Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells

By Jingkun Li, M. Sougrati, A. Zitolo, J. Ablett, I. Oğuz, T. Mineva, I. Matanovic, P. Atanassov, Ying Huang, Iryna Zenyuk and 6 more. Nature Catalysis, 2020

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While Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe–N–C catalysts initially comprising two distinct FeN x sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure–activity correlations drawn from end-of-test 57 Fe Mössbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ 57 Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeN x moieties in different electronic states, we identify S1 to be a high-spin FeN 4 C 12 moiety and S2 a low- or intermediate-spin FeN 4 C 10 moiety. These insights lay the groundwork for rational approaches towards Fe–N–C cathodes with improved durability in acidic fuel cells. Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells. Now, a 57 Fe Mössbauer study reveals that while these catalysts initially comprise two distinct FeN x sites, a high-spin FeN 4 C 12 and a low- or intermediate-spin FeN 4 C 10 , only the latter is durable in operating conditions.

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

View Paper: https://www.semanticscholar.org/paper/107c0a0f6a6dd52af7194ad488a12f14f1bdcc9a

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