SOFC Electrolyte Research Articles

Electrochemical performance of gadolinia-doped ceria (CGO) electrolyte thin films for ITSOFC deposited by spray pyrolysis

Solid Oxide Fuel Cell is an attractive, efficient, alternative source of power generation. However several challenges remained for this technology to be viable. These challenges include high power density, degradation rate, and cost. One way to decrease the SOFC cost is to use stainless steel interconnector. To be able to use a stainless steel interconnector one of the challenges is to find a way to produce an electrolyte, which does not need sintering at high temperature. This work presents the results of the process applied to gadolinia-doped ceria thin films deposited in cycles by spray pyrolysis. The aim of this work was to obtain thin, dense, and continuous CGO coatings, which has electrochemical performance suitable to be used as electrolyte for SOFC. The results obtained show that the air flow rate influenced the droplets size and hence the film quality. X-ray diffraction analysis showed that the films were crystalline after the deposition. Electrochemical tests showed maximum power density of 510 mW cm−2 at 650 °C with a thickness average of 3.30 μm when the film was deposited in 12 cycles showing that the film has a potential to be used as an electrolyte for ITSOFC on metal support.

Significant performance enhancement of yttrium-doped barium cerate proton conductor as electrolyte for solid oxide fuel cells through a Pd ingress–egress approach

Proton-conducting perovskite oxides are excellent electrolyte materials for SOFCs that may improve power density at reduced temperatures and increase fuel efficiency, thus encouraging the widespread implementation of this attractive technology. The main challenges in the application of these oxides in SOFCs are difficult sintering and insufficient conductivity in real cells. In this study, we propose a novel method to significantly enhance the performance of a yttrium-doped barium cerate proton conductor as an electrolyte for SOFCs through a Pd ingress–egress approach to the development of BaCe0.8Y0.1Pd0.1O3−δ (BCYP10). The capability of the Pd egress from the BCYP10 perovskite lattice is demonstrated by H2-TPR, XRD, EDX mapping of STEM and XPS. Significant improvement in the sinterability is observed after the introduction of Pd due to the increased ionic conductivity and the sintering aid effect of egressed Pd. The formation of a B-site cation defect structure after Pd egress and the consequent modification of perovskite grain boundaries with Pd nanoparticles leads to a proton conductivity of BCYP10 that is approximately 3 times higher than that of BCY under a reducing atmosphere. A single cell with a thin film BCYP10 electrolyte reaches a peak power density as high as 645 mA cm−2 at 700 °C.

Investigation on the Sr-doped ceria Ce1−xSrxO2−δ (x = 0.05–0.2) as an electrolyte for intermediate temperature SOFC

The effect of Sr doping in CeO2 for its use as solid electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been explored here. Ce1−xSrxO2−δ (x=0.05–0.2) are successfully synthesized by citrate-complexation method. XRD, Raman, FT-IR, FE-SEM/EDX and electrochemical impedance spectra are used for structural and electrical characterizations. The formation of well crystalline cubic fluorite structured solid solution is observed for x=0.05 based on XRD and Raman spectra. For compositions i.e., x>0.05, however, a secondary phase of SrCeO3 is confirmed by the peak at 342cm−1 in Raman spectra. Although the oxygen ion conductivity was found to decrease with increase in x, based on ac-impedance studies, conductivity of Ce0.95Sr0.05O2−δ was found to be higher than of Ce0.9Gd0.1O2−δ and Ce0.8Gd0.2O2−δ. The decrease in conductivity of Ce1−xSrxO2−δ with increasing dopant concentration is ascribed to formation of impurity phase SrCeO3 as well as the formation of neutral associated pairs, SrCe″-Vo. The activation energies are found to be 0.77, 0.83, 0.85 and 0.90eV for x=0.05, 0.1, 0.15 and 0.20, respectively.

21119 レーザ焼結法によるセリア系電解質上へのSOFC用空気極膜の作製

21119 レーザ焼結法によるセリア系電解質上へのSOFC用空気極膜の作製

Nonstoichiometric (La0.95Sr0.05)xGa0.9Mg0.1O3−δ electrolytes and Ce0.8Nd0.2O1.9–(La0.95Sr0.05)xGa0.9Mg0.1O3−δ composite electrolytes for solid oxide fuel cells

A-site nonstoichiometric electrolytes (La0.95Sr0.05)xGa0.9Mg0.1O3−δ (LSGM, x = 0.97, 1.00, 1.03), and their composites Ce0.8Nd0.2O1.9 (NDC)–LSGM, were synthesized and investigated. The nonstoichiometry efficiently enhanced the total conductivity of LSGM electrolyte, and the A-site deficient composition showed the highest total conductivity above 550 °C (σt,LSGM(x=0.97) = 0.880 S m−1 > σt,LSGM(x=1.03) = 0.808 S m−1 > σt,LSGM(x=1.00) = 0.582 S m−1 at 600 °C). The cubic fluorite and perovskite structures were adopted by all the composites. The LSGM additive significantly promoted the grain growth of the composite electrolyte. The grain boundary conductivities of the composite electrolytes were more or less 5–10 times higher than that of NDC electrolyte at 500 °C. The optimum A-site nonstoichiometry was found to be x = 0.97 in composite electrolytes. This study provides a possible route to design high performance single phase or composite electrolytes for SOFCs.

Galliosilicate glasses for viscous sealants in solid oxide fuel cell stacks: Part II: Interactions with yttria stabilized zirconia and stainless steel coated with alumina

Optimized boro-galliosilicate viscous glass sealants were successfully tested against SOFC electrolytes and interconnect materials for up to 1500 h. The glasses exhibit glass transition temperatures near 620 °C and thus offer viscous sealing at temperatures as low as 650 °C. No interaction of the selected glasses occurred at 650 °C after 500 h, though testing at 850 °C against both 8 mol% yttria-stabilized zirconia (8YSZ) and alumina coated stainless steel (Al-SS) results in partial bulk and interfacial crystallization. The glasses penetrate into the grain boundaries of 8YSZ, but the penetration depth remains less than 10 μm from the original interface even after 1500 h at 850 °C. Dissolved 8YSZ completely inhibits bulk nucleation and crystallization in the interfacial region within 20–100 μm of the 8YSZ surface. Glasses with 25 and 30 mol% SrO and no CaO form dendritic SrZrO3 crystal phases at the interface, while a glass with 20 mol% SrO and 10 mol% CaO retains an amorphous interface even after 1500 h at 850 °C. Complex K–Ga–Si–O crystals are formed at the interface with Al-SS, and test vehicles of 8YSZ/glass/Al-SS remain intact on cooling from 850 °C to room temperature and without Cr diffusion into the glass.

Investigations on the Structural and Electronic Properties of Pure and Doped Bulk Pr2NiO4 through First Principles Calculations

Pr2NiO4-based ceramic oxides are known promising MIEC cathodes material for SOFC, owing to its experimentally found good ionic and electronic conductivity compared with other known materials. In this study, the structural and electronic properties of pure and doped Pr2NiO4-based oxides are investigated through first principles calculations based on density functional theory (DFT) in an attempt to investigate the possibility of using this material as an SOFC electrolyte. Substitutional doping at the cation site [Pr2-yRyNi1-x-y-zExTzO4 (R=La, and E=Cu and T=Ga)] shows change in both structural and electronic properties. Higher concentration of La doping increases the lattice parameter along the c direction while maintaining the electronic property of the host system. On the other hand, Cu doping causes tilting of the nickelate octahedral substructures so as to stabilize the whole structure but accompanies emergence of states which can contribute to electronic conduction. However, Ga doping has indications of high O ion diffusion and lower electronic conductivity

Understanding the Relationship between Ink Rheology and Film Properties for Screen-Printed Nickel/Scandia-Stabilized-Zirconia Anodes

Optimization of SOFC electrode or electrolyte screen-printing ink processing conditions is critical to the reliable manufacture of high quality films with optimum properties. Hence, in this study, the relationship between the ink rheology and the screen-printed film properties were investigated using nickel/scandia-stabilized-zirconia (NiO/ScSZ) anode inks having various binder (0-5 wt%) and solid contents (20-35 vol%). Ink rheological tests showed increased particle network strength in the inks with increasing binder and solids content. The electronic conductivity and electrochemical performance of anode films also increased as a function of binder and solids content. These can be related to increased particle network strength within the inks, as confirmed from the rheological studies. In conclusion, from the perspective of ink rheology, screen-printability and performance, inks having 26 vol% solid with 3 wt% binder or 28-30 vol% solid with 2 wt% binder were determined as the most suitable for the manufacturing of high quality SOFC anode films with a thickness of around 10 μm.

Characterization of Doped Ceria Films as SOFC Electrolyte Prepared by Using Ultrasonic Spray Pyrolysis Method

Terbium and samarium co-doped ceria was studied as electrolyte membrane for intermediate temperature solid oxide fuel cell single cells. The ceramic doped ceria powders with different composition like Ce0.9Sm0.1O2- δ and Ce0.9SmxTbyO2- δ were synthesized using glycine-nitrate thermal decomposition process and the films were deposited using ultrasonic spray pyrolysis method. The samples were characterized using XRD, SEM, AFM and impedance methods. The conductivity measurements of the disk like samples were conducted at different temperatures and gas environments.

Application of Low Temperature Ceramic Coating Process for SOFC Electrolyte and Electrode Fabrication

Aerosol deposition, ceramic coating process by powder spray at room-temperature under low vacuum, is a promising process for preparation of SOFC cells and stacks’ component to minimize a detrimental reaction during cell production. Moreover, the process has flexibility of microstructure modification from gas-tight dense structure to porous structure with nano-scaled composites on various substrates including Ni-based cermet or even metal-support. In this study, we developed dense electrolyte coating of LSGM electrolyte on Ni-GDC, Ni, and Crofer22APU support, respectively, at room temperature, and achieved maximum power density of 1.31, 0.79, and 0.28 W/cm2 at 750oC, respectively. Nano-structured porous LSCF cathode were also prepared at room-temperature and polarization resistance of 0.11 Ω·cm2 was achieved.

First Principle Researches on Component Optimization Mechanism for the SOFC Solid Electrolyte Materials

First-principle calculations based on DFT method are performed on two theoretical phase structure for potential SOFC electrolyte materials--the fluorite structure for MO2 and corundum structure for M2O3 in which M represents 3rd~6th period metal elements. The energy stability diagram of the oxides is draw, whilst the distribution trend of valence, ionic radii and oxide melting points (experimental data) for each element is obtained. Next, the bulk modulus and band structure of MO2 phase are calculated and the results are discussed combining with the structure stability trend graph. With all the results above we can discussed the relationship between the structure stability and relevant properties, and may offer some theoretic advices on the optimization of this kind of materials.

Improved electrical properties of Nb doped BaCe0.9Y0.1O2.95 electrolyte for intermediate temperature SOFCs obtained by autocombustion method

Abstract Nb doped BaCe 0.9 Y 0.1 O 2.95 (BCY) powders were synthesized by both solid state reaction (SSR) and autocombustion reaction (ACR) method to investigate how different synthesis routes influence the properties of the powders and sintered samples. The powders were uniaxially pressed and sintered at 1550 °C to form dense electrolyte samples. Stability of BCY in CO 2 can be increased by doping with Nb, but at the same time its electrical conductivities are reduced to a certain extent. It was shown that conductivities depended on both mechanism of defect formation, lattice parameters and microstructure of the electrolytes. Finer, less agglomerated powders were obtained by the method of autocombustion. SEM analysis revealed somewhat larger average grain size of the sintered samples synthesized by the ACR method. Total electrical conductivities were determined by electrochemical impedance spectroscopy (EIS) measurements in temperature range from 550 °C to 750 °C in a wet argon atmosphere. Again, higher conductivities were observed for the samples obtained by the ACR method and in some cases the relative difference in conductivity exceeds 20%. Thus, a certain improvement of electrolyte properties can be achieved by this method compared to the classical solid-state reaction route.

The Stability and Ionic Conductivity in Cr Doped Y2Ti2O7 in the Search for Potential SOFC Electrolyte Materials

In this study, a series of Y2Ti2-xCrxO7 were synthesized by sol-gel method and characterized by powder X-ray diffraction (PXRD), scanning electron microscope energy dispersive spectroscopy (SEM-EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), temperature-programmed reduction (TPR), and AC-impedance to investigate their properties for the application of intermediate temperature solid oxide fuel cell (IT-SOFC). Y2Ti2-xCrxO7 existed single-phase in a range of 0 < x ≤ 0.8. The refined cell parameters decreased as the amount of doped Cr concentration increased. In addition, the TPR profiles showed that the as-prepared materials started to react with hydrogen in the temperature range of 350 - 450 ºC. The analyzed results of XPS suggested that the average oxidation state of Cr decreased with the increased Cr content. Measurements of ionic conductivities induced that the optimized performance was observed at x = 0.2 with 0.01 S/cm at 700 ºC due to an optimized condition of carrier concentration and carrier mobility. The ionic conductivity dropped under reducing condition, indicative of a p-type semiconductor. The as-prepared Y2Ti1.8Cr0.2O7 exhibits low electrical conductivity (0.0004 S/cm at 700 ºC) and good ionic conductivity, which is potentially suitable to be utilized as an electrolyte material for IT-SOFC.

Mechanical Properties of Gd‐CeO2 Electrolyte for SOFC Prepared by Aqueous Tape Casting

AbstractMechanical properties of gadolinium‐doped ceria (Ce0.9Gd0.1O1.95, 10GDC) green tape prepared by aqueous‐based tape casting process were characterized by tensile test and shear punch test (SPT). SPT was found to be a useful method for characterizing mechanical properties of green tapes. Microstructures and mechanical properties such as flexural modulus, bending strength, and microhardness of tapes sintered at 1,300–1,500 °C have been evaluated. Indentation fracture toughness was also determined by the method of Palmqvist cracks at different applied loads for tapes sintered at 1,500 °C. Grain size measurements showed that excessive grain growth occurred during sintering despite using 10GDC nanopowders as the starting material. However, mechanical properties of sintered tapes improved by increasing sintering temperature and the results are comparable with those reported for 10GDC in literature.

Preparation and characterization of Ce1−xGdxO2−δ (x=0.1–0.3) as solid electrolyte for intermediate temperature SOFC

Abstract The effect of Gd doping on the ionic conductivity of CeO2 for its use as a solid electrolyte material for the intermediate temperature solid oxide fuel cells (IT-SOFCs) has been explored here. Ce1−xGdxO2−δ (x = 0.1–0.3) samples are successfully synthesized by carbonate co-precipitation method. XRD, FT-IR, Raman spectroscopy, UV–Vis spectroscopy, SEM and impedance spectroscopy are used for structural and electrical characterization. From the XRD patterns, well-crystallite cubic fluorite structured solid solution is confirmed. As Gd3+ ions are doped into CeO2 lattice, the absorption spectrum exhibits a red shift when compared to CeO2. Raman spectra show a peaks at 463 cm−1, 546 and 600 cm−1, which are the characteristic peaks of doped ceria. Based on ac-impedance data, the total ionic conductivity is same for both Ce0.9Gd0.1O2–δ (GDC10) and Ce0.8Gd0.2O2–δ (GDC20) in the temperature range 473 K–623 K. Hence for Gd doped ceria solutions 10% and 20% of Gd, are proper doping level which leads to the maximum total conductivity. Above this doping level, the mobility of oxygen ions decreases the total conductivity due to blocking effect.

Conductivity of New Electrolyte Material Pr1-xM1+xInO4(M=Ba,Sr) with Related Perovskite Structure for Solid Oxide Fuel Cells

Successful application of related perovskite structure with K2NiF4-type materials, Pr1-xM1+xInO4 (M=Sr, Ba), as electrolyte for SOFCs is reported here for the first time. The highest open current voltage was 0.70 V for PrSrInO4 at 500°C and 0.77 V for Pr0.9Ba1.1InO4 at 600°C. Measured electromotive force using hydrogen and oxygen concentration cells reveal that Pr1-xSr1+xInO4 materials have proton conductivity, whereas Pr1-xBa1+xInO4 materials have both proton and oxide-ionic conductivity. Due to the high proton conductivity at lower temperature, the highest maximum power density in the Pr1-xSr1+xInO4 and Pr1-xBa1+xInO4 materials occurred at 500°C and 700°C, respectively. Both materials showed a decrease in OCV with increasing temperature, indicating that the optimal operating temperature was between 500°C to 700°C. Our results confirm that both materials are potential candidates as electrolyte materials for proton-conducting solid oxide fuel cells.

Correlation of Crystallinity of SOFC Electrolyte and Output Power

not Available.

Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Abstract Gd 0.1 Ce 0.9 O 1.95 and Gd 0.2 Ce 0.8 O 1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10 −2 S cm −1 at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.

On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCs

Abstract Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La 9.33 (RO 4 ) 6 O 2 with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La 9.33 Si 2 Ge 4 O 26 electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La 2 O 3 , SiO 2 and GeO 2 powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO 2 used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La 9.33 Si 2 Ge 4 O 26 integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO 2 do promote densification of these electrolytes thereby improving their structural integrity.

Reversible oxygen removal and uptake in the La2ZnMnO6 double perovskite: Performance in symmetrical SOFC cells

A polycrystalline oxygen-stoichiometric La2ZnMnO6 double-perovskite oxide has been prepared by soft-chemistry procedures, followed by annealing in air at 800 °C. A reduced specimen, with a La2ZnMnO6−δ composition, has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600 °C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Zn and Mn and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the monoclinic P21/n space group, exhibiting an antisite Zn/Mn disorder of about 15% and 14%, respectively. The partial reduction of Mn4+ to Mn3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range. Magnetic susceptibility measurements indicate an antiferromagnetic behaviour, confirming the presence of Mn3+ ions in the structure. The magnetic structure of the reduced phase, determined from NPD data at 3 K, shows an antiferromagnetic G-type coupling between Mn at 2c and 2d sites (promoted by the anti-site disorder); the ordered magnetic moment at Mn site is 0.789 μB at 3 K. Both phases display a semiconductor-like behaviour with a maximum conductivity of 0.052 S cm−1 for the reduced phase at 650 °C, due to the occurrence of Mn3+–Mn4+ mixed valence. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes. The reversibility and versatility of the present compounds as catalysts for oxygen reduction (cathode) or fuel oxidation (anode) were tested in single SOFC cells yielding power density spanning from 120 to 155 W/cm2 using these perovskites as anode, cathode and symmetric electrodes for SOFC.