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Advanced Functional Materials for Intermediate-Temperature Ceramic Fuel Cells
(Ceramics)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. , vii, 49 p.
National Category
Ceramics
Identifiers
URN: urn:nbn:se:kth:diva-51479ISBN: 978-91-7501-145-5 (print)OAI: oai:DiVA.org:kth-51479DiVA: diva2:464160
Public defence
2011-12-19, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111213Available from: 2011-12-13 Created: 2011-12-12 Last updated: 2011-12-13Bibliographically approved
List of papers
1. Comparative study of Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) electrolyte synthesized by different routes
Open this publication in new window or tab >>Comparative study of Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) electrolyte synthesized by different routes
2011 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 509, no 35, 8720-8727 p.Article in journal (Refereed) Published
Abstract [en]

In this work, four different methods, including polyvinyl alcohol (PVA)-assisted sol-gel process, polyethylene glycol (PEG)-assisted sol-gel process, citrate sol-gel process and oxalate coprecipitation process (OCP) are employed to synthesize the Sm and Nd co-doped ceria electrolyte with the composition of Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) (SNDC). The phase structure of the powders can be well indexed with the fluorite-type CeO(2) structure. The morphology of sintered samples indicates that the ceramics can be highly densified. The relative density and the average grain size vary with the synthesis processes and the sintering temperatures. The bulk conductivities are quite close and the OCP-SNDC yields highest grain-boundary conductivities and total conductivities. The results indicate that the OCP process for the powder synthesis results in higher relative density and conductivities, lower grain-boundary resistance and activation energy. Grain-boundary space charge potentials for different specimens are calculated based on the Mott-Schottky model. The synthesis process and sintering temperature have significant effect on the space charge potential and the specific grain-boundary conductivity. (C) 2011 Elsevier B.V. All rights reserved.

Keyword
Solid oxide fuel cells, Electrolyte, Co-doped, Ceria, Conductivity
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-39009 (URN)10.1016/j.jallcom.2011.06.040 (DOI)000293824600022 ()2-s2.0-80051792097 (Scopus ID)
Available from: 2011-09-07 Created: 2011-09-06 Last updated: 2017-12-08Bibliographically approved
2. Investigation of oxygen reduction reaction kinetics on Sm(0.5)Sr(0.5)CoO(3-delta) cathode supported on Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) electrolyte
Open this publication in new window or tab >>Investigation of oxygen reduction reaction kinetics on Sm(0.5)Sr(0.5)CoO(3-delta) cathode supported on Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) electrolyte
2011 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 196, no 22, 9195-9203 p.Article in journal (Refereed) Published
Abstract [en]

Sm(0.5)Sr(0.5)CoO(3-delta) (SSC) cathode prepared by a glycine-nitrate process (GNP) is investigated for solid oxide fuel cells (SOFCs) based on Ce(0.85)Sm(0.075)Nd(0.075)O(2-delta) (SNDC) electrolyte. SSC forms cubic perovskite structure after being annealed at 1100 degrees C for 5 h. SSC cathode and SNDC electrolyte can retain their own structure and there is no reaction between the two compositions. The microstructure of the cathode and the interfaces between cathodes and SNDC electrolytes are studied by scanning electron microscopy (SEM) after sintering at various temperatures. Impedance spectroscopy measurements reveal that area specific resistances (ASRs) of SSC-SNDC30 cathode are much lower than those of SSC cathode. Kinetics of oxygen reduction reaction (ORR) on porous SSC cathode is investigated by analysis of impedance spectra. Medium-frequency conductivities show no dependency on oxygen partial pressure (Po(2)), which can be attributed to the oxygen ions transfer across the electrode/electrolyte interface. The dependencies of low-frequency conductivities on oxygen partial pressure (Po(2)) vary in the range from ca. 0.31 to ca. 0.34 and increase with the increasing temperatures. The low-frequency electrode process is a mixing process involving oxygen reduction reaction related to atomic oxygen and oxygen ions conduction step together with total charge-transfer step. IR-compensated current density (i)-overpotential (eta) relationship is established and the exchange current densities i(0) originated from high-field approximations are much higher than those of low-field approximations and a.c. impedance data under OCV state. It demonstrates the polarization overpotential has great effect on the kinetics of ORR. The polarization current is observed to increase with time in the long-term stability measurement, which can be ascribed to the propagation process of oxygen vacancies.

Keyword
Solid oxide fuel cells, Cathode, Oxygen reduction reaction, Area specific resistance
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-45576 (URN)10.1016/j.jpowsour.2011.07.012 (DOI)000295602400013 ()2-s2.0-80052483639 (Scopus ID)
Note
QC 20111107Available from: 2011-11-07 Created: 2011-10-31 Last updated: 2017-12-08Bibliographically approved
3. Enhanced ionic conductivity of Ce0.8Sm0.2O2-delta by Sr addition
Open this publication in new window or tab >>Enhanced ionic conductivity of Ce0.8Sm0.2O2-delta by Sr addition
2012 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 208, 225-231 p.Article in journal (Refereed) Published
Abstract [en]

Sm and Sr co-doped ceria-based electrolyte with compositions of Ce-0.8(Sm1-xSrx)(0.2)O2-delta (x = 0, 0.3, 0.5, 0.7) are synthesized and investigated with the aim of improving the electrical properties of Ce0.8Sm0.2O2-delta. X-ray diffraction (XRD) and electron microscope (SEM and TEM) techniques are employed to characterize the microstructure of powders and sintered pellets. The ionic conductivity has been examined by the A.C. impedance spectroscopy in air. The Ce-0.8(Sm0.7Sr0.3)(0.2)O2-delta exhibits the highest bulk conductivity among the series, which can be mainly ascribed to the increase of oxygen vacancy concentration. The specific grain-boundary conductivities are observed to increase with the Sr doping content up to x = 0.5. Further increase in Sr concentration will lead to reduced specific grain-boundary conductivities. The total conductivities of all Sm and Sr co-doped ceria are higher than that of Ce0.8Sm0.2O1.9. The results indicate that Sr co-doping opens a new avenue to improve ionic conductivity in Ce0.8Sm0.2O1.9.

Keyword
solid oxide fuel cells, electrolyte, co-doped, ceria, conductivity, grain
National Category
Materials Engineering Ceramics
Identifiers
urn:nbn:se:kth:diva-51493 (URN)10.1016/j.jpowsour.2012.01.001 (DOI)000303286500028 ()2-s2.0-84858738255 (Scopus ID)
Note
QC 20120524Available from: 2011-12-13 Created: 2011-12-13 Last updated: 2017-12-08Bibliographically approved
4. Electrical properties of ceria co-doped with Sm3+ and Lu3+electrolyte materials
Open this publication in new window or tab >>Electrical properties of ceria co-doped with Sm3+ and Lu3+electrolyte materials
(English)Article in journal (Other academic) Submitted
Abstract [en]

Sm and Lu co-doped ceria with compositions of Ce1-x(Sm3Lu2)x/5O2-δ (SLDC, x=0.05, 0.1, 0.15, 0.2) are investigated to validate the concept of critical dopant ionic radius ( rc ), where the number-average dopant ionic radius is designed tomatch the critical dopant ionic radius ( c r ). A variety of techniques including X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to characterize the SLDC powders and the sintered pellets. Electrical properties of different specimens are investigated by using the impedance spectroscopy. The result sdemonstrated that the critical dopant ionic radius concept is not totally valid for Sm-Lu co-doping strategy, even that the co-doping with appropriate chemicalcomposition, Ce0.85(Sm3Lu2)0.03O2-δ, yields higher total conductivity than either Smor Lu-doped ceria. More co-doping strategies need to be studied to test the critical dopant ionic radius concept.

Keyword
solid oxide fuel cells, critical dopant ionic radius, co-doped, ceria, conductivity
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-51494 (URN)
Note
QS 2011 QS 20120326Available from: 2011-12-13 Created: 2011-12-13 Last updated: 2012-03-26Bibliographically approved
5. Novel BaZr0.1Ce0.7Y0.2O3-δ (BZCYO)-Ce0.8Y0.2O2-δ (YDC) composite ceramic electrolyte for low-temperature SOFCs
Open this publication in new window or tab >>Novel BaZr0.1Ce0.7Y0.2O3-δ (BZCYO)-Ce0.8Y0.2O2-δ (YDC) composite ceramic electrolyte for low-temperature SOFCs
Show others...
(English)Article in journal (Other academic) Submitted
Abstract [en]

Novel BaZr0.1Ce0.7Y0.2O3-δ (BZCYO)-Ce0.8Y0.2O2-δ (YDC) composite ceramic electrolyte possessing both proton and oxygen ion vacancies conduction was first reported. There are no chemical reactions between the two compositions. The composite ceramic electrolyte shows excellent chemical stability to reduction. BZCYO2-YDC8 exhibits the highest bulk ionic conductivities in the temperature range of 450 oC to 650 oC and total ionic conductivities when the temperatures are higher than 550 oC. The conductivity enhancement mechanism has been discussed. BZCYO-YDC composite ceramics may be promising electrolytes in low-temperature solid oxide fuel cells.a)

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-51495 (URN)
Note
QS 2011 QS 20120326Available from: 2011-12-13 Created: 2011-12-13 Last updated: 2012-03-26Bibliographically approved

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