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Customizing Experiments in Explain

Case Study: How to turn the cell on in the middle of the experiment.

Introduction

The standard experiments available in Gamry’s electrochemical data acquisition software, the Framework, are all that is required for many users.  But for a small percentage of users, they would like to perform slightly more unusual experiments.  Some of these experiments can be achieved by small modifications to existing scripts, but for some there is a requirement for more major modifications.

In general, for most experiments, turning the cell switch on just before the start of the experiment and turning it off after it has completed is the standard requirement.  For potential controlled experiments, this avoids the current spike that can accompany the sudden polarization of the surface.  Sometimes, however, experimenters have a requirement to turn the cell on and off during the data acquisition phase because they want to monitor the effect of this action on the device under test.  This can be accomplished in the Framework with some special experimental scripting.

In this particular example, a customer wanted to get the peak current of the discharge of their capacitor.  One way to achieve this is to short circuit the capacitor through the potentiostat and monitor the current that flows.   A standard chronoamperometry experiment could be used, but it is difficult to achieve with a standard Framework experiment because you need to match the initial voltage step with the open circuit potential of the capacitor prior to the start of the experiment.  While this is possible, it is better to leave the cell turned off until the moment you want to start the discharge of the capacitor.

The main changes to the experimental script that is built to perform this experiment are related to the Signal that is used.  The waveforms that the potentiostat uses to create the experiments are called Signals. In this case we use a signal called UNIV which has a lot of features that allow you to create unusual signals for custom experiments.  The signal is created by adding sections together that have different properties (for instance is the cell is switched on or off in the section).  In this case study, we only need 2 sections; the first where the cell is turned off and the second where the cell is turned on at the selected potential.

For information about the Explain scripting language, look at the application notes Customizing Experiments with Explain and Explain Programming Reference. The text below shows the part of the experimental script where the signal is created from 3 sections. 

;Section 1
              VectorSignal[0] = FirstPoint
              VectorRLE[0] =  1
              VectorCTRL[0] = CTRL_DPT
              VectorSections[0] = 0
             
              ;Section 2
              VectorSignal[1] = 0.0
              VectorRLE[1] = Index(RestTime1/Sample )
              VectorCTRL[1] = CTRL_DPT
              VectorSections[1] = 0

              ;Section 3
              VectorSignal[2] = PulseOn1
              VectorRLE[2] =  Index(PulseTime1/Sample )
              VectorCTRL[2] = CTRL_CELLSSON + CTRL_DPT
              VectorSections[2] = 0

Figure 1 shows a graph for a 0.1F capacitor where the cell is turned on halfway through the experiment. The capacitor had previously been charged to 2.7 V. An initial period of open circuit, 10 seconds in length, is recorded, followed by the short circuit discharge of the capacitor.

fig1 current Time plot for capacitorFigure1. The Current-Time Plot for the Capacitor

Figure 2 below shows the voltage behavior of the capacitor.

fig2 voltage time pot for capacitorFigure 2 The Voltage-Time Plot for the Capacitor

Gamry customers interested in custom experimental scripting should contact Gamry or a Gamry distributor for information and pricing.