Electrochemical impedance spectroscopy (EIS) is one of the principle technique which is capable of in situ analysis of electrode – electrolyte interface and of probing electrochemical reaction mechanism. Typically, small amplitude perturbations are employed and the response is analyzed assuming that the system behavior is linear and stable [1, 2]. If large amplitude perturbations are employed, the resulting current will have significant higher harmonic signals, which can provide more information about the system examined [3]. This is often referred to as nonlinear EIS (NLEIS).

In this work, we present NLEIS analysis of an unstable system, where potential drifts and concentration changes are introduced. The classical Fe²⁺/Fe³⁺ redox couple is chosen for experimental and numerical analyses. A setup comprising of three instruments (Potentiostat, Lock-in amplifier and signal analyzer from Stanford Research System) to acquire the NLEIS data with the help of an in-house developed software was established. The applied perturbation frequency was varied from 100 kHz to 125 mHz and the amplitude was varied from 20 mV to 100 mV. The current magnitude and phase at fundamental and higher harmonics were measured using FFT analyzer. Instabilities were introduced by continuously changing the dc potential or the active species concentrations, during the NLEIS measurements. All the solution contained 100 mM Na₂SO₄ as supporting electrolyte. The initial concentration of Fe²⁺/Fe³⁺ species was 5 mM each. Figure 1 shows the experimental results with fixed as well as changing concentrations. In case of experiments denoted as ‘changing concentration’, the active species concentrations at any time ‘t’, min, were given by C = (200+ 3 t)^-1 M. The governing equations were solved without linearization, using numerical methods. Both mass transfer and reaction kinetics effects were taken into consideration and the experimental results were compared with the numerical solutions.

Keywords: nonlinear EIS, system instabilities, potential drift, mass transfer effect

References

[1]. Orazem, M., Tribollet, B., Electrochemical Impedance Spectroscopy, John Wiley & Sons, NJ, (2008)

[2]. Wilson, J. R., Sase, M., Kawada, T. Adler , S.B., Electrochem. Solid-state Lett. 10 (5) (2007) B81-B86.

[3]. Fathima Fasmin and Ramanathan Srinivasan, J Electrochem. Soc., 164 (7) H443-H455 (2017)