NOVA Technical Note 1 Automatic data analysis Case study: automatic iV curve and power curve from fuel cell measurements Fuel cell characterization is usually performed by measuring the iV and power curve of the fuel cell. Two different strategies are possible: • • Constant potential mode: this method works in potentiostatic mode (using the Autolab or an electronic load in combination with the Autolab). Starting at the open-circuit potential (OCP), negative over-potential steps are applied and the discharge current is monitored. Constant current mode: this method works in galvanostatic mode (using the Autolab or an electronic load in combination with the Autolab). Starting a zero current (OCP), negative current steps are applied and the potential of the cell is monitored. This case study illustrates how to use the data analysis commands of NOVA in order to perform fully automatic data handling and analysis in fuel cell measurements. Note The procedure described in this technical note is a simulation of the experiment which can be tested on the Autolab dummy cell (a). 1 – The experiment The constant potential method is used in the experiment. The procedure and the data analysis tools used in this procedure can be easily adjusted for a constant current method. The experiment uses a Repeat for each value 1 loop to apply a sequence of overpotentials (relative to the open-circuit potential) on a fuel cell and measure the discharge current, cell potential and power for each over-potential. Each measurement takes 12 s, the interval time is 0.1 s. The signals recorded during each step are defined in the signal sampler (see Figure 1). Please refer to the Command list document, available from the Help menu in Nova, section 3.5 for more information on the Repeat for each value command. 1 1|Page NOVA Technical note 1 Figure 1 – Specifying the signals to be recorded during the measurement Figure 2 shows a simulation example on the Autolab dummy cell (a), for one of the steps in the repeat loop. Figure 2 – Current (blue) and Potential (red) measurements on the dummy cell (a) At the end of the experiment, the average value of the current, potential and power are calculated using the last 30 data points for each step. The average potential and the power values are plotted versus the averaged current values. 2|Page NOVA Technical Note 1 All this can be done completely automatically using the following commands 2: • • • • • The Windower command to select the last 30 points for each overpotential The Calculate signal command to calculate the average values of the Current, Potential and Power values provided by the Windower The Nested procedure command to create a sequence at the end of the procedure to separate the measured data from the calculated data The Build signal command to filter all the average values of current and potential and the power values for each over-potential The Custom plot (twice) command to build the V versus i plot and the P vs i plot. Please refer to the Command list document, available from the Help menu, for more information on the commands used in this technical note: section 11.3 (Windower), 11.4 (Calculate signal), 11.2 (Build signal), 16 (Custom plots). 2 3|Page NOVA Technical note 1 2 – The procedure Figure 3 provides an overview of the simulation procedure, to be used on the Dummy cell (a). Figure 3 – Overview of the simulation procedure This procedure starts by determining the open-circuit potential of the cell (using the OCP determination command). The measured OCP is then used as reference potential, using the Set reference potential command. The OCP value is then applied, using the Set potential command (0 V – relative to the OCP). The cell is switched on and there is a delay of five seconds. The next part of the experiment is located into a Repeat for each value command. The over-potential values (relative to the OCP) are predefined in the repeat loop (in Figure 3, from 0 V to -1 V, 100 mV step, 11 values). The values of the repeat for each value loop are linked to the Set potential command. 4|Page NOVA Technical Note 1 This command is followed by a Record signals (>1 ms) command, which measures the potential, current and power during 12 seconds, with an interval time of 0.1 s. Three pre-defined plots are used (E vs t, P vs t and i vs t). The final part is used to switch the cell off. At the end of the experiment, the user has a total of 11 record signals items in the data presentation. Each Record signals item provides three plots: i vs t, E vs t and P vs t. Figure 4 shows the data recorded using the procedure on dummy cell (a). Figure 4 – Overlay of all the data recorded using the procedure 3 – Modification of the procedure In this section, we will illustrate how the data analysis command in NOVA can be used to perform complex data handling automatically. 3.1 – Data extraction using the Windower command Since the user wants to calculate the average of the current and the potential over the last 30 data points for each value, the first step is to extract the 30 last data points from each measurement. This can be done using the Windower command. More information on this command can be found in the User manual, chapter 4 and in the Command list document. The Windower command is directly accessible through the quick access toolbar. Click the Record signal (>1 ms) command in the procedure editor. The quick access toolbar will be displayed showing 8 icons corresponding to individual tasks that can be performed on this command. In the quick access toolbar, click the icon to add a Windower command (see Figure 5). 5|Page NOVA Technical note 1 Figure 5 – Adding the Windower to the Record signals (>1 ms) command Note It is also possible to add a Windower command by dragging the command from the Data handling group of commands onto the Record signals (> 1 ms) command. When the Windower is added, a popup menu will appear (see Figure 6). Figure 6 – The Windower dialog window will appear when the Windower command is added to the procedure This dialog window is used to specify the source of the Windower and the boundaries of the source signal. The source is the signal that is going to be used 6|Page NOVA Technical Note 1 to extract the data points required for the analysis. Since we want to select the 30 last data points, we are going to set the source signal to Index, using the dropdown list (see Figure 6). Once the source has been defined, it is possible to specify the boundaries for the selected source. Each Record signals command provides a total of 120 data points, indexed from 1 to 120. Since we want to extract the final 30 points from each measurement, the boundaries will be from index 91 to index 120. These values can be entered in the Begin and End fields of the Windower dialog (see Figure 7). Figure 7 – Specifying the boundaries of the Windower Click the button of the dialog to validate the settings. The Windower command will be added to the procedure editor. The original signals provided by the Record signals command are now also located below the Windower (see Figure 8). However, these signals are shown in blue, indicating that they are generated by a data handling command. These signals will only contain the values of the last 30 data points. 7|Page NOVA Technical note 1 Figure 8 – The Windower is added to the procedure editor Note Measured signals are shown in red in the procedure editor. Calculated signals and signals generated by data handling commands are shown in blue. 3.2 – Signal calculation Now we can calculate the average value of the current and the potential using the Calculate signal command. More information on this command can be found in the User manual, chapter 4 and in the Command list document. The Calculate signal command is directly accessible through the quick access toolbar. Click the Windower command in the procedure editor. The quick access toolbar will be displayed showing 6 icons corresponding to individual tasks that can be performed on this command. In the quick access toolbar, click the icon to add a Calculate signal command (see Figure 9). 8|Page NOVA Technical Note 1 Note Since we want to perform the calculation on the last 30 points, the Calculate signal command must be added to the Windower command in the procedure editor. Figure 9 – Adding the Calculate signal to the Windower command Note It is also possible to add a Calculate signal command by dragging the command from the Data handling group of commands onto the Windower command. When the Calculate signal is added, a popup menu will appear (see Figure 10). 9|Page NOVA Technical note 1 Figure 10 – The Calculate signal dialog window will appear when the Calculate signal command is added to the procedure Click the Single value checkbox to force the calculation to return a single value. Define the Name of the signal (Average current), the units (A), the expression MEAN(Current) 3. Click the Parameters frame to validate the expression. NOVA will identify the Current parameter as the only parameter of the equation (see Figure 11). It is possible to copy and paste this expression into the Expression frame of the Calculate signal window 3 10 | P a g e NOVA Technical Note 1 Figure 11 – Specifying the expression (1/2) Double click the WE(1).Current signal, listed in the Signals field, in the Windower group, to assign the parameter Current to this signal. The parameter Current will be displayed between brackets next to the WE(1).Current signal to indicate the link (see Figure 12). 11 | P a g e NOVA Technical note 1 Figure 12 – Specifying the expression (2/2) Click the button to close the Calculate signal window. The Calculate signal command will be added to the procedure, and the result of the calculation, the Average current signal will be added to the signals available in the Windower. A link between the WE(1).Current signal from the Windower and the Current parameter of the Calculate signal command will be automatically added to the procedure (see Figure 13). 12 | P a g e NOVA Technical Note 1 Figure 13 – The Calculate signal command is now added to the procedure Using this approach, the average value of the last 30 WE(1).Current values will be calculated and the result of the calculation, called Average current, will be added to the available signals. Add another Calculate signal command to the Windower in the procedure to calculate the Average potential signal, using the same approach (see Figure 14). 13 | P a g e NOVA Technical note 1 Figure 14 – Calculating the Average potential value Click the button to close the Calculate signal window. The procedure will now also calculate the Average potential using the last 30 WE(1).Potential values. Add another Calculate signal command to the Windower in the procedure to calculate the Average power signal, using the same approach (see Figure 15). 14 | P a g e NOVA Technical Note 1 Figure 15 – Calculating the Average power value Click the button to close the editor. The procedure will now also calculate the Average power using the last 30 WE(1).Power values. The procedure will be updated has shown in Figure 17. 15 | P a g e NOVA Technical note 1 Figure 16 – The three Calculate signal commands have been added to the procedure At this point, the procedure will extract the values of the last 30 points from each Record signals step in the experiment, calculate the average value of the current, potential and power. 3.3 – Gathering the data from all the measurements Now we need to extract all the values of Average current, Average potential and Average Power in order to build the required plots. In order to separate measured data from calculated data, it is convenient to add a Nested procedure command at the end of the procedure. The Nested procedure command can be used to create a separate sequence in the procedure tree. This command can be found in Control group of command. Switch to the Commands browser in the setup view, locate the Nested procedure command in the Control group and add it to the procedure, at the very end of the sequence (see Figure 18). 16 | P a g e NOVA Technical Note 1 Figure 17 – Adding a Nested procedure command to the procedure To gather all the signals into a single place, the Build signal command will be sued. The Build signal command is directly accessible through the quick access toolbar. Note More information on this command can be found in the User manual, chapter 4 and in the Command list document. Click the Nested procedure command in the procedure editor. The quick access toolbar will be displayed showing 2 icons corresponding to individual tasks that can be performed on this command. In the quick access toolbar, click the icon to add a Build signal command (see Figure 19). 17 | P a g e NOVA Technical note 1 Figure 18 – Adding the Build signal command to the Nested procedure command Note It is also possible to add the Build signal command manually to the procedure by dragging and dropping the command from the Data handling group onto the Nested procedure command at the end of the procedure editor. When the Calculate signal is added, a popup menu will appear (see Figure 20). 18 | P a g e NOVA Technical Note 1 Figure 19 – The Build signal editor The Build signal command can be used to gather all values with the same name in a single location. The Build signal editor shows the available signals, listed below the name of the commands supplying these signals. Expand the Data handling group in the Build signal window interface and expand the Windower group. The calculated signals (Average current, Average potential and Average Power) will be displayed in the list (see Figure 21). 19 | P a g e NOVA Technical note 1 Figure 20 – The calculated signals are listed in the Data handling – Windower group Double click the Average current signal to add it to the frame on the right-hand side (see Figure 22). Figure 21 – Adding the Average current value to the Build signal 20 | P a g e NOVA Technical Note 1 Repeat this procedure for the Average potential and the Average Power signals (see Figure 23). Figure 22 – Adding the Average potential and Average power to the Build signal Finally, adjust the value for the Search from X level(s) up parameter, to 2 in order to indicate that the signals should be gathered from the available values outside of the Nested procedure command (see Figure 24). 21 | P a g e NOVA Technical note 1 Figure 23 – Adjusting the search depth parameter Click the button to close the interface. The three signals provided by the Build signal command will be added to the Nested procedure command (see Figure 25). 22 | P a g e NOVA Technical Note 1 Figure 24 – The signals generated by the Build signal command are listed at the top of the procedure editor 3.4 – Plotting the data The data can now be plotted at the end of the experiment. Two Custom plots need to be added to the procedure. The Custom plot command is directly accessible through the quick access toolbar. Click the Nested procedure command in the procedure editor. The quick access toolbar will be displayed showing 5 icons corresponding to individual tasks that can be performed on this command. In the quick access toolbar, click the icon to add a Custom plot command to the Nested procedure (see Figure 26). 23 | P a g e NOVA Technical note 1 Figure 25 – Adding the Custom plot command to the Nested procedure command Note It is also possible to add a Custom plot command by dragging the command from the Plots - general group of commands onto the Nested procedure command. Add two Custom plot commands to the procedure to build the required plots (see Figure 27). Links towards the signals provided by the Build signal command are used. Set the Measurement plot number of the two additional plots on plot #4 (see Figure 27). 24 | P a g e NOVA Technical Note 1 Figure 26 – Adding two custom plots to the procedure The two plots can be customized (name, plotting style, axis location, etc…). When this procedure is performed on the dummy cell (a), the following results are obtained (see Figure 28). Note Click the button in the toolbar to display all plots in the measurement view. 25 | P a g e NOVA Technical note 1 Figure 27 – Final plots 26 | P a g e