Scientists discover a new proton conductor for next-generation fuel cells

By | June 6, 2023

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Credit: Prof. M. Yashima, Tokyo Institute of Technology

The discovery of Ba2LuAlO5 How a promising proton conductor paints a bright future for proton ceramic fuel cells, Tokyo Tech scientists report. Experiments show that this new material has remarkably high proton conductivity even without further chemical modifications, and molecular dynamics simulations reveal the underlying reasons. These new insights can pave the way for safer and more efficient energy technologies.

When it comes to sustainability, the ways in which a company generates energy are some of the most important factors to consider. Eager to eventually replace traditional energy sources such as coal and oil, scientists around the world are trying to develop environmentally friendly technologies that produce energy safely and more efficiently. Among them, fuel cells have been gaining ground since the 1960s as a promising approach to producing electricity directly from electrochemical reactions.

However, typical fuel cells based on solid oxides have a significant drawback in that they operate at high temperatures, usually above 700°C. This is why many scientists have instead focused on proton ceramic fuel cells (PCFCs). 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 than most other fuel cells. Unfortunately, only a few proton-conducting materials with reasonable performance are currently known, which is slowing progress in this field.

To address this challenge, a team of researchers, including Professor Masatomo Yashima of Tokyo Institute of Technology (Tokyo Tech) in Japan, has been looking for good proton-conducting candidates for PCFCs. In their latest study, published in Communication materialsthe team reported the remarkable properties of Ba2LuAlO5a new hexagonal perovskite-related oxide that has provided interesting insights into proton conduction.

Yashima and colleagues discovered Ba2LuAlO5 focusing on finding compounds with numerous intrinsic oxygen vacancies. This was motivated by the results of previous studies highlighting the importance of these vacancies in the conduction of protons. Experiments on Ba2LuAlO5 samples revealed that this material has high proton conductivity in its mass at low temperatures, its conductivity was 102 S cm1 at 487°C and 1.5103 S cm1 to 232C even without additional chemical refinements, such as doping.

Next, the team set out to find out the reasons behind this property. Through molecular dynamics simulations and neutron diffraction measurements, they learned about two important features of Ba2LuAlO5. The first is that this oxide absorbs a large amount of water (H2O), compared with other similar materials, to form Ba2LuAlO5*xh2Or (with x=0.50). This large absorption of water, which occurs within two opposing layers of AlO4 tetrahedra, is made possible by a high number of intrinsic oxygen vacancies in the compact hexagonal layers of h’ BaO. In turn, the higher water content of the oxide increases its proton conductivity through various mechanisms, such as higher proton concentration and increased proton hopping.

The second important feature is related to the way protons move through Ba2LuAlO5. The simulations revealed that protons mainly diffuse along LuO interfaces6 layers, which form cubic compact BaO3 layers, rather than through the AlO4 layers. This information could be crucial in the search for other proton-conducting materials, as Yashima explains: ‘Our work provides new design guidelines that open uncharted avenues for the development of high-performance proton conductors in the future.’

The researchers expect to find other Ba-based proton-conducting materials2LuAlO5 in future studies. “By changing the chemical composition of Ba2LuAlO5further improvements in proton conductivity can be expected,” comments Prof. Yashima, “For example, the perovskite-related Ba oxide2InAlO5 it can also exhibit high conductivity as its structure is quite similar to that of Ba2LuAlO5.”

More information:
Riho Morikawa et al, High proton conduction in Ba2LuAlO5 with highly oxygen deficient layers, Communication materials (2023). DOI: 10.1038/s43246-023-00364-5. www.nature.com/articles/s43246-023-00364-5

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