Double-layer Capacitor Electrode Research Articles

Adsorption and two-dimensional condensation of 5-methylcytosine

Purine and pyrimidine derivatives occurring in nucleic acids posses an extraordinary high ability of self-association at the electrode surface and can form there by a two-dimensional (2D) condensation a monomolecular compact film (self-assembled monolayer—SAM). The effects of methyl substituent on the 2D condensation were studied using the 5-methylcytosine molecule which is involved in gene silencing and has a great biological impact. At acid pHs, 5-methylcytosine forms at the mercury electrode a physisorbed self-assembled 2D layer at potentials close to the potential of electrocapillary maximum. From the temperature dependence of the electrode double layer capacitance, the standard Gibbs energy of adsorption (Δ G m = − 12.7 kJ mol − 1 ), lateral interaction coefficient of the Frumkin adsorption isotherm ( a c = 2.05) and area occupied by one molecule ( A = 1.31 nm 2) in the 2D layer were determined. Measurements performed on a single-crystal Au(111) surface show that the 2D condensation can take place on other substrates as well.

Structure and EDLC characteristics of pitch-based carbon aerogels

Abstract Morphology and porous structure of the pitch-based carbon aerogels (CAs) were characterized by scanning electron microscopy (SEM) and N2 adsorption–desorption measurement. Their electrochemical performances as electric double-layer capacitor (EDLC) electrodes were studied by cyclic voltammetry and galvanostatic charge–discharge measurements. It was found that the particle size of the CAs decreased with the increase of the volume ratio of toluene to acetic acid (T/A ratio). The BET surface areas of the obtained CAs were in the range of 290–450 m2 g−1. CA electrodes had good electrochemical performance and appropriate specific capacitance. Moreover, the electrochemical performance of the CAs was enhanced by the existence of extensive mesoporous networks of the CAs. CA with 2.23 of T/A ratio had the highest specific capacitance, i.e., 131.9 F g−1 for cyclic voltammetry. Thus, it was thought that the pitch-based CAs were suitable electrode materials for EDLCs.

Control and behaviors of resorcinol-acetaldehyde carbon cryogel porous structures as electrodes for electric double-layer capacitors

The porous structures of the mesoporous carbon materials, which are known to be effective energy storing systems, were manipulated and their behaviors as electric double-layer capacitor electrodes were evaluated. Resorcinol (R) as the raw material and acetaldehyde (A) as the cross-linking agent were used to synthesize the RA hydrogels via sol-gel polycondensation in a slightly basic aqueous solution. The RA hydrogels were freeze-dried and subsequently pyrolyzed at 1073 K for 30 min (heating rate, 10 K/min) to obtain the RA carbon cryogels. The obtained samples were confirmed to have a high mesoporous carbon content, with the primary pore size diameter distribution of 20 to 40 nm. Due to a high ratio of ion adsorption and desorption capacitances, the samples were effective electrode materials capable of high-speed electrical charge and discharge for use as electrode materials of electric double layer capacitors.

Comparison Between Electrochemical Properties of Aligned Carbon Nanotube Array and Entangled Carbon Nanotube Electrodes

Carbon nanotubes (CNTs) are promising electrochemical double-layer capacitor electrode materials because they have excellent conductivity and mesopores dominated pore structures. Electrochemical properties of an ultralong aligned carbon nanotube array (ACNTA) electrode and an entangled CNT (ECNT) electrode in ionic liquid electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance. The ACNTA electrode obtains higher specific capacitance, lower equivalent series resistance, and better rate capability than the ECNT electrode. The reason is that ACNTA electrode possesses larger pore size and more regular pore structure and conductive paths, which are revealed by adsorption and scanning electron microscopy results, than ECNT electrode.

Rate Capability of Electric Double-Layer Capacitor (EDLC) Electrodes According to Pore Length in Spherical Porous Carbons

A series of spherical porous carbons were prepared via resorcinol-formaldehyde (RF) sol-gel polymerization in the presence of cationic surfactant (CTAB, cetyltrimethylammonium bromide), wherein the carbon sphere size was controlled by varying the CTAB introduction time after a pre-determined period of addition reaction (termed as "pre-curing"). The sphere size gradually decreases with an increase in the pre-curing time within the range of 30-150 nm. The carbons possess two types of pores; one inside carbon spheres (intra-particle pores) and the other at the interstitial sites made by carbon spheres (inter-particle pores). Of the two, the surface exposed on the former was dominant to determine the electric double-layer capacitor (EDLC) performance of porous carbons. As the intra-particle pores were generated inside RF gel spheres by gasification, the pore diameter was similar for all these carbons, thereby the pore length turned out to be a decisive factor controlling the EDLC performance. The charge-discharge voltage profiles and complex capacitance analysis consistently illustrate that the smaller-sized RF carbons deliver a better rate capability, which must be the direct result of facilitated ion penetration into shorter pores.

Resonance impedance sensing of human blood cells

A challenging problem in alternating current (AC) impedance sensing of particles (e.g., blood cells in plasma) with micro electrodes is that with the shrinking of electrode surface area the electrode double layer capacitance decreases. This double-layer capacitor dominates the system impedance in low frequency range, while the parallel stray capacitor dominates the system impedance in high frequency range. Hence the sensitivity for particle sensing for micro impedance sensors decreases over a wide frequency range. In this paper, we propose an approach to solve the problem. The idea is to use resonant sensing by connecting an external parallel inductor to the system. At the resonant frequency, the capacitive components in the system are nullified by the inductor, leaving the channel impedance (including the particle impedance) to be a major component in the system impedance. We then successfully demonstrate this idea by sensing 5μm polystyrene beads. More important, this technique is extended to sensing blood cells in diluted human whole blood and leukocyte-rich plasma. The sensitivity can be improved by two orders of magnitude over more than three decades in frequency domain. The measured signal peak height histogram at low frequency matches well with known volume distribution of erythrocytes and leukocytes.

Electrochemical properties of ultra-long, aligned, carbon nanotube array electrode in organic electrolyte

The electrochemical properties of an electrochemical double-layer capacitor electrode based on an ultra-long (500 μm), aligned, carbon nanotube array (ACNTA) in Et 4NPF 6/propylene carbonate electrolyte are examined. The specific capacitance of the ACNTA electrode in an organic electrolyte is 24.5 F g −1, which is larger than that obtained in an aqueous electrolyte. The results of ac impedance measurements show that the ACNTA electrode gives a high power density and an excellent rate capability in an organic electrolyte. It is shown that the ACNTA electrode has a lower equivalent series resistance and a better rate capability than activated carbon electrode. This is due to the fact that ACNTA possesses a larger pore size and a more a regular pore structure. Both these features are conformed by scanning electron microscopic and nitrogen gas adsorption studies.

Supercapacitors from Activated Carbon Derived from Banana Fibers

Carbon materials were synthesized from banana fibers by treating the fibers with pore-forming substances such as ZnCl2 and KOH with an intention to improve the surface area and their electrochemical performance as electrical double-layer capacitor electrodes. The performance of these materials was studied in a neutral electrolyte for the first time. There has been a substantive increase in the specific surface area of the treated carbon material because of the effective pore generations. The structural and surface properties of the prepared carbon materials were studied using scanning electron microscopy and N2 adsorption/desorption studies. The surface area of the 10% ZnCl2 treated sample was found to be 1097 m2/g. The electrochemical properties of untreated and porogen treated carbons were evaluated by using cyclic voltammetry and galvanostatic charge−discharge studies, and the specific capacitance as high as 74 F/g in 1 M Na2SO4 neutral electrolyte was obtained for 10% ZnCl2 treated carbon as determined by constant current charge−discharge studies. The system showed excellent cyclability with a Coulombic efficiency of ∼88% at a high current density of 500 mA/g for 500 cycles. The electrochemical performance of the high surface area carbon in the neutral electrolyte medium is significantly high, and the reasons are discussed.

Matching ratio between positive and negative electrodes for double-layer capacitors

For double-layer capacitors in alkaline electrolyte, the specific capacitance of the positive electrode is not equal to that of the negative one. Thus, capacitor performance cannot be optimal with a positive/negative electrode matching ratio of 1. In this study nanoporous glassy carbons (NPGCs) were employed as the electrodes of capacitors, and the influence of matching ratio between positive and negative electrode on capacitor performance was systematically investigated. In aqueous KOH, the specific capacitance of the positive electrode is lower than that of the negative electrode. The matching ratio at which a maximum capacitance is obtained is dependent on the values of the positive and negative electrode capacitance. At low current rate, the highest specific capacitance is achieved at a matching ratio slightly higher than 1. At high current rate, a capacitor has the highest specific capacitance with the lowest resistance at a matching ratio of 1.5. This indicates that an optimum matching relationship between positive and negative electrodes is attained.

Mesoporous Carbon Materials as Electrodes for Electrochemical Double-Layer Capacitor

ABSTRACTNanostructured carbon materials with a regular array and narrow size distribution of mesopores have been synthesized at Oak Ridge National Laboratory via the self-assembly of block copolymers as soft templates. One promising application for these materials is as electrodes for electrochemical double-layer capacitors. To evaluate the performance, electrodes were prepared by coating the precursor onto fibrous carbon paper, followed by curing and pyrolysis at 850°C. The resulting mesoporous carbon has a surface area of 310 m2/g and an average pore size of 8.5 nm. Results from cyclic voltammetry and impedance spectroscopy experiments using a sulfuric acid electrolyte showed high specific capacitance values of up to 300 F/g. For comparison, a commercially available aerogel carbon coated paper was also examined.

Chemical treatment of carbon nanotubes as electrodes in electrochemical double-layer capacitors

Multi-walled carbon nanotubes with homogeneous diameters (40–60 ran), produced by chemical vapor deposition of hydrocarbon gas, are purified by nitric acids. Infrared and Raman studies indicate that oxygen containing surface groups, which are predominately carboxylic, phenolic and lactonic groups, are introduced into purified carbon nanotubes. Then three kinds of block-form porous tablets of carbon nanotubes are fabricated as electrodes in electrochemical double-layer capacitors. Using mounded mixture comprising carbon nanotubes and binder powders provides these tablets. Comparison of the effect of different processing on the structural performance of the capacitors is specifically investigated. Using chemically treated electrodes, electrochemical double-layer capacitors with a specific capacitance of about 33 F/g are obtained with 38 wt% H2 SO4 as the electrolyte.

The electrochemical reduction of VO 2+ in acidic solution at high overpotentials

An investigation has been carried out to attempt to understand the unusually low apparent symmetry factor observed during the reduction of V(5) at higher overpotentials at carbon electrodes (typically <0.13, or >460 mV decade −1). This reaction is of interest because it occurs in vanadium redox-flow batteries (VRBs) during discharge. Polarisation curves were measured using a rotating disk electrode (RDE). The reaction was not solution mass transport controlled, was pH independent (ca from 0 to 1), and the observed Tafel slope was unaffected by V(5) concentration over a range from 0.031 to 280 mM. Electrode double layer capacitance measurements were also carried out in sulphuric acid with and without vanadium. These tests showed that the presence of V(5) caused a suppression of the normal carbon surface quinone pseudocapacitance, as well as the appearance of two new pseudocapacitance peaks, one around 0.175–0.2 V and the other around 0.675–0.725 V versus SCE. The observed results do not appear consistent with a precipitated film causing diffusion limitations or causing IR drop. A model is developed to try to explain the data, which involves electron transfer through an adsorbed layer of vanadium.

Toluene-insoluble fraction of fullerene-soot as the electrode of a double-layer capacitor

Several electrochemical properties of the heat-treated toluene-insoluble fraction separated from fullerene-soot (Fs-TI) were investigated to find their application as an advanced electrical double-layer capacitor (EDLC) electrode material using an organic solvent electrolyte. The heat-treated Fs-TI at 900 °C (Fs-TI-900) showed as high as 58 F g −1 of capacitance, which is ascribed to its high capacitance per unit surface area as well as its reasonable surface area. In addition, the capacitor cell with Fs-TI-900 electrodes exhibited a better rate stability than that with conventional active carbon fiber electrodes due to its high electronic conductivity. The material heat-treated at 2400 °C (Fs-TI-2400) showed much smaller capacitance than Fs-TI-900 due to its smaller surface area. A unique rate dependence of capacitance was observed when the ion size of electrolyte was varied.

Assessment of Liquid Crystal Template Deposited Porous Nickel as a Supercapacitor Electrode Material

The high surface area porous nickel, obtained by template electrodeposition using a hexagonal liquid crystalline phase medium as a template was evaluated as a potential material for electrochemical capacitors using cyclic voltammetry ~CV! and electrochemical impedance spectroscopy ~EIS! studies in 6 M KOH. A single electrode double layer capacitance of 1.4 F/cm2 ~at 2 mV/s scan rate! was obtained using CV, which corresponds to a specific capacitance of 473 F/g. EIS studies show the typical behavior of a porous electrode and the data were analyzed in terms of complex capacitance and complex power from which the relaxation time constant stod has been determined. Nickel oxide electrode, obtained by the electrochemical oxidation of porous Ni, shows a double layer capacitance value of 171 mF/cm2, which corresponds to a specific capacitance of 57 F/g. The relaxation time constants for the template deposited porous nickel and nickel oxide were determined to be 1.35 s and 33 ms, respectively. The values of double layer capacitance and specific capacitance of the porous nickel with fast response time are the highest values reported for nickel in the literature so far. © 2005 The Electrochemical Society. @DOI: 10.1149/1.1911931# All rights reserved.

Investigation of Ig.G Adsorption and the Effect on Electrochemical Responses at Titanium Dioxide Electrode

The adsorption of Immunoglobulin G on a titanium dioxide (TiO(2)) electrode surface was investigated using (125)I radiolabeling and electrochemical impedance spectroscopy (EIS). (125)I radiolabeling was used to determine the extent of protein adsorption, while EIS was used to ascertain the effect of the adsorbed protein layer on the electrode double layer capacitance and electron transfer between the TiO(2) electrode and the electrolyte. The adsorbed amounts of Ig.G agreed well with previous results and showed approximately monolayer coverage. The amount of adsorbed protein increased when a positive potential was applied to the electrode, while the application of a negative potential resulted in a decrease. Exposure to solutions of Ig.G resulted in a decrease of the double layer capacitance (C) and an increase in the charge-transfer resistance (R(2)) at the electrode solution interface. As more Ig.G adsorbed onto the electrode surface, the extent of C and R(2) variation increased. These capacitance and charge-transfer resistance variations were attributed to the formation of a proteinaceous layer on the electrode surface during exposure.

Complex capacitance analysis on rate capability of electric-double layer capacitor (EDLC) electrodes of different thickness

The complex capacitance analysis is utilized to examine the thickness-dependent rate capability of electric double-layer capacitor (EDLC) electrodes. Based on the transmission line model, the theoretical imaginary capacitance is derived for porous carbon electrodes, where the resistance relevant to ion transport in pores of carbon particles (intra-particle pores) and within electrode layer (inter-particle pores) is assumed to be the major component for equivalent series resistance (ESR). The use of hexagonal mesoporous carbon (HMC) as the EDLC electrode material, which has a well-defined pore structure, allows us to estimate the number of intra-particle pores in the composite electrodes such that the two resistance components are separately analyzed as a function of electrode thickness. As the theoretical derivation suggests, the time constant for intra-particle pores is invariant against the electrode thickness, whereas the time constant for inter-particle pores becomes larger for thicker electrodes. The poorer rate capability observed in the thicker electrodes is thus ascribed to a larger time constant for inter-particle pores.

Scanning Electrochemical Microscopy, 52. Bipolar Conductance Technique at Ultramicroelectrodes for Resistance Measurements

The bipolar conductance, BICON, technique for the measurement of solution resistance, based on the application of microsecond current pulses, as originally described by Enke and co-workers for measurements with conventional electrodes, was extended for use with ultramicroelectrodes, with a focus on its application in scanning electrochemical microscopy (SECM). When the plateau time used to make the measurement lies within the BICON conditions, the solution conductance can be obtained directly from the output without the need for calibration curves. However, decreasing the size of the ultramicroelectrode decreases the range of values that satisfy these conditions, and one must resort to calibration curves to obtain solution conductance from the measured current, which was nevertheless found to be proportional to electrolyte concentration with electrodes as small as 5 mum in diameter. BICON/SECM approach curves over insulating substrates followed SECM negative feedback theory and approach curves in the presence of low (micromolar) or no added electrolyte are possible once the background conductivity is taken into account. Approach curves to a conducting substrate at open circuit potential are influenced by the solution time constant (solution resistance at the electrode tip x electrode double layer capacitance), which is a function of the tip/substrate distance, as well as the substrate size.

Simple Synthesis of Uniform Mesoporous Carbons with Diverse Structures from Mesostructured Polymer/Silica Nanocomposites

Uniform-sized mesoporous carbons having high surface areas and various structures were synthesized directly by the carbonization of P123 triblock copolymer/phenol−resin/silica nanocomposites, which were prepared from the sol−gel polymerization of silica in the presence of Pluronic P123 triblock copolymer and phenol and subsequent silica removal. Mesoporous carbon, with uniform 5.6 nm wormhole-like pores, was synthesized using sodium silicate as a silica source. When the amount of phenol relative to block copolymer was reduced in the synthesis, mesocellular carbon foams with bimodal pore structures were obtained. In addition, mesoporous carbons were synthesized using TEOS (tetraethyl orthosilicate) as a silica source. The pore size of the mesoporous carbons was controlled by varying the ratio of phenol and P123 triblock copolymer. At an optimal molar ratio of phenol to block copolymer, mesoporous carbon with a nanofiber morphology was obtained. The mesoporous carbons were successfully used as the electrodes for electrochemical double-layer capacitors.

Comparative studies of self-discharge by potential decay and float-current measurements at C double-layer capacitor and battery electrodes

Abstract Charged electrochemical capacitors and battery electrodes are in a state of high Gibbs energy in relation to that of their discharged states; hence there is a thermodynamic “driving force” for their self-discharge on open-circuit. Several mechanisms for self-discharge can be envisaged and diagnostically distinguished. They must take place by mixed cathodic/anodic electrochemical processes (as in corrosion) or, in some cases, by a surface-chemical process. Self-discharge can be characterized by two procedures: (a) measurement of open-circuit decline of electrode potential or state-of-charge with time or (b) by establishing the polarizing currents, so-called float-currents, at various potentials in the self-discharge process that are required just to maintain those respective potentials constant. The importance, for either case, of characterizing the self-discharge behavior individually for each electrode of a cell pair (using a third electrode as a reference) is stressed. Experimental data are presented for potentiostatic float-current measurements at porous C-cloth and glassy-C electrodes, and related to digital potential-decay measurements under the same conditions in aqueous H 2 SO 4 below the decomposition potential of the solution. Treatment of an equivalent circuit model enables the time dependence of components of double-layer charging and self-discharge under potentiostatic float conditions to be understood and evaluated.

Preparation of high surface area nickel electrodeposit using a liquid crystal template technique

We show in this work that template electrodeposition of nickel at room temperature from a nickel sulphamate bath prepared in a new hexagonal liquid crystalline phase of water–Triton X-100–poly (acrylic acid) results in a highly porous surface. The roughness factor value of about 3620 obtained for this coating is the highest value reported in the literature for any electrodeposited nickel. The scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) pictures show the formation of porous deposit with granular features in between the pores. The single electrode double layer capacitance value measured for the deposit is 338 mF cm −2, which translates into a specific capacitance of 50 F g −1 without any post-thermal treatment of the electrode, suggesting its utility in super capacitors. Electrochemical studies using cyclic voltammetry (CV), Tafel plots and electrochemical impedance spectroscopy (EIS) and comparison of these results with some existing high surface area Ni catalysts show that the material has potential application as an excellent hydrogen evolving cathode.