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Title: Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells

Abstract

Metallic cell interconnects (IC) made of ferritic stainless steels, i.e., iron-based alloys, have been increasingly favored in the recent development of planar solid oxide fuel cells (SOFCs) because of their advantages in excellent imperviousness, low electrical resistance, ease in fabrication, and cost effectiveness. Typical SOFC operating conditions inevitably lead to the formation of oxide scales on the surface of ferritic stainless steel, which could cause delamination, buckling, and spallation resulting from the mismatch of the coefficient of thermal expansion and eventually reduce the lifetime of the interconnect components. Various protective coating techniques have been applied to alleviate these drawbacks. In the present work, a fracture-mechanics-based quantitative modeling framework has been established to predict the mechanical reliability and lifetime of the spinel-coated, surface-modified specimens under an isothermal cooling cycle. Analytical solutions have been formulated to evaluate the scale/substrate interfacial strength and determine the critical oxide thickness in terms of a variety of design factors, such as coating thickness, material properties, and uncertainties. In conclusion, the findings then are correlated with the experimentally measured oxide scale growth kinetics to quantify the predicted lifetime of the metallic interconnects.

Authors:
 [1];  [2];  [3];  [3]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Computational Mathematics Group, Physical and Computational Sciences Directorate
  2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Computational Engineering Group, Physical and Computational Sciences Directorate
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Energy & Environment Directorate
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1368407
Report Number(s):
PNNL-SA-120828
Journal ID: ISSN 0960-1481; PII: S0960148117306079
Grant/Contract Number:
AC05-76RL01830
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Renewable Energy
Additional Journal Information:
Journal Volume: 113; Journal Issue: C; Journal ID: ISSN 0960-1481
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; Solid oxide fuel cell (SOFC); Ferritic stainless steel interconnect; Coating; Fracture mechanics

Citation Formats

Xu, Zhijie, Xu, Wei, Stephens, Elizabeth, and Koeppel, Brian. Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells. United States: N. p., 2017. Web. doi:10.1016/j.renene.2017.06.103.
Xu, Zhijie, Xu, Wei, Stephens, Elizabeth, & Koeppel, Brian. Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells. United States. doi:10.1016/j.renene.2017.06.103.
Xu, Zhijie, Xu, Wei, Stephens, Elizabeth, and Koeppel, Brian. 2017. "Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells". United States. doi:10.1016/j.renene.2017.06.103.
@article{osti_1368407,
title = {Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells},
author = {Xu, Zhijie and Xu, Wei and Stephens, Elizabeth and Koeppel, Brian},
abstractNote = {Metallic cell interconnects (IC) made of ferritic stainless steels, i.e., iron-based alloys, have been increasingly favored in the recent development of planar solid oxide fuel cells (SOFCs) because of their advantages in excellent imperviousness, low electrical resistance, ease in fabrication, and cost effectiveness. Typical SOFC operating conditions inevitably lead to the formation of oxide scales on the surface of ferritic stainless steel, which could cause delamination, buckling, and spallation resulting from the mismatch of the coefficient of thermal expansion and eventually reduce the lifetime of the interconnect components. Various protective coating techniques have been applied to alleviate these drawbacks. In the present work, a fracture-mechanics-based quantitative modeling framework has been established to predict the mechanical reliability and lifetime of the spinel-coated, surface-modified specimens under an isothermal cooling cycle. Analytical solutions have been formulated to evaluate the scale/substrate interfacial strength and determine the critical oxide thickness in terms of a variety of design factors, such as coating thickness, material properties, and uncertainties. In conclusion, the findings then are correlated with the experimentally measured oxide scale growth kinetics to quantify the predicted lifetime of the metallic interconnects.},
doi = {10.1016/j.renene.2017.06.103},
journal = {Renewable Energy},
number = C,
volume = 113,
place = {United States},
year = 2017,
month = 7
}

Journal Article:
Free Publicly Available Full Text
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  • Oxidation reaction of the ferritic stainless interconnects in a typical SOFC working environment is unavoidable and the thickness of the oxide scale will continue to grow with operating time, even with protective coatings. The interfacial strength of the various interfaces for the uncoated and coated ferritic interconnects is crucial to long term performance of SOFCs. In this paper, we employ an integrated experimental/modeling approach to quantify the interfacial strength and to further predict the life of Crofer 22 APU as SOFC interconnect under isothermal cooling condition. The life of Crofer 22 APU was predicted by comparing the predicted interfacial strength,more » interfacial stresses induced by the cooling process from the operating temperature to room temperature, together with the growth kinetics of oxide scale with and without spinel coating. It was found that the interfacial strength between the oxide scale and Crofer 22 APU substrate decreases with the growth of the oxide scale. The interfacial strength of the oxide scale and spinel coating is much higher than that of the oxide scale and Crofer 22 APU substrate. With the spinel coating, the predicted life of the Crofer 22 APU is significantly longer than that of the uncoated Crofer 22 APU.« less
  • To minimize electrical resistance, contact layers are often included between interconnects and electrodes during construction of a SOFC stack. In this work, simulated cathode/interconnect structures were used to investigate the effects of different contact materials on the contact resistance between a LSF cathode and a Crofer22 APU interconnect.. The results from the resistance measurements are reported and correlated to interfacial interactions occurring between the metallic interconnect and the contact materials, particularly perovskites. The materials requirements for the contact layers between cathodes and metallic interconnects in intermediate temperature SOFCs are also discussed.
  • With the steady reduction in solid oxide fuel cell (SOFC) operating temperatures into a range of 650-800C, cost effective metallic materials, in particular high temperature oxidation resistant alloys, have become promising candidates, in place of ceramic counterparts, for construction of interconnects in SOFC stacks. For a lifetime of thousands of hours, the alloys must demonstrate excellent surface, structural and electrical stability, and compatibility with other components in SOFC stacks in a very challenging environment. The paper provides a comprehensive critical review of metallic interconnects for SOFCs with a focus on recent progress in materials development, as well as advances inmore » understanding materials degradation and interfacial phenomena under the SOFC operating conditions. It also attempts to provide guiding principles for development of optimized, cost-effective metallic interconnect materials that can demonstrate satisfactory life-time stability and performance.« less
  • The oxidation properties of potential SOFCs materials Crofer 22 APU, Ebrite and Haynes 230 exposed in coal syngas at 800 °C for 100 h were studied. The phases and surface morphology of the oxide scales were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis (EDX). The mechanical endurance and electrical resistance of the conducting oxides were characterized by indentation and electrical impedance, respectively. It was found that the syngas exposure caused the alloys to form porous oxide scales, which increased the electrical resistant and decreased the mechanical stability. As for short-term exposure in syngas, neither carbide normore » metal dusting was found in the scales of all samples.« less
  • Ferritic stainless steels are promising candidates for interconnect applications in low- and mid-temperature solid oxide fuel cells (SOFCs). A couple of issues however remain for the particular application, including the chromium poisoning due to chromia evaporation, and long-term surface and electrical stability of the scale grown on these steels. Application of a manganese colbaltite spinel protection layer on the steels appears to be an effective approach to solve the issues. For an optimized performance, Mn{sub 1+x}Co{sub 2-x}O{sub 4} (-1 {le} x {le} 2) spinels were investigated against properties relative for protection coating applications on ferritic SOFC interconnects. Overall it appearsmore » that the spinels with x around 0.5 demonstrate a good CTE match to ceramic cell components, a relative high electrical conductivity, and a good thermal stability up to 1,250 C. This was confirmed by a long-term test on the Mn{sub 1.5}Co{sub 1.5}O{sub 4} protection layer that was thermally grown on Crofer22 APU, indicating the spinel protection layer not only significantly decreased the contact resistance between a LSF cathode and the stainless steel interconnects, but also inhibited the sub-scale growth on the stainless steels.« less