Project Title:

Carbon Nanotube Enhanced Ultracapacitors (poster)

Investigators:

John Kassakian, Joel Schindall, Riccardo Signorelli

Introduction:

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.

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

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.

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.

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

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