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Electrolytic cells are composed of a vessel used to do electrolysis, containing electrolyte, usually a solution of water or other solvents capable of dissolving various ions into solution, and a cathode and anode. The electrolyte in the cell is inert unless driven by external voltage into a redox reaction with the anode and cathode. The term literally means a cell that splits up or loosens ions.
An electrolytic cell is activated by applying electrical potential across the anode and cathode to force an internal chemical reaction between the ions in the solution and the anode or the cathode. Ions in the solution flow toward the electrodes. If the ions are positively charged cations they flow toward the cathode and are reduced. If the ions are negatively charged anions they flow to the anode and are oxidized. This reaction can be reversed to produce electricity in fuel cells.
Galvanic cells compared to electrolytic cells
In contrast, a galvanic cell, also known as a voltaic cell or an electrochemical cell, generates electricity using two metals that have different reaction potentials to the electrolyte which is ionically bridged. The ions are allowed to flow between the two solutions created when the two metals contact the electrolyte; the reaction difference of the two metals generates electricity. The difference between the two types of cells is that a galvanic cell must use dissimilar metals which are separated, except for ions, to produce a charge whereas an electrolytic cell has both anode and cathode suspended in the same solution and is driven by an external electrical charge; further, an electrolytic cell may use the same metal for cathode and anode.
Anode and cathode marking compared to charging
The anode and cathode in both types of cell are marked with the polarities of the current flow. In actual practice, the anode and the cathode in a galvanic cell swap functions during the charge portion of the cycle. Thus the electrode marked as anode for discharge is really acting as the cathode while charging and the electrode marked as cathode is actually acting as the anode during charging. In an electrolytic cell, the anode is where the electrons come up from the solution and the oxidation reaction creates electrons from reaction with the negative anions. The cathode reduces the ionic solution, drawing positive cations to the cathode and using electrons to reduce the cation.
The electrolysis of salt water or acidic water is an easy example of electrolysis. The ionic solution is driven by current and produces hydrogen gas in a reduction reaction at the cathode using electrons to complete the hydrogen ions. At the anode oxygen gas is produced in an oxidizing reaction and electrons are given off as the oxygen ions combine. These electrons then flow up the anode and through the circuit.
In molten salt (sodium chloride), when a current is passed through the salt the anode oxidizes chlorine which gives off electrons to the circuit. The cathode reduces sodium metal which deposits on the cathode using electrons to complete the sodium metal valence shell.
Commercially, electrolytic cells are used in electrorefining and electrowinning of several non-ferrous metals. Almost all high-purity copper, zinc and lead is produced industrially in electrolytic cells.
- Concentration cell
- Electrochemical cell
- Galvanic cell