(633c) Ab Initio Molecular Dynamics of Molecular Ions Contacting Carbon Nanotubes

Ab Initio Molecular Dynamics of Molecular Ions Contacting Carbon
Nanotubes

W. Zhang, G. G.
Hoffman and L. R. Pratt

Department of
Chemical & Biomolecular Engineering,

Tulane University,
New Orleans, LA 70118

Recently proposed electric
double-layer capacitors with electrodes of aligned, single-wall carbon
nanotubes can have capacitances exceeding conventional EDLCs.¹ They
offer possibilities for deliberate nanoscale mechanical and chemical design.
The natural goal for such capacitors would be to achieve energy densities
comparable to those of current battery systems, but also to retain the
traditional advantages of capacitors, namely faster response and longer cycle
lifetimes.²

An important start in
developing a molecular understanding of EDLCs based on nanotubes forests has
been made on the basis of classical molecular simulations.²
Nevertheless, some characteristics of particular interest are intrinsically
quantum mechanical in nature; chemical damage and quantum (or more properly
intrinsic) capacitances³ are examples of such characteristics. Ab initio molecular dynamics (AIMD) are
thus an essential step in the development of a molecular understanding of these
systems.

Here we present electronic
structure results, and AIMD simulation, on non-neutral systems of carbon
nanotubes (CNTs) and molecular ions, including the tetra-methyl ammonium ion. Traditional
electronic structure calculations show that CNTs prefer negatively charged
states. In contrast, AIMD (utilizing the CPMD package) shows substantial charge
transfer to a tetra-methylammonium cation in contact with a carbon nanotube. The
system studied (below) includes 80-carbon-atom nanotube and one
tetra-methylammonium cation, with periodic boundary conditions in all three
spatial dimensions, and utilizes the PBE-D electron density functional model, a
norm-conserving pseudo-potential (NCPP), and a plane-wave basis cut-off of 85
Ry. The charges on the ion (right below) for nine different configurations,
chosen from a high temperature AIMD trajectory, depend substantially with the
distance from the CNT, indicating charge transfer between the ion and CNT.

We further investigate the
interactions involving other ions essential for modeling these systems,
including tetraethylammonium (cation), tetrafluoroborate (anion), singly and in
combination.  Finally, we will
consider non-neutral charge states for the CNT electrode material.

[1] K. Hata, D. N. Futaba,
K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306, 1362-1364(2004).

[2] L. Yang, B. H. Fishbine,
A. Migliori, and L. R. Pratt, J. Am. Chem. Soc. 131, 12373-12376 (2009).

[3] G. G. Hoffman and L. R.
Pratt, Proc. LA EPSCoR RII LA-SiGMA 2011 Symp., p 101-104. (2011).