Understanding Galvanic vs. Electrolytic Cells

Electrochemical cells can operate in two primary modes: as galvanic cells or as electrolytic cells. A galvanic cell (also known as a voltaic cell) is a device that converts chemical energy into electrical energy through a spontaneous redox reaction. The cell potential, denoted as $E_{cell}$, for a galvanic cell is inherently positive ($E_{cell} > 0$), indicating spontaneity.

Conversely, an electrolytic cell converts electrical energy into chemical energy, driving a non-spontaneous redox reaction. This requires an external source of electrical energy (like a battery or power supply) to provide a voltage, denoted as $E_{ext}$. For a non-spontaneous reaction to occur, the applied external voltage must be sufficient to overcome the inherent tendency of the system to react spontaneously.

Switching Cell Behavior: Galvanic to Electrolytic

Consider a galvanic cell that is producing electricity. If we connect an external voltage source ($E_{ext}$) in opposition to the galvanic cell's natural voltage ($E_{cell}$), the behavior of the cell changes depending on the magnitude of $E_{ext}$:

• If $E_{ext} < E_{cell}$: The external voltage is not strong enough to counteract the cell's spontaneous reaction. The galvanic cell continues to act as a source of power, and the net voltage available is $(E_{cell} - E_{ext})$.
• If $E_{ext} = E_{cell}$: The external voltage perfectly balances the cell's voltage. The net potential difference across the cell is zero ($E_{cell} - E_{ext} = 0$). At this point, the reaction is at equilibrium, and there is no net flow of current. The cell neither produces nor consumes significant power.
• If $E_{ext} > E_{cell}$: The external voltage is greater than the cell's intrinsic voltage. This external voltage forces the redox reaction to proceed in the reverse direction compared to its spontaneous galvanic operation. The cell now acts as an electrolytic cell, consuming electrical energy from the external source to drive the non-spontaneous reaction. The net voltage driving the electrolysis is $(E_{ext} - E_{cell})$.

Condition for Galvanic Cell Acting as Electrolytic Cell

Based on the analysis above, a galvanic cell begins to behave like an electrolytic cell only when the externally applied voltage ($E_{ext}$) is greater than the cell's own potential ($E_{cell}$). This condition forces the reaction to run in reverse, consuming energy rather than producing it.