Development of low-cost and cobalt-free efficient cathode materials for oxygen reduction reaction (ORR) remains one of the paramount motivations for material researchers at a low temperature (<650 °C). In particular, iron-based perovskite oxides show promise as electrocatalysts for ORR because Fe metal is cheaper and naturally abundant, exhibit matched thermal expansion with contacting components such as electrolytes, and show high tolerance in a CO-containing atmosphere. Herein, we demonstrated a new mechanism, the in situ formation of alkali metal carbonates at the cathode surface. This new mechanism leads to an efficient and robust cobalt-free electrocatalyst (SrAFeNbTaO, SAFNT5, A = Li, Na, and K) for the application of low-temperature solid oxide fuel cells (LT-SOFCs). Our results revealed that the formation of Li\K carbonates boosts the ORR activity with an area-specific resistance as low as 0.12 and 0.18 Ω cm at 600 °C, which show the highest performance of the cobalt-free single-phase cathode that has been ever reported so far. We also find that the chemical stability and tolerance of tested cathodes toward CO poisoning significantly improved with alkali carbonates, as compared to the pristine SrFeNbTaO (SFNT) at 600 °C. This work demonstrates the conclusive role of alkali carbonates in developing highly efficient and stable cobalt-free cathodes for LT-SOFCs and CO neutralization.