Iron porphyrin-based cathode catalysts for polymer electrolyte membrane fuel cells: Effect of NH3 an
Ten different catalysts were prepared by loading 66 wt% ClFeTMPP on N330, a furnace grade carbon black, and pyrolyzing this catalyst precursor for 10 min at 950 °C in a NH3/Ar gas mixture with various NH3 volume fractions (from 0% to 100%). The activity and stability of these catalysts were measured in a fuel cell and compared. The only stable catalyst, although the least active, among these was the one pyrolyzed in pure Ar. A notable leap in catalytic activity, but drop in stability, was observed for all catalysts pyrolyzed in gas mixtures containing NH3, even with a mere volume fraction of 1.3% NH3in the pyrolysis gas mixture. Catalytic activities increased, while stability decreased with increasing volume fraction of NH3. The physicochemical properties of these catalysts were correlated with their electrochemical behaviour observed in fuel cell tests. It was found that a volume fraction of only 1.3% NH3 was enough to double the micropore surface area, the surface nitrogen and iron concentrations in the resulting catalyst. Since the active sites are believed to be of the Fe/N/C type, the sharp increase in catalytic activity with as little as 1.3% NH3 is attributed to the concurrent increase in microporous surface area, N and Fe surface contents in these catalysts. The only property that apparently correlates with stability is the degree of graphitization of the catalyst, which was estimated either from either X-ray diffraction and Raman spectroscopy measurements. Lastly, it was found that the catalysts’ peroxide yield, resulting from the partial reduction of O2, does not correlate with their degree of stability.