Effect of Mn-Co spinel coating for Fe-Cr ferritic alloys ZMG232L and 232J3 for solid oxide fuel cell interconnects on oxidation behavior and Cr-evaporation

Metallic materials, especially Fe-Cr ferritic alloys, are promising as interconnect materials of solid oxide fuel cells (SOFCs) operated at around medium temperatures. ZMG232L is one of the developed Fe-Cr ferritic alloys for SOFC metallic interconnects.These metallic materials are usually machined or pressed into various shapes of interconnect parts, and thickness of these parts is often thin. However, the oxidation rate of thin sheet was much higher than that of thick one because Cr content decreased under oxide layer of edge part of thin sheet. Such accelerated oxidation behavior could be improved by reducing Mn, increasing Cr, and adding W in ZMG232L.It is also very important to reduce Cr-evaporation from the oxidized surface of ferritic alloys in cathode side. The aim of this study is to reduce the Cr-evaporation from oxidized alloy surface in air by coating with Mn-Co spinel oxide. In this study, oxidation behavior and Cr-evaporation of ZMG232L and improved Fe-Cr alloy, 232J3, coated with Mn-Co spinel oxide were investigated at elevated temperature in air. MnCo₂O₄ spinel coating on the pre-oxidized Fe-Cr ferritic alloy surface improved oxidation resistance and Cr-evaporation.     

Evaluation of Ni–Cr-base alloys for SOFC interconnect applications

To further understand the suitability of Ni–Cr-base alloys for solid oxide fuel cell (SOFC) interconnect applications, three commercial Ni–Cr-base alloys, Haynes 230, Hastelloy S and Haynes 242 were selected and evaluated for oxidation behavior under different exposure conditions, scale conductivity and thermal expansion. Haynes 230 and Hastelloy S, which have a relatively high Cr content, formed a thin scale mainly comprised of Cr₂O₃ and (Mn,Cr,Ni)₃O₄ spinels under SOFC operating conditions, demonstrating excellent oxidation resistance and a high scale electrical conductivity. In contrast, a thick double-layer scale with a NiO outer layer above a chromia-rich substrate was grown on Haynes 242 in moist air or at the air side of dual exposure samples, indicating limited oxidation resistance for the interconnect application. With a face-centered-cubic (FCC) substrate, all three alloys possess a coefficient of thermal expansion (CTE) that is higher than that of candidate ferritic stainless steels, e.g. Crofer22 APU. Among the three alloys, Haynes 242, which is heavily alloyed with W and Mo and contains a low Cr content, demonstrated the lowest average CTE at 13.1 × 10⁻⁶ K⁻¹ from room temperature to 800 °C, but it was also observed that the CTE behavior of Haynes 242 was very non-linear.     

Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr₂O₃-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H₂O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr₂O₃-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H₂O to provide further guidance for SOFC BOP component alloy selection.     

High-fidelity stack and system modeling for tubular solid oxide fuel cell system design and thermal management

Effective thermal integration of system components is critical to the performance of small-scale (<10 kW) solid oxide fuel cell systems. This paper presents a steady-state design and simulation tool for a highly-integrated tubular SOFC system. The SOFC is modeled using a high fidelity, one-dimensional tube model coupled to a three-dimensional computational fluid dynamics (CFD) model. Recuperative heat exchange between SOFC tail-gas and inlet cathode air and reformer air/fuel preheat processes are captured within the CFD model. Quasi one-dimensional thermal resistance models of the tail-gas combustor (TGC) and catalytic partial oxidation (CPOx) complete the balance of plant (BoP) and SOFC coupling. The simulation tool is demonstrated on a prototype 66-tube SOFC system with 650 W of nominal gross power. Stack cooling predominately occurs at the external surface of the tubes where radiation accounts for 66-92% of heat transfer. A strong relationship develops between the power output of a tube and its view factor to the relatively cold cylinder wall surrounding the bundle. The bundle geometry yields seven view factor groupings which correspond to seven power groupings with tube powers ranging from 7.6-10.8 W. Furthermore, the low effectiveness of the co-flow recuperator contributes to lower tube powers at the bundle outer periphery.     

Chromium deposition and poisoning of cathodes of solid oxide fuel cells – A review

Intermediate temperature solid oxide fuel cells (IT-SOFCs) using chromia-forming alloy interconnect requires the development of cathode not only with high electrochemical activity but also with the high resistance or tolerance towards Cr deposition and poisoning. This is due to the fact that, at SOFC operating temperatures, volatile Cr species are generated over the chromia scale, poisoning the cathodes such as (La,Sr)MnO₃ (LSM) and (La,Sr)(Co,Fe)O₃ (LSCF) and causing a rapid degradation of the cell performance. Thus, a fundamental understanding of the interaction between the Fe–Cr alloys and SOFC cathode is essential for the development of high performance and stable SOFCs. The objective of this paper is to critically review the progress and particularly the work done in the last 10 years in this important area. The mechanism and kinetics of the Cr deposition and Cr poisoning process on the cathodes of SOFCs are discussed. Chromium deposition at SOFC cathodes is most likely dominated by the chemical reduction of high valence Cr species, facilitated by the nucleation agents on the electrode and electrolyte surface and/or at the electrode/electrolyte interface, i.e., the nucleation theory. The driving force behind the nucleation theory is the surface segregation and migration of cationic species on the surface of perovskite oxide cathodes. Overwhelming evidences indicate that the surface segregation plays a critical role in the Cr deposition. The prospect of the development in the Cr-tolerant cathodes for SOFCs is presented.     

Evaluation of several low thermal expansion Fe–Co–Ni alloys as interconnect for reduced-temperature solid oxide fuel cell

Several low thermal expansion Fe–Co–Ni alloys including HRA 929C, Thermo-Span, EXP 4005 and Three-Phase were evaluated as interconnect for reduced-temperature solid oxide fuel cell (SOFC). The isothermal oxidation behaviors of the four alloys were determined at 800 °C in air corresponding to the SOFC cathode environment. The results indicate that the mass gains of HRA 929C and Thermo-Span increased continuously with oxidation time, and were higher than those of EXP 4005 and Three-Phase, both of which exhibited low oxidation rate after the first-week exposure due to the formation of a semi-continuous Al₂O₃ inner layer. Compared to the ferritic alloy Crofer 22 APU, these low thermal expansion alloys exhibited inferior oxidation resistance; however, the area specific resistance (ASR) of the oxide scales thermally grown on these alloys was lower than that for Crofer 22 APU, as a result of the formation of a highly-conductive, Cr-free surface spinel layer. The promises and problems of these low thermal expansion alloys were discussed with regard to SOFC interconnect application.     

EB-PVD deposition of spinel coatings on metallic materials and silicon wafers

Spinel oxides are promising materials as protective coatings on metallic interconnects to reduce the area specific resistance (ASR) at high operating temperature in solid oxide fuel cells (SOFC). In this work, the deposition of MnCo₂O₄ (MC) and MnCo_1.9Fe_0.1O₄ (MCF10) materials (1 μm) on Si substrates and commercial alloys (Crofer 22 APU, SS430 and Conicro 4023 W 188) by electron beam physical vapour deposition (EB-PVD) was studied. Optimisation of deposition, the effectiveness of MC and MCF10 protective layers and the influence of the deposition method were investigated after oxidation at 800 °C for 100 h in air. Significant improvements in Cr poisoning of the cathode and in ASR were observed in cells assembled with coated versus uncoated samples. The best results were obtained with cells assembled with MC/Conicro 4023 W 188 with MC deposited by EB-PVD.     

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

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

Investigation of Mn/Co coated T441 alloy as SOFC interconnect by on-cell tests

T441 has been identified as the candidate for SOFC interconnect material because it is assumed that with the addition of Nb, Ti in T441, the formation of continuous silica sub-layer could be avoided or delayed due to Nb and Si rich secondary phase formation stabilizing silicon migration. Previously, electrodeposition Mn/Co alloys followed by oxidation has been proved as a simple and cost effective method to fabricate (Mn, Co)₃O₄ coatings. In this work, Mn/Co coated T441 interconnects were tested as the cathode current collector of solid oxide fuel cells. For comparison, uncoated and 500 h pre-oxidized T441 interconnects were tested as well. The cell with coated interconnect shows stable performance during total 850 h test, even after severe thermal cycles (heating rate 26.7 °C/min). The coating shows good adhesion with substrate and it can prevent Cr poisoning on SOFC cathode. While the cell with uncoated and pre-oxidized T441 interconnects degrade rapidly. XRD results show the coating peaks shifted from mainly Co₃O₄ with some little Mn before test to MnCo₂O₄ after test due to Mn diffusion from substrate. No Cr penetrated to the coating layer, as further proved by EDX linescan. The effect of laves phase on the Cr₂O₃ sub-layer formation and coating thickness was further discussed.     

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

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

Potential suitability of ferritic and austenitic steels as interconnect materials for solid oxide fuel cells operating at 600 °C

The oxidation behavior of a number of commercially available ferritic and austenitic steels was tested in air and in two simulated anode gases of a solid oxide fuel cell (SOFC) to evaluate the potential suitability as construction materials for interconnects in SOFC's operating at 600 °C. During air exposure all studied materials showed excellent oxidation resistance due to formation of a protective, double layered chromia/spinel surface scale even if the steel Cr content was as low as 17%. However, in the anode side gases the presence of water vapour (and possibly CO/CO₂) increased the tendency to form poorly protective Fe-base oxide scales, in combination with internal oxidation of Cr. The occurrence of this adverse effect could be suppressed not only by increased Cr contents of the alloy but also by a small alloy grain size either in the bulk of the material or in the specimen/component surface. The latter can be promoted by cold work e.g. introduced by specimen/component grinding. As high Cr contents may lead to undesired σ-phase formation and defined surface treatments of an interconnect will not be possible in all designs, the relatively low operating temperature of 600 °C, resulting in low Cr diffusivity in the alloy grains, may require the use of a fine grained interconnect material to obtain and sustain protective chromia base surface scale formation during long-term operation.     

Oxidation of ferritic stainless steel interconnects: Thermodynamic and kinetic assessment

Planar solid oxide fuel cells with yttria-stabilized zirconia electrolytes typically operate at temperatures in the range of 700-850 °C. The maximum temperature is limited by the use of ferritic stainless steel interconnects which offer significant advantages such as low cost and high thermal and electronic conductivity over traditional ceramic interconnects. However, these alloys rely on the formation of a protective chromia scale for oxidation resistance that results in an increase in ohmic resistance and can volatilize leading to a loss of cathode catalytic activity. To better understand the oxidation behavior of chromia forming ferritic stainless steels, thermodynamic modeling was performed in conjunction with experimental oxidation testing and empirical kinetic evaluation. The phase stability and oxidation behavior of the Fe-Cr-O ternary system were assessed using thermodynamic calculations. Calculated ternary phase diagrams were validated against the experimental oxidation data of Fe-20Cr and Fe-18Cr ferritic stainless steels, GE-13L and AL-441HP, respectively. Results indicate that the use of accelerated testing, such as exposing the system to higher temperatures, can lead to changes in phase equilibria and the oxidation kinetics of the alloys. Through combined thermodynamic assessment and controlled oxidation experiments, the oxidation behavior of high-chromium ferritic stainless steels is presented and discussed.     

Metallic interconnects for solid oxide fuel cell: A review on protective coating and deposition techniques

With the reduction of solid oxide fuel cells (SOFCs) operating temperature to the range of 600 °C–800 °C, metallic alloy with high oxidation resistance are used to replace traditional ceramic interconnects. Metallic interconnects is advantageous over ceramic interconnects; in terms of manufacturability, cost, mechanical strength, and electrical conductivity. To date, promising candidates for metallic interconnects are all Cr-containing alloys, which are susceptible to volatile Cr migration that causes cell degradation. As such, protective coatings have been developed to effectively inhibit Cr migration; as well as maintain excellent electrical conductivity and good oxidation resistance. This article reviews the progress and technical challenges in developing metallic interconnects; different types of protective coatings and deposition techniques for metallic interconnects for intermediate-temperature SOFC applications.     

Chromium transport by solid state diffusion on solid oxide fuel cell cathode

Iron-chromium ferritic stainless steel is widely used in solid oxide fuel cell (SOFC) components. At 650-800 °C, stainless steels form a protective chromia oxide scale. This low conductivity catalytic compound can degrade SOFC cathode performance. The migration of Cr species onto the cathode occurs through vapor transport and/or solid state diffusion, and electrochemical reactions may affect the migration.It is important to understand the relative Cr transport and reaction rates to evaluate the most viable commercially available cathode material. This study characterizes the migration of Cr species through solid state diffusion and vapor deposition. Chromia blocks and chromia-forming stainless steel interconnects were held in contact with LSM (Lanthanum Strontium Manganese Oxide), LSCF (Lanthanum Strontium Cobalt Ferrite) and LNF (Lanthanum Nickel Ferrite) perovskite pellets in Cr-saturated air at 700 °C for 300 h. XRD (X-ray Diffraction), SEM (Scanning Electron Microscope), EDS (Energy Dispersive X-ray Spectroscopy) and Ion Milling by FIB (Focused Ion Beam) were used to detect Cr on and within the perovskite pellets. Cr transport and reaction on LSCF is the most severe, followed by LSM. Cr transport is observed on LNF, but without noticeable reaction.     

Oxidation resistance of novel ferritic stainless steels alloyed with titanium for SOFC interconnect applications

Chromia (Cr₂O₃) forming ferritic stainless steels are being developed for interconnect application in Solid Oxide Fuel Cells (SOFC). A problem with these alloys is that in the SOFC environment chrome in the surface oxide can evaporate and deposit on the electrochemically active sites within the fuel cell. This poisons and degrades the performance of the fuel cell. The development of steels that can form conductive outer protective oxide layers other than Cr₂O₃ or (CrMn)₃O₄ such as TiO₂ may be attractive for SOFC application. This study was undertaken to assess the oxidation behavior of ferritic stainless steel containing 1 weight percent (wt.%) Ti, in an effort to develop alloys that form protective outer TiO₂ scales. The effect of Cr content (6–22 wt.%) and the application of a Ce-based surface treatment on the oxidation behavior (at 800 °C in air + 3% H₂O) of the alloys was investigated. The alloys themselves failed to form an outer TiO₂ scale even though the large negative ΔG of this compound favors its formation over other species. It was found that in conjunction with the Ce-surface treatment, a continuous outer TiO₂ oxide layer could be formed on the alloys, and in fact the alloy with 12 wt.% Cr behaved in an identical manner as the alloy with 22 wt.% Cr.     

Visualisation of water droplets during the operation of PEM fuel cells

A transparent proton exchange membrane fuel cell (PEMFC) has been designed to enable visualisation of water droplets during its operation. Images of the formation of droplets on the surface of the gas diffusion layer (GDL) on its cathode side, which result in water accumulation and blockage to the airflow channels, were recorded using a CCD camera. Measurement of the cell current and droplet characterisation have been carried out simultaneously and the effect of the airflow and external resistive load has been quantified. The droplet images show that water accumulation occurs first in the middle channels of a serpentine reactant-flow fuel cell design and that no droplets are formed at the bends of the flow channels. Water blockage to the airflow path was caused by the overlapping of two land-touching droplets developing on each side of the channel. Flooding was found to be more susceptible to the airflow than the other test operating conditions.     

A short review of cathode poisoning and corrosion in solid oxide fuel cell

Solid oxide fuel cell (SOFC) has been recognized as a promising energy conversion device that is expected to play a critical role in solving the global energy and environmental challenges, however, the durability of SOFC under practical working conditions has limited its wide spread deployment and commercialization. Specifically, SOFC cathode often suffers from various contaminations such as Cr and Si arising from the interconnect and sealing materials, respectively, as well as humidity and CO₂ which are inherent in ambient air, resulting in serious issues in long-term performance degradation. In this review, the impacts of certain poisoning and corrosions on SOFC cathode are introduced, and the latest results of durability research on the corrosion resistant properties of cathode under CO₂, humidity, Cr and Si-containing conditions are reviewed. The poisoning and corrosion mechanism and durability of these aspects are systematically assessed and discussed.     

Solid oxide fuel cell: Materials for anode,cathode and electrolyte

Solid oxide fuel cell technology is the technology which can be driving force to change the course of action of the modern era due to its optimal power generation features with maximum electrical efficiency for automobiles and household devices. Fuel cells can be best described as electrochemical devices that make use of fuel oxidation to convert chemical energy into electrical energy and also lower the amount of oxidant simultaneously.A typical SOFC consists of a cathode, anode and an electrolyte constituting a single cell. These single cells are stacked together for a bigger assembly to produce higher degree of power. The solid electrolyte fills the gap between the cathode and anode transporting O²⁻ ions only. This leaves out electrons as transporting medium, which then pass through the cell via external circuit. Out of the two electrodes, oxidation of fuel takes place at the anode and reduction of oxygen takes place at the cathode. The SOFCs operate at higher temperatures of 600–1200°C producing heat as a byproduct of high quality, actively encouraging quick electrocatalysis utilizing non-precious metals and allowing internal restructuration. The SOFC can also work with high purity hydrogen for proton transport other than O²⁻ ion transport. There are many ceramic materials which have been engineered to act as efficient electrolyte materials. Yttria-stabilized zirconia (YSZ) is the most widely used material as solid electrolyte in SOFC.The present review presents a detailed overview of the SOFC related materials and devices and is an effort to present various reported works in a concise manner.     

Comparison of oxidation behaviours among three Fe–Cr based alloys for solid oxide fuel cell interconnect

Abstract

Two metallic alloys, containing comparable amounts of Cr, underwent oxidation in hot air simulating (the solid oxide fuel cell cathode atmosphere) for various periods. The results demonstrated that the oxidation kinetics of Crofer22 APU and equivalent ZMG232 followed the parabolic rate law and oxidation rates increased with temperature. Typical oxidation rates of Crofer22 APU and ZMG232 upon annealing treatment are approximately 0·21 orders of magnitude lower than that of ZMG232. An oxide scale electron probe microanalyser, a scanning electron microscope and X-ray diffractometer were adopted to verify the applicability of Fe–Cr based alloys in the solid oxide fuel cell interconnect component. Two alloys contain comparable amount of Cr, Mn and Fe, and their surface oxides as analysed are indicated to be Cr₂O₃ and (Mn,Fe)Cr₂O₄ spinel compound. In summary, Crofer22 APU had the best oxidation resistance of any of the alloys of interest.     

Factors of cathode current-collecting layer affecting cell performance inside solid oxide fuel cell stacks

Factors of cathode current-collecting layer (CCCL) affecting cell performance are studied by investigation of solid oxide fuel cell (SOFC) stacks with various (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) as CCCL in-suit. A larger real contact area between cathode and interconnect appears when the LSM is coated on cathode side as CCCL through characterization of a 2-cell stack. The result reveals that the real contact area depends on the surface roughness match (SRM) between CCCL and its neighboring components (active cathode and interconnect). A 6-cell stack using CCCLs with various levels of surface roughness is assembled and characterized further. The results show a higher electrical output performance of the stack repeating unit can be obtained when the surface roughness of the CCCL matches that of its neighboring components better, i.e. the surface roughness match (SRM) is the factor of cathode current collector affecting cell performance inside stack. Accordingly, the cell performance inside SOFC stack can be regulated by designing the SRM to its neighboring components.