A team of researchers led by Tokyo Tech reports that the oxide Ba₂LuAlO₅ is a promising proton conductor for protonic ceramic fuel cells. Experiments show that this novel material has a remarkably high proton conductivity even without any additional chemical modifications, and molecular dynamics simulations reveal the underlying reasons. An open-access paper on the work is published in Communications Materials.

Proton conductors have found diverse applications, such as electrolytes in proton ceramic fuel cells, which require high ionic conductivity at low temperatures and high chemical stability. Here, we report the oxide, Ba₂LuAlO₅, which exhibits proton conductivities of 10⁻² S cm⁻¹ at 487 °C and 1.5 × 10⁻³ S cm⁻¹ at 232 °C, high diffusivity and high chemical stability without chemical doping.

Ba₂LuAlO₅ is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba₂LuAlO₅·x H₂O. Ab initio molecular dynamics simulations and neutron diffraction show the hydration in the h′ layer and proton migration mainly around cubic close-packed c layers existing at the interface of octahedral LuO₆ layers. These results demonstrate that the high proton conduction allowed by the highly oxygen-deficient and cubic close-packed layers is a promising strategy for the development of high-performance proton conductors.

—Morikawa et al.

Typical solid oxide fuel cells have a notable drawback in that they operate at high temperatures, usually above 700 °C. Many scientists as a result have focused on protonic ceramic fuel cells (PCFCs) instead. These cells use special ceramics that conduct protons (H⁺) instead of oxide anions (O²⁻). Due to a much lower operating temperature in the range of 300 to 600 °C, PCFCs can ensure a stable energy supply at a lower cost, compared to most other fuel cells. Unfortunately, only a few proton-conducting materials with reasonable performance are currently known, which is slowing down progress in this field.

Professor Masatomo Yashima from Tokyo Institute of Technology (Tokyo Tech) and colleagues discovered Ba₂LuAlO₅ while focusing on finding compounds with a lot of intrinsic oxygen vacancies. This was motivated by the results of previous studies highlighting the importance of these vacancies in proton conduction.

Experiments on Ba2LuAlO5 samples revealed that this material has a high proton conductivity in its bulk at low temperatures even without additional chemical refinements such as doping.

Afterwards, the team sought to find out the underlying reasons for this property. Through molecular dynamics simulations and neutron diffraction measurements, they learned two important characteristics of Ba₂LuAlO₅. The first is that this oxide absorbs a lot of water (H₂O), compared to other similar materials. This large water uptake, which occurs within two opposing layers of AlO₄ tetrahedra, is made possible by a high number of intrinsic oxygen vacancies in the hexagonal close-packed h´ BaO layers. In turn, the oxide’s higher water content increases its proton conductivity through various mechanisms, such as higher proton concentration and enhanced proton hopping.

The second important characteristic is related to how protons move through Ba2LuAlO5. Simulations revealed that protons diffuse mainly along the interfaces of LuO₆ layers, which form cubic close-packed c BaO₃ layers, rather than through the AlO₄ layers. This information could be critical in the search for other proton conducting materials.

The researchers expect to find other proton-conducting materials based on Ba₂LuAlO₅ in upcoming studies.

By modifying the chemical composition of Ba₂LuAlO₅ , further improvements in proton conductivity can be expected. For example, the perovskite-related oxide Ba₂InAlO₅ may also exhibit high conductivity since its structure is quite similar to that of Ba₂LuAlO₅.

—Prof Yashima

Resources

• Morikawa, R., Murakami, T., Fujii, K. et al. (2023) “High proton conduction in Ba2LuAlO5 with highly oxygen-deficient layers.” Commun Mater 4, 42 doi: 10.1038/s43246-023-00364-5