Components of a Cell

Figure 7.1 shows a simple electrochemical cell known as the Daniell cell (1839). Here zinc is oxidized at one electrode and copper ions reduced at the other:

The main components of a cell are the negative electrode (zinc bar), the positive electrode (copper bar), and the electrolyte. The electrolyte is an ion conductor and an electronic insulator. As mentioned in Chapter 4, batteries frequently have a physical separator to permit placement of the electrodes in close proximity without shorting. That separator can also serve as the electrolyte (e.g., a solid Li-ion conductor) or can house the electrolyte in a porous structure where current flows by ion movement through the pores. In the cell shown (Figure 7.1), the two electrodes are isolated electronically by a separator (here shown as two beakers connected by a salt bridge of KNO₃). This, of course, is not a configuration practical for commercial use, but it does illustrate the important components of a battery cell. During discharge, electrons travel from the anode to the cathode through the external circuit.

We can estimate the thermodynamic or equilibrium potential for this cell using the methods of Chapter 2.

From Appendix A, the standard potential is

(7.1)

The oxidation of zinc and reduction of copper takes place spontaneously at standard conditions; and electrons travel through the external circuit. The chemical nature of the materials used in the cell determines its thermodynamic potential. The chemical energy is stored in the two electrodes and sometimes in the electrolyte. The size of these components dictates the amount of energy stored in the cell.

Often the electrode that undergoes oxidation during discharge is referred to as the anode, and the one where reduction occurs during discharge is called the cathode. This practice can be confusing because for a rechargeable battery the process is reversed on charge, and the electrode that was the anode during discharge is now the cathode during charge. In contrast, it is unambiguous to refer to the electrode that is the anode upon discharge (e.g., zinc) as the negative electrode and the electrode that is the cathode on discharge (e.g., copper) as the positive electrode. Thus, the terms negative and positive electrode will be used, and the terms anode and cathode will be avoided for batteries.

The construction of a commercial cell is represented better by Figure 7.2, which shows a silver–zinc coin or button cell. The separator is a porous film or matrix that electronically isolates the positive and negative electrodes. The electrolyte fills the pores of the separator. This thin separator reduces the ohmic resistance of the cell. The electrolyte is ionically conductive and contains ions that are involved in the two electrochemical reactions. The electrode contains the active material. Quite often it also contains conductive materials to improve the electronic conductivity over that of the active material itself, and a binder to provide structural integrity, as well as other support materials. Typically, the electrodes are porous structures that increase the active area in contact with the electrolyte, as was introduced in Chapter 5. Current passes to and from the electrodes through their respective current collectors. For the silver–zinc cell, also known as a silver oxide cell, the overall reaction is

(7.2)

The methods of Chapter 2 can be used to determine the standard potential, U^θ, from the Gibbs energies of formation, found in Appendix C.

(7.3)

These electrodes are separated from each other with a porous cellophane film. Silver oxide is reduced at the positive electrode on discharge:

(7.4)

The other electrode is a powder of Zn, which reacts as follows during discharge:

(7.5)

The standard potential for each of these reactions is shown to the right of the corresponding half-cell reaction. Because the silver electrode has a higher potential, it is referred to as the positive electrode; the zinc electrode is the negative electrode. The pores of the electrodes and the separator are filled with concentrated KOH to provide a conduction path and to transport OH⁻. Also, for this cell, hydroxide ions are involved in the reactions at both electrodes. At the same time, water is consumed at the positive electrode and produced at the negative electrode during discharge. As the reaction proceeds, the net amount of water and hydroxide ions remains constant. This characteristic of the silver–zinc cell will influence its operation as we will see in subsequent sections.