Enhancement of electrochemical performance and thermal compatibility of GdBaCo2/3Fe2/3Cu2/3O5+δ cathode on Ce1.9Gd0.1O1.95 electrolyte 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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Equal contribution of work.