Evaluation of several low thermal expansion Fe–Co–Ni alloys as interconnect for reduced-temperature solid oxide fuel cell

Several low thermal expansion Fe–Co–Ni alloys including HRA 929C, Thermo-Span, EXP 4005 and Three-Phase were evaluated as interconnect for reduced-temperature solid oxide fuel cell (SOFC). The isothermal oxidation behaviors of the four alloys were determined at 800 °C in air corresponding to the SOFC cathode environment. The results indicate that the mass gains of HRA 929C and Thermo-Span increased continuously with oxidation time, and were higher than those of EXP 4005 and Three-Phase, both of which exhibited low oxidation rate after the first-week exposure due to the formation of a semi-continuous Al₂O₃ inner layer. Compared to the ferritic alloy Crofer 22 APU, these low thermal expansion alloys exhibited inferior oxidation resistance; however, the area specific resistance (ASR) of the oxide scales thermally grown on these alloys was lower than that for Crofer 22 APU, as a result of the formation of a highly-conductive, Cr-free surface spinel layer. The promises and problems of these low thermal expansion alloys were discussed with regard to SOFC interconnect application.     

Fabrication of solid oxide fuel cells (SOFCs) by solvent-controlled co-tape casting technique

A co-tape casting process has an advantage of cost-effectiveness for mass production. To fabricate solid oxide fuel cells (SOFCs) with high electrochemical performance by co-tape casting, high solid loading and binder content of tape cast slurry are required to improve particle network strength. However, high solid loading and binder content cause high viscosity of the slurry, which makes removal of air bubbles and handling difficult. In this study, a new method to fabricate uniform green tapes with high solid loading and binder content by controlling solvent ratio under vacuum condition is proposed. As a result, high solid loading and binder content with 39% improved storage shear modulus, 26% improved LVR length, tensile strength of 5.0 MPa, and packing density of 57.5% were achieved at solvent ratio of 22 wt%. To fabricate unit cells using the green tapes, thermal decomposition and shrinkage behavior are characterized, and heat treatment steps at 250 °C, 350 °C, and 500 °C and co-sintering temperature were determined at 1250 °C. A fabricated unit cell showed open circuit voltage (OCV) of 1.10 V and the maximum power density of 1.20 W cm^?2 at 800 °C. To fabricate crack-free Ф5.0 cm unit cells, the mechanical strength of the anode support tapes after thermal decomposition was measured to determine the tape compositions that can minimize cracks at the unit cell. As a result, a crack-free unit cell with a diameter of 5.0 cm was fabricated, achieving OCV of 1.05 V and power of 4.3 W at 800 °C.     

Semiconductor electrolyte for low-operating-temperature solid oxide fuel cell: Li-doped ZnO

Semiconductors have been successfully demonstrated as the electrolytes for solid oxide fuel cells (SOFCs) in recent years. Many such semiconductors have shown their potentials as a competent ionic conductor for fuel cell operation, indicated by the appreciable ionic conduction and electrochemical performance. In the present study, we depart from traditional electrolyte concept to introduce a new semiconductor electrolyte, Li-doped ZnO to low-operating-temperature SOFCs. The used material was synthesized via a co-precipitation method and investigated from phase structure, morphology and UV–vis absorption perspectives. Utilizing Li-doped ZnO as electrolyte layer, we found the corresponding fuel cell exhibited a remarkable maximum power density of 443 mW cm^?2 along with open circuit voltage (OCV) of 1.07 V at 550 °C, and represented a lower-temperature operation feasibility with power outputs of 138–165 mW cm^?2 at 425–450 °C. Besides, high ionic conductivities of 0.028–0.087 S cm^?1 and low activation energy of 0.5 eV were also found in the synthesized Li-doped ZnO at 425–550 °C. Our investigation in terms of electrochemical impedance spectra (EIS) analysis manifested that Li-doped ZnO as the electrolyte layer boosted the electrode reactions of the device, which resulted in rather small polarization resistances and eventually realized good low-temperature performances. Further study based on the rectification characteristic of Ni/Li-doped ZnO contact verified the Schottky junction formation of Li-doped ZnO with anodic Ni, which can avoid the underlying electronic short-circuiting problem. These findings show a profound significance of using doped semiconductor for the future exploitation of SOFC electrolytes.     

Long-term stability of Ni–Sn porous metals for cathode current collector in solid oxide fuel cells

Ni–Sn porous metals with different concentrations of Sn were prepared as potential current collectors for solid oxide fuel cells (SOFCs). The weight increase of these species was evaluated after heat-treatment under elevated temperatures in air for thousands of hours to evaluate the long-term oxidation resistance. Ni–Sn porous metals with 5–14 wt% of Sn exhibited excellent oxidation resistance at 600 °C, although oxidation became significant above 700 °C. Intermetallic Ni₃Sn was formed at 600 °C due to phase transformation of the initially solid solutions of Sn in Ni in the porous metals. For the porous metal with 10 wt% of Sn, the oxidation rate constant at 600 °C in air was estimated to be 8.5 × 10^?14 g² cm^?4 s^?1 and the electrical resistivity at 600 °C was almost constant at approximately 0.02 Ω cm² up to an elapsed time of 1000 h. In addition, the gas diffusibility and the power-collecting ability of the porous metal were equivalent to those of a platinum mesh when applied in the cathode current collector of a SOFC operated at 600 °C. Ni–Sn porous metals with adequate contents of Sn are believed to be promising cathode current collector materials for SOFCs for operation at temperatures below 600 °C.     

Novel alkaline earth silicate sealing glass for SOFC: Part II. Sealing and interfacial microstructure

This is the second part of a study of a novel Sr–Ca–Ni–Y–B silicate sealing glass for solid oxide fuel cells (SOFC). Part I of the study addresses the effect of NiO on glass forming, thermal, and mechanical properties, and is presented in the preceding paper. In this paper (part II), candidate composite glass with 10 vol.% NiO was tested for sealing standard coupons of Ni/YSZ anode-supported YSZ electrolyte bilayer and metallic interconnect Crofer22APU at various temperatures. Samples sealed at the highest temperature (1050 °C) showed hermetic seal after fully reduction and 10 thermal cycles. The interfacial microstructure characterization showed no distinct reactions at the interfaces of glass/YSZ or glass/metal, though some segregation of Ni was found along the glass/metal interface. Possible reactions were discussed. Overall the composite glass with 10 vol.% NiO appeared to be a good candidate for SOFC sealing.     

Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell

In this study, the phase inversion-based co-extrusion method was employed to fabricate a structural-improved electrolyte/anode dual-layer hollow fiber (HF) precursor, which was then co-sintered at 1450 °C. The electrolyte structures were thoroughly investigated by varying the loading of electrolyte material (i.e. Yttria-stabilized zirconia, YSZ) with differing particle sizes (i.e. micron, sub-micron, and nano-sized) during suspension preparation. The results showed that the most promising electrolyte layer with thin, dense, gas-tight, and defect-free properties was obtained by mixing 70% submicron-YSZ and 30% nano-YSZ in electrolyte suspension (E-0.7sub0.3nano). This electrolyte formulation co-extruded with a thick nickel-oxide-YSZ (NiO-YSZ) anode layer yielded the highest bending strength of 85 MPa, providing major mechanical strength to the HF. Besides that, the nitrogen permeability value at 2.87 × 10^?6 mol m^?2 s^?1 Pa^?1 suggested that the electrolyte was gas-tight, preventing fuel and oxidant transport. The fiber was then reduced to nickel (Ni)-cermet anode. It was developed to be a complete micro-tubular solid oxide fuel cell (MT-SOFC) by depositing the lanthanum strontium cobalt ferrite (LSCF)/YSZ cathode via brush painting on the dual-layer HF. The cell was fed with hydrogen gas and yielded an open-circuit voltage (OCV) as high as 1.06 V with maximum power density of 0.243 W cm^?2, at 875 °C. Based on this test, it was found that the electrolyte structural-modified dual-layer hollow fiber-based MT-SOFC using mixed particle sizes may result in a promising OCV. However, the relatively low value for power density may be due to a less porous anode; thus, improvements in the anode's structure are required in future research.     

The investigation of Ag & LaCo0.6Ni0.4O3−δ composites as cathode contact material for intermediate temperature solid oxide fuel cells

The high interface resistance between cathodes and interconnects is a major cause for performance degradation of solid oxide fuel cells (SOFCs). Ag particles were mixed to LaCo_0.6Ni_0.4O_3?δ (LCN) matrix which prevented the silver densification and demonstrated porosity microstructure. The composites with different Ag content were evaluated as cathode contact materials with SUS430 alloy as interconnects. The area specific resistance (ASR) of SUS430/10%Ag & LCN/SUS430 showed the optimal performance in which the ASR was 73 mΩ cm² after 50 h at 750 °C and showed stable property in 10 thermal cycles from 200 °C to 750 °C. The excellent performance of 10%Ag & LCN is attributed to the high conductivity of silver, the stable microstructure of LCN and its good interface adhesion with the interconnect alloy. With 10%Ag & LCN as cathode contact materials, the power density of a single cell reached 0.623 W/cm² at 750 °C and the average degradation is lower than 1% in 3 thermal cycles.     

Experimental characterization of glass-ceramic seal properties and their constitutive implementation in solid oxide fuel cell stack models

This paper discusses experimental determination of solid oxide fuel cell (SOFC) glass-ceramic seal material properties and seal/interconnect interfacial properties to support development and optimization of SOFC designs through modeling. Material property experiments such as dynamic resonance, dilatometry, flexure, creep, tensile, and shear tests were performed on PNNL's glass-ceramic sealant material, designated as G18, to obtain property data essential to constitutive and numerical model development. Characterization methods for the physical, mechanical, and interfacial properties of the sealing material, results, and their application to the constitutive implementation in SOFC stack modeling are described.     

Porous nickel-iron alloys as anode support for intermediate temperature solid oxide fuel cells: II. Cell performance and stability

Porous nickel–iron alloy supported solid oxide fuel cells (SOFCs) are fabricated through cost-effective ceramic process including tape casting, screen printing and co-sintering. The cell performance is characterized with humidified hydrogen as the fuel and flowing air as the oxidant. Effects of iron content on the cell performance and stability under redox and thermal cycle are investigated from the point of view of structural stability. Single cells supported by nickel and nickel–iron alloy (50 wt % iron) present relatively high discharge performance, and the maximum power density measured at 800 °C is 1.52 and 1.30 W cm^?2 respectively. Nickel supported SOFC shows better thermal stability between 200 and 750 °C due to its dimensional stable substrate under thermal cycles. Posttest analysis shows that a dense iron oxide layer formed on the surface of the nickel-iron alloy during the early stage of oxidation, which prevents the further oxidation of the substrate as well as the functional anode layer, and thus, making nickel-iron supported SOFC exhibits better redox stability at 750 °C. Adding 0.5 wt % magnesium oxide into the nickel-iron alloy (50 wt% iron) can inhibit the metal sintering and reduce the linear shrinkage, making the single cell exhibit promising thermal stability.     

Numerical simulation and analysis of thermal stress distributions for a planar solid oxide fuel cell stack with external manifold structure

A three-dimensional numerical model based on the finite element method (FEM) is constructed to calculate the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack with external manifold structure. The stack is composed of 5 units which include cell, metallic interconnect, seal and anode/cathode current collectors. The temperature profile is described according to measured temperature points in the stack. It can be clearly seen that the maximum stress concentration area appears at the corner of the components when the stack is heated from room temperature (RT) to 780 °C. The effects of stack components on maximum stress concentration have been investigated under the operation temperature, as well as the thermal stress simulation results. It is obvious that the coefficient of thermal expansion (CTE) mismatch between the interconnect and the seal plays an important role in determining the thermal stress distribution in the stack. However, different compressive loads have almost no effect on stress distribution, and the influence of glass-based seal depends on the elastic modulus. The simulation results can be applied for optimizing the structural design of the stack and minimizing the high stress concentration in components.     

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.     

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.     

The hydrophilicity of carbon for the performance enhancement of direct ascorbic acid fuel cells

As a representative of low-temperature direct biomass fuel cells, direct ascorbic acid fuel cells (DAAFCs) carry many advantages, including renewable fuel, easy transportation and storage, and high safety. However, a major challenge of DAAFCs confronting us is relatively low power density. Herein, to deal with this challenge, we treat carbon black (BP 2000) with nitric acid at an optimal concentration (4 M), which is further employed as anodic electrocatalyst for AA oxidation with improved hydrophilicity. Consequently, hydrophilic AA molecules can more readily access the surface of the carbon electrocatalyst and donate electrons. Furthermore, the electrocatalytic effect of acid-treated carbon for AA oxidation reaction is quantitatively evaluated by the determination of activation energy, which has not been assessed prior to this study. In a similar way, nitric acid treatment is also applied to gas diffusion layer (GDL) at the anode side. In addition, Nafion content in anodic electrocatalyst layer, single cell operating temperature, and hot pressing conditions for the fabrication of membrane electrode assembly (MEA) as well as membrane thickness are also optimized. A maximum power density of 31 mW cm^?2 is eventually attained at 80 °C with anode ionomer content of 9.2% and hot pressing at 130 °C and 6 MPa for 2 min. This power density is 1.72 times of that reported previously with carbon black as the anode electrocatalyst.     

Electrochemical properties and catalyst functions of natural CuFe oxide mineral–LZSDC composite electrolyte

In this study, a new functional composite based on CuFe-oxide mineral (CF) was prepared. This material was first investigated as a novel electrolyte material for low-temperature solid oxide fuel cells (LTSOFCs). The CF and an oxygen ion conducting Li_xZnO–Sm_0.2Ce_0.8O_2?δ (LZSDC) composite were prepared via a solid-phase blending method. The fuel cell device was fabricated by using the CF–LZSDC composite as an electrolyte layer sandwiched between symmetric electrodes of Ni_0.8Co_0.2Al_0.5Li (NCAL) coated Ni foam. The results showed that device performance increased with increasing compaction pressure. When the compaction pressure was 450 MPa, the maximum output power was 637 mW cm^?2, and the lowest ohmic resistance was 0.58 Ω. The electrochemical catalytic activity and the optimal design of LTSOFCs based on the mineral composite materials were investigated.     

Microstructure evolution and mechanical properties of Co coated AISI 441 ferritic stainless steel/ YSZ reactive air brazed joint

This study investigates the microstructure evolution and mechanical properties of bare and Co coated AISI 441 ferritic stainless steel/YSZ ceramic reactive air brazed joints achieved by Ag–CuO braze for Solid Oxide Fuel/Electrolysis Cells applications. Interfacial microstructure of steel/YSZ joints is analyzed by SEM and TEM with EDS. A thick and porous oxide layer rich in Fe, Cr, and Cu is found in bare steel/YSZ ceramic joints, which is induced by the severe oxidation and its intense reaction with CuO during the brazing process. For the coated steel/YSZ ceramic joint, a comparably dense and uniform (Co, Fe, Cu)₃O₄ spinel layer is formed on the steel surface, which is tightly bonded with Ag–CuO braze without visible bonding defects. Meanwhile, the oxidation of steel substrates and its interaction with CuO is significantly suppressed. Co coated steel/YSZ joints possess reliable mechanical properties with the shear strength of 51 MPa, which is 54.5% higher than that of bare steel/YSZ ceramic joints (33 MPa). Besides, the microstructure evolution of coated steel/YSZ ceramic joints during brazing is schematically illustrated by a physical model.     

Effect of aluminizing of Cr-containing ferritic alloys on the seal strength of a novel high-temperature solid oxide fuel cell sealing glass

A novel high-temperature alkaline earth silicate sealing glass was developed for solid oxide fuel cell (SOFC) applications. The glass was used to join two metallic coupons of Cr-containing ferritic stainless steel for seal strength evaluation. In previous work, SrCrO₄ was found to form along the glass/steel interface, which led to severe strength degradation. In the present study, aluminization of the steel surface was investigated as a remedy to minimize or prevent the strontium chromate formation. Three different processes for aluminization were evaluated with Crofer22APU stainless steel: pack cementation, vapor-phase deposition, and aerosol spraying. It was found that pack cementation resulted in a rough surface with occasional cracks in the Al-diffused region. Vapor-phase deposition yielded a smoother surface, but the resulting high Al content increased the coefficient of thermal expansion (CTE), resulting in the failure of joined coupons. Aerosol spraying of an Al-containing salt resulted in the formation of a thin aluminum oxide layer without any surface damage. The room temperature seal strength was evaluated in the as-fired state and in environmentally aged conditions. In contrast to earlier results with uncoated Crofer22APU, the aluminized samples showed no strength degradation even for samples aged in air. Interfacial and chemical compatibility was also investigated. The results showed aluminization to be a viable candidate approach to minimize undesirable chromate formation between alkaline earth silicate sealing glass and Cr-containing interconnect alloys for 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.     

Pr2NiO4-Ag composite as cathode for low temperature solid oxide fuel cells: Effects of silver loading methods and amounts

Active cathode materials for low temperature solid oxide fuel cells (SOFC) below 600 °C are urgently required due to the sluggish oxygen reduction reaction (ORR) kinetics at reduced temperature. In this work, a detailed experimental fabrication and characterization of silver modified Pr₂NiO₄ composite material for low temperature SOFC cathode catalyst with superior ionic conducting ceria-carbonate composite electrolyte was carried out. Pr₂NiO₄ was prepared by a co-precipitation method with NaOH as precipitant, and it was composited with silver to improve the electrode activity toward ORR through three various methods of impregnation, solid-state mixing and freeze-drying, respectively. Effects of Ag loading on the electrochemical activity were systematically investigated. It was found that composite materials originated from impregnation method presented the optimal material microstructure, and 15% Ag loaded composite gave the lowest area specific resistance of 0.45 Ω cm² at 600 °C, which is reduced by around 300% compared with previous work, indicating that impregnated Pr₂NiO₄-15Ag composite is a promising cathode catalyst for low temperature SOFC with ceria-carbonate composite electrolyte.     

Thermal stress analysis of planar solid oxide fuel cell stacks: Effects of sealing design

A three-dimensional multi-cell model based on a prototypical, planar solid oxide fuel cell (pSOFC) stack design using compliant mica-based seal gaskets was constructed in this study to perform comprehensive thermal stress analyses by using a commercial finite element analysis (FEA) code. Effects of the applied assembly load on the thermal stress distribution in the given integrated pSOFC stack with such a compressive sealing design were characterized. A comparison was made with a previous study for a similar comprehensive multi-cell pSOFC stack model but using only a rigid type of glass-ceramic sealant instead. Simulation results indicate that stress distributions in the components such as positive electrode-electrolyte-negative electrode (PEN) plate, PEN-supporting window frame, nickel mesh, and interconnect were mainly governed by the thermal expansion mismatch rather than by the applied compressive load. An applied compressive load of 0.6 MPa could eliminate the bending deformation in the PEN-frame assembly plate leading to a well joined structure. For a greater applied load, the critical stresses in the glass-ceramic and mica sealants were increased to a potential failure level. In this regard, a 0.6 MPa compressive load was considered an optimal assembly load. Changing the seal between the connecting metallic PEN-supporting frame and interconnect from a rigid type of glass-ceramic sealant to a compressive type of mica gasket would significantly influence the thermal stress distribution in the PEN plate. The critical stress in the PEN was favorably decreased at room temperature but considerably increased at operating temperature due to such a change in sealing design. Such differences in the stress distribution could be ascribed to the differences in the constrained conditions at the interfaces of adjacent components under various sealing designs.     

Review: Influence of alloy addition and spinel coatings on Cr-based metallic interconnects of solid oxide fuel cells

Solid oxide fuel cell (SOFC) is the modern eco-friendly technology of fuel cell power generation system. It generates electricity from a redox chemical reaction without producing hazardous gases. It consists of anode, cathode and electrolyte. It is operated in the form of stack connected by interconnects to boost-up power output. The recent development of low-temperature (600 °C–800 °C) brings an opportunity to use metallic interconnects over ceramics. Cr-based metallic interconnects are one of the prominent metallic interconnects. They offer chemical inertness, thermal stability, compatible coefficient of thermal expansion and highly dense structure. However, the Cr-migration towards the cathode side is the major problem in them which adversely affect the SOFCs performance. Therefore a good oxidation resistance without sacrificing electrical conductivity is required. To resolve this issue, several alloying elements and spinel coatings have experimented. These spinel coatings are the thin solid films of Mn, Co, Cu and rare earth metals. This review concluded that the Mn–Co based spinal coating showed excellent performance in reducing the Cr-migration in specially designed expensive Crofer 22 APU interconnect. However, the emerging low-cost ferritic interconnects also show their best results with Cu–Fe based spinel coating. Among them, the SUS-430 interconnect shows the equivalent performance of Crofer 22 APU interconnect after surface treatment and appropriate Cu–Fe based spinel coating. Therefore, it can replace the Crofer 22 APU interconnect on a cost basis.