Zahner 01/2012. Steady State I/E Recording Fixed Sampling Dynamic Scan

Transcription

1 Zahner 0/0 Steady State I/E Recording Fixed Sampling Dynamic Scan

2 I/E - -

3 I/E Recording Main Menu 5. Edit Parameters 6. Control Potentiostat 6.3 Start Recording 6.4 Display Diagram 6.5 File Operations 6.6 I/E - Analysis 6. Edit Parameters 7. Edge Potentials 7. Scan Mode 8.. Steady State Sampling 8.. Fixed Sampling 9..3 Dynamic Scan 9.3 General Parameters 0 3. Control Potentiostat 4. Next Measurement 5. Display Diagram 3 5. Save Measurement 4 5. Create Data List Export Drawing Hardcopy Import Data List Select Diagram Enter Crosshair Mode 6 6. File Operations 7 6. Open 7 6. Save 8 7. Steady State I/E Analysis 9 8. Analysis Main Menu 0

4 I/E File Operations 0. Select Measure Files. Define Diagram Type. Selection of Diagram Type. Setting of Display Limits 3.3 Plot Diagram 3. Plot Diagram 4. Create Data List 4. Export Drawing 5.3 Hardcopy 5.4 Select Diagram 5.5 Enter Crosshair Mode 6 3. Tafel s Slope 7 4. Butler-Volmer Analysis 9 5. Range Definition Mode 3

5 I/E I/E Recording I/E applies a defined potential curve to the sample and measures the current response. The results are displayed as a I/E curve. With different scan modes and settings the following standard methods can be easily done: Chronoamperometry with Triangular Potential Sweep Cyclic Triangular Wave Voltammetry Linear Sweep Voltammetry Stationary Electrode Voltammetry Steady State Current/Voltage Curves Tafel Scan. Recording Main Menu I/E starts up with the graphical menu shown below. The installed submenus can be called up by activation of the corresponding button. The submenus can easily be recognized through their titles and symbols which are plotted within the button. The main menu manages various submenus, which will be described briefly.

6 I/E Edit Parameters setting of control parameters edge potentials scan rate resolution scan mode tolerances delay times current limiting. Control Potentiostat calls up submenu 'test sampling' of the impedance measurement program EIS.3 Start Recording starts next measurement.4 Display Diagram displays last measurement.5 File Operations disc i/o operations of I/E data (save and load measurement, select i/o device and data path).6 I/E - Analysis calls I/E evaluation program

7 I/E Edit Parameters The actual control parameters and the selected scan mode are indicated in the main menu within two boxes.. Edge Potentials Four edge potentials can be defined independent from each other, wherever two successive potentials define a ramp. These potentials can be changed by activating the corresponding button and inputting the new value. The edge potentials can be defined in two ways: a) absolute potentials reference E o =0V b) relative potentials reference E o =OC (open circuit potential) The selected mode is indicated by a blue tick within the checkbox. It can also be indicated during input by means of appending the corresponding abbreviation (rel) or (abs) to the actual value. The relative mode function offers the possibility to investigate different probes in a defined potential range relative to the open circuit potential (ocp). The ocp is set to the actual value through a change to the submenu 'test sampling' of the ZENNIUM impedance measurement program. In addition, the potentiostat has to be switched off. There is no ocp control during an I/E recording. Relative and absolute potential settings can be mixed for any ramp. The setting of two successive edge potentials to the same value reduces the number of ramps and the shape of the control curve. It is possible to make a choice between bidirectional and mono-directional potential control.

8 I/E E(,,3,4)=(0V,V,-V,V) E E(,,3,4)=(-V,V,V,0V) E E3 E E4 E4 E3 E. Scan Mode The function 'scan mode' offers three operational modes. Different sets of parameters define the control conditions. The following modes can be selected: steady state sampling fixed sampling dynamic scan.. Steady State Sampling The potential swings between the defined edge potentials. The potential is changed in steps of ± E (resolution) in the scan direction correspondingly. A step of potential usually perturbs the investigated system and current oscillations may occur. In the 'steady state' mode the induced current is continuously observed until certain steady state conditions will hold. Current steady state is observed by four control parameters: In In absolute current tolerance I = t In In relative current tolerance I = t In minimum delay maximum delay The flowing current is measured after each step of the potential. Current steady state is recognized if the current fluctuations will be at least less than the absolute current tolerance or less than the relative current tolerance. In case of steady state, however, potential variation will pause for the minimum delay time relative to the last measurement. Maximum delay time will have to be provided to bypass steady state control if the system does not become stationary after a step of the potential. The system will continue with the next step after the maximum delay time has run out. The following diagram describes steady state control graphically. The current slope and the tolerances represent model functions. Each diagram has been plotted in an arbitrary unit.

9 I/E I rel I abs minimum delay time not valid t I rel = I n - I n- t I valid t I abs = I n - I n- t.i n- I l E E l + E t I l :last current El : last valid potential E : resolution E l E l - E t l t l +t s t l +t d t t l t s = t-t l t d : measuri ng time of last measurement : time af ter steady state has been reached : minimum delay time.. Fixed Sampling Fixed sampling' works without steady state control. The steps of the potentials are printed out after the defined delay time has run out. Then the next measurement will be carried out. 'Fixed sampling' results in equidistant time resolution of the I/E data. The step width of the potential is defined through resolution. E o 4E d E o 3E d E o E d E o E d E o E o E d E o E d E o 3E d E o 4E d E d = resolution = measuring time t d = delay time 3t d t d t o t d t d 3t d 4t d t t o t o t o t d t o t o t o t o..3 Dynamic Scan A dynamic scan works like a CV measurement but with a reduced slew rate (0uV/s...0mV/s). The control parameters are slew rate and resolution. The ramps are driven by the defined slew rate and the data is registered in steps of the actual resolution of the potential. 'Dynamic scan' does not work with the steady state control of the current.

10 I/E E o E o E d E o E d E o E d E o E d E d = resolution = interval of integration E(t)=E + slewrate*t s = measuring time t s = slewrate/resolution t o t s t o t s t o t o t s t o t s t.3 General Parameters Losses in potentials caused by, e.g., the electrolyte resistance or ohmic resistances of the connecting lines can be compensated by positive feedback. A drop potential is calculated by the actual cell current. This potential is added to the actual ramp potential. Ohmic drop can be compensated up to a resistance of R max =kω. 'Resolution' defines the step width of the potential in the modes steady state and fixed sampling. The dynamic mode resolution defines the step width of the measured I/E data. The anodic as well as the cathodic current range can be selected independently. The exceeding of one of the limits would cause the actual ramp to be interrupted and the corresponding end potential to be corrected. Finally, the next ramp will be started with respect to the corrected edge potentials

11 I/E Control Potentiostat This option starts the test-sampling menu of the EIS impedance measurement program. In this menu different potentials may be applied to obtain information about the induced current. In addition, the potentiostat can be set to the I/E starting potential. This offers the possibility to start the I/E scan faster, otherwise the potentiostat automatically changes from the actual potential to the starting potential by a ramp of de/dt=4*slew rate. Now the final condition of the potentiostat can be selected. If a I/E measurement is started with the potentiostat switched on, the potentiostat will remain operational after the I/E measurement has been finished. If the potentiostat has been switched off before starting a I/E measurement it will be switched off at the end of the measurement.

12 I/E Next Measurement This function starts the next measurement. The measured E-I data is plotted online within a data window. On the upper right of the recording window three 'digital' instruments indicate the numeric values of the actual ones measured. A running measurement can be terminated with use of <ESCAPE>.

13 I/E Display Diagram The 'display diagram' function recalls the last spectrum measured. This spectrum is then displayed on the screen. Data can be exported to other programs by means of the standard i/o routines.

14 I/E Save Measurement Save the recorded data + the measurement parameters + the comments to the hard disk and return to the I/E page. Save the recorded data + the measurement parameters + the comments to hard disk and pass them directly to the SIM software for analysis. You will end up in the SIM page. When clicking on either of the buttons a description box opens where you may input your measurement parameters and comments. Some lines will be filled automatically by the software (e.g. potential, current and measuring time) others are free for the user to fill. User parameters are not used by the software for any calculation. They are for the user s information only. Accept the inputs by clicking on the button in the upper right corner or reject the inputs by clicking on the button. Click on the button to call the calculator. In the following browser navigate to the desired path, input a file name and click on the button to save the data or click on the button to cancel the saving. 5. Create Data List Creates an ASCII list of the data: and copies it to the Windows clipboard or saves it as plain text-file or copies it to ZEdit

15 I/E Export Drawing Passes the actual graph as high quality vector graphic to the Windows clipboard saves graph as an EMF graphic file Transfer the graph to the CAD graphic editor section of Thales. Edit the graph manually in the CAD section of Thales for later clipboard or EMF export. 5.4 Hardcopy Create a bitmap copy of the screen for export or printing: Perform the selected output operation Do a form feed Select the output properties: landscape = big format (DIN-A4) to printer upright = small format (DIN-A5) to printer to clipboard = sends screen to the Windows clipboard save as file = saves the screen as a bitmap file 5.5 Import Data List - inactive within I/E -

16 I/E Select Diagram Show current data (y-axis) versus potential (x-axis) Show potential (y-axis) versus current data (x-axis) Compensate ohmic share from actual diagram 5.7 Enter Crosshair Mode Move cursors through the curves with the mouse. The course leads through the history of the data source: moving from left to right means moving from the first samples recorded to later. Use the cursor-left/right keys for fine steps. You leave the crosshair mode by clicking the middle mouse key. If more than one data set is displayed, use the cursor-up/down keys to change from one curve to the next. Click the right mouse button to open the option box: Increase cursor key stepwidth Decrease cursor key stepwidth Set cursor to next / previous curve Passes the data of the selected measurement point to ZEdit Not implemented, for future expansion

17 I/E File Operations The 'display diagram' function will recall the last record. The measured data will be displayed on the screen. 6. Open This function allows you to load/save I/E data from/to the hard disk of your PC. Click on the icon and decide in the submenu, whether you want to open previously stored I/E data or save actual I/E data. If there are no data in memory, this sub-menu is skipped. After your decide to Open, a file browser will open displaying only I/E data files (file extension.iss). Select a path and a file and click on the LOAD button to load the file to the Thales software. Click on the Display Spectrum icon on the IE main page to display the data in a graphical form. The next box shows the measurement parameters of the loaded data. Click on it if you want to close it. NOTE: Opening a file will switch off the potentiostat and configures the potentiostat according to the loaded measurement-settings automatically. Loading a measurement file is an easy way to perform a new measurement with previously used settings.

18 I/E Save If there are data in memory, usually after an IE data measurement, you may want for Save them on hard disk for storage. A description box opens where you may input your measurement parameters and comments. Some lines will be filled automatically by the software (e.g. potential, current and measuring time) others are free for the user to fill and are not used for any calculation. The last three lines are reserved for series recording information. In a series EIS run they will be filled automatically. You may use these lines to define spectra for series analysis even if they were created as single measurements. Accept the inputs by clicking on the button in the upper right corner or reject the inputs by clicking on the button. Click on the button to call the calculator. In the following browser navigate to the desired path, Input a file name and Click on the button to save the data. Click on the button to cancel the saving.

19 I/E Steady State I/E Analysis This button calls up the I/E analysis software. The actual data will not be present there at the moment. In order to be on the safe side a measurement has to be saved in the recording program which is to be loaded first. Besides, it is evaluated by the analysis program.

20 I/E I/E Analysis 8. Analysis Main Menu I/E analysis begins with the menu shown below. The offered functions and submenus are represented by graphic primitives.

21 I/E File Operations There are commonly known disc i/o functions to load measure data (see I/E recording chapter 6) but with one restriction: The save option has been blocked. The reason is that it is possible to use the I/E analysis to manipulate the original data. If this data is saved, the original one will be overwritten. A file browser will open displaying only IE data files (file extension.iss). You can also select more than one data file to open. 0. Select Measure Files 'select measure files' offers the possibility to select different sets of measurement for evaluation. When this function is activated a file list including all files which have already been loaded will be opened. A blue background color indicate whether a file has been selected. To select or deselect a data file simply click on the file name. NOTE: Up to ten measurements can be loaded at the same time depending on the memory space available. If a loading operation fails because of an overflow message, the analysis program will have to be initialised with the 'INIT' function of the SIM program.

22 I/E - -. Define Diagram Type I/E measurements store data triplets of the format (E,I,t). The data can be plotted in different graphical representations. During I/E recording data is automatically plotted in the log I(E) mode. 'Define diagram' offers selection of different display modes with linear and logarithmic scaling, either manually or automatically. An activated 'define diagram type' function offers the menu shown below. This submenu offers all definitions to create a plot of the data measured. The diagram can be affected - by typing, log I vs E, I vs E, E vs I or E vs log I or - by data ranges and display limits The settings which are organized in a graphical control panel can be carried out by chosing the corresponding option and by entering some parameters.. Selection of Diagram Type The diagram type is selected by activating the corresponding button. The activated mode is indicated by a green bar within this button and the symbolized terminal will change its graphic.

23 I/E Setting of Display Limits The display limits can be set in accordance with the selected diagram type. Each magnitude can be scaled automatically (auto) or manually (user). In order to redefine the display limits one of the buttons has to be selected and the new values have to be input with help of the i/o box. The values in brackets indicate the extreme values of the corresponding data track..3 Plot Diagram Redrawing the graphic without changing menus is effected by moving the cursor to the symbolized terminal. The actual graphic is then overlaid by the 'plot' button. The next diagram will be plotted after confirmation with <ENTER> or mouse click.

24 I/E Plot Diagram The diagram can be plotted after all settings have been made. Output is initiated by activating the button 'plot diagram'. When the plotting procedure has been finished, five additional buttons will appear on the right side of the screen and offer the options.. Create Data List Creates an ASCII list of the data: and copies it to the Windows clipboard or saves it as plain text-file or copies it to ZEdit

25 I/E Export Drawing Passes the actual graph as high quality vector graphic to the Windows clipboard saves graph as an EMF graphic file Transfer the graph to the CAD graphic editor section of Thales. Edit the graph manually in the CAD section of Thales for later clipboard or EMF export..3 Hardcopy Create a bitmap copy of the screen for export or printing: Perform the selected output operation Do a form feed Select the output properties: landscape = big format (DIN-A4) to printer upright = small format (DIN-A5) to printer to clipboard = sends screen to the Windows clipboard save as file = saves the screen as a bitmap file.4 Select Diagram I/E measurements store data triplets of the format (E,I,t). This data can be plotted in different graphical representations. During the recording, the data can be plotted in the I(E) mode automatically. The 'select diagram' function allows to select between the modes. Scaling can be done manually or automatically (see I/E analysis chapter ).

26 I/E Enter Crosshair Mode Move cursors through the curves with the mouse. The course leads through the history of the data source: moving from left to right means moving from the first samples recorded to later. Use the cursor-left/right keys for fine steps. You leave the crosshair mode by clicking the middle mouse key. If more than one data set is displayed, use the cursor-up/down keys to change from one curve to the next. Click the right mouse button to open the option box: Increase cursor key stepwidth Decrease cursor key stepwidth Set cursor to next / previous curve Passes the data of the selected measurement point to ZEdit Not implemented, for future expansion

27 I/E Tafel s Slope The Tafel plot ( log I vs E) is a useful tool to determine kinetic parameters through I/E measurements. In general, there is an anodic and a cathodic branch: ( α) nfη log I anod. 3RT α nf log I cath The linear segments can be extrapolated to η = E-E 0 = 0 and will intercept at logi 0. When being close to η = 0, the plots will deviate from the linear behaviour because of influences of the back reactions. At high values of η diffusion processes and ohmic behaviour may well dominate the current slope. The Tafel evaluation defines the 'linear' segments. Two-line segments are calculated by linear regression. Eventually, the point of interception is calculated. After Tafels's slope has been called up, four markers have to be set. The program automatically prompts the following when the Tafel's slope is entered for the first time. Define the anodic and the cathodic branch, each of them is limited by two markers. Move the markers by using the mouse or the cursor keys. There is no strict order of input as the markers are sorted by the program in accordance with rising potential. The range markers can be changed for further calculations within different range(s) after the evaluation. The range(s) can be changed during any graphical output on the screen with the crosshair.

28 I/E A marker is changed by moving it to its last position and by activating the motion with <ENTER>. Now the marker has to be moved to the new position and this position is fixed by pressing <ENTER> one more time. The step width can be changed by the <HELP> function which calls up the mouse help routine. The linear segments can be defined monofold or multifold, see section I/E analysis chapter 5. Finally, the lines of regression and the point of intersection is plotted. The corresponding parameters are printed in the data window to the right of the window of the graphic. The regression is calculated by the familiar chi-square test ξ N = [ y i= ( a + i bx i )] = min and results in b = N ( xi x)( yi y) N N a = y bx ( x = xi; y N N i= N i= ( x x) i= = i= i y ) i The coefficient of correlation is r xy = N i= ( x x)( y i N N ( xi x) i= i= i y) ( y y) i The slopes are denoted in units of mv/dec. Current and potential indicate the point of intersection.

29 I/E Butler-Volmer Analysis I/E data is fitted by the Butler-Volmer equation αzfη ( α ) zfη I( η) = I ( e e ) η = E E0 reduced potential 0 Free parameters are: E 0 zero count potential I 0 current at zcp α coefficient of symmetry z effective charge number f = F RT The fitting procedure is a least squares algorithm and minimizes the chi-square sum ξ = N i= ( I meas I theo ) The Butler-Volmer fit allows to enter starting values of the running parameters. Default values are calculated by means of the I/E scan.

30 I/E Eventually, the fit is displayed and the standard i/o routines offer output as a hardcopy, as a data list or a CAD file. Fitting can be effected in the monofold or multifold mode. The fitting limits can be changed and another fit can be started by the crosshair routine.

31 I/E Range Definition Mode When an I/E curve is measured with edge potentials ± E relative to the ocp, as shown below, the results of the descending branch will differ from those of the ascending branch. Relativpotentiale E(,,3,4)= (-V,V,-V,-V) E/V logi/a ocp - E a a E b b E3 E4 a a b b Potential E/V I/E analysis presents two modes of 'range definition', either the monofold or the multifold one. In the multifold mode all measure points within the limits are selected for evaluation, whereas the monofold mode offers the possibility to select only one branch for calculations. Fitting can be done in the monofold as well as in the multifold mode. The fitting limits can be changed and another fit can be started by the crosshair routines.