Revealing the Mechanism for Boosted Oxygen Reduction Reaction of Distorted Iron Single Atom Electrocatalyst

ABSTRACT The widespread application of fuel cells and zinc air batteries calls for the development of noble metal‐free electrocatalysts (e.g., Fe single atom, Fe‐N ₄ ) for oxygen reduction reaction (ORR) with high activity, durability, and selectivity. Recently, the distortion in the D _4h symmetric structure of the Fe‐N ₄ site has been reported to boost the ORR performance. However, the performance improvement is still limited and the mechanism is not clearly interpreted, especially at atomic level. In this study, we synthesize Fe‐N ₃ S/SNC electrocatalyst with broken D _4h symmetric structure (distortion degree of 1.08) and demonstrate outstanding ORR performance. The half‐wave potential (E _1/2 ) reaches 0.924 V vs. RHE for Fe‐N ₃ S/SNC, corresponding to 24 and 62 mV increase in E _1/2 value as well as 1.7 and 5.8 times higher kinetic current density compared with Fe‐N ₄ /NC and commercial Pt/C in alkaline ORR test. Significantly, the promoted ORR activity is further witnessed in aqueous and all solid‐state rechargeable zinc air battery (ZAB) with ultrahigh power density of 231.8 mW cm ⁻² and 134.9 mW cm ⁻² , respectively, which outperforms the devices implementing commercial Pt/C by 1.8 and 3.5 fold, as well as the current state‐of‐the‐art all‐solid‐state ZAB. The boosted performance of Fe‐N ₃ S/SNC is dynamically explicated by in situ X‐ray photoelectron spectroscopy and e _g filling of Fe during ORR. Stable Fe valence during ORR process is observed for Fe‐N ₃ S/SNC due to the electron buffer effect from sulfur atoms. Theoretical computations further validate the electron filling in e _g orbital is stable at 1 for Fe atom under ORR conditions due to the broken D _4h symmetry and the electron buffer effect, leading to lower energy barrier for Fe‐N ₃ S/SNC.