Coulometric Efficiency Research Articles

Electrochemical preparation of standard solutions of Pb(II) ions in ionic liquid for analysis of hydrophobic samples: The olive oil case

In this paper we present an electrochemical approach to prepare standard solutions of metal ions in a room temperature ionic liquid (IL), which can find useful application for analysis in hydrophobic matrices. The method, developed here for the case of lead ions, is based on the galvanostatic dissolution of a lead anode dipped directly in a suitable IL, namely tri-hexyl(tetradecyl)phosphonium bis (trifluoromethylsulfonyl) imide ([P14,6,6,6]+[NTf2]-). After each oxidation step, the metal dissolution process in the IL solutions was monitored by cyclic voltammetric measurements at a glassy carbon disk electrode. The results indicated that the peak current relevant to the reduction of the electro-generated Pb(II) increased linearly while increasing the oxidation time. By varying the oxidation time from 200 to 6000s, a set of Pb(II)/[P14,6,6,6]+[NTf2]- solutions at concentrations ranging between 10 and 300μgg−1 was prepared. To validate the efficiency of the electrochemical procedure to produce metal ion standard solutions, the Pb content was quantified by developing a microwave digestion procedure specifically suitable for the IL medium, followed by ICP-QMS analysis in the digested standards. The results indicated a satisfactory agreement between concentrations found by ICP-QMS and calculated from electrochemical data, with a coulometric efficiency of Pb(II) generation in ionic liquid ≥95.6%. Finally, the applicability of the Pb(II)/IL solutions as standards for analyses in hydrophobic media was tested by determining, by ICP-QMS, the Pb content in an extra-virgin olive oil spiked with known amounts of a Pb(II)/IL standard. Satisfactory Pb recoveries, ≥96%, were measured.

Silicon/Carbon Nano-Composite Based Anodes for Advanced Lithium-Ion Batteries

In the recent years, rechargeable lithium-ion batteries have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of lithium-ion batteries. Especially for high energy and power densities, the electrode materials must possess high specific storage capacities and coulometric efficiencies. However, state-of-the-art anode and cathode electrode materials, e.g. graphite and LiFePO4 exhibit high coulometric efficiencies but rather low theoretical storage capacities (372 and 170 mAh/g, respectively). In the last decade silicon has become a promising anode material due to its high theoretical specific capacity of 3579 mAh/g at ambient temperature. However, this high specific storage capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 280 % during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nano-sized active material or carbon coated silicon composite material. In addition to the materials science the electrode structure is of particular importance for the electrochemical performance. Electrode composition, binding mechanism due to the use of suitable binder polymers or particle size distribution of the active material are some exemplary parameters to stabilize the electrode structure and to handle such high mechanical stress during lithiation/delithiation. Here we present electrochemical investigations of high capacity and high efficiency graphene coated silicon nanocomposite based electrodes prepared by using a wet chemical doctor blade manufacturing process. This active material provides a capacity of >2000 mAh/g with efficiencies >99% over more than 500 cycles. Investigations focus on influence of the stability of the electrode structure by using impedance spectroscopy, scanning electron microscopy, confocal microscopy and estimation of the coating adhesion strength. Cyclic voltammetry and galvanostatic cycling will show the applicability of improved Si/C composite based electrodes compared to conventional graphite based electrodes for lithium-ion batteries both in half cells as well as full cells in combination with commercially available cathode material.

Low temperature process modified MCMB for lithium ion batteries

The negative electrode of a lithium-ion battery should have the characteristics of large charge capacity, excellent charge and discharge cycle, steady discharge voltage, low irreversible charge capacity, and stable electrolyte. Natural graphite and mesocarbon microbeads (MCMB) are the most common materials used for the negative electrodes of such batteries. This study examines the effect of the use of homemade non-modified MCMB (HMM), modified MCMB (HMMQ), and natural graphite (NG) to produce electrodes on the batteries' high current charge–discharge behavior. In this experiment, the charge–discharge results showed that the voltage plateaus of HMM and HMMQ sloped due to short-range structural order, which enabled the intercalation and deintercalation of lithium ions to occur at different voltages. When HMMQ was charging and discharging high current at a rate of 2C, the battery's charge capacity reached 160mAh·g−1 and the coulometric efficiency exceeded 96%. Following the start of charging and discharging, a stable solid electrolyte interface (SEI) layer formed on HMMQ. The charge capacity of HMMQ was approximately 167mAh·g−1 after 100 charge–discharge cycles at 1C, and the cycles were stable. According to FTIR analysis, the content of the SEI layer produced within HMMQ consisted of Li2CO3 and (CH2OCOLi)2, and was stable, non-soluble, and passive. Negative electrodes made using modified material displayed good stability throughout many cycles when charging and discharging at a high current.

Development and characterization of a novel semiautomated arrangement for electrochemically assisted injection in combination with capillary electrophoresis time‐of‐flight mass spectrometry

Electrochemically assisted injection (EAI) is an attractive injection concept for CE that enables the separation of neutral analytes via electrochemical generation of charged species during the injection process. A new semiautomated EAI configuration was developed and applied in conjunction with CE-MS (EAI-CE-MS). The EAI cell arrangement consists of an integrated buffer reservoir for CE separations and a compartment holding screen-printed electrodes. A drop of sample solution (50 μL) was sufficient to cover the three-electrode structures. A piezo motor provided a fast and precise capillary positioning over the screen-printed electrode assembly. Using ferrocene methanol as a model system, the EAI arrangement was characterized regarding coulometric efficiency, precision, and sensitivity of electrospray ionization-time-of-flight-MS. The formation of the cationic oxidation product of ferrocene methanol enhanced the sensitivity of CE-MS determination by two orders of magnitude and the electrochemically formed product showed a migration time corresponding to its individual electrophoretic mobility. Preliminary studies of EAI-CE-MS in the field of the analysis of nitroaromatic compounds were carried out. The formation of corresponding hydroxylamines and amines paved the way for selective and sensitive CE-MS determinations without the need of adding surfactants to the electrophoresis buffer.

Electrophotocatalysis of aqueous methyl tert-butyl ether on a titanium dioxide coated electrode

In this study, TiO2 was immobilized onto a titanium electrode for conducting MTBE degradation in aqueous solution. The TiO2 coated electrode could possess photocatalytic characteristics, and thus the application of the TiO2 coated electrode is able to perform photocatalysis and/or electrolysis under various controls. A bench-scale reactor system consisting of an electrolytic cell, an electrochemical analyzer, an electrical multimeter, and a UV-light source was employed to perform electrochemical experiments and electrophotocatalytic experiments.The results of electrochemical experiments showed that an optimum electrolytic condition was operated at 3.00V in a 1M Na2SO4 electrolyte solution, and the initial 20mg/L MTBE was reduced by 67% within 180min. Comparatively, the photocatalytic removal of MTBE was only 52% within 180min. The electrophotocatalytic experiments have demonstrated that the photocatalytic removal of MTBE was raised to 65% as applying a potential at −0.25V. The recombination of photoinduced electrons and holes could be significantly suppressed because the generated electrons by TiO2 excitation were taken away from the electrode as applying a negative potential. On the contrary, the improvement on MTBE removal is not obvious with the assistance of a positive electrical potential. Even though the mechanisms of electrochemical oxidation and electrochemically assisted photocatalysis to produce radicals for the breakdown of MTBE might be different, both processes followed the identical degradation pathway of forming acetate and CO2 based on the identification of GC/MS. According to the comparison of coulometric efficiency, photocatalysis with the assistance of a slight negative potential is the most effective and efficient approach to treat MTBE contaminated water.

Analysis of pressure variations in a low-pressure nickel–hydrogen battery: Part 1

A low pressure nickel–hydrogen battery using either a metal hydride or gaseous hydrogen for H2 storage has been developed for use in implantable neuroprosthetic devices. In this paper, pressure variations inside the cell for the gaseous hydrogen version are analyzed and correlated with oxygen evolution side reaction at the end of charging, the recombination of oxygen with hydrogen during charging and a subsequent rest period, and the self-discharge of the nickel electrode. About 70% of the recombination occurred simultaneously with oxygen evolution during charging and the remaining oxygen recombined with hydrogen during the 1st hour after charging. Self-discharge of the cell varies linearly with hydrogen pressure at a given state of charge and increased with increasing battery charge levels. The coulometric efficiency calculated based on analysis of the pressure–time data agreed well with the efficiency calculated based on the current–time data. Pressure variations in the battery are simulated accurately to predict coulometric efficiency and the state of charge of the cell, factors of extreme importance for a battery intended for implantation within the human body.

Operational Limits of Controlled Current Coulometry with Ion‐Selective Polymeric Membranes

AbstractPlasticized polymeric membrane electrodes containing selective chemical receptors (ionophores) that are used in potentiometric and optical sensors have recently been shown to be attractive in controlled current coulometry for calibration free reagent delivery. For this purpose, a galvanostatic pulse of fixed duration is applied across an ion‐selective membrane containing added ion‐exchanger sites, resulting in the release of a calculated amount of ions from the membrane into the sample phase. This paper evaluates the operational limits of such coulometric actuators with chronopotentiometry, using silver‐selective membranes as model systems. Diffusion theory predicts the depletion of ionophore and ion‐exchanger at one of the two interfaces in a matter of minutes, owing to the relatively small diffusion coefficients in such membranes. Chronopotentiometry on membranes containing lipophilic cations at either side of the membrane, rather than silver ions, confirms that the ionophore depletes at the inner side of the membrane. At the sample side, ionophore is expected to increase in concentration and does not result in a loss of selectivity. Chronopotentiometric responses show a drastic transition at long times, typically 30 min, which cannot plausibly be explained by the depletion of added lipophilic ion‐exchanger at the sample side since the diffusion coefficients are similar to that of ionophore. It is postulated that the intrinsic anion‐exchanger sites of the PVC matrix are relatively immobile and do not easily concentration polarize upon application of a transmembrane current pulse, in agreement with Buck's earlier work on fixed site membranes. Indeed, silver and calcium‐selective membranes fabricated with increased concentrations of such fixed sites, by using carboxylated PVC, exhibited chronopotentiometric breakdown times larger than 60 min and no loss in coulometric efficiency. The results obtained here will help in designing coulometric actuators with improved characteristics on the basis of hydrophobic polymeric ion‐selective membranes.

In-channel modification of electrochemical detector for the detection of bio-targets on microchip

The coulometric efficiency ( C eff) of an amperometric detector integrated on PDMS/glass capillary electrophoresis microfluidic device (microchip) has been enhanced by in-channel electrochemical modification. In-channel electrochemical deposition of gold particles was performed in order to vertically increase the surface area of the Au sensing microelectrode. The roughness of the electrodes was characterized using scanning electron microscopy and profilometric analysis. The degree of electrode modification was also characterized by roughness factor determination. Separation processes including detection potential was optimized and the analytical performance of the microchip was tested using a mixture of dopamine (DA) and catechol (CA). The modified electrochemical detector provided well-resolved separation of DA and CA in less than 60 s with enhanced sensitivity; no peak broadening was observed. The limit of detection using in-channel modification of working electrode for DA and CA are 60 and 110 nM, respectively. Thus, in-channel electrochemical deposition of metallic particles should be used to enhance the C eff of integrated amperometric detection of analytes with good redox properties in order to obtain lower LODs.

Electrochemistry of ionophore-coordinated Cs and Sr ions in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid

The selective extraction of Cs + and Sr 2+ from aqueous solutions by using the ionophores calix[4]arene-bis( tert-octylbenzo-crown-6) (BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6), respectively, was demonstrated in the hydrophobic, room-temperature ionic liquid (RTIL), tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Tf 2N −). The electrochemistry of Cs + coordinated by BOBCalixC6 and Sr 2+ coordinated by DCH18C6 was examined at a mercury film electrode (MFE) in this ionic liquid by using cyclic staircase voltammetry, sampled current voltammetry at a rotating electrode, and chronoamperometry. Both BOBCalixC6·2Cs + and DCH18C6·Sr 2+ exhibit well-defined reduction waves at approximately −2.4 and −2.9 V versus the ferrocene/ferrocenium (Fc/Fc +) couple, respectively, in which the coordinated ions are reduced to their respective amalgams, permitting the recycling of the ionophores. The diffusion coefficients of BOBCalixC6·2Cs + and DCH18C6·Sr 2+ are (2.7 ± 0.1) × 10 −9 and (2.1 ± 0.1) × 10 −9 cm 2 s −1, respectively, at 30 °C. The coulometric efficiency for the reduction and stripping of Cs at mercury pool electrodes was about 90% and was independent of the deposition time, whereas the efficiency for Sr was slightly less than 90% at short times and decreased with the deposition time, probably due to the formation of a passive layer of Sr(Tf 2N) 2.

Etched electrochemical detection for electrophoresis in nanometer inner diameter capillaries.

Capillary electrophoresis in nanometer inner diameter capillaries allows for the analysis of extremely small volume samples, such as the contents of single cells. However, the utilization of these ultrasmall capillaries requires a very sensitive and low volume detector. An improved method for end-column amperometric detection for capillary electrophoresis in nanometer inner diameter (i.d.) capillaries is presented. This new method involves etching both the electrode and the detection end of the capillary. These design improvements allow for better alignment between the capillary bore and the electrode. As a result, dead volume in the detector is minimized. The etched method for electrochemical detection in ultrasmall capillaries provides average coulometric efficiencies of 70 +/- 10% for dopamine and 40 +/- 20% for catechol in 770 nm i.d. capillaries. Furthermore, this technique provides peak efficiencies as high as 100,000 theoretical plates and detection limits as low as 340 zmol for both dopamine and catechol.

The improvement of the cycle life of Li 2.6Co 0.4N as an anode of Li-ion secondary battery

Graphite has been widely used as the anode material of commercial Li-ion secondary battery. With the growing demands of high-capacity secondary battery, the low capacity of graphite (theoretical capacity: 372 mA h/g) has been thought to be the limiting factor in wide applications and a new anode material with a high capacity has been sought. Of the many materials, Li 2.6Co 0.4N showed the best anode performance. It showed very high capacity of 1024 mA h/g, good rate capability (1 C/0.2 C=94.94%) and extraordinary initial coulometric efficiency (96%). In addition to the excellent capacity of this material, its rate capability was much superior to that of graphite (82–88%). However, it cannot be commercialized because its capacity loss after 30 cycles is around 40%. As a result of various analyses, it was confirmed that the formation of passivating surface film caused by the decomposition reaction between electrolyte and Li 2.6Co 0.4N is the main reason of its cyclic degradation. As a method of improving the cyclic degradation, iron doping in Li 2.6Co 0.4N was suggested and we attempted to restrain the formation of passivating film on the surface of Li 2.6Co 0.4N. Li 2.6Co 0.35Fe 0.05N had a bit lower capacity (about 900 mA h/g) than Li 2.6Co 0.4N (1024 mA h/g) but showed much better cycle life than Li 2.6Co 0.4N (35%→60% after 50 cycles).

Electrochemical Behavior of La2Cu4 in Aqueous Media

The dissolution behavior of has been examined by electrochemical techniques in aqueous media under open‐circuit conditions at 298 K. is dissolved by aqueous acid at proton‐mass transport limited rates in 1 M NaCl. Rotating ring‐disk electrode (RRDE) techniques were employed in collection (product detection) and shielding (reactant removal) measure modes to monitor the surface chemistry, including a new approach not involving separate disk electrode reduction measurements as required heretofore. Related techniques monitoring the competitive oxygen evolution reaction were used to study the coulometric efficiency of anodic formation of the superconductor phases, , in alkaline medium. Solid‐state diffusion severely limits growth. © 1999 The Electrochemical Society. All rights reserved.

Comparison of μm and mm sized disk electrodes for end-column electrochemical detection in capillary electrophoresis

End-column electrochemical detection based on either the use of a 25 μm microdisk electrode or a 0.5 mm macrodisk electrode has been compared with respect to performance and influence on non-aqueous capillary electrophoretic separations. Despite the much higher coulometric efficiency obtained with the larger disk electrode, the microdisk electrode configuration offers comparable limits of detection (LOD) for the neutral and positively charged ferrocene compounds employed in conjunction with a non-aqueous acetonitrile-based buffer. The LODs for ferrocene were found to be 4.0 × 10–8 M and 6.7 × 10–8 M for the microdisk and macrodisk detector, respectively. In addition, both detector arrangements showed different relative responses for neutral and positively charged analytes. The macroelectrode-based detector introduced additional zone broadening while this was not found to be the case with the microelectrode arrangement. Using the microelectrode detector, the band broadening in an electro-osmotically driven flow system was compared to that in a gravity flow-based system. It was demonstrated that the zone broadening under gravity flow conditions was approximately twice as large as under electro-osmotic flow conditions for a typical set of experimental parameters.

Conducting phenylenevinylene blends as electrode materials in LC amperometric detection

AbstractA novel amperometric electrode based on a conducting polymer is presented and its properties are compared with those of a glassy carbon electrode. The electrode material was made by mixing 2,5‐dimethoxy‐1,4‐bis[2‐(3,4,5‐trimethoxy‐phenyl)ethenyl]benzene with iodine and polycarbonate. The blend was coated on a glassy carbon electrode by evaporation from a chloroform solution. Catecholamines were used as test substances in LC and FIA measurements. They were oxidized at a potential of +0.7 V (vs. SCE). The electrode exhibited a linear response for injected concentrations from 2.5 × 10−6M to 2.5 × 10−3M. It had a response time of 1.2 s and a coulometric efficiency of 0.74% at a linear flow rate of 1 mL/min. The electrode material is stable for several months under normal operating conditions. Hydrodynamic voltammograms showed E1/2 values shifted by approximately 100 mV as compared to glassy carbon. The novel redox‐conducting coating is proposed as an alternative material for the development of enzyme electrodes and chemically modified electrodes.

Capillary zone electrophoresis with end-column amperometric detection for the determination of glutathione at a gold/mercury amalgam microelectrode without deoxygenation

Capillary zone electrophoresis was employed to the analysis of glutathione by using end-column amperometric detection with a gold/mercury amalgam microelectrode at a constant potential of 0.02~0.06 V vs. SCE. In this procedure deoxygenation is not necessary. The calibration plot was found to be linear over 4 orders of magnitude and the limit of detection was 6.3 × 10 −7 mol l −1 (or 126 amol). The effect of the concentration of the buffer, and the high separation voltage across the capillary on the migration time and separation efficiency, and the dependence of the injection conditions on the detection signal, the separation efficiency and the coulometric efficiency have been investigated. The method was applied to the determination of glutathione in human blood and erythrocytes.

Determination of cysteine by capillary zone electrophoresis with end-column amperometric detection at a gold/mercury amalgam microelectrode without deoxygenation

Capillary zone electrophoresis was employed for the determination of cysteine using an end-column amperometric detection with a gold/mercury amalgam microelectrode, at a constant potential of 0.02–0.06 V vs. saturated calomel electrode. In this procedure deoxygenation is not necessary. The electrochemical characteristics at the microelectrode, the effect of the concentration of the buffer and the separation voltage across the capillary on the migration time and separation efficiency, and the dependence of the injection voltage and time on the detection signal, the separation efficiency and coulometric efficiency has been investigated. The calibration plot was found to be linear over four orders of magnitude and the limit of detection was 5.8·10 −8 mol/l (or 14. 5 amol). The method was applied to the determination of cysteine in human plasma, blood and urine.

The coulometric efficiency and steady state current for the end-column amperometric detector in capillary zone electrophoresis

Abstract The equations for the coulometric efficiency η and steady state current Is (A) for the end-column amperometric detector in capillary zone electrophoresis have been investigated; they can be expressed as follows: η=1−m 1 exp (−m 2 DA Ub ) and I s =nFcU[1−m 1 exp (−m 2 DA Ub )] where D (cm2 s−1) is the diffusion coefficient of the species detected, A (cm2) is the area of the electrode, U (cm3s−1) is the mean volumetric velocity, b (cm) is the distance between the electrode and the capillary outlet, c (mol cm−3) is the concentration of the species detected in solution, and n and F have their normal significance. m1 and m2 are constants which can be determined by experiment. The two equations have been verified experimentally.

Electrochemical and Spectroscopie Evaluation of Lithium Intercalation in Tailored Polymethacrylonitrile Carbons

AbstractDisordered polymethacrylonitrile (PMAN) carbon monoliths have been studied as potential tailored electrodes for lithium ion batteries. A combination of electrochemical and surface spectroscopie probes have been used to investigate irreversible loss mechanisms. Voltammetric measurements show that Li intercalates readily into the carbon at potentials IV positive of the reversible Li potential. The coulometric efficiency rises rapidly from 50% for the first potential cycle to greater than 85% for the third cycle, indicating that solvent decomposition is a self-limiting process. Surface film composition and thickness, as measured by x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS), does not vary substantially when compared to more ordered carbon surfaces. Li+ profiles are particularly useful in discriminating between the bound states of Li at the surface of solution permeable PMAN carbons.

Improved end-column amperometric detection for capillary electrophoresis

An amperometric detector for end-column detection in capillary electrophoresis is described. The influence of the high voltage on the detection performance is studied. As a consequence of these studies it is shown that the end-column detection strategy operates up to capillary dimensions of 50 μm I.D. without using any electrical-field decoupler desig. The shift of the working electrode potential resulting from the presence of the high voltage electrical field is compensated by selecting a more positive potential setting via the potentiostat.The detector performance is characterized in terms of attainable separation efficiencies (300 000 theoretical plates for homovanillic acid), limits of detection (5·10−8 mol/l for serotonin) and coulometric efficiencies (8.9% for homovanillic acid). Practical aspects associated with the use of microdisk electrodes instead of carbon fiber electrodes are addressed.

Predictive model for coulometric operation in a thin-layer amperometric flow cell

A theoretical model is presented that may be usefully employed to establish the limiting geometric and hydrodynamic conditions for a thin-layer cell to attain 100% coulometric efficiency. The thin-layer cell, based on a glassy-carbon electrode 5 cm in length (the flow path) and 2 mm wide was tested for three different channel heights at high and low flow rates (1 and 60 ml/h) using 1 mM potassium hexacyanoferrate(II) as the electrochemical probe in conjunction with a flow injection system.