High frequency study of electropolymerised conducting polypyrrole thin film

The polypyrrole (PPY) thin films were synthesized by electropolymerisation in potassium nitrate solution. The substrate used was stainless steel. DC conductivity, microwave reflection, microwave conductivity and microwave dielectric constant of the conducting PPY thin films are reported. DC conductivity was between 1.6 × 10⁻² S/cm and 42.3 × 10⁻² S/cm. Microwave conductivity was between 10 S/cm and 160 S/cm. The ?′ generally decreases as frequency increases similarly ?″ also decreases with increases in frequency. The measurements have been carried over the frequency range 8.2-12 GHz. These polypyrrole thin films were characterized by FTIR. The polypyrrole thin film increases the reflectivity of the stainless steel.     

EPR study of the mechanism of chemical dedoping of conducting polypyrrole

The mechanisms of chemical dedoping of conductive polypyrrole with the initial conductivity values differing by an order of magnitude have been studied using EPR spectroscopy. An analysis of the temperature dependences of the electrical resistance of the initial highly conductive samples and dedoped samples has revealed a transition from three-dimensional conduction in the initial samples to one-dimensional conduction in the dedoped samples. This transition is caused by the transformation of transverse bipolarons into longitudinal bipolarons and ipolarons. It has been established that transverse and longitudinal quasiparticles are stabilized by counterions involving iron ions in different spin states: the spin of iron in the counterion of a transverse bipolaron is 2 and iron is inactive in the EPR spectrum, whereas in the counterion of longitudinal quasiparticles, the spin of iron is 5/2 and iron becomes paramagnetic.     

Proton modification of conducting polypyrrole

Experimental resluts on the protonic modification of the conducting polymer polypyrrole are reported. It is shown that the conductivity of the electrochemically generated polymer decreases over four orders of magnitude on treatment with strong alkalies. The effect is reversed upon treatment with proton acids. This process has been studied through electrical conductivity, optical absorption and XPS results. The observations indicate that the NH bond in the pyrrole units is disrupted in alkaline solution, leading to deprotonation. This effect is reversed in acid solution, where the protons are attached again. The deprotonation leads to shorter conjugated segments on the polymeric backbone and increasing hopping lengths, decreasing the conductivity.     

Electrochemical redox supercapacitors using PVdF-HFP based gel electrolytes and polypyrrole as conducting polymer electrode

Electrochemical redox supercapacitors have been fabricated using polymeric gel electrolytes polyvinylidene fluoride co-hexafluoropropylene (PVdF-HFP)–ethylene carbonate (EC)–propylene carbonate (PC)–MClO₄: M = Li, Na, (C₂H₅)₄N and electrochemically deposited polypyrrole as conducting polymer electrode. The performance of the capacitors have been characterized using a.c impedance spectroscopy, cyclic linear sweep voltammetry and galvanostatic charge–discharge techniques. The capacitors shows larger values of overall capacitance of about 14–25 mF cm^− 2 (equivalent to a single electrode specific capacitance of 78–137 F g^− 1 of polypyrrole), which corresponds to the energy density of 11–19 W h kg^− 1 and power density of 0.22–0.44 kW kg^− 1. The values of capacitance have been found to be almost stable up to 5000 cycles and even more. A comparison indicates that the capacitive behaviour and the capacitance values are not much affected with the size of cations of the salts incorporated in gel electrolytes, rather predominant role of anions is possible at the electrode–electrolyte interfaces. Furthermore the coulombic efficiencies of all the cells were found to be nearly 100% that is comparable to the liquid electrolytes based capacitors.     

MnO2-polypyrrole conducting polymer composite electrodes for electrochemical redox supercapacitors

Redox supercapacitors using electrochemically synthesised MnO₂-polypyrrole composite electrodes have been fabricated with different electrolytes, namely polymer electrolyte film (polyvinyl alcohol, PVA-H₃PO₄ aqueous blend), aprotic liquid electrolyte (LiClO₄-propylene carbonate, PC) and polymeric gel electrolyte [poly methyl methacrylate, (PMMA)-Ethylene carbonate (EC)-Propylene carbonate (PC)-NaClO₄]. The capacitors have been characterised using galvanostatic charge-discharge methods. The cell with aqueous PVA-H₃PO₄ shows non-capacitive behaviour owing to some reversible chemical reaction of MnO₂ with water while the MnO₂-polypyrrole composite is found to be a suitable electrode material for redox supercapacitors with aprotic (non-aqueous) electrolytes. The solid state supercapacitor based on MnO₂-polypyrrole composite electrodes with gel electrolyte gives stable values of capacitance of 10.0–18.0 mF cm⁻² for different discharge current densities.     

Optical investigations of conducting polypyrrole under pressure

Abstract

The changes in the characteristics of vibrational and optical properties of the conducting form of polypyrrole under pressure is studied.     

A study on the electrochemical behaviour of polypyrrole films in concentrated aqueous alkali halide electrolytes

The electrochemical behaviour of polypyrrole films doped with dodecyl benzene sulfonate (PPy/DBS) in LiCl aqueous electrolytes has been investigated in order to find the electrolyte concentration suitable for the operation of PPy/DBS-based soft actuators. For this investigation, PPy/DBS films deposited on gold-coated quartz crystals by electropolymerization and simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance techniques were used. During the first redox cycle, while large water movement is observed along with the counter ions in dilute electrolytes, such water transport in concentrated electrolytes is found to be very low. In dilute electrolytes, water molecules accompany counter ions as solvated molecules and due to osmotic effect. In concentrated electrolytes, water movement is less due to limited availability of free water as well as a smaller osmotic pressure difference. In highly concentrated aqueous electrolytes, the mass of the PPy/DBS film at the end of each redox cycle is found to drift, which can be controlled by changing the concentration of the electrolyte. The PPy/DBS films were also cycled at different scan rates in various alkali halide aqueous electrolytes of concentrations 0.1 and 1 M to determine the effective diffusion coefficients of alkali ions in the films. The effective diffusion coefficients were found to increase with the concentration of the electrolytes and decrease with the increase in size of cations.     

Diffusion processes in the doping of polymer films

A critical review is presented of the various diffusion models and the associated experimental techniques that have been put forth to describe the mass transfer limitations on the overall processes by which polymer films are doped to develop enhanced electrical conductivity. The discussion emphasizes comparative features and their connections with data interpretation. The strengths and weaknesses associated with particular approaches are stressed in order to explain the wide range of diffusion coefficients reported by the many prior investigators, and to suggest further steps that may eventually resolve some contradictions and provide a more satisfactory understanding of the doping and undoping processes for conductive polymers.     

Conformational defects in a conducting polymer

Electrically conducting organic polymers represent serious candidates for some of the electronic materials of tomorrow. The role of certain charge-carrying self-localized states, such as solitons, polarons and bipolarons, in determining the nature of the optical and electronic transport properties of these conjugated polymers is well established. These self-localized states are commonly referred to as ‘defects'. Recently, the list of possible ‘defects’ in these conducting polymers has expanded to include real defects which degrade the electrical properties, but lead to other interesting and potentially useful properties, such as colour changes.     

Quasi-one-dimensional transport in conducting polymer nanowires

Recent advances in the synthesis of nanowires based on conducting conjugated polymers are described. The results of recent experimental studies of the electrical properties of polymer nanowires are critically analyzed. The applicability of various theoretical models of tunneling in one-dimensional conductors (variable-range hopping conductivity, Luttinger liquid, Wigner crystal, etc.) to the interpretation of the experimental data on the electronic transport in polymer nanosystems is discussed. A phenomenological model is proposed describing the mechanism of transport in polymer nanowires with allowance for the quasi-one-dimensional nature of molecules of conjugated polymers. The first applications of polymer nanowires in nanoelectronics and prospects for the future are discussed.     

Polymer electrolyte based solid state redox supercapacitors with poly (3-methyl thiophene) and polypyrrole conducting polymer electrodes

Types I and II solid state redox supercapacitors have been constructed using polypyrrole (pPy) and poly (3-methyl thiophene) (pMeT) conducting polymer electrodes with lithium ion conducting polymer electrolyte poly(ethylene oxide) (PEO)-LiCF₃SO₃ plasticised with poly (ethylene glycol) (PEG). The performance of the capacitors has been characterised by a.c. impedance, linear sweep voltammetry, galvanostatic charge-discharge methods and long term cycling tests. The asymmetric type II capacitors with p-doped pPy and pMeT electrodes give a capacitance value ∼ 2 mF cm⁻² (equivalent to 18 Fg⁻¹ of the total mass of the electrodes) and can be charged up to the voltage of 1.7 V. The symmetric type 1 capacitors of the configuration pPy | polymer electrolyte | pPy and pMeT | polymer electrolyte | pMeT show comparable values of capacitance but they are limited to the working voltage of <1.0 V. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

An experimental set-up for the study of electromechanical properties of conducting polymer films

An experimental set-up have been designed and fabricated to study the electromechanical properties of a thick film of conducting polymer under load. Extension of the films versus voltage has been measured in terms of change in capacitance of parallel plate capacitor constituted by metal pan and a fixed metal plate. HP 4284A impedance analyzer measures absolute value of the capacitance. Change in capacitance is related to change in distance by pre-calibrating the assembly using traveling microscope. A computer programme is developed to convert the capacitance value with the corresponding distance and simultaneously plotting the graph between changes in length of polymer film versus applied voltage.The assembly has been used to study the electrochemomechanical behavior of solution cast polyaniline films (∼50 μ thick). During first cycle the length is enhanced by about 6% of original value, while repetitive value of extension is ≅2.8%, in subsequent cycles. This value closely matches to that reported in literatures.     

Neutron-transmutation doping of GaAs — as studied by ESR

Neutron (n⁰) transmutation doping of GaAs has been monitored by electron spin resonance (ESR). Strong evidence was obtained that, apart from fast neutron impact, As_Ga antisite defects are also created by the γ- and β-emissions following thermal n⁰-capture. The As_Ga defects, forming deep midgap states, anneal out at 500°C.     

Carrier dynamics in conducting polymers: case of PF6 doped polypyrrole

The carrier dynamics in PF6 doped polypyrrole has been probed by dielectric spectroscopy (from 10(-4) to 4 eV), down to 4.2 K. The phase-sensitive sub-THz data have assisted to resolve the discrepancies in Kramers-Kronig analysis in earlier studies. Even in metallic samples, just 1% of the carriers are delocalized, at 300 K; the fraction drops down considerably as a function of disorder, carrier density, and temperature. This subtle metallic feature and the anomalies in carrier dynamics are attributed to coherent and incoherent transport between short conjugated segments.     

基态非简并导电聚合物——坐标空间研究

通过引入简并破缺项,建立了非等势垒Kronig-Penney方势阱模型,在实坐标空间中研究了基态非简并聚合物——顺式聚乙炔的基态及其激发态,并与紧束缚模型所得到的结果进行了比较.给出了体系的电子态、电荷密度等在实坐标空间的分布特征.发现在坐标空间研究可以更加准确地反映体系的电子态密度、电子空间密度分布等特征. 关键词： 聚合物 方势阱 基态 极化子     

Quantitative assessment of solid oxide electrochemical doping

Quantitative analysis of metal cation doping by solid oxide electrochemical doping (SOED) has been performed under galvanostatic doping conditions. A M–β″-Al₂O₃ (M=Ag, Na) microelectrode (contact radius: about 10 μm) was used as cation source to attain a homogeneous solid–solid contact between the β″-Al₂O₃ and doping target. In Ag doping into alkali borate glass, the measured dopant amount closely matched the theoretical value. High Faraday efficiencies of above 90% were obtained. This suggests that the dopant amount can be precisely controlled on a micromole scale by the electric charge during electrolysis. On the other hand, current efficiencies of Na doping into Bi₂Sr₂CaCu₂O_y (BSCCO) ceramics depended on the applied constant current. Efficiencies of above 80% were achieved at a constant current of 10 μA (1.6 A cm⁻²). The relatively low efficiencies were explained by the saturation of BSCCO grain boundaries with Na. By contrast, excess Na was detected on the anodic surface of ceramics at a constant current of 100 μA (16 A cm⁻²). In the present study, we demonstrate that SOED enables micromole-scale control over dopant amount.     

Site-selective electron nuclear double resonance in the exchange-narrowed regime of the ESR in conducting solids

Electron nuclear magnetic double resonance on conduction electrons reveals the hyperfine interaction hidden by the fast electron spin exchange. We used the Overhauser shift technique to investigate the electron spin density of the conduction band of gallium oxide, beta-Ga(2)O(3). Due to the monoclinic structure, the conduction band of beta-Ga(2)O(3) is anisotropic and it is dominated by contributions from the two nonequivalent Ga sites. The large quadrupole couplings of the two gallium isotopes (69)Ga and (71)Ga (both with I = 3/2) are completely resolved in our double-resonance experiments. This resolved quadrupole interaction allows the determination of the electric field gradients at both gallium sites with high precision and high sensitivity. The resolved quadrupole splitting is the key to the site-selected determination of the hyperfine interaction. The concepts behind these double-resonance techniques are rather general and should be applicable in similar semiconductor systems.