Electrolysis and Electrodeposition of Metals

When an increasing DC voltage is applied to a solution of metal ions, the metal, in some cases, will begin to deposit the deposition material at the cathode at a minimum voltage. This is illush ated as D in Fig. 9.1. The deposition potential depends on the metal, the surface, the current density, the concentration of the metal, and other ions in solution. The electrodeposition or electroplating reaction 

The theoretical minimum voltage required for the electrolysis of water is 1.23 V at 25°C. However, the process has an activation energy which is referred to as the overvoltage or polarization which depends on the current, temperature, and materials used for the electrodes. Some typical overvoltages are given in Table 9.2. 

The industrial preparation of hydrogen by the electrolysis of water on nickel electrodes requires a voltage of more than 1.50 V (1.23 V + 0.210 V + 0.060 V) since it is necessary to add the RI drop (due to the internal resistance of the electrolyte). However, at very high current densities, the polarization is much higher, and higher temperatures are used to reduce the excess power 

Metal surface Cathode polarization dilute H2SO4 Anode polarization dilute KOH 

Cu (partially hydrated and adsorbed onto the surface), the ad ion, diffuses across the electrode surface to a crystal building site. 

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