Polypyrrole decomposition

Cyclic voltammetric studies of polypyrrole obtained by using FeC as oxidant showed no evidence of polypyrrole decomposition after repetitive cycling [24]. Voltammograms also showed that after the oxidation reaction a high capacitive current remained. A cyclic voltammogram of polypyrrole synthesized by oxidation with CuCl2 showed different and irregular shapes, probably influenced by the presence of copper ions incorporated into the polymer. 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

Polythiophene films can be electrochemically cycled from the neutral to the conducting state with coulombic efficiencies in excess of 95% [443], with little evidence of decomposition of the material up to + 1.4 V vs. SCE in acetonitrile [37, 54, 56, 396,400] (the 3-methyl derivative being particularly stable [396]), but unlike polypyrrole, polythiophene can be both p- and n-doped, although the n-doped material has a lower maximum conductivity [444], Cyclic voltammetry shows two sets of peaks corresponding to the p- and n-doping reactions, with E° values at approximately + 1.1 V and — 1.4 V respectively (vs. an Ag+/Ag reference electrode)... 

Electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy) and po 1 y(3.4-cthy 1 cncdi oxyth iophcnc) (PEDOT) have been applied in supercapacitors, due to their excellent electrochemical properties and lower cost than other ECPs. We demonstrated that multi-walled carbon nanotubes (CNTs) prepared by catalytic decomposition of acetylene in a solid solution are very effective conductivity additives in composite materials based on ECPs. In this paper, we show that a successful application of ECPs in supercapacitor technologies could be possible only in an asymmetric configuration, i.e. with electrodes of different nature. 

In 1985 Jakobs et al. studied polypyrrole (PPy) covered platinum and gold electrodes for the ORR,167,168 One interesting result of the work was that, compared to a bare gold electrode, the PPy covered gold reduced oxygen at a lower overpotential.168 Further, the PPy covered electrodes, when in the oxidized state, catalyzed peroxide decomposition and thus improved selectivity to water.168... 

Amount of deposited material - The difference in weight loss between coated and untreated silica corresponds to the weight of the plasma-polymerized film deposited on the surface. For the plasma-treated silicas, decomposition of the coating starts at 265°C for poly acetylene, 200°C for polypyrrole, and 225°C for poly thiophene, and is complete at 600°C. Between 265 and 600°C, PA-silica shows 6 wt% weight loss, and PPy- and PTh-silicas show 4.5 wt% and 5 wt% loss, respectively. 

Takenaka et al. 595) used XPS to study deterioration of a thin polythiophene film on an ITO electrode, such as might be used in a display device. After 105 dopingundoping cycles at 0.5 Hz with BF4" counter-ions there was evidence of extensive fluorination of the polymer, probably due to decomposition of the counter-ion or its hydrolysis by traces of water in the electrolyte. Corradini et al. 596) carried out a similar study of polypyrrole, polythiophene and their analogues with C104 counterions and concluded that all of the polymers are unstable to cycling or to standing in contact with the electrolyte, although polypyrrole performed best. 

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

PEC catalysis using polymeric assistance is considered to be a natural outgrowth of polymer assisted electrochemical and photochemical catalysis. Early experiments, mostly combining electroactive polymer films with small band gap semiconductors have demonstrated the feasibility of using such systems in the PEC decomposition of water. Both gaseous H2 and O2 have been separately generated from polymer coated photoelectrodes H2 from poly-Mv2+ films on p-Si and O2 from polypyrrole films on n-CdS. 

Dong and Jones Jr. reported on the preparation of submicron electrically conductive polypyrrole/poly(methyl methacrylate) coaxial fibers and conversion to polypyrrole tubes and carbon tubes [47]. In this study, PMMA fibers with an average diameter of 230 nm were initially fabricated by electrospinning as core materials. The PMMA fibers were subsequently coated as templates with a thin layer of PPy by in situ deposition of the conducting polymer from aqueous solution. Hollow PPy nanotubes were produced by dissolution of the PMMA core from PPy/PMMA coaxial fibers. Furthermore, high temperature (1000 °C) treatment under an inert atmosphere can be used to convert PPy/PMMA coaxial fibers into carbon tubes by complete decomposition of the PMMA fiber core and carbonization of the PPy wall (Figure 4.13). 

Second type of self-healing process came about in tantalum capacitors with polypyrrole cathode (Harada 1997). This material thermally decomposes at about 300°C. When increased current is flowing through the defect region, heat is generated causing the decomposition of polypyrrole, which becomes less-conductive and finally to be insulating. 

Potential Electropolymerization is carried out at moderate potentials to prevent the oxidative decomposition of the solvent, electrolyte and polymer film. The polymerization potential also determines the stability of intermediate species. The formation of a polypyrrole film, for example, occurs via cation intermediates whose stability favours the radical coupling reaction. The reactive cations may also react with solvent and other nucleophiles in the vicinity of the electrode surface, minimizing the polymer forming reaction. Some of the monomers which have been electropolymerized are listed in Table 2.3 along with their respective peak potentials and the apparent electrochemical stoichiometry of the reaction. 

In another experiment, a variation of this method was used to prepare an interpenetrating network of polypolypyrrole and poly(vinylacetate) [66]. Pyrrole monomer was added to stirred solutions of poly(vinylacetate) and FeCla in different concentrations. Films were cast from these solutions before pyrrole polymerization was complete and washed to remove excess oxidant. A 5% percolation threshold was observed, with a plateau conductivity of 10 S cm. Polypyrrole forms a network by spinodal decomposition and poly(vinylacetate)... 

Kojima and co-workers [52] identified the major thermal decomposition products of plasma polymerised polypyrrole as nitriles with less than four carbons and alkyl pyrroles. Evolution of only monosubstituted alkyl pyrroles, such as 2-methylpyrrole and 2-ethylpyrrole, suggests that polypyrroles consist of monosubstituted pyrrole rings. This is also supported by the result that the IR spectrum of polypyrroles differs from that of the electrochemically polymerised pyrrole, which consists of disubstituted pyrrole rings. Evolution of linear nitriles shows evidence that a polypyrrole molecule has the main chain containing nitrogen atoms. The mechanism of polymerisation of pyrrole in the discharge is considered to be similar to that of aromatic hydrocarbons, which mechanism involves a process of production of acetylene. 

The morphology of poly(vinylacetate)-polypyrrole composite film is very different from that of polymer blend prepared from the solution polymerization. Polypyrrole aggregates are connected with each other in the former case, which results in a polypyrrole network throughout the composite, while polypyrrole aggregates are separated from each other in the latter case. It is expected in the former case that a spinodal decomposition occurs during the phase separation process because of sudden changes in the concentration of pyrrole and ferric chloride as well as the viscosity of poly (vinylacetate). These sudden changes are caused by the evaporation of solvent. 

The poly(vinylacetate)-polypyrrole composite film also showed high electrical conductivity of about 10 S/cm which resulted from the polypyrrole network structure formed from the spinodal decomposition during the casting process. 

In the poly(vinylacetate)-polypyrrole composite film polypyrrole forms a network structure by spinodal decomposition and poly(vinylacetate) is cross-linked by ferric chloride during the polymerization process which results in excellent chemical stability of the composite. 

Kojima and co-workers [31] applied Py-GC-MS and infrared spectroscopy to a study of the structure and thermal plasma polymerised decomposition of polypyrrole. 

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