Coupling of Electron and Ionic Charge Transport

The data obtained for Ru(bpy)3 illustrates such a situation. Based on the results from potential step chronoamperospectrometiy, Kaneko et al. concluded that the oxidation of Ru(bpy)3+ to Ru(bpy)3+ in Nation films takes place via electron hopping, but physical diffusion plays a key role in the reduction [194], which is in accordance with earlier findings [2]. 

The electron transfer distance, which includes the physical vibration of the redox species around its anchoring position (called bounded motion [26]) and the distance of the electron exchange reaction, increases as a function of potential due to the increase in the center-to-center distance, which is 1.13 mn at 1.1V and 1.47 nm at 1.5 V vs. SCE. The bounded motion distance, which is estimated as 0.25-0.31 nm, remains unchanged. 

The bimolecular rate coefficient of the electron transfer reaction (ke) also increases with increasing potential. The apparent diffusion coefficient (Dapp) for the reduction is higher than that measured for the oxidation. The relationship between 

An exponential decrease in the rate coefficient of the electron transfer (ke) as a function of the distance was assumed. As can be seen in Fig. 6.18, the rate coefficient corresponding to the redox complexes in close contact (kg) increases strongly with the potential, so increasing the electric field enhances both the electron hopping distance and the electron propagation rate [42,43]. 

The increase in the rate of electron transfer was assigned to the enhancement of the counterion migration rate [194]. The rate of the reduction increased linearly with the redox center concentration, while Dapp was independent of c, which in- 

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The solid solution KCl-RbCl differs basically from the solid solution NiO-MgO in two ways. Firstly, the system KCl-RbCl exhibits purely ionic conduction. The transport numbers of electronic charge carriers are negligibly small. Secondly, a finite transport of anions occurs. Because of these facts, the atomic mechanism of the solid state reaction between KCl and RbCl is essentially of a different sort than that between NiO and MgO. Once again, the diffusion profile exhibits an asymmetry (see Fig. 6-1). However, in this case the asymmetry arises not so much because of the variation of the defect concentration with composition, but rather because of the different mobilities of the ions at given concentration. Were the transport number of the chloride ions negligible, then the diffusion potential (which would be set up because of the different diffusion velocities of potassium and rubidium) would ensure that the motion of the two cations is coupled. If, on the contrary, the transference number of the chloride ions is one, then there is no diffusion potential, and the motion of the two cations is decoupled. 

Phonons are of special importance for the transport of charge carriers (Section 6.2). The lattice vibrations — determining the solid s breathing frequency [90] are indispensable for the phenomenon of ionic conduction, and also set an upper limit for it. As far as electron conduction is concerned, the phonon scattering of electrons limits the mobility of these carriers, on the other hand electron-phonon coupling represents the basic mechanism of superconductivity. 

The thermodynamics of insertion electrodes is discussed in detail in Chapter 7. In the present chapter attention is focused mainly on the general kinetic aspects of electrode reactions and on the techniques by which the transport of species within electrodes may be determined. The electrodes are treated in a general fashion as exhibiting mixed ionic and electronic transport, and attention is concentrated on the description of the coupled transport of these species. In this context it is useful to consider that an electronically conducting lead provides the electrons at the electrodes and compensates the charges of the ions transferred by the electrolyte. 

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