Achieving high performance in solid oxide fuel cells (SOFCs) under low-temperature operation remains hindered by the lack of efficient electrolytes that can simultaneously ensure fast ion transport and negligible electron leakage. Here, we introduce a ternary multi-heterojunction composite electrolyte based on p-type NiO, n-type Li_0.25Sn_0.75O_2−δ (LS), and Sm_0.2Ce_0.8O_2−δ (SDC) to testify its high-performance capability in low-temperature SOFCs. The unique configuration integrates two p–n and one n–n junctions, which can promote the oxygen ion transport while eliminating electronic short-circuiting. The optimized NLS–SDC 6:4 heterostructure (in mass ratio, and NLS stands for the NiO–LS composite precusor prepared in molar ratio of 8:9) achieves a record of an ionic conductivity around 0.348 S·cm⁻¹ and a peak power density (PPD) of 916 mW·cm⁻² at 550 °C, and a long-term stability of over 280 h operation at 150 mA·cm⁻². Multi-scale characterizations coupled with the density functional theory analyses confirm that the interfacial band alignment and oxygen vacancy enrichment synergistically drive the exceptional ion transport properties. This study highlights multi‐heterojunction engineering of simple oxides as a versatile and scalable strategy for next-generation SOFC electrolytes, with broad implications for solid oxide electrolysis, solar energy conversion, and selective ion-conducting membranes.

A series of single-phase double perovskite Pr_1-xGd_xBaCo_2-yFe_yO_5+δ (x = 0, 0.5 and 1, 0 ≤ y ≤ 1) materials were engineered through A/B site co-doping strategy to improve the mechanical, electrical and electrochemical properties as potential cathode materials for the application of intermediate solid oxide fuel cells (IT-SOFCs). The corresponding thermochemical stability, thermal expansion behavior, electrical conductivity and cathodic polarization resistance of the materials were systematically investigated. It was found that the A-site dual lanthanide doped Pr_0.5Gd_0.5BaCo₂O_5+δ (PGBCO) exhibits improved electrical conductivity, reduced thermal expansion, and comparatively low electrochemical polarization resistance versus single lanthanide double perovskite, PrBaCo₂O_5+δ (PBCO) and GdBaCo₂O_5+δ (GBCO) materials. Further investigation on the effect of B-site Fe-doping on Pr_0.5Gd_0.5BaCo_2-yFe_yO_5+δ (PGBCF-y, 0 ≤ y ≤ 1) reveals that all the PGBCF-y compositions exhibit excellent chemical stability with Gd-doped ceria (GDC) at operating temperatures not higher than 1 100 ℃. Besides, doping of Fe in B-site can effectively reduce the thermal expansion coefficients (TECs) of the Pr_0.5Gd_0.5BaCo₂O_5+δ ceramics at 30–1 000 ℃. And the electrochemical impedance spectra (EIS) results show that the PGBCF-y|GDC| PGBCF-y symmetric cells have acceptable low area specific polarization resistances. Further examination of the cathodic polarization and characteristic capacitance from the AC impedance spectra by employing the relaxation time distribution (DRT) method demonstrated that charge transfer is the dominating sub-process for the oxygen transport through the materials.