Symmetric capacitors

Figure 3. Galvanostatic discharge curves (three-electrode cell) at 2 mA ofaPPy/CNTs pellet electrode (m=6.7 mg) before (2) and after (1) galvanostatic cycling in a symmetric capacitor (two electrode cell) at U=0.8 V. After the discharge (1), the electrode was charged up to 0.2 V (curve 3) and then discharged (curve 4).

ECP based electrode of a symmetric capacitor would not occur when a-Mn02 will be used as a positive electrode in an asymmetric configuration. 

Figure 29. Depiction of the charging process of a symmetric capacitor.

As the capacity of the two electrodes is different, even in a symmetric capacitor, Equation 8.1 indicates that the value of C is determined by the electrode with the smallest capacity value. Moreover, the later electrode operates in a larger potential window than the other one, which consequently reduces the voltage range of the device. This drawback can be circumvented by balancing the respective masses of the electrodes or by using, for each electrode, different materials working in their optimal potential range. 

FIGURE8.27 Impedance spectra of a symmetric capacitor based on PPy/CNT composite electrodes (20 wt% of CNT) in 1 mol L 1 H2S04, before (a) and after (b) performing 500 galvanostatic charge-discharge cycles at a maximum voltage of 0.8 V. (From Khomenko, V., et al., Appl. Phys. A, 82, 567, 2006. With permission.)... 

As it was already described in Section 8.2.1, the total capacitance of a supercapacitor is given by Equation 8.1. In a symmetric capacitor with equal values of capacitance for the positive (Cj) and negative (f2) electrodes, the total capacitance of the system is half the capacitance of one electrode. In the asymmetric device, as the capacitance of the battery electrode is much higher than the capacitive one, the capacitance of the system approaches that of the electrode with the smallest value. In other words, the capacitance of the asymmetric configuration combining a battery-like electrode with a capacitive one will be close to the value of the capacitive electrode, i.e., twice larger than that for a symmetric configuration with two capacitive electrodes. 

FIGURE 8.40 Cyclic voltammograms of an optimized asymmetric AClox/AC2ox capacitor in lmol L-1 H2S04 electrolyte in comparison with the symmetric capacitors AClOI/AClOI and AC2ox/AC2ox. (Adapted from Khomenko, V., et al., J. Power Sources, 2008.)... 

FIG U RE 2.15 CVs at 5 mV s of symmetric capacitor based on the (a) Glu-(NH4)3P04-800 and (b) GIU-NH4CI-8OO in 6 M KOH. (Reprinted from Journal of Power Sources, 239, Wang, C. et al., Sustainable synthesis of phosphorus- and nitrogen-co-doped porous carbons with tunable surface properties for supercapacitors, 81-88, Copyright 2013, with permission from Elsevier.)... 

Abbas, Q., D. Pajak, E. Fr ckowiak, and F. Beguin. 2014. Effect of binder on the performance of carbon/carbon symmetric capacitors in salt aqueous electrolyte. Electrochimica Acta 140 132—138. 

Because of the different size of cations and anions in the electrolyte, the capacitance of the electrodes is generally different, even in a symmetric capacitor in that case, according to Equation 8.4, the capacitance of the capacitor is determined by the electrode with the smallest capacitance. 

Fig. 12.14 Cyclic voltammograms of LN600- and LN750-based symmetric capacitors in 0.5 mol L Na2S04 [60]...

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