IV.A.17 SOFC Interconnect Materials Development at PNNL
Zhenguo “Gary” Yang (Primary Contact), Guanguang Xia, Jeff Stevenson and Prabhakar Singh
Pacific northwest national Laboratory (PnnL) P.O. Box 999, MS K2-44 Richland, WA 99352 Phone: (509) 375-3756; Fax: (509) 375-2186 Email: zgary.yang@pnl.gov
development of candidate materials for interconnects and their interfaces with electrodes.
Approach
Oxidation/corrosion behavior of candidate alloys has been investigated in air, air/hydrogen and air/ simulated reformate environments, typical of SOFC interconnect operation conditions. Alloys evaluated include traditional compositions, newly developed alloys, and recently ferritic stainless steels with a relative low Cr content, but a unique alloy chemistry. Candidate alloys are surface-modified by application of conductive oxide protection layers to improve surface, chemical, and electrical stability. Conductive oxides have been investigated and optimized for application as contact layers between electrodes and interconnects. novel processing approaches have been designed and optimized for fabrication of the electrical contact layers or interfaces between electrodes and metallic interconnects.
DOE Project Manager: Travis Shultz
Phone: (304) 285-1370 E-mail: Travis.Shultz@netl.doe.gov
Objectives
• Develop cost-effective, optimized materials for intermediate temperature solid oxide fuel cell (SOFC) interconnects and interconnect/electrode interfaces applications. identify and understand degradation processes in interconnects and at interconnect/electrode interfaces.
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Results
Due to space limitations, this report will focus on: i) evaluation and identification of ferritic stainless steels that are more cost-effective than the state-of-theart compositions, but potentially offer comparable or improved performance, and ii) development and longterm evaluation of (Mn,Co)3O4 spinel protection layers. The first task is a collaborative effort between PnnL and Allegheny Technologies, with a goal to identify and develop a ferritic stainless steel that is more cost-effective than recently developed compositions such as Crofer22APu, while demonstrating at least comparable performance for interconnect applications. The current approach focuses on evaluation and modification of ferritic stainless steels that have a similar level of Cr to AiSi430, but are alloyed with minor additions so that the formation of an insulating silica layer at the scale/metal interface during high temperature exposures can be avoided. in other words, potential negative effects of residual Si on the electrical stability of metallic interconnects are avoided by an inexpensive alloying approach, instead of high cost vacuum refining that is commonly used for the state-ofthe-art compositions such as Crofer22APu. Preliminary evaluation of one alloy in particular, T-441, has been encouraging. Minor additions of Ti and nb appear to prevent the build-up of silica, an insulating phase, along the scale/metal interface (see Figure 1), even though the stainless steel contains ~0.5% residual Si. The presence of residual Mn (~0.5%) led to growth of a scale on T-441 similar to that on Crofer22APu, consisting
Accomplishments
• • • • Evaluated ferritic stainless steels based on AiSi430 for interconnect applications. Completed isothermal kinetics study on bare and spinel-coated Crofer22APu, AiSi430, and T-441. Completed one-year stability tests on bare and spinel-coated Crofer22APu. investigated and developed conductive oxides and cermets for cathode-side contact layer applications.
Introduction
With the reduction in SOFC operating temperatures, low-cost high temperature oxidation resistant alloys have become promising candidates to replace lanthanum chromite, a ceramic that can withstand operating temperatures in the 1,000°C range. However, the metallic materials face challenges including chromia scale evaporation, scale electrical resistivity, oxidation/ corrosion under interconnect dual exposure conditions, and scale adherence and compatibility with adjacent components, such as seals, electrodes and/or electrical contact materials. To improve the understanding of the advantages and limitations of alloy interconnects, PnnL has been engaged in systematic evaluation and
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Zhenguo “Gary” Yang
IV.A SECA Research & Development / Materials & Manufacturing
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Fe Fe ASR, mOhm.cm2 Cr Cr Mn Mn
20 15 10 5 0
Bare T-441
Si
82Fe, 17Cr, 0.5Si 82Fe, 17Cr, 0.5Si
T-441 with Mn1.5Co1.5O4 protection layer T-441 with modifed protection layer 0 200 400 600 800 1000
Time, h
FiguRe 1. Cross-Section Scanning Electron Microscopy/Energy Dispersive Spectoscopy (SEM/EDS) of 441 with LSM Contact Paste after 500 hours of Oxidation at 800°C in Air FiguRe 2. Contact area specific resistance (ASR) of uncoated T-441 and T-441 with two versions of the spinel protection layer. The tests were carried out in air at 800°C.
Scale thickness (microns)
of a dual layer structure: a (Mn,Cr)3O4 top layer and a Cr2O3 sub-layer. The absence of the silica interfacial layer and formation of the dual layer scale structure resulted in low scale electrical resistance (see Figure 2). Application of Mn1.5Co1.5O4 spinel protection layers on T-441 significantly improved its oxidation resistance. Further investigations are in progress to develop a more extensive understanding of the properties and performance of uncoated and coated T-441. Spinel protection layers developed at PnnL are intended to: i) protect metallic interconnects from environmental attack and improve the metallic interconnect surface stability; ii) serve as a barrier to Cr migration from the chromia-forming alloy interconnect; and iii) minimize the interfacial contact resistance. Following previous success in fabrication of the protection layers on ferritic steel substrates and short- and mid-term evaluation of their performance, a yearlong isothermal evaluation was completed in this fiscal year. After ~9,000 hours in air at 800oC, the oxide scale grown on Crofer22APu beneath a spinel protection layer fabricated via a slurry coating approach was much thinner (~4 µm) than the scale (~14-15 µm) grown on uncoated Crofer22APu. importantly, there was no evidence of Cr penetration through the spinel protection layer after one year. Thus, the spinel protection layer acted as an effective barrier to both oxygen inward and chromium outward diffusion. Also, the high conductivity of the spinel protection layer may have helped to minimize the interfacial contact resistance, as indicated by data presented in Figure 2. The average thickness of the scale grown on coated and uncoated Crofer22APu during numerous oxidation and electrical resistance tests is shown in Figure 3. Overall, it appears that protective coatings or other surface modifications
18
15
Bare Crofer 22APU: 8,850 hrs, 800°C, air
12 Bare Crofer 22APU: after thermal cycling
9
125 cycles, 12 hrs at 800°C in air
6
T (mirons) = 0.0277t0.5 + 0.751
R2 = 0.9961
3
0
0
20
40
60
80
100
t0.5 (h0.5)
FiguRe 3. Thickness of Scales on Uncoated and Spinel-coated Crofer22APU, as a Function of Time during Oxidation in Air at 800°C
will be required for ferritic stainless steel interconnects operating at ~800oC.
Conclusions
Preliminary evaluation of T-441, a ferritic stainless steel with ~17% Cr and minor additions of nb and Ti, proved the feasibility of using an alloying approach to
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avoid the formation of insulating silica layers along the scale/metal substrate. Thermally grown spinel protection layers fabricated via a slurry approach demonstrated long-term benefits, including improved surface stability, reduction of Cr outward migration, and reduction of electrical resistance of ferritic stainless steel interconnects.
Zhenguo “Gary” Yang
5. “High Temperature Corrosion Behavior of Oxidation Resisitant Alloys under SOFC interconnect Dual Exposures,” Z.G. Yang, G.-G. Xia, J.W. Stevenson, P. Singh, Ceram. Eng. & Sci. Pro., 27, 211, 2007. 6. “Properties of (Mn,Co)3O4 Spinel Protection Layers for SOFC interconnects,” Z.G. Yang, X.S. Li, G.D. Maupin, P. Singh, J.W. Stevenson, G.-G. Xia, X.D. Xhou, Ceram. Eng. & Sci. Proc., 27, 231, 2007. 7. “Advanced interconnect Development at PnnL,” Z.G. Yang, G.-G. Xia, G.P. Maupin, S. Simner, X. Li, J.W. Stevenson, P. Singh, Fuel Cell Seminar Abstracts, Courtesy Associates, 2006.
Future Directions
• Continue to evaluate and modify cost-effective ferritic stainless steels for SOFC interconnect applications. Modify (Mn,Co)3O4 spinel protection layers and optimize coating processes for further performance improvement and cost reduction of ferritic stainless steel interconnects. Develop and optimize materials and processing approaches for contact layer applications.
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Presentations
1. “Volatilization of Cr Vapor Species from Coated and uncoated SOFC interconnects,” J.W. Stevenson, G.D. Maupin, P. Singh, Z.G. Yang, G.-G. Xia, int. Conf. Metal. Coatings & Thin Films (iCMCTF 2007), San Diego, CA, 2007. 2. “Corrosion Behavior of Metals and Alloys under SOFC interconnect Exposure Conditions,”Z.G. Yang, G.-G. Xia, J.W. Stevenson, P. Singh, Advanced Ceramics and Composites Meeting, Daytona Beach, FL, 2007. 3. “Properties of (Mn,Co)3O4 Spinel Protection Layers for SOFC interconnect Applications,” Z.G. Yang, G.-G. Xia, X.S. Li, G.D. Maupin, S.P. Simner, C.M. Wang, X.D. Xhou, Y.S. Chou, J.W. Stevenson, Advanced Ceramics and Composites Meeting, Daytona Beach, FL, 2007. 4. “Metallic interconnect and its Degradation in SOFCs,” Z.G. Yang, P. Singh, J.W. Stevenson, M.S. Walker, G.-G. Xia, 136th TMS Conference, Orlando, FL, 2007. 5. “Electrical Contacts and interfacial Resistance between Electrodes and Metallic interconnects in SOFCs,” G.-G. Xia, Z.G. Yang, Z. nie, J.F. Bonnet, S.P. Simner, J.W. Stevenson, 136th TMS Annual Conference, Orlando, FL, 2007.
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FY 2007 Publications/Presentations
Publications
1. “investigation of Modified ni-Cr-Mn Base Alloys for SOFC interconnect Applications,” Z.G. Yang, P. Singh, J.W. Stevenson, G.-G. Xia, Journal of the Electrochemical Society, 153, A1873, 2006. 2. “Evaluation of ni-Cr-Base Alloys for SOFC interconnect Applications,” Z.G. Yang, G.G. Xia, J.W. Stevenson, Journal of Power Sources, 160, 1104, 2006. 3. “Conductive Protection Layers on Oxidation Resistant Alloys for SOFC interconnect Applications,” Z.G. Yang, G.G. Xia, G.D. Maupin, J.W. Stevenson, Surface & Coatings Technology, 201, 4476 (2007). 4. “Evaluation of Perovskite Overlay Coatings on Ferritic Stainless Steels for SOFC interconnect Applications,” Journal of the Electrochemical Society, Z.G. Yang, G.-G. Xia, G.D. Maupin, J.W. Stevenson, 153, A1852, 2006.
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