Electrochemical investigations and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) and poly(acenaphtho[1,2-c]pyrrole)

The electrochemical characteristics and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) (PDBNP) and poly(acenaphtho[1,2-c]pyrrole) (PANP) films prepared by controlled potential oxidation in acetonitrile containing 0.002 M monomer and 0.1 M tetrabutylammonium perchlorate (TBAP) have been studied. The impedance of PDBNP and PANP films coated onto glassy carbon electrodes measured in 0.1 M TBAP shows that the ionic conductivities of these two films increase with increasing electrode potential (oxidation level) as ClO₄^? ions are incorporated. It is concluded that anion transport is primarily responsible for the ionic conductivities. PANP has a 45° Warburg region at all electrode potentials. However, for PDBNP, the 45° Warburg region is seen only at low electrode potentials. The difference in the mode of charge transport shows that the value of electronic resistance of PDBNP at higher doping levels is similar to the ionic resistance, but for PANP, the electronic resistance is much smaller than the ionic resistance at all doping levels. The apparent electrochemical reversibility was seen to be higher for PDBNP than for PANP from cyclic voltammetry. Evidence for this interpretation is that the ionic conductivities increase dramatically with electrode potentials for PDBNP, indicating that the counterion ClO₄^? moves more easily in PDBNP than PANP. The higher low-frequency capacitance obtained from impedance spectroscopy for PANP is discussed in the light of in situ atomic force microscopy (AFM) observation of the film structure morphology.     

Electrochemical oxidation of poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] films

Electrochemistry of σ–π-conjugated polymers, poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] has been investigated in acetonitrile by using cyclic voltammetric and spectroelectrochemical techniques. Cyclic voltammograms of these polymers films display generally two pairs of main redox peaks, indicating the doping and dedoping processes of the polymers. The electrochemical oxidation of the polymer films results in their partial decomposition, besides the doping, which is shown by electrochemical quartz crystal microbalance and spectroelectrochemical measurements. The decomposition products are characterized by UV-visible absorption spectroscopy, fluorescence spectroscopy, FT-IR, and GPC measurements. It is concluded that the decomposition originates mainly from electrochemical cleavage of silicon–silicon bonds in the polymer chains, and the electrochemical stability of the σ–π-conjugated polymers may be improved by decreasing the oligosilanylene block size in the polymer chains.     

Electrochemical doping of poly(vinyl chloride) obtained by photodehydrochlorination from laminated poly(vinyl chloride) + polypyrrole films

The poly(vinyl chloride) (PVC) in laminated PVC + polypyrrol (PPy) films has been converted to an electrically conducting structure by photodehydrochlorination and subsequent electrochemical doping. The maximum conductivity (6 × 10^?1 S cm^?1) of the PVC film was obtained by potentiostatic oxidation at 0.3 V vs. SCE in HCl solution. The reflection spectra of the PVC film showed a new band around 600 to 800 nm after the electrochemical doping, indicating the formation of a charge-transfer complex. The maximum conductivity of the PVC film was reached with the highest concentration of the charge transfer complex, polyene⁺ … X^?, formed between the photogenerated polyenes and dopant anions.     

The electrochemistry of fluorine-substituted polyphenylthiophenes for charge storage applications

A series of thiophene-based conducting polymers has been synthesized. The properties of these polymers were characterized using electrochemical techniques. The polymerization potentials of the monomers were found to be related to the corresponding Hammett σ values. The maximum n-doping levels of the polymers were also found to be correlated with the Hammett σ values. No correlation was found with the maximum p-doping levels of the polymers. The doping densities were found to decrease on repeated cycling and were correlated with irreversible morphological changes in the films. Test cells were fabricated using conducting polymer films for both anode and cathode. These cells exhibited discharge voltages of about 2.5 V and capacities of 9.5 to 11.5 mA h g^?1.     

The electrochemical behavior of p-sulfonated calix[4]arene

The electrochemical behavior of p-sulfonated calix[4]arene was studied. In aqueous solution, p-sulfonated calix[4]arene can be oxidized when the potential is more than 0.7 V versus SCE. It is confirmed that the reaction is an irreversible two-electron transfer electrochemical reaction. The anodic peak potential, E_p, is affected by the acidity of the solution. E_p shifts in the negative direction when the pH increases. The electron transfer coefficient, α, is 0.65. At 25 °C, the diffusion coefficient of p-sulfonated calix[4]arene is 3.1 × 10^?5 cm² s^?1. The activation energy, E_a, for the electrochemical reaction is (18.8 ± 0.2) kJ mol^?1.     

Diamond-like carbon electrodes in electrochemical microgravimetry

Conducting diamond-like carbon films on quartz crystal electrodes were prepared by laser ablation of graphite. They were stable in many solvents including water, dichloromethane, and acetonitrile. Their potential window is much wider than those of the metal electrodes used usually in quartz oscillators and the electron transfer rate at the electrode surface is fast enough to perform electrochemistry. In dichloromethane with 0.1 M (n-Bu)₄NClO₄, DLC electrodes could scan the potential range between +2.0 and ?2.0 V to record cyclic voltammetric (CV) curves of [Ru(bpy)₃]^3+/2+. We also showed that the DLC works satisfactorily as a useful electrode material for electrochemical microgravimetry by examining the deposition–dissolution of [Ru(bpy)₃]³⁺ in dichloromethane solutions. It was found that the oxidized [Ru(bpy)₃]³⁺ have two different fates, that is, some of them make deposits on the DLC electrode and the rest diffuse out of the electrode surface instead of depositing. Based on the frequency data representing the amount of deposit only and the oxidation charge data reflecting the amount of oxidized species, deposits were found to have the average composition of [Ru(bpy)₃](ClO₄)₃?·?1.7(n-Bu)₄NClO₄.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Using Pt microelectrodes in liquid ammonia for studying proton reduction

The electrical response of Pt microelectrodes, with a radius of 10 μm, was studied in liquid ammonia (?50°C). The question of a suitable time scale for steady state measurements in liquid ammonia is important. The proton reduction currents were measured for different scan rates. A suitable experimental time scale of 20 mV s^?1 was determined to reach a stationary state. The diffusion coefficient of protons in liquid ammonia was deduced from these electrochemical results: D(NH₄⁺)_NH3=(3.8±0.4)×10^?9 m² s^?1.     

Semiconductor and electrochromic properties of electrochemically deposited nickel oxide films

Nickel oxide films have been prepared by electrochemical deposition on a conducting substrate such as Au or ITO from a basic solution with nickel sulfate and an amino acid. The NiO films thus prepared exhibit typical p-type semiconductor and electrochromic properties. From a Mott–Schottky plot, the flatband potential, V_FB and the acceptor density, N_A are determined as 0.47 V versus Ag|AgCl (at pH 7.0) and 1.8 × 10²¹ cm^?3, respectively. The optical band gap, E_g obtained from measurement of the absorption coefficient of the NiO at a particular photon energy is estimated as 3.31 eV. The absorption spectrum of a NiO|ITO electrode exhibits a maximum at a wavelength of 460 nm in the potential region above +0.4 V. The colors of the nickel oxide are dark blue and transparent in the potential regions above and below this potential, respectively.     

Pseudopolarography of trace metals. Part II. The comparison of the reversible,quasireversible and irreversible electrode reactions

The theoretical and experimental pseudopolarographic curves of reversible, quasireversible and irreversible electrochemical reactions were compared and evaluated. The measurements were performed on a stationary mercury drop electrode (SMDE, PAR 303A), using differential pulse anodic stripping voltammetry (DPASV). A good agreement between the theoretical and the experimental shift of the half-wave potential with an increasing accumulation time was obtained for the reversible pseudopolarograms of 10^?7 mol dm^?3 Cd(II) (in 0.1 mol dm^?3 NaClO₄, pH ～2). As compared with the curve of the logarithmic analysis of the polarogram, the corresponding curve of the pseudopolarogram is steeper in the region of the half-wave potential. It has been shown that even though the pseudopolarograms are quasireversible or irreversible, there is a range at the foot of the curves with a reversible slope (usually below 10% of the total/limiting current). It has been verified that the range of this reversible slope can be extended by increasing the accumulation time, lowering the mercury drop size and diminishing the thickness of the diffusion layer. The estimated value for its approximative evaluation is about 1% of the total/limiting current. This is essential for the determination of the corresponding electrochemical parameters, such as: the formal potential (E°^′), transfer coefficient (α) and rate constant (k_s). From the experimentally obtained reversible slope of the (pseudo)polarographic curves of Zn(II) (in 1 mol dm^?3 NaClO₄, pH 4.7 ± 0.1), the parameters for the quasireversible electrochemical reactions were estimated as follows: E°^′=?0.964 ± 0.002 V, α=0.24 ± 0.02 and k_s～2–3×10^?3 cm s^?1. It is shown that an accurate transfer coefficient can be calculated from the curves of the logarithmic analysis of the quasireversible pseudopolarograms, which is not the case for the polarographic curves. The irreversible system, tested on the electrochemical reaction of the CdNTA complex (in 0.1 mol dm^?3 NaClO₄, pH 7.9 ± 0.1), shows relatively good agreement between the experimental and the theoretical dependences. The (pseudo)polarographic measurements enabled approximate estimation of the electrochemical parameters (E°^′=?0.835 ± 0.010 V, α=0.55 ± 0.02 and k_s=1.0 ± 0.4 × 10^?4 cm s^?1) which are in fairly good agreement with the literature data.     

Characterization of a new spinel Li–Cr–Mn–O for secondary lithium batteries

A new chromium-substituted Li₂O–yMnO₂ (y=4) spinel phase has been prepared at low temperature using a sol–gel process. The electrochemical behavior of the obtained material is investigated in liquid-electrolyte lithium cells. This material presents a plateau at around 3 V versus Li ∣ Li⁺ with a large discharge capacity of 140 mAh g^?1 and fairly good cycle ability.     

Electrochemical study of the redox dyes Nile Blue and Toluidine Blue adsorbed on graphite and zirconium phosphate modified graphite

Solid graphite electrodes were modified with the redox dyes Nile Blue (NB) and Toluidine Blue (TB), following irreversible adsorption from ethanol solution, and a droplet evaporation procedure. The shift of redox potential for both dyes to the negative direction upon adsorption on graphite was estimated from cyclic voltammetry, and from extrapolation of the dependence of E_p versus I_p to I_p=0, to vary between 60 and 110 mV. For electrodes containing a controlled amount of NB or TB, heterogeneous charge transfer rate constants of 31 and 11.6 s^?1, respectively, were estimated by extrapolation of the data to zero coverage of the electrode surface. The rate constants obtained are several times higher than those obtained for multilayer coatings with the dyes. The modification of solid graphite electrodes with zirconium phosphate (ZrP), followed by the adsorption of NB and TB was performed. For NB, two forms of the dye were found to be present, one of them presenting an usual adsorbate of NB on graphite, and another one having E_pc shifted by ca. 0.15–0.30 V to more positive potential values. The latter form is ascribed to NB, included in the ZrP host matrix, as follows by comparing its electrochemical behavior with that observed for NB, included in carbon paste containing ZrP.     

Application of multi-walled carbon nanotubes functionalized with hemin for oxygen detection in neutral solution

Functionalization of multi-walled carbon nanotubes (MWNTs) is of paramount importance for developing new sensors. In this study, a hemin-modified MWNT electrode was successfully constructed. Upon saturation, the amount of adsorbed hemin is estimated as 2.7 × 10^?9 mol cm^?2, which is 39 times larger than the value of the hemin monolayer. The electrochemical behavior of the hemin-modified MWNT electrode has been characterized by cyclic voltammetry (CV). The electron-transfer coefficient (α) is found to be 0.38 with a heterogeneous electron transfer rate (k) of 2.9 s^?1 for the adsorbed hemin. Both MWNT and hemin-modified MWNT electrodes show ideal reversibility in 5 mM K₃[Fe(CN)₆] in the range of 0.02–1.00 V s^?1, indicating fast electron-transfer kinetics. CV of the hemin-modified MWNT electrode in pH 7.4 phosphate buffer solution (PBS) clearly shows the dioxygen reduction peaks close to 0 V (vs. Ag|AgCl). These results are useful in the development of a novel oxygen sensor for working at a relatively low potential.     

Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δ_o^wφ_1/2 on pH and Na⁺ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me_(w)²⁺+HX_(o)?MeHX²⁺_(o)) and (Me_(w)²⁺+X^?_(o)?MeX⁺_(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔE_p=0.030 V while at intermediate pH the electrochemical exchange reaction (Me²⁺_(w)+HX_(o)?MeX⁺_(o)+H⁺_(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔE_p=0.060 V for the peak observed ΔE_p in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba²⁺ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.     

Use of electrochemical techniques for the study of solubilization processes of cerium–oxide compounds and recovery of the metal from molten chlorides

This work presents a study on the chemical and electrochemical properties of CeCl₃ in two molten chloride mixtures with different oxoacidity properties, the eutectic LiCl–KCl at 723 K and the equimolar CaCl₂–NaCl melt at 823 K. The E–pO^2? (potential–acidity) diagram for Ce–O compounds stable in both melts has been constructed by combining both theoretical and experimental data. The stable oxidation states of Ce have been found to be (III) and (0) in both melts; Ce(IV) is only stable in the form of solid CeO₂. The standard potential value of the Ce(III)/Ce(0) system has been determined by potentiometry, giving values ?3.154±0.006 V and ?3.036±0.009 V versus Cl₂/Cl^? in the eutectic LiCl–KCl at 723 K and the equimolar CaCl₂–NaCl melt at 823 K, respectively. On the other hand, from the calculated activity coefficient values it was possible to say that the Ce(III) ions form stronger complexes with the chloride ions in the alkaline melt. This is probably due to the smaller amount of free chloride ions in the calcium molten mixture. Identification of the Ce–O compounds that are stable in the melts as well as the determination of their solubility products was easily carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that Ce₂O₃ is a strong oxobase and that CeOCl is a solid stable compound in the melts studied. CeO₂ is also a stable compound that can exist under oxobasic conditions. The best chlorinating conditions could be extracted from the comparison of the E–pO^2? diagram corresponding to the Ce–O compounds and that of some chlorinating mixtures. Experimental solubilization tests allowed us to verify the thermodynamic chlorinating predictions. Moreover the electrochemical behaviour of CeCl₃ solutions was also investigated. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species on metallic substrates such as tungsten and molybdenum. The results showed that the Ce(III)/Ce(0) system is quasi-reversible with values of the charge transfer rate constant, k°, and transfer coefficient, α, around 10^?3.7 cm s^?1 and 0.4, respectively. The diffusion coefficient of Ce(III) ions was also determined by different electrochemical techniques, obtaining a value in the order of 1×10^?5 cm² s^?1. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T.     

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi₂S₃ and Bi₂O₃ semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies (D≈ 10^?16 cm² s^?1).Solid-state transformation of sulfide to the oxide film is a consequence of OH^? ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.     

Enhanced electrochemical and electrocatalytic activity of a new supramolecular manganese-porphyrin species containing four bis(bipyridine)(aqua)ruthenium(II) complexes

A novel supramolecular species [MnTPyP{Ru(bipy)₂H₂O}₄](PF₆)₉ containing four cis-bis(bipyridine)(aqua)ruthenium(II) complexes attached to the peripheral pyridyl groups of meso-tetra(4-pyridyl)porphyrin manganese(III) (MnTPyP) has been synthesized, and employed as the precursor of a rather promising multinuclear oxo-transfer catalyst. This complex forms molecular films by dip coating. Such films are soluble in water, but can be stabilized by exchanging the PF^?₆ anions by [Fe(CN)₆]^4? or [Zn–TPPS]^4?. They exhibit remarkable electrochemical and electrocatalytic activity, as demonstrated in the oxidation of nitrite ions. In this case, experimental evidence supports the involvement of electrochemically generated [ORu^IV]²⁺ groups in the oxidation reaction, reflecting the characteristic reactivity of high-valence metal–oxo complexes.     

Preparation,characterization, and electrocatalytic properties of copper hexacyanoferrate film and bilayer film modified electrodes

Copper(II)hexacyanoferrate films have been prepared from various electrolyte aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of copper(II)hexacyanoferrate films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions from solutions of ten cations: Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, H⁺ and Al³⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the copper(II)hexacyanoferrate films. The copper(II)hexacyanoferrate film showed a single redox couple that exhibited a cation effect (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and an anion effect (F^?, Cl^? and Br^?) in the cyclic voltammograms and formal potential of the redox couple. The electrochemical and EQCM properties of the film indicate that the redox process was confined to the immobilized copper(II)hexacyanoferrate, and the interaction between the copper(II)hexacyanoferrate film with K⁺ (monovalent cation) and Ca²⁺ (divalent cation). The electrocatalytic oxidation properties of NADH, NH₂OH, N₂H₄, SO₃^2? and S₂O₃^2? were also determined. The electrocatalytic reduction properties of SO₅^2? and S₂O₈^2? by monolayered iron, nickel, and cobalt hexacyanoferrate films, and by bilayered metal–copper hexacyanoferrate films were determined. Two-layered modified electrodes and hybrid films composed of a copper(II)hexacyanoferrate film with iron(II)hexacyanoferrate, cobalt(II)hexacyanoferrate, or nickel(II)hexacyanoferrate film were prepared, and these films caused the electrocatalytic reduction of SO₅^2? and S₂O₈^2?.     

Anodic electrodeposition of copper oxide/hydroxide films by alkaline solutions containing cuprous cyanide ions

A novel procedure of anodic electrodeposition of copper oxides and hydroxides on glassy carbon and noble metals electrodes in an alkaline medium is described. The deposited films have been investigated by electrochemical and by X-ray photoelectron spectroscopy (XPS) techniques. The copper films are deposited on the electrode surface as Cu(OH)₂ and CuO after the dissociation of the Cu(CN)_n^1?n complex caused probably by the formation of Cu^III species at relatively high applied potentials (i.e. 0.5 V). The resulting active copper films, independently of the electrochemical procedure adopted (i.e. potentiostatic or potentiodynamic conditions), appear to be uniform, smooth and with a good degree of adhesion on the electrode surface The electrocatalytic properties of the copper deposited films were investigated in an alkaline medium for the electrooxidation of several classes of organic compounds.     

Comparative studies on the electrogenerated chemiluminescent and amperometric behavior of the Ru(bpy)32+ system on a paraffin-impregnated graphite electrode and a glassy carbon electrode

The electrogenerated chemiluminescent (ECL) and amperometric behavior of Ru(bpy)₃²⁺ system on a paraffin-impregnated graphite electrode (PIGE) and a glassy carbon electrode (GCE) was studied by different electrochemical techniques. Under conventional cyclic voltammetric (CV) conditions, two anodic ECL peaks (EP1 and EP2) (vs. SCE) were observed at 1.18 and 1.37 V for a PIGE, and at 1.20 and 1.44 V for a GCE. The EP1 normally occurred, but not the EP2. A complicated mechanism was involved in the formulation of EP2. A detection limit as low as 1×10^?9 mol l^?1 of C₂O₄^2? was obtained on the PIGE by the CV method at 100 mV s^?1. Strong ECL signals were found on both electrodes in either an oxalate-containing aqueous solution or an organic solution when a cyclic square wave (CSW), between two suitable potentials was used. However, the GCE showed higher reproducibility than the PIGE for continuous CSW (n=10) measurements. Chronoamperometry was also applied by using a potential step from 0.15 to 1.85 V. It took 389 ms on a PIGE and 837 ms on a GCE to reach each maximal ECL intensity.