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New proton conductor for next-generation protonic ceramic fuel cells: Ba2LuAlO5

Scientists from Tokyo Tech have discovered Ba2LuAlO5 as a promising proton conductor for protonic ceramic fuel cells. The oxide 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. These new insights may pave the way to safer and more efficient energy technologies.

An open-access paper on the work is published in Communications Materials.

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.



Typical fuel cells based on solid oxides have a notable drawback in that they operate at high temperatures, usually over 700 °C. That is why many scientists have focused on protonic ceramic fuel cells (PCFCs) instead. These cells use special ceramics that conduct protons (H+) instead of oxide anions (O2−). Thanks 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.

To address this challenge, a team of researchers, including Professor Masatomo Yashima from Tokyo Institute of Technology (Tokyo Tech) in Japan, has been on the lookout for good proton conductor candidates for PCFCs.

Prof. Yashima 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.

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, to form Ba2LuAlO5.0.5H2O. 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 Ba2LuAlO5 may also exhibit high conductivity since its structure is quite similar to that of Ba2LuAlO5.

—Prof. Yashima


  • Riho Morikawa, Taito Murakami, Kotaro Fujii, Maxim Avdeev, Yoichi Ikeda, Yusuke Nambu, and Masatomo Yashima (2023) “High Proton Conduction in Ba2LuAlO5 with Highly Oxygen Deficient Layers” Communications Materials doi: 10.1038/s43246-023-00364-5


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