Potentiostat Working Principles and Fundamentals
Potentiostats are voltage sources that vary their output potential in response to changes in the resistance across the circuit. In doing so, they supply more or less current so that the potential across a cell remains constant as per Ohm’s Law. Potentiostats are used in electrochemistry to measure redox events taking place in a solution, generally contained within a three electrode cell. Analysis of the data reveals various intrinsic electrochemical properties of the material in question, depending on which method is used.
Types of Potentiostat
Potentiostat specifications and features vary depending on their intended application.
|Type of Potentiostat||Specific Features||Applications|
|Portable potentiostats||Small, battery powered, able to perform specific functions and give a quick result.||Field work, environmental measurements such as testing for lead in drinking water or medical tests such as measuring the amount of iron in blood.|
|USB potentiostats||Suitable for use in the lab, more powerful than handheld units and produce data that can be analysed in much greater depth.||Voltammetric techniques such as linear sweep voltammetry and cyclic voltammetry.|
|Specialist potentiostats||As above, but supplied with bespoke software designed to make it quicker and easier to run specific electrochemical experiments.|
|Bipotentiostats||Two channels||For performing experiments that involve hydrodynamic flow such as hydrodynamic voltammetry. Typically used with a rotating ring-disk electrode (RRDE).
|Polypotentiostats||Three or more channels||As above. Each channel can also usually be run independently with separate cells so that multiple experiments can be run simultaneously.|
Potentiostats are often discussed alongside similar electrochemical devices such as galvanostats (also known as amperostats). Galvanostats operate on similar principles to potentiostats but maintain a constant current across the cell rather than a constant potential. The most common application for galvanostats is as battery chargers.
Working Principles of a Potentiostat
A simplified potentiostat circuit diagram is shown in figure 1. It consists of several key sections, each of which is detailed below: a signal generator, feedback amplifier, control amplifier, current amplifier, and a signal converter.
The signal generator outputs variable DC (direct current) voltages. To achieve this a digital-to-analogue converter (DAC) is used, which converts a computer-generated signal into a voltage. This allows a user to be able to accurately control the output voltage of a potentiostat via a computer. It is this component which determines the applied voltage resolution of the system.
The voltage feedback amplifier is perhaps the most important part of the circuitry of a potentiostat. It measures the voltage between the working and reference electrodes, passing it to the signal converter to be sent to the computer. However, it also feeds this voltage to the negative terminal of the control amplifier. This enables the control amplifier to keep the set voltage stable relative to the reference electrode.
It is important that the feedback amplifier not overload the cell’s electrode voltage and disturb the electrochemical reaction. Therefore, it requires a high input impedance with very low input current (pA). It also needs to be fast enough to follow rapid changes that can occur in electrochemical reactions and provide the feedback voltage to output amplifier.
The control amplifier takes the voltage output of the signal generator and the feedback amplifier and outputs the voltage that will be passed between the working and counter electrodes. Here, any loss of voltage due to electrochemical reactions is accounted for. By using the output of the feedback amplifier as an input of the control amplifier the signal is increased or decreased, ensuring that voltage between the working and reference electrodes is stable.
The specifications of the control amplifier will determine the maximum voltage and current that the potentiostat system can output.
Current amplifier and current ranges
This section of the circuit is for measuring the current passing through the working electrode and counter electrode. It consists of a single resistor or a set of resistors (the current ranges) and an amplifier. Each resistor corresponds to a different current range and can be toggled on or off to select the desired current range. Therefore, the number and resistances of these resistors will determine the currents that can be measured by a potentiostat.
When the current passes through a resistor it generates a voltage. The voltage is increased by the amplifier according to the selected current range and passed into the signal converter where it is converted to a current measurement.
The signal converter uses an analogue-to-digital converter (ADC) to convert the output of the voltage feedback amplifier and the current amplifier into a digital signal that can be interpreted by a computer connected to the instrument.
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Electrochemical Cell Set-Up
Potentiostats can be used to control two, three or four electrode configurations (and as discussed above, multi-channel potentiostats can control rotating ring-disk electrode systems). The three electrode set-up is by far the most common and consists of a working electrode, a counter electrode, and a reference electrode. Each of these three functions also exists in a two electrode cell, only a single interface both provides a reference potential and allows current to flow across the cell. The primary issue with such a set-up is that it renders it impossible to accurately control the potential at the working electrode.
The working electrode is the primary electrode in an electrochemical system. It is where the applied voltage enters the system, and where most electrochemical reactions and electron transfer take place.
Measurements of potential and current in an electrochemical system involve the working electrode for both two and three-electrode systems. In two-electrode systems the potential and current are measured between the working and counter electrodes. In three-electrode systems, a potentiostat measures the potential between the working and reference electrodes whilst the current is measured between the working and counter electrodes.
Platinum is the most common material used for working electrodes due to its electrochemical stability and ease of fabrication. Other commonly used materials include gold, carbon, and mercury.
By having a stable, known, and well-defined electrochemical potential, reference electrodes provide a constant for an electrochemical measurement.
They are used in a three-electrode system to maintain a stable potential against which the working and counter electrodes can be measured and controlled. This is achieved using materials with well-defined electrochemical potentials, typically chemically separated from the reactions occurring during the measurement. When a voltammogram of such a system is plotted, the potential is that which is measured between the working and reference electrodes.
In an ideal electrochemical system zero current will flow through the reference electrode, enabling accurate measurements and control of the potential at the working electrode. This is achieved by the reference electrode having a very low impedance, ideally zero.
Counter electrodes, also known as auxiliary electrodes, complete the circuit of a two or three-electrode system. As with the working electrode, platinum is the most common material used for counter electrodes due to its electrochemical and mechanical stability, and high electrical conductivity.
In two-electrode systems, the counter electrode is also used as the reference electrode. This requires it to have a much greater surface area than the working electrode to ensure that reaction kinetics at the working electrode are not inhibited by those at the counter electrode.
In three-electrode systems, the current is measured between the working and counter electrodes whilst the potential is not, instead being adjusted by the potentiostat to balance the electrochemical reactions occurring. Instead, the potential is measured between the working and reference electrodes, ideally with zero current passed between the counter and reference electrodes.
Sometimes the counter electrode is separated from the working electrode in a similar manner to the reference electrode to reduce the influence of reactions taking place at the counter electrode on those at the working electrode.
Potentiostat Specifications In-Depth
Accuracy, precision, and resolution
Before we can discuss the meanings of the various specifications of the potentiostat, we must first clarify three properties that are often misunderstood: accuracy, precision, and resolution.
Accuracy is how close a measured value is to the true value. This is represented either as an absolute offset, a percentage of the measured value, or a combination of the two.
Precision is the repeatability of a measurement, i.e., how close together repeated measurements of the same point will be. Typically, this is represented as an absolute value.
Resolution is the smallest change in a value that can be perceived by a measurement. Typically, this is represented as an absolute value.
A useful analogy to help clarify the differences of these three properties is to think of a target such as would be used in archery, and how each of these properties would apply, as shown in figure 2.
As we can see, the accuracy determines how close to the centre of the target the arrows (or in the case of a measurement, the data points) are, whilst the precision determines the grouping. The four possible outcomes of the figure are given below:
- High accuracy, high precision – arrows are tightly grouped around the centre of the target.
- High accuracy, low precision – arrows are sparsely grouped around the centre of the target.
- Low accuracy, high precision – arrows are tightly grouped away from the centre of the target.
- Low accuracy, low precision – arrows are sparsely grouped away from the centre of the target.
How does resolution factor into this? Using the same analogy of a target, the resolution would be the width of each ring, with lower resolutions having thicker rings and higher resolutions having thinner rings, as shown in figure 3.
The potential range is the potential window that can be applied and measured by a potentiostat between the working and reference electrodes. This typically applies for both positive and negative potential, and in such a case is represented with a plus/minus symbol.
The potential compliance is the maximum limit for the potential that a potentiostat is capable of outputting between the working and counter electrodes. As with the potential range, it applies for both positive and negative potentials and as such is represented with a plus/minus symbol.
Electrochemical reactions that occur during a measurement often result in decreased potential at the reference electrode. This means that a greater output potential must be applied to the cell to achieve the desired potential.
It is important to note the distinction between the potential range and the potential compliance. Potential range is the applied and measured potential between the working and reference electrodes, whilst the potential compliance is the absolute maximum potential that can be applied between the working and counter electrodes.
Applied potential accuracy
The applied potential accuracy is the maximum amount the output potential of a potentiostat can vary from the set potential. Specifically, this refers to the potential between the working and reference electrodes. It is represented as a plus/minus offset.
Applied potential resolution
As a potentiostat uses digital signals to determine the potential to output, any change in potential will appear as a step, and a scan profile a series of steps over time, as shown in figure 4. The applied potential resolution determines how small these steps can be, as it is the smallest change in potential that can be output by a potentiostat.
The maximum current is the highest electrical current that can be measured by a potentiostat. It applies for both positive and negative currents and as such is represented with a plus/minus symbol. If a measured current is outside of this range, a potentiostat will turn off the output potential to prevent damaging the unit.
To achieve a consistent level of accuracy and precision for current measurements at both milliamps and nanoamps, current measurements are split into a set of ranges. These ranges are typically separated by an order of magnitude, and whilst one range can usually measure the currents encompassed by the range below it, the accuracy and precision of the measurement will be worse.
Measurement of current works by measuring the voltage across a known resistor when the current passes through it. Therefore, to measure current at different orders of magnitude a series of resistors are used, with lower currents passed through higher resistance resistors.
The accuracy, precision, noise, and resolution of each range typically follows the same order of magnitude differences as the currents, as these are most heavily influenced by the measurement of the voltage, rather than the resistors used.
Current measurement accuracy
The current measurement accuracy is the maximum amount that a measured current value can vary from the actual current value. It is important to note that most measurements will vary by less than this amount. This value scales with the order of magnitude of the current ranges.
Current measurement resolution
The current measurement resolution is the smallest change in current that can be measured by a potentiostat. This value scales with the order of magnitude of the current ranges.
Ossila Potentiostat Specifications
|Potential range||±7.5 V|
|Potential compliance||±10 V|
|Applied potential accuracy||±10 mV offset|
|Applied potential resolution||333 µV|
|± 150 mA||± 200 µA||50 µA|
|± 20 mA||± 20 µA||5 μA|
|± 2 mA||± 2 µA||500 nA|
|± 200 µA||± 200 nA||50 nA|
|± 20 µA||± 20 nA||5 nA|
|Overall Dimensions||Width: 125 mm Height: 55 mm Depth: 175 mm|
- Chris Bracher
- Omar Sayidmarie