Active electrode material, such as fibrillized blend of activated carbon, polymer, and conductive carbon, is pretreated by immersion in a sealing coating. After the active electrode material is dried, the coating seals micropores of the activated carbon or another porous material, thus preventing exposure of water molecules or other impurities trapped in the micropores to outside agents. At the same time, the sealing coating does not seal most mesapores of the porous material, allowing exposure of the mesapores' surface area to the outside agents. The pretreated active electrode material is used for making electrodes or electrode assemblies of electrical energy storage devices. For example, the electrodes may be immersed in an electrolyte to construct electrochemical double layer capacitors. Pretreatment with the sealing coating reduces the number of water molecules interacting with the electrolyte, enhancing the breakdown voltage of the capacitors.