How can an electrochemical cell be converted into an electrolytic cell?

We need to determine how an electrochemical (galvanic) cell can be converted into an electrolytic cell.

Key Concepts:

A galvanic cell converts chemical energy to electrical energy through spontaneous redox reactions, producing an EMF equal to $$E^\circ_{cell}$$.

An electrolytic cell uses external electrical energy to drive non-spontaneous reactions.

Analysis of each option:

Option 1: Applying an external opposite potential lower than $$E^\circ_{cell}$$ -- This would reduce the current from the galvanic cell but not reverse the reaction. The cell would still function as a galvanic cell (albeit with reduced output). Incorrect.

Option 2: Reversing the flow of ions in the salt bridge -- The salt bridge maintains electrical neutrality. You cannot simply reverse ion flow without an external driving force. Incorrect.

Option 3: Applying an external opposite potential greater than $$E^\circ_{cell}$$ -- When the external potential exceeds the cell's EMF and is applied in the opposite direction, it overcomes the natural tendency of the cell and forces the reaction to proceed in the reverse (non-spontaneous) direction. This is exactly the principle of electrolysis and charging of batteries. Correct.

Option 4: Exchanging the electrodes -- Simply swapping electrode positions does not change the underlying chemistry or provide the energy needed to drive a non-spontaneous reaction. Incorrect.

The correct answer is Option 3: Applying an external opposite potential greater than $$E^\circ_{cell}$$.