| 1. |
- Blennow Tullmar, Peter, et al.
(författare)
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Defect and electrical transport properties of Nb-doped SrTiO3
- 2008
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 179:35-36, s. 2047-2058
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Tidskriftsartikel (refereegranskat)abstract
- This study reports the defect and electrical transport properties of Nb-doped SrTiO3, Samples with various A/B-ratios were synthesized by a modified glycine-nitrate combustion process and evaluated as a constituent in a SOFC anode. The phase purity and defect structure of the materials have been analyzed with SEM, XRD, TGA, and XANES. The electrical conductivity of Nb-doped strontium titanate (Sr0.94Ti0.9Nb0.1O3 - sintered in 9% H-2/N-2 at 1400 degrees C for 12 h) decreased with increasing temperature and showed a phonon scattering conduction mechanism with (sigma>120 S/cm at 1000 degrees C (in 9% H-2/N-2). The results were in agreement with the defect chemistry model of donor-doped SrTiO3 where the charge compensation changes from Sr vacancy compensation to the electronic type when samples are sintered in reducing atmosphere. XANES in combination with TGA indicated that Ti is the only species that is reduced to a lower oxidation state (from Ti4+ to Ti3+). The pre-edge fine structure (PEFS) from the XANES results indicated that Nb improved the overlap of the Ti atomic orbitals and thereby provided one more explanation for the positive effect of Nb on the electronic conductivity of Nb-doped SrTiO3. (C) 2008 Elsevier B.V. All rights reserved.
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| 2. |
- Blennow Tullmar, Peter, et al.
(författare)
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Effects of Sr/Ti-ratio in SrTiO3-based SOFC anodes investigated by the use of cone-shaped electrodes
- 2006
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686. ; 52:4, s. 1651-1661
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Tidskriftsartikel (refereegranskat)abstract
- In this work the A-site deficiency limit for undoped SrTiO3, calcined in air or reducing atmosphere, was investigated. XRD and SEM analyses showed that when using the solid-state synthesis route it was difficult to achieve any A-site deficiency. Even very small deviations from unity in the Sr/Ti-ratio resulted in impurity phases. The effects of Sr/Ti-ratio on the performance of SrTiO3 anodes for solid oxide fuel cells (SOFCs) was investigated by impedance spectroscopy with cone-shaped electrodes. The electrocatalytic activity of the SrTiO3 cone samples, with various amount of reduced titanium dioxide phases (Magnéli phases), was investigated for oxidation of H2 at 700 °C. High temperature treatment at 1000 °C, between the measurements at 700 °C, showed that high content of Magnéli phases leads to faster degradation of the activity. The suitability for using cone-shaped electrodes as a tool for investigating electrode materials is discussed.
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| 3. |
- Blennow Tullmar, Peter, et al.
(författare)
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Electrochemical characterization and redox behavior of Nb-doped SrTiO3
- 2009
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 180:1, s. 63-63
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Tidskriftsartikel (refereegranskat)abstract
- Sr-vacancy compensated Nb-doped SrTiO3 with the nominal composition Sr0.94Ti0.9Nb0.1O3 has been evaluated as a solid oxide fuel cell (SOFC) anode material in terms of redox stability and electrochemical properties. Sr0.94Ti0.9Nb0.1O3 has been synthesized with a recently developed modified glycine-nitrate process. The phase purity and redox behavior have been analyzed with XRD and TGA. The electrochemical properties of Sr0.94Ti0.9Nb0.1O3 and a composite electrode of Sr0.94Ti0.9Nb0.1O3/YSZ have been investigated by electrochemical impedance spectroscopy (EIS) on cone shaped electrodes and on electrodes in a symmetrical cell configuration. The experiments indicated that the Nb-doped SrTiO3 electrodes were redox stable and showed a potential ability to be used as a part of a SOFC anode. The electrochemical activity appeared to be governed by the concentration of defect species (especially Ti3+ and V-0(++)) in the vicinity of the triple phase boundaries. However, the electrocatalytic activity of the materials was not sufficient and it needs to be improved if Nb-doped SrTiO3 based materials are to be realized as SOFC anodes. (C) 2008 Elsevier B.V. All rights reserved.
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| 4. |
- Blennow Tullmar, Peter, et al.
(författare)
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Niobium-doped strontium titanates as SOFC anodes
- 2008. - 4
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Ingår i: Advances in Solid Oxide Fuel Cells III. - 9780470196359 ; 28, s. 203-214
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Konferensbidrag (refereegranskat)abstract
- Sr-vacancy compensated Nb-doped SrTiO3 with the nominal composition Sr0.94Ti09Nb0.1O3 has been evaluated as part of a solid oxide fuel cell (SOFC) anode material in terms of redox stability, electrical conductivity, as well as electrochemical properties. Sr0.94Ti0.9Nb0.1O3 has been synthesized with a recently developed modified glycine-nitrate process. The synthesized powders have been calcined and sintered in air or in 9% H 2 / N2 between 800 - 1400 °C After calcination the samples were single phase Nb-doped strontium titanate with grain sizes of less than 100 nm in diameter on average. The phase purity, defect structure, and microstructure of the materials have been analyzed with SEM, XRD, and TGA. The electrical conductivity of the Nb-doped titanate decreased with increasing temperature and showed a phonon scattering conduction mechanism with σ > 120 S/cm at 1000 °C (in 9 % H2 / N2). The electrochemical properties of Sr0.94Ti0.9Nb 0.1O3 and a composite electrode of Sr 0.94Ti0.9Nb0.1O3/YSZ have been investigated by using a symmetrical cell configuration. The tests indicated that the electrodes were redox stable and showed a potential ability of the Nb-doped titanates to be used as a part of a SOFC anode. However, the catalytic activity of the materials was not sufficient and it needs to be improved if titanate based materials are to be realized as constituents in SOFC anodes.
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| 5. |
- Blennow Tullmar, Peter, et al.
(författare)
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Synthesis of Nb-doped SrTiO3 by a modified glycine-nitrate process
- 2007
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219. ; 27:13-15, s. 3609-3612
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Tidskriftsartikel (refereegranskat)abstract
- The objective of the present investigation was to develop a technique to synthesize submicronic particles of Nb-doped strontium titanate with a homogeneous composition. This was achieved by a modified glycine-nitrate process, using Ti-lactate, Nb-oxalate, and Sr(NO3)(2) as starting materials. A combination of both citric acid and glycine was needed in order to integrate the useful features of both complexation and combustion natures of citric acid and glycine, respectively. The amount of citric acid, glycine, and nitrates in the starting solution, as well as the source for extra nitrates, and the uniformity of heating during the thermal dehydration step were found to have significant influence on the final phase purity of the material. Calcination at 1100 degrees C in 7% H-2 in N-2 produced single phase Nb-doped strontium titanate with grain sizes of about 100 nm in diameter on average. (c) 2007 Elsevier Ltd. All rights reserved.
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| 6. |
- Bu, Junfu, 1982-
(författare)
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Advanced BaZrO3-BaCeO3 Based Proton Conductors Used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs)
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts:1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics.2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs.3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity.4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (< 1400 ℃) and a very fast cooling rate (> 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors.5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity.In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications.
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| 7. |
- Dalslet, Bjarke, et al.
(författare)
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Assessment of doped ceria as electrolyte
- 2006
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Ingår i: Journal of Solid State Electrochemistry. - : Springer Science and Business Media LLC. - 1433-0768 .- 1432-8488. ; 10:8, s. 547-561
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Tidskriftsartikel (refereegranskat)abstract
- A model describing the performance of a fuel cell based on 10 mol% gadolinia-doped ceria, Ce0.9Gd0.1O1.95−x (CG10), was formulated. The total electrical conductivity of CG10 was measured under very reducing conditions in the temperature range of 753 K to 948 K. Oxygen permeation experiments were carried out to measure the leak current through a ceria electrolyte. The results of the measurements are compared with predictions of the formulated model. Furthermore, the response of a fuel cell to changing operating conditions such as external load, temperature, electrode polarization resistances, and defect chemistry is investigated using the model. It is found that the maximum achievable efficiency of a CG10-based fuel cell is increased when (1) the temperature is decreased, when (2) the electrolyte thickness is increased, or when (3) the cathode polarization resistance is decreased. The efficiency can also in certain circumstances be increased by an increase of anode polarization resistance. Finally, the efficiency is reduced if the vacancy formation enthalpy is decreased to the level of fine-grained CG10. The performance of a CG10-based cell is evaluated by comparing it with a state-of-the-art zirconia-based cell. At 873 K, the efficiency of a fuel cell with a 10-μm CG10 electrolyte was limited to 0.74, whereas a cell with a perfect electrolyte would have an efficiency of 1. The power output of the CG10 cell at this efficiency is, however, four times larger than the zirconia-based cell at the same efficiency. This is due to the much lower cathode polarization resistance of (La0.6Sr0.4)zCo0.2Fe0.8O3-delta - CG10 cathodes on CG10 compared to the (La0.75Sr0.25)0.95MnO3 cathodes on stabilized zirconia.
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| 8. |
- Ehn, Andreas, et al.
(författare)
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Electrochemical Investigation of Model Solid Oxide Fuel Cells in H2/H2O and CO/CO2 atmospheres using Nickel Pattern Electrodes
- 2010
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 157:11, s. B1588-B1596
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Tidskriftsartikel (refereegranskat)abstract
- In this study, nickel pattern electrodes were electrochemically investigated in a three-electrode setup, operating both with H 2 / H 2 O and CO/ CO 2 atmospheres. Heating introduced structural differences in the nickel layer among the pattern electrodes, which appear to affect the electrode performance. Both dense and porous nickel pattern electrodes were formed by heating. Holes appeared in the nickel layer of the porous pattern electrodes, where the open cavity triple phase boundaries exhibited different limiting processes than open triple phase boundary electrodes of the dense electrode. As the temperature was raised in the experiment, the electrodes stabilized, with a degraded behavior that seemed to be strongly coupled to the structural changes in the electrode. It was possible to compare literature results with high temperature impedance measurements in H 2 / H 2 O presented here, while new results at lower temperatures in H 2 / H 2 O are also presented. Impedance spectroscopy measurements were performed, and the gas dependence of the polarization resistance was observed as the mixture ratios and temperatures were varied in both atmospheres. A positive relation between the polarization resistance and the partial pressure of CO was determined for the dense nickel pattern electrode, which agrees with previous results using nickel point electrodes. © 2010 The Electrochemical Society.
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| 9. |
- Gao, Zhan, 1981-
(författare)
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Advanced Functional Materials for Intermediate-Temperature Ceramic Fuel Cells
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low to intermediate-temperature solid oxide fuel cell (SOFC, 500 oC-700 oC) based on doped ceria (DCO) electrolyte has attracted much attention during the last decade. However, DCO suffers from electronic conduction caused byreduction of Ce (IV) to Ce (III) at high temperatures and low oxygen partial pressures and has a high grain-boundary resistivity. Co-doping has been chosen as the focus of this PhD study to investigate the feasibility of overcoming these technical problems. For different purposes, two co-doping strategies have been implemented to improve the properties of the singly doped ceria. Sr addition has been used with the aim of enhancing the ionic conductivity of Ce0.8Sm0.2O2-δ. The Sr addition greatly improves the microstructure of the space charge layers and the space charge potentials. The total conductivity of Sm and Sr co-doped ceria is higher than that of Ce0.8Sm0.2O1.9, and Ce0.8(Sm0.7Sr0.3)0.2O2-δ has the highest total conductivity. Sm and Lu co-doped ceria with composition of Ce1-x(Sm3Lu2)x/5O2-δ was investigated to validate the concept of critical dopant ionic radius. The elastic strain and critical dopant ionic radius may have an immediate effect on the grain bulk ionic conduction characteristics. As the basis of the ceramic electrolyte processing, the effect of the powder synthesis routes, including Polyvinyl alcohol (PVA)-assisted sol-gel process,Polyethylene glycol (PEG)-assisted sol-gel process, citrate sol-gel process and oxalate co-precipitation process (OCP), on the microstructure and the ionic conductivity of the Ce0.85Sm0.075Nd0.075O2-δ (SNDC) electrolyte has beeninvestigated. OCP process results in higher relative density and ionic conductivity, lower grain-boundary resistance and activation energy. Sm0.5Sr0.5CoO3-δ (SSC) cathode was investigated for SOFCs based on Ce0.85Sm0.075Nd0.075O2-δ (SNDC) electrolyte. Kinetics of oxygen reduction reaction (ORR) on porous SSC cathode was investigated by AC impedance spectra. Finally, novel BaZr0.1Ce0.7Y0.2O3-δ (BZCYO)-Ce0.8Y0.2O2-δ (YDC) composite ceramic electrolyte having both proton and oxygen ion conduction was studied. The composite ceramic electrolyte shows an enhanced ionic conductivity and chemical stability against reduction.
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| 10. |
- Li, Shuai, 1981-
(författare)
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Preparation and characterization of perovskite structure lanthanum gallate and lanthanum aluminate based oxides
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present work was initiated to study the synthesis and properties of lanthanum gallate based oxides as intermediate temperature electrolyte for solid oxide fuel cells. The wet chemical method, polymer complexing route, was used to prepare the precursor powders. To further investigate the polymer complexing method, it was also applied to the preparation of lanthanum aluminate based oxides. Single perovskite phase La0.8Sr0.2Ga0.83Mg0.17O2.815 can be prepared by the polymer complexing method using PVA as complexing agent. The thermal decomposition of the precursor powder undergoes three stages. While complete decomposition of the precursor is obtained at 1000°C. Further investigation of LaGaO3 doped with various amounts Sr or/and Mg was conducted. Three secondary phases were identified by X-ray diffraction, e.g. LaSrGaO4, LaSrGa3O7 and La4Ga2O9. The relative amount of these secondary phases depends on the doping compositions. Sr doping produced more Sr rich secondary phases with increasing content, while enhanced solid solubility was observed with Mg addition. Sintered samples showed dense microstructures with well-developed equiaxed grains, and the secondary phases were mainly in the grain boundaries. The oxygen ionic conductivity was enhanced by doping with Sr and Mg. Mg doping showed the increased activation energy of conductivity. Preliminary study showed that the lanthanum gallate and ceria composite electrolyte is mainly fluorite CeO2 phase after sintering. The minority secondary phase, Sm3Ga5O12, was also detected by XRD. The composite electrolyte showed superior electrical performance. It exhibited the highest conductivity in the temperature range of 250–600°C, compared with lanthanum gallate and ceria specimens. The phase pure perovskite La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 powders can easily be obtained by the polymer method using PVA as complexing agent. No secondary phase was detected after calcination at various temperatures (500–1100°C). The fully crystallized LaAlO3 phase was prepared after calcination at 900°C. Meanwhile the secondary phases were difficult to eliminate in the Sr- and Mg- doped LaGaO3 powder prepared by the same polymer method. It is thus concluded that the polymer, PVA in this work, provides more homogeneous mixing for cations of lanthanum aluminate based oxides, compared with the one for doped lanthanum gallate. The influence of different complexing agents, e.g. PVA and PEG, was investigated in the synthesis of lanthanum aluminate powders. Minority impurity La2O3 existed in the PEG powder, but it could be eliminated after sintering at high temperatures. Although the pure phase LaAlO3 can be easily obtained in PVA powders calcined at 950°C, more seriously aggregated particles existed. PEG showed advantages over PVA in terms of better densification and microstructure control in the sintered products. To select proper polymers in complex oxide synthesis, the agglomeration and morphology of the powder are the most important factors to be considered.
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