In recent years, the scientists have designed the cells which convert chemical energy of a fuel directly into electrical energy. Such cells are called fuel cells. These are the voltaic cells in which, the fuels such as H2, CO, CH4, C3H8, etc., are used to generate electrical energy without the intervention of thermal devices like boiler, turbines, etc.
The conventional method of conversion of chemical energy of a fuel into electrical energy involves combustion of a fuel to liberate heat. The heat energy so produced is used to generate steam for spinning the turbines which are coupled to electrical generators. This process is approximately 40% efficient.
Fuel cells are designed in such a way that the materials to be oxidised and reduced at the electrodes are stored outside the cell and are constantly supplied to the electrodes. In fact, fuel cell is a flow battery that continues to operate as long as the reactants from outside are fed into it. One of the most successful fuel cells uses the reaction of hydrogen and oxygen to form water and is known as H2O2 fuel cell. The H2-O2 fuel cell is also called Bacon cell after the name of its inventor and it had been used to power the Apollo space Missions. The water vapours produced during the reaction were condensed and added to drinking water supply for the astronauts. The experimental arrangement is shown in Fig. 33.13.
The cell consists of porous carbon electrodes which are impregnated with catalyst (Pt, Ag or CoO). Hydrogen and oxygen are bubbled through the electrodes into electrolyte which is an aqueous solution of NaOH or KOH. The electrode reactions are:
The cell runs continuously as long as the gases hydrogen and oxygen are supplied at the temperature 525 K and 50 atm. pressure.
ADVANTAGES OF FUEL CELLS
Some prominent advantages of fuel cells are being described as follows:
(i) Pollution Free Working. There is no harmful or objectionable product formed in fuel cells. Hence, they do not cause pollution problems.
(ii) High Efficiency. The efficiency of fuel cells is approximately 70-75%, which is much higher than the conventional cells.
(iii) Continuous Source of Energy. Unlike conventional batteries, energy can be obtained from the fuel cell continuously so long as the supply of fuel is maintained.
The H2/O2 cell has been used for generating electrical power in the Apollo space programmes.