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

Abstract

Electrochemical 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.

Investigations on porous silicon as electrode material in electrochemical capacitors

“…1b). The capacitance measurements were realized with an Impedance Analyzer Agilent 4294A, at 40 Hz with an amplitude of 100 mV to avoid the aqueous electrolyte dissociation [2]. Measurements were made both in equivalent serial and parallel modes, in order to extract the Equivalent Series Resistor (ESR) and the parallel resistor R p .…”

“…Electric double layer capacitors have been widely used during this past decade for energy storage, notably for electric vehicle applications [2]. A unit cell in these super-capacitors is composed of a pair of activated carbon electrodes with a large surface area, an electrolyte and an ion permeable electronic insulating separator [3].…”

A comparative study of sequentially layer-deposited and co-deposited Co–Mn oxides as potential redox capacitors

“…The second sample (M|C) was cathodically deposited at −1 V/ SCE from a 2M Co NO 3 ð Þ 2 Á 6H 2 O solution onto the surface of manganese oxide previously anodically deposited on SS at 1 V/SCE from a 2 M solution of…”

“…Supercapacitors or electrochemical capacitors take advantage of (1) the charge stored in the electrochemical double layer and (2) the charge stored due to Faradic processes [1][2][3][4] that can provide high-specific capacitance around 1,000 F g −1 .…”

“…These types of capacitors are currently under extensive investigation because of both high power and high energy characteristics [1][2][3][4]. Hydrous RuO 2 providing capacity of around 720 F g −1 in aqueous acidic electrolyte is the best system so far realized [5,6] where charge/ discharge is accompanied by the release/uptake of proton to/from the electrolyte [7].…”