Elongated Fe–N–C containing trace atomic Co dopants for high power density PEMFCs†

Abstract

Developing single-atom Fe–N₄/C catalysts is crucial for the large-scale implementation of proton exchange membrane fuel cells (PEMFCs). While Fe–N₄/C catalysts are inherently active in accelerating the slow ORR process, their performance is still inferior to that of Pt/C. Herein, a trace Co-doped Fe single-atom catalyst (Fe(tCo)–N–C) containing more active Fe₂N₈ sites has been synthesized. Interestingly, compared with typical FeN₄ sites in an Fe–N–C electrocatalyst, the Fe₂N₈ sites generate a larger Fe–N bond length due to Co-doping. The elongated Fe–N bond in Fe₂N₈ lowers the d-band center and charge density of iron sites, enhancing the ORR process by facilitating the formation of *OOH and generation and desorption of *OH. Fe(tCo)–N–C manifested excellent acidic and alkaline ORR activity, with a half-wave potential (E_1/2) of 0.80 V in HClO₄ solution and 0.89 V in KOH medium. More importantly, high peak power densities (P_max) were realized by applying Fe(tCo)–N–C in PEMFCs, with the P_max reaching 890 mW cm⁻² in H₂–O₂ and 380 mW cm⁻² in H₂–air. Additionally, trace Co dopants in the catalyst improved carbon graphitization and provided high ORR catalytic stability. This research introduces an innovative approach to engineering highly active Fe₂N₈ sites, providing valuable insights for the sustainable progress of PEMFC technology.

Keywords: Proton exchange membrane fuel cells; Oxygen reduction reaction; Platinum-group-metal-free catalysts; Single-atom catalysts; Bimetallic active sites.