Double-layer capacitors energy density

ECs are another promising electrical energy storage device with a higher energy density than electrical capacitors, and a better rate capability and cycling stability than LIBs [32]. Carbon-based electric double layer capacitors and metal oxide- or polymer-based pseudocapacitors are two main types of ECs. The charge-... 

Highly porous carbons can serve as electrodes in -> super (EDL = electric double layer) capacitors. Their very wide electrochemical window allows their use in nonaqueous (relative) high energy-high power density super (EDL) capacitors. 

The double-layer capacitor is one of the electrochemical capacitors showing intermediate performances between conventional capacitors and rechargeable batteries from the viewpoint of energy and power densities. Although the terms supercapacitor and ultracapacitor are often used for double-layer capacitors, in a sense that they have higher capacitance than conventional capacitors (ceramic, film, aluminum electrolytic, or tantalum electrolytic capacitors), these terms are not to be used because they are the trademarks of certain companies products. 

Based on these properties, the double-layer capacitor comprising from a pair of the activated carbon electrodes and the nonaqueous liquid electrolyte is the most favorable one from the viewpoint of energy density. Ionic liquids are a kind of nonaqueous liquid electrolytes. 

However, in double layer capacitors this equation does not consider the changes of electrolyte concentration during the charge and discharge cycles. Therefore, the maximum energy density should be expressed as in Eq. (5) [21,22] ... 

Electrodes of various types are used in hybrid (asymmetric) supercapacitors (HSCs). For example, one of the electrodes is highly dispersed carbon, that is, a double-layer electrode, and the other electrode is a battery one or one of the electrodes is carbon and the other one is a pseudocapacitor, for example, based on electron-conducting polymer (ECP). The main advantage of HSCs as compared EDLCs is an increase in energy density because of the wider potential window. The main fault of HSCs, meanwhile, as compared to electric double-layer capacitors (EDLCs), is a decrease in cyclability following the limitations posed by the nondouble-layer electrode. 

Zheng, J. R 2003. The limitations of energy density of battery/double-layer capacitor asymmetric cells. Journal of the Electrochemical Society 150 A484-A492. 

Xing, L. D., J. Vatamanu, G. D. Smith, and D. Bedrov. 2012. Nanopatteming of electrode surfaces as a potential route to improve the energy density of electric double-layer capacitors Insight from molecular simulations. Journal of Physical Chemistry Letters 3 1124-1129. 

Fan, L. Q., J. Zhong, J. H. Wu, J. M. Lin, and Y. F. Huang. 2014. Improving the energy density of quasi-solid-state electric double-layer capacitors by introducing redox additives into gel polymer electrolytes. Journal of Materials Chemistry A 2 9011-9014. 

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