Electroplated Ni–Fe2O3 composite coating for solid oxide fuel cell interconnect application

Ni–Fe₂O₃ composite coating was applied onto ferritic stainless steel using the cost-effective method of electroplating for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. By comparison, the coated and bare steels were evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the coated steel was close to that of the bare steel after initially rapid mass gain. The mass gain of the coated steel was higher than that of the bare steel owing to the formation of double-layer oxide structure with an outer layer of (Ni,Fe)₃O₄/NiO atop an inner layer of Cr₂O₃. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr₂O₃ scale thermally grown on the bare steel.     

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.     

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.     

CuFe2O4 protective and electrically conductive coating thermally converted from sputtered CuFe alloy layer on SUS 430 stainless steel interconnect

An inexpensive CuFe alloy layer with an atomic ratio (1:2) of Cu to Fe is coated on SUS 430 stainless steels via magnetron sputtering for solid oxide fuel cells interconnect application. The coated steels are thermally exposed to air at 800 °C for 15 weeks. The CuFe alloy layer is converted to CuFe₂O₄ spinel layer atop Cr₂O₃ layer developed from steel substrate. The outer layer of CuFe₂O₄ spinel not only retards Cr outward migration and reduces oxidation rate but also significantly lowers area specific resistance of the surface scale which is predicted for solid oxide fuel cells lifetime by a parabolic law. The sputtered CuFe alloy layer demonstrates a promising prospect for the application of steel interconnects coatings.     

NiO/NiFe2O4 dual-layer coating on pre-oxidized SUS 430 steel interconnect

A Ni/NiFe₂ dual-layer coating is deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, followed by thermal exposure in air at 800 °C for 1680 h. The thermally grown oxide scales exhibit tri-layer structure with inner Cr₂O₃ layer, middle NiO layer and outer NiFe₂O₄ spinel layer. The oxide coating converted from Ni/NiFe₂ coating not only inhibit the growth of Cr₂O₃ and the outward diffusion of Cr species but also improve the electrical performance of the surface scale. In addition, pre-oxidation treatment for the steel before Ni/NiFe₂ coating deposition prevents the interdiffusion between steel substrate and coating in the oxidation process.     

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.     

Interfacial failure via encapsulation of external particulates in an outward-growing thermal oxide

A Cr₂O₃-forming Ni-base superalloy and this alloy coated with a Pt-modified aluminide coating were exposed to SiO₂ powder and cyclically oxidized at 950 °C. The uncoated alloy showed a considerable amount of spallation and buckling whereas the Pt-NiAl coated alloy remained protective throughout hundred 1 h-cycles. The interfacial failure is mainly ascribed to the increased thermal strain by the encapsulation of external SiO₂ particulates in an outward-growing Cr₂O₃ layer. However, the particles were not embedded in the thermally grown oxide of the Pt-NiAl coated alloy due to the slow inward-growing characteristics of Al₂O₃ scales. The buckling of the Cr₂O₃ scale with embedded SiO₂ was analyzed with (1) a classical buckling criterion using the instantaneous coefficients of thermal expansion of the constituents, and (2) finite element analyses (FEA) to estimate the local interfacial shear stresses. It turns out that the thermal strain with embedded SiO₂ is larger than the experimentally determined critical thermal strain (?_b) explaining the buckling of the oxide scale observed in the experiment. The FEA results demonstrate that local shear stresses at the metal/oxide interface are significantly amplified near the SiO₂ particles showing that the buckling of oxide can be readily initiated especially in the vicinity of the embedded particles.     

Electrodeposited Ni/CeO2 mulriple coating on SUS 430 steel interconnect

Ni/CeO₂ mulriple coating has been fabricated on SUS 430 steel via electrodepositing approach. 100-h initial and 3-week long-term thermal exposing to air at 800 °C has enunciated that the oxide scale grown on the Ni/CeO₂ coated steel contains an external oxide layer of NiFe₂O₄ spinel, a middle oxide layer of NiO and an internal oxide layer of Cr₂O₃. Simultaneously, dispersive CeO₂ particles embed in the oxide scale. Compared to the Ni coated steel on which the same tri-layer oxide structure without discrete CeO₂ particles grows in the same exposing environment, growth rate of the internal Cr₂O₃ layer on the Ni/CeO₂ coated steel has been profoundly suppressed, which subsequently lowers the oxide scale area specific resistance (ASR). Enhancement of the oxidation resistance and reduction of the oxide scale ASR are attributed to the presence of CeO₂.     

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.     

CuFe2O4/CuO coating for solid oxide fuel cell steel interconnects

CuFe_0.8 (Fe:Cu = 0.8:1, atomic ratio) alloy layer is fabricated on both bare and pre-oxidized SUS 430 steels by direct current magnetron sputtering, followed by exposing at 800 °C in air to obtain a protective coating for solid oxide fuel cell (SOFC) steel interconnects. The CuFe_0.8 alloy layer is thermally converted to CuFe₂O₄/CuO coating, which effectively suppresses the out-migration of Cr. Pre-oxidation treatment not only initially accelerates the formation of CuFe₂O₄/CuO coating but also further inhibits the Cr and Fe outward diffusion. Suppressing outward diffusion of Cr could improve electrical property of oxide scale and decrease the risk of cathode Cr-poisoning. Blocking out-diffusion of Fe is beneficial to stabilize the CuO layer. After 2520 h oxidation, the scale ASR at 800 °C is 66.9 mΩ cm² for coated bare steel, 43.4 mΩ cm² for the coated pre-oxidized steel.     

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.     

Effect of reactive elements on oxidation behaviour of Fe–22Cr–0.5Mn ferritic stainless steel for a solid oxide fuel cell interconnect

Ferritic stainless steel has become a promising material for metallic interconnects for solid oxide fuel cells (SOFCs) operating in an intermediate temperature range (650–800 °C). Ferritic stainless steels containing reactive elements (REs) such as Crofer22APU and ZMG232 have been developed for SOFC interconnects. Nevertheless, the effectiveness of REs on the growth kinetics of the chromia-rich scale that forms on the ferritic stainless steels is not yet well understood. The current study focuses on the investigation of the effect of REs such as Y, Ce and La on the oxidation behaviour and scale properties of Fe–22Cr–0.5Mn stainless steel. The results show that Y is the most effective reactive element for reducing the scale growth kinetics and area-specific resistance of the chromia scale which forms on this stainless steel. The growth kinetics of the chromia-rich scale can be effectively reduced by the dominant segregation of Y at the interface between the oxide scale and alloy substrate, and by the formation of a thin SiO₂ and MnO layer underneath the Cr₂O₃-rich oxide.     

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.     

Effect of Ti addition on the electric and ionic property of the oxide scale formed on the ferritic stainless steel for SOFC interconnect

Ferritic stainless steels with Ti addition are considered as promising candidates for SOFC interconnect application. In this study, the effect of Ti addition on the electrical conductivity and Cr evaporation resistance was discussed in terms of microstructure and ionic property of the oxide scale by using TEM analysis and asymmetry polarization method. Ti addition induced the generation of ionic defects in the oxide layer and modified the growth kinetics of Cr₂O₃ and MnCr₂O₄, but in different manner depending on Ti amount. Ti content in a range of 0.05–0.07 wt% was effective for reducing the oxidation rate and electrical resistance. Addition of 1 wt% Ti promoted fast Cr₂O₃ growth due to the excess ionic defect in Cr₂O₃ matrix. However, the formation of the outermost MnCr₂O₄ layer was accelerated by Ti segregation near the scale/alloy interface and it reduced Cr evaporation effectively. Co-addition of a small amount of Ti and La enhanced Ti segregation without generation of excess ionic defect and improved both the electric conductivity and Cr evaporation resistance.     

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.     

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.     

Performance evaluation of several commercial alloys in a reducing environment

Several commercial alloys including Ebrite, Crofer 22 APU, Haynes 230 and Haynes 242, which are candidates for intermediate-temperature solid oxide fuel cell (SOFC) interconnect materials, were isothermally and cyclically oxidized at 900 °C in the reducing atmosphere of Ar + 5 vol.% H₂ + 3 vol.% H₂O corresponding to the SOFC anode environment. Results indicate that these alloys exhibited good scale spallation resistance with the Ni-base alloys possessing better oxidation resistance over the Fe-base alloys. Both Mn–Cr spinel and Cr₂O₃ were formed in the oxide scales of these alloys. For Crofer 22 APU and Haynes 242, a continuous protective MnO and Mn–Cr spinel layer formed outside on the inner layer of Cr₂O₃. The increase in scale ASR after longer-term thermal exposure in the reducing environment was relatively slower for the Ni-base alloys than for the Fe-base alloys.     

Investigation of iron–chromium–niobium–titanium ferritic stainless steel for solid oxide fuel cell interconnect applications

As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, both bare AISI441 and AISI441 coated with (Mn,Co)₃O₄ protection layers were studied in terms of its metallurgical characteristics, oxidation behavior, and electrical performance. The addition of minor alloying elements, in particular Nb, led to formation of Laves phases both inside grains and along grain boundaries. In particular, the Laves phase which precipitated out along grain boundaries during exposure at intermediate SOFC operating temperatures was found to be rich in both Nb and Si. The capture of Si in the Laves phase minimized the Si activity in the alloy matrix and prevented formation of an insulating silica layer at the scale/metal interface, resulting in a reduction in area-specific electrical resistance (ASR). However, the relatively high oxidation rate of the steel, which leads to increasing ASR over time, and the need to prevent volatilization of chromium from the steel necessitates the application of a conductive protection layer on the steel. In particular, the application of a Mn_1.5Co_1.5O₄ spinel protection layer substantially improved the electrical performance of the 441 by reducing the oxidation rate.     

Ruddlesden–Popper nickelate as coating for chromia-forming stainless steel

A Ruddlesden–Popper nickelate, La₂NiO_4+δ (LN), is examined as a coating material for metallic interconnects of a solid oxide fuel cell (SOFC) operating at intermediate temperatures. Isothermal oxidation of the coated and uncoated SS430 alloy with and without pre-annealing at 850 °C in air suggests that the coating reduces significantly the oxidation rate and impact of thermal cycle on the integrity of the scale. After an annealing in air at 850 °C for 50 h, the LN coating has turned into to La(Cr,Ni)O₃ (LCN) perovskite, but no spalling is observed because of the matching thermal expansion of LN and LCN to SS430. A low area specific resistance (ASR) of 2.5 × 10⁻³ Ω cm² is observed for the sample with the resultant LCN coating, indicating LN can be a good candidate as a coating for interconnects of SOFC.     

Investigation of oxidation and electrical behavior of AISI 430 steel coated with Mn–Co–CeO2 composite

Ferritic stainless steels, under the working conditions of solid oxide fuel cells, form a chromium oxide layer. This layer has a low electrical conductivity and consequently reduces the efficiency of these energy converters. An action to improve the properties of the connecting plates is to use a conductive and protective layer of coating. In this study, AISI 430 stainless steel was coated with Mn–Co–CeO₂ through electroplating technique. To evaluate the oxidation behavior, isothermal and cyclic oxidation tests were used at 800 °C. Area specific resistance (ASR) of uncoated and coated specimens was also compared as a function of time during oxidation at 800 °C. Coating microstructure and oxidized samples were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) device. In isothermal oxidation, uncoated samples had more weight gain than the Mn–Co–CeO₂ coated samples. The coating layer improved oxidation resistance by limiting the diffusion of chromium cation and oxygen anion. The cyclic oxidation results showed that the Mn–Co–CeO₂ coated samples had a very good resistance to cracking and spallation. Also, the results of ASR showed that formation of MnCo₂O₄ and MnFe₂O₄ spinels and also the presence of CeO₂ resulted in reduction of area specific resistance. ASR for samples coated with Mn–Co–CeO₂ and uncoated samples was 12.4 mΩ.cm² and 38.7 mΩ.cm², respectively after 200 h of oxidation at 800 °C.