Elsevier

Electrochimica Acta

Volume 78, 1 September 2012, Pages 466-474
Electrochimica Acta

Systematic evaluation of Co-free LnBaFe2O5+δ (Ln = Lanthanides or Y) oxides towards the application as cathodes for intermediate-temperature solid oxide fuel cells

https://doi.org/10.1016/j.electacta.2012.06.073Get rights and content

Abstract

Co-free oxides with a nominal composition of LnBaFe2O5+δ, where Ln = La, Pr, Nd, Sm, Gd, and Y, were synthesized and phase structure, oxygen content, electronic conductivity, oxygen desorption, thermal expansion, microstructure and electrochemical performance were systematically investigated. Among the series of materials tested, LaBaFe2O5+δ oxide showed the largest electronic conductivity and YBaFe2O5+δ oxide had the smallest thermal expansion coefficient (TEC) of 14.6 × 10−6 K−1 within a temperature range of 200–900 °C. All LnBaFe2O5+δ oxides typically possess the TEC values smaller than 20 × 10−6 K−1. The oxygen content, electronic conductivity and TEC values are highly dependent on the cation size of the Ln3+ dopant. The lowest electrode polarization resistance in air under open circuit voltage condition was obtained for SmBaFe2O5+δ electrode and was approximately 0.043, 0.084, 0.196, 0.506 and 1.348 Ω cm2 at 800, 750, 700, 650 and 600 °C, respectively. The SmBaFe2O5+δ oxide also demonstrated the best performance after a cathodic polarization. A cell with a SmBaFe2O5+δ cathode delivered peak power densities of 1026, 748, 462, 276 and 148 mW cm−2 at 800, 750, 700, 650 and 600 °C, respectively. The results suggest that certain LnBaFe2O5+δ oxides have sufficient electrochemical performance to be promising candidates for cathodes in intermediate-temperature solid oxide fuel cells.

Introduction

A range of new energy conversion technologies are being exploited to help counteract the greenhouse effect and to meet the increasing energy demand resulting from the rapid development of the world economy. Solid oxide fuel cells (SOFCs) with high efficiency and low emissions are particularly competitive candidates. Conventional SOFCs feature a porous La0.8Sr0.2MnO3 (LSM) cathode layer, a dense yttrium-stabilized zirconia (YSZ) electrolyte film supported on a porous Ni + YSZ anode substrate that can transfer chemical energy directly into the desired electric power and high-level hot stream while operating at 800–1000 °C [1].

Reducing materials and fabrication costs and prolonging lifetime will greatly accelerate the commercialization of this attractive technology. Therefore, an intermediate-temperature SOFC (IT-SOFC) operating between 600 and 800 °C has been proposed. However, because the oxygen reduction reaction (ORR) in Mn-based perovskites is strictly limited to the triple-phase boundary (TPB) in the electrolyte, cathode and gas phase, its activation energy and polarization resistance are too large to maintain acceptable performance while operating at intermediate temperatures. Therefore, there is a need for new cathode materials with enhanced ORR activity to extend the active sites to the entire surface of the cathode layer beyond the TPB and thereby, achieve low polarization losses. Perovskite-type mixed ionic-electronic conductors (MIECs), particularly Co-containing oxides such as LaxSr1−xCoyFe1−yO3−δ (LSCF), SmxSr1−xCoO3−δ (SSC), BaxSr1−xCoyFe1−yO3−δ (BSCF) and LnBaCo2O5+δ, can be applied as cathodes in IT-SOFCs because of their high mixed conductivity and excellent electrocatalytic activity for the ORR [2], [3]. However, there are some doubts about such cathodes for practical long-term applications because of their high thermal expansion coefficients (TECs), low chemical stability because of their flexible redox behavior, easy evaporation, as well as the high cost of compositional elements. The cathodes also have high reactivity with Zr-based electrolytes, potentially forming insulating zirconate phases at the cathode–electrolyte interface during high-temperature sintering [4]. Therefore, it is of great interest to develop Co-free cathodes with satisfactory performance at reduced temperatures. Fe-based perovskite oxides could serve as promising alternatives to Co-based cathode materials because of the less flexible redox behavior of iron to expect their low TEC and low price of iron compared to cobalt. To date, several Co-free Fe-based compounds such as LaxSr1−xFeO3−δ, Ba0.5Sr0.5Zn0.2Fe0.8O3−δ, LaBaCuFeO5+δ, BixSr1−xFeO3−δ and Sr0.9K0.1FeO3−δ, perovskite oxides have been reported as IT-SOFC cathodes with acceptable electrochemical performance compared to Mn-based perovskites [5], [6], [7], [8], [9]. These findings suggest Fe-based composite oxides might be potential alternative cathode materials for reduced-temperature operation.

Recently, there have been some reports suggesting remarkable ORR activities in oxygen-deficient layered perovskites in the intermediate temperature range, which opens the possibility of developing a new class of materials suitable for application as cathodes in IT-SOFCs, which require high oxygen diffusion rates and surface exchange kinetics at intermediate temperatures [10], [11], [12], [13], [14]. In typical A-site ordered perovskites, LnBaCo2O5+δ, oxygen can move easily through the LnO plane, which was observed through neutron diffraction and molecular dynamics simulations [15], [16]. However, these developing layered perovskite cathode materials often have cobalt cations or the partial substitution of cobalt with other transitional metal cations at the B-site. Therefore, to reduce the drawbacks of Co-based cathodes and to take advantage of the enhanced oxygen mobility in the LnO plane, we present a systematic investigation of the properties of LnBaFe2O5+δ (Ln = La, Pr, Nd, Sm, Gd, and Y) with full iron ion occupying the B-site as cathodes of IT-SOFCs, including a study of the influence of the Ln3+ ions on the phase structure, oxygen content, electronic conductivity, oxygen desorption property, thermal expansion behavior and electrocatalytic activity for ORR, The performance of single cells with LnBaFe2O5+δ cathodes, a YSZ electrolyte, a Sm0.2Ce0.8O1.9 (SDC) buffering layer, and a Ni + YSZ anode was also evaluated.

Section snippets

Synthesis of powders

LnBaFe2O5+δ powders were synthesized via a combined EDTA-citrate complexing sol–gel process. Ln(NO3)3·xH2O and Fe(NO3)3·xH2O were first prepared as an aqueous solution around 1 M with its precise concentration determined by standard EDTA titration technique. Details of the synthetic procedure can refer to the published paper [11]. To obtain powders with the desired perovskite crystal structure, the solution was heated at 90 °C to evaporate water to form a transparent gel, which was then pre-fired

Phase structure and oxygen content

To ensure the formation of a stable phase, samples prepared from the EDTA-citrate complexing sol–gel process were all calcined at 1000 °C for 10 h in air. XRD patterns of the various perovskite-based LnBaFe2O5+δ are shown in Fig. 1. In the present study, Ln = La and Pr samples could be well indexed with a simple cubic perovskite structure, suggesting that the rare-earth and alkali-earth ions were randomly intermixed and a cation-ordered structure failed to form under such conditions. These findings

Conclusions

The rare earth elements had a significant effect on the phase structure, oxygen content, electronic conductivity, oxygen desorption property, thermal expansion behavior, microstructure and electrochemical performance of composite oxides with the nominal composition of LnBaFe2O5+δ (Ln = La, Pr, Nd, Sm, Gd, and Y). LaBaFe2O5+δ and PrBaFe2O5+δ showed a simple cubic perovskite structure, whereas LnBaFe2O5+δ (Ln = Nd, Sm and Gd) exhibited a cation-ordered layered structure. Decreasing the ionic radius

Acknowledgments

This work was supported by the “National Science Foundation for Distinguished Young Scholars of China” under contract no. 51025209.

References (49)

  • S.P. Simner et al.

    Journal of Power Sources

    (2003)
  • B. Wei et al.

    Journal of Power Sources

    (2008)
  • Q.J. Zhou et al.

    Electrochemistry Communications

    (2009)
  • S. Hou et al.

    Journal of Power Sources

    (2010)
  • D.J. Chen et al.

    Journal of Power Sources

    (2009)
  • F. Zhao et al.

    Journal of Power Sources

    (2010)
  • Y. Lee et al.

    Solid State Ionics

    (2011)
  • S.L. Zhang et al.

    Journal of Magnetism and Magnetic Materials

    (2010)
  • A. Ecija et al.

    Solid State Ionics

    (2011)
  • A. Maignan et al.

    Journal of Solid State Chemistry

    (1999)
  • J.C. Burley et al.

    Journal of Solid State Chemistry

    (2003)
  • A. Tarancón et al.

    Solid State Ionics

    (2008)
  • S. Carter et al.

    Solid State lonics

    (1992)
  • Q.J. Zhou et al.

    Journal of Power Sources

    (2010)
  • H.Y. Tu et al.

    Solid State Ionics

    (1999)
  • H. Lv et al.

    Solid State Ionics

    (2006)
  • W. Zhou et al.

    Journal of Power Sources

    (2008)
  • D.J. Chen et al.

    Journal of Power Sources

    (2010)
  • D.J. Chen et al.

    Journal of Power Sources

    (2010)
  • Q.J. Zhou et al.

    Electrochemistry Communications

    (2010)
  • Z. Gao et al.

    Journal of Power Sources

    (2011)
  • Y.M. Guo et al.

    Electrochimica Acta

    (2011)
  • H.P. Ding et al.

    Journal of Power Sources

    (2010)
  • D.J. Chen et al.

    Electrochemistry Communications

    (2011)
  • Cited by (110)

    View all citing articles on Scopus
    View full text