Nitric Oxide as a Probe Molecule for Understanding the Nature and Density of Active Sites in Fe-N-C Oxygen Reduction Reaction Catalysts

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).^1-3These 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, using nuclear resonance vibrational spectroscopy, to bind to Fe 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 which can in turn guide the synthesis of catalysts with improved activity and durability. To study this interaction, we have used aqueous electrochemical measurments of the impact of nitric oxide on the Fe redox features and on the ORR activity and we have evaluated the selectivity of the nitric oxide for Fe sites using electrochemical stripping, temperature programmed desorption, and in situ X-ray absorption spectroscopy. The implication of these results on the nature and density of ORR active sites in Fe-N-C catalysts will be discussed.<br/><br/>1. X.X. Wang, M.T. Swihart, and G. Wu, “Achievements, challenges, and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation”, <i>Nature Catalysis</i>, 2 (2019) 578-589.<br/>2. M. Chen, Y. He, J.S. Spendelow, and G. Wu, <i>ACS Energy Letters</i>, 4 (2019) 1619-1633.<br/>3. S.T. Thompson and D. Papageorgopoulos, <i>Nature Catalysis</i>, 2 (2019) 558-561.<br/>4. J.L. Kneebone, et al., “A Combined Probe-Molecule, Mossbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst”, J. Phys. Chem. C 121 (2017) 16283-16290.<br/>5. D. Myers and P. Zelenay, “ElectroCat (Electrocatalysis Consortium)”, Department of Energy Hydrogen and Fuel Cells Program Annual Merit Review, 2019. https://www.hydrogen.energy.gov/pdfs/review19/fc160_myers_zelenay_2019_o.pdf<br/>6. P. Boldrin, et al., “Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O₂”, Appl. Cat. B: Environ., 292 (2021) 120169-120180.