie the development of advanced science.
