Fe–N-modified multi-walled carbon nanotubes for oxygen reduction reaction in acid

Abstract

We report a facile synthesis of Fe–N–C catalysts based on the surface functionalization of multi-walled carbon nanotubes (MWCNTs), which show high activity and stability for oxygen reduction reaction (ORR) in acid. Fe–N-MWCNT catalysts, whose ORR mass activities could vary by 3–4 times depending on the choice of Fe precursors, were found to have considerably higher ORR mass activity and higher stability than N-modified MWCNTs (N-MWCNTs). The Fe–N-MWCNT catalyst with a dominant Fe–N_x moiety (with x ≈ 4) and a surface Fe/C ratio of ∼0.004 exhibits the highest ORR mass activity in acid (∼0.7 mA mg⁻¹_Fe–N-MWCNT at 0.8 V vs. RHE), where the lower mass activity of other Fe–N-MWCNT catalysts can be attributed to lower Fe/C ratios and Fe–N_x moieties (with x smaller than 4) as revealed from X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Moreover, the enhanced stability of Fe–N-MWCNTs in comparison to N-MWCNTs can be attributed to less H₂O₂ production during ORR as determined from rotating ring disk electrode (RRDE) measurements, and higher activity for H₂O₂ electro-reduction by rotating disk electrode (RDE) measurements. The large surface Fe/C ratio and Fe–N_x moiety corresponding to high ORR activity and stability of Fe–N-MWCNTs demonstrate that surface functionalization can be very helpful to graft active catalytic sites onto carbon nanostructures, and to provide insights into the ORR mechanism of non-noble metal catalysts (NNMCs) for proton exchange membrane fuel cells (PEMFCs).