Enhancement of electrochemical performance and thermal compatibility of GdBaCo2/3Fe2/3Cu2/3O5+δ cathode on Ce1.9Gd0.1O1.95 electrolyte for IT-SOFCs

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Abstract

Transition-metal doped double-perovskite structure oxides GdBaCo2/3Fe2/3Ni2/3O5+δ (FN-GBCO), GdBaCo2/3Fe2/3Cu2/3O5+δ (FC-GBCO), GdBaCoCuO5+δ (C-GBCO) and pristine GdBaCo2O5+δ (GBCO) were synthesized via a citrate combustion method. The thermal-expansion coefficient (TEC) and electrochemical performance of the oxides were investigated as potential cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The TEC exhibited by the FC-GBCO cathode up to 900 °C is 14.6 × 10−6 °C−1, which is lower than the value of GBCO (19.9 × 10−6 °C−1). Area specific resistances (ASR) of 0.165 Ω cm2 at 700 °C and 0.048 Ω cm2 at 750 °C were achieved for the FC-GBCO cathode on a Ce0.9Gd0.1O1.95 (CGO) electrolyte. An electrolyte supported (300 μm thick) single-cell configuration of FC-GBCO/CGO/Ni-CGO attained a maximum power density of 435 mW cm−2 at 700 °C. The unique composition of GBCO co-doped with Fe and Cu ions in the Co sites exhibited reduced TEC and enhancement of electrochemical performance and good chemical compatibility with CGO, and this composition is proving to be a potential cathode for IT-SOFCs.

Introduction

Intermediate-temperature solid oxide fuel cells (IT-SOFCs) with high working efficiency at 500–750 °C have gained considerable importance in the area of power fabrication [1], [2], [3], [4]. Recently, simple perovskite-type mixed ionic-electronic conducting (MIEC) oxides, such as doped BaCoO3, LaCoO3 and LaFeO3 have attracted attention as candidate cathode materials. In MIEC oxides, the penetration of conducting oxygen-ions into the electrode bulk greatly increases the active sites for oxygen-reduction, which lowers interfacial polarization [5]. More recent studies on new types of MIEC oxides, i.e. cation ordered LnBaCo2O5+δ (Ln = Gd, Pr, Y and La) with two dimensional ion-diffusion channels have shown them to be attractive potential cathode materials for IT-SOFCs. In particular, GdBaCo2O5+δ (GBCO) has been reported to exhibit ionic conductivity due to the oxygen vacancies that are mainly located in the rare-earth planes [GdO]x and high electronic conductivity due to the metal–insulator transition (at 87 °C) [6], [7], [8].

The GBCO exhibits a high thermal-expansion coefficient (TEC) 20.1 × 10−6 °C−1 [9], as commonly observed in cobalt-based oxides, which is undesirable for practical use as a cathode for SOFCs [10]. Hence, a systematic investigation on the optimization of TEC and electrochemical performance is essential for the double-perovskite oxides to be used as potential cathodes in IT-SOFCs. In the present study, oxides of Fe, Ni and Cu doped GdBaCo2O5+δ were synthesized via a citrate combustion method. The thermal and chemical compatibilities of the cathodes with Ce0.9Gd0.1O1.95 (CGO) and the area specific resistance (ASR) by electrochemical impedance spectroscopy were studied. A single-cell electrochemical performance of the FC-GBCO cathode on the CGO electrolyte support and Ni-CGO as the anode was also investigated.

Section snippets

Experiment

Double-perovskite oxides of GdBaCo2/3Fe2/3Ni2/3O5+δ (FN-GBCO), GdBaCo2/3Fe2/3Cu2/3O5+δ (FC-GBCO), GdBaCoCuO5+δ (C-GBCO) and pristine GdBaCo2O5+δ (GBCO) were synthesized via the citrate combustion method. Analytical grade Gd(NO3)3⋅6H2O (>99.9%), Ba(NO3)2 (99 + %), Co(NO3)2⋅2.5H2O (98 + %), Fe(NO3)3⋅9H2O, Ni(NO3)2⋅6H2O and Cu(NO3)3⋅6H2O were used as precursors for the synthesis of oxide powders, and citric acid (99%) was used as fuel for the combustion reaction.

Initially, stoichiometric amounts of

Result and discussions

Fig. 1a shows the XRD patterns of the doped GBCO powders calcined at 900 °C for 20 h. The XRD patterns of pristine and doped GBCO oxides exhibit phase pure, high crystalline double-perovskite structure with no peaks attributable to impurities detected [11], [12], [13]. All the diffraction peaks of GBCO and FN-GBCO can be indexed based on the JCPDS #53–0135 with an orthorhombic symmetry, space group Pmmm. On the other hand, FC-GBCO and C-GBCO exhibited tetragonal symmetry, space group P4/mmm [11],

Conclusions

The double-perovskite FN-GBCO, FC-GBCO C-GBCO and GBCO cathode materials were synthesized via the citrate combustion method. The FC-GBCO cathode exhibits a relatively reduced TEC value of 14.6 × 10−6 °C−1 compared with that of GBCO. The doping ions Fe and Cu retain the oxygen stoichiometry within the range of 0.25 < δ < 0.45, even above 700 °C and possess equivalent or better ASR values compared with GBCO. The maximum power densities of the electrolyte supported single-cell configuration of

Acknowledgements

This work was financially supported by the Korea Research Foundation Grant funded by the Korean government (MOEHRD; KRF-2008-005-J00903). This work was partially supported by Brain Korea 21 (BK21) program from Korean Ministry of Education.

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