Metal-Precursor Adsorption Effects on Fe-Based Catalysts for Oxygen Reduction in PEM Fuel Cells

Fe-based electrocatalysts for the reduction of oxygen in polymer electrolyte membrane (PEM) fuel cells have been prepared by adsorbing either Fe2+ or Fe(CN)(6)(4-) ions on two carbon blacks to determine if their maximum activity is limited by (i) the maximum number of micropores available to host catalytic sites in the support or by (ii) the maximum number of ions able to be adsorbed on the carbon. The two carbon supports having the same microporous surface area, one etched in air and the other in NH3, were derived from the same carbon black (N234). Air-etched N234 is more acidic in nature as it possesses carboxylic functionalities that can adsorb Fe2+, while NH3-etched N234 possesses both pyridinic and carboxylic functionalities that can adsorb either Fe(CN)(6)(4-) or Fe2+. Catalysts were prepared by heat-treating, in pure NH3, the materials resulting from Fe2+ or Fe (CN)(6)(4-) adsorption on both etched carbons. It is concluded that, when catalysts are prepared in pure NH3, the catalytic activity is only governed by the number of micropores, having a size between 0.8 and 2.0 nm, that are available to host the catalytic sites in the porous volume of the carbon support, because Fe ion uptake by adsorption is never the limiting factor. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3089369] All rights reserved.