Oxygen reduction on the electrocatalysts based on pyrolyzed non-noble metal/poly-o-phenylenediamine/carbon black composites: New insight into the active sites

Poly-o-phenylenediamine (PPDA) was synthesized by oxidative polymerization. Catalysts for oxygen reduction were prepared by pyrolyzing PPDA/C, Fe- or Co-PPDA composites supported on carbon black. Their activity, selectivity and surface elemental states were characterized by cyclic voltammetry (CV), steady-state polarization, rotating disc electrode (RDE) technique and X-ray photoelectron spectroscopy (XPS), respectively. The iron-based catalyst is more active than that of the cobalt-based one and pyrolyzed PPDA/C alone but lower than that of Pt/C for O₂ reduction reaction (ORR). The RDE results reveal that ORR on pyrolyzed Fe-PPDA/C shows an apparent four-electron mechanism, while on pyrolyzed Co-PPDA/C, the two-electron pathway and the four-electron one parallel in the reaction mechanism. The results on surface analysis demonstrated that the role played by transition metal Co or Fe in the catalyst procursor is only to catalyse the formation of active sites of catalyst for oxygen reduction reaction. It is not the component of active site itself. The active site is closely associated with pyridinic nitrogen. The activity and selectivity of the catalysts depend on the surface density of this group.