Ohmic iR drop

In NOVA and in INTELLO, it is possible to manually calculate the uncompensated resistance using EIS and then introduce this value in the appropriate section of the Apply Settings (INTELLO) or Autolab Control commands (NOVA).

The EIS measurement was conducted at 0 V vs OCP (open circuit potential), with an amplitude of 5 mV (V_TOP). A frequency range of 100 kHz to 100 Hz was used.

Usually, the uncompensated resistance can be obtained from both the Nyquist and Bode plots. In the former, it is the intersection of the data with the x-axis at the high frequency end, while in the latter, it is the plateau at the high frequency end in the |Z| vs log f plot. In Figure 1, the annotated Nyquist plot is shown with the high frequency end highlighted.

This returns an uncompensated resistance of 87 Ω. If for any reason it is not possible to read off the value from the plots, then the value can also be obtained from the Fit and Simulation tool in NOVA (Figure 2).

Once measured, R_u (the uncompensated resistance) should be introduced in the Apply Settings command in INTELLO (Figure 3). It is recommended to apply a maximum of between 80% and 90% of the total value in order to avoid having the system go into oscillation. In INTELLO, the desired percentage can be entered alongside the measured value, and the compensated resistance to be applied (R_c) will be automatically calculated.

A new command is available in INTELLO called «Measure iR Drop» (Figure 4). This command automatically measures and then compensates for the iR drop at the start of the measurement, assuming that the iR compensation option is activated in a subsequent Apply Settings command.

In order to obtain the uncompensated resistance (R_u), the command executes a single-frequency EIS measurement. The frequency is user-defined, though the default is 30 kHz. It is important to ensure that the chosen frequency is appropriate for the system by first performing a complete EIS measurement. For this measurement, it is recommended to set the DC potential to a value near the OCP and allow enough stabilization time (no superimposed sine) at this potential for the most accurate measurement. In order to avoid overcompensation and therefore oscillations of VIONIC, it is possible to set an acceptable range of values (safety limits) for R_u. Maximum and minimum values can be input. It is recommended to provide at least a maximum value for the uncompensated resistance (usually an estimated value plus 10%). If the measured value of R_u is not between the specified values, a warning message will appear. It is possible to stop the procedure or continue with the maximum input value. If the measured value of R_u is within the specified range, either no message will appear and the procedure will continue, or the acceptance window will appear for validation depending on the user’s preference and settings.

In Figure 5, the acceptance window is shown. This window shows the real part of the impedance (WE.Z’), the imaginary impedance (WE.-Z”) and the total impedance (WE.|Z|). The real part of the impedance is analogous to the uncompensated resistance, R_u. The percentage of this value to be compensated is also shown. Finally, R_c, the value which will be compensated, is shown here. If these values are acceptable to the user, the measurement can continue by pressing the Accept button. Note that the values should be considered acceptable if WE.Z’ and WE.|Z| are very close to each other, indicating that most of the impedance is purely resistive. This can also be observed in the phase (WE.-Phase), which should be close to 0.

The effect of iR drop compensation is illustrated in Figure 6 with the CV of K₄[Fe(CN)₆] in ultrapure water. In blue, the iR drop is not compensated for, resulting in an increase in the peak-to-peak separation and a lower peak current. In extreme cases, without iR drop compensation it may be wrongly assumed that a redox process is irreversible, when in fact it is reversible. Shown in orange is the iR-drop corrected cyclic voltammogram, where the peak-to-peak separation is consistent with a one-electron reversible process.