- Galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy are called fuel cells.
Working of fuel cell
- In the cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution.
- Catalysts like finely divided platinum or palladium metal are incorporated into the electrodes for increasing the rate of electrode reactions.
- The electrode reactions are given below:
- The cell runs continuously as long as the reactants are supplied.
- Fuel cells produce electricity with an efficiency of about 70 % compared to thermal plants whose efficiency is about 40%.
Advantages of fuel cells
- A hydrogen-oxygen fuel cell is a non-polluting clean fuel.
- Fuel cells do not produce pollutants like carbon monoxide, sulfur dioxide and nitrogen oxides.
- Fuel cells could replace larger batteries which are not easily recycled and contain highly toxic metal compounds.
- In the secondary cells, the reactions can be reversed by an external electric energy source. Therefore, these cells can be recharged by passing electric current and used again and again.
- These are also called storage cells.
- Examples of secondary cells are lead storage battery and nickel-cadmium storage cell.
Lead storage battery
· It consists of a lead anode and a grid of lead packed with lead dioxide (PbO2 ) as cathode. A 38% solution of sulphuric acid is used as an electrolyte.
· It is also known as accumulator cell.
· It is a rechargeable device.
A nickel-cadmium (Ni-Cd) battery is a type of rechargeable battery that uses nickel oxide hydroxide and metallic cadmium as electrodes. Ni-Cd batteries have been used widely in various applications such as portable electronics, power tools, and electric vehicles.
The Ni-Cd battery operates based on a redox reaction between the nickel oxide hydroxide and metallic cadmium electrodes in an alkaline electrolyte. During charging, a current is passed through the battery in the opposite direction of the discharge current, which causes the cadmium electrode to oxidize and form cadmium hydroxide, while the nickel oxide hydroxide electrode is reduced to form nickel hydroxide.
During discharge, the opposite reaction occurs, and the cadmium hydroxide and nickel hydroxide are oxidized and reduced, respectively, producing an electric current. Ni-Cd batteries have a relatively low energy density compared to other types of rechargeable batteries, but they have a high discharge rate, can operate in a wide range of temperatures, and have a long cycle life.
One drawback of Ni-Cd batteries is that they contain toxic cadmium, which can cause environmental pollution if not disposed of properly. Additionally, Ni-Cd batteries can suffer from memory effect, which occurs when the battery is repeatedly charged and discharged without being fully discharged. This can result in a decrease in the battery's capacity.
Comparison between primary, secondary and fuel cells-
1) It only acts as galvanic or voltaic cell. i.e., produces electricity
1) It acts as galvanic or voltaic cell while discharging (produces electricity) and acts as electrolytic cell (consumes electricity)
1) It is a simple galvanic or voltaic cell. i.e., produces electricity
2) Cell reaction is not reversible.
2) Cell reaction is reversible.
2) Cell reaction is reversible.
3) Can't be recharged.
3) Can be recharged
3) Energy can be withdrawn continuously
4) Can be used as long as the active materials are present
4) Can be used again and again by recharging.
4) Reactants should be replenished continuously. it does not store energy.
eg: Leclanche cell or Dry cell, Lithium cell.
Uses: In Pace makers watches, Transistors, radios ect.
eg: Lead storage battery, NiCd battery, Lithium ion cell
Uses: In electronic equipments, automobile equipments, digital cameras, laptops, flash light.
eg: Fuel cell