A New Oxychloride Demonstrates Exceptional Stability and Conducts Oxide Ions via Interstitial Oxygen Site.

Solid oxide fuel cells (SOFCs) are a promising solution to the contemporary problem of the impending global energy crisis. SOFCs exhibit high efficiency, lower emissions, and low operating costs, making them an ideal energy source for a fossil fuel-free society.

Conventional SOFCs containing YSZ electrolytes have high operating temperatures (700 °C–1000 °C), and their widespread adoption has been limited by their degradation problems and high cost. Therefore, there is a need to search for new materials that show high conductivity and stability at low temperatures (100°C-300°C). While some bismuth (Bi)-containing materials exhibit high oxide ion conductivity through the conventional vacancy diffusion mechanism, they are not very stable under lower atmospheres. As an alternative, the mechanism of interstitial migration, which involves transverse movement of interstitial and lattice oxide ions, has attracted considerable attention. However, it is rarely observed in Bi-containing materials.

A team of researchers from Japan led by Professor Masatomo Yashima of the Tokyo Institute of Technology (Tokyo Technology) put their heads together to find a solution to these problems. In a recent hack published in Advanced functional materialsthe team reported a new Bi-containing compound, LaBi_1.9t_0.1a_4.05Cl, as the oxide ions migrate through the interference mechanism. The team showed that LaBi_1.9t_0.1a_4.05Cl shows high stability and a high oxide ion conductivity that is superior to that of the best conductors of oxide ions at low temperatures (below 201 °C).

When asked how the team managed to discover Lapi_1.9t_0.1a_4.05Professor Yashima Cl explains, “Most known Bi-containing materials exhibit high conductivity of oxide ions via the conventional vacuum diffusion mechanism. The alternative mechanism, namely interstitial diffusion, is rare in these materials. Thus, we specifically looked for Bi-containing materials with an interstitial oxygen site that It could enable interstitial proliferation.”

The location of the interstitial oxygen refers to the empty space within a crystal structure that is partially occupied by oxide ions. Prof. Yashima’s group chose a siline oxychloride-containing compound, Lapi₂a₄Cl, with a fluorite-like triple layer to ensure the presence of these interstitial oxygen sites. Then they partially replaced binary³⁺ The cation with a high percentage of doped valency, tellurium(T)T cation⁴⁺at the stage of Sillén LaBi₂a₄Cl to increase the amount of interstitial oxygen atoms (x/2) in LaBi_{2 – S}t_xa_{4 + x / 2}Cl. The chemical structure of Abe_1.9t_0.1a_4.05Cl (x = 0.1 in LaBi_{1 – S}t_xa_{4 + x / 2}Cl), for detailed experimental and computational studies since the bulk conductivity of LaBi_1.9t_0.1a_4.05Cl was the highest among all the other compositions, i.e., LaBi_{2 – S}t_xa_{4 + x / 2}Cl (0 ≤ x ≤ 0.2).

The team found that Lapi_1.9t_0.1a_4.05Cl shows high chemical and electrical stability at 400 °C in an oxygen partial pressure region between 10⁻²⁵ to 0.2 atm, as well as a high chemical stability in carbon dioxide₂wet H.₂ bar₂And air with normal humidity. Moreover, Lapi_1.9t_0.1a_4.05Cl showed a high conductivity of the 2.0 × 10 oxide ion^-2 sesame^-1 at 702°C. The bulk conductivity of the material was much higher than that of the best oxide ion conductors such as Bi₂Fifth_0.9copper_0.1a_5.35 At temperatures ranging from 96°C – 201°C.

To elucidate the underlying mechanism of superoxide conduction, the team performed neutron diffraction experiments, molecular dynamics simulations, and DFT calculations. The results indicated that the extremely high oxide-ion conductivity is explained by interstitial transport of oxide ions through the lattice and interstitial sites, which is rare in Bi-containing materials.

Discovery of the high oxide ionic conductivity along with the high chemical and electrical stability of LaBi_1.9t_0.1a_4.05Cl, and the unique mechanism underlying its high conductivity will open the door for further research on Bi-containing compounds and Sillén phases, and eventually on high-performance low-temperature SOFC electrolytes.

“There have been studies on photocatalysis and fluorescence of Sillén phases in the research literature. In our study, we have now shown that CO-containing Sillén Bi oxychloride can also serve as promising electrolytes for SOFCs and can contribute to the fuel cell revolution,” Professor Yashima.