Oxygen-Vacancy-Engineered Tm-Doped CeO₂ Electrolyte for Efficient Low-Temperature Solid Oxide Fuel Cells

Realizing solid electrolyte materials that deliver excellent performances within a low-temperature window remains a key challenge for low-temperature solid oxide fuel cells (SOFCs). In this study, we report thulium-doped ceria (TDC, Ce_0.75Tm_0.25O_2-δ) as a high-performance electrolyte for low-temperature SOFCs. TDC is synthesized via a solid-state reaction and systematically characterized using XRD, Raman spectroscopy, FESEM, HRTEM, and XPS. Structural analysis confirms a stable cubic fluorite phase with homogeneous Tm incorporation and the oxygen vacancy formation. Microstructural investigations reveal nanoscale grains and lattice expansion, both of which are favorable for enhanced oxide-ion transport. Electrochemical testing of the cell demonstrates superior performance, achieving a peak power density of 897 mW cm^-2 at 550 °C with an open-circuit voltage of 1.10 V, confirming the absence of any short circuit. Impedance spectroscopy confirms low ohmic and reduced polarization resistances, correlating with a high ionic conductivity (0.14 S cm^-1 at 550 °C). First-principles calculations further clarify the electronic structure, highlighting spin polarization, strong p-d hybridization, and localized d states as key contributors to TDC's multifunctional behavior. The synergistic effects of defect engineering and electronic structure modulation suggest that TDC is a promising electrolyte material for next-generation LT-SOFCs and multifunctional energy devices.