Supercapacitors have occupy an indispensable role in today's energy storage systems due to their high power density and long life. The introduction of car- bon nanotube (CNT) forests as electrode offers the possibility of nano-scale design and high capacitance. We have performed molecular dynamics simulations on a CNT forest-based electrochemical double-layer capacitor (EDLC) and a widely used electrolyte solution (tetra-ethylammonium tetra-fluoroborate in propylene carbonate, TEABF4 /PC)
We compare corresponding primitive model and atomically detailed model of TEABF4 /P, emphasizing the significance of ion clustering in electrolytes. The molecular dynamic simulation results suggests that the arrangement of closest neigh- bors leads to the formation of cation-anion chains or rings. Fuoss's discussion of ion-pairing model provides the approximation for a primitive model of 1-1 electrolyte is not broadly satisfactory for both primitive and atomically detailed cases. A more general Poisson statistical assumption is shown to be satisfactory when coordina- tion numbers are low, as is likely to be the case when ion-pairing initiates. We examined the Poisson-based model over a range of concentrations for both models of TEABF4 /P, and the atomically detailed model results identified solvent-separated nearest-neighbor ion-pairs.
Large surface areas plays an essential role in nanomaterial properties, which calls for an accurate description of interfaces through modeling. We studied propylene carbonate, a widely used solvent in EDLC systems. PC wets graphite with a contact angle of 31°. The MD simulation model reproduced this contact angle after reduction 40% of the strength of graphite-C atom Lennard-Jones interactions with the solvent. The critical temperature of PC was accurately evaluated by extrapolating the PC liquid-vapor surface tensions. PC molecules tend to lie flat on the PC liquid-vapor surface, and project the propyl carbon toward the vapor phase. Liquid PC simulations also provide basic data for construction of accurate information to assist in device- level modeling of EDLCs.
The most serious uncertainty with previous simulations of CNT based EDLCs was definition of the actual composition of the pores. Therefore, direct simulations of filling of CNT forest based electrochemical double-layer capacitors with TEABF4 /P solution was performed. Those calculation characterize the charging and discharg- ing process, including rates of charging responses, the possibility of bubble forma- tion, and kinetic properties with confinement. The mobilities of ions and solvent was investigated through mean square displacement (MSD) and velocity autocorrela- tion functions (VACF). The memory kernel was extracted from VACF by discretized linear-equation solving and a specialized Fourier transform method, results implies the existence of dielectric friction.
With the interest in chemical features of EDLCs, a multi-scale theory was de- veloped to embed high resolution ab initio molecular dynamics (AIMD) methods into studies of EDLCs. This theory was based on McMillan-Mayer theory, potential dis- tribution approach, and quasi-chemical theory. The quasi-chemical theory allow us to break-up the free energies into packing, outer-shell and chemical contributions, where the last part can be done by AIMD directly. For the primitive model of TEABF4 /P, Gaussian statistical models are effective for these outer-shell contributions. And the Gaussian approximation is more efficient than the Bennett method in achieving an accurate mean activity coefficient.
|Advisor:||Pratt, Lawrence R.|
|Commitee:||John, Vijay, Pesika, Noshir, Rick, Steven|
|School:||Tulane University School of Science and Engineering|
|Department:||Chemical and Biomolecular Engineering|
|School Location:||United States -- Louisiana|
|Source:||DAI-B 76/06(E), Dissertation Abstracts International|
|Subjects:||Physical chemistry, Chemical engineering, Nanotechnology, Materials science|
|Keywords:||Carbon nanotube, Energy storage, Simulation model, Supercapacitors|
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