Electric double layer capacitance of highly pure single-walled carbon nanotubes (HiPco™ Buckytubes™) in propylene carbonate electrolytes

The double layer capacitance of highly pure single-walled carbon nanotubes (SWCNTs) prepared by the HiPco(TM) process was measured in 1.0 mol dm(-3) LiClO4/propylene carbonate solution. The unpurified SWCNT electrode was mainly composed of a bundle structure of SWCNTs with around 1.0 nm tube diameter, small amount of amorphous carbons, and Fe catalyst particles. The Fe catalysts in the surface of the SWCNT were removed by immersion in HClaq. The alpha(s)-SPE analysis of the N-2 adsorption isotherms revealed that both the SWCNTs before and after the immersion in HClaq had relatively high specific surface areas of similar to500 m(2) g(-1) without microporosity although the tube ends were closed. The SWCNTs showed a gravimetric capacitance of around 45 F g(-1). Thus, the specific capacitance per unit surface area was estimated to be around 10 muF cm(-2), which was higher than that of conventional activated carbon fibers. Furthermore, the capacitance of the SWCNTs did not decrease even at high current density. This good rate property of the SWCNTs is related to the large area of the external surface (similar to 400 in 2 g(-1)) on which ion adsorption/desorption can proceed fast because of no ion sieving. On the other hand, most of the Fe catalyst in the SWCNT could be removed by thermal oxidation followed by immersion in HClaq. However, the gravimetric capacitance of this purified SWCNT was not as great as that expected by the correlation of 10 PF cm(-2). This is related to the formation of amorphous carbons caused by the thermal oxidation. (C) 2002 Elsevier Science B.V. All rights reserved.