Here, we report an oxygen ion-proton-electron-conducting nanocomposite, BaCo0.7(Ce0.8Y0.2)0.3O3-δ (BCCY), derived from a self-assembly process, as a high-performance protonic ceramic fuel cell (PCFC) or mixed O2−/H+ dual-ion conducting fuel cell (dual-ion FC) cathode. Self-assembly during high-temperature calcinations results in the formation of a nanocomposite consisting of a mixed H+/e− conducting BaCexYyCozO3-δ (P-BCCY) phase and mixed O2−/e− conducting BaCoxCeyYzO3-δ (M-BCCY) and BaCoO3-δ (BC) phases. The interplay between these phases promotes the oxygen reduction reaction (ORR) kinetics of this composite cathode and improves its thermo-mechanical compatibility by tempering the mismatch in thermal expansion coefficient (TEC). When tested as the cathode in anode-supported dual-ion FCs and PCFCs, peak power densities (PPDs) of 985 and 464 mW cm−2, respectively, are achieved at 650°C while maintaining a robust operational stability of 812 h at 550°C. This material is ideally suited for high-performance cathodes for PCFCs and dual-ion FCs, greatly accelerating the commercialization of this technology. Display omitted •A multi-phase nanocomposite PCFC cathode is derived from the self-assembly process•The nanocomposite shows sufficient oxygen ion, proton, and electron conductivity•Direct experimental measurement method is shown for specific proton conductivity values•The interplay in multi-phase promotes ORR kinetics and reduces thermal expansion Protonic ceramic fuel cells (PCFCs) have attracted more attention than solid oxide fuel cells based on oxygen-ion-conducting electrolytes that operate at intermediate temperatures due to the lower activation energy for proton conduction than for oxygen ions in oxide electrolytes. However, the practical application of PCFC technology is hindered by the lack of suitable cathode materials. Here, we report our rationally designed triple conducting nanocomposite cathode BaCo0.7(Ce0.8Y0.2)0.3O3-δ with sufficient oxygen ion-proton-electron transfer capability for PCFCs. This work develops the highly active and stable cathode material and simultaneously measures the specific values of oxygen ion, proton, and electron conductivity of the cathode material. This work also highlights the design strategy of perovskite-based electrocatalysts for other energy conversion and storage systems such as water splitting, metal-air batteries, and dye-sensitized solar cells. A nanocomposite cathode is composed of a mixed H+/e− conducting BaCexYyCozO3-δ (P-BCCY) phase, a mixed O2−/e− conducting BaCoxCeyYzO3-δ (M-BCCY) phase, and a mixed O2−/e− conducting BaCoO3-δ (BC) phase. The P-BCCY phase could promote proton diffusion, the M-BCCY phase could facilitate oxygen ion diffusion, and the BC phase could enhance the electronic conduction of the electrode; the interfaces between the three phases in nano-domain greatly increases the number of active sites for electrochemical reactions.