Oxygen reduction catalysts for polymer electrolyte fuel cells from the pyrolysis of Fe-II acetate ad

ELECTROCHIMICA ACTA Volume: 44 Issue: 15 Pages: 2589-2603 Published: 1999

Catalysts for O2 reduction in acidic media were prepared from the pyrolysis of Fe acetate adsorbed on perylenetetracarboxylic dianhydride (PTCDA) in Ar:H2:NH3 ambient. The amount of adsorbed Fe acetate ranged from 0 to 25600 ppm Fe nominal. Catalysts are obtained when the pyrolysis temperature is at least 8008C, but 9008C gives the best results. RDE analysis demonstrates that the catalytic activity increases ®rst with the Fe content but levels o€ for Fe contents larger than 5000 ppm (0.5 wt% Fe). GDE measurements in a single membrane electrode assembly indicate that the polarization curve obtained with a catalyst containing 0.2 wt% Fe is similar to the one obtained for 2 wt% Pt. The prepared catalysts have been characterized by neutron activation analysis, XRD, TEM, ToF SIMS and XPS. From these characterizations, one may conclude that an oxidized state of Fe and pyridinic type nitrogen are both involved in the composition of the catalytic site for O2 reduction obtained at high pyrolysis temperature. Pyridinic type nitrogens account for about 1/3 of the N1s signal detected by XPS at the surface of the catalyst. Beyond 0.5 wt% Fe, a plateau is reached in the catalytic activity because Fe in excess begins to form aggregates containing metallic and/or carbidic iron that are detectable by TEM and XRD. These aggregates have no catalytic activity for O2 reduction.