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Highly efficient and stable cathodes are significantly vital for solid oxide fuel cells (SOFCs). However, one of key impediment to the cathode development is the undesirable oxygen reduction activity and stability with exposure upon steam. Herein, we present a novel nanoparticles-decorated cathode, comprised of O2−/e−/H+ triple-conducting K+-doped BaCo 0.7 Fe 0.2 Y 0.1 O 3-δ (Ba 0.9 K 0.1 Co 0.7 Fe 0.2 Y 0.1 O 3-δ , BKCFY) matrix oxide and the mixed O2−/e− conducting nanoparticles of BaCoO 3-δ (BCO) through water-mediated exsolution process, exhibiting high activity and robust stability with exposure upon the steam. The cathode shows a low polarization resistance of 0.048 Ω cm2 at 700 °C when exposed upon humid air with 3% H 2 O, approximately 24% lower than that in dry air (0.065 Ω cm2 at 700 °C). Such improvement is mainly ascribed to the exsolved BaCoO 3- δ nanoparticles decorated on the porous BKCFY, which enlarged the triple phase boundary for oxygen reduction reaction. The increased oxygen vacancy concentration through K doping can accelerate the Ba and Co ions segregation to form BaCoO 3-δ with exposure upon the wet condition. Furthermore, K+ dopant with higher basicity than Ba2+ is beneficial for hydration behavior, which effectively promotes the catalytic activity under the water vapor. Moreover, the exsolved BaCoO 3-δ nanoparticles are socketed on the BKCFY matrix oxide, which ensures high stability for this hybrid cathode operated at 700 °C. The findings provide a promising approach to design the triple-conducting cathode for solid oxide fuel cells via water-mediated exsolution, which can also be applied for solid oxide electrolysis cells and membrane reactors. [Display omitted] [ABSTRACT FROM AUTHOR]