Working Principle And Types Of Dry Cells

The dry cell is a simple electrochemical cell that converts chemical energy into electrical energy. They were developed by George Leclanche in 1866, and are also called Leclanche Cells. 

Dry cells or batteries are very commonly used in everyday life. We use batteries in clocks, watches, toys, cameras, etc. 

Types of Dry Cells

• Primary Cells
• Secondary Cells

Primary Cells

• In these cells, the chemical reaction is non-reversible.
• Once the chemical reagents are consumed, the cell is said to be dead. It cannot be used again.
• They cannot be recharged.
• They have a fixed output voltage of 1.5 V
• These cells are small in size and hence, they are easy to move around and transport.

Secondary Cells

• These cells involve a reversible chemical reaction. 
• These cells can be reused multiple times.
• Once the cell is discharged, it can be recharged by passing electricity in the opposite direction.
• They do not have a fixed output voltage. It depends upon the load attached to it.
• These cells can also be transported even though they are not as small as primary cells.

Working Principle And Types of Dry Cells 

•  Primary Cells

• Zinc Carbon Cell

In the primary cell, the chemical reaction happens only once. In this cell, the function of the anode is performed by the zinc case. There is a graphite/carbon rod that touches the brass cap on the top but does not touch the zinc case at the bottom. It acts as the cathode. 

The zinc case is protected on the sides but is not protected at the bottom. The space between the zinc case and the carbon rod is packed with a moist paste of ammonium chloride (NH4Cl), zinc chloride (ZnCl2), manganese dioxide (MnO2), and black carbon. Now let’s see the reactions that happen at the Anode and Cathode respectively.

Anode: Oxidation happens at the anode. Oxidation is the loss of electrons. The Zinc case acts as the anode. The oxidation of Zinc happens and as a result, the zinc case corrodes over time.

Zn(s) → Zn2+ + 2e– 

Cathode: Reduction happens at the cathode. The process of gain of electrons is called Reduction. Here, the Manganese is reduced from the +4 oxidation state to +3 oxidation state. Ammonia released in the process is trapped by Zn+2 ions to form a complex [Zn (NH3)] +2. This complex is soluble in nature and is in the aqueous form. 

Mn+4O2 + NH4 ++ e– → Mn+3O (OH) + NH3

Zn+2 + NH3 → [Zn (NH3)]+2(aq)

This cell has an electric potential of 1.5V.

• Mercury Cell

In the Mercury cell, the anode is made of Zinc Amalgam and a paste of Mercury oxide and carbon is the cathode. The electrolyte is the paste of potassium hydroxide and zinc oxide. 

The overall potential of this cell is 1.35V. It is used in small appliances like hearing aids, watches, etc. It is not rechargeable.

Anode: The oxidation reaction happens at the anode. It is the loss of electrons or the addition of oxygen.

Zn (Hg) + 2OH– → ZnO + H2O + 2e–

Cathode: HgO + H2O + 2e– → Hg (l) + 2OH–

Thus, the overall reaction is as follows:

Zn (Hg) + HgO(s) → ZnO(s) + Hg (l)

2. Secondary Cells

• Lead Storage Cell

The secondary batteries are capable of undergoing a large number of charging and discharging cycles. Once they are used, they can be recharged by passing electricity in the opposite direction. The lead storage cell is the most common type of secondary battery that is used in automobiles and inverters.

In this cell, the anode is made of lead (Pb). The cathode comprises a grid of lead packed with lead dioxide (PbO2). It has a 38% concentrated solution of sulfuric acid H2SO4 as the electrolyte in between the anode and cathode grid.

When the battery is in use, the following reactions occur in the cell:

Anode: The oxidation reaction happens at the anode, in which the lead (Pb) is oxidised or loses electrons when it reacts with the sulphate ions from the H2SO4 solution.

Pb(s) + SO4 2–(aq) → PbSO4 (s) + 2e–

Cathode: The reduction of the lead ions from +4 to +2 state happens in the presence of the electrolyte.

Pb+4O2 (s) + SO4 2–(aq) + 4H+ (aq) + 2e– → Pb+2SO4 (s) + 2H2O (l)

The overall reaction is:

Pb(s) + PbO2 (s) + 2H2 SO4 (aq) → 2PbSO4 (s) + 2H2O (l) + Energy (which is converted to electricity)

As the cell is used, it gets discharged. In the forward direction, it is called the Galvanic cell. Once the cell is fully used, i.e. it is completely discharged, then we pass electricity through it in the opposite direction. This reverses the chemical reaction in the cell. In this condition, it is called the Electrolytic cell. During this process, the cell is recharged.

• Fuel Cells

The galvanic cells that directly convert the chemical energy of the highly combustible fuels like hydrogen, methane, etc. into electric energy are called fuel cells, and they are highly efficient. 

In these cells, the electrodes are continuously fed with the reactants, and the products are continuously removed from the electrolyte compartment. Such fuel cells are used to power spaceships etc.

In this cell, we push the hydrogen gas from one end and oxygen gas from the other end through the porous graphite rods into the concentrated electrolyte solution of sodium hydroxide (NaOH). 

The reactions at the anode and cathode are as follows:

Anode: Oxidation of hydrogen happens at the anode, i.e. addition of oxygen happens here.

2H2 (g) + 4OH– (aq) → 4H2O (l) + 4e–

Cathode: Reduction of Oxygen happens at the cathode, i.e. addition of hydrogen happens here.

O2 (g) + 2H2O (l) + 4e– → 4OH– (aq)

The overall reaction is –

2H2 (g) + O2 (g) → 2 H2O (l) + energy (which is converted to electricity)

Applications of Dry Cells

• Primary cells are used in electronic gadgets as they are much smaller and easily transportable.
• Lead storage cells are used in automobiles and inverters
• Fuel cells are used in space programs

Conclusion

The dry cells convert the stored chemical energy into electrical energy. They undergo reduction-oxidation (Redox) reactions at the cathode and anode. Some dry cells can be recharged while others can be used only once. The secondary fuel cells are pollution-free and are extremely efficient in generating electricity.