Selection and Evaluation of Heat-Resistant Alloys for SOFC Interconnect Applications

Abstract

Over the past several years, advances in the design and fabrication of planar solid oxide fuel cells ͑SOFCs͒ have led to a steady reduction in the temperatures necessary for their operation. Consequently, it appears more realistic now to use low cost heatresistant alloys for interconnect 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 the preselection of heat-resistant compositions, such as Ni-, Fe-, and Co-based superalloys, Cr-based alloys, and stainless steels. The proposed criteria have been employed to establish a database of heat-resistant alloys at Pacific Northwest National Laboratory, where a systematic approach has been initiated to evaluate and modify and/or develop alloys for SOFC interconnect applications. The preselected compositions are further evaluated by referring in-house studies and reference to published data. It appears that it would be difficult for traditional alloys to fully satisfy the materials requirements for long-term operation at temperatures higher than 700°C. However, the applicability can be improved via surface/bulk modification and by the implementation of novel stack designs.

Mn–Co spinel protective–conductive coating on AL453 ferritic stainless steel for IT-SOFC interconnect applications

“…However, the main obstacle on the way to the commercialization of the IT-SOFC is the high cost of the stack and its durability related to the long-term stability of stack/ cell components, such as interconnects [1,2]. Bipolar interconnects, the role of which is to connect a large number of single cell elements into a planar-type IT-SOFC stack, are currently developed from high-chromia ferritic steels, such as AL453 [3][4][5][6]. The following factors provide arguments in favor of selecting this type of steel: a thermal expansion coefficient closely matching that of the YSZ solid electrolyte, high resistance against high-temperature corrosion, and, furthermore, its chemical stability with respect to the glass sealant used for the construction of the cell stack [5,7].…”

“…Bipolar interconnects, the role of which is to connect a large number of single cell elements into a planar-type IT-SOFC stack, are currently developed from high-chromia ferritic steels, such as AL453 [3][4][5][6]. The following factors provide arguments in favor of selecting this type of steel: a thermal expansion coefficient closely matching that of the YSZ solid electrolyte, high resistance against high-temperature corrosion, and, furthermore, its chemical stability with respect to the glass sealant used for the construction of the cell stack [5,7]. Since the AL453 steel is exposed to both air and a reducing (H 2 -H 2 O) atmosphere at the predicted IT-SOFC operation temperature of about 1,073 K, surface modification is required in order to improve the electrical conductivity of the chromia scale or the chromia-rich sublayer [5].…”

“…The following factors provide arguments in favor of selecting this type of steel: a thermal expansion coefficient closely matching that of the YSZ solid electrolyte, high resistance against high-temperature corrosion, and, furthermore, its chemical stability with respect to the glass sealant used for the construction of the cell stack [5,7]. Since the AL453 steel is exposed to both air and a reducing (H 2 -H 2 O) atmosphere at the predicted IT-SOFC operation temperature of about 1,073 K, surface modification is required in order to improve the electrical conductivity of the chromia scale or the chromia-rich sublayer [5]. The second issue encountered with chromia is their potential transformation into volatile chromium species such as CrO 2 (OH) 2 , leading to the loss of their protective properties and the poisoning of the cathode material, and the subsequent degradation in the electrochemical performance of the cell, irrespective of the type of steel used for the construction of SOFC steel interconnects [8,9].…”

Effect of Oxidation on Creep Strength and Resistivity of Porous Fe-26Cr-1Mo

“…Potential metals for use in metal-supported cells, as in the case of dense substrates, remain limited due to the harsh environmental conditions imposed by simultaneous exposure to an oxidizing and reducing atmosphere at temperatures as high as 1073 K (800°C), [10] Consequently, ferritic stainless steels remain one of the most studied candidate families of alloys due to their coefficient of thermal expansion matching with the stack, electrically conducting oxide (chromia, Cr 2 O 3 ), ease of fabrication, and low cost. Several studies have characterized the oxidation behavior, thermal expansion, and area-specific resistance (ASR) of dense ferritic steels for SOFC operating conditions.…”

“…Several studies have characterized the oxidation behavior, thermal expansion, and area-specific resistance (ASR) of dense ferritic steels for SOFC operating conditions. [10][11][12] Porous ferritic stainless steels suitable for metal-supported cells have been successfully prepared by a variety of methods including: laser drilling, [13][14][15] tape-casting, [16] and prealloyed powder isostatic pressing. [7,17,18] Porosities are typically in the range of 30 to 50 pct so as to allow sufficient gas flow across the electrodes.…”