Unveiling the Promoting Effect of Trace CO ₂ on the Performance of Aqueous Fe–Air Batteries

Fe–air batteries hold potential for large-scale energy storage due to iron’s abundance, safety, and considerable theoretical capacity. However, their performance in ambient air is generally believed inferior to that in pure oxygen, as environmental CO2 participates in reactions and generates irreversible carbonates. In this work, we observed that the Fe–air battery outperforms its Fe–O2 counterpart and unveiled that trace CO2 acts as a performance promoter by facilitating the formation of FeCO3 intermediates, which guide the discharge product FeOOH to grow into rod-like morphology rather than dense passivation layer. This unique mechanism effectively mitigates cathode blockage and enhances the reaction kinetics. Consequently, the battery achieves a full discharge capacity of 8.43 mAh cm–2, a low polarization gap of 0.35 V, and consistent operation over 80 cycles in ambient air. This work resolves the critical challenges of carbonate precipitation and cathode passivation, paving the way for advanced air-breathing batteries.