Electrodeposition is a shortened form of “electrolytic deposition,” which is the use of an electrolytic cell to deposit metal(s) or other material(s) on the target electrode. Therefore, like other electrolytic processes, electrodeposition consumes power to produce the desired product. For the moment, we focus our discussion on the electroplating of metals from aqueous solutions. Figure 13.1 illustrates the basic components of an electroplating cell. The metal of interest is deposited on the cathode. Consequently, the cathode must be conductive in order to transfer the electrons needed to reduce the metal over the entire surface of the cathode. Variation of the potential across the metal surface will affect the local deposition rate as we will see later in this chapter. To plate on nonconductive materials, such as plastic, a conductive seed layer must be deposited prior to electrodeposition of the metal.
Figure 13.1 Illustration of an electroplating cell with a consumable anode.
The electrolyte contains the soluble form of the metal to be deposited. This may simply be the metal ion in solution, or may be a complex that contains that metal. A salt is frequently added to enhance conductivity. Buffers can also be added to stabilize the pH of the plating solution. What’s more, the electrolyte typically contains a variety of additives, often proprietary, that influence factors such as the morphology and local deposition rate of the metal on the cathode surface.
The anode may be either consumable (soluble) or inert (insoluble), depending on the system. Ideally, a soluble anode that is made up of the same metal that is being deposited at the cathode can be used. If this is possible, the metal of interest would enter the solution due to oxidation at the anode and then deposit at the cathode. Under these ideal conditions, the average composition of the electrolyte bath would remain constant with time. The cell voltage would also be minimized since the equilibrium potential of the cell would be zero for two electrodes of the same type in the same solution. Unfortunately, these ideal conditions are rarely present in real systems. For example, the current efficiency at one or both of the electrodes is frequently less than 100%, disrupting the balance between the amount of metal added to and plated from the bath. The anode may contain impurities that remain in the bath or plate onto the cathode. Dissolution of the anode may not readily occur in the plating solution due to passivation.
An insoluble or nonconsumable inert anode may also be used. In such cases, a reaction other than dissolution of the metal to be deposited occurs, the most common of which is oxygen evolution. The metal ions in the electrolyte bath need to be regularly replenished under these conditions. The pH at the depositing electrode will also change with time unless the electrolyte is adequately buffered or unless a membrane is used to isolate the anode compartment. Contamination of the bath can also be of concern when using this type of anode, as it is with soluble anodes.
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