Capacitance-potential Curves Research Articles

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor.

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Electrified interfaces of deep eutectic solvents

Many theoretical and experimental studies have been focused on the physicochemical properties of dense ionic fluids such as ionic liquids (ILs). However, less attention has been given to interfacial properties involving deep eutectic solvents (DES). The impact of the DES composition, hydrogen bond donor (HBD) structure, temperature, and electrode nature material on the DES-electrode vertical interactions remain vague. The lack of knowledge imposes significant constraints in proposing a suitable Electrical Double Layer model (EDL) to describe the DES at electrified interfaces. Measuring differential capacitance-potential curves is a strategy to assess the EDL structure and understand how ions interact with the electrode surface, which knowledge is fundamental to designing and optimizing electrochemical systems for various applications (e.g., energy storage devices). Accordingly, a set of choline chloride-based DESs was assessed containing distinct HBD at their eutectic composition (the polyalcohol's 1,2-ethanediol, 1,2-propylene glycol, 1,3-propylene glycol, and the amide urea) against glassy carbon (GC), gold (Au), and the platinum (Pt) electrode at different temperatures.The differential capacitance-potential curves were found to vary significantly in shape in the three different electrode surfaces studied, ranging from camel shape (Au electrode), U-shape (GC), and asymmetric bell shape (polycrystalline Pt). The carboxylic malonic and oxalic acids were also assessed for a proper comparison to understand better the role of the HBD's functional group in shaping the electrode-electrolyte structure against the trend found with diol isomers. A suitable EDL model must inevitably accommodate interfacial properties assessed at the capacitive region, namely the influence of the surface chemistry, potential dependence, DES structure molecules, and temperature in shaping the electrified interfacial anatomy.

The adsorption of 4,4ʹ-bipyridine at a Cd(0001)|ionic liquid interface – The descent into disorder

In this study, electrochemical impedance spectroscopy and in situ STM measurements have been used to investigate the interface between a Cd(0001) single-crystal electrode and an ionic liquid electrolyte containing an organic additive. The addition of 4,4ʹ-bipyridine clearly influenced the interfacial structure and properties by forming an adlayer on the surface of the electrode during the experiment. Furthermore, in the potential range under study, the organic adlayer of adsorbed 4,4ʹ-bipyridine undergoes structural changes, as seen in both cyclic voltammograms and capacitance–potential curves. The processes occurring on the interface were further visualised using in situ STM, and the formation of 4,4ʹ-bipyridine agglomerates on the electrode’s surface was observed. The results underline the impact of the electrolyte characteristics (water vs. ionic liquid) on the adsorption of organic additives and the formation of adlayers.

Membrane Potentials Trigger Molecular-Scale Rearrangements in the Outer Membrane of Gram-Negative Bacteria.

The structural complexity of the cell envelope of Gram-negative bacteria limits the fabrication of realistic models of bacterial cell membranes. A vertical Langmuir-Blodgett withdrawing was used to deposit a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) monolayer on the Au(111) surface. The second leaflet composed of di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) was deposited using Langmuir-Schaefer transfer. The use of an electrode material as a support for the POPE-KLA bilayer allowed electrochemical control of the membrane's stability, compactness, and structure. Capacitance-potential curves showed a typical pattern for the supported lipid bilayers electrochemical characteristic. The minimum membrane capacitance was ∼4 μF cm-2 and did not change in the following desorption-adsorption cycles, indicating the presence of a stable bilayer structure with an asymmetric composition of both leaflets. However, at a molecular scale, as elucidated in spectroelectrochemical experiments, large differences in the response of both leaflets to electric potentials were observed. The acyl chains in POPE and KLA existed in a liquid state. The quantitative analysis of the CH stretching modes indicated potential-driven reorientations in the hydrophobic fragment of the bilayer, already in the adsorbed state. To assign observed rearrangements to POPE and KLA lipids in both leaflets, per-deuterated d31-POPE was transferred into the inner leaflet. Since no potential-dependent changes of the CD2 stretching modes in the d31-POPE-KLA bilayer were observed, reorientations in the acyl chain region were assigned to the KLA molecules. Mg2+ ions were bound to the polar head groups of KLA. The strength of electrostatic interactions in the polar head group region of KLA was dependent on the direction of the electric field. At negative electric potentials, the binding of divalent cations weakened, which gave the KLA molecules increased orientational flexibility. This behavior in electric fields is peculiar for the outer membrane and indicates that the microbial cell membranes have different electrochemical properties than phospholipid bilayers.

Ionic liquid–metal interface: The origins of capacitance peaks

The complex nature of electrode charge screening is well-known for ionic liquids (ILs). Due to strong ion–ion correlations, these electrolytes form a distinctive layered structure at interfaces. Variations in electrode potential cause structural changes that are reflected in a peculiar shape of differential capacitance–potential dependence with characteristic peaks. Although the differential capacitance for various ILs in conjunction with metal electrodes accessed via molecular dynamics (MD) simulations has been reported in the literature, retrieving a capacitance-potential curve, C(U), from the MD trajectories is not a trivial task. In this work, we present the results of the MD simulations of the IL 1-butyl-3-methylimidazolium hexafluorophosphate at a single-crystalline Au (100) surface. The discussion focuses on the simulation data treatment for C(U) curve fitting. It is shown that the resulting C strongly depends on the fitting method used. Four capacitance peaks and three structural reorganization types were identified in the studied system. With the help of a semi-quantitative approach in the framework of the original bilayer model of electric double layer (EDL), it is argued that the ions’ reorientation is in the origin of the capacitance peaks. Also, it is shown that under the conditions of this study, the multilayer structure, characteristic of EDL in ILs on the whole, is far from the “lattice saturation” regime. The multilayer structure possesses a steric packing effect that impedes structural changes, decreasing the capacitance.

Do Deep Eutectic Solvents Behave Like Ionic Liquid Electrolytes? A Perspective from the Electrode-Electrolyte Interface

The differential capacitance as a function of potential and composition for choline chloride (ChCl) and ethylene glycol (EG) at 1:2, 1:4, 1:6, and 1:20 molar ratios were studied using electrochemical impedance spectroscopy (EIS) on glassy carbon. In addition, ChCl:EG at the known ethaline DES composition (1:2) and 1:20 molar ratio were studied on Au and Pt electrodes, and compared to glyceline (ChCl:glycerol, 1:2). The capacitance-potential curves on glassy carbon were best interpreted by the modified Gouy-Chapman model. A dampened U-shape was observed similar to dilute electrolytes, however, the non-negligible ionic and H-bonding interactions in these electrolytes resulted in ambiguity about the point of zero charge where capacitance was weakly dependent on potential. Additionally, the differential capacitance was independent of composition on glassy carbon due to limited availability of mobile free ions and constraints by solvated ion sizes within the interface. Increases in capacitance are observed with Au as a result of desolvation and specific adsorption of Cl, and with Pt upon decreased Cl concentrations. These results indicate DESs and H-bonded electrolytes with high salt concentrations are distinct from ionic liquids and present no pronounced overscreening or crowding with applied potential.

Employment of electrostriction phenomenon for label-free electrochemical immunosensing of tetracycline

The presented work describes a simple label-free electrochemical immunosensor for the determination of tetracycline (TC). The functioning of the sensor was based on the electrostriction of a antibody-terminated thiol layer self-assembled on a gold electrode surface, serving as a dielectric membrane. The intensity of electrostriction was correlated with the amount of TC captured through an affinity reaction with its specific antibody, and was followed in the form of capacitance-potential curves. The process of the immunosensor construction was optimized using electrochemical impedance spectroscopy (EIS). The chemisorption time of the thiol, the duration of the TCAb immobilization and its concentration were optimized. The developed immunosensor exhibited a linear response in two concentration ranges: from 0.95 to 10 μmol L–1 and from 10 to 140 μmol L–1, with the mean sensitivity of 6.27 nF μmol−1 L (88.67 nF μmol−1 L cm−2) and 0.56 nF μmol−1 L (7.84 nF μmol−1 L cm−2), respectively. The limit of detection was evaluated as 28 nmol L−1. The specificity of the proposed sensor toward other antibiotics, amoxicillin and ciprofloxacin, was examined. The immunosensor was successfully employed to quantify TC in a tablet form and in a matrix of river water.

Pulse Electrodeposition Synthesis and Characterization of Sns Thin Films Obtained from Dimethyl Sulfoxide Solutions

Thin film materials for solar photovoltaic energy conversion have been considered a good option to provide a low cost power supply for a progressive increasing demand across the world. The most intensively studied and commercialized thin film solar cells are those based on (CIGS), Cu(In,Ga)Se2 and CdTe. Concerns regarding the cadmium because its toxicity and the limited abundance of indium and gallium could limit the future of these technologies. For these reasons alternative materials to develop new photovoltaic solar cells are currently investigated around the world. From the point of view of earth abundance, cost and toxicity SnS appears as a promising candidate. It has a direct optical band gap value (1.5 eV) which is within the optimum range for photovoltaic solar energy conversion. It exhibits p-type conductivity and high optical absorption coefficient meaning that only some microns of thickness could absorb most of the incident light. Theoretical calculations predict a conversion efficiency of 24% for materials with the same band gap1, however the reported efficiency for solar cells based on SnS is less than 2%2. Such low efficiencies were attributed to factors like poor quality of the films, grain boundaries, lower thickness employed, insufficient diffusion lengths in the layers and large band discontinuities. Hence, in order to obtain better efficiencies in SnS based solar cells the quality of the layers need to be substantially improved. Electrodeposition is one of the economical methods for large surface coating and has been successfully used to prepare semiconductor films. The method allows the control of the bandgap width and the doping level through the control of variables such as solution composition, applied potential, pH and working temperature. Furthermore, monitoring the circulating charge it is also possible to control the thickness of the deposited layers. Metal chalcogenides have been generally electrodeposited by the cathodic co-reduction of the metal and chalcogenide ions. However, compositional heterogeneity is usually a problem with this approach. An attractive option involves the use of aprotic deposition baths such as dimethylsulfoxide which presents a high boiling point and also allows the solubility of the chalcogene precursor in nonionized, molecularly dissolved form. In this solvent it is possible to form binary compounds starting from the electrochemical reduction of the elementary chalcogen precursor and the consecutive formation of a chalcogenide anion that further precipitates heterogeneously onto the surface of a convenient substrate for the formation of II-VI semiconductors. In current work SnS thin films were prepared on molybdenum substrates by pulsed electrodeposition at differents synthesis times. The structure, morphology and composition were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and Raman spectroscopy. The optical and electrical properties were studied by UV–vis absorption and capacitance-potential curves, respectively. The electrodeposited SnS films showed p-type semi-conductive characteristics and exhibited cathodic photocurrent under visible light illumination. Highly compact, dense, homogenous and almost stoichiometric films were obtained by properly selecting the electrodeposition parameters. These results show that the as grown films show composition and thickness compatible with the requirements of SnS based solar cell devices Acknowledgements: ToEuropean Commission, NanoCIS project FP7-PEOPLE-2010-IRSES grant 269279 and to CONICYT (Chile), FONDECYT (Fondo de Desarrollo Cientifico y Tecnológico, Chile) through Project N° 3160217.

Adsorption of 2,2′-bipyridine at an Au(111)|ionic liquid electrified interface

The adsorption of added 2,2′-bipyridine (2,2′-BP) from 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethanesulfonyl)imide (EMMImNTf2) at an Au(111) electrode has been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Addition of 2,2′-BP to the ionic liquid clearly modifies the interfacial region as a result of the competition between 2,2′-BP and EMMImNTf2 to occupy the electrode surface. Within the region of ideal polarizability, the 2,2′-BP adlayer undergoes structural changes, shown by the presence of peaks in the CV curves. Between −0.2V and +0.9V, the capacitance–potential curves obtained from EIS data present a capacity maximum depending strongly on the ac frequency, which is typical pseudo-capacitive behavior indicative of a reorganization of the interfacial layer. At more positive potentials a true capacity value close to 10μF.cm−2 and invariant with the potential suggests that the 2,2′-BP molecules adopt a perpendicular orientation with the nitrogen atoms facing the electrode surface, similar to their adsorption on gold from aqueous solutions.

Characteristics of Capacitors Based on Ionic Liquids: From Dielectric Polymers to Redox-Active Adsorbed Species

The development of novel devices for energy recuperation and storage systems is of paramount importance for the realization of an energetically sustainable society. Capacitors play a huge role in mentioned complexes, being assumed as versatile devices for power applications. Ionic liquids (IL) have been tabbed as “designer electrolytes”, thus offering a multitude of unique and advantageous properties, such as negligible vapour pressure, high electrochemical and temperature stability, good ionic conductivity and the ability to modify the salt for novel applications. The presentation reviews the recent developments regarding the use of ILs as electrolytes for capacitors, starting from the formation of the electrical double layer (EDL) and ending up with some unique applications of ILs. Capacitor technology is fundamentally based on the formation of the EDL and the storage of charge within the EDL. It is thus important to understand the probable mechanism of charge storage and the formation of the EDL in ILs at different electrode surfaces. Experiments with highly oriented pyrolytic graphite, carbide derived carbons and bismuth electrodes in ILs have shown that the systems are indeed ideally polarizable [1], thus ideally suited for capacitor applications. It has been shown that the value of double layer capacitance in ILs is of the same order of magnitude as that for organic electrolyte based systems and the capacitance-potential curve of carbon electrodes shares the same principal V-shape as those seen for aqueous and organic electrolyte solutions. Although many theoretical studies have been published about the subject, the agreement between experimental and theoretical results is limited. One of the most widely studied IL applications as electrolytes are AC/AC supercapacitors (SC). SCs based on ILs have been shown to possess higher energy density, owing to their wider electrochemical window, but lower power density due to higher viscosity and lower ionic conductivity than more conventional organic electrolyte based SCs[2]. Although extremely wide electrochemical stability has been suggested for many ILs, this has not been realized in SCs, allowing only slightly higher voltage than commercial SCs. This has been explained by IL decomposition at extreme potentials confirmed by both infrared and photoelectron spectroscopy studies[1,3]. Because of this, SCs based on ILs are not competitive in the mainstream, however, serving many alternative applications in high or low temperature systems that require higher security with regards to failure. In order to increase the energy density of SCs based on ILs, the addition of redox species or redox-active organic compounds has been explored. The specific adsorption and subsequent oxidation of halide species has been tested for both bismuth and carbon electrodes. It has been shown that redox and surface active ions can significantly increase the gravimetric capacitance of the system with the drawback of limiting the applied potential range . Small additions of organic solvents to IL has been shown to increase the power density of SCs with negligible effect on the voltage limit and gravimetric capacitance, while the addition of specifically adsorbing organic compounds to ILs has been observed to produce systems with properties similar to those for dielectric capacitors [5]. The electrochemical breakdown of ILs can also be advantageous with regards to specific capacitor applications. It has been shown that some nitrile group-containing anions have the tendency to electropolymerize at high positive potentials, forming thick dielectric layers on the electrode surface [6]. This process has been applied in order to produce dielectric capacitors based porous carbon electrodes with high surface area and has been shown to possess extremely high energy density compared to that of traditional electrolytic capacitors based on electro-oxidized aluminium and tantalum electrodes [6]. Aknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT55 and PUT1107), and Estonian Centres of Excellence (projects no. TK117T and TK141).

Spectroscopy study of ionic liquid restructuring at lead interface

Cyclic voltammetry, electrochemical impedance spectroscopy and surface enhanced infrared absorption (SEIRA) spectroscopy have been applied for the study of the electrical double layer of the magnetron sputtered thin film Pb|1-ethyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquid system. A wide potential range of ideal polarizability has been established for the system between −1.6 and −0.1V vs. Ag|AgCl in the same ionic liquid, in which the differential capacitance-potential curve is shown to have two maxima with a minimum at the perceived potential of zero charge. The first application of the in situ SEIRA technique for the study of Pb electrodes in an electrolyte solution is shown and the results are put into context of the latest developments concerning screening of metal interfaces in ionic liquid medium. Slow kinetics of oxide reduction reaction is demonstrated, which may limit the application of dry ionic liquids in lead-acid batteries.

Photoelectrochemical Studies and Capacitance Measurements during the Nitride Passivation of InP in Liquid Ammonia (-55°C)

This work presents a fundamental photo-electrochemical process at the interface SC/electrolyte. The photoelectrochemical passivation of n-InP is reported in liquid ammonia (-55 °C) during an anodic treatment[1]. The extraction of InP sample from the solvent is successfully performed at ambient temperature. Thanks to XPS (X-ray Photoelectron Spectroscopy), a reproducible polyphosphazene like film is reported on InP (-[(H2N)-P=N]n-)[2]. A soft anodic electrochemical treatment under illumination is applied at the interface to provide a gradual controlled coverage ratio of the surface by the film. This quantitative study requires a chronopotentiometry process with a constant anodic current of 1µA.cm-2 under illumination. An anodic charge around 0.3 mC.cm-2 is enough to ensure a complete coverage of the surface by the film. According to the anodic charge a gradual striking flat band potential shift is observed (Fig. 1). A flat C-2 = f(V) is extended over 1 volt leading to a high interfacial capacitance (» 2µF.cm-2). From the analysis of capacitance-potential curves, the interface appears to be under accumulation conditions after the film formation. A significant drop of the resulting photo potential is then observed (Fig. 2). However, under a strong positive overvoltage (» 1V), a stationary anodic photocurrent is recovered. [1] A-M. Gonçalves, N. Mézailles, C. Mathieu, P. Le Floch, A. Etcheberry, « «Fully Protective yet functionalizable monolayer onto InP” Chemistry of Materials.22(2010) 3114-3120. [2] A-M. Gonçalves, C.Njel, C. Mathieu, D.Aureau, A. Etcheberry. « Phosphazene like film formation on InP in liquid ammonia (223K) . Thin Solid Films 538 (2013) 21–24. Figure 1

Investigation of Nickel-Rare Earth Electrodes for Sodium Borohydride Electrooxidation

Sodium borohydride (NaBH4) is seen as a promising fuel of the future due to its chemical stability, ease in storage and handling, non-toxicity of its oxidation products and possibility of their recycling [1]. Additionally, fuel cells operating with NaBH4as the anodic fuel and oxygen or hydrogen peroxide as the oxidant, known as direct borohydride fuel cells (DBFCs) or direct borohydride-peroxide fuel cells (DBPFCs), respectively, have higher energy density compared to other types of fuel cells [2].However, at most electrode materials, parallel with borohydride oxidation reaction (BOR) proceeds borohydride (BH4 -) hydrolysis reaction that results in lower BOR coulombic efficiency. Consequently, there is a wide search for anode materials for DBFCs that would catalyze only BOR and suppress BH4 -hydrolysis.Herein, nickel-samarium (Ni-Sm) and Ni-dysprosium (Ni-Dy) alloys, with different amounts of RE metal (from 5 to 10 at.%), were tested as electrode materials for BH4 - oxidation in alkaline media. The alloys were prepared by arc-melting starting from the stoichiometricamounts of the parent metals and, subsequently, their microstructure and composition were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy.Preliminary study of the alloys activity for BOR was performed using cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). CV and CA data obtained made possible evaluation of different reaction parameters including number of electrons exchanged. Influence of temperature on BOR and its parameters was studied in the 25 – 65 ºC range. This investigation further enabled calculation of the activation energy for BOR at the studied alloy electrodes.EIS measurements were carried out mainly in the potential region of the faradaic processes of BH4 -oxidation. Resulting capacitance-potential curves for the tested Ni-RE electrodes helped in the interpretation of the electroactivity of the materials for the BOR. Impedance diagrams at different overpotentials in the BOR region indicated the nature of the reaction mechanisms and a possible change of the overall reaction mechanism.Furthermore, laboratory DBPFCs were constructed employing the studied Ni-RE alloy electrodes as fuel cell anodes. Main cell parameters, namely maximum power density and cell potential were evaluated for temperatures ranging from 25 to 65 ºC.The obtained parameters were compared to those obtained for DBPFCs employing pure Ni anode, as well as with those employing other Ni-RE anodes [3].ReferencesD.M.F. Santos, C.A.C. Sequeira, Sodium borohydride as a fuel for the future, Renew. Sustain. Energy Rev. 15 (2011) 3980.D.M.F. Santos, P.G. Saturnino, R.F.M. Lobo, C.A.C. Sequeira, Direct borohydride/peroxide fuel cells using Prussian Blue cathodes, J. Power Sources 208 (2012) 131.D.M.F. Santos, B. Šljukić, L. Amaral, D. Macciò, A. Saccone, C.A.C. Sequeira, Nickel and nickel-cerium alloy anodes for direct borohydride fuel cells, J. Electrochem. Soc. 161(5) (2014) F599.

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

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.

Surface chemistry of carbon electrodes in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid – an in situ infrared study

Three different carbon materials were studied in very dry 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) ionic liquid using infrared and electrochemical spectroscopy methods. Advantages of in situ infrared measurements with magnetron sputtered thin carbon films are demonstrated over thicker porous carbide-derived carbon and pyrolytic graphite electrodes. Differences in the double layer structure exist for the three carbon materials, confirmed by the different shape of capacitance-potential curves and infrared spectra. It was established that imidazolium cation dimerizes at potential E < −2V (vs. Ag|AgCl wire in the same ionic liquid), whereas anion as well as cation decomposition takes place at E > +1.6V. An explanation is given why EMImBF4 with a potential window of 4.1V for the carbon film electrode gives symmetric supercapacitor voltage of only 2.8V.

Ion effects at electrode/solid polymer electrolyte membrane interfaces

The differential capacity and the potential distribution at electrode/solid polymer electrolyte membrane/solution interfaces are calculated through an analytical approach. The model considers coions' and counterions' permeation through the membrane from the solvent phase and the ions' partitioning equilibrium at the SPEM/solution interface. The latter effects are included by incorporating the Donnan equilibrium, the steric hindrance, the solvation energy change when ions move from water to membrane pores and ion electrostatic interactions. It is shown that capacitance maxima in capacitance-potential curves may appear because of the acid-base dissociation process inside the membrane and the change in the ions' total interaction energy with the applied potential. For low dielectric constants inside membrane pores, εp, sharp peaks can be obtained. These peaks broaden, decrease in magnitude and shift to positive potentials once εp is increased. Finally, model predictions are discussed in light of recent experimental data obtained on Nafion® covered Pt(111) electrodes, providing a theoretical framework for the qualitative electroanalysis of these systems.

Molecular Insights into Carbon Nanotube Supercapacitors: Capacitance Independent of Voltage and Temperature

Molecular dynamics (MD) simulations of supercapacitors with single-walled carbon nanotube (SWCNT) electrodes in room-temperature ionic liquids were performed to investigate the influences of the applied electrical potential, the radius/curvature of SWCNTs, and temperature on their capacitive behavior. It is found that (1) SWCNTs-based supercapacitors exhibit a near-flat capacitance–potential curve, (2) the capacitance increases as the tube radius decreases, and (3) the capacitance depends little on the temperature. We report the first MD study showing the influence of the electrode curvature on the capacitance–potential curve and negligible dependence of temperature on capacitance of tubular electrode. The latter is in good agreement with recent experimental findings and is attributed to the similarity of the electrical double layer (EDL) microstructure with temperature varying from 260 to 400 K. The electrode curvature effect is explained by the dominance of charge overscreening and increased ion density per unit area of electrode surface.

Electric double layer studies at the interface of mercury–binary ionic liquid mixtures with a common anion

To gain deeper insight into the structure of the electrode–ionic liquid interface differential capacitance–potential curves have been acquired through electrochemical impedance measurements at the mercury electrode. Understanding the role of the nature and size of the ionic species in the ionic liquid, namely the influence of different lengths of alkyl side chains of the imidazolium cation on the structure of the double layer, is the aim of the work being carried out. Differential capacitance–potential curves were obtained at the mercury electrode in contact with various mixtures of room-temperature ionic liquids of varying cation chain length (1-alkyl-3-methylimidazolium, [CnMIM]+ where n = 2, 6 and 12) with the same anion (bis(trifluoromethylsulfonyl)imide, [Tf2N]−). For comparison purposes we have also carried out electrochemical measurements of the pure ionic liquids under the same conditions. Systematic variations in the electrocapillary drop-time curves and capacitance were found with increasing amounts of the cation with longer alkyl chains in the mixture. The electrocapillary curve results may be interpreted as indicating the preferential adsorption of the longer alkyl group on the mercury surface.

A Computational Study of the Interfacial Structure and Capacitance of Graphene in [BMIM][PF6] Ionic Liquid

A combination of graphene-like electrodes and ionic liquid (IL) electrolytes has emerged as a viable and attractive choice for electrochemical double layer (EDL) capacitors. Based on combined classical molecular dynamics and density functional theory calculations, we present the interfacial capacitance between planar graphene and [BMIM][PF6] IL, with particular attention to the relative contributions of the electric double layer capacitance at the graphene/IL interface and the quantum capacitance of graphene. The microstructure of [BMIM][PF6] near the graphene electrode with varying charge densities are investigated to provide a molecular description of EDLs, including BMIM/PF6 packing and orientation, cation-anion segregation, and electrode charge screening. Although the IL interfacial structures exhibit an alternative cation/anion layering extending a few nanometers, the calculated potential profiles provide evidence of one-ion thick compact EDL formation. The capacitance-potential curve of the EDL is convex- or bell-shaped, whereas the quantum capacitance of graphene is found to have concave- or U-shaped characteristics with a minimum of nearly zero. Consequently, we find that the total interfacial capacitance exhibits a U-shaped trend, consistent with existing experimental observations at a typical carbon/IL interface. Our work highlights the importance of the quantum capacitance in the overall performance of graphene-based EDL capacitors.

A New Capacitive Sensor based on Electrostriction Phenomenon. Application for Determination of Ionic Surfactants

Abstract The aim of the work was to develop a new capacitive sensor based on the electrostriction phenomenon observed in membranes deposited on solid electrodes in contact with an electrolyte solution. The functioning of the sensor was checked in determination of ionic surfactants. The sensor was based on self-assembled monolayers (SAM's) of thiol supported on the surface of a gold electrode. Two thiols, 1-octadecanethiol and 1-hexadecanethiol, and two surfactants, anionic sodium dodecyl sulfate and cationic hexadecyltrimethylammonium bromide, were chosen for studies. Different shapes of capacitance-potential curves with hysteresis were revealed. After an addition of a surfactant, the height of hystheresis changed, enabling construction of linear relationship between the hystheresis’ height and concentration of surfactant.