Alkaline Capacitors Based on Nitride Nanoparticles

John H. Glenn Research Center, Cleveland, Ohio

High-energy-density alkaline electrochemical capacitors based on electrodes made of transition-metal nitride nanoparticles are undergoing development. Transition-metal nitrides (in particular, Fe₃N and TiN) offer a desirable combination of high electrical conductivity and electrochemical stability in aqueous alkaline electrolytes like KOH. The high energy densities of these capacitors are attributable mainly to their high capacitance densities, which, in turn, are attributable mainly to the large specific surface areas of the electrode nanoparticles. Capacitors of this type could be useful as energy-storage components in such diverse equipment as digital communication systems, implanted medical devices, computers, portable consumer electronic devices, and electric vehicles.

Cyclic Voltammetry is a good technique for examining the capacitance, stability, and voltage window of a capacitor. This is a 100-cycle voltammogram, made at a scan rate of 100 mV/s, of a capacitor containing electrodes of nanoparticulate TiN. This plot indicates, among other things, that the electrode material is stable during cycling in the alkaline electrolyte. This cycle exceeds the voltage limits typical of prior capacitors that contain prior carbon or ruthenium oxide electrodes in electrolytes based on sulfuric acid.

Although the desirable properties of the transition-metal nitrides were known prior to the present development, realization of the inherent electrochemical stability of these materials and of the large specific surface areas and high electrical conductivities needed for high-energy-density capacitors was prevented by side effects of processing:

Synthesis of these materials involved thermal conversion at temperatures so high (>600 °C) as to cause nucleation of larger particles from smaller ones, with consequent reduction of specific surface areas.
The nature of the synthesis was such as to yield oxynitrides and oxides in addition to the desired pure nitrides. As a result, electrochemical series resistance (ESR) values were excessive.
Unlike the nitrides, the oxynitrides and oxides are not sufficiently chemically stable in alkaline electrolytes.

The present development effort follows a multifaceted approach in addressing the aforementioned issues as well as others. In this approach, transition-metal nitride nanoparticles are synthesized at room temperature under conditions that exclude oxygen and thereby prevent the formation of oxynitrides: A synthesis according to this approach involves radio-frequency-plasma-assisted conversion of a nanoparticulate precursor material (e.g., iron acetate, titanium hydride, or titanium chloride) in the presence of anhydrous ammonia gas flowing at a suitable low pressure.

Current collectors for the electrodes of the developmental capacitors are made of films of an electrically conductive composite material that consists mostly of TiN nanoparticles in an elastomeric matrix. To ensure highly electrically conductive interfaces with the electrode materials, thin (250-Å thick) coats of TiN can be sputtered onto the surfaces of the current-collector films.

Capacitors designed and fabricated according to the present approach have been characterized in a variety of tests (for example, see figure). A specific capacitance in excess of 150 Farads/gram (F/g) or 800 F/cm² has been observed. Capacitors of this type, containing both anodes and cathodes made of transition-metal nitride nanoparticles, have withstood potentials >1.75 V. Tests of single-cell and multiple-cell stacks have yielded encouraging results, and significant improvements are expected in future efforts.

This work was done by Matt Aldissi of Fractal Systems, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.

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