This article is part of the AfterMath Data Organizer Electrochemistry Guide
Like most of the other electrochemical techniques offered by the AfterMath software, this experiment begins with an induction period. During the induction period, a set of initial conditions is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. The default initial condition involves holding the working electrode current at the Initial Current for a brief period of time (i.e., 3 seconds).
After the induction period, the current of the working electrode is swept linearly at the specified Sweep Rate between an upper and lower current.
After the sweeps have finished, the experiment concludes with a relaxation period. The default condition during the relaxation period involves holding the working electrode current at the final current for an additional brief period of time (i.e., 1 seconds).
The working electrode potential is plotted as a function of the current applied to the working electrode, resulting in a cyclic potentiogram.
You can click on the “I Feel Lucky” button, located at the top of the setup (see Figure 1), to fill in all the parameters with typical default values. You will no doubt need to change the parameters to values which are appropriate for the electrochemical system being studied.
Figure 1: CCP Basic Parameters Setup
The Electrode Range on the Basic tab is used to specify the expected range of potentials. The potential of the electrode is dictated according to the Nernst equation, assuming a fully reversible system (see Theory section in CRP), and will change to the value necessary to maintain the specified current. Therefore, if the choice of potential range is too small, actual potential may go off scale and the results will be truncated. If the potential range is too large, the potentiogram may have a noisy, choppy, or quantized appearance. Please see the ugly duckling wiki for an analogous situation in a voltammogram. .
The actual waveform that is applied to the electrode is linear but not truly analog (see Figure 2A). The flat portions at the beginning and end of the waveform are the induction and relaxation periods, respectively. For a typical two sweep experiment, the sweep starts at an Initial current and sweeps to a Vertex current, then sweeps to the Final Current. The sweep is not perfect; in reality is consists of many small steps (see Figure 2B, orange trace) which are generated using the maximum available resolution of the potentiostat's digital-to-analog converter (DAC).
Figure 2: CCP Waveform Details Showing A) the Total Waveform and B) a Magnified Waveform that shows the Applied Current (Orange) and Measured Current (Red)
While cyclic current ramp potentiograms are typically two segments or one cycle, it is possible to choose any number of segments for an experiment. If you choose any odd number of segments greater than two, the parameters that must be entered are a little different than the two segment case (see Figure 3). You must choose an Initial Current, Upper Current, Lower Current, and Final Current. You must also choose whether the Initial direction is rising (sweep towards Upper Current) or falling (sweep towards Lower Current). If the Initial direction is rising, the Final Current must be different than the Lower Current. If the Initial direction is falling, the Final Current must be different than the Upper Current.
Figure 3: Basic setup for three segment CCP.
If you choose any even number of segments greater than two (see Figure 4), you must enter an Initial current, Upper current, Lower current, and Final current. You must also choose whether the first sweep is initially rising (sweep towards Upper current) or falling (sweep towards Lower current). If the Initial direction is rising, the Final current must be different than the Upper current. If the Initial direction is falling, the Final current must be different than the Lower current.
Figure 4: Basic setup for four segment CCP.
The Advanced Tab for this method (see Figure 5) allows you to change the behavior of the potentiostat during the induction period and relaxation period. By default, the current applied to the working electrode during the induction and relaxation period will match the initial current and final current, respectively, as specified on the Basic Tab. You may override this default behavior, and you may also change the durations of the induction and relaxation periods if you wish.
Other important parameters on the Advanced tab are found in the Sampling Control area. This area contains two parameters, Alpha and Threshold which control when and how samples are acquired during the sweep portion of the experiment.
Figure 5: Advanced parameters setup for Cyclic Current Ramp Chronopotentiometry
As mentioned previously, the waveform applied to the electrode (see Figure 2) is not truly linear. The actual waveform is a staircase of small current steps. The duration of each small step is called the step period, and the step period is automatically chosen to take full advantage of the resolution of the potentiostat's digital-to-analog converter.
The Alpha parameter controls the exact time within the step period at which the potential is sampled. A alpha value of zero means the potential is sampled at the start of the step period, immediately after a new current step is applied. An alpha value of 100 means the potential is sampled at the end of the step period, immediately before the next current step is applied. Please see the Advanced Parameter Tab section of CV for an analogous situation in a voltammogram.
The Threshold parameter helps you to limit the amount of data retained as the potentiogram is acquired. The threshold parameter controls the interval between samples as the current is swept from one limit to another. As shown in the example in Figure 3, a data point is acquired every time the sweep moves . You could change the threshold from to a smaller interval (if you want to acquire more data) or to a greater interval (if you want to acquire less data).
It is likely that you will have to change the default value based on your experimental setup in order to obtain the best possible data. Extreme values for the threshold parameter can lead to undesirable results. A smaller threshold value, typically less than or equal to , produces smoother curves and larger data files. If you set the threshold parameter to zero, then the maximum number of data points is acquired (and the size of the resulting data file will be quite large). If you set the parameter to a very large interval, then the number of points acquired will be so small that the potentiogram appears sharp and jagged. Please see the CV article for an analogous situation in a voltammogram.
Post Experiment Conditions Tab
The chronopotentiogram for a solution of ferrocene look similar to the results for a series of successive titrations except that the current is not ramped through the transition points. Potential can be plotted against time (see Figure 6A) or against current (see Figure 6B, specific parameters were: Ferrocene in , OD Pt WE, sweep rate ).
Figure 6: Chronopotentiograms Depicting A) Potential vs. Time and B) Potential vs. Current for a Ferrocene Solution.