June 29, 2013

Battery - Working Principle - Types and why electrolyte is Used in batteries

A battery is a source of electrical energy. It is consisted by two substances, and represented by two electro-chemically active electrodes of different composition, both of which are immersed in an electrolyte that provides a conductive medium between them.
Principle of Batteries:
Electro-chemical reactions form the basis of conversion of chemical energy into electrical energy in batteries. The anode undergoes what is known as an oxidation reaction: during discharge two or more ions from the electrolyte combine with the anode to form a compound and release one or more electrons. Simultaneously, the cathode undergoes a reduction reaction.
Principle / Working  of Battery
Types of Battery:
What is the difference between primary and secondary battery?
Primary Battery: A primary battery can discharge once only. A primary battery internal structure is more simple, it doesn't have to accommodate reversible volume changes. The primary battery has bigger quality ratio and volume ratio than rechargeable battery.
Secondary Battery: a second battery is rechargeable. When discharge a rechargeable battery, its electrode volume and structure cause a reversible change. An external source of direct electrical current supplies electrons to the anode and removes them from the cathode, forcing the chemical reactions into reverse until the cell is recharged.
How a Battery works?
During battery discharge, the electrons released at the negative electrode (anode) travel through an external circuit and reach the positive electrode (cathode). 
What is the purpose of electrolyte in a battery?
Charge neutrality inside the battery is established by movement of ions in an electrolyte. Thus, the electrolyte is required to be a good conductor of ions. The electrolyte in a battery may or may not participate in the reactions occurring at the electrodes, but must support the reactions. 
For example, the electrolyte does not participate in the cell reactions in nickel-cadmium and lithium-ion batteries.
In the case of the zinc-manganese dioxide dry cells, a variety of neutral/alkaline electrolytes such as ammonium chloride, zinc chloride and potassium hydroxide can be employed.
Depending on the chemicals used, the zinc-manganese dioxide cell with potassium hydroxide electrolyte can be formulated as a primary cell or a secondary cell.
Why is only sulphuric acid used in batteries? Why not hydrochloric acid or nitric acid?
The working of the lead-acid battery (commonly used for starting-lighting-ignition operations in automobiles) involves 
  • formation of lead sulphate by oxidation of lead at the anode and
  • reduction of lead dioxide at the cathode.

Because the product of battery discharge is lead sulphate on both the electrodes, the total cell reaction is sometimes referred to as double sulphate reaction.
The formation of the lead sulphate products involves sulphuric acid, which is the electrolyte used in these batteries. During battery charge, lead sulphate is converted back to lead and lead dioxide, releasing sulphuric acid into the electrolyte.
Because sulphuric acid is 
  • consumed during the discharge process and 
  • released during the charge process.

The basic reactions in the battery involve uptake and release of sulphuric acid molecules, an electrolyte of sulphuric acid is used in these batteries.
Other acids that can be used in Batteries include:
It is, however, possible to use other electrolytes with the lead-lead dioxide couple. They include perchloric acid, fluoroboric acid, fluorosilicic acid, etc. 


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