From thermodynamic calculations we have find out the minimum voltage at which an aluminum electrolysis cell has to work in reversible conditions:
But in the real world the electrolysis cell voltage is higher than the reversible potential.
Overall, the voltage of a pot is the sum of various components:
- Erev is the voltage to apply in reversible conditions for the basic reaction to occur
- The terms indicated with η are the so called “overvoltages”. In electrochemistry, overvoltage indicates an extra voltage, respect to the reversible potential, to be applied to the pot to force the cell reaction to proceed at a required rate. This extra energy is a energy loss which is transformed into heat inside the cell. In detail, we have:
- ηcc: concentration overvoltage at cathode
- ηaa: concentration overvoltage at anode
- ηac: reaction overvoltage at anode (0.6 ÷ 0.9 V)
- The terms indicated with the letter V denotes ohmic voltage drops. In detail:
- VA is the voltage drop in the anodes
- VB is the voltage drop in the electrolyte
- VC is the voltage drop in the cathode
- VX is the voltage drop in the buss bars external to the pot but which contributes to the total pot voltage
every of these three overvoltages follow a classical Tafel equation:
being i the current density.
The term VB, the voltage drop in the electrolyte, is the only term that can be immediately changed in the overall voltage of a pot, varying the distance of the anodes from the metal pad (the so-called ACD: Anode to Cathode Distance).