Oxygen reduction at Fe–N-modified multi-walled carbon nanotubes in acidic electrolyte

Multi-walled carbon nanotubes (MWCNTs) modified with iron tetramethoyxphenyl-porphyrin chloride (FeTMPP-Cl) and heat treated are active towards electrocatalytic oxygen reduction in acidic media. The activity slightly depends on the heat treatment temperature (850 < 550 °C) and the amount of porphyrin deposited onto the nanotubes before the heat treatment step. In comparison with as-received MWCNTs no increase in activity has been found with iron phenanthroline or iron acetate impregnated and heat treated MWCNTs. When MWCNTs are pretreated in an oxidation step using HNO3, there is only a slight increase in activity after FeTMPP-Cl modification and heat treatment compared to the not pretreated MWCNTs. The HNO3 treatment itself, however, leads to an increase in activity of the unmodified MWCNTs. TEM-measurements revealed an amorphous layer surrounding the MWCNTs after HNO3 treatment, while XPS showed an increased amount of oxygen functional groups. It is suggested that there are different kinds of active sites at the catalyst surface, the first ones consisting of oxygen functionalities or other entities introduced by the HNO3 treatment, and the second ones containing nitrogen (and probably iron) introduced via the porphyrin. Pyridine-type nitrogen has been found by XPS after heat treatment at both temperatures, indicating that the active sites are already formed at 550 °C.