## 3 Results and Discussion  
Some synthesized N-heterocyclic chemical salts (i.e., [DI]\({}^{+}\)[Br]\({}^{-}\), [DMP]\({}^{+}\)[Br]\({}^{-}\), and [BMTA]\({}^{+}\)[I]\({}^{-}\) of the aromatic selection, and [MPy]\({}^{+}\)[I]\({}^{-}\), [DMpy]\({}^{+}\)[I]\({}^{-}\), and [E-TEDA]\({}^{+}\)[Br]\({}^{-}\) of the aliphatic selection) could not be used as electrolyte supporters under the charge-discharge process in a non-aqueous electrochemical capacitor because the [Br]\({}^{-}\) and [I]\({}^{-}\), as counter anions, could behave as Lewis bases and react with the vanadium cation. Furthermore, a self-redox reaction of the [Br]\({}^{-}\) and [I]\({}^{-}\) could occur under the charge-discharge process as shown in reactions (1) and (2); therefore, this study did not use them as electrolyte supporters in the organic electrochemical capacitor. \[3\text{Br}^{-}\overset{+2\text{e}}{\leftrightarrow}\text{Br}_{2}+\text{Br}^{-} \rightleftharpoons\text{Br}_{3}^{-} \tag{1}\]  
\[3\text{I}^{-}\overset{+2\text{e}}{\leftrightarrow}\text{I}_{2}+\text{I}^{-} \rightleftharpoons\text{I}_{3}^{-} \tag{2}\]  
Figure 3: Chemical structure of the commercial anion-exchange membrane (Neosepta\({}^{\text{\text{\textregistered}}}\) AFN) and images of the single cell for electrochemical capacitor performance. We evaluated the oxidation (O) and reduction (R) peaks of V(acac)\({}_{3}\) under acetonitrile with the aromatic and aliphatic N-heterocyclic salts to determine the electrochemical potential of V(acac)\({}_{3}\). The CVs of the V(acac)\({}_{3}\) under acetonitrile with aromatic N-heterocyclic salts ([MPy]\({}^{+}\)[BF4]\({}^{-}\), [MPy]\({}^{+}\)[PF6]\({}^{-}\), [DI]\({}^{+}\)[BF4]\({}^{-}\), [DI]\({}^{+}\)[PF6]\({}^{-}\), [BMTA]\({}^{+}\)[BF4]\({}^{-}\), and [BMTA]\({}^{+}\)[PF6]\({}^{-}\)) were obtained on the glassy carbon working electrode with a scan rate of 100 mV/s, as shown in Figure 4. The redox couple's voltage ranges were observed to be \(-2.5\) to 1.0 V, depending on the electrolytes (i.e., salts), and the current peaks were observed according to reactions (3) and (4) below:  
\[\mathrm{R}_{1}/\mathrm{O}_{1}:\mathrm{V(III)(acac)}_{3}+\mathrm{e}^{-}\rightleftharpoons \mathrm{V(II)(acac)}_{3}^{-} \tag{3}\]  
\[\mathrm{R}_{2}/\mathrm{O}_{2}:\mathrm{V(III)(acac)}_{3}\rightleftharpoons \mathrm{V(IV)(acac)}_{3}^{+}+\mathrm{e}^{-} \tag{4}\]  
The electrochemical potential of active species V(acac)\({}_{3}\) was determined to be about 2.2 V for each aromatic N-heterocyclic salt electrolyte (i.e., [MPy]\({}^{+}\)[BF4]\({}^{-}\), [MPy]\({}^{+}\)[PF6]\({}^{-}\), [BMTA]\({}^{+}\)[BF4]\({}^{-}\), [BMTA]\({}^{+}\)[BF4]\({}^{-}\), [DI]\({}^{+}\)[BF4]\({}^{-}\), and [DI]\({}^{+}\)[PF6]\({}^{-}\)). This result means that the energy storage capacity of the V(acac)\({}_{3}\) can achieve a maximum of 2.2 V for each supporting electrolyte. However, the CVs of the V(acac)\({}_{3}\) under acetonitrile with aliphatic N-heterocyclic salts (i.e., [DMP]\({}^{+}\)[BF4]\({}^{-}\), [DMP]\({}^{+}\)[PF6]\({}^{-}\), [DMPy]\({}^{+}\)[BF4]\({}^{-}\), [DMPy]\({}^{+}\)[PF6]\({}^{-}\), [E-TEDA]\({}^{+}\)[BF4]\({}^{-}\), and [E-TEDA]\({}^{+}\)[PF6]\({}^{-}\)) did not obtain because the aliphatic N-heterocyclic salts could degrade during the redox process. Figures 5 and 6 show the electrochemical capacitor performance for 0.01 M V(acac)\({}_{3}\) under acetonitrile electrolytes in the presence of 0.1 M aromatic and aliphatic N-heterocyclic salts. The experimental condition used was as follows: voltage = 2.5 V, \(\mathrm{I}_{\mathrm{c}}\) = 0.1 mA, and \(\mathrm{I}_{\mathrm{d}}\) = \(-0.1\) mA, and was applied using a commercial Neosepta-AFN membrane in a flow cell (see Figure 3). Regarding charge, the counter anions were transferred through the anion-exchange membrane from the reduction cell to the oxidation cell, while the electrolyte cations and the charged V(acac)\({}_{3}\)\({}^{+}\) could not be transferred through the anion-exchange membrane because of the Donnan exclusion effect [16].