I realized many papers use scan rate 50mV/s or 100mV/s. But how do actually they determine the right scan rate to use throughout the experiment? What are the measurements that I should take into account in choosing it?
Choosing the scan rate in cyclic voltammetry depends on what purpose you want it to serve. Imagine you want to study two consecutive oxidation processes using cyclic voltammetry. You get one peak at, say, +0.6 V, and the product of this reaction is then further oxidized at +1.2 V. Now, if you choose a little too slow scan rate, you might miss the second oxidation peak as the product of the first oxidation (+0.6 V) may diffuse away. By contrast, scan rates too fast might distort the course of your cyclic voltammograms, and they are usually useful in, e.g., liquid flow techniques, where you need to record the entire cyclic voltammogram almost instantaneously since the composition of the solution changes in time.
Scan rates between 50 and 100 mV/s are usually used to "probe" the given compound's electrochemical behavior, and then you can vary it depending on the information you aim to obtain. Moreover, for analytical purposes (biosensors), other voltammetric techniques, such as differential pulse voltammetry, may be appropriate.
Jan
Jan is quite right, the correct experimental approach, as ever, depends on the question you are asking. That said, there are upper and lower limits for the scan rate, limited by capacitance and mass transport boundary layers respectively. At the lower end, the conventional analyses assume homogeneous concentrations at the start of the scan and only diffusional mass transport throughout the scan. In any fluid, there will always be natural convection due to temperature gradients and density changes (which can arise from the electrode reaction). Bockris suggests that natural convection boundary layer thickness is around 0.05 cm (Bockris & Reddy Vol 2. He gives no basis for this but it is probably about right). When the electrode reaction starts, a concentration gradient will be formed at the electrode-solution interface. The thickness of this concentration gradient is sqrt(2Dt). For the conventional analysis to be correct, this sqrt(2Dt), where D is the diffusion coefficient (typically 10^-10 m^2 s^-1 in aqueous solutions), this needs to be small compared with 10^-4 m. So, the scan needs to be over before sqrt(2Dt), around 10-100 s or so. So if you scan 1 V up and down, the slowest scan rate that will be unaffected by convection, 2 V needs to be scanned in under 100 s at best, 20 mV/s, ideally faster. The upper scan rate is limited by capacitance, typically 20-50 uF cm^-2 for Au or Pt, higher for most carbons (except BDD). If your CV experiment is on a species in solution (rather than immobilised), the peak current will scale with sqrt(scan rate). Capacitance scales linearly with scan rate, so at fast scans, the faradaic current will ultimately end up being lost in comparison with the capacitance (it needs to be significantly larger than the resolution of the analog-to-digital converter). In typical circumstances, without specialised instrumentation (as used by the Wightman group for example), scan rates faster than 10 V/s become problematic. Given how long it takes to set up an experiment, it is always worth looking at half a dozen scan rates and plotting i vs sqrt(scan rate) see if you get a straight line and zero intercept.
The choice of scan rates depends upon the information sought:
1.If the aim is to look for anodic and cathodic reversible peaks, scan rate of about 5 mV per sec is suitable.
2.If the aim is to obtain approximate kinetic parameters, scan rate of 100 mV per sec is adequate.
3.If a complete elucidation of the reaction mechanism is required, the convolution potential sweep voltammetry is the best since this will provide the exact values of transfer coefficients and yield new insights into the validity of Butler-Volmer equations.
Dear Nur Zafirah Mohd. Izham
This depends on the nature of the material used for the measurement. It is possible to check to what extent it can be chosen as the best rate of scanning speed by trying 50 as a first step and observing the extent to which the material is formed through the resulting current, then you can try the rate of the scanning rate at 100 By comparing the output current in both cases, you can determine the best scanning rate
Hello dear, my comment for that is you should make optimization for different scan rates. Obliviously, on many reverse systems as you increase scan rate you will get a higher current peak. However, there might be a deviation on the current at maximum current mainly greater than 0.1 v/s scan rate. That is why people select below 100 v/s. Thank you
May I ask is it okay have a wide potential range (-1 to 1V) in my CV experiment? Is there a disadvantage in using a wide potential range?
Hi Miguel, the goal of cyclic voltammetry is typically to do initial exploration of the redox system of interest in a semi-quantitative way. The potential range is constrained by solvent (actual solvent, pH, background electrolyte) and electrode materials. On the latter, electrocatalytically active materials e.g. Pt are usually fairly promiscuous in the redox reactions that they catalyse. Consequently, they will have a smaller potential range. If you're working with aqueous solvents, it is generally a bad idea to allow solvent decomposition to occur as this will definitely cause pH changes (which will be substantial in the electrode-electrolyte region) and possibly lead to bubbles on the electrode and changes in mass transport, increased noise. The experimental process therefore will involve preliminary scoping experiments to identify the potential range that characterises your system. The wider the potential range, the more likely that other redox reaction and their products will get involved in the system that is your primary interest and add needless complications.
Scan rate is usually used to study the diffusion controlled process of the that sensor. Increased the scan rate till you get linear response
Hi. I hope the following link might help you:
https://www.ossila.com/pages/cyclic-voltammetry
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