The present invention relates to methods of manufacturing an electrochemical energy storage device, such as a hybrid capacitor. The method comprises saturating a porous electrically conductive material in a solution comprising an organic solvent and a metal complex or a mixture of metal complexes; assembling a capacitor comprising the positive electrode made of porous electrically conductive material saturated with a metal complex, a negative electrode, and a separator in a casing; introducing the electrolyte solution into the casing; sealing the casing; and subsequent charge-discharge cycling of the capacitor. The charge-discharge cycling deposits a layer of an energy-accumulating redox polymer on the positive electrode. The electrolyte solution for filling the hybrid capacitor contains an organic solvent, a metal complex, and substances soluble to a concentration of no less than 0.01 mol/L and containing ions that are electrochemically inactive within the range of potentials between −3.0 V to +1.5 V.