Oxide Fuel Cell Interconnect Applications Research Articles

Evaluation of selected Fe–Cr steels under single- and dual-atmosphere conditions for intermediate-temperature solid oxide fuel cell interconnect applications

For Intermediate-Temperature SOFC, the lower operating temperature allows for a broader range of steel grades to be used as interconnects. This work compares the oxidation performances of six Cr2O3-forming steels and one Al2O3-forming steel, under single- and dual-atmosphere conditions.Single-atmosphere exposures in combination with Cr(VI) evaporation measurements were carried out at 600 °C. Except for AISI 430 all the Cr2O3-forming steels showed protective behaviour with similar Cr(VI) evaporation values and low area specific resistance (ASR) values. Kanthal EF101 showed the lowest level of Cr(VI) vaporisation but substantially higher ASR values.Dual-atmosphere exposures were carried out at 600 °C in Ar-5%H2–3%H2O//air-3%H2O. All Cr2O3-forming steels suffered various degrees of breakaway corrosion. Therefore, operation under dual-atmosphere conditions is likely the life-time-limiting failure mode.

Metallic Co with Reactive Element Oxide Composite Coatings for Solid Oxide Fuel Cell Interconnect Applications

Metallic Co with Reactive Element Oxide Composite Coatings for Solid Oxide Fuel Cell Interconnect Applications

11–23% Cr steels for solid oxide fuel cell interconnect applications at 800 °C – How the coating determines oxidation kinetics

The present work investigates the low-cost steels AISI 441, AISI 430, and AISI 444 against the tailor-made high Cr steel Crofer 22 APU (22.9 wt% Cr) at 800 °C in simulated solid oxide fuel cell (SOFC) cathode conditions. Furthermore, a low Cr steel, AISI 409 (11.4 wt% Cr) is included in the study. The oxidation, chromium evaporation, and area-specific resistance (ASR) of the uncoated and Ce/Co-coated steels are studied for up to 3000 h. Ce/Co-coated steels showed significant improvement in behaviour compared to their uncoated counterparts. The oxidation and chromium evaporation behaviour between the uncoated steels varied substantially while the Ce/Co coated steels exhibited highly similar behaviour. The area-specific resistance of the coated low-cost steels was on par with Crofer 22 APU. However, 430 formed a continuous silica layer, resulting in a higher ASR after 3000 h. Cross-sections of the uncoated and Ce/Co-coated steels were analysed using a scanning electron microscope and energy dispersive X-ray spectroscopy.

Ce-doped (Mn,Co)3O4 coatings for solid oxide fuel cell interconnect applications

In order to improve the electrical conductivity of solid oxide fuel cell (SOFC) interconnect coatings, a series of Ce-doped (Mn,Co)3O4 (Mn1.5-xCo1.5-xCe2xO4+δ, x = 0, 0.025, 0.05, and 0.1) coatings are prepared. The chemical composition, lattice structures, morphology, electrical properties, and weight gain of the coatings are characterized. Experimental results show that doping Ce lowers the area-specific resistance of (Mn,Co)3O4 coatings from 6.83 to 2.62 mΩ cm2 at 800 °C. The electrical conductivity improvement can be attributed to the ion substitution by Ce3+ that turns (Mn,Co)3O4 into (Mn,Co)2CeO4, an N-type semiconductor with a higher electrical carrier density. However, the coatings with heavier doping of Ce exhibit more structural defects due to the lattice parameter reduction and volume shrinkage, which weaken the oxidation resistance of the coatings. This study verifies that doping Ce can effectively improve the electrical conductivity of spinel coatings. The negative influence of doping Ce can be mitigated by modifying the coating preparation technique or by changing the SOFC operating temperature.

Fe doped Ni–Co alloy by electroplating as protective coating for solid oxide fuel cell interconnect application

To obtain higher electrical conductivity and lower area specific resistance (ASR), the 10% Fe doped Ni–Co (NCF) alloy was prepared on SUS 430 steel substrate by electroplating for solid oxide fuel cells (SOFCs) interconnects application. Then, the SUS 430 steels and NCF coated steels were oxidized at 800 °C. The microstructure and oxide phase of samples were tested by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). These results proved that the NCF coated steel achieved the lower oxidation rate of 9.28 × 10−14 g2cm−4s−1 and ASR of 14.72 mΩ cm2.

Cu doped Ni–Co spinel protective coatings for solid oxide fuel cell interconnects application

Four different amount of Cu doped Ni–Co alloy coatings were fabricated on SUS430 substrate by electroplating for solid oxide fuel cells (SOFCs) interconnects application. After oxidation at 800 °C, the microstructure and oxide phase of samples were tested by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Our experimental results indicated that the Cu addition improved the electrical behavior of Ni–Co alloy coating. Cu doping reduced the activation energy (Ea) of electrons hopping and inhibited the growth of Cr2O3 oxide layer. Furthermore, the oxidation kinetics and electrical properties of the alloy coatings were obtained. These results showed that the 9% Cu doped Ni–Co coated steels achieved the minimum parabolic rate constant (2.05 × 10−14 g2cm−4s−1) and area specific resistance (14.11 mΩ cm2) after the thermostatic oxidation process.

Electrophoretic deposition of (Cu,Mn,Co)3O4 spinel coating on SUS430 ferritic stainless steel: Process and performance evaluation for solid oxide fuel cell interconnect applications

This study aimed to investigate the colloidal stability of (Cu,Mn,Co)3O4 spinel in various suspension media and suitable applied voltage for electrophoretic deposition (EPD). The performance of the coated SUS430 was evaluated in terms of oxidation behaviour, Cr migration and electrical conductivity. The (Cu,Mn,Co)3O4 suspension prepared using a mixture of 50/50 isopropanol and acetylacetone presented the highest zeta potential (41.4 mV), which indicated a stable and well-dispersed suspension. An applied voltage of 80 V is recommended to produce a coating surface with uniform particle arrangement. The electrophoretically deposited (Cu,Mn,Co)3O4 spinel coating could reduce the oxidation kinetics by three orders of magnitude and exhibited stable electrical conductivity during 200 h of oxidation. Cr element was not detected on the surface of the coated SUS430 and LSCF cathode after 200 h of oxidation. Therefore, the electrophoretically deposited (Cu,Mn,Co)3O4 spinel coating could successfully inhibit Cr migration from the stainless steel interconnects to the cathode.

Oxidation Behavior and Electrical Properties of De-siliconized AISI 430 Alloy with Mn1.5Co1.5O4 Coating for Solid Oxide Fuel Cell Interconnect

The effect of de-siliconization on the oxidation behavior of AISI 430 stainless steel used for solid oxide fuel cell interconnect application was investigated. De-siliconization treatment was conducted via heating steel parts in an H2 environment. The de-siliconized substrates were then coated with a Mn1.5Co1.5O4 spinel coating, using wet spray method. For comparison, a similar coating process was applied on the as-received AISI 430 stainless steel specimens. Oxidation kinetics of coated interconnects were evaluated at 700, 800 and 900 °C in air. Results showed that the de-siliconization surface treatment decreased oxidation rates, with kinetic rates (g2 cm−4 s−1) of 4.39 × 10−14 at 700 °C, 1.71 × 10−13 at 800 °C and 1.62 × 10−12 at 900 °C, i.e., 55, 48 and 22% lower compared to the as-received coated substrates, respectively. Scanning electron microscopy images and EDS analysis confirmed the formation of SiO2-rich layer beneath the surface oxide scale for the oxidized specimens. The thickness of the formed scale for the de-siliconized specimens was noticeably lower than that of as-received ones. Step-wise electrical conductivity measurements showed that the de-siliconized specimens, compared to the as-received specimens, had higher electrical conductivity (~ 30–40%), implying lower oxidation degrees of the substrate and thus, better electrical performance under cathodic service condition of SOFC.

Long-term (4 year) degradation behavior of coated stainless steel 441 used for solid oxide fuel cell interconnect applications

The present work aims to investigate the long-term stability of Ce/Co coated AISI 441 used as an interconnect material in solid oxide fuel cells (SOFC). Being a commercially available alloy the use of AISI 441 would greatly reduce the cost of SOFCs in comparison to tailor-made interconnect materials such as Crofer 22 APU. To analyze the long-term stability Ce/Co coated AISI 441 is exposed in air at 800 °C for up to 38 000 h. Mass gain values are recorded continuously. After 7 000, 23 000, and 35 000 h area specific resistance (ASR) measurements are performed, and cross-sections are prepared and analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy. Cr-evaporation measurements are conducted on samples exposed for up to 38 000 h.

Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediate-temperature solid oxide fuel cell interconnect applications

Chromia scale growth and Cr evaporation of ferritic stainless steel interconnects are known to be major causes of serious degradation of the solid oxide fuel cell (SOFC) stack. The development of suitable ceramic coating materials on the metallic interconnects has been demonstrated as an effective way to address these challenges. Herein, we developed a Mn1.35Co1.35Cu0.2Y0.1O4 (MCCY) spinel material via a facile glycine-nitrate process as a protective coating on a metallic interconnect (SUS 441). Crystal structure and surface charge state analysis of the MCCY material revealed that co-doping of Y and Cu into the (Mn,Co)3O4 spinel resulted in redistribution of the Mn ions (Mn3+ and Mn4+) into the octahedral site, which increased the electrical conduction by enhanced small polaron hopping. Accordingly, the MCCuY-coated interconnect exhibited ∼8 times lower area specific resistance (ASR) than that of the undoped Mn1.5Co1.5O4 (MCO) coated interconnect. Moreover, time-dependent ASR behavior of MCCuY-coated sample was monitored in-situ using electrochemical impedance spectroscopy at 650 °C, showing excellent stability with no observable change for >1000 h, while the ASR of the MCO-coated sample was raised by ∼71%. After 1000 h operation, we found strong adhesion between the MCCuY coating and the metallic interconnect as well as remarkably restricted Cr diffusion into the coating layer. Furthermore, the parabolic constant associated with the oxidation kinetics of the MCCuY-coated substrate (8.25 × 10−11 mg2 cm−4 s−1) was ∼1 order of magnitude lower than that of the MCO-coated one (7.34× 10−10 mg2 cm−4 s−1) at 650 °C after 1000 h measurement. These results demonstrate that the MCCuY is a highly promising coating material of metallic interconnects for intermediate-temperature SOFC applications.

Oxidation Behavior of NiFe2O4 Spinel-Coated Interconnects in Wet Air

Ferritic stainless steels are promising materials for solid oxide fuel cell interconnect applications. However, volatilization of Cr from the Cr-containing oxide scales can result in cathode poisoning, especially in wet air, and thus greatly degrades the performances of the cell. In the present study, a protective and adhesive NiFe2O4 spinel coating was applied on a type 430 stainless steel (430SS) to isolate Cr2O3 from gas environment. The performance of the uncoated and NiFe2O4-coated samples has been evaluated in the air-7%H2O at 700, 800, and 850 °C, respectively. The bare steel lost mass at 700 and 800 °C and underwent breakaway oxidation at 850 °C after ~ 75 h, including scale spallation and Cr evaporation. The NiFe2O4 coating could significantly increase the oxidation resistance of 430SS and effectively retard the outward diffusion of Cr and inhibit the Cr evaporation.

Promising cermets of TiN-Ni for intermediate temperature solid oxide fuel cell interconnects application

Abstract Novel TiN-Ni cermets as potential interconnects for intermediate temperature solid oxide fuel cells (IT-SOFCs) are proposed in this study. The cermets with 30, 50 and 70 vol% Ni are fabricated by optimizing the process parameters of the hot-pressed sintering method. With the increase of Ni content in the cermets, the relative density, electrical conductivity, and thermal conductivity are pronounced improved, and the maximum values reach 99.6%, 1.7 × 10 4 S cm −1 and 45 W m −1 K −1 , respectively. In addition, the coefficient of thermal expansion (CTE) of the TiN-Ni cermets in the range of (9.7–13.2) × 10 −6 K −1 shows a superior match with other components of IT-SOFCs. Furthermore, the cermets with 70 vol% Ni exhibit excellent oxidation resistance and electrical conductivity retention with a weight gain of only 0.7% and a high conductivity of 1.6 × 10 4 S cm −1 after oxidizing at 800 °C for 100 h. Our work demonstrates that the TiN-Ni cermets can be used as promising interconnects for the practical application of IT-SOFCs.

Sputtered MnCu metallic coating on ferritic stainless steel for solid oxide fuel cell interconnects application

MnCu (Mn:Cu = 1:1, atomic ratio) metallic coatings have been deposited by magnetron sputtering on bare and on 100 h pre-oxidized SUS 430 steel for planar solid oxide fuel cells interconnects application. After oxidation at 800 °C in air, the MnCu coating directly deposited on the bare steel has been thermally converted to (Mn,Cu)3O4 spinel with Fe, containing discrete CuO on the outer surface. Nevertheless, the converted (Mn,Cu)3O4/CuO layer from the MnCu coating deposited on the pre-oxidized steelis almost free of Fe. A double-layer oxide structure with a main (Mn,Cu)3O4 spinel layer atop a Cr-rich oxide layer has been developed on the bare and pre-oxidized steel samples with MnCu coatings after thermal exposure. The outer layer mainly consisted of (Mn,Cu)3O4 spinel has not only significantly suppressed Cr outward migration to the scale surface, but also effectively reduced the area specific resistance (ASR) of the scale. The sputtered MnCu metallic coating is a very promising candidate for steel interconnect coating material.

500 Hour Investigation of Electroless Ni-Plated AISI 441 Steel for Solid Oxide Fuel Cell Interconnect Applications Under Operating Conditions

Solid Oxide Fuel Cells (SOFCs) are high-temperature (600-800°C) electrochemical devices that produce heat and electricity by oxidizing hydrogen or short chain hydrocarbon fuels. Due to their high efficiency, fuel flexibility, and low emissions, SOFCs have gained popularity as a method of both large- and small-scale power production. In planar SOFC technology, individual cells are stacked in electrical series using ferritic stainless steel interconnects (ICs), which separate the fuel (anode) and oxidant (cathode) chambers, facilitate gas distribution, and provide mechanical support. The ICs are simultaneously exposed to the fuel (H2) on one side and the oxidant (air) on the other, known as a “dual atmosphere exposure.” Recent advances in SOFC technology have allowed ferritic stainless steels (FSSs) to replace the cost-prohibitive ceramic IC components that were previously used. However, FSSs experience accelerated and/or anomalous corrosion in dual atmosphere environments (relative to single atmosphere exposure – e.g., air only) that leads to SOFC performance degradation. In this study, electroless Ni-plated FSS AISI 441 was evaluated under standard operating conditions, namely 700°C dual atmosphere with and without a current flow of 0.5 A for 500 hours. Morphology of the surfaces and cross sections of each sample were analyzed using a Field Emission Scanning Electron Microscope (FE-SEM) and the composition was assessed using Energy Dispersive X-Ray Spectroscopy (EDX). Anomalous and accelerated oxidation were observed with and without current flow involving massive outward iron diffusion phenomena, as well as inward diffusion of nickel plating. A layered oxide structure was found with and without current flow; however, substantial differences in elemental distribution throughout these layers were observed. These findings, along with mechanism hypotheses, will be presented and discussed in the context of developing durable interconnects for long-term high-performance SOFC systems. Figure 1: EDX linescan of the anode side (exposed to hydrogen) of Electroless Ni-plated AISI 441 cross sections after 500 hours of exposure at 700°C with current flow of 0.5 A (A) and without current flow (B) Figure 1

Formation and performances of spinel reaction layers on Co-40Mn coatings under an oxygen pressure of 105 Pa for solid oxide fuel cell interconnect application

Because of good high temperature conductivity (Mn,Co)3O4 is a promising oxide coating for solid oxide fuel cell interconnect. High-temperature oxidation of Co-Mn alloys at 750 °C and 850 °C at 105 Pa oxygen reveals that formation of a continuous (Co,Mn)3O4 spinel layer significantly decreases oxidation rate constant but not the same for the discontinuous spinel layer. Co-40Mn fine grain alloy coatings are prepared by high-energy micro-arc alloying process (HEMAA). Microstructures and compositions after preparation and oxidation were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). When pre-oxidized under an oxygen pressure of 105 Pa at 850 °C for 10 h, Co-40Mn coatings are transformed into the external oxide which is mainly composed of (Co,Mn)3O4 and Mn3O4. The results disclose Cr outward transport from the stainless steel is significantly suppressed by the as-prepared (Co,Mn)3O4 oxide coatings, when coatings oxidized at 800 °C for 100 h in wet air. The composited spinel coatings have excellent high temperature oxidation resistance and electrical performance.

CeO2-doped (Co,Mn)3O4 coatings for protecting solid oxide fuel cell interconnect alloys

Reactive element-doped (Co,Mn)3O4 spinel is considered as the most promising coating system to protect ferritic alloys for solid oxide fuel cell interconnect application. In this paper, a CeO2-doped (Co,Mn)3O4 coating was synthesized on a Crofer 22 APU alloy substrate via electrolytic codeposition of a composite layer consisting of a Co matrix and embedded Mn3O4/CeO2 particles, followed by thermal conversion of the deposited layer in air at elevated temperatures. After oxidation at 800°C in air for 250h, no Cr penetration was detected in both the CeO2-doped and CeO2-free spinel coatings. However, CeO2 doping significantly reduced the Cr2O3 scale growth at the coating/substrate interface. An area specific resistance of 8mΩ·cm2 at 800°C was achieved for the CeO2-doped coating sample, which was much lower than that of the CeO2-free coating sample (13mΩ·cm2) or the bare substrate (24mΩ·cm2) at the same temperature. CeO2 doping in the spinel coating also improved the performance stability of the anode-supported cell in contact with the alloy interconnect.

Mn1.4Co1.4Cu0.2O4 spinel protective coating on ferritic stainless steels for solid oxide fuel cell interconnect applications

In an attempt to reduce the oxidation and Cr evaporation rates of solid oxide fuel cells (SOFCs), Mn1.4Co1.4Cu0.2O4 spinel coating is developed on the Crofer22 APU ferritic stainless steel substrate by a powder reduction technique. Doping of Cu into Mn–Co spinels improves electrical conductivity as well as thermal expansion match with the Crofer22 APU interconnect. Good adhesion between the coating and the alloy substrate is achieved by the reactive sintering process using the reduced powders. Long-term isothermal oxidation experiment and area specific resistance (ASR) measurement are investigated. The ASR is less than 4 mΩ cm2 even though the coated alloy undergoes oxidation at 800 °C for 530 h and four thermal cycles from 800 °C to room temperature. The Mn1.4Co1.4Cu0.2O4 spinel coatings demonstrate excellent anti-oxidation performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.

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 Mn1.5Co1.5O4 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.

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

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

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

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