Catalysis

Identification of the catalytically dominant iron environment in iron- and nitrogen-doped carbon catalysts for the oxygen reduction reaction

Authors

Abstract

For a large scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal and nitrogen doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the iron ion is coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando 57Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures, implying a direct structural correspondence. Pyrrolic N-coordination, i.e. FeN4C12, is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coor-dination. These findings thus overcome the previously conflicting structural assignments for the active site, and moreover identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

Content

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Supplementary material

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Supporting Information for 'Identification of the catalytically dominant iron environment in iron- and nitrogen-doped carbon catalysts for the oxygen reduction reaction'
Additional experimental and theoretical information as referred to in the main text: extended computational details, discussion of model geometries and electronic structures, TEM images, additional Mössbauer data, additional electrochemistry data, additional com-puted Mössbauer and thermodynamic data, detailed information on geometric and electronic structures, possible assignments of compu-tational models to experiment when relying on quadrupole splitting only, experimental estimation of average spin states