Solid Oxide Fuel Cell Interconnect Research Articles

Comparison of oxidation behaviours among three Fe–Cr based alloys for solid oxide fuel cell interconnect

Two metallic alloys, containing comparable amounts of Cr, underwent oxidation in hot air simulating (the solid oxide fuel cell cathode atmosphere) for various periods. The results demonstrated that the oxidation kinetics of Crofer22 APU and equivalent ZMG232 followed the parabolic rate law and oxidation rates increased with temperature. Typical oxidation rates of Crofer22 APU and ZMG232 upon annealing treatment are approximately 0·21 orders of magnitude lower than that of ZMG232. An oxide scale electron probe microanalyser, a scanning electron microscope and X-ray diffractometer were adopted to verify the applicability of Fe–Cr based alloys in the solid oxide fuel cell interconnect component. Two alloys contain comparable amount of Cr, Mn and Fe, and their surface oxides as analysed are indicated to be Cr2O3 and (Mn,Fe)Cr2O4 spinel compound. In summary, Crofer22 APU had the best oxidation resistance of any of the alloys of interest.

Interaction mechanisms between slurry coatings and solid oxide fuel cell interconnect alloys during high temperature oxidation

Six different coatings consisting of fluorite-, corundum-, spinel- or perovskite-type oxides were deposited on a Fe22Cr alloy (Crofer 22APU) and oxidized at 900°C in moisturized air.Five of the coatings prevented break-away oxidation otherwise observed for the uncoated alloy, and the parabolic oxidation rate constant was reduced with 50–90% of that for uncoated alloy. One coating consisting of MnCo2O4 did not significantly affect the oxidation rate of the alloy, and just as for uncoated samples break-away oxidation occurred for MnCo2O4 coated samples. The interaction mechanisms between the growing oxide scales and applied coatings can be classified according to three types.

Optimization of Alloy-Coating Compositions for Use as Solid Oxide Fuel Cell Interconnects

ABSTRACTReduction in the operating temperature of solid oxide fuel cells (SOFCs) allows for the use of ferritic stainless steels as the interconnect material. However, cathode poisoning due to volatilization of chromium from the oxide scale formed on these alloys requires the use of a ceramic coating on the alloy. The reaction layer formed between the coating and the alloy affects the performance of the fuel cell, so the composition and properties of this layer must be considered in selecting the alloy and coating compositions. In this paper, the factors relevant to selection of the alloy/coating materials and the effect of the alloy-coating interaction on these factors are discussed.

Preparation and properties of a MnCo2O4 for ceramic interconnect of solid oxide fuel cell via glycine nitrate process

MnCo2O4 powder was prepared by a wet chemistry method using metal nitrates and glycine in an aqueous solution. The phase stability, sintering behavior, thermal expansion and electrical conductivity were examined to characterize powder suitability as an interconnect material in solid oxide fuel cells (SOFCs). X-ray diffraction indicated that the MnCo2O4 spinel synthesized by the glycine nitrate process was stable until 1100 °C and it was possible to obtain a fully densified single phase spinel. On the other hand, the MnCo2O4 synthesized by a solid state reaction decomposed into a cubic spinel and CoO after being sintered at 1100 °C. This might be associated with the reduction of Co3+ in the octahedral site of the cubic spinel phase. MnCo2O4 showed a thermal expansion coefficient comparable to that of other SOFCs components, as well as good electrical conductivity. Therefore, MnCo2O4 is a potential candidate for the ceramic interconnects in SOFCs, provided the phase instability under reducing environments can be improved.

Development of the spinel powder reduction technique for solid oxide fuel cell interconnect coating

Mn 0.9Y 0.1Co 2O 4 spinel coatings are developed for solid oxide fuel cell (SOFC) alloy interconnects by a novel powder reduction technique. Material properties, electrical performance and long-term stability of the coatings are explored. The coating is about 9 μm in thickness and adheres well to the alloy substrate without any cracking or delamination. The area specific resistance (ASR) remains almost unchanged and is less than 3 mΩ cm 2 even though the coated alloy undergoes oxidation at 800 °C for 1017 h and ten thermal cycles from 800 °C to room temperature. The coated alloy presents excellent electrical performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.

Oxidation behavior of (Co,Mn) 3O 4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co 1.5Mn 1.5O 4 spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented and discussed.

Effects of Nb and W additions on high-temperature creep properties of ferritic stainless steels for solid oxide fuel cell interconnect

Tensile and creep properties of a newly developed ferritic stainless steel (Crofer 22 H) with additions of Nb and W for planar solid oxide fuel cell (pSOFC) interconnect are investigated at 25–800 °C. These properties are compared with those of another ferritic stainless steel (Crofer 22 APU) without Nb or W. A significantly improved tensile and creep strength of Crofer 22 H over Crofer 22 APU for pSOFC interconnect is found and attributed to a precipitation strengthening effect of the Laves phase. According to the creep stress exponent, activation energy, and microstructural observations, the superiority of Crofer 22 H over Crofer 22 APU in creep resistance at 650–800 °C involves a power-law dislocation creep mechanism interacting with an in situ precipitation strengthening mechanism. The relation between creep rupture time and applied stress for Crofer 22 H is well described by a simple power law. Larson–Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 H steel. A significant coarsening of the Laves phase is responsible for the reduced improvement of creep resistance in Crofer 22 H at the low-stress, long-term region of 800 °C.

A combined SEM and CV study of solid oxide fuel cell interconnect steels

Scanning electron microscopy and cyclic voltammetry were used to investigate the high-temperature oxidation behavior of two solid oxide fuel cell interconnect steels. One alloy had a low content of manganese; the other alloy had a high content of manganese. Four reduction and four oxidation peaks were observed in the voltammograms of the low manganese alloy at a temperature of 800 °C. The voltammograms of the alloy with a high content of manganese were different. At 600 °C, three reduction peaks and two oxidation peaks were observed. At 800 °C, additional peaks were observed in the voltammogram for this alloy.

Magnetron-sputtered cobalt-based protective coatings on ferritic steels for solid oxide fuel cell interconnect applications

Co, Co–Mn (67:33 at.%) and Co–Cu (67:33 at.%) coatings were fabricated using magnetron sputtering on two kinds of ferritic stainless steels (Crofer22APU and F17TNb) in order to form spinel protective coatings on metallic interconnects for solid oxide fuel cells. Despite the thickness unevenness at different regions, dense metallic coatings were successfully applied onto all necessary surfaces of the channelled interconnect substrates. Upon oxidation, spinel oxide coatings with very low Cr content were formed, reducing effectively the Cr release. Among the three protective coatings, Co–Cu coating showed the lowest area specific resistance (<15 mΩ cm 2 at 800 °C).

Effect of glass contents on the electrical and sintering property of La 0.8Ca 0.2CrO 3/glass composite interconnects for solid oxide fuel cells

In order to improve the sintering ability and electrical conductivity of La 0.8Ca 0.2CrO 3 (LCC), LCC/glass composite interconnect materials for high temperature solid oxide fuel cells (SOFCs) were studied in this paper. Glass is known as a sintering aid for improving sintering ability. It promotes liquid phase sintering and improves densification during the sintering process. The components of the glass used in this study are B 2O 3, SrO, La 2O 3, SiO 2 and Al 2O 3.The phase stability, microstructure, electrical conductivity and thermal expansion coefficient (TEC) were measured to determine the optimal glass content in the composite materials. All of the tested composite materials showed perovskite structures and dense microstructures. It was found that the addition of up to 5 wt.% glass increased the sintering ability and the electrical conductivity in both air and hydrogen atmospheres. The glass powder enhances the sintering behavior because it acts as a liquid phase sintering aid and the Sr 2+ ion in glass powder generates [ S r L a ′ ] and [ C r C r • ] . These lead to improvement in the electrical conductivity of the material. The TEC of the composites indicated compatibility with other cell components. The above results present that LCC/glass composite materials are suitable to be used as interconnects for SOFCs.

Nanocoatings for SOFC Interconnects - Mitigating Chromium Volatilization and Improving Corrosion Properties

Two important degradation mechanisms in Solid Oxide Fuel Cells (SOFC) are directly related to the metallic interconnects. The formation of volatile chromium oxides from metallic interconnects commonly causes fast degradation in cell performance due to poisoning the cathode. Secondly is the ability of the metallic interconnect to form a thin protective oxide one of the most important lifetime limiting factors for SOFC. Chromium volatilization of various uncoated steels is studied as a function of temperature by a recently developed denuder technique which allows time resolved quantification of volatile chromium species. The inhibition of Cr evaporation by Co thin film coatings (800nm) is investigated; it will be shown that these coatings are more effective than much thicker ceramic coatings that are commonly used for this purpose. In order to increase the lifetime of the metallic components in SOFC nano-coatings of reactive elements (RE) have been investigated as well. The application of such coatings can reduce the corrosion rates substantially and thus increase the lifetime of the fuel cell stack. It will be shown that it is possible to combine the positive effects of RE with the beneficial effects of a Co coating and thus to obtain an interconnect material with low Cr evaporation and increased oxidation resistance.

Manganese-Cobalt Spinel Coating on Alloy Interconnects for SOFCs

Manganese-cobaltite spinel coatings were produced on Fe-Cr alloys for the improvement of the chemical and mechanical stability of solid oxide fuel cell (SOFC) interconnects. It was found by thermal investigation, i.e., the microscopy of various samples that were heat-treated in air at 800 °C, that the screen-printed coating more effectively inhibited oxide scale growth than the sputtered coating. The reason why the manufacturing method of the spinel coating affected the oxide scale growth rate was investigated. It was demonstrated that the oxide scale in both the samples after annealing in air at 800 °C for 5000 h comprised MnCr2O4 and Cr2O3 with no difference in composition. However, the interface between the alloy and the oxide scale was deeper and rougher and had a larger grain size because of the high oxygen diffusivity in the sputtered coating. In contrast, in the screen-printed sample, the dense spinel layer above the oxide scale blocked oxygen diffusion into the alloy, so the oxide scale growth rate was lower and the interface between the alloy and oxide scale remained flat even after thermal treatment. Introducing a reduction treatment in the manufacturing process made the Mn-Co spinel layer denser and further inhibited the oxide scale growth. Moreover, the addition of Li as a sintering aid into the Mn-Co spinel was found to even more effectively inhibit the oxide scale growth.

Sputtered Ni coating on ferritic stainless steel for solid oxide fuel cell interconnect application

Ni coating was deposited on ferritic stainless steel by means of magnetron sputtering method for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. The steel with the sputtered Ni coating was evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the steel with Ni coating was close to that of the bare steel after 100 h thermal exposure, in spite of its mass gain being higher than that of the bare steel. The oxide scale formed on it consisted of double layers containing an inner layer of Cr 2O 3 and an outer layer of NiO with (Ni,Fe,Cr) 3O 4. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr 2O 3 scale thermally developed on the bare steel.

Oxidation and electrical behavior of AISI 430 coated with cobalt spinels for SOFC interconnect applications

Stainless steel can be used as interconnect plates in solid oxide fuel cells (SOFCs) below operating temperature of 800°C. Unwanted reactions between the alloy and other SOFC components decrease the efficiency of these energy convertors. One approach to improving interconnect properties is to apply a surface coating to them. In this study, AISI 430 ferritic stainless steel interconnect is coated in a cobalt-base pack mixture using the pack cementation method. Isothermal oxidation, cyclic oxidation and oxidation at different temperatures (400–900°C) are applied to evaluate the role of the coating layer during oxidation. Area-specific resistance (ASR) of the coated substrates has also been tested as a function of temperature and time. The surface morphology was examined by SEM, the chemical composition and structure of oxide formed were analysed by EDS and XRD. Results showed that the coating layer transforms into MnCo2O4, CoCr2O4 and CoFe2O4 spinels during isothermal oxidation. This scale is protective and acts as an effective barrier against chromium migration into the outer oxide layer and prevents weight gain. The mass gain and spallation indicated that the formation of spinel significantly improved the high temperature oxidation. These spinels also cause a reduction in ASR for coated substrates (9.7mΩcm2) as compared to uncoated substrates (36.1mΩcm2) after 200h of isothermal oxidation at 800°C.

A-Site and B-Site Non-stoichiometry and Sintering Characteristics of (Sr1−xLax)1−yTi1−zO3 Perovskites

The A-Site and B-Site non-stoichiometry and sintering characteristics of (Sr1−xLax)1−yTi1−zO3 perovskites were studied using samples prepared by solid-state mixing. A-Site or B-Site deficiency was observed for La-doped SrTiO3 perovskites, and this increased with increasing La content. For A-Site deficient (Sr1−xLax)1−yTiO3 perovskites, the cell volumes decreased although the reverse was observed for B-Site deficient (Sr1−xLax)Ti1−zO3 perovskites. This may be related to number of B-Site vacancies in the perovskites through charge compensation. It was found that A-Site and B-Site deficiencies increase the density of (Sr1−xLax)1−yTi1−zO3 perovskites. These characteristics may prove useful in the development of solid oxide fuel cell interconnect materials.

Electrophoretic deposition of (Mn,Co)3O4 spinel coating for solid oxide fuel cell interconnects

We discuss here our attempt to develop (Mn,Co)3O4 spinel coatings on the surface of Cr-containing steel through electrophoretic deposition (EPD) followed by reduced-atmosphere sintering for solid oxide fuel cell (SOFC) interconnect application. The effects of EPD voltages and sintering atmospheres on the microstructure, electrical conductivity and long-term stability of the coated interconnects are examined by means of scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and four-probe resistance techniques. For the spinel coatings generated using smaller voltage than 400V, the interconnect surfaces exhibit good packing behavior and high conductivity. The reduced atmosphere during sintering has a beneficial impact on the minimizing chromia subscale formation and thus reducing the area specific resistance (ASR) of the coated interconnects. Moreover, it is interesting to note that a more stable long-term performance is achieved for the spinel coating sintered in H2/H2O atmosphere with thin chromia sub-scale and no Cr penetration. Based on the current results, EPD followed by reduced-atmosphere sintering is a fast and economic way to deposit (Mn,Co)3O4 coating for SOFC interconnect applications.

The effect of Mn on the oxidation behavior and electrical conductivity of Fe–17Cr alloys in solid oxide fuel cell cathode atmosphere

Four Fe–17Cr alloys with various Mn contents between 0.0 and 3.0 wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe–Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1 min) at 750 °C in air, Cr is preferentially oxidized to form a layer of Cr 2O 3 type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr 2O 3 type oxide layer to form Mn-rich (Mn, Cr) 3O 4 and Mn 2O 3 oxides on the top. After 700 h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0 wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0 wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe–17Cr alloy with conducting Cr free spinel coatings is preferred.

Oxide Modification by Alloying Molybdenum to Fe-22Cr-0.5Mn for Solid Oxide Fuel Cell Interconnect

The effect of molybdenum addition to Fe-22Cr-0.5Mn ferritic stainless steel on the oxidation, electric property and Cr evaporation was investigated for applications to the solid oxide fuel cell interconnect. Mass gain was measured at 800oC to evaluate the oxidation rate and a 4-probe DC method was used to measure the area specific resistance. Cr and Mo evaporation rate was evaluated by the condensation method. Addition of 0.1 - 4 wt% Mo improved oxidation resistance and electric conductivity. The result suggests that Mo is doped into Cr2O3 layer and changes the defect structure of the layer. The evaporation rate of Cr vapor species was not changed significantly by Mo addition.

Low-Chromium Alloys for Solid Oxide Fuel Cell Interconnects

Ferritic stainless steel alloys have a combination of good oxidation resistance and appropriate coefficient of thermal expansion that makes them attractive for use as the interconnect in solid oxide fuel cells. However, chromia can be oxidized to vapor species, which can lead to poisoning of the cathode, so ceramic coatings are applied to reduce chromium volatilization. With the use of coatings, alloys with lower chromium contents could potentially be used. In this work, the oxidation behaviors of a series of ferritic steel alloys, Mn1.5Co1.5O4-coated and uncoated, with chromium contents from 13-18% and other alloying additions typical of those of AISI 441 (i.e. small amounts of niobium and titanium) were evaluated.

Oxide Protective Coatings for Solid Oxide Fuel Cell Interconnects

The long-term stability of SOFC stacks is a critical hurdle to their commercial success. To maintain low stack and system costs, metallic interconnect materials are required; to achieve long life, protective coatings must stabilize metal interconnect performance. Further, these coatings must exhibit sufficient bulk conductivity to achieve low stack area specific resistance. To exploit the cost advantages offered by metallic interconnects, coating approaches must be size-scalable and allow high volume throughput at low capital cost. NexTech has systematically developed cost-effective approaches to synthesizing and depositing oxide coatings through value-conscious materials processing and deposition processes. Recently, manganese cobalt spinel synthesis and deposition has been demonstrated using production process designed to allow high throughput production.