Potentiastat Terminology

What is a Potentiostat and how does it work?

 The name “potentiostat” describes its function. “Potentio” refers to electrical potential, and “stat” stems from the Greek word “stato,” which means standing or set.

As you can control the temperature of a room using a “thermostat,” a potentiostat can control the potential of a single working electrode in a 3-electrode system, and is sometimes referred to as a “single-channel potentiostat.”  A potentiostat where the potential of 2 working electrodes can be controlled independent of each other is referred to as a “bipotentiostat.”  Potentiostats with more than two working electrode channels are referred to as a “Multichannel Potentiostat.”  A galvanostat refers to an instrument that controls the current at an electrode rather than the potential.  Most modern potentiostats have galvanostat functionality built in.  The phrase "electrochemical workstation" also refers to a potentiostat, and to our knowledge, the phrase is only used for marketing purposes.

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1What is a Potentiostat?

A potentiostat is an analytical instrument designed to control the working electrode's potential in a multiple electrode electrochemical cell.  The potentiostat contains many internal circuits that allow it to function in this capacity.  The circuits generate and measure potentials and currents.  External wires in a cell cable connect the potentiostat circuit to the electrodes in the electrochemical cell.  In a conventional three-electrode cell, the cell cable connects to the working, counter (auxiliary), and reference electrodes on one end and the potentiostat cell cable connector on the other end.  The internal circuitry of the potentiostat controls the applied signal.  In the case of potential controlled methods, for example, the working electrode's potential is held with respect to the reference electrode.  At the same time, current flows between working and counter electrodes.  The potentiostat circuit design prevents all but a tiny current from flowing between the working and the high impedance reference electrode.  While modern potentiostats are more complicated than can be explained in this article, a brief introduction to potentiostat circuitry will be discussed to help you understand how a potentiostat works.

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1.1Terminology

It's not uncommon that terminology gets confusing when discussing electrochemistry.  Below is a short note on potentiostat terminology.  This can be useful if you are purchasing a potentiostat and want to know all the equipment the potentiostat comes with.  If you are interested in purchasing a potentiostat, you might want to check out our guide to purchasing a potentiostat article.  Additionally we offer a variety of potentiostats for purchase.

 

The name “potentiostat” describes its function. “Potentio” refers to electrical potential, and “stat” stems from the Greek word “stato,” which means standing or set.  As you can control the temperature of a room using a “thermostat,” a potentiostat can control the potential of a single working electrode in a 3-electrode system, and is sometimes referred to as a “single-channel potentiostat.”  A potentiostat where the potential of 2 working electrodes can be controlled independent of each other is referred to as a “bipotentiostat.”  Potentiostats with more than two working electrode channels are referred to as a “Multichannel Potentiostat.”  A galvanostat refers to an instrument that controls the current at an electrode rather than the potential.  Most modern potentiostats have galvanostat functionality built in.  The phrase "electrochemical workstation" also refers to a potentiostat, and to our knowledge, the phrase is only used for marketing purposes.

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2How does a Potentiostat work?

The core electronic component of a potentiostat is the operational amplifier, or op-amp for short.  When part of the op-amp output signal is fed into itself, it creates a feedback mechanism.  Feedback is the mechanism by which the potentiostat can control the potential of the working electrode.  Conceptually, one can think about feedback like driving a car.  While driving, you want to maintain a constant distance between you and the car in front of you.  As the distance between you and the car changes, you adjust your speed to maintain a safe distance.  You are using the visual feedback from the car to adjust your speed.  In this analogy, the distance between you and the car is the potential, and changing your car's speed (via slowing down and accelerating) is the op-amp.

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2.1Operational Amplifiers (Op-Amps)

An operational amplifier, is an electronic circuit component powered by a DC voltage capable of amplifying an input voltage.  Schematically, the op-amp symbol is a sideways triangle with inputs along the vertical line and the output at the point opposite the inputs (Figure 1).  The voltage input to the op-amp is a differential value between two potential inputs, ∈₊ (non-inverting input) and ∈_- (inverting input), both of which are referenced to ground potential.  The open loop gain of the amplifier is a scalar factor (β), and common amplifications are on the order of 10⁵ - 10⁶, dictated by the integrated circuit construction.  The configuration of the external feedback circuitry can lower the gain to a user-selectable value.  Some amplifiers have a fixed gain of 1, and only output the difference between the two input potentials.  The output voltage, V_o , is equal to the product of the gain and the difference in the input voltages (Equation 1).  In practice, several specific details of the op-amp circuit diagram are omitted for clarity (Figure 2).  Notice in the simplified diagram, power supply voltages and circuit common (ground) are omitted and are assumed.

 

(1)

 

Figure 1: Circuit Diagram of an Operational Amplifier.  In most op-amp circuit diagrams, the positive and negative Power Supply (+PS and -PS) potentials are assumed.  Therefore, in circuit diagrams of op-amps, they are most often omitted.

 

Figure 2: Simplified Circuit Diagram of an Operational Amplifier.

 

 

Op-amps have several important characteristics, as follows:

 

• Small footprint, often measuring < 1 cm²
• Generally inexpensive and widely available
• Typically have large gains (β = 10⁴ - 10⁶)
• High input impedance (Z_i ≥ 10⁶ MΩ)
• Low output impedance (Z_o ≤ 1 - 10 Ω)
• Ideally, zero output for zero input. In practice, there may be an offset voltage, which can be corrected (i.e., trimmed)
• Intrinsically have high bandwidth, allowing for a wide range of frequency input and output

 

If an op-amp merely amplifies a voltage input, the voltage output can be driven too high.  Due to the large gain, even a small differential voltage input (e.g., V_i = 0.001 V) can amplify the signal to a highly unusable (and possibly dangerous) voltage, limited by the amplifier supply voltage.  To overcome this possibility, the output signal is routed back into the ∈_- input, which creates a feedback loop.  When an op-amp uses feedback, it forces ∈_- to equal ∈₊.  This circuit is called a voltage follower (Figure 3).  In the voltage follower circuit, if ∈₊ input increases, so does ∈_-.  Since ∈_- and V_o are tied together, V_o follows ∈₊.  V_i = V_o, thus the reduced gain, β, is 1.

 

Figure 3: Circuit Diagram of a Voltage Follower

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2.2The Potentiostat Circuit Diagram

By understanding how an operational amplifier works we can describe how a potentiostat functions based on the figure below.

 

Figure 4. Simplified Potentiostat Circuit Diagram.

Let's start with reviewing the components in the potentiostat diagram above.  V_i represents a user-controlled applied potential called the potentiostatic set point that is referenced to ground.  V_i enters the inverting input (∈_-) on the High Gain Op-Amp.  The output (V_o) connects to the counter electrode lead (CTR).  The reference electrode lead (REF) and the working sense lead (WRK_sense) connect to a true difference amplifier called the electrometer.  The output voltage (V_feedback) from the electrometer feeds into a voltmeter (V) and into the non-inverting input (∈₊) on the High Gain Op-Amp.  The working electrode drive lead (WRK_drive) connects to a sense resistor (R_wrk) that connects to ground.  The voltmeter (E/I), measures the potential across R_wrk, and converts it to a current.  WRK_sense and WRK_drive are shorted together and connect to the working electrode of interest.  CTR, REF, WRK_sense, and WRK_drive are all cables that come out of the potentiostat and connect to the electrochemical cell.

 

During potentiostat operation, the user applies a voltage V_i to the electrochemical cell.  An output voltage, V_o, comes from the High Gain Op-Amp into the electrochemical cell through the counter electrode (CTR).  The counter electrode passes current (i_ctr) through the solution to REF, WRK_sense, and WRK_drive.  Remember that the inputs to an operational amplifier have a high impedance, hence no current passes through them.  Since the REF and WRK_sense are connected to a high impedance operational amplifier, the only viable path for the current to flow is through WRK_drive.

 

The electrometer output voltage is the voltage difference between the REF and WRK_sense.  As the current travels between the CTR and WRK_drive, the voltage drops across a gradient that is proportional to the bulk solution resistance.  This process affects the potential of REF and WRK_sense.  However, reference electrodes are designed to maintain a stable potential, and the potential at REF remains constant.  This means that any measured changes in the potential come from WRK_sense.  The voltage difference between REF and WRK_sense is the output voltage of the electrometer, V_feedback.  V_feedback feeds into a voltmeter (V) to measure the difference between the REF and WRK_sense.  It also feeds into the non-inverting (∈₊) input of the High Gain Op-Amp.  Recall, that the High Gain Op-Amp amplifies the difference between ∈₊ and ∈_- (equation 1).  If V_feedback is not equal to V_i, the op-amp will either increase or decrease V_o until they are equal.

 

Finally, note that in the design of the potentiostat, working electrode current is not directly measured.  Rather, the voltage is measured across R_wrk which is located between WRK_drive and ground.  R_wrk is a known resistor, so based on the voltage measured across it, we can calculate the measured current via Ohm's Law.

 

 

(2)

 

Where i is the current, V is the voltage, and R_wrk is the known resistor between WRK_drive and ground.  In summary, the current at the working electrode is measured by voltmeter (E/I) across R_wrk, the potential at the working electrode is measured by the electrometer (V), and the applied potential is determined by the potentiostatic set point.  By utilizing the op-amp's feedback mechanism, the potential of the working electrode with respect to the reference electrode can be adjusted.  Simultaneously adjusting potential while measuring the current at the working electrode is the heart of voltammetry techniques.

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2.3Other Electronic Components

The operational amplifier and its feedback mechanism are critical for proper potential control of the working electrode with respect to the reference electrode (in controlled potential methods).  Additional electronic components of a potentiostat include a function generator, analog to digital (A/D) converters, and digital to analog (D/A) converters.

 

Modern instrumentation routinely translates between true analog responses and their digital equivalent.  Electrochemical experiments generate analog data (current and voltage), data which are continuous and can take on any value.  Computers require digital data, represented with zeroes and ones.  A/D converters rely upon the instrumentation calibration to accurately translate analog signals to digital signals.  The opposite is true, that D/A converters translate digital signals into continuous analog signals.  In the case of a potentiostat, current response (an analog signal) passes through an A/D converter, where the computer is able to plot and manipulate data.  At the same time, digital waveforms generated by the computer pass through a D/A converter, such that the waveform is applied as an analog signal to the electrochemical cell.

 

Modern potentiostats include all electronic components—op-amps, waveform generators, D/A and A/D converters—in one instrument.  In general, a researcher only requires a potentiostat, which is often USB-interfaced with a computer equipped with the potentiostat software.   In summary, D/A and A/D converters allow users to apply and measure potential and current values using a computer.

Open Circuit Potential: an incredibly versatile measurement

 Nguồn: https://maciassensors.com/open-circuit-potential/

Table of Contents

• What is Open Circuit Potential?
  • What does the Open Circuit Potential represent?
  • What generates the Open Circuit Potential?
  • What are the factors influencing the Open Circuit Potential?
    • Influence of the Electrode Material on Open Circuit Potential
    • Influence of the Electrolyte Composition on Open Circuit Potential
• How do you measure Open Circuit Potential?
  • How to measure Open Circuit Potential with a potentiostat
  • How to measure Open Circuit Potential with a multimeter
• How to interpret the Open Circuit Potential
  • Stable Open Circuit Potential
  • Erratic Open Circuit Potential
  • Noisy Open Circuit Potential
    • Random Noise in Open Circuit Potential
    • Periodic Noise in Open Circuit Potential
  • Drifting Open Circuit Potential
  • Evolving Open Circuit Potential
• Applications of Open Circuit Potential
  • Open Circuit Potential in Corrosion
  • Open Circuit Potential in Sensing
  • Open Circuit Potential in Electrochemical Energy Storage
• Pros and Cons of Open Circuit Potential Measurements

In electrochemistry, one of the parameters that can be measured is the so called open circuit potential. This reading can have tremendous significance and importance depending on the application. In this post, we will dig deep into this versatile measurement and how to make the most of it every time.

What is Open Circuit Potential?

In a 3 electrode setup, the Open Circuit Potential, also known as OCP, is the electrode potential that can be measured versus the reference electrode when there is no current flowing through the working electrode of an electrochemical system.

In general, however, it can be simply referred to as the potential measured between two electrodes immersed in an electrolyte when there is no current flowing.

What does the Open Circuit Potential represent?

The OCP of an electrochemical system represents its thermodynamic equilibrium. That is, the rates of oxidation and reduction are balanced.

What generates the Open Circuit Potential?

The OCP occurs due to the charge separation at the electrode-electrolyte interface. This charge separation occurs due to the difference in the electrochemical activity between the electrode material and the electrolyte.

What are the factors influencing the Open Circuit Potential?

There are mainly two factors that influence the resulting OCP: the electrode material and the electrolyte composition.

Influence of the Electrode Material on Open Circuit Potential

One of the most important factors that influence the OCP of an electrochemical system is the material of the electrodes.

Each material has a different tendency to oxidize or reduce. This means that each material has a different standard electrode potential, which has a direct impact on the open circuit potential.

Influence of the Electrolyte Composition on Open Circuit Potential

The other most important factor in the OCP of an electrochemical system is electrolyte in contact with the electrodes.

The different components that make up the electrolyte can affect the standard electrode potential of the electrode materials. For example, pH affects the OCP of carbon electrodes and ionic chloride concentration affects the OCP of AgCl electrodes (thus the need for a fixed and known concentration of KCl in reference electrodes) the Factors such as the concentration of ionic species, pH and oxidation state of any redox analytes, they all contribute to the measured OCP.

How do you measure Open Circuit Potential?

OCP is a very easy parameter to measure. In fact, while most research labs utilize potentiostats to measure OCP, in reality you only need a traditional voltmeter to measure it.

However, there are slight differences when measuring OCP with a potentiostat and a voltmeter. We explain it in detail below.

How to measure Open Circuit Potential with a potentiostat

When using a potentiostat to measure OCP one must remember that, with this instrument, all potentials are always measured versus the reference electrode lead. So, from the 3 leads that most potentiostats have to connect to electrochemical cells, you would only really need 2 of them:

• the working electrode (WE) lead
• the reference electrode (RE) lead

So to measure the OCP you would only need to connect the WE lead to the electrode that you want to measure the OCP from and the RE to your reference electrode in the system (often a Ag/AgCl electrode).

In general, the counter electrode (CE) lead is not needed at all. Yet some researchers may argue that having a 3rd counter electrode may help obtain more stable readings.

In most cases, what scientists in the lab would do when purely measuring OCP in an electrochemical system is keep their measurement setup the same. So if a 3 electrode setup is being used at the time, the lab operator would connect the potentiostat in the same way they would connect it for any other electrochemical measurement. This is useful because oftentimes an OCP measurement would only be part of the characterisation experiments. So this approach avoids having to rearrange the connections after the measurement.

How to measure Open Circuit Potential with a multimeter

While most of our readers probably work with potentiostats, we think it is interesting to know how to measure OCP with a digital multimeter too.

To measure OCP with a digital multimeter:

First set your device in potential measurement mode. In most cases, this would require turning the measurement knob to DC voltmeter mode and selecting the appropriate potential range. However, always refer to the user manual of your specific multimeter. For aqueous electrochemical measurements, the best potential range would be in the Volts range, as in most cases you would be measuring from a few mV up to 1.25 V.

Once the multimeter is in voltmeter mode in the appropriate current range it is time to connect to the respective electrodes. For this, connect the black lead of the multimeter to the reference electrode and the red lead to the working electrode. Please note that inverting these would invert the measured potential.

How to interpret the Open Circuit Potential

OCP is normally collected as a series of potential measurements during a specific period of time, and it can be interpreted in multiple ways.

As we explained above, OCP represents the equilibrium state of the electrochemical system we are studying. Therefore, depending on the application, the OCP reading can be transformed into actionable data. (We will explain the applications of OCP in detail in the next section.)

For now, let’s discuss the different ways that the OCP vs time curves can present themselves: stable, erratic, noisy, drifting and evolving.

Stable Open Circuit Potential

Example of a stabe OCP signal. Acquired from 0.5 mM Methylene Blue in PBS on a gold PCB sensor.

A stable OCP is characterized by a consistent and repeatable potential value over time. This type of behaviour is normally achieved after some time, once the electrodes have been in the electrolyte long enough that any side reactions, such as those arising from electrode impurities or double layer capacitance development, have terminated.

This type of OCP behaviour is often a requirement in measurement protocols. For example, in corrosion studies, electrodes are normally required to reach a stable OCP before proceeding with the measurements.

OCP stability in itself is a fuzzy term. Since most aqueous experiments are prone to evaporation effects, and thus variations in electrolyte composition, an OCP measurement may appear to be unstable if the measurement is long enough. Yet in most cases, the OCP was stable, but the system was changing.

To avoid such mistakes, OCP stability is often determined using a certain stability criterion. This stability criterion depends on the application, but it normally ranges in the mV/s to the µV/s.

Erratic Open Circuit Potential

An erratic OCP is characterized by quick (within a few seconds), high (few hundreds of mVs) and often random (without any particular pattern) variations of potential over time.

Such a behaviour is not common of electrochemical systems.

Therefore, if you encounter such an OCP vs time curve, you likely have a connection problem in your setup. In these cases, the best course of action is to verify all the connections in your setup.

Noisy Open Circuit Potential

A noisy OCP has a clear average value that varies slowly and according to the conditions of the reactions at the electrode and/or electrolyte. However, it has a superimposed disturbance that reduces the quality of the data acquired.

While some level of noise is unavoidable, it is best to try and keep it to a minimum and as close to the noise level of the potentiostat instrument as possible.

There are two main types of noise that should be avoided: random and periodic.

Random Noise in Open Circuit Potential

Random noise in OCP is characterized by sudden spikes in the signal. When it is due to the noise level of the potentiostat, the data quality is hardly affected and can be improved in post-processing using data smoothing algorithms.

But when the data starts showing a lot of large spikes, then it is time to troubleshoot.

When large spikes in the OCP data appear consistently, it is a sign of 2 potential issues:

• A faulty spring connection
• Mechanical vibration

Nowadays most connections to SPEs or electrochemical cells are performed with spring loaded connectors. The most common are crocodile clips, edge connectors or pogo-pins.

These types of connections suffer from wear and tear. And when the spring starts to loose its elasticity, or the contact surfaces rust, it can lead to a faulty connection. If this is the case, the best course of action is to exchange them with a new connection.

With regards to mechanical vibrations, these are normally less common. They may cause connection issues by physically detaching the connections to the electrochemical cell (thus exacerbating the effects of a spring connector starting to fail) or by inducing significant movements of the electrolyte. So, if the connections are working properly and you still see a high noise level, look for vibrations. Vortexers and spinners are common sources of mechanical vibrations in laboratories.

Periodic Noise in Open Circuit Potential

Periodic Noise in OCP signals are characterized by a pattern that repeats over time. In most cases, this noise is sinusoidal in nature.

This type of noise is most of the times electronic. Especially if it matches the frequency of the mains power, that is 50 or 60 Hz, depending on where you are located. Moreover, this noise can also present itself as harmonics of these frequencies.

In these case, there are two potential approaches to troubleshoot.

• Adjust the datapoint acquisition rate to avoid picking up this noise
• Filter out the noise signal in post-processing

If possible, it’s always best to remove the noise at the data acquisition stage.

One final potential source of periodic noise could be the connecting cables themselves.

Long cables can effectively behave as an antenna, introducing noise into the system. Most potentiostat manufacturers sell their connecting cables with an electromagnetic shield to avoid this from happening. However, it is common that glass reference electrodes, especially the economic ones, do not have shielding on their connecting cable. As a result an unsightly periodic noise is introduced in all measurements. In this case, it is best to try to shield the cable, reduce its length, or acquire a new glass reference electrode from a reputable manufacturer.

Drifting Open Circuit Potential

Example of a Drifting OCP. Acquired from a gold PCB sensor coated with a polymer with entrapped methylene blue. This data shows a ongoing increase in OCP as a result of changes within the polymer layer.

A drifting OCP can be identified by a constant and slow slope that is not related to environmental condition changes such as evaporation effects. This slope can either be upwards or downwards. And it will persist even when a change in the test conditions occurs. For example, when a concentration of an analyte is changed as part of a measurement.

This drifting slope is normally a result of degradation of the electrode material and/or the formulation immobilized on top.

As materials attach, such as during biofouling, or dissolve, from an electrode, the measured OCP will change.

For sensing applications, while not desired, a drifting OCP is an annoying effect that has to be managed during data post-processing of one-off short measurements. But for continuous and long measurements, it is a sign that the device still requires more development. This is because this degradation will eventually lead to the device no longer responding to the target analyte.

Evolving Open Circuit Potential

Example of an evolving OCP. Acquired from 0.5 mM Methylene Blue in PBS upon adding a chemical reducer on a gold PCB sensor. The curve shows the chemical reduction of methylene blue to methylene white.

An evolving OCP is a curve that shows a variation in the signal as a result of an external stimulus.

This is the type of curve that is expected of active measurements, such as pH measurements and other ion selective electrodes.

These types of OCP curves are normally characterized by an initial flat region (baseline), an exponential increase region (when the external stimulus is applied), and a final plateau (when the effect of the external stimulus ends and the signal stabilize again).

Applications of Open Circuit Potential

OCP is a parameter of interest in multiple applications. Below we cover some of its most common applications.

Open Circuit Potential in Corrosion

In corrosion science, electrochemistry can help determine corrosion rates accurately and quickly compared to other, more conventional methods.

In any electrochemical corrosion study, the first step is to determine the OCP. In this application, the OCP relates to the corrosion potential of the system and serves as a starting point for the characterisation of corrosion.

Moreover, OCP in corrosion is also partly related to the corrosion resistance of the electrode in a given electrolyte. With higher OCP values being an indicative of higher corrosion resistance.

Open Circuit Potential in Sensing

OCP is a common and desirable method for developing sensors. Since OCP measurements arise from a type of potentiometry that drives no current, this type of measurement has a low power consumption. This is why the vast majority of potentiometric sensors operate with OCP.

While there are some glucose sensors in the literature that work with OCP, most commercial applications focus on ion sensing. Some of the most common commercial ion selective electrodes (ISE) that operate in OCP mode are the pH meter, sodium and potassium ISEs.

One particular application of OCP in sensing that is very common in analytical chemistry laboratories is titration. While there are several types of titrations, most of them can be monitored via OCP measurements using appropriate electrodes that, in this application, are normally referred to as “titrodes”.

Open Circuit Potential in Electrochemical Energy Storage

OCP has applications in electrochemical energy storage too. In batteries and supercapacitors, the OCP, also referred to as Open Circuit Voltage or OCV in this field, is an important parameter used to determine the state of charge (SoC) of the device.

Therefore, OCP has two practical applications in energy storage:

• Calibrating the cell to predict SoC by measuring the potential across
• Determining self-discharge rates

Both of these applications are crucial for the development of energy storage devices.

The first one enables manufacturers a quick and straightforward way to measure charge percentage so that device developers can accurately control battery charging.

And the second one allows researchers to determine if the battery self-discharges too quickly and adjust the design accordingly to mitigate this unwanted phenomena.

Pros and Cons of Open Circuit Potential Measurements

Pros	Cons
The equipment required to measure OCP is simple and economic.	Since OCP is a passive measurement the acquired data has less features compared to other electrochemical measurements such as voltammetry.
OCP measurements consume very low power.	Since OCP can change due to several factors, experimental conditions must be closely monitored.
OCP can easily be transformed into actionable information such as concentration of an analyte or a state of charge	Reliable OCP-based assays require either calibration, a special surface chemistry, or both.

Pros and Cons of OCP measurements

We hope you have enjoyed our comprehensive post on Open Circuit Potential. In this guide we have covered everything there is to know about OCP: what it is, what influences it, how to measure it, and its real-world applications. We have also looked into its pros and cons as an electrochemical measurement. This way we expect that you will be able to make an informed decision of whether this is a parameter of interest for your investigation.

If you would like to know more about electrochemistry and its techniques, check our blog! We post regularly and have an extensive knowledge base.