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

Abstract

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 β 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 nearedge structure (XANES) analysis has shown that the majority (>95%) of Fe centers are in the Fe 3+ 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 (∼2.5 Å) 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 ∼3.1 Å were shown to be a feature of Fe 3+ 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' Fe 3+ centers, as well as with graphene layers protecting the Fe 0 nanoparticles, and extracts some Fe ions into the Nafion matrix.

Metal-Dependent Electrocatalytic Oxygen Reduction in Surface-Conjugated Macrocyclic Electrodes

“…Importantly, these electrodes show electrocatalytic responses similar to those observed for the glassy carbon-conjugated catalysts. Note that addition of Nafion changes the spectroscopic Fe XAS data in a single site Fe-N-C catalyst, which was interpreted as a stron catalyst ionomer interaction and Fe binding to the Nafion . An unmodified Pt ring electrode with its potential fixed at 1.2 V vs RHE was used to reoxidize all of the reduced products (except water) to O 2 .…”