Thermal Expansion Matching as a Key Criterion for Developing High‐Performance Mn‐based Mullite Cathodes in SOFCs

Thermal expansion matching is crucial for solid oxide fuel cell (SOFC) cathode design to prevent interfacial delamination or cracking. This work adopts thermal expansion coefficient (TEC) matching as the primary design principle to enable both structural stability and high catalytic activity. A Mn‐based mullite‐type cathode, SmMn 2 O 5 (SMO), is developed, and composite electrodes with Gd 0.1 Ce 0.9 O 1.95 (GDC) are fabricated. To reveal the structure–property relationships, TEC and electrochemical performance measurements are combined with in situ X‐ray diffraction (XRD), Raman spectroscopy, and density functional theory (DFT) analysis. SMO exhibited a low TEC (8.12 × 10 −6 K −1 ) due to anisotropic lattice expansion and phonon scattering confirmed by XRD and Raman. The SMO–GDC composite displayed extremely well‐matched TECs with yttria‐stabilized zirconia, with only 2.36% deviation across the operating range. Electrochemically, the composite cathode achieved 580.9 mW cm −2 with polarization resistance of 0.193 Ω cm 2 and maintained stable operation for 300 h. DFT further revealed that GDC addition facilitated interfacial charge transfer and shortened Mn–Mn dimers, explaining the enhanced catalytic activity. This cathode material selection strategy, prioritizing TEC matching as the primary principle, provides a new insight into SOFC cathode development.