Electrolytes electron transfer

Finally, it should be noted that non-Butler-Volmer behaviour may be observed in the analysis of cyclic voltammetric data. For example, particularly in the presence of low concentrations of supporting electrolyte, electron transfer kinetics of charged species may be significantly modified due to the double layer or Frumkin effects [79]. Under these conditions, (i) the potential experienced by the reactant at the point of closest approach to the electrode can be different from the applied potential, and (ii) an additional energy barrier for the approach of charged reactants to the electrode may exist. Corrections to account for Frumkin effects have been proposed. Deviations from Butler-Volmer behaviour may also be interpreted in terms of the Marcus theory [80]. A further interesting case of non-Butler-Volmer voltammetric characteristics is observed with semiconducting elecfrode materials [81]. 

Finally, it should be noted that non-Butler-Volmer behaviour may be observed in the analysis of cyclic voltammetric data. For example, particularly in the presence of low concentrations of supporting electrolyte, electron transfer kinetics of charged species may be significantly modified due to the double layer or Frumkin effects... 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

The situation in figure C2.8.5(b) is different in that, in addition to the mechanism in figure C2.8.5(a), reduction of the redox species can occur at the counter-electrode. Thus, electron transfer tlirough the layer may not be needed, as film growth can occur with OH species present in the electrolyte involving a (field-aided) deprotonation of the film. The driving force is provided by the applied voltage, AU. 

The rate of an electrochemical process can be limited by kinetics and mass transfer. Before considering electrode kinetics, however, an examination of the nature of the iaterface between the electrode and the electrolyte, where electron-transfer reactions occur, is ia order. 

Reductants such as zinc or sodium dithionite generate the semidione from diketones. Electrolytic reduction can also be used. Enolates can reduce diones to semidiones by electron transfer. 

When equimolar quantities of 80a and its dication 110 are combined in acetonitrile, single electron transfer occurs and the coproportionation product was obtained (95TL2741).Tliis deeply red-colored, air-sensitive radical cation 111 showed a strong ESR signal (g = 2.0034). On the other hand, the excellent electron donor 80a could be prepared by electrolytic reduction starting from 110. It was necessary to carry out the reduction with scrupulous exclusion of oxygen. Tlius, the electrolysis of 110 at -1.10 V initially gave rise to an intense red color, which was presumably due to the formation of 111. Upon further reduction, the red color faded and the tetraaza-fulvalene 80a was isolated at a 62% yield (Scheme 45). 

Lithium carbonate and hydrocarbon were identified in XPS spectra of graphite electrodes after the first cycle in LiPF6/EC-DMC electrolyte [104]. Electrochemical QCMB experiments in LiAsF6/EC-DEC solution [99] clearly indicated the formation of a surface film at about 1.5 V vs. (Li/Li+). However the values of mass accumulation per mole of electrons transferred (m.p.e), calculated for the surface species, were smaller than those of the expected surface compounds (mainly (CF OCC Li ). This was attributed to the low stability of the SEI and its partial dissolution. 

Electronic conductivity of thin-film solid electrolytes. Besides having low electronic transference numbers, it is essential for thin films of the order of 1 jim that the magnitude of the electronic resistance is low in order to prevent self-discharge of the battery. For this reason, specific electronic resistances in the range of 1012-1014 Qcm are required for thin-film solid electrolytes. Often the color may be a valuable indication of the electronic conductivity. In this regard, solid electrolytes should preferably be transparent white [20]. 

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

The reversible voltage is 2.8-3.0 V and the operating voltage is >7 Y. Details about electron transfer from the bulk electrolyte into the carbon base of the anode are not clear. 

Electron transfer rate and its exponential increase at zinc oxide-electrolyte interfaces, 512 Electronic conductivity... 

It is apparent that, as in chemical systems, the magnitude of these effects will become useful and interesting from a practical viewpoint only when the pressure is increased above one kilobar. Thus for a typical electron transfer reaction with JF"=—20 cm mole , AE will be 211 mV when the pressme is ten kilobars. This shift could be important in the not uncommon situation where, at atmospheric pressure, the oxidation of a neutral substrate occurs at around the same potential as the anion of the base electrolyte. An increase in the pressure to ten kilobars will result in a separation of the processes... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Electrochemical reductions and oxidations proceed in a more defined and controllable fashion because the potential can be maintained at the value suitable for a one-electron transfer and the course of the electrolysis can be followed polarographically and by measurement of the esr or electronic spectra. In some cases, conversion is low, which may be disadvantageous. Electrolytic generation of radical ions is a general method, and it has therefore become widely used in various applications. In Figures 3 and 4, we present electrochemical cells adapted for esr studies and for measurements of electronic spectra. Recently, electrochemical techniques have been developed that permit generation of unstable radicals at low temperatures (18-21). 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

Fig. 5.2 The n-Cd(Se,Te)/aqueous Cs2Sx/SnS solar cell. P, S, and L indicate the direction of electron flow through the photoelectrode, tin electrode, and external load, respectively (a) in an illuminated cell and (b) in the dark. For electrolytes, m represents molal. Electron transfer is driven both through an external load and also into electrochemical storage by reduction of SnS to metaUic tin. In the dark, the potential drop below that of tin sulfide reduction induces spontaneous oxidation of tin and electron flow through the external load. Independent of illumination conditions, electrons are driven through the load in the same direction, ensuring continuous power output. (Reproduced with permission from Macmillan Publishers Ltd [Nature] [60], Copyright 2009)...

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

Koval CA, Howard JN (1992) Electron transfer at semiconductor electrode-liquid electrolyte interfaces. Chem Rev 92 411 33... 

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