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.     

Improving sealing performance of borosilicate glass-ceramics for solid oxide fuel cell applications: Effect of AlN

Glass and glass-ceramic are one of the key sealing materials for solid oxide fuel cells (SOFCs) and they need to meet stringent requirements for long-term operation at high temperatures. Here, we report for the first time the incorporation of aluminum nitride (AlN) dopant into borosilicate glasses and glass-ceramics so as to tailor their basic properties and sealing performance. The results show the AlN-doped glass-ceramics exhibit remarkably enhanced thermal stability and chemical compatibility when adhering to Y₂O₃-ZrO₂ electrolyte. The electrical conductivity is also significantly reduced by the AlN doping, and the conductivity of 15 wt.% AlN-doped glass-ceramic is nearly two orders of magnitude lower than that of the undoped glass-ceramic. This work indicates that AlN doping is an effective strategy to obtain a reliable borosilicate glass-ceramics for SOFCs.     

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.     

Room and elevated temperature shear strength of sealants for solid oxide fuel cells

The shear strength of glass-ceramic sealants was measured in torsion to aid the developments of robust sealants that can withstand the combined complex stress situation typical for solid oxide fuel cell stacks. The specimens consisted of hour-glass-shaped samples, with the sealant layer in typical application relevant thickness between two steel plates. Partially crystallized sealant materials with either silver particles or 8YSZ fibers as filler material were tested and compared with respect to their shear strength and failure behavior at room and elevated temperatures up to 800 °C. The results emphasize the importance of interfacial bonding as well the effect of creep properties of sealant and steel substrates onto the testing results. An outlook on improvements of testing procedure and specimens’ geometry is given.     

Effects of solid loading on joining and thermal cycling performance of glass-ceramic sealing pastes for solid oxide fuel cells

The variation of the joining performance of glass-ceramic sealants in the form of a paste as a function of the solid powder content in the sealing paste after the formation and a number of thermal cycles are experimentally studied. Three different sealing pastes having 40, 50 and 60 wt % solid loadings are prepared and tested for this purpose. The pastes are applied between two metallic interconnector plates and subjected to a glass formation step for the joining. The fracture strengths of 24 samples prepared for each case are determined via tensile tests. Similarly, the mechanical performances of the sealants after 3, 6 and 9 thermal cycles are also obtained. The results reveal that the joining strength tends to increase with the amount of solid powder content in the paste. This can be attributed to increased number of crystalline phases in the sealants with increasing the solid loading. The thermal cycles, on the other hand, are shown to have an adverse effect on the joining performance regardless of the solid loading. However, the rate of decrease in the fracture strengths is found to decrease with the solid powder contents in the pastes. This can be elucidated by the amount of glassy phases in the sealants, which can be expected to increase with the solid loading and provide self-healing ability. The microstructures of the fracture surfaces of all samples are also investigated by a scanning electron microscopy. The obtained images confirm the tensile test results.     

Chromium poisoning for prolonged lifetime of electrodes in solid oxide fuel cells - Review

Solid Oxide Fuel Cells (SOFCs) offer low carbon emission and high efficient energy conversion systems. For the wide commercial distribution of this system, one of the technological issues and challenges is prolonged durability: the SOFC systems should have a long lifetime of more than 10 years. The volatile chromium species poisoning is the key degradation factor to overcome at the functional ceramics of air electrode (cathode)/interlayer/electrolyte interfaces in the SOFC system among many degradation factors. This paper reports recent degradation mechanisms, especially on the chromium (Cr) vapors poisoning at the perovskite oxide cathode. The Cr-concentration levels at cathodes were evaluated from the reported data at small cells and practical cell-stacks. The interactions of volatile Cr species and perovskite oxide cathode surface were evaluated by the chemical reaction of cathode materials with Cr-vapors to form SrCrO₄ and the electrochemical induced Cr-vapors reduction (Cr⁶⁺ to Cr³⁺) to form Cr₂O₃ at (La,Sr,Ca)MnO₃-based and (La,Sr)(Co,Fe)O₃-based materials. Recovery mechanism from Cr-poisoning was reanalyzed at the (La,Sr)(Co,Fe)O₃/ceria-based interlayer/YSZ electrolyte interfaces by Cr-cleaning reaction with the evaporation of Cr₂O₃/SrCrO₄ and nano-meter level cation migration/rearrangement effects with phase separation and new phases formation. This paper is covering not only the elucidation of degradation mechanism but also the fundamentals of physical and chemical analyses on perovskite oxide cathode surface and interfaces. An insight for new materials combination for the next-generation SOFCs is also included.     

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.     

Fabrication of electrolyte materials for solid oxide fuel cells by tape-casting

In this research, solid oxide fuel cell electrolytes were fabricated by aqueous tape-casting technique. The basic compositions for SOFC electrolyte systems were focused on yttria-stabilized zirconia (YSZ) system. The powders used in this study were from different sources. ZrO₂-based system doped with 3, 8, and 10 mol% of Y₂O₃, and 8YSZ electrolyte tape illustrated the desirable properties. The grain size of the sintered electrolyte tapes was in the range of 0.5–1 μm with 98–99% of theoretical density. Phase and crystal structure showed the pure cubic fluorite structure for 8–10 mol% YSZ and tetragonal phase for 3 mol% doped. The electrolyte tapes sintered at 1450 °C for 4 h had the highest ionic conductivity of 30.11 × 10⁻³ S/cm which was measured at 600 °C. The flexural strengths were in the range of 100–180 MPa for 8–10 mol% YSZ, and 400–680 MPa for 3 mol% YSZ.     

Boosting the electrocatalytic performance of Fe-based perovskite cathode electrocatalyst for solid oxide fuel cells

The development of cathode materials with excellent electrocatalytic activity and CO₂ tolerance is an important direction for the wide application of solid oxide fuel cells. Herein, the cobalt-free perovskite oxides Bi_0.5Sr_0.5Fe_1-xV_xO_3-δ (BSFVx, x = 0.025, 0.05 and 0.075) are developed as the efficient cathode electrocatalysts for SOFCs. The V-doping strategy is beneficial to improve the thermal stability, CO₂ tolerance and electrochemical performance of undoped Bi_0.5Sr_0.5FeO_3-δ. Among all samples, Bi_0.5Si_0.5Fe_0.95V_0.05O_3-δ (BSFV0.05) cathode presents excellent oxygen reduction reaction activity, achieving a lower polarization resistance of 0.076 Ω cm² and the peak power density of the single cell with the BSFV0.05 cathode reaches to 1.16 W cm⁻² at 700 °C, which can be comparable to those of the representative cobalt-based cathodes. Furthermore, the improved CO₂ tolerance of the BSFV0.05 cathode can be ascribed to the high acidity of the V⁵⁺ and the larger average bonding energy in the oxide.     

Synergistic effects of sulfur poisoning and gas diffusion on polarization loss in anodes of solid oxide fuel cells

Poisoning effects of sulfur compounds on the performances of solid oxide fuel cells are non‐trivial. However, the synergistic effects of gas diffusion, adsorption, desorption and reaction in anodes are typically neglected. In this work, an analytical model is derived to quantitatively evaluate the poisoning effects of H₂S. The results show that sulfur poisoning correlates closely with inefficient gas diffusion for small anode pore size, small porosity/tortuosity, and low working temperatures. As compared with concentration polarization, H₂S‐diffusion‐induced activation polarization in thin anodes with a large is detrimental, especially for low‐temperature operations with a high H₂S concentration and a low current density. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1127–1134, 2018     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.     

Oxidation studies of anodes layer microstructures in metal supported solid oxide fuel cells

The effect of oxidation on the microstructural and mechanical stability of ceramic layers in metal supported solid oxide fuel cells is reported. Half-cells that are produced with a reduced nickel based anode are oxidized for different times and temperatures in order to assess stability limits. Samples are analyzed in terms of the effective cell curvature and microstructure, where further insight is obtained via the observation of microstructures before and after oxidization. The interpretation is aided by a comparison to the behavior of structures without electrolyte layer. Electrolyte cracking and anode delamination are observed after oxidation, where the latter is absent in case of oxidation experiments without electrolyte layer, highlighting the failure relevance of strain induced by electrolyte deposition.     

Tailoring the properties of yttria-stabilized zirconia powders prepared by the sol-gel method for potential use in solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) powders have been prepared by the sol-gel method, following two alternative procedures: a series of powders was obtained by drying the sol-gel solutions in air at 100 °C until dry residue, and another series of powders was obtained by scratching the thin films deposited on cylindrical wide flat glassy surfaces after evaporating to dryness in air at 100 °C for 2 h. Samples were characterized by Scanning Electron Microscopy (SEM), nitrogen adsorption at −196 °C and Fourier Transform Infrared (FT-IR) spectroscopy. In general, a noticeable contraction of the pores is observed as the molecular size of the alcohols used grows. Powders prepared by conventional drying of sol-gel solutions at 100 °C exhibit remarkably high values of specific surface area (up to 148 m² g^− 1). On the contrary, samples prepared by scratching of the deposited thin films show a noticeable decrease in their specific surface area. Values of fractal dimension follow the same trend and indicate that, in general, the texture of the samples is mainly microporous for the first series of samples and more ordered for the second one. Finally, in order to investigate the effect of the calcination temperature on the morphological and textural properties of 3 mol% yttria-stabilized zirconia powders, once the 3YSZ powders were dried at 100 °C they were subjected to calcination at different temperatures. The experimental results suggest that the removal of residual water and alcohol occluded within the powder particles as well as the elimination of gases produced during the calcination stage play a very important role in the development of the porosity and surface area of the samples.     

A highly conductive Ce0.8Nd0.2O1.9 electrolyte with double sintering aids for intermediate-temperature solid oxide fuel cells

In this paper, the influence of Bi/Zn mass ratio on the phase composition, microstructure, sintering properties, and electrical properties of Bi/Zn co-added Nd_0.2Ce_0.8O_1.9 (NDC) used for intermediate-temperature solid oxide fuel cells (SOFCs) was investigated. At 700 °C, the total conductivity of the NDC-based electrolyte (3Bi/1Zn-NDC) with the mass ratio 3:1 for Bi₂O₃ and ZnO was as high as 5.89 × 10^?2 S cm^?1, 4.60 and 4.51 times higher than the single addition of 4 wt% Bi₂O₃ and 4 wt% ZnO, respectively. In addition, the 3Bi/1Zn-NDC electrolyte exhibited a good physical and chemical compatibility with the electrode materials. The open circuit voltage (OCV) of the cell supported by the 3Bi/1Zn-NDC electrolyte was 0.67 V, and the output power density could reach 402.25 mW cm^?2 at 700 °C. It showed stable power output and OCV in the long-term stability test within 50 h. Overall, the combination of 3 wt% Bi₂O₃ and 1 wt% ZnO was a very effective dual sintering aid for NDC electrolyte.     

Properties of CuMn1.5Ni0.5O4 spinel as high-performance cathode for solid oxide fuel cells

Spinel oxide cathode has made great progress in solid oxide fuel cells (SOFCs) because of its special characteristics different from perovskite. In this study, a spinel-structured SOFC cathode, CuMn_1.5Ni_0.5O₄ (CMN), is proposed. Rietveld refinement shows that CMN takes the cubic structure of the space group of P4₃32. CMN shows a high conductivity of about 70.0–91.2 S cm⁻¹ at 600–800 ºC in the air and exhibits good catalytic activity for oxygen. A symmetric cell with CMN-GDC composite cathode demonstrates a low Rp of 0.047 Ω cm² at 800 ºC. The charge transfer of oxygen is the rate-limiting process at lower temperatures. The performance test results of the button cell with CMN-GDC composite cathode are excellent, with high power densities of 1342.4 mW cm⁻² at 800 ºC. After a110h long-term test, the cell runs stably, and no microstructure damage is observed.     

Thermo-mechanical analysis of 3D manufactured electrodes for solid oxide fuel cells

Additive manufacturing has widened the scope for designing more performing microstructures for solid oxide fuel cells (SOFCs). Structural modifications, such as the insertion of ceramic pillars within the electrode, facilitate ion transport and boost the electrochemical performance. However, questions still remain on the related mechanical requirements during operation. This study presents a comprehensive thermal-electrochemical-mechanical model targeted to assess the stress distribution in 3D manufactured electrodes. Simulations show that a dense pillar increases the stress distribution by ca. 10 % compared to a flat electrode benchmark. The stress is generated by the material thermal contraction and intensifies at the pillar-electrolyte junction while external loads have negligible effects. An analysis on manufacturing inaccuracies indicates that sharp edges, surface roughness and tilted pillars intensify the stress; nonetheless, the corresponding stress increase is narrow, suggesting that manufacturing inaccuracies can be easily tolerated. The model points towards robust design criteria for 3D manufactured electrodes.     

Sr and Fe co-doped Ba2In2O5 as a new proton-conductor-derived cathode for proton-conducting solid oxide fuel cells

BaSrInFeO₅ (BSIF), a new cathode material for proton-conducting solid oxide fuel cells (SOFCs), is designed based on the modification of the Ba₂In₂O₅ proton conductor with Sr and Fe cations. Compared with the Ba₂In₂O₅ proton conductor tailored with only Fe cations (Ba₂InFeO₅, BIF), doping Sr can improve the chemical stability and also benefit the formation of oxygen vacancies. The proton mobility is also improved with Sr-doping, which is confirmed by first-principles calculations and experimental studies. An H-SOFC using the BSIF cathode generates a relatively high peak power density of 1192 mW cm^-2 at 700 ^oC, which is superior to many cells in previous reports. First-principles calculations find that the cathode oxygen reduction reaction (ORR) energy barrier for BSIF is significantly lower than that for BIF. Although Ba₂In₂O₅ is less studied, the derived cathode materials can still present decent performance, probably offering new material selections for H-SOFCs.     

Effect of cathode fabrication method on characteristics of anode-supported tubular solid oxide fuel cells

We have adopted three different methods: dip-coating, brush pen painting, and ion-impregnating, to fabricate cathodes for anode-supported tubular solid oxide fuel cells; and studied the performances of the cells using cathodes fabricated by these three different methods. The cell with ion-impregnated cathode presented the best electrochemical performances in these three cells, and it generated a maximum power density of 446 mW cm⁻² at 850 °C, when operating with humidified hydrogen. The cells with dip-coated cathode and brush pen painted cathode produced acceptable electrochemical performances; they generated maximum power densities of 403 and 405 mW cm⁻² at 850 °C, respectively, when running on humidified hydrogen; also, they represented more stable, much easier processes and lower cost.     

Enhancing thermal-stability of metal electrodes with a sputtered gadolinia-doped ceria over-layer for low-temperature solid oxide fuel cells

Cathodic activation loss is the dominant loss mechanism in the operation of low-temperature solid oxide fuel cells (LT-SOFCs). The thermal degradation of metallic cathodes decreases the performance of LT-SOFCs, causing practical issues in long-term operation. In this paper, we investigate the effect of the sputtered gadolinia-doped ceria (GDC) over-layer on the thermal stability of platinum (Pt) cathodes. The thermal stability of Pt cathodes with 23 nm-thick GDC over-layers significantly increased compared to that of the Pt-only cathodes after 2hrs’ operation at 450 °C. (<4% vs. 17% performance degradation, respectively).