Effect of nitrogen and iron in carbon nanowalls on oxygen reduction reaction

The effect of nitrogen (N) and iron (Fe) on oxygen reduction reaction (ORR) was investigated by using carbon nanowalls (CNWs) as carbon matrix. The dopings of N and Fe into CNWs are carried out by N plasma and Fe sputtering, respectively. The onset potentials corresponding to ORR activities are evaluated from steady-state curve of cyclic voltammogram. The composition and structure of CNW catalysts before and after ORR measurements are characterized by X-ray photoelectron spectroscopy (XPS). The onset potential increases in the order of non-, N- and Fe-N-doped CNW catalysts. The ORR activity for N-doped CNW catalysts is ascribed to pyridinic N stabilized near the surface of carbon matrix. The higher ORR activity for Fe-N-doped CNW catalysts is attributed to trace Fe-N rather than pyridinic N near the surface. Additionally, it is shown that such ORR activities due to pyridinic N and Fe-N near the surface are further promoted by Fe and Fe-N incorporated beneath the surface. These knowledges provide effective strategy for the synthesis of Fe-N-doped carbon catalysts with higher ORR activities.