Supercapacitors energy density

Fig. 9.29 shows a plot of power density versus energy density for supercapacitors in comparison with some conventional and advanced batteries where it may be seen that supercapackors typically operate in the very high power density range (i.e. 400-1000 W/kg) but with energy densities of only a few Wh/kg. This is confirmed by Table 9.2 which lists the characteristics of some prototypes presently under development. 

Fig. 9.29 Power and energy densities of double-layer and electrochemical supercapacitors in comparison wiLh typical rechargeable batteries and fuel cells...

The performance of electrochemical systems is compared in a Ragone plot (Figure 8.4). Supercapacitors have a higher power density than any battery by contrast, their energy density is much lower. The main research effort is now oriented to improving the energy. 

The capacitance of carbon electrodes in acidic and alkaline solutions is quite high, yet the narrow electrochemical window of aqueous solutions is a main disadvantage when used in supercapacitors. Hence, it is possible to increase the -> energy density of supercapacitors by enlarging their working potential window. For that, polar aprotic solutions can be suitable. Indeed, many apro-... 

In conclusion, supercapacitors are robust devices better capable of energy storage in electrical vehicle applications, backups, and highly reversible power sources for computer storage, than conventional capacitors. For the former application, supercapacitors can be a part of hybrid power sources, which also include high-energy-density batteries in which the supercapacitors provide the high power required. 

The porous electrode theory was developed by several authors for dc conditions [185-188], bnt the theory is usually applied in the ac regime [92,100,101,189-199], where mainly small signal frequency-resolved techniques are used, the best example of which are ac theory and impedance spectra representation, introdnced in the previons section. The porous theory was first described by de Levi [92], who assumed that the interfacial impedance is independent of the distance within the pores to obtain an analytical solution. Becanse the dc potential decreases as a fnnction of depth, this corresponds to the assnmption that the faradaic impedance is independent of potential or that the porons model may only be applied in the absence of dc cnrrent. In snch a context, the effect of the transport and reaction phenomena and the capacitance effects on the pores of nanostructured electrodes are equally important, i.e., the effects associated with the capacitance of the ionic donble layer at the electrode/electrolyte-solntion interface. For instance, with regard to energy storage devices, the desirable specifications for energy density and power density, etc., are related to capacitance effects. It is a known fact that energy density decreases as the power density increases. This is true for EDLC or supercapacitors as well as for secondary batteries and fnel cells, particnlarly due to the distributed nature of the pores... 

Miller, J., B. Duim, T. Tran, and R. Pekala, Deposition of mthenium nanoparticles on carbon aerogels for high energy density supercapacitor electrodes. J. Electrochem. Soc. 1997,144(12), L309. 

Fan, Z. et al.. Asymmetric supercapacitors based on graphene/MnOj and activated carbon nanofiber electrodes with high power and energy density. Adv. Funct. Mater. 2011,27(72 , 2366-2375. 

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