Carbon Nano-Onion Ultracapacitor Model
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Carbon Nano-Onion Ultracapacitor Model Fabio Parigi, Tanya Gachovska, Yang Gao, Yunshen Zhou, Jerry L. Hudgins, and Yongfeng Lu Department of Electrical Engineering, University of Nebraska-Lincoln 209N Walter Scott Engineering Center Lincoln, NE 68588-0511 USA ABSTRACT This paper describes a novel ultracapacitor made from carbon nano-onions. Characterization of the material was performed including measurement of the impedance spectra and cyclic voltammetry. A new ultracapacitor model composed of an LRC circuit and a constant phase element was developed along with a parameter extraction procedure. The model was validated using experimental data and simulation. INTRODUCTION Ultracapacitors (UCs) are energy storage devices that can deliver energy quickly, have hundreds of thousands of cycle lives, can be charged in seconds, and can withstand cold temperatures. UCs have been implemented in multiple applications in conjunction with power electronics [1]. Additionally, UC models have been used for circuit simulation in power electronics applications. Systems composed of electrodes and electrolytes are frequently represented by lumped element circuits [2] and by N-cascaded resistance and capacitor (RC) circuits [3]. Considerable work has been done on the modeling of double-layer capacitors for activated carbon by a capacitance, an equivalent series, and parallel resistance [4]; by three RC branches, one of them with a voltage-dependent capacitance [5]; and by the study of the impedance spectroscopy [6]; as well as for carbon nanotubes (CNTs) utilizing molecular dynamics to solve Poisson’s equations (and hence knowing the charge density distribution) within the UC’s cell [7]. Up to now, a model for a carbon-nano-onion (CNO)-based UC has not been reported. Therefore, the objective of this work is to develop a novel model, for application engineering use, of the dynamic behavior of the porous structure of a carbon nano-onion UC and, hence, extend the work accomplished on activated carbon [6,8] to CNOs. MATERIALS AND METHODS Electrode Fabrication CNO material grown through a laser-assisted combustion process in open air was used for the study [23]. The CNO electrodes were fabricated by depositing CNOs on nickel (Ni) foam sheets using the electrophoretic deposition (EPD) technique. EPD has the following advantages: short formation time, simple apparatus, and suitability for mass production [9]. It has been successfully used for depositing uniform films of CNTs [10]. A solution containing 20 mg of CNO powder, 10 ml of acetone, and 10 ml of ethanol, 0.03-0.05 wt.% of aluminum nitrate nonahydrate Al(NO)3·9H2O, was used for EPD. The solution was placed in a 40 ml beaker and dispersed by sonication for 1 hour at room temperature (21 °C). Two Ni foam sheets (thickness: 1.6 mm; surface density: 346 g/m2; porosity: ≥ 95%, 80-110 pores per square inch, Marketech International, Inc. U.S.) were washed by ultrasonication in acetone for 10 min. and dried for 24 hr. at room temperature. During the coating, one of the electrodes (10 x 10 mm)
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