Thermodynamic electrodeposition

These considerations have been based entirely on thermodynamics and take no account of the overpotential, which is dependent on the rate of the process and the nature of the surface at which the reaction occurs. For this reason, the rate of reduction of HjO or HjO is usually low, and remains so to potentials from 0-5 to 1-OV below that given in equation 12.1. Even so, the instability of water is an insuperable obstacle to electrodepositing... 

Given the thermodynamic properties of a system, judicious variation of the different plating parameters to assist in manufacturing the desired electrodeposit should be based on an accurate kinetic model. Engelken and Van Doren [6, 7] proposed... 

A comprehensive work on the electrodeposition chemistry and characterization of anodically synthesized CdTe thin films has been presented by Ham et al. [98]. In this work, along with the electrolytic anodic synthesis of CdTe by using Cd anodes in alkaline solutions of sodium telluride, an electroless route of anodizing a Cd electrode held at open circuit in the same solution was also introduced. The anodic method was expected to produce CdTe with little contamination from Te on account of the thermodynamic properties of the system the open-circuit potential of Cd anodes in the Te electrolyte lies negative of the Te redox point, so... 

The electrodeposition of tellurium and silver has been investigated in dilute aqueous solutions of tellurous acid and Ag " ions (concentrations in the order of 10 to 10 " M) in 0.1 M HCIO4 [164], In particular, cyclic voltammetry experiments were conducted with rotating glassy carbon disk electrodes in baths with various concentration ratios of Ag(I) and Te(IV) precursors, and their outcome was discussed in terms of the voltammetric features. For a Ag(I)/Te(IV) ratio close to 0.8, formation of quasi pure silver telluride, Ag2Te, was reported. The authors, based on their measurements and on account of thermodynamic predictions, assumed that silver is deposited first on the electrode (Ag" + e Ag), and then Te(IV) is reduced on the previous silver deposit with formation of Ag2Te according to the reaction... 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

Fig. 20. Variation of the composition of Ni-Al alloy electrodeposits as a function of the applied potential in the 66.7 m/o AlCl3-EtMeImCl melt the Ni(n) concentrations were ( ) 10.0, (+) 25.0, ( ) 35.0, and (x) 50.0 mmol L 1. The dotted line represents the theoretical composition assuming an fee lattice at 40 °C, following the thermodynamic treatment of Moffat [80], Adapted from Pitner et al. [47] by permission of The Electrochemical Society.

Moffat [80] reported the electrodeposition of Ni-Al alloy from solutions of Ni(II) in the 66.7 m/o AlCl3-NaCl melt at 150 °C. The results obtained in this melt system are very similar to those found in the AlCh-EtMcImCI melt. For example, Ni deposits at the mass-transport-limited rate during the co-deposition of Al, and the co-deposition of Al commences several hundred millivolts positive of the thermodynamic potential for the A1(III)/A1 couple. A significant difference between the voltammetric-derived compositions from the AlCl3-NaCl melt and AlCl3-EtMeImCl melt is that alloy composition is independent of Ni(II) concentration at the elevated temperature. Similar to what has been observed for room-temperature Cu-Al, the rate of the aluminum partial reaction is first order in the Ni(II) concentration. Moffat s... 

Cr-Al, Mn-Al, and Ti-Al alloys can be obtained from acidic melt solutions containing Cr(II), Mn(II), or Ti(II), respectively, only if the deposition potential is held very close to or slightly negative of the thermodynamic potential for the electrodeposition of aluminum, i.e., 0 V. From these observations it can be concluded that the formal potentials of the Cr(II)/Cr, Mn(II)/Mn, and Ti(II)/Ti couples may be equal to or less than E0 for the A1(III)/A1 couple. Unlike the Ag-Al, Co-Al, Cu-Al, Fe-Al, and Ni-Al alloys discussed above, bulk electrodeposits of Cr-Al, Mn-Al, and Ti-Al that contain substantial amounts of A1 can often be prepared because problems associated with the thermodynamic instability of these alloys in the plating solution are absent. The details of each of the alloy systems are discussed below. 

Although the concept of phase is well defined thermodynamically, here phase refers to a mechanically separable homogeneous part of an otherwise heterogeneous system. The concept of phase change refers here to a change in the number present or in the nature of a phase or phases as a result of an imposed condition such as temperature or pressure. To clarify and illustrate the topic at hand, we use the specific cases of electrolessly deposited nickel and electrodeposited cobalt. 

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electrodeposition, and other phenomena in aqueous solutions.From the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

It was John Wilkes who realized that room-temperature molten salts would only experience a widespread interest and uptake if they were stable under environmental conditions. Wilkes group published details of the first such liquid in 1992 using the BF]j" and the PFj anions, the latter showing a miscibility gap with water. Thus these liquids could, in principle, be made water free. (Today we know that ionic liquids containing BFJ and PF are subject to decomposition in the presence of water.) Electrochemical studies showed that even these early ionic liquids had wide electrochemical windows of about 4 V with cathodic limits of-2 to -2.5 V. vs. NHE. This cathodic limit should, from the thermodynamic point of view, be wide enough to electrodeposit many reactive elements. 

Electrodeposition could be a pseudo-S-L type process, although definitive proof is lacking. The element to be ionized, uranium [16] or plutonium [17], is coelec-trodeposited with a platinum metal layer, then covered with an additional layer of platinum. The U or Pu is believed to be electrodeposited as an oxide, and platinum is electrodeposited as the metal. Hence there is thought to be a U or Pu oxide buried in the metal matrix. When this deposit is heated, after a sufficient length of time atomic cations of U or Pu begin to sublime from the surface without measurable metal oxide ions. Metal oxide ions should be readily observable if they are present in the matrix. Thermodynamic calculations indicate that the hot platinum matrix will not reduce the U and Pu oxides to the metallic state, and yet the observed species are atomic ions and not oxide molecular ions. 

Here, AH(A-B) is the partial molar net adsorption enthalpy associated with the transformation of 1 mol of the pure metal A in its standard state into the state of zero coverage on the surface of the electrode material B, ASVjbr is the difference in the vibrational entropies in the above states, n is the number of electrons involved in the electrode process, F the Faraday constant, and Am the surface of 1 mol of A as a mono layer on the electrode metal B [70]. For the calculation of the thermodynamic functions in (12), a number of models were used in [70] and calculations were performed for Ni-, Cu-, Pd-, Ag-, Pt-, and Au-electrodes and the micro components Hg, Tl, Pb, Bi, and Po, confirming the decisive influence of the choice of the electrode material on the deposition potential. For Pd and Pt, particularly large, positive values of E5o% were calculated, larger than the standard electrode potentials tabulated for these elements. This makes these electrode materials the prime choice for practical applications. An application of the same model to the superheavy elements still needs to be done, but one can anticipate that the preference for Pd and Pt will persist. The latter are metals in which, due to the formation of the metallic bond, almost or completely filled d orbitals are broken up, such that these metals tend in an extreme way towards the formation of intermetallic compounds with sp-metals. The perspective is to make use of the Pd or Pt in form of a tape on which the tracer activities are electrodeposited and the deposition zone is subsequently stepped between pairs of Si detectors for a-spectroscopy and SF measurements. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

Complexation has two effects on electrodeposition a thermodynamic effect, or shift in the equilibrium potential, and a kinetic effect, or alteration of the exchange current. The thermodynamic effect is always in the direction of a more negative potential and so makes deposition more difficult [Equation (12-53)]. 

Thermodynamically, it is possible for metallic Mo to be codeposited in a host lattice provided its Gibbs energy is thereby sufficiently lowered. An analogous situation is the deposition of Na into Hg at some 1.2 V less negative than its normal (standard) electrode potential. The extensive coevolution of Hj during Mo electrodeposition with Ni or Co indicates that the overall process of alloy or composite metal deposition is far from efficient and that electrosorbed H may easily also be codeposited into the joint metal lattice, providing a hydride phase. 

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