Elsevier

Journal of Power Sources

Volume 274, 15 January 2015, Pages 318-323
Journal of Power Sources

Effect of the symmetric cell preparation temperature on the activity of Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as cathode for intermediate temperature Solid Oxide Fuel Cells

https://doi.org/10.1016/j.jpowsour.2014.10.064Get rights and content

Highlights

  • BSFCu is prepared in one step at 850 °C by a modified gel-combustion route using EDTA.

  • The cubic nanocrystalline material is used to prepare symmetrical cells with CGO.

  • The effect of the cell preparation temperature on the cathode activity was evaluated.

  • The ASR of cells prepared, follows the tendency: ASR900°C < ASR950°C < ASR1000°C.

  • Cells prepared at 900 °C shows ASR values as low as 0.035 (1) Ω cm2 at 700 °C.

Abstract

In this work we studied the electrochemical performance of Ba0.5Sr0.5Fe0.8Cu0.2O3-δ (BSFCu) as cathode for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) with Ce0.9Gd0.1O1.95 (CGO) electrolyte and the effect of the symmetric cell preparation temperature on the oxygen reduction reaction (ORR) activity. Symmetrical cells with the configuration BSFCu/CGO/BSFCu were prepared at 900 °C, 950 °C and 1000 °C to perform the electrochemical characterization in the 500–700 °C temperature range. The resultant area specific resistance (ASR) of the cells with different preparation temperatures followed the tendency: ASR900°C < ASR950°C < ASR1000°C. The symmetric cell constructed at 900 °C showed ASR values of 0.18, 0.078 and 0.035 Ω cm2 at 600, 650 and 700 °C respectively, which demonstrated superior electrochemical activities than previous reports. Additional, X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM) techniques were used to characterize the microstructure of the original and fired BSFCu materials and correlate it with the cell preparation temperature.

Introduction

Solid Oxide Fuel Cells (SOFCs) convert hydrogen and hydrocarbon fuels into electricity and heat, through an electrochemical reaction, with high efficiency and low emission of pollutants [1], [2]. These devices are considered as one of the most promising candidates for power generation from large stationary plants to small and portable distributed generation applications with superior benefits than other fuel cells. However, the high manufacturing cost and materials long term stability have prevented its commercial viability. Nowadays, one of the research targets for SOFCs is lowering the operation temperature which can improve the lifetime of the cell/stack, allows the use of cheaper interconnecting and structural components (such as stainless steel) and facilitates the gas sealing [3].

The current research target is focused on the development of electrode materials for intermediate temperature SOFCs (so called IT-SOFCs) operated between 500 and 700 °C. Lowering the operating temperature, however, reduces electrode kinetics and increases the interfacial polarization resistances, particularly on the cathode side of the cell. For this reason, a significant effort is being made to prepare cathode materials with high activity for the oxygen reduction reaction (ORR) in this temperature range [4]. The best cathode materials that have been described are based on the simple perovskite structure ABO3-δ, with cation disorder in the A and B sites and a significant proportion of disordered oxygen vacancies. Generally, these materials are mixed ionic–electronic conductors (MIECs) favoring the reduction of the cathode area specific resistance (ASR) [5]. The lowest ASR values have been reported for cathodes with mixed alkali–earths and/or lanthanides in the A site and mixed cobalt and iron in the B site of the perovskite structure such as (Ba,Sr)(Co,Fe)O3-δ (BSCF) [6], [7], [8], (La,Sr)(Co,Fe)O3-δ (LSFC) [9], [10], [11], [12], [13] and LnBaCo2O5+δ [14], [15], [16], [17].

Unfortunately, these cathode compositions with high cobalt content show large thermal expansion coefficients (TEC), much larger than those of state-of-the-art electrolytes and interconnector materials which are available for IT-SOFC applications (i.e. gadolinia-doped ceria (CGO), (La,Sr)(GaMg)O3-δ (LSGM), yttria-stabilized zirconia (YSZ) and ferritic stainless steels) limiting the cell lifetime [18]. Moreover, the easy reduction and evaporation, and the high cost of cobalt are also significant issues. For these reasons, several efforts have recently been made to develop cobalt-free cathodes with high electrocatalytic activity for the ORR process, with similar TEC to the other cell components and structural stability in the operation conditions.

Iron-based perovskite Ba0.5Sr0.5FeO3-δ has also attracted much attention due to its lower TEC and superior structural stability than cobalt-based materials [19]. However, this material presents a lower activity due to the low electrical conductivity and oxygen permeation [20]. Efimov et al. [21] and Zhao et al. [22], [23] were the first to report Ba0.5Sr0.5Fe0.8Cu0.2O3-δ (BSFCu) as a novel cobalt-free perovskite for IT-SOFC. These authors report a high electrical conductivity with a maximum value between 45 and 57 S cm−1 at 600 °C, which exceeds the acceptable limit for cathode applications (≈10 S cm−1). Zhao et al. reported a good electrochemical performance with an ASR value of 0.137 Ω cm2 at 700 °C working with samaria-doped ceria electrolyte (CSO) and a maximum power density of 718 mW cm2 at 700 °C in a BSFCu/CSO/NiO–CSO cell.

However, the effect of the cell preparation temperature has not been studied for BSFCu and its influence on the cathode activity is poorly discussed in the literature for SOFCs. From our point of view this is a key parameter in the SOFCs construction process that needs to be evaluated in detail, mainly when nanostructure materials are used in the cell. The preparation temperature may affect the grain size of the cathode material and the cathode/electrolyte interface, so it is important to optimize this parameter to achieve high ORR activities that enable SOFCs to operate at lower temperatures.

In this work we studied the electrochemical performance of the Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as cathode for IT-SOFC and the effect of the symmetric cell preparation temperature on the cathode ORR activity. The materials Ba0.5Sr0.5Fe0.8Cu0.2O3-δ and Gd0.1Ce0.9O1.95 were synthesized by a modified gel-combustion route using EDTA as chelating agent and NH4NO3 as combustion promoter. A detailed structural and microstructural characterization of the BSFCu powder was performed by XRD and TEM. The main purpose of this work is to demonstrate that optimizing a parameter often neglected as the cell preparation temperature is important to improve the cathode performance.

Section snippets

Preparation of cathode material: Ba0.5Sr0.5Fe0.8Cu0.2O3-δ

Nanocrystalline Ba0.5Sr0.5Fe0.8Cu0.2O3-δ (BSFCu) powder was synthesized by a modified gel-combustion route using Ba(NO3)2, Sr(NO3)2, Fe(C5H7O2)2 and Cu(NO3)2.3H2O (all >99.9%, from Sigma–Aldrich) as metal sources. The synthesis of BSFCu was performed by dissolving stoichiometric amounts of the mentioned compounds in distilled water in a pyrex-glass beaker. Excess EDTA (99.4–100.6 %, Sigma–Aldrich) was added in a 1.1:1 EDTA/metal ion ratio. In this synthesis route the EDTA plays a role as fuel

Structural and morphological characterization

Ba0.5Sr0.5Fe0.8Cu0.2O3-δ has a perovskite-type structure that crystallizes in the cubic Pm-3m (#221) space group. In the ideal cubic perovskite structure, the B cations are 6-fold and the A cations 12-fold coordinated, with the oxygen anions. In this compound, however, Basbus et al. [28] reported a presence of ∼1/8 oxygen vacant sites (δ ≈ 0.375) that reduces the average coordination number of Sr/Ba to 10 and of Fe/Cu to 5 and determines the average charge of Fe/Cu site to be 3.25+. The large

Conclusions

We synthesized nanostructured Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode material by an alternative gel-combustion route at low temperature. The cathode electrochemical performance was studied as function of the symmetric cell preparation temperature and the cathode ASR was calculated for cells prepared at 900, 950 and 1000 °C. The ORR activity of the Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode was enhanced by lowering the symmetric cell preparation temperature (ASR900°C < ASR950°C < ASR1000°C). The symmetric cell

Acknowledgments

This work was supported by Agencia Nacional de Investigación e Innovación (Uruguay) through grant FSE_2009_1_51 and Laboratorio Nacional de Luz Synchrotron (LNLS, Campinas, SP, Brazil) through a grant to perform research proposal XRD1-15297 at D10B-XPD beamline. LS is also indebted to PEDECIBA. SV acknowledges the financial support of Espacio Interdisciplinario-Universidad de la República. The authors are grateful to Dr. Alberto Caneiro for the use of his laboratories for symmetrical cell

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