Formation Of Electrical Double Layer Research Articles

Controlling Clay Swelling–Shrinkage with Inorganic Nanoparticles: A Molecular Dynamics Study

The control of clay swelling can be achieved by using nanoparticles. This paper describes the use of atomistic molecular dynamics simulations to study the interaction of SiO2 nanoparticles (NPs) with Na-montmorillonite (MMT) clay platelets. The NPs@MMT interfaces were simulated by taking into account different aqueous solutions (NaCl and CaCl2) and three different coverages for NPs (hydroxylated, PEGlyated, and sulfonated). The formation of electric double layers (EDL) was observed on the NP and MMT surfaces. The free energies as a function of the NP–platelet distance were determined for each interface, while global minima near MMT surfaces and local minima in the middle path were observed. The presence of NPs in the local (dispersed) or global energy minima (adsorbed) leads to a broadening and compressing of the EDLs, respectively. Accordingly, a mechanism for this swelling–shrinkage can be proposed, based on changes in the EDLs caused by NPs. Because of overlap between the EDLs, for the adsorbed NP the ...

Flexible Polyaniline/Reduced Graphene Oxide/Carbon Fibers Composites Applied As Electrodes for Supercapacitors

Introduction Reduced graphene oxide has received extraordinary attention due to its many special properties, such as good conductivity, mechanical resistance, high accessible surface area (2630 m2 g-1), and good flexibility. These properties makes it the best candidate for flexible electrode materials. Nevertheless, according the literature, the facile agglomeration of graphene, due to the strong Π-Π interactions between their individual sheets, decreases the performance of graphene based materials [1]. In this context, the carbon fibers have been shown to be an ideal support for reduced graphene oxide (rGO). The carbon fiber (CF) offers the advantage for using as electrodes because its 3D structure decreases the packaging of the graphene sheets. In addition, it is not necessary any substrates or binders to prepare the binary composite rGO/CF. Thus, the use of CF is a plausible strategy to maximize the energy density of supercapacitors due to its high surface area as well as its good thermal and chemical stabilities. On the other hand, Polyaniline (PAni) has been considered as a promising pseudocapacitive material due to its high theoretical specific capacitance, low cost, high conductivity, and easy synthesis. In this work were proposed a combination of both double layer capacitance (binary composite rGO/CF) and ternary composite of PAni/rGO/CF associating the PAni pseudocapacitance to enhance this electrode capacitive response. The rGO/CF and PAni/rGO/CF samples were evaluated electrochemically, aiming their performance as electrodes in supercapacitors. Experimental Part CF samples were produced from polyacrylonitrile (PAN) precursor at heat treatment temperature of 1000 ºC using temperature steps of 330 ºC/h under inert atmosphere of nitrogen, reaching the maximum during 30 min up to its cooling down to room temperature. CF were cut in size of 1 cm2 and previously weighed. The rGO electrodeposition on CF was performed under potentiostatic mode, at a fixed potential of -1.25 V vs. Ag/AgCl/KCl(sat) for 15 min in a 25 mL of the graphene oxide (3mg mL-1) + 0.1 mol L-1 LiClO4 aqueous solution. In sequence, the PAni was electrodeposited on rGO/CF binary composite under galvanostatic mode with current density of 5 mA cm2 for 10 min in 0.5 mol L-1 H2SO4 aqueous with 0.1 mol L-1 aniline. The obtained ternary composite PAni/rGO/CF was washed in acid solution and vacuum dried. The morphological and structural analysis were obtained by Scanning Electronic Microscopy with field emission gun (SEM-FEG) and Raman spectroscopy, respectively. The electrode electrochemical characterizations were studied by cyclic voltammetry (CV) and charge/discharge tests. These measurements were performed in 1.0 mol L-1 H2SO4solution using a conventional three electrode cell. Results and discussion The SEM-FEG images showed aggregates with irregular and flakes-like shape and high particle density on CF substrate, which is a characteristic of the rGO. After electrochemical deposition, an uniform PAni coating was observed on binary rGO/CF composite, as shown in Figure 1 (b). Their Raman spectrum showed the characteristic C-Ṅ+ stretching vibration bands of the radical cations observed between 1319 cm-1 and 1340 cm-1 related to PAni emeraldine salt. The ternary composite CV curves presented a quasi rectangular shape with apparent redox peaks characteristic of emeraldine salt, indicating that the charge storage occurs by two process, the electric double layer and pseudocapacitive reactions. The charge-discharge profile of the PAni/rGO/CF electrodes, carried out at a current density of 1 mA cm-1, showed linear charge/discharge profile which indicates that capacitance is mainly due to electric double layer formation. The curves of the PAni/rGO/CF ternary composite showed a long time of the charge/discharge, improving the capacitance value, probably attributed to the electrode high surface area associated with the PAni pseudocapacitive behavior. Conclusion The PAni/rGO/CF composite electrodes were synthesized by electrochemical polymerization. The ternary composite presented a better performance in terms of specific capacitance and electrochemical reversibility than those of the conventional PAni films. The results showed that this ternary composite has a great potential as new promising electrode for applications in supercapacitors. Acknowledgments This work was supported by FAPESP 2016/13393-9, CAPES 086/2013 e CNPq. References S. Wang et al. Journal of Power Sources 299 (2015) 347-355. Figure 1

Electric Double Layer Dynamics in Poly(ethylene oxide) LiClO4 on Graphene Transistors

Formation of an electric double layer (EDL) is a powerful approach for exploring the electronic properties of two-dimensional (2D) materials because of the ultrahigh capacitance and induced charge in the 2D materials. In this work, epitaxial graphene Hall bar devices are gated with an EDL using a 1 μm thick solid polymer electrolyte, poly(ethylene oxide) and LiClO4. In addition to carrier density and mobility, ion dynamics associated with the formation and dissipation of the EDL are measured as a function of temperature over a gate bias range of ±2 V. The room temperature EDL formation time (∼1–100 s) is longer than the dissipation time (∼10 ms). The EDL dissipation is modeled by a stretched exponential decay, and the temperature-dependent dissipation times are described by the Vogel–Fulcher–Tammann equation, reflecting the coupling between polymer and ion mobility. At low temperatures, approaching the glass transition temperature of the electrolyte, the dissipation times of both cations and anions exceed...

Specific adsorption from an ionic liquid: impedance study of iodide ion adsorption from a pure halide ionic liquid at bismuth single crystal planes

Electrical double layer (EDL) formation and adsorption of ions from ionic liquids is important for the fundamental understanding of interfacial processes and their dependence on both the electronic and ionic surface structure. Electrochemical impedance spectroscopy coupled with electrical equivalent circuit modelling has been applied to analyse the interfacial processes that govern the interface between three single-crystal bismuth electrodes with a pure iodide ionic liquid. Major differences in the double layer capacitance are observed for the three bismuth planes in the region of iodide specific adsorption, dependent on both the electronic structure and metallic characteristics of the electrode interface, also observed in case of aqueous iodide solutions. It is shown that this is due to the strong influence of both the electrode material and specifically adsorbed ion layer on the capacitive behaviour, rather than the bulk electrolyte composition. These results suggest a new way of describing the EDL in ionic liquids as well as show how the reactivity of the interface is determined.

Understanding the Capacitance of PEDOT:PSS

Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is the most studied and explored mixed ion‐electron conducting polymer system. PEDOT:PSS is commonly included as an electroactive conductor in various organic devices, e.g., supercapacitors, displays, transistors, and energy‐converters. In spite of its long‐term use as a material for storage and transport of charges, the fundamentals of its bulk capacitance remain poorly understood. Generally, charge storage in supercapacitors is due to formation of electrical double layers or redox reactions, and it is widely accepted that PEDOT:PSS belongs to the latter category. Herein, experimental evidence and theoretical modeling results are reported that significantly depart from this commonly accepted picture. By applying a two‐phase, 2D modeling approach it is demonstrated that the major contribution to the capacitance of the two‐phase PEDOT:PSS originates from electrical double layers formed along the interfaces between nanoscaled PEDOT‐rich and PSS‐rich interconnected grains that comprises two phases of the bulk of PEDOT:PSS. This new insight paves a way for designing materials and devices, based on mixed ion‐electron conductors, with improved performance.

A Study of Mediators As Ion Providers in Supercapactors

Doping mediators (soluble redox compounds) in polymer electrolyte contained in the porous electrodes is an effective method to increase the capacitance of supercapacitors. Mediators not only provide additional capacitance but also ions to electrolyte. The total capacity of a supercapacitor is dominated by the minor of the electrode capacity and the number of ions in electrolyte. In this research, we focused on revealing the effect of on the extra ions provided by the mediators during the charge/discharge processes. Two types of supercapacitors were fabricated. For the first type, the active electrode material consisted of 50 wt% mediators (in solid state) and 50 wt% high specific surface area active carbon (AC). For the other type of supercapacitor, the active electrode material consisted of AC only. Acetonitrile was used as liquid phase for both types. Under this circumstance, ions are provided by mediator only. The calculation of specific current was based on total weight of two electrodes and cyclic voltammetry (CV) curves. As shown in Figure 1, supercapacitor (a) (AC only) is unable to store any charges and energy without the ions in the solution. However, supercapacitor (b) can store ten times more energy with the mediator enhancement. In this situation, we can consider mediators as ‘charge harbor’. They can release ions into the liquid phase. These ions then participate in formation of electric double layers (EDLs) on AC. This capability of providing ions into the liquid phase can increase capacity of supercapacitor. CVs (b) and (c) also indicate that different mediators are different in the kinetics of providing ions. Taking supercapacitor (b) as an example, when we charge the supercapacitor, the K2CuFe(CN)6 can release K+ cation into the liquid phase and [CuFe(CN)6]- will still be hold on the solid phase because of the electrostatic force. On the other hand, we can use VOSO4 as a mediator too. V4+ can be reduced and releases SO4 2- anion. Thus, it is preferred to use K2CuFe(CN)6 in the positive electrode while use VOSO4 in the negative electrode. An series of mediators were evaluated using the same method. Figure 1

(Invited) An Integrated Microbial Desalination Cell-Driven Capacitive Deionization System as an Electrochemical Means for Wastewater Treatment, Electricity Generation and Desalination

Due to unstainable use of natural water resources, alternative water resources such as brackish water and seawater desalination have been an emerging solution. However, development of desalination capacity is limited due to the high energy requirements for removing salt ions from water. Currently, capacitive deionization technology (CDI), following the working principle of supercapacitors, has attracted considerable attention from academia, industry, and government agency. As compared to conventional desalination technologies, CDI has several advantages including low energy consumption, easy regeneration, high water recovery, and no secondary waste. In CDI, by applying an external electric filed between two parallel of nanoporous carbon electrodes (i.e., carbon aerogel, activated carbons, carbon nanotubes, and graphene), ions can be stored at the electrode/solution interface via electrical double layer (EDL) formation. Additionally, microbial desalination cell (MDC) is a new bioelectrochemical technology for seawater desalination with simultaneous electricity generation and wastewater treatment. Basically, a MDC reactor contains an anode chamber, a desalination chamber, and a cathode chamber. In MDC, microorganisms can oxidize organic waters in wastewater to harvest electric energy, and meanwhile, salt ions can be removed during the electricity generating process. In this study, we propose a hybrid electrochemical desalination system for seawater desalination by coupling CDI device with a MDC reactor. As a result, MDC produced electricity with open circuit voltage of 0.8 V and a current of 3 mA by using bacteria to degrade organic contaminants through anode bacterial oxidation and cathode reduction. In MDC, 91% removal of chemical oxygen demand (COD) in synthetic wastewater can be achieved, and the solution conductivity can be reduced from 17,000 µS/cm to about 200 µS/cm. More importantly, CDI device can be driven by electricity harvesting from the two MDCs in parallel, and as the downstream desalination process to further desalinate salt water. The results of this study can demonstrate the feasibility of the integrated electrochemical MDC-CDI system for simultaneous wastewater treatment, power production, and water desalination. .

Unveiling the Dynamic Processes in Hybrid Lead Bromide Perovskite Nanoparticle Thin Film Devices

Hybrid and all‐inorganic perovskite (PK) materials are a promising next generation of semiconducting materials due to their outstanding light‐harvesting features, as well as their color‐tunablility and efficient luminescent properties that lead to highly efficient photovoltaic and lighting devices. Bulk PK films are both ionic and electronic conductors under the presence of an externally applied electric field. In this work, the internal ion motion behavior is demonstrated within PK nanoparticles in thin‐film devices by means of different long‐time poling scheme assays and both static and dynamic electrochemical impedance spectroscopy measurements. In particular, the existence of a dynamic device behavior is related to the migration and rearrangement of different ionic species upon applying different driving schemes. The latter resembles the well‐known signatures of the ionic motion in light‐emitting electrochemical cells (LECs), that is, (i) the formation of electrical double layers due to the ionic distribution at the electrodes' interfaces, (ii) the growth of the doped regions once the charge injection is effective, and (iii) the subsequent formation of a non‐doped region in the bulk of the device. Hence, this comprehensive study opens up an alternative route toward understanding the dynamics inside hybrid perovskite materials based on the large body of knowledge of LECs.

Influence of combined electromagnetohydrodynamics on microchannel flow with electrokinetic effect and interfacial slip

We investigate analytically the combined consequences of electromagnetohydrodynamic forces and interfacial slip on streaming potential mediated pressuredriven flow in a microchannel. Going beyond traditional Debye-Huckel limit, we first derive a closed-form analytical solution for velocity field by considering nonlinear electrical potential distribution, wall slip effects, externally imposed transverse magnetic field, and laterally applied electric field in the plane of flow. The effects of electrical double-layer (EDL) formation and the consequent interfacial phenomena are critically examined under such situations. An expression for induced streaming potential in the microchannel is deduced considering EDL formation and the consequences of finite conductance of the immobilized Stern layer. This simplified analytical expression is later on critically assessed against three-dimensional simulation paradigm of streaming potential mediated flows, which is a first effort of this kind. We demonstrate that flow rate increases progressively with increasing surface potential and eventually approaches to a limiting value. Combination of electromagnetohydrodynamic effect with liquid slip is shown to amplify the flow rate, even at lower values of surface potential. Our study brings out the possibility of achieving an optimum flow rate by judicious application of combined electromagnetohydrodynamics. The present analysis has significant consequence in the design of advanced microfluidic devices with improved efficiency and functionality.

Reversible Heating in Electric Double Layer Capacitors.

A detailed comparison is made between different viewpoints on reversible heating in electric double layer capacitors. We show in the limit of slow charging that a combined Poisson-Nernst-Planck and heat equation, first studied by d'Entremont and Pilon [J. Power Sources 246, 887 (2014)JPSODZ0378-775310.1016/j.jpowsour.2013.08.024], recovers the temperature changes as predicted by the thermodynamic identity of Janssen etal. [Phys. Rev. Lett. 113, 268501 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.268501], and disagrees with the approximative model of Schiffer etal. [J. Power Sources 160, 765 (2006)JPSODZ0378-775310.1016/j.jpowsour.2005.12.070] that predominates the literature. The thermal response to the adiabatic charging of supercapacitors contains information on electric double layer formation that has remained largely unexplored.

A Study of Dip-Coatable, High-Capacitance Ion Gel Dielectrics for 3D EWOD Device Fabrication.

We present a dip-coatable, high-capacitance ion gel dielectric for scalable fabrication of three-dimensional (3D) electrowetting-on-dielectric (EWOD) devices such as an n × n liquid prism array. Due to the formation of a nanometer-thick electric double layer (EDL) capacitor, an ion gel dielectric offers two to three orders higher specific capacitance (c ≈ 10 μF/cm2) than that of conventional dielectrics such as SiO2. However, the previous spin-coating method used for gel layer deposition poses several issues for 3D EWOD device fabrication, particularly when assembling multiple modules. Not only does the spin-coating process require multiple repetitions per module, but the ion gel layer also comes in risks of damage or contamination due to handling errors caused during assembly. In addition, it was observed that the chemical formulation previously used for the spin-coating method causes the surface defects on the dip-coated gel layers and thus leads to poor EWOD performance. In this paper, we alternatively propose a dip-coating method with modified gel solutions to obtain defect-free, functional ion gel layers without the issues arising from the spin-coating method for 3D device fabrication. A dip-coating approach offers a single-step coating solution with the benefits of simplicity, scalability, and high throughput for deposition of high-capacitance gel layers on non-planar EWOD devices. An ion gel solution was prepared by combining the [EMIM][TFSI] ionic liquid and the [P(VDF-HFP)] copolymer at various wt % ratios in acetone solvent. Experimental studies were conducted to fully understand the effects of chemical composition ratios in the gel solution and how varying thicknesses of ion gel and Teflon layers affects EWOD performance. The effectiveness and potentiality of dip-coatable gel layers for 3D EWOD devices have been demonstrated through fabricating 5 × 1 arrayed liquid prisms using a single-step dip-coating method. Each prism module has been individually controlled to achieve spatial beam steering without the need for bulky mechanical moving parts.

In Situ Mass Spectrometric Monitoring of the Dynamic Electrochemical Process at the Electrode-Electrolyte Interface: a SIMS Approach.

The in situ molecular characterization of reaction intermediates and products at electrode-electrolyte interfaces is central to mechanistic studies of complex electrochemical processes, yet a great challenge. The coupling of electrochemistry (EC) and mass spectrometry (MS) has seen rapid development and found broad applicability in tackling challenges in analytical and bioanalytical chemistry. However, few truly in situ and real-time EC-MS studies have been reported at electrode-electrolyte interfaces. An innovative EC-MS coupling method named in situ liquid secondary ion mass spectrometry (SIMS) was recently developed by combining SIMS with a vacuum compatible microfluidic electrochemical device. Using this novel capability, we report the first in situ elucidation of the electro-oxidation mechanism of a biologically significant organic compound, ascorbic acid (AA), at the electrode-electrolyte interface. The short-lived radical intermediate was successfully captured, which had not been detected directly before. Moreover, we demonstrated the power of this new technique in real-time monitoring of the formation and dynamic evolution of electrical double layers at the electrode-electrolyte interface. This work suggests further promising applications of in situ liquid SIMS in studying more complex chemical and biological events at the electrode-electrolyte interface.

Designing of experiments for evaluating the interactions of influencing factors on the photocatalytic activity of NiS and SnS2: Focus on coupling, supporting and nanoparticles

Synergistic photocatalytic activity of SnS2 and NiS supported was observed by coupling and supporting the semiconductors onto clinoptilolite nanoparticles (NC). Formation of electrical double layer at the semiconductors’ interface in the coupled semiconductors significantly prevented from electron/hole recombination which in turn enhanced the photodegradation activity. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) confirmed enhancement in charge transfer efficiency of the coupled/supported system with respect to the mono-component systems. Consequently the coupled/supported system showed the best photodegradation activity in degradation of metronidazole (MZ). The response surface methodology (RSM) showed that irradiation time, solution pH and interactions of time-CMZ and time-pH are the most important factors in MZ degradation. The best degradation extent was obtained at pH 4, catalyst dosage of 3gL−1, 3mgL−1 of MZ at irradiation time of 270min. R2 of 0.9743 confirmed that the data predicted by RSM well agree with the experimental results. Catalyst dosage and irradiation time had synergistic effect and initial pH and CMZ had an antagonistic effect on the response, meaning that increasing in time and catalyst dosage and decreasing in pH and CMZ decreased MZ photodegradation. Sharp decrease in the UV–Vis absorption at 319nm and increase in the intensity at 240 and 206nm during the process confirmed converting the nitro group and nitrogen existing in MZ to nitrite ion.

Fluid manipulation on the micro-scale: Basics of fluid behavior in microfluidics.

Fluid manipulation on the micro-scale (microfluidics) is bringing new potential applications in a number of fields, including chemistry, biology and medicine. At sub-millimeter channel scale, some phenomena, unimportant at the macroscale, become an important force to consider when designing a microfluidics system. For example, the decrease in fluid mass causes the effects of viscosity to overcome the influence of inertia. Turbulent flow cannot be achieved at any realistic fluid velocity, making mixing a challenging task. The only phenomenon capable of blending liquids at microscale is diffusion and liquid streams can be flowed side-by-side for tens of minutes before they completely fuse together. The decrease in the channel size also leads to an increased surface-to-volume ratio, which increases the importance of surface effects, including adsorption, capillary action and surface wetting and/or electric double layer formation with related electrokinetic phenomena. While rivers cannot flow uphill, a stream of liquid can easily flow up against gravity inside a capillary. Similarly, the formation of electric double layer near the charged surface of a micro-channel or capillary can be applied for electrokinetic actuating. This review summarizes selected physical phenomena related to liquid-based (water solutions) microfluidics as described recently.

N-doped porous reduced graphene oxide as an efficient electrode material for high performance flexible solid-state supercapacitor

Supercapacitors (SCs) are energy storage devices with an immense potential to resolve energy related issues. Being a low cost, durable and multifunctional material, graphene and its derivatives are the most promising candidates for SCs. Here, we report the synthesis of N-doped porous reduced graphene (N-pGr) and its use for the fabrication of all-solid-state supercapacitor (ASSC) device. The N-pGr exhibited a specific capacitance of 230Fg−1 at 1Ag−1 current density and good capacitance retention up to ∼88% even after 10,000 galvanostatic charge–discharge cycles recorded at 5Ag−1 fabricated in a solid-state supercapacitor. The synergistic effect of the porosity and N-doping is believed to be the underlying principle for the improved charge storage ability when compared to other reports. Significantly, the porous nature of N-pGr contributes in increasing the electroactive surface area for the formation of electrical double layer as well as facilitating the faster movement of electrolyte ions inside the electrode network. N-doping is believed to change the electrical properties of the electrode material and also to participate in the pseudo-capacitance which helped to boost the overall capacitance of the fabricated device. The good capacitance retention provides the room for making robust flexible solid-state supercapacitor (FSSC) with the N-pGr material.

First-principles thermal modeling of hybrid pseudocapacitors under galvanostatic cycling

Abstract This study presents a theoretical framework developed from first principles for predicting the spatiotemporal thermal behavior of hybrid pseudocapacitors under galvanostatic cycling. It accounts for irreversible and reversible heat generation in the electrolyte and in the electrodes due to Joule heating, electric double layer (EDL) formation, and redox reactions. Detailed numerical simulations were performed to investigate the different local heat generation rates and the temperature as functions of time and cycling current. Numerical predictions showed good qualitative agreement with the limited experimental data available in the literature. Such numerical simulations can be used to physically interpret experimental measurements. Indeed, the present results suggest that a distinctive endothermic peak observed in the experimental heat generation rate resulted from cation starvation in the electrolyte reducing the faradaic current density. In addition, heating due to EDL formation significantly affected the local temperature and must be accounted for. The thermal model and the present results will help define safe modes of operation and develop thermal management strategies for pseudocapacitors.

Electroformation of giant unilamellar vesicles in saline solution

Giant unilamellar vesicle (GUV) formation on indium tin oxide (ITO) electrodes in saline solution and from charged lipids has proven to be difficult in the past. Yet the best cell membrane models contain charged lipids and require physiological conditions. We present a way to overcome this problem by using plasma cleaned ITO electrodes. GUVs from zwitterionic lipids, lipid mixtures and even pure charged lipids could be electroformed under physiological conditions and even higher concentrations of NaCl. The hydrophilic ITO surface may facilitate the hydration of the solid lipid film and the formation of lipid bilayers that subsequently bend and form vesicles. The formation of GUVs in saline solution is influenced by different parameters. The influences of the amplitude and frequency of the used AC field, the NaCl concentration, and the temperature were investigated. Finite element analysis simulating the effect of the electric field on GUV formation in saline solution could well explain the experimental results. Frequencies in the kHz-range favored for GUVs formation in saline solution, as they suppress the formation of electric double layer, while higher frequencies could again impair the effect of electric field and impede GUV formation. The diameters of the GUVs increased gradually with NaCl concentration from 0mM to 200mM and subsequently decreased from 200mM to 2M. High yields of GUVs were also formed in PBS solution and cell culture medium, which indicates this method is a promising way to prepare GUVs on a large scale in physiological relevant conditions.

Characterization of the Electric Double Layer Formation Dynamics of a Metal/Ionic Liquid/Metal Structure.

Although ionic liquids (ILs) have been used extensively in recent years as a high-capacitance "dielectric" in electric double layer transistors, the dynamics of the double layer formation have remained relatively unexplored. Better understanding of the dynamics and relaxation processes involved in electric double layer formation will guide device optimization, particularly with regard to switching speed. In this paper, we explore the dynamical characteristics of an IL in a metal/ionic liquid/metal (M/IL/M) capacitor. In particular, we examine a Au/IL/Au structure where the IL is 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate. The experiments consist of frequency-dependent impedance measurements and time-dependent current vs voltage measurements for applied linear voltage ramps and abrupt voltage steps. The parameters of an equivalent circuit model are determined by fits to the impedance vs frequency data and subsequently verified by calculating the current vs voltage characteristics for the applied potential profiles. The data analysis indicates that the dynamics of the structure are characterized by a wide distribution of relaxation times spanning the range of less than microseconds to longer than seconds. Possible causes for these time scales are discussed.

Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces.

We evaluate the influence of Li-salt doping on the dynamics, capacitance, and structure of three ionic liquid electrolytes, [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4], using molecular dynamics and polarizable force fields. In this respect, our focus is on the properties of the electric double layer (EDL) formed by the electrolytes at the electrode surface as a function of surface potential (Ψ). The rates of EDL formation are found to be on the order of hundreds of picoseconds and only slightly influenced by the addition of Li-salt. The EDLs of three electrolytes are shown to have different energy storage capacities, which we relate to the EDL formation free energy. The differential capacitance obtained from our computations exhibits asymmetry about the potential of zero charge and is consistent with the camel-like profiles noted from mean field theories and experiments on metallic electrodes. The introduction of Li-salt reduces the noted asymmetry in the differential capacitance profile. Complementary experimental capacitance measurements have been made on our three electrolytes in their neat forms and with Li-salt. The measurements, performed on glassy carbon electrodes, produce U-like profiles, and Li-salt doping is shown to strongly affect capacitance at high magnitudes of Ψ. Differences in the theoretical and experimental shapes and magnitudes of capacitance are rationalized in terms of the electrode surface and pseudocapacitive effects. In both neat and Li-doped liquids, the details of the computational capacitance profile are well described by Ψ-induced changes in the density and molecular orientation of ions in the molecular layer closest to the electrode. Our results suggest that the addition of Li+ induces disorder in the EDL, which originates from the strong binding of anions to Li+. An in-depth analysis of the distribution of Li+ in the EDL reveals that it does not readily enter the molecular layer at the electrode surface, preferring instead to be localized farther away from the surface in the second molecular layer. This behavior is validated through an analysis of the free energy of Li+ solvation as a function of distance from the electrode. Free energy wells are found to coincide with localized concentrations of Li+, the depths of which increase with Ψ and suggest a source of impedance for Li+ to reach the electrode. Finally, we make predictions of the specific energy at ideal graphite utilizing the computed capacitance and previously derived electrochemical windows of the liquids.

Mapping the Transmission Functions of Single-Molecule Junctions.

Charge transport phenomena in single-molecule junctions are often dominated by tunneling, with a transmission function dictating the probability that electrons or holes tunnel through the junction. Here, we present a new and simple technique for measuring the transmission functions of molecular junctions in the coherent tunneling limit, over an energy range of 1.5 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different exposed areas, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction's transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.