Area Specific Resistance Measurements Research Articles

Oxidation kinetics and electrical properties of oxide scales formed under exposure to air and Ar–H2-H2O atmospheres on the Crofer 22 H ferritic steel for high-temperature applications such as interconnects in solid oxide cell stacks

A 100 h isothermal oxidation kinetics study for Crofer 22H was conducted in air and the Ar–H2-H2O gas mixture (p(H2)/p(H2O) = 94/6) in the range of 973–1123 K. The parabolic rate constant was independent of oxygen partial pressure in the range from 6.2 × 10−24 to 0.21 atm at 1023 and 1073 K, while at 973 and 1123 K it was higher in air than in Ar–H2-H2O. The scales consisted of Cr2O3 and manganese chromium spinel with an Mn:Cr ratio dependent on the oxidation conditions. Cross-scale resistance was evaluated with regards to the application of the steel in solid oxide fuel cells using a number of methods, including X-ray diffraction, scanning electron microscopy, confocal Raman imaging and area-specific resistance measurements.

CuMn1.7Fe0.3O4 – RE2O3 (RE = Y, Gd) bilayers as protective interconnect coatings for solid oxide cells

Efficient replacement of materials based on critical elements such as cobalt is one of the greatest challenges facing the field of solid oxide cells. New generation materials, free of cobalt show potential to replace conventional materials. However, these materials are characterised by poor ability to block chromium diffusion. This article described the study of CuMn1.7Fe0.3O4 (CMFO) spinel combined with single metal oxide (Y2O3 or Gd2O3) thin films as protective coatings for steel interconnects. CMFO was examined using XRD and TPR. Coated steel samples were oxidised in an air atmosphere at 700 °C for 4000 h. The coatings and oxide scale microstructures and cross-sections were examined by CRI, XRD, and SEM-EDX. The electrical properties of the steel-coating system were evaluated using Area Specific Resistance measurements. Based on the results obtained, it can be concluded that the use of thin layers of rare earth oxides allowed for better blocking of chromium diffusion.

Fe-modified Mn2CuO4 spinel oxides: coatings based on abundant elements for solid oxide cell interconnects

The current state of the art steel interconnect coating materials are based on critical raw material - Co-oxide spinels. Replacing Co-oxide spinels with alternative, abundant materials can reduce the dependence on the critical raw materials. Cobalt-free coatings with the general formula Mn2-xCuFexO4, where x = 0, 0.1, 0.3, were electrophoretically deposited on a ferritic stainless-steel support and evaluated. Prior to deposition, the powders were prepared by a soft chemistry process and studied in terms of crystallographic phase analysis, electrical conductivity, thermal expansion, and sinterability behaviour. Coated steel samples were oxidised in an air atmosphere at 750 °C for 3000 h. In parallel, a state-of-the-art MnCo2O4 spinel oxide was tested as a reference. The coatings and oxide scale microstructures of the surfaces and cross-sections were examined by XRD, and SEM-EDX. TEM-EDX, XRF, and micro-XRD were also performed on the cross-section lamellae. The electrical properties of the steel-coating system were evaluated by Area Specific Resistance measurement. The results confirm that Mn–Cu–Fe oxides exhibit higher conductivity and lower TEC than Mn–Co oxide. Based on the obtained results, it might be concluded that the proposed coatings are a promising alternative to coatings that contain cobalt.

Highly dense (Mn,Co)3O4 spinel protective coating derived from Mn–Co metal precursors for SOFC interconnect applications

The major degradation issues of solid oxide fuel cells (SOFC) are associated with the Cr2O3 scale growth and Cr diffusion of the Cr-based ferritic stainless steel (FSS) interconnects. Although (Mn,Co)3O4 has been proved as a suitable material for protecting FSS interconnects, the porous structure of the coatings prepared with the pre-synthesized spinel weakens the protective capability of the coatings. In this paper, the widely-used pre-synthesized spinel is replaced with metal precursors (Mn and Co powders). Due to the low melting point (≤1290 °C) and the volume expansion during oxidation, the metal precursors, can be effectively sintered at 900 °C in a reducing atmosphere and form dense, well-protective coatings at 850 °C in the air. The samples are characterized with X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and a 4-probe area-specific resistance (ASR) test. Compared with the coatings derived from pre-synthesized spinel, the metal-derived coatings present denser structures with better electrical conductivity (ASR = 5.76 mΩ cm2). The weight gain and ASR measurement results indicate that the metal-derived coatings significantly mitigate the increase of weight gain and ASR by inhibiting scale formation and growth, showing better protective capability for SOFC applications.

Evaluation of Titanium Based Alloys as Interconnects for the Light Weight SOFC System

Ti alloys were evaluated as new candidates for interconnects of a lightweight SOFC which is to be mounted on aircrafts. Oxygen permeation test was performed by flowing air and He gas. No notable oxygen leak was observed. Area specific resistance (ASR) of oxide-scales formed between the alloy the electrode contacts was investigated by impedance spectroscopy. Two different electrodes, Pt and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), attached on the surface of Ti alloys were tested with time in air at 873 K. Although ASR at the contact with Pt electrode was small, the oxide scale grown under LSCF showed a large ASR increasing with time. After the ASR measurement, cross section between the alloy and the electrode contacts were observed by scanning electron microscope and analyzed by energy-dispersive X-ray spectroscopy. Growth of the oxide scale was found to be promoted by LSCF.

Method to Measure Area Specific Resistance and Chromium Migration Simultaneously from Solid Oxide Fuel Cell Interconnect Materials

AbstractChromium evaporation is identified as a major degradation mechanism in solid oxide fuel cell (SOFC) stacks. The major chromium source is the commonly used stainless steel interconnects, thus raising a need for protective coatings on the interconnect steel. Ex situ characterization methods of protective coatings involve chromium evaporation measurements, area specific resistance (ASR) measurements and long‐term exposure tests. To replicate stack conditions, commonly used ASR measurement setups should be further developed. This work presents an improved characterization method for steels and coatings and aims to be an extension to state‐of‐the‐art characterization methods. The studied steel samples, bare or coated, are placed adjacent to palladium foils with a screen‐printed lanthanum‐strontium‐cobalt (LSC) layer and the resistivity over the pair is measured. The method offers similar contact materials, chromium migration mechanisms, electrical contacts and chemical interactions, as seen in stacks. Further, it enables post‐test chromium migration analysis with electron microscopy. Demonstration of the method validated that protective coatings hindered both oxidation and chromium migration from the substrate steels. The presented method could aid in accelerating protective coating development.

Does the conductivity of interconnect coatings matter for solid oxide fuel cell applications?

The present work aims to quantify the influence of typical interconnect coatings used for solid oxide fuel cells (SOFC) on area specific resistance (ASR). To quantify the effect of the coating, the dependency of coating thickness on the ASR is examined on Crofer 22 APU at 600 °C. Three different Co coating thicknesses are investigated, 600 nm, 1500 nm, and 3000 nm. Except for the reference samples, the material is pre-oxidized prior to coating to mitigate the outward diffusion of iron and consequent formation of poorly conducting (Co,Fe)3O4 spinel. Exposures are carried out at 600 °C in stagnant laboratory air for 500 h and subsequent ASR measurements are performed. Additionally the microstructure is investigated with scanning electron microscopy (SEM). On all pre-oxidized samples, a homogenous dense Co3O4 top layer is observed beneath which a thin layer of Cr2O3 is present. As the ASR values range between 7 and 12 mΩcm2 for all pre-oxidized samples, even though different Co3O4 thicknesses are observed, the results strongly suggest that for most applicable cases the impact of the coating on ASR is negligible and the main contributor is Cr2O3.

Oxidation behavior and area specific resistance of La, Cu and B alloyed Fe-22Cr ferritic steels for solid oxide fuel cell interconnects

Two Fe-22 wt% Cr ferritic stainless steels containing varying concentrations of La (0.14 or 0.52 wt%), Cu (0.17 or 1.74 wt%) and B (48 or 109 ppm) are investigated with respect to oxidation behavior and high temperature area specific resistance (ASR) of the surface oxide scales. To determine the oxidation resistance of developed steels, continuous isothermal oxidation is carried out at 800 °C in air, for 2000 h, and their thermally grown oxide scale is characterized using dynamic SIMS, SEM/EDX, XRD and GI-XRD techniques. To assess their electrical performance, the ASR measurement by four-point probe method is conducted at 800 °C in air, for 400 h. In higher La content steel, the La-oxides at the scale/alloy interface promotes the oxygen transport which resulted in sub-surface oxidation of Mn, Cr, Ti and Al. Moreover, the inward growth of oxides contributes to increase of Fe-Cr alloy protrusions within the scale, which reduced the ASR. In contrast, sub-surface oxidation is reduced in high Cu-alloyed steel by segregated Cu at the scale/alloy interface. Thus, addition of Cu is effective to oxidation resistance and also to better electrical performance. However, no obvious impact of B on the scale sequence and/or ASR is observed.

Oxidation and electrical behavior of CuFe2O4 spinel coated Crofer 22 APU stainless steel for SOFC interconnect application

In an attempt, CuFe2O4 spinel coating was investigated with the purpose of increasing the performance of Crofer 22 APU ferritic stainless steel as a potential SOFC interconnect material. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) and 4-probe area specific resistance (ASR) tests. CuFe2O4 spinel layers were deposited via screen-printing method on the bare and pre-oxidized samples. Good bonding between the coating and the substrate was achieved by the reactive sintering process of the reduced coating. Long term oxidation experiments and ASR measurements are investigated. The results indicated that the spinel protection layers not only significantly decreased the ASR, but also inhibited the subscale growth by acting as a barrier to the inward diffusion of oxygen. An ASR of 13.8 and 26.7mΩ⋅cm2at 800°C after 400h oxidation were achieved for the bare and pre-oxidized CuFe2O4 spinel coated samples, respectively, which were much lower than that of the bare substrate (48mΩ⋅cm2) at the same temperature. The ASR of 14.3mΩ⋅cm2was reached after 600h of exposure at 800 for the coated sample. The high electrical conductivity of CuFe2O4 spinel and its doping by Mn, inhibition of the resistive Cr2O3 oxide scale growth, and good adherent between the coating and substrate are supposed to be responsible for the substantial electrical conductivity improvement.

Evaluation of Protective Coatings for SOFC Interconnects

Chromium poisoning is a widely recognized degradation mechanism in solid oxide fuel cells (SOFC). Stainless steel interconnects (IC), in a direct contact with the cathode, have been identified as the main chromium source. Chromium evaporation from the bulk steel may also cause break-away oxidation of the IC plate, which is equally deleterious for the stack. Thus, protective coatings are needed to prevent chromium from migrating from the steel. A low electrical resistivity and high chemical and physical stability are required of the coatings. This work compares manganese-cobalt and cobalt-cerium protective coatings. The evaluated coatings are fabricated on thin steel samples by commercial companies and research facilities with differing manufacturing methods. The chosen steels are Sandvik Sanergy HT and K41/441 as they are common in SOFC applications. Area specific resistance (ASR) and overall stability were determined with an innovative measurement setup. The steel samples were stacked in a sandwich structure adjacent to thin palladium foils with a screen-printed lanthanum strontium cobalt (LSC) layer. This setup offers two advantages. First, a realistic electrical contact with the cathode material is obtained since the LSC layer is manufactured with the same methods as real SOFC cathodes. Second, the adoption of palladium spacers instead of steel enables electron microscopy analysis on chromium migration into the LSC layer as well as on oxide scale growth. ASR measurements were conducted in a humid atmosphere at 700 °C for a period of 1000 hours. The paper compares the coating solutions in terms of ASR and stability and discusses their usability in SOFC applications.

A duplex β″-Al2O3 solid electrolyte consisting of a thin dense layer and a porous substrate

To improve the performance of Na-beta batteries at intermediate temperatures (≤200°C) where much improved cyclability and reduced degradation can be achieved, there is a need to lower the resistance/polarization originated from β″-Al2O3 solid electrolyte (BASE) while maintaining its good strength. In this paper, the concept of a duplex BASE consisting of a thin dense electrolyte and a porous support was proposed as a solution to achieve low area-specific resistance (ASR) with good mechanical strength supported by the porous substrate. The effects of various factors including porosity, composition, and homogeneity of ingredients on the flexural strength of duplex BASEs were examined. In summary, lower porosity, higher YSZ content in the structure, and attrition milling of raw powders resulted in improved strength. The ASR measurement exhibited that the resistance of duplex BASEs was mainly originated from a dense layer. Overall, the maximum strength of 260MPa and the ASR value of 0.31Ωcm2 (at 350°C) was achieved from a duplex BASE consisting of a 50μm thick dense layer (Al2O3:YSZ=7:3 in volume) and a 500μm thick porous support (Al2O3:YSZ=4:6 in volume with 19% open porosity). The effects of various factors on the properties of duplex BASEs will be discussed in detail.

Reduced long term electrical resistance in Ce/Co-coated ferritic stainless steel for solid oxide fuel cell metallic interconnects

The present study is focused on the influence of selected coatings on a ferritic stainless steel (Sanergy HT™, Sandvik) on the evolution of the area specific resistance (ASR) as a function of time at high temperature. The samples are exposed in humidified air at 850 °C for up to 4200 h. It combines long-term ASR measurements with the thermogravimetric behavior and microstructural analysis of the cross sections by scanning electron microscopy. The results show that uncoated and Co-coated Sanergy HT™ exhibit similar oxidation kinetics and comparable ASRs, while a combined Ce/Co coating improves oxidation resistance and, consequently, reduces the ASR significantly. Other reports have earlier shown that Co- (and Ce/Co)-coated Sanergy HT™ reduces the evaporation of volatile chromium species. Overall, the study indicates that Ce/Co-coatings will render substantially improved performance for ferritic steel interconnects for solid oxide fuel cells.

Effect of Oxidation on Creep Strength and Resistivity of Porous Fe-26Cr-1Mo

To investigate its application as a material for solid oxide fuel cell interconnects, oxidation rates of replicated E-Brite (Fe-26Cr-1Mo, wt pct) foams with 43 and 51 pct open porosity were measured in static laboratory air for up to 200 hours. Results correlate well with previously reported values for dense material when normalized by surface area. Area-specific resistance measurements, taken in the range of 823 K to 1073 K (550 °C to 800 °C) after 24 hours of oxidation at 1123 K (850 °C), yield activation energies in the range 69 to 82 kJ mol−1 for porous E-Brite that closely match dense E-Brite. Compressive creep properties, measured at 1073 K (800 °C) for pristine and oxidized porous E-Brite, show that pre-oxidation (10 hours at 1073 K (800 °C)) led to a ~100-fold decrease in creep rate. This is due to strengthening of the alloy foam by the formation of a continuous network of oxide, which coats the internal pore surface and reduces porosity by as much as 10 pct after 200 hours of oxidation at 1073 K (800 °C). Choking of the fenestrations between the pores, however, leads to an increase in closed porosity. Strengthening and pore filling effects should be taken into account in the design of the SOFC stack when using E-Brite as a porous interconnect material.

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 Mn1.5Co1.5O4 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.

Effects of using (La0.8Sr0.2)0.95Fe0.6Mn0.3Co0.1O3 (LSFMC), LaNi0.6Fe0.4O3−δ (LNF) and LaNi0.6Co0.4O3−δ (LNC) as contact materials on solid oxide fuel cells

Three lanthanum-based perovskite ceramic compounds were studied as contact materials, (La0.8Sr0.2)0.95Fe0.6Mn0.3Co0.1O3 (LSFMC), LaNi0.6Fe0.4O3−δ (LNF) and LaNi0.6Co0.4O3−δ (LNC), between a Crofer22APU interconnect and a La0.6Sr0.4FeO3 (LSF) cathode. The layers were deposited using in all cases wet colloidal spray technique. Phase structures of materials were checked by X-ray Diffraction (XRD) measurements. Electrical conductivity and thermal expansion coefficient (TEC) for these selected compounds were also determined.The important properties of the resulting {interconnect/contact layer/cathode} systems; including area specific resistance (ASR), reactivity, and adhesion of contact materials to the interconnect and to the cathode were investigated. Moreover, the electrical resistance and reactivity of the system without a contact layer, {steel/LSF/LSF} system, was measured for comparison. The contact resistance is strongly influenced by the conductivity of selected contact materials, showing the lowest ASR values for {Crofer22APU/LNC/LSF} assembly. The point microanalysis on cross-section of the systems, after ASR measurements, reveals that there is chromium enrichment in the contact and cathode layers which allows the formation of phases like SrCrO4 and Cr-containing perovskite in short exposure times. An adequate integrity and low reactivity is achieved when LNF contact coating is applied between Crofer22APU and LSF cathode without compromising the contact resistance of the system.

Chemical Compatibility and Electrical Contact of LaNi0.6Co0.4O3–δ (LNC) between Crofer22APU Interconnect and La0.6Sr0.4FeO3 (LSF) Cathode for IT‐SOFC

AbstractIn order to simulate the contact situation of interconnect/contact layer/cathode in SOFC stacks, contact resistance and chemical compatibility of LaNi0.6Co0.4O3–δ (LNC) as contact layer between Crofer22APU interconnect and La0.6Sr0.4FeO3 (LSF) cathode was investigated at 800 °C in air for more than 1300 h using X‐ray diffraction (XRD), scanning electron microscopy (SEM) set‐up equipped with an energy dispersive X‐ray analyser (EDX) and area specific resistance (ASR) measurements. The XRD analysis reveals that multiple phases were formed during ASR test. The point microanalysis on cross‐section of Fe–Cr/LNC/LSF system, after ASR measurements, shows chromium within the porous contact material mainly concentrated close to interconnect, but no Cr, Ni, or Co was detected in the cathode. It was found between LNC and LSF cathode, a thin and uniform layer which contains Sr, La, Cr, Co, Ni, and Fe. The contact between layers could act as a physical barrier for element migration and thus can suppress degradation of the cathode for these systems. The area specific resistance slope depends on the interactions between the contact material and/or cathode and the interconnect. Co‐containing spinels formed during ASR test can be responsible of the resistance decrease of the system, related to the low degradation of the cell.

Study of Conductivity of K41X Chromia Forming Alloy in High Temperature Electrolysis Environment

Alloy K41X has been proposed as interconnect material for high temperature vapor electrolysis (HTVE) devices. This chromia forming alloy (alloy K41X) was oxidized at 800 °C in a thermobalance in oxidizing (synthetic air) and reducing (Ar–1 %H2–9 %H2O) environments for 250 h. The evolution of the contact resistance was evaluated using a dedicated device under the same conditions. There were higher oxidation kinetics rate in air than in Ar–1 %H2–9 %H2O but surprisingly, the corresponding area specific resistance (ASR) values were 20 times higher in Ar–1 %H2–9 %H2O mixture than in air. Additional tests and analyses (exposure in Ar–D2–H2O environment, GD-OES and SIMS analyses) clearly showed that the higher ASR value can be attributed to the presence of hydrogen in the oxide scale when exposed in Ar–H2–H2O mixture. In situ changes of atmosphere during ASR measurement showed the rapid kinetics for hydrogen desorption.

Effects of electroless nickel plating on 441 stainless steel as SOFC interconnect

Metal nickel was electrolessly deposited in thickness of approximately 6, 8, and 10 μm on ferritic stainless steel 441. Oxidation behaviour of plated samples was investigated after isothermal ageing at 750 °C for 400 h in an air atmosphere. Microstructure examination showed that nickel oxide formed on the surface, (Fe, Ni)3O4 spinel layer at the subsurface, and chromite at the oxide scale/steel interface. No chromium species was readily detected on the outer surface. Area specific resistance measurement indicated 2.0, 3.3, and 6.3 mΩ cm2 for 6, 8, and 10 μm plating layers, respectively, all lower than that of the uncoated case, 28 mΩ cm2. Results suggested nickel plating of thickness 8 μm would be sufficient for effectively preventing Cr out-diffusion while maintaining a low electric resistivity.

Investigation of the Effects of Catalyst Loading and Gas Flow Rate on Polymer Electrolyte Membrane (PEM) Fuel Cell Performance and Degradation

In this paper, the effects of gas flow rates and catalyst loading on polymer electrolyte membrane fuel cell (PEMFC) performance was investigated using a 50 cm2active area fuel cell fixture with serpentine flow field channels machined into poco graphite blocks. Membrane electrode assemblies (MEAs) with catalyst and gas flow rates at two levels each (0.5 mg/cm2, 1 mg/cm2; 0.3 l/min, 0.5 l/min, respectively) were tested at 60 °C without humidification. The cell performance was analyzed by taking ac impedance, Tafel plot, open circuit voltage, and area specific resistance measurements. It was observed that MEAs with lower gas flow rate had lesser cell resistance compared to MEAs with a higher gas flow rate. Tafel plot shows the highest exchange current density value of 10−2.05 mA cm2 for MEA with 0.5 mg/cm2 catalyst loading tested at reactant gas flow rate of 0.3 l/min signifying it had the least activation loss and fastest reaction rate. Open circuit voltage-time curve shows a higher output voltage and lesser voltage decay rate for MEAs tested at higher gas flow rates.

Strontium transport and conductivity of Mn 1.5Co 1.5O 4 coated Haynes 230 and Crofer 22 APU under simulated solid oxide fuel cell condition

Interaction of current collectors with coated interconnect alloys was examined using dual-coating specimens. Oxidation kinetics of (La,Sr)(Co,Fe)O 3 (LSCF)/Mn 1.5Co 1.5O 4 (MCO) and (La,Sr)(Mn)O 3 (LSM)/MCO coated Haynes 230 (H230) was determined at 750 °C in air via a gravimetric method. The LSCF/MCO dual coating samples exhibited a rate constant 2 times larger than that of the LSM/MCO coated H230. SrCrO 4 that formed at the LSCF/MCO coated H230 interface contributed to the increased rate constant. No SrCrO 4 was detected at the interface of the LSM/MCO coated H230. Chemical stability of LSCF in air at 750 °C was confirmed by TGA. Area specific resistance (ASR) of MCO coated Haynes 230 (H230) and Crofer 22 APU with LSCF and LSM (coated H230 only) current collectors was measured at 800 °C for 1100 h. No gross difference in ASR evolution was observed between the LSCF/MCO-H230 and LSM/MCO-H230 assemblies. Activation energy of conduction of the LSCF/MCO-H230 and LSCF/MCO-Crofer 22 samples evolved from 0.63 eV to 0.80 eV and from 0.53 eV to 0.63 eV respectively over a period of ca. 1100 h, suggesting somewhat different interface chemistry evolution. Sr transport was observed in the LSCF/MCO-H230 samples subjected to both exposure test and ASR measurement, whereas it was not detected on the LSCF/MCO-Crofer 22 sample. Mechanism of the Sr transport is discussed on a qualitative basis to account for the observations.