Reducing the resistance for the use of electrochemical impedance spectroscopy analysis in materials chemistry

Abstract

Electrochemical impedance spectroscopy (EIS) is a highly applicable electrochemical, analytical, and non-invasive technique for materials characterization, which allows the user to evaluate the impact, efficiency, and magnitude of different components within an electrical circuit at a higher resolution than other common electrochemical techniques such as cyclic voltammetry (CV) or chronoamperometry. EIS can be used to study mechanisms of surface reactions, evaluate kinetics and mass transport, and study the level of corrosion on conductive materials, just to name a few. Therefore, this review demonstrates the scope of physical properties of the materials that can be studied using EIS, such as for characterization of supercapacitors, dye-sensitized solar cells (DSSCs), conductive coatings, sensors, self-assembled monolayers (SAMs), and other materials. This guide was created to support beginner and intermediate level researchers in EIS studies to inspire a wider application of this technique for materials characterization. In this work, we provide a summary of the essential background theory of EIS, including experimental design, signal responses, and instrumentation. Then, we discuss the main graphical representations for EIS data, including a scope of the foundation principles of Nyquist, Bode phase angle, Bode magnitude, capacitance and Randles plots, followed by detailed step-by-step explanations of the corresponding calculations that evolve from these graphs and direct examples from the literature highlighting practical applications of EIS for characterization of different types of materials. In addition, we discuss various applications of EIS technique for materials research.

Fig. 1. (A) Graphical representation of the equations E(t) = |E₀| sin(ωt) + E_app and I(t) = I₀ sin(ωt + φ), depicting their time dependence. (B) E(t) graphically presented on the same axis of rotation as I(t) to make visible the phase angle (φ)'s impact on the time-scale position of I(t) compared to E(t).

Fig. 2. Sample simple circuits: (A) without faradaic process and (B) with a faradaic process.

Fig. 3. Randles equivalent circuit model.

Fig. 4. Transmission line model (TLM) of a porous working electrode.

Fig. 5. (A) Nyquist plot with key regions labelled. (B) Expansion of Nyquist plot region 2 with typical observed Warburg impedances occurring for different types of electrode materials. Inset of (B) shows how Nyquist plot would appear when displaying capacitive behaviour. (C) Calculation of R_Σ using a linear line fit of the Warburg region.

Fig. 6. Diagram of the partial charge blocking across a SAM on a conductive substrate where (A) demonstrates the SAM layer formation with increasing surface coverage, and (B) gives the correlated Nyquist diagram.

Fig. 7. (A) Capacitance plot featuring two separate impedance measurements where bias 1 corresponds to a measurement with a faradic response, and bias 2 represents a measurement with no faradaic response in the double layer. (B) Sample cyclic voltammogram from which the bias potentials of C_dl and C_T can be determined. (C) Normalized capacitance plot demonstrating how the capacitance measurements from (A) would appear if normalized to their maximum value.

Fig. 8. Example of a Randles plot with the line fitting in the low-frequency region.

Fig. 9. (A) Sample curve responses for a Bode phase angle plot. (B) Sample Bode magnitude plot.