Chemical Sources of Electric Current

(batteries), devices that generate electric energy through the direct conversion of the chemical energy of oxidation-reduction reactions. The first chemical sources were created in the 19th century (voltaic pile, 1800; Daniell cell, 1836; Leclanché cell, 1865). Prior to the 1860’s, chemical sources were the sole sources of electric energy for electrical equipment and laboratory research.

The heart of chemical sources of electric current consists of two electrodes—one containing the oxidant, the other the reductant —in contact with an electrolyte. A potential difference is established between the electrodes, constituting an electromotive force (emf) corresponding to the free energy of the oxidation-reduction reaction. The functioning of chemical current sources is based on the occurrence across a closed external circuit of spatially separated processes: at the negative electrode the reductant is oxidized to form free electrons that travel along the external circuit (creating a discharge current) to the positive electrode, where they participate in the reduction of the oxidant.

Depending on the operating characteristics and the electrochemical system (the combination of reagents and electrolyte), chemical sources of electric current are divided into primary cells, which usually become nonfunctional after the reagents are consumed (after discharge), and secondary, or storage, cells, in which the reagents are regenerated during charging (the input of current from an external source). The division is arbitrary, since some cells can be partially charged. Among the important and promising chemical sources of electric current are fuel cells (electrochemical generators), which are suitable for long periods of continuous operation by virtue of the fact that the electrodes are continuously provided with a fresh supply of reagents and the reaction products are removed. The design of reserve batteries permits the batteries to be stored in an inactive state for 10–15 years.

Since the beginning of the 20th century the production of chemical sources of electric current has expanded steadily in conjunction with the development of motor vehicle transport and electrical engineering and the increasing use of electronic and other equipment with independent power supplies. In commercially available chemical current sources, the most commonly used oxidants are PbO₂, NiOOH, and MnO₂; Pb, Cd, Zn, and other metals serve as the reductant. The electrolytes are aqueous solutions of alkalis, acids, or salts.

The major characteristics of several chemical sources of electric current are shown in Table 1. Those sources under development that are based on more active electrochemical systems display the most desirable characteristics. Thus, in nonaqueous electrolytes (organic solvents, fused salts, or solid compounds with ionic conductivity), alkali metals can be used as the reductant. Fuel cells permit the use of high-energy liquid or gas reagents.