Electrode polarization effect

Values of s and s" are calculated using equations in which the contributions of dipolar effects, ionic displacements, and electrode polarization effects are additive. If conduction effects predominate, i.e., when neither interfacial effects nor dipolar effects are significant, the loss factor is given by (Kranbuehl et al., 1986) ... 

In the continued electrolysis of each of the following solutions at pH 7.0 and 25°C, predict the main product at each electrode if there are no (irreversible) electrode polarization effects (a) 1 M NiS04 with palladium electrodes (b) 1M NiBr2 with inert electrodes (c) 1M Na2SC>4 with Cu electrodes. 

Kinetic processes in an electrochemical reaction must be considered when current is extracted from a battery, that is, when the current passed through an external circuit is different from zero. In this case, while current is removed from the battery, the equilibrium voltage or open circuit voltage falls because of electrode polarization or overvoltage [6,8,10,66,123,124], This electrode polarization effect occurs owing to kinetic limitations experienced by the reactions, and because of other processes taking place during the production of current flow. 

Ionic conductivity has another important implication. The resin system acts like an electrolyte thus, all of the electrode polarization effects that can be observed in conventional electrolytes can also be observed in resins. The effect of electrode polarization is discussed in Section 3.2. 

Figure 11 a illustrates the frequency dependence of e for Eq. (3-6). Note that e is midway between eu and er when co = l/id. The corresponding plots for s" are more complex, because one must assess the relative contributions of a and the dipole loss. The simplest case is for cr = 0 (Fig. lib), where the characteristic dipolar loss peak of amplitude (sr — eu)/2 is observed at frequency co = l/td. For non-zero ct, however, the 1/co dependence of e" greatly distorts the e" curve from the ideal Debye peak. Log-log scales are helpful, as illustrated in Fig. 12. The ct = 0 case is replotted from Fig. lib also plotted are the frequency dependences of e" for CTTd/Eo having various values relative to er — eu. Asct increases, it becomes increasingly difficult to discern the dipole loss peak. Roughly speaking, for CTTd/Eo greater than about three times er, the observed e" is entirely dominated by ct. (Ideally, even when cr dominates the dipolar contribution to e", it should still be possible to observe the dipolar contribution to e however, when o is large, electrode polarization effects tend to dominate the e measurement as well. See Sec. 3.2.1).

The first attempt to use these ideas in epoxy cure was by Fisch and Hofmann 66), but their assignment of permittivity changes to changes in polar group concentrations was marred by what we interpret as electrode polarization effects. Blyakhman et al. 51 52), examined the post-cure dielectric permittivity and loss tangent of anhydride-... 

Electrical conductivity is easily determined by measuring the impedance of the sample in the low frequency range (see Section 22.6.3), but at a frequency above the range in which errors arise owing to electrode polarization effects. For example, measurement of the electrical conductivity of cream, which can be carried out by measuring the impedance between a pair of stainless steel or platinum electrodes immersed in the cream sample, should be done at a frequency of 105 Hz or higher (Lawton and Pethig, 1993). 

Both alternating and direct current techniques can be used (see also impedance spectroscopy), but the electrode polarization effects should be minimized or taken into account in all cases. For this goal, a four-electrode method where the potential probes are placed between current probes, is often used. 

Neglecting electrode polarization effects, predict the principal product at each electrode in the continued electrolysis at 25 °C of each of the following (a) 1 M Fe2(S04)3 with inert electrodes in 0.1 M H2SO4. (b) 1 M LiCl with silver electrodes, (c) 1 M FeS04 with inert electrodes at pH 7.0, (d) molten NaF with inert electrodes. 

Tertiary current distribution. This method of analysis applies to those systems where there is significant mass transport and electrode polarization effects. Electrode kinetics is considered, with electrode surface concentrations of reactant and/or products that are no longer equal to those in the bulk electrolyte due to finite mass transfer resistance. The analysis of tertiary current distributions is complex, involving the solution of coupled... 

The application of a dc voltage V will result in the flow of an ionic current /ion from the anode to the cathode. If one assumes that electrode polarization effects can be ignored, then the ratio is a measure of the ionic... 

In this process, ions migrate to the electrode surfaces, producing localized concentrations that increase with a decrease in co. If a chemical reaction between the electrode material and the ions also occurs, an irreversible, nonequilibrium process sets in, and the effects of the reaction products begin to contribute. Such effects are observed in liquids and solids and are relatively small at high oj or low Udc values. But when <7ionic process exceeds 1 /rS/m, the electrode polarization effects begin to contribute significantly to the e and e" values in the low-frequency part of the spectrum, and its magnitude needs to be determined. 

Figure 9 Equivalent circuit accounting for the electrode-polarization effect. Fq(0 is a rapidly increasing voltage step I(i) is a current Zq is the coaxial line impedance Cp is the capacitance of electrode polarization Cq is an empty cell capacitance filled with a dielectric sample of permittivity e and conductivity 1/R Vp(t) and Fg(i) are the voltages at the appropriate parts of the circuit. (From Ref. 72. With permission from Elsevier Science B.V.)...

The parameter X2 may be obtained from the tail of the signal where only the electrode polarization effect takes place ... 

Actually, by noise spectrography measurements, electrode polarization effects are considerably eliminated and there is not an induced moment due to an external applied field. 

These deviations are caused by electrode polarization owing to double layer formation in electrode-electrolyte interface. Only the semicircle refleets the bulk conductivity of the polymer electrolyte. In contrast to PEO, ENR-25 displays even at high salt contents dramatic electrode polarization effects. This dominating electrode polarization disappears only at high salt concentrations, Y > 0.2. 

Formerly, conductivity was frequently measured by d.c. techniques using either reversible electrodes (e.g. H, Pd, H MoGj, H Ti) and low voltages or just by electrolysing the sample at voltages above the decomposition potential. The former often suffers from electrode polarization effects, whereas the latter yields reliable results only if proton conductivity is the rate determining step for the overall current. 

In addition, ionic conduction causes a charge qiit) to pass through the material, which rises from zero and then becomes linearly dependent on time to give a constant current I(t) = dgi/d. In contrast to qp(t), which becomes constant at long times (since the dipolar system reaches thermodynamic equilibrium in the presence of the steady field), qiit) increases without limit in the absence of interfacial and electrode polarization effects, giving a continual dissipation of energy via the conduction process. The fact that the steady conduction current takes time to get established means that the ac real conductivity a ico) is constant at low frequencies and becomes /-dependent at high frequencies. 

These considerations are the reason behind the phenomenological approach taken in Chapter 2 with respect to alternating-current electrode polarization effects. It is virtually impossible to determine rate constants and surface-chemical environments under the conditions of applied broad-spectrum signals. 

It is suggested that a 1-kHz ac voltage be used for this determination to minimize electrode-polarization effects. Stray capacitance should enter into the measurement only as a phase shift in relative to F. Since voltage amplitudes are used, this is of no consequence. 

In Fig. 4, we see how temperature influences the shape and the values of the dielectric loss and the corresponding conductivity spectra of PAH/PSS PEM. At a given frequency, the conductivity increases with temperature. One also sees that the transition into the dispersive regime shifts to higher frequencies when the temperature is raised. Electrode polarization effects are visible at the lowest frequencies. 

---