Dual single-atom sites coupled with graphene-encapsulated core-shell Fe-Cu nanoalloy for boosting the oxygen reduction reaction

Replacing platinum-based electrocatalysts with iron single-atom catalysts (Fe-N₄-C) for the oxygen reduction reaction (ORR) remains challenging due to the symmetric electronic structure of atomically dispersed Fe-N₄ sites and sluggish kinetics. To address this issue, we introduce Cu-N_x sites and graphene-encapsulated core-shell Fe-Cu nanoalloy (FeCu@G) particles into the Fe-N_x site surroundings through the self-assembly and pyrolysis of a metal-organic framework (MOF). This strategy leverages synergistic interactions with the associated species to modify the uniform electronic structure of Fe single-atom sites, thereby enhancing oxygen-adsorption/desorption kinetics. Density functional theory (DFT) calculations reveal that the incorporation of Cu-N_x sites and FeCu@G nanoalloy particles significantly alters the electronic structure of Fe-N_x sites, leading to improved ORR activity. Consequently, the optimized FeCu-DSAs@CNT, comprising dual single-atom sites (DSAs: Fe-N_x and Cu-N_x) and FeCu@G nanoalloy within MOF-derived nitrogen-doped carbon nanotubes (CNTs), exhibits a significantly improved half-wave potential (E_1/2 = 0.91 V) and feasible ORR kinetics (Tafel slope = 48.15 mV dec⁻¹), surpassing the Pt/C benchmark (E_1/2 = 0.847 V and Tafel slope = 56.76 mV dec⁻¹). Notably, FeCu-DSAs@CNT shows a 58 mV more positive E_1/2 compared to monometallic Fe-SAs@CNT, attributed to synergistic interactions with Cu species. Moreover, it demonstrates excellent power density, specific capacity, and cycling stability in a lab-made zinc-air battery, outpacing the Pt/C-battery. This study addresses gaps in understanding Fe-N_x site interactions with associated species, providing valuable insights for the advancement of Fe-N_x-C electrocatalysts.