Overvoltage (electrochemistry)

In electrochemistry, an overvoltage is a potential difference between the thermodynamic redox potential of a half-reaction and the potential at which the redox reaction actually takes place.

Overvoltage components of some gases at vers. Electrode materials at 25 ° C
Electrode material	hydrogen	oxygen	chlorine
Platinum (platinum-plated)	−0.07 V	+0.77 V	+0.08 V.
palladium	−0.07 V	+0.93 V
gold	−0.09 V	+1.02 V
iron	−0.15 V	+0.75 V.
Platinum (smooth)	−0.16 V	+0.95 V	+0.10 V
silver	−0.22 V	+0.91 V
nickel	−0.28 V	+0.56 V.
graphite	−0.62 V	+0.95 V	+0.12 V
lead	−0.71 V	+0.81 V
zinc	−0.77 V
mercury	−0.85 V

The overvoltage is therefore a kinetic phenomenon . It occurs in metastable redox systems. Processes that should be based solely on thermodynamic considerations do not take place due to kinetic inhibition.

A well-known example is the overvoltage of hydrogen: According to thermodynamic calculations, iron should dissolve in neutral water, but this does not happen. The metal only dissolves in an acidic environment, where the concentration of oxonium ions is several orders of magnitude higher.

Overvoltage in electrolysis

General

The occurrence of metastability is very common in redox processes: Almost every organic substance is metastable and, after sufficient stimulation, e.g. B. lighting with a match, go into a more stable state. Complete combustion produces water, carbon dioxide and other gases. Correspondingly, overvoltage as a form of metastability can also occur during electrolysis , i.e. redox reactions that are forced by an electric current . Overvoltage occurs particularly frequently in reactions that lead to the formation of gases such as hydrogen .

The excitation, which nevertheless leads to the course of the reaction, can take place through an increased voltage which is applied in addition to the decomposition voltage . The level of excitation required depends on various aspects, such as the type of gas produced or the material of the electrodes.

Microscopic reasons for overvoltage

Overvoltages occur when at least one single step of the electrolysis reaction is kinetically inhibited, i.e. slowed down. All sub-steps of the overall process come into question:

The diffusion of the reactants to the electrode
Any reaction of the reactant before it reaches the electrode
At least partial stripping of the solvation shell of the reactant
Adsorption of the reactant
Electron transfer from the reactant to the electrode ( oxidation of the reactant) or vice versa ( reduction of the reactant)
Desorption of the product or looking for a low-energy position on the surface (if metals are deposited on the same metal) or nucleation (if a layer is deposited on the electrode)
If the product desorbs: formation and / or restructuring of the solvation shell,
Possible reaction of the product in front of the electrode
Diffusion of the product into the solution

Other reasons for overvoltage

Ohmic overvoltage
Transport overvoltage (diffusion, migration and convection overvoltage)
Crystallization overvoltage

example

Example of the electrolysis of an aqueous hydrochloric acid solution with graphite electrodes. Two reactions can take place at both the anode and the cathode:

Reaction location	equation	potential	Potential + overvoltage
Anode ( oxidation / electron release)	${\ displaystyle \ mathrm {4 \ Cl ^ {-} \ to 2 \ Cl_ {2} +4 \ e ^ {-}}}$	${\ displaystyle E = 1 {,} 359 \, \ mathrm {V}}$	${\ displaystyle E = 1 {,} 359 \, \ mathrm {V} +0 {,} 12 \, \ mathrm {V} = 1 {,} 479 \, \ mathrm {V}}$
Anode ( oxidation / electron release)	${\ displaystyle \ mathrm {2 \ H_ {2} O \ to O_ {2} +4 \ H ^ {+} + 4 \ e ^ {-}}}$	${\ displaystyle E = 1 {,} 229 \, \ mathrm {V}}$	${\ displaystyle E = 1 {,} 229 \, \ mathrm {V} +0 {,} 95 \, \ mathrm {V} = 2 {,} 179 \, \ mathrm {V}}$
Cathode ( reduction / electron uptake)	${\ displaystyle \ mathrm {4 \ H ^ {+} + 4 \ e ^ {-} \ to 2 \ H_ {2}}}$	${\ displaystyle E = 0 \, \ mathrm {V}}$	${\ displaystyle E = 0 \, \ mathrm {V} -0 {,} 62 \, \ mathrm {V} = -0 {,} 62 \, \ mathrm {V}}$
Cathode ( reduction / electron uptake)	${\ displaystyle \ mathrm {4 \ H_ {2} O + 4 \ e ^ {-} \ to 2H_ {2} +4 \ OH ^ {-}}}$	${\ displaystyle E = -0 {,} 413 \, \ mathrm {V}}$	${\ displaystyle E = -0 {,} 413 \, \ mathrm {V} -0 {,} 62 \, \ mathrm {V} = -1 {,} 033 \, \ mathrm {V}}$

Experimental setup for an electrolysis

If the overvoltage is not taken into account, the following redox reaction would have to take place (electrolysis of the water):

	equation	potential
Overall reaction redox reaction	${\ displaystyle \ mathrm {2 \ H_ {2} O + 4 \ H ^ {+} \ to O_ {2} +4 \ H ^ {+} + 2 \ H_ {2}}}$	${\ displaystyle \ Delta E = 1 {,} 229 \, \ mathrm {V}}$

In the execution of the experiment, however, chlorine and hydrogen are produced, since when the overvoltage is taken into account, this electrolysis of HCl has a lower total voltage than the electrolysis of water (see tables). The same result is obtained with a sodium chloride solution. The sodium ions are not reduced to metallic sodium because of the strongly negative potential of sodium (-2.71 V). The water molecules are not oxidized to oxygen because of the overvoltage .

	equation	Potential + overvoltage
Overall reaction redox reaction	${\ displaystyle \ mathrm {2 \ H ^ {+} + 2 \ Cl ^ {-} \ to Cl_ {2} + H_ {2}}}$	${\ displaystyle \ Delta E = 2 {,} 099 \, \ mathrm {V}}$

	equation	Potential + overvoltage
Overall reaction redox reaction	${\ displaystyle \ mathrm {2 \ H_ {2} O + 4 \ H ^ {+} \ to O_ {2} +4 \ H ^ {+} + 2 \ H_ {2}}}$	${\ displaystyle \ Delta E = 3 {,} 212 \, \ mathrm {V}}$

Web links

Electrolysis - example of overvoltage

Individual evidence

↑ Faulkner, Larry R., 1944-: Electrochemical methods: fundamentals and applications . Wiley, 2001, ISBN 978-0-471-04372-0 .

[1] Faulkner, Larry R., 1944-: Electrochemical methods: fundamentals and applications . Wiley, 2001, ISBN 978-0-471-04372-0 .