DFT Study of the Oxygen Reduction Reaction Activity on Fe-N4-Patched Carbon Nanotubes: The Influence of the Diameter and Length

The influences of diameter and length of the Fe-N-4-patched carbon nanotubes (Fe-N-4/CNTs) on oxygen reduction reaction (ORR) activity were investigated by density functional theory method using the BLYP/DZP basis set. The results indicate that the stability of the Fe-N-4 catalytic site in Fe-N-4/CNTs will be enhanced with a larger tube diameter, but reduced with shorter tube length. A tube with too small a diameter makes a Fe-N-4 site unstable in acid medium since Fe-N and C-N bonds must be significantly bent at smaller diameters due to hoop strain. The adsorption energy of the ORR intermediates, especially of the OH group, becomes weaker with the increase of the tube diameter. The OH adsorption energy of Fe-N-4/CNT with the largest tube diameter is close to that on Pt(111) surface, indicating that its catalytic property is similar to Pt. Electronic structure analysis shows that the OH adsorption energy is mainly controlled by the energy levels of Fe 3d orbital. The calculation results uncover that Fe-N-4/CNTs with larger tube diameters and shorter lengths will exhibit better ORR activity and stability.