Electrolytic cells an introduction 

Electrolytic cells are a type of electrochemical cell. An electrochemical cell is a system or circuit which converts external electrical energy into chemical energy or vice versa.

Thus, an electrolytic cell converts the electrical energy provided to its circuit into chemical reactions, the circuit consists of two electrodes one of which is an anode and the other is a cathode. 

Based on the usage of an Electrolytic cell it can be formed into various types.

Electrolytic cells types

1. Electroplating cell 

It is an application of electrolytic cells used to deposit or cover the metal with other metals. It is done to protect metal or decorate it or improve its appearance. 

Commonly this cell is used in making silver-coated articles like spoons, cups, bowls, etc.

Structure

• The arrangement consists of a beaker or a container cell that has a silver strip and a spoon made from different materials connected by the means of a wire.
• The following circuit is completed only when these elements are finally attached to a power supply (in this figure a battery ).
• The silver strip and the spoon both are dipped into the silver salt solution like AgNO3 .
• When the current is passed the silver strip connected to the positive terminal acts as an anode while the spoon connected to the negative terminal acts as a cathode.
• The electrons flow toward the cathode (spoon), while the anode releases the electrons.
• When the electrons are released from the anode, the silver metal turns into its ionic form. These ions move towards the spoon and get deposited on it.
• In this way, the silver strip gets dissolved and the silver is deposited on the spoon, making its appearance as required by the manufacturer.

Reactions 

• At anode,  Ag  →  Ag⁺  + e^–

At the anode, silver gets oxidized into its ionic form releasing an electron. Therefore, the electron, as well as the silver ion, travels toward the cathode i.e. the spoon (for this example).

• At cathode,  Ag⁺ + e^–  →  Ag (deposited on the spoon)

At the cathode, the spoon which is to be coated with silver accepts the electrons as well as the silver ions which combine to form a silver layer on it.

2. Electrorefining

Electrorefining is the process of purifying impure metals i.e. the metal elements containing unwanted particles which are responsible for the change of properties of any metal into its pure metallic form with the help of a circuit of the electrolytic cell.

• The structure of an electrorefining electrolytic cell consists of elements identical to that of an electrolytic cell.
• The difference here is, the material or the impure metal is connected to the positive terminal of the external power source or the battery.
• Hence, it acts as an anode and releases electrons and pure metal ions.
• While a pure metallic element is also connected to the circuit, it is attached to the negative terminal of the power source.
• It is important to note that both pure metallic elements and impure elements must contain the same metal.
• When the current flows through the circuit impure metallic element starts to release the metallic ion and also releases electrons in the solution.
• These metal ions travel toward the cathode and then combine with the electrons to form the metallic deposition on the pure metal element.
• In this way, with the help of an electrolytic cell, we can extract, pure metal from its impure form or from its ore as well.

Reactions 

Let us take any metal M, for this process, its impure form is connected as an anode while its pure form is acting as a cathode.

• At the anode, M  →  Mn⁺  + ne^–

n is the oxidation number of the metal M.

• At the cathode, Mn⁺  + ne^–  →  M (deposited at the pure metallic element)

3. Electrolyzer 

Electrolyzer is a type of electrolytic cell that is used to produce hydrogen and oxygen gases from water. An electrolytic cell consists of a cathode that releases hydrogen gas at its end while the anode releases oxygen at its end. The medium or electrolytes in the electrolyzer are of two types due to which electrolyzers are can be made of two different types. The electrolytes are- 

1. Alkaline Electrolyte 
2. PEM (Polymer Electrolyte Membrane)

Alkaline Electrolyte 

In this, an alkaline solution is taken as an electrolyte and due to which the hydrogen, as well as oxygen gases, are released.

This electrolyzer is not very efficient in the production of hydrogen as it consumes more electricity. Also after some time or when there is a fluctuation in the power supply the electrolyzer stops producing the product gases.

PEM (Polymer Electrolyte Membrane)

In this type of electrolyzer instead of an aqueous solution a polymer membrane is used as a medium. In this electrolytic cell instead of cathode and anode rods, cathodic and anodic sections are used. And hence the PEM (polymer electrolytic membrane) also acts as the separator between the two sections.

• In a PEM electrolyzer there are anodic and cathodic sections separated by a polymer membrane this membrane allows the hydrogen ion to pass through it, while it deflects the oxygen ion back
• Now when electricity is passed through the electrolyzer it dissociates the water molecules into the hydrogen and oxygen ions. The anodic region accepts oxygen ions and forms oxygen gas through oxidation.
• Whereas, the hydrogen atoms move toward the cathode due to reduction from hydrogen gas.

Reactions 

• At anode,   O₂^–  → ½  O₂ (g) + 2e^–
• At cathode, 2H⁺ + 2e^–→ H₂ (g)

Q&A

What are the process occurring at anode and cathode of an electrolytic cell?

At the anode, oxidation takes place, while at the cathode reduction takes place.

What are the two types of electrolytes used in electrolyzer?

The two electrolytes used in the electrolyzer are

• Alkaline Electrolyte
• Polymer electrolyte membrane (PEM).

Reference

https://www.sciencedirect.com/book/9780444519580/handbook-of-electrochemistry

Written By: Bharat Awasthi