Nitric Oxide Probe Molecule Studies of Iron-Nitrogen-Carbon PEMFC Oxygen Reduction Reaction Electrocatalysts

Abstract

The highest oxygen reduction reaction (ORR) activities in acidic environments for platinum group metal-free electrocatalysts have been achieved for materials derived from iron, nitrogen, and carbon-containing precursors such as iron-substituted zinc-based zeolitic imidazolate frameworks (ZIF).[1-3] These precursors are typically heat treated in an inert atmosphere at temperatures ranging from 900 to 1100°C to form the ORR-active material. In addition to their high intrinsic activity, under certain preparation conditions these catalysts are free of crystalline iron species, with iron atomically dispersed in a nitrogen-doped carbon matrix, as determined through characterization by high-resolution electron microscopy and X-ray absorption spectroscopy. While the exact nature of the active site in these materials is still a matter of debate, the probe molecule, nitric oxide, has been shown to bind to Fe, using nuclear resonance vibrational spectroscopy, and also to poison the ORR.[4-6] An in-depth study of the interaction of nitric oxide with a variety of Fe-N-C catalysts can thus provide insight into the nature of the ORR active site in this class of catalysts. This presentation will discuss the application of multiple characterization techniques to understand the interaction of nitric oxide with Fe-N-C catalysts, including nuclear resonance vibrational spectroscopy, temperature-programmed desorption, and in situ X-ray absorption spectroscopy as a function of potential in aqueous acidic electrolyte. The implications of the results of these studies on the nature of the ORR active site will be discussed.