Charge transfer region resistance

By EIS analysis of the corresponding lithium ion cells, Zhang et al. showed that the impact of SEI resistance on total cell impedance was rather negligible, and hence, they attributed the superior low-temperature behavior of LiBF4-based electrolytes to the lower resistance associated with the so-called charge-transfer processes , which are usually represented in impedance spectra by the semicircle at the lower frequency region. This suggestion could be viewed as a further extension of the conclusion... 

Figure 6.61 shows the four parameters (Rf, Rct, Rc. and f) with potential. In the region of 370-600 mV versus RHE, the inverse of the Faradaic resistance and charge-transfer resistance versus electrode potential gives the Tafel slope. The changing features in these plots indicate a change in the mechanism of methanol electrooxidation. 

The resistor R and capacitor C shown in Table 16.1 can take on different meanings for different electrochemical systems. The resistance may, for example, be associated with the charge-transfer resistance of an electrochemical reaction, with the resistance of an oxide or porous layer, or with the electronic resistance of a semiconductor. The capacitor C may be associated with the double layer for an electrode in electrolyte, with surface capacitance of a film, or with the space-charge region of a semiconductor. The resistor Re may be associated with the Ohmic resistance of the electrolyte or with the frequency-independent resistance of a solid. 

Figure 2.20 Reduction current vs. potential 4>sci across the space-charge region at -GaAs in acetonitrile with cobaltocenium (CoCp2 ) R = charge-transfer resistance, Ug = electrode potential (Meier etal., 1999).

This raises some important possibilities, which have not escaped the attention of the electroplating community. For example, while metal deposition is conducted in fairly concentrated solutions of the metal being plated, and at current densities well below the mass-transport limit, additives acting as inhibitors for metal deposition are often introduced at concentrations that are several orders of magnitude lower, to ensure that their supply to the surface will be mass-transport limited. In this way, the tendency for increased rate of metal deposition on certain features on the surface, such as protrusions, will be moderated by the faster diffusion of the inhibitor to the very same areas. Furthermore, if deposition occurs in the region of mixed control, which is usually the case, it must be remembered that the relevant roughness factor is quite different for the charge-transfer and the mass-transport processes, and this may well be a function of current density, since the Faradaic resistance is inherently potential dependent. 

Equation (50) forms the basis upon which v can be evaluated (e.g. (1) by the radioactive tracer method to evaluate simultaneously and ), (2) by comparing i values at appropriate potentials for different reactant activities (3) coupling information from high and low overpotential regions of steady-state polarization curves " (extrapolated io and charge-transfer resistance, Rcr, respectively) (4) or by back-reaction correction analysis. 2 qqie first two methods involve determination of v at any single potential while the latter two procedures must assume that the same mechanism (and hence v) applies at different potentials (at which individual measurements are required) and that the reverse reaction occurs by the same path and has the same transition state and thus rate-determining step [for both forward (cathodic) and reverse reactions]. 

In the CP-rich bath type, experimental transfer coefficients were ttc= a - 0.5 0.05, while in the H -rich baths the values were ttc= 0.3 0.05 and = 0.65 0.05. The stoichiometric number for the process was calculated by coupling the charge-transfer resistance with the extrapolated exchange current density from low and high overpotential regions, respectively, and by back-reaction correction. " It was determined to be 3 by both methods and for all bath compositions. It has been suggested that... 

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