Potential Step Experiments Research Articles

Cryoelectrochemistry: electrochemical reduction of 2( RS)-methyl 1-( tert-butoxycarbonyl)-2-iodomethyl-2,5-dihydropyrrole-2-carboxylate

Cryoelectrochemisty with cyclic voltammetry and chronoamperometry has been applied to give an insight into a reductive pyrroline ring opening reaction, and has allowed the number of electrons participating in the reaction to be deduced from potential step experiments.

Investigating Metalloenzyme Reactions Using Electrochemical Sweeps and Steps: Fine Control and Measurements with Reactants Ranging from Ions to Gases

Protein film voltammetry is a powerful method for probing the chemistry of redox-active sites in metalloproteins. The technique affords precise potential control over a tiny quantity of material that is manipulated on an electrode surface, providing information on ligand- or metal-exchange reactions coupled to electron transfer. This is illustrated by examples of transformations of the iron-sulfur clusters in ferredoxins. Protein film voltammetry is particularly advantageous in studies of metalloenzymes for which the current response is proportional to catalytic activity: kinetic data of extremely high signal/noise ratio are obtained for highly active enzymes. We present a series of interesting examples in which catalytic activity varies in unusual ways with applied potential, surveying information that can be obtained from cyclic voltammetry and then looking beyond this method to controlled potential-step experiments that yield kinetic and mechanistic details. Recent results on the voltammetry of the highly active [NiFe]-hydrogenase from Allochromatium vinosum illustrate how it is possible to use the precise kinetic information from potential-step experiments to diagnose subtle details of transformations between catalytically active and inactive states of an enzyme. Protein film voltammetry thus complements spectroscopic techniques and other physical methods, revealing the chemistry of systems that might appear intractable or convoluted by other means.

Solid-state electroanalytical characterization of the nonaqueous proton-conducting redox gel containing polyoxometallates

A novel polymetacrylate-based redox-conducting polymeric gel, into which Keggin-type polyoxometallate, phosphododecatungstic acid (H 3PW 12O 40), had been incorporated, was electrochemically characterized in the absence of external liquid supporting electrolyte using an ultramicrodisk-working electrode. The phosphotungstate component (15 wt.% of the gel block) was entrapped as the polar organic solvent solution within pores of the polymer matrix. H 3PW 12O 40 plays bifunctional role: it provides well-behaved redox centers and serves as strong acid (source of mobile protons). The solid-state voltammetric properties of the system are defined by the reversible one-electron transfers between phosphotungstate redox centers. The following parameters have been determined from the combination of potential step experiments performed in two limiting (radial and linear) diffusional regimes: the concentration of heteropolytungstate redox centers, 6 × 10 −2 mol dm −3, and the apparent diffusion coefficient, 5 × 10 −7 cm 2 s −1. The room temperature ionic (protonic) conductivity of the bulk gel was equal to 1.6 × 10 −3 S cm −1. The charge propagation mechanism was found to be primarily controlled by physical diffusion of heteropolytungstate units within the gel pores rather than by electron hopping (self-exchange) between mixed-valence sites.

In situ conductance studies of p- and n-doping of poly(3,4-dialkoxythiophenes)

We report unique in situ conductance results with comparable conductance values in p- and n-doping ranges for a series of poly(3,4-dialkoxythiophenes). The polymers with different lengths of alkyl chain in the side alkoxy groups (1, 3 and 8 carbon atoms) were synthesized electrochemically from acetonitrile solutions of monomers on arrays of Pt microelectrodes or on a two-band Au electrode and studied in acetonitrile solution of 0.1 M TBAPF 6. The differences in the height of the conductance responses in the n-doping range as well as in the width of a high conductance window for oxidized polymers were discussed in terms of different conjugation lengths resulting from different lengths of substituents. The results obtained in the p- and n-doping ranges and the STM images of poly(3,4-dioctyloxythiophene) film were compared with those for poly(3,4-ethylenedioxythiophene) (PEDOT). The electrochemical stability of poly(3,4-dialkoxythiophenes) at different oxidation potentials was studied by in situ UV–Vis spectra recorded during double potential step experiments.

Relaxation kinetics of poly(3,4-ethylenedioxythiophene) in 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide ionic liquid during potential step experiments

The relaxation kinetics poly(3,4-ethylenedioxythiophene) (PEDOT) electrodeposited on platinum electrode surface were studied in a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (EMITFSI), by means of large amplitude potential step experiments. The influence of the applied potential and the film thickness were analyzed. We have developed a kinetic model allowing the determination of the kinetic features. Accordingly, two time or kinetic constants, for both oxidation and reduction that were in relation to PEDOT film morphology were determined. These kinetic constants seemed independent to the applied potential. We showed that the kinetic constants for the fast process depended upon the thickness of the polymer film. However, the time constant for the slow process was virtually independent of the film thickness. We have introduced dimensionless variables that can be used to analyze the global process. Accordingly, the kinetics for the oxidation were faster than that of reduction. These results were in agreement with the ions transfer mechanisms during the redox switching of PEDOT in EMITFSI in which a departure of cation occurred during the oxidation.

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: a chronoamperometric study

The CO electro-oxidation reaction has been studied on Rh[ n(1 1 1) × (1 1 1)]-type electrodes in 0.5 M H 2SO 4 using chronoamperometry. The transients recorded on Rh(1 1 1), Rh(5 5 4), Rh(5 5 3) and Rh(3 3 1) are characterized by a current plateau, visible directly after charging of the double layer, followed by a main oxidation feature, which consists of two peaks, a pre-peak and a main peak. The current density in the plateau region is nearly constant over time and, thus, is of (quasi) zeroth order in CO coverage. A plot of log( j plateau) vs. E final gives a linear relationship with a slope of ca. 45 mV dec −1, suggesting a second electron transfer as the rate determining step. Analogously to platinum, the current density plateau was ascribed to a Langmuir–Hinshelwood type reaction between CO ads and OH ads with no effective freeing of sites for OH adsorption due to relaxation of the CO adlayer. The presence of two peaks, rather than one, in the main oxidation feature can be explained by assuming a low surface mobility of CO and high oxidizability of rhodium surfaces. Indeed, dual step chronoamperometry shows that the mobility of CO on rhodium surfaces in aqueous media is very low. Since rhodium surfaces are known to oxidize readily, the pre-peak and main peak can be ascribed to CO reacting with OH, which adsorbs fast at the steps and more slowly at terrace sites. Since the geometry of the steps is nearly the same on each surface, the pre-peak appears structure insensitive, while the main peak shifts considerably with the step density. Introducing randomly distributed crystalline defects by cycling the electrodes repeatedly up to the surface oxidation region prior to each potential step experiment, results in a negative shift of the main peak, while the position of the pre-peak remains fixed. From the data presented, we conclude that the reaction nucleates at the steps and grows in the direction of the terraces.

EQCM Study of Anodic Film Growth on Valve Metals

A comparative study of anodic film growth on niobium, zirconium, and tungsten in was performed using the rotating electrochemical quartz crystal microbalance (EQCM). Potential sweep as well as potential step and hold experiments were performed in the range between open circuit and 8 Significant differences in the mass transients between the three metals were observed. During the formation of anodic films on niobium and zirconium, the dissolution was negligible and the fraction of the current serving for film growth was always close to unity. On tungsten, anodic dissolution in combination with film growth led to a growth fraction that diminished with time. The results obtained by the double potential step technique were compared with numerical simulations based on models considering either high field conduction or an interface reaction as limiting the kinetics of film growth. The growth of anodic films on niobium and zirconium was found limited by high field conduction, whereas film growth on tungsten was interface controlled. Film growth on tungsten was accompanied by important anodic dissolution, while dissolution of niobium and zirconium was negligible. © 2004 The Electrochemical Society. All rights reserved.

Growth of RuO 2 by electrochemical and gas-phase oxidation of an Ru(0 0 0 1) surface

Rectangular monolayer stripes of RuO 2(1 1 0) obtained in a simple gas-phase oxidation of an Ru(0 0 0 1) surface in argon (Ar) containing 115 ppm of O 2 were compared with the electrochemical oxidation of an Ru(0 0 0 1) surface in sulfuric acid at high positive potentials. In situ scanning tunneling microscopy (STM) showed that the electrochemical oxidation of the Ru(0 0 0 1) surface starts at step edges, and proceeds through the formation of randomly distributed RuO 2 islands. The nucleation process is instantaneous, as verified by potential step experiments. The oxidation of Ru(0 0 0 1) in a mixture of O 2 (115 ppm) and Ar produces a layer of ordered RuO 2(1 1 0) oxide in the form of rectangular stripes of monolayer height. The growth of these stripes, globally initiated at the step’s edges, is unidirectional, forming a 60° angle to the steps’ direction. The entirely different morphologies of RuO 2 grown in electrochemical- and gas-phase oxidation environments probably reflect the difference in the reaction temperature of two processes. In the latter, the elevated temperatures (600–800 K) appear to confer mobility on the reaction species that facilitates ordering of the oxide layer.

An electrochemical quartz crystal microbalance study of the etching of gold surfaces in the presence of tetramethylthiourea.

The oxidation of tetramethylthiourea (TMTU) at gold electrodes in acetonitrile, leading to dissolution of the electrode, has been studied by electrochemical methods and by an electrochemical quartz crystal microbalance (EQCM). TMTU in acetonitrile readily adsorbs at gold electrodes and an estimated coverage of 5.5 x 10(-10) mol cm(-2) (30 A2 per molecule) was measured electrochemically. Nevertheless, the oxidation of TMTU in solution is a diffusion-controlled process and is strongly influenced by the electrode material, as observed by comparison of gold electrodes with glassy carbon and platinum working electrodes. In the absence of TMTU, EQCM cyclic voltammetry experiments showed dissolution of gold through a 1e- oxidation process at potentials more positive than 1.20 V vs saturated calomel electrode (SCE). Potential step and cyclic voltammetry EQCM experiments performed using gold surfaces in the presence of TMTU revealed TMTU-assisted etching of gold at potentials as low as 0.35 V vs SCE. In the potential region from 0.35 to 1.20 V the current response of TMTU oxidation mimics the response expected for a redox-active species in solution, including the presence of a mass-transfer-limited region, which supports the conclusion that the etching process in this potential region is initiated by the oxidation of TMTU at the gold surface. The current efficiency of the TMTU-assisted etching was found to vary between 12 electrons per gold atom dissolved (e/Au) (E = 0.50 V vs SCE) and 2 e/Au (0.90 V < E < 1.20 V). At potentials <0.90 V the dominant electrochemical process is the formation of TMTU+, whereas at higher potentials the etching of the gold surface by formation of a Au(I)-TMTU+ species becomes equally important. At potentials above 1.20 V the etching is no longer dependent on the diffusion of TMTU and the e/Au value approaches 1.

The influence of platinum on the performance of silver–platinum anti-bacterial coatings

Abstract Anti-bacterial silver coatings were deposited on thermally sensitive polymeric substrates using a combination of magnetron sputtering and neutral atom beam (Saddle Field) plasma sources. The anti-bacterial activity of silver is dependent on the release of Ag+ ions, which act by displacing other essential metal ions such as Ca2+ or Zn2+. This study reports some initial electrochemical experiments intended to evaluate the use of platinum to enhance the release of silver ions from the silver coating. In the galvanic series in seawater silver is more active than platinum and therefore probably enhances Ag+ ion formation through galvanic action when the two metals are in electrical contact. In order to evaluate this several electrochemical methods were employed using magnetron target samples as working electrodes. Cyclic voltammetric, open circuit potential measurements and potential step (chronoamperometric) experiments were performed on several silver–platinum alloys containing from 0.5% to 3.0% Pt and on a pure silver target electrode. In the case of the chronoamperometric measurements, the resulting current–time curves demonstrated that Ag+ formation was enhanced with platinum addition by up to 100%. Results of this preliminary work are discussed with reference to various reactions that can occur on the surface of silver in chloride-containing environments.

Spatially limited diffusion coupled with ohmic potential drop and/or slow interfacial exchange: a new method to determine the diffusion time constant and external resistance from potential step (PITT) experiments

We have analyzed chronoamperometric curves, I( t), after small-amplitude potential steps Δ E (PITT technique) for the model of linear diffusion of a species inside an electroactive film, taking into account ohmic effects in the external media (solution and electrode) as well as a finite rate of the interfacial exchange. For its short-time interval, t≪ τ d ( τ d is the diffusion time constant, corresponding to unlimited diffusion from the interface), three approximate analytical expressions have been proposed. One of these represents an interpolation formula between the value of the current at the start of the diffusion process, I(0)=Δ E/ R ext (after the end of the EDL charging), and the Cottrell equation: I≅I(0)/(1+Λ( π t/τ d ) 1/2) , Eq. (9) where Λ= R d/ R ext is the ratio of diffusion and external (solution, electrode, etc.) resistances. Its comparison with the exact analytical solution derived recently by [Montella, J. Electroanal. Chem. 518 (2002) 61] shows the ability of this simple approximation to reproduce qualitatively the current-time dependence within the short-time interval for a wide range of Λ including the case where the external resistance is dominant. Another similar formula, Eq. (10), but with fractional exponents results in even better agreement with the exact result. Both analytical expressions enable one to evaluate the parameters, τ d and Λ, of a real system by linear fitting of its experimental data in the corresponding coordinates, e.g., in coordinates, [ I( t) t 1/2] −1 vs. t −1/2 for the above analytical expression. The treatment of experimental data in these coordinates also allows one to determine the upper limit of the “short-time range” in which the analytical approximation is applicable so that only the points for this time interval are used for the fitting procedure. We have also derived an analytical expression (11) for the same short-time interval, which reproduces the exact solution with a much higher precision. Since the use of this formula does not allow one to extract parameters of the process from experimental data by the simple linear fitting, we have proposed an original procedure of the data treatment to determine τ d and Λ without complicated calculation or optimization schemes.

Electrochemical in situ STM study of Al and Ti–Al alloy electrodeposition on Au(111) from a room temperature molten salt electrolyte

The electrodeposition of Al and Ti–Al alloys on Au(111) from an acidic aluminium chloride-1-methyl-3-butylimidazolium chloride (AlCl3–MBImC) room-temperature molten salt electrolyte containing TiCl4 was studied by cyclic voltammetry and chronoamperometry and in particular, nucleation and growth processes were monitored in situ by means of electrochemical scanning tunneling microscopy (EC-STM) for the first time. Two underpotential deposition (UPD) states for Al were observed in the cyclic voltammogram at 0.51 and +0.22 V vs. Al/Al3+. At corresponding UPD conditions STM images show different nucleation behavior for Al: first a growth of clusters all over the Au surface, and later preferentially along the step edges leading to a step edge decoration. Potential step experiments imply progressive nucleation and growth and are evidenced by STM images obtained at different time intervals showing the growth of Al clusters of up to 8 nm diameter in size. After anodic dissolution of the deposited Al film, the appearance of circular holes of a size comparable with the Al clusters notably along Au step edges indicates Al–Au surface alloy formation. Tunneling current vs. bias voltage (I–U) curves of the Al clusters exhibit a reduced tunneling barrier height (Φ) of 0.23 eV. A distinct two-dimensional nucleation of Ti–Al clusters begins to occur at 0.5 V vs. Al/Al3+ exclusively on the Au terraces. They grow in the form of chains reaching a size of about 2–3 nm diameter in size, smaller in comparison with Al clusters. X-Ray photoelectron spectra of the potentiostatically deposited film under overpotential deposition (OPD) conditions confirm the formation of a Ti–Al alloy phase.

A fractal analysis of colloidal Au nanoparticle electrodes

An electrochemical fractal analysis of three types of colloidal Au nanoparticle electrodes is described. One type of electrode comprises multilayers of 12-nm diameter colloidal Au particles on glass substrates; another type consists of analogous films grown on conductive Au substrates. Plots of log( I) vs. log( t) data from electrochemical potential step experiments were used to determine D f, the fractal dimension. On the Au substrate, D f ranged from 2.16 to 2.47 for films consisting of 7–13 colloidal Au layers, respectively, while for the glass substrate D f varied from 1.54 to 2.33 over the same range of layers. A third type of Au nanoparticle-based electrode was prepared by enlargement of nanoparticles in Au colloid monolayers by exposure to NH 2OH/Au 3+. For these samples, D f varied from ca. 1.2 to 2.0. D f determination by atomic force microscopy gave values in the range of 2.3–2.4 for both types of nanoparticle electrodes on glass substrates, regardless of Au film thickness.

Analysis of the diffusion layer thickness, equivalent circuit and conductance behaviour for reversible electron transfer processes in linear sweep voltammetry

The parametric dependence of the diffusion layer thickness upon electrode potential and scan rate has been derived for reversible electron transfer processes in linear sweep voltammetry (LSV) using a simple analytical expression for the dimensionless current function. The quantitative variation of the conductance with potential and scan rate has been illustrated and the equivalent circuit pertaining to LSV has been proposed. Several earlier results pertaining to potential step experiments, thin layer voltammetry, surface bound LSV, radial flow at micro-ring electrodes, etc. are shown to arise from the general expression for the current function obtained here, thus indicating an isomorphism among various potential perturbation techniques.

Electrochemical redox reaction at silicate based electrode–silicate based electrolyte interface

The electrochemical redox reactions of ferrocene or ferrocenedimethanol were studied at silicate based electrode–silicate based electrolyte interface. The electrode was prepared from methyltrimethoxysilane sol and graphite particles. The electrolyte consists of silicate matrix obtained from tetramethoxysilane with embedded redox probe and salt solution in propylene carbonate or water. It was prepared with electrodes immersed before gelation occurred. Stable slow scan peak shaped voltammograms were obtained after tens of days elapsed after gel formation. Their shape depends on whether electrode is immersed in liquid electrolyte, sol and gel. For organic solvent based gels the weak dependence of the magnitude of the peak current on time elapsed after gelation, whereas for aqueous based system its substantial decrease is observed. On the results of potential step experiments some features typical for spherical diffusion as well as these indicating penetration of the electrode matrix by organic solvent were observed.

Electrochemical kinetics of hydrogen intercalation in gadolinium switchable mirrors

Potential-step experiments on gadolinium thin-film electrodes provide current and optical transmission transients, whose time constants are related to effective hydrogen diffusion coefficients according to a standard diffusion model. The diffusion coefficients and switching kinetics depend on temperature, hydrogen concentration, potential, and film thickness. A thickness dependence of the kinetics is observed in which the diffusion coefficient and optical switching time are linearly dependent.

A surface resistance study of lead underpotential deposition on epitaxial silver thin film electrodes

The method of dc surface resistance is used as a tool to measure trace amounts of Pb2+ in an aqueous ClO4 electrolyte. Long term stability is obtained with epitaxial Ag(111) films grown on Si(111). Various potential step and linear scan experiments were applied, yielding nearly the same resistance dependence on the surface coverage. The limit of detection is below 10−6 M of Pb2+.

A High-Performance Supercapacitor/Battery Hybrid Incorporating Templated Mesoporous Electrodes

A supercapacitor/battery hybrid was fabricated using a mesoporous positive electrode and a mesoporous Pd negative electrode, both fabricated by electrodeposition of the metal within the hexagonal phase of a liquid crystalline template. Because we are predominantly interested in the performance of the electrode, we have designed the device such that performance is positive electrode limited. Use of rather than NiO allows us to derive a significant proportion of the capacity from bulk redox processes and the designed porous electrode structure allows us to access this capacity very rapidly. Cyclic voltammetry and potential step experiments show that we are able to deliver or (of Ni electrode) in 50 ms at a mean discharge voltage of 1.18 V using an aqueous electrolyte. This translates into energy and power densities of and respectively, for the electrode. Furthermore, device capacity has been shown to resist decay and actually increase by 10% over 15,000 cycles. © 2003 The Electrochemical Society. All rights reserved.

Influence of anode material on the electrochemical oxidation of 2-naphthol: Part 1. Cyclic voltammetry and potential step experiments

The anodic oxidation of 2-naphthol has been studied by cyclic voltammetry and chronoamperometry, using a range of electrode materials such as Ti–Ru–Sn ternary oxide, lead dioxide and boron-doped diamond (BDD) anodes. The results show that polymeric films, which cause electrode fouling, are formed during oxidation in the potential region of supporting electrolyte stability. IR spectroscopy verified the formation of this organic film. While the Ti–Ru–Sn ternary oxide surface cannot be reactivated, PbO 2 and BDD can be restored to their initial activity by simple anodic treatment in the potential region of electrolyte decomposition. In fact, during the polarization in this region, complex oxidation reactions leading to the complete incineration of polymeric materials can take place on these electrodes due to electrogenerated hydroxyl radicals. Moreover, it was found that BDD deactivation was less pronounced and its reactivation was faster than that of the other electrodes.

Electrochemical behaviour of poly(3,4-ethylenedioxythiophene) in a room-temperature ionic liquid

The cyclic voltammetry responses and the redox switching dynamics of poly(3,4-ethylenedioxythiophene) (PEDOT) in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (EMImTf 2N), were investigated. The shape of the cyclic voltammograms showed two anodic and two cathodic peaks. These peak currents varied linearly with the scan rate indicating a thin-layer behaviour. No memory effects were observed during the cyclic voltammetry experiments in this ionic liquid. On the other hand, the redox switching dynamics of PEDOT were studied by means of potential step experiments. The analysis of chronocoulograms in term of RC-circuits indicated that the time dependence of the charge transferred during the potential step showed two time constants. These results were consistent with the postulated structure or morphology of the PEDOT film which contained two types of coexisting zones: a compact and an open structures.