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:

$\eta_c=\dfrac{RT}{nF}\ln\left(\dfrac{a_e}{a_0}\right)$
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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electrode_overpotentials · electrode_overvoltage

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