Abstract A thin interlayer of samarium doped ceria (SDC) is applied as diffusion barrier between La 1 − x Sr x Co y Fe 1 − y O 3 x = 0.1–0.4, y = 0.2–0.8 (LSCF) cathode and La 1.8Dy 0.2Mo 1.6W 0.4O 9 (LDMW82) electrolyte to obstruct Mo–Sr diffusion and solid state reaction in the intermediate temperature range of SOFC. We demonstrate the effectiveness of the diffusion barrier through contrasting the clearly defined interfaces of LSCF/SDC/LDMW82 against a rugged growing product layer of LSCF/LDMW82 in 800 °C thermal annealing, and analyze the product composition and the probable new phase. In addition, the measured polarization resistance is considerably lower for the half-cell with a diffusion barrier. Therefore, the electrochemical performance of the LSCF cathode is investigated on the SDC-protected LDMW82. The cell with LSCF ( x = 0.4) persistently outperforms the one with x = 0.2 in polarization resistance because of its small low-frequency contribution. The activation energy of polarization resistance is also lower for La 0.6Sr 0.4Co y Fe 1 − y O 3 (112–135 kJ/mol), than that for La 0.8Sr 0.2Co y Fe 1 − y O 3 (156–164 kJ/mol). La 0.6Sr 0.4Co y Fe 1 − y O 3 y = 0.4–0.8 is the proper composition for the cathode interfaced to SDC/LDMW82.