Effect of Iron Precursors on the Structure and Oxygen Reduction Activity of Iron-Nitrogen-Carbon Catalysts

To understand the effect of an iron precursor on the structure and oxygen reduction activity of an Iron-Nitrogen-Carbon (Fe-N-C) catalyst, several iron precursors, FeCl3, FeCl2, Fe(C2H3O2)(2), and FeSO4, were mixed with polyaniline (PANI) and carbon black and then pyrolyzed at 700 degrees C under a nitrogen atmosphere; the obtained nonprecious metal catalysts were denoted as FeCl3-PANI/C, FeCl2-PANI/C, Fe (C2H3O2)(2)-PANI/C, and Fe2SO4-PANI/C, which were compared for the activity of oxygen reduction reaction (ORR) in acidic media and characterized through X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption near-edge spectroscopy, and extended X-ray absorption fine structure. The ORR activity of the catalysts was ranked in the following order: FeCl3-PANI/C, FeCl2-PANI/C, Fe (C2H3O2)(2)-PANI/C, and Fe2SO4-PANI/C, where the electron-transfer numbers were 3.99, 3.98, 3.94, and 3.83, respectively. FeCl3-PANI/C and FeCl2-PANI/C exhibited Fe-N-4 structures and Fe3+ chelates, in which a large amount of quaternary-type nitrogen and pyrrolic-type nitrogen but a small amount of cyanide were formed in the catalysts. Fe(C2H3O2)(2)-PANI/C showed that Fe-N-4 structures and Fe2+ chelates were formed mostly in the catalyst, but showed the lowest nitrogen content among all catalysts. Fe2SO4-PANI/C was composed of FeS and Fe2O3 compounds. These results indicated that iron precursors affected the carbon-nitrogen composition, iron oxidation state of iron chelate, and chelate structure of Fe-N-C catalysts, resulting in the change of the ORR activity. (C) 2016 Elsevier Ltd. All rights reserved.