[Show abstract][Hide abstract] ABSTRACT: Dielectric characteristics of a molecular model of liquid propylene carbonate are evaluated for utilization in molecular scale simulation of electrochemical capacitors based on nanotube forests. The linear-response dielectric constant of the bulk liquid, and its temperature dependence, is in good agreement with experiment. Dielectric saturation is studied by simulations with static uniform electric fields as large as 4 V/nm. The observed polarization is well described by the Langevin equation with the low-field/high-field crossover parameter of 0.09 V/nm. Simulation of liquid propylene carbonate confined between charged parallel graphite electrodes yields a capacitance that depends on the electric potential difference across those thin films. An effective dielectric constant inferred from the capacitance is significantly less than the uniform liquid dielectric constant, but is consistent with the nonlinear dielectric response at the strong fields applied to the electrode film. Those saturation effects reduce the weak-field capacitance.
Full-text · Article · Jan 2010 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Described here are the first simulations of electric double-layer capacitors based on carbon nanotube forests modeled fully at a molecular level. The computations determine single-electrode capacitances in the neighborhood of 80 F/g, in agreement with experimental capacitances of electric double-layer capacitors utilizing carbon nanotube forests or carbide-derived carbons as electrode material. The capacitance increases modestly with the decrease of the pore size through radii greater than 1 nm, which is consistent with recent experiments on carbide-derived carbon electrodes. Because the various factors included in these simulations are precisely defined, these simulation data will help to disentangle distinct physical chemical factors that contribute to the performance of these materials, e.g., pore geometry, variable filling of the pores, pseudocapacitance, and electronic characteristics of the nanotubes.
Full-text · Article · Sep 2009 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Depletion of fossil fuels, increased energy consumption, and the desirability of low CO2 emissions have heightened the need for efficient, clean, and renewable energy sources. However, many carbon-neutral renewable energy resources, e.g. solar and wind, are intermittent and require effective energy storage. Achieving a significant increase in the energy density of capacitive energy storage devices, while retaining their characteristic advantages of high power and extraordinary cyclability will open up great possibilities for the use of these devices across the entire energy sector. At present, there is a lack of fundamental understanding of the molecular interactions in both bulk electrolyte and the electrode-electrolyte interface, which has lead to an Edisonian approach to improvements in these systems. Here I will describe the first molecular scale calculations on the performance of supercapacitors based on carbon nanotube forests as electrodes, systems proposed to respond to these effective energy storage needs. To provide baseline information for modeling the capacitance of nanotube forests, we first calculated the dielectric constant of propylene carbonate, both as a uniform bulk liquid and in thin films confined between oppositely charged parallel graphite electrodes. We then calculated the capacitance for a fully realistic simulation of the experimental system. We find that the computational results are in good agreement with experiment. Calculations for different spacings of the nanotubes confirm an anomalous dependence of capacitance on pore radius that has been reported previously for supercapacitors. The present results therefore offer a basis for better understanding of capacitive energy storage and provide new insight into device design.