Introduction:

Matrix of vertically aligned CNTs [R.H.Baughman et al., Science, 297, 787-792, 2002]

Introduction Ultracapacitors or double layer capacitors (DLCs) are energy storage devices whose operation is based on the double layer effect. By utilizing highly porous carbon material with a surface area up to 2000m2/g as electrodes (as in Fig. 3) commercial DLCs can achieve a energy density (6Wh/kg) much greater than the energy density of a conventional capacitor. However, this figure is much lower than the energy density reached by Lithium-Ion batteries (120Wh/kg). Our analysis shows that the utilization of a matrix of vertically aligned CNTs as electrode structure, can lead to an ultracapacitor characterized by a power density greater than 100kW/kg

(three orders of magnitude higher than batteries), a lifetime longer than 300,000 cycles, and an energy density higher than 60Wh/kg.

Double Layer Principle:

The significant energy density improvement of DLCs over other types of capacitors arises from the higher specific capacitance achieved with DLCs that can be up to 180 F/g. This result can be explained by the double layer principle discovered by Helmholtz in 1853. According to the Helmholtz model, when two electrodes, between which a potential is established, are immersed in an ionic solution, ions from the electrolyte migrate to the interface between the oppositely charged electrode and the solution.

Nanotube Enhanced Ultracapacitor:

A matrix of vertically aligned carbon nanotube (CNT) has been investigated as a DLC electrode. Our analysis shows that this configuration can provide a combination of high power density (more than four orders of magnitude greater than fuel cells) and energy density (comparable to Li-Ion batteries). The significant enhancement in the achievable DLC power density derives from the high conductivity obtainable with CNTs, which in the limit of a few microns in length present ballistic conduction. The energy density improvement of a “nanotube enhanced electrode” is due to the higher effective surface area obtainable with a structure based on vertically aligned nanotubes over activated carbon.

SEM of a SWNT surrounded by catalyst particle and impurities on the surface of the Si substrate.

Recently, we have been able to grow straight single wall nanotubes (SWNT) with diameters varying from 0.7 to 2nm and a length of several tens of microns. We grew SWNTs via thermal chemical vapor deposition (CVD) on a silicon substrate coated with a catalyst consisting of nanocolloids of aluminum oxide (AlO2) coated with iron nitrogen oxide (Fe(NO3)3). The average diameter of the catalyst seeds was 3nm. The substrate coated with catalyst was processed at 900°C at atmospheric pressure by CVD in an environment saturated with hydrogen (H2) and argon (Ar). As stockfeed gas we used methane (CH4).

Single wall nanotubes grew from the Fe(NO3)3 seeds via decomposition of methane at the catalyst interface.