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

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 K-beta 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-N-4 centers (>95%) and centers with short (similar to 2.5 & Aring;) Fe-Fe interactions of Fe metal and/or Fe-carbide nanoparticles (<5%) featuring the Fe-0 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 similar to 3.1 & Aring; 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 Fe-0 nanoparticles, and extracts some Fe ions into the Nafion matrix.