In Situ and Ex Situ Spectroscopic Studies of the Inhibition of Active Sites in Iron-Nitrogen-Carbon Electrocatalyst for Oxygen Reduction Reaction

Abstract

One of the most promising classes of PGM-free materials for oxygen reduction reaction (ORR) is based on graphene-like carbon containing nitrogen and transition metal (metal-nitrogen-carbon, MNC) 1, which show promise as replacement of Pt in both alkaline exchange membrane (AEM) and proton exchange membrane (PEM) fuel cells. Understanding the specific roles of nitrogen and metal in the activity and durability of MNC-based catalytic materials is a prerequisite for the rational design of ORR electrocatalysts with improved performance. The mechanism of ORR in MNC catalysts has been studied previously by a combination of spectroscopic and theoretical structure-to-activity studies 2-4. Using inhibitors that have unique spectral signatures and have strong binding to the active sites allows elucidating the relationship between the chemistry of active sites and activity. We report results of synchrotron near ambient pressure X-ray photoelectron spectroscopic (NAP-XPS) analysis performed at synchrotron beamlines for series of electrocatalysts belonging to iron-nitrogen-carbon (FeNC) family. In-situ monitoring of oxygen binding to different nitrogen and metal moieties existing at the surface of these materials is performed with and without complexing inhibiting agent based on phosphonate (etidronic acid HEDP). From the spectroscopic analysis of catalyst with and without adsorbed HEDP we observe that major sites blocked by HEDP are hydrogenated and protonated nitrogens. Using near-ambient pressure XPS we have studied how oxygen adsorption in humidified atmosphere is affected by presence of the HEDP potentially blocking N-H/N+ active sites. These spectroscopic observations of inhibited active sites is correlated to an electrocatalytic performance from rotating ring disk electrode (RRDE) tests.