Indirect contribution of transition metal towards oxygen reduction reaction activity in iron phthalocyanine-based carbon catalysts for polymer electrolyte fuel cells

The electronic structure of the residual metal atoms in Fe phthalocyanine (FePc)-based carbon catalysts, prepared by pyrolyzing a mixture of FePc and phenolic resin polymer at 800 °C, before and after acid washing have been investigated using X-ray absorption fine structure (XAFS) spectroscopy to clarify the role of Fe in the oxygen reduction reaction (ORR) activity. The Fe K X-ray emission intensity suggests that the acid washing process reduces 36% of the total amount of residual Fe in the FePc-based catalysts. The decomposition analyses for the XAFS spectra reveal that the composition ratio of each Fe component is unaltered by the acid washing, indicating that the residual Fe components were removed by the acid washing irrespective of their chemical states. Because the oxygen reduction potential was approximately unchanged by the acid washing, the residual Fe itself does not seem to contribute directly to the ORR activity of the samples. The residual Fe is composed mainly of metallic Fe components (Fe metal and iron carbide Fe3C), which can act as a catalyst to accelerate the growth of the sp2 carbon network during pyrolysis. The results imply that light elements such as C and N are components of the ORR active sites in the FePc-based carbon catalysts pyrolyzed at high temperatures where the metal-N4 structures in the macrocycles are mostly decomposed.

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