Two-dimensional ZIF-L derived dual Fe/FeN<inf>x</inf> sites for synergistic efficient oxygen reduction in alkaline and acid media

Abstract

Fe–N–C catalysts have emerged as the most promising alternatives to commercial Pt/C catalysts for oxygen reduction reaction (ORR) due to their cost-effectiveness and favorable activity. Herein, a dual-site Fe/FeNx-NC catalyst was synthesized via a green, in situ doping strategy using two-dimensional Fe-doped ZIF-L as a nitrogen-rich precursor. The catalyst integrated Fe nanoparticles (NPs) and FeNx sites anchored on carbon nanotubes, intertwined with nitrogen-doped porous carbon nanosheets, achieving a high active site density and graphitisation. Electrochemical tests revealed that the optimized Fe/FeNx-NC-1 exhibited significant ORR activity, with a half-wave potential of 0.92 V and 0.80 V for alkaline and acidic medium, respectively. Zn-air batteries employing Fe/FeNx-NC-1 delivered a peak power density of 168 mW·cm−2 and a specific capacity of 790 mAh·g−1, outperforming those of Pt-based catalysts. Density functional theory calculations demonstrated a reduced free energy barrier for the rate-determining step (0.48 eV) compared to single-site Fe–N4 models (0.79 eV). The synergy between Fe NPs and FeNx optimized ORR intermediate adsorption and facilitated charge/mass transfer. This study offers valuable insights for the development of advanced energy conversion systems.