Sponsored by AsyntReviewed by Louis CastelJun 10 2024
During an electrochemical reaction, electrons are manipulated to move through a circuit using an external power supply. This circuit includes the reaction mixture with two electrodes separated by a certain distance.
The current, measured in amperes, relates to the flow rate of these charged particles and is immediately associated with the rate of electron flow around the circuit. Calculating the moles of flowing electrons for a particular current and time is possible. The interactive modeler (below) demonstrates the relationship between these. This article will outline how this can be used to guide your experiments.
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Image Credit: Asynt
The voltage corresponds to the work done to push electrons around the circuit. Some circuits have low resistance and, therefore, only need a small application of voltage to propel the electrons, while a circuit with high resistance requires higher voltages: the potential difference (measured in volts) is greater.
The resistance is contingent on the reactants and products, the solvent and any electrolytes added (to impede the resistance), and the surface area and distance between electrodes (which is why it is important to control these with ElectroReact). It may also change during the reaction.
Voltage, current, and resistance are interconnected. Since the solution mixture and reactor influence resistance, it is impossible to control voltage and current independently across a circuit as they are proportional to each other – so if the current is doubled, so is the voltage.
More generally, V=IR, where V corresponds to the voltage (volts), I represents the current (amperes), and R is equal to the resistance (ohms). Typically, you set either the voltage or the current to remain constant throughout the reaction. The reaction can be conducted in one of two ways: constant current (galvanostatic) or constant potential (potentiostatic) conditions.
Performing a reaction under constant current (galvanostatic conditions) makes it possible to understand the precise rate of electron flow. Since the number of electrons is known, this can be tallied up with the number of moles of required electrons in your chemical reaction.
The interactive modeler below uses the known relationship between current and the resulting flow of electrons to approximate the time for a reaction to complete. The number of electrons required per mol of substrate must be specified to generate a result relative to the final product. This modeler assumes 100% Faradaic efficiency (see Determining the efficiency of a reaction).
While one major advantage of constant current is that it reveals the number of electrons flowing, a drawback is that the potential is not controlled, as this automatically varies to preserve the rate of electrons flowing in the circuit.
If the potential is too low (due to a low resistance), very mild conditions are created under which the reaction might not be possible. Too high, and unwanted reactions such as decomposition or side reactions may arise. This can be regulated by modifying the resistance (e.g., electrolyte type, concentration, etc.) or the reactor assembly (e.g., adjusting the distance between electrodes, charge, density, etc.).
When working under constant potential, it is possible to ensure that the potential lies in a band to generate a powerful reaction without resulting in sub-reactions (e.g., within the solvent). Tables of standard reduction potentials can be used to set the potential within a reaction.
Moreover, it is possible to measure these using cyclic voltammetry. However, this comes at the cost of losing track of how long the reaction might take since the current is not being controlled (flow of electrons), and unless the current is logged as a function of time, it is impossible to determine efficiency.
Finally, regardless of how the reaction is conducted, efficiency can be established by calculating the conversion of the starting material (conversion) or formation of the product (yield) in relation to the number of electrons supplied. Asynt provides relative guidance on measuring efficiency.
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This information has been sourced, reviewed and adapted from materials provided by Asynt.
For more information on this source, please visit Asynt.