FUEL CELLS

FUEL CELLS
Definition
Fuel cell is a voltaic cell, which converts the chemical energy of the fuels directly into electricity without combustion. It converts the energy of the fuel directly into electricity. In these cells, the reactants, products and electrolytes pass through the cell.

Fuel + Oxygen → Oxidation products + Electricity

Examples:
Hydrogen-oxygen fuel cell, Methyl alcohol-oxygen fuel cell, Propane – oxygen fuel cell.

1. Hydrogen-Oxygen fuel cell
Hydrogen-oxygen fuel cell is the simplest and most successful fuel cell, in which the fuel-hydrogen and the oxidiser-oxygen and the liquid electrolyte are continuously passed through the cell.
It consists of two porous electrodes anode and cathode. These porous electrodes are made of compressed carbon containing a small amount of catalyst (Pt,Pd,Ag). In between the two electrodes in electrolytic solution such as 25% KOH or NaOH is filled. The two electrodes are connected through the voltmeter.

Working
Hydrogen (the fuel) is bubbled through the anode compartment, where it is oxidised. The oxygen (oxidiser) is bubbled through the cathode compartment, where it is reduced.

At cathode
The electrons produced at the anode pass through the external wire to the cathode, where it is absorbed by oxygen and water to produce hydroxide ions.

O2 + 2H2O + 4e¯ → 4OH¯

At anode
Hydrogen molecules are oxidised at the anode with the liberation of electrons, which then combine with hydroxide ion to form water.

2H2+4OH¯ → 4H2O + 4e¯

Cell reaction
At anode: 2H2O + 4OH¯ → 4H2O + 4e¯ E = +0.83V
At cathode: O2 +2H2O+4e¯ → 4OH¯ E = +0.40V
Overall cell reaction: 2H2 + O2 → 2H2O E = +1.23V

Fuel Battery
When a large number of fuel cells are connected in series, it form a battery called fuel cell battery or fuel battery.

Advantages
1. H2-O2 fuel cells are used as auxiliary energy source in space vehicles, submarines or other
military-vehicles.
2. The energy conversion (from chemical to electrical) is highly efficient.
3. They have low maintenance cost.
4. H2-O2 fuel cell produces drinking water.
5. Noise and thermal pollution are low.
6. In case of H2-O2 fuel cells, the product of water is proved to be a valuable source of fresh
water by the astronauts.

Disadvantages
1. Their initial cost is high.
2. Life time of fuel cells is not known accurately.

Methanol Fuel cell
In this fuel cell, platinum black catalyst is deposited on a 10 sq. inch nickel plate which acts as fuel electrode. Silver catalyst is deposited on a nickel plate. This plate acts as oxygen electrode. Both of these electrodes are placed in 25% NaOH or KOH with air bubbling up the silver catalyst surface. Now 35 cc of pure methyl alcohol (methanol) is added and the fuel cell is activated. This fuel cell produces enough power to run a simple transistorized radio even without bubbling of air.