Electrochemical Cells: Galvanic Cells and Electrolytic Cells

An electrochemical cell is any device in which a redox reaction is coupled to the flow of electric current through an external circuit. Electrochemical cells are classified into two fundamentally different types, based on the direction in which energy is converted.

A galvanic cell (or voltaic cell) contains a spontaneous redox reaction ($\Delta G^{\circ} < 0$) and uses the chemical energy released by that reaction to generate electrical energy, which can then do useful work in an external circuit — powering a bulb, a motor, or a measuring instrument. The Daniell cell, described in detail in the next section, is the classic teaching example: it spontaneously converts the chemical energy of zinc displacing copper into electrical energy.

An electrolytic cell works in the opposite direction: it contains a non-spontaneous redox reaction ($\Delta G^{\circ} > 0$) and uses externally supplied electrical energy (from a battery or power supply) to force that reaction to occur anyway. Electroplating (depositing a thin layer of one metal onto another using an applied current) and the industrial extraction of reactive metals like sodium and aluminium from their molten salts/oxides are both electrolytic processes. …