Tafel-like behavior

At high electrical field, azFx > RTl, and thus a Tafel-like behavior is obtained ... 

A Tafel-like behavior for the intrinsic redox behavior of the Li/Li+ could be measured, providing i0 values of 31.6 mA/cm2 (at 25°C) and (3 = 0.67 [89], These high values of the i0 of the Li/Li+ couple mean that the charge transfer resistance (/ CT) of Li electrodes is negligible compared with the surface film resistance (for Li+ migration). 

Anodic polarization of Ca electrodes in TC leads to current passage and dissolution of the active metal at high efficiency. As expected for SEI electrodes, a Tafel-like behavior connects the current and the overpotential applied [see Eqs. (5)—(11) in Section V.C.3], It is assumed that upon anodic polarization the anions (Cl-) migrate from the surface film s solution interface to the surface film s metal interface. Two processes can thus occur ... 

It is interesting that, despite the high nonuniformity of currents along a CER (Fig. 36), a plot of In versus 6 often yields a Tafel-like behavior from which an apparent transfer coefficient and rate constant can be extracted (60-62). Thus, potential-current density data are not sufficient to indicate multiple reactions. At long retention times in the reactor, however, an unusual maximum and subsequent decrease of the average current density with potential occurs for series reactions (60). This results from fast depletion of species A and B with potential at long space-times, but it is not related to zero concentrations or mass transport-limited reactions. Such maxima or limiting currents have been observed in the stepwise oxidation of unsaturated... 

Assuming Tafel-like behavior via Equation 21.32, that is, = k [A 2,... 

Assumption of Tafel-like behavior permits combining this equation with... 

Within this polarization interval the plateau is retained, but its potential becomes strongly dependent on Therefore a Tafel-like behavior is expected in this case for redox reactions, both in cathodic and anodic directions, taking place at the polymer/solution interface. This is similar to reactions at the metal electrode, though with a more complicated... 

The surface films discussed in this section reach a steady state when they are thick enough to stop electron transport. Hence, as the surface films become electrically insulating, the active electrodes reach passivation. In the case of monovalent ions such as lithium, the surface films formed in Li salt solutions (or on Li metal) can conduct Li-ions, and hence, behave in general as a solid electrolyte interphase (the SEI model ). See the basic equations 1-7 related to ion transport through surface films in section la above. The potentiodynamics of SEI electrodes such as Li or Li-C may be characterized by a Tafel-like behavior at a high electrical field and by an Ohmic behavior at the low electrical field. The non-uniform structure of the surface films leads to a non-uniform current distribution, and thereby, Li dissolution from Li electrodes may be characterized by cracks, and Li deposition may be dendritic. The morphology of these processes, directed by the surface films, is dealt with later in this chapter. When bivalent active metals are involved, their surface films cannot conduct the bivalent ions. Thereby, Mg or Ca deposition is impossible in most of the commonly used polar aprotic electrolyte solutions. Mg or Ca dissolution occurs at very high over potentials in which the surface films are broken. Hence, dissolution of multivalent active metals occurs via a breakdown and repair of the surface films. 

Although Butler s model was successful in describing the photoelec-trochemical kinetics at certain semiconductor electrodes, it is now clear that the assumptions made in his model are valid only for very limited cases.The following experimental results suggest that the surface steps control the electrochemical reaction rate under illumination. They are (1) the Tafel-like behavior of the photocurrent-potential relations, " (2) the nonlinear relation between the photocurrent and the light intensity, (3) the photocurrent enhancement by addition of an easily reactive compound,and (4) the photocurrent enhancement by the modification of an electrode surface with metals or metal ions which catalyze the electrochemical reactionor passivate the surface recombination. Thus, only a part of photogenerated... 

The use of galvanostatic transients enabled the measurement of the poten-tiodynamic behavior of Li electrodes in a nearly steady state condition of the Li/film/solution system [21,81], It appeared that Li electrodes behave potentio-dynamically, as predicted by Eqs. (5)—(12), Section III.C a linear, Tafel-like, log i versus T dependence was observed [Eq. (8)], and the Tafel slope [Eq. (10)] could be correlated to the thickness of the surface films (calculated from the overall surface film capacitance [21,81]). From measurements at low overpotentials, /o, and thus the average surface film resistivity, could be measured according to Eq. (11), Section m.C [21,81], Another useful approach is the fast measurement of open circuit potentials of Li electrodes prepared fresh in solution versus a normal Li/Li+ reference electrode [90,91,235], While lithium reference electrodes are usually denoted as Li/Li+, the potential of these electrodes at steady state depends on the metal/film and film/solution interfaces, as well as on the Li+ concentration in both film and solution phases [236], However, since Li electrodes in many solutions reach a steady state stability, their potential may be regarded as quite stable within reasonable time tables (hours —> days, depending on the system s surface chemistry and related aging processes). 

Despite these successes, important process parameters, like bath agitation, bath constituents and particle type are disregarded. The constants k, 0 and B inherently account for these constants, but they have to be determined separately for every set of process parameters. Moreover, the postulated current density dependence of the particle deposition rate, that is Eq. (2), is not correct. A peak in the current density against the particle composite content curve, as often observed (Section III.3.H), can not be described. The fact that the peak is often accompanied by a kink in the polarization curve indicates that also the metal deposition behavior can not be accounted for by the Tafel equation (Eq. 4). Likewise, the (1-0 term in this equation signifies a polarization of the metal deposition reaction, whereas frequently the opposite is observed (Section 111.3,(0 It can be concluded that Guglielmi s mechanism... 

Note that lezyl should not be too small—the Tafel law holds only beyond a bias that satisfies kz > k[jT. When rj —> 0 the net current which results from the balance between the direct and reverse reactions, must vanish like rj. This imphes that the Tafel behavior is always preceded by a low bias Ohmic regime. 

The anodic Tafel line should, therefore, become very steep and show a limiting current behavior when 0 approaches zero, or a +2 approaches zero, as indeed is observed on Pd (Figure 13). It seems likely that the same concept also applies to analogous behaviors on some other metals, ° although similar experiments would no longer be possible for obtaining a +2. 

The ORR mechanism on Ag in alkaline media shows many similarities with that on Pt, with similar Tafel behavior and reaction order in O2, with both two-electron and four-electron pathways occurring, and with the reaction most likely proceeding via a superoxide intermediate [2]. On clean Ag surfaces, the four-electron ORR pathway predominates, but this selectivity is sensitive to surface contamination. On iodine-poisoned Ag surfaces, the two-electron ORR pathway predominates. 

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