Interconnectors In Solid Oxide Fuel Cells Research Articles

Microstructure and electrical property of composite Co-W/CuO multilayered oxide coating for solid oxide fuel cell interconnector

To overcome the challenges related to the electrical resistance owing to the growth of the Cr-containing oxide layer in the SOFC system at high temperature, a composite coating of Co-W/CuO was fabricated on ferritic stainless steel for interconnectors by composite electroplating and pre-oxidation treatment. Compared to the Co-W alloy coating, the addition of CuO led to the formation of Cu-Co spinels and Cu-W composites, which affected the surface microstructure of the coating. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) investigations shows that CuWO4 nano-particles formed in the Cr-rich layer beneath the CoWO4 layer, of which the lattice deviation leads to the formation of oxygen vacancies. The CuWO4 acted as conductive pathways, reducing the electrical resistance. Long-term oxidation exposure further demonstrated that the Co-W/CuO coated SUS 430 exhibited a stable electrical conductivity of 10.23 mΩ cm2.

Interface Design Development for High Durability of SOFC Interconnectors Using Electrodeposition Coating

ENE-FARM type S equipped with solid oxide fuel cell (SOFC) stack has been commercialized since 2012. We have been working on improving the power generation efficiency, reducing costs, and the making them more compact by considering that market expansion and cost reduction are critical for the continued market spread of ENE-FARM type S. However, there is a trade-off between cost reduction and performance improvement and issues related to achieving both have emerged during the development process. In order to reduce costs, significant reduction of the number of cells were required. By reducing the number of cells, the current per cell increased, and the stainless steel connecting the cells as the SOFC interconnector was required to have further improvement in durability at high temperature. The degradation modes of the stainless steel used for SOFC mainly include the growth of an insulating oxide layer such as Cr2O3, Cr poisoning of the SOFC cathode, and anomalous oxidation due to decrease in the Cr concentration in the stainless steel. Therefore, ceramic coating is indispensable for the SOFC interconnector in order to suppress these degradations. Material properties of the coating film and coating manufacturing conditions are critical to the ceramic coating equipped on the SOFC interconnector. In addition, it has excellent physical properties such as high Cr diffusion suppression, barrier performance of oxygen gas diffusion, and good physical properties such as high electrical conductivity and good matching of thermal expansion coefficient with the SOFC components. In general, spinel-based oxide materials have been reported as the coating materials for SOFC interconnector, and many binary oxides containing transition metals such as MnCo2O4, MnCu2O4, and NiCo2O4. We have investigated (Zn, Mn, Co)3O4 because of matching the thermal expansion coefficient to the ferritic stainless steel and improving the sinterability at lower temperature. The total electrical conductivity and the thermal expansion coefficient were changed due to the Mn / Co ratio. In terms of these properties, the optimum composition in ZnMnCoO4 was found to be the Mn / Co ratio of around 1 / 1. We have evaluated the long-term durability of SOFC stack components for 12 years in a short period of time by temperature acceleration evaluation method. The voltage changes with the constant current density of 0.3 A cm-2 at a furnace temperature of 973 K shows no degradation over 45000 hours. Moreover, ZnMnCoO4 coated on the 22Cr-Fe alloy shows higher durability over 20000 hours at 1123 K. In 2016, we released the ENE-FARM type S equipped with the electrodeposition coating technology and achieved the highest power generation efficiency and the smallest size at that time. The electrodeposition coating technology has been used in all of ENE-FARM type S sold in Japan. Furthermore, this electrodeposition technology has contributed significantly to the increase in sales volume since 2016. The electrodeposition coating technology can be applied not only to SOFC but also SOEC and is expected to be used more and more widely in the future.

Interface Design Development for High Durability of SOFC Interconnectors Using Electrodeposition Coating

Solid Oxide fuel cell (SOFC) mounted in ENE-FARM type S has been commercialized since 2012. For further market expansion of ENE-FARM type, higher efficiency, higher durability, and lower cost of SOFC stacks are required. A key technology to advance its performance is a coating on SOFC metallic interconnector for the long-term durability of SOFC stacks. We achieved significant reduction in resistance and improved durability by improving the highly mass producible electrodeposition coating method. In this research, the coating technology of the SOFC interconnector has been developed. Long-term durability was confirmed through high-temperature acceleration evaluation, and the electrodeposition coating on the SOFC interconnector was installed in the ENE-FARM type S.

Optimization of Plasma-Sprayed Lanthanum Chromite Interconnector Through Powder Design and Critical Process Parameters Control

For tubular solid oxide fuel cells (SOFCs), individual cells are connected in series by the interconnector to obtain desired voltage and power output. As the most widely used ceramic interconnector for high-temperature SOFCs, lanthanum chromite-based material is quite difficult to sinter to high density due to the high vapor pressure of the chromium component. In this paper, dense strontium-doped lanthanum chromite (LSC) membranes with enhanced lamellar interface bonding were achieved at an elevated deposition temperature over the critical bonding temperature by atmospheric plasma spraying (APS). The effects of molten droplet particle size on the chemical composition and phase structure of LSC deposits were studied. The preferential vaporization of Cr during plasma spraying was of the greatest interest here. When the particle size of molten droplet was smaller than 30 μm, the vaporization loss of Cr increased remarkably with the decrease in particle size, resulting in impurity phases. When the particle size was larger than 30 μm, the vaporization loss of Cr was greatly reduced. The electrical conductivity of plasma-sprayed LSC deposits reached 8.7 and 2.1 S/cm at 850 °C in oxidizing and reducing atmospheres, respectively. The optimized LSC deposits exhibited good stabilities of phase structure and properties under operation conditions, which is adequate for SOFC interconnector application.

Coating layer and influence of transition metal for ferritic stainless steel interconnector solid oxide fuel cell: A review

Progressive efforts on lowering working temperature of Solid Oxide Fuel Cell (SOFC) to approximately 600 °C enable application of stainless steel as interconnector instead of expensive ceramic. Prolonged exposure of stainless steel to SOFC operating conditions can lead to chromium poisoning owing to migration of chromium (IV) species to cathode and significantly reduces electrical conductivity of cells. Ferritic stainless steel is potential candidate as interconnector because it has appropriate chromium content and is less expensive compared other stainless steels. Protective coating layer on interconnector is essential in minimizing chromium poisoning phenomenon in aspect of area specific resistance (ASR), thermal expansion coefficient (TEC) and coating uniformity. Addition of transition metal to coating layer is an enhanced method to improve coating behavior. Nickel, copper, manganese and silver are promising metals that can be used as coating layer to inhibit chromium (IV) species from diffusing outward and improve electrical conductivity and excellent oxidation resistance. This paper reviews oxidation behavior of coating layer of interconnector of SOFC, sintering effect, protective coating technique and influence of transition metal in the coating layer.

Oxidation resistance, thermal expansion and area specific resistance of Fe-Cr alloy interconnector for solid oxide fuel cell

It is promising for metal especially ferritic stainless steel (FSS) to be used as interconnector when the solid oxide fuel cell (SOFC) is operated at temperature lower than 800 °C. However, there are many challenges for FSS such as anti-oxidant, poisoning to cathode and high area specific resistance (ASR) when using as SOFC interconnector. The effect of Cr content (12 — 30 mass%) in Fe-Cr alloys on thermal expansion coefficient (TEC), oxidation resistance, microstructure of oxidation scale and ASR was investigated by thermo-gravimetry, scanning electron microscopy, energy dispersive spectroscopy and four-probe DC technique. The TEC of Fe-Cr alloys is (11 — 13) × 10−6 K−1, which excellently matches with other SOFC components. Alloys have excellent oxidation resistance when Cr content exceeds 22 mass% because of the formation of chromium on the surface of alloy. The oxidation rate constants kd and ks decrease rapidly with increasing the Cr content and then increase slowly when the Cr content is higher than 22 mass%. The kinetic results indicate that Cr evaporation must be considered at high temperature for Fe-Cr alloys. After the alloys were oxidized in air at 800 °C for 500 h, log(ASR/T) (T is the absolute temperature) presents linear relationship with 1/T and the conduct activation energy is 0.6 — 0.8 eV (Cr16-30). Optimal Cr content is 22 — 26 mass% considering the oxidation resistance and ASR.

Development of advanced metallic alloys for solid oxide fuel cell interconnector application

Topologically closed packed (TCP) phases bearing Ni-based alloys have been developed as candidate materials for interconnector application in solid oxide fuel cell (SOFC). The high contents of W and Mo have promoted the formation of TCP phases and decreased the coefficient of thermal expansion (CTE) of these alloys. Furthermore, newly developed alloys can form continuous chromia for protection against oxidation, and exhibit low area specific resistance. They also show strong resistances against softening and Cr poisoning at 800 °C. The potentials of these low CTE Ni-based alloys for SOFC interconnector application are discussed in present article.

Development of an advanced bond coat for solid oxide fuel cell interconnector applications

An advanced bond coat has been developed for solid oxide fuel cell interconnector applications; a low thermal expansion superalloy has been selected as the substrate, and the newly developed bond coat is applied between the substrate and the LSM top coat. The bond coat composition is designed to be near thermodynamic equilibrium with the substrate to minimize interdiffusion with the substrate while providing oxidation protection for the substrate. The bond coat exhibits good oxidation resistance, a low area specific resistance, and a low thermal expansion coefficient at 800 °C; experimental results indicate that interdiffusion between the bond coat and the substrate can be hindered.

Mass transfer enhancement of a spiral-like interconnector for planar solid oxide fuel cells

In order to mitigate the fuel shortage in porous electrode of planar Solid Oxide Fuel Cell (SOFC), this work presents a novel spiral-like SOFC interconnector which could achieve a tight gas sealing and sufficient electrical contact between the interconnector and electrode or gas supply line. A naphthalene sublimation mass transfer experiment at room temperature is applied to verify the superiority of the new structure. A 3-D model is set up by COMSOL 3.5A and the cell is operated with the mixture of H2 and H2O as fuel at 1023K. The experimental results and simulation results show that this new structure could improve the mass transfer performance of SOFC greatly. Comparing with traditional direct channel interconnector, this new design could not only improve the gas velocity in porous electrode which parallel to the triple phase boundary (TPB), but also enhance the gas velocity perpendicular to it. The H2 molar fraction in porous anode with this spiral-like interconnector is almost two orders of magnitude higher than that with direct channel interconnector both at room temperature and high temperature. These improvements would be helpful to increase the fuel availability and enhance the electrical performance of SOFCs.

Numerical Study on Mass Transfer Performance of a Spiral-like Interconnector for Planner Solid Oxide Fuel Cells

In order to transfer more fuel of a planner SOFC (Solid Oxide Fuel Cell) from gas channel into porous anode, this paper has designed a novel spiral-like SOFC interconnector, a 3-D model is made by COMSOL 3.5a and the cell was operated with the mixture of H2 and H2O as fuel at 1023K. The result shows that, compared with conventional direct channel interconnectors, the new interconnector in this paper could not only improve the gas velocity parallel to the TPB(Triple Phase Boundary), but also with much higher gas velocity perpendicular to it, which has led to the H2 molar fraction close to the TPB in anode is almost two orders of magnitude higher than that of director channel interconnector SOFC, which would be helpful to improve the electrical performance of SOFCs.

Fabrication of Sr- and Co-doped lanthanum chromite interconnectors for SOFC

Many studies have been performed dealing with the processing conditions of electrodes and electrolytes in solid oxide fuel cells (SOFCs). However, the processing of the interconnector material has received less attention. Lanthanum chromite (LaCrO 3) is probably the most studied material as SOFCs interconnector. This paper deals with the rheology and casting behaviour of lanthanum chromite based materials to produce interconnectors for SOFCs. A powder with the composition La 0.80Sr 0.20Cr 0.92Co 0.08O 3 was obtained by combustion synthesis. Aqueous suspensions were prepared to solids loading ranging from 8 to 17.5 vol.%, using ammonium polyacrylate (PAA) as dispersant and tetramethylammonium hydroxide (TMAH) to assure a basic pH and providing stabilization. The influence of the additives concentrations and suspension ball milling time were studied. Suspensions prepared with 24 h ball milling, with 3 wt.% and 1 wt.% of PAA and TMAH, respectively, yielded the best conditions for successful slip casting. Sintering of the green discs was performed in air at 1600 °C for 4 h leading to relatively dense materials.

Evaluation of Fe-Cr Systems Metallic Interconnectorby Spark Plasma Sintering

Fe based SOFC(Solid Oxide Fuel Cell) interconnector was fabricated by using the spark plasma sintering process and its microstructure and mechanical properties were investigated in this study. To fabricate the interconnector, the Fe-26Cr powder was mixed with the Ag (5, 10, 20wt.%) particles. In the Fe-26Cr-Ag sintered bodies, the Ag particles were almost dispersed at the grain boundary of the Fe-26Cr. The sintered bodies have the density of 87.2-97.5%, the density increases with increasing Ag content at sintering temperature of <TEX>$850^{\circ}C$</TEX>. Also, the compressive yield strength increases with increasing Ag content at the same sintering temperature.

Metallic interconnectors for solid oxide fuel cells – a review

For planar solid oxide fuel cell (SOFC) designs, ceramic as well as metallic materials are being considered as construction materials for the interconnectors. Compared to the ceramics, mostly compounds on the basis of La-chromite, metallic materials have the advantage of easier fabricability, lower costs as well as higher heat and electrical conductivity. Based on the requirements in respect to oxidation resistance, low thermal expansion coefficient and electrical conductivity of surface oxide scales, Cr-based alloys and high-Cr ferritic steels seem to be the most promising metallic interconnector materials. Whereas Cr-based alloys have recently especially been developed for SOFC application, a large number of ferritic steels are commercially available in a wide range of compositions. However, it seems that the specific combination of properties required for a SOFC interconnector will necessitate the development of a new, specifically designed steel or the modification of an existing commercial steel composition.

Application of Fe–16Cr ferritic alloy to interconnector for a solid oxide fuel cell

Iron-base scaling-resistant alloys (Fe–Cr) as materials for interconnectors of planar-type solid oxide fuel cells (SOFC) are proposed for application in automobile industry because of their advantages in comparison with other Ni- or Co-based alloys and ceramic materials (e.g. (La,Sr)CrO 3). The oxidation kinetics of Fe–16Cr alloy (SUS 430) has been studied in H 2–H 2O gas mixtures ( p H 2 / p H 2O =94/6 and 97/3), and in air in the temperature range of 1023–1173 K for 3.6 up to 1080 ks, in the conditions simulating the anode and cathode environments in SOFC. It has been found that the oxide scale, composed mainly of Cr 2O 3, grows in accordance with the parabolic rate law. The dependence of the parabolic rate constant, k p, on temperature can be described as k p=6.8×10 −4 exp(−202.3 kJ mol −1/ RT) for the H 2–H 2O gas mixture with p H 2 / p H 2O =94/6. The determined parabolic rate constant is independent of the oxygen partial pressure in the experimental range of 5.2×10 −22 to 0.21 atm at 1073 K, which means that the growth rates of scale on Fe–16Cr alloy in the above-mentioned atmospheres are comparable. The increase in electrical resistance of the chromia scale growing on Fe–16Cr alloy vs. time, calculated from k p and the specific resistance of Cr 2O 3 scale, in comparison with the constant electrical resistance of a ceramic interconnector, made of (La,Sr)CrO 3, indicates the necessity to modify the studied alloy surface. At 1073 K, the resistance of the Fe–16Cr alloy coated with La 0.6Sr 0.4CoO 3 by a spray-pyrolysis method is low, the average of 45 mΩ cm 2 in the H 2–H 2O gas mixture ( p H 2 / p H 2O =94/6) and the average of 20 mΩ cm 2 in air, in comparison with the ceramic interconnector, La 0.85Sr 0.15CrO 3, 0.5 cm thick. This indicates the applicability of SUS 430 alloy as interconnector for SOFC.