Overpotential Co-Deposition OPCD — Electrodeposition of Alloys

The generalized thermodynamic condition for electrodeposition of A Bi binary alloy is defined as  

Ei (/ = A, B) is the equilibrium potential of the metal-component i in the alloy A Bi- defined by modified Nernst expression  

The activity of the metal in the alloy is always less than 1 which yields different values of the equilibrium potential for the component in the alloy as compared to the corresponding elemental bulk metal electrode. According to Eq. (6), in order to form an AB alloy, the applied potential to the electrode surface has to be such that both components of the alloy are at overpotential conditions with respect to the equilibrium potentials of constituents in the alloy (Eq. (7)). In practice, the term OPCD is related to electrodeposition of alloys in the potential range where the applied potential is such that both components of the alloy are at overpotential conditions with respect to the equilibrium potentials of their elemental phase, defined by Eq. (1). In this case, the resulting composition of the electrodeposited alloys is controlled by the combination of several effects (1) kinetics of the electrodeposition of each component itself, (2) transport limitations, (3) conditions at the electrochemical interface, and (4) mutual interaction of adsorbed intermediates. 

The schematic indicating the potential region where OPCD occurs is shown in Fig. 2 using the example of CoFe alloy. The schematic considers electrodeposition process from the solution which is at standard conditions (P°, Cqq2+ = Cpg2+ = 1 mol). In order to obtain desired composition of CoFe (50 50) alloy, the concentrations of Co and Fe ions in the solution have to be appropriately adjusted together with the potential (overpotential) or current at which the alloy deposition occurs. Typical approach towards the solution and deposition potential (current) design involves experiments where the concentration of more noble metal, Co, is such that C(-q2+ Cp 2+, so that Co deposition occurs under mixed control for a 

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