To enhance the performance and widespread use of solid oxide fuel cells (SOFCs), addressing the low-temperature (< 650 °C) electrochemical performance and operational stability issues of cathode materials is crucial. Here, we propose an innovative approach to enhance oxygen ion mobility and electrochemical performance of perovskite oxide by substituting some oxygen sites with chlorine anions. The designed SrTa_0.1Fe_0.9O_3−δ−xCl_x (x = 0.05 and 0.10) exhibits improved performance compared to SrTa_0.1Fe_0.9O_3−δ (STF). SrTa_0.1Fe_0.9O_2.95−δCl_0.05 (STFCl0.05) shows the lowest area-specific resistance (ASR) value on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. At 600 °C, STFCl0.05 achieves an ASR value of 0.084 Ω·cm², and a single cell with STFCl0.05 reaches a higher peak power density (PPD) value (1143 mW·cm⁻²) than that with STF (672 mW·cm⁻²). Additionally, besides exhibiting excellent oxygen reduction reaction (ORR) activity at lower temperatures, the STFCl0.05 cathode demonstrates good CO₂ tolerance and operational stability. Symmetrical cell operation lasts for 150 h, and single cell operation endures for 720 h without significant performance decline. The chlorine doping approach effectively enhances ORR activity and stability, making STFCl0.05 a promising cathode material for low-temperature SOFCs.

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.