Preparation and Performance Study of <em>In-Situ</em> Self-Assembled Biphasic SmMn<sub>2</sub>O<sub>5</sub>-NiMn<sub>2</sub>O<sub>4</sub> Composite Cathode

Mullite-structured oxides exhibit excellent oxygen reduction reaction activity, possess a low thermal expansion coefficient due to their unique crystal structure, and can eliminate the need for a barrier layer and simplify the preparation process as they contain no alkaline earth elements. Thus, they hold great promise as novel cathode materials for solid oxide fuel cells. In this work, a mullite-spinel-structured SmMn2O5-NiMn2O4 (SMO-NMO) composite cathode was one-step synthesized via a solid-liquid composite route, and its in-situ self-assembly enabled good compatibility with the electrolyte without any barrier layer. Characterization results showed that the SMO:NMO = 5:5 (SN55) composite cathode overcame the bottlenecks of single-phase materials-namely, the low conductivity of pure SMO (only 0.035 S·cm–1 at 800 °C) and the insufficient oxygen reduction reaction activity of pure NMO-through a synergistic effect between the two phases. At 800 °C, the single cell delivered a peak power density of 1069.82 mW·cm–2, which was 3.24-fold and 1.20-fold higher than that of pure SMO (329.92 mW·cm–2) and NMO (890.20 mW·cm–2), respectively. Under galvanostatic operation at 750 °C (current density corresponding to 540 mA·cm–2), the SN55-based cell runs for ~150 h with only 1.6% voltage loss, corresponding to a degradation rate of 5.62%/kh, and no defects appeared at the cathode-electrolyte interface. This study provided a new route for designing barrier-layer-free cathodes for solid oxide fuel cells.