Nguồn: https://maciassensors.com/open-circuit-potential/
Table of Contents
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
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
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
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. |
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.
Không có nhận xét nào:
Đăng nhận xét