Use In Electrochemical Capacitors Research Articles

Characterizing the Dopant Behavior of Functionalized Carbon Nanotubes in Conducting Polymers

ABSTRACTThe electrochemical polymerization of conducting polymers, such as polypyrrole, generally requires the incorporation of an anionic dopant to balance the positive charge on the oxidized conducting polymer chains. The susceptibility of multiwalled carbon nanotube (MWNT) surfaces to functionalization makes them exciting candidates for a new class of dopant for conducting polymers. In this work, the doping of polypyrrole with functionalized MWNTs is investigated using a combination of electrochemical impedance spectroscopy, scanning electron microscopy, and quartz crystal microbalance work. The findings described here are particularly relevant in light of recent reports indicating that carbon nanotube-conducting polymer composites hold great promise for use in electrochemical capacitors, also known as supercapacitors [1,2].

Cyclic Voltammetric Deposition of Hydrous Ruthenium Oxide for Electrochemical Capacitors

Hydrous ruthenium oxide‐coated titanium electrodes with high pseudocapacitance were prepared by cyclic voltammetry from an aqueous chloride solution in the −200 to 1000 mV range. The growth rate of , represented by (peak current) of the cyclic voltammograms, was constant up to cycle 120, but it decreased slightly between 120 and 240 cycles. Voltammetric responses studied by cyclic voltammetry as well as the charging and discharging behavior examined by chronopotentiometry in demonstrated the suitability of for use in electrochemical capacitors. X‐ray diffraction spectra exhibited an amorphous structure of this hydrous oxide film. The oxide consisted of mixed oxyruthenium species with various oxidation states as demonstrated by X‐ray photoelectron spectroscopy, and the surface showed a porous morphology. © 1999 The Electrochemical Society. All rights reserved.

Deposition of Ruthenium Nanoparticles on Carbon Aerogels for High Energy Density Supercapacitor Electrodes

The preparation and characterization of high surface area ruthenium/carbon aerogel composite electrodes for use in electrochemical capacitors is reported. These new materials have been prepared by the chemical vapor impregnation of ruthenium into carbon aerogels to produce a uniform distribution of adherent ≈20 Å nanoparticles on the aerogel surface. The electrochemically oxidized ruthenium particles contribute a pseudocapacitance to the electrode and dramatically improve the energy storage characteristics of the aerogel. These composites have demonstrated specific capacitances in excess of 200 F/g, in comparison to 95 F/g for the untreated aerogel.

Nonaqueous Electrolytes for Electrochemical Capacitors: Imidazolium Cations and Inorganic Fluorides with Organic Carbonates

Electrolytes based on 1‐ethyl‐3‐methylimidazolium cation and either the hexafluorophosphate or tetrafluorborate anion in organic alkyl carbonate solvents have been evaluated for use in electrochemical capacitors The conductivity capacitance, limiting oxidation and reduction potentials, and thermal stability were assessed. High conductivity and capacitance values were found regardless of whether cyclic (high viscosity/high dielectric constant) or acyclic (low viscosity/low dielectric constant) alkyl carbonates were used. The best correlation with conductivity for the salt was found to be the molecular weight (κ ∝ 1/Mw) and to a lesser degree the viscosity (κ ∝ 1/η) of the solvent. The high specific capacitance (130 F/g) and excellent stability (>3.5 V, >130°C) make these electrolytes well suited for use in electrochemical double‐layer capacitors.

Material Characteristics and the Performance of Electrochemical Capacitors for Electric/Hybrid Vehicle Applications

ABSTRACTElectrochemical capacitors (ultracapacitors) are one approach to meeting the high power requirements for the energy storage system in an electric vehicle. Energy is stored in an electrochemical capacitor by charge separation in the double layer formed in the micropores of a very high surface area electrode material, which does not undergo chemical change as in a battery. Consequently, the material requirements for capacitors are very different from those of batteries. In the last several years, a number of promising material technologies have been identified for use in electrochemical capacitors. These include activated carbon fibers, foams, and composites, doped conducting polymers, and mixed metal oxides. The most important material property is its specific capacitance (F/gm or F/cm3). Carbon materials with specific capacitances of 100 to 300 F/gm have been developed. Doped polymer materials having specific capacitances of 300 to 400 F/gm are also being studied. In addition to high specific capacitance, the electrode material must also have a low electronic resistivity ( < 0.1 Ω-cm) in order that charge can be distributed with minimum voltage drop in the electrode. Electrochemical capacitor cells have been fabricated using the various material technologies with both aqueous and organic electrolytes. Tests of the cells have shown near ideal charge/discharge characteristics — that is, the voltage versus time curves are nearly linear for constant current tests. The energy densities of 1 V cells, using aqueous electrolytes, are 1 to 1.5 W-h/kg and those of 3 V cells, using organic electrolytes are 7 to 10 W-h/kg. Most of the cells have high power densities of 1 to 3 kW/kg. Numerous new materials for electrochemical capacitors have been identified, processed, and tested in electrodes and cells in recent years and progress is rapid in this relatively new field of research.

A study of the electrochemical properties of conducting polymers for application in electrochemical capacitors

Conducting polymers can be doped and dedoped rapidly to high charge density and as a result are potential active materials for use in electrochemical capacitors. We discuss three schemes by which conducting polymers can be utilized in electrochemical capacitors and in the third of these, which employs a conducting polymer that can be both n- and p-doped, high energy and power densities are demonstrated. Polythiophenes can be both n- and p-doped reversibly and we have tested a number of poly-3-arylthiophenes in order to achieve the best n-doping at thick polymer films. We have found that poly-3-(4-fluorophenyl)-thiophene, in a solution of 1 mol dm −3 tetramethylammonium trifluoromethanesulfonate in acetonitrile, can be n- and p-doped, both reversibly and to high charge density. Such key properties for high power, energy storage devices have not been demonstrated before with conducting polymer active materials. We propose that the improvement in polymer n-dopability that results from derivitization of thiophene with aryl substituents in the 3-position, is probably achieved thanks to electron transfer from the negatively charged polythiophene backbone to the aryl substituent. This proposal is discussed in the light of modeling information and of voltammetric data. Further supporting information is provided by electron microscopy, impedance spectroscopy and electrochemical quartz crystal microbalance results.