Mott-Schottky Setup Parameters

The Mott-Schottky  Setup dialog box has the following parameters associated with the experiment.

Mott-Schottky Setup

Initial Voltage

  • The starting point for the potential sweep during data-acquisition. The allowed range is ± 8 V with a resolution of 1/8 mV. Its accuracy depends on the setting.

Final Voltage

  • The ending point for the potential sweep during data-acquisition. The allowed range is ± 8 V with a resolution of 1/8 mV. Its accuracy depends on the setting.The Sweep Range, defined as the absolute value of Final Voltage minus Initial Voltage, must be less than 8 V.

Voltage Step

  • The spacing between data points, and helps determine the number of points in the plot. Each point in the sweep is one Voltage Step away from the neighboring points. Therefore, the Nth point in the sweep is recorded at a voltage calculated by:
    • Voltage = Initial Voltage + (N × Voltage Step)
    • The number of points in the sweep can be calculated as:
    • Sweep Range = Final Voltage – Initial Voltage
    • Number of Points = Sweep Range / Voltage Step
  • The plot cannot contain more than 32000 data points. This is not a serious limitation, because Mott-Schottky plots with more than a few hundred points are recorded too slowly for most users’ patience.

AC Voltage

    • The amplitude of the AC signal applied to the cell. The units are rms (root mean square) mV. To convert the entered value into a peak-to-peak value, multiply by 2√2 (or ~2.83). The resolution of the AC Voltage parameter and its range vary from system to system. In general, you can enter values between 1 mV and 1 V.The system does not control the AC Voltage exactly. At higher frequencies, the potentiostat often cannot maintain a one-to-one ratio between the AC signal at the potentiostat input and the resulting signal between the working and reference electrodes. The input signal may need to be 10 or even 100 times larger than the output signal. The EIS system automatically compensates for this effect. When it does so, it adjusts the applied signal such that the measured AC potential is close to being correct. It does not attempt to keep the measured E signal exactly equal to the AC Voltage parameter.In some cases, the potentiostat simply cannot apply the requested AC voltage. This occurs at high frequency on cells with high solution-resistance. An error message such as Unable to Control AC Cell Voltage should appear on the real-time display. If you see this message, try lowering your frequency, lowering the setting on the AC Voltage parameter, or lowering your cell’s solution resistance.

Frequency

  • The measurement frequency. The frequency is entered in Hertz. The allowed range for Frequency depends upon the potentiostat.
  • For a Reference 600, the allowed frequency-range is 10 mHz to 1 MHz.
  • For an Interface 1000T or 5000P, the maximum frequency is 20 kHz.

Estimated Z

  • A user-entered estimate of the cell’s impedance at the Initial Voltage. It is used to limit the number of trials required before acquiring the first data point in the Mott-Schottky Plot. Before taking the first data point, the EIS software sets up the potentiostat and FRA to measure an impedance equal to Estimated Z, and tries to measure the cell’s impedance. If the estimate is fairly accurate, the first (or second) attempt to measure the impedance will succeed. If the estimate is poor, the system may take five or more trial readings before it finds the correct settings. It is generally sufficient if Estimated Z is within a factor of 5 of the cell’s impedance.After the first data point, the last measured impedance is used to calculate new measurement settings, so the entered Estimated Z becomes unimportant.An accurate Estimated Z is more valuable at lower initial frequencies, where each measurement takes a long time. Remember, 1 mHz is 1000 seconds per cycle. Each impedance reading requires at least 3 cycles at a given frequency, so five readings to find a range at 1 mHz will take over 4 hours! The units for Estimated Z are ohms. There is no reason to enter values larger than 1012Ω or smaller than 0.01 Ω, because these values drive the system settings to their most sensitive and least sensitive settings, respectively.