The Overpotential Effect
[Taken from: https://cpb-ca-c1.wpmucdn.com/blog44.ca/dist/a/4/files/2013/04/Unit-5-part-3-notes24hpoa9.pdf]
In practice, it’s been found that a higher potential than calculated must be applied in order for electrolysis
to occur. The causes of this are things such as the nature of the electrodes, temperature, current density,
and time.
The difference between the actual potential required and the calculated potential is termed the “half-cell
overpotential”. This phenomenon requires two adjustments to electrolysis theory:
1) Because of the high hydrogen overpotential, it’s possible to plate out metals that have half-cell
potentials lower than the one for the reduction of water, down to and including zinc. (See the attached BC
Chemistry 12 data table; the overpotential effect is labeled in the bottom left).
2) Because of the high oxygen overpotential, it’s possible to oxidize Br‒(aq) to Br2(g) and Cl‒(aq) to Cl2(g)
before water will oxidize to produce O2(g). That is, the overpotential for oxygen is higher than that for
chlorine. (See the attached BC Chemistry 12 data table; the overpotential effect is labeled in the top right).
Overpotential: This is defined as the difference in the electrode potential of an electrode between its
equilibrium potential and its operating potential when a current is flowing. The overpotential represents
the extra energy needed (an energy loss that manifests as heat) to force the electrode reaction to proceed
at a required rate (or its equivalent current density). Consequently, the operating potential of an anode is
always more positive than its equilibrium potential, while the operating potential of a cathode is always
more negative than its equilibrium potential. The overpotential increases with increasing current density.
In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction’s
thermodynamically determined reduction potential and the potential at which the redox event is
experimentally observed. The term is directly related to a cell’s voltage efficiency. In an electrolytic cell
the existence of overpotential implies the cell requires more energy than thermodynamically expected to
drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered (generated)
than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of
overpotential is specific to each cell design and varies across cells and operational conditions, even for the
same reaction. Overpotential is experimentally determined by measuring the potential at which a given
current density (typically small) is achieved. Due to irreversibility, sustaining a given current requires that
a penalty be paid in terms of electrode potential. The penalty is called “overvoltage”. (See “overvoltage”
on page 2.)
The value of the overpotential also depends on the “inherent speed” of the electrode reaction: a slow
reaction (with small exchange current density) will require a larger overpotential for a given current density
than a fast reaction (with large exchange current density). (See “Exchange current” and “Exchange current
density” on page 2.) These are also referred to as “polarization” of the electrode. An electrode reaction
always occurs in more than one elementary step, and there’s an overpotential associated with each step.
Even for the simplest case, the overpotential is the sum of the concentration overpotential and the
activation overpotential.
Activation Overpotential (Overvoltage): This is defined as the contribution to the total overpotential due
to the charge transfer step at the electrode interface.
Concentration Overpotential (Polarization): This is defined as the overpotential (alternatively called
the “polarization”) associated with the diffusional transport of the reactants to the electrode surface from
the bulk of the electrolyte and the reverse transport of the products. The diffusion is an elementary step in
the overall electrode reaction. It’s sometimes referred to as the “diffusion overpotential” or “mass-transport
overpotential”.
Overvoltage: This is defined as the difference between the cell voltage (when current is flowing) and the
open-circuit voltage (“ocv”: when no current is flowing). The overvoltage represents the extra energy
needed (an energy loss that appears as heat) to force the cell reaction to proceed at a required rate.
Consequently, the cell voltage of a voltaic/galvanic cell, e.g., a rechargeable battery during discharging, is
always less than its ocv, while the cell voltage of an electrolytic cell, e.g., a rechargeable battery during
charging, is always more than its ocv. This is occasionally referred to as “polarization” of the cell. The
overvoltage is the sum of the overpotentials of the two electrodes of the cell and the ohmic losses of the
cell. (See “Ohmic losses” below.)
Unfortunately, the terms “overvoltage” and “overpotential” are sometimes used interchangeably. While
related, they are not the same.
Exchange current: This is the rate of either oxidation or reduction at the equilibrium electrode. It’s
expressed as a current. It’s also technically a misnomer, since there is zero (no) net current flowing.
Exchange current represents the rates of both oxidation and reduction for a given single electrode at
equilibrium.
Exchange current density: This can be defined as the current density that flows equally at equilibrium
and in both directions. The larger the exchange current density, the faster the reaction, and vice versa.
When we talk of equilibrium, it means that there is no gain or loss experienced by the electrode material.
The reaction in the electrode proceeds at equal rates. This is where the reaction proceeds both in the
forward and in the reverse directions to result in a zero net current and a zero net reaction rate.
Ohmic losses represents the voltage drop due to the transfer of electrons in the electric circuit and the
movement of ions through the electrolyte (and any membrane which may be present). The losses usually
take the form of heat generated during the transfer of electrons and the movement of the ions. The electric
circuit (within wires) offers resistance to the transfer of electrons, while the electrolyte offers resistance to
the movement of ions.