Redox-cycling of poly

Fig. 7.2 The redox cycling of poly(pyrrole) involving intercalation and expulsion of (a) the anion or (b) the cation from the electrolyte to effect charge balance.

Redox cycling of poly(thionine) films in aqueous acetic acid solution involves not only electron transfer (coupled with proton transfer to maintain electroneutrality), but also film solvation and acetic acid coordination... 

Redox cycling of poly(vinylferrocene) films involves not only coupled electron/anion transfer, but also solvation and polymer configuration changes. The situation is further complicated by both reversible and irreversible elements of solvation and configuration changes, commonly referred to as break-in", immediately following deposition. Here, a dou-... 

Fig. 13.16. Redox cycling of a broken-in poly(vinylferrocene) film on short time scales. Reconfiguration is assumed faster than (de-)soivation in the lower cube.

As the degradation of polyaniline occurs via an imine intermediate [281,284], Kim et al. [285] prepared self-doped polymer by alkylsulphonate substitution in the polymer backbone, Besides self-doping for a facile redox process, the perceived advantage of this bulky substituent includes the protection of nitrogen centres from nucleophiles responsible for irreversible degradation of polyaniline. Poly(aniline N-butylsulphonate) retained its reversible electrochromic response up to 150 000 cycles when scanned between its oxidized and reduced states (between 0.2 and 0.5 V) then started diminishing slowly. The excellent redox cyclability of poly(aniline N-butylsulphonate) over unsubstituted polyaniline was also confirmed by chronoabsorptom-etry by Kim et al. [285],... 

The cyclic voltammograms and the changes that occur to them during repetitive cycling are similar to those of 3-methylthiophene oxidation in acetonitrile. When a platinum electrode is used, the color change (red-blue) due to the redox transformation of poly (3-methylthiophene) is easily visible. A visual inspection also reveals that the electropolymerization reaction starts at the three-phase junction, as theoret-... 

During continuous redox cycling, the first cycle usually differs from the following ones. This effect is referred as break-in. In poly(vinylferrocene), PVF, films this has been related to the incorporation of solvent and ions into the film, decreasing its resistivity [132]. This effect has been observed for several polyelectrolyte and polymer-modified electrodes, for example, polyaniline [155]. 

ZnO displays similar redox and alloying chemistry to the tin oxides on Li insertion [353]. Therefore, it may be an interesting network modifier for tin oxides. Also, ZnSnOs was proposed as a new anode material for lithium-ion batteries [354]. It was prepared as the amorphous product by pyrolysis of ZnSn(OH)6. The reversible capacity of the ZnSn03 electrode was found to be more than 0.8 Ah/g. Zhao and Cao [356] studied antimony-zinc alloy as a potential material for such batteries. Also, zinc-graphite composite was investigated [357] as a candidate for an electrode in lithium-ion batteries. Zinc parhcles were deposited mainly onto graphite surfaces. Also, zinc-polyaniline batteries were developed [358]. The authors examined the parameters that affect the life cycle of such batteries. They found that Zn passivahon is the main factor of the life cycle of zinc-polyaniline batteries. In recent times [359], zinc-poly(anihne-co-o-aminophenol) rechargeable battery was also studied. Other types of batteries based on zinc were of some interest [360]. 

Our published studies [18, 19] of poly(thionine) redox switching showed that, in acetic acid buffer, the leuco (reduced) form (L) is oxidized, losing two electrons and two protons, to generate the oxidised form (7 ). Furthermore, acetic acid is coordinated only to the 7-form so that reduction causes 7 to lose an acetic acid molecule. However, reduction of 7 to L follows a different path than the reverse of the oxidation path. Water transfer is also involved in the redox cycle. We can (in simplified form) write the overall redox switching process as ... 

Fig. 13.6. Cube representing the poly(thionine) redox mechanism. Shaded (white) circles at the corners of the cube indicate species that are (are not) accessed in the redox cycle.

Fig. 13.9. EQCM data obtained during the "break-in" of a poly(vinylferrocene) film (T = 23.7 nmol cm 2). Frames a and b, respectively, correspond to the current- and mass change-potential curves for the first two redox cycles in aqueous 0.1 mol dm-- NaCI04 immediately following deposition from CH.CI ,. In frame a, the current (electron flux) was converted to the equivalent mass flux of counter ions, for subsequent correlation with the total (observed) gravimetric response of frame b. Potential scan rate 5mVs l. The star symbol denotes the first anodic scan responses. (Adapted from Ref. [37] with permission.)...

The use of the ETSM to study polymeric systems, especially redox and conducting polymers, is a rapidly growing area of research. It has been used to elucidate die mechanisms of film formation, ion and solvent transport phenmnena, and compositional changes that occur in these films upon redox cycling. Among the redox polymer systems that have been studied are poly(vinylferrocene)... 

NQOl is a homodimer with a flavodoxin fold (5). This enzyme does not stabilize the semiquinone state. The obligate two-electron transfer mechanism prevents the generation of quinone radicals and redox cycling, which would result in oxidative stress. The NADPH and quinone substrates occupy the same site, consistent with the observed ping-pong bi-bi mechanism. NQOl is inhibited by many (poly)aromatic compounds including the anticoagulant dicoumarol and the phytoalexin resveratrol (5). 

Fig. 18 Current, beam deflection, and frequency change responses (panels (a-c), respectively) of a poly(l-hydroxyphenazine) film to a redox cycle under cyclic voltammetric conditions. Electrodes glassy carbon (probe beam experiment) and Au (area = 0.36 cm ) on 5-MHz AT-cut quartz crystals (QCM experiment). Solution 1 mol dm HCIO4. Potential scan rate 50 mV s (Reproduced from Ref. [122] with permission from Elsevier.)...

Borjas, R., and D.A. Buttry. 1991. EQCM studies of film growth, redox cycling, and charge trapping of n-doped and p-doped poly(thiophene). Chem Mater 3 872. 

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