Solid Oxide Fuel Cell Interconnect Research Articles

High temperature low cycle fatigue properties of 24Cr ferritic stainless steel for SOFC applications

The low cycle fatigue (LCF) properties of 24Cr ferritic stainless steel for solid oxide fuel cell interconnects were investigated. The fatigue strength of 24Cr stainless steel decreased with increasing temperature, but the fatigue life increased at 600°C and 700°C. The fatigue behavior at room temperature (RT) was characterized as cyclic hardening followed by saturation and cyclic softening while marginal cyclic hardening was observed at 600°C and 700°C. The superior oxidation resistance of 24Cr stainless steel allows preventing the fatal impact of oxidation on the high-temperature fatigue life. Microstructural analysis showed that persistent slip bands (PSBs) developed prevalently at RT but not at 600°C and 700°C. Such temperature-dependent microstructural difference retarded the crack initiation and prolonged fatigue life at high temperatures.

Effect of Sr and Ca dopants on oxidation and electrical properties of lanthanum chromite-coated AISI 430 stainless steel for solid oxide fuel cell interconnect application

Abstract Lanthanum chromite coating occupies a noticeable position as a ceramic coating on metallic interconnects in solid oxide fuel cells because of its excellent electrical conductivity, high oxidation resistance and desirable chemical stability in both oxidizing and reducing atmospheres. In the present work, a sol–gel process based on the dip-coating technique was used to prepare dense and uniform coatings on metallic alloy for interconnect application (AISI 430 type). The effect of strontium and calcium doping into lanthanum chromite structure on electrical conductivity and oxidation behavior of the coated sample has been investigated. The oxidation behavior was evaluated by cyclic oxidation test at 800 °C in lab air. In addition, the area specific resistance (ASR) of the coated and uncoated samples after long-term oxidation was measured through a two-point, four-wire probe method. It was found that, for the coated sample, when compared to the bare metallic one, the addition of Ca into lanthanum chromite coating drastically lowered the oxidation rate and electrical resistance by approximately 2 and 6 times, respectively.

Evaluation of Protective La0.67Sr0.33MnO3–δ Coatings on Various Stainless Steels Used for Solid Oxide Fuel Cell Interconnects

Four metallic alloys, namely 2205 duplex stainless steel (2205DSS), ZMG232, and stainless steels SS430 and SS304 are investigated for use as interconnects in solid oxide fuel cells (SOFCs). A La0.67Sr0.33MnO3–δ (LSMO) film is deposited on these metallic-alloy substrates using a pulsed-DC magnetron sputtering system in the reactive mode, leading to the formation of a cubic perovskite structure. The coated alloys are then subjected to oxidizing heat treatments in air at 600 °C, 700 °C, 800 °C, and 900 °C, and their microstructures as well as electrical resistances are evaluated. The electrical resistance measurements are performed at 800 °C, and the area-specific resistance (ASR) of the film-coated 2205DSS alloy is found to be less than that of the uncoated alloy. This is because a thick layer of Cr2O3 and a (Mn, Fe)Cr2O4 spinel phase layer are formed, and some divalent metallic ions migrate into the Cr2O3 layer. It is found that alloys coated with a thin film of LSMO are more suitable for use as metallic interconnects in SOFCs with intermediate-temperature operating ranges.

Studies on elements diffusion of Mn/Co coated ferritic stainless steel for solid oxide fuel cell interconnects application

The spinel structure of manganese cobalt oxide (Mn,Co)3O4 is one of the most promising coatings for solid oxide fuel cell (SOFC) stainless steel interconnects. The stoichiometric Mn1.5Co1.5O4 composition has properties that are preferable to other Mn/Co ratios, for example a higher conductivity and a thermal expansion coefficient that matches the typical steel substrate. However, previous work showed the Mn/Co ratio changes during operation due to the diffusion of Mn from the substrate. The results presented here are on three coatings with different compositions (namely; pure Co, Mn20Co80, and Mn40Co60) with each coating composition deposited to a thickness of 800 nm, 1500 nm, and 3000 nm. The coatings were applied by DC magnetron sputtering and then machine cut into coupons for isothermal annealing at 800 °C in air using a batch-type furnace for 2, 10, 50, 250, and 1000 h. The morphology, chemical composition (including surface and cross sections of the layers) and structures of the oxides formed were analyzed by SEM, EDS and XRD. Analysis of the element diffusion (Mn, Co, Cr, Fe) shown here points to an optimized coating recipe of Mn40Co60.

Development of low-temperature sintered Mn–Co spinel coatings on Fe–Cr ferritic alloys for solid oxide fuel cell interconnect applications

To reduce the sintering temperature of spinel coatings to a safe temperature for Fe–Cr ferritic alloys, an MnCo2O4–MnO2 bicomponent coating has been prepared on SUS430 ferritic alloy by slurry coating, followed by air sintering at a temperature of 800 °C. The resultant coating is stable during cyclic oxidation tests and is effective at improving the oxidation resistance of SUS430 alloy. The area-specific resistance of coated samples remains nearly constant during long-term oxidation at 800 °C and is less than 6 mΩ cm2 even after oxidation for 1000 h. These results indicate that the low-temperature sintered MnCo2O4–MnO2 coating is a promising protective material for Fe–Cr ferrite alloys, especially SUS430, which are seriously oxidized at temperatures ≥850 °C.

Sputtered nanocrystalline coating of a low-Cr alloy for solid oxide fuel cell interconnects application

Sputtered coating of a low-Cr Fe–Co–Ni alloy with 0.36wt.% Si has been prepared on the same cast alloy via magnetron sputtering method. The sputtered low-Cr alloy coating with columnar nanocrystalline structure is thermally oxidized in air at 800°C corresponding to the cathode atmosphere of solid oxide fuel cell (SOFC). It is found that the oxidation resistance of the sputtered low-Cr alloy coating is significantly improved in comparison with the cast low-Cr alloy. A double-layer oxide scale is thermally developed on the sputtered low-Cr alloy coating after oxidation for 12 weeks. The outer layer is (Co,Fe)3O4 spinel containing small amount of Ni and Cr. The inner layer is a continuous Cr2O3 layer, followed by oxides of CrNbO4 and Nb2O5. The surface scale formed on the sputtered coating after 12-week thermal exposure demonstrates an area specific resistance (ASR) of 31.97mΩcm2. The sputtered low-Cr alloy nanocrystaline coating exhibits a promising perspective for intermediate temperature SOFC interconnects application.

Long-term oxidation behavior of spinel-coated ferritic stainless steel for solid oxide fuel cell interconnect applications

Long-term electrical resistance tests were performed to evaluate the performance of AISI 441 ferritic stainless steel coated with a Mn–Co spinel protection layer as a candidate material for intermediate temperature solid oxide fuel cell interconnect applications. The tests indicated that, while uncoated AISI 441 shows a substantial increase in area-specific electrical resistance (ASR), spinel-coated AISI 441 exhibits much lower ASR values. The spinel coatings reduced the oxide scale growth rate and blocked outward diffusion of Cr from the oxide scale that grew between the coating and the steel substrate during the long-term tests. The oxide scale consisted of a chromia layer containing discrete regions of Mn–Cr spinel distributed throughout the layer. The presence of Ti in the chromia scale matrix and/or the presence of regions of Mn–Cr spinel within the scale may have increased the scale electrical conductivity, which would explain the fact that the observed ASR in the tests was lower than would be expected if the scale consisted of pure chromia.

Development and Application of HVOF Sprayed Spinel Protective Coating for SOFC Interconnects

Protective coatings are needed for metallic interconnects used in solid oxide fuel cell (SOFC) stacks to prevent excessive high-temperature oxidation and evaporation of chromium species. These phenomena affect the lifetime of the stacks by increasing the area-specific resistance (ASR) and poisoning of the cathode. Protective MnCo2O4 and MnCo1.8Fe0.2O4 coatings were applied on ferritic steel interconnect material (Crofer 22 APU) by high velocity oxy fuel spraying. The substrate-coating systems were tested in long-term exposure tests to investigate their high-temperature oxidation behavior. Additionally, the ASRs were measured at 700 °C for 1000 h. Finally, a real coated interconnect was used in a SOFC single-cell stack for 6000 h. Post-mortem analysis was carried out with scanning electron microscopy. The deposited coatings reduced significantly the oxidation of the metal, exhibited low and stable ASR and reduced effectively the migration of chromium.

Structural optimization of (La, Sr)CoO3-based multilayered composite cathode for solid-oxide fuel cells

In the present study, (La, Sr)CoO3 material was screen-printed as a cathode and as current-collecting layers for planar-type low-temperature solid-oxide fuel cells (SOFCs). The compositional effects of the cathode functional and current-collecting layer on unit-cell performance were investigated using AC and DC polarization experiments. Two different cathode (La, Sr)CoO3 materials, La0.6Sr0.4CoO3 and La0.8Sr0.2CoO3, were examined, and La0.6Sr0.4CoO3 was found to be more effective for increasing the electrocatalytic activation and collection efficiency of the current produced in the cathode reaction. Furthermore, a well-designed architecture for the current-collecting layer was very effective in increasing unit-cell performance by reducing the contact resistance between SOFC cathodes and interconnects.

Oxidation of Co- and Ce-nanocoated FeCr steels: A microstructural investigation

The effect of novel Co and Ce nanocoatings on oxidation behaviour and chromium volatilization from a commercial Fe–22Cr steel (Sanergy HT) developed for solid oxide fuel cell interconnect applications is investigated. Three different coatings (10nm Ce, 640nm Co and 10nm Ce+640nm Co) are studied. Uncoated and nanocoated samples are exposed isothermally at 850°C in the air with 3% H2O for 168h. The detailed microstructure of the different coatings is investigated. The surface morphology and microstructure of the oxide scales are characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX). Cross-section TEM thin foils are prepared by using a combined FIB/SEM (focused ion beam/scanning electron microscope) instrument. A 640nm cobalt coating strongly inhibits Cr volatilization but has only minor effects on oxidation rate. In contrast, a 10nm Ce coating decreases the oxidation rate but has no significant effects on chromium volatilization. Combining the two coatings, i.e., applying a 640nm Co coating on top of the 10nm Ce, effectively reduces Cr evaporation and slows down the rate of alloy oxidation.

LaCrO3 composite coatings for AISI 444 stainless steel solid oxide fuel cell interconnects

Doped lanthanum chromite-based ceramics are the most widely used interconnector material in solid fuel cells (SOFC) since they exhibit significant electrical and thermal conductivity, substantial corrosion resistance and adequate mechanical strength at ambient and high temperatures. The disadvantage of this material is its high cost and poor ductility. The aim of this study is to determine the mechanical and oxidation behavior of a stainless steel (AISI 444) with a LaCrO3 deposition on its surface obtained through spray pyrolisis. Coated and pure AISI 444 materials were characterized by mechanical properties, oxidation behavior, X-ray diffraction and scanning electronic microscopy. Results indicated that the coated material displays better oxidation behavior in comparison to pure stainless steel, but no improvement in mechanical strength. Both materials indicate that deformation behavior depends on testing temperatures.

Formation of spinel reaction layers in manganese cobaltite – coated Crofer22 APU for solid oxide fuel cell interconnects

The microstructural development of Mn1.5Co1.5O4-coated Crofer22 APU has been studied using cross-sectional transmission electron microscopy. Alloy samples were coated via a slurry process involving consolidation by reduction and re-oxidation, and these samples were then oxidized at 800 °C for times of up to 1000 h. All samples exhibited a thin chromia scale at the alloy/coating interface plus spinel phases as a reaction layer between the chromia and the manganese cobaltite coating. The oxidized samples also exhibited pockets of stoichiometric MnCr2O4 spinel at the chromia/alloy interface and internal Ti-rich oxides in the alloy below the chromia. The reaction layer spinels exhibit remarkable changes in thickness, morphology and composition, and these effects are explained on the basis of changes in the diffusive fluxes during the different stages of coating application and subsequent exposure. The possible consequences of these observations for the degradation mechanisms that could affect SOFC interconnects produced from MCO-coated Crofer22 APU are discussed.

Long term study of Cr evaporation and high temperature corrosion behaviour of Co coated ferritic steel for solid oxide fuel cell interconnects

The oxidation behaviour of the uncoated ferritic Fe-22Cr steel Sanergy HT is compared with an 640 nm Co coated version of the same material. The materials have been subject to corrosion and Cr volatilization measurements in air for up to 3000 h at 850 °C. Oxidation tests have been carried out both isothermal and discontinuously. The volatilization measurements were carried out using a recently developed denuder technique, which allows to quantify Cr evaporation in a time resolved manner. The oxidation process is studied from very initial phases (>15 s) to long term behaviour (3000 h). The formed oxide scales are analysed by XRD, SEM/EDX as well as TEM/EDX.The results show that both materials form an oxide scale with an inner layer of Cr2O3 and a spinel layer on top. In the case of the uncoated material, the spinel layer is of (Cr,Mn)3O4 type while in the presence of a Co coating a (Co,Mn,Fe)3O4 is formed. The Cr evaporation measurements show that despite the fact that the Co coating is very thin (640 nm) it effectively blocks Cr evaporation for at least 3000 h. This is in line with TEM analysis showing that after 3000 h there is only a low Cr content in the outer oxide scale. This long term stability indicates the suitability of the coated material as solid oxide fuel cell (SOFC) interconnect.

Influence of precipitation on initial high-temperature oxidation of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy

Oxidation phenomena on Laves phase forming Ti–Nb stabilized ferritic stainless steel (EN 1.4509) were studied at 650 °C by electron microscopic and electron spectroscopic methods. These investigations reveal a strong competition between Nb and Si for interfacial oxidation at the oxide–metal interface that is affected by different segregation rates of Nb and Si at elevated temperatures. In particular, formation of Si containing Laves (FeNbSi)-type intermetallic compounds in the bulk results in non-uniform distribution of Si oxide at the interface. This has direct implications to the electrical properties of this alloy in solid oxide fuel cell (SOFC) applications. Furthermore, these results provide better understanding to the controversial role of second phases (e.g. Laves, chi) on high-temperature oxidation (as recently discussed by Dae Won Yon, Hyung Suk Seo, Jae Ho Jun, Jae Myung Lee, Do Hyuong Kim, Kyoo Young Kim in Int J Hydrogen Energy 2011;36:5595–5603).

Thermo-mechanical fatigue properties of a ferritic stainless steel for solid oxide fuel cell interconnect

Thermo-mechanical fatigue (TMF) behavior of a newly developed ferritic stainless steel (Crofer 22 H) for planar solid oxide fuel cell (pSOFC) interconnect is investigated. TMF tests under various combinations of cyclic mechanical and thermal loadings are conducted in air at a temperature range of 25oC–800 °C. Experimental results show the number of cycles to failure for non-hold-time TMF loading is decreased with an increase in the minimum stress applied at 800 °C. There is very little effect of maximum stress applied at 25 °C on the number of cycles to failure. The non-hold-time TMF life is dominated by a fatigue mechanism involving cyclic high-temperature softening plastic deformation. A hold-time of 100 h for the minimum stress applied at 800 °C causes a significant drop of number of cycles to failure due to a synergistic action of fatigue and creep. Creep and creep–fatigue interaction mechanisms are the two primary contributors to the hold-time TMF damage. The creep damage ratio in the hold-time TMF damage is increased with a decrease in applied stress at 800 °C and an increase in number of cycles to failure.

Electroplated Ni–Fe2O3 composite coating for solid oxide fuel cell interconnect application

Ni–Fe2O3 composite coating was applied onto ferritic stainless steel using the cost-effective method of electroplating for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. By comparison, the coated and bare steels were evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the coated steel was close to that of the bare steel after initially rapid mass gain. The mass gain of the coated steel was higher than that of the bare steel owing to the formation of double-layer oxide structure with an outer layer of (Ni,Fe)3O4/NiO atop an inner layer of Cr2O3. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr2O3 scale thermally grown on the bare steel.

Mechanical performance of LaCrO3 doped with strontium and cobalt for SOFC interconnect

Abstract Doped lanthanum chromite was investigated as ceramic interconnect materials to be used in high-temperature solid oxide fuel cells (SOFCs). In this work, a La0.80Sr0.20Cr0.92Co0.08O3 powder produced by combustion synthesis was used to obtain dense specimens. Powders were uniaxially compacted under a load of 30 MPa and sintered at 1500 °C for 4 h in air. The sintered specimens were characterized in terms of crystalline phases, density and porosity, strength and hardness measurements, deformation behavior, fracture mode and microstructure aspects. The results have shown dense samples (6.09 g/cm3) with strength and hardness values of 62 MPa and 5.98 GPa, respectively. Surface micrograph indicates a mixture of inter- and transgranular fracture modes, a brittle deformation behavior and grain size around 3 μm.

In Situ Electrochemical X-ray Spectromicroscopy Investigation of the Reduction/Reoxidation Dynamics of Ni–Cu Solid Oxide Fuel Cell Anodic Material in Contact with a Cr Interconnect in 2 × 10–6 mbar O2

Following our systematic investigations on the durability of solid oxide fuel cell (SOFC) components (Bozzini, B.; Tondo, E.; Prasciolu, M.; Amati, M.; Kazemian, M.; Gregoratti, L.; Kiskinova, M. ChemSusChem, http://dx.doi.org/10.1002/cssc.201100140), the present in situ scanning photoelectron microscopy study is focused on the redox behavior of Ni–Cu bilayers in contact with Cr, representing the anodic material and interconnects for SOFCs, respectively. The experiments with this model cell, using yttria-stabilized zirconia (YSZ) electrolyte, were carried out in 2 × 10–6 mbar O2 at 650 °C at open circuit potential (OCP) and under applied potential. The elemental images and the spectra from selected parts of the cell have revealed dramatic compositional and morphological changes under OCP conditions, yielding Ni–Cu islands covered with NiO in the electrode region and a NiO network in the YSZ electrolyte region. The Ni reduction dynamics as a function of applied potential is followed by continuous monitorin...

Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling

Research and development efforts continue to improve oxidation resistance and electrical conductivity of solid oxide fuel cell (SOFC) interconnects. This research evaluates the performance of a ferritic stainless steel (AISI 430 — a candidate SOFC interconnect material) with and without a Co coating (via sol–gel dip coating technique) with various pre-treatments, during cyclic oxidation exposures up to 750°C in laboratory air. A pre-treatment exposure of Co coated samples to a 750°C reducing atmosphere (prior to oxidation exposures) led to the formation of an effective Co-based spinel oxide coating that lowers oxidation rates by more than 40 times, and substantially lowers area specific resistance (ASR) in comparison to uncoated specimens. The development of effective sol–gel Co coatings with reducing pre-treatments, and their evolution within simulated SOFC interconnect environments is presented and discussed.

Ionic Conductivity Method for measuring vaporized chromium species from solid oxide fuel cell interconnects

A novel method of measuring vaporized chromium species from metallics specimens, in real time, was developed to enable more rapid alloy and coating screening for use as solid oxide fuel cell (SOFC) interconnects. This method, the Ionic Conductivity Method (ICM), simulates an SOFC environment and captures vaporized species into a de-ionized water (DI) solution where total amount of vaporized chromium can be measured. The development, design, and final implementation of this method is presented with calculations of expected theoretical accuracy and comparison with alternative methods. This method will allow for further investigation into vaporization kinetics, more rapid coating research, and facilitate inquiry into secondary vaporized species. Finally, ICM was used to measure the vaporization rates of bulk chromia, CroferAPU22, E-Brite, ZMG-232, and 441 Stainless Steel for comparison with current methods.