Impedance spectroscopy of Gd-doped ceria analyzed by genetic programming (ISGP) method
Introduction
Impedance spectroscopy (IS) is one of the key characterization tools in the fields of electrochemical energy technology and electro-ceramics. Equivalent circuit models have been widely used for the analysis of IS. In the case where the relaxation time constant (τ) of two different phenomena are very close, their responses to applied AC field appear as overlapping semicircles or a single semicircle in the impedance spectra. In that case, it is difficult to distinguish between two different processes accurately by using an equivalent circuit model, and in general, many different configurations of equivalent circuits can provide very similar impedance spectra.
Our group has developed an evolutionary programming technique for the analysis of IS. The details of this so-called impedance spectroscopy by genetic programming (ISGP) are discussed in previous reports [1], [2], [3]. This method provides an analytic form of the underlying distribution function of relaxation times (DFRT, a.k.a. Γ) without the need to employ any filtering. In this work we have studied the relaxation time distribution in nanocrystalline 10 mol% Gd doped ceria (GDC) and CoO (1–3 mol%) co-doped GDC prepared by precipitation method, to understand the ionic transport processes through DFRT using ISGP analysis. A comparative study is performed based on ISGP results and that of equivalent circuit model.
Section snippets
Experimental
The nanocrystalline GDC powder was prepared by co-precipitation method. For the co-doped samples, 1 & 3 mol% CoO was added to the GDC powder by deposition precipitation method (abbreviated here as GDCCo1 and GDCCo3, respectively) [4], [5]. Details of the sample preparation are described in a previous report [6]. Surface area of nanoparticles (obtained by BET) was about 78.9, 119.6, and 124.5 m2/g for GDC, GDCCo1 and GDCCo3, respectively. The powders were pressed into pellets by applying a
Results and discussion
Typical Nyquist plots for GDC, GDCCo1 and GDCCo3 at various temperatures are shown in Fig. 1(a–f), which appear to consist of two semicircles at higher and intermediate frequencies and a spike at the lower frequency range. In a previous work, an equivalent circuit model was used to fit the imepdance spectra. The semicircles at higher and intermediate frequencies with capacitance values in the range of ~ 10− 12–10− 11 and 10− 10–10− 8 F, were attributed to the bulk and grain boundary conductivity,
Conclusions
Grain boundary properties of GDC and Co co-doped samples are studied thoroughly by ISGP analysis. DFRT indicates that at lower temperatures, both the space charge effect and defect association between oxygen vacancies and dopants contribute to the grain-boundary activity of doped ceria. Relaxation times differ between the two effects by about one order at 200 °C, which reduces at higher temperatures. Conductivities (σb and σgb) and activation energies (Eb and Egb) obtained previously from
Acknowledgements
We acknowledge the support from the Nancy and Stephen Grand Technion Energy Program (GTEP), Israel Science Foundation (ISF) under grants 938/15 and the 2nd Israel National Research Center for Electrochemical Propulsion (INREP 2).
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2023, Chemical Engineering JournalCitation Excerpt :It is an electrochemical AC impedance analysis method that does not depend on the characteristics of the object, and can be used to separate the information of different components in the impedance spectrum of electrochemical system [27,28]. Currently, it is implemented in electrochemical systems such as solid oxide fuel cell [29], solid oxide water electrolyzer [30], battery [31], PEMFC [32,33], Gd-doped ceria [34] and supercapacitors [35] with ISGP methods. Weber et al. [33] applied DRT technology to the research of low-temperature PEMFC for the first time.