The Limitations of Energy Density for Electrochemical Capacitors

Abstract

Electrochemical capacitors can be divided into two types depending on whether the salt concentration in the electrolyte changes during charging and discharging. In the first type of capacitor, such as double-layer capacitors, the salt concentration in the electrolyte reduces during the charging of the capacitor. The maximum energy density of this type of capacitor will depend not only on the specific capacitance and the operating voltage, but also on the salt concentration of the electrolyte. In this paper, a formula describing the dependence of energy density on specific capacitance, operating voltage, and salt concentration is given based on the optimized weight (or volume) ratio of the electrode material and the electrolyte. It shows that f or electrochemical capacitors using nonaqueous electrolytes, the maximum energy density of the capacitor will be limited mainly by the low salt concentrations of the electrolyte. The relationship between the energy density and the mass density of the electrode is also given. The optimum mass density of the electrode can be obtained based on the value of the theoretical energy density for capacitors with different electrolytes. In the second type of capacitor, such as pseudocapacitors with metal oxide electrodes, the salt concentration in the electrolyte remains constant during charging and discharging. The maximum energy density of this type of capacitor will be limited mainly by specific capacitance and operating voltage.

Carbon Nanotube Supercapacitors

“…In addition, electrolyte depletion is another limitation of the currently used electrolytes. Upon charge, ions of the electrolyte are transported into the double layers at the electrode/electrolyte interfaces, resulting in the decrease of salt concentration in the electrolyte (the so-called electrolyte depletion) and hence the limit of energy density of the capacitor (Zheng et al, 1997). Also, this electrolyte depletion increases the cell resistance and Typically, ionic liquids consist of nitrogen (or phosphorus)-containing organic cations and inorganic anions.…”

Ideally Pseudocapacitive Behavior of Amorphous Hydrous Cobalt-Nickel Oxide Prepared by Anodic Deposition

“…Because transition metal oxides ͑e.g., oxides of Co, Ni, Mn, Mo, V, Cr, W, Re, etc.͒ and conducting polymers have several oxidation states or structures that lead to redox transitions within the potential region of water decomposition, 1,3,7-13 energy ͑i.e., pseudocapacitance͒ can be stored within the reversible redox transitions of these electroactive materials. Because the performance of an electrochemical ͑EC͒ supercapacitor is mainly determined by the electrochemical characteristics of the superficial electroactive species, 1,[3][4][5][6] the electrochemical reversibility as well as the pulse power property of these potential candidates should be examined systematically.Hydrous cobalt and nickel oxides prepared by the sol-gel or electrochemical techniques have been proposed to be suitable materials for the application of EC supercapacitors.10,11 However, the sol-gel technique is relatively complicated and the electrochemical properties as well as electric conductivity of the electroactive materials may be affected by the introduction of binders ͑e.g., polyvinyldifluoride, PVDF͒ during the electrode preparation process. Accordingly, an alternative preparation method, which can directly coat hydrous oxides with high reversibility and pulse power density in the potential window of water decomposition, is worth seeking.…”

“…1,3,4 The former devices are also called double-layer capacitors because of the storage of charge within the electrical double layer at the electrodeelectrolyte interface. On the other hand, the latter are generally called pseudocapacitors since faradaic processes at/within the electroactive materials exhibit capacitive-like responses.…”

Ionic Liquids for the Electric Double Layer Capacitor Applications

“…Generally, an organic electrolyte that is a solid quaternary ammonium salt, such as N,N,N,N-tetraethylammonium tetrafuluoroborate (TEA-BF 4 ), dissolved in the high dielectric constant solvent propylene carbonate (PC) has been used for high voltage EDLCs of 2V or more. This device stores electricity physically, and lacks the chemical reactions found in rechargeable batteries during charging and discharging (Zheng et al, 1997). Therefore, compared to rechargeable batteries, the EDLC has a remarkably long cycle life and high power density.…”