Electrochemical cell
A galvanic cell converts chemical energy to electrical energy. Two different metals sit in electrolyte solutions connected by a salt bridge. The more reactive metal dissolves, releasing electrons that flow through an external wire to the less reactive metal, where ions from solution deposit as solid metal. This is how batteries work.
Zn(s) → Zn2+(aq) + 2e− | Cu2+(aq) + 2e− → Cu(s) | E°cell = E°cathode − E°anode
The galvanic cell
A galvanic (or voltaic) cell is the simplest type of battery. It was invented by Alessandro Volta in 1800 and refined by John Daniell in 1836. The Daniell cell uses zinc and copper electrodes, each immersed in a solution of its own ions, connected by a salt bridge that allows ion migration while preventing the solutions from mixing directly.
How it works
At the anode (the more reactive metal, e.g., zinc), metal atoms lose electrons and dissolve into solution as ions: Zn → Zn2+ + 2e−. This is oxidation. The freed electrons travel through the external wire to the cathode (the less reactive metal, e.g., copper), where metal ions in solution gain those electrons and plate onto the electrode: Cu2+ + 2e− → Cu. This is reduction.
The salt bridge
Without a salt bridge, the reaction would quickly stop. As zinc dissolves, the anode solution builds up positive charge (excess Zn2+). As copper deposits, the cathode solution becomes negatively charged (excess SO42−). The salt bridge — typically a tube filled with KNO3 or KCl gel — allows negative ions to migrate toward the anode and positive ions toward the cathode, maintaining electrical neutrality in both half-cells.
Cell voltage
The voltage of a galvanic cell is determined by the difference in standard reduction potentials of the two half-reactions: E°cell = E°cathode − E°anode. For Zn/Cu, this is +0.34 − (−0.76) = 1.10 V. More reactive anode metals or more noble cathode metals produce higher voltages. The Nernst equation adjusts this for non-standard concentrations: E = E° − (RT/nF) ln Q.
From cells to batteries
A single galvanic cell produces modest voltage. Real batteries stack multiple cells in series: a 9V battery contains six 1.5V cells. Rechargeable batteries (like lithium-ion) reverse the reaction by applying external voltage, replating the anode and dissolving the cathode. The fundamental electrochemistry is the same — just the materials and engineering change.