In battery, overpotential is the potential difference (or voltage measure) between a theoretical or thermodynamically determined voltage and the actual voltage under operating conditions. To understand this clearly, let us recall the operating principle behind an electrolytic cell and a galvanic cell or voltaic cell.
An electrolytic cell is an electrochemical cell that undergoes a redox (short for reduction/oxidation) reaction when electric current is applied. A galvanic cell is an electrochemical cell that generates electric current from redox reactions occurring within the cell. In an electrolytic cell, overpotential means that more energy is required than thermodynamically determined in order to drive a redox reaction. In a galvanic cell overpotential means that less energy is recovered than thermodynamically determined. In both cases, the extra or missing energy is lost as heat.
Graphical Representation of Overpotential
Image Source: Royal Society of Chemistry Website
In a more practical sense, a rechargeable battery acts as a galvanic cell when it is discharging as it converts chemical energy to electrical energy. On the other hand, it acts as an electrolytic cell when it is being charged as it converts electrical energy to chemical energy. What happens during these conversions is an electrochemical process known as redox reaction. The voltage measured in a redox reaction is called reduction potential and it is expressed in terms of volts (V) or millivolts (mV).
Redox Reaction in Electrolysis
To further understand how overpotential occurs in batteries, let us re-examine the chemical process known as electrolysis that occurs in an electrolytic cell. Electrolysis occurs when DC current is applied through the electrolyte, resulting in a chemical reaction between electrodes and the separation of elements (molecules, atoms and ions). During this process, a transfer of electrons also occurs at the anode and cathode.
The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit via the electrodes. When an electrode loses electrons, oxidation occurs. When it gains electrons, reduction occurs. The voltage that is needed for electrolysis to occur is called decomposition potential. In such a case, the potential difference between decomposition potential (actual voltage) and the reduction potential (thermodynamically determined) is now the so-called overpotential.
Overpotential and Voltage Efficiency
Overpotential is directly related to a cell’s voltage efficiency. In a galvanic cell, Voltage Efficiency is the ratio of the actual voltage under operating conditions to the theoretical cell voltage.
Voltage Efficiency (galvanic cell) = Actual Voltage/Theoretical Voltage
It’s the opposite ratio for the electrolytic cell.
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