New glass and glass–ceramic sealants for planar solid oxide fuel cells

This work describes the design of three new glass and glass ceramic compositions to join the ceramic electrolyte (YSZ wafer) to the metallic interconnect (Crofer22APU) in planar SOFC stacks. The designed sealants are low-sodium, barium free and boron-oxide free silica-based glasses.The sealing process was optimized for the most promising composition and joined Crofer22APU/glass–ceramic sealant/YSZ samples were morphologically characterized and tested for 300 h in humidified hydrogen atmosphere at the fuel cell operating temperature of 800 °C. The study showed that the use of the glass–ceramic was successful in joining the YSZ ceramic electrolyte to the Crofer22APU metallic interconnect and in preventing severe corrosion effects at the Crofer22APU/glass–ceramic interface after static treatments in humidified hydrogen at 800 °C for 300 h.     

Flexural strength of glass–ceramic for structural applications

In order to characterise the design strength of an innovative type of glass–ceramic with a view to structural applications, 4-point-bending and Ring-on-Ring biaxial bending tests have been performed. Strength is comparable with that of tempered glass but remarkably, the material breaks into large pieces like annealed glass. Interpreted through Weibull-type probability distributions, the data show much lower dispersion with respect to glass. Additional tests performed at different load rates have allowed an evaluation of the effects of static fatigue. Using a phenomenological model of equivalent-crack growth, a method is presented to calculate the decay of strength with loading time. As expected, this material is proven to be less sensitive than glass to this type of degradation.     

Lowering the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular solid oxide fuel cells

To prevent undesirable reactions between the cathode and electrolyte materials in cathode-supported solid oxide fuel cells (SOFCs), the co-sintering temperature of these two layers must be lowered. In the present work, we employed different strategies to lower the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular SOFCs by increasing the cathode sintering shrinkage and adding sintering aids to the electrolyte. Strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ) were used as the cathode and electrolyte materials, respectively. To facilitate densification of the electrolyte layer by controlling the shrinkage of the cathode support, the particle size of the LSM powder was reduced by high-energy ball milling and different amounts of micro-crystalline cellulose pore former were used. Sintering aids, namely NiO and Fe₂O₃, were also added to the YSZ electrolyte to further improve its low-temperature sintering. Our results indicate that with the improvement in the cathode support shrinkage and use of the small amounts of sintering aids, the cathode–electrolyte co-sintering temperature can be reduced to 1250–1300 °C. It was also observed that the presence of the sintering aids helps to reduce the reactivity between the LSM cathode and YSZ electrolyte.     

Effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze

The effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze was investigated using a glass–ceramic frit in the K₂O–ZnO–MgO–CaO–Al₂O₃–SiO₂ system and a granite waste glass. Measurements by X-ray diffraction (XRD) combined with scanning electron microscopy (SEM) and EDX microanalysis showed that the distribution of Fe³⁺ ions among different crystalline phases such as franklinite (ZnFe₂O₄) and hematite Fe₂O₃ depends on the iron content in the original diopside mixture. Thus, the original glaze crystallises to franklinite or hematatite when iron content is greater than 2 and 15%, respectively.     

Electrochemical performance of La2NiO4+δ cathode for intermediate-temperature solid oxide fuel cells

The application of the La₂NiO_4+δ (LNO), one of the Ruddlesden–Popper series materials, as a cathode material for intermediate temperature solid oxide fuel cells is investigated in detail. LNO is synthesized via a complex method using ethylenediaminetetraacetic acid (EDTA) and citric acid. The effect of the calcination temperature of the LNO powder and the sintering temperature of the LNO cathode layer on the anode-supported cell, Ni–YSZ/YSZ/GDC/LNO, is characterized in view of the charge transfer resistance and the mass transfer resistance. Charge transfer resistance was not significantly affected by calcination and sintering temperature when the sintering temperature was not lower than the calcination temperature. Mass transfer resistance was primarily governed by the sintering temperature. The unit cell with the LNO cathode sintered at 1100 °C with 900 °C-calcined powder presented the lowest polarization resistance for all the measured temperatures and exhibited the highest fuel cell performances, with values of 1.25, 0.815, 0.485, and 0.263 W cm⁻² for temperatures of 800, 750, 700, and 650 °C, respectively.     

Development of Ni–YSZ cermet anode for solid oxide fuel cells by electroless Ni coating

Nickel–Yttria-stabilized zirconia (Ni–YSZ) cermet (ceramic–metal composite) anodes have been prepared from a simple electroless Ni bath without hypophosphite. Ni–YSZ powder having varying amounts of Ni has been prepared. The effect of two different reducing agents has been evaluated with respect to stability of the bath. Hydrazine can be effectively used as a reducing agent up to 30 vol.% Ni. However beyond 30% Ni, the hydrazine bath loses its stability. Formaldehyde is found to be a very effective reducing agent for higher Ni concentration. The Ni–YSZ powder obtained is characterized by SEM and XRD. When the powder is oxidized for calculating actual amount of Ni deposited, it turns to complete green due to the formation of NiO. The XRD results also show distinct peaks of NiO. The powder is pressed and sintered in air and reduced in hydrogen atmosphere to convert NiO back to Ni. The sintered microstructure shows a well-defined network of Ni around YSZ particles and the fracture surface shows porosity. These features indicate the effectiveness of the technique in producing the essential microstructural elements necessary for effective functioning of the anode.     

Highly electrocatalytic activity Ruddlesden−Popper type electrode materials for solid oxide fuel cells

To promote the viability of commercial solid oxide fuel cell (SOFC), developing novel oxygen electrodes with high electrochemical activity is essential. Herein, a series Ruddlesden-Popper oxides, Sr_3?xLa_xFe₂O_7?δ (SLFx), are successfully synthesized and evaluated as potential cathode materials for SOFC. The oxygen desorption behavior, electrochemical activity and oxygen reduction reaction (ORR) kinetics of the SLFx cathodes are systematically discussed. The Sr_2.9La_0.1Fe₂O_7?δ (SLF10) cathode exhibits highest oxygen vacancy concentration and excellent electrocatalytic performance, as evidenced by a low polarization resistance of 0.14 Ω cm² and high maximum power density of 0.77 W cm^?2 at 700 °C. From electrochemical impedance spectra and distribution of relaxation times analysis, the oxygen adsorption/desorption process is the rate-limiting step toward ORR at the cathode interface. Furthermore, SLF10 shows considerable polarization overpotentials in both SOFC and solid oxide electrolysis cell (SOEC) modes, indicating that SLF10 is a promising bifunctional electrode for electrocatalytic oxygen reaction.     

Processing of ZrO2 scaffolds coated by glass–ceramic derived from 45S5 bioglass

Three dimensional, highly porous, ZrO₂ scaffolds coated by glass–ceramic derived from 45S5 bioglass were fabricated. The surface reactivity of 45S5 in aqueous solution was investigated as a function of the immersion time. The influence of the solid loading on the rheological behavior of 45S5 aqueous slips with ammonium polyacrylate (NH₄PA) was studied; besides the effect of poly(vinyl)alcohol (PVA) on the relative viscosity was determined. The structure and microstructure of uncoated and coated ZrO₂ scaffolds were characterized. The high ionic exchange capability of 45S5 was demonstrated by the pH rise, the significant weight loss and the amorphous calcium phosphate nucleation, upon its immersion in aqueous solution. The addition of PVA did not affect the dispersion properties of the 45S5 powder, which were basically controlled by its negative surface charge. 30 wt% 45S5 slips with 4 wt% PVA exhibited a yield stress and an adequate viscosity in the low shear rate range, to produce a bioglass coating into the ZrO₂ scaffold. The glass-ceramic coating was distributed along the strut surfaces, forming a thin film without altering the porosity and the strut thickness of the original ZrO₂ scaffold structure.     

Effect of GDC addition method on the properties of LSM–YSZ composite cathode support for solid oxide fuel cells

Equal amounts of Gd_0.1Ce_0.9O_2−δ (GDC) were added to La_0.65Sr_0.3MnO_3−δ/(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM/YSZ) powder either by physical mixing or by sol–gel process, to produce a porous cathode support for solid oxide fuel cells (SOFCs). The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm⁻² at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by sol–gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs.     

Electrochemical performance of Ni1−xFex–Ce0.8Gd0.2O1.9 cermet anodes for solid oxide fuel cells using hydrocarbon fuel

Ni_1?xFe_x bimetallic-based cermet anodes were investigated for hydrocarbon-fueled solid oxide fuel cells. Ni_1?xFe_x–Ce_0.8Gd_0.2O_1.9 cermet anodes were synthesized using a glycine nitrate process, and their electrical conductivity and the amount of carbon deposits were found to decrease with increasing Fe content. The anode polarization resistance for the CH₄ fuel was significantly reduced by Fe alloying, which was strongly related to the carbon deposition behavior. The maximum power density of the single cell with Ni_0.85Fe_0.15–Ce_0.8Gd_0.2O_1.9 in CH₄ at 800 °C was 0.27 W/cm². Fe alloying significantly improved the electrochemical performance of solid oxide fuel cells in CH₄ fuel by suppressing carbon deposition.     

Statistical design of experiments for evaluation of Y–Zr–Ti oxides as anode materials in solid oxide fuel cells

Abstract

Mixed conducting anode materials for solid oxide fuel cells are desirable in order to extend the electron transfer reaction zone for fuel gas conversion and to minimise the nickel content for achieving a redox stable anode. Partial substitution by titania in yttria stabilised zirconia (YSZ) is known to increase the electronic conductivity in reducing atmospheres. Nine different compositions were selected from the quasi ternary phase diagram according to principles used in statistical design of experiments covering the whole stoichiometric regime relevant for ionic applications. The dc electrical conductivity values increase strongly with high Ti contents under reducing (Ar–4%H₂) conditions, whereas they decrease continuously with increasing Ti content under oxidising conditions (air). The results clearly show that the chosen screening process for materials selection can considerably reduce the number of samples. For solid oxide fuel cell anodes, the compositions in the YO_1.5–ZrO₂–TiO₂ system should be restricted to low Ti contents.     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

PrBaFe2O5+δ promising electrode for redox-stable symmetrical proton-conducting solid oxide fuel cells

The proton conduction behavior and electrochemical performance of PrBaFe₂O_5+δ (PBF) are firstly studied for use as promising anode materials in symmetrical proton-conducting solid oxide fuel cells (PCFCs). This study focuses on the investigation of protonic defect formation in PBF and its properties as an electrode, including its chemical stability, electrochemical performance and redox stability. PBF is found to have proton conductivity at intermediate temperatures, which contributes to improved hydrogen oxidation reaction and water formation reaction at the anode and cathode, respectively. Hence, the symmetrical PCFC exhibits a peak power density of 301 mW?cm^?2 and total resistance of 0.77 Ω?cm² at 700 °C. Further, it shows excellent redox-cycle stability upon cycling between fuel and air under a current load. The electrochemical analysis and redox cycling tests of the PBF anode demonstrate that PBF is an attractive candidate for alternative material for symmetrical PCFCs.     

Preparation and characterisation of diopside-based glass–ceramic foams

Foaming and crystallisation behaviours of compacted glass powders based on a diopside glass–ceramic composition were investigated using the sintering route. The foaming agent was 2 wt.% SiC particles. The effect of PbO on the foaming ability of glasses was investigated. The results showed that the addition of PbO not only improved the foaming ability, by improving the wettability of the glass–SiC particles but also increased the crystallisation temperature and widened the temperature interval between the dilatometric softening point and the onset of crystallisation. The glass–SiC wetting angle was decreased from 85° for the lead-free glass, to 55° for the glass that contains 15 wt.% PbO.     

Preparation and characterization of vancomycin releasing PHBV coated 45S5 Bioglass®-based glass–ceramic scaffolds for bone tissue engineering

Porous 45S5 Bioglass^®-based glass–ceramic scaffolds with high porosity (96%) and interconnected pore structure (average pore size 300 μm) were prepared by foam replication method. In order to improve the mechanical properties and to incorporate a drug release function, the scaffolds were coated with a drug loaded solution, consisting of PHBV and vancomycin. The mechanical properties of the scaffolds were significantly improved by the PHBV coating. The bioactivity of scaffolds upon immersion in SBF was maintained in PHBV coated scaffolds although the formation of hydroxyapatite was slightly retarded by the presence of the coating. The encapsulated drug in coated scaffolds was released in a sustained manner (99.9% in 6 days) as compared to the rapid release (99.5% in 3 days) of drug directly adsorbed on the uncoated scaffolds. The obtained drug loaded and bioactive composite scaffolds represent promising candidates for bone tissue engineering applications.     

Carbon-tolerance effects of Sm0.2Ce0.8O2−δ modified Ni/YSZ anode for solid oxide fuel cells under methane fuel conditions

Samarium-doped ceria (SDC) is coated onto a Ni/yttria-stabilized zirconia (Ni/YSZ) anode for the direct use of methane in solid-oxide fuel cells. Porous SDC thin layer is applied to the anode using the sol–gel coating method. The experiment was performed in H₂ and CH₄ conditions at 800 °C. The cell performance was improved by approximately 20 % in H₂ conditions by the SDC coating, due to the high ionic conductivity, the mixed ionic and electronic conductive property of the SDC, and the increased triple phase boundary area by the SDC coating in the anode. Carbon was hardly deposited in the SDC-coated Ni/YSZ anode. The cell performance of the SDC-coated Ni/YSZ anode did not show any significant degradation for up to 90 h under 0.1 A cm^?2 at 800 °C. The porous thin SDC coating on the Ni/YSZ anode provided the electrochemical oxidation of CH₄ over the whole anode, and minimized the carbon deposition by electrochemical carbon oxidation.     

Synthesis and characterization of Ca3-xLaxCo4-yCuyO9+δ cathodes for intermediate temperature solid oxide fuel cells

The calcium cobaltite Ca_3-xLa_xCo_4-yCu_yO_9+δ with x and y = 0 and 0.1 were synthesized and the electrical, thermal, and catalytic behaviors for the oxygen reduction reaction (ORR) for use as air electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) were evaluated. X?ray diffraction confirms the Ca_3-xLa_xCo_4-yCu_yO_9+δ samples were crystallized in a monoclinic structure and scanning electron microscopic image shows lamella-like grain formation. Introduction of dopants decreases slightly the loss of lattice oxygen and thermal expansion co-efficient. The Ca_3-xLa_xCo_4-yCu_yO_9+δ samples exhibit good phase stability for long-term operation, thermal expansion, and chemical compatibility with the Ce_0.8Gd_0.2O_2-δ electrolyte. Among the studied samples, Ca_2.9La_0.1Co₄O_9+δ shows a maximum conductivity of 176 Scm^?1 at 800 °C. Although the doped samples exhibit a higher total electrical conductivity, an improved symmetrical cell performance is displayed by the undoped sample. Comparing the sintering temperatures, the composite cathode Ca₃Co₄O_9+δ + Ce_0.8Gd_0.2O_2-δ sintered at 800 °C exhibit the lowest area specific resistance of 0.154 Ω cm² at 800 °C in air. In the Ca_3-xLa_xCo_4-yCu_yO_9+δ + GDC composite cathodes, the charge-transfer process at high frequencies presents a major rate limiting step for the oxygen reduction reaction.     

Evaluation of GdBaCuCo0.5Fe0.5O5+δ as cathode material for intermediate temperature solid oxide fuel cells

The GdBaCuCo_0.5Fe_0.5O_5+δ (GBCCF) layered perovskite oxide was evaluated as novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Its electrical conductivity was 9–13 S cm^?1 at 650–800 °C in air. The average thermal expansion coefficient (TEC) of GBCCF was 14.4 × 10^?6 K^?1, which was close to that of the typical electrolyte material. The cathode polarization resistance of GBCCF was 0.650 Ω cm² at 750 °C and it decreases to 0.118 Ω cm² when Ce_0.9Gd_0.1O_1.95 (GDC) was added to form a GBCCF–GDC composite cathode. Preliminary results indicated that layered perovskite GBCCF was a promising alternative cathode material for IT-SOFCs.     

Preparation and characterization of alkali-free glass substrates with enhanced properties for TFT–LCDs applications

To obtain an alkali-free glass substrate with enhanced properties for thin-film transistor–liquid crystal displays (TFT–LCDs) applications, we chose a base glass composed of 3B₂O₃-15Al₂O₃-58SiO₂-22MgO-0.5SrO-1.5MgF₂ (mol%) for nucleation–crystallization. The results show that when the nucleation–crystallization processes of the base glass are 810 °C/6 h + 880 °C/6–9 h, the prepared GC/6–GC/9 glass-ceramics exhibit enhanced properties because of the precipitation of nano-sized cordierite. The transmittances in the visible range of the GC/6–GC/9 glass-ceramics exceed 85%, the densities are 2.564–2.567 g/cm³, thermal expansion coefficients are 2.934–3.059 × 10^-6/°C (25–300 °C), compressive strengths are 417–589 MPa, bending strengths are 141–259 MPa, Vickers hardnesses are 6.8–7.8 GPa, and strain points are approximately 735 °C. Considering these properties, the prepared GC/6–GC/9 glass-ceramics have good potential as candidate materials for alkali-free glass substrates. Additionally, these results demonstrate that it is feasible to improve the properties of alkali-free glass substrates by nucleation–crystallization.