title: Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells.
abstract: Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by (57)Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH(3) at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN(4)-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (Fe(x)N, with x≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN(4)-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e(-) per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.
authors: Institut National de la Recherche Scientifique, Énergie, Matériaux et Télécommunications Varennes, Québec, Canada. kramm@tu-cottbus.de
authors: Ulrike I
authors: UI
authors: Kramm
authors: Juan
authors: J
authors: Herranz
authors: Nicholas
authors: N
authors: Larouche
authors: Thomas M
authors: TM
authors: Arruda
authors: Michel
authors: M
authors: Lefèvre
authors: Frédéric
authors: F
authors: Jaouen
authors: Peter
authors: P
authors: Bogdanoff
authors: Sebastian
authors: S
authors: Fiechter
authors: Irmgard
authors: I
authors: Abs-Wurmbach
authors: Sanjeev
authors: S
authors: Mukerjee
authors: Jean-Pol
authors: JP
authors: Dodelet
mesh_terms: D000641:Ammonia / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D002384:Catalysis; D004566:Electrodes; D004573:Electrolytes / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*
chemical_list: D004573:Electrolytes; D002244:Carbon; D000641:Ammonia; D007501:Iron; D009584:Nitrogen; D010100:Oxygen