How does overpotential affect the observed cell potential during electrolysis versus galvanic operation?

• A. Overpotentials have no effect on either mode.
• B. Overpotentials make the reaction spontaneous in galvanic cells.
• C. In electrolysis, the applied potential must overcome overpotentials; in galvanic operation, the observed cell potential is decreased by overpotentials.
• D. In electrolysis, overpotentials increase the measured cell potential; in galvanic, they increase it further.

Answer: (C)

Overpotential is the extra potential required at an electrode to drive a reaction at a given current beyond its thermodynamic value due to kinetic barriers at the interface.

In electrolysis, you are forcing a non-spontaneous reaction to proceed. To do this, you must supply enough energy to overcome both the thermodynamic threshold and the kinetic losses (overpotentials), plus any IR drop in the circuit. So the applied potential must be higher than the ideal cell potential by the amount of overpotential.

In galvanic operation, the reaction is spontaneous, but once current flows, the electrode surfaces polarize and you lose some of the driving force to these kinetic barriers. The observed cell potential under load is therefore reduced by the overpotentials (and IR losses) from the ideal thermodynamic value.

Thus, you apply extra potential in electrolysis to overcome overpotentials, while the galvanic cell’s observed potential is diminished by overpotentials.