High-temperature oxidation process analysis of MnCo2O4 coating on Fe-21Cr alloy

Even though the operation temperature of solid oxide fuel cells (SOFCs) stacks has been reduced (∼750 °C), stainless steel interconnect within the stacks still requires protection by high conductive coatings to delay the growth of oxide scales and reduce chromium evaporation. Manganese cobaltite spinel protective coating with a nominal composition of MnCo₂O₄ was produced on Fe-21Cr stainless steel. Electrical, microstructural and compositional analysis were performed to investigate the interfacial reaction of MnCo₂O₄ protective coating with the stainless steel substrate during 750 °C oxidation process. The spinel coating not only acts as a barrier to Cr outward transport, but also improves the electrical conductivity of the alloy interconnect during long-term oxidation. The coated alloy demonstrates good electrical conductivity with an area specific resistance (ASR) of about 5 mOhm cm² after oxidation for 1000 h at 750 °C, which is about 1/4 of the ASR of bare Fe-21Cr alloy. The reduction of ASR might be caused by the fact that Cr migrated from the steel substrate interact with MnCo₂O₄ coating and generated Mn-Co-Cr spinel phase, which has higher electrical conductivity than that of Cr₂O₃.     

Effects of cathode thickness and microstructural properties on the performance of protonic ceramic fuel cell (PCFC): A 3D modelling study

Protonic Ceramic Fuel Cells (PCFCs) are promising power sources operating at an intermediate temperature. Although plenty of experimental studies focusing on novel material development are available, the design optimization of PCFC through numerical modelling is limited. In this study, a 3D PCFC model focusing on the cathode thickness and microstructure design is developed due to the high overpotential loss of the cathode. Unlike the 1D/2D models, the rib-size effects on the PCFC performance are fully considered when optimizing the cathode structure. Different from 1D/2D models suggesting thin cathode thickness, this study finds that the optimal cathode thickness is about 120–200 μm. In a thin cathode, weak O₂ diffusion from the channel to the rib-covered cathode can lead to O₂ depletion under the rib and very low local cell performance. By adjusting the cathode porosity from 0.3 to 0.5, nearly 9% performance improvement and 22.5% improvement in gas distribution uniformity can be achieved. When the cathode particle size changes from 0.1 μm to 0.2 μm, the O₂ concentration under the rib increases nearly 50%. The optimal electronic phase volume fraction is suggested to be around 50–60% for achieving a balance between ohmic resistance and reaction sites. This model elucidates the relationship between cathode microstructure and PCFC performance comprehensively and can serve as a guiding tool for cell fabrication and future novel interconnect structure design.     

Controlling chromium vaporization from interconnects with nickel coatings in solid oxide devices

Vaporization of Cr-rich volatile species from interconnect materials is a major source of degradation that limits the lifetime of planar solid oxide devices (solid oxide fuel cells and solid oxide electrolysis cells) with metallic interconnects. Some metallic coatings (Ni, Co, and Cu) may significantly reduce the Cr release from interconnects and slow down the oxide scale growth on the steel substrate. To shed additional light upon the mechanisms of such protection and find a suitable coating material for ferritic stainless steel materials widely used for interconnects, we used a combination of first-principles calculations, thermodynamics, and diffusion modeling to investigate which factors determine the quality of the Ni metallic coatings. We found that Cr migration in Ni coatings is determined by a delicate combination of the nickel oxidation, Cr diffusion, and phase transformation processes. Although the formation of Cr₂O₃ is more exothermic than that of NiO, the kinetic rate of the chromia formation in the coating layer and its surface is significantly reduced by the low mobility of Cr in nickel oxide and in NiCr₂O₄ spinel. These results are in a good agreement with diffusion modeling for Cr diffusion through the Ni coating layer on the ferritic 441 steel substrate and available experimental data.     

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.     

Performance of stainless steel interconnects with (Mn,Co)3O4-Based coating for solid oxide electrolysis

Mixed transition-metal oxide coatings are commonly applied to stainless steel interconnects for solid oxide cell stacks. Such coatings reduce oxidation and Cr evaporation rates, leading to improved degradation rate and stack lifetime. Here, the ChromLok? MCO-based composition (Mn,Co)₃O₄ is applied to Crofer 22 APU stainless steel and evaluated specifically for application in solid oxide electrolyzer stacks operating around 800 °C and utilizing oxygen-ion-conducting solid oxide cells. The MCO coating is found to decrease the stainless steel oxidation rate by about one order of magnitude, and decrease the Cr evaporation rate by fourfold. The coating also dramatically lowers the rate of area-specific resistance increase for stainless steel coupons oxidized for 500 h with constant current applied, from 33 mΩ1cm² kh^?1 for an uncoated coupon to less than 4 mΩ1cm² kh^?1 for coated coupons. The coating is demonstrated on full-scale interconnects for single-cells, where the coating dramatically reduces degradation rate, and for a stack, which displays stable operation for 700 h.     

Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect

Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at operating temperatures 600–800 °C. Chromium (Cr)-forming stainless steel as an interconnect plate possesses a low oxidation resistance at high temperature and electrical conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective strategy is to use a surface coating to improve interconnect performance. This work is to form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic stainless steel. The area specific resistance of samples was measured as a function of time. The area specific resistance of coated sample with 2% of activator content and holding time of 2 h is 90.21 and 108.32 mΩ cm² after 450 h of oxidation in air, respectively. Additionally, the coated sample with 2% of activator content and holding time of 2 h has a weight change of merely 0.299 and 0.231 mg/cm² after 650 h of isothermal oxidation at 800 °C, separately. The results displayed that the formation of CoFe₂O₄ spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions.     

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 Mn_1.35Co_1.35Cu_0.2Y_0.1O₄ (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 Mn_1.5Co_1.5O₄ (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.     

Corrosion behavior of TiN-coated stainless steel as bipolar plate for proton exchange membrane fuel cell

Stainless steel is a potential material to be used as the bipolar plate for proton exchange membrane fuel cell (PEFC) because of its suitable physical and mechanical properties. Several coating techniques have been applied to improve its corrosion resistance. But seldom study is focused on the microstructure evolution with corrosion. In the present study, the use of TiN-coated stainless steel as the bipolar plate is evaluated. Two surface coating techniques, pulsed bias arc ion plating (PBAIP) and magnetron sputtering (MS), are adoped to prepare the TiN-coated stainless steel. Their corrosion resistances and electrical conductivities of the coated substrates are evaluated. The performance shows strong dependance on microstructural characteristics. The corrosion of SS304/Ti₂N/TiN prepared by MS mainly occurs on the grain boundary. The corrosion of SS304/TiN prepared by PBAIP mainly takes place from the large particles on the coating. The Ti₂N/TiN multilayer coating provides superb corrosion protective layer for stainless steel. Both the TiN and Ti₂N/TiN coatings provide low contact resistance.     

Improved oxidation resistance of a nanocrystalline chromite-coated ferritic stainless steel

An economical dip coating process was developed to synthesize uniform, crack-free, and adherent thin nanocrystalline LaCrO₃ films on a ferritic stainless steel substrate for the solid oxide fuel cell interconnect applications. LaCrO₃ perovskite phase was formed after annealing in air at 800 °C for 1 h for both the LaCrO₃ and La₂O₃ precursors. The effectiveness of the coating in improving the oxidation resistance of the alloy was demonstrated by both isothermal and cyclic oxidation tests. The LaCrO₃ coatings were found to cause a pronounced reduction in oxidation rate of the alloy, especially with low La-content precursors. The area-specific resistance of the oxide scales formed on the bare and coated alloy substrates was also evaluated and discussed.     

(Mn,Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications

(Mn,Co)₃O₄ spinel with a nominal composition of Mn_1.5Co_1.5O₄ demonstrates excellent electrical conductivity, satisfactory thermal and structural stability, as well as good thermal expansion match to ferritic stainless steel interconnects. A slurry-coating technique was developed for fabricating the spinel coatings onto the steel interconnects. Thermally grown layers of Mn_1.5Co_1.5O₄ not only significantly decreased the contact resistance between a LSF cathode and stainless steel interconnect, but also acted as a mass barrier to inhibit scale growth on the stainless steel and to prevent Cr outward migration through the coating. The level of improvement in electrical performance and oxidation resistance (i.e. the scale growth rate) was dependent on the ferritic substrate composition. For E-brite and Crofer22 APU, with a relatively high Cr concentration (27 wt% and 23%, respectively) and negligible Si, the reduction of contact ASR and scale growth on the ferritic substrates was significant. In comparison, limited improvement was achieved by application of the Mn_1.5Co_1.5O₄ spinel coating on AISI430, which contains only 17%Cr and a higher amount of residual Si.     

Investigation of MnCu0.5Co1.5O4 spinel coated SUS430 interconnect alloy for preventing chromium vaporization in intermediate temperature solid oxide fuel cell

The MnCu_0.5Co_1.5O₄ spinel coating is proposed as a protective coating for SUS430 alloy to improve its oxidation resistance and prevent chromium vaporization. The coated alloy is exposed to dual atmosphere (Air/H₂–3%H₂O) at 750 °C for 200 h, exhibiting a stable spinel structure on the air side, but reduced to MnO, Cu and Co on the fuel side. The coating layer could maintain integrated and dense with a thickness of 13–14 μm. The experiment results shown that the MnCu_0.5Co_1.5O₄ coating is an effective diffusion barrier that can inhibit oxidation and chromium vaporization of metallic interconnect. The relatively low amount of Cr deposition on LSM cathode on coated condition is considered associating with the stable electrochemical performance under current density of 400 mA cm^?2. The above results indicate that MnCu_0.5Co_1.5O₄ spinel is a promising coating for interconnect alloy of solid oxide fuel cell.     

Novel coatings for protecting solid oxide fuel cell interconnects against the dual-atmosphere effect

A key component of a Solid Oxide Fuel Cell (SOFC) is the interconnect, which connects individual fuel cells in series to form a fuel cell stack to reach a desired electrical potential. The interconnect is exposed to air and fuel in parallel, these so-called dual-atmosphere conditions give rise to especially severe corrosion on the air-side. This work investigates coatings to mitigate this effect. Physical Vapour Deposition (PVD) CeCo-coated AISI 441 samples on the air-side and PVD metallic Al- and Al₂O₃-coated AISI 441 samples on the fuel-side were exposed under dual-atmosphere conditions for up to 7000 h. The evolution of the corrosion products was followed every 1000 h with an optical microscope. Scanning electron microscopy and energy-dispersive x-ray spectroscopy were performed on cross-sections of the samples after 3000 h of exposure. The SEM analysis showed that coating on the air-side improved the sample's life-time by reducing the level of Cr evaporation even in a dual-atmosphere. The use of fuel-side coatings suppressed the dual-atmosphere effect since the coatings formed a barrier to hydrogen permeation. The best results were observed with metallic Al and Al₂O₃ coating on the fuel-side, which drastically reduced the dual-atmosphere effect. However, the poor conductivity of Al₂O₃ makes its use as a coating challenging.     

LaCrO3-based coatings deposited by high-energy micro-arc alloying process on a ferritic stainless steel interconnect material

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO₃-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO₃-based coating using an electrode rod of LaCrO₃-20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO₃-based coatings showed a three-layered microstructure with a NiFe₂O₄ outer layer, a thick LaCrO₃ sub-layer and a thin Cr₂O₃-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.     

Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells

Cr₂O₃ evaporation from Cr₂O₃-forming metallic interconnects during operation of the solid oxide fuel cells (SOFC) can poison other cell components and cause degradation. Protective NiFe₂O₄ spinel coatings on interconnect alloys were developed by electroplating and screen printing, respectively. Results indicate that NiFe₂O₄ coatings can significantly improve the oxidation resistance of the alloy while providing effective conducting path with inherent low resistance, and also are expected to serve as a diffusion barrier to effectively reduce the Cr₂O₃ evaporation. Two coating techniques were evaluated in terms of the performances of the coatings. A very interesting and smart coating structure was reported.     

MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the interconnection plates can be made from stainless steel. This is a big economic advantage, but energy losses can be caused by undesirable reactions between the alloys and other SOFC components. The use of coatings on interconnect stainless steels can reduce this degradation. A MnCo_1.9Fe_0.1O₄ (MCF) spinel not only significantly decreases the contact resistance between a La_0.8Sr_0.2FeO₃ cathode and a stainless steel interconnect, but also acts as a diffusion barrier to prevent Cr outward migration through the coating. The level of improvement in electrical performance depends on the ferritic substrate composition. For Crofer22APU and F18TNb, with a Mn concentration of 0.4 and 0.12 wt%, respectively, the reduction in contact resistance is significant. In comparison, limited improvement is achieved by application of MCF on IT-11 and E-Brite containing no Mn. No influence of the minor additions of Si or Al is observed on contact resistance. The MCF protection layer bonds well to the stainless steel substrates under thermal cycling, but the thermal expansion difference is too large between the La_0.8Sr_0.2Co_0.75Fe_0.25O₃ contact layer used and Crofer22APU and IT-11.     

Cu- and Ni-doped Mn1.5Co1.5O4 spinel coatings on metallic interconnects for solid oxide fuel cells

An interconnect in solid oxide fuel cells electrically connects unit cells and separates fuel from oxidant in the adjoining cells. Metallic interconnects are usually coated with conductive oxides to improve their surface stability and to mitigate chromium poisoning of a cathode. In this study, Mn_1.5Co_1.5O₄ (MCO) spinel oxides doped with Cu and Ni are synthesized and applied as protective coatings on a metallic interconnect (Crofer 22 APU). Doping of Cu and Ni into MCO improves sintering characteristics as well as electrical conductivity and thermal expansion match with the Crofer interconnect. The dense layers of Cu- and Ni-doped MCOs are fabricated on the interconnects by a slurry coating process and subsequent heat-treatment. The coated interconnects exhibit area-specific resistances as low as 13.9–17.6 mΩ cm² at 800 °C. The Cu-doped MCO coating acts as an effective barrier to evaporation and migration of Cr-containing species from the interconnect, thereby reducing Cr poisoning of a cathode.     

Development of novel metal-supported proton ceramic electrolyser cell with thin film BZY15–Ni electrode and BZY15 electrolyte

Metal supports for planar MS-PCEC were manufactured using tape-casting of low-cost ferritic stainless steel. A coating protecting the metal support against oxidation was applied by vacuum infiltration and a buffer layer of La_0.5Sr_0.5Ti_0.75Ni_0.25O_3–δ (LSTN) was further deposited to smoothen the surface. The BaZr_0.85Y_0.15O_3–δ–NiO (BZY15–NiO) cathode and the BaZr_0.85Y_0.15O_3–δ (BZY15) electrolyte were applied by pulsed laser deposition (PLD) at elevated substrate temperatures (at 700 °C and 600 °C, respectively). The main challenges are related to the restrictions in sintering temperature and atmosphere induced by the metal support, as well as strict demands on the roughness of substrates used for PLD. Reduction treatment of the half cells confirmed that NiO in the BZY15–NiO layer was reduced to Ni, resulting in increased porosity of the BZY15–Ni cathode, while keeping the columnar and dense microstructure of the BZY15 electrolyte. Initial electrochemical testing with a Pt anode showed a total resistance of 40 Ω·cm² at 600 °C. Through this work important advances in using metal supports and thin films in planar PCEC assemblies have been made.     

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a CrC alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of Cr_xC_y@Cr₂O₃/Al₂O₃ was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.     

Thin chromium nitride PVD coatings on stainless steel for conductive component as bipolar plates of PEM fuel cells: Ex-situ and in-situ performances evaluation

In this paper, two types of chromium PVD coatings (100 nm) have been elaborated on 316L stainless steel (SS) by adjusting the nitrogen flow rate. The first coating is a mixture of Cr₂N and Cr, the second one is a single phase CrN. It is shown that the performances of the material are strongly dependant of the nature of the passive film formed on the chromium nitride layers due to the galvanic coupling between the coating and the substrate. The CrN coated SS shows very good corrosion resistance in simulated PEMFC media. The surface conductivity of the SS is also greatly improved and the CrN coated SS shows an interfacial contact resistance of 10 mΩ cm² at 140 N cm⁻². Five single cells of stainless steel bipolar plates coated with the CrN film were assembled for performance test. This 5 cell stack does not show any mean voltage degradation over 200 h dynamic cycling. Moreover, the performances of the CrN coated SS bipolar plates are very close to the Au-coated SS bipolar plates.     

Lanthanum oxide-coated stainless steel for bipolar plates in solid oxide fuel cells (SOFCs)

Solid oxide fuel cells typically operate at temperatures of about 1000 °C. At these temperatures only ceramic interconnects such as LaCrO₃ can be employed. The development of intermediate-temperature solid oxide fuel cells (IT-SOFCs) can potentially bring about reduced manufacturing costs as it makes possible the use of an inexpensive ferritic stainless steel (STS) interconnector. However, the STS suffers from Cr₂O₃ scale formation and a peeling-off phenomenon at the IT-SOFC operating temperature in an oxidizing atmosphere. Application of an oxidation protective coating is an effective means of providing oxidation resistance. In this study, we coated an oxidation protective layer on ferritic stainless steel using a precursor solution prepared from lanthanum nitrate, ethylene glycol, and nitric acid. Heating the precursor solution at 80 °C yielded a spinable solution for coating. A gel film was coated on a STS substrate by a dip coating technique. At the early stage of the heat-treatment, lanthanum-containing oxides such as La₂O₃ and La₂CrO₆ formed, and as the heat-treatment temperature was increased, an oxidation protective perovskite-type LaCrO₃ layer was produced by the reaction between the lanthanum-containing oxide and the Cr₂O₃ scale on the SUS substrate. As the concentration of La-containing precursor solution was increased, the amount of La₂O₃ and La₂CrO₆ phases was gradually increased. The coating layer, which was prepared from a precursor solution of 0.8 M, was composed of LaCrO₃ and small amounts of (Mn,Cr)O₄ spinel. A relatively dense coating layer without pin-holes was obtained by heating the gel coating layer at 1073 K for 2 h. Microstructures and oxidation behavior of the La₂O₃-coated STS444 were investigated.