Oxide scale formation on different metallic interconnects for solid oxide fuel cells
Introduction
Solid oxide fuel cells (SOFC) are an alternative technology for the conversion of chemical fuels, such as hydrogen and hydrocarbon, directly into electrical power. The future of SOFC is associated with the reduction of the working temperature to 600–800 °C [1], [2], [3], [4], [5].
Interconnect materials connect the anode side of a single cell with the cathode side of an adjacent single cell, acting as a physical barrier to prevent any contact between the reducing and oxidising atmospheres [6]. Interconnects must exhibit the following properties [7], [8], [9]:
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Good electrical conductivity.
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Adequate chemical and thermal stability at operating temperatures approximately 800 °C in both reducing and oxidising atmospheres.
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Its thermal expansion coefficient (TEC) between ambient and operating temperatures should be comparable to those of the electrodes and electrolyte.
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No reaction or inter-diffusion between the interconnect and its adjoining components.
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A low permeability to reactant gases, good mechanical strength and corrosion resistance.
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Ease of manufacture and low cost.
In recent years, metallic materials have received progressively more attention in SOFC as possible replacements of ceramics as interconnects. Metallic interconnects are easier to fabricate and cost less than oxide ceramics, but their lifetimes under SOFC operating conditions must be improved [10], [11]. Stainless steels are commonly used because of their resistance to oxidation. According to their crystal structure, stainless steels are typically divided into three groups: ferritic, austenitic and martensitic steels. The ferritic stainless steels (FSS) are the most attractive metallic interconnect material for solid SOFC because they can be used at temperatures between 600 and 800 °C, they show a high strength and have a good machinability, their TEC is closely matched with electrode materials and they have a low manufacturing cost [12].
In commercial FSS, the Cr content for high-temperature applications varies between approximately 12 and 28 wt.%. FSS have a body-centred cubic (bcc) structure at all temperatures, and the oxidation resistance of FSS generally increases with increasing Cr content. FSS have a lower thermal expansion compared to austenitic alloys, and their structure can be easily deformed and machined [6], [8], [9], [13].
In addition to Ni- and Fe-, Co-based super alloys have been developed for elevated temperature applications, usually based on group VIIIB elements that have a γ-austenitic face-centred-cubic (fcc) structure [14]. Due to their properties, they can also be good candidates as interconnects in SOFC.
However, the metallic interconnects have two main disadvantages: the release of volatile chromium species (CrO3 (gas) or CrO2(OH)2 (gas)) and the precipitation of chromia, (Cr2O3(s)) phase, which blocks the cathode/electrolyte interface, where the reduction of the oxidant occurs. Chromia inhibits the oxygen reduction necessary for the operation of SOFC and may lead to polarisation losses [15], [16], [17]. On the other hand, the anomalous growth of oxide scales under SOFC interconnect operating conditions can lead to accelerated corrosion and thus may affect the stability of metallic interconnects [18].
The oxidation of steels could be improved with the addition of some reactive elements, such as Mn, Ti, Si or Al, Mn and Ti form oxides with spinel and rutile structures in the outer oxide scale, reducing the chromia volatilisation. The increase of the Mn content from 1% to 2% appears to enhance the formation of the spinel, which improves the scale conductivity. Moreover, the addition of trace elements, such as La, Ce or Y, may improve oxide scale adhesion and reduce the corrosion resistance of the alloy [19], [20], [21], [22].
To determine the most appropriate metallic material to improve the behaviour of SOFC, a detailed study was conducted investigating the evolution of the oxide scale microstructure as a function of time (i.e., 100 and 1000 h) at the operation temperature of 800 °C. An oxidation study of Crofer 22 APU and Conicro 4023 W188 with a Cr concentration of 22 wt.% was compared to the oxidation evolution of SS430, which has a Cr concentration of 17 wt.%. The oxidation kinetics, structure and the microstructure of the oxide scale were determined to ascertain the oxidation processes after a long exposure and thermal cycling at 800 °C.
The crystal structures of the compounds after 100 and 1000 h were determined using X-ray diffraction (XRD); the microstructural changes were evaluated using a scanning electron microscopy (SEM) set-up equipped with an energy dispersive X-ray analyser (EDX); and the oxidation behaviour was investigated using thermogravimetric analysis.
Section snippets
Sample preparation
The oxidation behaviour of two Fe–Cr based alloys, Crofer 22 APU (Thyssenkrupp VDM) and SS430 (Hamilton Precision Metals), and a Co based superalloy, Conicro 4023 W188 (Thyssenkrupp VDM), were compared in this study. Nominal compositions given by the technical specifications are listed in Table 1.
Prior to the oxidation experiments, the sheets were cut into 10 × 10 mm squares with thicknesses of 1 mm for SS430 and Crofer 22 APU and 1.5 mm for Conicro 4023 W188. All samples were ground using SiC
Results and discussion
To ensure the chemical composition of the samples (i.e., Crofer 22 APU, SS430, Conicro 4023 W188) matched the values in the technical specifications, the surface of the samples was semiquantitatively analysed using a JEOL-6400 scanning electron microscope equipped with a tungsten filament gun and an Oxford Inca Pentafet X3 energy dispersive X-ray analyser (EDX). The measured values of the major elements were checked on different points to obtain the average composition grade (Table 3).
Conclusions
This paper reports a comparative study of three alloys used as metallic interconnects for SOFC application operating at 800 °C. To determine the most promising alloy, the samples were oxidised for 100 and 1000 h at 800 °C in air. The nature and the distribution of the formed phases and the thickness of the oxide scale have been analysed by XRD and SEM/EDX. The following can be concluded from the obtained results:
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The oxidation kinetics of the alloys used as metallic interconnects follows a
Acknowledgements
This work has been financially supported by the Consejería de Industria, Innovación, Comercio y Turismo of the Basque Government (SAIOTEK 2011 programmes), the Consejería de Educación, Universidades e Investigación of the Basque Government (IT-177-07) and the Ministerio de Ciencia e Innovación (PSE-120000-2008-1, MAT2010-15375 and Consolider-Ingenio 2010 CSD2009-00013). The authors wish to thank Ikerlan´s Fuel Cell group and the technical and human support provided by SGIker (UPV/EHU, MICINN,
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