Solid Oxide Fuel Cell Interconnect Research Articles

Oxidation behavior and area specific resistance of La, Cu and B alloyed Fe-22Cr ferritic steels for solid oxide fuel cell interconnects

Two Fe-22 wt% Cr ferritic stainless steels containing varying concentrations of La (0.14 or 0.52 wt%), Cu (0.17 or 1.74 wt%) and B (48 or 109 ppm) are investigated with respect to oxidation behavior and high temperature area specific resistance (ASR) of the surface oxide scales. To determine the oxidation resistance of developed steels, continuous isothermal oxidation is carried out at 800 °C in air, for 2000 h, and their thermally grown oxide scale is characterized using dynamic SIMS, SEM/EDX, XRD and GI-XRD techniques. To assess their electrical performance, the ASR measurement by four-point probe method is conducted at 800 °C in air, for 400 h. In higher La content steel, the La-oxides at the scale/alloy interface promotes the oxygen transport which resulted in sub-surface oxidation of Mn, Cr, Ti and Al. Moreover, the inward growth of oxides contributes to increase of Fe-Cr alloy protrusions within the scale, which reduced the ASR. In contrast, sub-surface oxidation is reduced in high Cu-alloyed steel by segregated Cu at the scale/alloy interface. Thus, addition of Cu is effective to oxidation resistance and also to better electrical performance. However, no obvious impact of B on the scale sequence and/or ASR is observed.

Evaluation of electroplated copper coating on ferritic stainless steel for solid oxide fuel cells interconnects

A Cu coating was deposited on SUS 430 ferritic stainless steel by a low-cost method of electroplating which was employed for the protection of solid oxide fuel cell metallic interconnects. After oxidation, a two-layer oxide scale consisting of an external layer of CuO and an internal layer of Cr2O3 was primarily formed on the coated steel, and a spinel layer consisting of Cu, Cr, Fe and Mn was formed at the CuO/Cr2O3 interface. The CuO outer layer not only suppressed the Cr2O3 scale growth but also served as an effective obstacle against Cr outward diffusion. The coated steels had lower scale area-specific resistances (ASRs) compared to the uncoated steel. The oxidation mechanism and surface scale electrical properties of the coated steel were discussed.

Tape casting of lanthanum chromite for solid oxide fuel cell interconnects

A method, based on tape-casting, is developed for the manufacture of ceramic interconnects for SOFCs. The development of an optimized process is shown with a discussion of the effects on the precursor-slurry quality and viscosity of, among other things, the concentrations of ammonia, binder and solid particles. Furthermore the effects of some processing modifications, such as the drying rate in the tape casting machine, the speed of the transport band and the doctor blade height on the quality (density and homogeneity) of the green tapes and the finished product are discussed. Improving the degree of dispersion of the primary particles in the slurry was found to make possible improved densification and quality of the thick plates produced as did reducing the drying rate of the wet tapes during the casting process.

One new route to optimize the oxidation resistance of TiC/hastelloy (Ni-based alloy) composites applied for intermediate temperature solid oxide fuel cell interconnect by increasing graphite particle size

TiC/hastelloy composites with suitable thermal expansion and excellent electrical conductivity are promising candidates for IT-SOFC interconnect. In this paper, the TiC/hastelloy composites are fabricated by in-situ reactive infiltration, and the oxidation resistance of composites is optimized by increasing graphite particle size. Results show that the increase of graphite particles size from 1 μm to 40 μm reduces TiC particle size from 2.68 μm to 2.22 μm by affecting the formation process of TiC. Moreover, the decrease of TiC particles size accelerates the fast formation of dense and continuous TiO2/Cr2O3 oxide layer, which bring down the mass gain (800 °C/100 h) from 2.03 mg cm−2 to 1.18 mg cm−2. Meanwhile, the coefficient of thermal expansion decreases from 11.15 × 10−6 °C-1 to 10.80 × 10−6 °C-1, and electrical conductivity maintains about 5800 S cm−1 at 800 °C. Therefore, the decrease of graphite particle size is one simple and effective route to optimize the oxidation resistance of composites, and meantime keeps suitable thermal expansion and good electrical conductivity.

Self-Healing of Precoated AISI 441 for Solid Oxide Fuel Cell Interconnects at Intermediate Temperatures

Sandvik Materials Technology develops nanocoatings for SOFC stainless steel interconnects. The properties of cerium cobalt coated AISI441 (EN 1.4509) have previously been studied extensively (1-4). However, the trend in SOFC have been to decrease working temperatures from above 800°C and today commercial systems are sold that operate below 650°C. The lower operating temperature opens up new possibilities in selection of component materials. In this study we have examined precoated AISI 441 used in solid oxide fuel cell interconnects at 650°C and compared to previous studies at 800°C. The coating characteristics after different oxidation times were studied by SEM. Also, the variation of cobalt coating thickness and the development of the surface oxides were examined by SEM. By using the double trench approach (3) samples with and without priming at 900°C for 5 hours followed by heat treatment at 650°C, were compared. These samples were also compared to the results from (3) with a sample prepared at 800°C for a total of 504 hours. Figure 1. Mass gain of pre-coated cerium cobalt AISI441 (EN 1.4509) from cyclic oxidation at 650°C and 800°C in ambient air. The mass gains are not corrected for Cr evaporation, which shows for the 650°C as it displays flat or negative mass gains. References J. Froitzheim, S. Canovic, M. Nikumaa, R. Sachitanand, L.G. Johansson and J. E. Svensson J. of Power Sources, 220, 217 (2012)J.G. Grolig, J. Froitzheim, J.E. Svensson, J. of Power Sources, 248 1007 (2014)R. Berger, M. W. Lundberg, J. Westlinder, H. Holmberg, ECS Trans. 68 1649 (2015)H. Falk-Windisch, J. Claquesin, M. Sattari, J.-E. Svensson, J. Froitzheim, J. of Power Sources, 343 1 (2017) Figure 1

MCO-Coated Interconnects for Mitigation of Cr-Poisoning in Solid Oxide Fuel Cells

Chrome species evaporated from stainless steel interconnect are considered to be the major poisoning source for the cathode of solid oxide fuel cells (SOFCs). The poisoning effect is extremely severe when humidity is present in the incoming air. High performance perovskite-oxide type cathodes, such as LSCF and BSCF are subject to deactivated by Cr. To eliminate/mitigate the Cr poisoning effect, thin film coatings on the surface of stainless steel (SS) are employed to block Cr evaporation. In the present study, a Mn-Co oxide (MCO) thin film coating on SS using the dip coating method will be reported. Dip coating was found to be very suitable to fabricate MCO coatings on SS, which in the case of SOFC interconnects are usually specially designed for gas delivery with complex structure. The initial results indicate MCO thin film is very effective in blocking Cr evaporation, which was examined in both single cell and stack testing. Single cell tests using MCO-coated cathode SS test hardware resulted in a very low degradation rate of < 4 mV / 1000 hours when tested over thousands of hours with up to 10% cathode humidity. This result is very similar to tests without humidity. Stack testing indicates dip coating is feasible for coating specially designed, complex parts such as SOFC interconnects. Key words: SOFC, dip coating, MCO

Modelling Microstructural and Chemical Degradation of Ferritic Stainless Steels for SOFC Interconnects

Ferritic Fe-based (FeCrX) alloys forming a protective chromium oxide film are used for providing electrical interconnection in high temperature Solid Oxide Fuel Cells (SOFC) are susceptible to degradation mechanisms in the presence of high temperatures (>600oC), and humid air. These translate by the increase in interconnect electrical contact resistance and the formation of volatile chromium species causing poisoning of the adjacent cathode. Modelling tools and techniques are increasingly becoming popular to gain better understanding of the degradation mechanisms that lead to the formation of volatile chromium species. This research work examines the oxidation behaviour of uncoated ferritic stainless steels system operated at temperature of 700oC in both wet (3% humidity) and dry conditions. A model is presented based on the physical processes involved. The model is able to predict the weight gain of the samples upon oxidation, the oxide film thickness, the volatilization of chromium and the useable lifetime of interconnects based on chromium depletion. Parameter determination from standard experiments is discussed along with the physical and design insights from the model. Perhaps most importantly the model provides recommendations regarding accelerated testing of FeCrX interconnects and control of the Chromia film thickness. Acknowledgements: This work was supported by the European FCH JU in project SCORED 2:0 under contract no. 325331 and the EPSRC.

Long Term (4 Years) Performance of Co/Ce Coated 441 for SOFC Interconnect Applications

Ferritic stainless steel interconnectors are widely used in solid oxide fuel cells (SOFC) due to a combination of low cost, compatible thermal expansion properties and ease of manufacturing. However, their viability is hindered by some key technical hurdles. Most stainless steels suggested for SOFC applications rely on the formation of a fairly protective Cr2O3 scale. However, in air side environments Cr species evaporation leads to material failure and insufficient lifetimes. Coatings offer a way to mitigate Cr evaporation and in some cases decrease oxide scale growth. Numerous research papers have investigated different coating compositions and deposition techniques, however, the most commonly suggested material is (Mn,Co)3O4 (MCO). Instead of depositing MCO metallic Co can be applied. During operation Co is converted into Co3O4 and subsequently enriched in Mn from the underlying steel substrate. It has been shown that this type of coating effectively impedes Cr volatilization, even when only very thin layers (600 nm) are applied. Moreover the major advantage of metallic Co coatings are cost savings, since Co can be applied in a large scale roll-to-roll process before stamping the interconnect into its final shape. Furthermore, our previous work has shown that an additional layer of 10 nm Ce in combination with the Co coating results in the aforementioned mitigation of volatilization of Cr species, and also effectively reduces oxide scale growth. Oxide scale growth is detrimental for two reasons. First, a depletion of oxide scale forming element (Cr) can result in the formation of poorly protective iron oxide. Second, even a protective Cr2O3 scale grows with time. Since Cr2O3 is only a moderate electronic conductor this results in an increase of Area Specific Resistance (ASR) as the oxide scale grows in thickness. This eventually creates prohibitively high resistances. Both effects can be mitigated by addition of a thin Ce layer. In the present work Ce/Co (10/600 nm) coated AISI 441 has been exposed for 4 years (35 000 h) at 800 °C. The mass gain of the samples over the course of exposure has been recorded at selected intervals. After 35 000 h a mass gain of 3.76 mg/cm2 has been measured. Over the entire exposure parabolic mass gain kinetics are observed indicating the absence of a poorly protective iron oxide. After 35 000 h the samples were moved to a different setup to measure Cr(VI) evaporation. It was found that despite the fact that the coating was only 600 nm in thickness, it effectively inhibits volatilization of Cr even after 4 years of exposure. Furthermore, samples were removed at selected intervals to measure ASR.

The oxidation resistance optimization of titanium carbide/hastelloy (Ni-based alloy) composites applied for intermediate-temperature solid oxide fuel cell interconnects

Titanium carbide/hastelloy (TiC/hastelloy) composites are potential candidates for intermediate-temperature solid oxide fuel cell interconnects. In this work, TiC/hastelloy composites with suitable coefficient of thermal expansion are fabricated by in-situ reactive infiltration method, and their properties are optimized by adjusting TiC particle size (dTiC). The oxidation process of TiC/hastelloy composites is comprehensive performance of TiC and Ni-Cr alloy and determined by outward diffusion of Ti and Ni atoms and internal diffusion of O2. The oxidation resistance of composites could be improved by the decrease of dTiC through accelerating the formation of continuous and dense TiO2/Cr2O3 oxide scale. Moreover, the electrical conductivity of composites at 800 °C for 100 h is 5600–7500 S cm−1 and changes little with the prolongation of oxidation time. The decrease of dTiC is favorable for the properties optimization, and composites with 2.16 μm TiC exhibits good integrated properties.

Self-Healing of Precoated AISI 441 for Solid Oxide Fuel Cell Interconnects at Intermediate Temperatures

Sandvik Materials Technology develops nanocoatings for SOFC stainless steel interconnects. The properties of cerium cobalt coated AISI441 (EN 1.4509) have previously been studied extensively. However, the trend in SOFC have been to decrease working temperatures from above 800°C and today commercial systems are sold that operate below 650°C. The lower operating temperature opens up new possibilities in selection of component materials. In this study we have studied precoated AISI 441 used in solid oxide fuel cell interconnects at 650°C. The coating characteristics after different oxidation times were studied by SEM. Also, by using the double trench approach samples with and without pre-oxidizing at 900°C for 5 hours followed by heat treatment at 650°C, were compared.

MCO-Coated Interconnects for Mitigation of Cr-Poisoning in Solid Oxide Fuel Cells

Chrome species evaporated from stainless steel interconnect are considered to be the major poisoning source for the cathode of solid oxide fuel cells (SOFCs). The poisoning effect is extremely severe when humidity is present in the incoming air. High performance perovskite-oxide type cathodes, such as LSCF and BSCF are subject to deactivation by Cr. To eliminate/mitigate the Cr poisoning effect, conductive coatings on the surface of stainless steel (SS) are employed to block Cr evaporation. In the present study, a Mn-Co oxide (MCO) coating on SS using the dip coating method will be reported. Dip coating was found to be very suitable to fabricate MCO coatings on SS, which in the case of SOFC interconnects are usually specially designed for gas delivery with complex structure. The initial results indicate MCO coating is very effective in blocking Cr evaporation, which was examined in both single cell and stack testing. Single cell tests using MCO-coated cathode SS test hardware resulted in a very low degradation rate of < 4 mV / 1000 hours when tested over thousands of hours with up to 10% H2O in flowing air. This result is very similar to tests without humidity. Stack testing indicates dip coating is feasible for coating specially designed, complex parts such as SOFC interconnects.

Modelling Microstructural and Chemical Degradation of Ferritic Stainless Steels for SOFC Interconnects

Ferritic (FeCrX) alloys forming a protective chromium oxide layer (Cr2O3) are the preferred materials for the interconnect component in solid oxide fuel cells (SOFC). Due to their oxidation rate in moist air, these alloys undergo degradation causing an increase in interconnect electrical contact resistance and formation of (poisonous) volatile chromium species. Developing coating agents and techniques that protect these alloys and lower their degradation rates is the current approach to extending SOFC lifetime . In this paper we examine the oxidation behaviour of an uncoated ferritic stainless steel interconnect for a supposed SOFC system operating at 850oC. A precursor numerical model with respect to the oxidation and evaporation kinetics of the steel has been developed. The model is able to predict the weight gain of the samples upon oxidation, the oxide film thickness, the volatilisation of chromium, and the useable lifetime of interconnects based on chromium depletion calculations.

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.

Determination of the bonding strength in solid oxide fuel cells’ interfaces by Schwickerath crack initiation test

An adaptation of the Schwickerath crack initiation test (ISO 9693) was used to determine the bonding strength between an anode support and three different cathodes with a solid oxide fuel cell interconnect. Interfacial elemental characterization of the interfaces was carried out by SEM/EDS analysis on fracture surfaces to investigate the bonding mechanisms. SEM/EDS of fresh fractures were also performed to determine the cohesion/adhesion mechanism of bonding. Calculations of the residual stresses were determined by finite element simulation using ANSYS, based on thermo-mechanical properties of the materials obtained by measurement, calculation or literature.

Influence of Alloying Elements on the Behavior of Different Ferritic Steels as Candidate Materials for SOFC Interconnect

The efficiency of a solid oxide fuel cell (SOFC) can be improved by using a Ni-mesh between the ferritic steel interconnect and the Ni/YSZ anode. However, interdiffusion processes can lead to internal oxidation within the Ni-mesh and some microstructural changes, i.e., formation of an austenite zone and accelerated formation of σ-phase at the ferrite/austenite interface. These changes may adversely affect the performance of the cell during long-term operation. The present work focused on the influence of certain alloying elements on the overall behavior of the ferritic steel under simulated SOFC operating conditions to define conditions to minimize σ-phase formation without degrading the oxidation resistance and mechanical properties of the steel. The experimental results indicate that decreasing the amount of Cr and adjusting the amount of Nb, Si and W of the steel is a possible way to achieve that goal.

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo1.7Fe0.3O4 spinel coatings of different density. The coating density is shown to have minor influence on the long-term oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88% reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H2O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97%.

Long‐term oxidation and electrical behavior of Nb‐doped Ti3SiC2 as solid oxide fuel cell interconnects

AbstractNb‐doped Ti3SiC2 compounds ((Ti1‐xNbx)3SiC2, x=0%, 2%, 5%, 7%, and 10%) as novel interconnect materials of intermediate temperature solid oxide fuel cell (IT‐SOFC) were studied in the simulated cathode atmosphere. The long‐term oxidation behaviors and area‐specific resistance (ASR) of these compounds have been investigated at 800°C up to 700 hours. Among these compounds, (Ti0.95Nb0.05)3SiC2 shows the best oxidation resistance and lowest postoxidation ASR (5.6 mΩ·cm2 after exposure at 800°C in air for 700 hours), endowing it a great promising material in the application as interconnect of IT‐SOFC. After oxidation, Nb is mainly doped uniformly into the lattice of rutile‐TiO2 (r‐TiO2) grains formed on the tested compounds. Nb doping could decrease the concentrations of both oxygen vacancies and titanium interstitials in r‐TiO2. As a result, the oxidation rate of (Ti,Nb)3SiC2 decreases remarkably, the structure of the oxide scale changes from a duplex layer of TiO2 outer layer and TiO2+SiO2 mixture inner layer to a single mixture layer. Nb doping also increases the amount of semifree electrons, causing the significant reduce of the postoxidation ASR of (Ti,Nb)3SiC2.

Formation of sol–gel derived (Cu,Mn,Co)3O4 spinel and its electrical properties

An (Mn,Co)3O4 spinel layer effectively suppresses Cr outward diffusion, but the occurrence of spallation between coating and metal interfaces and the increasing oxidation rate over time significantly deteriorate cell performance. A transition metal cation, particularly Cu, is added into the (Mn,Co)3O4 to improve the long-term stability of the interconnect materials for low-to-intermediate-temperature solid oxide fuel cells (SOFCs). In this work, fine-crystalline (Cu,Mn,Co)3O4 spinel powders with an average crystallite size of 21nm were successfully synthesized via the citric acid–nitrate method. The potential for use as a coating material for low-to-intermediate-temperature SOFC interconnects was explored. According to the TG curves, IR spectra, and XRD patterns, 800°C is recommended as the minimum calcination temperature for (Cu,Mn,Co)3O4 spinel materials. Compared with (Mn,Co)3O4 spinel materials, the addition of Cu does not induce significant changes in the crystal structure but markedly improves the electrical conductivity (116 Scm−1) and the activation energy (0.394eV) of the (Cu,Mn,Co)3O4 spinel. Overall, the sol–gel derived (Cu,Mn,Co)3O4 spinel can be used as a coating material for low-to-intermediate-temperature SOFC interconnects and is superior to (Mn,Co)3O4 spinel because high electrical conductivity is preferred to minimize ohmic losses between the electrodes of adjacent cells.

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.

Investigation on the properties of Ta doped Ti3SiC2 as solid oxide fuel cell interconnects

Reported herein demonstrate Ta doping greatly improves the oxidation resistance and electrical conductivity of Ti3SiC2 as interconnect of SOFC.