The Electrode
An electrochemical reaction takes place on the surface of the electrode, hence the material, quality of material, size and shape can affect the reaction. The material used must be conducting and is typically chemically and electrochemically inert. Common electrode material includes platinum and carbon-based (graphite, glassy carbon, reticulated vitreous carbon). Sacrificial electrodes may be used, for example, copper, zinc or iron anodes will oxidise and release ions into solution. Hydrogen evolution is common at the cathode, hence if the reaction of interest is a reduction, then electrode material with a large hydrogen overpotential will be desirable e.g. carbon based materials. As well as the information in Making electrochemistry easily accessible to the synthetic chemist, there is a more in-depth review in Modern Organic Electrochemistry where you can find much more information.
The Electrolyte
An electrolyte is added to most electrochemical reactions to reduce the resistance between the electrodes and therefore the potential at a given current (we've put together a whole page on voltage and current). The electrolyte must be fully soluble in the reaction solvent and should be inert under the reaction conditions. Tetraalkylammonium salts are often used due to their good solubility in organic solvents, with short alkyl groups providing lowest resistance which increases with increasing alkyl chain length. PF6 and BF4 are usually used as anions due to their inertness under most electrochemical conditions. Halides may be employed although they can be oxidised at high potentials. The quality of the electrolyte can affect the reaction hence it is advisable to recrystallise this prior to use, especially when used for analytical applications.
The Solvent
In common with chemical reactions, the solvent affects electrochemical reactions. Factors to consider include solubility of reagents and products, stability of intermediates and products, inertness under reaction conditions and sustainability factors. Specifically for electrochemical reactions the solvent window i.e. the range of potentials over which the solvent does not oxidise or reduce, requires consideration. If the solvent is more easily oxidised or reduced than the substrate of interest, then this will become a competing reaction. However, solvent reduction can be a useful counter-reaction to an oxidation as it can reduce to hydrogen on the cathode (providing the source of electrons). Protic solvents such as alcohols serve as good proton sources for this, which can also be enhanced through the addition of acids. Typical aprotic solvents (generally used when you want the solvent to be inert) used in electrochemistry include acetonitrile, dichloromethane and tetrahydrofuran. Polar solvents help lower the resistance as they have higher conductivity than non-polar solvents, in addition to being better for dissolving the electrolyte.
Voltage, current and time
We've provided a separate page for understanding the electro- part of electosynthesis..