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Spectroscopic Characterization of Highly Active Fe–N–C Oxygen Reduction Catalysts and Discovery of Strong Interaction with Nafion Ionomer

Publication Type
Journal Name
ACS Applied Energy Materials
Publication Date
Page Numbers
604 to 613

Scaling up clean-energy applications necessitates the development of platinum group metal (PGM)-free fuel cell electrocatalysts with high activity, stability, and low cost. Here, X-ray absorption (XAS) at the Fe K-edge and Fe Kβ X-ray emission (XES) spectroscopies were used to study the electronic structure of Fe centers in highly active Fe–N–C oxygen reduction catalysts with significant commercial potential. X-ray absorption near-edge structure (XANES) analysis has shown that the majority (>95%) of Fe centers are in the Fe3+ oxidation state, while extended X-ray absorption fine structure (EXAFS) detected a mixture of single site Fe–N4 centers (>95%) and centers with short (∼2.5 Å) Fe–Fe interactions of Fe metal and/or Fe-carbide nanoparticles (<5%) featuring the Fe0 oxidation state. Surprisingly, addition of Nafion, the most widely used ionomer, resulted in pronounced changes in the XAS spectra, consistent with a strong catalyst–ionomer interaction where long Fe–Fe interactions at ∼3.1 Å were shown to be a feature of Fe3+ ions bound with the Nafion. We conclude that exposure to Nafion during the device formulation has a different effect from the aggressive acid leaching typically used in the preparation of Fe–N–C catalysts. It was hypothesized that the polymer interacts with single sites’ Fe3+ centers, as well as with graphene layers protecting the Fe0 nanoparticles, and extracts some Fe ions into the Nafion matrix.