Charge storage mechanism of nanostructured anhydrous and hydrous ruthenium-based oxides

The charge storage mechanism of nanostructured anhydrous and hydrous ruthenium-based oxides was evaluated by various electrochemical techniques (cyclic voltammety, hydrodynamic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy). The effects of various factors, such as particle size, hydrous state, and structure, on the pseudocapacitive property were characterized. The electric double layer capacitance (C_dl), adsorption related charge (C_ad), and the irreversible redox related charge (C_irr) per unit mass and surface area of electrode material has been estimated and the role of structural water within the material either in micropores or interlayer are discussed.     

Preparation, characterization and electrocatalytic properties of an iodine|lignin-modified gold electrode

Hydrolytic lignin (HL) was adsorbed from an aqueous/organic solution on bare and iodine-modified gold electrode. Subsequent electrooxidation of the lignin adsorbate generated redox-active quinone-based groups in the biopolymer structure, exhibiting high reversibility during potential cycling and fast electron transfer kinetics. The presence of the chemisorbed iodine layer on the supporting gold electrode had a pronounced effect on the electrochemical properties of the final modified electrode in terms of double-layer capacitance (C_dl) and the observed surface coverage (Γ_obs). The high electrochemical activity in connection with low C_dl made it possible to apply the Au|I_(ads)|HL electrode as a fast-responding and sensitive electrochemical sensor for NADH. When tested in the amperometric mode at a constant potential of +0.4 V vs. Ag/AgCl, the modified electrode showed a linear current-concentration response over the range of 5-120 μM with a sensitivity of 2.39 nA μM⁻¹ cm⁻² and a detection limit of 1.0 μM (S/N = 3). Kinetic studies using the rotating disk electrode revealed that the mediated oxidation of NADH on the Au|I_(ads)|HL electrode was limited by the second order reaction of the analyte molecules with o-quinone moieties with a rate constant of ca. 4.7 × 10² M⁻¹ s⁻¹ (C_NADH → 0). The modified electrode showed high resistivity against fouling and retained ca. 65% activity after storage in phosphate buffer (pH 7.4) at room temperature for 1 week.     

Inhibition of copper corrosion in aerated hydrochloric acid solution by amino-acid compounds

The effect of two amino-acid compounds, dl-alanine and dl-cysteine, on copper corrosion in an aerated 0.5 mol l⁻¹ HCl solution was studied by weight-loss measurements, potentiodynamic polarisation curves, and electrochemical impedance spectroscopy. A conventional benzotriazole (BTA) inhibitor was also tested for comparison. dl-cysteine was shown to be the most effective inhibitor among those tested inhibitors. Potentiodynamic polarisation results revealed that both the dl-alanine and dl-cysteine acted as an anodic inhibitor; however, dl-cysteine, in particular, was more effective, as it strongly suppressed anodic current densities. The improved inhibition efficiency of dl-cysteine in the 0.5 mol l⁻¹ HCl solution was due to its adsorption on the copper surface via the mercapto group in its molecular structure.     

The electrochemical characteristics of lithium—sulphur dioxide cells

The current—potential and impedance—potential characteristics of some commercial lithium—sulphur dioxide cells have been investigated. An equivalent circuit and parameter curves C_dlE, R_ctE, σ—E, R_wE, which describe the electrochemical response over a limited potential range, are given and discussed.     

Monitoring the degradation of a high solids epoxy coating by means of EIS and EN

This paper reports a study of the degradation processes suffered by steel samples painted with a high solid content epoxy coating. Because this coating shows a high resistance when exposed to NaCl solutions, HCl solutions were employed to accelerate the corrosion processes. Macroscopic images were used to observe the coating degradation. Then electrochemical techniques, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) were employed to monitor the corrosion behaviour of the system studied. A close correlation was found between EIS and EN data. Several parameters were estimated using these techniques: R_po, R_ct, C_c, C_dl and Z_0.1 Hz using EIS, and R_n using EN. In addition, a new parameter estimated by means of EN was employed, Z_0.1 Hz(EN). The evolution of all these parameters with time enabled the effective monitoring of the degradation stage of the coating.     

Novel electrochemical surface modification method of carbon fiber and its utilization to the preparation of functional electrode

The electrooxidative and -reductive methods for the surface modification of carbon fiber were developed. The carbon fiber surface was first oxidized under anodic conditions to introduce phenolic hydroxyl groups on the carbon fiber, and then the resulting oxidized carbon fiber was treated under cathodic conditions in the presence of various kinds of electrophiles such as alkyl halides and alkyl tosylates introducing the alkyl groups on the carbon fiber. The changes of the functional groups on the carbon fibers were confirmed by the X-ray photoelectron spectroscopy (XPS) study and the observation of the hydrophilicity. The functional carbon fiber electrode introduced β-cyclodextrin (β-CD) was then prepared by this electrochemical method. The electroreduction of acetophenone (1) was carried out using the resulting carbon fibers as cathodes. The use of the carbon fiber modified with β-CD gave relatively high dl selectivity of the reductive coupling products 2 (the dl/meso ratio: 5.2), while the dl/meso ratio using the untreated carbon fiber was 3.0, and the formation of 1-phenyl ethanol (3) was observed only by using the β-CD-modified carbon fiber. The observed products selectivity is discussed from the viewpoints of the interaction between the electrogenerated species and the functional groups on the carbon fibers.     

Electrocatalysis and the chlorine evolution reaction

The chlorine evolution reaction has been investigated on D.S.A. type electrodes in 5.13 M NaCl solution. A large amount of potentiostatic current—potential and impedance—potential data has been collected over a wide potential and frequency range. The current—potential and impedance—potential data has been reduced by curve fitting to a series of a parameter curves. The impedance—potential data were analysed by two methods: an equivalent circuit and the parameter curves C_dlE, R_ctE, R_ωE reported, and a reaction mechanism and the parameter curves C_dlE, kSHE, R_ωE reported. The two ways of analysing the impedance data are equivalent. However the analysis into a reaction parameter has the advantage that all the data, current—potential and impedance—potential, can be tested using a single set of parameters (kSH, R_ω, a, b, D_A, D_B, δ).In this work the data is compared to that expected for a redox reaction. Any deviation from the expected behaviour of a perfect redox reaction is ascribed to Cl₂ bubble formation and to electrode structure effects. It is suggested that the parameter curves are related also to bubble formation characteristics and the ‘real’ area of the electrode.     

Importance of Time-Dependent Wetting Behavior of Gas-Diffusion Electrodes for Reactivity Determination

Tin foil and SnO_x/C gas-diffusions electrodes (GDEs) were investigated via electrochemical impedance spectroscopy (EIS) to extract the differential double-layer capacitance (C_dl) as a measure of the wetted surface area. Time-dependent C_dl values revealed an immediate stationary wetting for tin foil electrodes while a distinct increase of C_dl – which becomes stationary with time – was observed for GDEs. The time-dependent wetting behavior of the GDEs was substantiated by physical post-mortem characterization. Since the wetted surface area determines the number of reachable active sites the performance of GDEs should be normalized to the wetted surface area for evaluation of reactivity.     

Automation of electrode kinetics—V. The dissolution of Al in Cl− and F− containing aqueous solutions

The dissolution of Al in Cl^? and F^? containing aqueous solutions has been investigated over a wide potential range. The steady state current-potential curves and the impedance have been measured using a new electrochemical measurement system. Using an equivalent circuit the C_dl,-E,θ-E, σ_A1-E and R_w-E curves are derived and with the primary i-E and Z(w)-E data are compared for each anion.     

Effect of electroless nickel interlayer on the electrochemical behavior of single layer CrN, TiN, TiAlN coatings and nanolayered TiAlN/CrN multilayer coatings prepared by reactive dc magnetron sputtering

The electrochemical behavior of single layer TiN, CrN, TiAlN and multilayer TiAlN/CrN coatings, deposited on steel substrates using a multi-target reactive direct current (dc) magnetron sputtering process, was studied in 3.5% NaCl solution. The total thickness of the coatings was about 1.5 μm. About 0.5 μm thick chromium interlayer was used to improve adhesion of the coatings. With an aim to improve the corrosion resistance, an additional interlayer of approximately 5 μm thick electroless nickel (EN) was deposited on the substrate. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to study the corrosion behavior of the coatings. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the corroded samples. The potentiodynamic polarization tests showed lower corrosion current density and higher polarization resistance (R_p) for the coatings with EN interlayer. For example, the corrosion current density of TiN coated steel was decreased by a factor of 10 by incorporating 5 μm thick EN interlayer. Similarly, multilayer coatings of TiAlN/CrN with EN interlayer showed about 30 times improved corrosion resistance as compared to the multilayers without EN interlayer. The porosity values were calculated from the potentiodynamic polarization data. The Nyquist and the Bode plots obtained from the EIS data were fitted by appropriate equivalent circuits. The pore resistance (R_pore), the charge transfer resistance (R_ct), the coating capacitance (Q_coat) and the double layer capacitance (Q_dl) of the coatings were obtained from the equivalent circuit. Multilayer coatings showed higher R_pore and R_ct values as compared to the single layer coatings. Similarly, the Q_coat and Q_dl values decreased from uncoated substrate to the multilayer coatings, indicating a decrease in the defect density by the addition of EN interlayer. These studies were confirmed by examining the corroded samples under scanning electron microscopy.     

Studies of double layer capacitance and electron transfer at a gold electrode exposed to protein solutions

Electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical behaviour of a gold electrode exposed to proteins prepared in phosphate buffer. Exposure to solutions of human serum albumin (HSA) and immunoglobulin G (Ig.G) resulted in a decrease of the double layer capacitance (C_dl) and an increase in the charge transfer resistance (R_ct) at the gold electrode solution interface. The greatest capacitance decrease for both proteins was observed when exposure occurred at or more positive to the electrode open circuit potential (OCP). Exposure to Ig.G resulted in a greater decrease in capacitance as compared to HSA under identical conditions. These capacitance and charge transfer resistance variations were attributed to the formation of a proteinaceous layer on the electrode surface during exposure.     

Catalytic synthesis ofdl-serine and glycine from glycerol

dl-serine (dl-Ser), glycine (Gly), serinol (SerOH) and related compounds were catalytically derived from glycerol (GLY). Catalytic oxidation in the presence of ammonia of GLY, followed by hydrogen treatment over noble metals, to achieve direct synthesis of amino acids is not practical. Reductive amination over an Ru-Pd/C catalyst of glyceric acid, one of the oxidation products of GLY, was effective fordl-Ser synthesis. Gly was catalytically formed fromdl-Ser by dehydrogenation and decarbonylation in hydrogen atmosphere. Catalytic oxidation of SerOH prepared by reductive amination over an Rh-Pd/C catalyst of dihydroxyacetone, another oxidation product of GLY, was also effective fordl-Ser synthesis. A new route fordl-Ser and Gly from GLY is presented.dl-alanine and γ-aminobutyric acid also formed, depending on reaction conditions.     

Evaluation and origins of the difference between double-layer capacitance behaviour at Au-metal and oxidized Au surfaces

In studies of processes at oxidized compared with unoxidized electrode surfaces by transient methods corrections for double-layer charging are usually required and have often been made by extrapolation of double-layer capacitance (C_dl) data for the metallic surface, e.g. at Au or Pt, into the potential region of oxide-film formation. Voltammetry and impedance spectroscopy provide direct information on C_dl values determined at unoxidized, i.e. metallic, Au surfaces compared with those of anodic oxide films generated potentiostatically to various extents that are stable in time, and characterized by reductive linear-sweep voltammetry. C_dl is derived from constant-phase element (CPE) values and the CPE parameter, ?, which is near unity for most conditions. At oxidized Au surfaces C_dl depends on potential for various extents of oxide formation; it increases from 15 (±1) μF cm⁻² at 1.75 V (RHE) to 25 (±1) μF cm⁻² at 1.45 V (RHE) and is independent of added Cl⁻ or Br⁻ for concentrations 0-10⁻³ M of both anions, while, at unoxidized Au electrodes in the absence of halide anions, C_dl has a maximum value of 60 (±2) μF cm⁻² at 0.80 V (RHE) and is now dependent on concentration of added Cl⁻ or Br⁻ ion. These major differences of C_dl for the oxidized and unoxidized Au surfaces indicate that double-layer charging corrections cannot be made simply by extrapolation of C_dl data for unoxidized Au metal surfaces into the potential region for oxide formation.     

AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nafion® as solid polymer electrolyte

The AC impedance characteristics of planar solid-state electrochemical carbon monoxide (CO) sensors with Nafion® as the solid polymer electrolyte are reported. For a re-cast Nafion® sensor design, Nyquist plots indicated very high impedances (10⁷-10⁸ Ω), which slightly decreased with an increase in the working electrode surface area (0.25-0.50 cm²). Only one enlarged semi-circle was observed, relating to an equivalent circuit with a geometrical capacitance (C_g) in parallel with a bulk resistance (R_b). In contrast, a sensor design using a commercial membrane produced a system with a much lower impedance (<10³ Ω), with Nyquist plots showing a flattened semi-circle at high frequencies (due to C_g and R_b in parallel), followed by the start of a large semi-circle (due to the double-layer capacitance, C_dl, in parallel with the charge-transfer resistance, R_ct) over a lower frequency range. On addition of CO, the second circular arc reduced in size, indicating that the interfacial impedance of the electrochemical system had decreased.     

Corrosion inhibition of iron in 1 M HCl by some gemini surfactants in the series of alkanediyl-α,ω-bis-(dimethyl tetradecyl ammonium bromide)

Three new gemini surfactants in the series of alkanediyl-α,ω-bis-(dimethylalkyl ammonium bromide) were synthesised and tested as corrosion inhibitors of iron in hydrochloric acid medium using gravimetric, electrochemical polarisation and electrochemical impedance spectroscopy (EIS) measurements. Results obtained show that the surfactants studied are good cathodic inhibitors and act on the cathodic hydrogen reaction without modifying its mechanism. EIS results show that the changes in the impedance parameters (R_T and C_dl) with concentration of surfactants studied is indicative of the adsorption of molecules of surfactant leading to the formation of a protective layer on the surface of iron. The effect of the temperature on the iron corrosion in both 1 M HCl and 1 M HCl with addition of various concentrations of 1,2-ethane bis-(dimethyl tetradecyl ammonium bromide) in the range of temperature 20–60 °C was studied. The associated apparent activation corrosion energy has been determined.     

The automation of electrode kinetics — III. The dissolution of Mg in Cl−, F− and OH− containing aqueous solutions

The dissolution of Mg in Cl^?, F^?, and OH^? containing aqueous solutions has been investigated over a large potential range, from far into H₂ evolution at ? 3000 mV (sce) to the passive state at ? 1400 mV (sce). The steady state current-potential and the impedance have been measured using a new automatic electrochemical measurement system. An equivalent circuit and a least squares fitting procedure have been used to analyse the data. The resulting C_dl-E, θ-E, σ_Mg-E, R_w,-E, and the primary i-E and Z(ω)-E data are compared for each anion, and reveal details of the electrode kinetics of these complex dissolution reactions.     

Comparison of electrolyte effects for poly(3,4-ethylenedioxythiophene) and poly(3-octylthiophene) by electrochemical impedance spectroscopy and polymerization parameters with morphological analyses on coated films

3,4-Ethylenedioxythiophene and 3-octylthiophene were electropolymerized on glassy carbon electrodes (GCE) to compare with four different electrolytes [lithium perchlorate (LiClO₄), sodium perchlorate, tetraethylammonium tetrafluoroborate, and tetrabutylammonium tetrafluoroborate] in a solvent of acetonitrile (CH₃CN). Modified electrodes were characterized by cyclic voltammetry, attenuated total reflectance–Fourier transform IR spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, atomic force microscopy, and electrochemical impedance spectroscopy (EIS). Nyquist and Bode plots for magnitude, phase, admittance, and capacitance on both polymer-modified electrodes were comparatively investigated in detail. The highest low-frequency capacitance (C _LF) and double-layer capacitance (C _dl) were obtained in 0.1 M LiClO₄/CH₃CN for poly(3,4-ethylenedioxythiophene) and poly(octylthiophene)/GCE. EIS data were fitted to the equivalent circuit model of R(Q(R(C(R(C(RW))))))(CR), which is used to investigate circuit parameters.     

Etude theorique de mecanismes E.C. par chronopotentiometrie en couche mince. Application a la reduction du nitrosobenzene

A theoretical study of E.C. mechanisms in thin layer chronopotentiometry is presented for an electrochemical reaction O + ne ? R coupled with an irreversible chemical reaction. The following schemes are considered: R → Z (first order reaction) 2R → Z (dimerization), O + R → Z (coupling reaction). The theory allows the determination of the rate constant of the chemical reaction in every case if the electrochemical reaction is reversible but only in the case of a coupling reaction if the electrochemical reaction is irreversible. The method is applied to the study of the condensation of nitrosobenzene with phenylhydroxylamine.     

Thermodynamic analysis for a solid oxide fuel cell with direct internal reforming fueled by ethanol

In the present study, a detailed thermodynamic analysis is carried out to provide useful information for the operation of solid oxide fuel cells (SOFC) with direct internal reforming (DIR) fueled by ethanol. Equilibrium calculations are performed to find the ranges of inlet steam/ethanol (H₂O/EtOH) ratio where carbon formation is thermodynamically unfavorable in the temperature range of 500-1500 K. Two types of fuel cell electrolytes, i.e., oxygen-conducting, and hydrogen-conducting electrolytes, are considered. The key parameters determining the boundary of carbon formation are temperature, type of solid electrolyte and extent of the electrochemical reaction of hydrogen. The minimum H₂O/EtOH ratio for which the carbon formation is thermodynamically unfavored decreases with increasing temperature. The hydrogen-conducting electrolyte is found to be impractical for use, due to the tendency for carbon formation. With a higher extent of the electrochemical reaction of hydrogen, a higher value of the H₂O/EtOH ratio is required for the hydrogen-conducting electrolyte, whereas a smaller value is required for the oxygen-conducting electrolyte. This difference is due mainly to the water formed by the electrochemical reaction at the electrodes.     

Electrochemical copolymerization of carbazole and 2,2′:5′-2″ terthiophene: characterization and micro-capacitor application

In this paper, a copolymer of carbazole (Cz) and 2,2′:5′,2″-terthiophene (TTh) was electropolymerized in 0.1 M sodium perchlorate (NaClO₄)/acetonitrile (CH₃CN) on glassy carbon electrode. The optimum conditions of resulting homopolymers of Cz, TTh and copolymer of Cz and TTh in the initial feed ratio of [Cz]₀/[TTh]₀ = 1/10 were characterized by cyclic voltammetry, Fourier-transform infrared-attenuated total reflectance, scanning electron microscopy, energy dispersive X-ray analysis, and electrochemical impedance spectroscopy. Morphological analysis of copolymer shows that a micro-spherical and web-like morphology was formed for copolymer at different initial feed ratios of [Cz]₀/[TTh]₀ = 1/2, 1/5 and 1/10. The capacitive behavior of the modified electrodes was defined via Nyquist, Bode-magnitude, and Bode-phase plots. The highest low-frequency capacitance (C _LF) was obtained as 4.11 mFcm^?2 in the initial feed ratio of [Cz]₀/[TTh]₀ = 1/10. Double-layer capacitance (C _dl) and phase angles (θ) were obtained for homopolymer and copolymer systems. The highest C _dl was obtained as 2.01 mFcm^?2 for the copolymer in the initial feed ratio of [Cz]₀/[TTh]₀ = 1/2. The highest phase angle of copolymer was obtained as θ = ~75° in the initial feed ratio of [Cz]₀/[TTh]₀ = 1/1. These capacitance results confirmed that films of copolymer Cz/TTh are promising materials for micro-capacitor applications.