Version 1.11.0 NOVA iR compensation tutorial 1 – The ...

1 Version NOVA iR compensation tutorial 1 The uncompensated resistance Potentiostats are instruments that are designed to control the potential of the working electrode (WE) relative to a non-polarizable reference electrode (RE). In order to achieve this potential control, the potentiostat will change the potential of the counter (CE) relative to the WE (which is at virtual ground), in normal operating conditions. The potential of the CE is set to the required voltage within the compliance voltage limits in order to keep the potential difference between the RE. and the WE equal to the user-defined value. For the following basic electrochemical cell (see Figure 1): Figure 1 An overview of an electrochemical cell (simplified).

2 R corresponds to the solution resistance and Rp is the polarization resistance (also known as the charge transfer resistance ). In order to keep the potential of the WE. at +1 V relative to the RE, the CE is set to V relative to the WE. Note A complete description of the working principle of a potentiostat can be found in the Electrochemical Methods Fundamentals and Applications handbook by L. Faulkner and A. Bard, 2nd edition, Wiley, New York, 2000. In practice, this means that the potentiostat will always compensate the solution resistance , within the limits of the compliance voltage. Unfortunately, in a real electrochemical cell, the reference electrode is always located at a distance relative to the working electrode.

3 This means that an additional resistance , the uncompensated resistance , Ru, can never be avoided completely (see Figure 2). 1|Page NOVA iR compensation tutorial Figure 2 An overview of an electrochemical cell (complete). The uncompensated resistance leads to an additional voltage drop, called the iR. drop, given by: = . Therefore, whenever a current is passed through the circuit described in Figure 2, there is always a potential control error due to the uncompensated resistance . If a cathodic current flows, the true potential of the working electrode is less negative than the specified potential. The opposite holds in the case of an anodic current. Even with a very low uncompensated resistance value, the voltage drop can become significant when the current is high 1.

4 How to minimize the uncompensated resistance Although iR drop cannot be avoided, it is possible to minimize its value. The following strategies can be used: 1. Use a supporting electrolyte with high conductivity: this will reduce the total resistance of the solution (and therefore also the uncompensated resistance ). 2. Reduce the size of the working electrode: the total current depends on the surface of the working electrode. 3. Use a Luggin capillary: this can be used to reduce the distance between the reference electrode and the working electrode, therefore reducing the uncompensated resistance . The iR compensation circuit An additional strategy that can be used to reduce the effects of the uncompensated resistance is to use the so-called positive feedback built into the Autolab potentiostat in order to partially compensate the iR drop.

5 The iR drop functionality is available in the Autolab PGSTAT series 2. In the Autolab PGSTAT, the iR compensation circuit is fitted with a DAC that can be used to compensate the iR drop. The voltage range of the compensation circuit is 0. 1. An Ru of 1 with a current of 100 mA leads to a voltage drop of 100 mV. 2. The option is not available with the PGSTAT10 and the Autolab II/III. 2|Page NOVA iR compensation tutorial 2 V. This means that the resistance range than can be compensated through this circuit is: ( ). =.. where is the current range. Automatic current ranging restrictions When the iR compensation circuit is used, the Automatic current ranging option cannot be used.

6 This restriction is verified during the validation process and an error will be displayed by the procedure validation if this situation is detected (see Figure 3). Figure 3 Automatic current ranging is not possible when the iR compensation circuit is used Using the iR compensation circuit in NOVA. It is possible to define the value of the resistance to compensate using the iR. compensation circuit in the Autolab control interface. Using the Autolab control command, it is possible to switch the iR compensation circuit on and off and to specify the value of the resistance to compensate. The Autolab control window displays the list of hardware settings for the potentiostat/galvanostat.

7 Using this interface, it is possible to set the current range and switch the iR compensation On or Off (see Figure 4). 3|Page NOVA iR compensation tutorial Figure 4 The Autolab control command can be used to switch the iR compensation circuit On or Off The iR compensation circuit can also be controlled using the Autolab display. Clicking the iR-C label in the Autolab display switches the iR compensation circuit On or Off (see Figure 5). Figure 5 Clicking the iR-C label in the Autolab control switches the iR compensation circuit On or Off The value of the resistance to compensate using the iR compensation circuit is also specified through the Autolab control window (see Figure 6).

8 4|Page NOVA iR compensation tutorial Figure 6 Defining the value of the compensated resistance Note The range of resistance values that can be compensated depends on the active current range. The value of the resistance to compensate can also be specified using the provided slider (see Figure 7). 5|Page NOVA iR compensation tutorial Figure 7 The compensated resistance can also be specified using the provided slider Note It is common practice to compensate about 90-95 % of the uncompensated resistance . 6|Page NOVA iR compensation tutorial 2 Determination of the uncompensated resistance , Ru In order to use the iR compensation circuit efficiently, the uncompensated resistance value must be estimated.

9 Several techniques can be used in order to determine this value. The following methods are available in Nova: i-Interrupt: the current interrupt method (i-Interrupt) is a quick method that can be used to determine the uncompensated resistance value. The technique involves switching off the current and measuring the decay of the cell potential. As soon as the current is switched off, the potential difference across the uncompensated resistance is zero and the charged double layer is discharged. By extrapolating the measured voltage decay to the beginning of the current interruption, the iR-drop can be calculated. Two i-Interrupt methods are available in NOVA and a standard Autolab procedure is available for both methods: o i-Interrupt: in this case, the voltage decay is measured using the ADC164 module.

10 O i-Interrupt high speed: measures the voltage decay using the fast sampling ADC module (ADC10M or ADC750). Positive feedback: the positive feedback technique is an interactive method based on the measurement of current response following the application of a potential pulse. The current response depends on the actual values of the uncompensated resistance and the double layer capacitance. compensation of Ru results in a faster decay of the charging current. When the compensation is near 100%, the measured current response will show damped oscillation. This method is therefore based on a trial and error approach. A standard Autolab procedure is provided for this method.