Overpotentials on various electrodes table
Table shows overpotential values on selected electrodes.

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Electrodes overpotential [V]

 Electrode / secreted substance Al Zn Cr Fe Cd Ni Sn Pb Bi Cu Ag Hg Pt C H2 acid environment 0.8 0.7 0.5 0.5 1.2 0.4 1 1.3 0.8 0.6 0.4 1.2 0.1 0.5 H2 alkaline environment 0.4 0.4 - 0.2 - 0.4 0.8 0.9 0.5 - - 1.3 0.1 - O2 - - - 0.25 0.4 0.1 - 0.3 - 0.2 0.4 - 0.4 0.3 Zn - 0.2 - - 0.55 - - - - 0.4 - - 0.5 - Cr - - 0.6 - - - - - - - - - - - Fe - - - 0.6 - - - - - - - - - - Cd - - - - 0.2 - - 0.35 - 0.35 - - 0.35 - Co - - - - - - - - - - - - 0.3 - Ni - - - - - 0.6 - - - - - 0.6 - - Sn - - - - - - 0.2 0.3 - 0.3 - - - - Pb - - - - - - - 0.2 0.12 0.02 - - - - Bi - - - - - - - 0.2 0.35 - - - - - Cu - - - - - - - - 0.2 - - - 0.3 - Ag - - - - - - - - - - 0.2 - 0.8 -

Some facts

• Overpotential is a difference between the potential of the electrode polarized by the flow of electric current and its potential in the equilibrium state.
$\eta = E - E_0$
where:
• $\eta$ - overpotential of electrode,
• $E$ - potential of the electrode in a polarized state,
• $E_0$ - electrode potential in the equilibrium state.
• Overpotential is the polarization measure of the electrode.
• Overpotential is measured in volts (V).
• The author of the overpotential concept is W.A. Caspari. This concept appeared for the first time in 1899.
• The total overvoltage on the electrode consists of partial overvoltages:
$\eta = \eta_\Omega + \eta'_\Omega + \eta_a + \eta_c$
where:
• $\eta_\Omega$ - resistive overvoltage, related to the potential drop across the electrode-solution interface,
• $\eta'_\Omega$ - pseudo-resistance overvoltage, associated with the resistance of the electrolyte layer separating the test electrode and the reference electrode,
• $\eta_a$ - activation overvoltage, associated with an additional expenditure of electric potential to overcome the activation energy of the electrode reaction,
• $\eta_c$ - concentration overvoltage, associated with additional electrical work associated with changes in concentrations of electroactive substances in the immediate vicinity of a polarized electrode.
• Concentration overpotential can be calculated using Nernst equation:
$\eta_c=\frac{RT}{nF}\ln \frac{a_e}{a_0}$
where:
• $a_e$ – depolarizer activity at the surface of a polarized electrode,
• $a_0$ – the activity of the depolarizer in a state of equilibrium,
• $n$ – number of electrons transferred in the transition reaction,
• $R$ – gas constant,
• $T$ – absolute temperature in kelvins,
• $F$ – Faraday's constant.

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