Polarization curve reversible

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

Figure 6.7 shows a typical special feature of the polarization curves. In the case of reversible reactions (curve 1), the anodic and cathodic branches of the curve form a single step or wave. In the case of irreversible reactions, independent, anodic and cathodic, waves develop, each having its own inflection or half-wave point. The differences between the half-wave potentials of the anodic and cathodic waves will be larger the lower the ratio fH. ... 

Reversible polarization curve (voltammogram). If the value of k° is so large that the first term on the right-hand side of Eq. (5.4.12) is much larger than the second term, even when j approaches d, then... 

This is the equation of a reversible polarization curve. The anodic polarization curve of the reduced form obeys an identical equation... 

These reactions proceed very rapidly, so that the overall reaction corresponds to the transfer of two electrons. As reaction (5.7.9) is very slow in acid and neutral media, the electrode reaction is irreversible and the polarization curve does not depend on the concentration of hydrogen ions. In weakly alkaline media, reoxidation of H02 begins to occur. At pH > 11, the polarization curve at a dropping mercury electrode becomes reversible. In this way, the process proceeds in water and water-like solvents. On the other hand, for example in carbonate melts, the step following after the reaction (5.7.9) is the slow reaction 02 + e = 022-. 

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

Polarization curves are recorded at temperatures with intervals of 1 C around the expected CPT. A new specimen is used for each polarization. The specimen and the electrolyte (usually neutral 1 mole/liter NaCl) are thermostatted at the selected temperature. Referring to Fig. 11, when the temperature has stabilized, the anodic polarization is started at 300 mV SCE and reversed at a given current density (inv = 5 mA/cm ) the polarization is stopped when the current density has decreased to a level of 10 fi A/crn. ... 

Figure 4. Typical polarization curve of the electrooxidation of hydrogen and electroreduction of oxygen the exchange current density, io, determined by extrapolation of E vs, log i to the reversible potential...

Based on these assumptions the measurement of the large signal ferroelectric hysteresis with additional measurements of the small signal capacitance at different bias voltages are interpreted in terms of reversible and irreversible parts of the polarization. As shown for ferroelectric thin films in Figure 1.24, the separation is done by substracting from the total polarization the reversible part, i. e. the integrated C(V)-curve [18]. 

Because the flow of electric current always involves the transport of matter in solution and chemical transformations at the solution-electrode interface, local behavior can only be approached. It can be approximated, however, by a reference electrode whose potential is controlled by a well-defined electron-transfer process in which the essential solid phases are present in an adequate amount and the solution constituents are present at sufficiently high concentrations. The electron transfer is a dynamic process, occurring even when no net current flows and the larger the anodic and cathodic components of this exchange current, the more nearly reversible and nonpolarizable the reference electrode will be. A large exchange current increases the slope of the current-potential curve so that the potential of the electrode is more nearly independent of the current. The current-potential curves (polarization curves) are frequently used to characterize the reversibility of reference electrodes. 

Such cathodic loop behavior is often observed on the reverse scans of polarization curves in which pitting does not occur as shown in Fig. 10 (9). During the initial anodic scan, the oxide is thickening and the anodic line is moving to the left. Thus, upon the return scan, the unchanged cathodic line now intersects the anodic line at several places, leading to the appearance of cathodic loops. Cathodic loops do not pose fundamental problems they merely conceal the passive current density at potentials near the active-passive transition. 

Figure 10 Polarization curve for Type 302 stainless steel in 0.5% HC1. Note the presence of a cathodic loop on the return scan due to the greatly reduced passive current density. Also, note the lowered critical current density on the reverse scan due to incomplete activation of the surface. (From Ref. 9.)...

Figure 23 Schematic polarization curve for metal that spontaneously passivates but pits upon anodic polarization. A hysteresis loop, which can appear during a reverse scan, is shown ending at Erp. One dotted line shows behavior for anodizing conditions, while the other shows transpassive dissolution.

The -> polarization curves for irreversible and quasireversible systems are shown in Figure (a). The respective -> Tafel plots are presented in Figure (b). Tafel plots can be constructed only for electrochemically irreversible systems, and kinetic parameters can be determined only when irreversible kinetics prevails. A switching from reversible to irreversible behavior and vice versa may occur. It depends on the relative values of ks and the -> mass transport coefficient, km. If km ks irreversible behavior can be observed. An illustration of the reversibility-irreversibility problem can be found in the entry -> reversibility. 

CLs), resulting in a drastic drop in cell performance [17], Figure 3.13 also shows the difference between the theoretical cell potential (1.23 V) and the thermoneutral voltage (1.4 V), which represents the energy loss under reversible conditions (the reversible loss) [18], Very often, polarization curves are converted to power density versus current density plots by multiplying the cell voltage by the current density at each point of the curve. 

Typical curves schematically showing the current-voltage characteristics of a redox system are shown in Fig. 2. Curves A and C are the polarization curves for the anodic and cathodic reaction, respectively. Eeq is die reversible potential for the various concentrations of the cathodic reactant - tne metal ion in the case of a metal - and Cp C2 and the polarization curves for decreasing concentration. 

A kinetic analysis is not complete without determination of the temperature effects and activation energies. Figure 6 summarizes some of the polarization curves for the ORR recorded at 333 K and 298 K for details, see [41]. Clearly, results obtained at 333 K are qualitatively similar to the curves recorded at room temperature, and the order of activity remains the same as at room temperature, i.e., Pt(lll)elevated temperatures in both the mixed diffusion-kinetic potential region and the hydrogen adsorption potential region. These higher currents reflect the temperature dependence of the chemical rate constant, which is approximately proportional to jRT where is the apparent enthalpy of activation at the reversible... 

Kinetic data extracted from the foot of RDE dynamic polarization curves for 02 reduction yielded for pH < 11 linear log[i/(ilim — i)] versus E, or Tafel plots, with a slope of around 120 mV per decade, and thus consistent with the first electron transfer as being rate determining for the reduction of the CoPI-02 adduct. As expected for a reversible (Nerstian) two-electron redox couple, the Tafel slope at pH = 14, decreased to 30 mV per decade. It is interesting to note that an oxidized form of the closely related CoOEP displays extraordinary reversibility for the 02—H202 couple in solutions of pH < 1 [63]. 

During the past few years it has been reported that the reversible electron electrode can be realized in solutions of solvated electrons in hexamethylphospo-triamide against a background of lithium 3-i65) sodium 21,165,166) the system is reversible in these solutions is evidenced by the fact that the polarization curve in linear coordinates passes trough the origin of coordinates without any kink When the potential is more positive than the equilibrium potential,... 

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]. 

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