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Title: Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications

Abstract

Conductive oxide coatings are used as protection layers on metallic interconnects in SOFCs to improve their surface stability and electrical performance, as well as to mitigate or prevent chromium poisoning to cells. This paper discusses materials requirements for this particular application and summarizes our systematic study on varied conductive oxides as potential candidate materials for protection layers on stainless steel substrates. Overall, it appeared that chromites such as (La,Sr)CrO3 improved surface stability, but might not be good candidates for the protection layer applications due to chromium vaporization, albeit at a lower rate than Cr2O3, from these oxides at high temperatures in air or moist air. The application of non-chromite perovskite (La,Sr)FeO3 protection layers resulted in improved oxidation resistance and electrical performance. It is doubtful, however, that LSF can be an effective barrier to prevent chromium release during long term SOFC stack operation due to chromium diffusion through the LSF coatings. With a high oxygen ion conductivity, the coatings of Sn-doped In2O3 failed to provide protection to the metal substrate and are thus not suitable for the protection layer applications. The best performance was achieved using thermally-grown (Mn,Co)3O4 spinel protection layers that substantially improved the surface stability of the metal substrates,more » and prevented chromium outward migration.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
898640
Report Number(s):
PNNL-SA-50052
AA2530000; TRN: US200706%%246
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Surface & Coatings Technology, 201(7):4476-4483
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; 36 MATERIALS SCIENCE; ALLOYS; CHROMITES; CHROMIUM; COATINGS; DIFFUSION; EVAPORATION; OXIDATION; OXIDES; OXYGEN IONS; PEROVSKITE; POISONING; SOLID OXIDE FUEL CELLS; SPINELS; STABILITY; STAINLESS STEELS; SUBSTRATES; SOFC; interconnect applications; stack operation; metallic interconnects

Citation Formats

Yang, Zhenguo, Xia, Guanguang, Maupin, Gary D., and Stevenson, Jeffry W.. Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications. United States: N. p., 2006. Web. doi:10.1016/j.surfcoat.2006.08.082.
Yang, Zhenguo, Xia, Guanguang, Maupin, Gary D., & Stevenson, Jeffry W.. Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications. United States. doi:10.1016/j.surfcoat.2006.08.082.
Yang, Zhenguo, Xia, Guanguang, Maupin, Gary D., and Stevenson, Jeffry W.. Wed . "Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications". United States. doi:10.1016/j.surfcoat.2006.08.082.
@article{osti_898640,
title = {Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications},
author = {Yang, Zhenguo and Xia, Guanguang and Maupin, Gary D. and Stevenson, Jeffry W.},
abstractNote = {Conductive oxide coatings are used as protection layers on metallic interconnects in SOFCs to improve their surface stability and electrical performance, as well as to mitigate or prevent chromium poisoning to cells. This paper discusses materials requirements for this particular application and summarizes our systematic study on varied conductive oxides as potential candidate materials for protection layers on stainless steel substrates. Overall, it appeared that chromites such as (La,Sr)CrO3 improved surface stability, but might not be good candidates for the protection layer applications due to chromium vaporization, albeit at a lower rate than Cr2O3, from these oxides at high temperatures in air or moist air. The application of non-chromite perovskite (La,Sr)FeO3 protection layers resulted in improved oxidation resistance and electrical performance. It is doubtful, however, that LSF can be an effective barrier to prevent chromium release during long term SOFC stack operation due to chromium diffusion through the LSF coatings. With a high oxygen ion conductivity, the coatings of Sn-doped In2O3 failed to provide protection to the metal substrate and are thus not suitable for the protection layer applications. The best performance was achieved using thermally-grown (Mn,Co)3O4 spinel protection layers that substantially improved the surface stability of the metal substrates, and prevented chromium outward migration.},
doi = {10.1016/j.surfcoat.2006.08.082},
journal = {Surface & Coatings Technology, 201(7):4476-4483},
number = ,
volume = ,
place = {United States},
year = {Wed Dec 20 00:00:00 EST 2006},
month = {Wed Dec 20 00:00:00 EST 2006}
}
  • In intermediate solid oxide fuel cells, the use of cost effective chromia forming alloy interconnects such as ferritic stainless steels can lead to severe degradation in cell performance due to chromium migration into the cells at the cathode side. To protect cells from chromium poisoning and improve their performance, a Mn1.5Co1.5O4 spinel barrier layer has been developed and tested on the ferritic stainless steel Crofer22 APU. Thermal and electrical tests confirmed the effectiveness of the spinel protection layer as a means of stopping chromium migration and decreasing oxidation, while promoting electrical contact and minimizing cathode/interconnect interfacial resistance. The thermally grownmore » spinel protection layer was well-bonded to the Crofer22 APU substrate and demonstrated stable performance under thermal cycling.« less
  • Over the past several years, advances in the design and fabrication of planar SOFCs have led to a steady reduction in the temperatures necessary for their operation. Consequently, it appears more realistic now to use low cost heat resistant alloys for interconnect sub-components in the SOFC stack. However, no specific criteria or inclusive study are available as a reference to help select and evaluate suitable candidates from the hundreds of available heat resistant alloy compositions, which overall demonstrate oxidation resistance at high temperatures. In this work, composition criteria have been proposed for pre-selection of heat resistant compositions, such as Ni-,more » Fe-, and Co-base superalloys, Cr-base alloys and stainless steels. The proposed criteria have been employed to establish a database of heat resistant alloys at PNNL, where a systematic approach has been initiated to evaluate and modify/develop metallic alloys for SOFC interconnect applications. The pre-selected compositions are further evaluated by referring our screening studies and published data. It appears that it would be very difficult for traditional alloys to fully satisfy the materials requirements for long-term operation at the intermediate temperature range of 700~850 degrees C. The applicability however can be improved through surface/bulk-modification and implementation of novel stack designs.« less
  • Chromia (Cr2O3) forming ferritic stainless steels are being developed for interconnect application in Solid Oxide Fuel Cells (SOFC). A problem with these alloys is that in the SOFC environment chrome in the surface oxide can evaporate and deposit on the electrochemically active sites within the fuel cell. This poisons and degrades the performance of the fuel cell. The development of steels that can form conductive outer protective oxide layers other than Cr2O3 or (CrMn)3O4 such as TiO2 may be attractive for SOFC application. This study was undertaken to assess the oxidation behavior of ferritic stainless steel containing 1 weight percentmore » (wt.%) Ti, in an effort to develop alloys that form protective outer TiO2 scales. The effect of Cr content (6–22 wt.%) and the application of a Ce-based surface treatment on the oxidation behavior (at 800° C in air+3% H2O) of the alloys was investigated. The alloys themselves failed to form an outer TiO2 scale even though the large negative {delta}G of this compound favors its formation over other species. It was found that in conjunction with the Ce-surface treatment, a continuous outer TiO2 oxide layer could be formed on the alloys, and in fact the alloy with 12 wt.% Cr behaved in an identical manner as the alloy with 22 wt.% Cr.« less
  • Two Ni-Cr-W-Mn base alloys based on Haynes 230 were developed and evaluated against criteria relevant to SOFC interconnect applications, which included oxidation behavior under SOFC operating conditions, scale electrical conductivity, and thermal expansion. It was found that, similar to the ferritic stainless steel Crofer22 APU, additions of Mn led to the formation of a unique scale that was comprised of a M3O4 (M=Mn, Cr, Ni, …) spinel-rich top layer and Cr2O3-rich sub-layer. The modified alloys demonstrated reasonable oxidation resistance under SOFC operating conditions, though the Mn additions increased the scale growth rate and thus sacrificed to some extent the oxidationmore » resistance of the base alloy (Haynes 230). The formation of a spinel-rich top layer improved the scale conductivity, especially during the early stages of oxidation, but the higher scale growth rate resulted in a higher rate of increase in the area-specific electrical resistance. Due to their FCC crystal structure, the Ni-Cr-W-Mn base alloys demonstrated a CTE that was higher than that of anode-supported cells and candidate ferritic stainless steels such as Crofer22 APU.« less
  • To further understand the suitability of Ni-Cr-base alloys for solid oxide fuel cell (SOFC) interconnect applications, three commercial Ni-Cr-base alloys, Haynes 230, Hastelloy S and Haynes 242 were selected and evaluated for oxidation behavior under different exposure conditions, scale conductivity and thermal expansion. Haynes 230 and Hastelloy S, which have a relatively high Cr content, formed a thin scale mainly comprised of Cr2O3 and (Mn,Cr,Ni)3O4 spinels under SOFC operating conditions, demonstrating excellent oxidation resistance and a high scale electrical conductivity. In contrast, a thick double-layer scale with a NiO outer layer above a chromia-rich substrate was grown on Haynes 242more » in moist air or at the air side of dual exposure samples, indicating limited oxidation resistance for the interconnect application. With a face-centered-cubic (FCC) substrate, all three alloys possess a coefficient of thermal expansion (CTE) that is higher than that of candidate ferritic stainless steels, e.g. Crofer22 APU. Among the three alloys, Haynes 242, which is heavily alloyed with W and Mo and contains a low Cr content, demonstrated the lowest average CTE at 13.1x10-6 K-1 from room temperature to 800oC, but it was also observed that the CTE behavior of Haynes 242 was very nonlinear.« less