Oxygen reduction activity of pyrolyzed polypyrroles studied by 15N solid-state NMR and XPS with principal component analysis

Abstract

New analytical method for characterizing the nitrogen sites in N-doped carbon catalysts is proposed by employing 15N solid-state NMR. 15N labeled polypyrrole is prepared as a precursor of N-doped carbon catalysts and is pyrolyzed at several different temperatures in a nitrogen atmosphere. The oxygen reduction reaction is evaluated by rotating disk electrode experiments, and solid-state 15N NMR and XPS spectra are measured. The relationship between oxygen reduction activity and the chemical structure, combined with principal component analysis is discussed. The iron-free pyrolyzed polypyrrole samples display quite poor catalytic activity for oxygen reduction, whilst the iron-containing pyrolyzed polypyrrole samples display better oxygen reduction activity. 15N solid-state NMR spectra show that the pyrolyzed polypyrroles contain pyridinic, quaternary, pyrrolic nitrogens at edges or at defects in graphitic sheets. This is the first observation of quaternary nitrogens at edges or defects in graphitic sheets using 15N solid-state NMR. Using principal component analysis of the XPS and 15N solid-state NMR spectra, it is found that most pyridinic, quaternary, and pyrrolic nitrogen atoms are not related to oxygen reduction reaction. However, the samples which contain a larger proportion of some particular type of pyridinic nitrogen atoms show a higher activity for the oxygen reduction reaction.