Double layer capacitors made with activated carbon electrodes and filled with nonaqueous electrolytes at different salt concentrations have been studied. It was found that the performance of the capacitor is strongly dependent on the salt concentration in the electrolyte. For electrolytes with high salt concentrations, the maximum energy stored in the capacitor is limited by the capacitance of the electrode material. For electrolytes with low salt concentrations, the maximum operating voltage as well as the maximum energy decreases with decreasing salt concentration. It has been demonstrated from ac impedance measurements that the decay of the maximum energy is due to the depletion of free ions in the electrolyte. The maximum energy storage in double layer capacitors with electrolytes at different salt concentrations was measured and was found to agree with the theory developed previously. From the study of dc charge and discharge cycles at different constant current rates, it was found that the power performance of the capacitor is also strongly dependent on the salt concentration in the electrolyte.

Infroduction The mechanism for energy storage in double layer capacitors is based on the separation and accumulation of charge at the interface between the electronically conductive electrode and the ionically conductive electrolyte. The capacitance built at this interface is called the double layer capacitance. High surface area activated carbon is one of the best examples of a double layer capacitor mate-rial. The electrochemical processes and the theoretical maximum energy density for a double layer capacitor have been described previously.'Briefly, when a capacitor is charged, the ionic species are separated and accumulated at the surface of the electrodes. The quantity of ions removed from the bulk electrolyte equals the charge devel-oped on the electrode surface. Therefore, the maximum energy stored in a capacitor is not only limited by the capacitance of the capacitor based on the electrode material, C, and the operating voltage, V, but also by the quantity of free ions in the electrolyte. From the energy density's point view, the maximum energy density of a capacitor is dependent on the specific capacitance of the electrode, c5, the operating voltage, and the ion concentration of the electrolyte, c0.