Improved cathode/electrolyte interface of SOFC

doi:10.1016/j.ssi.2010.01.028

Solid State Ionics

Volume 192, Issue 1, 16 June 2011, Pages 12-15

https://doi.org/10.1016/j.ssi.2010.01.028 Get rights and content

Abstract

The electrochemical performance of porous La_0.6Sr_0.4Co_0.2Fe_0.8O₃ ₋ _δ (LSCF) cathodes is improved by inserting a dense LSCF layer. A 200 nm thin layer is deposited on the electrolyte substrate by pulsed laser deposition, prior to the screen printing process. This procedure enhances the adherence of the porous cathode layer to the electrolyte and allows a lower sintering temperature, which reduces grain growth during sintering. In air a decrease in polarization resistance with a factor of 3 is observed for electrodes sintered at 1100 °C. The apparent electrolyte resistance is also reduced with the dense PLD layer. A remarkable change in Po₂ dependence is observed for the Gerischer parameters that describe part of the electrode impedance, indicating a possible change in the oxygen transfer mechanism.

Introduction

The optimization of the SOFC performance at intermediate temperatures (600–800 °C) depends strongly on efficient cathode materials and structures. The La_0.6Sr_0.4Co_0.2Fe_0.8O₃ ₋ _δ perovskite material (LSCF) is an interesting candidate. Its reactivity with the standard zirconia electrolyte can be suppressed by using an yttria-stabilized ceria interlayer. The polarization resistance of the cathode is also dependent on the microstructure and thus on the preparation process [1], [2]. In a separate report the influence of the sintering temperature on microstructure and polarization resistance of screen printed LSCF cathodes is presented [3]. A very strong sintering temperature dependence was noted, cathodes sintered at 1100 °C showed poor adherence, while cathodes sintered at 1300 °C suffered from excessive grain growth, leading to a low porosity structure and hence a high polarization resistance. The optimum in cathode properties was found for a sintering temperature of 1200 °C. Lowering the sintering temperature has the advantage of smaller grain sizes, which will increase the active surface area for the oxygen exchange and reduction reaction. To improve the adherence of the porous electrode to the electrolyte it is proposed to apply a thin, dense LSCF layer to the electrolyte before the screen printing process. Pulsed laser deposition (PLD or laser ablation) has the advantage of stoichiometric transfer and a strong bonding with the substrate of the deposited composition. In this report the performance of a PLD-enhanced cathode structure is studied with electrochemical impedance spectroscopy (EIS) as function of temperature and oxygen partial pressure. The results are compared with the performance of the regular screen printed cathode.

Section snippets

Experimental

Symmetrical two-electrode cells were prepared by tape-casting and screen printing processes. Tape-casted yttria-partially stabilized zirconia (3YSZ, Tosoh) was used as electrolyte. Yttria-doped ceria (YDC, Praxair) was provided as interlayer by screen printing and sintered at 1400 °C on both sides of the tape-casted 3YSZ electrolyte. Two identical porous electrodes of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ ₋ _δ (LSCF, Praxair) were then screen printed on both sides of the electrolyte and sintered at 1100 °C to

Microstructure

The cross section of a typical symmetrical cell is shown in the SEM micrograph in Fig. 2a. The electrolyte thickness is 92 μm, the electrodes on both sides of the cell are 53 μm thick, the electrode and the electrolyte are separated by a YDC interlayer approximately 3 μm thick. Mean grain size of the porous LSCF cathode sintered at 1100 °C is 0.50 μm. In the DP sample the dense PLD-LSCF layer is 200 nm thick, completely covering the rough YDC surface. Fig. 3 shows that there is a very good adhesion

Conclusion

The cathode/electrolyte interface is playing an important role in the overall performance of the cathode. Introduction of a thin, dense La_0.6Sr_0.4Co_0.2Fe_0.8O₃ ₋ _δ layer is introduced by Pulsed Laser Deposition at the electrolyte/cathode interface improves the adherence of the cathode to the electrolyte and allows a lower sintering temperature during the process. A thin, dense LSCF layer results in a decrease of the polarization resistance of the cathode by a factor 3. The oxygen partial pressure

Acknowledgement

The authors gratefully acknowledge financial support from SenterNovem, an agency of the Dutch Ministry of Economic Affairs promoting sustainable development and innovation.

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Improved cathode/electrolyte interface of SOFC

Abstract

Introduction

Section snippets

Experimental

Microstructure

Conclusion

Acknowledgement

J. Power Sources

Solid State Sciences

Solid State Ionics

J. Power Sources

Solid State Ionics

Solid State Ionics

Solid State Ionics