Boron in the Second Coordination Sphere of Fe Single Atom Boosts the Oxygen Reduction Reaction

Metal single atoms coordinated with four nitrogen atoms (M 1 N 4 ) are regarded as tremendously promising catalysts for the electrocatalytic oxygen reduction reaction (ORR). Nevertheless, the strong bond intensity between the metal center and the O atom in oxygen-containing intermediates significantly limits the ORR activity of M 1 N 4 . Herein, the catalytically active B atom is successfully introduced into the second coordination sphere of the Fe single atom (Fe 1 N 4 -B-C) to realize the alternative binding of B and O atoms and thus facilitate the ORR activity. Compared with the pristine Fe 1 N 4 catalyst, the synthesized Fe 1 N 4 -B-C catalyst exhibits improved ORR catalytic capability with a half-wave potential ( E 1/2 ) of 0.80 V and a kinetic current density ( J K ) of 5.32 mA cm –2 in acid electrolyte. Moreover, in an alkaline electrolyte, the Fe 1 N 4 -B-C catalyst displays remarkable ORR activity with E 1/2 of 0.87 V and J K of 8.94 mA cm –2 at 0.85 V, outperforming commercial Pt/C. Notably, the mechanistic study has revealed that the active center is the B atom in the second coordination shell of the Fe 1 N 4 -B-C catalyst, which avoids the direct bonding of Fe–O. The B center has a moderate binding force to the ORR intermediate, which flattens the ORR energy diagram and thereby improves the ORR performance. Therefore, this study offers a novel strategy for tailoring catalytic performance by tuning the active center of single-atom catalyst.