Engineering spatial electron bridges in molecular heterostructure single-atom catalysts for oxygen electroreduction

Abstract

Molecular single-atom catalysts (SACs) offer tunable and well-defined active sites, rendering them ideal model systems to explore the fundamental concepts of the oxygen reduction reaction (ORR). However, the high-efficiency molecular SACs are still plagued by easy aggregation, planar symmetry of their 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 the electronic structure, and enhance interfacial coupling. Through dual-descriptor (ΔG_*OH and (ΔG_*O–ΔG_*OH)) analysis correlating the activity with theoretical overpotentials, we systematically decoded the structure–activity relationships in symmetry-broken X–Fe–N₄ (X = O, S, and N) sites. Molecular heterostructure SACs were constructed by tethering an iron pyridinic hexaazacyclophane macrocycle (Fe(Phen)₂) to electron bridge (phenol, thiophenol, and pyridine)-functionalized carbon nanotubes (CNTs), forming precisely controlled CNT–X–Fe architectures. Combined spectroscopic studies and DFT calculations revealed that the phenol bridge triggered a low-to-medium spin-state transition via an electron bridge-to-metal charge transfer, facilitating rapid electron shuttling between Fe(Phen)₂ and the CNT. This optimized the Fe d-band center occupancy and enhanced 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.