Fe/Fe-Nx-based carbon nanotubes with isolated active sites by ionic-state mediated synthesizing for highly efficient ORR

Abstract

Herein, Zn ionic-mediated Fe/Fe-N_x based N-doped carbon nanotubes (Fe/Fe-N_x based CNTs-m) were prepared by using Fe-TPc with Fe-N₄ structure as a source of iron, in which Fe/Fe-N_x based CNTs-0.2 exhibit the best ORR performance. The carbon nanotubes encapsulate Fe-based nanoparticles located at the tips of the carbon nanotubes, while Fe-N_x is exposed in the walls of the carbon nanotubes and in the carbon layer at the tips of the carbon nanotubes. The Fe/Fe-N_x based CNTs-0.2 demonstrated a half-wave potential (E_1/2) of 0.87 V vs RHE in 0.1 M KOH and an onset potential (E_onset) of 0.93 V vs RHE, surpassing the ORR performance of commercial 20 % Pt/C. Additionally, the material exhibited excellent long-term stability and maintained its superior ORR performance even after 10,000 CVs. The yield of H₂O₂ was less than 5 %, and the electron transfer number was greater than 3.9. The significant improvement in the ORR performance of Fe/Fe-N_x based CNTs-0.2 may be attributed to the synergistic effect of Fe-based nanoparticles encapsulated in carbon nanotubes located at the tip and Fe-N_x exposed in the carbon layer at the tip of carbon nanotubes. Excitingly, through the separation of active sites, Fe/Fe-N_x based CNTs-0.2 performs stable ORR performance over a long period of time.