Structural Origin of the Improved Ionic Conductance in CeO ₂ –ZrO ₂ –CeO ₂ Triple-Layer Electrolytes for Thin-Film Solid Oxide Fuel Cells

To lower the operation temperature of solid oxide fuel cells, thin-film electrolytes have attracted considerable attention, but their chemical instability and mechanical defects hinder their performance. To address these issues, gadolinia-doped ceria (GDC)-yttria-stabilized zirconia (YSZ)-GDC trilayer (CZC) thin-film electrolytes have been developed, wherein the fuel-side GDC layer provides high ionic conductivity and densifies the YSZ, and the cathode-side GDC chemically protects the YSZ electrolyte from the lanthanum strontium cobaltite cathode. Electrochemical impedance spectroscopy shows that the measured ionic conductance of CZC electrolytes is 3-4 times higher than the simple sum of the single-layer conductance of YSZ and GDC in series. This enhancement does not depend on the growth temperature, as low-temperature-deposited then annealed films perform comparably to their high-temperature-grown counterparts. X-ray diffraction and electron microscopy reveal that the YSZ layer in the CZC electrolytes forms highly continuous columnar grains with a markedly decreased grain boundary density, minimizing lattice discontinuities along the ion transport pathways. Grain-orientation mapping confirms predominantly low-angle columnar boundaries rather than the highly misoriented interfaces that would impede ion transport. These observations suggest that the CZC trilayer conductance is enhanced by the single-crystal-like columnar growth of YSZ, facilitated by its crystallographic compatibility with the underlying GDC. Collectively, these findings establish the practicality of CZC trilayers as a high-performance thin-film electrolyte operable at lower temperatures, providing microstructural design guidelines for interface-engineered solid oxide ion conductors. This approach highlights how interface-guided microstructural engineering can unlock unexpected transport properties in such conductors, offering broad implications for next-generation energy devices.