Engineering Spatial Electron Bridge in Molecular Heterostructure Single-Atom Catalyst for Oxygen Electroreduction

Abstract

Molecular single-atom catalysts (SACs) offer tunable and well-defined active sites, rendering them ideal model systems to explore fundamental concepts in oxygen reduction reaction (ORR). However, the high-efficiency molecular SACs are still plagued by easy aggregation, planar symmetry of active sites, suboptimal adsorption/desorption of oxygen intermediates, and poor conductivity. Herein, we propose spatial electron bridge engineering as a universal strategy to disrupt the planar configuration of Fe-N4 moieties, modulate electronic structure, and enhance interfacial coupling. Through dual-descriptor (ΔG*OH and (ΔG*O-ΔG*OH)) analysis correlating activity with theoretical overpotentials, we systematically decode structureactivity relationships in symmetry-broken X-Fe-N4 (X=O, S, N) sites. Molecular heterostructure SACs are constructed by tethering iron pyridinic hexaazacyclophane macrocycle (Fe(Phen)2) to electron bridges (phenol, thiophenol, pyridine) functionalized carbon nanotubes (CNT), forming precisely controlled CNT-X-Fe architectures. Combined spectroscopic studies and DFT calculations reveal that the phenol bridge triggers a low-to-medium spin state transition via electron bridgeto-metal charge transfer, facilitating rapid electron shuttling between Fe(Phen)2 and CNT. This optimizes the Fe d-band center occupancy and enhances antibonding orbital hybridization, yielding the best ORR performance. This work establishes spatial electron bridges as orbital-coupling hubs bridging quantum-level d-p hybridization to macroscopic catalytic performance, offering a universal design framework for molecularly precise electrocatalysts.