Phosphorus-Driven Electron Delocalization on Edge-Type FeN4 Active Sites for Oxygen Reduction in Acid Medium

Precise tuning of the chemical environment of neighboring atomic FeN4 sites is extremely important for optimizing Fe-N-C catalysts to produce the fast oxygen reduction reaction (ORR) kinetics both in acidic and alkaline media, but it is actually very challenging. Heteroatoms could affect the metal charge of the active center through long-range electron delocalization; however, there are a few studies on it. Herein, density functional theory (DFT) calculations demonstrate that the addition of long-range P into edge-type FeN4 can drive the electron delocalization and decrease the band gap of the FeN4 center, leading to a substantial decrease in the free energy barrier to direct four-electron ORR kinetics compared to P-free edge-type FeN4, indicating superior intrinsic ORR activity. Experimentally, by incorporating P in edge-rich FeN4 supported on N,P-doped carbon (Fe-N-C-P/N,P-C), the created Fe-N-C catalyst presents the greatly increased acidic ORR activity, with a half-wave potential (E-1/2) of 0.80 V (vs a reversible hydrogen electrode), which approaches that of commercial Pt/C and also has a high half-wave potential of 0.87 V, beyond Pt/C for alkaline ORR. In addition, it shows higher proton exchange membrane fuel cell and Zn-air battery performances than the pristine Fe-C-N catalyst (Fe-N-C/N-C). This work will guide the rational design of highly active metal atomic scale catalysts with optimized chemical surroundings in terms of P incorporation as a chemically tunable method.