Electrochemical characterization of high-surface-area catalysts and other nanoscale electroactive materials at sticky-carbon electrodes

Direct quantitative electroanalysis of submilligram quantities of nanoscale, high-surface-area electroactive powders is achieved by hand-pressing the solids onto the surface of a carbon–wax composite electrode. Since this ‘sticky carbon’ method does not require solvents or polymer binders to create electrode structures, such as cast films, one may now examine the innate electrochemical character of nanoscale materials. Electrode preparation with sticky carbon is rapid and simple and offers a convenient means to screen high-surface-area electrode materials whose inherently high RC time constants can compromise the resolution of the faradaic information. The effects on the electrochemical character of electrocatalysts, active materials in batteries, electrochromic materials, etc., of the common constituents (e.g. solvents, polymer binders, ionomers) and processes/forces (temperatures, pressures) necessary to create practical electrode structures may now be methodically explored.     

Electrodissolution of Ti and p-Si in acidic fluoride media: formation ratio of oxide layers from electrochemical impedance spectroscopy

The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterising the oxide layers covering both materials. The impedance diagrams have been analysed to obtain the low frequency capacitance C_lf, the high frequency capacitance C_hf and the product R_hfI (where R_hf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from C_lf, C_hf and R_hfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.     

An improved algorithm for the numerical simulation of cyclic voltammetric curves affected by the ohmic potential drops and its application to the kinetics of bis(biphenyl)chromium(I) electroreduction

Previously published algorithms for the modeling of cyclic voltammetric curves affected by ohmic potential drops, in terms of the classical explicit finite differences method, are discussed briefly. A fast and efficient numerical procedure suitable for such simulations is described. This approach exhibits high numerical stability for both high scan rates and large uncompensated ohmic resistances. The procedure is based on the calculation of the faradaic current as a root of the non-linear equation, with a simultaneous calculation of the capacitive current. Comparison of the cyclic voltammograms obtained using this method with those calculated using alternative published procedures proves its validity. For comparison with the experimental data, the cyclic voltammetric response for the reduction of bis(biphenyl)chromium(I) tetraphenylborate in N,N-dimethylformamide is shown and the kinetic parameters of this process are fitted. Compared to earlier modelings, they show better concordance with the results of studies at microelectrodes. In conjunction with earlier successful applications of the analogous numerical procedure to the realistic modeling of electrochemical oscillations and multistability at a constant external voltage, the algorithm presented appears to be one of the most applicable methods of calculation of the electrochemical responses affected by the ohmic potential drops.     

Linear sweep voltammetric and chronopotentiometric charge/potential curves for non reversible redox monolayers

A simple equation for the charge/potential (Q/E) dependence of an electroactive monolayer is presented. This equation is valid for any chronopotentiometric or voltammetric electrochemical technique, whatever the degree of reversibility of the molecular film. Particular expressions corresponding to linear sweep and cyclic voltammetry and to chronopotentiometry with a power time programmed current have been deduced, and analogies with the current/potential curves obtained in electrochemical systems under semi-infinite diffusion mass transport control under transient and steady state conditions has been established. Reversible Q/E curves present a time-independent and universal behaviour, which does not depend on the electrochemical technique employed to obtain them. The irreversible charge/potential curves exhibit different behaviour depending on which electrochemical method has been used. Easy methods for estimating thermodynamic and kinetic parameters of the electroactive film in both chronopotentiometric and voltammetric techniques are proposed.     

Determination of diffusion coefficient of hydrogen in metals and their elastic modulus by generalised impedance

Transport of hydrogen in elastic metals has been analysed in terms of an impedance-like non-linear transfer function (TF). The equilibrium in a membrane specimen is perturbed by a sinusoidal signal of the concentration of hydrogen, of large amplitude, applied to one of its surfaces. The fundamental harmonics of periodically reproducible flux at the same surface is considered as a response. The TF is defined as the ratio of the signal to the response. The power of the said TF in determining the diffusion coefficient of hydrogen in a metal–hydrogen system and its bulk elastic modulus was analysed for the example of α-phase Pd–H and Pd₈₁Pt₁₉–H systems. It has been found that such a TF is suitable for determination of the above parameters. Also, the sensitivity of these estimates to the uncertainty of concentration of hydrogen in the metal, its partial molar volume and the thickness of the specimen has been explored.     

Glucose microsensor based on electrochemical deposition of iridium and glucose oxidase onto carbon fiber electrodes1

Miniaturized glucose biosensors, prepared by electrochemical deposition of iridium and glucose oxidase (GOx), are characterized. The iridium network offers good retention of GOx and efficient preferential electrocatalytic detection of the liberated hydrogen peroxide at potentials lower than those of common interfering substances (the ascorbic acid signal actually shifts to a higher potential). The remarkable selectivity thus achieved towards the detection of glucose is coupled to a very fast response. Unlike analogous preparations of noble metal carbon fiber biosensors, a two-step electrodeposition process is required for the fabrication of Ir/GOx microelectrodes. The dependence of the biosensor response upon electrodeposition parameters, such as amounts of GOx and iridium or plating time is examined and optimized. Scanning electron microscopy is used to characterized the growth patterns of the iridium and Ir/GOx layers. The high selectivity associated with electrodeposited iridium matrices makes them very attractive for localizing other hydrogen-peroxide-liberating oxidases.     

Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption

The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles.     

Double potential step chronoreflectometry approach to bi-stable potential region of the faradaic phase transition of heptyl viologen at a HOPG electrode

The phase transition of an adsorption layer of heptyl viologen (HV) at a basal plane HOPG electrode was investigated using double potential step chrono-reflectometry in the visible wavelength region. For the first order faradaic phase transition between an expanded adsorption layer of oxidized form (HV²⁺) and a two-dimensional (2D) condensed monolayer of reduced form (HV⁺), the nature of the bi-stable potential region was targeted to be explored. The merit of chrono-reflectometry capable of directly tracking the change of 2D superficial fractions of the two phases on the electrode surface was demonstrated experimentally. The double potential step measurements made it possible to gain access to the steady-state coexistence of the two phases in the bi-stable potential region. After the secondary reverse potential step to the positive direction into the bi-stable potential region was given from a transient nucleation state at the first step potential, the superficial fraction of the condensed phase monitored by reflectance was saturated. The plot of this saturated value (φ_dbl) as a function of potential (E) in the bi-stable potential region was found to be dependent on the time period spent at the transient nucleation potential. Therefore, the φ_dbl–E curve thus obtained does not necessarily represent a unique, true equilibrium relationship.     

Novel electrochemically approach for probing in situ self-assembly by tracking shift of I–E curve of Ohmic circuit

A novel electrochemically approach for in situ probing the self-assembly is proposed by tracking the shift of the I–E curve of an Ohmic circuit. In the proposed approach, the external electrolysis circuit of three-electrode potentiostat is re-equipped to be an Ohmic circuit by connecting in series a conductive substrate between two clips of working and counter electrodes, and a reference electrode is still connected with the clip of reference electrode. Moreover, the conductive substrate and the reference electrode are immersed in a supporting electrolyte containing adsorbate, a linear sweep potential with respect to the reference electrode is applied to the substrate, at the same time an I–E curve is recorded. With the self-assembly processing, interfacial potential at the substrate/solution interface gradually changes, resulting into the gradual shifts of the recorded I–E curves along potential axis. The shift of the I–E curve is confirmed by employing zero current potential E_zcp that circuit current I is equal to zero in the I–E curve. Tracking the shift of zero current potential E_zcp can probe in situ the self-assembly continuously. Herein, the fundamental of the proposed approach is illustrated briefly and the self-assembly of l-cysteine on the gold surface as an example is probed by the proposed approach. The obtained results showed the proposed approach is very simple, rapid and sensitive.     

A correlation study between the conformation of the 1,4-dithiane SAM on gold and its performance to assess the heterogeneous electron-transfer reactions

A gold electrode spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species has been characterized by cyclic voltammetry of [Fe(CN)₆]^3?/4? and cytochrome c (cyt c) probe molecules, SERS (surface-enhanced Raman scattering), and EIS (electrochemical impedance spectroscopy), using [Fe(CN)₆]^3?/4? as the redox-active molecule in solution. The cyclic voltammograms showed a decrease in the faradaic current response with an increase of the immersion time of the gold substrate into the 1,4-dt modifier solution, suggesting that the heterogeneous electron-transfer (hET) process of the [Fe(CN)₆]^3?/4? probe molecule occurs through the surface pinholes. This observation is an indication that the 1,4-dt chemisorption mechanism must involve a slow step that is responsible for the final configuration of the modifier molecules on the surface. The SERS spectra acquired for the surface as a function of immersion time of the gold electrode in the modifier solution presented an intensity enhancement of the bands assigned to the trans and gauche conformation of the 1,4-dt on the surface. The apparent rate constant, k_app, estimated by the EIS results, decreases with increase of the immersion time indicating that the pinholes on the surface act as a microarray. This result is consistent with those observed by cyclic voltammetry of the probe molecules, i.e., the pinhole density decreases with increase of the immersion time of the gold electrode in the 1,4-dt modifier solution, and a strong attenuation of the probe molecules interfacial electron-transfer response is observed.     

The electrochemical behavior of p-sulfonated calix[4]arene

The electrochemical behavior of p-sulfonated calix[4]arene was studied. In aqueous solution, p-sulfonated calix[4]arene can be oxidized when the potential is more than 0.7 V versus SCE. It is confirmed that the reaction is an irreversible two-electron transfer electrochemical reaction. The anodic peak potential, E_p, is affected by the acidity of the solution. E_p shifts in the negative direction when the pH increases. The electron transfer coefficient, α, is 0.65. At 25 °C, the diffusion coefficient of p-sulfonated calix[4]arene is 3.1 × 10^?5 cm² s^?1. The activation energy, E_a, for the electrochemical reaction is (18.8 ± 0.2) kJ mol^?1.     

Investigation of the cathodic electropolymerization of acrylonitrile,ethylacrylate and methylmethacrylate by coupled quartz crystal microbalance analysis and cyclic voltammetry

The cathodic electropolymerization of acrylonitrile (AN), ethylacrylate (EA) and methylmethacrylate (MMA), has been monitored for the first time by coupled electrochemical quartz crystal microbalance (QCM) and cyclic voltammetry analyses. These data have been compared to the previously published analyses for methacrylonitrile (MAN). At the potential E_p1 of the less cathodic of the two voltammetric peaks observed (peak I), the polymer formed is anchored firmly to the cathode even in a good solvent for it, and it remains adsorbed when the voltammetric scan is repeated up to E_p1. In parallel, a linear relationship is observed between the frequency change recorded in situ by the QCM up to E_p1 and the PAN film thickness measured ex-situ by ellipsometry. However, when potentials more negative than peak I are scanned, the polymer desorption (degrafting) occurs as assessed by solubilization in a good solvent. Polymerization is also resumed but in solution and no longer as grafted chains. The major difference between the acrylic monomers (AN and EA) and the methacrylic ones (MMA and MAN) is that part of the methacrylic chains are not grafted at E_p1. This coexistence of adsorbed and desorbed chains is not observed for the polyacrylic chains in that potential range. The mass of PAN deposited onto the cathode has been approximated from the film thickness and the Sauerbrey equation, so allowing the M_n of the grafted chains to be estimated.     

Photocorrosion of polycrystalline CdSe in aqueous KOH: an in situ study by photocalorimetry

Photocalorimetry in conjunction with thermodynamical analysis has been employed to characterize the photocorrosion reaction of a semiconductor electrode. The potential dependence of the heat-monitoring signal and of the photocurrent has been measured at a CdSe electrode (polycrystalline film on Ti) in 0.1 mol l^?1 aqueous KOH under 600 nm illumination. From these measurements a characteristic potential (E*=?0.21±0.02 V versus SHE) is obtained which connects the reversible potential and the Peltier coefficient of the corrosion reaction. These quantities are specific for a given reaction. Values of E* have been calculated for thermodynamically possible corrosion reactions of CdSe from the respective standard potential, standard entropy change and heat of transport. Good agreement between the measured and calculated values of E* and the photocalorimetric detection of a light-absorbing corrosion film indicate that CdSe+2OH^?→Cd(OH)₂+Se+2e^? is the predominating corrosion reaction under the present conditions.     

Electrochemical properties of cation sensitive polypyrrole films

The influence of the electrochemical pretreatment (potentiodynamic cycling) and previous conditioning of thick cation sensitive PPy films (30 μm) doped with sulphate, naphthalene-2-sulfonate (N-2-S) and naphthalene-1,5-disulfonate (N-1,5-DS) on the electrochemical properties were studied. The changes in the chemical composition of the PPy films were determined by electron probe microanalysis (EPMA). The poor mobility of dopant anions in the film and participation of cations in the PPy film redox process is the basis of their cation sensitivity. The influence of potentiodynamic cycling on the properties of these PPy films is correlated with the change of several factors, such as the redox activity in the potential interval of the potentiometric measurements, the redox capacity, the ionic content and the quality of contact with the substrate. The long-term conditioning of the PPy electrode in a certain solution is necessary for the achievement, maintenance or re-establishment of the stable quasi-equilibrium state of PPy film characterized by a quasi-equilibrium potential E_r. The conditioning process of the untreated PPy|N-2-S and PPy|N-1,5-DS electrodes proceeds very slowly. It accelerates after potentiodynamic cycling, enabling the use of the electrodes for potentiometric measurements after only 2 days of conditioning. The potentiodynamic cycling of the PPy electrodes doped with big organic anions improves also their potentiometric response, extending considerably the linear parts of E–log a plots towards low concentrations. The nature of this effect consists in the change of a thick PPy film to become redox active in the potential region of the potentiometric measurements.     

The influence of porosity and the nature of the charge storage capacitance on the impedance behaviour of electropolymerized polyaniline films

Impedance spectra of electropolymerized polyaniline films were analysed by the theory of the impedance of macrohomogeneous porous electrodes. Specific ohmic and charge transfer resistivities were calculated, and their potential and pH dependence is discussed in terms of structural changes of the polymer during redox and protonation processes. Furthermore it is shown that a correct understanding of the high frequency limit of the impedance is achieved only if the porosity is implemented and thus the discrepancies inherent in the inappropriate assumption of a compact polymer film are avoided. Another not well understood electrochemical phenomenon of conducting polymers is the apparently capacitive current plateau in the cyclic voltammogram and the large series ac capacitance. It will be demonstrated that this supposed `capacitive' behaviour is related to continuous faradaic charge transfer at potentials beyond the first oxidation wave in the cyclic voltammogram.     

Surface electromyography, force and single motor-unit data for inhibitory reflex responses in human masseter at two levels of excitatory drive

The inhibitory reflex response evoked by mildly noxious lip stimulation was tested in up to 100 trials at two different levels of pre-stimulus excitation. To regulate that excitation, the subject voluntarily controlled the firing rate of one motor neurone at 10 Hz, and then at 15 Hz while the reflex was tested. When the frequency of the motor-unit action potential (MUAP) in the controlled unit increased or decreased, the frequency of other active units' MUAPs also changed in parallel: that is, the frequency of one MUAP was an index of whole muscle excitation. The inhibitory response in the surface electromyogram (EMG) was quantitated by rectifying and averaging the signal, and by measuring its depth and area. The best correlation between the responses evoked in single units and the response in the whole muscle EMG at the two levels of excitation was obtained when the area of inhibition in the surface record was normalized to the pre-stimulus EMG level. The inhibitory response to lip stimulation in the masseter EMG decreased when the level of pre-stimulus excitation in the muscle was increased. This was confirmed using the reflex response in the averaged force data. The findings suggest that controlling one motor unit's firing rate is a useful method for controlling the whole-muscle excitation in quantitative reflex studies.     

Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.     

Electrochemical properties of natural carotenoids

Carotenoids are widely distributed among plants, certain bacteria and animals, serving for light harvesting and photoprotection in photosynthesis and as antioxidants. Electrochemical data (such as oxidation potentials, reaction rate constants, kinetic equilibrium constants) for 12 naturally occurring carotenoids are summarized in this paper. Electrochemical parameters were estimated by simulating experimental cyclic voltammograms (CVs). Conventional electrochemical techniques (cyclic voltammetry and Osteryoung square-wave voltammetry) were used in these studies. CVs of some natural carotenoids are presented and discussed. The dependence of redox potentials and other kinetic parameters on the structures of carotenoids is summarized. The oxidation potential of neutral carotenoid (E°₁), which corresponds to the formation of cation radicals, varies from 0.50 to 0.72 V versus SCE. The oxidation potential of carotenoid cation radical (E°₂), which corresponds to the formation of a dication, usually lies in the region of 0.52–0.95 V versus SCE. The electrochemical data presented in this article should be helpful in the study of reaction processes in model systems or actual photosynthesis reaction centers.     

Electrochemical properties of a gold electrode modified with a mixed monolayer

The electrochemical properties of a gold electrode modified with a mixed thiol monolayer containing both a polar and a non-polar head group have been investigated in aqueous Fe(CN)^4?₆, flavin adenine dinucleotide (FAD) and ubiquinone-0 (2,3-dimethoxy-5-methyl-1,4-benzoquinone, UQ₀) solutions. The cyclic voltammetric current—potential (i — E) response of Fe(CN)^4?₆ was found to be affected considerably by the polarity of the head group contained in the mixed monolayer assembly, as compared with those of FAD and UQ₀. It was also found that in the cases of UQ₀ and FAD the i — E responses for the modified electrode were affected by their own molecular size rather than the polarity of the mixed monolayer head group. Furthermore, compared with Fe(CN)^4?₆ ion, these biologically related molecules are able to permeate readily into the well-organized and hydrophobic alkyl chains of the monolayer assembly. The voltammetric profile of UQ₀ was improved by the modification of aminoethanethiol, as compared with those of bare gold and the electrode modified with other polar thiols. Further, two different permeation paths of the electrode species into the mixed monolayer are suggested from the variation of the i — E response with the cycle of the potential scan.