A study of the charge propagation in nanoparticles of Fe2O3 core-cobalt hexacyanoferrate shell by chronoamperometry and electrochemical impedance spectroscopy

A composite electrode comprised of graphite, Nujol, and nanoparticles of Fe₂O₃ core–cobalt hexacyanoferrate shell was prepared and the charge transport processes in the bulk of this composite were studied. The electrode/solution interface was assumed as a binary electrolyte whose charge transport occurred between redox sites of the nanoparticles present in the composite and counter cations present in the solution. Using cyclic voltammetry, the diffusion of counter cation in the shell was investigated. Using chronoamperometry, an effective diffusion coefficient and its dependency on the applied potential was obtained. In the Nyquist diagrams, different time constants were appeared with relation to different physical and electrochemical processes. Percolation of electron in the shell of the nanoparticles appeared at very high frequencies and exhibited the feature of a diffusion process with a transmissive boundary condition at interface of core–shell structure/graphite particles. The diffusion coefficients of electron and counter cation and the standard rate constants of each individual electrochemical reaction were obtained.     

Gas diffusion hindrances on Ni cermet anode in contact with Zr<Subscript>0.84</Subscript>Y<Subscript>0.16</Subscript>O<Subscript>1.92</Subscript> solid electrolyte

The electrochemical behavior of Ni cermet electrode with CeO_{2 ? x} additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H₂, H₂O, CO₂, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σ_lf) is characterized with the following regularities: (a) temperature dependence of σ_lf has a positive slope in Arrhenius coordinates; (b) σ_lf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σ_lfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σ_lf vs. 1/P _{CO 2} (P _CO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σ_lf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.     

掺杂双(N-2-甲基苯基-水杨醛亚胺)合铜(II)配合物的碳糊电极电催化测量过氧化氢HosseinKhoshro,HamidR.Zare,RasoulVafazadeh简

The electrochemical behavior of a bis(N-2-methylphenyl-salicyldenaminato)copper(Ⅱ) complex spiked in a carbon paste electrode (BMPSCu-CPE) and its electrocatalytic reduction of H₂O₂ were examined using cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Cyclic voltammetry was used to study the redox properties of BMPSCu-CPE at various potential scan rates. The apparent charge transfer rate constant and the transfer coefficient for the electron transfer between BMPSCu and the carbon paste electrode (CPE) were 1.9±0.1 s^-1 and 0.43, respectively. BMPSCu-CPE had excellent electrocatalytic activity for H₂O₂ reduction in 0.1 mol/L phosphate buffer solution (pH 5.0), and it decreased the overpotential by 300 mV as compared to CPE alone. The diffusion coefficient and kinetic parameters such as the heterogeneous catalytic electron transfer rate constant and electron transfer coefficient for the reduction of H₂O₂ at the BMPSCu-CPE surface were also determined using electrochemical methods. Differential pulse voltammetry showed two linear dynamic ranges of 1.0-10.0 and 10.0-300.0 μmol/L and a detection limit of 0.63 μmol/L H₂O₂. The BMPSCu-CPE has excellent reproducibility and long term stability, and it was successfully applied for the determination of H₂O₂ in two pharmaceutical samples: an antiseptic solution and a hair dying cream.     

Amperometric sensor for hydrogen peroxide based on direct electron transfer of spinach ferredoxin on Au electrode

A protein-based electrochemical sensor for hydrogen peroxide (H₂O₂) was developed by an easy and effective film fabrication method where spinach ferredoxin (Fdx) containing [2Fe–2S] metal center was cross linked with 11-mercaptoundecanoic acid (MUA) on a gold (Au) surface. The surface morphology of Fdx molecules on Au electrodes was investigated by atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were employed to study the electrochemical behavior of adsorbed Fdx on Au. The interfacial properties of the modified electrode were evaluated in the presence of Fe(CN)₆^3?/4? redox couple as a probe. From CV, a pair of well-defined and quasi-reversible redox peaks of Fdx was obtained in 10 mM, pH 7.0 Tris–HCl buffer solution at ?170 and ?120 mV respectively. One electron reduction of the [2Fe-2S]²⁺ cluster occurs at one of the iron atoms to give the reduced [2Fe-2S]⁺. The formal reduction potential of Fdx ca. ?150 mV (vs. Ag/AgCl electrode) at pH 7.0. The electron-transfer rate constant, k_s, for electron transfer between the Au electrode and Fdx was estimated to be 0.12 s^?1. From the electrochemical experiments, it is observed that Fdx/MUA/Au promoted direct electron transfer between Fdx and electrode and it catalyzes the reduction of H₂O₂. The Fdx/MUA/Au electrode displays a linear increase in amperometric current for increasing concentration of H₂O₂.The sensor calibration plot was linear with r² = 0.998 with sensitivity approximately 68.24 μAm M^?1 cm^?2. Further, the effect of nitrite on the developed sensor was examined which does not interfere with the detection of H₂O₂. Finally, the addition of H₂O₂ on MUA/Au electrode was observed which has no effect on amperometric current.     

Kinetic and thermodynamic studies of hydrogen storage alloys as negative electrode materials for Ni/MH batteries: a review

This paper reviews the present performances of intermetallic compound families as materials for negative electrodes of rechargeable Ni/MH batteries. The performance of the metal-hydride electrode is determined by both the kinetics of the processes occurring at the metal/solution interface and the rate of hydrogen diffusion within the bulk of the alloy. Thermodynamic and electrochemical properties for each hydride compound family will be reported. The steps of hydrogen absorption/desorption such as charge-transfer and hydrogen diffusion for evaluating the electrochemical properties of hydrogen storage alloys are discussed. Exchange current density (I ₀) and hydrogen diffusion coefficient (D _H) are the two most important parameters for evaluating the electrochemical properties of metal hydride electrode. The values of the two parameters for a number of hydrogen storage alloys are compared. The relationship between alloy composition and electrochemical properties is noted and evaluated.     

Unexpected role of electronic coupling between host redox centers in transport kinetics of lithium ions in olivine phosphate materials

The discrepancy between the trend in the diffusion coefficient of a lithium ion (D_Li⁺) and that in the activation energy of ion hopping signals hidden factors determining ion transport kinetics in layered olivine phosphate materials (LiMPO₄). Combining density functional theory (DFT) calculations and the Landau–Zener electron transfer theory, we unravel this hidden factor to be the electronic coupling between redox centers of the host materials. The ion transport process in LiMPO₄ is newly described as an ion-coupled electron transfer (ET) reaction, where the electronic coupling effect on D_Li⁺ is considered by incorporating the electronic transmission coefficient into the rate constant of the transfer reaction. The new model and DFT calculation results rationalize experimental values of D_Li⁺ for various LiMPO₄ (M = Fe, Mn, Co, Ni) materials, which cannot be understood solely by the calculated activation barrier of ion hopping. Interestingly, the electronic coupling between host redox centers is found to play an essential role. Particularly, the sluggish ion mobility in LiFePO₄ is due to a very weak electronic coupling. The obtained insights imply that one can improve the rate performance of intercalation materials for metal-ion batteries through modifying the electronic coupling between redox centers of host materials.

The discrepancy between the trend in the diffusion coefficient of lithium ions (D_Li⁺) and that in the activation energy of ion hopping signals hidden factors determining ion transport kinetics in layered olivine phosphate materials (LiMPO₄).     

The transient response of electrodes coated with membrane-type polymer films under conditions of diffusion and migration of the redox ions

Membrane transport processes involving diffusion and migration of the electroactive species A and product B are analysed. The transport equations (Nernst-Planck equation) are solved by digital simulation techniques with boundary conditions appropriate for the system electrode/membrane-type polymer coating. A large potential step leads to a zero surface concentration of A at time t = 0. Time-dependent concentration gradients of A and B, and field strengths are derived. The current transients are shown to follow Cottrell's law; however, the observed “effective” diffusion coefficients, D_eff, differ from D_A.     

Amorphous carbon nitride a-CNx microelectrode: Fabrication and characterization

Fabrication and characterization of amorphous carbon nitride a-CN_x microelectrodes are reported. These electrodes were prepared by DC-sputtering of a thin carbon layer on sharpened glass tip. The kinetic parameters (k⁰ and α) and the diffusion coefficient of the ferri-ferrocyanide redox probe were determined by steady-state voltammetry (CV) and by electrochemical impedance spectroscopy (EIS), and were used for characterizing both the electrochemical sensitivity of microelectrodes and their dimensions. The cathodic activation procedure of the electrode resulted in an increase of the electron rate constant. This procedure provides a new way for the fabrication of carbon microelectrodes for local electrochemical measurements.     

The electrochemical Peltier effect observed with electrode reactions of Fe(II)/Fe(III) redox couples at a gold electrode

The electrochemical Peltier effect was studied at a gold electrode in solutions containing some Fe(II)/Fe(III) redox couples by measuring the local temperature change in the electrode/solution interphase under controlled-potential and controlled-current polarization. Relative values of the electrochemical Peltier coefficient for the cathodic process at equilibrium potential, which is denoted by (Π_c)_I=0, were determined by analyzing the observed temperature change as a function of current. The values of (Π_c)_I=0 were found to be positive for the Fe(H₂O)₆²⁺/Fe(H₂O)₆³⁺ systems in HClO₄ (1 M), HNO₃ (1 M), H₂SO₄ (0.5 M), and HCl (1 M), their magnitudes being very similar in the first three acid solutions, but smaller in the HCl solution. On the other hand, a negative value of (Π_c)_I=0 was obtained in the case of a Fe(CN)₆^4?/Fe(CN)₆^3? couple in a H₂SO₄ (0.5 M) solution. Such a difference in the Peltier coefficient is considered to be due to the difference in the ionic species of iron involved in the electrode reaction.     

Determination of diffusion coefficient of lithium in substituted LiMn1.95Cr0.05O4 spinel using impedance technique

Electrodiffusion properties of chromium-substituted lithium-manganese spinel Li _x Mn_1.95Cr_0.05O₄ intended for application as a cathodic material for lithium-ion batteries is studied. The studies are carried out at 25°C using the electrochemical impedance spectroscopy technique in alkyl-carbonate electrolyte. In the analysis of impedance spectra, the apparatus of electric equivalent circuits was employed to determine surface layer resistances, double electric layer capacitance, differential intercalation capacity, chemical diffusion coefficient D of lithium, and other electrode characteristics. The issues of substantiating the choice of electric equivalent circuits and correct interpretation of their elements are discussed; dependences of the calculated model parameters on the electrode potential (lithium concentration in the electrode) are analyzed. The chemical diffusion coefficient of Li⁺ in Li _x Mn_1.95Cr_0.05O₄ found on the basis of the impedance spectra is in the range of 10^?9 to 10^?12 cm²/s under electrode potential variation in the range of 3.5–4.5 V (vs. Li/Li⁺) with a pronounced minimum of D in the middle of this range. Repeated cycling of the electrode is accompanied by a gradual increase in resistance of the solid-electrolyte interphase (SEI).     

Solid-state electrochemistry of silicotungstic acid immobilized by sol-gel chemistry

Voltammetry of silicotungstic acid (STA), H₄SiW₁₂O₄₀, that was encapsulated in silica was performed in the absence of a contacting liquid phase. Two one-electron reductions that are separated by 200?mV were observed, which is the same behavior as in aqueous solution. At scan rates, v, below 10?mV s^?1 with a 10?μm dia. carbon fiber indicator electrode, plateaus with limiting currents which are independent of v were observed, which is indicative of spherical diffusion from a field that is much larger than the electrode area. At v?>?20?V?s^?1, peaks were observed with currents directly proportional to v ^½. For a gel aged for 2 days, an effective diffusion coefficient, D _eff, of 3?×?10^?7?cm² s^?1 was estimated by voltammetry and chronoamperometry; the concentration of the redox sites thereby determined was about 0.5?M. The D _eff values that were obtained in this study were larger than expected for a solid electrolyte, which suggests an important role of residual water. In support of this model, gels that were aged in a humidistat at 33% humidity at room temperature for 2 and 5 days lost 16% and 13%, respectively, of their mass when dried at 120°.     

Direct electrochemistry of hemoglobin in a nafion and CuS microsphere modified carbon ionic liquid electrode and its electrocatalytic behavior

Direct electrochemistry of hemoglobin (Hb) was realized on a Nafion and CuS microsphere composite film modified carbon ionic liquid electrode (CILE) with N-butylpyridinium hexafluorophosphate (BPPF6) as binder. Scanning electron microscopy (SEM), UV-Vis absorption spectroscopy and cyclic voltammetry were used to characterize the fabricated Nafion/CuS/Hb/CILE. Experimental results showed that a pair of well-defined quasi-reversible redox peaks appeared with the formal potential as ?0.386 V (vs. SCE) in pH 7.0 Britton-Robinson (B-R) buffer solution, which was attributed to the Hb heme Fe(III)/Fe(II) redox couples. The electrochemical parameters of Hb in the composite film were carefully investigated with the charge transfer coefficient (α), the electron transfer number (n) and the electron transfer rate constant (k _s) as 0.505, 1.196 and 0.610 s^?1, respectively. The composite film provided a favorable microenvironment for retaining the native structure of Hb. The presence of CuS microspheres showed great improvement on the electron transfer rate of Hb with the CILE, which maybe due to the contribution of specific characteristics of CuS microspheres and the inherent advantages of ionic liquid on the modified electrode. The fabricated Hb modified electrode showed good electrocatalytic ability in the reduction of H₂O₂. The proposed bioelectrode can be used as a new third generation H₂O₂ biosensor.     

Novel Fe2O3@CeO2 Coreshell‐based Electrochemical Nanosensor for the Voltammetric Determination of Norepinephrine

A new selective carbon paste electrode (CPE), was applied as an electrochemical sensor for the detection of norepinephrine (NOE). The sensor was modified with 6‐amino‐4‐(3,4‐dihydroxyphenyl)‐3‐methyl‐1,4‐dihydropyrano[2,3‐c],pyrazole‐5‐carbonitrile (ADPC) assisted Fe₂O₃@CeO₂ coreshell nanoparticles (CNs) synthesized by simple method. To identify the redox properties of the modified electrode, and to examine its electrochemical properties, cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV) were conducted. Through electrochemical investigations, the coefficient of electron transfer between ADPC and the CNs/CPE (i. e. carbon paste electrode which was modified with CNs), the apparent charge transfer rate constant (k_s), and the diffusion coefficient (D) were calculated. The NOE oxidation occurred at the optimum pH of 7.0 and a potential that was about 235 mV less positive than that of the unmodified carbon paste electrode. The interaction between the two metals in the Fe₂O₃@CeO₂ coreshell led to an increase in the surface area and, consequently a sharp increase in the current. The differential pulse voltammogram of NOE showed two linear dynamic ranges an excellent detection limit (3σ) of 40 nM. In addition, NOE, AC and Trp were simultaneously determined at the modified electrode. Finally, NOE was quantitated in a number of real samples.     

UV-ozone dry-cleaning process for indium oxide electrodes for protein electrochemistry

The present study reports, for the first time, on electrochemical responses of cytochrome c at a UV-ozone treated indium oxide electrode. Results from surface tension measurements indicate that UV-ozone treatment is an efficient cleaning procedure to remove organic species contamination on surfaces. Well-defined redox responses for cytochrome c were observed at a UV-ozone treated fully hydrophilic indium oxide electrode. Electrochemical parameters, including the diffusion coefficient, the heterogeneous electron transfer rate constant and the redox potential, were in good agreement with those previously reported. However, decrease in peak current for cytochrome c and [Fe(CN)₆]⁴⁻ were observed at a UV-ozone treated electrode. From XPS results, this behavior would be understood to indicate a decrease in homogeneous active electrode surface area by a decrease in conductivity of the indium oxide surface by UV-ozone treatment. Simple and effective UV-ozone treatment methods are useful for surface contamination sensitive electrochemistry.     

The electrodissolution of magnetite: Part I. The electrochemistry of Fe3O4/C discs-potentiodynamic experiments

A simple technique is reported for the preparation of Fe₃O₄ compact electrodes by cold pressing of powdered magnetite admixed with carbon. The electrochemical behaviour of these compacts was shown to be similar with that of polycrystalline magnetite in respect of their potentiodynamic polarisation response. The variation of rest potential with solution composition is complex and indicates that there is oxidation of the surface to Fe₂O₃. Linear potentiodynamic polarisation curves are reported corresponding to unit concentration of ions for the pH range 3–9. The results provide evidence of an electrode process which is an irreversible single electron redox reaction. The current passing through the electrode is generally controlled by a solid-state diffusion process. It is postulated that the diffusing species is the H⁺ ion.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Electrodeposited nanogold decorated graphene modified carbon ionic liquid electrode for the electrochemical myoglobin biosensor

In this paper, a carbon ionic liquid electrode (CILE) was fabricated using ionic liquid 1-hexylpyridinium hexafluorophosphate as modifier, which was further in situ electrodeposited with graphene (GR) and gold nanoparticles step by step to get an Au/GR nanocomposite modified CILE. Myoglobin (Mb) was further immobilized on the Au/GR/CILE surface with Nafion film to get the modified electrode denoted as Nafion/Mb/Au/GR/CILE. Cyclic voltammetric experiments indicated that a pair of well-defined quasi-reversible redox peaks appeared in pH 3.0 phosphate buffer solution with the formal potential (E ^0′) located at ?0.197 V (vs. saturated calomel electrode), which was the typical characteristics of Mb heme Fe(III)/Fe(II) redox couples. Thus, the direct electron transfer rate between Mb and the modified electrode was promoted due to the high conductivity and increased surface area of Au/GR nanocomposite present on electrode surface. Based on the cyclic voltammetric data, the electrochemical parameters of Mb on the modified electrode were calculated. The Mb-modified electrode showed excellent electrocatalytic activities towards the reduction of trichloroacetic acid and H₂O₂ with wider linear range and lower detection limit. Using GR and Au nanoparticles modified CILE, a new third-generation electrochemical Mb biosensor was constructed with good stability and reproducibility.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Strategy for Low Background‐Current Levels in the Electrochemical Biosensors Using Horse‐Radish Peroxidase Labels

This article describes an electrochemical strategy to achieve low background‐current levels in horse‐radish peroxidase (HRP)‐based electrochemical immunosensors. The strategy consists of (i) the use of an HRP substrate/product redox couple whose formal potential is high and (ii) the use of an electrode that shows moderate electrocatalytic activity for the redox couple. The strategy is proved by a model biosensor using a catechol/o‐benzoquinone redox couple and an indium tin oxide (ITO) electrode. The combined effect of high formal potential and moderate electrocatalytic activity allows o‐benzoquinone electroreduction with minimal catechol electrooxidation and H₂O₂ electroreduction. The detection limit for mouse‐IgG is 100 pg/mL.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part 2. Evaluation of low-temperature isotropic carbon for use as an electrode material

Unalloyed Pyrolyte, a low-temperature isotropic carbon (LTIC) material is evaluated electrochemically for use as an electrode material in h.p.l.c. thin-layer electrochemical detection cell systems. LTIC is compared to carbon paste electrode material in all systems studied, and to glassy carbon impregnated with ceresin wax in the hexacyanoferrate(II)/(III) redox system. Electrode areas, residual currents, and potential windows are described. Quantitative kinetic studies are reported for the hexacyanoferrate(II)/(III) redox pair in 2.0 M KCl, and rate constants are given. Quantitative rate constant studies are also reported for o-dianisidine in 1.0 M H₂SO₄. Finally, qualitative rate comparisons are made by cyclic voltammetric techniques between carbon paste and LTIC electrode materials for maetanephrine, normetanephrine, adrenaline, noradrenaline, dopamine, 3,4-dihydroxyphenylalanine, 5-hydroxytryptophan, and chlorpromazine in chemical systems reflecting previous applications of h.p.l.c. with electrochemical detection.