Fe-based catalysts for the reduction of oxygen in polymer electrolyte membrane fuel cell conditions: determination of the amount of peroxide released during electroreduction and its influence on the stability of the catalysts
Fe-based catalysts have been prepared by pyrolyzing ClFeTMPP (Cl–Fe tetramethoxyphenyl porphyrin) or Fe acetate adsorbed on PTCDA (perylene tetracarboxylic dianhydride) or on prepyrolyzed PTCDA (p-PTCDA). The catalysts which were already well characterized in terms of active FeN4/C and FeN2/C catalytic sites (J. Phys. Chem. B 106 (2002) 8705) are now characterized by RRDE experiments to determine the values of the apparent number of electron transferred (n) and the percentage of peroxide (%H2O2) released during the oxygen reduction reaction (ORR) in H2SO4 at pH 1. A direct correlation is found between the relative abundance of the FeN2/C catalytic site in these materials, their catalytic activity and the value of n. The correlation is inverse for %H2O2. The best catalysts at their maximum catalytic activity are characterized by n>3.9 and %H2O2<5%, equivalent to a value of %H2O2 released by a 2 wt.% Pt/C catalyst. It is shown that even low peroxide levels of the order of 5 vol% in H2SO4 are able to decompose the catalytic sites releasing iron ions in the H2SO4solution. The loss of catalytic activity correlates directly with the loss of iron ions by these catalysts. All the catalysts have been tested at the cathode of single membrane electrode assemblies (MEAs). The slow decrease in performance in fuel cell stability tests is interpreted as the result of the detrimental effect that has H2O2, released during ORR, on the chemical integrity of the nonnoble metal catalytic sites at work at the fuel cell cathodes.