Boosting the ORR activity in PEM fuel cells: tailored electron-withdrawing properties of Fe-based catalysts via optimizing fluorine doping

Abstract

Fe-based oxygen reduction reaction (ORR) catalysts are promising for low-cost proton exchange membrane fuel cells (PEMFCs). However, the Fe-N₄ sites show strong adsorption interaction with the ORR intermediates, which slows down the catalytic kinetics and reduces the catalytic activity. Herein, we synthesized Fe/N and F co-doped carbon nanotubes (Fe-N/F-CNTs) by pyrolyzing the metal–organic framework with fluorine doping precursors. Fluorine doping, especially CF₂ doping, was demonstrated to drive the electron transfer from the Fe surface to the nearby N atoms, which weakened the adsorption strength of Fe to the intermediate OH* and boosted the ORR catalytic activity. The Fe-N/F-CNT-2 with an optimized Fe-N₄ electron structure boosted the 4e⁻ catalytic selectivity significantly. The optimized fluorine doping increased the half-wave potential from 0.71 V to 0.80 V. In the PEMFCs, the Fe-N/F-CNT-2 exhibited a peak power density of 0.48 W cm⁻² and a high durability of 75% over 30 000 voltage cycles, which was much higher than the 0.36 W cm⁻² and 35% of the control sample, respectively. This work provides an efficient fluorine doping approach for boosting the Fe-based ORR catalytic activity of practical PEMFCs.