Determination of Oxygen Reduction Reaction Process of La0.6Sr0.4CoO3-
δ
Porous Electrode Using Nonlinear Frequency Response Analysis
Authors: Sakai Marika , Budiman Riyan Achmad , Yamaguchi Mina , Yashiro Keiji , Kawada Tatsuya 
Published: December 2024

Electrochemical impedance spectroscopy (EIS) is a method to measure the performance of an electrochemical cell where linear responses are obtained when a small perturbation of current or voltage is applied to the cell. It is so often that different limiting steps of electrochemical processes give similar responses of the impedance spectrum. Therefore, the typical linear EIS measurements are not sufficient to understand the electrode reaction mechanism. On the other side, the frequency response analysis including nonlinearities can provide more information in addition to EIS, and is considered an advanced method to understand the reaction mechanism as reported by the research group of S. Adler [1,2]. In EIS, the small perturbation is applied to the cell so that the input and output can be regarded as a linear relationship. Meanwhile, in the non-linear electrochemical impedance spectroscopy (NLEIS) large perturbation is applied to the cell. In this way, NLEIS can visualize not only the fundamental waveform but also the changes in the harmonics, which is expected to enable more detailed analysis and reaction modeling. Using the above principles in potentiostatic mode, this study aims to elucidate the ORR process of La0.6Sr0.4CoO3-

δ
(LSC) porous electrode for SOFC cathode.
A symmetrical cell was prepared by depositing LSC porous electrodes on both sides of a Gd0.1Ce0.9O1.95 electrolyte. The experiments were performed in I-V measurements and NLEIS as a function of temperature (873 – 1073 K) under various p(O2) (1 – 10-4 bar). An electrochemical workstation (Zahner Zennium, Zahner-Elektrik GmbH & Co. KG, Germany) was used for the measurement to obtain the non-linear response. The obtained harmonics are normalized by the amplitude of the fundamental wave after Fourier transformation, and data are output up to the fifth harmonic. NLEIS was performed at frequencies from 0.1 Hz to 1M Hz.
The experimental results showed clear response at the second harmonic with the magnitude increasing and phase changing as the frequency decreased. Harmonic components tended to increase with decreasing p(O2) and increasing temperature. In addition to experiments, the simulation using an electric circuit simulator (LTspice, Analog Devices) was performed. As an equivalent circuit of a mixed-conducting porous electrode, a ladder circuit has been applied to analyze the nonlinear response. The currents related to ion diffusion were defined based on the Nernstian Einstein equation, and the currents related to surface reactions were defined based on an empirical model of oxygen adsorption and dissociation[3]. The result showed that the 1st harmonic and 2nd harmonic responses from the experimental result is in good agreement with the simulation results from LTspice at high p(O2) (1 – 10-2 bar) in the terms of the shape and phase of the response. At low p(O2) (10-3 – 10-4 bar), the effect of gas diffusion resistance could not be ignored.. The complete analysis on ORR process as a function of the p(O2) and temperature will be discussed after resolving technical issues related to computational convergence when gas diffusion resistance is included into the equivalent circuit.

References

[1] J. R. Wilson, D. T. Schwartz, S. B. Adler, Electrochim. Acta, 51, 1389–1402 (2006).
[2] J. R. Wilson, M. Sase, T. Kawada, S. B. Adler, Electrochem. Solid-State Lett., 10, B81-B86 (2007).
[3]T. Kawada, Current Opinion in Electrochemistry,
21, 274-282(2020)

Acknowledgement

This study is supported by ministry of environment, Japan, for Carbon Neutral Technology Research and Development Program.