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

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 H 2 /NH 3 /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 FeN x C y + 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 FeN 4 /C, is at the origin of three families of FeN x C y + ions: FeN 4 C y +, FeN 3 C y +, and FeN 1 C y + . The most representative ion of that site is FeN 4 C 8 + . The other site, labeled FeN 2 /C, is at the origin of the family of FeN 2 C y + ions. The most representative ion of that site is FeN 2 C 4 +; (iii) the abundance of FeN 2 /C goes through a maximum for catalysts pyrolyzed between 700 and 900 °C. When Fe acetate is the Fe precursor, FeN 2 /C may represent up to 80% of the catalytic sites, while this falls to a maximum of about 50% when Fe porphyrin is the precursor; (iv) FeN 2 /C is more electrocatalytically active than FeN 4 /C; (v) at high porphyrin loading (2.0 wt % Fe), the catalytic sites bound to the carbon support are covered with a porous layer of pyrolyzed Fe porphyrin.