Solvent effects on electrochemical behavior of poly(ferrocenylsilane) films

The electrochemical behaviors of poly(ferrocenylsilane) (PFS) films in organic solutions were investigated by means of cyclic voltammetry (CV) and electrochemical quartz crystals microbalance (EQCM). The influences of solvent on the electrochemical behavior of the films were discussed. In “good” solvents, the supporting electrolytes dissociated completely, the films were solvent-swollen moderately which provided a favorable condition for the electrolyte ions penetrating through, and the CV behavior of the films exhibited reversible or nearly reversible features. With increasing the carbon chain length of solvent molecule, however, the polarity of solvent reduced, which conduced to decrease the dissociation of electrolyte and the swelling of the polymer film. The efficiency of electrochemical reaction in the film was depressed, and the CV behavior of the film exhibited low reversibility. The solvent effects on the oxidation process of films exhibited more noticeable than the reduction process. The results supported the viewpoint that penetration of the electrolyte anions played an important role on the charge balance and transfer in the films during the CV process. The different electrochemical behaviors of the two PFS films in various organic solutions indicated that molecular structure of polymer had important influence on the electrochemical properties of the PFS.     

Towards TiO2-conducting polymer hybrid materials for lithium ion batteries

Nanocomposites of TiO₂ (anatase) with polypyrrole (Ppy) or poly(3,4-ethylenedioxythiophene) (PEDOT) were prepared via electrochemical routes. The deposition process of the conducting polymer films was performed in the presence of perchlorate, p-toluenesulphonate (TOS) or bis(trifluoromethylsulphonyl)imide (TFSI) anions in propylene carbonate (PC). The obtained electrode materials were characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). An improvement of lithium ion intercalation/de-intercalation properties of titanium(IV) oxide as a result of its interfacing with the polymers is evidenced. This effect was strongly dependent on the thickness of the polymer layer and closely related to the polymer facility for transporting of lithium ion. Polypyrrole properties, in contrast to the PEDOT case, are very sensitive to selection of the substrate material (Pt or Pt/TiO₂) during electropolymerization. Polypyrrole deposited on a rough surface exhibits an improvement in its ion exchange abilities. The impact of underlying TiO₂ layers on Ppy properties has an indirect (synergic) influence on the effectiveness of lithium ion intercalation into the oxide too. The properties of the composites were discussed also in view of the comparative electrochemical quartz crystal microbalance (EQCM) study focussing on ion transport properties of Ppy and PEDOT.     

The role of ion and solvent transport during the redox process of conducting polymers

The understanding of the redox behavior of conducting polymers is essential for a successful application of these so-called synthetic metals as functional coatings. The redox process involves the exchange of ions and solvent molecules. This so called doping/dedoping process involves changes of the mechanical and the electronic structure of the polymer. This paper discusses investigations at poly(3,4-ethylenedioxythiophene (PEDOT) and poly(pyrrole) (Ppy) by the electrochemical quartz crystal microbalance (EQCM) technique and electrochemical impedance spectroscopy (EIS). In the case of PEDOT a determination of the anion and the solvent fluxes was possible, and it was found that most anions replace solvent molecules upon their incorporation. The doping/dedoping mechanism of Ppy is more complicated. Here, the first redox cycles are characterized by a complex interplay of cation, anion and solvent fluxes with irreversible changes of the polymer structure. However, in combination with EIS new insights of the ion and solvent exchange and its influence on the electronic properties can be achieved.     

On the p-doping of PEDOT layers in various ionic liquids studied by EQCM and acoustic impedance

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been polymerized in several ionic liquids on the polycrystalline gold electrode of an electrochemical quartz crystal microbalance (EQCM) at 25 °C and 85 °C. Under cyclic potential variation, the EQCM resonance frequency decreased in the anodic potential region, indicating that the p-doping process is accompanied by the incorporation of anions. Elastic shear moduli G′, G″ - calculated from acoustic impedance measurements - were about 10⁷ Pa, values that are 2 orders of magnitude higher than for PEDOT polymerized in acetonitrile solutions. This difference is explained by the stiffening of the film by incorporated charged species because of the absence of a neutral molecular solvent plasticizing the film.     

Investigation of electrochemical behavior of Mn–Co doped oxide electrodes for electrochemical capacitors

The electrochemical properties of nanocrystalline Co-doped Mn oxide electrodes were investigated to determine the relationship between physicochemical feature evolution and the corresponding electrochemical behavior of synthesized electrodes. Co-doped Mn oxide electrodes with a rod-like morphology and antifluorite-type structure were synthesized by anodic electrodeposition on Au coated Si substrates from a dilute solution of 0.01 M Mn acetate (Mn(CH₃COO)₂) and 0.001 M Co sulphate (CoSO₄).Electrochemical characterization of synthesized electrodes, with and without a conducting polymer (PEDOT) coating, was performed with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) at different scan rates. In addition, structural characterization of as-deposited and cycled electrodes was conducted using SEM, TEM and XPS.Capacitance values for all deposits increased with increasing scan rate to 100 mV s⁻¹, and then decreased after 100 mV s⁻¹. The Mn–Co oxide/PEDOT electrodes showed improved specific capacity and electrochemical cyclability relative to uncoated Mn–Co oxides. Mn–Co oxide/PEDOT electrodes with rod-like structures had high capacitances (up to 310 F g⁻¹) at a scan rate of 100 mV s⁻¹ and maintained their capacitance after 500 cycles in 0.5 M Na₂SO₄ (91% retention). Capacitance reduction for the deposits was mainly due to the loss of Mn ions by dissolution in the electrolyte solution.     

Correlation between conductance and mass changes during p-doping of 2-methoxynaphthalene films

The redox response of 2-methoxynaphthalene films electrosynthesized in two different organic solvents, acetonitrile (ACN) and nitrobenzene (NB) has been studied by different in situ electrochemical techniques: the in situ conductance technique, electrochemical quartz crystal microbalance (EQCM) and electrochemical voltage spectroscopy (EVS). In situ measurements of conductance as a function of the potential during p-doping of 2-methoxynaphthalene films, electrosynthesized in TBAPF₆-ACN and TBAPF₆-NB, show that the conductance properties are strongly dependent on the solvent used during electrosynthesis, resulting in higher conductance values for films electrosynthesized in NB solutions. The EQCM technique has been used to correlate the frequency changes (mass changes) at the electrode surface with conductance changes during p-doping of the different films. The molar mass of the species involved in the charging-discharging reactions has been estimated from the EQCM results. For the determination of the electrochemical bandgap of the two different films, cyclic voltammetry (CV) and EVS were applied. Films electrosynthesized in NB solutions have a lower value of the bandgap (1.34 eV) than films electrosynthesized in ACN solutions (2.00 eV).     

Electrochemical behaviors of poly(ferrocenylsilane) solutions

The electrochemical behaviors of two kinds of poly(ferrocenylsilanes) (PFS) with different substituent groups in CH₂Cl₂ solutions were investigated by means of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance. The results demonstrated that the CV processes of the PFS on the glass carbon electrode surface in CH₂Cl₂ solutions are the diffusion‐controlled reversible processes. The oxidative state of PFS forms an adsorption layer on the electrode surface during the oxidation process. The interaction of active ferrocene centers along the main chain induces the stepwise redox process, and makes the CV waves of the PFS solutions present two couples of peaks. The diffusion coefficients of PFS in CH₂Cl₂ solution are much larger than those of PFS in films. The different electrochemical behaviors of the two PFS solutions indicated that the molecular structure of polymer has influence on the electrochemical properties of the PFS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 789–794, 2007     

A comparison of charge transport behavior in functionalized and non-functionalized poly 3,4-(ethylenedioxythiophene) films

Poly 3,4-(ethylenedioxythiophene) (PEDOT) films electropolymerized from an aqueous micellar solution containing sodium dioctyl sulfosuccinate and the monomer were functionalized with 1-fluoro-2-nitro-4-azidobenzene (FNAB) molecules by a photochemical nitrene insertion reaction. The variation in redox activity and the changes in the charge transfer and diffusion (through bulk) behavior of the functionalized and the non-functionalized PEDOT films have been followed by electrochemical impedance spectroscopy and cyclic voltammetry. While the functionalized film allows a reversible insertion and extraction of guest cations and anions, the non-functionalized film is capable of exchanging only anions. The higher level of oxidation attained in the functionalized film is also reflected in the longer diffusion length (l_D) observed for the ions in this film. In both films the barrier to charge transfer is resistive rather than capacitive. Both charge transfer and diffusion resistance (R_CT and R_D) are lower for the functionalized film, a consequence of a higher surface roughness and a more nodular morphology and therefore higher optical contrast and faster color-bleach kinetics are achieved in this film. For the functionalized and the non-functionalized films, both R_CT and R_D are greatly enhanced during reduction than for oxidation. In particular, in the low frequency regime, the hindered diffusion-controlled extraction of anions from the bulk of the film is also evident from the larger R_D as compared to R_CT and the difference in their magnitudes is more pronounced for the functionalized film thus confirming that functionalization is a useful method for controlling the redox response of conducting polymer films.     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

Synthesis and electro-optical properties of a new copolymer based on EDOT and carbazole

A new copolymer of 3,4-ethylenedioxythiophene (EDOT) and 5-(2-ethylhexyl)-1,3-bis(9-methyl-9H-carbazol-3-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione (CzPDICz) was electrochemically synthesized using different monomer feed ratios. The resulting copolymer films were investigated in terms of their electrochemical and electro-optical behaviors. Properties of the obtained copolymer films through different monomer feed ratios were compared to each other and to individual poly(ethylenedioxythiophene; PEDOT) and homopolymer of CzPDICz in order to observe the differences in the properties with respect to PEDOT and P(CzPDICz). Copolymers exhibited well adherence on the electrode surface with having non-diffusional redox process. The monomer feed ratios were prepared as 9:1; 4:1, and 1:1 (EDOT:CzPDICz) and changes in the electrochemical and spectroelectrochemical behavior were noted with increasing CzPDICz ratio in the monomer mixture. Although no appreciable change in the optical band gap values of the copolymers was noted as compared to PEDOT, the neutral blue copolymers exhibited grayish color in their semi-oxidized states and transparent green in their fully oxidized states.     

Structural, chemical and electrochemical characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) prepared with various counter-ions and heat treatments

Electrochemical deposition of the conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) forms thin, conductive films that are especially suitable for charge transfer at the tissue-electrode interface of neural implants. For this study, the effects of counter-ion choice and annealing parameters on the electrical and structural properties of PEDOT were investigated. Films were polymerized with various organic and inorganic counter-ions. Studies of crystalline order were conducted via X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to investigate the electrical properties of these films. X-ray photoelectron spectroscopy (XPS) was used to investigate surface chemistry of PEDOT films. The results of XRD experiments showed that films polymerized with certain small counter-ions have a regular structure with strong (100) edge-to-edge correlations of PEDOT chains at ∼1.3 nm. After annealing at 170 °C for 1 h, the XRD peaks attributed to PEDOT disappeared. PEDOT polymerized with LiClO₄ as a counter-ion showed improved impedance and charge storage capacity after annealing at 160 °C.     

Electrochemical immobilization of ascorbate oxidase in poly(3,4‐ethylenedioxythiophene)/multiwalled carbon nanotubes composite films

Immobilization of ascorbate oxidase (AO) in poly(3,4‐ethylenedioxythiophene) (PEDOT)/multiwalled carbon nanotubes (MWCNTs) composite films was achieved by one‐step electrochemical polymerization. The PEDOT/MWCNTs/AO modified electrode was fabricated by the entrapment of enzyme in conducting matrices during electrochemical polymerization. The PEDOT/MWCNTs modified electrodes were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The experimental results showed that the composite films exhibited better mechanical integrity, electrochemical activity, higher electronic and ionic conductivity, and larger redox capacitance compared with pure PEDOT films, which would be beneficial to the fabrication of PEDOT/MWCNTs/AO electrochemical biosensors. The scanning electron microscopy studies revealed that MWCNTs served as backbone for 3,4‐ethylenedioxythiophene (EDOT) electropolymerization. Furthermore, the resulting enzyme electrode could be used to determine L ‐ascorbic acid successfully, which demonstrated the good bioelectrochemical catalytic activity of the immobilized AO. The results indicated that the PEDOT/MWCNTs composite are a good candidate material for the immobilization of AO in the fabrication of enzyme‐based biosensor. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Electrochemical properties of benzylmercapto and dodecylmercapto tetra substituted nickel phthalocyanine complexes: Electrocatalytic oxidation of nitrite

Nickel tetrakis(benzylmercapto)phthalocyanine (NiTBMPc) and nickel tetrakis(dodecylmercapto)phthalocyanine (NiTDMPc) complexes were synthesized and their spectral and electrochemical properties reported. The CV showed four or five redox processes for NiTBMPc and NiTDMPc, respectively. For the first time, spectroelectrochemistry gave evidence for the formation of Ni^II/Ni^I process in a NiPc complex. The rest of the processes were ring based. The NiTBMPc complex was successfully deposited on both gold and glassy carbon electrodes by electropolymerisation while NiTDMPc complex was deposited on gold electrode only. The films were electro-transformed in aqueous 0.1 M NaOH solution to the O–Ni–O oxo bridged form. The modified electrodes were characterized using electrochemical impedance spectroscopy and the results showed typical behavior for modified electrodes. Electrodes with poly-Ni(OH)Pcs films exhibited higher charge transfer resistance values, R_p than their corresponding poly-NiPcs films counterparts. All the modified electrodes showed improved catalytic activities than the unmodified electrodes towards nitrite ions electrooxidation. Better catalytic activities were observed for the modified electrodes when they were transformed to O–Ni–O oxo bridge form. All the modified electrodes exhibited high resistance to electrode surface passivation.     

In-situ QCM-D analysis reveals four distinct stages during vapour phase polymerisation of PEDOT thin films

Gaining a deeper understanding of the growth of poly(3,4-ethylenedioxythiophene) (PEDOT) films by vapour phase polymerisation (VPP) is essential for the rational design and optimization of such films. The VPP process was used to synthesise films of PEDOT on oxidant-coated substrates. Atomic force microscopy images showed that the morphology of the films changed considerably with time. Utilising a quartz crystal microbalance with dissipation measurement (QCM-D), we found that the kinetics of polymerisation and the viscoelastic properties of the films varied. The data reveal four distinct stages in film growth. Each stage produces a layer having different conductivity values, from a low of 276 S cm⁻¹ to a high of 1196 S cm⁻¹. Conductivity and electrochromic optical contrast, Δ%Tx, can thus be maximized by appropriate termination of the polymerisation reaction. Factors determining the polymerisation rate and changes in conductivity and optical performance are discussed.     

Electrosyntheses and characterizations of novel electrochromic copolymers based on pyrene and 3,4-ethylenedioxythiophene

The copolymers based on pyrene and 3,4-ethylenedioxythiophene (EDOT) are electrochemically synthesized in acetonitrile (ACN) containing tetrabutylammonium perchlorate (TBAP) via the direct oxidation of pyrene/EDOT mixtures. FT-IR and ¹H NMR characterizations confirm that the obtained polymers contain both pyrene and EDOT units. Elemental analyses results show that the ratio of pyrene/EDOT units in polymers decreases as the feed ratio of pyrene/EDOT decreases in ACN. The effects of pyrene/EDOT feed ratio and polymerization potential on the electrochemical properties of the obtained copolymers are studied by cyclic voltammetry (CV). Compared with polypyrene, the copolymers perform reversible redox process, and their UV–vis absorption peaks exhibit obvious red-shift. It is interesting that the obtained copolymer films present a property of tunable electrochromism, smooth morphology and good thermal stability.     

Electro-reduction of diazonium salts on gold: Why do we observe multi-peaks?

Multi-peaks have been observed when reducing diazonium salts by cyclic voltammetry (CV). In order to explain that behavior, we studied the electrochemical properties of a fluorinated diazonium salt that exhibits a strongly passivating behavior on various micro-crystalline gold electrodes. We propose here that each sub-peak observed in CV corresponds to the reduction of the compound on a distinct crystallographic site.     

Improved capacitive behavior of electrochemically synthesized Mn oxide/PEDOT electrodes utilized as electrochemical capacitors

A sequential synthetic approach and a one-step method were adopted to synthesize Mn oxide/PEDOT electrodes through anodic deposition on Au coated Si substrates from aqueous solutions. In the former case, free standing Mn oxide rods (about 10 μm long and less than 1.5 μm in diameter) were first synthesized without a template through anodic deposition from a dilute solution of Mn acetate, then coated by electro-polymerization of a conducting polymer (PEDOT) giving coaxial rods. The one-step, co-electrodeposition method produced agglomerated Mn oxide/PEDOT particles. The electrochemical behavior of the deposits depended on the morphology and crystal structure of the fabricated electrodes, which were affected by the pH of electrolyte, deposition potential, current density and polymer deposition time. Structural characterization of as-deposited and cycled electrodes was conducted using XPS, SEM, TEM and AES.The Mn oxide/PEDOT coaxial core/shell electrodes prepared by the sequential method showed significantly better specific capacity and redox performance properties relative to both uncoated Mn oxide rods and co-electrodeposited Mn oxide/PEDOT electrodes. The best specific capacitance for Mn oxide/PEDOT rods produced sequentially was ∼285 F g⁻¹ with ∼92% retention after 250 cycles in 0.5 M Na₂SO₄ at 20 mV s⁻¹.     

Preparation and characterisation of poly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedioxythiophene)/poly(neutral red) modified carbon film electrodes, and application as sensors for hydrogen peroxide

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been prepared for the first time on carbon-film electrodes (CFE) in aqueous solution using electropolymerisation by potential cycling, potentiostatically and galavanostatically. Characterisation of the modified electrodes was done by cyclic voltammetry and electrochemical impedance spectroscopy and the stability of the polymer films was probed. The coated electrodes were tested for application as hydrogen peroxide sensors, by oxidation and reduction. A novel polymer film was also formed by modification of CFE by co-electropolymerisation of EDOT and the phenazine dye neutral red (NR) – (PEDOT/PNR) with a view to enhancing the properties for sensor applications. It was found that hydrogen peroxide reduction at the PEDOT/PNR coated electrodes could be carried out at a less negative potential, the sensor performance comparing very favourably with that of other polymer-modified electrodes reported in the literature.     

Interaction between myoglobin and hyaluronic acid in layer-by-layer structures—An electrochemical study

The layer-by-layer (LBL) self-assembled film construction of the biocompatible polymer hyaluronic acid (HA) and single heme redox protein, myoglobin (Mb) is described. The films were built upon gold electrode substrates, both gold quartz crystal electrodes and bulk gold (Au(bulk)) electrodes, and formation of the LBL films was gravimetrically monitored by an electrochemical quartz crystal microbalance. The electrochemical properties of the hyaluronic acid/myoglobin films ({HA/Mb}_n) were investigated after each deposition step using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV response presented an oxidation peak at +0.3 V vs. SCE, not characteristic of the redox protein myoglobin, and, the peak current decreased slightly with each additional bilayer. CV at Au(bulk) electrodes in pH 5.0 acetate buffer solution, containing Mb, presented the same oxidation peak as observed at {HA/Mb}_n modified electrodes, confirming the presence of the same electroactive species. The Mb oxidation peak current depends linearly on scan rate, characteristic of adsorbed thin-layer electrochemical systems, attributed to free adsorbed heme. Impedance spectra, recorded after deposition of each bilayer, were in agreement with the cyclic voltammetry observations.     

Electropolymerization of a poly(3,4-ethylenedioxythiophene) and functionalized, multi-walled, carbon nanotubes counter electrode for dye-sensitized solar cells and characterization of its performance

Composite films of poly(3,4-ethylenedioxythiophene) and functionalized, multi-walled, carbon nanotubes (PEDOT–MWCNT) were fabricated by a simple oxidative electropolymerization method. These films were formed on fluorine-doped, tin oxide, glass substrates as counter electrodes (CEs) of platinum-free, dye-sensitized solar cells (DSSCs). The surface morphology, formation mechanism and electrochemical nature of PEDOT–MWCNT films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), cyclic voltammetry (CV) and alternating current (AC) impedance spectroscopy. The SEM and AFM images showed that PEDOT–MWCNT films were more porous than PEDOT films. CV and AC impedance spectroscopy revealed that the PEDOT–MWCNT electrode had higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction and a smaller charge transfer resistance than the PEDOT electrodes. The energy conversion efficiency of the DSSC with a PEDOT–MWCNT CE was 13.0% higher than with a PEDOT CE using the same conditions with a ruthenium sensitizer.