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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, E p, 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 E p 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.

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Three synthetic paths have been used to prepare iron-based catalysts for the electrochemical reduction of oxygen in solid polymer fuel cells. The catalyst precursor used in the first synthetic path was a dispersion of Fe(OH)2 on carbon black (Vulcan) that was reduced at 600°C in H2. The second synthetic path involved a method to intercalate FeC13 that was first used with graphite and then extended to Vulcan. The oxidized iron was reduced by either heating the resulting materials at 600°C in 112 or by reacting them at room temperature with K-naphthalene in a nonaqueous solution. In the third synthetic path, Fe particles were generated directly on Vulcan by the reduction in a nonaqueous solution of FeC12 with triethylborohydride. All these iron containing materials were then activated at 1000°C in the presence of acetonitrile in order to transform them into active catalysts for °2 reduction. The best catalyst was obtained by extending the intercalation method of FeC13 to carbon black (Vulcan) and then reducing the oxidized iron with K-naphthalene. Based on electrochemical and physicochemical analysis (mostly scanning electron microscopy and x-ray diffraction analysis) it is suggested that iron is included or intercalated in the carbon black particles. The higher catalytic activity of this material compared to the others is explained by the progressive release of the iron from the carbon black particles and its reaction with acetonitrile during the final thermal activation step at 1000°C.