Development of ceramic fiber reinforced glass ceramic sealants for microtubular solid oxide fuel cells

Ceramic fibers in various forms with different fiber sizes are tested to improve the sealing performance of glass ceramic seals for microtubular solid oxide fuel cell applications. In this regard, several sealing pastes are prepared by mixing each ceramic fibers type with glass ceramics at 1.25 wt %. Five layered microtubular anode supported cells are also fabricated by extrusion and dip coating methods to evaluate the sealing performance of the composite sealants. The pastes are applied between the cells and gas manifolds made of Crofer22 APU. The electrochemical and sealing performances at an operating temperature of 800 °C under hydrogen are investigated after the glass forming process. Microstructures of the sealants are also examined by a scanning electron microscope. Experimental investigations reveal that the cells sealed by the pastes with ceramic bulk fiber and ceramic fiber rope gasket show acceptable open circuit potentials close to the theoretical one. These cells can be also pressurized up to around 150 kPa back pressure in the sealing performance tests. On the other hand, the pastes without any filler, with ceramic rope and with ceramic blanket exhibit poor sealing performance due to gas leakage originated from flowing of the main glass ceramic matrix from the joints. Therefore, ceramic bulk fiber and ceramic fiber rope gasket are found to behave as a stopper and can be used to prevent glass ceramics from flowing for microtubular solid oxide fuel cells or similar applications.     

Fabrication of glass ceramic sealants with ceramic fiber filler for solid oxide fuel cells

In this study, ceramic fibers are used as a filler material for glass ceramic sealant in solid oxide fuel cells to improve the thermal cycle behavior. Beside the bare glass ceramic sealant for comparison, multilayered sealants with different ceramic fiber contents are fabricated to investigate the effect of ceramic fiber quantity also. The mechanical performances of the samples are measured via tensile tests by placing them between two metallic interconnector plates after the glass formation process as well as after 1, 5 and 10 thermal cycles. The results show that the mechanical strength in general tends to decrease with increasing the ceramic filler content, which can be attributed to poor adhesion due to reduced glass ceramic composition. On the other hand, thermal cycle behavior of the samples with ceramic fibers is found to be improved at some extend. This may be due to the behavior of ceramic filler network and relatively slow crystallization with increasing the amount of the filler as proven by microstructural observations. Especially for the sample including 4 ceramic fiber interlayers each having 0.030 g ceramic fibers, the mechanical strength shows an increasing trend with the number of thermal cycles.     

Shear strength tests of glass ceramic sealant for solid oxide fuel cells applications

Different approaches are used for the integration of ceramic components in solid oxide fuel cells stacks, where dissimilar materials (ceramics and metals) have to be joined and coupled for a reliable long term operation. This work focuses on the mechanical characterisation of a glass ceramic sealant used for the joining of Crofer22APU metallic interconnect samples as well as the interaction with a preoxidised Crofer22APU. Crofer22APU–glass ceramic sealant joined samples are tested by two different mechanical tests. Hourglass samples with different geometries were tested using an in-house developed torsion test machine at room temperature. In addition, their mechanical strength was also evaluated according to the ISO?13124 standard. The comparison of the two different testing methods, with particular focus on the shear strength of the joined samples, are reviewed and discussed.     

Processing of Foturan® glass ceramic substrates for micro-solid oxide fuel cells

The microfabrication of Foturan^® glass ceramic as a potential substrate material for micro-solid oxide fuel cells (micro-SOFC) was investigated. Foturan^® was etched in 10% aqueous hydrofluoric (HF) acid solution at 25 °C with a linear rate of 22 ± 1.7 μm/min to create structures with an aspect ratio of 1:1 in 500 μm-thick Foturan^® substrates for micro-SOFCs. The concentration of the HF etchant was found to influence the etching rate, whereas the UV-exposure time creating nuclei in the glass for subsequent crystallization of the amorphous Foturan^® material had no significant influence on the etching rates. The surface roughness of the crystallized Foturan^® was determined by the crystallite size in the order of 10–15 μm. Free-standing micro-SOFC membranes consisting of a thin film Pt cathode, an yttria-stabilized-zirconia electrolyte and a Pt anode were released by HF etching of the Foturan^® substrate. An open-circuit voltage of 0.57 V and a maximum power density of 209 mW/cm² at 550 °C were achieved.     

Effect of nickel doping on structure and suppressing boron volatility of borosilicate glass sealants in solid oxide fuel cells

The high volatility of boron from borosilicate glass sealants often leads to boron deposition and poisoning of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode, presenting a challenge for the development of reliable solid oxide fuel cells (SOFCs). In this paper, we report that boron volatilization from borosilicate glass at 700 °C can be significantly suppressed by appropriate NiO dopant, mainly due to the increase of Si-O-B linkages in the combining B-O^? and Si-O^? network. Also, the formation of boron-containing phase in NiO-doping glass-ceramics has been studied, which suppresses the reaction between glass and LSCF cathode after heat treatment at 700 °C for 1000 h. Moreover, the change of crystalline phases leads to an improvement in thermal and electrical properties. We believe that our findings will open a new way for the design and development of the reliable sealing glass for SOFCs applications.     

Co-extrusion of multilayered ceramic micro-tubes for use as solid oxide fuel cells

Current methods to manufacture tubular solid oxide fuel cells (SOFCs) involve multiple steps of extrusion, layer deposition and sintering, leading to high manufacturing costs. The aim of the work presented in this paper is to reduce the cost of manufacturing SOFCs. This is achieved by developing a method for manufacturing a five-layered micro-tubular structure by a multi-billet co-extrusion process. With the implementation of continuous screw extrusion equipment, this co-extrusion process could easily be adapted into a fully continuous manufacturing process.The co-extrusion process presented initially involves rheologically unifying five pastes made up of individual powder compositions. It is shown that it is possible to formulate the pastes with in an optimum solids loading region where the die land rheological properties are relatively insensitive to small variations in solids loading, thus allowing for a more stable process.These pastes are then extruded as billets from separate extrusion barrels through a single nozzle. This uses a novel die design which does not require the use of a central mandrel to form the tubular structure. The sintered structure comprises four Ni/YSZ anode layers and a YSZ electrolyte layer, each layer being approximately 60 μm thick, forming a tube with an outer diameter of 3 mm and an inner diameter of 2.4 mm.     

Crack formation in ceramic films used in solid oxide fuel cells

The manufacture of solid oxide fuel cells (SOFCs) involves fabrication of a multilayer ceramic structure, for which constrained sintering is a key processing step in many cases. Defects are often observed in the sintered structure, but their formation during sintering is not well understood. In this work, various ceramic films were fabricated by screen printing and a variety of defects observed. Some films showed “mud-cracking” defects, whereas others presented distributed large pores. “Mud cracking” defects were found to originate from a network of fine cracks present in the green film and formed during drying and binder burn-out. Control of these early stages is essential for producing crack-free films. In order to investigate how defects evolve during sintering, artificial cracks were introduced in the green films using indentation. It was observed that crack opening always increased during constrained sintering. In contrast, similar initial cracks could be closed and healed during co-sintering.     

High gas tightness ZrO2-added silicate glass sealant with low thermal stress for solid oxide fuel cells

A low leakage rate sealant of 10 wt% ZrO₂-added CaO–K₂O–Na₂O–BaO silicate glass for SOFC has been studied. The structure of the sealant is stable at high temperatures with leakage rates less than 10⁻⁴ sccm∙cm⁻¹, and no crystal except for ZrO₂ is found in XRD analysis after heating at 800 °C for 100 h. ZrO₂ is distributed in the glass matrix and plays a supporting role in avoiding over-softening at operating temperature. Good compatibility in both oxidizing and reducing atmospheres between the sealant and SUS430 interconnect was proved by SEM at 750 °C for 100 h. A fully coupled 3D Multiphysics button SOFC is constructed for mechanical analyses. The results show that the increase of ZrO₂ in the sealant will decrease the stress and displacement in the SOFC. Besides, the width of the sealant also affects the stress value and distribution. The results show that GZ10 is a competitive sealing material compared with other ZrO₂-added sealants.     

Tuning ORR electrocatalytic functionalities in CGFO-GDC composite cathode for low-temperature solid oxide fuel cells

Mixed ionic and electronic conduction (MIEC) in the composite cathode can alter oxygen stoichiometry and other physiochemical properties, eventually promoting the electrocatalytic functionalities for oxygen reduction reaction (ORR) at low operational temperatures (<650 °C). Here, we demonstrate a composite cathode of CoGd_0.8Fe_1.80O₄ /Gd_0.10Ce_0.9O_2?δ (CGFO-GDC), which delivers low electrode polarization resistance of 0.60 Ω cm² at 550 °C. The best-performing sample CGFO-GDC exhbits the peak power density (PPD) of 611-343 mW cm^?2 at 550-470 °C under a fuel cell conditions. Moreover, durability measurement verifies CGFO-GDC as a chemically stable cathode with improved ORR catalytic functionality. Additionally, first principle calculations using density function theory (DFT) were also conducted to analyze the ion diffusion mechanism of fabricated CGFO-GDC cathode. Our findings certify that introducing ionic conducting GDC into CGFO sample improves the catalytic functionalities. As a result, the composite CGFO-GDC based SOFC delivers minimum electrode polarization resistance with improved power output owing to its enhanced oxygen vacancies and fast catalytic reactions at 550 °C.     

Synthesis and long-term test of borosilicate-based sealing glass for solid oxide fuel cells

A series of borosilicate-based glasses, ternary BaO-SiO₂-B₂O₃ and quaternary BaO-SiO₂-B₂O₃-Al₂O₃ systems, are prepared as sealing materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion, crystalline phases, glass forming ability, and thermo-chemical stabilities of the glasses are characterized. Additionally, the effect of the B₂O₃/SiO₂ ratio and Al₂O₃ and BaO contents on the coefficient of thermal expansion (CTE) are discussed and compared. Test results show that one glass (G6) can fully wet various substrates at the sealing temperature of 1000 °C and match thermal expansion. Possible interfacial reactions between the glass and those cell components aging up to 5000 h are investigated by element mapping, XRD, SEM, and EPMA. Leakage testing was also performed at temperatures up to 650 °C. The results show that the glass (G6) remains amorphous after 5000 h test and is stable under these conditions and compatible with the other fuel cell components.     

Efficient and robust ceramic interconnects based on a mixed-cation perovskite for solid oxide fuel cells

To achieve a required output voltage, a solid oxide fuel cell (SOFC) stack has multiple unit cells connected in series through interconnects (ICs). A dual-layer IC film comprising p- and n-type conducting perovskite oxides is considered to be a promising design in terms of electrical properties and stability; however, its fabrication is materials-intensive and time-consuming. Here, we propose an advanced design of a single-layer IC film inspired by the interfacial nature of the dual-layer IC film. Based on an understanding of cross-diffusion phenomena across the interface between Sr_0.7La_0.2TiO₃ and La_0.8Sr_0.2MnO₃ during the co-sintering process revealed by secondary ion mass spectroscopy, we design and synthesize mixed-cation perovskite oxides, (Sr_0.6La_0.4)_0.9(Ti_1–xMn_x)O₃ (SLTM), for IC applications in SOFCs. Among various compositions, Ti-rich SLTM (x?=?0.25) shows high sinterability, leading to the formation of a thin, gas-tight IC film on a porous anode support via simple screen printing and sintering. When exposed to both reducing and oxidizing atmospheres, the fabricated SLTM-IC film is highly conductive and remains stable during continuous operation. The ceramic IC design proposed in this work may provide great benefits in terms of reduced costs of IC materials and fabrication, while maintaining the IC performance required for SOFCs.     

Physicochemical properties of Ni-loaded yttrium stabilized zirconia nanotubes for solid oxide fuel cells

Yttrium-stabilized zirconia nanotubes (YSZNTs) were prepared using a conventional hydrothermal method, and their characteristics were compared with those of yttrium-stabilized zirconia nanoparticles (YSZNPs) synthesized in this study and with those of commercial YSZNPs (CYSZNPs). YSZNTs had widths and lengths of 20–30 nm and 100–700 nm respectively. The electrical conductivity of NiO (60.0 wt%)-loaded YSZNTs (40.0 wt%) was higher than those of NiO/YSZNPs and NiO/CYSZNPs at the same NiO loading. The zeta-potentials of YSZNTs in aqueous solution, determined by electrophoretic light scattering (ELS), indicated high positive surface charges at lower pH values, which is known to be related to surface stability, but negative values at high pH. The results of cyclic voltammetry (CV) and H₂-temperature-programmed reduction (H₂-TPR) confirmed that NiO(60.0 wt%)/YSZNTs (E_red = −0.445 mV) were more reduced than NiO/YSZNPs (E_red = −0.517 mV) and NiO/CYSZNPs (E_red = −0.516 mV).     

Performance of Al2O3 particle reinforced glass-based seals in planar solid oxide fuel cells

Glass-based materials are usually considered as excellent seals for jointing adjacent components in planar solid oxide fuel cells, but the uncontrollable crystallization in the glass may cause delamination and micro-cracks in such seals. To solve this problem, Al2O3 ceramic particles were added to a BaO–CaO–Al2O3–B2O3–SiO2 glass system to reduce negative effects caused by crystalline phase on the gas tightness and the joint strength in the seals. At an operating temperature of 750 °C, the glass-based seals with 20 wt% Al2O3 addition (GA80) exhibited extremely low leakage rates (~0.002 sccm/cm under an input gas pressure of 13.6 kPa) and higher shear strength (3.31 MPa). The Al2O3 ceramic addition and the crystalline phase BaAl2Si2O8 reinforced the glass matrix. Further thermal cycle analyses indicated that leakage rates for the GA80 seals remained at around 0.0025 sccm/cm after 10 thermal cycles, which was consistent with minor microstructural change and good interface bonding. Single cell testing with of GA80 seals was performed and the results demonstrated stable electrochemical performance through 6 thermal cycles at an open circuit voltage of 1.16–1.18 V, as well as a power density above 546 mW/cm2 at a current density of 925 mA/cm2. These results showed the high thermal cycle stability of the glass/Al2O3 composite seals in intermediate temperature planar solid oxide fuel cells.     

Laboratory techniques for evaluating solid oxide fuel cells

The construction of a small solid oxide fuel cell laboratory is described in terms of required materials, measuring techniques and equipment design. Details of various electrode deposition techniques and ways of making contacts to the electrodes are also outlined.     

Bi-layer glass-ceramic sealant for solid oxide fuel cells

A bi-layered concept of glass-ceramic (GC) sealant is proposed to overcome the challenges being faced by solid oxide fuel cells’ (SOFCs). Two separated layers composed of glasses (Gd-0.3 and Sr-0.3) were prepared and deposited onto interconnect materials using a tape casting approach. After heat treating the bi-layered structure at 850 °C for 1–100 h, smooth and void free interfaces over the entire cross-section of joint were obtained. Micro-Raman analysis confirmed the presence of a higher amount of residual glassy phase in Gd-0.3 in comparison to Sr-0.3. The bi-layered GC showed good wetting and bonding ability to the Crofer22APU metallic plate. Slight increase of electrical conductivity with increasing annealing time was observed due to partial crystallization of the glass, but the overall conductivity levels of GC bi-layers were low enough to grant good electrical insulation. This set of relevant properties makes the investigated bi-layered sealants suitable for SOFC applications.     

Interconnect materials for next-generation solid oxide fuel cells

Highly-efficient solid oxide fuel cell (SOFC) systems are gaining increased attention for future energy conversion applications. Many planar SOFC stack designs utilise ferritic stainless steel (FSS) interconnect components. During operation, surface corrosion of FSS interconnects degrades stack operation by increasing electrical resistance and introducing other deleterious material interactions. To minimise these effects, various surface modifications and coatings are currently under investigation. Two of these methods under development for this application are: metal organic chemical vapour deposition (MO-CVD); and, large area filtered arc deposition (LAFAD). SOFC interconnect-relevant corrosion behaviour of an MO-CVD coating on Crofer 22 APU, AL453, Fe30Cr and Haynes230, and complex, amorphous LAFAD AlCrCoMnTiYO coatings on FSS 430 were investigated. Both of these surface modifications and coatings exhibit significantly improved corrosion protection as compared with uncoated FSS samples.     

A support layer for solid oxide fuel cells

The use of a perforated, ceramic layer, laminated on top of a porous electrode, as a support structure for solid oxide fuel cells is described. The support layer can be used on either the anode or cathode side and allows for the fabrication of mechanically robust, impregnated electrodes. Using fuel cells based on yttria-stabilized zirconia (YSZ), we demonstrate that the support layer can be made from YSZ or from a composite of YSZ and La_0.3Sr_0.7TiO₃ (LST). It is shown that there is no solid-state reaction between LST and YSZ, even after calcination at 1550 °C. Unlike pure LST, LST–YSZ composites can form a laminated structure on porous YSZ. Unlike pure YSZ, a dense LST–YSZ composite with 60-wt% LST exhibited an electronic conductivity of 7.6 S/cm at 700 °C after reduction in H₂ at 1000 °C, making the composite layer attractive for current collection in anode-supported cells.     

Tuning interfacial chemistry and electrochemical properties of solid oxide cells via cation interdiffusion

Electrochemical performance of solid oxide cells is sensitive to electrode/electrolyte interfacial chemistry. Herein, taking SmBaCoCuO_5+δ (SBCC) oxygen electrode and Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte as an example, the role of cation interdiffusion in tuning interfacial chemistry and electrochemical properties of solid oxide cells was studied. An increase in cation diffusion between SBCC and GDC layers results in bigger and better-connected SBCC particles with slight shrinkage in unit cell and denser GDC layer with distorted crystal structure. Cation diffusion is also responsible for abnormal deviation in electrochemical impedance spectroscopy that can be used to characterize catalytic properties of the electrode for oxygen redox reactions in SBCC/GDC/SBCC symmetric cell. The internal mechanism that contributes to this phenomenon was revealed through a series of well-designed tests and theoretical modeling. Single cell, SBCC/GDC/GDC-NiO, has high cation interdiffusion, works well in both solid oxide fuel cell and solid oxide electrolysis cell model; it also exhibits decreased catalytic activity and open-circuit voltage. This work provides helpful guides for the design of advanced solid oxide cells and other related devices.