Solid oxide fuel cells convert the chemical energy of fuel directly into electricity. They can run on hydrogen, natural gas and biogas, but widespread use is hindered by high operating temperatures: standard electrolytes such as yttria-stabilized zirconia (YSZ) require heating to roughly 700-1000 degrees Celsius. This accelerates material wear and makes production more expensive.
Scientists have long sought electrolytes capable of working efficiently at 200-500 degrees. This range is called the "Norby gap." In it, it is difficult to simultaneously achieve high oxygen ion conductivity, chemical stability and minimal change in the material's dimensions during operation.
A group led by Professor Masatomo Yashima of the Institute of Science Tokyo (Science Tokyo, Japan) synthesized a new oxybromide with the composition Bi1.9Te0.1LaO4.05Br. At 233 degrees, its bulk oxygen ion conductivity reaches 0.01 siemens per centimeter. For comparison: the common electrolytes YSZ and LSGM (lanthanum-strontium-gallium-magnesium oxide) achieve the same conductivity at temperatures of 644 and 534 degrees respectively. The results were published in the journal Nature Communications.
The material was obtained by replacing the chloride ion in a related compound with a larger and more polarizable bromide ion. The new oxybromide retains chemical stability in dry and humid oxidizing, reducing and carbon dioxide atmospheres when heated to 300 degrees and hardly expands when the environment changes.
Structural analysis showed that the crystal consists of conducting layers similar to fluorite, which are separated by bromine layers. This layered architecture suggests a new principle for creating fast oxygen ion conductors.
The most unexpected part is the ion movement mechanism. Despite the excess oxygen, the ions move not through interstitial sites, as previously thought, but via vacancies - empty places in the crystal lattice. This contradicts previous ideas about materials with excess oxygen and may set a new direction in conductor design.
The authors emphasize that the sample's total conductivity is lower than the bulk conductivity because of resistance at grain boundaries. To bring the material closer to practical application, microstructure optimization is needed - grain growth and increased density. In addition, the study is laboratory-based, and there is no talk of a commercial product yet.
Nevertheless, the discovery expands the family of oxyhalides for low-temperature fuel cells and electrolyzers. Scientists have already obtained other oxybromides and iodides with high electrical conductivity, which indicates the potential of an entire class of such materials.