Probing carbon edge exposure of iron phthalocyanine-based oxygen reduction catalysts by soft X-ray absorption spectroscopy

Recently carbon-based catalysts have attracted much attention for its high ability to catalyze oxygen reduction reaction (ORR) in a cathode for polymer electrolyte fuel cells. Among active site models of the catalyst, edge region of sp2 carbon network is expected to demonstrate high ORR activity comparable to platinum catalysts. However, it is difficult to directly identify the chemical structure of the edge sites by applying conventional structural or electronic probes to actual catalysts. Here, we used C 1s X-ray absorption spectroscopy to observe electronic structure of carbon in iron phthalocyanine-based catalysts, and found a signature of edge exposure below the π* edge, whose intensity is well correlated with the ORR activity. Combined with information about the chemical structure of nitrogen, ORR activity of the catalyst is characterized in terms of the edge exposure. ► O2 reduction site of FePc-based catalysts was characterized by C 1s X-ray absorption. ► Low energy shoulder of the π* peak becomes a good indicator of active edge carbons. ► The method can be applied to characterize a wide variety of carbon-based catalysts.