Ferritic Interconnects Research Articles

Cu- and Ce-doped MnCo2O4 spinel coatings on ferrite interconnects by electrophoretic deposition

MnCo2O4 spinel coating is considered as one of the most promising protective coatings on the surface of ferritic stainless steel interconnects in solid oxide fuel cells (SOFCs). And the doping of metallic elements and active rare earth elements may further improve the electrical conductivity and antioxidant properties of spinel coatings. In this work we firstly prepare the Cu-doped and Ce-doped MnCo2O4 spinel coatings (abbreviated as MCCu, MCCe) on the surface of SUS430 substrate by Sol-Gel and electrophoretic deposition methods. The optimal doped ratio of Cu and Ce based on the results of oxidized mass gain and area-specific resistance (ASR) tests are determined respectively. Then we deposit Cu-Ce co-doped MnCo2O4 spinel coatings (abbreviated as MCCuCe) accordingly. The successful deposition of the target modified spinel coating on the substrate surface is confirmed using SEM-EDS, XRD, XPS characterization. The performance test results of the MCCuCe coating on the SUS430 surface show that the mass gain after 500 h oxidation at 800 °C in air is 1.3564 × 10−3 mg cm−2, and the ASR at 800 °C after 500 h oxidation is 9.80mΩ cm2, which is much lower than those of the bare SUS430 substrate and the substrate with deposited MnCo2O4 spinel coating. And the MCCuCe protective coating plays a better role in preventing the diffusion of Cr from the substrate. Further analysis prove that the doping Cu mainly contributes to the electrical conductivity and adhesion to the substrate while the doping Ce mainly improve the anti-oxidation and Cr diffusion resistance. This work determines the optimal doped ratio of Cu and Ce and provides a simple way to prepare the Cu-Ce co-doped MnCo2O4 spinel coatings on the ferritic interconnects, which effectively improve comprehensive performances of MnCo2O4 spinel coatings and helps to accelerate the industrial application of SUS430 ferrite in the SOFCs.

Mitigation of Chromium Poisoning of Ferritic Interconnect of SOFC from Annealed Spinel of CuFe2O4

Low-temperature solid oxide fuel cells revitalizing usage of metallic interconnects over former ceramic materials. However, chromium poisoning is reducing the performance of ferritic interconnects. To resolve this problem, an attempt to layer metal-oxide coating has been considered. This article encloses the fabrication of optimized slurry composition of CuFe2O4 spinel over Crofer 22 APU and SUS 430 by using the dip-coating technique. These coated samples were dried, heated at 800oC in reducing environment (5% hydrogen and 95% nitrogen), oxidized in the open air and lastly annealed. SEM-EDS results are confirming that spinel protection layer has been fabricated over steel substrates, avoiding any spallation. Annealing reduced thickness of oxide layer from 7.25µm to 4.69µm and improved density of spinel. Spinel coated Crofer 22 APU and SUS 430 formed three and four layers with weight gain rate of 1.65 x 10-12 g2 cm-4.s-1 and 2.597 x 10-12 g2cm-4.s-1 respectively. Decarburization was also observed in both substrates during operational heating and annealing process. The research reveals that the spinel is the promising protective layer for interconnects of SOFC and annealing is the better processing technique for achieving the preferred properties.

Mitigation of Chromium Poisoning of Ferritic Interconnect from Annealed Spinel of CuFe2O4

Low-temperature solid oxide fuel cells permit the possibility of metallic interconnects over conventional ceramic interconnects. Among various metallic interconnects, the ferritic interconnects are the most promising. However, chromium poisoning in them adversely affects their performance. To resolve this issue, various coatings and pretreatment methods have been studied. Herein, this article encloses the coating of CuFe2O4 spinel over two prominent ferritic interconnects (Crofer 22 APU and SUS 430). The CuFe2O4 spinel layer coating has been developed by the dip-coating of both samples in CuFe2O4 slurry, followed by heat treatment at 800 °C in a reducing environment (5% hydrogen and 95% nitrogen). Additionally, both samples were annealed to further enhance their spinel coating structure. The morphological and crystallinity analysis confirmed that the spinel coating formed multiple layers of protection while annealing further reduced the thickness and improved the densities. Moreover, the area-specific resistance (ASR) and weight gain rate (WGR) of both samples before and after annealing was calculated using mathematical modeling, which matches with the experimental data. It has been noted that CuFe2O4 spinel coating improved the ASR and WGR of both samples which were further improved after annealing. This research reveals that the CuFe2O4 spinel is the promising protective layer for ferritic interconnects and annealing is the better processing technique for achieving the preferred properties.

Post‐test Characterization of Metallic Materials and Adjacent Components in an SOFC Stack After 34,000 h Operation at 700 °C

AbstractThe present paper describes the electrochemical results and the post exposure microstructural characterization by means of SEM/EDS of a four layer SOFC stack after 34,000 h operation under load at 700 °C, showing low voltage degradation rate less than 0.3% per 1,000 h. Emphasis was put on the behavior of the ferritic interconnect steel and its interaction with glass sealants as well as contacting and coating materials. The interconnect steel had formed thin protective chromia scales. Gas distributing steel foils of 200 µm showed in some locations breakaway type oxidation. It was related to local overheating caused by locally occurring combustion of a defective cell. The interaction of the steel with the glass sealant showed good adhesion. Interdiffusion at the joint between nickel contact and steel interconnect at the anode side resulted in minor oxide formation on the Ni wires and in austenite formation in the steel without formation of σ‐FeCr formation at the steel/austenite interface. The steel showed excellent compatibility with the Manganese‐Cobalt‐Ferrite (MCF) chromium retention coatings. Chromium enriched phases were found near the interface between MCF coating and perovskite contact layer. Excess glass sealing material interacted with the contact layer without showing obvious detrimental effects.

Oxidation behaviour and phase transformations of an interconnect material in simulated anode environment of intermediate temperature solid oxide fuel cells

The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition. The kinetics of σ-phase formation and the depth of the interdiffusion zone were found to follow a traditional diffusion relationship. It was observed that as the Ni-concentration increases the sigma-phase re-dissolves and thus the zone which, contains sigma phase moves deeper into the ferritic steel with exposure time. Interdiffusion processes between the nickel layer and the ferritic steel result not only in accelerated formation of σ-phase but also in the formation of Cr-rich oxides within the nickel layer.

Behavior of Metallic Components During 4,000 h Operation of an SOFC Stack with Carbon Containing Fuel Gas

AbstractIn the present study the behavior of the ferritic interconnect steel Crofer 22 APU and the nickel contacting material during exposure in anode gas of a solid oxide fuel cell (SOFC) stack was investigated. The stack had been operating for 4,000 h and was ten times subjected to thermal cycling within this time period. A temporary high carbon activity in the fuel gas combined with temperature changes resulted in local disintegration of the nickel mesh. Additionally, nickel diffusion from the nickel mesh into the steel resulted in the formation of an austenitic zone. Diffusion of steel constituents into the nickel mesh lead to the formation of Cr,Mn‐oxides in the latter. Presence of the nickel/steel contact allows transport of carbon from the gas into the steel, resulting in local internal carburization of the steel. In areas which were not in direct contact with the nickel mesh, the Crofer 22 APU interconnect formed a protective surface oxide scale and no indications for carbon uptake were found. Mechanisms for the experimentally observed effects, including the local disintegration of the nickel mesh, are presented.

Formation of chromium containing impurities in (La,Sr)MnO3 solid-oxide-fuel-cell cathodes under stack operating conditions and its effect on performance

In this study, we present an investigation of the chromium-related electrical performance degradation of anode-supported SOFCs with a LSM/YSZ composite cathode. A traditional ferritic interconnect steel with high chromium content is established as the primary chromium source and chromium poisoning of the cathode is carried out at relevant SOFC operating conditions. The prolonged influence of the gaseous chromium species on the cell performance and cathode microstructure under constant current conditions was examined quantitatively. Physical deposition of chromium(III) compounds (mainly spinel-type (Cr,Mn)3O4 phases) was observed at the electrochemically active region adjacent to the electrolyte only under realistic constant current conditions. The microstructural degradation associated with the formation of secondary phases correlated directly with the performance degradation, the effective chromium partial pressure and the current density. Furthermore, the influence of the presence of a number of protecting layers on the interconnect steel was evaluated with regard to the cathode poisoning. It was shown that chromium-induced degradation was reduced drastically when an additional manganese reservoir layer and a Cr getter layer were applied.

Mechanisms of Oxide Scale Formation on Ferritic Interconnect Steel in Simulated Low and High pO2 Service Environments of Solid Oxide Fuel Cells

The Laves phase strengthened ferritic steel Crofer 22 H has recently been proposed as construction material for interconnects in solid oxide fuel cells (SOFCs). In the present study, the oxidation kinetics of Crofer 22 H at 800 °C was compared with that of a Mn-free batch during exposure in air and Ar–H2–H2O, the latter simulating the anode gas of a SOFC. Main emphasis was put on studying the oxidation mechanisms of Crofer 22 H during the early stages of isothermal reaction as well as during long-term discontinuous exposure. Differences in oxidation mechanisms in air and simulated anode gas (SAG) were defined. Oxidation rates for Crofer 22 H in air were found to be initially slower than in SAG, but the opposite effect was observed after longer exposure times. This effect of atmosphere composition on the oxidation rate is shown to be governed by the time dependent extent to which the inner chromia scale layer is covered by an outer Cr/Mn spinel layer. In both test gases the minor addition of silicon to Crofer 22 H resulted in internal silica formation during the very early stages of oxidation; this effect vanished after longer exposure times. Incorporation of titanium in the scale differed for the two test atmospheres because the oxygen partial pressure affects titanium solubility in chromia and especially the spinel phase.

Evaluation of the oxidation and Cr evaporation properties of selected FeCr alloys used as SOFC interconnects

In recent years, a number of ferritic interconnect materials for use in solid oxide fuel cells (SOFC) have been developed and are now commercially available. Although similar, there are substantial variations in minor alloying elements. This study compares the oxidation performance of five such interconnect materials: Crofer 22 H, Crofer 22 APU (ThyssenKrupp VDM), Sanergy HT (Sandvik Materials Technology), ZMG232 G10 (Hitachi Metals) and E-Brite (ATI Allegheny Ludlum).1000 h exposures have been carried out in tubular furnaces at 850 °C, with 6 l/min airflow and 3% H2O to simulate the air side atmosphere in an SOFC. In addition to the oxidation tests, time resolved in-situ chromium evaporation measurements have been carried out using a novel denuder technique. It was found that higher Mn concentrations in the alloy lead to lower Cr evaporation. Nonetheless, all steels exhibit substantial Cr volatilization and coatings are needed for most SOFC applications. Furthermore, this study demonstrates that the mass gain data alone can be misleading, and the mass loss due to Cr volatilization needs to be taken into account. Neglecting Cr evaporation results in an underestimation of the oxidation rate by between 15% and 200% for the studied steel grades.

Chromate formation at the interface between a solid oxide fuel cell sealing glass and interconnect alloy

High-temperature interactions between glass–ceramic sealants and Cr-containing ferritic interconnects used in solid oxide fuel cells (SOFC) lead to the formation of detrimental chromate interfacial phases, such as BaCrO4 or SrCrO4, which can cause mechanical failure of the SOFC. In this work, these interactions are characterized by reacting Cr2O3 powders with a SrO-containing sealing glass and by characterizing representative reaction couples between the glass and 430 stainless steel. The extent of chromate formation depends on the reaction time and temperature, and the effect of the partial pressure of oxygen is modeled by thermochemical calculations.

High Temperature Oxidation of Plastically Deformed Ferritic Interconnect Steel

In this study, thin sheets of a 22% Cr ferritic steel, Sandvik Sanergy HT has been coated with a thin metallic cobalt film. Samples of the coated steel sheet were exposed to two different forming experiments simulating different aspects of sheet metal forming and post high temperature oxidation. The results show that the metallic cobalt film cracks when the steel sheet is subjected to biaxial straining but that the cracks heal at high temperature. Material subjected to modified scratch testing i.e. simulating the contact between sheet metal and forming tool will oxidize in the same manner as non-scratched material. Also, from high temperature corrosion point of view there is no difference between materials formed under lubricated or dry conditions when oxidized. However, from a tribological point of view a lubricated contact situation is preferred.

Effect of substrate thickness on oxide scale spallation for solid oxide fuel cells

In this paper, the effect of the ferritic interconnect thickness on the delamination/spallation of the oxide scale was investigated experimentally and numerically. At the operating environment of solid oxide fuel cells (SOFCs), a combination of growth stress with thermal stresses may lead to scale delamination/buckling and eventual spallation during SOFC stack cooling, even leading to serious degradation of cell performance. The experimental and numerical results show that the interfacial shear stresses increase with the growth of the oxide scale and also with the thickness of the ferritic substrate, i.e., the thick ferritic substrate can easily lead to scale delamination and spallation.

High Temperature Oxidation of Plastically Deformed Ferritic Interconnect Steel

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Interfacial Shear Strength of Oxide Scale and SS 441 Substrate

Recent developments on decreasing the operating temperature for solid oxide fuel cells (SOFCs) have enabled the use of high-temperature ferritic alloys as interconnect materials. Oxide scale will inevitably grow on the ferritic interconnects in a high-temperature oxidation environment of SOFCs. The growth of the oxide scale induces growth stresses in the scale layer and on the scale/substrate interface. These growth stresses combined with the thermal stresses induced after stacking cooling by the thermal expansion coefficient mismatch between the oxide scale and the substrate may lead to scale delamination/buckling and eventual spallation, which may lead to serious cell performance degradation. Hence, the interfacial adhesion strength between the oxide scale and the substrate is crucial to the reliability and durability of the metallic interconnect in SOFC operating environments. In this article, we applied an integrated experimental/modeling methodology to quantify the interfacial adhesion strength between the oxide scale and the SS 441 metallic interconnect. The predicted interfacial strength is discussed in detail.

Area specific resistance of oxide scales grown on ferritic alloys for solid oxide fuel cell interconnects

Planar solid oxide fuel cells (SOFC) are considered to be power generators with high efficiency and low emission at small power units (1–200 kW el). Many prototype systems are already successfully realized. For mass production the costs have to be reduced and the long-term stability has to be enhanced. Power losses <0.5%/1000 h is the target value for stacks in stationary SOFC-based power systems. To reach this goal, the factors influencing degradation have to be found and reduced. In this work the interaction between interconnect and different ceramic materials such as perovskites (La 0.8Sr 0.2(Mn ,Co)O 3, La 0.65Sr 0.3MnO 3, La 0.65Sr 0.3(Mn,Co)O 3) and spinels (Mn(Co,Fe)O 4, (Cu,Ni)Mn 2O 4) was investigated on the cathode (air) side of conventional ferritic interconnect materials (CroFer22APU, ITMLC, ZMG232L). The method to determine the value of the area specific resistance between interconnect and contact layer ( R #ICC) within a tolerance of 10% has been developed to provide reliable data for ASR values and their degradation. The R #ICC-value increases with annealing time. The degree of this increase depends on used materials and their combination. The spinel contact layers form a thin dense ceramic layer at the beginning of the annealing process. This layer reduces the oxidation rate of the alloy. Because of this protection layer a thinner oxide scale grows and the ASR aging rate is much lower (0.4–0.9 mΩ cm 2/1000 h). The comparison of the aging rates of different alloys with La 0.8Sr 0.2(Mn,Co)O 3 contact layer reveals remarkable differences: 3.1 mΩ cm 2/1000 h for CroFer22APU, 10.9 mΩ cm 2/1000 h for ITMLC and 21.2 mΩ cm 2/1000 h for ZMG232L. The degradation in a stack has been determined from the R #ICC-values and geometric factors. The impact of oxidation at the cathode side of interconnect is about one third of the total stack degradation. The method opens the possibility for comparing area specific resistances of special material combinations with high accuracy. By optimized material combinations the degradation in stacks can be reduced to <0.5%/1000 h.

Effect of Creep of Ferritic Interconnect on Long‐Term Performance of Solid Oxide Fuel Cell Stacks

AbstractHigh‐temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel‐ and air‐channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long‐term operations. The ferritic IC creep model was incorporated into software SOFC‐MP and Mentat‐FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long‐term steady‐state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel‐ and the air‐flow channels. In addition, stress re‐distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.

Reduction of chromate formation at the interface of solid oxide fuel cells by different additives

High temperature interactions between glass–ceramic sealants and ferritic interconnects used in solid oxide fuel cells (SOFCs) lead to the formation of detrimental chromate phases, such as BaCrO 4 or SrCrO 4, which lead to the mechanical failure of the cell. In present work, the interactions are characterized using model reactions between Cr 2O 3 and glass powders. It is found that the addition of MnO 2 reduces the formation of SrCrO 4 more significantly than that of ZnO and La 2O 3, in the SOFCs operational temperature range (e.g., 900 °C). In addition, the mechanism of the reduction of the chromate phase by different additives is discussed in terms of the possible competing reactions.

CoFe 2O 4 spinel protection coating thermally converted from the electroplated Co–Fe alloy for solid oxide fuel cell interconnect application

CoFe 2O 4 has been demonstrated as a potential spinel coating for protecting the Cr-containing ferritic interconnects. This spinel had an electrical conductivity of 0.85 S cm −1 at 800 °C in air and an average coefficient of thermal expansion (CTE) of 11.80 × 10 −6 K −1 from room temperature to 800 °C. A series of Co–Fe alloys were co-deposited onto the Crofer 22 APU ferritic steel via electroplating with an acidic chloride solution. After thermal oxidation in air at 800 °C, a CoFe 2O 4 spinel layer was attained from the plated Co 0.40Fe 0.60 film. Furthermore, a channeled Crofer 22 APU interconnect electrodeposited with a 40-μm Co 0.40Fe 0.60 alloy film as a protective coating was evaluated in a single-cell configuration. The presence of the dense, Cr-free CoFe 2O 4 spinel layer was effective in blocking the Cr migration/transport and thus contributed to the improvement in cell performance stability.

Ag-Perovskite Composite Materials for SOFC Cathode–Interconnect Contact

Silver-perovskite composites with the perovskite being either (LSCF) or (LSM) were evaluated as the contact material between the ferritic interconnect alloy Crofer 22 APU and the LSM cathode. The area-specific resistances (ASRs) of the test specimens were highly dependent on the volume percentage and the type of the perovskite present in the composite contact material as well as the amount of thermal cycling that the specimens were subjected to during testing. The chromium content present in both the contact layer and the adjacent cathode material was measured after testing, and the Ag–LSCF composite contact materials proved more effective in trapping Cr within the contact material and preventing Cr migration into the cathode than the Ag–LSM composites. Silver-perovskite composite contact materials are promising candidates for use in intermediate-temperature solid oxide fuel cell (SOFC) stacks with ferritic stainless steel interconnects due to their ability to maintain acceptably low ASRs while reducing Cr migration into the cathode material.

Investigation of Oxide-Nucleation on Ferritic FeCr Interconnect Alloys for Solid Oxide Fuel Cells Exposed to Anode and Cathode Gases at 850°C

In recent years solid-oxide fuel cell (SOFC) interconnect components have proven to be a key-component accountable for the functionality of high temperature fuel cells. Amongst adequate thermal expansion and high temperature strength, highest oxidation resistance in anode and cathode gases under thermal cycling conditions is required in order to reach long term durability, particularly when using thin film light-weight components with particular focus on automotive applications. In order to match the challenging parameter profile Plansee developed the mechanically alloyed ITM, a ferritic P/M Fe26Cr alloy strengthened with additions of Y2O3 dispersoids, since it has been observed that apart from their HT strengthening effect, which is of specific interest for thin sheets components, finest ODS particles reduce the growth and enhance the adhesion of the forming oxide layers. The latter effect is of particular interest in applications where alloys are exposed to HT cyclic conditions. In this work the nucleation phase of the oxide scale formation on P/M ODS Fe26Cr ITM is compared to that on a reference ingot steel Fe22Cr in typical anode gases containing significant amounts of H2, CO and approximately 3% H2O as well as in laboratory air at 850°C. Thermal cycling oxidation tests following the COTEST standard up to 168h are carried out in both environments. Moreover cyclic oxidation tests are performed in dry anode gas. Detailed studies of oxide scale formation and evolution by means of electron microscopy of cross sections as well as oxide surfaces are undertaken.