Potential Step Experiments Research Articles

A study of the lead dioxide electrocrystallization mechanism on glassy carbon electrodes. Part I: Experimental conditions for kinetic control

The dependence on chemical and electrochemical variables of lead dioxide electrodeposition on a glassy carbon electrode from Pb(II) aqueous solutions has been studied. Depending on the experimental conditions, the mechanism of lead dioxide electrodeposition may involve kinetic or mass transport control. Both modeling and morphological studies of the nucleation and growth process have been carried out. The analysis of the chronoamperometric curves obtained from potential step experiments required a previous study regarding (i) the experimental solution conditions (pH, Pb(II) concentration), (ii) range of the final step potential to be used, and (iii) the choice of theoretical models used to obtain the kinetic parameters. Direct microscopic observation of the electrode through the initial stages of oxide phase formation has provided independent values of nucleation kinetics parameters ( N 0 A), growth constants ( k), induction time ( t 0) and their dependence on the electrochemical variables. The fitting of the experimental curves using some of these independent values allowed other ones, with more realistic values, to be obtained. Further discussion on the mechanistic details of the lead dioxide electrodeposition is presented.

ChemInform Abstract: Pitting of Fe in pH 7.4 Acetate Buffer

Abstract The passivation and pitting of Fe in pH 7.4 acetate buffer solution has been investigated as a function of the acetate and chloride concentrations. The efficiency of passivation was found to depend on the concentrations of both solutes, with the highest efficiency in the more concentrated solutions. Cl− did not interfere with the establishment of passivity, either in potential sweep or potential step experiments, but was significantly less effective in promoting efficient passivation than acetate. Cl− only has a deleterious effect on passivation after the film has formed. Potentiostatic results obtained above the pitting potential in concentrated acetate solution suggest that the onset of pitting is associated with an interaction of the Cl− with a certain stage of development of the passive film, based on the similarity of results with those obtained previously in borate buffer solution. 1 M acetate acts as a more effective inhibitor for pitting than 0.2 M borate.

Plane-recessed disk electrodes and their arrays in transient generator–collector mode: The measurement of the rate of the chemical reaction of electrochemically generated species

A novel electrode geometry is proposed in which a disk microelectrode is recessed below a second electrode which covers an entire plane. Potential step experiments on such plane-recessed disk electrodes and arrays of these electrodes are considered. The recessed disks act as a generator and a plane electrode above acts as a collector. Electrochemical reactions at the generator electrode are assumed to be followed by chemical reaction. Both first order (EC 1) and second order (EC 2) mechanisms are considered. Current transients are found to be sensitive to the diffusion coefficient of both the reduced and oxidised species as well as to the rate of the chemical reaction and its mechanism. The rate constant of the chemical reaction can be measured over a range of six orders of magnitude. The rate of the second order chemical reaction can be measured up to the limit of diffusion controlled reaction.

Theory of partial diffusion-limited interfacial transfer/reaction on realistic fractals

The problems of diffusive transport to and across an irregular interface are of general interest, but are considered a difficult class of problems for theoretical understanding. In this paper we discuss theory of partial diffusion-limited interfacial transfer/reaction on a realistic fractal interface. The surface irregularity is modeled as a random surface fractal, which is characterized by statistically isotropic self-affine fractals on limited length scales. The power spectrum of roughness of such surface fractal is approximated in terms of power law function for the intermediate wave-numbers (or spatial frequency components in roughness). This description of roughness consists of four fractal morphological characteristic features. Results unravel the connection between the flux/current, surface morphology and its kinetics following a step of surface activity (like a potential step experiment). We show the dependence of reaction flux/current on various fractal roughness characteristics related to power spectrum, and discuss the dynamic crossover of charge transfer controlled regime to fractal morphology controlled diffusion regime to classical inverse square root of time regime.

Bismuth UPD on the modified Au(111) electrode

Abstract The bismuth UPD was studied on Au(1 1 1) electrodes modified by silver intermediates and adsorbed thymine. Cyclic voltammetry, potential step experiments and X-ray photoelectron spectroscopy were used for investigation. The results were compared to the results of the analogous experiments performed on bare Au(1 1 1) and on bulk silver. In the first part it is shown that the Bi UPD on AgML/Au(1 1 1) differs completely from the UPD on Au(1 1 1) as well as on bulk Ag due to changed electronic properties of the substrate. The 2nd part focuses on the Bi UPD in presence of the nucleobase thymine. Thymine undergoes a reorientation from the chemisorbed state to the physisorbed state on Au(1 1 1) at potentials where the Bi UPD takes place. During the UPD adsorbed thymine is replaced by Bi species on the electrode surface. It does not re-adsorb on top of the deposited Bi layer. An energetic advantage due to re-adsorption on topmost layer as described for the Cu UPD on thymine modified Au(1 1 1) does not occur.

Effects of applied stress and long-term stability on PPy(CF 3SO 3) linear actuators

The actuation performance of PPy(CF 3SO 3) films, in the free-standing form, has been characterized in aqueous NaPF 6 electrolytes during potential step experiments. Actuation strains of up to 7.5% were observed due to anion insertion at more positive potentials. The actuation strain decreased as the applied stress was increased up to 12 MPa, and then remained constant at above 4% up to the maximum applied stress of 28.8 MPa. A relatively large creep was observed in the case of an applied stress of 28.8 MPa. The film could be cycled more than 1500 times with the retention of 18% of the initial strain level.

Determination of Kinetic and Diffusional Parameters for Sodium Borohydride Oxidation on Gold Electrodes

Direct borohydride fuel cells are among the latest fuel cell types under considerable research and development. There is a clear need for further elucidation of the borohydride electrochemical oxidation. From this perspective, potential step experiments were carried out with a Au disk electrode in alkaline sodium borohydride solutions of varying concentrations and temperatures. Analysis of the current transients for the full time scale enabled the delimitation of the domains where diffusion, electron transfer, and mixed control prevail. Chronoamperometric measurements in the diffusion-controlled region led to current responses exhibiting Cottrellian behavior, from which accurate diffusion coefficients, , were estimated. Arrhenius plots of allowed determination of the diffusion activation energies and maximum diffusion coefficients. Chronocoulometric measurements generated Anson plots from which the kinetic rate constants and charge transfer activation energies were calculated. Analysis of the current densities in conditions of infinite mass transfer led to Tafel plots from which exchange current densities, transfer coefficients, standard rate constants, and standard activation energies for charge transfer were determined. Purely charge-transfer regimes were not recognized in the studied system under the imposed potentials.

A Mechanical Actuator Driven Electrochemically by Artificial Molecular Muscles

A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules (artificial molecular muscles), undergoes reversible deflections when subjected to alternating oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol-gold chemistry. An electrochemical cell was employed in the experiments, and deflections were monitored both as a function of (i) the scan rate (< or =20 mV s(-1)) and (ii) the time for potential step experiments at oxidizing (>+0.4 V) and reducing (<+0.2 V) potentials. The different directions and magnitudes of the deflections for the microcantilevers, which were coated with artificial molecular muscles, were compared with (i) data from nominally bare microcantilevers precoated with gold and (ii) those coated with two types of control compounds, namely, dumbbell molecules to simulate the redox activity of the palindromic bistable [3]rotaxane molecules and inactive 1-dodecanethiol molecules. The comparisons demonstrate that the artificial molecular muscles are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. The approximately 550 nm deflections were calculated to be commensurate with forces per molecule of approximately 650 pN. The thermal relaxation that characterizes the device's deflection is consistent with the double bistability associated with the palindromic [3]rotaxane and reflects a metastable contracted state. The use of the cooperative forces generated by these self-assembled, nanometer-scale artificial molecular muscles that are electrically wired to an external power supply constitutes a seminal step toward molecular-machine-based nanoelectromechanical systems (NEMS).

Hydrogen Absorption in Nanocrystalline Cubic Ti-Based Compounds

Hydrogen absorption in a series of compounds having the same crystallographic structure but different chemical compositions was studied. To this end, high-energy ball milling was used to prepare cubic nanocrystalline Ti-based compounds of general formula Ti−Ru−M, with M = Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. After extensive milling, the most important phase (>95 wt %) has a simple cubic structure (CsCl). Hydrogen absorption in these compounds was assessed by potential step and current efficiency measurements, as well as by in situ electrochemical X-ray diffraction experiments. In the case of potential step experiments, severe degradation of nanocrystalline Ti−Ru−V, Ti−Ru−Cr, and Ti−Ru−Ni electrodes was observed. This is because these measurements were obtained after a succession of hydrogen absorption/hydrogen desorption cycles, that emphasize the mechanism responsible for the degradation of the electrodes. This is not the case with current efficiency measurements, and an estimation of the amount of H atom absorbed was obtained for all electrode materials. It was shown that as much as 0.7 H atom per unit cell can be absorbed for M = V. In situ electrochemical X-ray diffraction patterns were recorded while the electrodes were negatively polarized. The change of the lattice parameter, Δa, and the change of the volume of the unit cell, ΔV, were measured as a function of M. For M belonging to the fifth period, it was shown that the nature of M does not have any marked influence on the stability of the electrode, all nanocrystalline Ti−Ru−M materials absorbing only small amount of hydrogen. This assertion does not hold true in the case of M coming from the fourth period. In that case, the amount of hydrogen absorbed is correlated to the unit cell volume of the material.

Hybrid sol–gel–conducting polymer synthesised by electrochemical insertion: tailoring the capacitance of polyaniline

The templated growth of polyaniline through porous sol–gel films on ITO substrates has been performed. The polyaniline–silica composites have been synthesized by electrochemical reactive insertion in potentiodynamic and potentiostatic conditions. When the polymer is synthesized by potential step experiments, the shape of the chronoamperograms indicates two regimes for the electrochemical polymerization of PANI. Electropolymerization happens faster within the silica pores than on bare ITO electrodes due to confinement effects of oligomeric species formed upon oxidation. The electrochemical capacitance of the silica–PANI hybrids is several time higher than the capacitance of ITO/PANI synthesized under the same conditions. The silica matrix avoids the electric collapse between vicinal conducting fibres in ITO/silica–PANI but allows the diffusion of ionic species that are in contact with the conjugated polymer, increasing therefore the conducting surface exposed to the electrolytic solution.

Investigating the concept of diffusional independence. Potential step transients at nano- and micro-electrode arrays: theory and experiment

Microelectrode arrays find broad application in electroanalysis offering the enhanced sensitivity associated with microelectrodes, but with a high total current output. Such arrays are often constructed to make the electrodes 'diffusionally independent'. To emphasize that this is a time dependent property, a two-dimensional simulation, in conjunction with the diffusional domain approach, is used to model potential step transient currents at microelectrode arrays. Two types of array, hexagonal and cubic, are considered. In both cases the absolute (not relative) microelectrode separation distance has a significant effect on transient current. Three different regimes of transient current versus time can be observed at microelectrode arrays. At short times the transient response of isolated microelectrodes is seen, then at intermediate times the steady-state response of independent electrodes can be observed. At longer times planar diffusion to the entire array takes over. It follows that only at timescales corresponding to the first two regimes can the electrodes be considered as diffusionally independent. To verify the theory the potential step experiment is performed at a regularly spaced hexagonal iridium microdisk array. Theory is found to be in a good agreement with the experimental results.

Voltammetry in Weakly Supported Media: The Stripping of Thallium from a Hemispherical Amalgam Drop. Theory and Experiment

A theoretical model for electrochemical processes in resistive media is applied to interpret the current measured for the stripping of thallium from a Tl/Hg amalgam. Chronoamperometry is presented for a potential step experiment at a 12.5 μm radius hemispherical mercury drop in which thallium is first deposited and then stripped. Unusual features are observed in the transient stripping current, and it is proposed that these occur when the concentration of thallium(I) cations in solution is so great that the inert electrolyte salt is no longer in excess and the system is only partially supported. The theoretical model uses the Nernst−Planck−Poisson equations, which take into account the effects of the electrical potential in the aqueous phase, and avoids making the approximation of electroneutrality. The numerically simulated current accurately predicts the unusual experimental behavior.

Determination of Kinetic and Diffusional Parameters for Sodium Borohydride Oxidation on Gold Electrodes

Potential step experiments were carried out with a Au disc electrode in alkaline sodium borohydride solutions of varying concentrations and temperatures. Analysis of the current transients for the full timescale enabled the delimitation of the domains where diffusion, electron transfer, and mixed control prevail. Chronoamperometric measurements in the diffusion controlled region led to current transients exhibiting Cottrellian behaviour, from which accurate diffusion coefficients, D, were estimated. Arrhenius plots of D allowed determination of the diffusion activation energies, Ed, and maximum diffusion coefficients, D0. Chronocoulometric measurements generated Anson plots from which the kinetic rate constants, k, and charge transfer activation energies, Ect, were calculated. Analysis of the current densities in conditions of infinite mass transfer led to Tafel plots from which exchange current densities, i0, transfer coefficients, α, standard rate constants, ks, and standard activation energies for charge transfer, Ect0, were determined. Purely charge transfer regimes were never observed for the studied applied potentials.

Probing the Structure of Operating Fuel Cell Cathode Catalysts using XAS

A proton exchange membrane fuel cell has been designed to enable correlation of cathode catalytic activity with structural parameters determined by in situ XAS investigations in transmission mode. The cell design is described and validated for oxygen reduction activity measurement using both Pt and PtCo alloy catalysts. Findings from in situ XAS investigations at the Pt LIII edge are presented for a 60% Pt/C cathode catalyst operating at OCV, under load with oxygen feed, and during time resolved potential step experiments with oxygen and nitrogen feeds. In nitrogen XANES and EXAFS analysis show increasing oxide formation and particle disruption with increasing potential up to 1.0 V. In oxygen the catalyst structure remained unchanged irrespective of the applied potential and therefore load drawn from the cell. Differences in coordination number at the same applied potential in oxygen compared to nitrogen indicate that a particle morphology change occurs during the ORR.

Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

Abstract The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br 2 /Br − in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge–discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge–discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm −2 . The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.

Participation of Aluminum Hydride in the Anodic Dissolution of Aluminum in Alkaline Solutions

The mechanism of anodic alkaline dissolution of aluminum was investigated through the analysis of cyclic voltammetry (CV) and potential step experiments. Attention was focused on the role of aluminum hydride as a reaction intermediate, as suggested by the recent detection of formation during open-circuit dissolution. Potential step experiments at pH 11.75 revealed that the potential at the metal–surface film interface was close to the Nernst potential of oxidation. This finding suggested a reaction mechanism in which an interfacial layer is formed continuously by reaction of cathodically formed H with Al, and is then oxidized to the dissolution product, aluminate ions. However, potential step experiments at pH 11 did not indicate the presence of interfacial ; instead, the metal–film interface was close to the equilibrium potential of Al oxidation. Analysis of the CV indicated an abrupt transition in dissolution behavior between the two pH values, from a relatively rapid dissolution controlled by diffusion and film conduction in highly alkaline solutions, to a slow dissolution at a lower pH controlled by a highly resistive surface film. The formation of interfacial occurs readily at the higher pH, but is suppressed as the pH approaches neutrality.

A new approach on the Cu UPD on Ag surfaces

The Cu UPD on different Ag substrates was investigated by cyclic voltammetry, potential step and X-ray photoelectron spectroscopy (XPS) experiments. Although it is known that a Cu UPD fails on bulk silver substrates, it could be proved that on silver monolayers copper ions are reduced in the underpotential region forming an Au–Ag–Cu sandwich structure. This process occurs only in absence of anions and thymine which are specifically adsorbed on the silver substrate. In this sense the Cu UPD on silver monolayers behaves completely different compared to the Cu UPD on Au(1 1 1) for which the process is supported only in presence of specifically adsorbed anions or thymine.

CO monolayer oxidation at Pt(1 0 0) probed by potential step measurements in comparison to Pt(1 1 1) and Pt nanoparticle catalyst

The oxidation of CO adsorbed to high coverage on Pt(1 0 0) electrodes in 0.5 M H 2SO 4 is investigated through comparative measurements with Pt(1 1 1). In potential step experiments, current–time transients recorded during CO oxidation on Pt(1 1 1) display the same peak times and adherence to a Langmuir–Hinshelwood (LH) model for adsorbed CO electrochemical oxidation as literature benchmarks. For potentials in the vicinity of 0.8–0.9 V (versus a reversible hydrogen electrode reference), CO oxidation was faster on ordered Pt(1 0 0) electrodes than on Pt(1 1 1), and responses for ordered Pt(1 0 0) were close to, but somewhat more complicated than those predicted by the LH model. On Pt(1 0 0) with defects intentionally introduced by eliminating H 2 gas from the cooling atmosphere, current–time transients recorded during CO monolayer oxidation showed tailing at long times and responses similar to those for the reaction over nanometer-scale (<10 nm) Pt catalyst particles. The kinetics for CO monolayer oxidation on Pt(1 0 0) is discussed in terms of properties of Pt(1 0 0)-(1 × 1) islands and possible changes in island size with electrode treatment.

Application of PEDOT layers for the electrogravimetric detection of sulphate and phosphate in aqueous media

Poly(3,4-ethylenedioxythiophene), PEDOT, films were electrochemically prepared from aqueous solutions containing different supporting electrolytes (ClO 4 −, SO 4 2− and H 2PO 4 −). Characterization of the resulting films was carried out using cyclic voltammetry and potential step experiments combined with the dissipative quartz crystal microbalance technique. The ion incorporation caused an increase of the film mass and below 10 −2 M depended on the concentration of the solutions. Potential step experiments showed that the pH of the solutions also has an influence on the mass change of the films, which can only be partially attributed to the pH dependence of the charge of the anions. In principle, the ratio of the mass and the passed electric charge (apparent molar mass) during the redox switching of the polymer can be used to identify the ionic species.

Spectroelectrochemical study of the adsorption of acetate anions at gold single crystal and thin-film electrodes

Acetate adsorption at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in-situ infrared spectroscopy. External reflection measurements, performed with gold single crystal electrodes, are combined with Surface Enhanced Infrared Reflection Absorption Spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) carried out with sputtered gold thin-film electrodes. Theoretical harmonic IR frequencies of acetate species adsorbed with different geometries on Au clusters with (1 1 1), (1 0 0) and (1 1 0) orientations have been obtained from B3LYP/LANL2DZ, 6–31 + G* calculations. The theoretical and experimental results confirm that, irrespective of the surface crystallographic orientation, bonding of acetate to the surface involves the two oxygen atoms of the carboxylate group, with the OCO plane perpendicular to the metal surface. DFT calculations reveal also that the total charge of the metal cluster–acetate supermolecule has small effect on the vibrational frequencies of adsorbed acetate species. Both the external and the internal reflection measurements show the co-adsorption of acetate and perchlorate anions. Step-scan measurements carried out with the gold thin-film electrodes have allowed the monitoring of the time-dependent behaviour of perchlorate, acetate and water bands in potential step experiments. Acetate adsorption under those conditions is shown to involve perchlorate desorption and to follow a Langmuir-type kinetics. The step-scan spectra also show the rise and decay of transient water structures with parallel time-dependent shifts of the background intensity in the infrared spectra.