A basic Daniels cell :
The reaction that follows:
The above Redox reaction releases electrical energy. Such an arrangement is often called a Galvanic or Voltaic cell. The protential developed is equal to 1.1V for 1 m dm^-3 conecentration of solution.
If an external opposite potential is applied in the galvanic cell above and increased slowly, the reaction continues to take place till the opposing voltage reaches the value 1.1 V. When the reaction stops altogether and no current flows through the cell. Any further increase in the external potential again starts the reaction but in the opposite direction. Thus converting electrical energy to chemical energy in the process. Such an arrangement is called An Electrolytic Cell.