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Optimization of Europium Dopant Concentration in Strontium Cerate Mixed Conducting Ceramic Membrane for Maximum Hydrogen...

Permanent Link: http://ufdc.ufl.edu/UFE0022071/00001

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Title: Optimization of Europium Dopant Concentration in Strontium Cerate Mixed Conducting Ceramic Membrane for Maximum Hydrogen Production
Physical Description: 1 online resource (143 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: europium, hydrogen, membrane, oxide, permeability, proton, stability, strontium, tubular
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The protonic-electronic mixed conductors have received great attention for their potential applications particularly for the hydrogen separation from hydrocarbon. Strontium cerate doped with europium has been investigated to maximize hydrogen production as proton conducting membrane. The optimal europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurement were performed. Among the three different compositions of Eu doped SrCe1-xEuxO3 (x = 0.1, 0.15 and 0.2) studied, SrCe0.9Eu0.1O3 showed highest total conductivity between 773K and 1173K under both dry/wet hydrogen conditions. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe0.85Eu0.15O3 to SrCe0.8Eu0.2O3 depending on temperature. A tubular type Ni-SrCeO3 support was developed for investigating the hydrogen permeability of various amounts Eu doped strontium cerate hydrogen membrane film. Appreciable and stable hydrogen permeation through this thin film membrane was observed under humid hydrogen condition and it's PH21/4 dependence of hydrogen permeation flux agrees with Norby and Larring's model which protons and electrons are dominating defects. Activation energy for hydrogen permeation flux suggests that the hydrogen permeation flux is limited by electrons. However, this configuration showed cracking due to chemical reaction with CO/CO2 under methane steam reforming condition. SrZrO3 show good chemical stability under same conditions. Therefore, Zr ion also introduced into membrane and support material to increase chemical stability. SrZr0.2Ce0.7Eu0.1O3 membrane coated into the Ni-SrCe0.8Zr0.2O3 support cell achieved stable hydrogen permeation flux and chemical stability under steam reforming condition. Based on the Wagner equation, hydrogen permeation flux is inversely proportional to the membrane thickness. We expected increased hydrogen permeation with increasing dopant level in the membrane material due to increased electronic conduction for ambipolar diffusion. Therefore SrZr0.2Ce0.8-xEuxO3 (x=0.1, 0.15 and 0.2) hydrogen membranes were investigated for their hydrogen permeation properties and mechanism according to their membrane thickness. Their inversely proportional linear dependence of membrane thickness indicates bulk diffusion is the rate-limiting factor for hydrogen permeation. With respect to water vapor formation, higher Eu doping in the membrane permeates more hydrogen, in which water vapor was formed by reaction of oxygen gas from oxide lattice.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Wachsman, Eric D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-11-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022071:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022071/00001

Material Information

Title: Optimization of Europium Dopant Concentration in Strontium Cerate Mixed Conducting Ceramic Membrane for Maximum Hydrogen Production
Physical Description: 1 online resource (143 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: europium, hydrogen, membrane, oxide, permeability, proton, stability, strontium, tubular
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The protonic-electronic mixed conductors have received great attention for their potential applications particularly for the hydrogen separation from hydrocarbon. Strontium cerate doped with europium has been investigated to maximize hydrogen production as proton conducting membrane. The optimal europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurement were performed. Among the three different compositions of Eu doped SrCe1-xEuxO3 (x = 0.1, 0.15 and 0.2) studied, SrCe0.9Eu0.1O3 showed highest total conductivity between 773K and 1173K under both dry/wet hydrogen conditions. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe0.85Eu0.15O3 to SrCe0.8Eu0.2O3 depending on temperature. A tubular type Ni-SrCeO3 support was developed for investigating the hydrogen permeability of various amounts Eu doped strontium cerate hydrogen membrane film. Appreciable and stable hydrogen permeation through this thin film membrane was observed under humid hydrogen condition and it's PH21/4 dependence of hydrogen permeation flux agrees with Norby and Larring's model which protons and electrons are dominating defects. Activation energy for hydrogen permeation flux suggests that the hydrogen permeation flux is limited by electrons. However, this configuration showed cracking due to chemical reaction with CO/CO2 under methane steam reforming condition. SrZrO3 show good chemical stability under same conditions. Therefore, Zr ion also introduced into membrane and support material to increase chemical stability. SrZr0.2Ce0.7Eu0.1O3 membrane coated into the Ni-SrCe0.8Zr0.2O3 support cell achieved stable hydrogen permeation flux and chemical stability under steam reforming condition. Based on the Wagner equation, hydrogen permeation flux is inversely proportional to the membrane thickness. We expected increased hydrogen permeation with increasing dopant level in the membrane material due to increased electronic conduction for ambipolar diffusion. Therefore SrZr0.2Ce0.8-xEuxO3 (x=0.1, 0.15 and 0.2) hydrogen membranes were investigated for their hydrogen permeation properties and mechanism according to their membrane thickness. Their inversely proportional linear dependence of membrane thickness indicates bulk diffusion is the rate-limiting factor for hydrogen permeation. With respect to water vapor formation, higher Eu doping in the membrane permeates more hydrogen, in which water vapor was formed by reaction of oxygen gas from oxide lattice.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Wachsman, Eric D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-11-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022071:00001


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OPTIMIZATION OF EUROPIUM DOPANT CONCENTRATION IN STRONTIUM CERATE MIXED-CONDUCTING CERAMIC MEMBRANE FOR MAXIMUM HYDROGEN PRODUCTION By TAKKEUN OH A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008 1

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2008 Takkeun Oh 2

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To my parents, wife Jieun and lovely sons Eric & Trevor. 3

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ACKNOWLEDGMENTS The fulfillment of the research work for this dissertation would be impossible without assistant. I especially thank my advisor, Prof. Eric D. Wachsman who introduced me to the field of solid-state electrochemistry, guided me through the entire course of study, and encouraged me to a higher level of success. I learned a lot from him, not only in science but also in life and I will always remember this valuable and rewarding graduate school experience over the years that I have worked with him. I would also like to thank Susan Sinnott, Wolfgang Sigmund, Juan Nino and Mark Orazem for their advice and participation as part of my supervisor committee. I would like to acknowledge Dr. Heesung Yoon for his valuable discussion on the various theoretical aspects of this field and his indispensable processing assistance. Thanks to Keith Duncan for his valuable comments, understanding, constant encouragement and friendship. I also wish to acknowledge my group members Sunju Song, Junyoung Park, Guoging, Martin Van Assa, Dohwon Jung, Jinsoo Ahn, Dongjo Oh and everyone else for numerous discussion and debates throughout the years. I thank to my parent and sister for their forever support and trust. I would also like to thank to my wifes parents for welcoming me into their family, entrusting their precious daughter to me. Finally, I am entirely grateful to my beautiful wife, Jieun An, for her constant encouragement and giving me two lovely sons, Yoochan (Eric) and Uichan (Trevor). 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 1 INTRODUCTION..................................................................................................................15 2 LITERATURE REVIEW.......................................................................................................20 2.1 Hydrogen Production Technology ....................................................................................20 2.2 Membrane Materials for Hydrogen Separation ................................................................20 2.3 Proton Conducting Ceramic Membrane ...........................................................................22 2.4 Crystal Structures of BaCeO and SrCeO 3 3 .......................................................................23 2.5 Proton Incorporation and Conduction Mechanism ...........................................................23 2.6 Chemical Stability of Perovskite Oxide ...........................................................................26 2.7 Electrical Properties of SrCeO System 3 ...........................................................................27 2.8 Hydrogen Permeation .......................................................................................................29 3 TOTAL CONDUCTIVITIES OF VARIOUS AMOUNT EU-DOPED STRONTIUM CERATES...............................................................................................................................40 3.1 Introduction .......................................................................................................................40 3.2 Experimental .....................................................................................................................40 3.3 Result and Discussion .......................................................................................................41 4 TRANSFERENCE NUMBER BEHAVIOUR OF DIFFERENT AMOUNT EUROPIUM DOPED STRONTIUM CERATES UNDER DRY/WET HYDROGEN ATMOSPHERE......................................................................................................................48 4.1 Introduction .......................................................................................................................48 4.2 Experimental .....................................................................................................................50 4.3 Result and Discussion .......................................................................................................51 4.3.1 Transference Number Behavior of SrCeEuOunder Dry/Wet H Atmosphere 1-x x 32 ..................................................................................................................51 4.3.2 Proton, Electron and Ambipolar Conductivity of SrCeEuO 1-x x 3 ............................52 5 FABRICATION OF TUBULAR TYPE MEMBRANE.........................................................64 5.1 Introduction .......................................................................................................................64 5.2 Membrane Supporting Tube: Materials and Fabrication ..................................................64 6 HYDROGEN PERMEATION THROUGH THE TUBULAR TYPE MEMBRANE...........76 5

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6.1 Introduction .......................................................................................................................76 6.2 Experimental .....................................................................................................................76 6.3 Result and Discussion .......................................................................................................77 6.3.1. Hydrogen Permeation Properties of 10ESC/Ni-SrCeO at Dry / Wet Hydrogen Atmosphere 3 .................................................................................................77 6.3.2. Hydrogen Permeation Properties from CH Steam Reforming 4 .............................80 6.3.3. Modification of Tubular-Type Hydrogen Permeation Membrane Cells ...............81 6.3.3.1. 10ESC / Ni-SrCeZr0O tubular-type supports 0.8 0.2 3 .....................................82 6.3.3.2. SrCeZrEuO (x = 0.1, 0.15 & 0.2) / Ni-SrCeZr0O tubular-type hydrogen membrane cells 0.7-x 0.2 x 30.8 0.2 3 .............................................................................82 7 EFFECT OF ZIRCONIUM ION DOPING ON ELECTRICAL PROPERTY AND CHEMICAL STABILITY OF 10 ESC PROTON CONDUCTORS...................................117 7.1 Introduction .....................................................................................................................117 7.2 Experimental ...................................................................................................................118 7.3 Result and Discussion .....................................................................................................119 8 CONCLUSIONS AND FUTURE WORKS.........................................................................131 APPENDIX A AMBIPOLAR DIFFUSION................................................................................................135 B GRTTHUSS MECHANISM AND VEHICLE MECHANISM........................................137 C LATTICE PARAMETER CALCULATION.......................................................................138 LIST OF REFERENCES.............................................................................................................139 BIOGRAPHICAL SKETCH.......................................................................................................143 6

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LIST OF TABLES Table page Table 2-1. Major hydrogen production processes..........................................................................38 Table 2-2. Properties of relevant hydrogen selective membranes.................................................39 Table 4-1. OCV data under the first condition.[100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar].........................................................................................................................62 Table 4-2. OCV data for hydrogen/water vapor concentration cell 1 and 2..................................63 Table 5-1. Slurry composition of SC containing nickel catalyst for tape casting process.............74 Table 5-2. Preparation steps of the tubular hydrogen membrane cell and related processes........75 Table 6-1. Activation energies of hydrogen flux from figure 6-9 for ESC membrane in 700-900................................................................................................................................114 Table 6-2. Change of support materials pre-sintering temperature and problems....................115 Table 6-3. SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.2 Ce 0.65 Eu 0.15 O 3membranes water vapor fluxes on the outlet side depending on thickness.............................................................................116 Table 7-1. Tolerance numbers of SrCe 1-x Zr x O 3.........................................................................130 7

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LIST OF FIGURES Figure page Figure 1-1. Integration of proton transport membranes in conversion of hydrocarbon fuels to H 2 .......................................................................................................................................19 Figure 2-1. Conductivity of some proton conductors as function of 1/T.......................................31 Figure 2-2. Crystal structures of an idealized cubic BaCeO 3 BaCeO 3 and SrCeO 3. .....................32 Figure 2-3. Predominant proton transfer between oxygen sites in the CeO 6 octahedra of orthorhombically distorted BaCeO 3 and SrCeO 3 ...............................................................33 Figure 2-4. Total conductivity vs. 1000/T (K), with various dopant SrCeO 3systems under dry conditions.....................................................................................................................34 Figure. 2-5. Arrhenius plots of total conductivity of SrCe 0.95 Eu 0.05 O 3with a various P H2 under dry condition and wet condition..............................................................................35 Figure 2-6. Hydrogen flux as a function of temperature for SrCe 0.95 Eu 0.05 O 3and SrCe 0.95 Sm 0.05 O 3..............................................................................................................36 Figure 2-7. Hydrogen fluxes as a function of applied hydrogen chemical potential gradients in 0 < P H2O < 0.086atm at 850........................................................................................37 Figure 3-1. XRD pattern of Eu doped SrCe 1-x Eu x O 3and lattice parameter/volume vs Eu dopant.................................................................................................................................43 Figure 3-2. Total conductivity of A) SrCe 0.9 Eu 0.1 O 3, B) SrCe 0.85 Eu 0.15 O 3, and C) SrCe 0.8 Eu 0.2 O 3as a function of P H2 ..................................................................................45 Figure 3-3. Total conductivity of different Eu doped SrCe 1-x Eu x O 3between 600 and 900 under dry and wet hydrogen atmosphere................................................................46 Figure 3-4. Activation energy behavior of different Eu doped SrCe 1-x Eu x O 3between 600 and 900 under dry and wet hydrogen atmosphere.........................................................47 Figure 4-1. Experimental set-up for transference number measurements.....................................55 Figure 4-2. Measured proton and electron transference number under the first condition. [100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar].................................................56 Figure 4-3. Measured proton and electron transference number depends on the different hydrogen chemical potential gradient................................................................................57 Figure 4-4. Transference number of each species in SrCe 1-x Eu x O 3under hydrogen/water vapor atmosphere...............................................................................................................58 8

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Figure 4-5. Proton and electron conductivity behavior of SrCe 1-x Eu x O 3with different temperatures under dry hydrogen atmosphere...................................................................59 Figure 4-6. Proton and electron conductivity behavior of SrCe 1-x Eu x O 3with different temperatures under hydrogen/water vapor atmosphere.....................................................60 Figure 4-7. Ambipolar conductivity behavior depends on the dopant concentration under dry hydrogen and hydrogen/water vapor atmospheres............................................................61 Figure 5-1. Tape caster for making ceramic green tape.................................................................67 Figure 5-2. Schematic process flow chart for Eu-doped SrCeO 3dense layers on porous Ni-SrCeO 3 tubular type supports.............................................................................................68 Figure 5-3. Process sequence for fabricating one end closed green body supports......................69 Figure 5-4. Shrinkage rates for 10 mol % Eu-doped and undoped SrCeO 3 prepared by using an uni-axial press technique and SrCeO 3 with 30wt% NiO green tape by tape casting, and microstructures for 10 mol % Eu-doped SrCeO 3 thin layers which were coated on NiO-SrCeO 3 support partially sintered at 1100 in air atmosphere according to the final sintering temperature...........................................................................................70 Figure 5-5. Cross-sectional view of A) coated 10ESC film on partially sintered NiO-SrCeO 3 and B) dense 10ESC film on a Ni-SrCeO 3 support after sintering and reduction in hydrogen atmosphere.........................................................................................................71 Figure 5-6. Photo-graphs of the 15cm long one end closed tubular-type hydrogen membrane cells according to the processing steps..............................................................................72 Figure 5-7. Pictures of a six-inch unit cell of 10 mol% Eu-doped SrCeO 3 membrane coated on the inner side of Ni-SrCeO 3 tubular support at each processing step...........................73 Figure 6-1. Schematic diagram for the hydrogen permeation reactor system...............................86 Figure 6-2. Configuration of the hydrogen permeation reactor using a 15cm long one end closed tubular-type hydrogen membrane cell....................................................................87 Figure 6-3. Experimental set-up for hydrogen permeation test.....................................................88 Figure 6-4. 6 tubular type hydrogen membrane cell and its temperature gradients at the set point temperature...............................................................................................................89 Figure 6-5. Scanning electron micrograph for the cross sectional view of 10ESC/Ni-SrCeO3 hydrogen membrane cell after sintered and reduced.........................................................90 Figure 6-6. Hydrogen flux as a function of P H2 at 850..............................................................91 Figure 6-7. Hydrogen flux and water vapor formation as a function of P H2 at 850...................92 9

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Figure 6-8. Hydrogen flux as function of P H2 according to T in 3% humid hydrogen..................93 Figure 6-9. Hydrogen flux as function of T in 3% humid hydrogen.............................................94 Figure 6-10. Hydrogen permeation properties with time at 850 in dry and humid hydrogen atmosphere.........................................................................................................................95 Figure 6-11. Thickness dependence of hydrogen permeation flux of SrCe 0.9 Eu 0.1 O 3/ Ni-SrCeO 3 hydrogen membrane cells under 3% humid hydrogen.........................................96 Figure 6-12. 2nd case membrane which 15ESC peel off from the Ni-SrCeO 3 support................97 Figure 6-13. Hydrogen flux in humid hydrogen/methane atmospheres as a function of temperature........................................................................................................................98 Figure 6-14. Thermodynamic calculation data for methane steam reforming according to CH 4 /H 2 ratio.....................................................................................................................102 Figure 6-15. Thermodynamic calculation of carbon formation for CH 4 /H 2 O ratio.....................103 Figure 6-16. Photograph of the 10ESC / Ni-SrCeO 3 and 10ESC / Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cells after exposed in methane with 18% steam............................................104 Figure 6-17. Hydrogen flux for 10ESC / Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cell as a function of P H2 in humid hydrogen at 900...................................................................105 Figure 6-18. SEM images of different thicknesss SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes.106 Figure 6-19. Hydrogen permeation flux of different thicknesss SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes depends on hydrogen partial pressure and temperature...............108 Figure 6-20. SEM images of different thicknesss SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes.......................................................................................................................109 Figure 6-21. Hydrogen permeation flux of different thicknesss SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes depends on hydrogen partial pressure and temperature...............111 Figure 6-22. Hydrogen permeation flux of SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.2 Ce 0.65 Eu 0.15 O 3membranes with thickness at 900................................................................................112 Figure 6-23. Permeated hydrogen from SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.2 Ce 0.65 Eu 0.15 O 3with counting hydrogen from water formation........................................................................113 Figure 7-1. X-ray diffraction patterns of SrCe 0.9 Eu 0.1 O 3after being exposed to dry hydrogen atmosphere and wet hydrogen atmosphere at 700 < T < 900 .....................123 Figure 7-2. Conductivity change of SrCe 0.9 Eu 0.1 O 3with time under dry/wet hydrogen atmosphere at 800 and 900......................................................................................124 10

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Figure 7-3. SEM & EDS image of A) before and B) after H 2 exposed SrCe 0.9 Eu 0.1 O 3............125 Figure 7-4. X-ray diffraction pattern of Zr substituted Eu doped SrCeO 3 ...................................126 Figure 7-5. Compared powder X-ray pattern of calcined SrCe 0.9 Eu 0.1 O 3, calcined SrZr 0.2 Ce 0.7 Eu 0.1 O 3and dry hydrogen exposed SrZr 0.2 Ce 0.7 Eu 0.1 O 3during 24 hr at 900................................................................................................................................127 Figure 7-6. Total conductivity of SrCe 0.9 Eu 0.1 O 3, SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.3 Ce 0.6 Eu 0.1 O 3under dry pure H 2 atmosphere........................................................128 Figure 7-7. Conductivity changes of SrCe 0.9 Eu 0.1 O 3and SrZr 0.2 Ce 0.7 Eu 0.1 O 3for the stability test under dry hydrogen atmosphere was performed at 900 for 3 days.........129 Figure A-1. Drift flux by electric field.........................................................................................136 Figure B-1. Illustration of Grtthuss mechanism and vehicle mechanism..................................137 11

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LIST OF ABBREVIATIONS SC Strontium Cerate ESC Europium doped Strontium Cerate 10ESC Europium doped Strontium Cerate SrCe 0.9 Eu 0.1 O 315ESC Europium doped Strontium Cerate SrCe 0.85 Eu 0.15 O 320ESC Europium doped Strontium Cerate SrCe 0.8 Eu 0.2 O 310EZSC Zirconium substituted Europium doped Strontium Cerate SrZr 0.2 Ce 0.7 Eu 0.1 O 315EZSC Zirconium substituted Europium doped Strontium Cerate SrZr 0.2 Ce 0.65 Eu 0.15 O 320EZSC Zirconium substituted Europium doped Strontium Cerate SrZr 0.2 Ce 0.6 Eu 0.2 O 3Ni-SC Ni and Strontium cerate support Ni-SZC Ni and Zirconium substituted strontium cerate 12

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy OPTIMIZATION OF EUROPIUM DOPANT CONCENTRATION IN STRONTIUM CERATE MIXED-CONDUCTING CERAMIC MEMBRANE FOR MAXIMUM HYDROGEN PRODUCTION By Takkeun Oh May 2008 Chair: Eric Wachsman Major: Materials Science and Engineering The protonic-electronic mixed conductors have received great attention for their potential applications particularly for the hydrogen separation from hydrocarbon. Strontium cerate doped with europium has been investigated to maximize hydrogen production as proton conducting membrane. The optimal europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurement were performed. Among the three different compositions of Eu doped SrCe 1-x Eu x O 3(x = 0.1, 0.15 and 0.2) studied, SrCe 0.9 Eu 0.1 O 3showed highest total conductivity between 500 and 900under both dry/wet hydrogen conditions. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe 0.85 Eu 0.15 O 3to SrCe 0.8 Eu 0.2 O 3depending on temperature. A tubular type Ni-SrCeO 3 support was developed for investigating the hydrogen permeability of various amounts Eu doped strontium cerate hydrogen membrane film. 13

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Appreciable and stable hydrogen permeation through this thin film membrane was observed under humid hydrogen condition and its [P H2 ] 1/4 dependence of hydrogen permeation flux agrees with Norby and Larrings model which protons and electrons are dominating defects. Activation energy for hydrogen permeation flux suggests that the hydrogen permeation flux is limited by electrons. However, this configuration showed cracking due to chemical reaction with CO/CO 2 under methane steam reforming condition. SrZrO 3 show good chemical stability under same conditions. Therefore, Zr ion also introduced into membrane and support material to increase chemical stability. SrZr 0.2 Ce 0.7 Eu 0.1 O 3membrane coated into the Ni-SrCe 0.8 Zr 0.2 O 3 support cell achieved stable hydrogen permeation flux and chemical stability under steam reforming condition. Based on the Wagner equation, hydrogen permeation flux is inversely proportional to the membrane thickness. We expected increased hydrogen permeation with increasing dopant level in the membrane material due to increased electronic conduction for ambipolar diffusion. Therefore SrZr 0.2 Ce 0.8-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membranes were investigated for their hydrogen permeation properties and mechanism according to their membrane thickness. Their inversely proportional linear dependence of membrane thickness indicates bulk diffusion is the rate-limiting factor for hydrogen permeation. With respect to water vapor formation, higher Eu doping in the membrane permeates more hydrogen, in which water vapor was formed by reaction of oxygen gas from oxide lattice. 14

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CHAPTER 1 INTRODUCTION People have looked forward to developing alternative fuels for replacing high price crude oil. In consideration for the environment, hydrogen has received increasing attention as a clean fuel. Conventionally, hydrogen has been produced by steam reforming reaction. Natural gas has received great attention as a H 2 source due to its abundance. A steam reforming process is the most cost effective way to produce hydrogen today. In the natural gas steam reforming process, natural gas is exposed to high temperature steam to produce hydrogen, carbon monoxide and carbon dioxide as follow. Step 1: CH 4 + H 2 O CO + 3H 2 (1-1) Step 2: CO + H 2 O CO 2 + H 2 (1-2) This hydrogen can then be separated from the synthetic gas mixture through the hydrogen separation membrane. The advancement of hydrogen separation technology has become important in energy applications for production of petrochemicals and hydrogen for fuel cells [1-4]. High temperature proton conductors based on perovskite-structured oxides have received increasing attention for hydrogen separation technologies. The separation process of protonic conductors is purely ionic transport, and results in nearly 100 % pure hydrogen. Dense membranes using proton conducting oxide can be used to extract pure hydrogen from hydrogen-containing gases such as fossil fuels and natural gases [5-9]. For extraction or production of pure hydrogen from natural gases using the proton conductors, some researchers have developed hydrogen pumping systems by applying a potential across proton conductors through its electrodes [10]. The other group studied non-galvanic cell systems [11]. Compared to the hydrogen pumping system, the non-galvanic cell for hydrogen permeation is a more cost 15

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effective and convenient system because electrodes on both sides of hydrogen permeation membrane are not required to compose a cell. Recently, a series of high-temperature proton conducting ceramics, typically doped with multivalent cations, have been studied, based on BaCeO 3[12-14], SrCeO 3[15-17], and complex perovskites in the form of A 2 BBO 6 and A 3 BB 2 O 9 [18-20]. BaCeO 3based oxide exhibit oxygen ion conductivity comparable to their proton conductivity and thus is not desirable for hydrogen separation applications where oxygen is present [21, 22]. According to previous reports by Knight and Bonanos et al., distorted orthorhombic structure of SrCeO 3inhibits oxygen ion conduction enabling high transference number for proton conduction [23, 24]. Therefore, SrCeO 3 based oxides have promise for selective hydrogen separation if their electronic conductivity can be improved by proper doping. Also, T. Norby and Y. Larring derived the flux of each carrier is inversely proportional to membrane thickness in a mixed conductor which will be handled detail on Chapter 2[25]. Therefore, our research has been directed towards the development of thin film hydrogen membranes using tubular-type porous supports for increasing the hydrogen production by reducing the thickness of the membrane. Figure 1 illustrates the basic design for thin film hydrogen membranes producing pure hydrogen through the methane steam reforming process. For a hydrogen production cell, a hydrogen permeable thin film is coated on the inner-side of a catalytic tubular-type porous support. A methane and steam mixture is flowed on the outer side of a reactor cell. At operating temperatures, CO, CO 2 and H 2 are formed, and due to the hydrogen partial pressure gradient, the produced H 2 permeates through the hydrogen permeable thin film. 16

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According to Strotz and Wagner, an increase in protonic conduction is expected by increasing the amount of acceptor dopant on the B site [26]. However, excess doping may lead to a decreasing conductivity via formation of secondary phase and defect clustering. Also, to achieve high H 2 permeability, electronic conductivity should be comparable to protonic conductivity. Therefore, the maximum hydrogen permeation can be achieved with the composition that results in maximum ambipolar conductivity which has more detail explanation on Appendix A. The overall goal of my research is to demonstrate the feasibility of producing hydrogen from hydrocarbon based fuels using advanced proton conducting membranes. To achieve this goal, many factors need to be considered including optimization of dopant level for electrical properties, stability under CO/CO 2 atmosphere, CH 4 /H 2 O ratio optimization for maximize hydrogen production and evade carbon deposition in same time, sealing etc. The objective of my research is the optimization of Eu dopant level in SrCeO 3 to maximize the ambipolar conductivity causing high hydrogen permeation flux. In addition, the effect of Zr ion substitution to electrical property, chemical stability and permeability was also investigated. Finally, permeation measurement was performed with thin film (micrometer thickness) membrane fabrication. Optimization of Eu dopant level in SrCeO 3 to maximize ambipolar conductivity: To find optimized dopant concentration in SrCeO 3 we conducted a study of the electrical properties of SrCe 1-x Eu x O 3(x = 0.1, 0.15 and 0.2) in various oxygen and hydrogen atmosphere. The total conductivity was measured by impedance spectroscopy which allows one to extract bulk resistance from complex impedance as a function of frequency (Chapter 3). To find the 17

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maximum ambipolar conductivity, transference number was calculated by measuring open circuit voltage under reduced atmosphere (Chapter 4). Improvement of structural stability of SrCeO 3 : The zirconate-based oxides, such as SrZrO 3 show good chemical stability and mechanical strength and they are more stable against carbon dioxide gas, which reacts with cerate materials below 800 [27-29], though the conductivity of zirconate is lower than that of the cerates. However, it is possible to achieve a compromise between the proton conductivity and chemical stability using a mixed solid solution of SrCeO 3 and SrZrO 3 In this study, four different zirconium dopant contents were attempted to improve the stability of SrCeO 3 under steam reforming reaction atmosphere. The possible SrCeO 3 decomposition mechanism was investigated using SrCe 0.8 Zr 0.2 O 3 (denoted SCZ82) powder. Also, the stability of SrCe 0.9 Eu 0.1 O 3and SrZr 0.2 Ce 0.7 Eu 0.1 O 3under hydrogen atmosphere was studied by investigating secondary phase formation (XRD), total conductivity, and permeability. (Chapter 7) Permeation measurement using thin film membrane fabrication: The fabrication of hydrogen separation membranes was performed by tape casting and rolling for end capped tubular type supports (Ni-SrCeO 3 ) and by slurry coating to form dense Eu doped SrCeO 3 hydrogen permeable membrane thin films on the inner-side of tubular type supports. For the development of the seals on hydrogen permeation membrane cells, our group has developed a one-end-closed tubular type ESC/Ni-SrCeO 3 hydrogen membrane cell via tape casting and colloidal coating techniques. (Chapter 5) In this work, hydrogen permeation properties were studied for 15cm long one end closed tubular type ESC/Ni-SrCeO 3 hydrogen membrane cells. Due to chemical stability issue under steam reforming condition, several different configurations 18

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of cell were fabricated and tested their performance in hydrogen permeation with introduction of Zr ion into membrane and support materials. (Chapter 6) Figure 1-1. Integration of proton transport membranes (including SEM of membrane) in conversion of hydrocarbon fuels to H 2 [adapted from E.D. Wachsman and M.C. Williams, Hydrogen Production for Fossil Fuels with Proton and Oxygen-Ion Transport Membranes, Interface (2004)]. 19

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CHAPTER 2 LITERATURE REVIEW 2.1 Hydrogen Production Technology A transition to hydrogen as a major fuel in the next 50 years could fundamentally transform the U.S. energy system, creating opportunities to increase self-supporting energy through the use of a variety of domestic energy sources for hydrogen production while reducing environmental impacts, including atmospheric CO 2 emission and criteria pollutants. However, hydrogen dose not exist alone in nature. Therefore, all hydrogen production processes are based on the separation of hydrogen from hydrogen-containing feedstock including biomass, coal, oil, natural gas, sunlight, wind, water, nuclear power and geothermal power. There are three primary methods to separate hydrogen: thermal, chemical and biological as shown in Table 2-1 [3, 29, 30]. Today, 95% of the hydrogen produced in the U.S. uses a thermal process with natural gas or the light hydrocarbons [4]. This process, called steam methane reformation (SMR) contains three main steps: steam reforming (equation 1-1), water gas shift reaction (equation 1-2) and purification. The hydrocarbons are initially reacted at high temperatures with steam to produce hydrogen, carbon monoxide and carbon dioxide. The water gas shift reaction produces more hydrogen and carbon dioxide after converting the carbon monoxide with steam. After these processes, membrane is required to extract high-purity hydrogen from the synthesis gas stream (mainly mixture of hydrogen and carbon dioxide). 2.2 Membrane Materials for Hydrogen Separation Hydrogen selective membranes can be classified into four categories: dense polymeric, dense metallic, porous carbon and micro porous or dense ceramic. Table 2-2 summarizes their properties [29, 31]. 20

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Polymeric membranes are dense, transporting species through a bulk of the material. The polymeric operates around 90-100 and shows good endurance against high pressure drops while achieving low cost. However, it shows limited resistance to certain chemicals, weak mechanical strength, and relatively low hydrogen selectivity. If very pure hydrogen (>99.99%) is required, dense metallic membranes is a proper membrane material. Palladium and palladium alloys usually used in this membrane are very selective to hydrogen [32-35]. Their operating temperature is in the range 300-600. However, they can be seriously damaged when palladium membranes are exposed to hydrogen at lower temperatures. Hydrogen becomes locked inside the palladium at low temperature and therefore causes stresses inside the membrane, increasing the likelihood of membrane failure. Their high sensitivity to chemicals such as sulphur, chlorine and even CO is also a drawback for the effective hydrogen flux. Carbon membranes can be used in non-oxidizing environments at 500 < T < 900. They are, however, brittle. Ceramic membranes can be either porous or dense. In dense ceramic membranes, so-called proton conducting membranes, hydrogen is transported through the solid phase as ions (protons). Preferred materials for this application are SrCeO 3and BaCeO 3since their H 2 selectivity is very high. They work generally at 600 < T < 900. Limited chemical stability in the presence of certain species (e.g., CO 2 and H 2 S) is a major concern. The temperature of steam reforming reaction is over 700. Therefore, the ceramic proton conducting membrane can be applied to this harsh condition. 21

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2.3 Proton Conducting Ceramic Membrane Some of solid oxides with perovskite (ABO 3 ) and related structures are of interest as proton selective membrane materials [1, 36-41]. The perovskite oxides should have both high electronic and protonic conductivity (mixed protonic electronic conductor), and be structurally stable in reducing atmospheres. The electronic conductivity is commonly given by mixed valence states of the B-site transition metal ions. The oxygen vacancies, typically created by acceptor doping, function as pathways for mobile proton containing species. At elevated temperature, when a gradient of hydrogen chemical potential is applied across the membrane, this mixed conductor works as a H 2 permeation membrane non-galvanically. The majority of these perovskite oxides are doped with low-valent cations. In the presence of water, proton conduction occurs via a transport of hydroxyl group through the oxygen vacancies. A transference number of proton conduction should be much higher than that of oxygen conduction to achieve a highly H 2 selective membrane. In addition, one can expect high permeability when transference number of electronic conduction is comparable to that of protonic conduction [15, 16]. Figure 2-1 shows the total conductivities of several electronic-protonic conducting ceramics, including SrCeO 3[15-17], SrZrO 3[28, 42], and BaCeO 3[12-14] doped with trivalent cations such as Eu, Tb and Yb. BaCeO 3based oxides exhibit oxygen ion conductivity comparable to their proton conductivity and thus is not proper for applications where oxygen is present [21, 22]. According to previous reports by Kinght and Bonanos et al., a distorted orthorhombic structure of SrCeO 3inhibits oxygen ion conduction, showing high transference number for proton condition [23, 24]. Therefore, SrCeO 3based oxides can be promising for selective hydrogen separation if their electronic conductivity can be improved by a proper doping. 22

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2.4 Crystal Structures of BaCeO 3 and SrCeO 3 The crystal structures of BaCeO 3 SrCeO 3 and an idealized cubic BaCeO 3 are shown in Figure 2-2. The CeO 6 octahedron is very regular with an average bond length of 2.240 in BaCeO 3 and 2.246 in SrCeO 3 The A site cations are eightfold coordinated in the center of a triangular prism. Figure 2-2 shows the principal difference between the two compounds lies in the magnitude of the rotations around the pseudo-cubic axes and the displacement of the A site cation from its ideal position. The smaller ionic radius of Sr 2+ (1.26 in eightfold coordination) results in larger tilts of the CeO 6 octahedron than in the case of Ba 2+ (1.42 in eightfold coordination). The larger rotations also give rise to a significantly greater spontaneous strain in SrCeO 3 at room temperature, which is an order of magnitude greater than BaCeO 3 [43]. A departure from cubic symmetry might be expected to increase the activation energy via the non-equivalence of oxygen lattice sites. Furthermore, BaCeO 3 is known to undergo a complex sequence of phase transitions, including a change from orthorhombic to rhombohedral symmetry where the two distinct oxygen sites become crystallographically equivalent. By contrast, SrCeO 3 undergoes no high temperature structural phase transitions up to 1000. This difference between these two materials may well be related to the A-site cation radius which results in such a large spontaneous strain in SrCeO 3 [44]. 2.5 Proton Incorporation and Conduction Mechanism The introduction of protons into a perovskite oxide lattice generally occurs by interaction with a moisture-containing atmosphere or hydrogen atmosphere. The most important reaction leading to the formation of proton defects at moderate temperatures is the dissociative absorption of water, which requires the presence of oxide ion vacancies, The vacancies may be formed OV 23

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intrinsically by varying the ratio of the main constituents or they may be formed extrinsically to compensate for an acceptor dopant. In order to form protonic defects, water from gas phase dissociates into a hydroxide ion and a proton; the hydroxide ion fills an oxide ion vacancy, and the proton forms a covalent bond with lattice oxygen. Reaction of these gas streams with the oxide can be written using Krger-Vink notation. In a moisture containing atmosphere, the dissolution of water vapor into the oxide lattice can be expressed as follows [45]; OxOO2OH2O+V+)gas(OH (2-1) Alternatively, in a hydrogen atmosphere, protons can be incorporated directly into the oxide lattice along with electrons [46]. e+OHO+)gas(H21OxO2 (2-2) With Krger-Vink notation, represents oxygen vacancies, represents oxide ions on an oxygen lattice site, represents protons associated with oxide ions on an oxygen lattice site, and represents electrons. Fundamental studies of oxide proton conductors have focused on the interaction of water vapor with oxides, rather than the direct reaction with hydrogen. The interaction of water vapor is treated as two processes: the absorption of a hydroxide ion by oxygen vacancies and the protonation of a lattice oxide ion. Experimental and simulation research describing the transport of protons through perovskite oxide lattices supports using the same treatment for protons incorporated directly from hydrogen atmosphere as is used for protons incorporated from water vapor [47-50]. OV XOO OOH e Two models were proposed to describe proton conduction in oxides: the vehicle mechanism and the Grtthuss or free migration mechanism [46, 51]. With the vehicle mechanism, the proton moves as a passenger on a vehicle molecule or ion (e.g., H 3 O + ). Diffusion 24

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of this vehicle, coupled with diffusion of an unprotonated molecule in the opposite direction, results in net transport of protons. With the Grtthuss mechanism, the proton attaches to a stationary ion (e.g., ), undergoes a reorientation step, and then transfers to another stationary ion. Further explanation about conduction mechanism is on Appendix B. XOO Over the last decade, general agreement has been formed that protons transfer between fixed oxygen sites via the Grtthuss mechanism at high temperatures in ABO 3 perovskites. Isotope effect (H + /D + ) measurements of perovskite oxides have suggested that the conduction mechanism is due to proton hopping between adjacent oxygen ions (Grtthuss mechanism) rather than by hydroxyl ion migration (vehicle mechanism) [52-55]. Molecular dynamic studies in both cubic and orthorhombic perovskites proton conducting oxides support for the Grtthuss mechanism. Munch et al determined that the reorientation process occurs relatively fast (10 -12 s) compared to the proton transfer process (10 -9 s) indicating the proton transfer process is the rate-limiting step [56, 57]. Kreuer and his colleagues have been studied for the fundamental understanding of proton conduction in ABO 3 perovskites [53, 58-62]. The structural oxygen separation, the oxygen separation coordinate, and the stiffness of the B-O bond were discussed as three important parameters. Cubic perovskites with large lattice constants that correspond to large structural oxygen separation show the highest proton diffusivities [60]. This high proton mobility can be explained by increased thermal vibrations of oxygen. These oxygen vibrations are influenced by the softness of the oxygen separation coordinate, and by the stiffness of the B-O bond. Materials with high thermal expansion coefficients may exhibit this softness, which is pivotal for rapid proton transfer [53]. The stiffness of the B-O bond is a result of the covalency of the B-O bond. Low covalency in a loosely packed oxide structure with a large oxygen separation, (such 25

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as BaCeO 3 ), allows for extended oxygen vibrations and an energetically low proton transition, which facilitates proton transfer [60]. In contrast with cubic perovskites where neighboring oxygen ions are treated as equivalent sites, orthorhombic perovskites that have a low symmetry (such as SrCeO 3 ) must be treated differently. The orthorhombic distortion of SrCeO 3 doped with yttrium has major effect on the arrangement of the lattice oxygen [61]. The cubic oxygen sites degenerate to two sites (O 1 and O 2 ) with site occupancies of 1/3 and 2/3 respectively. These oxygen sites have different chemical interactions with the cations and exhibit different binding energies for protons, due to different electron densities (related to the basicity). In SrCeO 3 the most basic oxygen site is O 1 ; while in BaCeO 3 O 2 is most basic [59]. Proton transfer between oxygen sites in both SrCeO 3 and BaCeO 3 is shown in Figure 2-3 [63]. The long-range transport between O 2 sites in BaCeO 3 should be easier than transport between O 1 and O 2 sites in SrCeO 3 This difference in proton transport is a possible reason for lower conductivity in SrCeO 3 than BaCeO 3 2.6 Chemical Stability of Perovskite Oxide A high basicity of the oxide is advantageous for the formation of protonic charge carriers but basic oxides are expected to react easily with acidic gases such as CO 2 and SO 2 (SO 3 ). The stability against to the formation of carbonates and hydroxides increases in the order cerate zirconate titanates with decreasing the stability of protonic defects. The reaction of an ABO 3 perovskite with CO 2 can be broken into two reactions involving the individual metal oxides: [60, 64, 65] 23BO+AOABO (2-3) 32ACOCO+AO (2-4) 26

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The formation enthalpy of the perovskite from the binary oxides, which mainly reflects the compatibility of the cations with the perovskite structure, and the stability of the carbonate with respect to AO help to understand its thermodynamics. The enthalpy for the formation of the perovskite from the individual oxides shows a correlation with the perovskite tolerance factor, t, defined as )rr(2rrtOBOA (2-5) which describes the extent of distortion of the perovskite structure from the ideal cubic structure due to mismatch between the A-O and B-O bond lengths [66]. As mentioned earlier, this tolerance factor also increases in the order cerate zirconate titanates. For perovskite oxides with alkaline earth metals on the A-site, it is believed that the stability with respect to the above reaction is mainly determined by the choice of the B-cation. Small B-cations increase the stability and the result in improved packing densities in perovskite oxides thereby reducing the water solubility limit, whereas the big B-cations results in poor compatibility with the perovskite structure and reduce the thermodynamic stability including the stability in acidic gases [60, 67]. Although both BaCeO 3 and SrCeO 3 forms carbonate, protonic defects are better stabilized in BaCeO 3 than in SrCeO 3 which has stronger orthorhombic distortion. It suggests that the choice of the acceptor dopant, which may have some local symmetry reducing effect, is also critical to the stability of the perovskite and the protonic defects. 2.7 Electrical Properties of SrCeO 3 System The total conductivity of Eu-doped SrCeO 3[68], SrCe 0.95 Yb 0.05 O 3[27] and SrCe 0.95 Tb 0.05 O 3[69] are shown in Figure 2-4. The conductivity increases with increasing 27

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temperature for all the samples. The conductivities of Eu-doped SrCeO 3are higher than those of Tb-doped one, but smaller than those of Yb-doped one. The differences in the conductivity between these samples are obviously attributed to the effect of dopant on the properties of SrCeO 3. Song et al. explained that the difference in conductivity on the kind of the dopant is due to a hopping mechanism, where charge transfer occurs between two neighboring ions of differing oxidation state and ionization potential of different dopant. Hopping distance is an important parameter for the hopping mechanism. The ionic radius of Eu (III) is 108.9 pm, intermediate between that of Tb (IV), 90 pm, and Yb (III), 116 pm. The hopping distance of Yb (III) is smallest among them. Therefore, the various values of the conductivity ( Yb, 0.05 > Eu, 0.05 > Tb, 0.05 ) in a dry atmosphere depend inversely on the ionic radius of the dopant [15]. Also, they explained that the dependence of conductivity as the valence change of Eu ions between trivalent and divalent. This behavior may be described as a small polaron model, because the electron transport in Eu can be can be understood in terms of a charge transferred from Eu ion in high oxidation state Eu 2OP 3+ to a neighbor ion in the low oxidation state Eu 2+ [16]. The ionization dopant concentration depends on the ionization potential of each dopant under a given thermodynamic condition. The third ionization potential of Eu is 24.8 eV, which is smaller than the fourth ionization potential of Tb (39.8 eV) and almost same as the third ionization potential of Yb (25.0 eV). Therefore, Eu-doped SrCeO 3shows higher electron/hole conductivity than that of Tb-doped SrCeO 3[16]. Previous studies about SrCe 0.95 Eu 0.05 O 3have been conducted in our group [15, 16, 70-75]. Thus, even though the electrical properties of Yb-doped SrCeO 3shows higher that of Eu-doped SrCeO 3, my research will be focus on the Eu-doped SrCeO 3to understand their 28

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electrical properties and permeability according to the dopant concentration. Later, these properties of Eu-doped SrCeO 3may be compared with Yb-doped SrCeO 3. The electrical conductivity of SrCe 0.95 Eu 0.05 O 3obtained in various H 2 /N 2 mixtures with/without = 0.038atm is plotted as a function of temperature in Figure 2-5 [16]. In dry condition, Figure 2-5(A), the conductivity increases with increasing. It can be explained by equation 2-2. Figure 2-5(B) shows that the total conductivity increases with an increasing / ratio at a fixed = 0.038atm. Such dependent conductivity change is related to the equation 2-1. OH2P 2HP 2HP OH2P OH2P 2HP Reaction 2-2 is expected to move forward with increasing and reaction 2-1 is also expected to shift to the product side with, leading to decreasing, and thus to an increase in electron concentration and total conductivity. Also, decreased conductivity and slight increased activation energy as a result of increasing the ratio of / with a fixed hydrogen flow was found. It is explained that the increased contribution of protons to total conductivity may not compensate for the decreased contribution of electronic conductivity to total conductivity. Therefore, it is reasonable to say that proton conduction increases at the expense of electron conduction. Since the membrane is to be used for hydrogen gas separation, (low), in this research, the electrical conductivity measurements will be done in reduced condition. 2HP 2HP 2OP OH2P 2HP 2OP 2.8 Hydrogen Permeation Once a hydrogen chemical potential gradient, a driving force for hydrogen permeation, is applied, hydrogen will permeate through the membrane via an ambipolar diffusion of protons and electrons [76]. The motion of electrons, the minority carrier, gives rises to the hydrogen permeation by charge compensated transport of protons in the same direction [29]. 29

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Generally, a hydrogen permeation flux across the oxide membrane can be calculated using the Wagner equation (equation 3) assuming that bulk diffusion is the rate limiting step. ''2'2''2'22'2Oln)(2ln41J22OHOOHHOOOOPPPPHeVOHtOVOHtPdtttFRTPdttFRTL (2-6) where t is the total conductivity; t i is the transference number of charged species (i = ); F is the Faraday constant and and are the chemical potential gradients of oxygen and hydrogen, respectively [68]. /OOe,V,OH 2OlnPd 2HlnPd Song et al. measured the hydrogen permeability of SrCe 0.95 Eu 0.05 O 3as a function of temperature at dry 100% and at = 0.972atm with = 0.028atm. A variation in hydrogen flux with temperature is shown in Figure 2-6 [74, 75]. 2H 2HP OH2P The hydrogen fluxes increase with temperature for both systems and the SrCe 0.95 Eu 0.05 O 3exhibits higher permeability in comparison with SrCe 0.95 Sm 0.05 O 3over the entire temperature range investigated under both dry and wet conditions. The influence of applied hydrogen chemical potential gradient at various on the hydrogen permeability of SrCe OH2P 0.95 Eu 0.05 O 3and SrCe 0.95 Sm 0.05 O 3is shown in Figure 2-7 [74]. The hydrogen permeation flux of the Eu-doped specimen shows higher hydrogen permeability than that of the Sm-doped one. At both dry and wet conditions, hydrogen permeability of the SrCe 0.95 Eu 0.05 O 3increases with increasing due to increased proton and electron concentrations according to the reaction (1). As increases, hydrogen permeability decreases because of an increase which leads to a decrease in concentration of electrons. 2HP OH2P 2OP 30

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Figure 2-1. Conductivity of some proton conductors as function of 1/T [adapted from H. Iwahara, T. Esaka, H. Uchida, T. Yamauchi and K. Ogaki, Solid State Ionics 18-9 (1986), p. 1003]. 31

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Figure 2-2. Crystal structures of an idealized cubic BaCeO 3 BaCeO 3 and SrCeO 3. 32

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Figure 2-3. Predominant proton transfer between oxygen sites (shown by arrows) in the CeO 6 octahedra of orthorhombically distorted BaCeO 3 and SrCeO 3 The degree of basicity is indicated by the color of the oxygen sites (purple = more basic) [adapted from K.D. Kreuer, E. Schonherr and J. Maier, Phase separation and grain boundary proton conductivity in BaCeO 3 based ceramics, 14 th Riso International Society Materials Science (1993), pp. 297-304]. 33

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0.91.01.11.21.3 -6-5-4-3-2-1 log(/ Scm-1)1000/T Eu(0.05) doped in O2 Eu(0.05) doped in N2 Eu(0.1) doped in air Yb(0.05) doped in air Tb(0.05) doped in air Figure 2-4. Total conductivity vs. 1000/T (K), with various dopant SrCeO 3systems (under dry conditions) [Reprinted from S.J. Song, E.D. Wachsman, S.E. Dorris and U. Balachandranb, J Electrochem Soc 150 (2003) (6), p. A790 with permission from ECS]. 34

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Figure. 2-5. Arrhenius plots of total conductivity of SrCe 0.95 Eu 0.05 O 3with a various P H2 (A) dry condition, (B) wet condition (P H2O = 0.038atm) [Reprinted from S.J. Song, E.D. Wachsman, S.E. Dorris and U. Balachandran, J Electrochem Soc 150 (2003) (11), p. A1484 with permission from ECS] 35

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0.0 1005.0 10-101.0 10-91.5 10-92.0 10-92.5 10-93.0 10-93.5 10-9600650700750800850900Hydrogen Flux (mol/cm2sec)Temperature ( oC)SrCe0.95Eu0.05O3-SrCe0.95Sm0.05O3Figure 2-6. Hydrogen flux as a function of temperature for SrCe 0.95 Eu 0.05 O 3and SrCe 0.95 Sm 0.05 O 3: closed symbols: dry 100% H 2 open symbols: P H2 = 0.972atm, P H2O = 0.028atm [Reprinted from S.J. Song, E.D. Wachsman, J. Rhodes, S.E. Dorris and U. Balachandran, Solid State Ionics 167 (2004) (1-2), p. 99 with permission from Elsevier]. 36

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5.0 10-101.0 10-91.5 10-92.0 10-92.5 10-93.0 10-93.5 10-900.20.40.60.81Hydrogen Flux (mol/cm2sec)PH2 (atm)SrCe0.95Sm0.05O3SrCe0.95Eu0.05O3PH2O = 0.051 atmPH2O = 0.086 atmPH2O = 0.028 atmDryDry Figure 2-7. Hydrogen fluxes as a function of applied hydrogen chemical potential gradients in 0 < P H2O < 0.086atm at 850 [ Reprinted from S.J. Song, E.D. Wachsman, J. Rhodes, S.E. Dorris and U. Balachandran, Solid State Ionics 167 (2004) (1-2), p. 99 with permission from Elsevier]. 37

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Table 2-1. Major hydrogen production processes [adapted from Hydrogen & Our Energy Future, US Department of Energy Hydrogen Program DOE/EE-0320]. Primary Method Process Feedstock Energy Emissions Steam Reformation Natural Gas High Temperature steam Some emissions Carbon sequestration can mitigate their effect. Thermochemical Water Splitting Water High temperature heat from advance gas-cooled nuclear reactors No emissions Gasfication Coal, Biomass Steam and oxygen at high temperature and pressure Some emissions. Carbon sequestration can mitigate their effect Thermal Pyrolysis Bomass Moderately high temperature steam Some emissions. Carbon sequestration can mitigate their effect Electrolysis Water Electricity from wind, solar, hydro and nuclear No emissions Electrolysis Water Electricity from coal or natural gas Some emissions from electricity production Electrochemical Photoelectrochemical Water Direct sunlight No emissions Photobiological Water and algae strains Direct sunlight No emissions Anaerobic Digestion Biomass High temperature heat Some emissions Biological Fermentative Microorganism Biomass High temperature heat Some emissions 38

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Table 2-2. Properties of relevant hydrogen selective membranes [adapted from Status Review on Membrane system for hydrogen separation, Intermediate report EU project MIGREYD NNE5-2001-670, S.C.A. Kluiters (December 2004).]. Dense Polymer Micro porous ceramic Dense metallic Porous carbon Dense ceramic Temperature range <100 o C 200-600 o C 300-600 o C 500-900 o C 600-900 o C H2 selectivity low 5-139 >1000 4-20 >1000 H2 flux ( 10 -3 mol/m 2 s) at dP=1 bar low 60-300 60-300 10-200 6-80 Stability issues Swelling, mechanical strength Stability in H 2 O Phase transition Brittle, oxidizing Stability in CO 2 Poisoning issues HCl, SO x (CO 2 ) H 2 S, HCl, CO Strong adsorbing vapors, organics H 2 S Materials Polymers Silica, alumina, zirconia, titania, zeolites Palladium alloy Carbon Proton conducting ceramics Transport Mechanism Solution/ diffusion Molecular sieving Solution/ diffusion Surface diffusion, molecular sieving Solution/ diffusion (proton conduction) Development status Commercial by Air Products, Linde, BOC, Air Liquide Prototype tubular silica membranes available up to 90 cm. Other materials only small samples Commercial by Johnson Matthey; prototype membrane tubes available up to 60 cm Small membrane modules commercial, mostly small samples (cm 2 ) available for testing Small samples available for testing 39

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CHAPTER 3 TOTAL CONDUCTIVITIES OF VARIOUS AMOUNT EU-DOPED STRONTIUM CERATES 3.1 Introduction According to Strotz and Wagner, an increase in protonic conduction is expected by increasing the amount of acceptor dopant on the B site [26]. However, excess doping may lead to a decreasing conductivity via formation of a secondary phase and/or defect clustering. Also, to achieve high H 2 permeability, electronic conductivity should be comparable to that of protonic conductivity. Therefore, the maximum hydrogen permeation can be achieved with the composition that results in maximum ambipolar conductivity [76]. In this chapter, the determination and understanding of optimal Eu dopant concentration for hydrogen permeation was performed by studying total conductivity of SrCe 1-x Eu x O 3(x=0.05 to 0.2) under dry/wet hydrogen and oxygen atmospheres. 3.2 Experimental Polycrystalline SrCe 1-x Eu x O 3(x = 0.1, 0.15 and 0.2) samples were prepared by solid-state reaction. SrCO 3 (99.99%, Alfa Aesar), CeO 2 (99.99%, Alfa Aesar), and Eu 2 O 3 (99.99%, Alfa Aesar) in the desired stoichiometric ratio were ball-milled for at least 24h and then calcined for 10h at 1300 in air. The calcined oxides were pressed into pellets and cold-isostatic-pressed. The pellets were sintered at 1450 for 5 hr in air and X-ray diffraction spectra was utilized to confirm a single phase with the orthorhombic perovskite structure. Pt electrodes were applied on the 10 mm diameter x 2 mm thickness dense disk pellets using Pt-paste (Engelhard 6926) and heated to 1273K for 1h for electrical measurement. Conductivity measurements were performed with a Solartron 1260 Impedance Analyzer in the frequency range of 0.1 Hz to 1 MHz and temperature range of 773 to 1173 K under dry/wet pure 40

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hydrogen atmosphere. Water vapor was obtained by bubbling the hydrogen gas through de-ionized water at room temperature. 3.3 Result and Discussion X-ray diffraction was utilized to determine the phase purity of Eu doped SrCe 1-x Eu x O 3(x=0.05 to 0.2) as shown in Figure 3-1 (A). Each diffraction pattern revealed a single phase with orthorhombic perovskite structure. Figure 3-1 (B) and (C) shows the lattice parameter and volume change, which decreases with Eu dopant levels. It is consistent with Vegards law. The electrical conductivity of SrCe 1-x Eu x O 3(x = 0.1, 0.15 and 0.2), under various hydrogen atmospheres, was also investigated. In figure 3-2, the plot of the electrical conductivity against in dry hydrogen atmospheres shows a good linear relationship between and which is consistent with Kosaci et al. for Yb-SrCeO 4/12HP tot 4/12HP 3 [77]. The hydrogen dependence of the conductivity can be explained if we assume that some degree of n-type conduction occurs because of the formation of electrons, as indicated by the equation. /2222)(eOHOgHOxO ; 4322212][KKKPnOHKHO (3-1) ; /222)()(eVgOHOgHOxO 2222][HOOHPnVPK (3-2) xOOOhgOHgOOH22)()(21222 ; 2122322][OOOHPOHpPK (3-3) The total conductivity of SrCe 0.9 Eu 0.1 O 3was higher than that of SrCe 0.85 Eu 0.15 O 3and SrCe 0.8 Eu 0.2 O 3, which shows similar values. Also, SrCe 0.9 Eu 0.1 O 3was more dependants on P H2 than others. It can be explained the dominant electrical conduction species of SrCe 0.9 Eu 0.1 O 341

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was proton, on the contrary the SrCe 0.85 Eu 0.15 O 3and SrCe 0.8 Eu 0.2 O 3showed the increased electronic conduction mechanism. Figure 3-3 shows total conductivity behavior of different Eu doped SrCe 1-x Eu x O 3under (A) dry H 2 atmosphere and (B) wet H 2 atmosphere. Every composition showed an increase in conductivity with increasing temperatures; the highest total conductivity was observed for 10 mol% Eu-doped strontium cerate between 600 and 900 in both conditions. Total conductivity for both conditions decreased as the dopant concentration increased. Although total conductivity in dry H 2 condition shows a higher value than in the wet H 2 condition, the latter was more affected than the former condition by dopant level. Figure 3-4 (A) shows similar activation energies (0.49 ~0.59eV) up to 15 mol% Eu doped strontium cerate. However, higher activation energy (0.89eV) for 20 mol% Eu doped strontium cerate was observed. On the contrary, under wet hydrogen atmosphere, activation energy was increased from 0.70eV to 0.77eV with dopant concentration as shown in Figure 3-4 (B). Generally, the mobility of proton defects and electrons in perovskite-type oxides has activation enthalpies of the order of 0.4-0.6eV and 1eV, respectively [78]. The decreased total conductivity and increased activation energy can be interpreted as evidence for increased electronic conduction at the expense of decreased ionic conduction. Though 10 mol% Eu doped Strontium cerate shows the highest total conductivity, maximum hydrogen permeation is achieved when the protonic conduction is comparable to the electronic conduction ( elecOHO ). Therefore, the protonic and electronic conduction for various amounts of Eu dopant was measured. However, the transference number might be measured to calculate proton and electron conduction. 42

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2030405060708090Relative Intensity2 SrCe0.95Eu0.05O3-SrCe0.9Eu0.1O3-SrCe0.85Eu0.15O3-SrCe0.8Eu0.2O3-(122)(240)(044)(252)(080)(400)(362)(166)(256)(124)(032)(002)(014) A 6.0556.066.0656.0712.2612.2712.2712.2712.285101520lattice parameter a (nm)lattice parameter b (nm)Eu Dopant B 8.48.428.448.468.488.56246266286306325101520lattice parameter c (nm)lattice volume V (nm3)Eu Dopant C Figure 3-1. A) XRD pattern of Eu doped SrCe 1-x Eu x O 3(x=0.05, 0.1, 0.15, and 0.2), B) a and b lattice parameter vs Eu dopant, and C) c lattice parameter and volume vs Eu dopant. 43

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00.0010.0020.0030.0040.0050.0060.70.750.80.850.90.951Conductivity [Scm-1]PH21/4 [atm1/4]900 oC800 oC700 oC600 oC A 00.0010.0020.0030.0040.0050.0060.70.750.80.850.90.951Conductivity [Scm-1]PH21/4 [atm1/4]900 oC800 oC700 oC600 oC B Figure 3-2. Continued. 44

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00.0010.0020.0030.0040.0050.0060.70.750.80.850.90.951PH21/4 [atm1/4]Conductivity [Scm-1]900 oC800 oC700 oC600 oC C Figure 3-2. Total conductivity of A) SrCe 0.9 Eu 0.1 O 3, B) SrCe 0.85 Eu 0.15 O 3, and C) SrCe 0.8 Eu 0.2 O 3as a function of P H2 45

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-4-3.5-3-2.5-2101520Log total (Scm-1)mol %900oC800oC700oC600oC A -4-3.5-3-2.5-2101520Log total (Scm-1)mol %800oC700oC600oC900oC B Figure 3-3. Total conductivity of different Eu doped SrCe 1-x Eu x O 3between 600 and 900 under A) dry and B) wet (P H2O = 0.038atm) hydrogen atmosphere. 46

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-4-3.5-3-2.5-20.80.850.90.9511.051.11.151.215 mol%10 mol%20 mol%[Eu] content[0.49 eV][0.89 eV][0.59 eV][Ea]Log total (Scm-1)1000/T (K) A -4-3.5-3-2.5-20.80.850.90.9511.051.11.151.215 mol%10 mol%20 mol%[Eu] content[0.70 eV][0.70 eV][0.77 eV][Ea]Log total (Scm-1)1000/T (K) B Figure 3-4. Activation energy behavior of different Eu doped SrCe 1-x Eu x O 3between 600 and 900 under A) dry and B) wet (P H2O = 0.038atm) hydrogen atmosphere. 47

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CHAPTER 4 TRANSFERENCE NUMBER BEHAVIOUR OF DIFFERENT AMOUNT EUROPIUM DOPED STRONTIUM CERATES UNDER DRY/WET HYDROGEN ATMOSPHERE 4.1 Introduction For a concentration cell exposed to different gases (I and II), OCP can be expressed as: kOC1VkIIIkkdztF (4-1) where the chemical potential of species k is ekkz ~ ~ k (4-2) k ~ is the electrochemical potential of defect k, e ~ is the electrochemical potential of electrons, is transference number for defect k, and is the charge number of defect k. kt kz In a concentration cell containing hydrogen and water vapor, the electrode reactions can be expressed as: e2+2H(g)H2 (4-3) e2+(g)O21+2HO(g)H22 (4-4) Integrating equation (4-2) over the thickness of the electrolyte, the experimental electromotive force in a hydrogen and water partial pressure gradient can be expressed as ]lnln[2E22..22alexperimentIOHIIOHVIHIIHionPPtPPtFRTO OOVOHionttt (4-5) due to the possibility of both proton and oxygen ion conductivity [78]. and are the ionic and oxygen ion transference number, respectively. The oxygen partial pressure in dry hydrogen concentration cells is around 10 iont OVt -24 ~10 -25 atm; therefore, proton concentration dominates in these cells consistent with the assumption of .iontOOHt where is the proton transference number. .OOHt 48

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When proton concentration dominates, the ionic transference number can be considered as the proton transference number because the mobility of the proton is much higher than that of oxygen vacancies. It can be expressed as eOHOOHOelecOHOHOHeneOHeOHtOOOOO][][t.....ion (4-6) where and are the proton and electron conductivity, respectively. is the concentration of protons, n is the concentration of electrons, OOH elec ][OHO .OOH is the mobility of protons and e is the mobility of electrons. The ionic transference number is not only related to the concentration of charge carriers, but also related to their mobility. OCP measurements were utilized to obtain the transference number of each mobile carrier. In OCP measurements, the material was used as an electrolyte in a concentration cell. If it was a pure ionic conductor, OCP of the cell is given by the Nernst equation (4-7), IHIIHPPnFRT22lnEth (4-7) where F is the Faraday constant and n is the number of Faraday equivalents, which flow through the cell. If this material also has electronic conductivity, the experimental value of the OCP is smaller than the theoretical one. Therefore, the ionic transference number can be obtained from equation (4-8). thionEt expE (4-8) Ionic transference number includes proton and oxygen ion transference number and can be determined as long as it is not too small, i.e. 1.0iont Therefore, the electronic transference number can be obtained since ionelectt 1 49

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4.2 Experimental OCP measurements were performed on dense disks 24 mm diameter x 1.7 mm thickness. Each disk was sintered at 1450 for 5hr with 2/min ramp rate. The planar surface of each disk was connected with two platinum lead wires to measure OCP. Figure 4-1 shows the schematic illustration of experimental set-up for transference number measurements. The samples were sealed to the alumina tube via softening of a glass sealant. The transference number of SrCe 1-x Eu x O 3in dry/wet hydrogen atmosphere was studied by employing the following cells between 500 and 900: 1) I, 100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar, II 2) I, 50% H 2 + balance Ar, Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar, II 3) I, 25% H 2 + balance Ar, Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar, II 4) I, 12.5% H 2 + balance Ar, Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar, II 5) I, 96.7% H 2 + 3.3% H 2 O, Pt / SrCe 1-x Eu x O 3/ Pt, 4% H 2 + 2.6% H 2 O +balance Ar, II 6) I, 96.7% H 2 + 10% H 2 O, Pt / SrCe 1-x Eu x O 3/ Pt, 4% H 2 + 2.6% H 2 O + balance Ar, II where I and II correspond to the two sides of the cell. The gas inlets in the set-up were positioned as close as possible to the test sample. A constant 40 cc/min H 2 /Ar mixture was controlled by BOC Edwards 825 mass flow controllers on both gas inlets. Water vapor was obtained by controlling the bubbler temperature and gas tubes were wrapped using heating tape to prevent water vapor condensation. OCP was measured using a multimeter (Keithley 2000-20). 50

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4.3 Result and Discussion 4.3.1 Transference Number Behavior of SrCe 1-x Eu x O 3under Dry/Wet H 2 Atmosphere The observed OCV under the first condition [100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar] are listed in Table 4-1, while the transference numbers calculated from the measured OCV and theoretical OCV, are presented in Figure 4-2. The measured OCVs decrease as the dopant level increases, indicating that the electronic transference number increases with Eu concentration. As previously mentioned, the oxygen partial pressure in dry hydrogen concentration cells is very low, which allows an assumption of where is the proton transference number. Therefore, and can be calculated under this dry H .iontOOHt .OOHt .OOHt elect 2 condition. The electron transference number increased with increasing Eu and temperature, with a corresponding decrease in proton transference number. This behavior is attributed to increased n-type electronic conduction by the change of Eu oxidation state [Eu 2+ Eu 3+ + e ] [15]. Greater hydrogen chemical potential gradients and higher temperature result in higher (Figure 4-3). Further, this effect increases with increasing Eu concentration due to the greater electronic conductivity affected by the Eu dopant. Greater dependence of electronic transference number upon hydrogen chemical potential is due to P elect O2 difference between cells (P O2 (1) >> P O2 (4) ). For a given total conductivity, ambipolar diffusion flux density for hydrogen reaches its maximum when the proton transference number equals the electronic transference number. SrCe 0.8 Eu 0.2 O 3shows the point of .iontOOHt at 850 under dry H 2 atmosphere. However, this measured transference number has an assumption of no oxygen conduction under dry H 2 51

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atmosphere. Therefore, another transference number measurement in hydrogen/water vapor atmosphere was conducted to investigate oxygen ion transference number. Transference numbers in hydrogen/water vapor atmosphere were measured by using the following two cells: 96.7% H 2 + 3.3% H 2 O, Pt / SrCe 1-x Eu x O 3/ Pt, 4% H 2 + 2.6% H 2 O +balance Ar (Cell 1). 90% H 2 + 10% H 2 O, Pt / SrCe 1-x Eu x O 3/ Pt, 4% H 2 + 2.6% H 2 O + balance Ar (Cell 2). The observed OCVs are listed in Table 4-2, while the transference number calculated from the OCV measurements of each species are presented in Figure 4-4. The electron transference number increases with Eu dopant level with a corresponding decrease in proton and ion transference number. It shows a higher proton transference number and a lower electron transference number than those in dry H 2 atmosphere, due to an increased Po 2 on both sides of the cell by the water vapor. Also, oxygen transference number does not change much compared with other species with Eu dopant level. This oxygen transference number is much lower than that in Y-doped BaCeO 3or Gd-doped BaCeO 3[78]. Therefore, this material is good for hydrogen separation membrane, and oxygen conduction can be ignored. 4.3.2 Proton, Electron and Ambipolar Conductivity of SrCe 1-x Eu x O 3 Proton and Electron conductivity was calculated from total conductivity data using equation 4-5 and 4-6. Figure 4-5 illustrates (A) proton and (B) electron conductivity behavior of different dopant levels in doped strontium cerate at different temperatures under dry H 2 atmosphere. Proton conductivity decreases with increasing dopant concentration due to a reduction of the proton mobility and an entropic destabilization of proton defects [59]. In contrast, electron conductivity increases with increasing Eu dopant. However, there is a slight 52

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decrease for x > 0.15 in SrCe 1-x Eu x O 3at low temperature due to the increased activation energy for electron conduction. Figure 4-6 shows (A) proton and (B) electron conductivity behavior of each sample in hydrogen/water vapor atmosphere. Both conductivities in hydrogen/water vapor atmosphere show a similar trend and lower values compared with under dry H 2 atmosphere. However, a decrease of electron conductivity in hydrogen/water vapor was greater than that in dry H 2 atmosphere due to an increased Po 2 This research is targeted for the maximum hydrogen permeation in strontium cerate. Generally, a hydrogen permeation flux across the oxide membrane can be calculated using the Wagner equation (4-9), assuming that bulk diffusion is the rate limiting step. ''2'2''2'22'2Oln)(2ln41J22OHOOHHOOOOPPPPHeVOHtOVOHtPdtttFRTPdttFRTL (4-9) where t is the total conductivity; t i is the transference number of charged species (i = ); F is the Faraday constant and and are the chemical potential gradients of oxygen and hydrogen, respectively [74]. This equation can be simplified with assumption of no oxygen conduction, with result /,,eVOHOO 2lnOPd 2lnHPd ''2'22'Oln21J2OHHHOPPHeOHtPdttFRTL (4-10) eOHeOHambOHOOOJ itotalit (4-11) Equation 4-11 shows that hydrogen permeation is proportional to the ambipolar conductivity. Figure 4-7 shows the ambipolar conductivity dependence on dopant concentration in (A) dry hydrogen atmosphere and (B) hydrogen/water vapor atmosphere. The maximum ambipolar conductivity increases with temperature and Eu dopant levels under dry hydrogen 53

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atmosphere. Therefore, the optimized dopant level which shows maximum ambipolar conductivity is obtained from 15 mol% Eu doped strontium cerate at 600 to 20 mol % Eu doped strontium cerate at 900. In contrast, ambipolar conductivity values in hydrogen/water vapor atmosphere were lower than that in dry hydrogen atmosphere, and the maximum ambipolar conductivity was observed on 20 mol % Eu doped strontium cerate in the range of investigated temperature. Therefore, maximum hydrogen flux can be achieved by controlling Po 2 dopant level, and temperature. 54

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Figure 4-1. Experimental set-up for transference number measurements. 55

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00.20.40.60.81550600650700750800850900950Transference numberProton [Eu10]Proton [Eu15]Proton [Eu20]Electron [Eu10]Electron [Eu15]Electron [Eu20]Temperature [oC] Figure 4-2. Measured proton and electron transference number under the first condition. [100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar] 56

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00.10.20.30.40.50.60.20.40.60.811.21.41.6Log (PH2II/PH2I)Electron Transference number10 mol% Eu15 mol% Eu700 oC800 oC900 oC20 mol% Eu600 oC700 oC800 oC900 oC600 oC700 oC800 oC900 oC600 oCPO2(1)PO2(2)PO2(3)PO2(4) >>> Figure 4-3. Measured proton and electron transference number depends on the different hydrogen chemical potential gradient. 57

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00.20.40.60.81550600650700750800850900950 Ion [Eu10] Electron [Eu10] Proton [Eu10] Oxygen [Eu10] Ion [Eu15] Electron [Eu15] Proton [Eu15] Oxygen [Eu15] Ion [Eu20] Electron [Eu20] Proton [Eu20] Oxygen [Eu20]Transference #Temperature Figure 4-4. Transference number of each species in SrCe 1-x Eu x O 3(x=0.1, 0.15, and 0.2) under hydrogen/water vapor atmosphere. 58

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-4.5-4-3.5-3-2.5-2101520mol %Log Proton (Scm-1)900oC800oC700oC600oC A -4.5-4-3.5-3-2.5101520mol %Log Electron (Scm-1)900oC800oC700oC600oC B Figure 4-5. Proton and electron conductivity behavior of SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) with different temperatures under dry hydrogen atmosphere. 59

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-4.5-4-3.5-3-2.5-2101520Log Proton (Scm-1)mol %900oC800oC700oC600oC A -5.5-5-4.5-4-3.5-3-2.5101520900oC800oC700oC600oCLog Electron (Scm-1)mol % B Figure 4-6. Proton and electron conductivity behavior of SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) with different temperatures under hydrogen/water vapor atmosphere. 60

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-5-4.5-4-3.5-3101520mol %Log amb (Scm-1)900oC800oC700oC600oC A -6-5.5-5-4.5-4-3.5-3101520Log amb (Scm-1)mol %900oC800oC700oC600oC B Figure 4-7. Ambipolar conductivity behavior depends on the dopant concentration under A) dry hydrogen and B) hydrogen/water vapor atmospheres. 61

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Table 4-1. OCV data under the first condition.[100% H 2 Pt / SrCe 1-x Eu x O 3/ Pt, 5% H 2 + balance Ar] OCV (mV) Temperature () SrCe 0.9 Eu 0.1 O 3SrCe 0.85 Eu 0.15 O 3SrCe 0.8 Eu 0.2 O 3600 -107.1 -85.9 -65.7 700 -118.8 -91.5 -68.1 800 -130.5 -96.6 -70.4 900 -139.7 -98.12 -73.6 62

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Table 4-2. OCV data for hydrogen/water vapor concentration cell 1 and 2 OCV (mV) SrCe 0.9 Eu 0.1 O 3SrCe 0.9 Eu 0.1 O 3SrCe 0.9 Eu 0.1 O 3Temperature () Cell 1 Cell 2 Cell 1 Cell 2 Cell 1 Cell 2 600 -119.8 -119.4 -118.5 -111.3 -103.9 -103.2 700 -133.2 -132.6 -124.5 -123.6 -115.6 -114.5 800 -145.0 -144.0 -135.2 -133.9 -125.3 -123.8 900 -150.3 -147.3 -141.1 -137.1 -131.9 -126.9 63

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CHAPTER 5 FABRICATION OF TUBULAR TYPE MEMBRANE 5.1 Introduction Tubular type membrane was fabricated for measuring the permeability of ESC hydrogen membrane. The biggest advantage of tubular type over planar type membrane is that they do not require any high-temperature seals to isolate permeated gas from input gas. Also, tubular type membrane have also shown the ability to be thermally cycled to room temperature from 900 without any mechanical damage [79]. Ni-SrCeO 3 tubular type supports for SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membrane thin film was prepared by tape casting as shown in Figure 5-1 and rolling techniques. Using tape casting and slurry coating, SrCeO 3 powder for a tubular type support material was prepared by a conventional solid state reaction method from SrCO 3 (99.9%, Alfa-Aesar) and CeO 2 powder (99.9%, Alfa-Aesar) as starting materials. The SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) powder was synthesized by a citrate process using 1 to 2 molar ratios of the total metal nitrates to citric acid in order to prevent second phase formation and to decrease the calcining temperature [80]. 5.2 Membrane Supporting Tube: Materials and Fabrication In order to make the NiO-SrCeO 3 slurry for tape casting, NiO and SrCeO 3 were mixed in EtOH/Toluene solvent, and then, a binder, two plasticizers and a dispersant were added. The slurry composition for the SC-NiO tape is shown in Table 5-1. Figure 5-2 shows the process flow design for the preparation of the SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) dense layer on porous Ni-SrCeO 3 tubular type supports. The process sequence for making a one end closed tubular-type green body support is shown in Figure 5-3. NiO-SrCeO 3 green tapes were rolled on a 1/4 inch diameter steel core 64

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PTFE rod for making tubular type supports. After making the support, the tubular type green body was partially sintered at between 1100 and 1200. To make the hydrogen permeation thin film membrane cell, 10ESC layer was coated on the inner-side of the partially sintered tubular type support by a colloidal coating, and then the SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membrane layer and NiO-SrCeO 3 support was sintered together. The tubular type hydrogen membrane cell is composed of a dense membrane film and a Ni-SrCeO 3 catalytic support. To preserve the integrity of the tube, the shrinkage rates between the membrane film and the support need to be matched so that cracking does not occur on membrane film. Figure 5-4 shows the linear shrinkage rates for NiO-SrCeO 3 support which was prepared by tape casting and Eu 2 O 3 doped and un-doped SrCeO 3 which were prepared by uni-axial pressing according to the firing temperature. By doping with Eu 2 O 3 the SrCeO 3 system was easily densified, and the shrinkage rate for SrCeO 3 was increased. For the NiO-SrCeO 3 tubular-type support, the shrinkage rate of the support was higher than that of the SrCeO 3 thin film due to the high concentration of organic vehicles. In order to increase the compatibility of the linear shrinkage of SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) membrane film and the support, the NiO-SrCeO 3 support was initially partially sintered at 1100 to 1200 for 4 h. The SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membrane film which and the partially sintered NiO-SrCeO 3 support were then sintered between 1350 1450. The microstructures of 10ESC film on NiO-SrCeO 3 supports according to sintering temperature are shown in Figure 5-4. From the SEM results, the 10ESC film was fully densified above 1400. Also, Figure 5-5 shows the cross-sectional view of the as-coated 10ESC layer on the inner side of the partially sintered NiO-SrCeO 3 tubular type support and the fully densified 10ESC, with apparent thickness of 30 m, on the Ni-SrCeO 3 support after sintering at 1450 65

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and reducing in a hydrogen atmosphere at 900. The porous Ni-SrCeO 3 was achieved from reduction of NiO-SrCeO 3 substrate at 900. Figure 5-6 shows the 15cm long end capped tubular type hydrogen membrane cell. According to our investigations, the final sintering temperature should be below 1450 to prevent a reaction between NiO and strontium cerate (SC), and above 1400 to obtain dense films of Eu-doped strontium cerate (ESC). In addition, the pre-sintering temperature for the NiO-SC support should be in the range of 1150 to 1200, in order to obtain compatible shrinkage rates between NiO-SC and ESC film at the sintering range. The cracking of the NiO-SC support was observed when the support was pre-sintered at temperatures below 1150. The optimal pre-sintering and sintering temperature ranges are given in Figure 5-4. As summarized in Table 5-2, the preparation of the ESC hydrogen membrane on NiO (or Ni)-SC tubular support consists of four steps; and the pictures of a six-inch unit cell thus prepared at each preparation step are shown in Figure 5-7. 66

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Figure 5-1. Tape caster for making ceramic green tape. 67

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Figure 5-2. Schematic process flow chart for Eu-doped SrCeO 3dense layers on porous Ni-SrCeO 3 tubular type supports. 68

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Figure 5-3. Process sequence for fabricating one end closed green body supports. 69

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Figure 5-4. Shrinkage rates for 10 mol % Eu-doped and undoped SrCeO 3 prepared by using an uni-axial press technique and SrCeO 3 with 30wt% NiO green tape by tape casting, and microstructures for 10 mol % Eu-doped SrCeO 3 thin layers which were coated on NiO-SrCeO 3 support partially sintered at 1100 in air atmosphere according to the final sintering temperature. 70

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A B Figure 5-5. Cross-sectional view of A) coated 10ESC film on partially sintered NiO-SrCeO 3 and B) dense 10ESC film on a Ni-SrCeO 3 support after sintering and reduction in hydrogen atmosphere. 71

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Figure 5-6. Photo-graphs of the 15cm long one end closed tubular-type hydrogen membrane cells according to the processing steps. 72

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Figure 5-7. Pictures of a six-inch unit cell of 10 mol% Eu-doped SrCeO 3 membrane coated on the inner side of NiO(or Ni)-SrCeO 3 tubular support at each processing step. 73

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Table 5-1. Slurry composition of SC containing nickel catalyst for tape casting process Process step Material Function Weight(g) SrCeO 3 80 NiO Powder 70 Solsperse Dispersant 1.5 Toluene 22 Mill for 24 hr (1 st stage) E.A Solvent 14 DBP 5 PEG Plasticizer 1.32 Add to above and mill for 24 hr (2 nd stage) PVB Binder 9.5 74

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Table 5-2. Preparation steps of the tubular hydrogen membrane cell and related processes Preparation step Related process Green state tube Slurry preparation, tape casting and rolling Pre-sintering NiO-SC support at 1150 to 1200 Citrate process/solid state reaction Partially sintered support and membrane coating Slurry coating ESC on NiO-SC support Sintered cell Sintering ESC/NiO-SC at 1400 to 1450 Reduced cell Reducing NiO to Ni in support by annealing in H 2 75

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CHAPTER 6 HYDROGEN PERMEATION THROUGH THE TUBULAR TYPE MEMBRANE 6.1 Introduction Based on the conductivity and transference number measurement, Eu-doped SrCeO 3 is promising since it provides high selectivity to protons. Wagner equation (6-1) shows that hydrogen permeation through a proton conducting membrane is inversely proportional to the thickness of membranes [74]. For enhancement of hydrogen permeation flux through hydrogen membranes, many works have tried to make thin films. Therefore, our research has been directed towards the development of thin film hydrogen membranes using tubular-type porous supports for increasing the hydrogen production by reducing the thickness of the membrane. IOIIOIHIIHOOOOOPPPPHeVOHtOVOHtOHPdttt F RTPdtt F RTLJ222222ln)(2ln4122 (6-1) For a hydrogen production cell, a hydrogen permeable thin film is coated on the inner-side of a catalytic tubular-type porous support. Methane and steam mixtures are flowed on the outer side of a reactor cell. At operating temperatures, CO, CO 2 and H 2 are formed and the produced H 2 permeates through the hydrogen permeable thin film, due to the hydrogen partial pressure gradient. 6.2 Experimental For the hydrogen permeation measurement, the tubular type hydrogen membrane cell was installed in a high temperature reactor apparatus as shown in Figure 6-1. The fabrication of these tubular type hydrogen membranes was described in chapter 5. One side of the membrane cell (feed) was exposed to H 2 (99.999%) diluted to the desired concentration using Ar (99.999%) with 20 cc/min total flow rate. For the wet gas flow, the feed gas was flow through a water bubbler at 25 to pick up 3vol % water vapor. The other side (sweep) was swept with He at 20 cc/min. Hydrogen permeation was measured using a mass spectrometer (Dycor QuadLink IPS Quadrupole Gas Analyzer). 76

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By assembling the unit cell with gas inlet and outlet systems, we obtained a hydrogen permeation reactor, the picture of which is shown in Figure 6-2. Installing the hydrogen permeation reactor in a furnace and connecting the reactant and sweep gases, we are now ready for testing the hydrogen permeability of the six-inch unit cell. Mass flow controllers (BOC Edward 825) are used to control the gas flow rate, and hydrogen permeation rate is measured by a mass spectrometer. The actual experimental set-up is shown in Figure 6-3. For the methane steam reforming condition, CH 4 gas balanced with Ar flow with 18% steam in my experiment which methane to steam ratio is 1.6:1 condition. Permeated hydrogen was measured by a mass-spectrometer which was connected with the sweep gas outlet. Gas-chromatography was installed for measuring the product gases, while CH 4 gas was used as a feed gas. Figure 6-4 shows the temperature gradient of 15cm tubular type hydrogen membrane cell at the set points in a reactor furnace. The effective area for hydrogen permeation was taken by the hot zone range because the insulator zone which was blocked by a thermal insulator was rapidly dropped the temperature. Effective membrane area in the hot zone range was 12cm 2 6.3 Result and Discussion 6.3.1. Hydrogen Permeation Properties of 10ESC/Ni-SrCeO 3 at Dry / Wet Hydrogen Atmosphere A SEM image of the cross sectional view for 10ESC / Ni-SrCeO 3 after sintering at 1450 o C for 5 hours was shown in Figure 6-5. The thickness of the 10ESC hydrogen permeable film was about 40 m. The hydrogen permeation of the 10ESC tubular type hydrogen membrane cell in dry and humid hydrogen condition at 850 according to the partial pressure of hydrogen was shown in Figure 6-6. Hydrogen permeation in humid hydrogen condition is slightly lower than that in dry hydrogen. The presence of water vapor reduces the concentration of oxygen vacancies 77

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() and increases that of proton defects (). However, the hydrogen permeability in humid hydrogen atmosphere is lower due to the lower electron concentration resulting from the higher P OV OOH O2 of the humid atmosphere. Figure 6-7 shows hydrogen flux and water vapor formation in dry hydrogen atmospheres at 900 as a function of hydrogen partial pressure. The water vapor flux increased with increasing hydrogen partial pressure. In general, if perovskite oxide is exposed in low oxygen partial pressure, then the oxide system can be reduced by producing O 2 gas. However, in the case of hydrogen presence, water vapor can be created instead of oxygen gas. If the water vapor comes from the reaction of the permeated hydrogen and the oxygen ion in 10ESC lattice, then the pure hydrogen permeation should be decreased as a result of reduction of the 10ESC membrane. The hydrogen permeation flux as a function of temperature according to hydrogen partial pressure on the feed side is shown in Figure 6-8. The result also shows a single tube of our current hydrogen membrane is able to produce about 6.9cc/min of pure H 2 The hydrogen flux for the tubular type SrCe 0.9 Eu 0.1 O 3thin film hydrogen membrane cell is proportional to [P H2 ] 1/4 agreeing well with Norby and Larrings model in which protons and electrons are dominating defects [25]. The hydrogen permeation as function of temperature according to hydrogen partial pressure on the feed side is shown in Figure 6-9. From these results, the activation energy (E a ) for the hydrogen flux (J H2 ) can be calculated by the Arrhenius equation: )/exp(2RTEKJaoH (6-2) where R is the gas constant and T is absolute temperature. The activation energies are about 0.89 to 0.97 eV in the temperature range of 700-900 as listed in Table 6-1. This result suggests that the hydrogen flux is limited by electrons as we expect. 78

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Figure 6-10 (A) shows hydrogen flux according to exposure time in dry hydrogen as feed gas at 850. In the case of 100% of dry hydrogen, the maxmum hydrogen flux was 4.97cc/min in Figure 6-6. Otherwise, this figure shows the hydrogen permeation was detected from 4.08cc/min because hydrogen permeation of 10ESC membrane was continuously degradaded following exposure to dry hydrogen atmosphere. The secondary phase formation may cause this hydrogen permeability degradation. CeO 2 phase are appeared at high temperature after exposing the reduced condition by XRD analysis. Reduced atmosphere will increase vacancy ( value) which may cause a disruption in the ABO 3 perovskite structure. Finally, CeO 2 phase formed along the grain boundary which will be explained more detailly on chapter. However, when the membrane was exposed to 3vol% humid hydrogen atmosphere, the permeation could be kept more stable than in dry hydrogen atmosphere shown in Figure 6-10 (B). Therefore, in order to keep a stable hydrogen permeation for the high temperature hydrogen membrane cell, water vapor should be introduced in feed gas. Hydrogen permeation with varying the membrane thickness in SrCe 0.9 Eu 0.1 O 3hydrogen membranes / Ni-SrCeO 3 tubular type support system was also investigated. Figure 6-11 shows permeated hydrogen is linearly increased with decreasing membrane thickness, which follows Wagners equation (6-1). Because the Wagner equation is based on the diffusion limited condition, we can conclude that proton conducting membrane for the hydrogen permeation is controlled by diffusion mechanism up to 30m thick not by surface reaction. These experiments were conducted using Ni-SrCeO 3 support. In addition to the 10ESC, 15ESC and 20ESC membrane were also coated inside the support tube to investigate the permeation trend whether it matched with previous ambipolar conductivity data or not. However, problems were encountered during the sintering process such as a mismatched thermal expansion 79

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coefficient (TEC) between support and. Therefore, I changed support materials pre-sintering temperatures. As shown in Table 6-2, several changes of pre-sintering temperature couldnt match with membrane sintering temperature. Figure 6-12 shows the 2 nd case membrane which 15ESC membrane peel off from the support material. When 20ESC membrane was coated on the support material, it was more severe. In addition, sintering temperature cause another problem with increasing Eu dopant concentration. As increase Eu dopant concentration in membrane material, Ni phase was observed along the grain boundary on the membrane surface due to Ni phase diffusion through the membrane during the sintering. This phenomenon can be explained more decreased eutectic point on the interface between membrane and support material where four metal components coexist. Therefore, other methods were required to measure the permeation of higher Eu dopant concentration membrane. 6.3.2. Hydrogen Permeation Properties from CH 4 Steam Reforming We have evaluated the hydrogen permeability for the 10ESC tubular type hydrogen membrane cell in a H 2 atmosphere on the feed side. The overall goal of the ESC hydrogen membrane cell is pure hydrogen production from hydrocarbon based fuel. Figure 6-13 shows hydrogen flux for CH 4 on the feed side (CH 4 /H 2 O=1.6). In order to use steam reforming of methane for hydrogen production using a hydrogen membrane cell, we have to consider methane conversion rates, partial pressure of product hydrogen, and solid state carbon formation rates. Using thermodynamic data for CH 4 and H 2 O, we calculated the conversion rate of methane, hydrogen partial pressure after methane conversion, and the amount of carbon formation according to the ratio of methane to steam. Figure 6-14 shows the thermodynamic calculation data for methane steam reforming according to the methane to steam ratio using thermo-cal. When the methane to steam ratio is 80

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increased, the conversion rate of methane and the hydrogen partial pressure are increased. As shown in Figure 6-13, the hydrogen flux through the ESC membrane for methane was higher than when pure hydrogen was introduced. This is due to the increase of hydrogen partial pressure after methane was converted to CO, CO 2 and H 2 by the steam reforming process. Also, the problem of coking needs to be considered for using the hydrogen membrane cells on Ni-based porous support materials for the steam reforming of the methane process. Coking can generate degradation of hydrogen permeability and cracking of oxide type membrane cells. Figure 6-15 shows the thermodynamically calculated carbon formation as a function of the methane to steam ratio. As the steam ratio for methane will increase, the formation of carbon decreases and the hydrogen permeation through the ESC membrane increases. For the steam reforming of methane, the methane to steam ratio should be below 0.6 to avoid possible coking and to increase the production of pure hydrogen. 6.3.3. Modification of Tubular-Type Hydrogen Permeation Membrane Cells During the permeation test of SrCe 0.9 Eu 0.1 O 3/ Ni-SrCeO 3 hydrogen membrane cells under steam reforming condition which is in methane with 8 % steam, cracking occurred around 700 o C region on the membrane tube as shown in Figure 6-16. The crack was possibly caused by the phase change of SrCeO 3 in CO/CO 2 atmosphere and/or the coking in methane steam reforming above the 0.6 ratio of CH 4 /H 2 O. Coking may not a problem under high steam content or high temperature. XRD analysis proved the phase change with SrCO 3 formation than carbon formation under our experimental condition. Therefore, ESC membrane can not be used for hydrogen production under steam reforming condition due to its chemically unstable property. By substitution of Zr onto the Ce-site in the membrane and/or support materials, it was possible to decrease the probability of cracking for the hydrogen membrane cell by supplementing the 81

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structural stability of SrCeO 3 Therefore, I approached two different configurations for stable permeation flux as follow. SrCe 0.9 Eu 0.1 O 3membrane / Ni-SrCe 0.8 Zr0 0.2 O 3 support SrZr 0.2 Ce 0.7 Eu 0.1 O 3membrane / Ni-SrCe 0.8 Zr0 0.2 O 3 support 6.3.3.1. 10ESC / Ni-SrCe 0.8 Zr0 0.2 O 3 tubular-type supports In order to improve the phase stability of supports in CO/CO 2 atmosphere derived from methane steam reforming, the Ni-SrCe 0.8 Zr0 0.2 O 3 system was used instead of the previous Ni-SrCeO 3 systems. Figure 6-16 showed the SrCe 0.9 Eu 0.1 O 3/ Ni-SrCeO 3 and SrCe 0.9 Eu 0.1 O 3/ Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cells after exposure in methane with 18 % steam. No cracking occurred on the SrCe 0.9 Eu 0.1 O 3/ Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cell. This result proved introduction of Zr ion onto Ce site of support increased its chemical stability Figure 6-17 shows the hydrogen flux through the tubular-type SrCe 0.9 Eu 0.1 O 3/ Ni-SrCe 0.8 Zr0 0.2 O 3 hydrogen membrane cell whose membrane thickness is 50m. However, the flux level is quite lower than that of the SrCe 0.9 Eu 0.1 O 3/ Ni-SrCeO 3 tubular-type cell and long term testing showed decreasing hydrogen permeation flux with time. This decreased permeation is assumed Zr ion diffuse into the membrane because Zr ion size is comparable with Ce ion in the membrane. And, it is well-known introduction of Zr ion may decrease the proton conductivity which cause permeation flux decrease. Though this configuration showed chemical stability under methane steam reforming condition, degradation of hydrogen permeation flux may impede this configuration utilize. 6.3.3.2. SrCe 0.7-x Zr 0.2 Eu x O 3(x = 0.1, 0.15 & 0.2) / Ni-SrCe 0.8 Zr0 0.2 O 3 tubular-type hydrogen membrane cells Inevitable Zr ion diffusion into the membrane material resulted on Zr substituted ESC membrane materials. Though permeation flux could decrease by Zr substitution, continuous 82

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stable hydrogen permeation flux and chemical stability under CO/CO 2 condition can be achieved. SrZr 0.2 Ce 0.8-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membrane thickness was dependant upon the number of coating. SEM images of three different thick SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes were shown in Figure 6-18. Each images show clear differences between the dense membrane and porous support. Figure 6-19 shows the hydrogen permeation flux of (A) 50m, (B) 26m and (C) 17m thick SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes depends on hydrogen partial pressure and temperature. Hydrogen permeation flux increases with temperature and hydrogen partial pressure. When the hydrogen permeation membrane was exposed to a hydrogen atmosphere, protons and electrons are the dominating defects which may show [P H2 ] 1/4 dependence of hydrogen permeation as in Norby and Larrings model. Each membranes hydrogen permeation flux is proportional to [P H2 ] 1/4 agreeing well with their model [25]. Figure 6-19 B) shows different temperature dependence of hydrogen permeation flux compared with others. Leakage during the experiment may cause a false hydrogen permeation flux increase. This leak can be caused by viton-o-ring sealing on the membrane tube. This viton-o-ring expand at high temperature but it shrink at low temperature. Therefore, leak can be severe at low temperature. The Wagner equation (6-1) shows that hydrogen permeation through a proton conducting membrane is inversely proportional to the thickness of membranes. Permeated hydrogen through SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membrane is also linearly increased with decreasing membrane thickness. SEM images of (A) 42.5m, (B) 30m and (C) 20m thick SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes were shown in Figure 6-20. These images also show clear differences 83

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between dense membrane and porous support. Figure 6-21 shows their hydrogen permeation flux depends on the hydrogen partial pressure and temperature. It also follows the same trend of hydrogen partial pressure dependence as SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes. However, leakage level of 15 mol% doped membrane was higher than that of 10 mol% doped one. It can attribute to the increased micro-crack on membrane material. This phenomenon has been more severe on 20 mol% doped membrane and cause membrane crack during sintering process due to increased value. The hydrogen permeation flux of different amount doped membranes with thickness at 900 was shown in Figure 6-22 (A) and (B), respectively. Both hydrogen permeation show linear dependence with membrane thickness which means bulk diffusion is the rate-limiting factor for permeation. However, hydrogen permeation fluxes are not distinguishably different when two plots are compared. This result does not match with our expectation in which hydrogen permeation may increase with increasing dopant level. In fact, SrZr 0.2 Ce 0.65 Eu 0.15 O 3membranes permeated more hydrogen than SrZr 0.2 Ce 0.7 Eu 0.1 O 3membranes. Table 6-3 shows both membranes water vapor fluxes on the outlet side depending on thickness. More water vapor forms with increasing dopant and decreasing thickness. The water vapor flux also increased with increasing hydrogen partial pressure. As previously mentioned, water vapor can be formed from the reaction with permeated hydrogen gas and oxygen gas which comes from oxide lattice under reduced atmosphere. This water vapor formation may reduce the permeated hydrogen pressure. Therefore, with respect to water vapor formation, a higher amount of Eu doped in the membrane may permeate more hydrogen, in which water vapor was formed by reaction of oxygen gas from oxide lattice. 84

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For the verification of hydrogen permeation dependence on Eu dopant level, I re-calculated permeated hydrogen from the both proton membrane (SrZr 0.2 Ce 0.7 Eu 0.1 O 3& SrZr 0.2 Ce 0.65 Eu 0.15 O 3) with counting on the hydrogen from water formation on the permeated side as shown in Figure 6-23 (A) and (B), respectively. Increased hydrogen permeation with Eu dopant concentration in membrane material was observed. This hydrogen permeation also shows same trend with ambipolar conductivity with Eu dopant level (Figure 4-7) by showing the incensement of hydrogen permeation decreases with temperature. Though 20 mol% doped membrane could not be constructed due to cracking during the sintering process, this experiment shows membrane properties and mechanism. hydrogen permeation flux increases with dopant amount. 85

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Figure 6-1. Schematic diagram for the hydrogen permeation reactor system. 86

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Figure 6-2. Configuration of the hydrogen permeation reactor using a 15cm long one end closed tubular-type hydrogen membrane cell. 87

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Figure 6-3. Experimental set-up for hydrogen permeation test. 88

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Figure 6-4. 6 tubular type hydrogen membrane cell and its temperature gradients at the set point temperature. 89

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Figure 6-5. Scanning electron micrograph for the cross sectional view of 10ESC/Ni-SrCeO3 hydrogen membrane cell after sintered and reduced. 90

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0123450.40.50.60.70.80.91 Humid hydrogen Dry hydrogenHydrogen flux [cc/min]PH21/4 [atm1/4] Figure 6-6. Hydrogen flux as a function of P H2 at 850. 91

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0.010.111000.20.40.60.81 H2O HydrogenFlux (cc/min)PH2 Figure 6-7. Hydrogen flux and water vapor formation as a function of P H2 at 850. 92

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012345670.650.70.750.80.850.90.951Hydrogen flux [cc/min][PH2] 1/4900oC850oC800oC700oC3% humid hydrogen Figure 6-8. Hydrogen flux as function of P H2 according to T in 3% humid hydrogen. 93

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0.11108.5 10-19 10-19.5 10-11 100 PH2=1 atm PH2=0.6 atm PH2=0.35 atm PH2=0.20 atm1000/T [/K]Hydrogen flux [cc/min]3% humid hydrogen Figure 6-9. Hydrogen flux as function of T in 3% humid hydrogen. 94

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0123450 1001 1042 1043 1044 1045 1046 1047 104Time (sec)50% Dry Hydrogen25% Dry Hydrogen5% Dry Hydrogen100% Dry HydrogenHydrogen Flux (cc/min) A 0123450 1005 1031 1041.5 104Hydrogen Flux (cc/min)Time (sec)50% Hydrogen with 3% H2O25% Hydrogen with 3%H2O5% Hydrogen with3%H2O12.5% Hydrogen with 3% H2O100% Hydrogen with 3% H2O B Figure 6-10. Hydrogen permeation properties with time at 850 in A) dry and B) humid hydrogen atmosphere. 95

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012345670.020.0250.030.035Flux (cc/min)1/thickness (m)900oC850oC800oC700oC50 m42 m30 m Figure 6-11. Thickness dependence of hydrogen permeation flux of SrCe 0.9 Eu 0.1 O 3/ Ni-SrCeO 3 hydrogen membrane cells under 3% humid hydrogen. 96

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Figure 6-12. 2nd case membrane which 15ESC peel off from the Ni-SrCeO 3 support. 97

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Figure 6-13. Hydrogen flux in humid hydrogen/methane atmospheres as a function of temperature. 98

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Figure 6-14. Continued. 99

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Figure 6-14. Continued. 100

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Figure 6-14. Continued. 101

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Figure 6-14. Thermodynamic calculation data for methane steam reforming according to CH 4 /H 2 ratio. 102

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Figure 6-15. Thermodynamic calculation of carbon formation for CH 4 /H 2 O ratio. 103

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Figure 6-16. Photograph of the 10ESC / Ni-SrCeO 3 and 10ESC / Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cells after exposed in methane with 18% steam. 104

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0123450.40.50.60.70.80.91Hydrogen Flux [cc/min]PH21/4 [atm1/4] Figure 6-17. Hydrogen flux for 10ESC / Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cell as a function of P H2 in humid hydrogen at 900. 105

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50 m A 26 m B 17 m C Figure 6-18. SEM images of A) 50m, B) 26m and C) 17m thicknesss SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes 106

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0123450.650.70.750.80.850.90.9511.05Permeated H2 (cc/min)PH21/4900oC850oC800oC750oC700oC A 0123450.650.70.750.80.850.90.9511.05900oC850oC800oC750oC700oCPermeated H2 (cc/min)PH21/4 B Figure 6-19. Continued. 107

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0123450.650.70.750.80.850.90.9511.05900oC850oC800oC750oC700oCPermeated H2 (cc/min)PH21/4 C Figure 6-19. Hydrogen permeation flux of A) 50m, B) 26m and C) 17m thicknesss SrZr 0.2 Ce 0.7 Eu 0.1 O 3hydrogen membranes depends on hydrogen partial pressure and temperature. 108

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42.5 m A 30 m B C Figure 6-20. SEM images of A) 42.5m, B) 30m and C) 20m thicknesss SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes 109

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0123450.650.70.750.80.850.90.9511.05Permeated H2 (cc/min)PH21/4900oC850oC800oC750oC700oC A 0123450.650.70.750.80.850.90.9511.05Permeated H2 (cc/min)PH21/4900oC850oC800oC750oC700oC B Figure 6-14. Continued. 110

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0123450.650.70.750.80.850.90.9511.05900oC850oC800oC750oCPermeated H2 (cc/min)PH21/4700oC C Figure 6-21. Hydrogen permeation flux of A) 42.5m, B) 30m and C) 20m thicknesss SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes depends on hydrogen partial pressure and temperature. 111

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11.522.533.544.50.020.030.040.050.061/thicknessPermeated H2 (cc/min)20 cc H2 (100% H2)15 cc H2 (75% H2)10 cc H2 (50% H2)5 cc H2 (25% H2) A 11.522.533.544.50.020.030.040.050.061/thickness20 cc H2 (100% H2)15 cc H2 (75% H2)10 cc H2 (50% H2)5 cc H2 (25% H2)Permeated H2 (cc/min) B Figure 6-22. Hydrogen permeation flux of A) SrZr 0.2 Ce 0.7 Eu 0.1 O 3B) SrZr 0.2 Ce 0.65 Eu 0.15 O 3membranes with thickness at 900 112

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12345670.010.020.030.040.050.06Permeated H2 (cc/min)1/thickness900oC850oC800oC A 12345670.010.020.030.040.050.061/thicknessPermeated H2 (cc/min)900oC850oC800oC B Figure 6-23. Permeated hydrogen from A) SrZr 0.2 Ce 0.7 Eu 0.1 O 3and B) SrZr 0.2 Ce 0.65 Eu 0.15 O 3with counting hydrogen from water formation. 113

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Table 6-1. Activation energies of hydrogen flux from figure 6-9 for ESC membrane in 700-900. P H2 (atm) 0.20 0.35 0.60 1 average value Activation energy, E (eV) 0.97 0.96 0.93 0.89 0.924 114

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Table 6-2. Change of support materials pre-sintering temperature and problems. Support (Ni-SrCeO 3 ) pre-sintering 1100 o C 1150 o C 1200 o C Membrane (SrCe 0.85 Eu 0.15 O 3) sintering 1450 o C 1450 o C 1450 o C Problem Crack Peel off Not Coated 115

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Table 6-3. SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.2 Ce 0.65 Eu 0.15 O 3membranes water vapor fluxes on the outlet side depending on thickness. SrZr 0.2 Ce 0.7 Eu 0.1 O 3SrZr 0.2 Ce 0.65 Eu 0.15 O 30.080.130.240.460.82100 % H20.080.130.240.450.8075 % H20.080.130.240.440.8050 % H20.080.120.240.430.7825 % H2700C750C800C850C900CH2O [ 17 m] 0.080.130.240.460.82100 % H20.080.130.240.450.8075 % H20.080.130.240.440.8050 % H20.080.120.240.430.7825 % H2700C750C800C850C900CH2O [ 17 m] 0.110.190.310.570.98100 % H20.110.180.300.560.9675 % H20.110.170.300.540.9150 % H20.110.170.290.510.8025 % H2700C750C800C850C900CH2O [ 20 m] 0.110.190.310.570.98100 % H20.110.180.300.560.9675 % H20.110.170.300.540.9150 % H20.110.170.290.510.8025 % H2700C750C800C850C900CH2O [ 20 m] 0.080.120.200.330.64100 % H20.080.110.190.330.6475 % H20.070.110.180.330.6350 % H20.070.110.170.340.5925 % H2700C750C800C850C900CH2O [ 27 m] 0.080.120.200.330.64100 % H20.080.110.190.330.6475 % H20.070.110.180.330.6350 % H20.070.110.170.340.5925 % H2700C750C800C850C900CH2O [ 27 m] 0.090.150.240.470.87100 % H20.090.140.240.450.8475 % H20.090.140.240.440.7550 % H20.080.130.240.420.7125 % H2700C750C800C850C900CH2O [ 30 m] 0.090.150.240.470.87100 % H20.090.140.240.450.8475 % H20.090.140.240.440.7550 % H20.080.130.240.420.7125 % H2700C750C800C850C900CH2O [ 30 m] 0.070.100.150.280.51100 % H20.070.100.150.260.4975 % H20.070.100.150.260.4750 % H20.070.090.150.240.4525 % H2700C750C800C850C900CH2O [ 50 m] 0.070.100.150.280.51100 % H20.070.100.150.260.4975 % H20.070.100.150.260.4750 % H20.070.090.150.240.4525 % H2700C750C800C850C900CH2O [ 50 m] 0.130.190.290.490.78100 % H20.110.170.270.460.7475 % H20.110.160.270.460.6850 % H20.110.150.240.430.6025 % H2700C750C800C850C900CH2O [ 42.5 m] 0.130.190.290.490.78100 % H20.110.170.270.460.7475 % H20.110.160.270.460.6850 % H20.110.150.240.430.6025 % H2700C750C800C850C900CH2O [ 42.5 m] 116

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CHAPTER 7 EFFECT OF ZIRCONIUM ION DOPING ON ELECTRICAL PROPERTY AND CHEMICAL STABILITY OF 10 ESC PROTON CONDUCTORS 7.1 Introduction The SrCeO 3 -based perovskite is a potential hydrogen membrane material that incorporates the steam reforming as a water gas shift reaction catalyst and hydrogen separation into one unit. However, many researchers found that these perovskites are reactive to CO 2 and H 2 O leading to mechanical disintegration of the electrolyte as a result of decomposition [17, 81, 82]. Additionally, during the conductivity measurement under dry H 2 atmosphere, chemical decomposition in SrCeO 3 -based perovskite proton membrane is observed. In order to commercialize the membrane manufacture, these issues need to be handled. It is well known the SrZrO 3 is chemically stable in CO 2 containing atmosphere, but SrZrO 3 is undesirable for proton membrane due to its relatively low proton conductivity. The improvement in chemical stability of strontium zirconate can be explained by the tolerance factor, which was introduced by Goldschmidt, being in essence a measure of the cubic-ness. The definition of this factor for the perovskite structure is [83, 84]: )rr(2rrtOBOA (7-1) where are the ionic radii of the A-site cation, the B-site cation and the oxygen anion, respectively. When t equals unity, the structure is predicted to be cubic. Lower values of t correspond to lower symmetry. Using ionic radii found in Shannons paper [43], the tolerance factor can be calculated and shown in Table 7-1. The tolerance factors of SrCeO Ar Br Or 3and SrCe 1-x Zr x O 3(x=2.5%, 5%, 10% and 20%) are below one, and increase with increasing zirconium dopant concentrations. 117

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Both the chemical stability and tolerance factor of SrCe 1-x Zr x O 3are increased with increasing dopant concentration, x, in accordance with Kamatas result [81]. They reported that the chemical stability of each perovskite was related to the tolerance factor. Generally, the closer the value of the tolerance factor to unity, the higher the chemical stability of the perovskite structure. In this study, the stability of SrCe 0.9 Eu 0.1 O 3 under wet/dry hydrogen atmosphere was investigated using XRD analysis and the effect of Zr ion substitution into 10ESC for electrical and permeability was also studied. 7.2 Experimental SrCe 0.9 Eu 0.1 O 3and SrZr 0.2 Ce 0.7 Eu 0.1 O 3samples were prepared by solid-state reaction. SrCO 3 CeO 2 ZrO 2 and Eu 2 O 3 (Alfa Aesar) with the desired stoichiometric ratio were ball-milled for at least 24h and then calcined for 10h at 1300 in air. The powders were then ground, and pervoskite structure was confirmed by X-ray diffraction. Each sample was exposed for 24hrs at both dry and wet (3vol% H 2 O) hydrogen atmospheres and their X-ray diffraction patterns were investigated after each exposure. Conductivity change was investigated under wet (3vol% H 2 O) and dry H 2 atmosphere and multiple temperatures. Pellets were coated with Pt-paste (Englehard 6926) and heated to 1000 for 1 hr. Conductivity measurement were performed with a Solartron 1260 Impedance Analyzer in the frequency range of 0.1 Hz to 1 MHz. Hydrogen permeation measurements were performed on a tubular type membrane made by tape casting and rolling processes whose detail explanation is in Chapter 5. Further details of the processing method of these tubular type membranes will be handled another paper. For the permeation measurement, one side of the membrane (feed) was exposed to H 2 (99.999%) diluted 118

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to the desired concentration using He (99.999%) and/or Ar (99.999%) with 20 cc/min total flow rate. Wet gas stream (3vol% H 2 O) was produced by bubbling through water at a controlled temperature. The other sweep side was flushed with He at 20 cc/min. Hydrogen permeation was measured using a mass spectrometer (Q100MS Dycor Quadlink Mass Spectrometer). Leakage of neutral gas through pores in the sample or through an incomplete seal was detected by measuring the argon tracer content of the permeate stream. 7.3 Result and Discussion The stability of SrCe 0.9 Eu 0.1 O 3was investigated by heat treatment at wet/dry hydrogen atmosphere. Each SrCe 0.9 Eu 0.1 O 3powder was exposed under wet/dry hydrogen atmosphere at different temperature during 24 hr. The X-ray powder diffraction patterns of dry and wet hydrogen atmosphere exposed SrCe 0.9 Eu 0.1 O 3is shown in Figure 7-1 (A) and (B), respectively. The CeO 2 peaks are marked by asterisks. Both Figures reveal that SrCe 0.9 Eu 0.1 O 3decomposes as CeO 2 at (A) dry and (B) wet hydrogen atmosphere and more intense CeO 2 peaks are observed as increased heat treatment temperature. Also, dry hydrogen atmosphere exposed SrCe 0.9 Eu 0.1 O 3shows more intense CeO 2 peaks compared with wet hydrogen exposed samples. Glckner et al. and Atsuko Tomita et al. studied the vaporization of BaO from the doped BaCeO 3surface at high temperatures [83, 84]. SrO peak didnt detected in XRD pattern which make it possible to assume SrO vaporization and decomposition of SrCe 0.9 Eu 0.1 O 3system in hydrogen atmosphere as a followed reaction (7-2). SrCe 1-x Eu x O 3ySrO (g) + yCe 1-x Eu x O 2-x/2 + (1-y)SrCe 1-x Eu x O 3(7-2) The X-ray pattern suggests that stability of SrCe 0.9 Eu 0.1 O 3is dependant on the P O2 Although the decomposition process is slow at relatively low temperature, the amount of CeO 2 keeps increasing according to the time. In addition, increased conductivity is expected with time because the CeO 2 phase shows relatively high electronic conduction compared with SrCeO 3 119

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The conductivity measurement of SrCe 0.9 Eu 0.1 O 3was conducted under dry/wet hydrogen atmosphere at different temperatures during the 3 days (72 hours). Figure 7-2 shows the conductivity change with time under dry/wet hydrogen atmosphere at 800 and 900 during the 3 days. The conductivity of SrCe 0.9 Eu 0.1 O 3under dry hydrogen atmosphere increases with time at 800 and 900. The increased conductivity is the evidence of CeO 2 phase formation by decomposition. On the other hand, the conductivity of SrCe 0.9 Eu 0.1 O 3under wet hydrogen atmosphere is more stable than that of dry hydrogen atmosphere. It is assumed that the oxygen source from wet hydrogen atmosphere supplies the SrCe 0.9 Eu 0.1 O 3lattice to prevent the decomposition. Although the total conductivity increases with the formation CeO 2 phase, this CeO 2 phase may block the protonic conduction in SrCe 0.9 Eu 0.1 O 3. SEM and EDS analysis of SrCe 0.9 Eu 0.1 O 3were conducted (A) before and (B) after dry hydrogen exposure as shown in Figure 7-3. SEM image of hydrogen exposed SrCe 0.9 Eu 0.1 O 3shows intergranular crack. It can be explained that some chemical reaction happened along the grain boundary which may have a different chemical composition between grain and grain boundary. EDS point analysis on grain and grain boundary of those samples was performed. There was no compositional difference between grain and grain boundary on sintered SrCe 0.9 Eu 0.1 O 3(not exposed to H 2 atmosphere). On the contrary, H 2 exposed SrCe 0.9 Eu 0.1 O 3shows the compositional difference between grain and grain boundary. Grain boundary has higher Ce and lower Sr intensity than grain in EDS analysis which result is consistent with reaction (7-2). Figure 6-10 shows the hydrogen flux according to the time in A)dry and B)humid hydrogen as feed gas. In the case of 100% of dry hydrogen, the maxmum hydrogen flux was 4.97cc/min, however, in this figure shows the hydrogen permeation was started from 4.08cc/min 120

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because the hydrogen permeation was degradaded continuously after the ESC membrane was exposed in dry hydrogen atmosphere. This degraded hydrogen permeation under dry hydrogen atmosphere can be interpreted as evidence for the secondary CeO 2 phase formation by decomposition, which are consistent with the data of conductivity and X-ray diffraction patterns. When the membrane was exposed in humid hydrogen atmosphere, the permeation could be kept more stable than in dry hydrogen atmosphere. Therefore, in order to keep the stability of hydrogen permeation for the high temperature hydrogen membrane cell, water vapor should be introduced in feed gas. Although hydrogen permeation membranes are usually not exposed to the upper extreme conditions, the development of more stable membrane material is still required. K. H. Ryu and Sossina M Haile synthesized Zr substituted barium cerate to increase chemical stability under CO 2 atmosphere [85]. Total conductivity monotonically decreased with Zr substitution. Chemically stable barium cerate achieved with 20 mol% Zr substitution because unit cell volume decreases due to the smaller ionic radius of Zr relative to Ce. Therefore, Zr substituted Eu doped SrCeO 3 was synthesized to improve the chemical stability. Figure 7-4 shows the XRD pattern of synthesized powders. It shows peak separation and shift from SrCe 0.6 Zr 0.3 Eu 0.1 O 3as increasing Zr substitution. Structural distortion due to the Zr substitution causes this peak to separate and shift on the XRD pattern. Therefore, an adequate Zr substitution amount without structural distortion is 20 mol %. Figure 7-5 shows the compared powder X-ray pattern of calcined SrCe 0.9 Eu 0.1 O 3, calcined SrZr 0.2 Ce 0.7 Eu 0.1 O 3and dry hydrogen exposed SrZr 0.2 Ce 0.7 Eu 0.1 O 3during 24 h at 900. The higher angle shifted peak position of calcined SrZr 0.2 Ce 0.7 Eu 0.1 O 3, with comparison to the calcined SrCe 0.9 Eu 0.1 O 3, is evident of the decreased perovskite unit cell volume by Zr 121

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substitution. After dry hydrogen heat treatment, the X-ray pattern of SrZr 0.2 Ce 0.7 Eu 0.1 O 3does not change meaning no secondary phase formation. It is also known that the conductivity decreased monotonically with increasing Zr content. Total Conductivity was measured on SrCe 0.9 Eu 0.1 O 3, SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.3 Ce 0.6 Eu 0.1 O 3under pure H 2 atmosphere as shown in Figure 7-6. SrCe 0.9 Eu 0.1 O 3shows higher conductivity value than Zr substituted samples and conductivity decreases with Zr content. On the contrary, activation energy of SrCe 0.9 Eu 0.1 O 3increased with Zr content due to the unit volume decrease by Zr substitution. Conductivity measurement of SrZr 0.2 Ce 0.7 Eu 0.1 O 3, for the stability test under dry hydrogen atmosphere, was performed at 900 for 3 days (72 hours) as shown in Figure 7-7. The total conductivity of SrZr 0.2 Ce 0.7 Eu 0.1 O 3is less than that of SrCe 0.9 Eu 0.1 O 3; on the contrary, increased stability under dry hydrogen atmosphere was observed compared with SrCe 0.9 Eu 0.1 O 3. 122

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20304050607080IntensityCeO2Standard [calcined]700oC00oC00oC A 20304050607080IntensityCeO2Standard [calcined]700oC00oC00oC B Figure 7-1. X-ray diffraction patterns of SrCe 0.9 Eu 0.1 O 3after being exposed to A) dry hydrogen atmosphere and B) wet hydrogen atmosphere at 700 < T < 900 123

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-3-2.5-2-1.5-101020304050607080Dry H2 900oCDry H2 800oCWet H2 800oCWet H2 900oCLog (Scm-1)Time (hr) Figure 7-2. Conductivity change of SrCe 0.9 Eu 0.1 O 3with time under dry/wet hydrogen atmosphere at 800 and 900. 124

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B A SEM oundary oundary Grain EDS Grain B SEM Grain EDS Grain B Figure 7-3. SEM & EDS image of A) before and B) after H 2 exposed SrCe 0.9 Eu 0.1 O 3. 125

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10203040506070Relative Intensity2 SrCe0.5Zr0.4Eu0.1O3-SrCe0.6Zr0.3Eu0.1O3-dSrCe0.7Zr0.2Eu0.1O3-SrCe0.8Zr0.1Eu0.1O3-SrCe0.9Eu0.1O3-200002211020202040411031113402231004422040611042 Figure 7-4. X-ray diffraction pattern of Zr substituted Eu doped SrCeO 3 [SrCe 1-y Zr y Eu x O 3, where x = 0.1 and y is 0.1, 0.2, 0.3 and 0.4]. 126

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102030405060702 Relative Intensity CalcinedSrCe0.9Eu0.1O3CalcinedSrZr0.2Ce0.7Eu0.1O3Dry H2 exposedSrZr0.2Ce0.7Eu0.1O3Figure 7-5. Compared powder X-ray pattern of calcined SrCe 0.9 Eu 0.1 O 3, calcined SrZr 0.2 Ce 0.7 Eu 0.1 O 3and dry hydrogen exposed SrZr 0.2 Ce 0.7 Eu 0.1 O 3during 24 hr at 900. 127

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-4-3.5-3-2.5-20.720.80.880.961.041.12Log s (Scm-1)1000/T (K)0.58 eV0.59 eV0.48 eVSrCe0.9Eu0.1O3-SrZr0.2Ce0.7Eu0.1O3-SrZr0.3Ce0.6Eu0.1O3Figure 7-6. Total conductivity of SrCe 0.9 Eu 0.1 O 3, SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.3 Ce 0.6 Eu 0.1 O 3under dry pure H 2 atmosphere 128

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-3.5-3-2.5-2-1.5-101020304050607080Time (hr) Dry H2 exposedSrZr0.2Ce0.7Eu0.1O3-Dry H2 exposedSrCe0.9Eu0.1O3-Log (Scm-1) Figure 7-7. Conductivity changes of SrCe 0.9 Eu 0.1 O 3and SrZr 0.2 Ce 0.7 Eu 0.1 O 3for the stability test under dry hydrogen atmosphere was performed at 900 for 3 days. 129

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Table 7-1. Tolerance numbers of SrCe 1-x Zr x O 3t values )XII(Sr2r () )VI(Ce4r () )VI(Zr4r () )VI(O2r () x=0 x=0.025 x=0.05 x=0.01 x=0.02 1.58 1.01 0.86 1.26 0.885 0.886 0.888 0.891 0.897 130

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CHAPTER 8 CONCLUSIONS AND FUTURE WORKS 10 mol% Eu doped Strontium cerate shows the highest total conductivity and lowest activation energy compared with other dopant concentration. Increased activation energy on higher dopant level can be explains as increased electronic conduction due to Eu dopant. The maximum hydrogen permeation is achieved when the protonic conduction is comparable to the electronic conduction ( elecOHO ). Therefore, the protonic and electronic conduction for various amounts of Eu dopant was calculated by measuring the transference number. The electron transference number increased with increasing Eu and temperature, with a corresponding decrease in proton transference number. This behavior is attributed to increased n-type electronic conduction by the change of Eu oxidation state [Eu 2+ Eu 3+ + e ]. SrCe 0.8 Eu 0.2 O 3shows the point of .iontOOHt at 850 under dry H 2 atmosphere. However, this measured transference number has an assumption of no oxygen conduction under dry H 2 atmosphere. Therefore, another transference number measurement in hydrogen/water vapor atmosphere was conducted to investigate oxygen ion transference number. It shows a higher proton transference number and a lower electron transference number than those in dry H 2 atmosphere, due to an increased Po 2 on both sides of the cell by the water vapor. Also, oxygen transference number does not change much compared with other species with Eu dopant level. The maximum ambipolar conductivity increases with temperature and Eu dopant levels under dry hydrogen atmosphere. Therefore, the optimized dopant level which shows maximum ambipolar conductivity is obtained from 15 mol% Eu doped strontium cerate at 600 to 20 mol % Eu doped strontium cerate at 900. In contrast, ambipolar conductivity values in hydrogen/water vapor atmosphere were lower than that in dry hydrogen atmosphere, and the 131

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maximum ambipolar conductivity was observed on 20 mol % Eu doped strontium cerate in the range of investigated temperature. Therefore, maximum hydrogen flux can be achieved by controlling Po 2 dopant level, and temperature. This ambipolar conductivity was obtained from total conductivity measurement and transference number measurement whose conditions were different. Therefore, I cannot calculate accurate hydrogen permeation flux with this ambipolar conductivity values using Wagner equation. However, this ambipolar conductivity shows the transport trends on these materials. Ni-SrCeO 3 tubular type supports for SrCe 1-x Eu x O 3(x=0.1, 0.15 and 0.2) hydrogen membrane thin film was prepared by tape casting and rolling techniques. Though tubular type membrane was fabricated successfully, thermal expansion coefficient mismatch and microcrak were observed on higher dopant levels membrane. It needs to be solved to achieve higher hydrogen permeation flux through the membrane without leak. Also, increased porosity on the support materials by adding pore former or increased NiO contents may increase the hydrogen permeability. A single tube SrCe 0.9 Eu 0.1 O 3thin film hydrogen membrane is able to produce about 6.9cc/min of pure H 2 The hydrogen flux is proportional to [P H2 ] 1/4 agreeing well with Norby and Larrings model in which protons and electrons are dominating defects. From activation energy calculation, hydrogen permeation flux is controlled by electron. Permeated hydrogen is linearly increased with decreasing membrane thickness, which follows Wagners equation. Because the Wagner equation is based on the diffusion limited condition, we can conclude that SrCe 0.9 Eu 0.1 O 3membrane for the hydrogen permeation is controlled by diffusion mechanism up to 30m thick not by surface reaction. 132

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ESC membrane can not be used for hydrogen production under steam reforming condition due to its chemically unstable property. By substitution of Zr onto the Ce-site in the membrane and/or support materials, it was possible to decrease the probability of cracking for the hydrogen membrane cell by supplementing the structural stability of SrCeO 3 No cracking occurred on the SrCe 0.9 Eu 0.1 O 3/ Ni-SrCe 0.8 Zr 0.2 O 3 hydrogen membrane cell. This result proved introduction of Zr ion onto Ce site of support increased its chemical stability SrZr 0.2 Ce 0.7 Eu 0.1 O 3and SrZr 0.2 Ce 0.65 Eu 0.15 O 3hydrogen membranes show increased hydrogen flux with temperature and hydrogen partial pressure. They also show [P H2 ] 1/4 dependence of hydrogen permeation as in Norby and Larrings model. Both hydrogen permeation show linear dependence with membrane thickness which means bulk diffusion is the rate-limiting factor for permeation. However, hydrogen permeation fluxes are not distinguishably different when two plots are compared. Water vapor can be formed from the reaction with permeated hydrogen gas and oxygen gas which comes from oxide lattice under reduced atmosphere. This water vapor formation may reduce the permeated hydrogen pressure. Hydrogen permeation increase with Eu dopant with count of water formation on the permeated side. SrCe 0.9 Eu 0.1 O 3decomposes as CeO 2 at dry and wet hydrogen atmosphere after heat treatment and more intense CeO 2 peaks are observed as increased heat treatment temperature. Also, dry hydrogen atmosphere exposed SrCe 0.9 Eu 0.1 O 3shows more intense CeO 2 peaks compared with wet hydrogen exposed samples. SrO peak didnt detected in XRD pattern which make it possible to assume SrO vaporization and decomposition of SrCe 0.9 Eu 0.1 O 3system in hydrogen atmosphere The increased conductivity and EDS analysis are the evidence of CeO 2 phase formation by decomposition on the grain boundary. The conductivity under wet hydrogen atmosphere is more 133

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stable than that of dry hydrogen atmosphere. It is assumed that the oxygen source from wet hydrogen atmosphere supplies the lattice to prevent the decomposition. Hydrogen permeation degradaded continuously in dry hydrogen atmosphere meanign the swecondary CeO 2 phase block the proton conduction. Improved stability property under reduced atmosphere was observed with Zr substitution in conductivity and permeation result. The objective of this research was the optimization of dopant concentration for maximum hydrogen production. During the investigation, many unexpected problems were encountered. Though my work couldnt cover all problems, I showed the possibility of improved hydrogen permeability by prototype membrane fabrication. This research was focused on the Europium dopant. However, increased hydrogen permeability is expected on different ion dopant which shows smaller ionic size dopant and lower ionization energy. In addition, further research is required to find adequate Zr ion content or other dopant to increase its chemical stability without decreasing hydrogen permeability. 134

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APPENDIX A AMBIPOLAR DIFFUSION Ambipolar diffusion is diffusion of positive and negative particle in plasma at the same rate due to their interaction via the electric field. In most plasma, the force acting on the ions are different from those acting on the electrons, so one would expect one species to be transported faster than the other, whether by diffusion or convection or some other process. If such differential transport has a divergence, then it will result in a change of the charge density, which will return create an electric field that will alter the transport of one or both species in such a way that they become equal. Both electrons and ions will stream outward with their respective thermal speed. If the ions are relatively cold, their thermal speed will be small. The thermal speed of the electrons will be faster due to their high temperature and low mass. As the electron leaves the initial volume, they will leave behind a positive charge density of ions, which will result in an outwardly-directed electric field. This field will act on the electrons to slow them down and on the ions to speed them up. The net result is that both ions and electrons stream outward at the sound speed which is much larger than that the ion thermal speed but much smaller than the electron thermal speed. The total flux of charged species such as electrons and ions is composed of a drift component and a diffusion component. The drift flux is that driven by the electric filed as shown figure A-1. In proton electron mixed conducting oxide membrane, hydrogen separated by this ambipolar diffusion. This hydrogen permeation flux through the oxide membrane can be calculated by the Wagner equation. We can find ambipolar conductivity term in this equation which is defined as. 135

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''2'2''2'22'2Oln)(2ln41J22OHOOHHOOOOPPPPHeVOHtOVOHtPdtttFRTPdttFRTL (A-1) eOHeOHambOO (A-2) Figure A-1. Drift flux by electric field. 136

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APPENDIX B GRTTHUSS MECHANISM AND VEHICLE MECHANISM The Grtthuss mechanism is the mechanism by which an 'excess' proton or protonic defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation/cleavage of covalent bonds The Grtthuss mechanism is now a general name for the proton hopping mechanism. The proton attached to a stationary oxygen ion site and undergoes a reorientation step. Then proton transfer to another stationary oxygen ion site. On the other hand, proton attached to oxygen ion and from hydroxyl ion. Then this hydroxyl ion diffuses. Two different mechanisms was illustrated in figure B-1. Figure B-1. Illustration of Grtthuss mechanism and vehicle mechanism. 137

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APPENDIX C LATTICE PARAMETER CALCULATION The X-ray Diffraction pattern analysis was conducted by XRD Philips APD 3720 in Major Analytical Instrumentation Center. Measured XRD pattern compared with SrCeO 3 pattern in JCPDS data (36-0980). Each diffraction patterns shows single phase orthorhombic perovskite structure patterns. For the lattice parameter and volume calculation, two equations were utilized as shown. Bragg Law: sin2d (C-1) Orthorhombic: 22222221clbkahd (C-2) The value of d, the distance between adjacent planes in the (h k l), can obtain from equation (C-1). Wavelength () was decided from the target material (Cu K: =1.5418). Therefore, three planes (122), (080), and (400) were chosen from XRD pattern to obtain the lattice parameter a, b and c. These three planes were chosen from high angle on the XRD pattern to get accurate value. However, more precise value may achieved by analyzing the all peaks of the XRD pattern 138

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BIOGRAPHICAL SKETCH Takkeun Oh was born in Taegu, Korea in 1975. In 1982 he moved to Seoul, Korea with his family and starts his education. He began attending Hanyang University, Korea in March 1993 and received his Bachelor of Science degree in Metallurgy and Materials Science and Engineering in February 2000. In 2001, he entered the graduate program at the University of Florida, USA, to pursue a masters degree in Materials Science and Engineering. While studying for masters degree, he experienced the field of polymer-clay nanocomposite under the guidance of Prof. Hassan El-Shall. He finished his masters degree in May 2004 and continued his studies for a degree of Doctor of Philosophy under Prof. Eric D Wachsmans guidance.