Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells
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- 21 June 2012
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Physical Chemistry Chemical Physics
- Vol. 14 (33), 11673-11688
- https://doi.org/10.1039/c2cp41957b
Abstract
Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe 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 NH3 at 950 °C. 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 ≤ 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 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.Keywords
This publication has 51 references indexed in Scilit:
- Effect of an Ammonia Treatment on Structure, Composition, and Oxygen Reduction Reaction Activity of Fe–N–C CatalystsThe Journal of Physical Chemistry C, 2011
- Iron porphyrin-based cathode catalysts for polymer electrolyte membrane fuel cells: Effect of NH3 and Ar mixtures as pyrolysis gases on catalytic activity and stabilityElectrochimica Acta, 2010
- Fe-based electrocatalysts made with microporous pristine carbon black supports for the reduction of oxygen in PEM fuel cellsElectrochimica Acta, 2008
- Fe/N/C non-precious catalysts for PEM fuel cells: Influence of the structural parameters of pristine commercial carbon blacks on their activity for oxygen reductionElectrochimica Acta, 2008
- Average turn-over frequency of O2 electro-reduction for Fe/N/C and Co/N/C catalysts in PEFCsElectrochimica Acta, 2007
- On Spin Hamiltonian fits to Mössbauer spectra of high-spin Fe(II) porphyrinate systemsHyperfine Interactions, 2006
- Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells from the Pyrolysis of Iron Acetate Adsorbed on Various Carbon SupportsThe Journal of Physical Chemistry B, 2003
- On the effect of a heat treatment on the structure of carbon-supported metalloporphyrins and phthalocyaninesElectrochimica Acta, 1988
- N4-chelates as electrocatalyst for cathodic oxygen reductionElectrochimica Acta, 1986
- Moessbauer and Raman spectra of carbon-supported iron phthalocyanineThe Journal of Physical Chemistry, 1980