Rubidium may emerge as a crucial element in oxide-ion conductors. Scientists at the Institute of Science Tokyo have identified a rare rubidium (Rb)-containing oxide-ion conductor, Rb5BiMo4O16, exhibiting exceptionally high conductivity. Discovered through computational screening and experimentation, its remarkable performance is attributed to low activation energy and structural characteristics such as ample free volume and tetrahedral motion. Its stability across diverse conditions presents exciting prospects for solid oxide fuel cells and clean energy technologies.

Enhancing Oxide-Ion Conductors: Rubidium-Based Materials for Sustainable Energy

Oxide-ion conductors facilitate the transport of oxide ions (O2-) in solid oxide fuel cells (SOFCs), which can operate on a variety of fuels beyond hydrogen, including natural gas, biogas, and even certain liquid hydrocarbons. This versatility makes them particularly valuable in the transition to a hydrogen-based economy. While SOFCs have the potential to revolutionize energy sustainability, their widespread adoption faces challenges related to cost, durability, and operating temperature range. Addressing these obstacles requires the development of advanced oxide-ion conductors, prompting researchers worldwide to explore new materials with diverse chemical compositions. Could rubidium (Rb) be the key to unlocking high-performance oxide-ion conductors?

Figure 1.Rubidium-Based Materials for a Sustainable Future.

A research team from the Institute of Science Tokyo (Science Tokyo), Japan, led by Professor Masatomo Yashima from the Department of Chemistry, School of Science, aimed to answer this question. Through a systematic and comprehensive approach, they investigated the untapped potential of rubidium (Rb) as a breakthrough in oxide-ion conductor technology. Their findings were published online in Chemistry of Materials on February 2, 2025. Figure 1 shows Rubidium-Based Materials for a Sustainable Future.

Since Rb+ is one of the largest cations, second only to the cesium ion, crystalline Rb-containing oxides are expected to feature a larger lattice and greater free volume, potentially reducing the activation energy for oxide-ion conductivity. Based on this concept, the researchers conducted a computational screening of 475 Rb-containing oxides using bond-valence-based energy calculations. Their analysis revealed that palmierite-type oxide materials, which share a crystal structure with the naturally occurring mineral palmierite, exhibited a relatively low energy barrier for oxide-ion migration.

Given that previous studies have shown high oxide-ion conductivity in several bismuth (Bi)- and molybdenum (Mo)-containing oxides, the team identified Rb5BiMo4O16 as a promising candidate. To verify their selection, they carried out a comprehensive set of experiments, including material synthesis, conductivity measurements, and tests for chemical and electrical stability. Additionally, they conducted detailed compositional and crystal structure analyses. To further investigate the underlying mechanisms behind the observed properties, they performed theoretical calculations and ab initio molecular dynamics simulations.

The results were highly promising. As Yashima noted, “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 the same temperature and comparable to the leading oxide-ion conductors with similar tetrahedral moieties.”

The research team identified several factors contributing to this exceptional oxide-ion conductivity. First, the presence of large Rb atoms facilitates a low activation energy for oxide-ion transport. Additionally, the rotation and arrangement of MoO4 tetrahedra within the crystal lattice further enhance conductivity. The material’s anisotropic large thermal vibration of oxygen atoms also plays a key role in promoting ion movement. Lastly, the inclusion of large Bi cations with a lone pair of electrons helps lower the activation energy required for oxide-ion migration.

Another remarkable characteristic of Rb5BiMo4O16 is its stability at high temperatures under various conditions, including exposure to CO2 flow, wet air flow, wet 5% hydrogen in nitrogen flow, and even in water at approximately 21 °C. “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,” says Yashima. “We anticipate that these advances will create new applications and markets for Rb while also helping to lower the operating temperature and reduce the cost of solid oxide fuel cells.”

Further research in this field could drive the development of improved oxide-ion conductors for sustainability-focused energy applications. Additionally, these advancements may benefit other technologies, including oxygen membranes, gas sensors, and catalysts.

Source: Institute of Science Tokyo

Cite this article:

Keerthana S (2025),Breakthrough in High-Performance Rubidium-Based Oxide-Ion Conductors, AnaTechmaz, pp.1099.