Modulating Fe spin state in FeNC catalysts by adjacent Fe atomic clusters to facilitate oxygen reduction reaction in proton exchange membrane fuel cell

Iron,nitrogen-codoped carbon (FeNC) has emerged as promising alternatives to precious metals for oxygen reduction reaction (ORR). Herein, we demonstrate that the ORR activity of FeNC can be markedly enhanced by the incorporation of adjacent Fe few-atom clusters (FeAC), where the octahedral field of FeAC boosts the splitting of the parallelogram field of FeN4, and facilitates the transition from high-spin (t2g 3 eg 2) Fe(III)N4 to medium-spin (t2g 5 eg 1) Fe(II)N4 and hence the interaction with the pi* antibonding orbitals of oxygen. This leads to a remarkable ORR performance due to optimized desorption of the OH* intermediate on FeN4, with a half-wave potential of +0.80 in 0.1 M HClO4, in comparison to that with only FeN4 single-atom moieties. In H2-O2 fuel cell tests, a high peak power density of 0.80 W cm-2 is obtained. Results from this work highlight the significance of spin en-gineering in the manipulation and optimization of the ORR activity of single-atom catalysts.