What Factors Control Electrochemical Reactions?

This article looks at the primary factors affecting electrochemical reactions and how these can be modified to influence your reaction. This information has been adapted from an open-access paper called Making electrochemistry easily accessible to the synthetic chemist. This paper has proved to be an extremely popular starting point for advancing electrochemistry.

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The Electrode

An electrochemical reaction occurs on the electrode’s surface. Hence, the importance of the material, such as quality, size, and shape, can influence the reaction. The material used can conduct while typically remaining chemically and electrochemically inert.

Frequently used electrode materials include platinum and carbon-based graphite, as well as glassy carbon and reticulated vitreous carbon. Sacrificial electrodes, such as copper, zinc, or iron anodes, can be used, as these will oxidize and release ions into the solution. Hydrogen evolution is common at the cathode; therefore, if the reaction of interest is a reduction, then electrode material with a large hydrogen overpotential, e.g., carbon-based materials, will be preferred.

In addition to the information in the paper, there is a complete review of modern organic electrochemistry.

The Electrolyte

An electrolyte is introduced to most electrochemical reactions to reduce the resistance between the electrodes, limiting the potential at a specific current. The electrolyte should be inert under the reaction conditions and fully soluble in the reaction solvent.

Tetraalkylammonium salts are typically used as they demonstrate excellent solubility in organic solvents. Short alkyl groups supply the lowest resistance, which increases with increasing alkyl chain length.

PF₆ and BF₄ are frequently used as anions as they remain inert when subjected to most electrochemical conditions. Halides may be employed, although they may oxidize at higher potentials. The quality of the electrolyte can impact the reaction; therefore, it is recommended to recrystallize it before use, particularly when used for analytical applications.

The Solvent

Similar to a chemical reaction, the solvent influences the outcome of an electrochemical reaction. Things to consider include the solubility of reagents and products, their inertness under reaction conditions, the stability of intermediates and products, and sustainability factors.

The solvent window consideration, i.e., the range of potentials over which the solvent does not oxidize or reduce, requires particular attention when observing electrochemical reactions. If the solvent is easily oxidized or appears reduced compared to the substrate of interest, the result will be a competing reaction.

However, reducing the solvent can be a useful counter-measure to react to oxidation as it can break down to hydrogen on the cathode (supplying the source of electrons). Protic solvents such as alcohols act as excellent proton sources for this, which can also be improved by introducing acids.

Commonly used aprotic solvents to ensure the solvent remains inert in electrochemistry applications include acetonitrile, dichloromethane, and tetrahydrofuran. Polar solvents play a key role in lowering the resistance as they have greater conductivity when compared to non-polar solvents, in addition to being better suited for dissolving the electrolyte.

This information has been sourced, reviewed and adapted from materials provided by Asynt.

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