A Cobalt‐Free Multi‐Phase Nanocomposite as Near‐Ideal Cathode of Intermediate‐Temperature Solid Oxide Fuel Cells Developed by Smart Self‐Assembly

An ideal solid oxide fuel cell (SOFC) cathode should meet multiple requirements, i.e., high activity for oxygen reduction reaction (ORR), good conductivity, favorable stability, and sound thermo-mechanical/chemical compatibility with electrolyte, while it is very challenging to achieve all these requirements based on a single-phase material. Herein, a cost-effective multi-phase nanocomposite, facilely synthesized through smart self-assembly at high temperature, is developed as a near-ideal cathode of intermediate-temperature SOFCs, showing high ORR activity (an area-specific resistance of ≈0.028 Ω cm² and a power output of 1208 mW cm^-2 at 650 °C), affordable conductivity (21.5 S cm^-1 at 650 °C), favorable stability (560 h operation in single cell), excellent chemical compatibility with Sm_0.2 Ce_0.8 O_1.9 electrolyte, and reduced thermal expansion coefficient (≈16.8 × 10^-6 K^-1 ). Such a nanocomposite (Sr_0.9 Ce_0.1 Fe_0.8 Ni_0.2 O_{3- δ} ) is composed of a single perovskite main phase (77.2 wt%), a Ruddlesden-Popper (RP) second phase (13.3 wt%), and surface-decorated NiO (5.8 wt%) and CeO₂ (3.7 wt%) minor phases. The RP phase promotes the oxygen bulk diffusion while NiO and CeO₂ nanoparticles facilitate the oxygen surface process and O^2- migration from the surface to the main phase, respectively. The strong interaction between four phases in nanodomain creates a synergistic effect, leading to the superior ORR activity.