Over potential metal deposition

The bath around the electrode was slightly coloured blue at a potential below 0.05 V. This indicates that metal fog was formed in precedence over Li metal deposition, but its amount seemed very small. Meanwhile, remarkable metal fog formation was observed below 0 V, that is, after the Li deposition occurred. The metal fog formation following Li deposition tended to occur at bigger cathodic overpotential and higher temperature, as shown in Figure 2.4.3. 

In addition to the establishment and understanding of activity trends on nanosegregated surfaces, it has been anticipated that finding relationships between chemical and electronic properties of thin metal films of Pt group metals deposited over 3d and 5d elements has the potential to open up new opportunities in the quest to... 

Electrochemical deposition has two main advantages over chemical reduction. First, it is much faster with most deposition completed within five minutes. Second, the size of the metal nanoparticles and their coverage on the nanocarbon can be controlled by the concentration of the metal salt and various electrochemical deposition parameters, including nucleation potential and deposition time [124,127,205]. 

The ability to produce a deposit over a given surface, including recesses, is called covering power. As a point of practical importance, it may be noted that in some cases the required potential for deposition of the metal may not be reached in recesses and vias. Other processes, such as H2 evolution or reduction of ions such as Fe to Fe, may occur instead. In such cases, a preliminary strike deposit may be the answer. This is made by an extremely high current density for a very short time or in a specially formulated bath. 

It should be understood that even for micro surface features the potential is uniform and the ohmic resistance through the bath to peaks and valleys is about the same. Also, electrode potential against SCE will be uniform. What is different is that over micro patterns the boundary of the diffusion layer does not quite follow the pattern contour (Fig. 12.3). Rather, it thus lies farther from depth or vias than from bump peaks. The effective thickness, 8N, of the diffusion layer shows greater variations. This variation of 8N over a micro profile therefore produces a variation in the amount of concentration polarization locally. Since the potential is virtually uniform, differences in the local rate of metal deposition result if it is controlled by the diffusion rate of either the depositing ions or the inhibiting addition (leveling) agents. 

Iron carbonyls have been also used to fabricate nanostructures of potential use in catalysis. In this context, the preparation at room temperature of nano-sized a-Fe single crystals over carbon micro-grid films has been reported. The particles were prepared by electron beam induced deposition using Fe(CO)s as precursor [77]. The use of a focused electron beam to induce metal deposition from carbonyl compounds opens a new route for the preparation of nano-sized metal particles. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

Although the deposition of metals from ionic liquids has been possible for over 50 years, to date no processes have been developed to a commercial scale. There are numerous technical and economic reasons for this, many of which will be apparent from the preceding chapters. Notwithstanding, the tantalizing prospect of wide potential windows, high solubility of metal salts, avoidance of water and metal/water chemistry and high conductivity compared to non-aqueous solvents means that, for some metal deposition processes, ionic liquids must be a viable proposition. 

Under standard conditions and in the absence of kinetic hindrance, the electrode potential (versus a hydrogen electrode) determines the potential at which the corresponding metal will be deposited out of an aqueous solution. Therefore, metals that have a more negative electrode potential than the hydrogen electrode cannot be deposited from aqueous electrolytes. Kinetic barriers often disfavor the production of hydrogen over metal deposition. Thus, technically important metals, such as tin, nickel, and zinc can be electrolytically deposited out of aqueous solutions without any problems, even though their electrode potentials are lower than that of the hydrogen electrode. 

The properties of the surface layers have a strong effect on the deposition process. The driving force of the electrochemical reaction is the potential difference over the electrochemical double layer. Adsorption of species can change this potential. For example, the additives used in electrodeposition adsorb in the Helmholtz layer. They can change the local potential difference, block active deposition sites, and so on. The thickness of the diffusion layer affects the mass-transfer rate to the electrode. The diffusion layer becomes thinner with increasing flow rate. When the diffusion layer is thicker than the electrode surface profile, local mass-transfer rates are not equal along the electrode surface. This means that under mass-transfer control, metal deposition on electrode surface peaks is faster than in the valleys and a rough deposit will result. 

Zn-Al LDHs can be prepared by electrodeposition from aqueous solutions of the nitrates of such metals (Yarger et al., 2008). Cathodic depositions, conducted at room temperature without stirring over noble metal-coated electrodes, were achieved by reducing nitrate ions to generate hydroxide ions on the working electrode, the optimal potential being -1.65 V vs. AgCl/Ag in 4 M KCl. The overall reactions involved are... 

The problem, in the view of the present authors, is that the partial current density for deposition of, say, nickel is determined from the total amount of nickel deposited per unit time. However, in a solution containing Ni , Mo04 , NH3 and Cit , there can be as many as nine different species from which nickel could be deposited (six complexes with 1-6 molecules of NH3, two with citrate, and one adsorbed mixed-metal complex). The reversible potential for deposition of nickel is, in principle, different for each complex (depending on the stability constants). Hence, although all these parallel paths occur at the same applied potential, the overpotential is different for each of them. Moreover, there is no basis to assume that the exchange current densities or the Tafel slopes would be the same. If the observed Tafel plot would, nevertheless, be linear over at least two decades of current density, it could be argued that one of these parallel paths for deposition of nickel happens to be predominant. However, in the work quoted here, the apparent linearity of the Tafel plots extends only over a factor of about three in current density, namely over half a decade (cf.. Fig. 4a in Ref. 97). 

In the various investigations on CP-driven metal deposition the timescale of the experiments has varied over a wide range starting from a few seconds to few minutes [163,219], hours [210-212,216], or even days [213]. Whereas short-timescale experiments in heterogeneous systems rely basically on the leveling off of the initial potential difference between the CP material and the metal ions in solution, long-timeseale experiments involve additionally so-called self-sustained deposition. Starting with the very first studies on the chemical deposition of Au in PPY and PANI [210-212] it has been established that in some particular cases chemical metal deposition in CPs may proceed for hours as a self-sustained process. This was found to occur in acidic solutions where spontaneous reprotonation of the deprotonated (in the course of metal ions reduction) CP (e.g. PANI or PPY)... 

When discharge of ions and the formation of adatoms at the surface is the rate-determining step, compact deposits are always obtained, since the steady and random supply of adatoms all over the surface enables an even incorporation into the crystal lattice. However, the structure and properties of the deposit are influenced by numerous factors such as the substrate, the crystallographic properties of the depositing metals, the potential of deposition and/or the corresponding current density, the presence of impurities or additives in solution, temperature, etc. 

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. 

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