Steel Interconnects Research Articles

Improving the stability of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ/Crofer 22 H glass-sealed joints in humid oxidizing environments with a Mn-Co spinel coating

Sealing the BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) electrolyte to stainless steel interconnect is necessary for proton ceramic fuel cell stack packaging. However, oxidation of the interconnect in the glass-sealed joint presents a significant challenge, especially in humid and oxidizing environments. In this work, a Mn-Co spinel coating was fabricated on Crofer 22 H using microwave heating to extend its service life. The coated Crofer 22 H was successfully sealed to BZCYYb by a SiO2-TiO2-K2O-Na2O-Al2O3-ZnO glass sealant, achieving a sufficient joining. The joint is defect-free and dense, with a shear strength of 20 MPa. Detailed analysis using transmission electron microscopy revealed the underlying sealing mechanism. The Mn-Co spinel coating not only enhances the corrosion resistance of Crofer 22 H but also significantly improves the service life of the joint without compromising strength. The joint remains robust and defect-free after aging in a humid oxidizing atmosphere at 600 °C for 300 h.

Influence of Cobalt and Cobalt–Manganese Oxide Coating Thickness Deposited by DLI-MOCVD as a Barrier Against Cr Diffusion for SOC Interconnect

The influence of cobalt and cobalt–manganese oxide coating thickness on its ability to be a good diffusion barrier against Cr outward diffusion was investigated for stainless steel interconnects (AISI 441) of a solid oxide cell (SOC). The coatings were all synthesized using a DLI-MOCVD (Direct Liquid Injection-Metal Oxide Chemical Vapor Deposition) hot wall reactor. The study shows that a minimum cobalt oxide thickness of 300 nm was needed to be a good diffusion barrier against Cr for the 500-h exposure test. This observation was linked to the Mn concentration reached in the cobalt spinel during exposure. Indeed, during exposure at high temperature, Mn diffused from the substrate into the cobalt coating and transformed cobalt spinel into Co-Mn spinel. Whereas pure cobalt spinel was a good Cr diffusion barrier, cobalt-manganese spinel, Co3-xMnxO4, was not when x > 2. The thickness of the cobalt coatings must be chosen so that the Mn quantity coming into it from diffusion from the substrate does not degrade the protectiveness of the coating.

In Situ Electrochemical Recovery: Sediment Transformation under Chromium Poisoning in Reversible Solid Oxide Cells with La0.6Sr0.4Co0.2Fe0.8O3-σ-Based Oxygen Electrodes.

Reversible solid oxide cells (RSOCs) are an all-solid-state electrochemical device, which can convert H2 into electricity in the fuel cell (SOFC) mode and electrolyze H2O into fuel gas in the electrolytic cell (SOEC) mode, exhibiting good application prospect in the development of carbon neutrality. However, the degradation of the air electrode caused by Cr-containing steel interconnects is a major obstacle that limits the broader application of RSOCs. Herein, the Cr poisoning effect on La0.6Sr0.4Co0.2Fe0.8O3-σ (LSCF)-based oxygen electrodes under the electrolysis mode was systematically investigated. The phase transition of the sediment during the chromium poisoning process was captured and monitored. When tested under the presence of Fe-Cr interconnects at 800 °C for 40 h, SrCrO4 on the surface of LSCF was clearly identified through XRD and Raman analysis as the main deposition, and with the prolonged operating time, LaCrO3 slowly emerged. Due to the much higher electrical conductivity of LaCrO3 compared to SrCrO4, the negative effect induced by Cr poisoning was offset along with test progressing due to the deposition transition phenomenon. Inspired by the interesting discoveries, transition from SrCrO4 to LaCrO3 can be artificially facilitated by switching the operating mode to the SOEC mode, which can partially recover the dramatic degradation caused by the Cr poisoning effect under the SOFC mode. The feasibility of the in situ electrochemical recovery method was also verified by the experimental results. The peak power density of the cells decreased from 0.829 to 0.505 W/cm2 when operating under the SOFC mode with an Fe-Cr metal connector, and after in situ electrochemical recovery in the SOEC mode, the peak power density recovered to 0.630 W/cm2. This study provides a new strategy for achieving high performance and stability of RSOCs.

CuMn1.7Fe0.3O4 – RE2O3 (RE = Y, Gd) bilayers as protective interconnect coatings for solid oxide cells

Efficient replacement of materials based on critical elements such as cobalt is one of the greatest challenges facing the field of solid oxide cells. New generation materials, free of cobalt show potential to replace conventional materials. However, these materials are characterised by poor ability to block chromium diffusion. This article described the study of CuMn1.7Fe0.3O4 (CMFO) spinel combined with single metal oxide (Y2O3 or Gd2O3) thin films as protective coatings for steel interconnects. CMFO was examined using XRD and TPR. Coated steel samples were oxidised in an air atmosphere at 700 °C for 4000 h. The coatings and oxide scale microstructures and cross-sections were examined by CRI, XRD, and SEM-EDX. The electrical properties of the steel-coating system were evaluated using Area Specific Resistance measurements. Based on the results obtained, it can be concluded that the use of thin layers of rare earth oxides allowed for better blocking of chromium diffusion.

Fabrication and Oxidation Behavior of the Cu–Fe Spinel Coating for SOFC Steel Interconnect Applications

Fabrication and Oxidation Behavior of the Cu–Fe Spinel Coating for SOFC Steel Interconnect Applications

Effect of the simultaneous addition of lanthanum and nickel on the oxidation behavior and related area-specific resistance of ferritic stainless steels for solid oxide fuel cell interconnects

The effects of simultaneous lanthanum and nickel addition on the oxidation behavior and area-specific resistance (ASR) of stainless steels for fuelcell interconnects were investigated. The La and Ni addition promoted the entire growth of the oxide layer and increased the roughness of the metal/oxide interface. Despite the thicker oxide scale, these alloys prevented the increase of ASR compared to the alloy with only Ni added. The void analysis indicated that simultaneous La and Ni addition could prevent void growth, leading to lower ASR. The above findings can be interpreted in terms of the fast La diffusion and convoluted metal/oxide interface.

Ni–Co spinel based oxide as new type of additive to glass seal for solid oxide fuel cells

Glass seals utilized in Solid Oxide Fuel Cells (SOFCs) can be subject to modification through the incorporation of oxide additives to bolster their mechanical strength. This research focuses on a novel class of additives, specifically protective coating materials designed for alloy interconnects. It is crucial to emphasize that the glass seal interfaces directly with the protective coating, bypassing direct contact with the alloy itself. The efficacy of this approach is exemplified through the utilization of Ni–Co spinel-based oxide, established as an effective protective substance for stainless steel interconnects. The introduction of a minute quantity of Ni–Co oxides does not induce alterations in the glass seal properties, encompassing phase structure, microstructure, thermal expansion coefficient (CTE), and softening behavior. The Ni–Co oxide additive serves to mitigate the glass devitrification tendency, leading to a substantial augmentation in shear strength at the seal-interconnect interface and a marked enhancement in sealing properties. The composite seal, enriched with 3.0 wt% Ni–Co oxide, showcases superior sealing performance, demonstrating exceptional stability throughout a rigorous 1200-h test. The average leakage rate during this test period stands at 0.0024 sccm⋅cm−1. The incorporation of Ni–Co oxide triggers the formation of needle-shaped crystals, playing a pivotal role in averting channel void formation and augmenting the bonding strength between the seal and interconnect. Consequently, this fortifies gas tightness, ensuring the maintenance of outstanding sealing performance over an extended duration.

Suspension and process parameters selection for electrophoretic deposition of Mn–Co spinel coating on steel interconnects

Metallic interconnect coatings, consisting of MnCo2O4 spinel, were effectively applied to Crofer 22 APU using the electrophoretic deposition (EPD) method in both H2O: ethanol and pure ethanol solvents. The primary goal of this method was to prevent chromium migration, minimize evaporation, and control the oxidation rate. The study aimed to assess the quality, adhesion, and thickness of the Mn–Co coating, with the objective of achieving a consistent and uniform layer. The results indicated a preference for pure ethanol solvent over H2O: ethanol (40:60 %Vol) for Mn–Co particles. Furthermore, the agglomeration of Mn–Co particles was notably lower (approximately 7 times) in ethanol compared to H2O: ethanol. The morphology and surface roughness of the sintered Mn–Co coating on the alloy were examined using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). A uniform and dense coating was successfully attained using pure ethanol solvent at a solid concentration of 10 g/L, with 0.5 g/L of iodine serving as a disperser. The deposition process, carried out at a voltage of 30V for 20 s, resulted in a coating with a thickness of 7.4 ± 0.3 μm and a surface roughness of 0.75 ± 0.5 μm.

The Preparation and Properties of Ti(Nb)-Si-C Coating on the Pre-Oxidized Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect.

Cr2O3 scale growth and volatilization are the main cause of the performance degradation of solid oxide fuel cells (SOFCs) with an Fe-based ferritic stainless steel (FSS) interconnect. In this work, an amorphous Ti(Nb)-Si-C coating is prepared on the pre-oxidized SUS430 with D.C. magnetron sputtering as the protective coating. The amorphous Ti(Nb)-Si-C coated alloy exhibits significantly enhanced oxidation resistance, and the oxidation kinetics obey the parabolic law with a low parabolic rate of 9.36 × 10-15 g2·cm-4·s-1. A dual-layer oxide scale is formed composed of an inner layer rich in Cr2O3 and an outer layer rich in rutile TiO2 and amorphous SiO2. MnCr2O4 appears at the interface between the inner and outer oxide layers. Meanwhile, the amorphous Ti(Nb)-Si-C coating also effectively blocks the outward diffusion of Cr. In addition, the coated steel presents good electrical properties with an area-specific resistance (ASR) of 13.57 mΩ·cm2 at 800 °C after oxidation at 800 °C in air for 500 h.

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.

Mathematical modeling and experiment verification for the Solid oxide Fuel Cell Mn1.5Co1.5O4 interconnect coating

This study presents a new mathematical modeling study for interconnect oxidation analysis. A protective coating Mn1.5Co1.5O4 with different thicknessessare applied on ferritic stainless steel interconnect to address the chromium poisoning issue. Different Mn1.5Co1.5O4 coating thicknesses are applied. The coatings are assessed by surface morphology and phase structure. Developing coatings with various thicknesses is successful while maintaining consistent crystalline phases and coating morphology. The average coating thickness achieved at 35v for deposition times of 20, 30, and 40 s are 53.38 μm, 68.13 μm, and 85.13 μm, respectively. The coating thickness, 68.13 μm yields the lowest area specific resistance of 0.0469 Ω cm2 after 400 h of oxidation at 800 °C, compared to 0.0532 Ω cm2 and 0.0477 Ω cm2 for 53.38 μm and 85.13 μm The weight gains of the coated samples are recorded mathematically.This modeling approach results in a low weight gain and low oxidation kinetic rate of 0.139 mg cm−2 and 21.55 × 10−15 g2 cm−4 s−1, respectively for 68.13 μm coating thickness It predicts an area-specific resistance of 0.0451 Ωcm2 for the 35v 30 s coating, which is close to 0.0469 Ωcm2 experimental data.

Co-WO3 composite coating on ferritic stainless steel interconnects

In order to apply Co-WO3 composite coating to SUS 430 steel interconnects for solid oxide fuel cell (SOFC) interconnects, a combination of electrophoresis and electroplating is used, followed by thermal energy exposure to ambient temperature at 800 ℃ for up to 5 weeks. The thermal growth of the oxide scale displays a tri-layer structure with a Cr2O3 layer in the inner part, a CoWO4 layer in the middle and (Co,Fe)3O4 and Co3O4 layer in the outer part. Co-WO3 coating produces an oxide layer that not only prevents Cr2O3 development and outward transport of Cr species but also enhances the electrical behavior of the surface scale.

Development of Mn-Co and Mn-Co-CeO2 Coatings on Crofer 22 APU Steel for Solid Oxide Fuel Cell Interconnects

The present study compared the performance of uncoated, Mn-Co and Mn-Co-CeO2-coated Crofer 22 APU ferritic stainless steel interconnects. The samples were oxidized for 500 h in an electric furnace at 800 °C. The surface morphology and phase structure of the samples before and after oxidation were examined using FESEM microscopy and X-ray diffraction (XRD) analysis, respectively. The electrical conductivity evaluation of the samples was also conducted by measuring the area specific resistance (ASR). The results indicated that the weight gain of the uncoated, Mn-Co and Mn-Co-CeO2-coated samples after 500 h of oxidation was 0.55, 0.58 and 0.27 mg.cm−2, respectively. Additionally, a comparison of the oxidation kinetics of the experimental samples revealed that the oxidation rate constant of the Mn-Co-CeO2-coated steel is 10 and 40 times lower than that of the Mn-Co-coated and uncoated steel, respectively. XRD analysis of the samples after oxidation confirmed the presence of MnCo2O4, Co3O4, and (Mn,Cr,Co)3O4 in the oxidized Mn-Co-coated, MnCo2O4, Co3O4, and CeO2 in the oxidized Mn-Co-CeO2-coated, and the chromium-containing phases (FeCr2O4, MnCr2O4, and Cr2O3) in the oxidized uncoated samples. The presence of phases with high electrical conductivity in the oxidized coated samples reduced their activation energy for conduction compared to the oxidized uncoated sample.

Area specific resistance of in-situ oxidized Mn-Cu and Mn-Co metal powders as contact layers for the solid oxide cell air side

Stacking of solid oxide cells (SOC) requires that a robust and durable electrical contact is established between the cell and the interconnect. In this work, we present a contact layer solution for the SOC air side based on the concept of reactive oxidative bonding in which metallic Mn-Co and Mn-Cu particles are oxidized in-situ during stack initiation or operation to form robust well-conductive spinel oxides. The long-term (3000 h) stability of the new contact layers is evaluated by measuring the area specific resistance (ASR) during aging in air at 750 °C and 850 °C, and during thermal cycling. Both Mn-Co and Mn-Cu layers are found to be well compatible with a CeCo coated 441 steel interconnect material, and do not significantly contribute to the resistance across the stack element. The resistance is dominated by the coated steel.

Efficient FeCoNi/CeO2 coatings for solid oxide fuel cell steel interconnect applications

A reactive element layer is introduced into the coating system to enhance the properties of FeCoNi coating for solid oxide fuel cell interconnects. An FeCoNi/CeO2 dual-layer coating is produced on stainless steel by sol-gel and magnetron sputtering, which is subsequently exposed to 800 °C for a duration of up to 20 weeks. The thermal treatment results in the formation of (Fe,Co,Ni)3O4 spinel and (Ni,Co)O, with CeO2 embedded in Cr2O3 that formed beneath the spinel. The FeCoNi/CeO2 coating demonstrates better effectiveness than FeCoNi coating in acting as a barrier against Cr out-diffusion. Moreover, adding CeO2 significantly suppresses the interfacial reaction of Cr2O3 and spinel. An area-specific resistance of 10 mΩ cm2 is obtained for the FeCoNi/CeO2 coated steel at 800 °C after 20 weeks.

Understanding early-stage oxidation mechanism of Crofer 22 APU solid oxide fuel cell steel interconnect using glow discharge optical emission spectroscopy and grazing incidence X-ray diffraction

Crofer 22 APU is used as metallic interconnects in stacking solid oxide fuel cells (SOFCs) operated at elevated temperatures (above 700 °C) owing to their excellent oxidation resistance. Understanding the protective layer formation in the initial oxidation stage would be useful for optimizing and designing protective coatings for extended life. Initial stage oxidation of Crofer 22 APU steel using surface analytical tools such as glow discharge optical emission spectroscopy (GD-OES), grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM) are studied in the paper. An oxidation test on as-received Crofer 22 APU steel was carried out in a controlled atmosphere (0.01 Pa) in an in-situ high-temperature X-ray diffraction (XRD) stage at 950 °C. Normal XRD showed no indication of oxidation, while GIXRD revealed the formation of two-layer oxides: Top layer spinel MnCr2O4 and fine-grained inner layer Cr2O3, which was confirmed and quantified by GD-OES depth profiling. The Cr2O3 formed initially led to the formation of MnCr2O4 spinel during the initial stage. The rapid diffusion of Mn through the fine-grained Cr2O3 layer results in an increased growth rate of MnCr2O4 spinel on the top of the fine-grained Cr2O3 layer.

Sr-doped LaFeO3 thin coatings for protection of ferritic stainless steel interconnects in solid oxide cells: A study on Cr-barrier and electrical properties

A high-temperature conversion process in molten carbonate baths was applied to produce thin Sr-doped LaFeO3 perovskite coatings on the surface of a K41 steel for interconnect applications in Solid Oxide Cells. Two conversion coatings with identical structure and phase composition, but slightly different thickness were produced in nitrate-accelerated baths containing the nitrate additive at two concentration levels. The as-prepared coatings consisted of a dual-layered perovskite-spinel structure with a top sub-micrometric Sr-doped LaFeO3 layer grown onto a micron-range thick inner Fe-Cr spinel oxide layer. Both coatings showed high protection against oxidation and chromia scale formation at 700°C. Further, excellent Area Specific Resistance (ASR) behavior was found for both coatings at 700°C, although an optimal balance of ASR and Cr-diffusion barrier properties was achieved with the coating produced in the lower nitrate content bath (N-02 coating). The causes for the enhanced barrier properties of the N-02 coating were ascribed not to a different coating thickness, but rather to the evolution of the pristine dual-layer into a more impervious three-layer structure during oxidation aging at 700°C. The evolved structure was formed by decomposition of the pristine sub-coating Fe-Cr spinel into two new spinel sublayers: an outer ferrite and an inner chromite layer, whereas the top thin-film perovskite layer remained essentially unaltered. Partial change in the Fe oxidation state and other possible causes involved in the observed decomposition of the pristine Fe-Cr spinel layer during the thermal aging are analyzed and discussed. Measurements of ASR activation energy indicated that the contact resistance behavior of the thin perovskite coatings was dominated by the internal spinel layer conductivity.

Electrochemically enhanced wetting of 3YSZ and Mn1.5Co1.5O4 by Ag−CuO and their application in flash joining

Ag−CuO is a widely used air braze sealant for joining ceramic electrolytes and stainless-steel interconnects in solid oxide fuel cells (SOFCs). Balancing the amount of CuO in a braze is challenging because it enhances wettability and also aggravates steel surface degradation. Manganese cobaltite spinel (MCO) is widely used as a protective coating for SOFC steel interconnects. Therefore, we herein propose a strategy for enhancing the wetting of an Ag−CuO alloy with a low concentration of CuO (4 mol%) on 3 mol% yttria-stabilized zirconia (3YSZ) and MCO by applying a direct current to the 3YSZ/Ag−CuO/MCO system. It was discovered that the wetting of 3YSZ improved regardless of the current polarity; however, the wetting of MCO was selectively improved by the current flowing from MCO to 3YSZ. The underlying mechanisms are discussed based on the current-induced interfacial electrochemical reactions and dissolution. Furthermore, the flash joining of 3YSZ to MCO-coated Crofer22APU was achieved, and an optimized shear strength of 62 ± 11 MPa was obtained. A correlation between the interfacial microstructure, mechanical properties, current polarity and current intensity was also established.

Improving functional properties of protective coatings obtained by electrophoretic deposition on steel interconnects for solid oxide cells by tuning particle size distribution

This paper analyses the effect of particle size distribution in the suspension on the quality of the resulting coating in the electrophoretic deposition of a protective coating on a steel interconnect for solid oxide cells (SOCs). Mn1.5Co1.5O4 commercial powder was subjected to grinding for 2, 4 and 18 h, and admixing it with the finest fraction was used to form thin layers on Crofer 22APU steel. The functional properties of the coatings, such as density and area-specific resistance (ASR) were evaluated over the respective temperature ranges. The resistivity of the coatings was measured in 60 and 1000 h tests. It was shown that the increase of the fine powder fractions yields a higher density layer, which contributes to slowing down the growth rate of the oxide scale and therefore can decrease ASR by 65%. The lowest resistivity was performed by the samples fabricated with the admixed powder. Post-test analysis by FIB/SEM/EDS did not reveal diffusion of chromium through the fabricated layers (after the 1000 h test), which is a key indicator of the reliability of the formed protective barrier.

Utilizing High Entropy Effects for Developing Chromium‐Tolerance Cobalt‐Free Cathode for Solid Oxide Fuel Cells

AbstractSolid oxide fuel cell (SOFC) is regarded as an environmentally friendly energy conversion device, which can directly convert the chemical energy stored in the fuel to the electrical energy. However, the degradation of cathodes caused by Cr‐containing steel interconnects is a major problem that limits the broader application of SOFC. Herein, a novel A‐site high entropy oxide, based on the cobalt‐free PrBaFe2O5+δ (PBF) cathode, La0.2Pr0.2Nd0.2Sm0.2Gd0.2BaFe2O5+δ (LPNSGBF), is proposed as a high catalyst activity and Cr‐tolerance cathode for SOFC. The anode‐supported cell with the LPNSGBF cathode exhibits an excellent peak power density of 1020.69 mW cm−2 at 800 °C, which is better than that of the PBF (794.96 mW cm−2). Moreover, under the Cr‐containing atmosphere, the outstanding stability of the single cell with the LPNSGBF for 100 h with a degradation rate of 0.17% h−1, is much lower than the 0.79% h−1 for that of the PBF cathode. The study provides a new strategy for achieving the enhanced oxygen reduction reaction and high Cr‐tolerance of the cobalt‐free cathode by high entropy doping.