Newswise —
SOFCs have emerged as a promising answer to the current challenge of the impending global energy crisis, as they offer high efficiency, lower emissions, and low operating costs. These attributes make them an optimal source of power for a society that aims to be free from reliance on fossil fuels.
The high operating temperatures (700-1000°C) of conventional SOFCs with yttria-stabilized zirconia (YSZ) electrolytes have limited their widespread use due to degradation issues and high costs. Consequently, there is a need to explore new materials with high conductivities and stability at lower temperatures (100-300°C). Although some Bi-containing materials have exhibited high oxide-ion conductivities via the conventional vacancy diffusion mechanism, their stability under reduced atmospheres is not satisfactory. Therefore, the interstitialcy migration mechanism, which involves the knock-on motion of interstitial and lattice oxide ions, has gained significant attention as an alternative. However, this mechanism is not frequently observed in Bi-containing materials.
To tackle these issues, a group of researchers, led by Prof. Masatomo Yashima from the Tokyo Institute of Technology (Tokyo Tech) in Japan, collaborated to find a solution. In their recent breakthrough, which was published in Advanced Functional Materials, the team introduced a new Bi-containing compound, LaBi1.9Te0.1O4.05Cl, where oxide ions migrate through the interstitialcy mechanism. The team demonstrated that LaBi1.9Te0.1O4.05Cl showcases both high stability and exceptional oxide-ion conductivity, surpassing that of the best oxide-ion conductors at low temperatures (below 201°C).
When questioned about how they discovered LaBi1.9Te0.1O4.05Cl, Prof. Yashima clarified that most of the Bi-containing materials known to date showcase high oxide-ion conductivities through the conventional vacancy diffusion mechanism. However, the interstitialcy diffusion mechanism is rare in these materials. Therefore, the team focused their search on Bi-containing materials with an interstitial oxygen site that could facilitate interstitialcy diffusion.
The interstitial oxygen site refers to an empty space within a crystal structure that oxide ions partially occupy. In this study, Prof. Yashima's team chose a Bi-containing Sillén oxychloride, LaBi2O4Cl, with a triple fluorite-like layer to ensure the existence of interstitial oxygen sites. They partially replaced the Bi3+ cation with a high valence dopant, the Te4+ cation, in the Sillén phase LaBi2O4Cl to increase the number of interstitial oxygen atoms (x/2) in LaBi2-xTexO4+x/2Cl. The chemical composition, LaBi1.9Te0.1O4.05Cl (x=0.1 in LaBi1-xTexO4+x/2Cl), was then selected for further experimental and computational studies as its bulk conductivity was the highest among all other compositions, i.e., LaBi2-xTexO4+x/2Cl (0 ≤ x ≤ 0.2).
The research team discovered that LaBi1.9Te0.1O4.05Cl exhibited remarkable chemical and electrical stability at 400°C, even at a wide range of oxygen partial pressure between 10−25 to 0.2 atm. Moreover, the material demonstrated high chemical stability when exposed to CO2, wet H2 in N2, and air with natural humidity. Additionally, LaBi1.9Te0.1O4.05Cl exhibited a high oxide-ion conductivity of 2.0 × 10−2 S cm−1 at 702°C. The bulk conductivity of the material was significantly higher than that of the best oxide-ion conductors, such as Bi2V0.9Cu0.1O5.35, at temperatures ranging between 96–201°C.
To understand the mechanism behind the high oxide-ion conduction observed in LaBi1.9Te0.1O4.05Cl, the research team conducted various experiments, including neutron-diffraction experiments, ab initio molecular dynamics simulations, and DFT calculations. Based on their findings, the team concluded that the high oxide-ion conduction in LaBi1.9Te0.1O4.05Cl is due to interstitialcy migration of oxide ions through both lattice and interstitial sites, which is a rare phenomenon in Bi-containing materials.
The discovery of LaBi1.9Te0.1O4.05Cl and its unique mechanism for high oxide-ion conductivity, along with its high stability, opens up new possibilities for further research on Bi-containing compounds and Sillén phases. This breakthrough could lead to the development of high-performance SOFC electrolytes at low temperatures and contribute to the fuel cell revolution. Despite previous studies on the photocatalysis and luminescence of Sillén phases, this study demonstrates a new promising application for Bi-containing oxychlorides in SOFCs. Overall, the discovery of LaBi1.9Te0.1O4.05Cl provides a promising solution for the development of more efficient and cost-effective fuel cells.
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