Alumina-forming austenitic stainless steel for high durability and chromium-evaporation minimized balance of plant components in solid oxide fuel cells

The chromium evaporation and oxidation behaviors of alumina-forming austenitic stainless steels are systematically investigated at 800 °C in air +10% H₂O relative to 310S for 5000 h. Cr evaporation rates of 310S are about 35 times higher than AFA alloys after 5000 h. Relatively rapid oxidation is observed on 310S after only one 500 h cycle, followed by a modest degree of mass loss and spallation, while the AFA alloys show high oxidation resistance throughout the entire test. Continuous inner alumina layer formed on AFA alloys stays compact and stable after 5000 h which greatly reduces the Cr evaporation.     

Investigation of coated FeCr steels for application as solid oxide fuel cell interconnects under dual-atmosphere conditions

Dual-atmosphere conditions are detrimental for the ferritic stainless steel interconnects used in solid oxide fuel cells, resulting in non-protective oxide scale growth on the air side. In this paper, low-cost steels AISI 441 and AISI 444 and the tailor-made Crofer 22 APU, were investigated at 800 °C and 600 °C under dual-atmosphere conditions: air-3%H₂O on one side and Ar-5%H₂-3%H₂O on the other side. At 800 °C, the uncoated and Ce/Co-coated steels formed protective layers of (Cr,Mn)₃O₄/Cr₂O₃ and (Co,Mn)₃O₄/Cr₂O₃ respectively on the air side after 336 h. However, at 600 °C, the Ce/Co-coated AISI 441 and AISI 444 showed ∼20–25 μm thick Fe₂O₃/(Fe,Cr)₃O₄ oxide scale on the air side after 336 h. Ce/Co coated Crofer 22 APU remained protective after 772 h at 600 °C, indicating better resistance to the dual-atmosphere. The effect of Ce/Co coatings on the air side and the need for coatings on the fuel side are discussed and compared with experimental data.     

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.     

Compatibility of a seal glass with (Mn,Co)3O4 coated interconnects: Effect of atmosphere

The interfacial compatibility of an alkaline earth silicate glass (SABS-0) with (Mn,Co)₃O₄ coated Crofer 22 APU and AISI 441 interconnects has been investigated at 800 °C for 100 h in argon, air, and H₂/H₂O atmospheres. Detailed microstructural and diffusion studies show that the interfacial compatibility is better for the (Mn,Co)₃O₄ coated Crofer 22 APU alloy but deteriorates from the H₂/H₂O to the air or the argon atmospheres for both alloys. Thermal treatment atmosphere mainly affects the inter-diffusion of Cr in the Crofer 22 APU alloy. The (Mn,Co)₃O₄ layer cannot eliminate the undesirable Cr diffusion for the Crofer 22 APU alloy regardless of the thermal treatment atmosphere. The interfacial reaction for the AISI 441/SABS-0 is so severe that the inter-diffusion of the elements is less significant. Oxidation of the AISI 441 alloy degrades the (Mn,Co)₃O₄ coating stability and increases the reaction of the coating with the SABS-0 glass, leading to inferior interfacial compatibility.     

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.     

Oxidation kinetics and phase evolution of a Fe–16Cr alloy in simulated SOFC cathode atmosphere

The oxidation behavior of a Fe–16Cr alloy containing a small amount of Mn oxidized in air for up to 500 h within the temperature range of 650–850 °C was examined. Two consecutive oxidation stages were found and these obeyed the parabolic rate law with various rate constants. Formation and growth of Cr₂O₃, MnCr₂O₄, surface nodules and oxide spallation were found to be responsible in the oxidation stages accordingly to various situations. The thin film X-ray diffraction, SEM and EDX confirmed the duplex oxide microstructure with MnCr₂O₄ on top of Cr₂O₃, Cr and Mn diffusion in Cr₂O₃ is considered to be responsible for the formation of each layer, respectively. The estimated area specific resistance (ASR) suggests the possibility of using this alloy as the interconnect material in reduced temperature SOFCs, however, surface modification to enhance its oxidation and spallation resistances is desired.     

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.     

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.     

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.     

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.     

Air exposure improving hydrogen desorption behavior of Mg–Ni-based hydrides

It is well established that H₂O and O₂ have an inauspicious influence on hydrogen reactivity of hydrogen storage alloys. In this work, an unexpected improvement of the desorption behavior was discovered by just exposing the magnesium rich Mg–Ni hydrides into the air for a certain period. Upon an exposure duration of 4 months, the dehydrogenation peak and onset temperature were sharply lowered by 150 °C and 130 °C. Furthermore, the air-exposed sample could quickly absorb 3.08 wt% H₂ and desorb 2.81 wt% H₂ within 400 s at 300 °C. Besides the refinement of the powders due to the spontaneous hydrolysis reaction, the in-situ formed magnesium hydroxide layer and Ni are thought to be responsible for the remarkable improvement. This work gives interesting insights that the self-generating surface passivation is not necessarily harmful in the solid-state hydrogen storage area, especially for the cases where active sites of catalysis are present.     

Structural peculiarities of La2Ge1-xCrxMgO6-δ (0

《International Journal of Hydrogen Energy》2023,48(33):12485-12492

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.     

Comparison of hydrogen consumption behavior of chromium and manganese promoted copper-based catalysts

CuO/ZnO/Al₂O₃/MgO–Cr and -Mn catalysts are synthesized using nitrate route via co-precipitation method. The precursors are characterized by XRD. The decomposition behavior of the precursors is analyzed by Air-TGA. The catalysts calcined at 250, 300, 350 and 450 °C are characterized by XRD and BET. CuO particle size reduction and surface area of the catalysts are investigated. Increasing the calcination temperature from 350 °C to 450 °C crystallite size increases about 3 nm, and BET surface area decreases about 30 m²/g. The reduction characteristics of the catalysts are analyzed via TPR and H₂-TGA, and H₂ consumption values of Cr and Mn containing catalysts is found as 40% and 60%, respectively. Peak temperatures of Mn containing catalysts (290–325 °C) are lower than peak temperatures of Cr containing catalysts (300–360 °C) as confirmed by H₂-TGA and H₂-DTG. The optimum H₂ consumption value of 52% is obtained with CuO/ZnO/Al₂O₃/MgO–Mn catalyst calcined at 350 °C.     

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.     

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.     

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.     

Heat resistant alloys as interconnect materials of reduced temperature SOFCs

Heat-resistant alloys, Haynes 230 and SS310, were exposed to air and humidified H₂ at 750 °C for up to 1000 h, respectively, simulating the environments in reduced temperature solid oxide fuel cells (SOFCs). The oxidized samples were characterized by using SEM, EDS and X-ray diffraction to obtain the morphology, thickness, composition and crystal structure of the oxide scales. A mechanism for the formation of metallic Ni-rich nodules on top of the oxide scale in Haynes 230 sample oxidized in humidified H₂ was established. Thermodynamic analysis confirmed that MnCr₂O₄ is the favored spinel phase, together with Cr₂O₃, in the oxide scales.     

Corrosion characteristics of a nickel-base alloy C-276 in harsh environments

The oxidation behaviors of alloy C-276 in two harsh environments: high-temperature air and supercritical water (SCW), respectively representing the working conditions of the external and internal surfaces of reactors for SCW gasification biomass to produce H₂, were investigated. In two environments, all oxidation kinetics followed parabolic laws, while the corrosion rate of alloy C-276 exposed to supercritical water gasification (SCWG) environments was 2.5–3 times higher than that in high-temperature air. The oxide scale formed in air at 500 °C consisted of an outer Fe-rich layer (Fe₂O₃ and NiCr₂O₄) and an inner layer of Cr₂O₃ and NiCr₂O₄, while the outer Fe-rich layer disappeared as the temperature increased to 550 °C. Compared to the scales formed on nickel-base alloys in near-pure SCW, the absence of NiO and Ni(OH)₂ phases within the scales formed on the C-276 samples in present SCWG environment may be due to higher molar proportion of hydrogen.     

Evaluation of electrodeposited Fe-Ni alloy on ferritic stainless steel solid oxide fuel cell interconnect

Fe-Ni alloy is electrodeposited on ferritic stainless steel for intermediate-temperature solid oxide fuel cell (SOFC) interconnects application. The oxidation behavior of Fe-Ni alloy coated steel has been investigated at 800 °C in air corresponding to the cathode environment of SOFC. It is found that the oxidation rate of the Fe-Ni alloy coated steel becomes similar to that of the uncoated steel after the first week thermal exposure, although the mass gain of the coated steel is higher than that of the uncoated steel. Oxide scale formed on the uncoated steel mainly consists of Cr₂O₃ with (Mn,Cr)₃O₄ spinel. However, a double-layer oxide structure with a Cr-free outer layer of Fe₂O₃/NiFe₂O₄ and an inner layer of Cr₂O₃ is developed on the Fe-Ni alloy coated steel. The scale area specific resistance (ASR) for the Fe-Ni alloy coated steel is lower than that of the scale for the uncoated steel.