EDLCs store energy in much the same way as a traditional capacitor, namely, by means of charge separation. The main difference is related to the higher capacitance values provided by EDLCs; achieved through the utilization of high-surface-area porous materials (usually AC) in contrast to the two-dimensional planar plates typically found in conventional capacitors.
EDLCs can store substantially more energy than a conventional capacitor (by several orders of magnitude) due to the following:
1) The increased amount of charge that can be stored on a highly extended electrode surface area (created by a large number of pores within a high surface-area electrode material)
2) The small thickness of the so-called electrical double layer at the interphase between an electrode and the electrolyte.
EDLC supercapacitor construction is similar to a battery in that there are two electrodes immersedin an electrolyte, with an ion-permeable separator located between the electrodes to prevent electrical contact (Figure a). In the charged state, the electrolyte anions and cations move toward the positive and negative electrodes, respectively, giving rise to two double layers, one at each electrode–electrolyte interface. The separation of ions also results in a potential difference across the cell (Figure b).
As each electrode–electrolyte interface represents a capacitor, the complete cell can be considered as two capacitors connected in series.
The types of electrolytes that can be used in EDLCs can be classified into three broad groups: (i) aqueous, (ii) salts dissolved in organic solvents, and (iii)ionic liquids (ILs). The advantages and disadvantages of each electrolyte system are summarized in Table 2.2. While early EDLCs were aqueous based, there has been a trend toward organic electrolytes in order to achieve higher operational voltages and therefore greater specific energy. Accordingly, another advantage of achieving high cell voltages is that the number of cells required to obtain a high-voltage module (by connecting cells in series) is reduced. This will also partially offset the higher cost of organic cells, reduce the burden on voltage balancing circuits, and improve reliability.
EDLCs take advantage of the numerous and frequently cited properties of carbon materials that include good chemical stability, good electrical conductivity, availability, and low-moderate cost. Carbon materials have long been incorporated into the electrodes of energy storage devices primarily as electroconductive additives, supports for active materials, electron transfer catalysts, intercalation hosts, for current leads, heat transfer, porosity control, surface area, and capacitance . The ultimate performance of carbon-based supercapacitors in EDLCs will be closely linked to the physical and chemical characteristics of the carbon electrodes. There are an enormous number of carbon materials produced from a variety of carbonization and activation procedures and materials ranging from conventional ACs to the more sophisticated carbon nanotubes (CNTs) that have been evaluated as electrode materials for EDLCs (Table 2.3).
—– Quote from：《Supercapacitors: Material, System, and Applications》
Energetic Performance and Discharging at Constant Power
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