Breakthrough Discovery: LaBi1.9Te0.1O4.05Cl, a new oxychloride material, shows promise as a high-performance SOFC electrolyte
A novel oxychloride has been discovered that demonstrates exceptional stability and oxide-ion conduction through interstitial oxygen sites, according to recent research. This finding has significant implications for the development of solid oxide fuel cells (SOFCs), which are increasingly viewed as a promising solution to the global energy crisis. SOFCs are highly efficient, have low operating costs, and produce fewer emissions, making them an ideal source of power for a fossil fuel-free world.
Unfortunately, the adoption of conventional SOFCs with yttria-stabilized zirconia (YSZ) electrolytes has been limited by their high operating temperatures (700–1000°C), degradation issues, and high cost. Therefore, scientists have been searching for new materials that exhibit high conductivities and stability at low temperatures (100–300°C). While some bismuth(Bi)-containing materials demonstrate high oxide-ion conductivities through the conventional vacancy diffusion mechanism, they are not stable under reduced atmospheres. As an alternative, researchers have been exploring the interstitialcy migration mechanism, which involves the knock-on motion of interstitial and lattice oxide ions. However, this mechanism has been rarely observed in Bi-containing materials until the discovery of this novel oxychloride.
In a bid to solve the challenges facing oxide-ion conductors, a team of researchers from Japan, led by Professor Masatomo Yashima from the Tokyo Institute of Technology, have made a groundbreaking discovery.
As reported in their latest study published in Advanced Functional Materials, the researchers have discovered a new compound containing Bi, known as LaBi1.9Te0.1O4.05Cl, which is noteworthy for its ability to facilitate the migration of oxide ions through the interstitialcy mechanism.
The researchers demonstrated that LaBi1.9Te0.1O4.05Cl not only exhibits exceptional stability but also has a higher oxide-ion conductivity than even the most efficient oxide-ion conductors at low temperatures (below 201 degrees Celsius).
During an interview, Professor Yashima was asked about how their team managed to uncover LaBi1.9Te0.1O4.05Cl.
The professor replied that many Bi-containing materials typically have high oxide-ion conductivities through the conventional vacancy diffusion mechanism, and interstitialcy diffusion is a rare occurrence in such materials.
“Thus, we specifically searched for Bi-containing materials with interstitial oxygen site that could enable the interstitialcy diffusion.”
To create interstitial oxygen sites, Professor Yashima’s group identified a Bi-containing Sillén oxychloride called LaBi2O4Cl with a triple fluorite-like layer. They then partially substituted the Bi3+ cation with a high valence dopant, Te4+ cation, in the Sillén phase to increase the number of interstitial oxygen atoms (x/2) in LaBi2–xTexO4+x/2Cl. The resulting composition, LaBi1.9Te0.1O4.05Cl (x = 0.1 in LaBi1–xTexO4+x/2Cl), was selected for detailed experimental and computational studies because it had the highest bulk conductivity among all other compositions.
LaBi1.9Te0.1O4.05Cl exhibited remarkable chemical and electrical stability at 400°C in a wide range of oxygen partial pressures, between 10−25 to 0.2 atm, and high chemical stability in CO2, wet H2 in N2, and air with natural humidity. Furthermore, the material showed a high oxide-ion conductivity of 2.0 × 10−2 S cm−1 at 702°C, significantly higher than the best oxide-ion conductors like Bi2V0.9Cu0.1O5.35 at temperatures between 96–201°C.
The team conducted several experiments and calculations, including neutron-diffraction experiments, ab initio molecular dynamics simulations, and DFT calculations, to understand the mechanism behind the high oxide-ion conduction in LaBi1.9Te0.1O4.05Cl. Their findings revealed that the exceptional oxide-ion conduction could be attributed to the migration of oxide ions through the lattice and interstitial sites via an interstitialcy mechanism, which is a rare phenomenon in Bi-containing materials.
The identification of LaBi1.9Te0.1O4.05Cl with its remarkable high oxide ionic conductivity, as well as its chemical and electrical stability, and the rare interstitialcy mechanism responsible for the high conductivity, paves the way for further research on Bi-containing compounds and Sillén phases. This breakthrough discovery offers promising possibilities for developing high-performance SOFC electrolytes at low temperatures.
“There were studies on the photocatalysis and luminescence of Sillén phases in the research literature. In our study, we have now demonstrated that Sillén Bi-containing oxychlorides can also act as promising electrolytes for SOFCs and could contribute to the fuel cell revolution,” adds Prof. Yashima.
Source: 10.1002/adfm.202214082
Image Credit: Tokyo Tech