Potential Step Technique Research Articles

An improved structure model to explain variations of electric properties of polyaniline film in the reduction process by using double potential step technique

The double potential step method was used to study the electric properties of polyaniline (PAn) film in acidic aqueous solutions. The resistance and capacitance of bulk film changed with prolonging of the reduction time and/or shifting of reduction potential in the negative direction. And also a shoulder appeared in the cathodic chronoamperometric curve indicate the two-stage reduction process of PAn. These experimental results have been satisfactorily explained by using an improved heterogeneous model, which takes the merits of both the percolation theory and the electrochemistry stimulated conformational relaxation model. This new model overcomes some difficulties that the previous two models may meet, and especially can quantitative explain the change of electric properties of PAn film in the reduction process.

A new route to synthesize high degree polythiophene in a room temperature melt medium

Conducting polymers suffer from folds and kinks because of random nucleation and solvation of a free radical cation to yield a cross linked/disordered polymer and therefore a solvent free electrochemical polymerization in a room temperature melt medium is adopted to yield a high degree polymer with high electronic conductivity. Electropolymerization of thiophene was performed on platinum/ITO substrates using cyclic voltametry or galvenostatic mode in chloroaluminate room temperature melt medium to obtain a reddish brown free standing film which can be peeled off from the electrode surface after a minimum of 10 cycles. The conductivity was found to be around 10 2 S/cm. The degree of polymerization was calculated to be around 44 from IR studies. A layered structure supportive for high degree of polymerization was witnessed from potential step technique. From UV spectra the charge carriers were found to be bipolarons. The morphology of the film was found to be crystalline from SEM and XRD studies. Capacitative impedance properties for doped samples were interpreted from impedance spectroscopy.

Study of multistep electrode processes in triple potential step techniques at spherical electrodes

In this paper, a reversible multistep electrode process is studied by the most popular triple pulse techniques as Reverse Differential Pulse Voltammetry and Double Differential Pulse Voltammetry. The equations presented here are valid for spherical electrodes of any size (including planar and ultramicrospherical electrodes as limit cases) and can be used to describe the electrochemical behaviour of molecules with multiple redox centers, whether they are interacting or not. The comparison between results in triple and double pulse techniques for reversible multistep processes allows us to draw important conclusions regarding the suitability of such techniques and the characterization of reversible processes.

In situ observation of the initial adsorption process of heptylviologen cation radicals by slab optical waveguide spectroscopy synchronized with electrode potential modulation methods

In situ observation of absorption spectra of the heptylviologen cation radical (HV +·) was performed by slab optical waveguide (SOWG) spectroscopy utilizing indium-tin-oxide (ITO) electrodes. On application of potential sweep and potential step techniques from −300 to −600 mV (vs. Ag|AgCl), we observed the initial adsorption of HV +· on the ITO electrode. Two specific absorption bands were found at around 380 and 400 nm, which indicated that HV +· existed as both monomer and dimer on ITO with a monolayer or a few deposition layers. The change in the intensity and the peak position of the absorption spectra from adsorption species of HV +· was correlated with the electrode potential. These two absorption bands became incorporated in one broad band after a 30 s interval at −600 mV.

Application of a Novel Refinement Method for Accurate Determination of Chemical Diffusion Coefficients in Electroactive Materials by Potential Step Technique

We describe application of a novel refinement method for an accurate determination of the chemical diffusion coefficient, D, and the generalized kinetic parameter, Λ, from experimental potentiostatic intermittent titration technique (PITT) data suited for a variety of electrochemically doped electroactive polymers and inorganic intercalation host materials. The proposed, simple, two-step refinement procedure, based on earlier derived analytical expressions for the PITT response, is exemplified by the analysis of chronoamperometric responses to small-amplitude potential perturbation for p- and n-doped poly(fluorenone-bithiophene) (PFDOBT-HH) thin film electrode. The initial p-doping and the entire region of n-doping of this polymer is characterized by a mixed kinetic control of the doping processes due to a combination of ohmic drops (in solution and in the bulk polymer), slow interfacial charge transfer, and diffusion kinetics. In these cases, the refinement procedure leads to meaningful values of the diffusion time constant, and hence, D. It is believed that this relatively simple refinement procedure will facilitate accurate determination of the chemical diffusion coefficients of practical, composite insertion electrodes. © 2005 The Electrochemical Society. All rights reserved.

A study of the initial stages of the electrochemical deposition of thallium on copper, Part IV. The potential step results: Underpotential deposition on (111), (110) and (100) oriented copper single c

The underpotential deposition and dissolution of thallium onto carefully chemically polished single crystal copper (111), (110) and (100) electrode surfaces from acetate, sulphate and perchlorate solutions have been investigated using single and double potential step techniques. It appeared that the different anions used did not change the current-time response characteristics significantly. Current-time responses to the potential steps applied strongly resemble those observed in the case of thallium underpotential deposition on silver single crystals [1]. The characteristics of i-t transients obtained by single potential step suggest very fast 2D crystal growth processes taking place. Sharp linear voltammetry peaks, which are observed for both thallium and lead deposition on Cu(111) [3,8] and on Ag(111) [1,3], probably always indicate nucleative phase formation processes but the rate of the lattice growth step will vary from system to system. Comparison of the obtained results with those for lead underpotential deposition [8-13] on Cu(111), (110) and (100) suggests that, in the present case, the rate of the 2D lattice building process for both thallium underpotential monolayers formed on three copper single crystal surfaces examined is so fast that it becomes controlled by diffusive processes; this will probably be surface diffusion [24,25] initially and, at longer times planar bulk diffusion. Double pulse experiments did not help significantly in attempts to obtain i-t transients capable of providing data suitable for showing conclusively the occurrence of 2D crystal formation processes. A similar situation occurred also with silver substrates [1].

A study of the initial stages of the electrochemical deposition of thallium on copper, Part V. The potential step results: Overpotential deposition on (111), (110) and (100) oriented copper single cry

The over potential deposition of thallium onto carefully chemically polished single crystal copper (111), (110) and (100) electrode from sulfate and perchlorate solutions have been investigated using single and double potential step techniques. It appears that the different anions used did not change the current-time response characteristics significantly. The charge corresponding to UPD monolayers of thallium (?210?10-6 Ascm-2 for the first, and ?400?10-6 Ascm-2 for both, first and second, monolayers on all three orientations of the copper substrates used) were observed under the initial falloff seen on the current-time transients obtained at very short times, preceding the currenttime relationships for the overpotential deposition. Indication of a rising i-t transient reflecting bulk thallium deposition on Cu(111) and Cu(100) were usually observed at ?=-9mVvs.Tl while an over potential of ?=-10mVvs. Tl was needed for similar effects on Cu(110). The observed i vs. t2 linearity is characteristic to 3D instantaneous nucleation and further growth of 3D centres. The results show no appreciable 3D nucleation occurring until the UPD monolayers were completed. It must be concluded, therefore, that thallium over potential deposition on copper single crystal surfaces starts with a charge transfer controlled 3D instantaneous nucleation and proceeds by 3D growth. After some time the individual 3D centres overlap and the rate of increase of current falls off until finally the limiting current is reached. Similarly to the cases of lead deposition on Ag18 and Cu30 crystals, bulk deposition of thallium commences only after the two UPD close-packed thallium monolayers have been completed on the underlying substrate. Formation of the two UPD thallium close-packed monolayers, in the case of copper (111), (110) and (100) substrates, is an essential precursor to thicker thallium deposits their influence on the initiation of the over potential deposition and its growth mechanism, was shown to be very significant.

Microelectrode studies of reversible Mg deposition in THF solutions containing complexes of alkylaluminum chlorides and dialkylmagnesium

A kinetic study of Mg deposition on Pt surfaces has been performed using Pt microelectrodes. The solutions chosen included tetrahydrofuran (THF) and several types of Mg–Al–R–Cl complex electrolytes from which magnesium can be deposited reversibly. The complexes included an all-inorganic, Cl-containing MgCl 2:AlCl 3 (1:1) complex compound, all-organic Bu 2Mg with different ratios of Bu 2Mg, and various combinations of AlEtCl 2 and AlEt 3. Tafel slopes of the resulting polarization curves obtained both by CV and potential step techniques were carefully analyzed within the framework of a complicated, three-stage electrocrystallization mechanism related to four consecutive and two parallel steps in the first stage (ad-atom formation on the metallic interface) followed by surface diffusion and embedding of Mg-ad-atoms in the growth kink sites. This mechanism not only explains the peculiar kinetic data on Mg deposition from these complex solutions, but also provides a natural route for the comparison of a Mg deposition mechanism with that of gold and Ni depositions from cyanide and AlCl 3–BPC–NiCl 2 room temperature molten salts, respectively. We also discuss the uniqueness of these THF-based complex solutions for metallic Mg deposition, and compare the electrodeposition mechanism in this electrolyte with the electrochemistry of Mg in thionyl chloride solutions.

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.

Studies on polypyrrole film in room temperature melt

A bluish-black shining free standing polypyrrole film (PPy) of electronic conductivity ∼130 S cm −1 has been prepared by electrochemical oxidative polymerization of pyrrole on Pt/transparent glass conducting electrode ∼ resistance 15 Ω cm −1, using a room temperature melt as an electrolyte, composed of 1:3 stoichiometric ratio of cetyl pyridinium chloride and anhydrous aluminum chloride at 0.58 V versus Al wire as a reference electrode. The film possessed a charge transfer resistance of 132 Ω, and showed two absorption peaks at 457 and 1264 nm in the UV-vis–NIR diffused reflectance spectra. The morphology of the film was hexagonal. The potential step technique suggested a layered structure. This thin film can easily be peeled off from the electrode surface after three cycles and can be used for various applications like dissipation of electrostatic charge, battery electrode materials, solid electrolytic capacitor, electrochromic windows and displays, microactuators etc. It was also characterized by IR, thermal and SEM studies.

Filter functions in pulse techniques: application to the study of a slow charge transfer reaction

A first class filter function is a mode of pulse voltammetry that removes completely the current response for a reversible charge transfer reaction without modifying the corresponding signal for a slow charge transfer reaction. Therefore, the application of these modes permits study without interferences of the response for an irreversible electron transfer in those cases where reversible processes are also present. In the double potential step technique, the mode defined by the linear combination c 1 I 1 + c 2 I 2, with I k ( k = 1,2) being the current response for the potential step k at the end of the corresponding pulse and where E 2 is set on the anodic diffusion current plateau, can be easily modulated as a first class filter function if the ratio of the coefficients c 1/ c 2 and the pulse durations are chosen appropriately. This polarization mode has interesting features that allow us to determine the kinetic parameters of the slow charge transfer reaction and, in addition, the curves obtained are well defined peak-shaped waves displaying zero baselines with the analytical advantages that this represents.

Kinetics of dissociative adsorption of ethylene glycol on Pt(1 0 0) electrode surface in sulfuric acid solutions

The dissociative adsorption of ethylene glycol (EG) on Pt(1 0 0) electrode surface cooled in air after flame annealing was investigated by using programmed potential step technique and in situ FTIR spectroscopy. The stable adsorbates derived from EG dissociative adsorption on Pt(1 0 0) were determined by in situ FTIR spectroscopy as linear- and bridge-bonded CO. The quantitative results demonstrated that the average rate of dissociative adsorption of EG on Pt(1 0 0) surface varies with electrode potential, yielding a volcano-type distribution with a maximum value located near 0.10 V versus SCE. From the variation of the quantity of CO adsorbates generated in EG dissociative adsorption with the adsorption time t ad, the initial rate ( ν i) of this surface reaction was evaluated quantitatively. The maximum value of ν i has been determined to be 2.64 × 10 −11 mol cm −2 s −1 in a solution containing 2 × 10 −3 mol L −1 EG. The influence of the surface structure of Pt(1 0 0) electrode obtained by different pretreatment as well as of the specific adsorption of (bi)sulfate anions on the kinetics of EG dissociative adsorption has been also investigated and discussed. In comparison with a Pt(1 0 0) surface cooled in air atmosphere after flame treatment, the Pt(1 0 0) surface cooled in an Ar–H 2 stream or subjected to a treatment of fast potential cycling decreased significantly the initial rate ν i of EG dissociative adsorption. Similar effect was also observed for the specific adsorption of (bi)sulfate anions. However, the maximum attainable coverage ( θ DA S ) of adsorbates derived from EG dissociative adsorption is not affected either by the surface structure of Pt(1 0 0) or by (bi)sulfate anions adsorption.

Measurement of Lateral Charge Propagation in [Os(bpy)2(PVP)nCl]Cl Thin Films: A Scanning Electrochemical Microscopy Approach

The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (DCT) in metallopolymers of the type [Os(bpy)2(PVP)nCl]Cl, bpy = 2,2‘-bipyridyl and PVP = poly(4-vinylpyridine), is described. In this approach, a triple potential step technique is employed in which the ultramicroelectrode (UME) tip of the SECM is used to electrogenerate a solution phase oxidant, Ru(CN)63-, in an initial potential step, via the oxidation of Ru(CN)64-. This moiety diffuses from the tip to the underlying polymer film where electron transfer occurs, causing the local oxidation of the polymer-bound complex of OsII to OsIII. The form of the current−time characteristic in this step provides information on the kinetics of the ET process between the solution species and the polymer-bound moiety, as well as the concentration of redox-active species in the polymer film. This process creates lateral concentration gradients of OsII and OsIII along the film. After the first potential step, a waiting period is introduced in which Ru(CN)63- is converted back to Ru(CN)64- at the UME and OsII can recover in concentration by electron self-exchange between OsIII and OsII moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN)63-. The current−time behavior associated with this step is influenced significantly by the extent of lateral electron hopping in the waiting period. It is shown that SECM is capable of measuring DCT values as low as 10-10 cm2 s-1 with good precision. We report experimental measurements on spin-coated films of [Os(bpy)2(PVP)nCl]Cl, where n = 5 or 10, which indicate that DCT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).

Study of multistep electrode processes in double potential step techniques at spherical electrodes

Analytical solutions for reversible multistep electrode processes in double potential step techniques at spherical electrodes of any size, including plane electrodes and ultra-microelectrodes, are derived. These solutions are valid when the diffusion coefficients of all species are equal, and they are applicable for any number of the species initially present in the solution, for any values of the formal potentials of the different steps and for any duration of the two potential steps. In differential pulse voltammetry (DPV) and in additive differential pulse voltammetry (ADPV) the effects of the applied potential and of the electrode sphericity on the current are found to be separable and, therefore, the position of the peaks and the cross potentials are not dependent on the size of the electrode. The convenience of using ADPV in determining the formal potentials when some electrochemical steps are not completely separated is shown. It is also demonstrated that the reproportionation/disproportionation homogeneous reactions have no effect either on the surface concentrations nor on the currents in any multipotential step technique under the conditions considered here.

Electrocatalytic Properties of Ni-Based Alloys Toward Hydrogen Evolution Reaction in Acid Media

The electrocatalytic performance toward the hydrogen evolution reaction (HER) of a series of Ni-based alloys prepared by electroless deposition was studied by means of linear polarization, cyclic voltammetry, and potential step techniques. Ni-W-P and Ni-Mo-P alloy electrodes exhibit good electrocatalytic activity toward HERs, with some Ni-Mo-P alloys approaching the performance of Pt foils. The electrocatalytic activity of Ni-based alloys decreases with increasing P content but increases with increasing content of W or Mo. The experimental results can be interpreted in terms of a pronounced synergy between Ni, which has internally paired d electrons, and W or Mo, which have empty or half-filled d orbitals and therefore, bind hydrogen atoms strongly. The absorption of hydrogen in these alloys is also found to have an important effect on their activity. Electrodes with higher activity are capable to incorporate larger amounts of hydrogen during cathodic charging. © 2003 The Electrochemical Society. All rights reserved.

In Situ Raman Spectroscopy of Single Particle Microelectrodes

An electrochemically sharpened W tip was used as a spear to capture and manipulate single, quasi-spherical particles of metallic Zn a few tens of micrometers in diameter, harvested from the anode of a commercial battery. This tactic made it possible to acquire in situ Raman spectra of Zn passive films formed by potential step techniques on individual Zn microparticles in 1 M KOH aqueous solutions using a shallow electrochemical cell placed directly beneath the objective of the Raman microscope. The results obtained were in good agreement with those reported earlier for Zn disk electrodes, a few millimeters in diameter, in alkaline electrolytes using the same Raman instrument without a microscope. More specifically, films formed by stepping the potential from −1.55 V to either −0.7 or −0.8 V vs. SCE, displayed a largely enhanced feature at ca. 565 cm−1 ascribed to the longitudinal optical (LO) phonon modes of ZnO, an effect associated with the presence of interstitial Zn and oxygen deficiencies in the lattice. In addition, significant amounts of crystalline ZnO could be detected only for passive films formed at the same two potentials after the electrodes had been microroughened by a single passivation-reduction step. © 2003 The Electrochemical Society. All rights reserved.

Kinetics of CuEtX film formation on copper piezoelectrodes

The interactions of ethylxanthate ions (EtX −) with copper electrodes were investigated using voltammetric, potential step, and nanogravimetric techniques. It has been found that chemisorption of EtX − ions on copper is followed by a multi-layer CuEtX film growth at sufficiently positive potentials. The growth mechanism of the anodic films has been elucidated and a kinetic equation proposed. The increasing irreversibility of the film formation/reduction processes has been found for films with thicknesses increasing from 2 to 10 monolayers (ML). The analysis of the phenomenological kinetic equation shows that a relatively large loss of the potential driving force, ca. 27 mV ML −1, strongly inhibits the CuEtX film growth at increasing film thickness. The electroreduction of thicker CuEtX films is also strongly retarded and the electrodissolution rates approach 0.42 ng cm −2 s −1 at E=−700 mV versus SCE. At high cathodic overpotentials ( E=−1200 mV), a dielectric breakdown of the film with instantaneous and complete film removal was observed. The film disintegration was manifested by a positive spike on the apparent mass transients. The implications of the appearance of apparent mass spikes in dynamic film conversions have been discussed and the use of an electrochemical quartz crystal nanobalance as a diagnostic tool in studies of film dynamics proposed.

Effects of Sb Adatoms on Kinetics of Electrocatalytic Oxidation of HCOOH at Sb-Modified Pt(100), Pt(111), Pt(110), Pt(320), and Pt(331) SurfacesAn Energetic Modeling and Quantitative Analysis

Electrocatalytic oxidation of formic acid on Pt(100), Pt(110), Pt(111), Pt(320), and Pt(331) surfaces modified with Sb adatoms (Pt(hkl)/Sb) was investigated using cyclic voltammetry, potential step technique, and in-situ FTIR spectroscopy. Emphasis has been placed on investigations of the effects of Sb adatoms at Pt single-crystal surfaces on the kinetics of HCOOH oxidation. Quantitative data of kinetics of HCOOH oxidation via reactive intermediates were obtained by applying a potential step technique in combination with a data processing method of integration transform of j−t transients, which has been described in our previous paper (Sun, S. G.; Yang, Y. Y. J. Electroanal. Chem. 1999, 467, 121). The current results illustrated that the kinetics of HCOOH oxidation is faster on bare surfaces of Pt(100), Pt(110), and Pt(320) than that on Sb-modified surfaces of these Pt single-crystal electrodes, but it is slower on bare surfaces of Pt(111) and Pt(331) than that on Sb-modified surfaces of the two Pt single-crystal electrodes. The peak potentials of HCOOH oxidation on Pt(hkl)/Sb were all converged in a narrow potential region between 0.20 and 0.30 V (SCE), implying that the adsorption of Sb on Pt(hkl) planes has altered the apparent activation energy of HCOOH oxidation. On the basis of quantitative results, we have proposed, for the first time, a rectifying factor of apparent activation energy (γ/kJ mol-1) that describes the alteration by Sb adatoms of the apparent activation energy of HCOOH oxidation on Pt(hkl)/Sb surfaces. The values of γ have been determined quantitatively for γPt(100) = 12.80 ± 0.317, γPt(110) = 18.74 ± 0.393, γPt(320) = 16.683 ± 0.349, γPt(111) = −7.886 ± 0.288, and γPt(331) = −11.69 ± 0.245. The transfer coefficient β of HCOOH oxidation on the five Sb-modified Pt(hkl) electrodes was determined to be within the range of 0.12 ± 0.03, which is less dependent on the orientation of a Pt single crystal when it is modified with Sb adatoms and signifies a stepwise transfer of two electrons involved in HCOOH direct oxidation.

Filter functions for reversible charge transfer reactions in pulse techniques

A filter function is a mode of pulse voltammetry that removes completely the current response for a reversible charge transfer reaction for all values of potential or gives a constant response (≠0) for all values of E. These modes will be designated as filter functions of the first and second class, respectively. With both kinds of functions it is a key condition to obtain the corresponding effect that the current is sampled at selected values of the pulse durations, i.e. if the times for the potential steps are chosen arbitrarily the filtering effect is not observed. In turn, and because for a slow charge transfer reaction the current response with a first class filter function is not zero (nor constant when a second class filter function is applied), we have that the use of these modes of pulse voltammetry provides a way of obtaining without interferences the signal for an irreversible electron transfer in those cases where reversible processes are also present. Thus, if reversible and irreversible electron transfers with close discharge potentials occur simultaneously the response observed when a first class filter function is applied shows only the contribution due to the irreversible process while with a second class mode this last response is superimposed to the constant output obtained for the reversible process. In this paper the theory for these kinds of filter functions is established, particularly for double and triple pulse potential step techniques, and the characteristics of the responses obtained with some of these modes is discussed. Finally, the characteristics of the response obtained with filter functions at spherical electrodes are also shown.

Electrochemical Study of Passive Layer Formation on Lead-Base Alloys Immersed in 5.31 M H 2 SO 4 Solution

The process for the anodic formation of films was studied over lead and two tin-containing lead-alloys electrodes with high- and low-tin contents, in 5.31 M sulfuric acid aqueous solution, using the potential step technique. It was found that the mechanism governing the formation of the lead sulfate anodic film was common for the overall electrodic process and that it takes place comprising three different contributions to the total current The individual contributions defined were the current density contribution due to 2D nucleation and growth of an insoluble conducting adlayer, the current density involved during electrodissolution of the lead electrode in the zones which were not covered yet by the lead sulfate film, and was the current due to the growth of the passive layer induced by electrodissolution of lead metal and transport of the resulting Pb(II) ions through the film. However, each stage contributed to the overall process in a different manner depending upon the Pb:Sn ratio. The results from voltammetry indicated that it is possible to establish the potential at which the lead sulfate film begins to form and that this was also a function of the ratio. The oxidation potential resulted most positive for the low-tin-containing alloy electrode. For all cases, the applied potential needed to form the passive layer influenced the relative importance of each contribution to the overall process. © 2002 The Electrochemical Society. All rights reserved.