Fe-based catalysts for oxygen reduction in PEMFCs - Importance of the disordered phase of the carbon support

Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) have been prepared with commercial and developmental carbon black powders containing initially between 0 and 0.8 atom % of nitrogen. The catalysts were obtained by adsorbing 0.2 wt % Fe from iron acetate on each carbon support, which is then pyrolyzed at 900 degrees C for 1 h in a NH3/H-2/Ar mixture. Under these conditions, N contents from 0 to 2.3 atom % are measured at the surface of the catalysts and increased N content leads to increased activity for the ORR. The N content correlates with the weight loss of the carbon support due to a reaction with NH3 during pyrolysis. It was found that NH3 reacts mainly with the disorganized carbon, leaving nitrogen at the surface of the support. The larger the amount of disorganized carbon in the pristine carbon black, the better the activity for ORR of the resulting catalyst. The most active non-noble catalyst was tested in fuel cells, where it was found that its specific activity (in A per cm(3) of electrode) is still about two orders of magnitude below the target of a non-noble catalyst for automotive applications. However, such catalysts could already compete with Pt in, e.g., methanol fuel cells because they are ORR-selective.