Durable and High-Performance Thin-Film BHYb-Coated BZCYYb Bilayer Electrolytes for Proton-Conducting Reversible Solid Oxide Cells

Proton-conducting reversible solid oxide cells are a promising technology for efficient conversion between electricity and chemical fuels, making them well-suited for the deployment of renewable energies and load leveling. However, state-of-the-art proton conductors are limited by an inherent trade-off between conductivity and stability. The bilayer electrolyte design bypasses this limitation by combining a highly conductive electrolyte backbone (e.g., BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb1711)) with a highly stable protection layer (e.g., BaHf_0.8Yb_0.2O_3−δ (BHYb82)). Here, a BHYb82-BZCYYb1711 bilayer electrolyte is developed, which dramatically enhances the chemical stability while maintaining high electrochemical performance. The dense and epitaxial BHYb82 protection layer effectively protects the BZCYYb1711 from degradation in contaminating atmospheres such as high concentrations of steam and CO₂. When exposed to CO₂ (3% H₂O), the bilayer cell degrades at a rate of 0.4 to 1.1%/1000 h, which is much lower than the unmodified cells at 5.1 to 7.0%. The optimized BHYb82 thin-film coating adds negligible resistance to the BZCYYb1711 electrolyte while providing a greatly enhanced chemical stability. Bilayer-based single cells demonstrated state-of-the-art electrochemical performance, with a high peak power density of 1.22 W cm^-2 in the fuel cell mode and −1.86 A cm^-2 at 1.3 V in the electrolysis mode at 600 °C, while demonstrating excellent long-term stability.