Metal Deposition on Solid Electrodes

The reduction of metal ions on solid electrodes is a process consisting of three main steps a deposition of adatoms, a two- and three-dimensional nucleation and a three-dimensional crystal growth [10]. The formation of the first mono-layer of metal atoms on the foreign substrate follows a quasi-Nernst equation  

The difference E1/2 - is related to the binding energy of the first monolayer. In cyclic voltammetry this is the difference between the potentials of the most positive peak and the peak corresponding to the three-dimensional metal phase. The latter difference is linearly correlated with the difference of work functions of the deposited metal and the electrode material [11]  

Amalgams are metallic systems in which mercury is one of the components. The solubility of the alkali metals, the alkaline earths, the rare earths and Au, Zn, Cd, Ga, In, Tl, Sn, Pb, Bi, Ru, Rh and Pt in mercury is higher than 0.1 atom % [1]. A reversible redox reaction of an amalgam-forming metal ion on a mercury electrode  

The accumulation of amalgams can be used in anodic stripping voltammetry if both the reduction of ions and the oxidation of metal atoms occur within the working window of the mercury electrode [21]. Ions that give the best responses are listed in Table II.7.4. 

When several metals are simultaneously electrodeposited in mercury, intermetallic compounds between them may be formed. In anodic stripping voltammetry the following compounds of copper, zinc and antimony may influence the measurements CuZn, CuSn, CuGa, SbZn, SbCd and Sbln. The values of their solubility products are between 2 x 10 (SbZn) and 4 x 10 (CuZn) [1]. Considering the low solubility of Sb and Cu in mercury, a concentration of Zn atoms higher than 10 % w/w may cause the precipitation of these compounds. Generally, the formation of intermetallic compounds is suppressed in diluted amalgams. 

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The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

Photoelectrochemical behavior of metal phthalocyanine solid films (p-type photoconductors) have been studied at both metal (93,94,95,96) and semiconductor (97,98) electrodes. Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) displayed photocurrents with signs depending on the thickness of film as well as the electrode potential. Besides anodic photocurrents due to normal dye sensitization phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with thicker films due to a bulk effect (p-type photoconductivity) of the dye layer. Meier et al. (9j>) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum electrodes where the dye layer acted as a typical p-type organic semiconductor. 

The effect of the solvent on the standard potential of electrode reactions can be derived from a thermodynamic cycle. In the case of the ion-transfer type of reaction, where the reduced form is the metal deposited on the solid electrode, we have... 

Several other attempts have been made by various authors to avoid anodic corrosion at n-type electrodes and surface recombination at p-type electrodes, by modifying the surface or by depositing a metal film on the electrode in order to catalyse a reaction. It has been frequently overlooked that the latter procedure leads to a semiconductor-metal junction (Schottky junction) which by itself is a photovoltaic cell (see Section 2.2) [14, 27]. In the extreme case, then only the metal is contacting the redox solution. We have then a pure solid state photovoltaic system which is contacting the solution via a metal. Accordingly, catalysis at the semiconductor electrode plays a minor role under these circumstances. 

The determination of a metal by selective deposition on an electrode, followed by weighing, is among the oldest of electroanalytical methods [Cruikshank (1801) W. Gibbs (1864)]. In controlled-potential methods, the potential of the solid electrode is adjusted to a value where the desired plating reaction occurs and no interfering reaction leading to the deposition of another insoluble substance takes place. 

Although one of the more complex electrochemical techniques [1], cyclic voltammetry is very frequently used because it offers a wealth of experimental information and insights into both the kinetic and thermodynamic details of many chemical systems [2], Excellent review articles [3] and textbooks partially [4] or entirely [2, 5] dedicated to the fundamental aspects and apphcations of cyclic voltammetry have appeared. Because of significant advances in the theoretical understanding of the technique today, even complex chemical systems such as electrodes modified with film or particulate deposits may be studied quantitatively by cyclic voltammetry. In early electrochemical work, measurements were usually undertaken under equilibrium conditions (potentiometry) [6] where extremely accurate measurements of thermodynamic properties are possible. However, it was soon realised that the time dependence of signals can provide useful kinetic data [7]. Many early voltammet-ric studies were conducted on solid electrodes made from metals such as gold or platinum. However, the complexity of the chemical processes at the interface between solid metals and aqueous electrolytes inhibited the rapid development of novel transient methods. 

Mamontov, G., Manning, D.L., and Dale, J.M. (1965) Reversieble deposition of metals on solid electrodes by voltammetry with linearly varying potential, J. of Electroanal. Chem. 9, 253-259. 

The main research interests of Vas ko were related to the electrochemistry of refractory metals where he was a well-known expert. He developed the process of galvanic coating of W-Ni and Mo-Ni alloys from aqueous electrolytes. Trying to explain the mechanism of this process, he assumed two stages of the formation of such alloys first, the deposition of solid film consisting of low-valency compounds and, second, electrochemical reduction of the film at the inner surface by solid mechanism. Thus, the solid non-metal film on the electrode surface for the first time became an active participant of the electrochemical process rather than simple passivative layer. This was a breakthrough. Further on, this electrochemical film system (EFS) concept was usefully applied not only in the aqueous electrochemistry but in the electrochemistry of molten salts and ionic liquids as well (see [7] for more details). 

As the concentrations of electro-active material become smaller and smaller a point is eventually reached where the behavior predicted by polarization curves is no longer obeyed because steady-state conditions cannot be maintained and because the amount of electroactive material is insuificient to provide uniform coverage of the electrode. Coche (15), for example, has shown that the critical potential for the deposition of heavy metals on solid electrodes depends primarily on the nature and state of the electrode surface. Similarly, the work of Rogers and co-workers (16-19) on the deposition of silver on platinum cathodes indicated that the deposition potential is shifted by several hundred millivolts from the potential predicted by the Nemst equation when the concentration of silver ion is insufficient to provide complete coverage of the electrode. This shift in potential values was found to depend on solution pH, electrode history and material. 

About 20 amalgam-forming metals, including Pb, Sn, Cu, Zn, Cd, Bi, Sb, Tl, Ga, In and Mn, are easily measurable by stripping strategies (ASV and PSA) based on cathodic deposition onto mercury electrodes. Additional metals, such as Se, Hg, Ag, Te and As are measurable at bare solid electrodes such as carbon or gold. 

Figure 1.5. Schematic representation of a metal electrode deposited on a 02 -conducting (left) and on a Na -conducting (right) solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover).

Consider the solid electrolyte cell shown in Figure 5.20. For simplicity we consider only a working (W) and reference (R) electrode deposited on a solid electrolyte, such as YSZ or p"-Al203. The two electrodes are made of the same metal or of two different metals, M and M. The partial pressures of 02 on the two sides of the cell are p02 and po2 Oxygen may chemisorb on the metal surfaces so that the workfunctions w and R(p 02). 

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