Impedance spectroscopy of Gd-doped ceria analyzed by genetic programming (ISGP) method

doi:10.1016/j.ssi.2017.04.003

Solid State Ionics

Volume 304, June 2017, Pages 145-149

https://doi.org/10.1016/j.ssi.2017.04.003 Get rights and content

Highlights

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Impedance analysis is performed by Genetic Programming (ISGP).
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DFRT confirms the co-existence of space charge effect and defect associates at grain boundaries.
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Lower sintering temperature of doped ceria leads to smaller grain boundary potential.

Abstract

This work presents the distribution function of relaxation time (DFRT) analysis of Gd doped ceria (GDC) and cobalt co-doped GDC prepared by precipitation method. Ionic transport properties and grain-boundary phenomena are discussed thoroughly based on the DFRT. The impedance results, especially the bulk and grain-boundary conductivities (σ_b and σ_gb) and activation energies (E_b and E_gb) obtained from the ISGP, are compared with the values obtained from the Equivalent Circuit Model. Grain boundary space charge (SC) effects discussed so far in the literature, generally do not consider the defect interaction between the oxygen vacancies and acceptor dopants in ceria and other oxide ion conductors. However, ISGP study clearly evidence the co-existence of SC effect and defect association in grain boundary regions, and both contribute to the grain boundary resistance (R_gb) at lower temperatures. The effect of sintering aid (Co) on the grain boundary activity is discussed considering both phenomena. Lower sintering temperature of the samples results in a relatively smaller grain boundary potential (Ф(0)) i.e., 0.15, 0.17 and 0.19 V at 300 °C in 0, 1 and 3 mol% Co co-doped GDC, respectively.

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 (σ_b and σ_gb) and activation energies (E_b and E_gb) 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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Impedance spectroscopy of Gd-doped ceria analyzed by genetic programming (ISGP) method

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Solid State Ionics

J. Alloys Compd.

J. Solid State Chem.

Ceram. Int.

Sources

Solid State Ionics

Solid State Ionics

Solid State Ionics

Physica B

Solid State Ionics

Solid State Ionics

Solid State Ionics

J. Electroceram.

Fuel Cells

J. Alloys Compd.