Molecular Oxygen Reduction in PEM Fuel Cells: Evidence for the Simultaneous Presence of Two Active Sites in Fe-Based Catalysts

Abstract

Three catalysts for the electroreduction of oxygen have been prepared by pyrolyzing between 400 and 1000 °C two iron precursors (Fe acetate or Fe porphyrin) adsorbed on a synthetic carbon made from the pyrolysis of PTCDA (perylene tetracarboxylic dianhydride) in a H2/NH3/Ar atmosphere. One Fe loading (0.2 wt %) has been used for the catalyst made from the salt precursor. Two Fe loadings (0.2 and 2.0 wt %) have been used for the catalyst made from the porphyrin precursor. These three catalysts have been analyzed by ToF SIMS and RDE (or GDE) in order to find correlations between ions detected by ToF SIMS and the catalytic activity. These correlations provide information about the number and the structure of the catalytic sites, which are active in these materials. By following the variation of FeNxCy+ ions, it is found that (i) two different catalytic sites exist simultaneously in all catalysts made with the Fe salt or the Fe porphyrin; (ii) one site, named FeN4/C, is at the origin of three families of FeNx...