Introduction to Electrochemical Cells

The purpose of this chapter is to provide the basic knowledge on batteries, which will allow for their general understanding. Therefore, after defi ning their components and structure, an overview of the quantities that characterize these storage devices will be given. Scientifi cally batteries are referred to as electrochemical or galvanic cells, due to the fact that they store electrical energy in the form of chemical energy and because the electrochemical reactions that take place are also termed galvanic. Galvanic reactions are thermodynamically favorable (the free energy difference, Δ G , is negative) and occur spontaneously when two materials of different positive standard reduction potentials are connected by an electronic load (meaning that a voltage is derived). The material with the lower positive standard reduction potential undergoes an oxidation reaction providing electrons by the external circuit to the material with the higher positive standard reduction potential, which in turn undergoes a reduction reaction. These half reactions occur concurrently and allow for the conversion of chemical energy to electrical energy by means of electron transfer through the external circuit. It follows that the material with the lower positive standard reduction potential is called the negative electrode or anode on discharge (since it provides electrons), while the material with the higher positive standard reduction is called the positive electrode or cathode on discharge (since it accepts electrons). In addition to the electrodes, the two other constituents that are required for such reactions to take place are the electrolyte solution and the separator. The electrolyte is an ion conducting material, which can be in the form of an aqueous, molten, or solid solution, while the separator is a membrane that physically prevents a direct contact between the two electrodes and allows ions but not electrons to pass through; it therefore ensures electrical insulation for charge neutralization in both the anode and cathode once the reaction is completed. Two fi nal parts required to complete a commercial galvanic cell are the terminals. They are necessary when applying the batteries to electrical appliances with specifi c holder 1