Evaluation of Ni80Cr20/(La0.75Sr0.25)0.95MnO3 dual layer coating on SUS 430 stainless steel used as metallic interconnect for solid oxide fuel cells

Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating is deposited on SUS 430 alloy by plasma spray for solid oxide fuel cell (SOFC) interconnect application. The phase structure, area specific resistance (ASR), and morphology of the coating are studied. A two-cell stack is also assembled and tested to evaluate coating performance in an actual SOFC stack. The NiCr/LSM coating adheres well to the SUS 430 alloy after oxidation in air at 800 °C for 2800 h. The ASR and its increasing rate of coated alloy are 25 mΩ cm² and 0.0017 mΩ cm²/h, respectively. In an actual stack test, the maximum output power density of the stack repeating unit increases from 0.32 W cm⁻² to 0.45 W cm⁻² because of the application of NiCr/LSM coating. The degradation rate of the stack repeating unit with no coating is 4.4%/100 h at a current density of 0.36 A cm⁻², whereas the stack repeating unit with NiCr/LSM coating exhibits no degradation. Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating can remarkably improve the thermal stability and electrical performance of metallic interconnects for SOFCs.     

NiO/NiFe2O4 dual-layer coating on pre-oxidized SUS 430 steel interconnect

A Ni/NiFe₂ dual-layer coating is deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, followed by thermal exposure in air at 800 °C for 1680 h. The thermally grown oxide scales exhibit tri-layer structure with inner Cr₂O₃ layer, middle NiO layer and outer NiFe₂O₄ spinel layer. The oxide coating converted from Ni/NiFe₂ coating not only inhibit the growth of Cr₂O₃ and the outward diffusion of Cr species but also improve the electrical performance of the surface scale. In addition, pre-oxidation treatment for the steel before Ni/NiFe₂ coating deposition prevents the interdiffusion between steel substrate and coating in the oxidation process.     

Magnetron-sputtered Mn/Co(40:60) coating on ferritic stainless steel SUS430 for solid oxide fuel cell interconnect applications

With recent progress in lowering the operating temperature, chromia-forming ferritic stainless steels are considered as promising interconnect materials for solid oxide fuel cells (SOFCs). To block the chromium evaporation and chromium poisoning, coatings with Mn/Co (40:60) was tested as the optimized recipe to maintain Mn_1.5Co_1.5O₄ composition after long term operation due to Mn diffusion from substrate. In order to study the coating thickness effect, Mn/Co (40:60) coatings were fabricated in thickness of approximately 800 nm, 1500 nm, 3000 nm on ferritic stainless steels SUS430 using magnetron sputtering. Oxidation behavior of sputtered samples was investigated after oxidized at 800 °C in air for 2 h, 250 h, 500 h, 1000 h, respectively. SEM, EDS, XRD and FIB are used to analyze the surface morphology, chemical composition and structures of the coatings. Area specific resistance measurement indicated the sputtered samples in thickness of 800 nm, 1500 nm, 3000 nm at 800 °C for various hours in air are in range of 15–36 mΩ cm², 12–25 mΩ cm², 10–23 mΩ cm² respectively. Eventually the optimized thickness of Mn/Co (40:60) coatings was suggested.     

A promising NiCo2O4 protective coating for metallic interconnects of solid oxide fuel cells

The NiCo₂O₄ spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol-gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800 °C for 200 h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr₂O₃ and the formation of conductive spinel phases in the oxide scale.     

Effect of CeO2 on oxidation and electrical behaviors of ferritic stainless steel interconnects with NiFe coatings

Ferritic stainless steels are promising materials for application in interconnects of solid oxide fuel cells (SOFC). The present problems to be solved urgently for using ferritic stainless steels as interconnects are their rapid increase in electrical resistance and the cathode poisoning caused by evaporation of chromia. In the present study, the NiFe and NiFeCeO₂ alloy coatings have been electro-deposited onto 430 stainless steels (430SS). During oxidation at 800 °C in air, an outer dense NiFe₂O₄ layer and an inner protective Cr₂O₃ layer have thermally grown on the coated samples. The NiFe₂O₄ layer retards the outward migration of chromium effectively. The addition of CeO₂ reduces the growth rate of Cr₂O₃ and decreases the number of pores near the oxide scale/alloy interface. Moreover, a higher electrical conductivity has been achieved by the addition of CeO₂.     

Evaluation of electrodeposited Fe-Ni alloy on ferritic stainless steel solid oxide fuel cell interconnect

Fe-Ni alloy is electrodeposited on ferritic stainless steel for intermediate-temperature solid oxide fuel cell (SOFC) interconnects application. The oxidation behavior of Fe-Ni alloy coated steel has been investigated at 800 °C in air corresponding to the cathode environment of SOFC. It is found that the oxidation rate of the Fe-Ni alloy coated steel becomes similar to that of the uncoated steel after the first week thermal exposure, although the mass gain of the coated steel is higher than that of the uncoated steel. Oxide scale formed on the uncoated steel mainly consists of Cr₂O₃ with (Mn,Cr)₃O₄ spinel. However, a double-layer oxide structure with a Cr-free outer layer of Fe₂O₃/NiFe₂O₄ and an inner layer of Cr₂O₃ is developed on the Fe-Ni alloy coated steel. The scale area specific resistance (ASR) for the Fe-Ni alloy coated steel is lower than that of the scale for the uncoated steel.     

Development of a Fe-Cr alloy for interconnect application in intermediate temperature solid oxide fuel cells

The oxidation behavior and electrical property of a newly designed Fe-Cr alloy with addition of 1.05 wt.% Mn, 0.52 wt.% Ti, 2.09 wt.% Mo and other elements, such as La, Y and Zr have been investigated isothermally or cyclically at 750 °C in air for up to 1000 h. With a coefficient of thermal expansion matched to SOFC cell components, the alloy demonstrates excellent oxidation resistance and low area specific resistance of the oxide scale. The thermally grown oxide scale presents a multi-layered structure with conductive Mn-Cr spinel in-between the underneath Cr₂O₃ and the top Mn₂O₃. The oxidation rate constants obtained under both isothermal and cyclic oxidation condition are in the range of 5.1 × 10⁻¹⁴ to 7.6 × 10⁻¹⁴ g² cm⁻⁴ s⁻¹, and the measured area specific resistance at 750 °C after 1000 h oxidation is around 10 mΩ cm², lower than that of the conventional Fe-Cr stainless steels and comparable with that of the Ni-based alloys. Thermal cycling seems to improve the oxide scale adherence and promotes the formation of the highly conductive Mn₂O₃, and in turn, to enhance the oxidation resistance and electrical property.     

Investigation of MnCu0.5Co1.5O4 spinel coated SUS430 interconnect alloy for preventing chromium vaporization in intermediate temperature solid oxide fuel cell

The MnCu_0.5Co_1.5O₄ spinel coating is proposed as a protective coating for SUS430 alloy to improve its oxidation resistance and prevent chromium vaporization. The coated alloy is exposed to dual atmosphere (Air/H₂–3%H₂O) at 750 °C for 200 h, exhibiting a stable spinel structure on the air side, but reduced to MnO, Cu and Co on the fuel side. The coating layer could maintain integrated and dense with a thickness of 13–14 μm. The experiment results shown that the MnCu_0.5Co_1.5O₄ coating is an effective diffusion barrier that can inhibit oxidation and chromium vaporization of metallic interconnect. The relatively low amount of Cr deposition on LSM cathode on coated condition is considered associating with the stable electrochemical performance under current density of 400 mA cm^?2. The above results indicate that MnCu_0.5Co_1.5O₄ spinel is a promising coating for interconnect alloy of solid oxide fuel cell.     

NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells

In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn₂O₄ is prepared on SUS 430 alloy substrate by the sol-gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3-4 μm thick is formed during cyclic oxidation at 750 °C in air for 1000 h, consisting of an inner layer of doped Cr₂O₃ and an outer layer of doped NiMn₂O₄ and Mn₂O₃; and the growth of Cr₂O₃ and formation of MnCr₂O₄ are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹. The area specific resistance at temperatures between 600 and 800 °C is in the range of 6 and 17 mΩ cm². The above results indicate that NiMn₂O₄ is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells.     

Effects of La0.67Sr0.33MnO3 protective coating on SOFC interconnect by plasma-sputtering

Two metallic alloys, namely, Crofer22 APU and equivalent ZMG23 were investigated as possible interconnect materials in SOFC fuel cells. A _{La0.67Sr0.33MnO3}${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSM) thin film is coated on these materials using pulsed DC magnetron sputtering. The as-deposited film is amorphous but is transformed into perovskite structure after annealing at different temperatures and times. The coating and uncoated structures and surface morphologies are analyzed using X-ray diffraction (XRD), electron Probe Micro Analyzer (EPMA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The result shows that the LSM thin film on Crofer22 APU is good for compaction and adhesion, but there are some stresses between the equivalent ZMG232 and the coating and then create some cracks on the coating. Thereby, the coefficients of thermal expansion (CTE) of the equivalent ZMG232 may be higher than the CTE of the LSM. The cross-section of equivalent ZMG232 did not allow diffusion of Cr element. Thus, coating by plasma-sputtering could prevent the growth of oxide and the diffusion of Cr element to avoid cathode poisoning and the decline of conductivity in SOFC at high temperature.     

LaNi0.6Fe0.4O3-Ce0.8Sm0.2O1.9-Ag composite cathode for intermediate temperature solid oxide fuel cells

LaNi_0.6Fe_0.4O₃ (LNF), LNF-Sm_0.2Ce_0.8O_1.9 (SDC), and LNF-SDC-Ag cathodes on SDC electrolytes were investigated at intermediate temperatures using AC impedance spectroscopy. Results show that adding 50 wt.% SDC into LNF yields a significant low area specific resistance (ASR) which was found to be 0.92 Ω cm² at 700 °C. Infiltrating 0.3 mg/cm² Ag into LNF-50 wt.% SDC can improve the electronic conductivity and oxygen exchange reaction activity, and thereby remarkably decrease the ASRs. The ASR value of the LNF-SDC-Ag cathode is as low as 0.18 Ω cm² at 700 °C, and 0.46 Ω cm² at 650 °C. The long-term test shows that the LNF-SDC-Ag cathode may be a promising candidate for solid oxide fuel cells operating at temperatures lower than 650 °C.     

LaCrO3-based coatings deposited by high-energy micro-arc alloying process on a ferritic stainless steel interconnect material

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO₃-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO₃-based coating using an electrode rod of LaCrO₃-20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO₃-based coatings showed a three-layered microstructure with a NiFe₂O₄ outer layer, a thick LaCrO₃ sub-layer and a thin Cr₂O₃-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.     

Promising alloys for intermediate-temperature solid oxide fuel cell interconnect application

The formation of a low Cr-volatility and electrically conductive oxide outer layer atop an inner chromia layer via thermal oxidation is highly desirable for preventing chromium evaporation from solid oxide fuel cell (SOFC) metallic interconnects at the SOFC operation temperatures. In this paper, a number of ferritic Fe–22Cr alloys with different levels of Mn and Ti as well as a Ni-based alloy Haynes 242 were cyclically oxidized in air at 800 °C for twenty 100-h cycles. No oxide scale spallation was observed during thermal cycling for any of these alloys. A mixed Mn₂O₃/TiO₂ surface layer and/or a (Mn, Cr)₃O₄ spinel outer layer atop a Cr₂O₃ inner layer was formed for the Fe–22Cr series alloys, while an NiO outer layer with a Cr₂O₃ inner layer was developed for Haynes 242 after cyclic oxidation. For the Fe–22Cr series alloys, the effects of Mn and Ti contents as well as alloy purity on the oxidation resistance and scale area specific resistance were evaluated. The performance of the ferritic alloys was compared with that of Haynes 242. The mismatch in thermal expansion coefficient between the different layers in the oxide scale was identified as a potential concern for these otherwise promising alloys.     

Evaluation of pulse electroplated cobalt/yttrium oxide composite coating on the Crofer 22 APU stainless steel interconnect

Ferritic stainless steels have been evaluated as favorable materials for utilization in SOFC interconnects. However, there are difficulties in utilizing these metallic interconnects, including the quick decrease of their electrical conductivity and cathode poisoning due to the evaporation of Cr species. In this work, Co and Co/Y₂O₃ composite coatings have been deposited onto Crofer 22 APU stainless steels by the pulse current electrodeposition method and the oxidation and electrical properties of uncoated and coated steels have been evaluated. Cyclic oxidation was performed in air at 800 °C for 500 h, oxidation rates were calculated, and oxide layer microstructures were examined. SEM–EDS and XRD investigations exhibited the created oxide layer on both coated samples made up of two scale after oxidation. The internal thin scale was composed of Cr and O and the external scale comprised of Co, Mn, Cr, Fe, and O. Y₂O₃ was observed as dispersed particles in the external oxide scale after the cyclic oxidation test. The thicknesses of internal oxide scale were reduced and oxidations rates also were meaningfully decreased for Co/Y₂O₃-coated steels relative to uncoated and Co-coated steels. Finally the ASR values of coated and uncoated substrates was also tested as a function of temperature and time in air. Results showed that the ASR value of the Co/Y₂O₃-coated steel was 13.1 mΩ cm² after 500 h of cyclic oxidation at 800 °C, which was significantly lower than that of bare steel and the Co-coated sample.     

Performance of CoNiO spinel oxide coating on AISI 430 stainless steel as interconnect for intermediate temperature solid oxide fuel cell

In order to decrease oxide growth kinetics, maintain suitable conductivity and prevent Cr-volatilization of AISI 430 stainless steels (430 SS) as the interconnect for intermediate temperature solid oxide fuel cells (SOFCs), a CoNiO spinel oxide protective coating has been successfully fabricated on the 430 SS specimen using a simple and cheap process with two steps: 1) electroplation of CoNi alloy layer and 2) pre-oxidation treatment to convert the CoNi alloy into spinel oxide. The CoNiO spinel layer on the 430 SS (CoNiO 430 SS) is dense and uniform with 8–10 μm thickness. And the CoNiO spinel oxide protective coating consists of a main face-centered-cubic (fcc) NiCo₂O₄ spinel phase and a minor fcc NiO phase. Compared with bare 430 SS, the oxidation resistance and the conductivity of the CoNiO 430 SS have been improved remarkably under simulated typical SOFC operating cathode conditions (at 800 °C in air). After an isothermal oxidation test at 800 °C, the area specific resistance (ASR) of CoNiO 430 SS is much lower and stable (0.1 Ω cm² for 100 h and 0.9 Ω cm² for 600 h) than that of bare 430 SS (1.2 Ω cm² for 100 h and 2.4 Ω cm² for 600 h). These performances of CoNiO 430 SS imply that it can be a promising candidate interconnect for solid oxide fuel cell.     

Fabrication of a double-layered Co-Mn-O spinel coating on stainless steel via the double glow plasma alloying process and preoxidation treatment as SOFC interconnect

To improve oxidation resistance, prevent Cr evaporation and maintain appropriate electrical conductivity of AISI 430 stainless steel (430 SS) as the solid oxide fuel cells' (SOFCs) interconnect, a double-layered Co-Mn-O spinel coating is fabricated successfully on 430 SS via a simple double glow plasma alloying process (DGPA) followed by heating in the air (preoxidation treatment). The double-layered Co-Mn-O spinel coating is composed of a thick MnCo₂O₄ spinel outlayer and a thin mutual-diffused (MnCoFe)₃O₄ oxide innerlayer. The isothermal and cyclic oxidation measurements are used to investigate the oxidation resistance, and the ASR test is performed to evaluate the conductivity for the coated and uncoated specimens. The coated specimen has a lower oxidation kinetics rate constant (9.0929 × 10⁻⁴ mg² cm⁻⁴ h⁻¹) than the uncoated one (1.900 × 10⁻³ mg² cm⁻⁴ h⁻¹) and the weight gain of the coated specimen (0.84 mg cm⁻²) is less than that of bare steel (1.29 mg cm⁻²) after 750 h oxidation. Meanwhile, the coated specimen holds a lower area specific resistance (0.029 Ω cm²) compared to the uncoated one (2.28 Ω cm²) after 408 h oxidation. Furthermore, the compact Co-Mn-O spinel coating can effectively impede Cr-volatilization. Additionally, the probable mechanism of the Co-Mn alloy conversion into spinel and the electronic conduction behavior in the spinel are discussed. The effects of mutual-diffused oxide innerlayer on oxidation behavior and conductivity are investigated.     

MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the interconnection plates can be made from stainless steel. This is a big economic advantage, but energy losses can be caused by undesirable reactions between the alloys and other SOFC components. The use of coatings on interconnect stainless steels can reduce this degradation. A MnCo_1.9Fe_0.1O₄ (MCF) spinel not only significantly decreases the contact resistance between a La_0.8Sr_0.2FeO₃ cathode and a stainless steel interconnect, but also acts as a diffusion barrier to prevent Cr outward migration through the coating. The level of improvement in electrical performance depends on the ferritic substrate composition. For Crofer22APU and F18TNb, with a Mn concentration of 0.4 and 0.12 wt%, respectively, the reduction in contact resistance is significant. In comparison, limited improvement is achieved by application of MCF on IT-11 and E-Brite containing no Mn. No influence of the minor additions of Si or Al is observed on contact resistance. The MCF protection layer bonds well to the stainless steel substrates under thermal cycling, but the thermal expansion difference is too large between the La_0.8Sr_0.2Co_0.75Fe_0.25O₃ contact layer used and Crofer22APU and IT-11.     

CuFe2O4/CuO coating for solid oxide fuel cell steel interconnects

CuFe_0.8 (Fe:Cu = 0.8:1, atomic ratio) alloy layer is fabricated on both bare and pre-oxidized SUS 430 steels by direct current magnetron sputtering, followed by exposing at 800 °C in air to obtain a protective coating for solid oxide fuel cell (SOFC) steel interconnects. The CuFe_0.8 alloy layer is thermally converted to CuFe₂O₄/CuO coating, which effectively suppresses the out-migration of Cr. Pre-oxidation treatment not only initially accelerates the formation of CuFe₂O₄/CuO coating but also further inhibits the Cr and Fe outward diffusion. Suppressing outward diffusion of Cr could improve electrical property of oxide scale and decrease the risk of cathode Cr-poisoning. Blocking out-diffusion of Fe is beneficial to stabilize the CuO layer. After 2520 h oxidation, the scale ASR at 800 °C is 66.9 mΩ cm² for coated bare steel, 43.4 mΩ cm² for the coated pre-oxidized steel.     

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 Cr₂O₃ 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 Cr₂O₃ type oxide layer to form Mn-rich (Mn, Cr)₃O₄ and Mn₂O₃ 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.     

High-temperature oxidation process analysis of MnCo2O4 coating on Fe-21Cr alloy

Even though the operation temperature of solid oxide fuel cells (SOFCs) stacks has been reduced (∼750 °C), stainless steel interconnect within the stacks still requires protection by high conductive coatings to delay the growth of oxide scales and reduce chromium evaporation. Manganese cobaltite spinel protective coating with a nominal composition of MnCo₂O₄ was produced on Fe-21Cr stainless steel. Electrical, microstructural and compositional analysis were performed to investigate the interfacial reaction of MnCo₂O₄ protective coating with the stainless steel substrate during 750 °C oxidation process. The spinel coating not only acts as a barrier to Cr outward transport, but also improves the electrical conductivity of the alloy interconnect during long-term oxidation. The coated alloy demonstrates good electrical conductivity with an area specific resistance (ASR) of about 5 mOhm cm² after oxidation for 1000 h at 750 °C, which is about 1/4 of the ASR of bare Fe-21Cr alloy. The reduction of ASR might be caused by the fact that Cr migrated from the steel substrate interact with MnCo₂O₄ coating and generated Mn-Co-Cr spinel phase, which has higher electrical conductivity than that of Cr₂O₃.