Description
The Get Ru utility includes a script that uses Electrochemical Impedance Spectroscopy (EIS) to make a determination of Uncompensated Resistance Ru.
Electrochemical test cells always have a solution resistance controlled by the cell's geometry and the composition of the cell's electrolyte. Current flow through this solution resistance can cause significant errors in the cell's measured potential. A three-electrode potentiostat compensates for some of the cell's solution resistance through placement of the cell's reference electrode. Because reference electrodes are not infinitely small and they cannot be placed infinitely close to the working electrode, this compensation is always incomplete. The portion of the total solution resistance that is left uncompensated is called the Uncompensated Resistance Ru.
You can use positive feedback iR-compensation to correct for the effects of Ru - if you know the Ru-value. In essence, positive feedback iR-compensation circuitry, at a desired potential Ed, applies a potential Ed + iRu (where i is the measured cell current). With this voltage applied between the working and reference electrodes, the electrochemical interface at the working electrode has a potential of Ed.
Unlike some other iR-compensation methods, ideally current feedback iR-compensation is fast, quiet, dynamically corrects for the iRu error as the cell current changes, and the actual voltage applied to the electrode interface is the same as the potentiostat's input voltage.
However, there are at least two problems with positive feedback iR-compensation:
•You must determine or calculate the value of Ru before you can apply the method.
•Excessive compensation can cause instability and oscillation in the potentiostat's electronics.
Run this utility by selecting Experiment > Utilities > Get Ru on the Framework™ menu bar. The Get Ru window opens:
Parameter |
Description |
Units |
|
|---|---|---|---|
Select the potentiostat/galvanostat to perform the experiment. Each radio button corresponds to an installed potentiostat. You can select only one potentiostat at a time. Potentiostats that are already in use are marked with an asterisk. They can be selected but cannot be used. |
|
||
The voltage applied to the working electrode during the measurement. Enter the voltage either versus the reference electrode potential (vs. Eref) or open-circuit potential (vs. Eoc). |
volts (V) |
||
A user-entered estimate of the cell's impedance at the Initial Freq. parameter. It is used to limit the number of trials required before acquiring the first data point in an impedance spectrum. It is generally sufficient if Estimated Z is within a factor of five of the cell's impedance.
|
ohm |
|
Carefully select the potential used for determining Ru should be free from electrochemical reactions. In most systems 0 V vs. Eoc (open-circuit potential) has no interference from reactions. |
For example, the figure below shows the Cyclic Voltammogram of ferrocyanide on a platinum electrode in 0.1 M KCl. Notice the reduction peak beginning at +0.3 V vs. SCE. Above this voltage, we see no faradaic current. For a system like this, we recommend determining Ru at 0.5 V vs. SCE.
In EIS, a small sinusoidal voltage is applied to the cell. The resulting current should be a sinusoid at the same frequency, but different amplitude and phase. If the cell impedance at the applied voltage and frequency is primarily resistive, the phase-angle between the voltage and current should be nominally zero. Phase-angles significantly more positive than zero indicate onset of potentiostat speed limitations or inductive behavior in the cell. Phase-angles significantly below zero are characteristic of capacitive, kinetic, or diffusional control of the cell impedance.
Ideally, measure Ru at the highest frequency at which the phase-angle of the cell impedance is zero degrees. At this frequency, Ru is the "real" portion of the cell impedance.
Note that many systems have quite a low Ru-value, indicating that iR-compensation is not required. The system shown in the figure above had a measured Ru of about 8 Ω. A typical CV current in this system was 10 µA. At this current, the error in the applied voltage in the absence of iR-compensation is 80 µV. In general, ignore iR errors less than 1 mV, as in this case.
The range and resolution of the positive feedback iR-circuit is limited. Assume for the moment that you need positive feedback iR-compensation for a Cyclic Voltammetry (CV) experiment. CV is normally done using a fixed current measurement range. There is an equivalent-current measurement resistance associated with each range. This resistance is known as Rm. The tables below list the Rm-values for all current ranges of each Gamry potentiostat:
Rm (Ω) |
|
|---|---|
10 nA |
1·107 |
100 nA |
1·106 |
1 μA |
1·105 |
10 μA |
1·104 |
100 μA |
1·103 |
1 mA |
100 |
10 mA |
10 |
100 mA |
1 |
1 A |
0.1 |
Rm (Ω) |
|
|---|---|
60 pA |
2·109 |
600 pA |
2·108 |
6 nA |
2·107 |
60 nA |
2·106 |
600 μA |
2·105 |
6 μA |
2·104 |
60 μA |
2·103 |
600 μA |
200 |
6 mA |
20 |
600 mA |
2 |
600 mA |
0.2 |
Rm (Ω) |
|
|---|---|
300 pA |
5·108 |
3 nA |
5·107 |
30 nA |
5·106 |
300 nA |
5·105 |
3 μA |
5·104 |
30 μA |
5·103 |
300 μA |
500 |
3 mA |
50 |
30 mA |
5 |
300 mA |
0.5 |
3 A |
0.05 |
In general, avoid entering the full measured Ru-value into tests that use positive feedback iR-compensation. A potentiostat is likely to oscillate when fully iR-compensated. We recommend that 75 to 95% of the measured Ru-value be applied as compensation. In our example system, with an 8 Ω Ru, the positive feedback value, if entered at all, would therefore be entered as a value between 6 Ω (at 80% compensation) or 7.6 Ω (at 95% compensation)
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