| Summary: | Triple (H<sup>+</sup>/O<sup>2−</sup>/e<sup>−</sup>) conducting oxides (TCOs) have been extensively investigated as the most promising cathode materials for solid oxide fuel cells (SOFCs) because of their excellent catalytic activity for oxygen reduction reaction (ORR) and fast proton transport. However, here we report a stable twin-perovskite nanocomposite Ba-Co-Ce-Y-O (BCCY) with triple conducting properties as a conducting accelerator in semiconductor ionic fuel cells (SIFCs) electrolytes. Self-assembled BCCY nanocomposite is prepared through a complexing sol–gel process. The composite consists of a cubic perovskite (Pm-3m) phase of BaCo<sub>0.9</sub>Ce<sub>0.01</sub>Y<sub>0.09</sub>O<sub>3-δ</sub> and a rhombohedral perovskite (R-3c) phase of BaCe<sub>0.78</sub>Y<sub>0.22</sub>O<sub>3-δ</sub>. A new semiconducting–ionic conducting composite electrolyte is prepared for SIFCs by the combination of BCCY and CeO<sub>2</sub> (BCCY-CeO<sub>2</sub>). The fuel cell with the prepared electrolyte (400 μm in thickness) can deliver a remarkable peak power density of 1140 mW·cm<sup>−2</sup> with a high open circuit voltage (OCV) of 1.15 V at 550 °C. The interface band energy alignment is employed to explain the suppression of electronic conduction in the electrolyte. The hybrid H<sup>+</sup>/O<sup>2−</sup> ions transport along the surfaces or grain boundaries is identified as a new way of ion conduction. The comprehensive analysis of the electrochemical properties indicates that BCCY can be applied in electrolyte, and has shown tremendous potential to improve ionic conductivity and electrochemical performance.
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