Deciphering Local Microstrain-Induced Optimization of Asymmetric Fe Single Atomic Sites for Efficient Oxygen Reduction

• Local microstrain from high-curvature carbon nanospheres enhances the intrinsic activity of asymmetric Fe–N3S1 sites for oxygen reduction. • Compressive strain on Fe–N bonds and tensile strain on Fe–S bonds downshift the d-band center, accelerating *OH reduction kinetics. • Strained Fe–N3S1 sites exhibit a high half-wave potential of 0.922 V vs. RHE and a turnover frequency of 6.2 e−1 s−1 site−1, outperforming flat counterparts. • Operando spectroscopies reveal dynamic transformation of strained Fe–N3S1 to Fe–N3 sites, mitigating *OH overadsorption and improving durability.