Development of a Fe-Cr alloy for interconnect application in intermediate temperature solid oxide fuel cells

The oxidation behavior and electrical property of a newly designed Fe-Cr alloy with addition of 1.05 wt.% Mn, 0.52 wt.% Ti, 2.09 wt.% Mo and other elements, such as La, Y and Zr have been investigated isothermally or cyclically at 750 °C in air for up to 1000 h. With a coefficient of thermal expansion matched to SOFC cell components, the alloy demonstrates excellent oxidation resistance and low area specific resistance of the oxide scale. The thermally grown oxide scale presents a multi-layered structure with conductive Mn-Cr spinel in-between the underneath Cr₂O₃ and the top Mn₂O₃. The oxidation rate constants obtained under both isothermal and cyclic oxidation condition are in the range of 5.1 × 10⁻¹⁴ to 7.6 × 10⁻¹⁴ g² cm⁻⁴ s⁻¹, and the measured area specific resistance at 750 °C after 1000 h oxidation is around 10 mΩ cm², lower than that of the conventional Fe-Cr stainless steels and comparable with that of the Ni-based alloys. Thermal cycling seems to improve the oxide scale adherence and promotes the formation of the highly conductive Mn₂O₃, and in turn, to enhance the oxidation resistance and electrical property.     

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.     

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.     

Evaluation of electroplated Co-Cu metallic coatings for intermediate-temperature solid oxide fuel cells

Co-Cu alloys have been co-deposited onto 430 ferritic stainless steels via electroplating with a citrate solution. At the initial oxidation stage, a three-layer scale composed of a thin CuO outer layer, a thick (Cu,Fe,Cr)-doped Co₃O₄ middle layer and a (Cu,Fe)-doped (Co,Cr)₃O₄ inner layer was formed on the coated steel. With extended oxidation, the (Co,Cr)₃O₄ inner layer has been transformed into a Cr-rich oxide inner layer. An obvious outward diffusion of Fe appeared, leading to the formation of an (Cu,Cr,Mn)-doped (Co,Fe)₃O₄ interaction zone between the Co₃O₄-based spinel and the chromia oxides. The Co-Cu coating effectively blocked the outward migration of Cr from the substrate. No Cr element could be found in the coupled La_0·8Sr_0·2MnO₃ (LSM) plate of the coated sample after oxidized at 800 °C in air for 500 h. The highly conductive coating with a structure of CuO/Co-based spinels significantly decreased the growth of the Cr-rich oxide scale, and thus a much lower scale area specific resistance (ASR). The electrical properties and the oxidation mechanism of the coated substrates were discussed.     

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.     

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.     

Evaluation of Ni80Cr20/(La0.75Sr0.25)0.95MnO3 dual layer coating on SUS 430 stainless steel used as metallic interconnect for solid oxide fuel cells

Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating is deposited on SUS 430 alloy by plasma spray for solid oxide fuel cell (SOFC) interconnect application. The phase structure, area specific resistance (ASR), and morphology of the coating are studied. A two-cell stack is also assembled and tested to evaluate coating performance in an actual SOFC stack. The NiCr/LSM coating adheres well to the SUS 430 alloy after oxidation in air at 800 °C for 2800 h. The ASR and its increasing rate of coated alloy are 25 mΩ cm² and 0.0017 mΩ cm²/h, respectively. In an actual stack test, the maximum output power density of the stack repeating unit increases from 0.32 W cm⁻² to 0.45 W cm⁻² because of the application of NiCr/LSM coating. The degradation rate of the stack repeating unit with no coating is 4.4%/100 h at a current density of 0.36 A cm⁻², whereas the stack repeating unit with NiCr/LSM coating exhibits no degradation. Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating can remarkably improve the thermal stability and electrical performance of metallic interconnects for SOFCs.     

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.     

Life prediction of coated and uncoated metallic interconnect for solid oxide fuel cell applications

In this paper, we present an integrated experimental and modeling methodology in predicting the life of coated and uncoated metallic interconnect (IC) for solid oxide fuel cell (SOFC) applications. The ultimate goal is to provide cell designer and manufacture with a predictive methodology such that the life of the IC system can be managed and optimized through different coating thickness to meet the overall cell designed life. Crofer 22 APU is used as the example IC material system. The life of coated and uncoated Crofer 22 APU under isothermal cooling was predicted by comparing the predicted interfacial strength and the interfacial stresses induced by the cooling process from the operating temperature to room temperature, together with the measured oxide scale growth kinetics. It was found that the interfacial strength between the oxide scale and the Crofer 22 APU substrate decreases with the growth of the oxide scale, and that the interfacial strength for the oxide scale/spinel coating interface is much higher than that of the oxide scale/Crofer 22 APU substrate interface. As expected, the predicted life of the coated Crofer 22 APU is significantly longer than that of the uncoated Crofer 22 APU.     

Magnetron-sputtered Mn/Co(40:60) coating on ferritic stainless steel SUS430 for solid oxide fuel cell interconnect applications

With recent progress in lowering the operating temperature, chromia-forming ferritic stainless steels are considered as promising interconnect materials for solid oxide fuel cells (SOFCs). To block the chromium evaporation and chromium poisoning, coatings with Mn/Co (40:60) was tested as the optimized recipe to maintain Mn_1.5Co_1.5O₄ composition after long term operation due to Mn diffusion from substrate. In order to study the coating thickness effect, Mn/Co (40:60) coatings were fabricated in thickness of approximately 800 nm, 1500 nm, 3000 nm on ferritic stainless steels SUS430 using magnetron sputtering. Oxidation behavior of sputtered samples was investigated after oxidized at 800 °C in air for 2 h, 250 h, 500 h, 1000 h, respectively. SEM, EDS, XRD and FIB are used to analyze the surface morphology, chemical composition and structures of the coatings. Area specific resistance measurement indicated the sputtered samples in thickness of 800 nm, 1500 nm, 3000 nm at 800 °C for various hours in air are in range of 15–36 mΩ cm², 12–25 mΩ cm², 10–23 mΩ cm² respectively. Eventually the optimized thickness of Mn/Co (40:60) coatings was suggested.     

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

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

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.     

Numerical simulation of intermediate-temperature direct-internal-reforming planar solid oxide fuel cell

A numerical model for an anode-supported intermediate-temperature direct-internal-reforming planar solid oxide fuel cell (SOFC) was developed. In this model, the volume-averaging method is applied to the flow passages in the SOFC by assuming that a porous material is inserted in the passages as a current collector. This treatment reduces the computational time and cost by avoiding a full three-dimensional simulation while maintaining the ability to solve the flow and pressure fields in the streamwise and spanwise directions. In this model, quasi-three-dimensional multicomponent gas flow fields, the temperature field, and the electric potential/current fields were simultaneously solved. The steam-reforming reaction using methane, the water-gas shift reaction, and the electrochemical reactions of hydrogen and carbon monoxide were taken into account. It was found that the endothermic steam-reforming reaction led to a reduction in the local temperature near the inlet and limited the electrochemical reaction rates therein. Computational results indicated that the local temperature and current density distributions can be controlled by tuning the pre-reforming rate. It was also found that a small amount of heat loss from the sidewall can cause significant nonuniformity in the flow and thermal fields in the spanwise direction.     

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.     

Study on GDC-LSGM composite electrolytes for intermediate-temperature solid oxide fuel cells

The electrolyte materials Ce_0.9Gd_0.1O_1.9 (GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) were synthesized by means of glycine-nitrate processes, respectively, then GDC-LSGM composite electrolytes were prepared by mixing GDC and LSGM. The GDC and LSGM powders were mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as GL9505, GL9010 and GL8515. Their structures and ionic conductivities were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and AC impedance spectroscopy. The grain sizes of GDC-LSGM composites could be increased distinctly and the grain boundary resistance could be significantly decreased by small addition of LSGM. The experimental results show that the GDC-LSGM composites exhibit excellent ionic conductivity and could significantly enhance the fuel cell performances. The open circuit voltages are higher in the cell with composite electrolytes than in the cell with single GDC as electrolyte at the working temperature. Among these electrolytes, GL9505 has the highest ionic conductivity and the maximum power density.     

Characterization and performance of (La,Ba)(Co,Fe)O3 cathode for solid oxide fuel cells with iron–chromium metallic interconnect

(La_0.6Ba_0.4)(Co_0.2Fe_0.8)O₃ (LBCF) is synthesized by a sol–gel method as a Cr-tolerant cathode for intermediate-temperature solid oxide fuel cells (ITSOFCs). The electrochemical performance and Cr deposition process for the O₂ reduction reaction on LBCF cathodes in the presence and absence of a Fe–Cr alloy interconnect are investigated in detail, in comparison with a (La,Sr)(Co,Fe)O₃ (LSCF) electrode. Cr deposition occurs for the O₂ reduction reaction on LBCF electrodes in the presence of Fe–Cr alloy. Very different from that observed for the reaction on the LSCF cathode, Cr deposition on the LBCF electrode/gadolinia-doped ceria (GDC) electrolyte system is very small and shows little poisoning effect for O₂ reduction on LBCF electrode. The results demonstrate that the LBCF electrode has a high resistance towards Cr deposition and high tolerance towards Cr poisoning.     

High-performance solid oxide fuel cells with fiber-based cathodes for low-temperature operation

Low-temperature operation of solid oxide fuel cells (SOFCs) results in deterioration in electrochemical performance due to sluggish oxygen reduction reaction (ORR) at the cathode. To enhance the reaction pathway for ORR, La_0.8Sr_0.2MnO₃ (LSM) nanofibers were fabricated by electrospinning and used for low-temperature solid oxide fuel cells operated at 600–700 °C. The morphological and structural characteristics show that the electrospun LSM nanofiber has a highly crystallized perovskite structure with a uniform elemental distribution. The average diameter of the LSM nanofiber after sintering is 380 nm. A symmetric cell of nanofiber-based LSM cathode on scandia-stabilized zirconia (SSZ) electrolyte pellet exhibits much lower area specific resistances compared to commercial LSM powder-based cathode. A single cell based on the nanofiber LSM cathode on yttrium-doped barium cerate-zirconia (BCZY) electrolyte exhibits a power density of 0.35 Wcm⁻² at 600 °C, which increases to 0.85 Wcm⁻² at 700 °C. The cell has an area specific resistance (ASR) of 0.46 Ωcm² at 600 °C, which decreases to 0.07 Ωcm² at 700 °C. The results indicate that the LSM electrode fabricated by the electrospinning process produces a nanostructured porous electrode which optimizes the microstructure and significantly enhances the ORR at the cathode of SOFCs.     

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.     

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.