Electrodeposition electrolytic cells

Electrodeposition. Electro deposition, the most important of the unit processes in electrorefining, is performed in lead- or plastic-lined concrete cells or, more recently, in polymer—concrete electrolytic cells. A refinery having an aimual production of 175,000 t might have as many as 1250 cells in the tank house. The cells are multiply coimected such that anodes and cathodes are placed alternately and coimected in parallel. Each cell is a separate unit and electrically coimected to adjacent cells by a bus bar. 

Figure 14.7 (a) A schematic diagram of the experimental set-up for the generation of an ultrathin electrolyte film and for electrodeposition. The cell for electrodeposition shown here has two parallel electrodes. 

It consists in a deposition of ions from an electrolyte onto the cathode in an electrolytic cell, under the influence of an applied potential. Usually the process is accompanied by material dissolution from the anode. The electrowinning from aqueous solutions is an important commercial method for the production (and/or refinement) of many metals, including, for instance, chromium, nickel, copper, zinc. As for the electrodeposition from non-aqueous solutions, the primary production of aluminium, electrodeposited from a solution of A1203 in molten cryolite, is a typical example. Other metals which may be regularly reduced in a similar way are Li, Na, K, Mg, Ca, Nb, Ta, etc. 

Having identified the main features of electrochemistry, the remainder of this chapter will focus on the use of electrolytic cells and will use as examples the electrodeposition (or electroplating) of metals such as copper, zinc, iron, chromium, nickel and silver. The chapter will also consider the electrochemistry of some organic molecules. Electroanalysis will not be considered since a full description is not within the scope of this chapter. Eor those interested readers, there is a review on the topic [2],... 

In order to produce current flow through an electrolytic cell for the discharge (or electrodeposition) of any metal, a potential, at least equal to if not greater than the zero current or reversible potential must be applied. For zinc ions this would be 0.763 V [3], The potential at which continuous deposition of material (or discharge of ions) commences is called the discharge or decomposition potential (Fig. 6.6). 

Figure 2.1. Electrolytic cell for electrodeposition of metal, M, from an aqueous solution of metal salt, MA.

The basic components of an electrolytic cell for electrodeposition of metals from an aqueous solution are, as shown in Figure 2.1, power supply, two metal electrodes (Mj and M2), water containing the dissolved ions, and two metal-solution interfaces Mj-solution and M2-solution. An electrolytic cell for electroless deposition is shown in Figure 8.1. A comparison of Figures 2.1 and 8.1 shows that in electroless deposition there is no power supply and the system has only one electrode. However, the solution is more complex. It contains water, a metal salt MA A ), and a reduc-... 

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. 

Toxic substances adsorbed on resins are removed during a regeneration procedure. The resulting spent regeneration solution has a higher concentration of the toxic substance than the stream from which it was removed by the resin. Toxic material in the spent regenerating solution can usually be precipitated, electrodeposited as in an electrolytic cell, or made insoluble by other acceptable procedures. 

However, in many situations, water is hardly the ideal solvent. Take the electrolytic production of sodium metal, for exanple. If an aqueous solution of a sodium salt is taken in an electrolytic cell and a current is passed between two electrodes, then all that will happen at the cathode is the liberation of hydrogen gas there will be no electrodeposition of sodium (see Chapter 7). Hence, sodium cannot be electrowon from aqueous solutions. This is why the electrolytic extraction of sodium has taken place from molten sodium hydroxide, i.e., from a medium free of hydrogen. This ... 

The basic components of an electrolytic cell for the electrodeposition of metals... 

Popov KI, Maksimovic MD, Totovski DC, Nakic VN (1983) Some aspects of current dcmsily distribution in electrolytic cells I dendritic growth of cadmium at the cathode edge in galvanostatic electrodeposition. Surf Technol 19 173-180... 

The electrolytic cells consist of lucite cylinders which are threaded at one end for stainless steel caps which contact the stainless steel cathode plating surfaces. A beveled lucite disk fits between the cap and cylinder and defines the electrodeposition area. 

The construction of our electrolytic cell for deposition of hafiiium coatings has been described [7]. Distilled water was used to wash the cathodic deposits. Niobium and steel (St.3) substrates 1 mm thick were used for electrodeposition of coherent hafiiium coatings. These substrates had a mean deviation in the profile of Ra=0,l 1-0,13 pm. 

FIGURE Al Schematic diagram of an electrolytic cell that can be used to electrodeposit conducting polymers. 

Primary applications of coulometry, such as in the electrodeposition of metals, are limited by the voltage resolution available. In order to deposit one metal in the presence of others, the potential of the cathode must be controlled with precision, an operation requiring a three-electrode electrolytic cell together with a potentiostat, a method introduced by Hickling in 1942 (37). (The principles involved in the choice of potential and the degree of separation possible lie beyond the scope of this article.)... 

In the text, words in italics followed by (q.v.) indicate a reference to another entry which would be of help in some instances the reference to another entry is in parentheses. Thus in the sentence Tn an electrolytic cell (q.v.) electrical energy from an external source brings about a desired chemical reaction (see electrodeposition of met-als, electrolysis of water), the reader is referred to entries electrolytic cell , electrodeposition of metals and electrolysis of water , for further information. Words in italics, followed by t, e.g. free energyt, indicate a reference to that entry in the companion work A Dictionary of Thermodynamics, by A. M. James. 

---