Characteristics of the electric double-layer capacitors using organic electrolyte solutions containing different alkylammonium cations

Influences of quaternary ammonium salts in propylene carbonate (PC) solvent on the capacitor behavior of an activated carbon (AC) electrode have been investigated. The charge/discharge profiles of three types of AC/AC full cells, symmetric two-electrode, asymmetric two-electrode, and symmetric three-electrode cells, were measured to evaluate the specific capacitance and the potential variation at each (positive and negative) electrode separately. As a result, the cell performance of an AC/AC symmetric full cell is mainly controlled by the electrochemistry of the negative electrode, whose specific capacitance and rate capability depends on the cationic species of the salt. A cation having symmetric structure, tetraethyammonium (TEA+), gives lower specific capacitance than the others, triethylmethylammonium (TEMA+) and 1,1-ethylmethylpyrrolidinium (EMP+), while it gives wide operating voltage based on its electrochemical stability. A spiro-type cation, EMP+, shows higher specific capacitance and better rate capability than the others, but tends to decompose cathodically under high-voltage charging.