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

By U. Kramm, J. Herranz, Nicholas Larouche, T. Arruda, M. Lefèvre, Frédéric Jaouen, P. Bogdanoff, S. Fiechter, I. Abs-Wurmbach, S. Mukerjee and 1 more. Physical Chemistry, Chemical Physics - PCCP, 2012

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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 ion in low (D1), medium (D2) or high spin state (D3), 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− site−1 s−1 at 0.8V vs RHE. Moreover, all D1 sites and between 1/2 to 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.

Subfields: Electrical and Electronic Engineering, Renewable Energy, Sustainability and the Environment

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Citations in the corpus

Carbon as Catalyst and Support for Electrochemical Energy Conversion (2014, background): “Ag doped multi-layer graphene was synthesized by the reduction of silver salt (Ag(NH3)2) / graphene oxide under rapid heating (room temperature to 1000 oC at 100oCmin-1 under inert atmosphere; 15 min at 1000oC) [356].”
Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction (2016, result): “…than that of the out-of-plane Ox–Fe 3+–N4 moiety; and the dz2 orbital of the out-of-plane Fe 2+–N4 motif is only partially filled.14 This also supports the previous report by Kramm et al.12 that attributed the higher TOF of D1 to the higher electron density of the central Fe, as the longer Fe–”
Transition metal–nitrogen–carbon nanostructured catalysts for the oxygen reduction reaction: From mechanistic insights to structural optimization (2017, methodology): “[40], © Royal Society of Chemistry 2012.”