Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells

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 diff...

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Published in:Physical chemistry chemical physics : PCCP 2012, Vol.14 (33), p.11673-11688
Main Authors: KRAMM, Ulrike I, HERRANZ, Juan, DODELET, Jean-Pol, LAROUCHE, Nicholas, ARRUDA, Thomas M, LEFEVRE, Michel, JAOUEN, Frédéric, BOGDANOFF, Peter, FIECHTER, Sebastian, ABS-WURMBACH, Irmgard, MUKERJEE, Sanjeev
Format: Article
Language:English
Subjects:
Ammonia - chemistry
Applied sciences
Carbon - chemistry
Catalysis
Chemical Sciences
Chemistry
Colloidal state and disperse state
Electrodes
Electrolytes - chemistry
Energy
Energy. Thermal use of fuels
Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc
Exact sciences and technology
Fuel cells
General and physical chemistry
Iron - chemistry
Nitrogen - chemistry
Oxidation-Reduction
Oxygen - chemistry
Physical and chemical studies. Granulometry. Electrokinetic phenomena
Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
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Summary: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.
ISSN:1463-9076
1463-9084
DOI:10.1039/c2cp41957b