Determining the Onset Potential in Cyclic Voltammetry
Cyclic voltammetry measurements are fundamental for understanding the electrochemical behavior of systems like batteries.
The onset potential describes the potential in an electrochemical cell that drives the forward or reverse reaction.
The onset potential can be easily determined using a linear fit to voltammogram data.
Electrochemistry is an important field of science that makes batteries possible. Without a thorough understanding of electrochemistry, we wouldn’t have the mobile devices we all know and love. There are some fundamental measurements that will tell you a lot about electrochemical reactions, particularly in battery cells, photoelectrochemical systems, and fuel cells. An important experiment for examining reaction progress and measuring cell potentials is cyclic voltammetry.
One important quantity that should be extracted is the onset potential in cyclic voltammetry data. The process for calculating the onset potential is quite simple, but the information it reveals is important for designing batteries, electrolysis systems, and other electrochemical systems. Here’s how to extract the onset potential from your cyclic voltammetry data and what it tells you about your electrochemical system.
What is Onset Potential in Cyclic Voltammetry?
Cyclic voltammetry is an important electrochemical experiment that is related to linear voltammetry. A standard electrochemical cell involves three electrodes (working, counter, and reference), although two and four-electrode cells are common for different experiments. In the three electrode cell, the voltage applied between the reference electrode and working electrode can be adjusted to define a zero voltage, which puts the cell in equilibrium and suppresses any electrochemical reaction. When the voltage between the working and counter (or working and reference) electrodes changes, you can start to drive the cell’s electrochemical reaction.
This is where the onset potential in cyclic voltammetry becomes important. When the voltage at the working electrode starts increasing, the current starts increasing by a small amount. Eventually, the current measured at the working electrode experiences a large increase once the voltage passes a certain threshold. This threshold voltage (measured between the reference and working electrodes) is the onset potential in cyclic voltammetry.
This potential shows when the applied potential overcomes the activation energy for the electrochemical reaction in the three electrode cell. In other words, the electrochemical reaction is driven with a higher rate at this particular applied voltage. A cyclic voltammogram is shown in the graph below, where the voltage is swept from left to right. Note that this graph uses the US convention for plotting voltammograms.
Cyclic voltammogram showing current as the applied potential is swept back-and-forth.
The graph above shows a typical cyclic voltammogram as the voltage is swept back-and-forth. From the upswing in current as the voltage is increased, one can determine the onset potential in cyclic voltammetry. This is shown in the graph below. Here, we can interpolate the current back to zero. The point where a tangent line intercepts the initial current can be taken as the onset potential.
The common way to determine the onset potential in cyclic voltammetry is by interpolation back to zero current.
Aside from interpolation, there are some other ways to determine the onset potential from a voltammogram. The onset potential can also be quantified as follows:
The point where the forward current exceeds or changes by a certain amount
The voltage where a tangent line on the current curve has a 45 degree slope
These other two methods will give very similar estimates of the onset potential in cyclic voltammetry. The second method below is less common as it will underestimate the onset voltage determined from interpolation.
Multiple Reaction Peaks
Each peak in the voltammogram corresponds to the reduction or oxidation potential at each electrode in the system. This positive current peak is the cathodic potential, and it tells you when the species at the cathode (working electrode) has been fully reduced. Scanning in the other direction, the negative current peak is the anodic potential, and it tells you when the species at the cathode has been fully oxidized.
When the scanning voltage rises sufficiently high, you’ll find that the current in the electrochemical cell peaks, and then it falls back to zero beyond a specific potential. At this peak potential, for a diffusion-dominated electrochemical cell, the diffusion gradient primarily determines the reaction rate, thus the reaction rate decreases because the diffusion gradient no longer supplies sufficient reactants to the electrodes. The diffusion gradient will decrease as the diffusion layer thickness increases as scanning continues. This creates a barrier to further current flow in the diffusion-dominated cell.
Some cyclic voltammograms will contain multiple peaks corresponding to different reaction-dominated kinetics or side reactions during a voltammetry scan. Some examples are shown below. Note that the onset potential is only sufficiently defined for the first peak encountered. A good analogue for the onset potential for the second peak could be an inflection point (shown below in red).
Cyclic voltammograms with multiple oxidation and reduction peaks.
Once you’ve determined the onset potential in cyclic voltammetry sweeps, and you’ve determined its stability, you’ll know the potential to apply in the system to drive or suppress reversible electrochemical reactions. Depending on how your electrochemical system will make use of these different reactions, you would need to design a regulator system with measurement feedback to control the reaction kinetics. Simple feedback loops with a PID control loop can be easily used for this type of measurement and can be implemented in a PLC, which is commonly used in industrial systems.
Once you know the onset potential in cyclic voltammetry for your next electrochemical system, you can incorporate any design changes to a potentiostatic regulator circuit and other important components using the right PCB design and analysis software. The PCB design tools in Allegro PCB Designer give you everything you need for electronics design in a single platform. You’ll also have access to a set of powerful analysis and simulation features.
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