Rapid Biphasic Pulsing

Introduction

Occasionally you may need to make your potentiostat emit rapid pulses for your experimental work. For example, neuroscientists sometimes use such pulsing for experiments with neurons. Plating chemists can use such pulses when performing plating. In work with batteries, rapid pulsing may be more efficient at charging batteries than pure DC. What are some of the physical limitations on biphasic (positive, neutral, then negative) pulsing with Gamry Instruments’ potentiostats?

The Problem

What factors limit the speed of pulsing for your potentiostat?

For an introduction to how your Gamry Instruments potentiostat operates, see our Technical Note “Understanding the Specifications of your Potentiostat”. Among the specifications that make a difference for high-speed pulsing of voltage or current are:

Rise time, which is how fast a potentiostat can respond to an abrupt change in signal.
Minimum timebase, which is the fastest sampling rate a potentiostat can take of the incoming signal.
Speed settings, which allow control of fast changes in signal.

Obviously for rapid pulsing, a potentiostat must have a quick rise time, a small minimum timebase, high slew rate, and be set to high speed.

Parameters to Consider

Minimum Timebase

The minimum timebase for the Reference™ family of instruments is 3.3 µs. For the Interface™ family, the minimum timebase is 10 µs. For effective sampling, you need to span at least two or three times the minimum timebase. Therefore the smallest resolvable pulse or step is about 7 µs for the Reference potentiostats, and roughly 25 µs for the Interface family.

Size of Data Files

Besides that, Framework™ software for recording the data has a limit of 264,000 data points per file. At such a high speed of data-acquisition, 264,000 points adds up rapidly.

Size of Your Cell

Even if your potentiostat could react faster to changes in signals, there is a physical limit to how fast your sample or electrochemical cell itself can react. For example, a capacitative cell with a large surface area just will not be able to ramp up from 0 V to 10 V in 5 µs. A resistive electrode with a small surface area should be able to ramp up in voltage in a very short time.

Speed Settings

Gamry Instruments’ potentiostats have a speed setting adjustable in Framework software, called CA Speed. This is a parameter that affects the Control Amplifier inside the potentiostat. Faster CA Speed settings allow the control of fast signal changes. However, this also affects the potentiostat’s stability, which becomes even more apparent when capacitive cells or reference electrodes with higher impedances are connected.

If the CA Speed is set too fast, slowing down the Control Amplifier would likely benefit the signal. In general, galvanostats hate resistors but love capacitors. Potentiostats tend to love resistors, but do not handle capacitors as well. Figure 1 is an example of a CA Speed set too high.

Figure 1. Chronopotentiometry scan of a resistive cell with CA Speed set too high.
Note the extreme overshoot at the leading edge of each pulse.

Here the overshoot is clearly dramatic: the initial pulse shoots the voltage way beyond the intended value. Gradually the voltage settles down to the intended value.

If the CA Speed is too slow with a significantly rounded pulse, increasing the CA Speed should help. Increasing the speed allows the instrument to reach the set value faster. On the other hand, a CA Speed set too slow results in a sluggish response from the potentiostat, as you can see in Fig. 2:

Figure 2. Chronopotentiometry scan of a capacitative cell with CA Speed set too slow. Notice the slow rise-time, and how the different voltage levels are very difficult to distinguish.

The key difference with Fig. 1 is in how long the initial rise takes. The slope is unacceptable, and the different voltages blend into one another.

Finding the correct balance between speed and stability can be an art. The trade-off is stability. Higher speeds may result in more noise, loss of control, or overshooting. For more details on speed versus stability, see our Application Note “Tips and Techniques for Improving Potentiostat Stability.”

Creating Biphasic Pulses

Example

Gamry Instruments’ potentiostats can handle biphasic pulsing. Figure 3 is an example of biphasic pulses using five different current steps in a chronopotentiometry experiment.

Installing and Running the Scripts

From our website you can download special scripts called Galv Repeating Pulse.exp and Poten Repeating Pulse.exp to be used in our Framework software, to run this sort of experiment. The scripts are at

https://www.gamry.com/support/documentation-downloads/

Figure3 Chronopotentiometry

Figure 3. Chronopotentiometry (top) and Chronoamperometry (bottom) scans using several different voltages and currents.

Download the zip files and extract the scripts.
Place the scripts into the ProgramData\Gamry Instruments\Framework\Scripts folder.
Start your potentiostat and Framework software as usual.
In the Framework software, click the Experiment menu, and choose Named Script….
Scroll down to the desired script and click the Open button.

The scripts are ready to use. In the set-up window, you can change the number of different voltage- or current-levels you want to use, and specify the voltage or current for each level in the experiment.

Conclusion

This Application Note shows you how you can use Gamry Instruments’ potentiostats to deliver biphasic pulsing with voltage and current.

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