Axial Chlorine Induced Electron Delocalization in Atomically Dispersed FeN4 Electrocatalyst for Oxygen Reduction...

Atomically dispersed iron sites on nitrogen-doped carbon (Fe-NC) are the most active Pt-group-metal-free catalysts for oxygen reduction reaction (ORR). However, due to oxidative corrosion and the Fenton reaction, Fe-NC catalysts are insufficiently active and stable. Herein, w e demonstrated that the axial Cl-modified Fe-NC (Cl-Fe-NC) electrocatalyst is active and stable for the ORR in acidic conditions with high H₂ O₂ tolerance. The Cl-Fe-NC exhibits excellent ORR activity, with a high half-wave potential (E_1/2 ) of 0.82 V versus a reversible hydrogen electrode (RHE), comparable to Pt/C (E_1/2 = 0.85 V versus RHE) and better than Fe-NC (E_1/2 = 0.79 V versus RHE). X-ray absorption spectroscopy analysis confirms that chlorine is axially integrated into the FeN4. More interestingly, compared to Fe-NC, the Fenton reaction is markedly suppressed in Cl-Fe-NC. In situ electrochemical impedance spectroscopy reveals that Cl-Fe-NC provides efficient electron transfer and faster reaction kinetics than Fe-NC. Density functional theory calculations reveal that incorporating Cl into FeN4 can drive the electron density delocalization of the FeN4 site, leading to a moderate adsorption free energy of OH* (∆G_OH* ), d-band center, and a high onset potential, and promotes the direct four-electron-transfer ORR with weak H₂ O₂ binding ability compared to Cl-free FeN4, indicating superior intrinsic ORR activity.