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.