Enhancement of the electrical conductivity of polypyrrole films on applying higher voltage

The effect of voltage on the conductivity of polypyrrole films has been investigated. It is found that the conductivity of the films increases with some structural change after the applied voltage reaches a higher value. The degree and the stability of the conductivity enhancement are related to the nature of the counter-anions doped in the polypyrrole films. A 65% increase in conductivity of the film doped with TsO⁻ is observed on applying higher voltage, and it remains stable after the treatment. The conductivity of the film doped with NO₃⁻ increases 47% on applying higher voltage, but it drops back to its original value quickly after the higher voltage is turned off. The structural change of PPy(TsO⁻) is studied through Vis and NIR absorption spectra.     

Optical characterization of polypyrrole-polytungstate anion composite films

The polypyrrole (PPy)-polytungstate anion (H₂W₁₂O₄₀^6-, PW) composite film formed by electrochemical polymerization was studied by two in situ optical methods: absorption spectroscopy and Raman spectroscopy. The PPy doped by chloride anions was also studied as a comparison. The potential dependence of the absorption band and the Raman shift indicates that the PPy-Cl film reduces in turn from the bipolaron-loaded PPy to the polaron-loaded PPy, and further to the neutral PPy with decrease of potential. The PPy-PW film does not, however, reach the neutral PPy even at a relatively low potential, where the reduction of tungsten ions of W(V1) to W(V) in the PW preferentially takes place.     

Ion exchange behaviour and charge compensation mechanism of polypyrrole in electrolytes containing mono-, di- and trivalent metal ions

The electrochemical quartz crystal microbalance technique was used to examine the ion exchange behaviour and charge compensation mechanism of polypyrrole (PPy) doped with polystyrene sulfonate (PSS) immersed in electrolytes containing singly, doubly and triply charged metal ions. New insights were obtained concerning the mechanism of charge compensation and ion exchange behaviour of PPy/PSS films in electrolytes with triply and doubly charged transition metal ions. Understanding the ion exchange behaviour of conducting polymers immersed in these types of aqueous media could play a pivotal part in the development of methods for removing toxic metals from water supplies. Charge compensation occurred predominantly by means of cation movement for PPy/PSS films when the electrolyte was KNO₃, Ca(NO₃)₂, Mn(NO₃)₂ or Co(NO₃)₂. After prolonged redox cycling the electroactivity of the polymer decreases slightly, and the charge compensation mechanism becomes complex owing to movement of ions and neutral species in opposite directions. The charge compensation mechanism for PPy/PSS films immersed in aqueous Cr(NO₃)₃ and Al(NO₃)₃ solutions was also complex, with the ion exchange behaviour shifting towards anion movement to and from the polymer.     

On the porosity of polypyrrole films

Polypyrrole (PPy) films doped with anions of various size, charge and chemical nature (inorganic, surfactant, with aromatic ring) were electrochemically synthesized and investigated by low-temperature N₂ sorption experiments at −196 °C. The specific surface area, total pore volume, average pore radius, pore size distribution and other parameters for oxidized PPy films using dodecylsulfate, 2-naphthalene sulfonate, 1,5-naphthalene disulfonate, poly(4-styrenesulfonate), tosylate, perchlorate, nitrate and chloride as dopant ions, were calculated. The obtained data show that although the average pore radius of investigated mesoporous PPy films (17–19 Å) is practically independent of the dopant anion used, however the latter determines the total pore volume and specific surface area values in different PPy materials investigated. As the total pore volumes for PPy films doped with large amphiphilic anions show the smallest values, the corresponding values for PPy/small inorganic anions, are up to 2 times higher.     

Polypyrrole film on 55% Al–Zn-coated steel for corrosion prevention

For corrosion protection of 55% Al–Zn-coated steel, a dense polypyrrole (PPy) film is electrochemically formed on 55% Al–Zn-coated steel in an acidic tartrate solution under constant current control. The film potentially consists of an inner layer of aluminium and/or zinc oxide and an outer PPy layer doped with tartrate anions. The PPy layer can maintain passivation of 55% Al–Zn-coated steel in a 3.5 wt.% NaCl aqueous solution and protected the steel for several hours. The doping of molybdate anions into the PPy–tartrate film greatly improved the film’s protective properties.     

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E_h = α + β log t. The rate of oxide film thickening in the respective solutions was in the order Na₃PO₄ > Na₂SO₄ > NaCl > NaNO₃. The equilibrium potential was found to be independent of anion concentration in the case of Cl^- and NO^-₃ and was slightly dependent on the PO^3-₄ and SO^2-₄ concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.     

Influence of dopant ions on properties of conducting polypyrrole and its electrocatalytic activity towards methanol oxidation

The polypyrrole (PPy) films were deposited on vacuum metallized substrates by electro-oxidation of pyrrole monomer. These electrodes were then modified with a range of metal halides having different electronegativities. The modified polypyrrole electrodes were employed to investigate electrocatalytic activity towards methanol electrochemical oxidation by means of cyclic voltammetry in 0.1 M HClO₄ as supporting electrolyte. It was found that the electronegativity of the dopant ion incorporated in the PPy film governs the electrocatalytic activity towards methanol oxidation. Among different dopant anions used in the present work, the PPy doped with zirconium chloride gave the highest anodic current of 10 mA cm⁻² at the oxidation potential of methanol. Electrical property and the charge created due to doping in the polymers were measured using X-ray photoelectron spectroscopy (XPS) and Electron spin resonance spectroscopy (ESR). Electrocatalytic activity of the modified electrodes was correlated with various factors obtained from different polymer characterization experiments. The results were explained on the basis of the charge-transfer efficiency at the electrodeelectrolyte interface, which was associated with the acceptor state created by the dopant in the semi-conducting polymer.     

Doping mechanism of PPy/LiDS film in aqueous solutions

We studied electrochemical redox reaction of polypyrrole (PPy) films prepared in an aqueous lithium dodecylsulfate (LiDS) solution by means of cyclic voltammetry, Auger electron spectroscopy, electrochemical quartz crystal microbalance and atomic force microscope, and presented a new doping–dedoping mechanism of PPy/LiDS film associated with the movement of DS⁻. During a potentiodynamic polymerization of PPy/LiDS, an additional small cathodic peak appeared at −0.37 V in addition to the main redox peaks at −0.81 V. PPy/LiDS film prepared under a galvanostatic condition also showed the additional cathodic peak, too. EQCM results indicated that the main redox peaks were due to the fast movement of small Li⁺ while the additional peaks were associated with the movement of DS⁻ despite of its large molecular size. In potentiostatic condition the movement of DS⁻ could be found even in negative potential region (−0.8 V) where the movement of Li⁺ is dominant for potentiodynamic condition. Thus, both cations and anions were involved in the redox process of PPy/LiDS film but the mobility of DS⁻ was controlled by scan rate of potential due to its bulkiness.     

Reticulated vitreous carbon/polypyrrole composites as electrodes for lithium batteries: Preparation,electrochemical characterization and charge–discharge performance

Polypyrrole (PPy) composites were prepared via electrochemical deposition onto reticulated vitreous carbon (RVC) – a tridimensional material – and tested as cathodes in lithium batteries. Different RVC/polypyrrole electrodes were prepared and characterized: PPy doped with ClO₄^? (RVC/PPy) as an anion-exchanger film; PPy doped with a large anion – poly(styrenesulphonate) – (RVC/PPyPSS) as a cation-exchanger film; and a bi-layer polymeric film formed by an inner layer of the former and an outer layer of the latter (RVC/PPy/PPyPSS). Photomicrographies, voltammetric profiles and electrochemical impedance spectroscopy data have shown different morphological and electrochemical characteristics for polypyrrole doped with small or large anions, and a peculiar behavior for the bi-layer electrodes. Charge/discharge results indicated that the RVC/PPy electrode can attain specific capacities as high as 95 mAh g^?1. PSS-doped polypyrrole electrodes also presented high specific-capacity values when the bi-layer configuration (RVC/PPy/PPyPSS) and lower charge–discharge currents were used. Impedance data indicated that the counter-ion diffusion within the bi-layer is hindered, which explains the slower activation of this composite when compared to that of RVC/PPy.     

Effect of various parameters on the conductivity of free standing electrosynthesized polypyrrole films

Electrical characteristics of polypyrrole films electrodeposited in different aqueous electrolyte solutions including p-toluenesulfonate, naphtalenesulfonate, nitrate, tetrafluoroborate, and perchlorate anions were investigated using the Van der Pauw procedure. The polymer films were synthesized by electrochemical oxidation at a fixed potential. Experimental parameters including the pyrrole concentration, electrolyte, applied potential and substrate were shown to affect the electrical conductivity σ of polypyrrole films. Since the substrate contributes significantly to the overall conductivity of polypyrrole-coated electrodes, the results obtained with free standing polymer films appeared more reliable. The results indicated that the p-toluenesulfonate doped PPy film showed the highest average conductivity (σ_293 K = 4.5 × 10⁵ S m^?1) whereas the perchlorate doped one produced the lowest of all the films prepared (σ_293 K = 2 × 10⁴ S m^?1).     

Study of chemically (AsF5) and electrochemically doped aluminium polyfluorophthalocyanine, (PcAlF)n

Chemical AsF₅ and electrochemical AsF₆^? doping of aluminium polyfluorophthalocyanine, (AlPcF)_n, has been studied. Thermogravimetric analysis, infrared spectroscopy, X-ray diffraction and electrical conductivity measurements were used to characterize the doped materials. Particular attention has been devoted to the study of the most heavily doped samples obtained in the first case (between 4 and 5 AsF₅ molecules per phthalocyanine ring). A qualitative structural model is proposed to explain the low electrical conductivity of these samples. Results are compared with those for iodine-doped (AlPcF)_n and (GaPcF)_n.Cyclic voltammetry performed on polymer films revealed the existence of two oxidation peaks. While it is very likely that the second peak is accompanied by a chemical reaction taking place at the polymer electrode, it is possible to obtain a lightly AsF₆^?-doped sample for applied potentials lower than 0.6 V (versus Ag/Ag⁺). Very similar results are obtained with BF₄^? and ClO₄^? doping anions.     

The preparation and gas sensitivity study of polypyrrole/zinc oxide

Polypyrrole (PPy) was prepared by chemical oxidation polymerization, analyzed by FT-IR, elemental analysis and HRTEM, and studied for gas sensitivity. It suggested that PPy had sensitivity to NH₃, H₂S and NO_x, and showed irreversibility to NO_x gas. The organic–inorganic hybrid materials PPy/ZnO with different PPy weight percents were prepared by mechanical mixing, and the sensitivity study of the materials to toxic gases NH₃, H₂S, NO_x was carried out at different operating temperatures 30, 60, and 90 °C. It was found that PPy/ZnO materials (PPy(1%)/ZnO, PPy(3%)/ZnO, PPy(5%)/ZnO, PPy(10%)/ZnO, PPy(20%)/ZnO) had better selectivity and reversibility to NO_x than pure PPy, and much lower working temperature than the reported working temperature of ZnO (about 350–450 °C). Their sensitivity increased with the increasing concentration of NO_x at particular working temperature, and among them PPy(10%)/ZnO had the maximum sensitivity to NO_x in the same condition. They showed no response to 1000, 1500, 2000 ppm NH₃ or H₂S. The response mechanism of PPy/ZnO materials to NO_x was discussed.     

Electrical conductivity response of polyaniline films to ethanol–water mixtures

Polyaniline emeraldine base was chemically synthesized and converted to polyaniline emeraldine salts through the protonation doping process using HCl and CH₃COOH as the acid dopants. The doped polyanilines were characterized by ultraviolet–visible (UV–VIS) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, TGA, GPC. The four-point probe technique was used to evaluate the effect of dopant type and doping molar ratio on the specific conductivity. Weak acid doping, by CH₃COOH, produces films with the specific conductivity which depends solely on the degree of protonation, or the number of charge carriers. On the other hand, stronger acid doping by HCl can induce crystalline domains, a greater electron mobility and hence a greater specific conductivity value. The specific conductivity of the HCl-doped and CH₃COOH-doped polyaniline films responds with positive increments upon exposed to water and ethanol. The interchain H-transfer is suggested to be a common mechanism which increases electron mobility upon exposure to water and ethanol, whereas additional protonation occurs only with the exposure to water. No evidence for ethanol molecules to interact chemically with the doped polyaniline films was found. The film electrical conductivity sensitivity is inversely proportional to ethanol concentration, with a higher sensitivity to concentration found in the film doped with the acid with a lower pK_a value.     

Effect of polystyrenesulphonate and electrolyte concentration for electrical properties of polypyrrole film on aluminium alloy using conductive AFM

The electrochemical synthesis of polystyrenesulphonate (PSS) doped polypyrrole (PPy) film onto aluminium alloy (AA 2024-T3) under galvanostatic conditions at current densities of 1 mA cm^?2 was studied. In this study, conductive atomic force microscopy (C-AFM) was performed to investigate the electrical properties of PPy film on AA 2024-T3 depending on the concentration of PSS as dopant and nitric acid as electrolyte during electrochemical synthesis. The addition of HNO₃ to aqueous electrolyte solution is found to allow the electrochemical synthesis of well adhering homogeneous PPy film in the presence of PSS on AA 2023-T3. The PPy film in the presence of nitric acid alone can be synthesised, although this film showed the poor quality of the electrical properties of PPy film. According to C-AFM, the study confirmed that the conductivity of PPy film is significantly increased with increasing the PSS, nitric acid concentration and electrochemical deposition time.     

Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

We have successfully observed electron spin resonance (ESR) signals of radical anions in thin films of tris(8-hydroxyquinoline) aluminum (Alq₃), a compound widely used as electron transporting and luminescent layers in organic light-emitting diodes. To obtain definitely defined radical-anion states in Alq₃, we doped Alq₃ with Mg by co-evaporating these materials. The obtained g value and peak-to-peak ESR linewidth ΔH_pp of Alq₃ radical anions are 2.0030 and 2.19 mT, respectively. Theoretical g value and hyperfine interactions were calculated by density functional theory method, which are in good agreement with the experimental results. A quantitative evaluation of doping concentration was performed. We confirmed that doped charges are localized at deep trapping sites by the lineshape analysis and temperature dependence of the ESR signals. Morphological investigation using transmission electron microscopy clarified that the co-evaporated Mg atoms form clusters.     

Electrochemical synthesis of adherent polypyrrole films on zinc electrodes in acidic and neutral organic media

The electrodeposition of polypyrrole on zinc electrodes has been performed by anodic oxidation of pyrrole in acetonitrile, nitrobenzene and propylene carbonate media in the presence of para-toluene sulfonate counter-ions. When the zinc electrode has preferably undergone a chemical treatment by aqueous sodium sulfide solution, homogeneous and adherent PPy films are obtained. While, no PPy film grows (in the case of C₆H₅NO₂ and PC) or a non-homogeneous and poorly adherent PPy–zinc oxide composite forms (in the case of CH₃CN) when the zinc surface is simply polished. Among the electrochemical techniques used in these experiments, the galvanostatic mode was proved to be better adapted to perform the electropolymerization of pyrrole on the pretreated zinc electrodes using low applied current densities. The polypyrrole films synthesized by this technique has been characterized by several microscopic and spectroscopic techniques. In particular, SEM, XPS, FT-IR and Raman analyses showed that the obtained coatings have the same structure and morphological properties as those electrodeposited on Pt electrodes.     

Polypyrrole/thermally sensitive polyelectrolyte composite (I)

Polypyrrole (PPy) was prepared by electrochemical polymerization with the polyelectrolyte (PE) as a dopant. The PEs were copolymers of the water soluble polymers and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS). The water soluble polymers were thermally sensitive poly(N-isopropyl acrylamide), P(NiPAAm) and thermally insensitive poly(acrylamide), P(AAm). The surface of PPy/PE film observed by SEM was smoother than that of PPy film doped with a monoelectrolyte. PPy/PE composites were fairly conductive, whose electrical conductivities measured by the four-probe method were in the range of 10⁻¹ to 10⁻² S/cm. The characteristics of cation and anion exchange during redox processes were investigated by applying potential from −0.8 to 0.5 V vs. an Ag/AgCl reference electrode to the PPy. The effect of temperature on the doping–dedoping behavior and mass change of PPy composites was investigated by potentiodynamic voltammetry and electrochemical quartz crystal microbalance (EQCM). The electrochemical activity of the PPy/PE gradually decreased with increasing temperature. PPy/P(NiPAAm/AMPS) showed much greater mass change with increasing temperature than PPy/P(AAm/AMPS), mainly because of a decrease in the degree of swelling of thermally sensitive moiety. This confirms that PPy/P(NiPAAm/AMPS) possesses temperature-dependent electrochemical activity, which indicates that it can be utilized for many attractive applications. The thermal volume transition temperature of PPy/P(NiPAAm/AMPS) was estimated from the slope change of mass decrease. The transition temperature of the oxidized state of PPy/P(NiPAAm/AMPS) was higher than that of the reduced state.     

The effect of counter ions and substrate material on the growth and morphology of poly(3,4-ethylenedioxythiophene) films: Towards the application of enzyme electrode construction in biofuel cells

Poly(3,4-ethylenedioxythiophene) (PEDOT) films, with Cl^?, NO₃^?, poly(styrene sulfonate) (PSS^?) or carbon nanotubes (CNTs) as counter ions, were electrochemically deposited onto indium tin oxide (ITO) glass from aqueous electrolyte. PEDOT film, with tetrafluoroborate anions (BF₄^?) as counter ions, was also electrochemically deposited on ITO glass from a propylene carbonate-based electrolyte. The effect of counter ions on the morphology of PEDOT films was demonstrated by the characterizations with scanning electron microscope (SEM). Different carbon-based materials were explored as the substrate materials for PEDOT film generation in electropolymerization, including carbon ink paper, carbon paper, carbon felt, and reticulated vitreous carbon foam (RVC foam). The micro-structure and the surface morphology of the resulted PEDOT films were imaged with SEM. The elemental analysis of the films was performed with energy dispersive X-ray analysis (EDAX). The topography of the PEDOT films was characterized using confocal white light microscopy (COM). A biofuel cell cathode was prepared by immobilization of Trametes Hirsuta laccase (Th-laccase) in PEDOT and the performance of the electrode towards oxygen reduction was examined by chronoamperomentric measurements.     

Ions transport and self-doping in layer-by-layer conducting polymer films

The self-doping mechanism for charge transport is investigated in layer-by-layer (LBL) films from two conducting polymers, namely poly(o-methoxyaniline) (POMA) and poly(3-thiophene acetic acid) (PTAA). The efficiency of charge intercalation, defined as the ratio between the charge and the mass change, is twice for the POMA/PTAA LBL film in comparison with a cast POMA film. This is attributed to differences in the diffusion-controlled charge and mass transport, where distinct ionic species participate in the LBL films, as demonstrated with experiments using a quartz crystal microbalance. The doping efficiency for LBL film is the same, i.e., 3.93 × 10⁻⁴ and 3.56 × 10⁻⁴ g/C for the Li⁺ and (C₂H₅)₄N⁺ doped films, and is different for the cast POMA film, i.e., 11.3 × 10⁻⁴ for Li⁺ and 6.45 × 10⁻⁴ g/C for (C₂H₅)₄N⁺. Therefore, once no significant differences in the intercalation mechanism are observed when different cations, Li⁺ or (C₂H₅)₄N⁺, are used with the LBL films, this indicates that the self-doping mechanism is controlled by the exchange of anions.     

Analysis of electrochemical impedance of polypyrrole|sulfate and polypyrrole|perchlorate films

PPy|SO₄ and PPy|ClO₄ films have been synthesized and investigated in K₂SO₄, ZnSO₄ and NaClO₄ aqueous solutions by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and electron probe microanalysis (EPMA) methods. On the basis of obtained data and calculated impedance parameters as the potential functions, the role of different processes (diffusion of ions, double-layer charging, adsorption and charge transfer) in oxidized, partially reduced and reduced PPy films is estimated. The lowest pseudocapacitance values (from n × 10⁻⁶ to n × 10⁻⁴ μF cm⁻² for 1 μm film), independent of solution concentration, were established for PPy|SO₄ in ZnSO₄. This phenomenon is related with strongly aggravated film reduction process in the solution of double-charged cations. In the case of PPy|ClO₄ in NaClO₄ and PPy|SO₄ in K₂SO₄,where the mono-charged cations participate in redox process, the capacitance values are in the range from: n × 10⁻³ to n × 10⁻² μF cm⁻² and even somewhat higher for PPy|ClO₄ system at oxidized state. The calculated effective diffusion coefficients of ions D remain inside the range from n × 10⁻¹² to n × 10⁻¹⁴ cm² s⁻¹ for PPy|SO₄ in 0.1 M K₂SO₄ and PPy|ClO₄ in 0.1 M NaClO₄ aqueous solution. In the case of PPy|SO₄ film in ZnSO₄ solution the D values are essentially lower.