Behaviour of 54.4SiO2-13.7Na2O-1.7K2O-5.0CaO-12.4MgO-0.6Y2O3-11.3Al2O3-0.9B2O3 HT-SOFC glass sealant under oxidising and reducing atmospheres

The behaviour of the promising glass sealant 54.4SiO₂-13.7Na₂O-1.7K₂O-5.0CaO-12.4MgO-0.6Y₂O₃-11.3Al₂O₃-0.9B₂O₃ for solid oxide fuel cells (SOFCs) under SOFC operating conditions was studied. First, the kinetics of the crystallisation processes at the operating temperature (850 °C) was discussed (maximum exposure time of 1000 h), and the effect of crystallisation on the coefficient of thermal expansion (CTE) of the sealant was studied. Furthermore, the degradation processes at the interface of the glass sealant and functional SOFC materials (Crofer 22 APU, YSZ, and NiO(Ni)-YSZ) during exposure to 850 °C in oxidising and reducing atmospheres for 500 h were studied. The tests demonstrated good performance of the sealant studied and possibility of its application in SOFCs.     

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.     

Low leakage rate of silicate glass modified with Al2O3 for solid oxide fuel cell

The gas tightness of glass sealing materials is a big challenge for the solid oxide fuel cell (SOFC) stacks operating at high temperature. Thermal, sintering, crystallization behavior and gas tightness properties of the glass-based with two different Al₂O₃ contents sealants are evaluated and discussed. The study showed that the sealants avoid cracks at the interface on NiO-YSZ (NiO-yttria stabilized zirconia) and SUS430 stainless steel interconnect substrates. The Al₂O₃ embedded in the glass matrix as a second phase, and promoted crystallization of K[AlSi₃O₈] at the early stage. This may because some ultrafine Al₂O₃ particles whose structure is destroyed by prolonged high temperature treatment according XRD and TEM analysis. Especially, the sealant containing 5 wt% Al₂O₃ undergoes a thermal cycle and maintains a stable leakage rate below 10^?4 sccm?cm^?1 for about 1000 h at 750 °C. The above results prove the possibility of using the Al₂O₃-doped sealing glass for SOFC stacks.     

Creep rupture of the joint between a glass-ceramic sealant and lanthanum strontium manganite-coated ferritic stainless steel interconnect for solid oxide fuel cells

Creep rupture is investigated at 800?°C of a joint between a glass-ceramic sealant and a ferritic stainless steel interconnect coated with lanthanum strontium manganite for solid oxide fuel cell application. Results reveal the shear and tensile creep strength of the as-joined, non-aged joint at a rupture time of 1000?h is about 42% and 3% of the average shear and tensile bonding strength, respectively. A thermal aging of 1000?h at 800?°C enhances the creep strength. For both non-aged shear and tensile specimens with a short creep rupture time, fracture mainly takes place in an oxyapatite interlayer which is formed in the joining process. For a medium creep rupture time, fracture site changes to a mixed BaCrO₄/oxyapatite layer. Oxyapatite and BaCrO₄ dominate the creep failure mechanism for 1000?h-aged shear specimens, while (Cr,Mn)₃O₄ spinel plays a role in the creep failure of 1000?h-aged tensile specimens.     

Development of La(CrCoFeNi)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells

The purpose of this research is to develop interconnect and cathode materials for use in solid oxide fuel cells (SOFCs) which demonstrate desired properties of outstanding sintering properties, high electrical conductivity, and excellent chemical stability at high temperatures. Five different perovskite oxides of lanthanum in combination with chromium, iron, cobalt and nickel oxides powders, i.e. LaCr_0.7Co_0.1Fe_0.1Ni_0.1O₃(LCr7CFN), LaCo_0.7Cr_0.1Fe_0.1 Ni_0.1O₃(LCo7CFN), LaFe_0.7Cr_0.1Co_0.1Ni_0.1O₃(LFe7CCN), LaNi_0.7Cr_0.1Co_0.1Fe_0.1O₃(LNi7CCF), and LaCr_0.25Co_0.25Fe_0.25Ni_0.25O₃(LCCFN), were synthesized through the Pechini method. XRD results show that all materials are in single phase, either rhombohedral or orthorhombic crystal structure. The resulting powders were able to be sintered to a high relative density at a temperature of 1400 °C for 2 h in air. The electrical conductivity of the sintered sample was measured and evaluated from 300 °C to 800 °C. The LCCFN sample appears to have the best combination of sintering property (approximate 94% relative density) and electrical conductivity (88.13 Scm⁻¹ at 800 °C).     

Design and development of SiO2-Al2O3-B2O3-Na2O based glass sealant for the glass-metal joint

SiO₂-Al₂O₃-B₂O₃-Na₂O based glass sealant compositions were designed for the development of GMJ using extreme vertices methodology. The effect of the glass sealant individual constituent on wettability, Glass transition temperature (T_g), Crystallization temperature (T_c), thermal conductivity, and density was analyzed with the help of analysis of variance technique (ANOVA). The increase in SiO₂ constituent in glass sealant decreased the spreading area (mm²) of glass sealant and increased the value of contact angle (θ) formed by glass sealant over the SS304 metal substrate. The T_g and T_c of glass sealant also increased as SiO₂ content increased. The addition of B₂O₃ and Na₂O in glass sealant system reduced the T_g and T_c of glass sealant. The developed regression model was validated and it was observed that the experimental and predicted results nearly matched with an error less than 5% in most of the cases.     

Reducing the reaction between boron-containing sealing glass-ceramics and lanthanum-containing cathode: Effect of Bi2O3

The volatile boron from boron-containing sealing materials often reacts with lanthanum-containing cathode, leading to the formation of LaBO₃ and consequently significant degradation of cathode. The reaction between boron-containing sealing glass-ceramics and lanthanum-containing cathode thus presents a challenge for the development of solid oxide fuel cell (SOFC). Here we report for the first time that such a reaction can be significantly reduced by Bi₂O₃ dopant in sealing glass-ceramics. In particular, the formation of LaBO₃ can be prohibited in reaction couple between glass containing 9 mol.% Bi₂O₃ and lanthanum strontium cobalt ferrite (LSCF) cathode. The addition of Bi₂O₃ enhances the [BO₃] → [BO₄] transition in glass structure and therefore improves the thermal stability of boron species in glass matrix. In addition, Bi₂O₃ dopant also favors the formation of BiBO₃, which dramatically reduces boron volatility from sealing glass-ceramics. The reported results provide an effective approach for solving the sealing challenge.     

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

In order to simulate the contact situation of interconnect/contact layer/cathode in SOFC stacks, contact resistance and chemical compatibility of LaNi_0.6Co_0.4O_3–δ (LNC) as contact layer between Crofer22APU interconnect and La_0.6Sr_0.4FeO₃ (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.     

Correlation between yttria stabilized zirconia particle size and morphological properties of NiO–YSZ films prepared by spray coating process

A systematic approach was taken to investigate the morphology of NiO–yttria stabilized zirconia (YSZ) films deposited by a spray coating process. The final morphological aspects of anode films were influenced by the particle size of YSZ powders and the milling time of the slurries used for film deposition. YSZ powders with average particle size of 17 and 52 nm were obtained from powders calcined at 800 and 1000 °C, respectively. The results obtained by rheological studies pointed out that slurries prepared from YSZ powders calcinated at 1000 °C and milling time of 20 h had more stability. All slurries presented thixotropic and pseudoplastic behaviors.     

不同硫化氢流率与浓度对硫化氢固体氧化物燃料电池性影响

A solid state H2S/air electrochemical cell having the configuration of H2S, (MoS2 NiS Ag)/YSZ/Pt, air has been examined with different H2S flow rates and concentrations at atmospheric pressure and 750-850 ℃. Performance of the fuel cell was dependent on anode compartment H2S flow rate and concentration. The cell open-circuit voltage increased with increasing H2S flow rate. It was found that increasing both H2S flow rate and H2S concentration improved current-voltage and power density performance. This is resulted from improved gas diffusion in anode and increased concentration of anodic electroactive species. Operation at elevated H2S concentration improved the cell performance at a given gas flow rate. However, as low as 5% H2S in gas mixture can also be utilized as fuel feed to cells. Highest current and power densities, 17500mA·cm-2 and 200mW·cm-2, are obtained with pure H2S flow rate of 50ml·min-1 and air flow rate of 100ml·min-1 at 850℃.     

Microstructure evolution and bonding strength of the Al2O3/Al2O3 interface brazed via Ni-Ti intermetallic phases

In this study, Al₂O₃ workpieces were vacuum brazed by using Ni-45Ti binary alloy. The interfacial microstructure evolution of the joints obtained at different brazing temperatures was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The bonding strength of the joints was measured by shear testing. The results indicated that Ni₂Ti₄O and AlNi₂Ti were the main reaction products in the joint areas. Moreover, the Ti₂Ni intermetallic compound formed in the brazing seam. The typical layer structure of the brazed joints was Al₂O₃/AlNi₂Ti/Ni₂Ti₄O/Ti₂Ni + NiTi/Ni₂Ti₄O/AlNi₂Ti/Al₂O₃. With the brazing temperature increasing, the thickness of the Ni₂Ti₄O reaction layer adjacent to the Al₂O₃ substrate increased significantly, while the AlNi₂Ti phase had a tendency to dissolve with the brazing temperature increasing. The mechanism for the microstructure evolution was also discussed. The maximum shear strength of 125.63±4.87 MPa of the joints was obtained when brazed at 1350 °C for 30min. The fracture occurred hardly in the interface between Al₂O₃ and Ni-45Ti filler alloy.     

Effects of Nb2O5 and Gd2O3 doping on boron volatility and activity between glass seals and lanthanum-containing cathode

In planar Solid Oxide Fuel Cells (SOFCs), the boron species volatilize from glass seals, and react with lanthanum-containing cathodes (i.e., La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ, LSCF) to form LaBO₃ under cathodic polarization, which decomposes the perovskite structure and consequently decreases the electrochemical activity of cathode. In this study, Nb₂O₅ and Gd₂O₃ are added to an aluminoborosilicate glass to reduce the boron volatility from glass and the reaction between sealing glass and LSCF cathode. Both Nb₂O₅ and Gd₂O₃ doping increases the network connectivity, but Nb₂O₅ doping enhances the [BO₃] → [BO₄] transition and reduces the boron volatility from glass seals, thus effectively suppressing the deposition and poisoning of boron contaminants on the LSCF cathode. However, an obvious degradation of the electrocatalytic activity of LSCF occurs in the presence of Gd₂O₃-doped glass. The relationship between glass structure and glass/cathode interaction has been established to provide useful information for designing stable sealing materials for SOFC applications.     

Enhanced sintering behavior of LSGM electrolyte and its performance for solid oxide fuel cells deposited by vacuum cold spray

How to obtain dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte at low sintering temperature (<1300 °C) is a challenge to improve solid oxide fuel cell (SOFC) performance at intermediate operation temperature. In this study, a double-layer design method for vacuum cold spray (VCS) prepared-LSGM electrolyte assisted with two-step sintering at a low temperature was proposed. The sintering behavior of VCS deposited LSGM layers at 1200 °C was investigated. The LSGM layers became denser in most regions except the appearance of some cracks. Subsequently, the effect of a second LSGM layer on the sintered top layer was studied to block cracks. Results showed that the co-sintered layer with a thickness of approximately 5 μm presented a maximum open circuit voltage of ∼0.956 V at 650 °C and a maximum power density of 592 mW/cm² at 750 °C. Result indicates that the sintering assisted VCS is a promising method to prepare the LSGM electrolyte applied in intermediate temperature SOFCs.     

Anti-CMAS corrosion mechanism of Al2O3 pore sealing and laser remelting on thermal barrier coatings

The sol-dip-coating method and surface laser remelting technology are applied to form an Al₂O₃ layer on a YSZ coating surface to effectively block the environmental sediment CMAS. The behaviour and mechanism for CMAS corrosion of the coating are investigated, and the interfacial reliability of the coating and matrix is calculated by using the Monte Carlo method. The bonding force between the Al₂O₃ sol and YSZ coating can be effectively improved by laser surface treatment. Samples subjected to a laser pretreatment and posttreatment (YL-AL) of the YSZ coating are found to show the best interfacial bonding strength between Al₂O₃ and YSZ. Furthermore, the YL-AL sample shows a higher CMAS resistance than the laser posttreatment (Y-AL) samples, which effectively combines the chemical resistance of Al₂O₃ to CMAS and the physical resistance of the laser re-melted densification layer against CMAS penetration.     

Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi₂O₃ sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi₂O₃ sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi₂O₃ was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi₂O₃/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).     

Potential of pyrochlore structure materials in solid oxide fuel cell applications

Pyrochlore structure material (A₂B₂O₇) has gained interest in diverse applications like catalysis, nuclear waste encapsulation, sensors, and various electronic devices due to the unique crystal structure, electrical property, and thermal stability. This review deals with the ionic/electronic conductivity of numerous pyrochlore structure materials (titanates, zirconates, hafnates, stannates, niobates, ruthenates, and tantalite based pyrochlore) as electrolyte and electrode materials for solid oxide fuel cells (SOFCs). The impact of cation radius ratio (r_A/r_B) on the lattice constant and oxygen ‘x’ parameter of different pyrochlore structure materials obtained by various synthesis methods are reported. Higher ionic conductivity is essential for better ion transport in an electrolyte, and mixed ionic and electronic conductivity in electrode is essential for attaining higher efficiency in a typical SOFC. Gd_xTi₂O_7-δ, Gd_2-xCa_xTi₂O_7-δ, Nd_2-yGd_yZr₂O₇, Y₂Zr₂O₇, Y₂Zr_2-xMn_xO_7-δ, SmDy_1-xMg_xZr₂O_7-x/2, Gd_2-xCa_xTi₂O_7-δ pyrochlore are reported as electrolytes for fuel cell applications. Some pyrochlore material (La_2-xCa_xZr₂O₇, Sm_2-xM_xTi₂O₇ (M = Mg, Co, and Ni) pyrochlore) shows protonic conductivity at lower temperatures and ionic conductivity at higher temperature condition. Also, the mixed ionic-electronic conductivity behavior is reported in electrode materials for SOFC such as R₂MnTiO₇ (R = Er and Y), R₂MnRuO₇ (R = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), R₂Ru₂O₇ (R = Bi, Pb and Y), Y_2-xPr_xRu₂O₇, Ni-(Gd_0.9Ca_0.1)₂Ti₂O_7-δ, (Gd_0.9Ca_0.1)₂Ti₂O_7-δ, Gd₂(Ti_0.8Ru_0.2)₂O_7-δ, (Sm_0.9Ca_0.1)₂Ti₂O_7-δ and (Y_0.9Ca_0.1)₂Ti₂O_7-δ pyrochlore. The detailed study of the electronic behavior of these pyrochlore system confirms the necessity of defect structure with high oxygen mobility, lower activation energy, ionic radii ratio criterion should satisfy, and possess notable ion-ion interaction. Ionic conductivity in pyrochlore is increased by enhancing the oxygen migration through 48f-48f site with the formation of oxygen vacancy. Vacancy formation can be achieved by adding a suitable dopant that creates oxygen vacancy by charge compensation mechanism or as anion Frenkel defects. Similarly, the electrical conductivity is improved while adding suitable dopant (Ce, Pr, Ru, etc.) due to disordered structure and anti-Frenkel defect formation which leads to oxygen vacancy formation and thus improves conductivity.     

Creep behaviour of porous SOFC electrodes: Measurement and application to Ni-8YSZ cermets

A methodology is proposed in this study to investigate the creep properties of porous Ni-8YSZ cermet. Creep experiments have been conducted under reducing atmosphere at the typical SOFC operating temperatures. Specimens have been loaded in a four-point bending test bench. A special attention has been paid in this work to the analytical and numerical modelling of the mechanical test. It has been highlighted that Ni-8YSZ exhibits substantial creep strain rates even at relative low temperatures (700 °C < T < 850 °C). The creep exponent has been found to be just slightly higher than unity (1 < n < 2) while the activation energy has been determined equal to Q = 115 kJ mol⁻¹.High-temperature plastic strains of both Ni and 8YSZ phases have been estimated through the local stress acting on the cermet particles. This analysis indicates that creep behaviour of the Ni-8YSZ composite is not influenced by the metallic phase, but is controlled by the deformation of the 8YSZ matrix. It is also proposed that cermet creep mechanism involves Zr⁴⁺cations diffusion at the surface rather than in the bulk of the 8YSZ material.Impact of the Ni-8YSZ cermet creep on the internal stresses distribution in SOFC is discussed considering the anode supported cell (ASC) design. It is shown that cermet creep strain can induce a substantial stress decrease in the thin electrolyte.     

Fabrication of hierarchical lattice structures from zirconia stabilized ceramics by micro-SLA 3D printing approach

The need for the new materials and advanced manufacturing techniques for achieving the highest characteristics of energy devices is an obvious trend. One of the possible ways to create structures for energy applications is to introduce complex geometries and promising features with additive manufacturing (AM). Using this approach, it is possible to create complex geometry and moreover decrease the weight of materials. In this paper, we developed the layer-by-layer fabrication approach of ceramic hierarchical lattice structures via the (micro-SLA) technology. As a feedstock material, the novel composition of partly stabilized-zirconia ceramics (6ScSZ, 8YSZ) was developed. Materials were selected as a solid slurry component due to high ionic conductivity at the working temperature of modern solid oxide fuel cells (SOFCs). For the sintering, the green body heat treatment process was optimized to one step, which decrease the time and production cost. The data from scanning electron microscopy and micro-CT shows that 112-layer samples of the octet truss did not show any critical defects, and the achieved relative density was close to the theoretical one. Totally, 22 samples with the total size of 6.5 mm * 6.5 mm * 2.8 mm and the diameter of struts in the range of 240–250 μm were fabricated at a rate of only 56 min per sample, using two modifications of advanced doped zirconia-ceramics. This study opens new opportunities for the development and transfer to the production of additive manufacturing of ceramics to build energy systems devices such as solid oxide fuel cells.     

The properties of Al2O3 coated fine‐grain temperature stable BaTiO3‐based ceramics sintered in reducing atmosphere

Chemical coating, an effective doping modification method, was employed to fabricate fine‐grain BaTiO₃‐based ceramics. Based on the consideration of subsequently using base metal as inner electrodes in multilayer ceramic devices, green bodies are generally sintered in reducing atmosphere, which generates more charged point defects and thus affects the electric properties. According to the elements distribution analysis, Al element is greatly enriched in the grain boundary and shell region. Coating Al₂O₃ achieves not only a smaller grain size and narrower distribution but also a higher breakdown strength, discharge energy density and energy efficiency at ambient temperature. In addition, temperature dependences of dielectric and energy storage properties under a same field were also investigated. Over the whole measuring temperature range, the sample with Al₂O₃ remains higher discharge energy density and energy efficiency.     

SiO2/Al2O3 ratio dependence of microstructures and dielectric properties in barium strontium titanate glass ceramics

(Ba, Sr)TiO₃-Al₂O₃-SiO₂ glass ceramic system with various SiO₂/Al₂O₃ ratios was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), dielectric spectroscopy and impedance spectroscopy. The XRD results demonstrated that the proper SiO₂/Al₂O₃ ratio could promote the crystallization of the major crystalline phase from the glass matrix. The dielectric property investigations showed that the dielectric constant passes through a maximum value while the dielectric breakdown strength has a minimum value with increasing SiO₂/Al₂O₃ ratio. Meanwhile, the complex impedance analyses suggest the resistance of the glass-crystal interface rapidly decreases and the capacitance of the crystal slightly decreases with the increase of SiO₂/Al₂O₃ ratio. The relaxation mechanisms of the (Ba, Sr)TiO₃ glass ceramics changed from localized relaxation to long range conductivity as the SiO₂/Al₂O₃ ratio was increased from 1.43 to 1.83. The variations in the dielectric response and the activation energy of the glass-crystal interface in the (Ba, Sr)TiO₃ glass ceramics with the ratio of 2.40 could be attributed to the crystallization of fresnoite phase.