Researchers at Tokyo Institute of Technology and Tohoku University have reported hexagonal perovskite-related oxides with exceptionally high proton conductivity and thermal stability.
The team explained that these materials' unique crystal structure and large number of oxygen vacancies enable full hydration and high proton diffusion, making them ideal candidates as electrolytes for next-generation protonic ceramic fuel cells that can operate at intermediate temperatures without degradation.
Fuel cells offer a promising solution for clean energy by combining hydrogen and oxygen to generate electricity, with only water and heat produced as byproducts. They consist of an anode, a cathode, and an electrolyte. Hydrogen gas is introduced at the anode where it 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 fuel cells are solid oxide fuel cells (SOFCs), which use oxide ion conductors as electrolytes. However, a major challenge with SOFCs is the high operating temperatures required, leading to material degradation over time. To address this, protonic ceramic fuel cells (PCFCs) that use proton-conducting ceramic materials as electrolytes are being explored. These fuel cells can operate at intermediate, more manageable temperatures of 200-500 °C. However, finding suitable materials that exhibit both high proton conductivity and chemical stability at these intermediate temperatures remains a challenge.
In the recent study, the researchers made a significant breakthrough. They identified chemically stable hexagonal perovskite-related oxides Ba5R2Al2SnO13 where R represents rare earth metals Gd, Dy, Ho, Y, Er, Tm, and Yb) as promising electrolyte materials with a high proton conductivity of almost 0.01 S cm⁻¹, which is notably higher than that of other proton conductors around 300 oC.
The high proton conductivity of the material is attributed to the full hydration in highly oxygen deficient material with a unique crystal structure. The structure can be visualized as a stacking of octahedral layers and oxygen-deficient hexagonal close-packed AO3-δ(h') layers (A is a large cation such as Ba²⁺ and δ represents the amount of oxygen vacancies). When hydrated, these vacancies are fully occupied by the oxygens from the water molecules to form hydroxyl groups (OH⁻), releasing protons (H⁺) which migrate through the structure, enhancing conductivity.
In their study, the researchers synthesized Ba5R2Al2SnO13 (BEAS) using solid-state reactions. The material had a large amount of oxygen vacancies (δ = 0.2) and exhibited a fractional water uptake of 1, indicating its capacity for full hydration. When tested, its conductivity in a wet nitrogen environment was found to be 2,100 times higher than in a dry nitrogen environment at 356 °C. When fully hydrated, it achieved a conductivity of 0.01 S cm⁻¹ at 303 °C.
Moreover, the arrangement of atoms in the octahedral layers provides paths for proton migration, further increasing proton conductivity. In simulations of Ba5R2Al2SnO13·H2O, the researchers studied proton movement in a 2×2×1 supercell of the crystal structure, represented by Ba40Er16Al16Sn8O112H16. This structure included two h' layers and two octahedral layers. The researchers found that protons in the octahedral layer showed long-range migrations of protons, indicating fast proton diffusion.
In addition to its high conductivity, the material is also chemically stable at the operating temperatures of PCFCs. Upon annealing the material under wet atmospheres of oxygen, air, hydrogen, and CO2 at 600 °C, the researchers observed no changes in its composition and structure, indicating the material's robust stability and suitability for continuous operation without degradation.
