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A method involving HNO3/NH3 treatments of a carbon support (Vulcan XC-72R), addition of Fe from inorganic precursors, and a thermal treatment at 900 °C under an inert atmosphere for the preparation of Fe-based oxygen reduction electrocatalysts for use in polymer electrolyte fuel cells has been investigated. The prepared materials were evaluated in acidic medium by the rotating disk electrode technique using the potential at the oxygen reduction peak, Ep, as a measure of the electrocatalytic activity. Two sets of experiments were performed: (i) varying the conditions of the HNO3/NH3 treatments of Vulcan XC-72R, and (ii) varying Fe loadings added to the HNO3/NH3 pretreated C samples. In the first set of experiments, no Fe was adsorbed on the C support. It was shown that a combined HNO3 and NH3 treatment of the carbon support gave higher activity for oxygen reduction than either HNO3 or NH3 treatment alone. In the second set of experiments, it was shown that the activity of the HNO3/NH3-modified C support increases steadily as the iron content is increased from 10 to about 1000 ppm. Above that Fe content, the activity of the materials first levels off and then decreases for even larger Fe concentrations. The value of Ep at saturation increases with the total N surface content of HNO3/NH3-pretreated C support. X-ray photoelectron spectroscopy reveals that at maximal electrochemical activity, there is only 1 Fe atom per 100 N atoms at the surface of the carbon support. For larger Fe contents another mechanism, i.e., the aggregation of small iron-containing particles, probably accounts for all additional Fe ions adsorbed on the modified carbon black during the preparation procedure.
Wang et al. (Mon,) studied this question.