Introduction to Bulk Electrolysis
Unlike many other electrochemical techniques, which are limited to the diffusion layer, bulk electrolysis (sometimes referred to just ‘electrolysis’) changes the composition of the bulk solution. Bulk electrolysis experiments aim to generate a quantitative conversion such that the amount of substrate consumed is directly proportional to the total consumed charge. This is in contrast to techniques like cyclic voltammetry in which the amount of substance consumed/produced is insignificant. Bulk electrolysis also uses milliamperes of currents as opposed to microamperes or even nanoamperes in non-bulk electrolytic processes.
The reaction rate in bulk electrolysis is limited by the rate of mass transfer between the substrate and the surface of the electrode. For this reason, bulk electrolysis is generally performed with large counter and working electrodes (typically two orders of magnitude larger than those used in cyclic voltammetry or for other voltammetric techniques) to give a favourable ratio of electrode area to solution volume. To further aid with mass transfer, the solution is usually stirred.
From simple equipment to high-tech apparatuses, bulk electrolysis can be carried out with many types of instrumentation. However, all electrolysis setups need to include:
- The electrochemical cell, with or without a separator
- The electrolyte, made of soluble organic salts like tetrabutylammonium tetrafluoroborate
- Either two or three electrodes, depending on the type of electrolysis being performed
- A power supply, often in the form of a potentiostat
Common applications of bulk electrolysis include acid-base titrations and oxidation-reduction (redox) titration.
Coulometry vs. electrolysis
Coulometry and electrolysis use the same cell setup, and the two terms are often used interchangeably . In coulometry, the amount of charge transferred during an exhaustive electrolysis reaction (in which the analyte is completely oxidised or reduced) is measured in Coulombs. This is proportional to the amount of matter that is transformed by the reaction.
Basic Theory and Setup
Bulk electrolysis can be performed in various different ways, but the fundamental principles (based on Ohm’s Law) remain the same.
Electrochemical cells for bulk electrolysis consist of a counter (or auxiliary) electrode, a working electrode, and (in three electrode configurations) a reference electrode. Bulk electrolysis typically requires a large current at the working electrode, where redox reactions take place, in order to oxidise or reduce the electrolyte contained within the cell. To balance this, a large potential is maintained at the counter electrode.
When performing bulk electrolysis, the counter electrode and working electrode should be physically separated in order to prevent the by-products produced at the counter electrode from reaching the working electrode. This can be done by placing the counter electrode and the working electrode in separate compartments in the electrochemical cell.
Bulk electrolytic reactions can either be carried out directly at the electrode surface or indirectly through the use of a mediator. In direct electrolysis, electrons are transferred between a substrate molecule and an electrode, while in indirect or mediated electrolysis, there is electron transfer between a redox mediator (sometimes also known as catalyst) and an electrode; the electrochemically-generated mediator undergoes a chemical reaction with a substrate to produce the product.
In order to be effective, a mediator should be stable in various oxidation states for a sufficient period of time. In addition, the overpotentials of heterogeneous electron transfer from the electrode from/to the mediator, and the kinetic barriers involved in the homogeneous electron transfer from/to the start materials, must be low .
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Fundamental Equations and Relationships
Where n is the number of moles of electrons transferred in the half-cell reaction, F is Faraday’s constant (96487C/mol), and nA is the number of moles of the analyte.
Where E is the voltage or potential in Volts (V), I is the current in Amperes (A), and R is the resistance in resistance in Ohms (Ω). V is commonly used instead of E to represent voltage.
Ohmic potential and IR drop
The Ohmic potential is defined as the voltage, given by Ohm's law, required to force a current to flow through a cell. The value must be large enough to provide sufficient free energy to drive the chemical reaction in the cell and overcome any cell resistance.
Overpotential or overvoltage (η)
Overpotential results from the difference between the concentration of electroactive species on the electrode surface and those in the bulk solution. Electrochemical reactions at electrode surfaces occur at a faster rate than diffusion of electroactive species from bulk solutions to electrode surfaces.
The term “polarisation” refers to the departure of electrode potential from its theoretical value according to the Nernst equation as the current flows. Polarisation is influenced by electrode size, shape, and composition, electrolyte composition, temperature, level of current, and the state of species involved in the cell reaction.
Total charge in coulombs
For constant current electrolysis, the following equation applies:
For constant potential electrolysis, the following equation applies:
The total charge flowing through a cell during electrolysis can be determined by monitoring the current as a function of time and measuring the area under the curve.
Controlled Potential Electrolysis
In controlled potential electrolysis (CPE), the working electrode is maintained at a constant potential to allow the quantitative reduction or oxidation of the analyte without also concurrently reducing or oxidising other species’ in the solution. The current passing through the cell shows a direct proportional relationship to the concentration of the analyte, decreases as the reactants are depleted, and is negligible by the end of the reaction. The area under the curve of a plot of current versus time shows how many coulombs were used.
Two electrode controlled potential electrolysis
Two electrode controlled potential electrolysis uses only a working electrode and a counter electrode. The setup for this technique is therefore relatively inexpensive; reactions can be powered by simple electric batteries or even portable power sources . The main limitation of this configuration is that the working electrode and counter electrode potentials will vary with time, as will the ohmic drop. In addition, the two electrode configuration does not allow individual adjustment of the working electrode.
Three electrode controlled potential electrolysis
In a three electrode electrochemical setup, a potentiostat controls the potential between the working and reference electrodes while monitoring the current between the working and counter electrodes .
It should be noted that the potential between the working and reference electrodes will differ from the applied potential between the working and counter electrodes (the cell potential) due to the potential needed for the counter electrode reaction. In addition, the current through the solution between the working and counter electrodes will suffer from high solution resistance, resulting in an Ohmic drop potential as per Ohm’s Law.
Three-electrode controlled potential electrolysis is best suited for processes that require selective oxidation, or where it is important to avoid over-oxidising the products. The use of three electrodes is advantageous in electrocatalytic oxidation, where the substrate has a slightly higher redox potential than the catalyst .
Constant Current Electrolysis
In constant current electrolysis (CCE), also known as amperostatic/galvanostatic electrolysis, the current is held constant by an amperostat until an indicator signals completion of the analytical reaction.
Two electrode constant current electrolysis
With just two electrodes, a working electrode and a counter electrode, it is not possible to determine the potential at the working electrode, because both electrodes’ potentials vary during the reaction. Due to the simplicity of the two-electrode setup combined with the fact that a potentiostat is not necessary, however, this method is widely adopted by organic chemists for electrosynthesis. A cheap and easily available power supply that utilises direct current, such as DC power sources routinely used for portable electronic devices, can be utilised for two-electrode CCE .
Three electrode constant current electrolysis
In three electrode constant current electrolysis, a potentiostat maintains a constant cell current between the working electrode and the counter electrode. It does this by changing the cell potential, including the potentials of both the working and counter electrodes. The potential of the working electrode is monitored relative to the reference electrode throughout the course of the reaction, but the reference electrode has no influence on the control function. As a result, this mode of electrolysis can also be referred to as constant current electrolysis with potentiometric analysis.
With a constant current over time, there is no need to integrate the current-time curve, making it very quick to analyse the data using the basic relationship between charge, current, and time.
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- Stephen O Aderinto