Mechanism of anodic oxidation of tungsten in neutral sulphate-fluoride solutions

The anodic oxidation of tungsten has been studied in 1 M Na₂SO₄ solutions containing 0–0.25 M NaF. Steady-state currents measured in the passivation and passivity ranges increase significantly with increasing fluoride concentration, indicating enhanced dissolution of the oxide film. The electrochemical impedance response is dominated by the processes in the barrier layer and at its interface with the electrolyte. The presence of a pseudo-inductive loop in the impedance spectra at intermediate frequencies indicates point defect interaction during film growth and dissolution processes. A kinetic model including the recombination reaction between oppositely charged point defects at the film/solution interface as well as a kinetic scheme for tungsten dissolution through the film mediated by cation vacancies is proposed. It is found to reproduce satisfactorily the steady-state currents and the impedance spectra in the potential range 0.2–2 V. Such a model for the conduction mechanism in the barrier layer is believed to be an essential part of a modelling approach to the formation of a nanoporous overlayer on tungsten in fluoride-containing solutions.     

An electrochemical impedance and ac-electrogravimetry study of PNR films in aqueous salt media

Electrochemical impedance spectra and ac-eletrogravimetry of poly(neutral red) films are studied in different aqueous media. The dependence of these experimental data on the nature of alkaline cation and monovalent anion present in the solution and on the pH is analysed. During the electrochemical processes, it is observed that all three species—the salt cation, the salt anion and the hydronium ion—participate to balance the electrical charge within the film. It is also possible to conclude that the participation of anions takes place faster than hydronium ions participation. Besides, the relative participation of these species is related to the pH of the solution.     

Electrochemical Behavior and Nonlinear Mott-Schottky Characterization of a Stainless Steel Passive Film

The electrochemical behavior of 304-stainless steel in solutions with different pH values and chloride concentrations was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and a nonlinear Mott-Schottky analysis method. The results showed that the corrosion behavior of 304-stainless steel was affected by various factors, such as pH, chloride ion concentration, and dissolved oxygen. The pit initiation and propagation stage were observed in EIS plots through the addition of sodium chloride. In addition, the effect of pH on the passive film dominated one of the film growth kinetic reactions. The effect of chloride ion concentration was caused by the generation of more cation vacancies, leading the passive film more susceptible to pitting, which was also demonstrated by X-ray Photoelectron Spectroscopy. The results are consistent with the point defect model.     

On the origin of frequency dispersion at the interface between mercury electrode and aqueous solutions of alkali halides

The origin of frequency dispersion of electrochemical impedance is investigated at the interface of mercury and aqueous solutions of single alkali halides. It is found that in the presence of each one of KI, CsI, CsF and CsBr salts, the interface presents certain potential regions where frequency dispersion effects are detected and others where the ideal capacitor behavior is closely approximated. Frequency dispersion effects are contributed by interfacial processes such as anion and cation adsorption, mercury halide film formation and dissolution and charge transfer reactions. The discrimination between frequency dispersion due to charge transfer processes occurring at the Hg/solution interface and that due to reactant adsorption itself is generally difficult and depends on the reaction mechanism, provided that a discrete adsorption step is anticipated.     

Ion and water transports in Prussian blue films investigated with electrochemical quartz crystal microbalance

Water and ion transport in electrochemically prepared Prussian blue (PB, Iron(III) hexacyanoferrate(II)) films has been investigated with the electrochemical quartz crystal microbalance (EQCM) and the electrochemical/electrogravimetric impedance techniques. It is shown that the freshly prepared PB film is highly hydrated and that it undergoes an irreversible mass change during the first cathodic scan. The latter result supports the previously proposed structural reorganization scheme of the PB film from the insoluble form Fe₄[Fe(CN)₆]₃·6H₂O to the soluble form MFeFe(CN)₆ (M is a monovalent cation). It is also shown that, during the first cathodic scan, a substantial amount of water is excluded from the PB film. After the structural reorganization, ion transport during the redox reaction of the PB film is cation-dominant with a small fraction of accompanying water transport.     

Impedance and Photoadmittance of Passive Iron Electrodes in Different Solutions

Electric characteristics of the oxide film at the surface of an anodized iron electrode in 0.5 M Na₂SO₄, NaNO₃, and Na₂MoO₄ solutions are studied using the impedance and photoadmittance measurements and taking polarization curves. The impedance frequency spectra correspond to an equivalent circuit comprising two parallel (RC) chains connected in a series. The relaxation time of the high-frequency circuit approaches that of the photopotential; hence, it can be related to the impedance of the oxide film. The low-frequency component of the impedance describes the impedance of the film/solution interface. In the Na₂SO₄ solution, the oxide film resistance is significantly higher and the capacitance is lower than in two other solutions, which can be explained by the film thickening. The marked concurrent decrease in the resistance and increase in the capacitance of the film/solution interface at 1.2 V in all solutions is caused by oxygen adsorption, which precedes oxygen evolution.     

铬对碳钢在NaHCO3/NaCO3缓冲溶液中所成钝化膜半导体性能的影响

应用电化学阻抗谱技术研究铬对碳钢钝化膜半导体性能的影响.实验表明,在碳酸氢钠/碳酸钠缓冲溶液中碳钢形成的钝化膜具n型半导体特性,而含有3%铬的碳钢钝化膜则呈n-p型半导体特性,随着成膜电位的增加,以上两种钝化膜之Mott-Schottky直线部分的斜率均呈增大的趋势,表明成膜电位升高,膜内的杂质密度减小,但铬的加入使得碳钢钝化膜的施主密度增加约一个数量级,从而增加了碳钢点蚀发生的趋势.EIS分析表明:铬的加入降低了钝化膜的传递电阻R1和膜电阻R2,特别是膜电阻R2下降达3个数量级,这就有可能增加碳钢在高pH值环境中的腐蚀.     

Study on the sorption of water into epoxy resins by means of electrochemical impedance spectroscopy

In this work the relationship between structure and water sorption for epoxy films was studied using electrochemical impedance spectroscopy. It is shown that the electrochemical impedance spectroscopy can be applied to obtain information about the amount of water absorbed by the epoxy film and the water permeation through the film, while only the amount of water absorbed is obtained from the “sorption method”. The free volume is the decisive factor for both the amount of water in the cured epoxy film and water permeation through the film.     

电化学交流阻抗技术表征自组装多层膜

采用电化交流阻抗技术对一种新型电极表面修饰的自组装多层膜进行表征,通过阻抗谱分析,得出电荷传递电阻和双电层电容与膜层数的关系,证明该多层膜随数增加具有均匀增长,结构致密等特性.     

CTAB-promoted prussian blue-modified electrode and its cation transport characteristics for K+, Na+, Li+, and NH4+ ions

A Prussian blue (PB) film was deposited on a glassy carbon (GC) electrode by cyclic voltammetry in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). The electrode thus formed showed 4-fold enhancements in redox current and charge values in pure KCl electrolyte as well as greater stability than an electrode prepared in the absence of CTAB. This improved performance of a PB+CTAB electrode versus a PB electrode was further demonstrated using SEM, XRD, and electrochemical impedance spectroscopy (EIS) measurements. A comparative study was undertaken on the cation transport characteristics of PB and PB+CTAB electrodes for Na+, Li+, and NH4+ ions. We obtained a CV pattern for a CTAB-promoted PB film, which showed ideal Nernstian behavior at all scan rates from 5 to 140 mV s(-1). Conditions for the formation and preservation of these ideal and stable PB films are discussed. Possible mechanisms for the beneficial effects of CTAB are proposed.     

Effect of the Structure of Nonuniform Conducting Polymer Films on Their Electrochemical Impedance Response

Electrochemical impedance spectroscopy is used to characterize thin p-doped polypyrrole (PPy) films in propylene carbonate (PC) solutions and poly(trifluorophenyl)thiophene (PTFPT), in solutions based on sulfolane (SF). It appears that the latter film is much less swelled compared to the former one. One consequence of this difference is that the PTFPT film shows a much higher bulk resistance compared to that for the PPy film. Another important consequence is that the swelling of the PTFPT film is essentially physically non-homogeneous. Two parallel, uncoupled paths, with different chemical diffusion coefficients, model the experimental results adequately. In order to quantify the impedance spectra for both polymer films, we use a model proposed by Rubinstein et al. explaining the difference in the diffusion coefficients of Ru(bpy)^3+/2+ ₃ within a thin Nafion film. The model can also predict the impedance spectra for composite powdery electrodes containing different particle sizes, such as composite cathodes and graphite anodes used in lithium batteries.     

稀土铅钙合金与传统铅钙合金在硫酸溶液中的阳极行为研究

应用交流阻抗,交流伏安和循环伏安等方法研究了Pb-Ca-Sn-Re合金和Pb-Ca-Sn合金在1.28 V(vs.SCE)和4.5 mol/L硫酸溶液中的阳极行为.结果表明:稀土铅钙合金提高了合金的耐腐蚀性能,同时抑制其阳极膜中Pb(Ⅱ)化合物的生长,从而降低阳极膜的阻抗,提高膜的导电性能,这对改善电池的深循环性能十分有利.     

Electrochemical behavior of CoCrMo implant in Ringer's solution

CoCrMo has been successfully employed as an orthopedic and orthodontic material because of its excellent corrosion resistant and suitable biocompatibility. The purpose of this research was to investigate the susceptibility of CoCrMo in Ringer's solution at three different temperatures: 22 °C, 37 °C, and 60 °C. The corrosion behavior of CoCrMo was carried out by using common electrochemical methods such as open circuit potential, potentiodynamic measurement, electrochemical impedance spectroscopy (EIS), and Mott–Schottky measurements. CoCrMo was passivated by the air, forming a passive film, which was not destroyed during the immersion in electrolytes under different temperatures. Results from potentiodynamic tests showed that at high anodic potentials, the passive film did not demonstrate significant localized corrosion and rather exhibited overall passive film degradation corresponding to the general corrosion of the alloy in Ringer's solution at the three temperatures. EIS measurements showed the presence of the stable passive film on the alloy surface when tested at open circuit potential. Mott–Schottky test indicated that the preformed passive film is an n‐type semiconductor due to the presence of a donor species. This is implied by the existence of oxygen vacancies and interstitial metallic cations. As the potential increased, the Cr³⁺ oxidized and produced soluble Cr⁴⁺ species. This resulted in the film changing to a p‐type semiconductor owing to the dissolution and creation of cation vacancies (acceptor species). The passive film rupture was not due to p‐type characteristics but rather was a result of the considerable oxidative dissolution of the film at high anodic potential. Copyright © 2013 John Wiley & Sons, Ltd.     

Biophysical measurement of red blood cells by laboratory on print circuit board chip

Microfluidic impedance pulse sensor has emerged as an easily handled and low‐cost platform in the electrical analysis of biological cells. In the conventional method, impedance sensor demanded expensive patterning metal electrodes on the substrate, which are directly in touch with electrolytes in order to measure the microfluidic channel impedance change. In this article, a cost‐effective microfluidic impedance sensor built upon a dielectric film coated printed circuit board is introduced. Impedance electrodes are protected by a dielectric film layer from electrochemical erosion between electrodes and electrolyte. Human red blood cells from adult and neonatal were utilized to demonstrate the feasibility of the proposed device in the electroanalysis of biological cells.     

Electric cell-substrate impedance sensing with screen printed electrode structures

Impedance sensors in thick film technology have been tested as a tool for electric cell-substrate impedance sensing. The screen printed Pt electrodes have a width of 250-400 microm. Electrodes and the surrounding ceramic chip substrate could be homogeneously grown with L-929 and Hela cells. The performance of a screen printed interdigitated electrode structure (IDES) was compared with that of thin film structures with the same layout geometry. The thick film impedance sensors allowed to correctly record the morphological response of confluent Hela cell layers to stimulation with histamine. A thick film conductivity sensor also revealed impedance values which were dependent on cell growth on the electrode surface, even at a very low frequency range of approximately 1 Hz.     

Au nanoparticle-enhanced surface plasmon resonance sensing of biocatalytic transformations

N-(3-Aminopropyl)-N'-methyl-4,4'-bipyridinium is coupled to tiopronin-capped Au nanoparticles (diameter ca. 2 nm) to yield methyl(aminopropyl)viologen-functionalized Au nanoparticles (MPAV(2+)-Au nanoparticles). In situ electrochemical surface plasmon resonance (SPR) measurements are used to follow the electrochemical deposition of the bipyridinium radical cation modified Au nanoparticles on an Au-coated glass surface and the reoxidation and dissolution of the bipyridinium radical cation film. The MPAV(2+)-functionalized Au nanoparticles are also employed for the amplified SPR detection of NAD(+) and NADH cofactors. By SPR monitoring the partial biocatalyzed dissolution of the bipyridinium radical cation film in the presence of diaphorase (DP) NAD(+) is detected in the concentration range of 1x10(-4) M to 2x10(-3) M. Similarly, the diaphorase-mediated formation of the bipyridinium radical cation film on the Au-coated glass surface by the reduction of the MPAV(2+)-functionalized Au nanoparticles by NADH is used for the amplified SPR detection of NADH in the concentration range of 1x10(-4) M to 1x10(-3) M.     

Hybrid organic/inorganic films of conducting polymers modified with phthalocyanines. II. EIS studies and film characterization

Conducting polymers were modified with Cu-phthalocyanine or Co-phthalocyanine embedded in a sol–gel matrix. The resulting films were characterized using electrochemical impedance spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. Electrochemical impedance spectroscopy data showed that the application of the sol–gel layer to the conductive polymer caused a noticeable increase in the impedance of the film across the frequency ranges studied. The hydrophobic character of the film was greatly influenced by the sol–gel and caused an increase in its capacitance. A modified ‘Randles’ equivalent cell was used to correlate the electrochemical parameters of the films. Elemental analysis and infrared data confirmed the presence of the phthalocyanine moieties in the film and the empirical formula of the film was estimated. The surface morphology of the sol–gel-modified conducting polymer was distinctly amorphous compared to the poly(3-methyl thiophene).     

Effect of anodic potential on process of formation of polyporphyrin film in solutions of tetrakis(p-aminophenyl)porphin in dichloromethane

The electrode impedance spectroscopy technique was used to study the process of formation of a conducting polyporphyrin film on a Pt electrode from a 10^?3 M solution of tetrakis(p-aminophenyl)porphin in dichloromethane. An equivalent circuit is suggested for simulation of interface impedance in a wide range of working electrode potentials. It is shown that regions with a different mechanism of film formation are observed at an increase in potential from 0.0 to +1.0 V. The kinetics of film formation are studied at the potentials of +0.40, +0.60, and +0.80 V. It is found that good agreement is observed between the model and experimental data when the growing film is simulated using a Warburg element with a finite diffusion length. Conductivity and the diffusion coefficient of charge carriers in it are estimated on the basis of the suggested model for a film obtained at the potential of +0.40 V. It is shown that conductivity of a polyporphyrin film grows by more than an order of magnitude at an increase in deposition potential from +0.40 to +0.80 V.     

Electrochemically aided solid phase microextraction: conducting polymer film material applicable for cationic analytes

The cation uptake and release properties of a poly(pyrrole-sulfated β-cyclodextrin) (PPy-SβCD) film electrode have been investigated under both open circuit and controlled potential conditions for prospective applications in electrochemically aided solid-phase microextraction (EA SPME). The EDAX and ion chromatography results show that the K⁺ and Na⁺ cation uptake is enhanced if a small negative potential is applied to the electrode in the range where PPy is in its neutral form. These cations are released rapidly from the film if the applied potential is switched to the value at which PPy is converted to its positively charged form, i.e., oxidized state. The cation ingress and egress mechanism is affected both by the cation exchange at the negative sulfate moiety on the cyclodextrin sites and electrostatic interactions generated by the applied potential. The electrochemical "switching" capability increases the speed of the cation uptake and release, presumably due to electro migration, as compared to the open circuit ion exchange which is controlled solely by diffusion. Our preliminary fundamental results show that the PPy-SβCD film is suitable for the future design of EA SPME devices. Electronic Publication     

PREPARATION, CHARACTERIZATION AND ELECTROCHEMICAL PROPERTIES OF POLYPYRROLE-POLYSTYRENE SULFONIC ACID COMPOSITE FILM

Polypyrrole-polystyrene sulfonic acid (PPy-PSSA) composite films have been electrosynthesized in an aqueous solution of PSSA. The electro-active films exhibit cation exchange during the redox process. Infrared, Raman and energy-dispersive spectroscopic results demonstrated that the polyanion of PSS^- is co-deposited into the PPy matrix and couldn‘t be stripped from the film extensively by dedoping. The doping level together with dipolaron content of the PPy-PSSA composite film increases during electrochemical polymerization process. SEM images revealed that the composite film has smooth and compact morphology and AFM pictures suggested that PPy chains are possibly grown perpendicular to the electrode surface. TGA tests indicated that the composite films has much better thermal stability than that of pure PPy.Furthermore, electrochemical studies showed that the relaxation process at certain holding potential has great effect on the shape of the cyclic voltammetric curves of PPy-PSSA composite film. The composite film exhibits cation and anion exchange during the redox process after undergoing the relaxation step. It is more difficult for divalent anion to enter the polymer matrix than a univalent ion, and a large cation such as (CH3CH2CH2CH2)4N^ cannot be involved in the ion exchange process.