An impedance study of the doping of polypyrrole in LiClO4/PC

doi:10.1016/0167-2738(93)90290-J

Solid State Ionics

Volume 60, Issues 1–3, March 1993, Pages 153-159

https://doi.org/10.1016/0167-2738(93)90290-J Get rights and content

Abstract

The kinetics of insertion of perchlorate ions into polypyrrole (PPy) is investigated by electrochemical impedance spectroscopy, and a simple transmission line model for the coupled diffusive transport of ions and electrons/holes in these films is proposed. Two modifications of PPy prepared at different current densities are studied. The change of resistance of PPy caused by electrochemical oxidation in LiClO₄/propylene carbonate (PC) was followed, and the switch to high conductivity was found to occur at a lower potential in the modification prepared at the lower current density. This modification has previously been shown to have a high degree of conjugation along the polymer chains. Two types of transport kinetics were observed. At potentials just above the switching point, 2.7 < E <3.0 V versus Li, anomalous diffusion dominates the ionic transport, whereas the ionic transport kinetics at higher potentials approaches ideal diffusion. In the potential range 2.7 < E <3.4 V versus Li diffusion coefficients, D, for the dopant were extracted from the impedance data. The values of D were slightly dependent on potential and film thickness, but not on the current density used during synthesis.

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The fractal dimensions of polypyrrole (PPy) film electrodeposited on the glassy carbon (GC) electrode formed in the presence of ${ClO}_{4}^{-}$ is obtained by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods in different film thicknesses and various synthesis temperatures. Atomic force microscopy (AFM) technique was employed to analyze fractal structure of film. Anomalous diffusion through the film in which mean squared displacement of particles has not a linear dependence with time ( $〈 x^{2} 〉 \propto t^{γ}$ ) has been discussed and values of γ that is the characteristic of anomalous diffusion is obtained by EIS method and then the relation between fractal dimension and anomalous diffusion has been investigated.
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The literature gives some guidance as to how these aims may be realised. Thus, early papers focus on ordered films of polypyrrole deposited at low currents (see for example Ref. [12]), and mention is also made of “dual mode” or “switching behaviour”, where doping by apparently immobile surfactant anions ensures participation by cations in the doping/de-doping process [13–15]. Naoi et al. [16] took this strategy a stage further, by showing how depositing from surfactant micelles leads to the appearance of a pPy film with a striking columnar morphology.
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Polypyrrole (pPy) electrodes containing polysulfonated aromatic anions are investigated by cyclic voltammetry as electrodes for use in electrochemical supercapacitors. These ‘ladder-doped’ materials are deposited as thick films (ca. 10 μm) having open structures that permit the rapid insertion/ejection (up to 300 mV s⁻¹) of cations and anions from aqueous solution, and give an effective or ‘geometric’ capacitance of up to 0.40 F cm⁻². The ‘dual mode’ doping behaviour seems essential for good capacitive response. These electrodes show a remarkable tendency to perform better at high cycling rates, an effect attributed to the way the structure ‘self-organises’ during the self-doping process. Good electrode response depends on protecting the open structure containing hydrophilic ion-conducting channels.
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The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.

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¹: Permanent address: Department of Physics, University of Peradeniya, Sri Lanka.

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An impedance study of the doping of polypyrrole in LiClO4/PC

Abstract

J. Electroanal. Chem.

Mat. Sci. Eng. B

Solid State Ionics

Solid State Ionics

Solid State Ionics

J. Electrochem. Soc.

An impedance study of the doping of polypyrrole in LiClO₄/PC