Cr deposition on porous La0.6Sr0.4Co0.2Fe0.8O3 − δ electrodes of solid oxide cells under open circuit condition

doi:10.1016/j.ssi.2015.08.018

Solid State Ionics

Volume 281, 15 November 2015, Pages 29-37

https://doi.org/10.1016/j.ssi.2015.08.018 Get rights and content

Highlights

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Cr-poisoning of LSCF electrodes is investigated under open circuit condition.
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SrCrO₄ phase is preferably formed on the outer surface of LSCF electrodes.
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Cr deposition decreases significantly with the decrease of temperature.
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The results in this study indicate that Cr deposition on LSCF is not an electrochemical process.

Abstract

The effect of Cr deposition and poisoning on the electrochemical activity and microstructure of La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ (LSCF) electrodes is investigated in detail under open circuit conditions at 900 °C, 800 °C and 700 °C in the presence of a Fe–Cr alloy interconnect. The electrochemical performance of LSCF electrodes for the O₂ reduction reaction (ORR) under solid oxide fuel cells (SOFCs) mode and for the O₂ evolution reaction (OER) under solid oxide electrolysis cells (SOECs) mode is seriously degraded after exposure to Cr-containing air at open circuit voltage. SEM, XRD and Raman spectroscopy analyses reveal the formation of SrCrO₄ phase and the deposition of a ~ 1 μm thick Cr deposit layer on the outer surface of LSCF electrodes after exposure to Fe–Cr alloy interconnect at 900 °C for 20 h under open circuit conditions, greatly limiting the oxygen exchange and diffusion related processes. However, the magnitude of Cr deposition decreases significantly with the decrease in temperature. The results also confirm that the mechanism of Cr deposition on LSCF electrodes is a chemical process in nature that is initiated most likely by the nucleation reaction between surface segregated SrO_x and gaseous Cr species.

Introduction

Solid oxide fuel cell (SOFC) and its reverse operation mode, solid oxide electrolysis cell (SOEC), are the highly efficient and environment friendly energy conversion technologies, which will play a key role in future energy supply and storage. SOFC can generate electricity and heat using versatile fuels including hydrogen, methane and light hydrocarbons, while SOEC can convert the surplus electricity of nuclear power plants or from the renewable wind/solar power and steam to generate hydrogen fuel. In both SOFC/SOEC technologies, chromia-forming alloys are extensively developed and used as interconnect materials [1], [2], [3], [4], due to their high electronic and thermal conductivity, good mechanical property and low cost. Under oxidizing atmosphere, however, gaseous hexavalent chromium species like CrO₃ and CrO₂(OH)₂ can form from the oxide scale of Fe–Cr metallic alloys and further deposit on the oxygen electrodes, leading to a serious electrochemical performance degradation [5], [6], [7], [8], [9], [10]. Cr deposition and poisoning of oxygen electrode has been identified as one of the most critical issues for the long-term durability of SOFC/SOEC stacks [11], [12], [13].

La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ (LSCF) has been considered as a very promising oxygen electrode for both SOFC and SOEC technologies, because of its high mixed electronic and ionic conductivities and high electrocatalytic activities for O₂ reduction and O₂ evolution reactions (ORR and OER) [14], [15], [16]. However, the stability of LSCF electrodes in the presence of Fe–Cr alloy interconnect is a serious concern for its practical applicability in both SOFCs and SOECs. Cr deposition has been extensively studied on LSCF electrodes under the SOFC operation conditions, in particular under cathodic polarization conditions [8], [17], [18], [19]. For ORR on LSCF cathodes, Cr deposition occurs preferentially at the outer surface of the electrode rather than at the electrode/electrolyte interface region [17], [20], [21]. The main product of Cr deposition is SrCrO₄, and the segregated SrO under SOFC operation conditions plays a dominant role in Cr deposition on LSCF electrodes [17], [18], [20]. Lee et al. [22] investigated the effect of chromium amount on the electrochemical performance of LSCF cathodes. Even a very small amount of 0.01 wt.% Cr₂O₃ (100 ppm) can lead to a significant increase in the electrode polarization resistance. Different to the situations in SOFCs, the studies of Cr deposition under SOEC mode are relatively rare. Sharma et al. studied Cr deposition on bi-layered oxygen electrodes consisting of a (La,Sr)CoO₃ (LSC) contact layer and found that Cr deposition occurred throughout the whole LSC contact layer after polarization under SOEC conditions for 2000 h with the formation of La–Cr–O phases on the electrode's outmost surface [3]. Most recently, we studied the effect of Cr deposition on the activity and microstructure of LSCF oxygen electrode under SOEC operation conditions. Significant SrCrO₄ phase formation on LSCF surface and rapid degradation of the electrochemical activity of LSCF oxygen electrodes were observed [23].

The operation conditions play an important role to the Cr deposition process. In the case of (La,Sr)MnO₃ (LSM) electrodes, it has been well known that Cr species are preferentially accumulated and deposited at the LSM electrode/YSZ electrolyte interface region [24], [25], [26], [27]. Using secondary ion mass spectroscopy (SIMS), Horita et al. [24] found a significant Cr concentration (~ 1000 ppm) in the vicinity of LSM/GDC interface region and some Cr accumulation (~ 100 ppm) on the surface after polarization at a cathodic potential of 0.3 V for 300 h at 800 °C. A recent study by Kornely et al. found that Cr deposition and poisoning also occurs on LSM electrode at open circuit condition [28]. The fuel cell polarization resistance exhibited a severe degradation rate of 0.202 mΩ cm² h^− 1 in the presence of Cr species, much higher than 0.08 mΩ cm² h^− 1 measured in the absence of Cr. We studied in detail the interaction between the gaseous Cr species and dense LSCF bar samples at 800 °C and confirmed that the gaseous Cr species preferentially deposit on the segregated SrO and not on Co₃O₄ due to the exclusion effect of the presence of SrO on the reactivity between Cr and segregated Co₃O₄ [29]. The operation temperature and humidity of air also play important roles in Cr deposition on the cathodes of SOFC [30], [31]. Bucher et al. studied Cr deposition on the chemical surface exchange coefficient (k_chem) of La_0.6Sr_0.4CoO₃ (LSC) and LSCF at 600 °C by conductivity relaxation method and found that the k_chem decreases significantly in humid air, while in dry atmosphere, k_chem is relatively stable, indicating the enhanced gas phase transport of the volatile Cr species in the presence of water [8]. However, the studies of Cr deposition and poisoning on the electrochemical activity of LSCF electrodes under open circuit condition for the O₂ reduction reaction and O₂ evolution reaction under SOFC and SOEC operation conditions are relatively scarce.

In this study, Cr-deposition and poisoning behavior of porous LSCF electrode was systematically investigated under open circuit conditions in the presence of a Fe–Cr alloy metallic interconnect. The effect of Cr deposition on the electrochemical performance of LSCF electrode for the ORR and OER under SOFC and SOEC operation modes was studied at 900–700 °C. The results show that Cr deposition occurs on LSCF surface under open circuit in the temperature range studied, suggesting the importance of protecting LSCF electrodes under open circuit conditions for both SOFC and SOEC applications.

Section snippets

Electrode preparation and electrochemical evaluation

Dense gadolinia-doped ceria (Gd_0.1Ce_0.9O_1.95, GDC, AGC Seimi Chemical Co. Ltd., Japan) substrates were prepared by sintering the pressed pellets at 1500 °C for 5 h. The thickness of the GDC electrolyte pellets was ~ 0.8–1.0 mm. La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ powder (LSCF, Fuel Cell Materials, USA) was thoroughly mixed with organic ink at a weight ratio of 1:1 to form the electrode ink, which was then painted onto the center of the surface of GDC electrolyte pellets. The LSCF electrodes were sintered at

Electrochemical activity

Before the analysis of the electrochemical impedance data for the O₂ reduction reaction on LSCF electrodes in the presence of a Fe–Cr alloy interconnect, the compliance of the measured spectra was verified with the Kramers–Kronig transformation by using the Gamry Echem Analyst software. The Kramers–Kronig validation was carried out on selected electrochemical impedance spectra measured at 900 °C for t = 0 h, 900 °C for t = 20 h and 700 °C for t = 0 h, and the results are shown in Fig. 2. The relative

Conclusion

In this study, Cr deposition and poisoning of LSCF electrode was studied in detail under open circuit condition at different temperatures. The results demonstrate that the presence of Cr source can cause serious performance degradation of LSCF electrode for both ORR and OER of SOFC and SOEC modes after exposure under open circuit conditions. At OCV, Cr deposition mainly occurs on the outermost surface of the LSCF electrodes, forming a thick and dense SrCrO₄ layer and thus substantially

Acknowledgments

The work is supported by the Curtin Research Fellow Program, Australian Research Council (LP110200281) and the National Natural Science Foundation of China (21373071, U1134001). The authors acknowledge the support of the Chinese Scholarship Council and the facilities, scientific and technical assistance of the Curtin University Electron Microscope Facility and Curtin X-Ray Laboratory, both of which are partially funded by the University, State and Commonwealth Governments. Dr. Thomas Becker is

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Cr deposition on porous La0.6Sr0.4Co0.2Fe0.8O3 − δ electrodes of solid oxide cells under open circuit condition

Highlights

Abstract

Introduction

Section snippets

Electrode preparation and electrochemical evaluation

Electrochemical activity

Conclusion

Acknowledgments

Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process.

J. Power Sources

J. Power Sources

J. Power Sources

Int. J. Hydrog. Energy

Int. J. Hydrog. Energy

Solid State Ionics

J. Eur. Ceram. Soc.

Electrochim. Acta

Int. J. Hydrog. Energy

Solid State Ionics

Solid State Ionics

Electrochem. Commun.

J. Electroanal. Chem.

Int. J. Hydrog. Energy

J. Fuel Cell Sci. Technol.

J. Electrochem. Soc.

J. Electrochem. Soc.

Fuel Cells

J. Electrochem. Soc.

J. Electrochem. Soc.

Faraday Discuss.

Cr deposition on porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ electrodes of solid oxide cells under open circuit condition