Abstract: Na-doped CeO₂ (NDC) electrolytes with 0.05, 0.10, 0.15, and 0.20 molar ratios of Na ions (0.05NDC, 0.1NDC, 0.15NDC, and 0.2NDC) were synthesized and systematically evaluated for low-temperature solid oxide fuel cell (SOFC) applications. Density functional theory (DFT) calculations reveal that Na doping lowers the oxygen-vacancy formation energy. Structural analysis confirms progressive lattice expansion in NDCs and a maximum oxygen-vacancy concentration in 0.15NDC, while incomplete incorporation of Na in 0.2NDC yields residual Na₂CO₃. Conductivity studies demonstrate negligible electronic conductivity and a peak ionic conductivity in 0.15NDC, suggesting that moderate Na doping enhances ionic transport, whereas excessive dopant is detrimental. Two 0.15NDC-based SOFCs are fabricated by ceramic and dry-pressing methods, and their maximum power densities at 550°C are 208 and 778 mW·cm⁻², respectively, indicating the rapid ionic transport of the 0.15NDC electrolyte. These results demonstrate that Na doping is an effective route for developing advanced low-temperature SOFC electrolytes.