Electrochemical behavior of electrodes containing nanostructured carbon of various morphology in the cathodic region of potentials

Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.     

Adsorption characteristics of electrodes containing nanostructured carbon of different morphology

The effect of camphor adsorption on the differential capacitance of electrodes of nanostructured carbon of different morphology (single-walled carbon nanotubes, filiform carbon, and columnar structures) in aqueous electrolyte solutions and also on the electrochemical reactions in these systems is studied. It is shown that irrespective of the ac frequency, the differential capacitance of the nanopaper and columnar electrodes increases 3–5-fold throughout the studied potential range. This experimental fact is explained by the substantial increase in the electrode surface accessible for electrolyte, which is a manifestation of the Rehbinder effect in electrochemistry. The revealed different kinds of effects of camphor adsorption layers formed at the nanostructured carbon/electrolyte interface on the electron transfer processes are as follows: partial inhibition of both the electron injection and the K₃[Fe(CN)₆] reduction; complete suppression of the reduction of sodium nitrate and nitrite; the absence of effects on the OH radical reduction and solvated electron oxidation.     

Instrumental neutron activation analysis of ferromanganese oxide encrustations of Indian Ocean by the k0 NAA method

Hot electron injection into aqueous electrolyte solution was studied with electrochemiluminescence and electron paramagnetic resonance (EPR) methods. Both methods provide further indirect support for the previously proposed hot electron emission mechanisms from thin insulating film-coated electrodes to aqueous electrolyte solution. The results do not rule out the possibility of hydrated electron being as a cathodic intermediate in the reduction reactions at cathodically pulse-polarized thin insulating film-coated electrodes. However, no direct evidence for electrochemical generation of hydrated electrons could be obtained with EPR, only spin-trapping experiments could give information about the primary cathodic steps.     

Cathodic electrochemiluminescence at double barrier Al/Al2O3/Al/Al2O3 tunnel emission electrodes

Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al₂O₃/Al/Al₂O₃ electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence.     

Effects of carbon nanofiber composites on electrode processes involving liquid|liquid ion transfer

Composite electrodes were prepared from chemical vapor deposition grown carbon nanofibers consisting predominantly of ca. 100 nm diameter fibers. A hydrophobic sol–gel matrix based on a methyl-trimethoxysilane precursor was employed and composites formed with carbon nanofiber or carbon nanofiber—carbon particle mixtures (carbon ceramic electrode). Scanning electron microscopy images and electrochemical measurements show that the composite materials exhibit high surface area with some degree of electrolyte solution penetration into the electrode. These electrodes were modified with redox probe solution in 2-nitrophenyloctylether. A second type of composite electrode was prepared by simple pasting of carbon nanofibers and the same solution (carbon paste electrode). For both types of electrodes it is shown that high surface area carbon nanofibers dominate the electrode process and enhance voltammetric currents for the transfer of anions at liquid|liquid phase boundaries presumably by extending the triple-phase boundary. Both anion insertion and cation expulsion processes were observed driven by the electro-oxidation of decamethylferrocene within the organic phase. A stronger current response is observed for the more hydrophobic anions like ClO₄⁻ or PF₆⁻ when compared to that for the more hydrophilic anions like F⁻ and SO₄²⁻. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005     

Characterization of multiple carbon fibre microelectrodes in a flow-through cell and amperometric detection with dilute electrolyte for liquid chromatography

A simple, convenient and sensitive flow-through cell incorporating multiple carbon fibres in a polyvinyl chloride tube was constructed for high performance liquid chromatography. The voltammetric behaviour, electrode treatment, stability and hydrodynamic voltammograms of such electrodes are discussed. An in-situ electrochemical pretreatment is proposed for activation and recovery of the activity of the carbon fibre electrode in flow-stream detection. The desirable detector was found to be nearly flow-rate independent. Catecholamines can be detected at concentrations as low as 2 × 10⁻⁹ M. The cell could be used in the mobile phase with little electrolyte. It was found that the solution resistance in the flow pathway was the major source of distortion of the shape of hydrodynamic voltammograms and of high noise at dilute electrolyte. Parallel detection using dual working electrodes is demonstrated for reversed-phase chromatographic separation of catecholamines.     

Electrochemical quartz crystal microbalance analysis of the cathodic process in a lithium-ion battery

The surface processes at carbon and platinum electrodes have been studied using the electrochemical quartz crystal microbalance technique in organic electrolyte solutions for lithium ion batteries. The changes in resonance frequency were analyzed as a function of the electrode potential, indicating that the process depended not only on the electrode material but also on the cathode potential. In the solution containing LiBF₄ as the electrolyte, the main product at the platinum surface was Li₂CO₃ and LiF, whereas formation of lithium alkylcarbonates was the primary process at the platinum and carbon electrodes in LiPF₆ solution.     

New accelerated method of impregnation by aqueous electrolyte of carbon black gas-diffusion electrodes to study their structural and electrochemical characteristics

A new method of fast impregnation of carbon black gas-diffusion hydrophobized electrodes is suggested under their cathodic polarization in an alkaline aqueous solution of tetrabutylammonium bromide (TBAB) at the potentials of hydrogen evolution. As dependent on the quantitative content of polytetrafluoroethylene (PTFE) in the electrode, current density, time, TBAB concentration, various degree of electrode wetting is observed, up to nearly complete electrode flooding in just several hours. When electrodes are stored, their original electrolyte porosity is not recovered. the electrode with 8 wt % of PTFE was used to show the effect of the electrode flooding degree on the double layer capacity, average diameter of electrolyte pores, their surface area and activity in the case of oxygen reduction. This method may be used for simulation of the process of flooding of gas-diffusion electrodes by electrolyte in the course of their prolonged operation.     

Electroactive ceramic carbon electrode impregnated with organic liquid

The carbon ceramic electrodes impregnated with hydrophobic organic solvent (toluene, hexadecane, nitrobenzene) containing redox probe (decamethylferrocene) were prepared. The electrode material was obtained by sol–gel process. It consists of graphite powder homogeneously dispersed in hydrophobic silica matrix. After gelation and drying it was filled with organic liquid. The electrochemical properties of the electrode were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Approximately symmetric cyclic voltammograms were obtained with these electrodes immersed in aqueous electrolyte solution. Their shape and current magnitude and position on the potential scale depends on the organic solvent and the salt present in aqueous phase. It has been concluded that the mechanism of the electrode process involves electron transfer between graphite particle and the redox probe in organic phase, followed by anion transfer from the aqueous phase.     

Chemical Adsorption onto an ITO Substrate of Single‐Wall Carbon Nanotube Functionalized by Carboxylic Groups as an Efficient Support for Electrocatalyst

A single‐wall carbon nanotube functionalized by carboxylic groups (SWNT‐CA) was found to be adsorbed on an indium tin oxide (ITO) electrode by chemical interaction between carboxylic groups and the ITO surface. The adsorption experiments indicated that the narrow pH conditions (around pH 3.0) exist for its adsorption which is restricted by preparation of stable fluid dispersion (favorable at higher pH) and by the chemical interaction (favorable at lower pH). Atomic force microscopic (AFM) measurements suggest that fragmented SWNT‐CA are adsorbed, primarily lying on the surface. Electrochemical impedance analysis indicated that an electrochemical double layer capacitance of the SWNT‐CA/ITO electrode is considerably higher than that for the ITO electrode, suggesting that the interfacial area between the electrode surface and the electrolyte solution is enlarged by the SWNT‐CA layer. Pt particles were deposited as a catalyst on the bare ITO and SWNT‐CA‐coated ITO (SWNT‐CA/ITO) electrodes to give respective Pt‐modified electrodes (denoted as a Pt/ITO electrode and a Pt/SWNT‐CA/ITO electrode, respectively). The cathodic current for the Pt/SWNT‐CA/ITO electrode was 1.7 times higher than that for the Pt/ITO electrode at 0.0 V, showing that the Pt/SWNT‐CA/ITO electrode works more efficiently for O₂ reduction at 0.0 V due to the SWNT‐CA layer. The enhancement by the SWNT‐CA layer is also effective for electrocatalytic proton reduction. It could be ascribable to the enlarged interfacial area between the electrode surface and the electrolyte solution.     

On the interaction of carbon electrodes and non conventional electrolytes in high-voltage electrochemical capacitors

This study is essentially based on innovative electrolytes such as the organic salt N-methyl-N-butylpyrrolidinium tetrafluoroborate (Pyr₁₄BF₄) dissolved in propylene carbonate (PC) and the pure ionic liquid (N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI) and its solution in PC. Activated carbon cloths were used as self-standing binder-free electrodes. It is found that the presence of impurities in carbon electrodes may lead to electrolyte decomposition and electrode degradation which notably affect the electrochemical double-layer capacitor (EDLC) performance. Such processes greatly depend on the composition of both the electrode and the electrolyte, being much less significant with solvent-containing electrolytes. By raising the operation temperature to 60 °C, the EDLC performance in the ionic liquid Pyr₁₄TFSI is notably improved due to a relevant decrease in the viscosity and increase in ionic conductivity. By contrast, the presence of impurities, e.g., Zn and Al, in the electrodes remarkably reduces the electrolyte stability and a thick layer of decomposition products completely covers the carbon fibers after cycling at high temperature. The ionic liquid in solution maintains the high maximum operative voltage of the net ionic liquid whereas its viscosity and ionic conductivity are close to those of the conventional Et₄NBF₄/PC. Furthermore, the presence of propylene carbonate as solvent prevents to some extent the ionic liquid degradation.     

Electrochemical generation of solvated electrons from nanostructured carbon

Voltammograms for electrodes fabricated of nanostructured carbon of various morphology (nanotube paper, columnar and filament structures) in hexamethylphosphoric triamide (HMPA) solution have been obtained and analysed. Intensive dark-blue coloration near cathode surface at potentials as low as E ∼ −1.2 V (s.c.e.) has been observed. An ESR (electron spin resonance) spectrum of this frozen dark-blue solution was recorded. This spectrum coincides with one of free electrons. Experimental proofs of the existence of electron emission into electrolytic solutions at moderate cathodic potentials are present for all electrodes. This effect is established to be connected with the presence of atomically sharp areas on the electrode surfaces.     

电解液离子与炭电极双电层电容的关系

以酚醛树脂基纳米孔玻态炭(NPGC)为电极, 通过微分电容伏安曲线的测试, 研究了水相体系电解液离子与多孔炭电极双电层电容的关系. 结果表明, 稀溶液中, 多孔炭电极的微分电容曲线在零电荷点(PZC)处呈现凹点, 电容降低, 双电层电容受扩散层的影响显著；若孔径小, 离子内扩散阻力大, 电容下降更为迅速, 扩散层对双电层电容的影响增大. 而增大炭材料的孔径或电解液浓度, 可明显减弱甚至消除扩散层对电容的影响. 炭电极的单位面积微分电容高, 仅表明孔表面利用率高, 如欲获得高的电容量, 还要有大的比表面积. 离子水化对炭电极的电容产生不利影响, 选用大离子和增大炭材料的孔径, 可有效降低离子水化对炭电极电容性能的影响.     

Fast electrochemical triple-interface processes at boron-doped diamond electrodes

Two important mechanisms for electron transfer processes at boron-doped diamond electrodes involving the oxidation of tetramethylphenylenediamine (TMPD) dissolved in aqueous solution and the oxidation of tetrahexylphenylenediamine (THPD) deposited in the form of microdroplets and immersed into aqueous eletrolyte solution are reported. For TMPD, the first oxidation step in aqueous solution follows the equation: Remarkably slow heterogeneous kinetics at a H-plasma-treated boron-doped diamond electrode are observed, consistent with a process following a pathway more complex than outer-sphere electron transfer. At the same boron-doped diamond electrode surface a deposit of THPD undergoes facile oxidation following the equation: This oxidation and re-reduction of the deposited liquid material occurs at the triple interface organic droplet|diamond|aqueous electrolyte and is therefore an example of a facile high-current-density process at boron-doped diamond electrodes due to good electrical contact between the deposit and the diamond surface. Received: 3 February 2000 / Accepted: 18 February 2000     

Structure of electrolyte solutions sorbed in carbon nanospaces, studied by the replica RISM theory

The replica RISM theory is used to investigate the structure of electrolyte solutions confined in carbonized polyvinylidene chloride (PVDC) nanoporous material, compared to bulk electrolyte solution. Comparisons are made between the models of electrolyte solution sorbed in the carbonized PVDC material and a single carbon nanosphere in bulk electrolyte solution. Particular attention is paid to the chemical potential balance between the species of the sorbed electrolyte solution and the bulk solution in contact with the nanoporous material. As a result of the strong hydrophobicity of the carbonized PVDC material in the absence of activating chemical groups, the densities of water and ions sorbed in the material are remarkably low compared to those in the ambient bulk solution. The interaction between water molecules and cations becomes strong in nanospaces. It turns out that, in carbon nanopores, a cation adsorbed at the carbon surface is fully surrounded by the hydration shell of water molecules which separates the cation and the surface. Distinctively, an anion is adsorbed in direct contact with the carbon surface, which squeezes a part of its hydration shell out. The tendency increases toward smaller cations, which are characterized as "positive hydration" ions. In the bulk, cations are not hydrated so strongly and behave similarly to anions. The results suggest that the specific capacitance of an electric double-layer supercapacitor with nanoporous electrodes is intimately related to the solvation structure of electrolyte solution sorbed in nanopores, which is affected by the microscopic structure of the nanoporous electrode.     

Reversible Electrochemistry of Cytochrome c on Recompressed,Binderless Exfoliated Graphite Electrodes

The direct electrochemistry of cytochrome c (cyt‐c) has been investigated on exfoliated graphite (EG) electrodes. The as‐polished and roughened (using SiC emery sheet) EG surfaces are inactive for the direct electron transfer. However, when the EG electrode was sonicated before the experiment, a pair of redox waves were obtained for freely diffusing cyt‐c in the solution phase. The formal potential was found to be 0.01 V (vs. SCE) in 0.1 M phosphate buffer at a pH of 7.1. The electrochemical response for the adsorbed cyt‐c on sonicated EG electrodes, which is shown to have carbonyl functional groups on its surface, shows nearly reversible voltammograms in the same electrolyte. However, the formal potential in the adsorbed state is more negative than that observed for the solution phase cyt‐c. A structure based on an open heme conformation proposed by Hildebrandt and Stockburger is probably present on the EG surface. It is suggested that the electrochemistry at the EG electrode is essentially governed by favourable electrostatic interactions.     

Experimental determination of the ψ′-potential by means of electron photoemission from metals into electrolyte solutions

The measurements of the photoelectron emission current from a metal electrode into electrolyte solution can be used for direct determination of the ψ′-potential. The limits of applicability of this method are discussed. The values of the ψ′-potential measured at mercury and lead electrodes in HCl solutions are in good agreement with those calculated by means of the Gouy-Chapman theory.     

Hot electron-induced electrogenerated chemiluminescence of 1-aminonaphthalene-4-sulphonate at oxide-covered aluminium electrodes in aqueous solution

Electrogenerated chemiluminescence of 1-aminonaphthalene-4-sulphonate (ANS) provides a sensitive means for the detection of the ANS in aqueous solution when oxide-covered aluminium electrodes are used as tunnel emission electrodes and cathodic pulse polarisation is used for the excitation of the luminophore. During the pulse polarisation of insulating oxide film-coated aluminium cathodes, hot electrons are tunnel emitted from the aluminium to the aqueous electrolyte solution by direct field-assisted tunnelling (in the case of oxide films of thickness 2-6 nm) or by Fowler-Nordheim (FN) tunnelling in the cases of thicker oxide films. As a result of direct tunnel emission of these energetic electrons, the generation of hydrated electrons (e_aq⁻) becomes possible. These electrochemically generated, extremely strong reductants (dry hot or hydrated electrons) make the efficient excitation of various types of luminophores at thin insulating film-covered electrodes possible and provide a means for sensitive immunoassays and DNA-probing assays when these luminophores are used as label molecules.     

HBF4溶液中全铅液流电池

报道了一种HBF₄水溶液中的全铅液流电池,正、负电极电解液均采用Pb（BF₄）₂的HBF₄水溶液.在酸性的四氟硼酸铅电解液中考察了石墨电极和玻碳电极作为工作电极的循环伏安性能,石墨电极较适于用作全铅液流电池的正、负电极.采用石墨电极作为电池的正、负电极并在四氟硼酸铅酸性电解液中进行充放电实验,其中Pb（BF₄）₂浓度分别为0.5、1.0和1.5 mol·L^-1,且保持游离的HBF₄浓度为1.0 mol·L^-1.该电池为单液流电池,不需要隔膜分隔正、负极的电解液,电流密度为10、20和40 mA.cm^-2,当限定充电容量为7.0 mAh.cm^-2,放电电压截止到1.0 V时,平均库仑效率大于87%,平均能量效率大于68%;当电解液采用1.0或1.5 mol·L^-1 Pb（BF₄）₂+1.0 mol·L^-1HBF₄水溶液时,在10及20 mA.cm^-2电流下的能量效率最高可超过74%.     

Dynamics of copper electrodeposition onto fibrous carbon electrodes

Distribution of copper electrodeposited from a sulfuric acid solution onto fibrous carbon electrodes, copper deposition rate, and current efficiency by the metal were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The variation of the electrode thickness on which copper ions discharge at the limiting diffusion current at various solution flow rates and the electrode thickness on which the whole amount of oxygen dissolved in the electrolyte is reduced were calculated in relation to the solution flow rate and geometric current density. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters from the beginning of the process till ??clogging?? of a part of the electrode by the metal were determined.