Co- and Ce/Co-coated ferritic stainless steel as interconnect material for Intermediate Temperature Solid Oxide Fuel Cells

Chromium species volatilization, oxide scale growth, and electrical scale resistance were studied at 650 and 750 °C for thin metallic Co- and Ce/Co-coated steels intended to be utilized as the interconnect material in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). Mass gain was recorded to follow oxidation kinetics, chromium evaporation was measured using the denuder technique and Area Specific Resistance (ASR) measurements were carried out on 500 h pre-exposed samples. The microstructure of thermally grown oxide scales was characterized using Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-Ray Analysis (EDX). The findings of this study show that a decrease in temperature not only leads to thinner oxide scales and less Cr vaporization but also to a significant change in the chemical composition of the oxide scale. Very low ASR values (below 10 m? cm2) were measured for both Co- and Ce/Co-coated steel at 650 and 750 °C, indicating that the observed change in the chemical composition of the Co spinel does not have any noticeable influence on the ASR. Instead it is suggested that the Cr2O3 scale is expected to be the main contributor to the ASR, even at temperatures as low as 650 °C

The Effect of Metallic Co-Coating Thickness on Ferritic Stainless Steels Intended for Use as Interconnect Material in Intermediate Temperature Solid Oxide Fuel Cells

The effect of metallic Co-coating thickness on ferritic stainless steels is investigated. This material is suggested to be used as interconnect material in intermediate temperature solid oxide fuel cells. Uncoated, 200-, 600-, 1000-, and 1500-nm Co-coated Sanergy HT is isothermally exposed for up to 500 h in air at 650 A degrees C. Mass gain is recorded to follow oxidation kinetics, and area-specific resistance (ASR) measurements are conducted on samples exposed for 168 and 500 h. The microstructure of the thermally grown oxide scales is characterized utilizing scanning electron microscopy and energy-dispersive X-ray analysis on broad ion beam-milled cross sections. A clear increase in ASR as a function of Co-coating thickness is observed. However, the increase in ASR, as an effect of a thicker Co-coating, is correlated with thicker (Cr,Fe)(2)O-3 scales formed on these materials and not to an increase in Co spinel top layer thickness

Determination of the oxide scale growth mechanism using 18O-tracer experiments in combination with Transmission Electron Microscopy and nanoscale Secondary Ion Mass Spectrometry

Two-stage 18 O 2 / 16 O 2 exposures can be used to investigate the effect that alloying elements, secondary phases, or surface treatments have on the high temperature oxidation behaviour of certain materials. During subsequent exposures to 16 O 2 - and 18 O 2 -rich atmospheres, 16 O- and 18 O-rich layers are formed. Analysis of the layers using Seco ndary Ion Mass Spectrometry (SIMS) depth profiling allows for conclusions to be drawn about the oxide scale growth mechanism. The conclusions are, however, not entirely unambiguous due to the limited lateral resolution of the technology. Rough surface topography and the thickness variation of the oxide scale over the analysed volume add to the ambiguity of the findings. In this study, an Fe-20%Cr alloy was exposed to both 18 O- and 16 O-rich environments at 850 \ub0C. Two methods were used to analyse the thermally grown Cr 2 O 3 scale: (1) traditional SIMS depth profiling and (2) preparation of a cross-sectional lamellae for Transmission Electron Microscopy (TEM), which, subsequently, was analysed in a NanoSIMS. The NanoSIMS 16 O and 18 O elemental maps were then superimposed on the TEM image. In comparison with traditional SIMS depth profiling, the nanoSIMS elemental maps reveal detailed information about local oxide growth in different parts of an oxide scale. Moreover, a clear 16 O/ 18 O interface can be seen in the nanoSIMS maps, which is not the case in the sputter depth profiles. The findings of this study show that the aforementioned issues associated with sputter depth profiling can be eliminated by mapping a cross-section of an oxide scale using high resolution nanoSIMS

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo1.7Fe0.3O4 spinel coatings of different density. The coating density is shown to have minor influence on the long-term oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88% reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H2O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97%

Cr Vaporization and Oxide Scale Growth on Interconnects in Solid Oxide Fuel Cells

The vaporization of Cr(VI) species and the greater electrical resistance caused by a growing oxide scale are probably the two most detrimental degradation mechanisms associated with the use of Cr2O3-forming alloys as the interconnect material in a Solid Oxide Fuel Cell (SOFC). High electrical efficiency, clean emissions and the possibility to utilize several types of fuels, such as hydrogen, alcohols and hydrocarbons are some of the great advantages of SOFC technology. However, the limited lifetime and high production costs of a SOFC have limited the commercialization of this technology. Therefore, improving the component materials and reducing production costs is of great importance. This thesis has examined both mechanisms; Cr vaporization and oxide scale growth. The possibility to pre-coat large amounts of steel and deform the material to allow for gas distribution as an alternative method for reducing production costs was also investigated. To achieve these objectives, nano coatings of Co and Ce were applied to Cr2O3-forming ferritic stainless steels to decrease Cr vaporization and improve the oxidation resistance of the interconnect material. All exposures were carried out in a simulated cathode-side environment consisting of air-3% H2O. Cr vaporization was measured using a denuder technique and oxide-scale growth was studied mainly gravimetrically. For chemical, structural and microstructural analysis, SEM/EDX, FIB and XRD techniques were used.The results presented in this thesis show that high quality coatings that mitigate Cr vaporization are necessary, even if the SOFC operating temperature is decreased to temperatures as low as 650 \ubaC. By coating a ferritic stainless steel with 600 nm Co, the Cr vaporization rate can be decreased by almost 90 %, and by adding an extra 10 nm layer of Ce, the high temperature corrosion resistance of the interconnect material can be significantly improved, and electrical oxide-scale resistance can be reduced. When the pre-coated steel was mechanically deformed to allow for gas distribution, large cracks formed in the coating. However, upon exposure, the cracks healed and formed a continuous surface oxide rich in Co and Mn. As an effect of this rapid healing, no increase in Cr vaporization was detected for the pre-coated material

Cr Vaporization and Oxide Scale Growth on Interconnects in Solid Oxide Fuel Cells

The vaporization of Cr(VI) species and the greater electrical resistance caused by a growing oxide scale are probably the two most detrimental degradation mechanisms associated with the use of Cr2O3-forming alloys as the interconnect material in a Solid Oxide Fuel Cell (SOFC). High electrical efficiency, clean emissions and the possibility to utilize several types of fuels, such as hydrogen, alcohols and hydrocarbons are some of the great advantages of SOFC technology. However, the limited lifetime and high production costs of a SOFC have limited the commercialization of this technology. Therefore, improving the component materials and reducing production costs is of great importance. This thesis has examined both mechanisms; Cr vaporization and oxide scale growth. The possibility to pre-coat large amounts of steel and deform the material to allow for gas distribution as an alternative method for reducing production costs was also investigated. To achieve these objectives, nano coatings of Co and Ce were applied to Cr2O3-forming ferritic stainless steels to decrease Cr vaporization and improve the oxidation resistance of the interconnect material. All exposures were carried out in a simulated cathode-side environment consisting of air-3% H2O. Cr vaporization was measured using a denuder technique and oxide-scale growth was studied mainly gravimetrically. For chemical, structural and microstructural analysis, SEM/EDX, FIB and XRD techniques were used.The results presented in this thesis show that high quality coatings that mitigate Cr vaporization are necessary, even if the SOFC operating temperature is decreased to temperatures as low as 650 \ubaC. By coating a ferritic stainless steel with 600 nm Co, the Cr vaporization rate can be decreased by almost 90 %, and by adding an extra 10 nm layer of Ce, the high temperature corrosion resistance of the interconnect material can be significantly improved, and electrical oxide-scale resistance can be reduced. When the pre-coated steel was mechanically deformed to allow for gas distribution, large cracks formed in the coating. However, upon exposure, the cracks healed and formed a continuous surface oxide rich in Co and Mn. As an effect of this rapid healing, no increase in Cr vaporization was detected for the pre-coated material

Improved Oxidation Resistance and Reduced Cr Vaporization from Thin-Film Coated Solid Oxide Fuel Cell Interconnects

High electrical efficiency, clean emissions, and the possibility to operate on a great variety of fuels, such as hydrogen, alcohols and hydrocarbons are some advantages of Solid Oxide Fuel Cell (SOFC) technology. Too short lifetimes and high production costs have, however, limited the commercialization of this technology. Volatilization of Cr(VI) species and the increased electrical resistance caused by a growing oxide scale are two major degradation mechanisms associated with the use of Cr2O3-forming ferritic stainless steels as interconnect material in a SOFC. In this thesis the possibility to mitigate Cr vaporization and improve oxidation resistance in a cost-effective way, by the application of thin-film metallic Co- and Ce/Co-coatings, was investigated. Uncoated and coated ferritic stainless steels were exposed for up to 3300 h at 650-850 \ub0C. Cr vaporization, oxide scale growth, microstructural and chemical evolution of the oxide scales, as well as the effect these factors have on the electrical resistance of the oxide scale were studied. Cr vaporization was measured using the denuder technique. Oxide scale growth kinetics were studied mainly gravimetrically and the electrical scale resistance was measured ex-situ in an ASR setup using platinum as the electrode material. For chemical, structural and microstructural analysis, SEM/EDX, XRD, BIB/FIB, TEM/EELS and SIMS techniques were utilized. In order to study the oxide scale growth mechanism a two-stage, 18/16O-tracer exposure setup was used. Within the studied temperature interval it was shown that thin-film Co- and Ce/Co-coated ferritic stainless steels exhibit excellent properties as interconnect material in SOFCs. Cr vaporization can be mitigated with a Co-coating. The addition of a Ce-coating can improve the oxidation resistance, and thus decrease the electrical resistance of the oxide scale. The possibility to coat high volumes of steel using Physical Vapour Deposition (PVD) technology, and subsequently press the pre-coated steel into interconnects to allow for gas distribution, was also investigated. Cracks are formed within the coating as the coated steel is pressed into interconnects. The results in this study, however, show that these cracks are able to heal upon exposure at high temperatures

Metallic thin-film Co- and Ce/Co-coated steels as interconnect material in IT-SOFC

Cr vaporization, oxide scale growth, and the electrical scale resistance (ASR) were studied at 650\ub0C on coated interconnects intended for IT-SOFC applications. The custom-made SOFC interconnect steel Sanergy HT was coated with thin-film Co- and Ce/Co-coatings and compared to the less expensive substrate steel AISI 441 which was coated with Ce/Co and a 15-20 μm thick commercial MCO-coating. All coated materials mitigated Cr vaporization, and due to the very thin chromia scales formed at the low exposure temperature, ASR values below 10 mΩcm 2 were measured for all materials after 500 h at 650\ub0C. The results in this study show that thin-film metallic Co- and Ce/Co-coated steels show excellent properties as interconnect materials for IT-SOFCs. Thick MCO-coatings can also be used at low temperatures; however, the coating method might be a critical issue for lowtemperature applications

The effect of temperature on chromium vaporization and oxide scale growth on interconnect steels for Solid Oxide Fuel Cells

Chromium vaporization and oxide scale growth are probably the two most important degradation mechanisms associated with the interconnect in Solid Oxide Fuel Cells (SOFCs) when Cr2O3-forming alloys are used as the interconnect material. This study examines the influence of temperature on both mechanisms. Two commercially available steels; Crofer 22 H and Sanergy HT, were isothermally exposed at 650, 750 and 850 \ub0C in an air-3% H2O atmosphere with a high flow rate. Volatile chromium species were collected using the denuder technique. The microstructure of thermally grown oxide scales was characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDX) and X-Ray Diffraction (XRD). The findings of this study show that although Cr evaporation is reduced with lower temperature, its relative importance compared to oxide scale growth is greater

Influence of chromium evaporation and oxidation on interconnect steels at 650-850°C

Chromium evaporation and oxide scale growth are two important degradation mechanisms in SOFCs when chromia-forming alloys are used as the interconnect material. In this paper the influence of temperature on both mechanisms were studied. Isothermal exposures were carried out for 24, 168 and 500h at 650, 750 and 850°C in an air-3%H2O atmosphere. In the second part uncoated and metallic nano-coated samples were exposed for 3000h. This study clearly points out the relevance of Cr-evaporation at reduced temperatures and the importance of high quality coatings not only to protect the cell from cathode poisoning but also to reduce the risk of Cr-depletion within the interconnect steel