The electrochemical reactions that we have been discussing are called faradaic reactions since they involve electron transfer that is directly related to the consumption of reactants and the formation of products as described by Faraday’s law. To this point, we have only considered one reaction at each electrode. However, it is possible, and in fact common, for multiple reactions to take place at a single electrode in a real system. Frequently, one of the reactions is the desired reaction, and the other reactions are referred to as side reactions. In such cases, not all of the current goes into the desired reaction. We can define a faradaic efficiency to characterize the fraction of the total current that drives the desired reaction.

(1.13)

Use of this efficiency is demonstrated in Illustration 1.5.

ILLUSTRATION 1.5

Nickel is electrodeposited from a bath of NiSO₄. A current of 1.0 A is passed for 3 hours and 3.15 g of Ni are deposited. What is the faradaic efficiency of this deposition?

The reaction is .

First, use Faraday’s law to determine how much Ni could possibly be deposited.

One of the common side reactions for aqueous metal deposition is water hydrolysis to produce hydrogen gas.