Optimization of the Perturbation Amplitude for EIS Measurements Using a Total Harmonic Distortion Based Method

Abstract

[EN] Ohm's generalized law defines the concept of impedance. This law, and thus the definition itself, are only valid if the system fulfills the linearity condition. However, electrochemical systems are typically highly nonlinear. Consequently, the linearity condition can only be achieved in these systems if a low perturbation amplitude is used for performing EIS measurements. Nevertheless, the use of low amplitude perturbations leads to low signal-to-noise ratios, which result in high measurement errors. The concept of optimum amplitude arises from this tradeoff: the perturbation has to have an amplitude big enough in order to minimize the measurement errors (i.e. maximize the SNR), but at the same time, the perturbation has to have an amplitude small enough to avoid the generation of significant nonlinear effects that would distort the measured EIS spectra. In a previous work, a linearity assessment quantitative method based on the total harmonic distortion parameter was developed. In this work, the aforementioned THD method was applied for the perturbation amplitude selection for EIS measurements in a highly nonlinear model system: the cathodic electrode of an alkaline water electrolyzer. The THD method successfully obtained the optimum amplitudes both, for a constant amplitude strategy and for a frequency dependent strategy. The THD method also allowed to obtain the noise structure and to quantify the nonlinear effects. This method is slightly superior to the U-P method, a method based on the harmonic analysis of the output signal that was developed in earlier works. (C) 2018 The Electrochemical Society.