Influence of structural properties of pristine carbon blacks on activity of Fe/N/C cathode catalysts for PEFCs

Four series of carbon blacks, with various disordered carbon content, have been fabricated at the Sid Richardson Carbon Corporation and were used as supports to obtain heat-treated Fe/N/C electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). All catalysts were very active and differed by about one order of magnitude in their electrocatalytic activity. Two pristine carbon blacks were then selected to determine, by Raman spectroscopy and X-ray diffraction, which structural parameters of the pristine carbons are important in electrocatalysis. It was found that W-D (the full width at half-maximum of the D band in the Raman spectrum) is indicative of the disordered phase content in the pristine carbon black. The pristine carbon black having the largest W-D also yielded the best ORR electrocatalytic activity. L-a and L-c are structural parameters representing the lateral (L-a) and vertical (L-c) extensions of the graphitic crystallites. They do not change with the catalytic activity, at least not with a change of one order of magnitude of the catalytic site density. Only the distance between the graphene layers in the graphitic crystallites (d(002)) changes with the electrocatalytic activity, but only marginally. This is, however, believed to be an indirect effect related to the gasification of disordered carbon found between the graphitic crystallites in pristine carbon black. (c) 2007 The Electrochemical Society.