Overpotential surface film

This allows metal dissolution-deposition processes to occur at a relatively low overpotential. A typical example for such behavior is lithium in many polar aprotic systems. The nature of the surface films in these cases determines the following possibilities ... 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

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). 

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 ... 

Along with the anode reaction, the so-called anode effect, a phenomenon often observed in fused salt electrolysis (see Chapter 4), may occur. In the present case, it may be due to a surface film of the type CVX formed on the anode material. This film on the one hand protects the carbon against destruction (and is the reason for high anodic overpotentials) in normal operation and, on the other hand, under more or less known conditions may block electron transfer completely. These conditions depend strongly on the electrolyte composition (purity) [46,47]. Additives, such as lithium fluoride, may be helpful in preventing the anode effect by wetting the electrode material. 

The voltammetric behavior of calcium electrodes is controlled by the surface chemistry described in 1-6. In Ca(C104)2 solutions, the electrodes are strongly passivated, due to the formation of CaC. Hence, high overvoltage (>1 V versus Ca/Ca ) is required in order to drive any anodic process of calcium in the solvents/Ca(C104)2 solutions. In contrast, Ca electrodes dissolve at low overpotential in BF4 salt solutions of all of the above solvents. The lowest overpotential required to obtain a massive Ca dissolution was measured in AN/TBABF4 or AN/Ca(BF4)2 solutions (this being in line with the results in Ref. 448). As discussed in Ref. 449, the voltammetric response of Ca electrodes in these solutions reflects Ca dissolution via a breakdown and repair mechanism of the surface films. 

In the case of Cu, due to the lower exchange current density value, a surface film is practically formed by a smaller quantity of electricity (Fig. 2.20b). The value of the deposition overpotential is larger than in the case of Cd, and the crystallization overpotential is lower, resulting in a decrease of the zero nucleation zone radiuses. In the case of Cu, it is clear that a considerably larger nucleation rate is observed. 

It is obvious that the larger nucleus density, the thinner is the thickness of the metal film required to isolate the substrate from the solution. At the same time, a thinner surface film will be less coarse than a thicker one. This means that a smoother and thiimer surface film will be obtained at larger deposition overpotentials and nucleation rates, i.e., by electrodeposition processes characterized by high cathodic Tafel slopes and low exchange current densities. 

If mercury is used as the cathode, the discharge of sodium ions will not produce a surface film of sodium metal, so the value -2.71V is irrelevant. The sodium amalgamates with the mercury and diffuses away into the interior, so that the activity of sodium at the surface is extremely small. This, combined with the very high hydrogen overpotential (q.v.) at a mercury cathode, especially at fairly high current densities, ensures that sodium discharge is the predominant reaction. 

The relative proportions of oxygen and chlorine evolved will be dependent upon the chloride concentration, solution pH, overpotential, degree of agitation and nature of the electrode surface, with only a fraction of the current being used to maintain the passive platinum oxide film. This will result in a very low platinum consumption rate. 

In acid conditions oxide films are not usually present on the metal surface and the cathodic reaction is primarily that of hydrogen discharge rather than oxygen reduction. Thus, inhibitors are required that will adsorb or bond directly onto the bare metal surfaces and/or raise the overpotential for hydrogen ion discharge. Inhibitors are usually organic compounds... 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

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