Activated carbon is still the most attractive material used as a component of electrochemical capacitor irrespective of electrolyte. Actually organic medium (volatile, toxic) is generally used. However, aqueous electrolytic solutions start to be more and more interesting despite a low thermodynamic stability range (1.23V). Performance of electrochemical capacitors operating in aqueous electrolyte can be significantly improved by modification of electrode materials as well as by application of redox reactions from electrolytes. Different types of aqueous electrolytes with a redox activity based on halides (iodides, bromides) and pseudohalides have been used for capacitance enhancement. For the voltage extension, combinations of electrode/electrolyte interfaces with various pH and different composition for both negative and positive electrodes were proposed, e.g. magnesium nitrate and potassium iodide. Electrochemical capacitor with a high operating voltage (1.8V) was successfully built, especially if separation of electrolytes was realized. The detailed electrical examination of such capacitor (galvanostatic charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy, floating, self-discharge etc.) confirmed a good cycling, perfect charge dynamics as well as beneficial energy and power values. In-situ techniques (Raman spectroscopy, Mass spectrometry) allow the safe operating voltage of capacitor to be determined. Another strategy to enhance capacitor performance is based on the electrodes modification by ammonia treatment. In this case pH gradient between positive and negative electrode interfaces allows a beneficial voltage extension. High energy and power values as well as stable cycling of capacitor have been confirmed. Finally, the capacitor characteristics (Ragone plot) obtained in aqueous electrolytes are comparable to the parameters achieved in organic medium.