Influence of anion composition and size on the double layer capacitance for Bi(111)|room temperature ionic liquid interface

Cyclic voltammetry and electrochemical impedance spectroscopy have been applied for investigation of electrochemically polished Bi(111) electrode in 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate (EMImFAP), 1-ethyl-3-methyl imidazolium tetracyanoborate (EMImTCB) and 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMImBF₄) ionic liquids. The region of ideal polarizability, series resistance and capacitance, limiting high-frequency and low-frequency capacitances have been calculated. The lower series capacitance values at electrode potential less negative than the potential of the minimum in the capacitance vs. voltage curve for Bi(111)|EMImFAP than that for EMImBF₄, and especially for EMImTCB, have been explained by the bigger diameter of FAP^?, higher cation and anion sizes symmetry, and less expressed surface activity (i.e. lower closest approach of the FAP^? mass centre to an electrode surface) compared with BF₄^? and TCB^? anions.     

Resonance nature of instabilities at the boundaries of the region of the electrode ideal polarizability

For the gallium metals, a model is proposed that explains the mechanism of an anomalous increase in the differential capacitance of the EDL dense part of the metal-electrolyte interface at limiting anodic and cathodic polarizations. The considerable increase in the capacitance is explained by the resonance interaction between electronic states of the metal and the adjacent layer of solvent molecules. The potential dependence of the capacitance is interpreted in terms of the Breit-Wigner parameters of a resonant scattering.     

Electrochemical impedance spectroscopy studies of the Ag(111)/<Emphasis Type="Italic">n</Emphasis>-decanethiol monolayer/NaNO<Subscript>3</Subscript> interface

Surface properties of a modified single-crystal silver (111) face are studied in aqueous 1 M NaNO₃ solutions with different acidity in the presence of a monolayer n-decanethiol (DT) film by the methods of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry that employs the meniscus contact technique. It is shown experimentally that in the potential range from 0 to −0.5 V, a DT-modified silver electrode in NaNO₃ solutions with pH 6 behaves as an ideal capacitance in a wide frequency range (from 0.01 Hz to 100 kHz). It is found that with the increase in the solution acidity up to pH 4 and 2, the region of stable capacitance characteristics narrows down. The concomitant changes in the adsorbed monolayer structure lead to deviations of impedance characteristics measured using a low-frequency ac signal from those of the ideal capacitance. A potential shift to values more negative than −0.5 V leads to the destruction of the monolayer. Based on the analysis of experimental EIS characteristics, system’s deviation from the ideal behavior that may indicate violation of the layer continuity is assessed and the relaxation frequencies of the DT/Ag(111) interface in solutions of different acidity are calculated. Estimates of capacitance, resistance, and thickness of the adsorbed monolayer are obtained as a function of the solution pH.     

The breakdown of β-alumina at positive and negative potentials

The breakdown of β^″-alumina at blocking electrodes, over a range of potentials from ?3 V to +8 V relative to the I₂/NaI couple, has been studied. The dependence of current and of interfacial capacitance on potential indicates that oxygen reduction occurs at a less negative potential (ca. ?1.8 V) than sodium deposition (ca. ?2.9 V). No evidence of anodic decomposition could be found over the entire potential range, suggesting that there is a substantial kinetic barrier to formation of oxygen gas from the oxygen anions in the lattice.     

Simulation of Coadsorption of Inorganic Ions and Organic Molecules on Electrodes

Electrode coadsorption of two components, one of which reduces the dense-layer capacitance and the other leaves it unchanged, is examined within the framework of Alekseev–Popov–Kolotyrkin model supplemented by Frumkin isotherm. The adsorption behavior of model systems 0.1 M NaF + n-C₄H₉OH and 0.1 M NaCl + n-C₄H₉OH is analyzed, and the results of calculations are compared with experimental data. It is shown that, in choosing the electrical variable for the adsorption of components that reduce the dense-layer capacitance, the preference should be given to the potential drop in the dense layer. It is concluded that the height of anodic maximum in C vs. E curves can serve as a criterion of the lateral interaction between coadsorbed particles at the known parameters of individual adsorption.     

Corrosion behavior of the steel used as a huge storage tank in seawater

Open-circuit potential (OCP), polarization curve, and electrochemical impedance spectroscopy (EIS) measurement were used to investigate the corrosion behaviors of high-strength low-alloy (HSLA) steel and mild steel in seawater. Both steels were used in the construction of a huge oil storage tank. The OCP results show that the HSLA steel quickly reached more negative E _OCP values than the mild steel. Polarization curve results reveal that the HSLA steel exhibits higher corrosion currents and more negative corrosion potentials than the mild steel. EIS measurements reveal that both steels exhibit similar corrosion behaviors up to 144 h, one increased capacitance loop can be shown in EIS diagrams. The mild steel presents higher corrosion resistances than the HSLA steel at former stage, which is associated with the effect of the grain size. After 240 h of immersion, both steels present different corrosion behaviors. The EIS diagrams exhibit two capacitance arcs for the HSLA steel and one capacitance arc for the mild steel, which is due to the formation of intact corrosion scales on the electrode surface of the HSLA steel as to introduce a new reaction interface. The HSLA steel exhibits higher corrosion resistances than the mild steel at latter stage of experiment, which is ascribed to the synthetic actions of residual Fe₃C and the protective property of corrosion products.     

Nanoporous oxide formation by anodic oxidation of Nb in sulphate–fluoride electrolytes

The influence of hydrofluoric acid (HF) concentration and applied potential on the processes of anodic oxidation of Nb in sulphuric acid solution was studied by chronoamperometry, electrochemical impedance spectroscopy and scanning electron microscopy. During the first stage of the process, a compact barrier film is formed. On top of this film, a porous overlayer starts to form, then the nanopores grow into an ordered nanostructure. Subsequently, secondary 3D flower-shaped structures begin to form. These structures gradually spread all over the surface as an irregular multilayer film. The rates of the process of porous overlayer formation and subsequent growth of nanopore arrays increase with applied potential as well as with the HF concentration. The films have been characterised ex situ by electrochemical impedance spectroscopy at open circuit potential and capacitance vs. potential measurements to follow the different stages of nanoporous film formation with electrochemical methods. The impedance spectra and capacitance vs. potential curves have been interpreted using previously proposed models for the amorphous semiconductor/electrolyte interface. An attempt to rationalise the mechanism of nanoporous layer growth is presented by using the conceptual views of the mixed-conduction model and recent ideas for porous film formation on valve metals.     

Crystal Face Specificity of Incipient Oxidation of Ag Single Crystal Electrodes in Acidic Aqueous Solution

The onset of anodic oxidation of Ag single crystal face electrodes in acidic solution is investigated by means of capacitance and voltammetric curves. The potential of incipient oxidation is found to depend only slightly on the atomic density of the surface. On the other hand, a pre-monolayer oxidation peak is identified only for the (110) face in HClO₄ while in H₂SO₄ it is suppressed. Results are interpreted in terms of anodic oxidation vs. anodic dissolution interference and anion adsorption vs. water oxidation competition. The behavior of the (110) face is explained on the basis of a model for water adsorption proposed for UHV experimental data. The higher reactivity of the (110) face toward water molecules supports the hydrophilicity scale of Ag crystal faces based on ionic and non-ionic adsorption data.     

Electropolymerization of Ni(salen) on carbon nanotube carrier as a capacitive material by pulse potentiostatic method

The composites of poly[Ni(salen)] and multi-walled carbon nanotube(MWCNT) were synthesized by pulse potentiostatic method.The composites were characterized by field emission scanning electron microscopy,Fourier transform infrared spectra,and electrochemical impedance spectroscopy.The wrapping of carbon nanotubes with poly[Ni(salen)] varied significantly with anodic pulse duration.Variance of structure of poly[Ni(salen)] caused by anodic pulse duration affected the ability of absorption to solvent molecules or solvated ions,which was indicated by ν(C≡N) intensity.The ability to store/release charge of poly[Ni(salen)] caused by redox switching was evaluated in the form of low-frequency capacitance.Correlations of chargetransfer resistance/ionic diffusion resistance with potential and anodic pulse duration were investigated.     

Stripped Charge of Ag Less than Deposited one Owing to Negative Capacitance Caused by Redox Reactions

Anodic stripping voltammetry was made in AgNO₃ solution, here Ag was deposited under long term potentiostatic conditions to evaluate the reduction charge, q_r, and then was stripped by linear sweep voltammetry to determine the oxidation charge, q_o. The charges were unbalanced, satisfying ca. q_o=0.7|q_r|, where other possible reduction charge such as by dioxygen and dichlorosilver were subtracted. The 30 % loss of the anodic charge can be ascribed to the negative capacitance by the potential sweep generation of Ag⁺. The generated Ag⁺ forms a dipole with a counter ion, of which orientation is the same as the direction of the externally applied electric field and opposite to the dipoles of solvent. The redox dipole decreases the conventional double layer capacitance caused by solvent dipoles, and high concentrations of Ag⁺ takes the capacitance to be negative values. The unbalanced charge, however, has no influence on quantitative determination of concentrations Ag⁺ by use of a calibration line.     

Electrochemical dissolution of chalcopyrite studied by voltammetry of immobilized microparticles

The characteristics of anodic electrochemical dissolution of chalcopyrite (CuFeS₂) powder in hydrochloric acid medium with sodium chloride have been studied. Cyclic voltammetry and chronopotentiometry of immobilized microparticles using paraffin-impregnated graphite electrode was employed. Present work is focused on electrochemical identification of chalcopyrite cathodic and anodic reaction products within the potential range of −0.7 to +0.8 V (vs. SCE) in hydrochloric acid solution containing sodium chloride and/or copper(II) chloride.     

Sulfide-enhanced electrochemical capacitance of cobalt hydroxide on nanofibered parent substrate

Two approaches—substrate nanostructuring and incorporation of sulfide—were studied with the aim to increase electrochemical capacitance of cobalt (hydro)oxide. A fiber structure of cobalt was deposited electrochemically with the fibers in the order of tens of nanometers in thickness and hundreds of nanometers in length. Cobalt hydroxide film was formed on the nanostructured substrate by anodic polarization in an alkaline solution. The hydroxide formation and its electrochemical capacitance have been studied by cyclic voltammetry in conjunction with the electrochemical quartz crystal microbalance (EQCM). An irreversible behavior was typical of the first anodic polarization cycle; it turned gradually to a reversible one during subsequent cycling. EQCM measurements indicated exponential electrode mass growth during the first cycle, with subsequent transition to a quasipassive state. The redox transitions Co(II) → Co(III) → Co(IV), which determine pseudocapacitance, did not cause remarkable electrode mass change. The electrochemical capacitance of the nanofiber sample was found up to five times higher when compared to that formed on conventional cobalt (abraded surface). Specifics of “per 1 g” evaluation of capacitance performance is discussed. Measurements showed that about 10% of the entire hydroxide structure took part in the capacitive process. The capacitance value determined per 1 g of active Co(OH)₂ was in agreement with the limiting value predicted by the Faraday’s law (2,421 F g⁻¹) sulfide-enhanced system with 18% CoS exhibited up to three times higher capacitance when compared to that of the sulfide-free counterpart. The system shows promise for practical applications due to its low cost and technical simplicity.     

An ac voltammetry study of Pt oxide growth

AC voltammetry of polycrystalline Pt in sulfuric acid solutions has been used to study the growth kinetics of the thin anodic Pt oxide film. Data were collected from 2 Hz to 50 kHz, one frequency per cycle, and were analyzed in the complex impedance plane. The faradaic process was modeled as a resistance parallel to the double-layer impedance, with a value approximately independent of potential in the do voltammetry plateau region. The equivalent circuit for the known growth law is derived and is shown to be a series RC combination. The capacitance was not detected but is expected to have a negligible effect in the measured frequency range. The value of the resistance found was consistent with the growth law found in other experiments. Evidence for additional faradaic elements in the equivalent circuit was inconclusive. We found no additional features in the impedance spectra at higher frequencies that could be associated with the fast electrosorption of OH suggested by other workers. The reversibility of the early stages of growth is therefore associated with structural reversibility rather than a fast process.     

Electrochemical investigation of a glassy carbon electrode modified with carbon nanotubes decorated with (poly)crystalline gold

Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly)crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1?mM ferricyanide is 131?mV, but 60?mV only for the modified GCE. In addition, the oxidation of NADH (1?mmol?L^?1 solution) begins at negative potentials (around ?100?mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94?mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of ?33.5?mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either ?50 or +50?mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50?mV, the response to NADH is linear in the concentration range from 1 to 100???mol?L^?1, with an RSD of 6% (n?=?4).

Investigation of the Anodic Behavior of Al in Room Temperature Ionic Liquid Electrolytes: BMI‐BF4, PP14‐BF4 and BMI‐BF4·PC

The anodic polarization behavior of Al, Ta and Nb foil was investigated in 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid (BMI‐BF₄). Compared with that of Ta and Nb foil, it showed that a better passive film was formed on Al foil surface after the anodic polarization in BMI‐BF₄, which could resist the potential up to 94.58 V vs. Ag⁺/Ag. Besides, similar anodic behavior of Al foil was observed in N‐methyl‐N‐butylpiperidinium tetrafluoroborate ionic liquid (PP14‐BF₄), which indicated that the anodic polarization behavior of Al foil was independent of the cations of RTIL. In addition, the investigation of anodic polarization behavior of Al foil was carried out in the mixture electrolytes composed of BMI‐BF₄·PC. Differently, two breakdown potential processes of Al foil were presented compared to pure BMI‐BF₄. Further research showed that the passive film on Al foil was mainly composed of AlF₃ and Al₂O₃ after the first breakdown potential process, while the fluoride film increased with continual anodic polarization, which improved the anodic stability of Al foil and resisted higher breakdown potential. The high breakdown potential properties of Al foil in BMI‐BF₄, PP14‐BF₄ and the mixture of BMI‐BF₄·PC during the anodic polarization can be favored for R&D of the high performance electrochemical devices.     

Influence of applied potential on the impedance of alkanethiol SAMs

Self-assembled monolayers are generally considered to behave as dielectric layers with a capacitance that is dependent on the monolayer thickness and the relative permittivity that is determined by the hydrocarbon tail. We show that the impedance response of alkanethiol-modified gold surfaces can be modeled as a parallel network consisting of the capacitance and resistance of the monolayer over a wide potential range. At potentials positive to -0.3 V (Ag/AgCl), the monolayer resistance is greater than 10(6) Omega cm2; however, at more negative potentials, the monolayer resistance decreases exponentially with potential with an inverse slope of about 250 mV. Over the same potential range, the monolayer capacitance is independent of potential. Although the same behavior is observed on ultrasmooth, template-stripped gold, the resistance at any potential is larger than for evaporated gold. The progressive increase in permeability of the monolayer is associated with an increase in electric field at potentials negative to the potential of zero charge.     

Modeling of finite diffusion impedance by <Emphasis Type="Italic">RC</Emphasis> two-terminal circuit

The Maxwell model consisting of three series capacitive circuits (SCCs) was employed for constructing an equivalent circuit of finite diffusion impedance (BW). The parameters of a low-frequency SCC were determined using a power series expansion of frequency dependences of BW capacitance and conductivity. All the further SCCs are calculated on the basis of the coincidence between BW and Warburg impedance at high-frequencies. Exact formulas are presented that allow extending an equivalent circuit by adding new SCCs, which is necessary for extension of the frequency range, in which the model and BW yield similar results.     

Simulating Unstable States during the Coumarin Adsorption on a Mercury Electrode

Dependences of differential capacitance C on potential E of a stationary electrode (hanging mercury drop) in aqueous 0.1 M NaF solutions containing 4.6 × 10^–4 to 3 × 10^–3 M C₉H₆O₂ are obtained using an automatic impedancemeter. At coumarin concentrations below 0.001 M and potential slowly scanned near –1.1 V (SCE) the capacitance is unstable, which results in differently-shaped C vs. E curves in this potential range. The obtained results are attributed to nonequilibrium phase transitions in the adsorption layer, during which the orientation of coumarin molecules at the electrode surface alters. These phenomena are explained semiquantitatively on the basis of a developed theory.     

Ellipsometric and microwave reflectivity studies of current oscillations during anodic dissolution of p-Si in fluoride solutions

Periodic current oscillations during anodic dissolution of monocrystalline p-Si(100) in buffered ammonium fluoride solutions (0.1 mol dm⁻³ fluoride, pH 4.5) were investigated using a flow cell in order to eliminate mass transport limitations. The flow cell was designed to permit simultaneous in-situ ellipsometry, impedance and potential modulated microwave reflectivity measurements. Analysis of the ellipsometric response showed that the current oscillations are accompanied by a synchronous variation of the overall oxide thickness with an amplitude of 4.5±0.1 nm. Analysis of the relationship between the total oxide thickness and the current during the oscillation cycle shows that to a first approximation the rate of chemical dissolution of anodic oxide remains constant. Oscillations of the electrode admittance and potential modulated microwave reflectivity were also measured. The imaginary component of the admittance is related to the oscillation in thickness of a narrow inner region of ‘dry’ oxide and to changes in the accumulation capacitance. The oscillation in the potential modulated microwave reflectivity is interpreted in terms of the changes in the density of holes accumulated at the p-Si SiO₂ interface.     

Actual activation energy of electrode process under mixed kinetics conditions

In the case of a single-electron reaction with account for slow diffusion of reagents, equations for actual (experimentally determined) activation energies of two types were derived and analyzed: real energy A f, i.e., the energy measured at a constant electrode polarization value η = const) and formal energy (Ω_f, i.e., the value measured at a constant value of potential vs. an ambiguously chosen reference electrode E = const). It is found that under the conditions of a sufficiently significant deviation from equilibrium, the actual activation energy A _f is the weighted arithmetic mean of the diffusion activation energy and the sum of A ₀ + αFη (where A ₀ is the real activation energy of the discharge stage at polarization of η = 0); herewith, the weighting coefficients are the corresponding values of the current of the discharge stage and the limiting diffusion current. A similar relationship is also obtained for Ω_f. It is found that the A _f, η- and Ω_f, E-curves can in a number of cases feature regions with the negative A _f and Ω_f values in the mixed kinetics range.