CHEMICAL BATTERIES
Reading edition
Electricity from the Chemical Bond
If you vigorously knead a lemon to free the juices and then stick a strip of zinc in one end and a copper strip in the other, you can measure a voltage across the strips. Electrons will flow through the load without the inconvenience of having to supply heat. You have made yourself a chemical battery.
The chemical battery was the first direct conversion device. Two hundred years ago it was the scientists’ only continuous source of electricity.
Since the chemical battery does not need heat for its operation, it is logical to ask what makes the current flow. Where does the energy come from?
The battery has no semiconductors, but, like the thermoelectric couple and the thermionic diode, it uses dissimilar materials for its electrodes. A conducting fluid or solid is also present to provide for the passage of current between the electrodes. In the example of the lemon, the copper and zinc are the dissimilar electrodes, and the lemon juice is the conducting fluid or electrolyte that supplies positive and negative ions. The battery derives its energy from its complement of chemical fuel. The voltage difference arises 23 because of the different strengths of the chemical bonds. The chemical bond is basically an electrostatic one; some atoms have stronger electrical affinities than others.
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.
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