Guanguang Xia

Thermal Growth and Performance of Manganese Cobaltite Spinel Protection Layers on Ferritic Stainless Steel SOFC Interconnects

To protect solid oxide fuel cells (SOFCs) from chromium poisoning and improve metallic interconnect stability, manganese cobaltite spinel protection layers with a nominal composition of Mn 1 . 5 Co 1 . 5 O 4 were thermally grown on Crofer22 APU, a ferritic stainless steel. Thermal, electrical, and electrochemical investigations indicated that the spinel protection layers not only significantly decreased the contact area specific resistance (ASR) between a LSF cathode and the stainless steel interconnect, but also inhibited the subscale growth on the stainless steel by acting as a barrier to the inward diffusion of oxygen. A long-term thermal cycling test demonstrated excellent structural and thermomechanical stability of these spinel protection layers, which also acted as a barrier to outward chromium cation diffusion to the interconnect surface. The reduction in the contact ASR and prevention of Cr migration achieved by application of the spinel protection layers on the cathode side of ferritic stainless steel interconnects resulted in improved stability and electrochemical performance of SOFCs.

Structure and Conductivity of Thermally Grown Scales on Ferritic Fe-Cr-Mn Steel for SOFC Interconnect Applications

With the development of solid oxide fuel cells (SOFCs) that operate in the intermediate temperature range of 650-800°C, ferritic stainless steels have become promising candidate materials for interconnects in SOFC stacks. The SOFC interconnect requires that the alloy possess not only excellent surface stability, but also high electrical conductivity through the oxide scale that forms at elevated temperatures and contributes to the alloys surface stability. It appears that ferritic Fe-Cr-Mn alloys may be potential candidates due to the formation of an electrically conductive scale containing (Mn, Cr) 3 O 4 spinel. To improve the understanding of scale growth on manganese-containing ferritic stainless steels and evaluate their suitability for use in SOFC interconnects, the oxidation behavior (i.e., growth kinetics, composition, and structure of the oxide scale) and the scale electrical conductivity of a commercially available Fe-Cr-Mn steel developed specifically for SOFC applications were investigated. The results are reported and compared with those of conventional ferritic stainless steel compositions.

Mn1.5Co1.5O4 Spinel Protection Layers on Ferritic Stainless Steels for SOFC Interconnect Applications

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 grown spinel protection layer was well-bonded to the Crofer22 APU substrate and demonstrated stable performance under thermal cycling.

A new redox flow battery using Fe/V redox couples in chloride supporting electrolyte

A new redox flow battery using Fe2+/Fe3+ and V2+/V3+ redox couples in chloride-supporting electrolyte was proposed and investigated for potential stationary energy storage applications. The Fe/V redox flow cell using mixed reactant solutions operated within a voltage window of 0.5–1.35 V with a nearly 100% utilization ratio and demonstrated stable cycling with energy efficiency around 80% at room temperature. Stable performance was also achieved in the temperature range between 0 °C and 50 °C. The improved stability and electrochemical activity over a broader temperature range over the current technologies (such as Fe/Cr redox chemistry) potentially eliminate the necessity of external heat management and use of catalysts, making the Fe/V redox flow battery a promising option as a stationary energy storage device to enable renewable integration and stabilization of the electrical grid.

Evaluation of Perovskite Overlay Coatings on Ferritic Stainless Steels for SOFC Interconnect Applications

Conductive oxide coatings are used to improve electrical performance and surface stability of metallic interconnects, as well as to mitigate or prevent chromium poisoning in solid oxide fuel cells (SOFCs). To further understand materials suitability and shed light on mass transport, two conductive perovskites, were taken as examples and applied as dense coatings via radio frequency (rf)-sputtering on three stainless steels.

Ce-Modified (Mn,Co)3O4 Spinel Coatings on Ferritic Stainless Steels for SOFC Interconnect Applications

This paper reports the development of a coating approach that simultaneously achieves the advantages of conductive (Mn,Co) 3 O 4 spinel coatings and of rare earth (RE) surface treatment on ferritic stainless steels for solid oxide fuel cell (SOFC) interconnect applications. This approach involves the modification of (Mn,Co) 3 O 4 spinel coatings through addition of a RE element to the spinel composition. In particular, Ce-modified spinel coatings (Ce 0.05 Mn 1.475 Co 1.475 O 4 ) behaved similarly to unmodified Mn 1.5 Co 1.5 O 4 spinel coatings by acting as a Cr-outward and O-inward diffusion barrier, thus improving the surface stability and electrical performance of ferritic stainless steel. In addition, the RE addition appeared to alter the scale growth behavior beneath the coating, so that alloy samples with the Ce-modified spinel coating exhibited a more adherent scale/metal interface compared to samples with the unmodified spinel coating. As a result, it is anticipated that compared to unmodified spinel coatings, the RE-modified coatings may lead to improved structural stability and electrical performance for ferritic stainless steel-based SOFC interconnects.

Investigation of Modified Ni – Cr – Mn Base Alloys for SOFC Interconnect Applications

Two Ni-Cr-Mn base alloys based on Haynes 230 were developed and evaluated against criteria relevant to solid oxide fuel cell (SOFC) interconnect applications, which included oxidation behavior under SOFC operating conditions, scale electrical conductivity, and thermal expansion. It was found that, when exposed to air at both sides, the Ni-Cr-Mn alloys formed a scale, containing M 3 O 4 (M = Mn, Cr, Ni, etc.) spinels and Cr 2 O 3 as well as some NiO. When exposed to air at one side and hydrogen at the other, however, the scales grown on these alloys at the air side were similar to the ferritic stainless steel Crofer22 APU, comprised of only a M 3 O 4 (M = Mn, Cr, Ni , etc.) spinel-rich top layer and Cr 2 O 3 -rich sublayer. 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 oxidation 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. In addition, the Ni-Cr-Mn base alloys demonstrated a coefficient of thermal expansion higher than that of anode-supported cells and ferritic stainless steel candidates.

Investigation of AISI 441 Ferritic Stainless Steel and Development of Spinel Coatings for SOFC Interconnect Applications

As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, both bare and spinel coated AISI 441 were studied in terms of metallurgical characteristics, oxidation behavior, and electrical performance. The conventional melt metallurgy used for the bulk alloy fabrication leads to significant processing cost reduction and the alloy chemistry with the presence of minor alloying additions of Nb and Ti facilitate the strengthening by precipitation and formation of Laves phase both inside grains and along grain boundaries during exposure in the intermediate SOFC operating temperature range. The Laves phase formed along the grain boundaries also ties up Si and prevents the formation of an insulating silica layer at the scale/metal interface during prolonged exposure. The substantial increase in ASR during long term oxidation due to oxide scale growth suggested the need for a conductive protection layer, which could also minimize Cr evaporation. In particular, Mn1.5Co1.5O4 based surface coatings on planar coupons drastically improved the electrical performance of the 441, yielding stable ASR values at 800oC for over 5,000 hours. Ce-modified spinel coatings retained the advantages of the unmodified spinel coatings, and also appeared to alter the scale growth behavior beneath the coating, leading to a more adherent scale. The spinel protection layers appeared also to improve the surface stability of 441 against the anomalous oxidation that has been observed for ferritic stainless steels exposed to dual atmosphere conditions similar to SOFC interconnect environments. Hence, it is anticipated that, compared to unmodified spinel coatings, the Ce-modified coatings may lead to superior structural stability and electrical performance.

Development of (Mn,Co)3O4 Protection Layers for Ferritic Stainless Steel Interconnects

A spinel-based surface protection layer has been developed for alloy SOFC current collectors and bi-polar gas separators. The (Mn,Co)3O4 spinel with a nominal composition of Mn1.5Co1.5O4 demonstrates an excellent electrical conductivity and thermal expansion match to ferritic stainless steel interconnects. A slurry-coating technique provides a viable approach for fabricating protective layers of the spinel onto the steel interconnects. Thermally grown protection layers of Mn1.5Co1.5O4 have been found not only to significantly decrease the contact resistance between a LSF cathode and stainless steel interconnect, but also inhibits the sub-scale growth on the stainless steel. The combination of the inhibited sub-scale growth, good thermal expansion matching between the spinel and the stainless steel, and the closed-pore structure contribute to the excellent structural and thermomechanical stability of these spinel protection layers, which was verified by a long-term thermal-cycling test. The spinel protection layers can also act effectively to prevent outward diffusion of chromium from the interconnect alloy, preventing subsequent chromium migration into the cathode and contact materials. PNNL is currently engaged in studies intended to optimize the composition, microstructure, and fabrication procedure for the spinel protection layers.

Fabrication of (Mn,Co)3O4 Surface Coatings onto Alloy Substrates

Ferritic stainless steels are promising candidates for IT-SOFC interconnect applications due to their low cost and resistance to oxidation at SOFC operating temperatures. However, several challenges remain, including long term electrical conductivity and surface stability under interconnect exposure conditions and chromia scale evaporation. One means of extending interconnect lifetime and improving performance is to apply a protective coating, such as (Mn,Co)3O4 spinel, to the cathode side of the interconnect. These coatings have proven effective in reducing scale growth kinetics and Cr volatility. This report describes several procedures developed at PNNL for fabricating (Mn,Co)3O4 spinel coatings onto ferritic stainless steels.