Constructing\nAsymmetric Fe–Nb Diatomic Sites\nto Enhance ORR Activity and Durability

Iron–nitrogen–carbon\n(Fe–N–C) materials\nhave been identified as a promising class of platinum (Pt)-free catalysts\nfor the oxygen reduction reaction (ORR). However, the dissolution\nand oxidation of Fe atoms severely restrict their long-term stability\nand performance. Modulating the active microstructure of Fe–N–C\nis a feasible strategy to enhance the ORR activity and stability.\nCompared with common 3d transition metals (Co, Ni, etc.), the 4d transition\nmetal atom Nb has fewer d electrons and more unoccupied orbitals,\nwhich could potentially forge a more robust interaction with the Fe\nsite to optimize the binding energy of the oxygen-containing intermediates\nwhile maintaining stability. Herein, an asymmetric Fe–Nb diatomic\nsite catalyst (FeNb/c-SNC) was synthesized, which exhibited superior\nORR performance and stability compared with those of Fe single-atom\ncatalysts (SACs). The strong interaction within the Fe–Nb diatomic\nsites optimized the desorption energy of key intermediates (*OH),\nso that the adsorption energy of Fe–*OH approaches the apex\nof the volcano plot, thus exhibiting optimal ORR activity. More importantly,\nintroducing Nb atoms could effectively strengthen the Fe–N\nbonding and suppress Fe demetalation, causing an outstanding stability.\nThe zinc–air battery (ZAB) and hydroxide exchange membrane\nfuel cell (HEMFC) equipped with our FeNb/c-SNC could deliver high\npeak power densities of 314 mW cm<sup>–2</sup> and 1.18 W cm<sup>–2</sup>, respectively. Notably, the stable operation time\nfor ZAB and HEMFC increased by 9.1 and 5.8 times compared to Fe SACs,\nrespectively. This research offers further insights into developing\nstable Fe-based atomic-level catalytic materials for the energy conversion\nprocess.