Modeling of a carbon nanotube ultracapacitor

Abstract

The modeling of carbon nanotube ultracapacitor (CNU) performance based on the simulation of electrolyte ion motion between the cathode and the anode is described. Using a molecular dynamics (MD) approach, the equilibrium positions of the electrode charges interacting through the Coulomb potential are determined, which in turn yield the equipotential surface and electric field associated with the capacitor. With an applied ac voltage, the current is computed based on the nanotube and electrolyte particle distribution and interaction, resulting in the frequency-dependent impedance Z(ω). From the current and impedance profiles, the Nyquist and cyclic voltammetry (CV) plots are then extracted. The results of these calculations compare well with existing experimental data. A lumped-element equivalent circuit for the CNU is proposed and the impedance computed from this circuit correlates well with the simulated and measured impedances.

Optimization of carbon nanotube ultracapacitor for cell design

“…Using the method reported previously, 1 we solve for the electric field between the nanostructured metallic electrodes. The method is essentially a Poisson equation solver using the MD based techniques with a boundary condition defined at the electrode surface, whose potential takes designated values consistent with the CNU device voltage input.…”

“…The study of ultracapacitors (UC) or supercapacitors has been the subject of intense research in recent years. [1][2][3] Increasing the total capacitor electrode surface area S to maximize the device stored energy has always been a prohibitive challenge. The use of nanomaterials as capacitor electrodes has been studied recently, [4][5][6][7][8][9][10][11][12][13][14] as many of these materials are low-dimensional and, when properly integrated onto a metallic electrode substrate, S increases significantly.…”

“…This is critical in understanding UC response to an AC voltage input, which is known as the device Nyquist characteristics. 1 We examine these electrode non-uniformities in detail using examples of carbon nanotube ultracapacitors (CNUs) where long and dense CNT arrays protrude from conducting surfaces. For such a study, it is not appropriate to use the homogeneous neutral jellium model for the electrolyte which predicts a smooth DLC formation.…”

“…Thus, the existing UC models are not adequate for our study. To mitigate the scarcity of such approach, we have recently developed a particle-based physical model, using molecular dynamics (MD), 1,37,38 which yielded results that compared well with the existing experimental data. 39 Such a model enables us to calculate the electric field between the electrodes, keep track of the electrolyte ionic motion in the capacitor, and evaluate the device parameters and performance under an arbitrary set of geometrical parameters, electrolyte properties, and voltage excitation.…”

“…In this article, following the development of our MD algorithm in Ref. 1, we apply this model to investigate the relationship between CNU capacitance and electrode geometry. In particular, we examine the dependence of CNU linear capacitance density CD L (in farad per unit electrode width) on electrode-to-electrode separation L y , CNT length L CNT , and CNT linear packing density D CNT (ratio of CNT diameter d to CNU cell width W C ), and compare our simulated results with available experimental data.…”