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NOVA Technical Note 4 1 | Page Calculating the Current density, j Case study: how to calculate the current density using NOVA? 1 – Current density NOVA allows the user to sample any of the available signals through the sampler (WE(1).Current, Time, Potential applied, etc…). The current density is not available in the sampler because it depends on the electrode area. One of the frequently asked questions of NOVA users is how to calculate the current density for data analysis purposes. Although it is obvious that the current density can be calculated using a Calculate signal command in the procedure or using the calculate signal tool in the analysis screen, this technical note will explore a more elegant way of calculating j and using it in combination with data analysis tools. To illustrate this case study, a .nox procedure is provided (NOVA technical note 4 – Current density [NOVA 1.10].nox). Import this file using the Import procedure option available in the File menu in NOVA. This technical note will illustrate how this procedure is built, step by step. The procedure uses the Autolab Cyclic voltammetry potentiostatic as template. 2 – Different samples Different working electrodes have different surface areas. It is therefore convenient to be able to specify the area of the working electrode for each new measurement. An Input box command located at the beginning of a procedure can be used to specify the surface area of the sample for each new measurement. 2.1 – Adding the Input box at the beginning of the procedure Load the Cyclic voltammetry potentiostatic procedure in the procedure editor and add the Input box command at the beginning of the procedure (see Figure 1).

Calculating the Current density, j - Metrohm Autolab...2.2 – Calculating the current density In order to use the value specified by the user in the Input box, the procedure needs

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Page 1: Calculating the Current density, j - Metrohm Autolab...2.2 – Calculating the current density In order to use the value specified by the user in the Input box, the procedure needs

NOVA Technical Note 4

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Calculating the Current density, j

Case study: how to calculate the current density using NOVA?

1 – Current density

NOVA allows the user to sample any of the available signals through the sampler (WE(1).Current, Time, Potential applied, etc…). The current density is not available in the sampler because it depends on the electrode area. One of the frequently asked questions of NOVA users is how to calculate the current density for data analysis purposes.

Although it is obvious that the current density can be calculated using a Calculate signal command in the procedure or using the calculate signal tool in the analysis screen, this technical note will explore a more elegant way of calculating j and using it in combination with data analysis tools.

To illustrate this case study, a .nox procedure is provided (NOVA technical note 4 – Current density [NOVA 1.10].nox). Import this file using the Import procedure option available in the File menu in NOVA. This technical note will illustrate how this procedure is built, step by step. The procedure uses the Autolab Cyclic voltammetry potentiostatic as template.

2 – Different samples

Different working electrodes have different surface areas. It is therefore convenient to be able to specify the area of the working electrode for each new measurement. An Input box command located at the beginning of a procedure can be used to specify the surface area of the sample for each new measurement.

2.1 – Adding the Input box at the beginning of the procedure

Load the Cyclic voltammetry potentiostatic procedure in the procedure editor and add the Input box command at the beginning of the procedure (see Figure 1).

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Figure 1 – Adding the Input box at the beginning of the procedure

Once the Input box has been added to the procedure, its parameters can be edited. It is convenient to remind the user about the surface area unit (for example, in cm2). Furthermore, it is possible to specify a default value in the Input box. Figure 2 shows an example of Input box parameters.

Figure 2 – The edited Input box

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2.2 – Calculating the current density

In order to use the value specified by the user in the Input box, the procedure needs to know what to do with this value. In this example, we want to use the surface area value to calculate the current density using the values of the WE(1).Current measured during the CV staircase experiment.

In order to do this, we need to add the Current density signal to CV staircase command in the procedure. To do this, click the CV staircase command in the procedure editor and then click the button in the quick access toolbar (see Figure 3).

Figure 3 – Adding a calculate signal to the procedure editor

This will add a Calculate signal1 command to the procedure.

The Calculate signal dialog will be displayed (see Figure 4).

1 More information about the Calculate signal command can be found in the NOVA User manual.

Note

More information on the Calculate signal tool can be found in the NOVA User manual, chapter 4 and in the Command list document, available from the help menu.

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Figure 4 – The Calculate signal window

In this example, we will create a new signal called Current density – in A/cm2. Fill in the Calculate signal window, specifying the following items (see Figure 5):

• Name: Current density • Unit: A/cm2 • Expression: Current/Area

Click the Parameters field after the Expression has been entered to identify the two parameters (see Figure 5).

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Figure 5 – The Calculate signal window - note the two expression parameters (Current and Area)

Click the Current parameter in the Parameters frame and expand the CV staircase group in the Signals frame to reveal the available signals. Double click the WE(1).Current signal to assign the values of this signal to the Current parameter (see Figure 6).

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Figure 6 – Double click the WE(1).Current signal to assign the values of this signal to the Current parameter

The Current parameter will be displayed in between brackets in the Calculate signal dialog. Click the button to close the editor. A warning message will be shown, indicating that not all parameters are linked (see Figure 7).

Figure 7 – A warning is displayed when the editor is closed

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The warning message can be ignored. The procedure editor will be updated, showing the Calculate signal command below the CV staircase command. The WE(1).Current signal is already linked to the Current parameter of the Calculate signal command (see Figure 8).

Figure 8 – The procedure editor is updated with the Calculate signal command

Note

The symbol is shown next to the Calculate signal command because the Area parameter is not yet linked.

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Select the Area parameter of the Calculate signal command and the Value parameter of the Input box command and add a link between both parameters (see Figure 9).

Figure 9 – Linking the Value and the Area parameters

2.3 – Adding a Current density plot

Since the Current density signal is calculated at the end of the measurement, it cannot be plotted in the measurement view, in real time. It is, however, possible to add an additional plot to the CV staircase command in order to build a current density plot at the end of the measurement.

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Click the CV staircase command and click the button in the quick access toolbar to add a Custom plot to the command (see Figure 10).

Figure 10 – Adding a custom plot using the quick access toolbar

Change the name of the plot from Custom to Current density plot and link the X signal to the Potential applied signal provided by the CV staircase command and the Y signal to the calculated Current density signal. Link the Z signal to the Time signal. Set the measurement plot number to 2 (see Figure 11).

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Figure 11 – Creating the Current density plot at the end of the measurement

2.4 – Testing the procedure on the Autolab dummy cell

Connect the Autolab dummy cell (a) and start the measurement. A window will be displayed at the very beginning of the measurement, prompting you to enter the value of the surface area in cm2 (see Figure 12).

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Figure 12 – The Input box is displayed at the beginning of the measurement

In this example, we will use 10 cm2 as working electrode area. Once the OK button is clicked, the measurement will start. A typical cyclic voltammogram for the Autolab dummy cell will be displayed in the measurement view. At the end of the measurement, the current density will be calculated using the measured WE(1).Current values and the surface area value specified at the beginning of the measurement. The current density plot will be shown on plot number 2 in the measurement view (see Figure 13).

Figure 13 – The Current density plot created using the measured WE(1).Current values and the surface area

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Note

Click the button in the toolbar to display both plot in the measurement view.