Degradation of oxygen reduction reaction kinetics in porous La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes due to aging-induced changes in surface chemistry

doi:10.1016/j.jpowsour.2016.10.090

Journal of Power Sources

Volume 337, 1 January 2017, Pages 166-172

https://doi.org/10.1016/j.jpowsour.2016.10.090 Get rights and content

Highlights

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Surface La- and Sr-enrichment induced by aging LSCFO cathodes at 800 °C for 50 h.
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Cation redistribution can extend up to ∼400 nm depth under the LSCFO surface.
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Surface chemical changes detected by high resolution STEM/EDS maps at cathode pores.
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Oxygen exchange at LSCFO/gas interface hampered by surface chemistry modification.
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Oxygen ion conductivity remains constant upon aging according to impedance analysis.

Abstract

The modification of surface composition after long-term operation is one of the most reported degradation mechanisms of (La,Sr)(Co,Fe)O_3-δ (LSCFO) cathodes for Solid Oxide Fuel Cells (SOFCs). Nevertheless, its effect on the oxygen reduction reaction kinetics of porous LSCFO cathodes has not been yet reliably established. In this work, La- and Sr-enrichment at the LSCFO surface of porous cathodes has been induced after 50 h aging at 800 °C under air. Such cation redistribution can extend up to ∼400 nm depth under the LSCFO surface as detected by high resolution Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy maps acquired inside the cathode pores. The observed surface chemical changes hamper the oxygen surface exchange reaction at the LSCFO/gas interface. Accordingly, a suitable Electrochemical Impedance Spectroscopy analysis revealed that the oxygen ion conductivity remains practically unaltered during the aging treatment while the oxygen surface exchange resistance increases up to 1.8 times. As a result, the cathode impedance response deteriorates within the 10–0.1 Hz frequency range during the aging treatment, resulting in a total cathode area specific resistance increase of 150%. The methodology adopted has demonstrated to be very valuable for studying the degradation of SOFC cathodes produced by the modification of surface composition.

Graphical abstract

Introduction

Solid Oxide Fuel Cells (SOFCs) are ceramic devices used to convert chemical energy into electricity and heat [1], [2]. The operation of these devices involves the oxygen reduction at the cathode, the oxygen ion conduction through the electrolyte, and the oxidation of the fuel (e.g. hydrogen, methane, carbon monoxide) at the anode. SOFC efficiency and durability is mainly determined by the area specific resistance (ASR) and degradation of the cathode, the electrolyte and the anode. Since the cathode is typically the component with the highest ASR [3], it is very important to limit its degradation.

(La,Sr)(Co,Fe)O_3-δ (LSCFO) is one of the most used mixed ionic-electronic conductors for SOFC cathodes [2]. The degradation of the electrochemical response in these cathodes is usually ascribed in the literature to the modification of surface composition, mostly by Sr segregation [4], [5], [6]. This phenomenon is generally detected by using techniques with low lateral resolution as X-ray Photoelectron Spectroscopy (XPS) [7], [8], [9], [10], [11] or Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) [12], [13], [14] or by using indirect techniques such as the detection of water-soluble species by Inductively Coupled Plasma (ICP) Spectrometry [7], [15], [16], [17]. No appreciable microstructural changes, as the formation of precipitates at the LSCFO surface, are normally observed in porous cathodes [7], [8], [9]. Consequently, the investigation of such degradation mechanism turns out to be challenging. Independently, several authors have reported the observation of Sr-rich and/or Co-rich precipitates in dense LSCFO pellets sintered at high temperature (i.e. ≥ 1250 °C) [12], [13], [14], [18] or in dense La_1-xSr_xCoO_3-δ thin films prepared by Pulse Laser Deposition (PLD) [10], [11]. This suggests that surface compositional changes induced by aging are more likely to occur in dense samples than in porous ones. Nevertheless, it is not clear to what extent the results obtained with dense samples can be extrapolated to porous cathodes used in practical SOFCs. Therefore, it is necessary to investigate LSCFO cathodes under conditions which maximize the occurrence of surface chemistry degradation without losing the porous structure of practical cathodes.

In this work, we have investigated the degradation of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ cathodes induced by aging at 800 °C for 50 h in ambient air. The studied cathodes have an intermediate microstructure between that of a dense pellet and that of a porous film since they are indeed porous but are mainly composed of particles within the micrometric range (as the dense pellets). This particular microstructure was chosen because our preliminary results [19] suggest that it facilitates the occurrence of noticeable surface microstructural and compositional changes under the aging conditions mentioned above. The evolution of the cathode electrochemical properties was continuously monitored by Electrochemical Impedance Spectroscopy (EIS) during the aging treatment while the cation distribution was investigated by high resolution Scanning Transmission Electron Microscopy and Energy Dispersive Spectroscopy (STEM-EDS) before and after the aging treatment.

Section snippets

Symmetrical cells

Commercial Ce_0.9Gd_0.1O_2-δ (CGO) electrolyte powders (Fuelcellmaterials, USA) were uniaxially pressed and sintered in air at 1350 °C for 12 h for obtaining two dense substrates with 9.75 mm diameter and 1 mm thickness. Separately, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCFO) cathode powders were prepared by solid state reaction. Stoichiometric amounts of SrCO₃, La₂O₃, Co₃O₄ and Fe₂O₃ were ball milled and sintered in air at 1000 °C for 5 h. An ink was prepared by mixing the LSCFO powders with isopropyl

Microstructural characterization

Fig. 1a and b show cross-section TEM images of the non-aged and the aged cells. The LSCFO/CGO interface of the aged sample looks sharp, indicating that no cathode delamination has occurred even after the aging treatment. In addition, no measurable chemical reaction [24], [25], [26] between the cathode and the electrolyte were detected.

STEM images with higher magnification were acquired around the pores of the non-aged and the aged cathodes (Fig. 2). A region with a distinctive contrast can be

Discussion

The only cathode degradation mechanism detected within the limits of this study is the modification of the cation concentration at the LSCFO surface. While most research efforts focuses in studying only the top surface of the samples, the use of the high resolution STEM-EDS technique has allowed us demonstrating that such chemical phenomena can also occur at the LSCFO surface inside the pores. The formation of Sr- and/or Co-rich particles on the top surface of dense LSCFO samples has been

Conclusions

Cation redistribution at the LSCFO surface has been induced by aging porous cathodes at 800 °C in air for 50 h. Sr-enriched/La-depleted and La-enriched/Sr-depleted regions (extending up to ∼400 nm underneath the surface) were detected by high resolution STEM-EDS mapping at the LSCFO/gas interface inside the cathode pores. It demonstrates that this degradation mechanism is not only limited to the top cathode surface as usually reported in the literature. An adequate EIS analysis, derived from

Acknowledgements

Prof. S. A. Barnett is gratefully acknowledged for valuable discussion of the impedance results. This work was funded by University of Cuyo (Grant number: 06/C447 2), CONICET, CNEA and ANPCyT-PICT (Grant numbers: PICT 2013-1032 and PICT 2014-1849). The authors also thank LNNano-CNPEM (Campinas, Brazil) for the use of the JEOL JEM-2100F TEM microscope.

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Degradation of oxygen reduction reaction kinetics in porous La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes due to aging-induced changes in surface chemistry

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Symmetrical cells

Microstructural characterization

Discussion

Conclusions

Acknowledgements

Prog. Mater. Sci.

J. Eur. Ceram. Soc.

J. Power Sources

Solid State Ion.

Int. J. Hydrogen Energy

J. Power Sources

Solid State Ion.

Solid State Ion.

Solid State Ion.

Electrochimica Acta

J. Power Sources

Solid State Ion.

Solid State Ion.

Electrochimica Acta

Solid State Ion.

Solid State Ion.

Materials for fuel-cell technologies

Nature

Surface segregation and poisoning in materials for low-temperature SOFCs

MRS Bull.

Durability of solid oxide fuel cells

Green

Mechanisms of performance degradation of (La,Sr)(Co,Fe)O3-δ solid oxide fuel cell cathodes

J. Electrochem. Soc.

Effect of Sr surface segregation of La0.6Sr0.4Co0.2Fe0.8O3-δ electrode on its electrochemical performance in SOC

J. Electrochem. Soc.

Degradation of oxygen reduction reaction kinetics in porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ cathodes due to aging-induced changes in surface chemistry

Mechanisms of performance degradation of (La,Sr)(Co,Fe)O_3-δ solid oxide fuel cell cathodes

Effect of Sr surface segregation of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ electrode on its electrochemical performance in SOC