Tailoring the d-Orbital Splitting Manner of Single Atomic Sites for Enhanced Oxygen Reduction.

Regulating the electronic states of single atomic sites around the Fermi level remains a major concern for boosting the electrocatalytic oxygen reduction reaction (ORR). Herein, a Fe d-orbital splitting manner modulation strategy by constructing axial coordination on FeN 4 sites is presented. Experimental investigations and theoretical calculations reveal that the axial tractions induce the distortion of square-planar field (FeN 4 SP), up to the quasi-octahedral coordination (FeN 4 O 1 OC quasi ), thus leading to the electron rearrangement with a diluted spin polarization. The declined population of unpaired electrons in d z 2 , d x z and d yz states engenders a moderate adsorption of ORR intermediates, thereby reinforcing the intrinsic reaction activity. In situ infrared spectroscopy further demonstrates that the reordering of d-orbital splitting and occupation facilitates the desorption of *OH. The FeN 4 O 1 OC quasi exhibits a dramatic improvement of kinetic current density and turnover frequency, which are fivefold and tenfold higher than those of FeN 4 SP. This work presents a novel understanding on improving the electrocatalytic performance through the orbital-scale manipulation.