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 Sr₄Fe₄Co₂O_13+δ (SFC). SFC is composed of tetragonal perovskite phase (Sr₈Fe₈O_23+δ, I4/mmm, 81 wt.%) and spinel phase (Co₃O₄, $Fd\overline{3}m$, 19 wt.%). The SFC composite cathode displays an ultra-high oxygen ionic conductivity (0.053 S·cm⁻¹ at 550 °C), superior CO₂ tolerance, and suitable TEC value (17.01 × 10⁻⁶ K⁻¹). SFC has both the O²⁻/e⁻ conduction function, and the triple conducting (H⁺/O²⁻/e⁻) capability was achieved by introducing the protonic conduction phase (BaZr_0.2Ce_0.7Y_0.1O_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·cm² 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.