Fuel cells, which combine hydrogen and oxygen to generate electricity while producing only water and heat as byproducts, offer a clean energy solution. These cells consist of an anode, cathode, and electrolyte. Hydrogen gas at the anode splits into protons (H+) and electrons. The electrons create an electric current, while the protons migrate through the electrolyte to the cathode, where they react with oxygen to form water. Most existing fuel cells are solid oxide fuel cells (SOFCs) that use oxide ion conductors as electrolytes. However, SOFCs require high operating temperatures, which can cause material degradation over time. To mitigate this, PCFCs using proton-conducting ceramic materials as electrolytes are being explored, as they can operate at lower, more manageable temperatures of 200-500 C. The challenge remains to find materials that exhibit both high proton conductivity and chemical stability at these intermediate temperatures.
In a study published in the Journal of the American Chemical Society, a team led by Professor Masatomo Yashima from Tokyo Tech, in collaboration with Tohoku University researchers, identified hexagonal perovskite-related oxides Ba5R2Al2SnO13 (R representing rare earth metals Gd, Dy, Ho, Y, Er, Tm, and Yb) as promising electrolyte materials. These materials exhibit high proton conductivity of nearly 0.01 S cm?, significantly higher than other proton conductors at around 300 C.
"In this work, we have discovered one of the highest proton conductors among ceramic proton conductors: novel hexagonal perovskite-related oxide Ba5Er2Al2SnO13, which would be a breakthrough for the development of fast proton conductors," says Yashima.
The material's high proton conductivity is due to its full hydration in a highly oxygen-deficient crystal structure. This structure consists of octahedral layers and oxygen-deficient hexagonal close-packed AO3-d (h') layers (A is a large cation such as Ba+ and d represents the amount of oxygen vacancies). When hydrated, these vacancies are filled by oxygens from water molecules to form hydroxyl groups (OH-), releasing protons (H+) that migrate through the structure, enhancing conductivity.
The researchers synthesized Ba5Er2Al2SnO13 (BEAS) using solid-state reactions. The material showed a high level of oxygen vacancies (d = 0.2) and a fractional water uptake of 1, indicating its full hydration capacity. In a wet nitrogen environment, its conductivity at 356 C was 2,100 times higher than in a dry nitrogen environment. Fully hydrated, it achieved a conductivity of 0.01 S cm? at 303 C.
The atomic arrangement in the octahedral layers provides pathways for proton migration, further increasing conductivity. In simulations of Ba5Er2Al2SnO13-H2O, proton movement was studied in a 2+ 2+ 1 supercell of the crystal structure, represented by Ba40Er16Al16Sn8O112H16. The protons in the octahedral layer demonstrated long-range migration, indicating fast proton diffusion.
"The high proton conductivity of BEAS is attributed to its high proton concentration and diffusion coefficient," explains Yashima.
Besides its high conductivity, BEAS is chemically stable at the operating temperatures of PCFCs. Tests under wet atmospheres of oxygen, air, hydrogen, and CO2 at 600 C showed no changes in its composition and structure, indicating robust stability and suitability for continuous operation without degradation.
"These findings open new avenues for proton conductors. The high proton conductivity via full hydration and fast proton migration in octahedral layers in highly oxygen-deficient hexagonal perovskite-related materials would be an effective strategy for developing next-generation proton conductors," says Yashima. With its exceptional properties, this material could lead to efficient, durable, and lower-temperature fuel cells.
Research Report:High Proton Conduction in the Octahedral Layers of Fully Hydrated Hexagonal Perovskite-Related Oxides
