Multitechnique Characterization of a Polyaniline–Iron–Carbon Oxygen Reduction Catalyst

This paper summarizes a XANES, XPS, XRD, and Mössbauer study of an oxygen reduction reaction (ORR) catalyst obtained via a heat treatment of polyaniline, iron, and carbon black. The catalyst was characterized at several critical synthesis stages and following heat treatment at various temperatures. The effect of sulfur during the synthesis was also investigated. XANES linear combination fitting (XANES-LCF) was used to determine the speciation of iron using 16 iron standards. The highest ORR activity was measured with a catalyst heat-treated at 900 °C, with the largest Fe–Nx content, as determined by the XANES-LCF, also characterized by the highest microporosity. An absence or a reduction in the amount of a sulfur-based oxidant in the aniline polymerization was found to lead to an increase in the amount of iron carbide formed during the heat treatment and a decrease in the number of Fe–N4 centers, thus attesting to an indirect beneficial role of sulfur in the catalyst synthesis. Using principal component analysis (PCA), a good correlation was found between the ORR activity and the presence of Fe–Nx structures.