Why is only sulphuric acid used in batteries? Why not hydrochloric acid or nitric acid?
Electrochemical reactions form the basis of conversion of chemical energy into electrical energy in batteries. An electrochemical reaction is an oxidation-reduction reaction in which the electrons involved are cleverly routed through an external circuit where they do useful work.
During battery discharge, the electrons released at the negative electrode (cathode) travel through an external circuit and reach the positive electrode (anode). 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.
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 specific gravity of the electrolyte changes during battery use and charge.
Therefore, the specific gravity of the electrolyte is used as an indicator of the state-of-charge of lead-acid batteries. Because the basic reactions in the battery involve uptake and release of sulphuric acid molecules, an electrolyte of sulphuric acid is used in these batteries.
It is, however, possible to use other electrolytes with the lead-lead dioxide couple. They include perchloric acid, fluoroboric acid, fluorosilicic acid, etc. But problems with their use limit their applicability as electrolytes. For example, perchloric acid is a potential explosive in contact with organic substances.
T. SRI DEVI KUMARI
CSIR-Senior Research Fellow
Functional Materials Division
CSIR-Central Electrochemical Research Institute
Karaikudi, Tamil Nadu
(copy has been corrected for a factual error)