Vol. 43 No. z SCIENCE IN CHINA (Series E) April 2OOO Development of supercapacitors based on carbon nanotubes MA Renzhi (-~'=,~), WEI Bingqing (-,~,~,.7:~.), XU Cailu (~,#, ~ ~-~), LIANG Ji (~:;~,. ~) & WU Dehai (:r~,i';~:r~-) Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Correspondence should be addressed to Ma Renzhi (email: mrzdlc@ 263. net) Received August 9, 1999 Abstract Block-type electrodes made of carbon nanotubes were fabricated by different processes. The volumetric specific capacitance based on such electrodes reached 107 F/cm 3, which proves car- bon nanotubes to be ideal candidate materials for supercapacitors. The composite electrodes consisting of carbon nanotubes and RuO2 9 xH20 were developed by the deposition of RuO2 on the surface of car- bon nanotubes. Supercapacitors based on the composite electrodes show much higher specific capaci- tance than those based on pure carbon nanotube ones. A specific capacitance of 600 F/g can be achieved when the weight percent of RuO2" xHzO in the composite electrodes reaches 75%. In addi- tion, supercapacitors based on the composite electrodes show both high energy density and high power density characteristics. Keywords: carbon, nanotube, Ru02, supercapacitor. Supercapacitors refer to electrochemical capacitors based on high surface area activated car- bon or noble oxides such as RuO2, which have a large capacitance 20--200 times that of conven- tional ones [1-3] . They have been developed in two categories: (i) the so-called double layer ca- pacitor based on double layer capacitance of the carbon/electrolyte interface at high-area carbon materials and (ii) the so-called faradaic pseudocapacitance! based on reduction/oxidation (re- dox) in microporous transition metal hydrous oxides, e.g. RuO2. The pseudocapacitance is about 1--2 orders of magnitude higher than that of double layer capacitance. But double layer capaci- tors are suitable for high-power operation. As far as carbon electrodes are concerned, some pseu- docapacitance due to possible faradaic redox process at surface functional groups or RuO 2 adhered to the carbon surface can be obtained in addition to double layer capacitance. Supercapacitors with double-layer capacitance and pseudocapacitance exhibit both high energy and power densi- ties N'51 . Carbon nanotubes (CNTs) are predicted to have potential application advantages in the areas such as reinforcements in composite materials, catalyst and field emission for their novel hollow- core structure and nanometer dimensions [6-s~ . Furthermore, carbon nanotubes have high surface area and excellent electrical conductivity. They are considered as strong candidate materials for supercapacitors. Niu et al. [9] reported that a specific capacitance of 49--113 F/g or 39.2-- 90.4 F/cm 3 under different frequencies can be achieved based on the carbon nanotubes sheet-type electrodes with thickness of 25.4 ttm. Carbon nanotubes they used have uniform diameters of ~ 8 nm. The electrodes show a novel pore structure formed by entangled carbon nanotubes. So carbon nanotubes are more suitable for the fabrication of supercapacitors if full utilization of surface area and excellent power characteristics are considered. But there are some technical difficulties in the