Chemical Reactions Used in Batteries and Fuel Cells
Consider the following chemical reactions of common batteries together with some fuel cell reactions which will be discussed further in the next section.
Battery Reactions Pb + PbO₂ + 2H₂SO₄ ⇔ 2PbSO₄ + 2H₂O Fe + NiO₂ ⇔ FeO + NiO Zn + AgO + H₂O ⇔ Ag + Zn(OH)₂ Pb + Ag₂O ⇔ PbO + 2Ag Fuel Cell Reactions 2LiH ⇔ 2Li + H₂ 2CuBr₂ ⇔ 2CuBr + Br₂ 2H₂ + O₂ ⇔ 2H₂O (Bacon cell) PbI₂ ⇔ Pb + I₂
In principle all these reactions are the same as those going on inside the lemon, although each type of cell produces a slightly different voltage because of the varying chemical affinities of the atoms and molecules involved. There are literally hundreds of materials which can be used for electrolytes and electrodes.
No heat needs to be added as the electrostatic chemical bonds are broken and remade in a battery to generate electrical power. The chemical reaction energy is transferred to the electrical load with almost 100% efficiency. The Carnot cycle is no limitation here; only “cold” electrostatic forces are in action. The reactions cannot go on forever, however, because the battery supplies the energy converter with a very limited supply of fuel. Eventually the fuel is consumed and the voltage drops to zero. This deficiency is remedied by the fuel cell in which fuel is supplied continuously.
An Old Standby in Outer Space
Almost every satellite and space vehicle has a chemical battery aboard. It is not there so much for continuous power production but as a rechargeable electrical accumulator or reservoir to provide electricity during peak loads. The battery is also needed to store energy for use during the periods when solar cells are in the earth’s shadow and therefore inoperative. In this capacity the dependable old battery serves the most modern science very well indeed.
THE FUEL CELL: A CONTINUOUSLY FUELED BATTERY
Potential Fuels
The battery has a very close relative, the fuel cell. Unlike the battery the fuel cell has a continuous supply of fuel.