Showing papers by "Peter T. Cummings published in 2014"

Structural Origins of Potential Dependent Hysteresis at the Electrified Graphene/Ionic Liquid Interface

Abstract: We studied the potential and time-dependent changes in the electric double layer (EDL) structure of an imidazolium-based room temperature ionic liquid (RTIL) electrolyte at an epitaxial graphene (EG) surface. We used in situ X-ray reflectivity (XR) to determine the EDL structure at static potentials, during cyclic voltammetry (CV) and potential step measurements. The static potential structures were also investigated with fully atomistic molecular dynamics (MD) simulations. Combined XR and MD results show that the EDL structure has alternating anion/cation layers within the first nanometer of the interface and that these structures are distinct at the most positive and negative static potentials (1.0 and −0.4 V, respectively) applied in this study. The dynamical response of the EDL to potential steps has a slow component (>10 s) and the RTIL structure shows hysteresis during CV scans (e.g., at 100 mV/s scan rate). Our results reveal that both the slow kinetics and hysteresis are due to the reorganization ...

Densification of Ionic Liquid Molecules within a Hierarchical Nanoporous Carbon Structure Revealed by Small-Angle Scattering and Molecular Dynamics Simulation

Abstract: The molecular-scale properties of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C4mim+][Tf2N–], confined in nanometer-scale carbon pores have been investigated using small-angle X-ray and neutron scattering and fully atomistic molecular dynamics simulations. [C4mim+][Tf2N–] densities significantly higher than that of the bulk fluid at the same temperature and pressure result from the strong affinity of the RTIL cation with the carbon surface during the initial filling of slitlike, subnanometer micropores along the mesopore surfaces. Subsequent filling of cylindrical ∼8 nm mesopores in the mesoporous carbon matrix is accompanied by weak RTIL densification. The relative size of the micropores compared to the ion dimension, and the strong interaction between the RTIL and the slit-like micropore, disrupt the bulk RTIL structure. This results in a low-excluded volume, high-RTIL ion density configuration. The observed interfacial phenomena are simulate...

The Electrical Double Layer of Dicationic Ionic Liquids at Onion-like Carbon Surface

Abstract: The electrical double layer (EDL) structure formed by ions at a charged surface, which is the key to determining the performance of supercapacitors, has been extensively studied for many monocationic ionic liquids (MILs). However, it is not known what effect replacing MILs with dicationic ionic liquids (DILs) will have on the EDL structure. In this work, the interfacial structure and electrochemical performance of DILs [Cn(mim)2](BF4)2 (n = 3, 6, 9) and [C6(mim)2](Tf2N)2 were investigated using classical molecular dynamics (MD) simulation for comparison with their monocationic counterparts. Different EDL structures formed by DILs and MILs near an onion-like carbon (OLC) electrode were observed. The interfacial orientation of the imidazolium plane in dications was verified to be similar to that of monocations. Moreover, the dissimilar sizes of the cation/anion and the specific ion adsorption on OLC were found to contribute to the distinctive shape of the differential capacitance–electric potential (C–V) cu...

Effect of cation on diffusion coefficient of ionic liquids at onion-like carbon electrodes

Abstract: While most supercapacitors are limited in their performance by the stability of the electrolyte, using neat ionic liquids (ILs) as the electrolyte can expand the voltage window and temperature range of operation. In this study, ILs with bis(trifluoromethylsulfonyl)imide (Tf2N) as the anion were investigated as the electrolyte in onion-like carbon-based electrochemical capacitors. To probe the influence of cations on the electrochemical performance of supercapacitors, three different cations were used: 1-ethyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium and 1,6-bis(3-methylimidazolium-1-yl). A series of electrochemical characterization tests was performed using cyclic voltammetry (CV), galvanostatic cycling and electrochemical impedance spectroscopy (EIS). Diffusion coefficients were measured using EIS and correlated with quasielastic neutron scattering and molecular dynamics simulation. These three techniques were used in parallel to confirm a consistent trend between the three ILs. It was found that the IL with the smaller sized cation had a larger diffusion coefficient, leading to a higher capacitance at faster charge-discharge rates. Furthermore, the IL electrolyte performance was correlated with increasing temperature, which limited the voltage stability window and led to the formation of a solid electrolyte interphase on the carbon electrode surface, evident in both the CV and EIS experiments.

Interfaces of dicationic ionic liquids and graphene: a molecular dynamics simulation study

Abstract: Molecular dynamics simulations were performed to investigate the interfacial structure and capacitance of electrical double layers (EDLs) in dicationic ionic liquids (DILs) 1-alkyl-3-dimethylimidazolium tetrafluoroborate [Cn(mim)2](BF4)2 (n = 3, 6, 9), with respect to a baseline of a monocationic ionic liquid [C3mim][BF4], near planar carbon electrodes consisting of graphene sheets. The simulation results show that an adsorbed layer with double peaks is exclusively found for [C3(mim)2](BF4)2, while a single peak of the other three cations is observed at the neutral electrode, due to the difference in ion-wall interaction and cation-anion association. As the electrode becomes negatively charged, the second peak of [C3(mim)2](2+) is dramatically reduced, whereas those of [C6(mim)2](2+) and [C9(mim)2](2+) become non-trivial. The capacitance-potential curve of EDLs in DILs manifests a transition from camel shape to bell shape as the cation chain length increases, which is attributed to the enlargement of ion adsorption (per unit charge) on the electrode and the decrease of attractive interaction between ions.

Strain‐Based In Situ Study of Anion and Cation Insertion into Porous Carbon Electrodes with Different Pore Sizes

Abstract: Dr. J. M. Black, Dr. S. V. Kalinin, Dr. N. BalkeCenter for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge , TN 37831 , USAE-mail: balken@ornl.gov Dr. G. Feng, Prof. P. T. CummingsChemical & Biomolecular Engineering Vanderbilt University Nashville , TN 37235 , USAE-mail: guang.feng@vanderbilt.edu Dr. P. F. Fulvio, Dr. P. C. Hillesheim, Prof. S. DaiChemical Sciences DivisionOak Ridge National Laboratory Oak Ridge , TN 37831 , USA Prof. S. DaiDepartment of ChemistryUniversity of Tennessee Knoxville , TN 37996 , USA Prof. Gogotsi. YDepartment of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute Drexel University Philadelphia , PA 19104 , USA

Toward understanding the structural heterogeneity and ion pair stability in dicationic ionic liquids.

Abstract: The structural and dynamical properties of dicationic ionic liquids (DILs) [Cn(mim)2](Tf2N)2, that is, 3-methylimidazolium dications separated by an alkyl chain and with bis(trifluoromethylsulfonyl)amide as the anion, were investigated by molecular dynamics (MD) simulation in combination with small/wide-angle X-ray scattering (SWAXS) measurements. Enhanced spatial heterogeneity is observed as the DIL chain length is increased, characterized by the changes in the scattering and the increased heterogeneity order parameter (HOP). Temperature variation imposes only slight influences on the local structures of DILs compared to monocationic ionic liquids (MILs). The peaks at 0.9 A−1 and 1.4 A−1 of the structure function shift towards low Q as the temperature increases, in a similar manner to MILs, and changes in peak positions in response to temperature changes are reflected in HOP variations. However, the prepeak shift with increasing temperature is ∼3 times smaller in DILs compared to MILs, and both MD and SWAXS indicate a DIL-specific prepeak shifting. Furthermore, the high ion pair/ion cage stability in DILs is indicative of high thermal stability and relative insensitivity of structural heterogeneity to temperature variation, which might be caused by the stronger Coulombic interactions in DILs.

Phase Behavior and Percolation Properties of the Patchy Colloidal Fluids in the Random Porous Media

Abstract: The lack of a simple analytical description of the hard-sphere fluid in a matrix with hard-core obstacles is limiting progress in the development of thermodynamic perturbation theories for the fluid in random porous media. We propose a simple and highly accurate analytical scheme, which allows us to calculate thermodynamic and percolation properties of a network-forming fluid confined in the random porous media, represented by the hard-sphere fluid and overlapping hard-sphere matrices, respectively. Our scheme is based on the combination of scaled-particle theory, Wertheim’s thermodynamic perturbation theory for associating fluids and extension of the Flory–Stockmayer theory for percolation. The liquid–gas phase diagram and percolation threshold line for several versions of the patchy colloidal fluid model confined in a random porous media are calculated and discussed. The method presented enables calculation of the thermodynamic and percolation properties of a large variety of polymerizing and network-fo...

Simulating Phase Equilibria using Wang-Landau-Transition Matrix Monte Carlo

Abstract: We will examine the strengths and weaknesses of the Wang-Landau and transition matrix Monte Carlo methods for simulating phase equilibria of continuous molecular systems alone and as a combined Wang-Landautransition matrix Monte Carlo algorithm. Although a combined Wang-Landau transition matrix Monte Carlo algorithm has been previously reported in the literature, the details of the method and a discussion of its performance for phase equilibria simulations has not been presented. The hybrid method combines the rapid initial estimate of the density of states from the Wang-Landau algorithm with the continual improvement in convergence of transition matrix Monte Carlo. The hybrid Wang-Landau-transition matrix (WL-TM) algorithm is found to be more efficient and has much better convergence properties than the Wang-Landau algorithm and is more robust than the transition matrix algorithm, enabling the simulations to reach relatively low reduced temperatures with ease.

Modeling of Supercapacitors

Abstract: Supercapacitors, also called electrical double-layer capacitors (EDLCs) or ultracapacitors, have attracted significant attention in recent years as a new class of electrical energy storage devices. Supercapacitors have much higher energy density than conventional capacitors and much higher power density than most batteries. As such, they complement these energy storage devices in many applications. Typically, a supercapacitor consists of electrodes, electrolytes, a membrane separator, and current collectors. When an electrical potential is applied between the two electrodes of a supercapacitor, cations (anions) will migrate into the negatively (positively) charged electrode, and an electrical double-layers (EDLs) forms at the electrode/electrolyte interface. As an analogy to conventional capacitors, in each EDL, the electrode serves as one conductor surface and the counterions adsorbed on the electrode serve as the other conductor surface. Since the separation between the electrode and counterions adsorbed on it is quite small (on the order of 1 nm), the electrical fields within EDLs are very strong, and hence, a large amount of energy can be stored for each unit electrode surface area. By adopting porous electrodes featuring fine pores, the surface area of electrodes can be very high, thus further enhancing supercapacitor’s capacitance. The charge storage capability (i.e., capacitance) of a supercapacitor depends primarily on the choice of electrolyte and electrode materials. Three types of electrolytes have been widely used in supercapacitors. Aqueous electrolytes such as sulfuric acid and KOH solutions are most widely used due to their good electrical conductivity and low cost, but these electrolytes suffer from low decomposition voltage (<1.2 V) and corrosion issues. As an alternative, two types of nonaqueous electrolytes are increasingly being used due to

Web- and Cloud-based Software Infrastructure for Materials Design

Abstract: Molecular dynamics (MD) simulations play an important role in materials design. However, the effective use of the most widely used MD simulators require significant expertise of the scientific domain and deep knowledge of the given software tool itself. In this paper, we present a tool that offers an intuitive, component-oriented approach to design complex molecular systems and set up initial conditions of the simulations. We integrate this tool into a web- and cloud-based software infrastructure, called MetaMDS, that lowers the barrier of entry into MD simulations for practitioners. The web interface makes it possible for experts to build a rich library of simulation components and for ordinary users to create full simulations by parameterizing and composing the components. A visual programming interface makes it possible to create optimization workflows where the simulators are invoked multiple times with various parameter configurations based on results of earlier simulation runs. Simulation configurations including the various parameters, the version of tools utilized and the results are stored in a database to support searching and browsing of existing simulation outputs and facilitating the reproducibility of scientific results.

Interfacial Ionic Liquids: Connecting Static and Dynamic Structures

Abstract: It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent electric double layer (EDL) structure of an imidazolium-based RTIL on charged epitaxial graphene during potential cycling as a function of temperature. The results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by fully atomistic molecular dynamics (MD) simulations at various static potentials. The potential-driven transition between the two structures is characterized by an increasing width but with an approximately fixed hysteresis magnitude as a function of temperature. The results are consistent with the coexistence of distinct anion and cation adsorbed structures separated by an energy barrier (~0.15 eV).

Audibilization: Data Analysis by Ear

Abstract: As molecular dynamics simulations continue to grow in size and complexity, new techniques are needed to rapidly identify regions of data likely to benefit from further analysis. Audibilization, the conversion of data to sound, facilitates this task by taking advantage of the user's innate ability to identify anomalies in patterns of sound. Audibilization also complements visualization of a molecular simulation by allowing the user to easily correlate changes in numerical quantities with changes in the overall structure of the molecular system. Here we present three examples highlighting the utility of audibilization in the analysis of three different molecular simulations. First, we present a simulation of liquid water in which the lengths of the O-H bonds are calculated at each time step and audibilized. Interestingly, we find that anomalies in the pattern of bond vibration are due to intermolecular interactions but do not correlate with the formation of hydrogen bonds. Next, we present a simulation of the rupture of a gold nanowire. Here we audibilize the nanowire potential energy and illustrate that sharp changes in this value coincide with important structural events such as the formation of monatomic chains and dislocations. Finally, we present a simulation of single-stranded DNA passing through a nanogap. Here the bond angle is audibilized and used to illustrate the conformational changes of each base as it passes through the nanogap. This simulation also illustrates the use of more advanced audibilization techniques such as the multiplexing of audibilized signals and the weighting of certain segments of data relative to others.