High Surface Area Metal Carbide and Metal Nitride Electrodes
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M. R. WIXOM*, D. J. TARNOWSKI*, J. M. PARKER*, J. Q. LEE*, P.-L. CHEN*, I. SONG* AND L.T. THOMPSON** *T/J Technologies, Inc., P.O. Box 2150, Ann Arbor, MI 48106; "**Universityof Michigan, Department of Chemical Engineering, Ann Arbor, MI 48109-2136
ABSTRACT Processes for fabricating new high surface area ceramic electrode materials have been developed. These electrode materials have been applied in electrochemical capacitors and related energy storage and conversion devices. Several synthetic approaches have been developed for producing high surface area carbide or nitride active materials. The fabrication methods provide the capability to vary the composition and microstructure of the electrode material. A number of new candidate high surface area electrode materials have been synthesized. Compositional and microstructural information is presented. Electrodes have been evaluated by cyclic voltammetry, chronopotentiometry, and impedance spectroscopy in acidic and basic aqueous electrolyte systems. Single electrode and single cell performance data are presented. Intrinsic properties such as open circuit potential, electrochemical stability and specific capacitance are discussed with respect to electrode composition. The influence on performance of extrinsic factors such as electrode thickness, particle size, and pore structure is also discussed. The performance of these new materials is compared to carbon, with emphasis on advantages with respect to volumetric energy and power density. INTRODUCTION Ultracapacitors store charge at the interface between a polarized electrode and an electrolyte. Since the energy storage mechanism is an interfacial phenomenon, high power density ultracapacitors generally require the incorporation of high surface area electrode materials. In addition to possessing high surface areas, ultracapacitor electrode materials should also be low cost, electrically conductive, and electrochemically stable. Present commercially available ultracapacitors are based on high surface area carbon or ruthenium oxide electrode materials. Ruthenium oxide-based electrodes offer good performance, but ruthenium is expensive and reserves are limited. Ruthenium is probably not practical for high volume applications. Carbon electrodes are based on low cost raw materials, but improvements will be needed to increase volumetric power density and to minimize the need for costly processing steps. T/J Technologies is developing a new class of high surface area (HSA) electrode materials based on early transition metal nitrides, carbides, and related compounds [1]. These are interstitial compounds with nitrogen or carbon atoms generally occupying a substantial fraction of the octahedral sites in the metal host lattice. Early transition metal nitrides and carbides are metallic conductors possessing significantly higher bulk electronic conductivities than carbon (see Table I). The electrochemical properties of selected metal nitride and carbide electrode materials will be presented and discussed below.
643 Mat. Res. Soc.
