Model-based design of low-temperature carbon nanotube synthesis via catalytic oxidation for supercapacitor application.

Novel electrochemical double layer capacitors with carbon nanotube (CNT) electrode, often referred to as supercapacitors, have a potential to bridge a power and energy gap between traditional dielectric capacitors and chemical batteries. However, their future is uncertain because current fabrication technologies involve difficult-to-control post-growth manipulations of CNTs. This paper addresses this problem by introducing model-based design of low-temperature CNT synthesis that is suitable for in-situ fabrication of CNT-based supercapacitor electrode. The insight to the surface kinetics during low-temperature CNT synthesis via catalytic oxidation was obtained via coupled Molecular Dynamics and Quantum Semiempirical Hamiltonian simulations. It was determined that the presence of oxygen on the surface of catalyst increases, by several times, the time necessary for the decomposition of hydrocarbons as well as shifts the reaction zone from the surface of catalyst to the catalyst underlayer. Theoretical trends were confirmed by CNT growth experiments. A contact between conducting CNTs and zinc oxide binding layer was analyzed in detail since its properties strongly affect the performance of CNT electrode. It was demonstrated that the formed CNT-zinc oxide interface was free from unbonded oxygen atoms and/or clusters of zinc atoms and was weakly affected by defects in CNTs.

Direct growth of individual carbon nanotubes on a flexible plastic substrate via selective heating.

The deficiency of carbon nanotube (CNT) growth on temperature-sensitive substrates such as plastic tapes impedes the further progress in CNT electronics. We report a novel method that is able to offer the solution by selectively delivering energy via high radio frequency heating to carbon atoms for their catalytic assembly into individual CNTs. Direct growth of CNTs on a flexible kapton substrate was demonstrated. A mechanism of selective heating was discussed.

Multiscale modeling catalytic decomposition of hydrocarbons during carbon nanotube growth.

A molecular dynamics simulator coupled to a quantum semiempirical Hamiltonian model was applied to multiscale modeling of the catalytic decomposition of hydrocarbons during carbon nanotube (CNT) and carbon nanofiber (CNF) growth. It was found that catalytic decomposition of acetylene is accompanied by a large energy release and its rate weakly depends on temperature in the range from 20 to 700 degrees C. In contrast, the methane decomposition rate substantially decreases as the iron temperature drops. A comparative analysis of acetylene decomposition on a clean surface and on an oxidized Fe(100) surface showed that the presence of oxygen reduces the decomposition rate by an order of magnitude, but has very little influence on the amount of heat released by the reaction. We also found that oxygen absorbed on the surface of catalyst does not easily diffuse into the catalyst or desorb from the surface. This implies that the surface of the catalyst is quickly covered by oxygen during CNT/CNF growth even at low oxygen flow rates.

Nonhydrodynamic aspects of electron transport near a boundary: the Milne problem.

The nonhydrodynamic behavior of electrons near a boundary is studied with the Milne problem of transport theory. A system of electrons dilutely dispersed in a heat bath of atomic moderators is considered in the positive one-dimensional spatial half-space with an absorbing boundary at the origin which mimics an electrode. A flux of electrons is assumed to originate at an infinite distance from the boundary. The Fokker-Planck equation for the electron distribution function in space and velocity is considered. The density and temperature profiles are determined, and the departure from hydrodynamic behavior near the boundary is studied. Argon and helium are chosen as the moderators, and results with different cross sections are obtained. The Fokker-Planck equation is solved with an expansion in Legendre and Speed polynomials, and compared wherever possible with results obtained with a Monte Carlo simulation. The behavior near the boundary is shown to be strongly influenced by the Ramsauer-Townsend minimum in the electron-Ar momentum transfer cross section.

Model of space- and energy-dependent electron flux density.

Analytic representation for the space- and energy-dependent electron flux density in terms of a phenomenological model was developed. The model was applied to electron energy degradation in gaseous argon. Flux density data were also generated from Monte Carlo simulations for 0.3-3.0 keV incident electrons. These data were used to determine adjustable parameters in our phenomenological model. From the nature of the cross section input to this model, we expect that the scaled flux density for most atomic and molecular gases will be similar to that obtained in this study.

Space-dependent aspects of electron degradation.

Space- and energy-dependent flux densities were computed by a Monte Carlo method of electron degradation for 0.1-3.0 keV incident electrons in argon. These flux densities contain the basic information about the electron degradation process and can be used to calculate a yield for any inelastic state at any spatial position. Numerical results were analytically represented in terms of the recently introduced phenomenological model. It was found that flux densities multiplied by an absorption cross section in argon are approximately the same as flux densities multiplied by an absorption cross section in nitrogen. This feature provides a basis to use an analytical approximation of flux densities from this study for any gas.