O2 reduction in PEM fuel cells:: Activity and active site structural information for catalysts obtained by the pyrolysis at high temperature of Fe precursors

Catalytic activity for O-2 electroreduction in acidic medium has been studied. Catalysts have been produced by pyrolyzing perylene tetracarboxylic dianhydride (PTCDA) combined with Fe and N precursors. The Fe precursors used in this work are either Fe-II acetate or Cl-Fe-III Tetramethoxyphenyl porphyrin (Cl-FeTMPP). The N precursors are NH3 and/or Cl-FeTMPP. To be able to vary the catalytic activity, two synthesis procedures have been used, varying either the heat-treatment temperature or the Fe content of the pyrolyzed materials. All the catalysts have been studied by Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS) in order to find out if the relative intensity of one or several secondary ions was following the changes observed in the catalytic activity. Only one ion, FeN2C4+, may be considered as the signature of the catalytic site generated in this work. It has been detected in all prepared catalysts, whatever the Fe precursor or the synthesis procedure was used. It is proposed that this ion is produced under ToF SIMS analysis from part of a 1,10 phenanthrolinic-type structure, which is itself part of the catalytic site. This catalytic site is probably the same as the one obtained by the pyrolysis at high temperature (greater than or equal to 800 degreesC) of N-4-Fe macrocycles adsorbed on carbon black. RDE and PEM fuel cell tests demonstrate that the most performing catalysts are those containing 2 wt % Fe as Cl-FeTMPP and 0.2 wt % Fe as Fe acetate. Increasing the Fe content beyond these limits mainly generates catalytically inactive Fe clusters and interferes with the characterization of the high-temperature active site.