Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria

Soumitra Sulekar,Juan Claudio Nino,Ji Hyun Kim,Mehrad Mehr

doi:10.3390/inorganics9080063

Abstract

Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries.

Highlights

Rare-earth metal doped cerium oxide (CeO2 ) has received considerable attention in the past decades due to exciting possibilities for application as the electrolyte in intermediate temperature solid oxide fuel cells [1,2]
The ionic conductivity in CeO2 is mediated by the presence of oxygen vacancies obtained by doping with rare earth acceptor dopant oxides such as Gd2 O3, Nd2 O3, and Sm2 O3, which have been found to be some of the best dopants for ceria
The hypothesis is that by sintering pressed gadolinia doped ceria (GDC) pellets in a reducing atmosphere of 4% H2 -N2, the Ce+4 ions will get reduced to Ce+3

Summary

Introduction

Rare-earth metal doped cerium oxide (CeO2 ) has received considerable attention in the past decades due to exciting possibilities for application as the electrolyte in intermediate temperature solid oxide fuel cells [1,2]. Doped ceria affords enhanced surface kinetics for oxygen reduction reactions, and has been proposed for use as an electrolyte-cathode interlayer to improve operating temperatures for solid oxide fuel cells [6,7]. The ionic conductivity in CeO2 is mediated by the presence of oxygen vacancies obtained by doping with rare earth acceptor dopant oxides such as Gd2 O3 , Nd2 O3, and Sm2 O3 , which have been found to be some of the best dopants for ceria. The vacancies allow for hopping of oxide ions under a concentration gradient, giving ionic conductivity

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