Impedance Measurement using EIS Technique
Electrochemical impedance spectroscopy (EIS) is commonly used impedance measurement technique to examine the interfacial chemistry at surfaces such as corrosion interfaces or battery electrodes. This is typically performed by applying a small sinusoidal potential and measuring the current response at frequencies ranging from below 1 Hz to MHz.3
An electrochemical interface can be modeled with a combination of electrical circuit elements. The simplest model is a Randles circuit that contains two resistors and a capacitor. The Warburg element, which represents diffusion, is omitted as it has no equivalent electrical circuit element. The PalmSens dummy cell has three test circuits, including a Randles cell with the nominal values shown in Figure 4c. Here, Rs represents the solution (electrolyte) resistance, Cdl represents the double layer (interface) capacitance, and Rct represents the charge transfer (interface) resistance.
EIS data is typically presented as a Nyquist or a Bode plot and then mathematical circuit fitting is used to identify the values of the elements of the equivalent circuit.
The EmStat Pico, a collaboration between Analog Devices and PalmSens BV, is a tiny (30.5 mm × 18 mm × 2.6 mm) system on module (SOM) potentiostat which continues this trend of size reduction. The device is built using Analog Devices technology, including the ADuCM355, ADP166, ADT7420, and AD8606.
Electrochemical sensor system development requires a knowledge of firmware, analog and digital electronics, and a familiarity with electrochemistry. This combination of knowledge is often not present in engineering departments. The EmStat Pico module allows the designer to skip the learning curve and shortcut development time by facilitating the integration of standard electrochemical measurements such as linear sweep voltammetry (LSV), squarewave voltammetry (SWV), or electrochemical impedance spectroscopy (EIS) into a product with minimal development time and effort. Given the increasing competition in the electrochemical sensors market, the module gives the developer a strong time to revenue advantage.
This article shows the ease of integration of the device into a system and demonstrates the range of applications of the potentiostat module by detailing three different electrochemical measurements: OCP (pH), cyclic voltammetry, and EIS.
The EmStat Pico is designed to be integrated into any microcontroller-based system using just four wires (5 V, ground, transmit, receive). Figure 1 shows example setups, firstly using an Arduino MKR as a master controller, and secondly using a USB to UART convertor to interface to a PC. In both setups, the EmStat Pico is connected with a screen printed electrode (SPE) for common electrochemical measurements such as cyclic voltammetry (CV).
The EmStat Pico development board shown in Figure 2 breaks out the SOM connections and adds a range of functionality including: battery power and SD card for standalone operation, USB and Bluetooth® communication options, real-time clock (RTC) for time-stamping, EEPROM for calibration data storage, and a header for direct insertion of an Arduino MKR.
For laboratory and test bench applications, the EmStat Pico can be operated by PSTrace PC software via a USB connection.
For OEM applications, communication is via the UART and the master can use the MethodSCRIPT™ EmStat Pico scripting language to control the EmStat Pico. This is a human-readable script for programming the EmStat Pico to run electrochemical techniques and perform other functions such as loops, data logging to SD, digital I/O, reading auxiliary values (for example, temperature), and sleep or hibernate. Method script code can be generated in PSTrace or written manually.
EIS Measurements using EMStat Pico
Typical measurement parameters:
- Excitation voltage: 10 mV p-p sine
- DC offset voltage: 100 mV
- Frequency range: 0.1 Hz to 100 kHz
- Current response: ±100 nA to ±1 mA
- EmStat Pico on development board
- Sensor cable: PalmSens sensor cable
- Randles equivalent circuit: PalmSens dummy cell
The sensor cable was inserted into CON8 of the EmStat Pico development board and the crocodile clip connections were attached to the Randles dummy cell, as shown in Figure 3.
The EmStat Pico was setup to perform an EIS measurement on PSTAT_0 with the following parameters: dc voltage: +1 V; sine: 10 mV p-p; frequency range: 10 Hz to 200 kHz.
A PSTrace equivalent circuit fitting, which used the Levenberg-Marquardt algorithm, was used to calculate the values of the electrical elements in the circuit.
Figure 4a shows the Bode plot of the Randles circuit in Figure 4c. At low frequency, the 10 kΩ resistance is dominant as the capacitor effect is small. At higher frequencies, the impedance drops to match the solution resistance as the capacitor becomes almost a perfect short.
Figure 4b shows the Nyquist plot of the data in blue and the theoretical model fitted to the data in orange. The values of the equivalent circuit elements calculated from the model are presented in Figure 4d. These match closely with the nominal values of the dummy cell. Note: resistor tolerance is 0.1%, capacitor tolerance is 5%.
The EmStat Pico is a versatile, user-configurable Potentiostat module capable of performing most common electrochemical measurements. It is presented in a small form-factor system on module package suitable for integration into miniaturized sensing systems. The device is built on Analog Devices technology, including the ADuCM355, AD8606, ADT7420, and ADP166. The EmStat Pico is available on https://embed.palmsens.com/product/emstat-pico-development-kit/
1 Tim Meirose. Essentials of pH Measurement. Thermo Fischer Scientific, 2019.
2 Allen J. Bard and Larry R. Faulkner. Electrochemical Methods: Fundamentals and Applications, Vol. 2. New York: John Wiley & Sons, Inc., December 2000.
3 Evgenij Barsoukov and J. Ross Macdonald. Impedance Spectroscopy: Theory, Experiment, and Applications. John Wiley & Sons, Inc., March 2005.