Concept of electrochemical vs electrolytic cell

• An electrochemical (galvanic) cell runs a redox reaction spontaneously and produces electrical energy.
• An electrolytic cell makes a non-spontaneous reaction occur by supplying external electrical energy.

Role of cell potential

For a galvanic cell, the spontaneous reaction has a positive cell emf: $E_{\text{cell}}^0 > 0$

If we want to force the cell to run in the reverse direction, we must oppose the natural emf of the cell.

Condition for converting galvanic cell into electrolytic cell

To stop the spontaneous reaction and then reverse it, an external potential must be applied in the opposite direction.

• If external opposing potential is less than $E_{\text{cell}}^0$, the spontaneous reaction still dominates.
• If external opposing potential is equal to $E_{\text{cell}}^0$, ideally no net current flows.
• If external opposing potential is greater than $E_{\text{cell}}^0$, the reaction is forced in reverse, so the setup behaves like an electrolytic cell.

Hence, required condition is: $E_{\text{external}} > E_{\text{cell}}^0$ with opposite polarity.

Checking options

A: Applying an external opposite potential greater than $E_{\text{cell}}^0$
✅ Correct. This forces the reaction to proceed in the non-spontaneous direction.

B: Exchanging the electrodes at anode and cathode.
❌ Incorrect. Merely swapping labels or positions does not supply the required energy to reverse the reaction.

C: Applying an external opposite potential lower than $E_{\text{cell}}^0$
❌ Incorrect. This is insufficient to overcome the cell emf.

D: Reversing the flow of ions in salt bridge.
❌ Incorrect. Salt bridge only maintains electrical neutrality; it does not convert the cell into an electrolytic one.

Final answer

The correct option is: $\boxed{\text{A}}$