Side reactions are reactions that do not directly contribute to the formation of the desired deposit. For the deposition of metals from aqueous solutions, the most common side reaction is electrolysis of water or reduction of hydrogen ions, which results in the evolution of hydrogen gas at the cathodic potentials needed for deposition. The portion of the applied cathodic current that results in hydrogen evolution is current that does not result in metal deposition and, therefore, represents a loss of efficiency. Loss of efficiency is one of the primary impacts of side reactions. The loss of efficiency can have a significant impact on the economic viability of a plating process, and is an important factor in determining the conditions under which feasible deposition can be performed.
Side reactions may also have a beneficial effect on deposit uniformity and hence on the throwing power of the bath. Because such reactions occur preferentially where the cathodic overpotential is largest, the side reaction can, under certain conditions, “scavenge” the current at locations where the deposition rate of the metal would otherwise have been much higher. This scavenging occurs because the side reaction lowers the cathodic overpotential at those locations by contributing to the iR drop (by increasing the current) without adding to the metal deposit. A necessary condition for this beneficial effect is that the side reaction takes place to a varying extent over the surface upon which deposition is taking place. An example of how this might occur is illustrated by the curves shown in Figure 13.14, which identifies a plating region over which hydrogen evolution varies significantly. In the situation illustrated, hydrogen constitutes a higher fraction of the total current at high (cathodic) potentials, and would tend to reduce nonuniformities. Illustration 13.6 further explores efficiency and its impact on the local deposition rate.
Figure 13.14 Impact of side reaction on deposition showing how both metal deposition and hydrogen evolution can contribute to the total current.
Finally, gas evolution is frequently the result of side reactions. The gas bubbles themselves may impact deposition by influencing the conductivity of the solution. Specifically, the cell potential and local nonuniformity may increase to some degree due to a decrease in both the overall and local conductivity of the plating bath as a result of bubble formation. Bubbling can have the positive effect of mixing the solution to minimize concentration gradients.
ILLUSTRATION 13.6
Nickel is electrodeposited from a bath that is 1 M in Ni2+ at a pH of 4.5. The anode is also nickel, and the potential applied across the cell is −1.3 V. Due to the larger size and surface area of the anode, the anodic surface overpotential can be neglected for this problem (not true in general); concentration gradients can also be neglected. However, iR losses in solution are important. Because of this, the current density is not uniform. We are interested in the relative deposition rate at two specific points on the surface, and the impact of the side reaction (hydrogen evolution) on that relative rate, as well as on the current efficiency. The solution resistance from the anode to the cathode is 0.002 Ω·m2 at the first point of interest, and 0.003 Ω·m2 at the second. Please determine the value of the current density at each of the two points, as well as the relative rate of deposition. Next, include the hydrogen reaction and repeat the calculation. In this example, how did H2 evolution impact the absolute and relative rates of deposition? The following parameters are known:
Nickel reaction: Tafel slope = −0.06 V per decade; i0 = 0.1 A·m−2
Hydrogen evolution reaction: Tafel slope: −0.11 V per decade; i0 = 1.0 A·m−2
SOLUTION:
In the absence of hydrogen evolution, the total voltage drop is due to the sum of the kinetic overpotential and the iR drop:
where b is the Tafel slope and R is the resistance that corresponds to the point of interest. U is equal to zero since the potential is referred to the nickel anode. This equation can be solved for i at each point. The answers are shown in the table. The relative deposition rate is 1.49.
Resistance i (A·m−2)
Point 1 0.002 538
Point 2 0.003 362
With the hydrogen reaction, we now have two equations to solve:
The first is the same as the equation solved earlier, except that the hydrogen current has been added to the iR term. The second equation states that the voltage drop (not the overpotential) across the cathode interface is the same for the two reactions, which of course must be true since there is only one interface at which the reactions are occurring. is the difference between the potential of a hydrogen electrode and a nickel electrode at equilibrium. Since the concentration of the nickel ions is 1 M, the equilibrium potential of the nickel electrode versus SHE (neglecting activity coefficients) is just the standard potential or −0.257 V. At a pH of 4.5, the equilibrium potential of the hydrogen electrode is −0.2664 V. Therefore, . Solving yields
Resistance i [A·m−2] iH [A·m−2]
Point 1 0.002 459 81.6
Point 2 0.003 299 64.6
The relative deposition rate (high/low) is 1.53, which is close to that calculated without hydrogen evolution. In this case, the fraction of hydrogen did not change significantly between the two points of interest and, therefore, the relative rate was nearly the same (very slightly worse). Hydrogen evolution did impact the magnitude of the deposition rate, however, as more than 15% of the current went to gas evolution and did not contribute to the deposition rate.
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