Redox electron conducting

From Pure Redox to Redox-Electronic Conduction in the Same Molecule The... 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

Mixed potential resulting from an interfering redox reaction at membranes with finite electronic conductance... 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

Heller A. 2006. Electron-conducting redox hydrogels design, characteristics and synthesis. Curr Opin Chem Biol 10 664-672. 

As described in the introduction, certain cosurfactants appear able to drive percolation transitions. Variations in the cosurfactant chemical potential, RT n (where is cosurfactant concentration or activity), holding other compositional features constant, provide the driving force for these percolation transitions. A water, toluene, and AOT microemulsion system using acrylamide as cosurfactant exhibited percolation type behavior for a variety of redox electron-transfer processes. The corresponding low-frequency electrical conductivity data for such a system is illustrated in Fig. 8, where the water, toluene, and AOT mole ratio (11.2 19.2 1.00) is held approximately constant, and the acrylamide concentration, is varied from 0 to 6% (w/w). At about = 1.2%, the arrow labeled in Fig. 8 indicates the onset of percolation in electrical conductivity. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

Keywords n-d interaction, 3d transition metal, Electronic conductivity, Magnetism, Redox-active complexes, TTF... 

The second factor, electronic conduction through an assembly of redox centers, involves electron hopping between adjacent sites. It is analyzed so as to highlight the characteristics of this transport in terms of equivalent diffusion. Field effects will also be addressed. Although bearing some... 

The distribution of the exchange transfer current of redox electrons o(e), which corresponds to the state density curves shown in Fig. 8-11, is illustrated for both metal and semiconductor electrodes in Fig. 8-12 (See also Fig. 8-4.). Since the state density of semiconductor electrons available for electron transfer exists only in the conduction and valence bands fairly away from the Fermi level nsc), and since the state density of redox electrons available for transfer decreases remarkably with increasing deviation of the electron level (with increasing polarization) from the Fermi level CFciiEDax) of the redox electrons, the exchange transfer current of redox electrons is fairly small at semiconductor electrodes compared with that at metal electrodes as shown in Fig. 8-12. 

As shown in Fig. 8-12 and in Fig. 8-13, the transfer current of redox electrons can be divided into the transfer current, in(c), carried by electrons Ccb in the conduction band and the transfer current, ip(e), carried by holes hvB in the valence band as shown in Eqn. 8-41 ... 

The overall transfer currents of redox electrons can be obtained by integrating these equations with respect to electron energy e from the lower edge of the conduction band to infinity for the conduction band mechanism and firom minus infinity to the upper edge of the valence band for the valence band mechanism. 

It follows from Eqn. 8-53 that the ratio of participation of the conduction band to the valence band in the exchange reaction current depends on the standard Fermi level of the redox electrons relative to the middle level in the band gap at the interface of semiconductor electrode. 

Figures 8-16 and 8-17 show the state density ZXe) and the exchange reaction current io( ) as functions of electron energy level in two different cases of the transfer reaction of redox electrons in equilibrium. In one case in which the Fermi level of redox electrons cnxEDax) is close to the conduction band edge (Fig. 8-16), the conduction band mechanism predominates over the valence band mechanism in reaction equilibrium because the Fermi level of electrode ensa (= nREDOK)) at the interface, which is also dose to the conduction band edge, generates a higher concentration of interfadal electrons in the conduction band than interfadal holes in the valence band. In the other case in which the Fermi level of redox electrons is dose to the valence band edge (Fig. 8-17), the valence band mechanism predominates over the conduction band mechanism because the valence band holes cue much more concentrated than the conduction band electrons at the electrode interface.

From these illustrations it follows, in general, that Ihe transfer reaction of redox electrons at semiconductor electrodes occurs via the conduction band mechanism if its equilibrium potential is relatively low (high in the Fermi level of redox electrons) whereas, the transfer reaction of redox electrons proceeds via the valence band mechanism if the equilibriiun redox potential is high (low in the Fermi level of redox electrons). 

Fig. 8-16. Electron state density in a semiconductor electrode and in hjrdrated redox partides, rate constant of electron tunneling, and exchange redox current in equilibrium with a redox electron transfer reaction for which the Fermi level is close to the conduction band edge eF(sc) = Fermi level of intrinsic semiconductor at the flat band potential 1. 0 (tp.o) = exchange reaction current of electrons (holes) (hvp)) - tunneling rate constant of electrons (holes).

We consider a transfer of redox electrons at semiconductor electrodes polarized at an overvoltage t relative to the equilibrium redox potential (the Fermi level cfcredox)). The transfer current of redox electrons is given in Eqn. 8-54 by the arithmetic sum of the electron current via the conduction band, in(ti) - (0(11) > and the hole current via the valence band, ij(ii) - i (Ti) ... 

Next, we consider the anodic reaction current of redox electron transfer via the conduction band, of which the exchange reaction current has been shown in Fig. 8-16. Application of a slight anodic polarization to the electrode lowers the Fermi level of electrode fix>m the equilibrium level (Ep(sc)( n = 0) = eiiOTSDca)) to a polarized level (ep(8C)( n) = ep(REDox)- n)withoutchanging at the electrode interface the electron level relative to the redox electron level (the band edge level pinning) as shown in Fig. 8-20. As a result of anodic polarization, the concentration of interfacial electrons, n, in the conduction band decreases, and the concentration of interfadal holes, Pm, in the valence band increases. Thus, the cathodic transfer current of redox electrons, in, via the conduction band decreases (with the anodic electron im ection current, ii, being constant), and the anodic transfer current of redox holes, (p, via the valence band increases (with the cathodic hole injection... 

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