Overpotential polarization

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

See also crystallization overpotential (polarization), - nucleation and growth kinetics, - equilibrium forms of crystals and droplets, - half-crystal position, - Kaischew, - metal deposition, - supersaturation, - Stranski, - Zeldovich. 

Crystallization overpotential (polarization) — Figure. Schematic representation of the mechanism of a metal deposition... 

Theoretical prediction of the overpotential polarization at a given current density is generally not possible, and it is impossible to determine the current densities to be expected during corrosion, a measure of the corrosion rate, from the equilibrium potentials. The relationship between potential and current density can normally only be determined experimentally. 

In electrochemical studies, -q is usually called overpotential. Polarization voltage is a more general term, which includes voltage loss due to a transport of reactants to the catalyst sites. In the following, we will use both terms as synonyms. 

Unfortunately, mechanisms of elementary steps, e.g., electron transfer, in electrode processes were not at all well understood during the period of early development of electro-organic chemistry, and the significance of the fundamental relation between overpotential ( polarization ) and current density was not appreciated until the work of Volmer, Gurney, Bowden,... 

Figure 5.2 Current versus overpotential polarization plot of the ferric/ferrous ion reaction on palladium showing both the anodic and cathodic branches of the resultant current behavior.

As the Nemst equation suggests, concentration variations in the electrolyte lead to potential differences between electrodes of the same kind. These potential differences are concentration polarizations or concentration overpotentials. Concentration polarizations can also affect the current distribution. Predicting these is considerably more difficult. If concentration gradients exist, equations 25 and 27 through 29 must generally be solved simultaneously. 

Typical polarization curves for SOFds are shown in Fig. 27-67. As discussed earlier, the open-circuit potential of SOFds is less than 1 because of the high temperature, but the reaction overpotentials are... 

The ESCR model allows us to derive from both Eq. (51) and the above experimental results an expression for the interchain free volume (od) left inside the polymeric structure after polarization at a given cathodic overpotential rjc ... 

We define a nucleation overpotential rjN EN E0 (Fig. 36) required to make the N0 oxidation nuclei appear. The nucleation overpotential is related to the degree of closure (compaction) of the polymeric entanglement ( ), expressed as the fraction of interchain free volume destroyed after polarization at a given potential Ec, compared with the amount of free volume present at Es. 

D.Y. Wang, and A.S. Nowick, Cathodic and anodic polarization phenomena at platinum electrodes with doped Ce02 as electrolyte. I. Steady-state overpotential, J. Electrochem. Soc. 126(7), 1155-1165 (1979). 

For the case of Ag it was found that both anodic and cathodic polarization lead to the creation of small insulated particles on the YSZ surface. Both the Fermi level and the work function of the insulated Ag particles was found to change with Ag electrode overpotential but the changes are smaller than on the continuous Ag film.24... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

For an analysis of the polarization curves at low values of polarization (low overpotentials), we shall use the general polarization equation... 

For isolating the overpotential of the working electrode, it is common practice to admit hydrogen to the counter-electrode (the anode in a PEMFC the cathode in a direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode. Furthermore, the overpotential comprises losses associated with sluggish electrochemical kinetics, as well as a concentration polarization related to hindered mass transport ... 

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

Plotting the overpotential against the decadic logarithm of the absolute value of the current density yields the Tafel plot (see Fig. 5.3). Both branches of the resultant curve approach the asymptotes for r RT/F. When this condition is fulfilled, either the first or second exponential term on the right-hand side of Eq. (5.2.28) can be neglected. The electrode reaction then becomes irreversible (cf. page 257) and the polarization curve is given by the Tafel equation... 

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

At low overpotentials, the silver electrode prepared according to Budev-ski et al. behaves as an ideal polarized electrode. However, at an overpotential higher than —6 mV the already mentioned current pulses are observed (Fig. 5.48A). Their distribution in the time interval r follows the Poisson relation for the probability that N nuclei are formed during the time interval x... 

The voltage demand was the sum of the various overpotentials and ohmic loss, since there was little concentration difference in H2 across the membrane in these experiments. These polarizations were not individually identified. 

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