Impedance of ruthenium electrodes in sulphuric acid solution

Electrochemical impedance spectroscopy was employed to characterize three different types of Ru electrodes, anodically polarized and measured at +0.85 V versus SCE in 0.5 mol dm^?3 H₂SO₄, both in their non-electrochemically treated (NET states), and also following various potential/time treatment programmes (ET states). On the basis of experimental data and results of the complex non-linear curve fitting procedure, the impedance/frequency response is for all three Ru electrodes in either the NET or ET state, modelled generally as two impedance branches in parallel. The first, an almost pure capacitative impedance is ascribed to the fast and reversible charging/discharging of the oxide film surface, proceeding through oxidation/reduction of electron sites coupled by the surface deprotonation/protonation. The second impedance consists of the resistive impedance and two transmission line elements in the series connection. This impedance is ascribed to the oxide film equilibration process which proceeds mainly through diffusion-like transport of reaction species within a heterogeneous structure of the oxide film. In dependence on the type of Ru electrode and particularly on the electrode state (NET or ET) there are significant differences in contribution of each impedance to the overall impedance/frequency response. Thus predominance of diffusion-like impedance at medium and low frequencies is observed for all three Ru electrodes in their NET state. This indicates the contribution of the oxide film equilibration process in the overall electrode response. If, however, Ru electrodes are treated by defined potential/time treatment programmes, the electrode impedance/frequency response of each electrode is changed continuously, showing increase of the extent of the surface reaction and facilitated and accelerated oxide film equilibration. For some types of Ru electrodes, pure and low capacitative impedance behaviour in almost the whole frequency region is observed after only few potential steps applied.     

An electrochemical impedance study of Alloy-22 in NaCl brine at elevated temperature: II. Reaction mechanism analysis

The development of deterministic models for predicting the accumulation of corrosion damage to high level nuclear waste (HLNW) canisters requires the acquisition of values for various model parameters. In the present work, we describe the extraction of values for various parameters in the point defect model (PDM) for the growth of passive films on Alloy-22, from electrochemical impedance data for this alloy measured in saturated NaCl brine (6.2 m, m = mol kg^?1) at 80 °C, as described in Part I. The barrier layer of the passive film on Alloy-22 in the passive region (V < 0.6 V_SHE) is postulated to be defective chromic oxide (Cr_2 + xO_3 ? y) with n-type electronic character. The data suggest that the barrier layer is cation rich (x > y) with cation interstitials being the principal defect, but it is not possible to exclude oxygen vacancies, unequivocally, as being the principal defect. If the barrier layer is cation rich, the phase is estimated to have the composition Cr_2.028O₃ close to the metal|film interface, whereas if it is oxygen deficient, the stoichiometry is estimated to be Cr₂O_2.981 at the same location. The thickness of the barrier layer in the passive region is found to increase linearly with voltage while the passive current density is constant. In the transpassive range (V > 0.6 V_SHE), the passive film is found to be a p-type semi conductor, and the thickness of the barrier layer decreases, while the log of the passive current increases, with increasing applied potential. These observations are consistent with the predictions of the PDM for passive films on metals and alloys, including the transition from the passive to the transpassive states, which is predicted to coincide with a change in oxidation state of the chromium species being ejected from the barrier layer from Cr(III) to Cr(VI) and with the oxidative dissolution of the film. The principal defect in the barrier layer in the transpassive state appears to be the Cr(III) cation vacancy, which is generated by the oxidative ejection of cations from the barrier layer. Optimization of the PDM on the impedance data has yielded a set of parameter values for the passive state that can be used in deterministic models for predicting the accumulation of general corrosion damage to Alloy-22 in HLNW repositories.     

In situ tracing the process of human enamel demineralization by electrochemical impedance spectroscopy (EIS)

OBJECTIVE: The purpose of this study was to in situ characterize the demineralization (namely dissolution of hydroxyapatite) on the surface of the human enamel using electrochemical impedance spectroscopy (EIS) technology. METHODS: Fresh human third molars extracted without visible evidence of caries, were used in this study. After they were immersed in a demineralizing solution prepared from lactic acid and carboxy methyl cellulose sodium (Na-CMC) buffering at pH 4, demineralization happened on their surfaces. EIS of the specimens were measured at a series of immersed interval. X-ray diffractometer (XRD) were used to distinguish the microstructure of the surface layer of the specimens. The depositions that appeared in the demineralizing solution after 46 h immersion were analyzed by fourier transform infrared spectrometer (FTIR). RESULTS: XRD analysis revealed that the percentage of intensity (I%) of HAP gradually decreased with the elapsed immersing time, which indicated the dissolution of HAP columns of enamel. Nyquist spectra were fitted with an equivalent circuit characterized by some parameters, such as Q and R(p) (error<0.1%). Changes of the parameters' values revealed that the rate of demineralization accelerated at the forepart of the demineralization, but slowed down beyond 70 h immersion. CONCLUSIONS: The results suggested that EIS was proved to be a useful method for in situ investigating and in vivo detecting the demineralization of the enamel.     

One layer,two layers,etc. An introduction to the EIS study of multilayer electrodes. Part 1: Theory

In this theoretical EIS study of the redox reaction Fe³⁺ + e^? ? Fe²⁺ occurring at multilayer electrodes, four points are dealt with in detail: firstly, a recursive formulation allowing the exact calculation of the impedance whatever the number of layers is given (‘Russian dolls’ behaviour of the mass-transport functions). Secondly, the study of the two bilayer electrodes Pt/layer 1/layer 2 and Pt/layer 2/layer 1 shows that the layer order drastically influences the electrochemical response of the system, and proves that a multilayer electrode cannot be simply treated as a monolayer electrode. Thirdly, it is shown that all the key parameters (thicknesses, diffusion coefficients, and partition coefficients) cannot be only determined by EIS, another combined physicochemical technique being indispensable. Fourthly, an exhaustive study of bilayer electrodes in quiescent solution is made and all the possible impedance diagram shapes are plotted.     

EQCM study on the oxidation/reduction of bismuth sulfide thin films

The oxidation/reduction of bismuth sulfide thin films deposited on an Au electrode using the successive ionic layer adsorption and reaction method has been investigated by electrochemical quartz crystal microbalance combined with cyclic voltammetry and X-ray photoelectron spectroscopy. It has been shown that the oxidation/reduction of bismuth sulfide film depends on the nature of anion. In the acetate solution the bismuth sulfide film is oxidized at more negative potential values than that in sulfuric acid solution. During the oxidation of bismuth sulfide film the Bi present on the surface of the bismuth sulfide film is oxidized first. This process is followed by subsequent oxidation of bismuth sulfide film alongside the oxidation of Au. In the acetate background solution the oxidation of bismuth sulfide occurs with the whole film stripping and the calculated amount is related to the amount of deposit and corresponds to the thickness of ∼9–10 nm. In sulfuric acid the bismuth sulfide film is not completely stripped out in one anodic cycle. The evaluated amount during the first anodic scan corresponded to the thickness of deposit of ∼3–4 nm. The reduction of bismuth sulfide film took places at more positive potential values in the acetate background solution than that in sulfate background solution at pH 4. After the reduction of bismuth sulfide film to Bi⁰ and S²⁻ in the background solution, depending on the nature of anion, the oxidation of metallic Bi proceeds by two different mechanisms: in the sulfate solution Bi is oxidized to Bi(III) oxygen compounds while in the acetate solution Bi is oxidized by dissolution.     

EIS study of the FeIII/FeII redox couple at a Nafion®-coated Pt electrode

The redox reaction Fe^III + e ? Fe^II is studied at a Nafion^®-filmed Pt electrode using both steady-state and EIS methods. It is shown that the fitting of the EIS experimental data can be greatly improved by using the new concept of a ‘theoretical–experimental hybrid model’, i.e., by introducing experimental data obtained at a bare electrode in the theoretical model for a filmed electrode. It is also shown that a combined steady-state and EIS study cannot lead to a separate determination of the three key parameters of the system: the diffusion coefficient in the polymer film (D_f), the thickness of the film (L), and the partition coefficient between the solution and the polymer (γ), only the two parameters γD_f/L and L²/D_f being obtained from experimental data by a curve fitting procedure. Finally, an exhaustive theoretical EIS study in quiescent solution is also proposed.     

Cathodic behavior of bismuth. I. Ellipsometric study of the electroreduction of thin Bi2O3 films

The electroreduction of thin Bi₂O₃ films was characterized by employing electrochemical techniques and in situ ellipsometry. The electroreduction optical response is described in terms of a bilayer model. The optical analysis indicates that electroreduction proceeds yielding a rough/porous layer of metallic bismuth at the oxide/electrolyte interface, that grows inwards at the expense of the underneath oxide layer to completely exhaust it. At the end of oxide electroreduction, the aging process of the porous metallic bismuth layer could be detected. On this framework, changes in the oxide growth kinetics observed on electroreduced surfaces can be consistently explained.     

纯钛在仿真蛋白环境中腐蚀性能的电化学阻抗谱分析

目的 在高仿真环境中,采用电化学阻抗法分析纯钛种植材料的腐蚀性能。方法 以PBS液为对照组,细胞培养液为实验组,采用电化学分析仪测试钛在含血清蛋白体系中的电化学阻抗谱,利用EIS-Nyquist图、EIS-Bode图及等效电路的拟合分析,研究血清蛋白对钛的抗腐蚀性能的影响,采用SPSS11.0软件包对数据进行统计学分析。结果 纯钛材料在细胞培养液中的氧化膜阻抗值（24.8±3.9）×10⁵ Ω显著大于在PBS液中的阻抗值（6.7±0.8）×10⁵ Ω（P<0.05）。纯钛在PBS中低频区的相位角约为55°,而在细胞培养液中低频区的相位角约为75°。结论 机体环境中的血清蛋白能够影响钛金属在体液中的腐蚀性能。     

人工唾液中氯化钠浓度对钴铬、镍铬、含钛镍铬合金腐蚀行为的影响

目的：在人工唾液中氯化钠浓度升高的条件下,用电化学阻抗谱研究3种牙科常用合金（钴铬、镍铬、含钛镍铬合金）腐蚀行为的变化。方法：测定在人工唾液中氯化钠浓度分别为0.9%、2%、3%的条件下,3种合金的电化学阻抗谱,将各体系中合金界面电容和电荷转移电阻拟合,分析其腐蚀机制和耐腐蚀性能的变化。结果：随着氯化钠浓度升高,3种合金的界面电荷转移电阻（Rt值）都分别减小,界面电容（Qdl）增大,腐蚀反应过程加剧,抗蚀能力降低。但钴铬合金的Rt值是最大的。结论：在高氯环境中,3种合金腐蚀反应加快,抗腐蚀能力均降低,但钴铬合金抗腐蚀能力相对较强,这将影响它们的使用寿命。     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Investigation of potential inversion in the reduction of 9,10-dinitroanthracene and 3,6-dinitrodurene

In the case of two-electron reduction of a compound, potential inversion refers to the situation where introduction of the second electron occurs with greater ease than the first. That is E⁰₁?E⁰₂<0 where E⁰₁ and E⁰₂ are the standard potentials for the two steps of reduction. The extent of potential inversion in the reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, has been assessed by steady-state microelectrode voltammetry, cyclic voltammetry under conditions of near-reversible behavior, cyclic voltammetry under conditions of quasireversible behavior and electrochemical impedance spectroscopy (EIS). The studies were conducted in acetonitrile at 298 K. Cyclic voltammetry under quasireversible conditions and EIS were most sensitive to small changes in E⁰₁?E⁰₂. The value of E⁰₁?E⁰₂ for 1 was found to be ?107 mV and that for 2 was ?280 mV.     

Redox switching hysteresis in polyaniline–acetate systems: a search of molecular factors important for the dynamics of the polymer reaction

The CV redox hysteresis of three polyaniline–aqueous trihalogenoacetic acid systems has been analyzed. The polymer phases studied involve monocarboxylate monoanion bases characterized by similar pK_a values in water to narrow the differences in molecular interactions. The hysteresis diminishes by 70 mV with size of the anions – from trifluoroacetate, through trichloroacetate, to tribromoacetate. The diffusion coefficients of the trihalogenoacetate anions in the oxidized polyaniline do not differ significantly. Their values are of the order of 10^?10 cm²/s, which is one order of magnitude lower than for the chloride anion in polyaniline. Ex situ 1750–1050 cm^?1 Raman spectra of polyaniline–trihalogenoacetates show small noticeable systematic changes that correlate with the hysteresis behavior. Electrochemical impedance spectroscopy provides evidence for the chemical rate process involved in the redox switching of the polyaniline–tribromoacetate system.     

镍铬、钴铬合金和纯钛在人工唾液中的耐腐蚀性评价

目的:用电化学交流阻抗谱方法研究钴铬、镍铬合金和纯钛在人工唾液中的耐腐蚀性。方法:用电化学工作站测量出3种金属材料在人工唾液中的电化学阻抗谱(Bode图、Nyquist图),利用ZSimpWin软件对数据进行拟合分析,根据等效电路R(CR)拟合结果来评估3种金属的耐腐蚀性能。结果:从Nyquist图可见,3种材料的容抗弧半径大小依次为纯钛>钴铬合金>镍铬合金;从Bode图见,3种材料均为一个时间常数,即表面均是一个电容层。根据等效电路R(CR),3种材料的电容层无明显区别,但三者的阻抗值均很大,其中纯钛的阻抗值最大,钴铬合金的阻抗值居中,镍铬合金的阻抗值最小。结论:3种材料在人工唾液中均具有良好的耐腐蚀性,其耐腐蚀性能顺序是纯钛>钴铬合金>镍铬合金。     

Electrochemical impedance spectroscopy study of the adsorption behaviour of α-lactalbumin and β-casein at stainless steel

The electrocatalytic reduction of tert-butyl hydroperoxide and cumyl hydroperoxide at platinum and glassy carbon disc microelectrodes has been investigated in N,N-dimethylformamide and 1,2-dichloroethane in the presence of a small amount of iron (III) chloride in the solution. In the course of a negative scan, metallic iron was electrodeposited on the electrode, making possible the reduction of the hydroperoxide at the electrode surface. During the repetitions of the recordings, an unusual reproducibility of the reduction signal was observed. This reproducibility is due to an in situ reconstitution of the active surface of the electrode that allows the electrode reaction to take place, at each scan, on a clean non-contaminated new electrode surface.     

Electrochemical properties of ruthenised electrodes in the oxide layer region

The impact of increasing the amount of electrodeposited ruthenium and of potential/time treatment of ruthenium substrates on the electrochemical properties of anodically formed ruthenium oxide films, are investigated by use of cyclic voltammetry, electrochemical quartz crystal microbalance, anodic and cathodic current transients, and electrochemical impedance spectroscopy. In addition to the specific interpretation of results obtained by each of the techniques used, general discussion in terms of the essential charge storage characteristics of ruthenium oxide films is presented. It is shown that an increase in the ruthenium loading acts beneficially on the stability of oxide films over a range of potentials, but detrimentally on the kinetics of the overall charging/discharging reaction, which becomes proton mass transfer limited. On the other hand, potential/time treatment of ruthenium substrates acts detrimentally on the stability of related oxide films, but makes the kinetics of the oxide film charging/discharging reaction more reversible. Total capacitance values of oxide films presented per gram of electrodeposited ruthenium are found to be almost independent of ruthenium loading, and slightly increased by the potential/time treatment performed. This all points to the fact that an increase in the ruthenium loading and potential/time treatment of ruthenium substrates results primarily in beneficial changes in the morphology of related oxide films. The magnitudes of the total capacitances per gram of electroplated ruthenium, however, remain strongly determined by the method of preparation of ruthenium substrates, and cannot be significantly changed by either an increase in the ruthenium loading or by the potential/time treatment of the ruthenium substrates.     

Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.