Student authors: Han Le 2019
How impedance spectroscopy works
Impedance (Z) is quite similar to resistance: it is a measure of the ability of a circuit to resist current. Resistance is a concept for ideal resistors, but many circuits are more complex (they do not exactly follow Ohm's Law, independent of frequency, no phase shift between current and voltage signals), so impedance is used to replace resistance instead. Impedance takes into account all the considerations limited to an ideal resistor and other factors such as inductance, resistance, and capacitance. During electrochemical impedance spectroscopy (EIS), an AC voltage is applied to a sample at different frequencies and the electrical current is measured. Impedance (Z) can then be calculated as the ratio of the frequency-dependent potential (E) to the frequency-dependent current (I). This technique allows for multiple frequency measurements. It can be used to probe different electrochemical processes happening at the same time, electron transfer rate of reaction, diffusion-limited reactions, or capacitive behavior of a system. Some of the applications of EIS includes detecting corrosion of metals, characterizing aging of food, measuring bacterial concentration in label-free biosensors, and studying ion mobility in batteries and supercapacitors.
How to interpret the data
Since impedance can be expressed as a complex number, it can be plotted as a Nyquist plot (the real part on the x-axis, and the imaginary part on the y-axis). Most of the times this plot is a semicircle, which signifies a charge transfer process, and the size of the semicircle represents how much charge transfer resistance we have. However, this plot can also looks like a straight line with positive slope if it is a diffusion process (called Warburg impedance). This plot is also dependent on the circuit (parallel, series, or combinations). Apart from Nyquist plots, impedance can also be presented as a Bode plot.
Figure 1: Nyquist plot of a RC parallel circuit. The arrow indicates increasing angular frequencies. Image author: Enseeg (Wikipedia), used under Creative Commons Attribution-Share Alike 4.0 International license.
Good literature examples
- Long, D.; Li, W.; Ling, L.; Miyawaki, J.; Mochida, I.; Yoon, S.-H. Langmuir 2010, 26(20), 16096–16102.
- Basics of Electrochemical Impedance Spectroscopy. https://www.gamry.com/application-no...-spectroscopy/ (accessed Feb 8, 2019).
https://www.jlab.org/conferences/tfs...ro%20Reece.pdf (accessed Feb 8, 2019).
Useful resources for in-depth reading
Chang, B.-Y.; Park, S.-M. Annual Review of Analytical Chemistry 2010, 3(1), 207–229.
Park, S.-M.; Yoo, J.-S. Analytical Chemistry 2003, 75(21).
Chulkin, P.; Data, P. Journal of Visualized Experiments 2018, No. 140.