Electrochemical properties of early transition metal nitrides.

Abstract

In recent years, there has been increased pressure to discover and develop novel materials for use in electrochemical energy storage and conversion devices such as ultracapacitors and fuel cells. These materials should be electrically conductive, electrochemically stable, and should possess high surface areas. Recently, early transition metal nitrides have attracted interest due to their high electrical conductivities and high surface areas (up to 220 m<super>2</super>/g). These materials are excellent candidates for electrochemical applications. Work in this dissertation was aimed at determining the utility of these materials by studying their electrochemical and surface properties, as well as by assessing their electrocatalytic activity. Both low and high surface area Group IV, V and VI metal nitride powders were characterized using x-ray diffraction and nitrogen physisorption techniques, and then fabricated into Teflon<super>(c)</super>-bound electrodes. Cyclic voltammetry in 1 N H₂SO₄ was used first to survey the electrochemical properties of the electrodes, and then to evaluate the electrode behavior over periods of extended cycling. Electrodes subjected to extended cycling were analyzed using x-ray photoelectron spectroscopy in order to identify irreversible changes in species on the surface. The low surface area electrodes were stable from at least 0 mV to approximately 800 mV. This represents a significant operating potential regime for an electrochemical device employing aqueous electrolytes. Surfaces consisted of mixtures of nitrides, oxides and oxynitrides. Oxynitride species oxidized then dissolved from surfaces of the vanadium nitride and titanium nitride-based electrodes at potentials higher than 800 mV. The FIN and NbN electrode surfaces were electrochemically stable, demonstrating no irreversible changes in composition. The ZrN and TaN electrode surfaces converted to oxides at higher potentials. Observed trends may be explained with respect to the kinetics of oxidation. Oxidation of the period four nitrides was facile, while oxidation of the lower period nitrides was more difficult. The resistance to oxidation may also have allowed the lower period nitride electrodes to be more active for both hydrogen and oxygen evolution. The higher surface area materials were considerably less stable than their low surface area counterparts, generally due to higher reaction rates. For example, the high surface area titanium nitride and vanadium nitride-based materials reacted at potentials greater than only 200 mV. This potential was least 600 mV lower than where the low surface area counterparts began to show a surface reaction. The surfaces of the molybdenum nitride electrodes (delta-MoN and gamma-Mo₂N) oxidized to MoO₃ at potentials as low as 200 mV, and were incapable of oxygen evolution. Most of the Group IV and V materials did demonstrate oxygen evolution, with lower overpotentials for oxygen evolution apparent for higher surface area electrodes.