Highly active and durable triple conducting composite air electrode for low-temperature protonic ceramic fuel cells

Protonic ceramic fuel cells (PCFCs) are more suitable for operation at low temperatures due to their smaller activation energy ( E a ). Unfortunately, the utilization of PCFC technology at reduced temperatures is limited by the lack of durable and high-activity air electrodes. A lot number of cobalt-based oxides have been developed as air electrodes for PCFCs, due to their high oxygen reduction reaction (ORR) activity. However, cobalt-based oxides usually have more significant thermal expansion coefficients (TECs) and poor thermomechanical compatibility with electrolytes. These characteristics can lead to cell delamination and degradation. Herein, we rationally design a novel cobalt-containing composite cathode material with the nominal composition of Sr 4 Fe 4 Co 2 O 13+ δ (SFC). SFC is composed of tetragonal perovskite phase (Sr 8 Fe 8 O 23+ δ , I 4/ mmm , 81 wt.%) and spinel phase (Co 3 O 4 , Fd 3̄ m , 19 wt.%). The SFC composite cathode displays an ultra-high oxygen ionic conductivity (0.053 S·cm −1 at 550 °C), superior CO 2 tolerance, and suitable TEC value (17.01 × 10 −6 K −1 ). SFC has both the O 2− /e − conduction function, and the triple conducting (H + /O 2− /e − ) capability was achieved by introducing the protonic conduction phase (BaZr 0.2 Ce 0.7 Y 0.1 O 3− δ , BZCY) to form SFC+BZCY (70 wt.%:30 wt.%). The SFC+BZCY composite electrode exhibits superior ORR activity at a reduced temperature with extremely low area-specific resistance (ASR, 0.677 Ω·cm 2 at 550 °C), profound peak power density (PPD, 535 mW·cm −2 and 1.065 V at 550 °C), extraordinarily long-term durability (> 500 h for symmetrical cell and 350 h for single cell). Moreover, the composite has an ultra-low TEC value (15.96 × 10 −6 K −1 ). This study proves that SFC+BZCY with triple conducting capacity is an excellent cathode for low-temperature PCFCs.