Electrodeposition cell Electrode potential

In order to decrease the electrolysis cell voltage, the Pd-Ag cathode could be activated by electrodeposition of a third metal, such as Ni or Pt. Figure 16.11 compares the curves of the electrode potential vs the current density for a Pd-Ag cathode, a nickel and a Ni covered Pd-Ag cathode. It is evident that the HER is influenced by the metal surface, showing that it is possible to reduce the overpotential by choosing an appropriate metal coating for the cathode. 

What reaUy distinguishes electrodeposition from other deposition techniques is the applied potential. The applied potential controls the departure from equilibrium and, therefore, the rate of the reaction. The electrodeposition of metals only requires that the electrode potential be driven negative of the equilibrium potential. The difference between the applied potential and the equilibrium potential is called the overpotential. Because the electrode must be poised at a potential for deposition to occur, the substrate must be a conductor or semiconductor. In contrast, vapor phase deposition does not require a conducting substrate. This substrate constraint is not a limitation in the electrodeposition of catalysts for fuel cells, batteries, and photoelectrochanical solar cells. 

Traditionally, the electrochemical analysis of thin layers of electrodeposited nonequilibrium alloys has simply involved either galvanostatic or potentiostatic dissolution of the electrodeposit under conditions where passivation and/or replacement reactions can be avoided [194, 195]. A technique based on ALSY at a RDE has also become popular [196], To apply this technique, a thin layer (a 10 pm) of the alloy of interest is deposited on a suitable electrode in a solution containing the reducible ions of the alloy components. The plated electrode is then removed to a cell containing an electrolyte solution that is devoid of ions that can be reduced at the initial potential of the experiment, and the complete electrodeposit is anodically dissolved from the electrode surface using slow scan ALSV while the electrode is rotated. 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

Electrochemical cells may be one of two types. Should a current spontaneously flow on connecting the electrodes via a conductor, the cell is a galvanic cell. An electrolytic cell is one in which reactions occur when an external voltage greater than the reversible potential of the cell is applied. Simple examples involving copper are given in Figure 1. It is the electrolytic cell which is of interest in the electrodeposition of metals. 

It is not appropriate in this chapter to tabulate quantities of electrochemical data since that required may be obtained from texts on electrodeposition.1 2 However, a brief mention of sign conventions must be made since, particularly in the early literature, confusion can arise. Two conventions have been used the European and the American .3 It is sometimes erroneously stated that the conventions differ only in sign however, the real difference lies in the distinction between the potential of an actual electrode and the EMF of a half cell reaction. 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

Electroraffination — (see also electrorefining) Purification of metals by means of dissolution and subsequent electrodeposition. Common method in - electrometallurgy for the removal of impurities from raw metals. Upon anodic dissolution the metallic constituents of the anode are dissolved as cations, oxyanions, or complex ions. All impurities - whether metallic or not - are also dissolved or will fall to the bottom of the cell. At the cathode set to a suitable potential (in most cases only fractions of one volt are needed) the desired metal is deposited. Less noble metals stay in solution, they can be recovered by processing the electrode solution. Metals more noble than the metal under consideration are in most cases not dissolved anodically, instead they settle in the solid deposit at the cell bottom. From this residue they can be recovered. 

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. 

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