When two electronically-conducting materials (electrodes) are placed in a solution containing ions (electrolyte), an electrochemical cell is formed and the potential for redox reactions to occur at the interfaces between the electrodes and electrolyte exists. Redox reactions are those reactions which involve a change in oxidation state, that is a gain or loss of electrons. If reactions are allowed to occur by electrically connecting the electrodes outside the solution, current will flow—as electrons in the electrodes (and external connections) and as ions in the solution. At one electrode, the anode, electrons are accepted and oxidation takes place and at the other electrode, the cathode, electrons are discharged and reduction takes place. In the absence of current flow, the “potential” for reactions to occur can be measured as a voltage. This voltage, called the equilibrium cell potential, is related to the overall Free Energy, ΔG, of the cell reactions made up of the sum of the reactions at both electrodes. It is expressed as
where n is the number of electrons transferred and F is a constant (Faraday’s Constant).
In any electrochemical cell, the amount of reduction at the cathode is equal to the amount of oxidation at the anode. This is regardless of whether the cell is generating electrical energy, as for example in a battery or Fuel Cell, or is consuming electrical energy in the generation of specific chemicals (as an electrolytic cell). These two types of cells are distinguished by the polarity of the electrodes; for example, in the case of a battery the anode is at negative potential relative to the cathode and vice-versa for an electrolytic cell. It is common usage to define electrochemical cells as the summation of two half-cells, that is the two halves of the cell complete with electrolyte and electrode. Thus, the equilibrium potential is the sum of two half-cell potentials.