Molar enthalpy of the polypyrrole electrochemistry

The kinetics of the angular movement of a triple-layer muscle working potentiostatically under different temperatures allowed the obtention of both the activation energy and the molar enthalpy of the process. By compaction of a polypyrrole film under constant temperature and at a constant cathodic potential, followed by a subsequent oxidation at a constant anodic potential under different temperatures, the molar enthalpy was obtained by application of the ESCR model. Similar results, 27.1 kJ/mol, from muscles, versus 27.7 kJ/mol, from relaxation experiments, were obtained.     

Thermally enhanced conformational relaxation during electrochemical oxidation of polypyrrole

Oxidation of polypyrrole which was previously prepolarized at highly negative potentials is limited by the ability of polymeric chains to undergo conformational changes. As is well known, polymeric chains can vary their conformational state only when some energetic requirements are fulfilled. This allows the study of how conformational relaxation processes associated with electrochemical doping in conducting polymers are affected by changes in the experimental conditions used. In this work, the influence of temperature on the rate of conformational relaxation in polypyrrole is evaluated, on the basis of our electrochemical relaxation model. The comparison between experimental data and theoretical predictions from the model allows the activation energy ΔH* for the conformational changes associated with polypyrrole doping to be obtained, together with the evolution of structural and electrochemical features of the polymer with the temperature. A new technique of electrochemical thermocurrents is proposed to obtain information about interactions between polymer, solvent and ions.     

Evaluation of the potential dependence of 2D–3D growth rates and structures of polypyrrole films in aqueous solutions of hexafluorates

Polypyrrole hexafluorosilicate, PPYSiF₆, and polypyrrole hexafluoroaluminate, PPYAlF₆, were found to follow the mixed 2DI–3DI kinetic model of electrodeposition in the potentials range (+0.80 V, +0.60 V) and (+0.95 V, +0.55 V) vs. SCE, respectively. The potentiostatic depositions of polypyrrole were performed on polycrystalline gold and on the single layer or bilayer polypyrrole films. Only in the case of the third layer deposition of PPYSiF₆ (on PPYSiF₆) at +0.6 V, the experimental current was found to fit better with the 2DP–3DI model. For electrodeposition of PPYAlF₆ on gold, a slightly better quality fitting with 2DP–3DP or 2DP–3DI kinetic models was observed only for some samples synthesized at 0.55 V. In general, contribution of the 2D structure in the polypyrrole deposits was observed to decrease with an increase of the polarization potential. Conclusions of the mathematical analysis of the current–time functions are in accordance with a texture of the outer surface of the polymer layers visible in SEM micrographs. The apparent rate constants of the lateral and outward growths of polypyrrole increase with potentials in (+0.55, +0.80) V range. Under assumption of the classical electrochemical kinetics law, the anodic transfer coefficients were found lower than 0.5. Values of the outward growth rate constant were of the order of 10⁻⁸–10⁻¹⁰ mol/(s cm²); being approximately one order of magnitude higher for polypyrrole hexafluorosilicate than for polypyrrole hexafluoroaluminate. The lateral growth rates of PPYAlF₆ on gold were found to depend on the electrodeposition potential more significantly than the outward growth rate. An increase in the growth rates with the potential was observed to diminish/vanish at E > +0.80 V, probably due to concurrent degradation processes of polypyrrole chains. The analysis of the deposition currents was done under conditions of the masking double layer currents that were characterized by the relaxation times of the order of seconds or longer.     

Synthesis of iron–palladium binary alloy nanotubes by template-assisted electrodeposition from metal-complex solution

The ordered arrays of Fe–Pd binary alloy nanotubes were synthesized from the mixed metal-complex solution by a simple electrodeposition method with the assistance of nanoporous anodic alumina (AAO). The electrodeposition potentials of Fe²⁺ and Pd²⁺ can be controlled by complexing agents, which has allowed for the fabrication of the Fe–Pd nanotubes by the AAO-template-assisted electrodeposition. Furthermore, the adjustment of the cathodic current density has caused the variation of the inner diameters and Fe–Pd composition ratios of the alloy nanotubes. Although the as-prepared Fe–Pd nanotubes are comprised of f.c.c. crystal structures, the metastable phase structure has been converted to L1₂ supper-lattice structure by annealing. The results of this work have shown a potential applicability of AAO-template-assisted electrodeposition to the preparation of other new binary alloy nanotubes by selecting adequate complexing agents.     

Voltammetric studies of electrochemical pretreatment of rotating-disc glassy carbon electrodes in phosphate buffer

Cyclic voltammetry was used to study the formation of graphite oxide film on the surface of rotating-disc glassy carbon electrodes. The film was formed by continuous potential cycling in phosphate buffer electrolyte. It was found that when cycling between the totally oxidized and totally reduced potential ranges (between + 2.3 and ?1.2V vs. SCE), well-shaped redox peaks were obtained. Typical voltammograms comprised one cathodic and two anodic waves, where the anodic waves were directly related to the oxidation of surface-bound products associated with the cathodic process. All three processes were pH dependent. Freshly formed surface graphite oxide films were not stable and would undergo transformation to give the stable state upon potential cycling between potentials ± 1.0V. The ultimate stable voltammograms were characterized by two electrode processes. The quasi-reversible process involved surface quinone-like functional groups, while the other irreversible process might be related to the uptake and release of hydrogen ions at the porous graphite oxide film.     

Cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using carbon paste electrodes

The present work relates to the cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using solid carbon paste electrodes. The cyclic voltammetric data of the solid catalysts, which were reproducible, were in agreement with those of electro-deposited Pt and PtSn electrode systems. The reversibility of the H-adsorption–desorption system in representative 0.3 Pt–Al₂O₃ monometallic catalyst was confirmed by the difference between the anodic and cathodic peak potentials. The unit ratio of the cathodic to anodic peak currents observed, indicated the equality of the transfer coefficient in the kinetics for the respective processes. The O₂-adsorption–desorption reaction was found to be irreversible as expected. The lower area of the curve for H-adsorption–desorption in the case of bimetallic catalysts gives an indication for the formation of alloys. The use of a carbon paste electrode in studying the cyclic voltammetric behavior of heterogenous catalysts yielded qualitative information about the activity of the catalysts. Good correlations were found between cyclic voltammetric parameters and activity results in both monometallic and bimetallic catalysts.     

Graphite photoelectrochemistry 2. Photoelectrochemical studies of highly oriented pyrolitic graphite

Photoelectrochemical reduction of oxygen and other observations of sustained electrochemically generated photocurrents with graphitic (HOPG) electrodes in aqueous solutions are reported. The photocurrents were observed over a wide pH range: 0 (0.5 M H₂SO₄)–14 (1 M NaOH). Photocurrent-potential, capacitance-potential and photocurrent light action spectral measurements were performed with basal plane and edge plane HOPG electrodes in order to elucidate the origin of the observed photocurrent. The photocurrent is attributed to hot electron–hot hole pairs photogenerated by direct transition between π-electronic states of the valence and conduction bands of graphite. Photogenerated carriers, holes or electrons, are driven by the electric field in the space charge layer (scl) to the electrode ∣ electrolyte interface, where they react directly with species in the electrolyte inducing anodic or cathodic photocurrent. The potential corresponding to the change of the photocurrent sign from cathodic to anodic was attributed to the flat band potential (E_FB). The E_FB of the basal plane electrode was 0 V versus SHE regardless of pH, while for the edge plane electrode E_FB changed at a rate of 54 mV per unit of pH. The shift of E_FB of the edge plane electrode with pH is ascribed to a change of the pH dependent surface dipole formed by oxygen containing surface redox groups.     

Electrochemical reactions with adsorption of the reactants and electrosorption. Simple analytical solutions for a Henry isotherm

A theoretical treatment of an electrochemical reaction O + ne^? R with adsorption of the reactants is presented when the adsorption, at equilibrium, obeys a Henry isotherm. A simple analytical solution, valid for any degree of adsorption of O and or R, is established for a polarographic wave. In LSV, for slow sweep rates and weak adsorption, both the cathodic and anodic peaks have a ‘diffusional’ character; for high sweep rates and strong adsorption, they are both of a ‘surface’ nature. In the intermediate case, one peak has a ‘diffusional’ character (deposition process), while the other is of a ‘surface’ nature (redissolution process). Expressions of the peak potentials are given in all cases, for reversible or irreversible reactions. Study of their variations with log v allows some or all of the diverse constants (surface and heterogeneous rate constants, adsorption coefficients, transfer coefficients, etc.) to be determined. The case of normal pulse polarography is discussed briefly. Electrosorption is formally equivalent, as far as the diffusional problem is concerned, to the case where only one entity is adsorbed. Similarities and differences between the two cases are analyzed.     

High-response tri-layer electrochemical actuators based on conducting polymer films

Sandwich structured tri-layer films of polypyrrole/gold/polypyrrole have been electrochemically synthesized by successive oxidation of pyrrole in aqueous solutions and sputtering deposition of a thin gold layer, respectively. One of the polypyrrole layers was doped by a relatively large anion, dodecylbenzene sulfonate (PPy(DBS)) and the other one was doped by a smaller anion, benzenesulfonate (PPy(BS)). The tri-layer film could be deformed quickly by electrochemical reduction and oxidation of polypyrrole layers. The thin gold layer provided a highly conductive support for actuation. The tri-layered film could move from a position of 0° (vertical) to a position of +90° (oxidation) or ?90° (reduction) at potentials higher than 0.7 V or lower than ?0.7 V (vs. saturated calomel electrode (SCE)), respectively. The movement rate of the tri-layer film was ～260°/s at potentials of ±1.2 V vs. SCE, which was five times that of a PPy(DBS)/PPy(BS) bi-layer film. Furthermore, the tri-layer film could be driven at potentials as low as ±0.2 V vs. SCE. The bending motion rate of the tri-layer film increased quadratically with the increase of driving potential, while the motion rate of the bi-layer film, PPy(DBS)/PPy(BS), increased linearly with an increase of driving potential.     

Electrochemical anion doping of poly[(tetraethyldisilanylene) oligot 2,5-thienylene)] derivatives and their p-type semiconducting properties

Chemically synthesized poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] derivatives (DS/mT; m = 3 to 5) have been successfully anion-doped by electrochemical oxidation. Band-gap energies of 2.52, 2.65, 2.82 and 3.27 eV were evaluated for DS5T, DS4T, DS3T and DS2T respectively. The DS5T, DS4T and DS3T films exhibited electrical conductivities of the order of 10^?3 to 10^?4 S cm^?1 when doped with BF₄^?. The work functions of the films changed from 5.1 to ca. 5.5 eV with electrochemical anion doping. In cyclic voltammograms of the polymer films for anion doping and dedoping, an anodic peak potential and a cathodic one (E_pc.) shifted to the positive direction as the number m of thienylene units decreased. E_pcs at a sweep rate of 100 mV s^?1 were about 0.8, 0.9 and 1.0V for DS5T, DS4T and DS3T respectively. Reversible electrochemical doping and dedoping of the DS5T film were feasible when the potential was cycled between 0 and 1.2 V. At potentials more positive than 1.2 V, however, both overoxidation of the oligo(thienylene) unit and Si-Si bond cleavage took place, leading to decreases in conductivity and work function of the film.     

A new approach to metal electrodeposition at a periodically changing rate: Part I. The reversing overpotential method

A method of increasing the apparent deposition exchange current density value in reversing overpotential (RO) copper electrodeposition is described. In pulsating overpotential (PO) metal electrodeposition, the current density during the ‘off’ period is anodic, and proportional to the exchange current density value. The current during the ‘off’ period in PO copper electrodeposition can be increased by appropriate anodic polarization. This method is referred to as the RO method to distinguish it from the pulsating method. In order to maintain the average current density in this case, it is necessary to increase the cathodic current density during the ‘on’ period. Hence, the current density during the anodic period can be considered as proportional to the exchange current density for some other deposition process, characterized by a larger exchange current density value. The apparent deposition process exchange current density can be increased easily by using different current densities during the anodic period, and different morphologies for the same average current density can be expected. In this way, previously unknown morphologies of copper electrodeposits in the overpotential region used were obtained as spongy and needle-like forms.     

Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (Lindane) at carbon cathodes in dimethylformamide

Direct reduction of Lindane (1R,2r,3S,4R,5r,6S-hexachlorocyclohexane, 1) at carbon cathodes in dimethylformamide (DMF) containing 0.10 M tetra-n-butylammonium tetrafluoroborate (TBABF₄) has been explored by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1 at a glassy carbon electrode exhibit two cathodic peaks at −1.40 V and −2.10 V as well as an anodic peak at −1.93 V; the first cathodic peak is attributed to reduction of 1 itself, whereas the second cathodic peak is due to reduction of chlorobenzene that is derived from 1. Controlled-potential (bulk) electrolyses conducted with reticulated vitreous carbon electrodes held at −1.70 or −2.20 V reveal that reduction of 1 is essentially a six-electron process that affords benzene as major product (80–100% yield) along with small amounts of chlorobenzene (3–10% yield). To account for these products, a mechanism is proposed that is supported by the results of theoretical computations based on density functional theory.     

Uncovering the influence of antioxidants on polyphenol oxidation in wines using an electrochemical method: Cyclic voltammetry

The influence of sulfur dioxide, glutathione and ascorbic acid on the cyclic voltammograms of four representative wine polyphenols (catechin, caffeic acid, rutin and quercetin) and five different wines was investigated using a glassy carbon electrode in a model wine solution. Sulfur dioxide increased the anodic current and decreased the cathodic current for all four polyphenols and all wines, pointing to a rapid interaction of SO₂ with the oxidized polyphenol quinones. A similar trend was seen for glutathione, except that in the case of quercetin, addition of glutathione led to the formation of a second set of voltammetric peaks, corresponding to redox activity of a glutathione derivative. However, ascorbic acid produced no additional effect on the cyclic voltammograms of wine polyphenols and wines, beyond that expected for a simple sum of the polyphenol and ascorbic acid responses, with the exception that adsorption of quercetin and rutin on the carbon electrode caused a shift in the ascorbic acid oxidation peak to more positive potentials.     

Microenvironment effects on the electrochemical and photoelectrochemical properties of thionine loaded Nafion® films

The polarity of the microenvironments within a Nafion^® (Nf) film was studied by electrochemical and photoelectrochemical techniques using a phenothiazine dye, thionine (TH) as a probe. When tetrabutylammonium ion-exchanged Nf film (TBA-Nf) is immersed in TH solution, TH partly displaces TBA ions into hydrophobic regions and binds to the less polar ionic cluster region of the Nf film. Although TH is able to enter the TBA-Nf film, the amount of TH incorporated in the TBA-Nf film is lower than the amount of TH incorporated in the Nf film. This is because Nf shows higher selectivity for TBA ions over TH. TH loaded into TBA-Nf film (TBA-Nf/TH) shows a 50 mV negative shift in the E_1/2 value and most of the exchanged TH becomes electroinactive. The observed electrochemical properties of the TBA-Nf/TH film could be attributed to the less polar environment experienced by TH molecules in the dehydrated TBA-Nf film. In the dehydrated TBA-Nf/TH film, the movement of protons is hampered by the lack of water molecules. However, when the TBA-Nf/TH film is immersed in supporting electrolyte solution for long times, the dehydrated TBA-Nf/TH film becomes hydrated due to the entry of ions and water molecules into the TBA-Nf/TH film, which leads to a positive shift in the E_1/2 value with increased electroactive TH species. The less polar environment experienced by TH in TBA-Nf film was also reflected in the photogalvanic cell measurements. In the photogalvanic cell, when the Nf/TH film was exposed to visible light, a cathodic photocurrent was observed whereas an anodic photocurrent was observed for the TBA-Nf/TH film. The observed difference in the polarity of the photocurrent again reiterates the polar and less polar environments experienced by TH in Nf and TBA-Nf films, respectively. The polarity of the photocurrent changes from anodic to cathodic when the TBA-Nf/TH film is soaked in the photogalvanic cell solution for longer times.     

Analysis of the impedance spectra of Pt|poly(o-phenylenediamine) electrodes – hydrogen adsorption and the brush model of the polymer films

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films on platinum in contact with 0.5 M H₂SO₄ solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.     

Electrogenerated chemiluminescence of CdSe hollow spherical assemblies in aqueous system by immobilization in carbon paste

Electrogenerated chemiluminescence (ECL) of CdSe hollow spherical assemblies (HSAs) was studied in aqueous system by entrapping them in carbon paste. In air-saturated strong alkaline solution three ECL processes were obtained at ?1.2, +1.34 and +1.75 V versus Ag/AgCl without adding any special coreactants. The different ECL processes resulted from the electron transfer reactions between positively or negatively charged CdSe HSAs and some electrogenerated species. Selective quench of the anodic and cathodic ECL processes were achieved by controlling the solution pH value or the concentration of dissolved oxygen. Comparing the ECL processes of the entrapped CdSe nanocrystals with the entrapped HSAs, the architecture of the HSAs showed important effects on their ECL behaviors. The ECL features of HSAs entrapped in carbon paste depended on the applied potentials. CdSe HSAs could be developed as a new ECL system in aqueous system with some super points.     

Formation of ferryl porphyrin by electrochemical reduction of iron porphyrin in aqueous solution

The formation of ferryl porphyrin, induced by electrochemical reduction of Fe^IIITMPyP, was examined by measuring luminol chemiluminescence and recording the absorption spectrum using a flow-injection method. Both the anodic oxidation and the cathodic reduction of iron(III) porphyrin induced the formation of a ferryl species, which is widely accepted to be responsible for stimulating luminol chemiluminescence. Emission was observed both below the reduction potential of Fe^IIITMPyP (?0.08 V at pH 11, ?0.02 V at pH 7 and 0.15 V at pH 3) and above the oxidation potential (0.6 V at pH 11, 0.75 V at pH 7 and 1.1 V at pH 3). The emission was relatively greater below the reduction potential than in the oxidative region. It was significantly higher especially at lower pH (<pH 4). The cathodic emission was largely dependent on the concentration of dissolved molecular oxygen while the anodic emission was not. Emission on both sides was inhibited significantly by the addition of alkene solutions downstream of the working electrode. The spectra on both the positive and negative potential sides were shifted to longer wavelengths (>424 nm) compared to the original spectrum of Fe^IIITMPyP (422 nm). Computer analysis also showed that the spectra on both sides were perfectly reproduced by a combination of two spectra, those of Fe^IIITMPyP and OFe^IVTMPyP (436 nm in pH 11). We therefore believed that the ferryl species, formed by the autoxidation reaction of electro-generated ferrous porphyrin, was responsible for the emission. Although, ferryl formation by anodic oxidation has been reported, this is the first time, to our knowledge, that the cathodic formation has been reported.     

Voltammetric investigation of new polythiophene derivatives possessing electrochemically cleavable arylsulfonamide groups as precursors for solid phase electrosynthesis

New thiophene derivatives substituted by electrochemically cleavable sulfonamide groups have been synthesised and their electrochemical behaviour has been investigated in acetonitrile. The anodic oxidation of phenyl (I)- and naphthyl (II)sulfonamide-substituted thiophenes yielded corresponding polymer films exhibiting p- and n-doping processes. The cathodic cleavage of the SN bond grafted to the polymer backbone was performed by applying a potential lower than ?2.5 V (versus Ag ∣ 10^?1 M Ag⁺) to the polymer. The efficiency of this solid-state electrochemical reaction was found to be strongly lowered when the film thickness was increased. It was demonstrated that the decrease of this parameter was less important when a copolymer electroformed from II or a phenylsulfonamide-substituted terthiophene (III) was involved. The recycling properties of this type of material were illustrated by the grafting of an aminoferrocene moiety after the cathodic cleavage step to poly(II-co-3-methylthiophene).     

Characterization of anodic tin passive films formed in citrate buffer solutions

Voltammetry, chronoamperometry and SEM were employed to derive mechanistic information about the electroreduction processes that take place after cathodic polarization of a previously oxidized tin surface. This information proved useful to gain a deeper insight into the complex passivation process as well as into the passive layer changes generated by the anodizing procedure. Tin electrodes were anodized in citric–citrate buffers with pH 5. Three cathodic contributions were detected in the voltammetric response and they were correlated with three maxima present in the electroreduction current transients. Two electroreduction processes correspond to the electroreduction of tin surface species with different oxidation state and generated during the anodic potential sweep within the secondary passivity range. From these two processes only the electroreduction of the Sn(IV)-containing anodic film is dependent on anodizing time and hydrodynamic conditions of the electrolyte. Synchronized with the latter reaction occurs the electrodeposition of tin metal from a Sn(IV)-complex in solution.     

Observations on the direct electrochemistry of bovine copper-zinc superoxide dismutase

The cyclic voltammetric behavior of holo-superoxide dismutase (SOD) at pH 4.0 consists of an anodic peak at +0.270 V/SCE with no evidence of a cathodic feature on the forward (reduction) scan. Chronocoulometric data suggest that the reduced enzyme is strongly adsorbed in a close-packed arrangement. Background-subtracted differential pulse voltammograms yield poorly defined cathodic and anodic peaks at pH 5.0–7.0. Chemically reduced holo-SOD solutions exhibit markedly improved differential pulse voltammetric characteristics (especially the anodic), thus suggesting a conformational change occurring, possibly at the active site, consequent to electrochemical/chemical reduction that, in turn, enhances accessibility of the electrode to the active site.