Fabrication of a double-layered Co-Mn-O spinel coating on stainless steel via the double glow plasma alloying process and preoxidation treatment as SOFC interconnect

To improve oxidation resistance, prevent Cr evaporation and maintain appropriate electrical conductivity of AISI 430 stainless steel (430 SS) as the solid oxide fuel cells' (SOFCs) interconnect, a double-layered Co-Mn-O spinel coating is fabricated successfully on 430 SS via a simple double glow plasma alloying process (DGPA) followed by heating in the air (preoxidation treatment). The double-layered Co-Mn-O spinel coating is composed of a thick MnCo₂O₄ spinel outlayer and a thin mutual-diffused (MnCoFe)₃O₄ oxide innerlayer. The isothermal and cyclic oxidation measurements are used to investigate the oxidation resistance, and the ASR test is performed to evaluate the conductivity for the coated and uncoated specimens. The coated specimen has a lower oxidation kinetics rate constant (9.0929 × 10⁻⁴ mg² cm⁻⁴ h⁻¹) than the uncoated one (1.900 × 10⁻³ mg² cm⁻⁴ h⁻¹) and the weight gain of the coated specimen (0.84 mg cm⁻²) is less than that of bare steel (1.29 mg cm⁻²) after 750 h oxidation. Meanwhile, the coated specimen holds a lower area specific resistance (0.029 Ω cm²) compared to the uncoated one (2.28 Ω cm²) after 408 h oxidation. Furthermore, the compact Co-Mn-O spinel coating can effectively impede Cr-volatilization. Additionally, the probable mechanism of the Co-Mn alloy conversion into spinel and the electronic conduction behavior in the spinel are discussed. The effects of mutual-diffused oxide innerlayer on oxidation behavior and conductivity are investigated.     

Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co_1.5Mn_1.5O₄ spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented 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.     

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.     

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.     

CoFe2O4 spinel protection coating thermally converted from the electroplated Co-Fe alloy for solid oxide fuel cell interconnect application

CoFe₂O₄ has been demonstrated as a potential spinel coating for protecting the Cr-containing ferritic interconnects. This spinel had an electrical conductivity of 0.85 S cm⁻¹ at 800 °C in air and an average coefficient of thermal expansion (CTE) of 11.80 × 10⁻⁶ K⁻¹ from room temperature to 800 °C. A series of Co-Fe alloys were co-deposited onto the Crofer 22 APU ferritic steel via electroplating with an acidic chloride solution. After thermal oxidation in air at 800 °C, a CoFe₂O₄ spinel layer was attained from the plated Co_0.40Fe_0.60 film. Furthermore, a channeled Crofer 22 APU interconnect electrodeposited with a 40-μm Co_0.40Fe_0.60 alloy film as a protective coating was evaluated in a single-cell configuration. The presence of the dense, Cr-free CoFe₂O₄ spinel layer was effective in blocking the Cr migration/transport and thus contributed to the improvement in cell performance stability.     

The preparation and properties of Mn–Co–O spinel coating for SOFC metallic interconnect

To meet the performance requirements of solid oxide fuel cell (SOFC) metallic interconnect, the Mn–Co–O spinel coating is prepared on the surface of AISI430 by pack cementation method to reduce the growth kinetics of oxides and inhibit the outward diffusion of Cr. The microstructural characterization shows that a dense, uniform, defect-free spinel coating is successfully fabricated on the surface of AISI430. Under the simulated SOFC cathode environment, the weight gain of coated steel (0.608 mg cm⁻²) after oxidation at 800 °C for 800 h is significantly lower than that of uncoated (1.586 mg cm⁻²). In addition, the area specific resistance (ASR) of the coated steel oxidized for 500 h is 17.69 mΩ cm², much smaller than that of the bare steel, indicating that the oxidation resistance and electrical conductivity of AISI430 are significantly improved by Mn–Co–O spinel coating. Cross-sectional observations of the Mn–Co–O spinel coating are conducted to assess the compatibility of substrate with the adjacent coating and its effectiveness in reducing the growth of the Cr₂O₃ layer.     

The effect of Mn on the oxidation behavior and electrical conductivity of Fe-17Cr alloys in solid oxide fuel cell cathode atmosphere

Four Fe-17Cr alloys with various Mn contents between 0.0 and 3.0 wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe-Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1 min) at 750 °C in air, Cr is preferentially oxidized to form a layer of Cr₂O₃ type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr₂O₃ type oxide layer to form Mn-rich (Mn, Cr)₃O₄ and Mn₂O₃ oxides on the top. After 700 h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0 wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0 wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe-17Cr alloy with conducting Cr free spinel coatings is preferred.     

Co/LaCrO3 composite coatings for AISI 430 stainless steel solid oxide fuel cell interconnects

Rapidly decreasing electronic conductivity, chromium volatility and poisoning of the cathode material are the major problems associated with inevitable growth of chromia on ferritic stainless steel interconnects of solid oxide fuel cells (SOFC). This work evaluates the performance of a novel, electrodeposited composite Co/LaCrO₃ coating for AISI 430 stainless steel. The oxidation behaviour of the Co/LaCrO₃-coated AISI 430 substrates is studied in terms of scale microstructure and growth kinetics. Area-specific resistance (ASR) of the coated substrates has also been tested. The results showed that the Co/LaCrO₃ coating forms a triple-layer scale consisting of a chromia-rich subscale, a Co–Fe spinel mid-layer and a Co₃O₄ spinel top layer at 800 °C in air. This scale is protective, acts as an effective barrier against chromium migration into the outer oxide layer and exhibits a low, stable ASR of ∼0.02 Ω cm² after 900 h at 800 °C in air.     

A promising NiCo2O4 protective coating for metallic interconnects of solid oxide fuel cells

The NiCo₂O₄ spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol-gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800 °C for 200 h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr₂O₃ and the formation of conductive spinel phases in the oxide scale.     

Electrophoretic deposition of (Mn,Co)3O4 spinel coating for solid oxide fuel cell interconnects

We discuss here our attempt to develop (Mn,Co)₃O₄ spinel coatings on the surface of Cr-containing steel through electrophoretic deposition (EPD) followed by reduced-atmosphere sintering for solid oxide fuel cell (SOFC) interconnect application. The effects of EPD voltages and sintering atmospheres on the microstructure, electrical conductivity and long-term stability of the coated interconnects are examined by means of scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and four-probe resistance techniques. For the spinel coatings generated using smaller voltage than 400 V, the interconnect surfaces exhibit good packing behavior and high conductivity. The reduced atmosphere during sintering has a beneficial impact on the minimizing chromia subscale formation and thus reducing the area specific resistance (ASR) of the coated interconnects. Moreover, it is interesting to note that a more stable long-term performance is achieved for the spinel coating sintered in H₂/H₂O atmosphere with thin chromia sub-scale and no Cr penetration. Based on the current results, EPD followed by reduced-atmosphere sintering is a fast and economic way to deposit (Mn,Co)₃O₄ coating for SOFC interconnect applications.     

Yttrium,cobalt and yttrium/cobalt oxide coatings on ferritic stainless steels for SOFC interconnects

Ferritic stainless steels are being considered as potential interconnect materials for SOFCs, in part because of their low cost relative to alternatives. These materials are, however, susceptible to degradation over time. A primary source of degradation is an increase in the area specific resistance (ASR), which is due to the formation of poorly conducting oxides (Mn–Cr spinel and Cr₂O₃) on the surface. In this work, the influence of Y, Co and Y/Co oxide coatings on the oxidation behaviour of a ferritic stainless steel (16–18 wt% Cr) has been investigated. Samples were oxidized in air for up to 500 h at temperatures ranging from 700 to 800 °C. Coated and uncoated samples were characterized, before and after heat treatment, using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric (TG) analysis and four-point probe resistance measurements.Surface morphology investigations of coated and uncoated stainless steels showed differences for Y and Co in terms of oxide formation. In all cases, Cr–Mn spinel and Cr₂O₃ were the two main surface oxides; however, the morphology of the spinel phase was dependent on the type of coating. The lowest resistances were obtained for the Y/Co-coated samples, which had ASR values up to seven times lower than corresponding uncoated ferritic stainless steels.     

Development of the spinel powder reduction technique for solid oxide fuel cell interconnect coating

Mn_0.9Y_0.1Co₂O₄ spinel coatings are developed for solid oxide fuel cell (SOFC) alloy interconnects by a novel powder reduction technique. Material properties, electrical performance and long-term stability of the coatings are explored. The coating is about 9 μm in thickness and adheres well to the alloy substrate without any cracking or delamination. The area specific resistance (ASR) remains almost unchanged and is less than 3 mΩ cm² even though the coated alloy undergoes oxidation at 800 °C for 1017 h and ten thermal cycles from 800 °C to room temperature. The coated alloy presents excellent electrical performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.     

Influence of preoxidation on high temperature behavior of NiFe2 coated SOFC interconnect steel

NiFe₂O₄ spinel coating is promising for solid oxide fuel cell (SOFC) steel interconnects application. In this work, NiFe₂ alloy coating was sputtered on bare steel and preoxidized steel (100 h in air at 800 °C), respectively, followed by exposing in air at 800 °C for up to 15 weeks in order to investigate the influence of steel preoxidation on high temperature behaviors of the coated steels. The results indicated that an outer NiFe₂O₄ spinel layer atop an inner Cr₂O₃ layer formed on the coated samples after oxidation. The preoxidation enhanced the oxidation resistance of the coated sample and reduced Cr out-migration to NiFe₂O₄ spinel layer. After 15 weeks, the area specific resistance (ASR) of surface scale on the coated preoxidized steel was much lower than that on the coated bare steel. The mechanisms of the preoxidation influence on oxidation behavior and surface scale electrical property of the coated steels were discussed.     

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.     

Evaluation of the reactive-sintered (Mn,Co)3O4 spinel layer for SOFC cathode-side contact application

Simulated interconnect/contact/cathode cells are used to investigate the performance of different contact precursors that are reactively sintered into spinel contact layers with nominal compositions of Mn_1.5Co_1.5O₄ and MnCo₂O₄, respectively. Both bare and Mn_1.5Co_1.5O₄-coated interconnects are employed in this study, where the Mn_1.5Co_1.5O₄ interconnect coating is also prepared via reactive sintering. The isothermal area specific resistance (ASR) of the test cells is evaluated at 800 °C in air for 2000 h. All the test cells exhibited excellent electrical behavior with ASR values of less than 10 mΩ·cm² during the measurement. The presence of the dense Mn_1.5Co_1.5O₄ coating leads to a better ASR performance. Cross-sectional observation of the test cells is conducted to assess the compatibility of the contact layer with adjacent components as well as its effectiveness in suppressing chromia scale growth and blocking Cr migration from the interconnect to the cathode.     

Coated interconnects development for high temperature water vapour electrolysis: Study in anode atmosphere

High temperature water vapour electrolysis (HTE) is an efficient technology for hydrogen production. In this context, a commercial stainless steel, K41X (AISI 441), was chosen as interconnect. In a previous paper, the high temperature corrosion and the electrical conductivity were evaluated in both anode (O₂–H₂O) and cathode (H₂–H₂O) atmosphere at 800 °C. In O₂–H₂O atmosphere, the formation of a thin chromia protective layer was observed. Nevertheless, the ASR parameter measured was higher than the maximum accepted value. These results, in addition with chromium evaporation measurements, proved that the K41X alloy is not suitable for HTE interconnect application. In this study, two perovskite-type oxides La_0.8Sr_0.2MnO_3−δ and LaNi_0.6Fe_0.4O_3−δ were tested as coatings in O₂–H₂O atmosphere at 800 °C. Screen-printing and physical vapour deposition were used as coating processes. The high temperature corrosion resistance and the electrical conductivity were improved, especially with the LaNi_0.6Fe_0.4O_3−δ coating. Cr specie volatility was also reduced.     

“Fe doped Ni–Co spinel protective coating on ferritic stainless steel for SOFC interconnect application”

In an attempt to reduce the oxidation and Cr evaporation rates, various protective coating layers with a nominal composition of NiCo_2−xFe_xO₄ (x = 0, 0.5, 1) were deposited on the SUS 430 ferritic stainless steel substrate, as interconnect for solid oxide fuel cell application, by sol-gel dip coating method. Then, the coated samples were soaked at 750 °C for 2.5 h in N₂ and subsequently for 2.5 h in air. Phase composition and microstructure of the coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moreover, long-term isothermal oxidation experiment and area specific resistance (ASR) measurement were also carried out on the coated samples. Results showed that the addition of 1 mol of Fe into Ni–Co spinel (x = 1), led to a parabolic rate constant of 5.16 × 10⁻¹⁵(g² cm⁻⁴ s⁻¹) and an ASR of 17 mΩ cm².     

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.     

NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells

In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn₂O₄ is prepared on SUS 430 alloy substrate by the sol-gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3-4 μm thick is formed during cyclic oxidation at 750 °C in air for 1000 h, consisting of an inner layer of doped Cr₂O₃ and an outer layer of doped NiMn₂O₄ and Mn₂O₃; and the growth of Cr₂O₃ and formation of MnCr₂O₄ are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹. The area specific resistance at temperatures between 600 and 800 °C is in the range of 6 and 17 mΩ cm². The above results indicate that NiMn₂O₄ is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells.