Characterization of Surface Changes on Molybdenum Nitride Thin Films during the Oxygen Reduction Reaction Using Operando Grazing Incidence X-Ray Absorption Spectroscopy

Abstract

Coupled water electrolysis systems and proton-exchange membrane fuel cells (PEMFC) are a promising technology for clean energy storage and conversion, respectively.1 The efficiency of the hydrogen PEMFC is limited, in part, by the poor performance of catalysts for the oxygen reduction reaction (ORR). Due to problems with low activity and conductivity, the majority of non-platinum group catalysts for the ORR are conductive-carbon-supported nanoparticulate transition metal nitrides and/or carbides. It is difficult to determine the active surface of these catalysts during reaction conditions due to their high surface area, inhomogeneous size and distribution, complicated support-catalysts interactions, and surface oxidation in air. To overcome these complications, our work has focused on the operando structural and compositional characterization of the surface of active molybdenum nitride thin films. These films, prepared via reductive sputtering,2 are an ideal template for study due to their tunable thickness, lack of carbon support, moderate electrical conductivity, and well-defined surface. Using a specifically designed electrochemical cell for grazing incidence x-ray absorption and diffraction, we have been able to characterize changes in our Mo x N surface under ORR conditions. These same surface changes are not apparent at similar non-ORR reducing conditions, supporting our hypothesis that we are characterizing the active surface during ORR. ­ This work highlights the role of nitrogen, oxygen vacancies, and electrical conductivity on electrochemical activity and allows us to probe the structure and thickness of the active surface layer as well as the active site geometry. Understanding the role and importance of nitride bulk and the distinct surface structure will allow us to re-think our typical rational design of catalyst materials and help us design more active and stable catalysts in the future. References (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science (80-. ). 2017, 355, 1–12. (2) Hones, P.; Martin, N.; Regula, M.; Francis, L. Structural and Mechanical Properties of Chromium Structural and Mechanical Properties of Chromium Nitride , Molybdenum Nitride , and Tungsten Nitride Thin Films. J. Phys. D Appl. Phys. 2003, 36, 1023–1029.