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-N4O1 OCquasi), 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-N4O1 OCquasi 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.

Automated summary

A strategy of Fe d-orbital splitting modulation by constructing axial coordination on Fe-N-4 sites is presented to regulate the electronic states of single atomic sites around the Fermi level. The axial tractions induce the distortion of Fe-N-4 SP and up to the quasi-octahedral coordination (Fe-N4O1 OCquasi), leading to electron rearrangement and diluted spin polarization. This work provides a novel understanding for improving electrocatalytic performance through orbital-scale manipulation.