Figure 2. The V–Q plots of the direct charging cycle for a commercial capacitor with charging voltage V_C increased from 9.2 V to 59.9 V.

For these plots, V_C are (a) 9.2 V, (b) 19.4 V, (c) 30.0 V, (d) 39.4 V, (e) 50.6 V and (f) 59.9 V. The inset of a shows the TENG structure. The shaded areas are the stored energy per cycle E_C,direct.