Doping redox reaction

For example, the p-doping process of a typical heterocyclic polymer, say polypyrrole, can be reversibly driven in an electrochemical cell by polarising the polymer electrode vs a counterelectrode (say Li) in a suitable electrolyte (say LiC104-PC). Under these circumstances the p-doping redox reaction (9.15) can be described by the scheme ... 

Redox doping Red oxide Redox indicators Redox polymers REDOX process Redox reactions Red PDC [80-22-8]... 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

The stoichiometry of the redox reactions of conducting polymers (n and m in reactions 1 and 2) is quite variable. Under the most widely used conditions, polypyrroles and polythiophenes can be reversibly oxidized to a level of one hole per ca. 3 monomer units (i.e., a degree of oxidation, n, of ca. 0.3).7 However, this limit is dictated by the stability of the oxidized film under the conditions employed (Section V). With particularly dry and unreactive solvents, degrees of oxidation of 0.5 can be reversibly attained,37 and for poly-(4,4 -dimethoxybithiophene), a value of n = 1 has been reported.38 Although much fewer data are available for n-doping, it appears to involve similar stoichiometries [i.e., m in Eq. (2) is typically ca. 0.3].34,39"41 Polyanilines can in principle be reversibly p-doped to one... 

The electrocontractile response of a composite of a PVA-PAA gel doped with PPy has been investigated by a group in the University of Pisa [70], The composite strip (initial length 3.05 cm, thickness 0.016 cm) in a 0.05 N NaCl solution has acted as both an electrode and an actuator. When dc voltages up to 2 V were applied, PPy doped in the gel underwent a redox reaction to change the pH inside the gel, and the change in size was observed within 30 min. The measured length variation was about 2%. 

Leach-proof sol-gel entrapment can be exploited to carry out one-pot reactions with mutually destructive reactants while still allowing these reagents to activate or participate in desired reactions. For instance, three different one-pot redox reactions can be carried out in sequence in one pot over two separate sol-gel matrices doped with an oxidant (pyridinium dichromate) and with a reducing species (RhCl[P(C6H5)3]3) without their mutual destruction and with no need for separation steps (Figure 5.12).24... 

It has been shown that the chemical and electrochemical doping of polymers may be described as a redox reaction which involves the... 

Upon illumination, semiconductor particles become charged, allowing even for electrophoretic mobility under an applied electrical field When appropriately prepared, colloidal TiO2 can apparently accumulate charge to effect directly multiple quanta redox reactions The efficiency of such charge accumulation is surely related to doping level for the doping level can alter band positions and may improve the efficiency of photoinduced electron transfer. For example, the dispersal of FcjOa... 

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

Cyclic Voltammetric Behavior of the PPy-GOD Film. Figure 1 shows the cyclic voltammetric curves of a PPy-GOD film (4000 A) in phosphate buffer solution with pH 7.4 at different scan rates. Both anodic and cathodic peaks should correspond to the redox reactions of PPy chains. The peak potentials, which were recorded at the scan rate of 200 mV/s, were -380 mV and -200 mV for cathodic and anodic peaks, respectively. This is similar to the potential shifts of the PPy film doped with large anions (27) such as poly(p-styrenesulfonate). Enzyme protein molecules are composed of amino acid and have large molecular size, which can not move out freely from the PPy-GOD film by the application of the reduction potential. In order to balance the charge of the Pfy-GOD film, cations must move into the film, and redox potentials move toward a more negative potential. This behavior is different from the one observed for the PPy-GOD film, which was prepared in the solution of GOD... 

UV-Visible and Raman Spectroscopies. In situ UV-visible absorption spectra of a 5000 A PPy-GOD film, which was formed on an ITO coated glass, were recorded in the PB solution (pH 7.4). The spectra recorded at both the oxidation (0.4 V) and the reduction (-1.0 V) potentials showed an absorption peak near 380 nm, which is due to the PPy. When the PPy was reduced at -1.0 V, the absorbance in the wavelength range of 500-800 nm decreased, and the absorbance at 380 nm increased. The observed spectral changes of the PPy-GOD film during the redox reaction were similar to those of the PPy film doped with C104" (PPy-C104) (27). 

Polyaniline differs from other CPs in that the doping process may be associated with protonation of the N atoms in the chain (i.e., to a base <- salt equilibrium in the presence of acid, in addition to the usual redox reactions). This is discussed further in Chapter 13. The related processes are therefore more complex. On the other hand, the low cost and proces-sibility of PAni makes it a prime candidate for several applications. 

Basically, doping is a redox reaction (i.e., an electron transfer reaction) between the polymer chain and the counterions. For example, in the case of negative doping one can write... 

In particular, a peak current appears at Vox, the potential characteristic of the redox reaction. The integral of the current gives the charge transferred with the polymer for a given applied voltage V. This integral is also the doping level (1 /u) fvI(V ) dV = q(V). 

When the redox potential was high enough we found no difference between p- and n-type material. Nitric acid is not a very convenient system to study because its redox reaction is in itself very complicated. The reduction scheme for nitric acid contains a step which appears to be catalyzed at the surface. The catalytic activity of a germanium surface is probably influenced by the doping perhaps this could explain your results. We did not find any difference in p- and n-type when we used ferri-cyanide or ceric salts as oxidizers. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

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