4. Electrochemical cells

 

Electrochemical cell consists of an electrolyte solution, electrodes and an external circuit as defined in Chapter 1.1. Electrochemical cells can be divided into two main types depending on the spontaneity of the cell reaction. The cell reaction in galvanic cells is spontaneous, and chemical energy converts directly into electric energy. In other words, the \Delta G of the reaction is negative and therefore the cell potential according to equation (1.6) is positive. In electrolysis cells, electrical energy is converted to chemical energy. The cell reaction is hence forced towards the non-spontaneous direction and the \Delta G of an electrolysis cell is positive and cell potential negative.

Galvanic cells are used in everyday life as primary and secondary batteries and fuel cells. An unfortunate side effect of galvanic cell is electrochemical corrosion. Examples of electrolysis are, for example, charging of batteries, production of chlorine gas and sodium hydroxide from electrolysis of sodium chloride solution, and different electrochemical analysis methods. The most important analysis methods and the requirements for the electrochemical cell used in connection with them are introduced in Chapters 7 and 8.

We know from thermodynamics that the \Delta G of a reaction gives the maximum work available from a particular reaction. In practice, electrochemical cells are also more or less non-ideal, and efficiencies of 100% are not achieved. Deviations from ideal behavior stem from irreversible processes that take place when electric current flows, e.g. ohmic losses due to restricted electrolyte conductivity (Figure 4.1) and possible slowness of electrode reactions, which is seen as overpotentials (Chapter 6.3).

In this chapter, we discuss the description of electrochemical cells, the conventions associated with them and the formation of the cell potential. Applications of cells in equilibrium, potentiometry and pH-potential equilibrium diagrams are also briefly introduced. Finally, one of the most important cell structures for electrochemical analysis, the three-electrode set-up, is presented.