Active materials used for the ECDL capacitors have todevelop high specific capacitance per surface area and pervolume. Various electrodes based on CNT and a-C werestudied. An increase of the specific capacitance per surfacearea was achieved by increasing the active layer thicknessand particularly the mass density. The ECDL capacitors using CNT reached a specific capacitance of 0.8–280 mF cm−2and 1.1–16 F cm−3, respectively. As the CNT films producedby CVD have a maximum thickness of 30 m, the electrodesA do not make efficient ECDL capacitors