Chemical compatibility and properties of suspension plasma-sprayed SrTiO3-based anodes for intermediate-temperature solid oxide fuel cells

doi:10.1016/j.jpowsour.2014.04.094

Journal of Power Sources

Volume 264, 15 October 2014, Pages 195-205

https://doi.org/10.1016/j.jpowsour.2014.04.094 Get rights and content

Highlights

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Nano-sized LST and SDC powders are produced for SPS.
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Ni reacted with LSGM electrolyte at or above 600 °C.
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LST exhibits a good chemical compatibility with SDC and LSGM.
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SPS was employed to prepare the LST–SDC composite anode.
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Test cell shows a power density of 221 mW cm⁻² at 800 °C and increase by 37% with the annealed anode.

Abstract

La-doped strontium titanate (LST) is a promising, redox-stable perovskite material for direct hydrocarbon oxidation anodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, nano-sized LST and Sm-doped ceria (SDC) powders are produced by the sol–gel and glycine–nitrate processes, respectively. The chemical compatibility between LST and electrolyte materials is studied. A LST–SDC composite anode is prepared by suspension plasma spraying (SPS). The effects of annealing conditions on the phase structure, microstructure, and chemical stability of the LST–SDC composite anode are investigated. The results indicate that the suspension plasma-sprayed LST–SDC anode has the same phase structure as the original powders. LST exhibits a good chemical compatibility with SDC and Mg/Sr-doped lanthanum gallate (LSGM). The anode has a porosity of ∼40% with a finely porous structure that provides high gas permeability and a long three-phase boundary for the anode reaction. Single cells assembled with the LST–SDC anode, La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ electrolyte, and La_0.8Sr_0.2CoO₃–SDC cathode show a good performance at 650–800 °C. The annealing reduces the impedances due to the enhancement in the bonding between the particles in the anode and interface of anode and LSGM electrolyte, thus improving the output performance of the cell.

Introduction

Solid oxide fuel cells (SOFCs) have been widely investigated as an efficient and environment-friendly alternative to conventional power generation using fossil fuels [1]. To solve severe problems during high temperature operations, such as the material selection for current collector and sealing problems, one of the major concerns is to reduce the operating temperature from 1000 °C to <800 °C. Therefore, it is essential to use electrolytes with high ionic conductivity and electrodes with high catalytic activity for intermediate-temperature SOFCs (IT-SOFCs) [2]. Mg/Sr-doped lanthanum gallate (LSGM) is one of the representative electrolytes for applications in SOFCs at 600–800 °C owing to its high ionic conductivity and stability over a wide range of oxygen partial pressure [3], [4], [5]. However, the conventional Ni-based cermet anode materials are not suitable for LSGM electrolyte because of the interfacial reaction between the anode and electrolyte during the cell fabrication process, which results in significant performance degradation [6], [7], [8], [9]. When the sintering temperature is >1250 °C, LSGM electrolyte reacts with NiO to produce LaNiO₃ with low conductivity and catalytic activity. Moreover, Ni reacts with LSGM when the operation temperature is >1000 °C [10]. Until now, the long-time stability of Ni and LSGM at intermediate temperature range has not been reported. Therefore, it is desirable to develop alternative anode materials for the further development of IT-SOFCs with LSGM electrolyte. La-doped strontium titanate (LST) is one of the perovskite ceramic oxides with significantly high electronic conductivity; therefore, LST is considered suitable as anode material [11], [12], [13]. LST has a conductivity of >20 S cm⁻¹ at 973 K [14]. Moreover, in the SOFCs with LST anode, methane can be directly used as the fuel, and the cells exhibit high sulfur-tolerant ability [15]. Several studies showed that the cells with LSGM electrolyte and LST anode exhibit a good performance [16], [17]. Further, LST has attracted much interest owing to its thermal and chemical stability with ZrO₂-based electrolyte in fuel cells operating environment [18], [19].

Usually, to enhance the electrochemical performance, a composite anode of Gd or Sm-doped CeO₂ (GDC or SDC) and LST is designed [13], [16], [17]. For SOFC anode, a porosity of ∼30–40% and a uniform pore distribution are required. In addition, a long three-phase boundary (TPB) is also necessary for anode reactions. Therefore, the fabrication process of anode plays an important role in determining the performance of SOFCs and their commercial applications. Until now, several approaches have been used to fabricate SOFCs electrodes, such as sol–gel method [20], tape-casting [21], screen-printing [22], and plasma spraying [23], [24], [25]. Compared to other methods, plasma spraying is one of the cost-effective methods owing to its fast deposition rate and easy automation features. In particular, for tubular and other complex geometrical SOFCs, plasma spraying shows a high flexibility during the electrode fabrication process. As one of the variations of plasma spraying technologies, suspension plasma spraying (SPS) can be used to deposit ceramic coatings with fine grains from sub-micrometers to nanometers. The size of splats in the deposits is usually one to two orders of magnitude smaller than that deposited by conventional plasma spraying, thus providing more TPB advantageous for SOFCs electrode applications [26]. Moreover, homogenously distributed components are expected in the fabricated coatings because of the homogeneity of the suspension. In summary, the SPS process has great potential to fabricate a highly efficient anode.

Until now, LST-based composite anodes were mainly prepared by slurry coating [13] and screen printing methods [17]. To the best of our knowledge, there are no published reports describing the use of plasma spraying to prepare LST-based anodes. The purpose of this study was to fabricate an LST–SDC composite anode by the SPS method and investigate the microstructure of the composite anode and its effect on the anode performance. In this study, nano-sized LST and SDC powders that are suitable for SPS were prepared. The chemical compatibility between LST, LSGM, and SDC was studied. Moreover, the reaction of Ni anode with LSGM electrolyte at intermediate temperature was also investigated. The effects of the annealing treatment on the microstructure and electrochemical performance of the composite anode were evaluated.

Section snippets

Synthesis and powder characterization

In this study, 20% La-doped SrTiO₃ (La_0.2Sr_0.8TiO₃) powders were prepared by the sol–gel method. La(NO₃)₃·6H₂O (99.99%, Sinopharm Chemical Reagent Co., Ltd (SCRC), China), SrCl₂·6H₂O (99.99%, SCRC), and TiCl₄ (99.9%, SCRC) were used as the starting materials. Stoichiometric amounts of La(NO₃)₃·6H₂O and SrCl₂·6H₂O were dissolved in deionized water to prepare a nitrate and chloride mixture, namely, solution A. A stoichiometric amount of TiCl₄ was dissolved in dehydrated alcohol (99.7%, Ante) to

Thermal behavior of LST gel precursor

The differential scanning calorimetry (DSC) and thermogravimetry (TG) curves for the LST foam-like dried gel are shown in Fig. 1. The DSC curve shows a weak endothermic peak at ∼92 °C, which can be attributed to the volatilization of free water in the gel, leading to a mass loss of 5.0% between 50 °C and 150 °C. Between 150 °C and 450 °C, the TG curve showed a mass loss of 53.5%, which can be attributed to the redox reaction between nitrates, a part of chlorides, and citric acid; thus, an

Conclusions

In this study, nano-sized LST and SDC powders, suitable for use in SPS, were synthesized by sol–gel and glycine–nitrate processes, respectively. The study on the chemical stability of the conventional Ni anode and LSGM electrolyte showed that Ni reacts with LSGM electrolyte even at 600 °C. However, the results indicate that LST has a good chemical stability with LSGM and SDC electrolyte materials at high temperatures and in different atmospheres. Moreover, the LST–SDC composite anode fabricated

Acknowledgments

The present study is partially supported by National Basic Research Program (Grant No. 2012CB625100).

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