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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 57Fe Mo¨ ssbauer 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 NH3 at 950 1C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-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 (FexN, with x r 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents Z0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-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.
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Submitted on : Monday, October 8, 2012 - 2:32:59 PM
Last modification on : Tuesday, May 28, 2019 - 2:06:10 PM

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Ulrike Kramm, Juan Herranz, Nicholas Larouche, Thomas Arruda, Michel Lefèvre, et al.. Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2012, 14, pp.11673-11688. ⟨10.1039/c2cp41957b⟩. ⟨hal-00739549⟩

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