Development and characterization of functional composite materials for advanced...

Development and characterization of functional composite materials for advanced

energy conversion technologies

Liangdong Fan

Doctoral thesis

Stockholm 2013

Division of Heat and Power Technology Department of Energy Technology

School of Industrial Engineering and Management Royal Institute of Technology, KTH, Sweden

Postal address Department of Energy Technology

School of Industrial Engineering and Management Royal Institute of Technology, KTH SE 100 44, Stockholm, Sweden

Supervisor Dr. Bin Zhu

Email: [email protected] Prof. Torsten H Fransson Email: [email protected]

Printed in Sweden Universitetsservice US-AB Stockholm, 2013 TRITA KRV Report 13/10 ISSN 1100-7990 ISRN KTH/KRV/13/10-SE ISBN 978-91-7501-827-0 © Liangdong Fan, 2013

mailto:[email protected]

mailto:[email protected]

Doctoral Thesis Liangdong Fan Page I

ABSTRACT

The solid oxide fuel cell (SOFC) is a potential high-efficiency electrochemical device for vehicles, auxiliary power units and large-scale stationary heat and power plants. The main challenges of this technology for market acceptance are associated with cost and lifetime due to the high temperature operation (700-1000 oC) and complex cell structure, i.e. the conventional membrane electrode assemblies. Therefore, it has become a top R&D goal to develop SOFCs for lower temperatures, preferably below 600 oC. To address these problems, two kinds of innovative approaches are adopted within the framework of this thesis. One is developing functional composite materials with desirable electrical properties at reduced temperatures, which results in the research on the ceria-based composite based low temperature ceramic fuel cell (LTCFC). The other one is discovering novel energy conversion technology - Single-component/ electrolyte-free fuel cell (EFFC), in which the electrolyte layer of conventional SOFC is physically removed while this device still exhibits the fuel cell function. Thus, the focus of this thesis is then put on the development and characterization of materials physical and electrochemical properties for these advanced energy conversion applications. The major scientific content and contribution to these challenging fields are divided into four aspects: 1. Continuous development and optimization of advanced electrolyte materials, ceria-

carbonate composite, for LTCFC. An electrolysis study has been carried out on ceria-carbonate composite based LTCFC with inexpensive Ni-based electrodes. Both oxygen ion and proton conductance in electrolysis mode are observed. High current outputs have been achieved at the given electrolysis voltage below 600 oC. This study also provides alternative manner for highly efficient hydrogen production.

2. Compatible and highly active electrode development for ceria-carbonate composite electrolyte based LTCFC. A symmetrical fuel cell configuration is intentionally employed. The electro-catalytic activities of novel symmetrical transition metal oxide composite electrode toward hydrogen oxidation reaction and oxygen reduction reaction have been experimentally investigated. In addition, the origin of high activity of transition metal oxide composite electrode is studied, which is believed to relate to the hydration effect of the composite oxide.

3. A novel all-nanocomposite fuel cell (ANFC) concept proposal and feasibility demonstration. The ANFC is successfully constructed by Ni/Fe-SDC anode, SDC-carbonate electrolyte and lithiated NiO/ZnO cathode at an extremely low in-situ sintering temperature, 600 oC. The ANFC manifests excellent fuel cell performance (over 550 mWcm-2 at 600 oC) and a good short-term operation as well as thermo-cycling stability. All results demonstrate its feasibility and potential for energy conversion.

4. Fundamental study results on breakthrough research Single-Component/Electrolyte-Free Fuel Cell (EFFC) based on above nanocomposite materials research activities (ion and semi-conductive composite). This is also the key innovation point of this thesis. Compared with classic three-layer fuel cells, EFFC with an electrolyte layer shows a much simpler but more efficient way for energy conversion. The physical-electrical properties of composite, the effects of cell configuration and parameters on cell performance, materials composition and cell fabrication process optimization, micro electrochemical reaction process and possible working principle were systematically investigated and discussed. Besides, the EFFC, joining solar cell and fuel cell working

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principle, is suggested to provide a research platform for integrating multi-energy-related device and technology application, such as fuel cell, electrolysis, solar cell and micro-reactor etc.

This thesis provides new methodologies for materials and system innovation for the fuel cell community, which is expected to accelerate the wide implementation of these highly efficient and green fuel cell technologies and open new horizons for other related research fields. Keywords: Low temperature ceramic fuel cell; Ceria-carbonate composite; Electrolysis; Transition metal oxide; Symmetrical fuel cells; All-nanocomposite fuel cell; Electrolyte-free fuel cell; Solar cell; ion conductor and semiconductor

Doctoral Thesis Liangdong Fan Page III

SAMMANFATTNING

Fastoxidbränslecellen (SOFC) är ett potentiellt högeffektivt elektrokemisk omvandlingssystem för fordon, hjälpkraftenheter och storskaliga stationära kraftvärmeverk. De största utmaningarna för denna teknik rörande marknadens acceptans är förknippade med kostnad och livstid på grund av den höga driftstemperaturen (700-1000 oC), och komplicerade cellstrukturen, dvs. konventionella membranelektroder arrangemang. Därför har det blivit ett prioriterat FoU-mål att utveckla SOFCs för lägre temperaturer, företrädesvis under 600 oC. För att lösa ovanstående problem, har två typer av innovativa angreppssätt tillämpats inom ramen för denna avhandling. Den första är utveckling funktionella kompositmaterial med önskvärda elektriska egenskaper vid lägre temperatur, vilket resultera i forskning av cerium-komposit lågtemperatur keramiska bränsleceller (LTCFC). Den andra är att upptäcka ny energiomvandlingsteknik - Single-component/electrolyte-free-bränslecell (EFFC), i vilken elektrolytlagret av en konventionell SOFC är fysiskt borttaget medan enheten fortfarande fungerar som bränslecell. Fokus för denna uppsats är således sedan att lägga på karakteriseringen av materialfysiska och elektrokemiska egenskaper för dessa avancerade applikationer inom energiomvandling. De stora vetenskapliga bidragen till detta utmanande område är indelade i fyra områden: 1. Kontinuerlig utveckling och optimering av ett avancerat elektrolytmaterial, cerium-

karbonatkomposit, för LTCFC. En elektrolysstudie har genomförts på cerium-karbonatkompositbaserad LTCFC med billiga Ni-baserade elektroder. Både syrejon och proton-konduktans i elektrolys läge har observerats. De höga strömutgångar har uppnåtts vid en given elektrolysspänningen under 600 oC. Denna studie ger också alternativa sätt för högeffektiv vätgasproduktion.

2. Kompatibel och högaktiv elektrodutveckling för cerium-karbonat-komposit LTCFC. En symmetrisk bränslecellkonfiguration är avsiktligt tillämpad. De elektro-katalytiska aktiviteterna av nya symmetrisk övergångsmetalloxidkompositelektrod för väteoxidation och syrereduktion har experimentellt undersökts. Dessutom är ursprunget för hög aktivitet av övergångsmetalloxid kompositelektrod har studerat, som tros att relatera till hydratiseringseffekten av den sammansatta oxiden.

3. Ett nytt koncept för helnanokompositbränslecell (ANFC) har undersökts. Den ANFC is framgångsrikt konstruerades av Ni/Fe – SDC anod, SDC-karbonat elektrolyt och litierad NiO/ZnO katod vid en extremt låg in-situ sintringstemperatur, 600 oC. Den ANFC manifesterar utmärkta bränslecell prestanda (över 550 mWcm-2 vid 600 oC) och en bra kortsiktig drift samt termo-cykling stabilitet. Alla resultat visar sin genomförbarhet och potential för energiomvandling.

4. Grundläggande studieresultat rörande genombrottsforskning för Single-Component/Electrolyte-Free Fuel Cell (EFFC) baserat på ovanstående nanokompositmaterial (jon och halvledande komposit). Detta är också en innovationseffekt av denna avhandling. Jämfört med klassiska trelagers bränsleceller med ett elektrolytiskt skikt, EFFC visar en mycket enkel men mer effektivt sätt för energiomvandling. De fysiska - elektriska egenskaperna hos kompositmaterialet, effekterna av cellens konfiguration och parametrar på cellens prestanda, processoptimering av materialsammansättning och celltillverkning, och mikroelektrokemisk reaktionsprocessen och möjliga funktionssätt har systematiskt undersökts och diskuterats. Dessutom, kombinerar solceller och bränsleceller arbetar princip, den EFFC representerat en forskningsplattform för att integrera flera

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energirelaterade enheter och tekniker, såsom bränsleceller, elektrolys, solcell och mikro-reaktor etc.

Denna avhandling beskriver nya metoder för material- och systeminnovation som är av intresse för bränslecellsforskare och som väntas accelerera en bred implementation av den högeffektiva och gröna bränslecellstekniken och öppnar nya horisonter för andra forskningsområden. Nyckelord: Lågtemperaturkeramisk bränslecell, Ceria-karbonatkomposit, elektrolys, övergångsmetalloxid, symmetriska bränsleceller, All-nanokompositbränslecell, elektrolyt-fribränslecell, solcell, jonledare och halvledare

Doctoral Thesis Liangdong Fan Page V

PREFACE

This thesis is based on the following publications: 1. Fan L., Wang C., Zhu B. Low temperature ceramic fuel cells using all nano composite

materials. Nano Energy, 1 (2012) 631-639. 2. Fan L., Wang C., Osamudiamen O., Raza R., Singh M., Zhu B. Mixed ion and electron

conductive composites for single component fuel cells: I. Effects of composition and pellet thickness. J. Power Sources 217 (2012) 164-169.

3. Fan L., Zhang H., Chen M, Wang C., Wang H., Singh M., Zhu B. Electrochemical study of lithiated transition metal oxide composite as symmetrical electrode for low temperature ceramic fuel cells. Inter. J. Hydrogen Energy, 38(2013) 11398-11405.

4. Fan L., Zhu B. Effective hydrogen production by high temperature electrolysis with ceria-carbonate composite. Manuscript, 2013.

5. Zhu B., Fan L., Lund P. Breakthrough fuel cell technology using ceria-based multi-functional nanocomposites. Appl. Energy 106 (2013) 163-175.

6. Zhu B., Raza R., Liu Q., Qin H., Zhu Z., Fan L., et al. A new energy conversion technology joining electrochemical and physical principles. RSC Advances, 2 (2012) 5066-5070.

7. Zhu B., Qin H., Raza R., Liu Q., Fan L., Patakangas J., et al. A single-component fuel cell reactor. Int. J. Hydrogen Energy 36 (2011) 8536-8541.

List of papers not included in this thesis: Journal papers: 1. Fan L., Wang C., Chen M., Zhu B. Recent development of ceria-based

(nano)composite materials for low temperature ceramic fuel cells and electrolyte-free fuel cells. J. Power Sources 234 (2013) 154-174.

2. Tan W., Fan L., Raza R., Ajmal Khan M., Zhu B. Studies of modified lithiated NiO cathode for low temperature solid oxide fuel cell with ceria-carbonate composite electrolyte. Int. J. Hydrogen Energy 38 (2013) 370-376.

3. Zhu B., Ma Y., Wang X., Raza R., Qin H., Fan L. A fuel cell with a single component functioning simultaneously as the electrodes and electrolyte. Electrochem. Commun. , 13 (2011) 225-227.

4. Zhu B., Raza R., Qin H., Fan L. Single-component and three-component fuel cells. J. Power Sources 196 (2011) 6362-6365.

5. Liu Q., Qin H., Raza R., Fan L., Li Y., Zhu B. Advanced electrolyte-free fuel cells based on functional nanocomposites of a single porous component: analysis, modeling and validation. RSC Advances, 2 (2012) 8036-8040.

6. Qin H., Zhu B., Raza R., Singh M., Fan L., Lund P. Integration design of membrane electrode assemblies in low temperature solid oxide fuel cell. Int. J. Hydrogen Energy 37 (2012) 19365-19370.

7. Raza R., Qin H., Fan L., Takeda K., Mizuhata M., Zhu B. Electrochemical study on co-doped ceria–carbonate composite electrolyte. J. Power Sources 201 (2012) 121-127.

8. Zhu B., Raza R., Qin H., Liu Q., Fan L. Fuel cells based on electrolyte and non-electrolyte separators. Energy Environ. Sci. , 4 (2011) 2986-2992.

9. Zhu B., Lund P., Raza R., Patakangas J., Huang Q., Fan L., Singh M. Nano-redox and nano-device processes for a new energy conversion technology. Nano Energy, 2 (2013) 1179-1185.

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Conference papers: 1. Fan L., Chen M., Wang C. and Zhu B. Synthesis and characterization all nano-

composite materials for LTCFCs, European Fuel Cell Technology & Applications Piero Lunghi Conference & Exhibition, Dec. 14-16th, 2011, Rome, Italy.

2. Fan L., Raza R. and Zhu B. Optimized single component fuel cells, Grove Fuel Cells Conference 2012, 10-11st, April 2012, Berlin, Germany.

3. Fan L., Zhu B. Single component low-temperature fuel cell operated with bio-alcohol fuels, World Resources Forum 2012, 21-23, Oct. 2012, Beijing, China. (Session chairman)

4. Fan L., Singh M, Zhu B. Nanotechnology and multifunctional nanocomposites for Electrolyte-free fuel cells (EFFCs), International Conference on Energy and Environment-Related Nanotechnology (ICEEN2012), 21-24, Oct., 2012, Beijing, China.

5. Fan L., Zhu B. Ceria-based nanocomposite for high performance fuel cell and other advanced applications, NANOSMAT-Asia, 13-15, March, 2013, Wuhan, China (Invited speaker)

Contribution of the authors: Paper 1-4, Fan L. performed all experiments, evaluated the results and wrote the

manuscript. The other authors joined the experiments and data analysis. Wang C. and Zhu B. are the academic supervisors and first reviewer.

Paper 5, Fan L. made the whole literature survey and partial manuscript writing. Paper 6, Fan L. performed the cell testing and materials characterization and partial

mechanism study. Paper 7, Fan L carried out partial experiments, results evaluation and analysis and

manuscript writing. In all papers, Zhu B. is the corresponding author. The original publications in this thesis are reproduced with permission from the copyright owners.

Doctoral Thesis Liangdong Fan Page VII

ACKNOWLEDGEMENTS

First and foremost, to my supervisor, Doc. Bin Zhu, for the kind chance to invite me and work at the fuel cell group, also for his enormous help in my life, valuable discussions and professional guidance throughout my study and research at KTH. Without his kind support and supervision, it would be impossible for me to complete this PhD. I am also greatly grateful to Professor Torsten Fransson for letting me join the Division of Heat and power, Department of Energy Technology and for always kind consideration of my study in here, especially the lifelong learning (LLL). Thank you very much for the effort put in my thesis. I would like to thank all my past and present colleagues at the Department of Energy technology for their availability and suggestions, friendly atmosphere. I will never forget the Friday Fika time. Special thanks are given to the persons in fuel cell group for interesting discussions and valuable co-operation. To Dr. Xiaodi Wang, Ying Ma, Rizwan Raza, Qinghua Liu, Haiying Qin, Wenyi Tan, Xuetao Wang, Qiuan Huang, Wujun Wang, Jianyong Chen, Xiaoxiang Zhang, Bo Wei, Fan Yang, Manish Singh and Mohanmod Afzal, for their help on various occasions in the laboratory and life in Stockholm. My PhD student years would not have been so enjoyable without you all! Personal thanks go to the friends at Tianjin University of China. To Professor Chengyang Wang, Mingming Chen, Dr. Jing Di, Zhiqiang Shi, Guoquan Zhang, Jing Wang, Jiuzhou Wang and Wenbin Li, for their invaluable guide, discussion and talking both on project research and life time. I do really appreciate Prof. Dr. Andrew Martin for his valuable time to take the responsibility for internal review of my thesis. I own many thanks to my committee members for their time, effort and comments on this dissertation. I do appreciate the help and consideration from the administrative office and department secretaries, Mirjam Truwant, Alena Joutsen, Hedrenius Emma, Rytterholm Petra, and so on, during my study and research time at KTH. The Swedish Research Council (VR, No. 621-2011-4983), the Swedish VINNOVA Systems, European Commission (FP7 TriSOFC project), KIC Innoenergy project and Chinese Scholarship Council (CSC, No. 2010625060) are recognized for the financial support. I am indebted to my Father (Guoyou Fan), my mother (Sulian Guo), my sister (Yufeng Fan) and all my other family members for their love, encouragement and endless support. Last but not least, I would like to take this opportunity to express my deepest gratitude to my wife, Na Yin, for her unconditional love, support for so many years. I want to dedicate this thesis to my parents, whom I owe the most. Stockholm, 2013-06-01 Liangdong Fan

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LIST OF CONTENTS

ABSTRACT .......................................................................................................................... I

SAMMANFATTNING .......................................................................................................... III

PREFACE ........................................................................................................................... V

ACKNOWLEDGEMENTS ................................................................................................ VII

LIST OF CONTENTS ...................................................................................................... VIII

LIST OF FIGURES ............................................................................................................. X

LIST OF TABLES ............................................................................................................ XIII

NOMENCLATURE .......................................................................................................... XIV

1 INTRODUCTION .......................................................................................................... 1

1.1 Fuel cells ............................................................................................................... 1 1.2 Solid oxide fuel cells .............................................................................................. 1

1.2.1 Electrochemical reactions .................................................................................. 1 1.2.2 Efficiency............................................................................................................ 2 1.2.3 Key cell components and challenges for SOFC ................................................. 4

1.3 Advanced fuel cell materials and system ............................................................... 7 1.3.1 Ceria-based composite and nanocomposite ...................................................... 7 1.3.2 Novel composite electrode material ................................................................... 9 1.3.3 Electrolyte free fuel cell (EFFC) ....................................................................... 11

1.4 Comparison between different fuel cell technologies in this thesis ...................... 12 1.5 Motivation and objective ...................................................................................... 13

2 EXPERIMENTAL METHODS AND TECHNIQUES .................................................... 15

2.1 Raw materials ...................................................................................................... 15 2.2 Sample preparation ............................................................................................. 15

2.2.1 Powder synthesis and Composite preparation ................................................. 15 2.2.2 Cylindrical pellet/disk fabrication ...................................................................... 16 2.2.3 Sample holder .................................................................................................. 16

2.3 Material characterizations .................................................................................... 17 2.3.1 X-ray diffraction ................................................................................................ 17 2.3.2 Morphologies and texture microstructure ......................................................... 17

2.4 Materials electrical performance .......................................................................... 18 2.4.1 Conductivity measurement (dc and ac) ............................................................ 18 2.4.2 Single cell performance ................................................................................... 19

3 RESULTS AND DISCUSSION ................................................................................... 21

3.1 Electrolysis study of ceria-carbonate composite for effective H2 production (paper 4) 21

3.1.1 Electrolysis cells (EC) in oxygen ionic conduction mode ................................. 22 3.1.2 EC in proton conduction mode ......................................................................... 23

3.2 Transition metal oxide composite electrode for symmetrical LTCFC (paper 3) ... 25

Doctoral Thesis Liangdong Fan Page IX

3.2.1 Crystal structures ............................................................................................. 25 3.2.2 Electrical conductivity ....................................................................................... 26 3.2.3 Electro-catalytic activities ................................................................................. 27 3.2.4 Hydration effect ................................................................................................ 29

3.3 An all-nanocomposites LTCFCs (paper 1) .......................................................... 31 3.3.1 Material properties ........................................................................................... 32 3.3.2 Electrochemical Performances ........................................................................ 33

3.4 Fundamental study of single-component/electrolyte-free fuel cell (paper 2 and 5-7) 35

3.4.1 Materials choice and electrical properties ........................................................ 36 3.4.2 Fuel cell performances ..................................................................................... 37 3.4.3 Micro-electrochemical reaction process ........................................................... 40 3.4.4 Joint fuel cell and solar cell p-n junction principle ............................................ 42 3.4.5 Energy conversion device integration/network ................................................. 43

4 CONCLUSIONS ......................................................................................................... 45

5 FUTURE WORKS ...................................................................................................... 47

6 REFERENCES ........................................................................................................... 49

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LIST OF FIGURES

Figure 1-1: Schematic representations of SOFC with oxygen ionic conductor (A), proton (B) and hybrid oxygen ion and proton conductor (C). ......................................................... 2

Figure 1-2: Typical I-V curve and the polarization loss in real running ................................. 3

Figure 1-3: Electrical properties of common single-phase electrolyte materials for intermediate and high temperature SOFC [Haile 2003; Zuo et al. 2006]. The line parallel with x axis indicates the required electrical conductivity of 0.1 S∙cm-1for high fuel cell performances. .................................................................................................. 6

Figure 1-4: Core-shell nanocomposite (a) SDC-Na2CO3 [Wang et al. 2008], (c) LiZnO-SDC [Wu et al. 2012] and (b, d) their corresponding ionic conductivities in air. (With reproduction permission from Elsevier. Copyright @ Elsevier 2007). ........................... 8

Figure 1-5: Proposed oxygen and proton (a) transport path and (b) conductivities in SDC-Na2CO3 prepared by two-step wet chemical method based four-probe experimental method [Wang et al. 2011], (c) shemetic representation of core-shell SDC-Na2CO3 naschematicite and its numerical simulated hybrid proton and oxygen ionic conductivity [Liu et al. 2010]. Note: The intefacial conduction is suggested in both cases. (With reproduction permission from Elsevier 2011 and AIP Publishing LLC., respectively). ................................................................................................................ 9

Figure 1-6: Schematic representation of three-layer fuel cells and single-component/electrolyte-free fuel cells .......................................................................... 11

Figure 2-1: Preparation procedure for SDC-carbonate composite powder ........................ 15

Figure 2-2: Setup illustration of sample holder used in this thesis ..................................... 17

Figure 2-3: Principle of electrochemical pumps for DC conductivity measurement ........... 18

Figure 3-1: Schematic illustrations of H2O electrolysis using a ceramic fuel cell with oxygen ionic conductive (left) and proton conductive (right) electrolyte. ................................. 21

Figure 3-2: Temperature dependance of I-V curves of LTCFCs with ceria-carbonate composite electrolyte and Ni-based electrode using 3% humidified hydrogen fuel and air oxidant at different temperature under FC and EC modes. ................................... 22

Figure 3-3: Electrochemical impedance spectra of ceramic electrolysis cells under 1.2 V bias at different temperatures (Applied frequency: 100 kHz-0.1 Hz), 3 vol% H2O-H2/Air.................................................................................................................................... 23

Figure 3-4: Absolute humidity dependence of electrochemical impedance spectra of ceramic electrolysis cells at 550 oC (applied voltage bias: 1.4 V) ............................... 23

Figure 3-5: (a) EIS under 1.2 V voltage bias and (b) I-V curves of SOECs with 3 vol% water in anode and cathode chambers, respectively .................................................. 24

Figure 3-6: Scheme of low temperature ceramic fuel cells with symmetrical lithiated transition metal oxide composite electrode ................................................................. 25

Figure 3-7: Room temperature XRD patterns of LiNiCuZnO after heat treatments in oxidation gas (air) and in reducing atmosphere (5% H2 balanced with N2 for 10 h), respectively. ................................................................................................................ 26

Doctoral Thesis Liangdong Fan Page XI

Figure 3-8: Electrical conductivities of lithiated transition metal oxide composite in air and in hydrogen by DC conductivity measurement (solid point). The inset: temperature dependence of electrical conductivities. The linear fitting is also presented. Taken from [Fan et al. 2012b] with permission of Elsevier. ................................................... 26

Figure 3-9: Nyquist plots of symmetrical ceramic fuel cell in air at various temperatures. The EIS contain an inductance, an intermediate frequency semi-arcs and a tail. The insets are the employed equivalent circuit models proposed for fitting of impedance spectra. Solid dots are the original data; the lines are the fitted results. ..................... 27

Figure 3-10: EIS of anode supported symmetrical cells with LiNiCuZnO as electrode and SDC-carbonate as electrolyte at different temperatures in fuel cell condition. ............ 28

Figure 3-11: Electrochemical impedance spectra of symmetrical cells at 450 oC under different gas atmospheres. ......................................................................................... 29

Figure 3-12: Anode supported LTCFC (D) constructed by all nanocomposite components: anode Ni/Fe-SDC (A), SDC-carbonate nanocomposite electrolyte (B) and cathode lithiated NiO/ZnO (C). Reprinted from [Fan et al. 2012c] with permission. Copyright @ 2012 Elsevier. ............................................................................................................. 32

Figure 3-13: (a) Voltage/power density-current density characteristic at different temperatures with hydrogen as fuel (100 ml∙min-1) and air as the oxidation, (b) Nyquist curve at 600 oC (c) short-term stability testing (close to short-circuit condition) and (d) its peak power densities after 5 times of thermal cycling between 200 oC and 600 oC of all nanocomposite constructed LTCFC. Reprinted from [Fan et al. 2012c] with permission. Copyright @ 2012 Elsevier. ..................................................................... 33

Figure 3-14: Diagrammatic presentation of traditional three-layer fuel cell and novel electrolyte-free fuel cells. Reproduced from [Zhu et al. 2013a] with permission. Copyright @ 2013 Elsevier. ........................................................................................ 35

Figure 3-15: (A) XRD pattern and (B) SEM image of LNCZO-SDC nanocomposite sintered at 800 oC for 2 h. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. .................................................................................................. 36

Figure 3-16: Arrhenius curves of LNCZO-SDC nanocomposite in air and hydrogen, respectively. In which the LNCZO content is 40 wt%. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. ..................................... 37

Figure 3-17: EFFC Voltage-Current density and Power density-Current density characteristics as function of (a) electronic conductor content at the fixed powder weight of 0.5 g and (b) pellet thickness (in the form of total powder weight) at the fixed 40 wt% SDC-Na2CO3 in the composite. The thicknesses of pellets are 0.70, 0.88, 1.10 and 1.45 mm for the above four cases. Reprinted from [Fan et al. 2012b] with permission. Copyright @ 2012 Elsevier. ..................................................................... 37

Figure 3-18: SEM of the cross-section of fractured EFFC, (a) the whole cell and (b) enlarged fuel cell side and (c) magnified cathode side. Reprinted from [Fan et al. 2012b] with permission. Copyright @ 2012 Elsevier. ................................................. 38

Figure 3-19: (a) Comparative electrochemical performance of EFFC and the conventional three-layer fuel cell after the materials fabrication procedure optimization and (b) ever improved EFFC electrochemical performance when adding the Fe active redox catalyst. (Reprinted from [Zhu et al. 2011c] with permission from Professor T. Nejat Veziroglu on behalf of the International Association for Hydrogen Energy) ................ 39

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Figure 3-20: Electrochemical performance of optimized EFFC operated with biogas, methanol and ethanol, respectively. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. ...................................................... 40

Figure 3-21: A proposed redox reaction process for EFFC (a micro-view): hydrogen and oxygen are dissociated on transition metal oxide and migrate and meet at the ionic conductor to form water. (Reprinted from [Zhu et al. 2011c] with permission from Professor T. Nejat Veziroglu on behalf of the International Association for Hydrogen Energy, with slight modification) ................................................................................. 41

Figure 3-22: The suggested EFFC working principle, similar to a solar cell: (A) initiated by the hydrogen and oxygen dissociation to build up the voltage and the corresponding electrical field to separate the hole and electron pair as well as the ionic and electronic phase. (B) The built up electrical field forces the electron and hole moving to the counter electrode while the ionic phase still goes through the ionic conductor to give H2O and electricity. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. ........................................................................................ 43

Figure 3-23: A proposed integrated system combined with EFFC, solar cell and photolysis technologies built on a one-layer nanocomposite. The EFFC and solar cell are also suggested to operate at elevated temperature by integrating the solar heating technology. Reproduced from [Zhu et al. 2013a] with permission of Elsevier 2012. ... 44

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LIST OF TABLES

Table 1: Electrochemical performance of ceria-carbonate composite electrolyte based FC system with various cathode catalysts from the open literature with hydrogen fuel except as indicated. .................................................................................................... 10

Table 2: Overview of the major challenges and features for investigating ceria-carbonate composite based LTCFC, EFF, conventional SOFC and thin film SOFC. Reproduced from ref. [Zhu et al. 2013a] with permission. Copyright @ 2013, Elsevier. ................. 12

Table 3: The fitted results of EIS with different equivalent circuit model between 500 oC and 600 oC (unit: Ω cm2) ............................................................................................. 27

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NOMENCLATURE

FC Fuel cell

EC Electrolysis cell

SOFC Solid oxide fuel cells

SOEC Solid oxide electrolysis cells

CFC Ceramic fuel cell

SFC Symmetrical fuel cell

PEMFC Proton exchange membrane fuel cell or polymer membrane fuel cells

LTCFC Low temperature ceramic fuel cell

LTSOFC low temperature SOFC

EFFC Electrolyte-free fuel cell

SC/EFFC Single-component/Electrolyte-free fuel cell

SCD Single component device

TLFC Three-layer fuel cell

TPB Triple phase boundary

MEA Membrane electrode assembly

DCO Doped ceria oxide

SDC Samarium doped ceria oxide (20 wt%)

GDC Gadolinium doped ceria oxide (10 wt%)

NSDC SDC composited with Na2CO3 by one-step co-precipitation

LNSDC SDC composited with Li2CO3 and Na2CO3, normally 20 wt% of

carbonate, except indication

LNCZ Lithiated NiO/CuO/ZnO oxide composite

SEM Scanning electron microscope

TEM Transmission electron microscope

EIS Electrochemical impedance spectroscopy/spectra

OCV Open circuit voltage

XRD X-ray diffraction

ac Alternating current

dc Direct current

ASR Area specific resistance

Rp Electrode polarization resistance

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Ro Ohmic resistance

YSZ Yttria stabilized Zirconia, Y0.08Zr0.92O2

LSGM La0.9Sr0.1Ga0.8Mg0.2O3

BZCY BaZr0.1Ce0.7Y0.2O3-δ

BSCF Ba0.5Sr0.5Co0.8Fe0.2O3-δ

LSM La0.8Sr0.2MnO3

Z' Real part of impedance

Z'' Imaginary part of impedance

σ Conductivity (S∙cm-1)

Ea Activation energy

TEC Thermal expansion efficient xOO lattice oxygen atom

••OV Oxygen vacancy

tη Maximum theoretical thermodynamics efficiency

vη Voltage efficiency

fη Fuel utilization efficiency

( )rG T, P∆ Change of Gibbs free energy at given temperature and pressure

298rH θ∆ Standard enthalpy of formation of reaction

actV∆ Voltage loss caused by activation polarization

ohmV∆ Voltage loss induced by ohmic polarization

conV∆ Voltage loss caused by insufficient mass diffusion

fv The fuel velocity of flow (mole per minute)

l Thickness (cm)

S Area of pellet (cm2)

L Inductance

Q Constant phase element

h Hole

e Electron

Doctoral Thesis Liangdong Fan Page 1

1 INTRODUCTION

1.1 Fuel cells

The ever increasing electric power demand, fast fossil fuel consuming rate and environmental issue concerns require highly efficient and green energy conversion technologies. Alternatives to fossil fuel are wind power, hydro-power, wave power, solar energy and power from the renewable fuel such as biomass, waste gas, alcohol etc. However, the current technology developments and system cost of the above power sources are still not well addressed. Alternative conversion technologies are pursued for future applied energy. Fuel cells using hydrogen and hydrocarbon fuel with the distinct characteristics of high efficiency and green process have attracted great attention in recent years. In an intermediate-long term consideration, fuel cells, especially high temperature solid oxide fuel cells enabling to use hydrocarbon fuel could relieve the energy crisis by significantly improving the fuel utilization efficiency. Fuel cells are electrochemical devices, which can directly convert the chemical energy from a fuel into electric power. Since the discovery of the first fuel cell by Grove [1839], there have been developed of several kinds of fuel cells. Based on the electrolyte materials used, they are divided into alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PCFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) or ceramic fuel cell (CFC). While based on the operating temperature, they are listed as low temperature fuel cells (AFC, PEMFC and PCFC) and high temperature fuel cells (MCFC and SOFC). Among different kinds of fuel cells, PEMFC and SOFC have received particular attention since they stand for the typical operational temperature with wide application fields. Compared with PEMFC, SOFC possesses the advantages of fast reaction kinetics, low requirement for precious metal, direct utilization of hydrocarbon fuel and high valuable waste heat. As a matter of fact, it has been recognized as the bridge technology between current fossil fuel century and future hydrogen economy.

1.2 Solid oxide fuel cells

1.2.1 Electrochemical reactions

Solid oxide fuel cell is a type of fuel cell using a ceramic oxide electrolyte (hence also called a ceramic fuel cell). Because of the highest operational temperature, SOFC processes high fuel utilization efficiency, especially when the high value waste heat is reused. SOFC also shows great fuel flexibility, not only for hydrogen, but also hydrocarbons, CO, H2S, NH3, natural gas, solid carbon, liquid fuel such as bio-alcohol like methanol and ethanol. It also presents modularity advantage since all cell components are solid-state. It has wide use application fields, such as laptop power, vehicle electricity, heavy truck and stationary power generation plant. The key components of a SOFC contain a porous anode and cathode, where the fuel oxidation and oxidant reduction electrochemical reactions happen, and their sandwiched dense electrolyte layer, through which the ionic can pass while the electron is blocked and forced to transfer from the external circuit. The electrochemical reaction processes in SOFCs with different ionic conductor, i.e. O2-, H+ and mixed conduction, are shown in

Page 2 Doctoral Thesis Liangdong Fan

Figure 1-1. Taking the oxygen conductor based SOFC as an example, the reactions expressed by KrÖger-Vink notation are: Anode: ••

2 2 OH H O 2x -O+O (g)+V + e→ Eq. 1-1

Cathode ••

2 OO 2 - xO+ e V O+ → Eq. 1-2

The overall reaction: 2 2 20.5O H H O( )+ g→ Eq. 1-3 In which, x

OO is a lattice oxygen atom and ••OV is oxygen vacancy. Thus the reactions

produce steam while the chemical energy in H2 and O2 is extracted and converted into electricity for external loading application.

Figure 1-1: Schematic representations of SOFC with oxygen ionic conductor (A), proton (B) and hybrid oxygen ion and proton conductor (C).

1.2.2 Efficiency

As mentioned above, SOFC has distinctly higher energy efficiency compared with current energy conversion technologies, such as heat engines including combustion engine, photovoltaic cells, thermo electric generator and other fuel cells. For a fuel cell, an electrical efficiency contains three parts [Wachsman et al. 2011]:


× ×t v f=η η η η Eq. 1-4 In which tη , vη and fη are maximum thermodynamic efficiency, voltage efficiency, and fuel utilization efficiency, respectively. There is a maximum theoretical electrical efficiency calculated by the following equation: r 298=- ( ) /r rV G T, P H θ∆ ∆ Eq. 1-5 In which, the ( )rG T, P∆ is the Gibbs free energy of formation at a given temperature and pressure and 298rH θ∆ is the standard enthalpy of formation of reaction [Williams et al. 2009]. There is another part of the energy from the chemicals which is converted to heat during the reaction. About the voltage efficiency, we need to first study the voltage behavior and polarization resistance of the fuel cell during a “discharge” process, which are shown in Figure 1-2. There is one theoretical electromotive force (EMF), also called reversible voltage: r =- ( ) / nFrV G T, P∆ Eq. 1-6 Where n=2 for hydrogen oxidation, and F=96485 C∙mol-1. Since the Gibbs free energy is temperature and pressure depended, so it would be a little complex using the above equation. There is also another universal calculation method, i.e. Nernst equation:

2 2

2

0.5o

r = + ln( )H O

H O

p pRTE EZF p

Eq. 1-7

Where

2Hp ,2Op ,

2H Op and oE are hydrogen partial pressure in the anode chamber, oxygen partial pressure in the cathode chamber, steam pressure in the anode and the voltage at standard condition, respectively. In the real fuel cell, the EMF is normally reduced to open circuit voltage (OCV) because of the electrolyte layer internal short-circuit and possible gas leakage.

Figure 1-2: Typical I-V curve and the polarization loss in real running


As can be seen from the Figure 1-2, there are several parts of polarization resistances in a fuel cell. They are activation polarization resistance at the initial voltage dropping due to the insufficient electro-catalytic activity, ohmic polarization resistance due to the internal charge (ion through the electrolyte and electron in electrode) transport resistance and concentration polarization resistance at the large current density situation due to the mass transport limitation. These polarization resistance will reflect corresponding voltage polarization losses, actV∆ , ohmV∆ and conV∆ , thus the voltage efficiency should be counted as follows:

o

= ohm act conv

rev rev

E - V - V - VV=E E

η ∆ ∆ ∆ Eq. 1-8

In which =ohmV i ASR∆ × Eq. 1-9 And ASR is the ohmic resistance of the electrolyte. The fuel utilization efficiency ( fη ) is defined as the ratio of the fuel used for electricity production to the total fuel input. Based on the Faraday law, the fη can be calculated by:

=ff

i / nFv

η Eq. 1-10

Where i and fv are the current density (A∙m-2) and fuel velocity of flow (mol∙s-1). Thus the final electrical efficiency is:

298

( )= o ohm act conr

r f rev

E - V - V - VG T, P i / nFH v Eθη

∆ ∆ ∆∆× × ∆

Eq. 1-11

In the actual operation, the voltage is set at 0.7 V to get enough voltage efficiency and relative high fuel utilization efficiency. Besides, according to Eq. 1-11, in order to improve the total electrical efficiency, high active electrode catalyst for reduced activation voltage loss, and high ionic conductive electrolyte with thin film status to reduce the ohmic resistance and high fuel utilization ratio, sufficient porosity for gas diffusion in the electrode are considered. To address the above voltage loss, the material innovation is the key, and the optimization of cell microstructure or configuration will also help to reduce the aforementioned polarization resistances. 1.2.3 Key cell components and challenges for SOFC

Anode

The state of the art anode material in solid oxide fuel cells (SOFC) is Ni cermet, a composite of Ni and an ion conductor material, in which Ni acts as the catalyst and the electron conductor while the electrolyte material plays as the ionic conductor, the Ni metal agglomeration inhibitor at the elevated temperature, and thermal expansion alleviant with other cell component. Ni-cermet is adopted due to its adequate electro-catalytic activity for all of fuels, especially for hydrogen. But there are also some shortcomings for Ni-cermet anode, they are redox instability, high temperature sintering, catalytic carbon deposition and low resistance to sulfur or other contaminations leading to cell performance


degradation and cell structure failure. Thus the current anode materials research activities are focused on developing redox anode and for possible sulfur contained hydrocarbon utilization [Atkinson et al. 2004]. Cu-based cermet [Park et al. 2000], ceria-based anode [Perry Murray et al. 1999], Alloy materials [Lee et al. 2004], perovskite oxides [Tao et al. 2003; Huang et al. 2006], some (nano)composite materials [Yang et al. 2011; Yang et al. 2012] and novel ionic conductors [Yang et al. 2009] and novel cell configuration [Zhan et al. 2005] have been widely proposed and investigated. For example, Park et al. [2000] developed the Cu-CeO2 composite anode to replace Ni-based cermet for methane fuel with high resistance for carbon deposition. Tao [2003] and Huang [2006] separately synthesized perovskite oxides of La0.75Sr0.25Cr0.5Mn0.5O3 and Sr2Mg1-xMnxMoO6-δ to solve the redox instability of Ni cermet electrodes and improve the tolerance to carbon deposition and sulfur poisoning. Considering the most developed anode materials are inferior to Ni based electrode, nanostructured barium oxide/nickel (BaO/Ni) interfaces fabricated by a simple thermal evaporation coating process exhibited high power density and stability in C3H8, CO and gasified carbon fuels at 750°C, making it a potential anode for future application. Cathode

Compared with anode materials, the cathode materials research activity is much richer because great attention has been given to the much slower electrochemical reaction kinetics of oxygen reduction compared with the hydrogen oxidation [Adler 2004]. Consequently, the cathode polarization resistance takes up a major part of the total polarization, especially at the reduced temperature. Activities have tried to not only explore new cathode materials, but also to understand the electrode reaction mechanisms, elucidate structure-property-performance relationships, solve the instability problems of some promising cathode. Among all of the cathode materials, perovskite oxides with mixed ionic and electronic conduction have attracted plenty attention and exhibit the most potential prospective for SOFC. The cobalt-based perovskite oxides are the particulate interest because of their satisfying electro-catalytic activity for oxygen reduction above 600 oC [Shao et al. 2004; Yang et al. 2008]. However, the high thermal expansion efficient (TEC) compared with the current most used electrolyte materials has hindered their wide application. Fe doped LaNiO3 is also one kind of promising electrode with the characteristics of low TEC and high resistance toward Cr poisoning [Komatsu et al. 2008]. Some other kinds of analogical layer LnBaCo2O5+δ (Ln = Y, Gd, Pr, Nd) [Baek et al. 2008; Liu 2009] and layer K2NiF4+δ oxides have also been developed for reduced temperature operation. Composite cathodes are also extensively adopted since no one single material can meet the whole requirements for high performance cathode materials. Some precious metals like Ag, Pt and Pd are added into the prefabricated cathode to improve the oxygen surface adsorption and exchange process, which is the rate-determining step during the whole oxygen reduction process. Co3O4, a much cheaper additive, is also used in composite cathode with unexpected improved activity [Zhang et al. 2007]. Zhou et al [2011] tried to simply modify porous Ba0. 5Sr0.5Co0.8Fe0.2O3-δ (BSCF) backbone with microwave-plasma, the resulted hetero-structure cathode gave an activity improvement of 250% higher than the untreated BSCF backbone. There have many kinds of newly developed cathode materials, but most of them are not subjected to the real condition testing, especially for the long-term nonstop operation. Some of them have already shown some drawbacks, like high reaction active of BSCF cathode with CO2 and H2O, though it is currently thought to be the most promising electrode material for LTSOFC. In this context, Zhou et al. [2012] developed simple


coating of BSCF cathode with dense La2NiO4 membrane. Such as simple modification not only improves the electrode reactivity but also makes it work stably in CO2-containing air. Electrolyte

The electrolyte research represents one of the major streams in the SOFC field since the adoption of electrolyte determines the working temperature. The current major research interest is to reduce the operating temperature of SOFC to widen the materials choice and application fields, reduce the thermal and chemical compatibility problems and descend the system cost to facilitate the commercialization. Versatile electrolyte majorly the single electrolyte materials have been invented or discovered in the past several decades [Singman 1966; Lacerda et al. 1988; Ishihara et al. 1994; Nakayama et al. 1995; Steele 2000; Haugsrud et al. 2006; Zuo et al. 2006; Garcia-Barriocanal et al. 2008]. Most of them have been demonstrated to show improved electrical properties compared with the state of art electrolyte material-YSZ. The temperature dependence of the electrical conductivities of current commonly used electrolyte materials is shown in Figure 1-3. Here three major kinds of electrolytes will be presented.

Figure 1-3: Electrical properties of common single-phase electrolyte materials for intermediate and high temperature SOFC [Haile 2003; Zuo et al. 2006]. The line parallel with x axis indicates the required electrical conductivity of 0.1 S∙cm-1for high fuel cell performances. The first one is the fluorite structured ceria, sharing the same crystal structure with the YSZ. It however shows one order of magnitude of electrical conductivity than that of YSZ. In addition, it displays good catalytic functions for electrode reactions both in the anode and cathode. Furthermore, ceria-oxide electrolyte shows the most chemical compatibility with the commonly used electrode materials [Steele 2000]. One of the major shortcomings of doped ceria is its electronic conduction caused by the reduction of Ce4+ to Ce3+ in high temperature and reduced atmosphere [Mogensen et al. 2000]. This will reduce the open circuit voltage and the system energy efficiency. Fortunately, the reduction trendy will descend as operating temperature reduces. Thus it has become the most investigated electrolyte for intermediate and low temperature SOFC.


The second one is the La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) perovskite oxide, which was discovered by Ishihara and his colleagues [1994]. It shows higher electrical conductivity than doped ceria oxide at high temperature, slight lower than the highest electrical conductivity Bi2O3 based oxide ionic conductor. It is also gives unit oxygen ionic transport number in a wide oxygen partial pressure window (10-20 to 1 atm). However, this promising electrolyte presents difficulties to get pure phase and high reaction activity with Ni-based electrode, even during the cell fabrication process [Matraszek et al. 2004; Shaula et al. 2004]. The last one is proton conductive perovskite oxide after the pioneering work of Iwahara et al. [1981], including acceptor-doped perovskite-type alkaline earth Cerates, Zirconates, Niobates and Titanates. The substitution of B site of perovskites by the low value of elements will produce oxygen vacancy in the lattice, then the proton transports through this charge carrier. Perovskite proton conductors show lower proton transfer activation energy than that of oxygen ion because of the smaller ionic radius, thus it shows more opportunity than oxygen ion conductor at the reduced temperature. Good proton conductive oxide of BaZr0.1Ce0.7Y0.2O3-δ has been developed and shows excellent fuel cell performance in 500 oC [Zuo et al. 2006]. However, most of proton conductive electrolyte materials have the tradeoff effect between the proton conduction and chemical stability in CO2 or H2O. Thus the future research will focus on improving the resistance against CO2 and H2O while maintaining sufficient ionic conductivity. Much progress on development novel electrolyte materials and the investigation of ionic conduction mechanism and optimization of ionic conductivity has been made. But one critical issue is that all the above single-phase electrolyte materials have much low electrical conductivity (<< 0.1 S∙cm-1) below 600 oC as shown in Figure 1-3, while it is suggested to be the criterion for achieving high fuel cell performance [Etsell et al. 1970]. Thus the more effort is still needed to develop new electrolytes for practical applications.

1.3 Advanced fuel cell materials and system

1.3.1 Ceria-based composite and nanocomposite

As stated above, the electrical conductivities are lower than 0.1 S∙cm-1 for the current most investigated single-phase electrolytes. The origin of low electrical conductivity has been extensively studied from the aspects of the material crystal structure and ionic conduction mechanism. The major reasons include large grain boundary resistance induced by space charge layer and block effect of impurity and dopant aggregation at grain boundary [Iguchi et al. 2011; Li et al. 2011; Lee et al. 2012]. Thus, to improve the electrical conductivities of the current electrolyte materials, the modification of grain boundary is seen as the only solution. Actually, as indicated by Ivanova et al. [2008], heterogeneous ceramics with improved grain boundary conductivity has been demonstrated with improved grain boundary engineering, or composite approach. As a matter of fact, the first observation of electrical conductivity enhancement through composite approach was reported in the lithium battery field [Liang 1973], in which the lithium ion conductivity of LiI is improved by two orders of magnitude by adding about 35-45 wt% of Al2O3. The composite electrolytes have also been used in SOFC, but normally do not exhibit sufficient electrical conductivity enhancement until Zhu et al. reported that doped ceria-salt composites exhibit adequate ionic conductivity higher than 0.1 S∙cm-1 below 600 oC, which is interesting for industrial applications [Zhu 2003; Zhu et al. 2003; Wang et al. 2008; Raza et al. 2010; Wu et al. 2012].


Figure 1-4: Core-shell nanocomposite (a) SDC-Na2CO3 [Wang et al. 2008], (c) LiZnO-SDC [Wu et al. 2012] and (b, d) their corresponding ionic conductivities in air. (With reproduction permission from Elsevier. Copyright @ Elsevier 2007). Various ceria-based composites such as ceria-salt (salt = Carbonate, Halide, Nitrate, Sulfate, and Phosphate or their mixtures) and ceria-oxide (oxide = perovskite oxide, LiZnO, Y2O3 etc.) have been developed. Some TEM images of typical ceria-based composite like ceria-carbonate and ceria-LiZnO oxide composite and their ionic conductivities are shown in Figure 1-4. Both of them have typical core-shell microstructures, the SDC cores are homogeneously covered by the second phase. Such a unique structure gives impressive electrical conductivity higher than 0.1 S∙cm-1 above 300 oC, while they are 1000 oC for YSZ and 800 oC for single-phase doped ceria oxide. Especially, these ceria-based composites exhibit unique hybrid proton and oxygen ionic conduction in fuel cell condition, as demonstrated by experimental and simulation analysis, shown in Figure 1-5. Advanced fuel cells with ceria-based composite electrolytes have exhibited promising fuel cell performance below 600 oC. For example, peak power densities of 1085 and 690 mW∙cm-2

have been achieved at 600 and 500 oC, respectively, using ceria-carbonate as electrolyte [Huang et al. 2007a]. A recent work by Xia [2010] even revealed that an impressive maximum power density of 1704 mW∙cm-2 at a current density of 3000 mA∙cm-2 at 650 °C had been obtained in fuel cell condition with CO2 as cathode gas additive. The hybrid ionic (O2-, H+ and CO3

2-) conduction behavior has been also widely investigated from different groups of various countries using different characterization methods and numerical analysis, such as ac impedance spectroscopy comparative study, dc four-probe method, electrochemical pump, current interrupt method, production method and improved effective-medium model [Fan et al. 2013], . However, there still not yet form consensuses on the ionic transport mechanism. Instead, contradictory results had been revealed from different groups [Zhu et al. 2006b; Huang et al. 2008; Di et al. 2010; Wang et al. 2011;


Zhao et al. 2012a]. Effort is still needed to reveal the origin of the conductivity difference and exploit new method for precision determining the possible ionic conduction behavior in composite electrolyte since it will help to design and uncover new composite materials for future advanced energy and environmental applications, such as direct carbon fuel cell and CO2 separation.

Figure 1-5: Proposed oxygen and proton (a) transport path and (b) conductivities in SDC-Na2CO3 prepared by two-step wet chemical method based four-probe experimental method [Wang et al. 2011], (c) shemetic representation of core-shell SDC-Na2CO3 naschematicite and its numerical simulated hybrid proton and oxygen ionic conductivity [Liu et al. 2010]. Note: The intefacial conduction is suggested in both cases. (With reproduction permission from Elsevier 2011 and AIP Publishing LLC., respectively).

1.3.2 Novel composite electrode material

Compared with the research activity on ceria-based composite, the endeavor put on the electrode materials for advanced fuel cell has been largely delayed even though the electrode materials are also extremely important for achieving high fuel cell performance at the reduced temperature.


Table 1: Electrochemical performance of ceria-carbonate composite electrolyte based FC system with various cathode catalysts from the open literature with hydrogen fuel except as indicated.

Cathode catalyst Temp. (oC) Pmax (mW∙cm-

2) Refs.

LiNiO2 400-650 300-800 [Zhu 2003] Ag 590 716.2 [Hu et al. 2006]

Sm0.5Sr0.5Fe0.8Cu0.2O3-δ 525-600 250-370 [Zhang et al. 2011] La2Ni0.8Co0.2O4+δ 500-600 401-700 [Huang et al. 2010] La(Ni, Fe, Cu)O3 450 227 [Li et al. 2006]

Ba0.8Sr0.2Co0.5-Fe0.5O3-δ 500 860 [Sun et al. 2007] SrTixCo1-xO3-δ 600 613 [Gao et al. 2011]

Pr2NiO4-Ag 600 695 [Fan et al. 2012a] La0.6Sr0.4Co0.8Fe0.2O3 400-600 200-700 [Zhu 2001] Li-Ni-Cu-Zn oxide* 550 730 [Zhao et al. 2012b]

NiO/ZnO* 500 1107 [Raza et al. 2011a] Lithiated NiO/ZnO 600 808 [Fan et al. 2012e] Li-Ni-Cu-Zn oxide* 570 584 [Imran et al. 2011]a

Li0.2Ni0.7Cu0.1O* 580 215b 148c [Qin et al. 2011]

* Symmetrical fuel cell. a Bio-ethanol fuel b Glycerol-water mixture c bio-ethanol/water mixture

The same as SOFC, the development of electrode materials for composite electrolyte based advanced fuel cells has been majorly put on the cathode materials to match with the composite electrolytes’ high ionic conductivity and obtain good fuel cell performance and stability at the reduced operational temperature. Different cathode materials and their fuel cell performance on ceria-carbonate composite electrolytes with hydrogen fuel are given in Table 1. All of the cathodes have given good fuel cell performance, in which transition metal oxide composites cathodes show the most promising characteristics because of its cheap raw sources, easily fabrication process, highest electrochemical performance and some other merits that will be introduced in the following paragraphs [Raza et al. 2011a; Fan et al. 2012e; Zhao et al. 2012b]. The effort also has been devoted to develop symmetrical electrode materials for ceria-carbonate composite electrolyte based advanced fuel cells because the symmetrical structure would reduce the cell fabrication procedure, and benefit for carbon deposition and sulfur poison treatment, which will subsequently reduce the system cost and enhance stability compared with asymmetrical counterpart. The symmetrical electrodes are the transition metal oxides (NiOx, CuO, ZnO, FeOy, CoOz, MnOn etc.) composite, with or without lithiation. The major composition is NiO since it has been demonstrated to show excellent cathode performance on ceria-carbonate composite electrolyte. Doping or compositing with other transition metal oxides could not only improve the electrode activity but also reduce the Ni dissolution in the molten carbonate [Fan et al. 2012e]. Especially, these transition metal oxides or alloys give improved electro-catalytic for fuel oxidation and carbon deposition resistance [Qin et al. 2011; Raza et al. 2011b]. The above features make transition metal oxide composite as promising symmetrical electrodes for advanced fuel cells [Raza et al. 2011a; Zhao et al. 2012b]. Especially, the research on transition metal oxide composite with semi-conductive properties and the highly ionically conductive composite electrolyte has opened a new research field –the Single-Component/Electrolyte-Free Fuel Cells (SC/EFFC).


1.3.3 Electrolyte free fuel cell (EFFC)

Solid oxide fuel cells or ceramic fuel cells have been largely limited to widely application due to the complex cell structure and insufficient cell component activity and their resulted system cost and durability issues. Complex three-layer configuration brings many undesirable issues, like the thermal expansion/chemical incompatibility, large interfacial polarization resistance and complex fabrication process and so on. Significant research effort has been devoted to decreasing the SOFC operation temperature and to developing simplified cell configurations. However, the journey to fuel cell economy is still a little farther than we could expect. If FC could be liberated from the constraints of complex cell structures, especially the electrolyte layer, there would be a completely fuel cell technology. In this context, a radical new fuel cell type, joining the fuel cell and solar cell principles, the single-component/electrolyte-free fuel cell (EFFC) was successfully developed by KTH fuel cell group to overcome the current shortcoming of solid oxide fuel cell, a three-layer fuel cell (TLFC) [Zhu et al. 2011a; Zhu et al. 2011b; Zhu et al. 2011c; Zhu et al. 2011d; Zhu et al. 2011e; Zhu et al. 2011f; Fan et al. 2012b; Liu et al. 2012; Qin et al. 2012; Zhu et al. 2013a; Zhu et al. 2013b]. A schematic presentation of TLFC and EFFC is shown in Figure 1-6.

Figure 1-6: Schematic representation of three-layer fuel cells and single-component/electrolyte-free fuel cells Compared with the TLFC, EFFC shows much reduced cell structure. Actually, the EFFC is constructed from the electrode materials of the conventional TLFC, and it can be simply considered that all the fuel cell components have been integrated into one component. Thus it is called “3 in 1”, as highlighted by Nature Nanotechnology [2011a]. In fact, the concept of single layer fuel cell has already reported by He et al. [2000] with a La0.9Sr0.1InO3−δ (LSI) materials. The LSI shows different electrical conduction behaviors at various oxygen partial pressures, i.e., n-type conduction in anode gas, p-type conduction in oxidation atmosphere, and ion conduction in middle oxygen partial pressure. It should be noticed that the single layer fuel cell proposed by He et al. is not the same or even similar to the EFFC reported by Zhu and his colleagues since a dense electrolyte layer which allows ion conduction while electronic insulation still exists. It is the conventional


three-layer fuel cell, other than EFFC, in He’s report. Moreover, the electrochemical performance in He’s report is too low for practical application. Since it is a newly developed fuel cell technology, and quite different from the conventional TLFC, many works, the EFFC related research such as the materials choice and optimization, comparative study with TLFC, possible working principle, and performance improvement are extensively studied in progress.

1.4 Comparison between different fuel cell technologies in this thesis

In this thesis, several kinds of fuel cell technologies, such as conventional solid oxide fuel cell, thin-film fuel cell and ceria-based composite electrolyte based LTCFC and EFFC advanced fuel cell are mentioned. To better understand and effectively compare different technologies, their characteristics and challenges are detailed summarized in Table 2. Table 2: Overview of the major challenges and features for investigating ceria-carbonate composite based LTCFC, EFF, conventional SOFC and thin film SOFC. Reproduced from ref. [Zhu et al. 2013a] with permission. Copyright @ 2013, Elsevier.

Fuel cell types Challenges Features

Ceria-based composite LTCFCs

Compatible electrodes* Well established science

Scaling up and fabrication# Long-life test

Low temperatures, 300-600 oC Two-or multi-phase composites, typical example, Ceria-carbonate, ceria-LiZnO2

Dual or hybrid ion transport Interfacial conduction dominated Thick electrolyte, 200-1000 μm

Single component EFFC

Urgent new science Long-life test

Low temperatures, 300-600 oC Two-or multi-phase composites

of the ion and semiconductors, Multi-charge, H+, O2-, e-, h. transport

Interfacial or surface conduction and reaction

Synergic ion and semiconducting properties Single-component device Thickness 600-1000 μm

Easy engineering and scaling up

Solid oxide fuel cells (SOFCs)

Sufficient low temperatures Low cost

High temperature, 800-1000 oC Single phase and single ion, O2- typically, YSZ

electrolyte Structural bulk conduction Anode/electrolyte/cathode Thick electrolyte, 200 μm

Thin film SOFCs

Expensive technologies Scaling up and fabrication

Long-life test Low cost

Low temperatures, 500-700 oC Ceria single phase material

single ion, O2- Thin film, typically, few to 10 μm

* In conventional anode/electrolyte/cathode configuration, it needs to develop compatible electrodes, anode and cathode, with the functional nanocomposite electrolytes. However, in the latest breakthrough technology, single component fuel cell device, such challenge has been avoided. # Scaling up processes and fabrication technologies need to be developed since nanocomposites request special shaping processes. As can be seen, the ceria-based composite LTCFC and EFFC have worked out many barriers for industrial application. Especially the EFFC, taking the advantages of radical


design and multi-functionalities, will greatly promote the commercialization of fuel cell technology.

1.5 Motivation and objective

Ceramic fuel cell (CFC) provides a green and sustainable approach to future energy conversion and environmental protection. Especially, CFC with ceria-based composite electrolyte has exhibited appealing fuel cell performance for potential application. However, some basic understandings on ceria-based composite, such as the hybrid proton and oxygen ion conduction mechanism, need further effort. Besides, the improvement of fuel cell system electrical efficiency needs the co-contribution of electrode materials. Novel electrode materials with high catalytic activity and reliability in fuel cell condition are highly demanded. Advanced fuel cell configuration and system are also suggested to help advance current fuel cells. Hence novel strategies, such as the symmetrical fuel cell and all-nanocomposite constructed fuel cells are adopted in this thesis. In addition, the development of conventional SOFC has been hindered by the high system cost and low durability, which is attributed to the complex cell structure and inadequate material properties. Based on our continuous research activities on functional composite material, the concept of Single-component/Electrolyte-free fuel cell (EFFC) has been demonstrated. The EFFC is a breakthrough research result for nanocomposite approach. However, the research on EFFC is still in its infant stage. More fundamental studies on this novel fuel cell are desirable. It is expected that the developed novel composite materials with interesting electrical properties and simplified fuel cell configuration will reduce the capital cost and system stability and eventually promote the commercialization of this green technology. In this thesis, we are aiming at developed functional composite materials in ceramic cell and electrolyte free fuel cell. The physicochemical properties of the developed composites have been carefully studied. First, the hybrid proton and ionic conductive behaviors of ceria-carbonate composite is revealed in electrolysis mode. It is concluded that the oxygen ionic conductivity takes up the main part of total ionic conductivity. Enhanced electrolysis efficiency is achieved with the Ni-based electrode over literature reported Pt electrode. Second, the electrochemical properties of lithiated transition metal oxide composite electrode material in symmetrical cell configuration are examined. These transition metal oxides presented sufficient electro-catalytic activity for oxygen reduction and hydrogen oxidation reactions. Especially, an astonishing super low oxygen reduction activation energy of 42 kJ∙mol-1 is identified, vs. ca. 100 kJ∙mol-1 for perovskite oxide. The hydrate effect of the transition metal oxide composite is considered to contribute to the excellent electrochemical performance. Third, the feasibility of all-nanocomposite fuel cells is proved. FC with all nanocomposite shows the impressive power output and desirable stability. The last part of thesis aims to fundamental investigates the newly developed electrolyte-free fuel cell. Comparative study of EFFC and traditional three-layer fuel cell is carried out both from the electrochemical performance, electrode reaction process and working principle. EFFC performs similar even higher peak power density compared with three-layer fuel cell while overcoming many drawbacks of conventional three-layer fuel cell. A jointing fuel cell and solar cell working principle is suggested to explain no internal short circuit problem in EFFC. A self-sustainable and CO2-free energy conversion integrated system is also proposed based on the developed functional (nano)composite materials for next generation energy conversion technology application.


2 EXPERIMENTAL METHODS AND TECHNIQUES

2.1 Raw materials

In this thesis, two major kinds of materials are prepared, one is the ionic conductor, including two-step prepared SDC-carbonate composite and one-step synthesized SDC-Na2CO3 nanocomposite. The other is the electronic conducting/semi-conducting determined material(s) as electro-catalyst(s) for three-layer fuel cells (TLFCs) and electrolyte free fuel cells, containing lithiated transition metal (Ni, Cu, Zn, Fe, Co, etc.) oxide or their composite by solid-state reaction method. Thus the applied raw materials are Ce(NO3)2·6H2O, Sm(NO3)2·6H2O or Sm2O3, concentrated HNO3, Na2CO3, Li2CO3, oxalic acid, Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Fe(NO3)3·6H2O, Zn(NO3)2·6H2O, Co(NO3)2·6H2O and 2NiCO3·Ni(OH)2·4H2O, ZnCO3·2Zn(OH)2·H2O, and LiOH. All of the chemicals are analytical grade from Sigma-Aldrich and used without further purification.

2.2 Sample preparation

2.2.1 Powder synthesis and Composite preparation

SDC-(Li/Na)2CO3 composite electrolyte (LNSDC)

SDC-(Li/Na)2CO3 composite with an SDC/carbonate weight ratio of 4:1 and L2CO3/Na2CO3 molar ratio of 52:48 was prepared by two-step processes: co-precipitation of Sm0.2Ce0.8O1.9 using oxalic acid as the precipitant, and mixing with carbonate by ball milling and subsequently co-sintering at 680 oC for 40 min. The obtained samples were crushed with mortar and pestle and deposited for further usage. The detailed preparation procedure is presented in Figure 2-1.

Figure 2-1: Preparation procedure for SDC-carbonate composite powder


SDC-Na2CO3 (NSDC) A facile one step co-precipitation process was developed by Raza [2010], which effectively simplified the procedure while presented improved ionic conductive properties. In brief, cation ions of Ce3+ and Sm3+ are dissolved in de-ionic water and mixed finely with each other, and then the Na2CO3 solution was added to the above cation ion solution in a cation ions/Na2CO3 molar ratio of 1:2. The precipitant was further stirred for 2 h before subjecting to filtration without water washing and drying in oven at 120 oC for a whole night. The resulted dried powders are subsequently sintered at 800 oC for 2 h to get the final sample. NiO and lithiated transition metal oxide composite

NiO was used as the anode for TLFC and fabricated by heat-treatment of nickel carbonate at 700 oC for 1 h. Lithiated transition metal oxide and their composites were prepared by a facial solid-state reaction method. In brief, all required raw materials (lithium source and transition metal sources) with required stoichiometry are well blended by high-speed ball milling for 2 h and then heat-treated at 700 oC for 2-3 h. The received powders are crushed with mortar and pestle before further use. 2.2.2 Cylindrical pellet/disk fabrication

Varied from the conventional complex and high temperature required solid oxide fuel cells cell component fabrication process and technology, the cells used in this thesis are simply prepared by one step co-pressing and co-sintering process. For TLFCs, the anode, electrolyte and cathode powders are successively deposited in the stainless steel die (Φ13), and co-pressed in one-step at a pressure of 200 MPa for 2-3 min to form the green pellets/disks. The thickness of each layer can be carefully controlled by varying the powder weight. In a normal case, anode support configuration was adopted with anode thickness of 0.4-0.5 mm, electrolyte thickness of 0.2 mm, and cathode thickness of 0.1-0.2 mm. While in some case, electrolyte supported configuration was also used with electrolyte layer thickness of 0.5 mm, and electrode thickness of 0.1-0.2 mm. Both sides of the green pellets were coated with silver paste (DAD87, Shanghai Research Institute of Synthetic Resins, China) as current collector, and sintered in a muffle furnace at 650 oC for 30 min before testing. For electrolyte free fuel cell, the same procedure was also employed, but with much reduced steps because only one-layer powder was used. In general, 0.5 g powder (ionic and semiconducting material mixture) was co-pressed on a porous nickel foam. The green disks were directly used without any further treatment. 2.2.3 Sample holder

A stainless steel device, shown in Figure 2-2, was used as the sample holder and tester. The highest applied temperature is 700 oC. The pellet is set on the sample holder which is placed at the center of an electric tube furnace in a designed gas atmosphere. A K-type thermocouple placed adjacent to the cell holder was used to detect the temperature. To help fix the pellet on the sample holder, two pieces of nick form were used both side and painted with silver paste.


Figure 2-2: Setup illustration of sample holder used in this thesis

2.3 Material characterizations

2.3.1 X-ray diffraction

Room temperature X-ray diffraction patterns recorded on Rigaku D/Max-2500V/PC X-ray diffracto-meter with Cu Kα radiation (λ=1.5418 Å) are used to identify the compositions and their corresponding crystal structures. The crystallite size of the compounds is estimated from their three highest intensity peaks by the Scherrer equation: 0.9

( )D

cosλ

β θ=

× Eq. 2-1

Where λ is the x-ray wavelength (typically ~1.54 Å), θ is the Bragg angle and β is the line broadening at half the maximum intensity (FWHM) in radians, which can be calculated by the following equation: 2 2 2

m s –β β β= Eq. 2-2

In which, βm and βs are the measured and standard full widths at half maxima (FWHM) of the diffracted lines, respectively. 2.3.2 Morphologies and texture microstructure

The particle morphologies and pellet fracture cross-section microstructures are observed by Scanning electron microscopy (SEM, NANOSEM430, FEI, USA). The powder samples are first ultrasonic dispersed in ethanol for several minutes and deposited on the glasses or the conductive tape, which were then subjected to gold deposition before taking the


SEM observation. The cross sections of pellet are obtained by hand cutting with a glazier's diamond.

2.4 Materials electrical performance

2.4.1 Conductivity measurement (dc and ac)

Two typical measurement methods were used to determine the electrical conductivity. One is the two probe dc method, which is also called an electrochemical pump method. The other one is an AC impedance spectroscopy method. For dc method, a specific voltage or current was given to the samples, the measurement continued for at least 30 min so that all of the processes reached the steady state. Then a corresponding current density or voltage was obtained. The measurement setup of dc conductivity was shown in Figure 2-3. The setup contains one DC power instrument to provide the specific voltage or current through the pellet which was set in different gas atmosphere, such as reduced H2 gas, in oxidation gas air, or in inert N2 gas.

Figure 2-3: Principle of electrochemical pumps for DC conductivity measurement For the AC impedance spectroscopy method, the sample was set in a specific atmosphere for 30 min before taking the impedance spectra which were recorded on the electrochemical working station, Versa 4 or PARSTAT 2273, both of them are from the Princeton applied research, USA. The applied frequency range is 100 KHz-0.01 Hz with signal amplitude of 20 mV. Generally, the high frequency intercept of the impedance spectroscopy on the real axis was defined as the electrical resistance of the pellet (R), including the ionic and electronic transport resistances. Then the obtained resistances (R value) were used to calculate the conductivity (σ, S∙cm-1) of the samples using the following equation: l

R Sσ =

× Eq. 2-3

In which l and S are the thickness (cm) and the active area (cm2) of pellet, respectively. The activation energy (Ea) was calculated by Arrhenius equation: o exp( / )aT E RTσ σ= − Eq. 2-4


Where oσ is the pre-exponential factor, R is the universal constant and T is Kelvin temperature. 2.4.2 Single cell performance

Current-voltage curves

The electrochemical performances of the single cells using different fuels (hydrogen, biomass, biogas, methanol and ethanol etc.) were determined by the voltage - current density (V-i) and power density - current density (p-i) characteristics. The V-I curves, which is normally displayed as a line were recorded by a computerized fuel cell tester- SANMU fuel cell tester (SM-102) by change of the external loading or a PARSTAT 2273 electrochemical workstation with a scanning rate of 20 mv∙s-1. The power density of single cell was then calculated by p=V*i, the p-i relationship was presented as a parabolic curve. The peak value of the parabolic curve is used to determine the single cell’s performance for comparative study. Electrochemical impedance spectroscopy

Electrochemical complex impedance spectroscopy (EIS) is a very useful tool to understand single cell performance. Moreover, it enables us to analysis the different contributions of each cell component to the total impedance and study the basic steps of the electrochemical reaction process, which has been widely used in solid oxide fuel cell community [Huang et al. 2007b]. In this thesis, EIS were recorded on open circuit voltage (OCV) condition except for the electrolysis study, in which a voltage bias is used. The measurement procedure is same as the ac conductivity process, but the data interpreting processes are much more complicated because of the complex electrochemical reactions involved in the whole impedance process. In general, the high frequency intercept is ascribed to the ohmic resistance, including the ionic and electronic transport resistance in the electrolyte and electrode, and the contact resistance between current collector and the electrode and the ohmic resistance of the instrument leads. While the arc after the high frequency intercept relates to the electrode polarization resistance, which always determine the whole reaction rate of the fuel cell, especially at the reduced temperature. To better understand the whole process of electrochemical reaction, some simple but effective empirical equivalent circuit models containing the series RQ elements are used, and a inductance element L and high frequency resistance of R were also integrated, forming a circuit of LR(RQ)(RQ)n-1 to get the total electrode polarization resistance and analyze the electrochemical reaction rate determining step. The EIS simulation was conducted on registered Zsimpwin software (Version 3.20, Echem Software). By selecting suitable equivalent circuit, the simulation results can be well fitted with the original data in a standard error less than 10%.


3 RESULTS AND DISCUSSION

3.1 Electrolysis study of ceria-carbonate composite for effective H2 production (paper 4)

The ceria-carbonate composite has attracted considerable attention in the recent years because of its super-ionic conductivity and unique hybrid proton and oxygen ion conductive property for LTCFC and other advanced applications. The impressive electrolyte performances have been widely demonstrated by several research groups [Huang et al. 2007a; Raza et al. 2010; Xia et al. 2010]. In parallel, the multi-ionic conductive properties and its related interface property, ionic conduction path and ionic interaction have also been extensively investigated. However, not like the approved excellent fuel cell performance, the studies of ionic conduction mechanism have not reached the universal agreement. Instead, some contrary conclusions were made [Di et al. 2010; Wang et al. 2011; Zhao et al. 2012a]. Thus the investigation of ionic properties of ceria-carbonate is still an open question. Versatile techniques have been used to study the multi-ionic conduction behaviors of ceria-carbonate, such as the concentration cells, ac impedance spectroscopy, dc four-probe method even coupling with some advanced ex-situ instruments, like Fourier transform infrared spectroscopy. But as suggested by B. Zhu, most of them can not suitable to determine the universal value of the conductivity and transport properties in composite electrolyte with multi-ions conduction [Zhu 1999]. Then fuel cell, i.e. in situ technique, was suggested to study the property. At a later time, high temperature electrolysis, a reverse process of fuel cell, is also adopted to study the ionic conductive behaviors in the composite electrolyte since it is more close the in-situ condition/atmosphere of fuel cell than other analysis [Zhu et al. 2006a] . The working principles of electrolysis cell using H2O as a favorable chemical with different charge carriers (O2- and H+) are shown in Figure 3-1. And it should be noted that the high temperature electrolysis process is current the most highly efficient process for hydrogen production. Thus it is worthy to study the ionic conduction behavior of ceria-carbonate composite and simultaneously produce highly valuable and green hydrogen fuel.

Figure 3-1: Schematic illustrations of H2O electrolysis using a ceramic fuel cell with oxygen ionic conductive (left) and proton conductive (right) electrolyte.


3.1.1 Electrolysis cells (EC) in oxygen ionic conduction mode

The electrochemical reactions of EC in oxygen ionic conducting mode are: Cathode: - 2-

2 2H O + 2e H + O→ Eq. 3-1 Anode 2- -

2O - 2e 0.5O→ Eq. 3-2 The whole reaction: Electricity

2 2 2H O H + 0.5O→ Eq. 3-3 During the reaction, water is provided in the cathode, and the electrical energy is supplied to overcome the O2- migration resistance and electrode reaction overpotential. The electrochemical performances both in fuel cell (FC) and EC mode in oxygen ionic conducting mode are shown in Figure 3-2. In FC mode, OCVs close to 1.0 V are observed in the temperature range of 500-600 oC. The performance of FC increases with the rising of working temperature, a maximum current density around 0.8 A∙cm-2 has been achieved at 0.5 V at 600 oC. In EC mode, current density and applied voltage show a linear relationship. A maximum current density of 1.2 A∙cm-2 has been obtained at 600 oC with an applied voltage of 1.6 V, which is much higher than that of previous study [Zhu et al. 2006a], indicating the great room for electrolysis performance improvement even without microstructure optimization.

Figure 3-2: Temperature dependance of I-V curves of LTCFCs with ceria-carbonate composite electrolyte and Ni-based electrode using 3% humidified hydrogen fuel and air oxidant at different temperature under FC and EC modes. The effect of temperature on the electrochemical performances of EC under 1.2 V bias at the oxygen ionic conducting mode is shown in Figure 3-3. The electrode and total resistances are reduced with the increase of operational temperature. The electrolyte ohmic resistance (high frequency intercept on the real axis) takes up the major polarization resistance under EC mode due to the adopted electrolyte supported cell configuration, thus higher EC performance is expected to achieve by reducing the electrolyte thickness in an anode-supported cell configuration.


Figure 3-3: Electrochemical impedance spectra of ceramic electrolysis cells under 1.2 V bias at different temperatures (Applied frequency: 100 kHz-0.1 Hz), 3 vol% H2O-H2/Air 3.1.2 EC in proton conduction mode

Electrolysis cell in proton conduction mode shows similar performance as in oxygen conduction mode. But the reactions are different: Anode - +

2 2H O- 2e 2H + 0.5O→ Eq. 3-4 Cathode + -

22H + 2e H→ Eq. 3-5 The whole reaction Electricity

2 2 2H O H +0.5O→ Eq. 3-6 During the reactions, water is provided in anode and proton transports through the electrolyte. Temperature dependence of electrolysis cell at proton conducting mode shows similar trends as an oxygen ion conducting mode, suggesting hybrid proton and oxygen ionic conduction in ceria-carbonate composite.

Figure 3-4: Absolute humidity dependence of electrochemical impedance spectra of ceramic electrolysis cells at 550 oC (applied voltage bias: 1.4 V)


The effect of the water content in the cathode chamber on the EIS at 1.4 V bias is displayed Figure 3-4. It is found that both the electrode polarization resistance (diameter of the low frequency arc) and ohmic resistance reduce at higher water content. High content of steam in the anode facilitates the steam adsorption and diffusion on the electrode surface while it is the rate-determining step in the electrolysis process [Jin et al. 2011]. In addition, comparative studies of the EIS and I-V performance of EC with a stream at the anode and the cathode side respectively (shown in Figure 3-5) shows that the proton transfer resistance in ceria-carbonate composite electrolyte is higher than that of oxygen ion counterpart. In other word, the oxygen ionic conductivity through ceria-carbonate composite in EC mode is higher than that of proton conductivity, which is consist with results reported in references [Di et al. 2010; Zhao et al. 2012a] in the same material system.

Figure 3-5: (a) EIS under 1.2 V voltage bias and (b) I-V curves of SOECs with 3 vol% water in anode and cathode chambers, respectively In conclusion, a high H2O electrolysis rate is achieved with ceria-carbonate composite in oxygen ionic conducting mode with large room for performance improvement. The proton conduction in ceria-carbonate composite is also demonstrated, but the proton conductivity is slightly less than that of oxygen ion under electrolysis cell mode.


3.2 Transition metal oxide composite electrode for symmetrical LTCFC (paper 3)

Symmetrical SOFC or CFC (SFC) have been extensively studied after the pioneering work by [Bastidas et al. 2006] due to the distinct advantages compared to the asymmetrical SOFC/CFC: high resistance to carbon deposition and sulfur poison, as well as the reduced materials preparation and cell fabrication technology. However, material choices for SFC have been limited to Chromate or Titanate perovskite oxides due to the strict requirements of symmetrical electrode. In addition, the fuel cell performances of SFCs are also much lower than those of unsymmetrical cell due to low electrical conductivity and electro-catalytic activity both in the reduced and oxidation atmospheres [Ruiz-Morales et al. 2011]. Therefore, alternative electrode materials for SFC are highly desirable. Besides, high performance and reliable electrode materials are highly required for ceria-carbonate composite based LTCFC. In the past time, in KTH fuel cell lab, a major effort has been focused on exploiting cheap transition metal oxide composite as electrode materials for LTCFCs. Some potential composite oxide materials have been developed with excellent fuel cell performance and multi-functionalities [Mat et al. 2007; Raza et al. 2010; Raza et al. 2011a; Fan et al. 2012e]. However, most of them are investigated only on the fuel cell performance. The detailed electro-catalytic activities for symmetrical electrode are rarely studied. Thus deeper thought of the transition metal oxide electrode for ceria-carbonate electrolyte based SFC urgently needs to be revealed. Figure 3-6 gives a simple scheme of SFC with lithiated Ni/Cu/Zn oxide symmetrical electrodes and SDC-carbonate composite electrolyte.

Figure 3-6: Scheme of low temperature ceramic fuel cells with symmetrical lithiated transition metal oxide composite electrode 3.2.1 Crystal structures

The XRD patterns of reduced and oxidized composites are shown in Figure 3-7. In the oxidation gas, the composite was identified to be NiO and ZnO. The CuO and Li2O are not observed in the composite, which may be the results of solid solution formation, substitution or insertion because of small ion radius. While for reduced (5% H2 balanced with N2 for 10 h) sample, Ni/Cu alloy was detected except NiO and ZnO crystal phases. The average crystalline sizes of NiO and ZnO in the oxidation atmosphere are 55 nm and 57 nm, respectively, according to the Scherrer equation shown in Eq. 2-1. After reduction, the crystalline size changed to 30 nm and 65 nm. Thus it can be concluded that the


presence of ZnO could effectively suppress the particle grain size growth even during the alloying process.

Figure 3-7: Room temperature XRD patterns of LiNiCuZnO after heat treatments in oxidation gas (air) and in reducing atmosphere (5% H2 balanced with N2 for 10 h), respectively. 3.2.2 Electrical conductivity

Figure 3-8 displays the electrical conductivity of the composite in the both gas atmospheres.

Figure 3-8: Electrical conductivities of lithiated transition metal oxide composite in air and in hydrogen by DC conductivity measurement (solid point). The inset: temperature dependence of electrical conductivities. The linear fitting is also presented. Taken from [Fan et al. 2012b] with permission of Elsevier.


Both conductivities increase with the rise of the operating temperature, and the Arrhenius curves are fitted well by the linear simulation, indicating semiconductor conductive behaviors [Yoon et al. 2001]. The conductivity of LiNiCuZnO in air is 5.3 S∙cm-1 at 600 oC, similar to lithiated NiO at the same condition [Fan et al. 2012e]. The electrical conductivity in hydrogen is 21.7 S∙cm-1, which is higher than in air. The enhanced conductivity is related to high electronic conductivity of the resulted Ni/Cu nano alloy in the composite. Finally, the electrical conductivities of the composite meet the critical values (1 S∙cm-1) for achieving a good cell performance [Steele et al. 2001].

3.2.3 Electro-catalytic activities

As an ideal symmetrical electrode, the high electro-catalytic activities toward O2 reduction and H2 oxidation are desirable. Electrochemical complex impedance spectra (EIS) of electrolyte supported SFC in air and in hydrogen are shown in the Figure 3-9. The corresponding applied equivalent circuits LR1(R2Q2)- (R3Q3)(R4Q4) and LR1(R2Q2)(R3Q3) are also given as the inset. In the equivalent circuit, L is the conductance; R1 is the ohmic resistance, while R2, R3 and R4 are related with the electrode polarization [Adler 2004]. It can be seen that the impedance spectra dimensions decrease with the increase of operational temperature. The EIS in air contain one high frequency conductance, a relative large intermediate frequency arc, and a visible low frequency tail. While in hydrogen, only one small intermediate frequency arc and a small tail are observed. Besides, the simulation values fit well with the original spectra and the fitted results are given in Table 3.

Figure 3-9: Nyquist plots of symmetrical ceramic fuel cell in air at various temperatures. The EIS contain an inductance, an intermediate frequency semi-arcs and a tail. The insets are the employed equivalent circuit models proposed for fitting of impedance spectra. Solid dots are the original data; the lines are the fitted results. Table 3: The fitted results of EIS with different equivalent circuit models between 500 oC and 600 oC (unit: Ω cm2)

In air In Hydrogen 600 oC 550 oC 500 oC 600 oC 550 oC 500oC

R1 0.546 0.595 0.746 0.236 0.360 0.478 R2 0.0444 0.0896 0.227 0.00698 0.00866 0.0167 R3 0.000773 0.0694 0.00851 0.0871 0.210 0.244 R4 1.58 2.63 3.24 - - - Rp 1.63 2.79 3.48 0.0941 0.219 0.261


All the resistances increase with the reduction of the temperature, suggesting a thermal activation process for all polarizations. In air, R4, related to the oxygen surface diffusion, adsorption and dissociation processes, is much larger than other three processes, indicating that the related process is the rate-determining step in the overall oxygen reduction process [Adler 2004; Li et al. 2010]. The total electrode polarization resistances are 1.63, 2.79 and 3.46 Ω∙cm2 at 600, 550 and 500 oC, respectively, much lower than the current widely investigated perovskite oxides, such as La0.8Sr0.2MnO3 (LSM) [Jiang et al. 2009] and symmetrical electrode La0.75Sr0.25Cr0.5Mn0.5O3 [Ruiz-Moralesa et al. 2006], at the same operational temperature. In addition, the calculated activation energies for oxygen reduction and hydrogen oxidation are 42 kJ∙mol-1 and 56 kJ∙mol-1, respectively, which are lower than those of current perovskite oxide symmetrical electrodes, such as La0.75Sr0.25Cr0.5Mn0.5O3 [Ruiz-Moralesa et al. 2006]. Especially, the oxygen reduction activation is much smaller compared to the well-known perovskite oxide, around 100 kJ∙mol-1 at the similar operating condition [Adler 2004; Ding et al. 2010; Li et al. 2010; Hieu et al. 2011]. Thus these results demonstrated that the lithiated transition metal oxide composite displayed excellent activity for electrode reaction. The origin of exceptional oxygen reduction catalytic activity on ceria-carbonate composite electrolyte needs further effort. In hydrogen gas, obvious reduced polarization resistances were obtained compared with those in air; this is related to the much faster electrochemical reaction kinetics of hydrogen oxidation compared with the oxygen reduction. In fact, the existence of well dispersed Ni/Cu alloy in the composite will considerably improve the anode reaction. The total electrode polarization resistances in hydrogen are 0.0941, 0.219 and 0.216 Ω∙cm2 at 600, 550 and 500 oC, respectively, over one order of magnitude less than oxygen reduction polarization resistances. It is also much lower than those of Ni-SDC anode [Yin et al. 2006] as well as the symmetrical perovskite La0.75Sr0.25Cr0.5Mn0.5O3 at 900 oC (0.20 Ω cm2) [Bastidas et al. 2006], demonstrating excellent prospective for fuel cell anode.

Figure 3-10: EIS of anode supported symmetrical cells with LiNiCuZnO as electrode and SDC-carbonate as electrolyte at different temperatures in fuel cell condition. The electrochemical performances of anode supported single cell in 3% H2O-H2/air gas atmosphere were also performed by EIS at different temperatures and the results are


shown in Figure 3-10. All the impedance spectra contain one depressed arc, which is ascribed to the interfacial polarization resistance. And the polarization resistance (Rp) reduces when the working temperature rises. For example, the Rp is 0.35 Ω cm2 at 450 oC, while it is reduced to 0.25 Ω cm2 at 600 oC. Both of them are much lower than that of classical (La0.75Sr0.25)Cr0.5Mn0.5O3 symmetrical electrode (800 oC, Rp=0.46 cm2) [Jiang et al. 2007], making it a promising symmetrical electrode for LTCFC. 3.2.4 Hydration effect

From the above discussion, we can see that the lithiated transition metal oxide exhibit excellent electro-catalytic activities for SFC. Especially, the polarization resistance under fuel cell condition is much lower than the sum of polarization resistance in air and in hydrogen. In the viewpoint of much less polarization in hydrogen compared with in air, the improved electrochemical performance in a fuel cell is ascribed to the enhanced cathode performance. In other word, the total polarization resistance in fuel cell condition is 6 times less than that in air only. The much improved electrochemical performance therefore demands deeper study. In the previous work, this is considered to the proton conduction contribution of ceria-carbonate composite, which effectively improve the total ionic conduction and enhanced electrode/interface kinetics, subsequently promoting oxygen reduction process and fuel cell performance [Zhu et al. 2001]. However, according to other sources [He et al. 2009; Grimaud et al. 2012], proton conduction in the electrolyte will not always benefit the electrochemical process for oxygen reduction. Contrarily, the presence of water will dilute the oxygen pressure leading to performance degradation. Recent works found that a perovskite oxide electrode possessed of water uptake capability in high operating temperature will facilitate oxygen reduction, such as BSCF, PrBaCo2O5+δ and Pr2NiO4+δ, while not for La0. 6Sr0.4Fe0.8Co0.2O3−δ. Thus considering the changed working condition caused by the mixed oxygen ionically and proton conductive ceria-carbonate composite electrolyte compared in day air, we used the EIS to interpret the enhanced oxygen reduction process in humidified air. The results are shown in Figure 3-11.

Figure 3-11: Electrochemical impedance spectra of symmetrical cells at 450 oC under different gas atmospheres. As can be seen from Figure 3-11, the shape of EIS at 450 oC is different from the EIS at 500 oC or above, a high frequency incomplete semi-arc is additionally appeared, which is attributed to the grain boundary response. Comparing the EIS under day air and


humidified air, both the high frequency and intermediate frequency intercept on x-axis reduced in the presence of water, especially for later one. The hybrid proton and oxide ionic conduction property of ceria-carbonate is attributed to the reduced high frequency intercept, i.e. the electrolyte ohmic resistance. While for the reduced intermediate frequency intercept, since the cell fabrication process and operation temperature are same except the operation gas atmosphere, thus the change should be ascribed to the operating gas condition, or the water uptake capability or the hydration effect on the composite electrode. In fact, the proton has been observed to migrate in the layered oxide [Huggins et al. 1994; Tao et al. 1999] and incorporate into ZnO through an interstitial mechanism [Ip et al. 2003]. Thus, we suggest that the oxygen reduction process of lithiated transition metal oxide in fuel cell condition has been facilitated in the presence of water due to the hydration properties of transition metal oxide. In conclusion, the LNCZO presents adequate electrical conductivity, electro-catalytic activities as a symmetrical electrode for LTCFC, and the hydration effect and the mixed oxygen ion and proton conduction properties co-contribute the excellent electrochemical performance in LTCFC with ceria-carbonate composite electrolyte and LNCZO symmetrical electrode in H2/air gas atmosphere.


3.3 An all-nanocomposites LTCFCs (paper 1)

The reduction of the operating temperature of SOFC based on the functional materials revolution and new elaboration techniques, in one hand, has given birth to many new FC concepts and technologies, such as all-perovskite SOFC [Tao et al. 2005], metal supported SOFC [Lee et al. 2008], micro-SOFC [Howe et al. 2011] and single chamber SOFC [Shao et al. 2005], bringing forth great impact and new opportunities for the development of SOFC; on the other hand, has created more challenges because of the reduced electrochemical activities of the cell components at the lower temperature. In the past decades, a new and promising research branch has been taken as an alternative strategy to develop composite ionic conductors for low temperature CFC considering the inadequate ionic conductivity and instability problems of single-phase electrolytes. Two-phase composite electrolytes with impressive high ionic conductivity and unique hybrid oxygen ionic and proton conductive properties have been successfully developed [Zhu 2001, 2003; Zhu et al. 2003; Zhu 2006, 2009; Fan et al. 2013]. Among all the composite electrolyte materials, ceria-carbonate (nano)composite presents the most promising prospective a high ionic conductivity higher than 0.1 S∙cm-1 above 500 oC and excellent fuel cell performance at the temperature of 500-600 oC. For example, a maximum power density of 1100 mW∙cm-2 had been reached in recent works [Huang et al. 2007a; Xia et al. 2010]. Another unique property of ceria-carbonate (nano)composite is the much improved sinterability because of the binder, lubricant and sealant effect of carbonate in the composite. Sintering temperature as low as 650 oC has been used on single cell while maintains high OCV values and exceptional excellent performances. In this chapter, this unique property has been further adopted to fabricate the all nano-composite LTCFCs. Besides, high active electrode materials are desirable at the reduced temperature range, especially for the cathode catalyst. Great progress has been made on perovskite oxide with mixed ionic and electronic conductive properties in the past decades [Shao et al. 2004; Tao et al. 2005; Yang et al. 2011]. For example, BSCF developed by Shao [2004] is thought as the next generation cathode operated around 600 oC. It should be noted that the electrochemical activity of electrode depends not only the intrinsic catalytic activity but also the microstructure of the electrode, since the electrochemical reaction is taken place at the triple phase boundary (TPB) where the gas, ionic conductor and electronic conductor meet [Suzuki et al. 2009; Holzer et al. 2011]. Actually, a lot of research activity is being placed on the novel structured materials for high performance electrode. Nano structure electrodes, such as nanotubes [Bellino et al. 2007], nano-fibers [Zhi et al. 2011] and mesoporous materials [Mamak et al. 2000] have come up as innovative strategies in LTSOFCs because of the high specific surface area. However, one of distinct disadvantages-the high sintering activity, has delayed their wide implementation. The costive mass production further inhibits the scale-up of nano electrode. In this context, nanocomposite approach (NANOCOFC) has been developed in our lab to develop highly active, multi-functionality and stable materials, both the electrolyte and electrode materials for ceramic fuel cell. Those nanocomposites show outstanding performance and expected stability in fuel cell condition. Especially the development of nanocomposite electrolyte leads to low fabrication and operating temperature which could further stabilize the nano-structure. In this chapter, we reported and demonstrated the feasibility for the first time of a low temperature ceramic fuel cell constructed from all nanocomposite materials. Single cells were experienced an in-situ sintering process without any pre-heat treatment before


testing, which will evidently reduce the fabrication time and cost, making it an economic, green and highly efficient process for fabrication CFCs. 3.3.1 Material properties

Ceria-carbonate nanocomposites were prepared by co-precipitation coupling of mixing and heat treatment process. The nanocomposite anode Ni/FeOx-SDC is also synthesized by the co-precipitation method with Na2CO3 as the co-precipitant. The choice of Ni/Fe alloy as the anode is its improved electro-catalytic activity, cook tolerance and redox stability compared with Ni electrode. The mass ratio of Ni/FeOx (Ni/Fe = 9:1) to SDC is set at 1:1. Nanocomposite cathode catalyst LiNiZnO is obtained by the solid-state reaction method (700 oC for 2 h) [Fan et al. 2012e]. To get mixed electronic and ionic conduction, LiNiZnO was mixed with the nanocomposite electrolyte in a weight ratio of 11:9. The SEM images of all nanocomposite cell components are shown in Figure 3-12. For Ni/FeOx-SDC anode, high dispersion nanoparticles are observed. The calculated crystalline sizes of NiO and SDC deduced from the XRD measurements according to the Eq. 2-1 are 9 and 11 nm, respectively, agreed well with the SEM observation (Figure 3-12A), while for spherical nanocomposite cathode, wide nanoparticle size distribution, from 50 nm to 200 nm, has been viewed (Figure 3-12C). Interestingly, SDC-carbonate composite electrolyte features an impact structure, in which SDC nano-ceramic skeleton is well surrounded by carbonate and forms a good earlier stage sintering characteristics as shown in Figure 3-12B. In fact, such a unique microstructure will provide a continuous path for high speed ionic transport in fuel cell condition. Anode supported LTCFC are fabricated with these nanocomposite components as shown in Figure 3-12D.

Figure 3-12: Anode supported LTCFC (D) constructed by all nanocomposite components: anode Ni/Fe-SDC (A), SDC-carbonate nanocomposite electrolyte (B) and cathode lithiated NiO/ZnO (C). Reprinted from [Fan et al. 2012c] with permission. Copyright @ 2012 Elsevier.


3.3.2 Electrochemical Performances

The electrochemical performances of the in-site sintered all nanocomposite-based CFCs are shown in Figure 3-13. In our experiment, we found that the in-situ sintered cell shows a similar fuel cell performance as the pre-sintered cells (results not shown here; please refer to the paper 1). The i-V and i-p characteristics of in-situ sintered all-nanocomposite LTCFC are shown in Figure 3-13a. Ideal open circuit voltages around 1.0 V, close to the theoretical values by Nernst equation, have been obtained in the temperature range of 500-600 oC, indicating acceptable densification of composite electrolyte after in-situ sintering process. Peak power densities of 550, 500 and 400 mW∙cm-2 have been achieved at 600, 550 and 450 oC, respectively. The EIS of single cell at 600 oC under OCV condition is shown in Figure 3-13b, and it is well fitted by an equivalent circuit of LR1(R2Q2)(R3Q3). The ohmic resistance is around 0.25 Ω∙cm2, and the electrode polarization resistance is 0.4 Ω∙cm2. Such a low total polarization resistance (0.65 Ω∙cm2) ensures the high cell performance, suggesting high activities of the prepared nanocomposites for LTCFC.

Figure 3-13: (a) Voltage/power density-current density characteristic at different temperatures with hydrogen as fuel (100 ml∙min-1) and air as the oxidation, (b) Nyquist curve at 600 oC (c) short-term stability testing (close to short-circuit condition) and (d) its peak power densities after 5 times of thermal cycling between 200 oC and 600 oC of all nanocomposite constructed LTCFC. Reprinted from [Fan et al. 2012c] with permission. Copyright @ 2012 Elsevier. All-nanocomposite based CFC is also experienced short-term operation and thermal cycling stability testing in view of nanoparticle instability under severe fuel cell condition. The cell was operated in an accelerated testing condition, i.e. near to the short-circuit


discharging. As can be seen from Figure 3-13c, the cell keeps quite stable during 100 min testing period. In addition, no obviously peak power density degradation is observed during 5 times thermal cycling process between 600-200 oC (ramping rate of 13.3 oC min-1 and hold time: 30 min) as shown in Figure 3-13d. Excellent and stable fuel cell performances ensure promising application of all-nanocomposite based LTCFCs. In conclusion, all-nanocomposite LTCFC has been first proposed and demonstrated in this work, it gives excellent electrochemical performance and operational stability upon short-term discharging and thermal cycling process.


3.4 Fundamental study of single-component/electrolyte-free fuel cell (paper 2 and 5-7)

The wide implementation of SOFC has been hindered by the constraints of electrolytes and its resulted complex cell structure. Though different approaches have been adopted to solve the aforementioned challenges, the road to market is still a little farther way than we could expect. In the above chapters, we have developed our unique strategy to reduce the operational temperature of SOFC by developing high active composite cell components based a NANOCOFC concept initiated by Zhu et al. [2009; 2013a]. Ceria-carbonate composite with the characteristics of high ionic conductivity and unique hybrid ionic conductive properties has been successfully developed. And the CFCs with such novel electrolyte displayed excellent fuel cell performances. The research of composite ionic conductive properties also induced several interesting energy and environment related applications, such as direct carbon fuel cell, high temperature electrolysis, electrochemical ammonia synthesis and CO2 separation, making it a star material in the fuel cell community. In fact, more than 100 research papers and reports have been published since 2010 [Fan et al. 2013; Zhu et al. 2013a]. More importantly, the recent research on the development of high active electrode materials based on ready available transition metal oxide composite to match with high ionic conductive ceria-carbonate nanocomposite electrolyte has led to a breakthrough research result: single-component/electrolyte-free fuel cell (EFFC) [Zhu et al. 2011b; Zhu et al. 2011c; Zhu et al. 2011d; Zhu et al. 2011e; Zhu et al. 2011f; Fan et al. 2012b; Liu et al. 2012; Qin et al. 2012; Zhu et al. 2012; Zhu et al. 2013a]. A schematic representation of conventional three-layer fuel cell (TLFC) and EFFC are shown in Figure 3-14. By intentionally adjusting the composition of electrode in LTCFC, the constructed EFFC without an electrolyte layer unanticipated displays the same functions as the TLFC and shows comparable electrochemical performance to the conventional LTCFC. The much reduced cell configuration and streamlined cell design have made EFFC a hotspot research field and attracted substantial attention from academic institutes and industrial agencies.

Figure 3-14: Diagrammatic presentation of traditional three-layer fuel cell and novel electrolyte-free fuel cells. Reproduced from [Zhu et al. 2013a] with permission. Copyright @ 2013 Elsevier.


3.4.1 Materials choice and electrical properties

As mentioned above, the material used for EFFC are the electrode material for LTCFC. It contains the ionic conductive materials, such as doped ceria based nanocomposite and single-phase doped ceria oxide, and electronic conductive transition metal oxide, solid solution, or composite, such as NiOx, CuO, ZnO, CoOy and FeOz, with or without lithiation. Such a mixture of electronically and ionically conductive materials provide the electrochemical reaction sites (TPB at the interface between transition metal oxide and ionic conductor) and charge transport network (ions through doped ceria and electron in the transition metal oxide) like the conventional LTCFC. The XRD pattern of a typical used nanocomposite material-LiNiCuZnO-NSDC (LNCZ-NSDC) is shown in Figure 3-15a. The major diffraction peaks of SDC and NiOx and ZnO can be observed except very limited Li2CO3 content. Cu and major Li elements cannot be observed which may be the results of the low content and easily doping during high temperature treatment. The SEM image of LNCZ-SDC nanocomposite is also shown in Figure 3-15b. A slight agglomeration of nanoparticles is observed. Larger particles are belonged to the transition metal oxide, while the smaller particle is attributed to SDC because of the easy sinterability of transition metal oxides, which are commonly used as the sinter aids for the refractory ionic conductors, such as YSZ and SDC [Tao et al. 2006; Zhang et al. 2006].

Figure 3-15: (A) XRD pattern and (B) SEM image of LNCZO-SDC nanocomposite sintered at 800 oC for 2 h. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. The electrical conductivities of LNCZO-NSDC nanocomposite in air and in hydrogen are shown in Figure 3-16. Both the electrical conductivities increase with the rising of the operating temperature, and exhibit nearly linear relationship in the Arrhenius curves. Electrical conductivities higher than 0.1 S∙cm-1 are obtained both in air and in hydrogen above 400 oC, but much lower than the pure LNCZO conductivities as shown in Figure 3-8. Besides, the conductivity difference between the oxidation and reduction states decreases in the presence of NSDC. Nevertheless, the matching of the p-type conduction in air and n-type conduction (this will be detailed in a later section) in the reduced atmosphere is expected to balance and separate the charge during fuel cell reactions, improving the EFFC electrical efficiency.


Figure 3-16: Arrhenius curves of LNCZO-SDC nanocomposite in air and hydrogen, respectively. In which the LNCZO content is 40 wt%. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. 3.4.2 Fuel cell performances

The electrochemical performances of EFFC prepared by mechanical mixing of different compositions of ion and electronic conductors without pre-sinter process are shown in Figure 3-17a. The open circuit voltage (OCV) increases with the decrease of LNCZO content. For example, OCV is negligible for the pure LNCZO, but it gradually increases to 0.32 V at the content of 60 wt%, then around 0.6 V at 50 wt% and lastly rises to 0.92 V with pure SDC-Na2CO3 nanocomposite ionic conductor.

Figure 3-17: EFFC Voltage-Current density and Power density-Current density characteristics as function of (a) electronic conductor content at the fixed powder weight of 0.5 g and (b) pellet thickness (in the form of total powder weight) at the fixed 40 wt% SDC-Na2CO3 in the composite. The thicknesses of pellets are 0.70, 0.88, 1.10 and 1.45 mm for the above four cases. Reprinted from [Fan et al. 2012b] with permission. Copyright @ 2012 Elsevier. The peak performance of EFFC, however, does not follow the same tendency as the OCV. It comes from 0 to around 100 mW∙cm-2 at the LNCZO content of zero to 30 wt%. It then


rises and reaches the maximum power density of 350 mW∙cm-2 at the LNCZO content of 40 wt%. However, it declines with the further increase of ionic conductor content in the composite. Based on these results, we can conclude that an interpenetrating network is critically demanded to form continuous paths for both ion and electronic conduction in EFFC. Moreover, the balance the ionic conduction and electronic conduction is also considerably important to obtain a high fuel cell performance. Since the green pellets have not gone through high temperature sintering processes, a large number of resident pores exist even after in-situ sintering. The porous structure, on one hand, is required for completing FC reactions and reaching high power outputs like the conventional electrode; on the other hand, may cause gas crossover between fuel and oxidant to decline OCVs, leading to reduced electrical efficiency. Therefore, the optimization of the porous structure of EFFC will be one of the key parameters to solve the tradeoff between fuel cell performance and electrical efficiency. In this context, we carried out the initial work to investigate the effects of the pellet thickness on the fuel cell performance of EFFC. The results are given in the Figure 3-17b. The OCV increases when the pellet thickness increases. However, it is interesting to see that peak power density is not improved until the powder weight reaches 0.5 g, i.e. 1.10 mm in thickness. A highest power density of 350 mW∙cm-2 has been achieved at 600 oC. Then it is suddenly reduced to around 160 mW∙cm-2 at the thickness of 1.45 mm, comparable to the performance of EFFC with a pellet thickness of 0.7 mm. Future work will try to investigate the relationships between pore size, distribution and porosity and EFFC electrochemical performance.

Figure 3-18: SEM of the cross-section of fractured EFFC, (a) the whole cell and (b) enlarged fuel cell side and (c) magnified cathode side. Reprinted from [Fan et al. 2012b] with permission. Copyright @ 2012 Elsevier. Figure 3-18 displays the SEM images of EFFC after fuel cell testing. The whole cell fracture section-cross is obviously different from the conventional three-layer fuel cell


(TLFC), in which a dense electrolyte layer, sandwiched by two porous electrodes is indispensable. In EFFC, no clear difference is observed between anode and cathode at a total view of fractured cross section (Figure 3-18a), but many large pores are found. When it comes to magnified images, the morphology of anode is clearly distinguished from the cathode. The surface exposed to the air appears smoother than that uncovered to the fuel, which is caused by the nanoparticles partial reduction at the fuel side, creating more porosity compared to the air side. The porous structure is suggested to facilitate the fast mass transfer of the reactants and products. In addition, we can see that the air side looks brighter than fuel side, which is originated from the lower electrical conductivity of the LNCZO-SDC nanocomposite in air compared with that in hydrogen as shown in Figure 3-16. Thus, the electronic conductivity of air side composite should be improved or adoption of suitable current collector, such as Ag paste, is recommended to improve the electron transfer process at the air side.

Figure 3-19: (a) Comparative electrochemical performance of EFFC and the conventional three-layer fuel cell after the materials fabrication procedure optimization and (b) ever improved EFFC electrochemical performance when adding the Fe active redox catalyst. (Reprinted from [Zhu et al. 2011c] with permission from Professor T. Nejat Veziroglu on behalf of the International Association for Hydrogen Energy) One might find that the EFFC electrochemical performance shown in Figure 3-17 is much inferior compared with the conventional SOFC and LTCFC; only 350 mW∙cm-2 has been achieved in the initial attempt. Thus we try to improve the fuel cell performance by choosing materials composition and optimizing the materials and cell fabrication technologies as well as making a balance of the electronic (n type and p-type conduction) and ionic conduction. For example, adding some high active redox FeOx or Co3O4 into the composition and optimizing the materials fabrication procedure help improve the EFFC electrochemical performance to some extent [Zhu et al. 2011c; Zhu et al. 2011e; Zhu et al. 2012], as shown in Figure 3-19. The optimized EFFC with active FeOx reaches a peak power output higher than 600 mW∙cm-2 at 550 oC (Figure 3-19a), which is much higher than that of EFFC prepared by mechanical mixing approach and comparable with the traditional TLFC. Further increase of maximum power density up to 700 mW∙cm-2 is achieved by pre-heating of the nanocomposite at 700 oC before forming the pellet, as shown in Figure 3-19b. Moreover, an appealing peak electrochemical performance close to 400 mW∙cm-2 has been achieved at 400 oC on the same EFFC device, as shown in (Figure 3-19b). In particular, the OCVs of EFFC are also improved to higher than 1.0 V. in addition, some advanced cell fabrication technologies, such as Spark Plasma Sintering (SPS) technology [Stingaciu et al. 2012], could be employed to reduce the cell fabrication


time, maintain nanoparticle size and a good cell microstructure which will facilitate fuel cell performance. Thus, the EFFC holds large rooms for performance enhancement and shows super advantages for low temperature application. Similar to the conventional solid oxide fuel cell operating at the intermediate temperature, one of the unique features of EFFC is its fuel flexibility. Actually, the lithiated transition metal oxide composite electrode has already been employed and exhibited good electro-catalytic activities for hydrocarbon fuel oxidation, such as biogas, alcohol and glycerol, without carbon deposition problems under fuel cell operation condition [Qin et al. 2011; Raza et al. 2011b]. As expected, the EFFC also shows good fuel cell performance using biogas (20% H2, 54% CO, 12% CO2, and 14% CH4 at a flow rate of 75 ml min-1) and alcohol (mixed with water in a volume ratio of 1:1, 3 ml min-1) as shown in Figure 3-20. The biogas and ethanol presents a similar peak output of 350 mW∙cm-2, while methanol gives a maximum power density of around 480 mW∙cm-2 at 550 oC, similar to the electrochemical performance of LTCFC with NSDC nanocomposite and same electrode composite [Qin et al. 2011; Raza et al. 2011b], making the EFFC as a competitive technology for green and renewable energy application.

Figure 3-20: Electrochemical performance of optimized EFFC operated with biogas, methanol and ethanol, respectively. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry. 3.4.3 Micro-electrochemical reaction process

In the conventional TLFCs, the ion transports through the electrolyte in combination with electron migration from the external circuit is crucial to realize to fuel cell function. However in EFFC's the electrolyte is no longer being there, so a different mechanism must be presented. Here we give some simple analysis, as shown in Figure 3-21. It has already been proven that the lithiated transition metal oxide composite has double functions: electro-catalytic oxidation of hydrogen and reduction of oxygen molecules. Thus when the fuel and oxidant pass through the surface of EFFC from two different chambers as the conventional TLFC, the hydrogen will lost two electrons and form H+ as Eq. 3-7: Anode: + -

2H 2H 2e→ + Eq. 3-7


The oxygen molecules will get the electrons and be dissociated and ionized to O2- (Eq. 3-8): Cathode: - 2

20.5O + 2e O −→ Eq. 3-8 Thus the whole reactions are: 2

2 2H 0.5O O 2H− ++ → + Eq. 3-9

Figure 3-21: A proposed redox reaction process for EFFC (a micro-view): hydrogen and oxygen are dissociated on transition metal oxide and migrate and meet at the ionic conductor to form water. (Reprinted from [Zhu et al. 2011c] with permission from Professor T. Nejat Veziroglu on behalf of the International Association for Hydrogen Energy, with slight modification) Once the hydrogen and oxygen are dissociated and ionized, the electricity has been extracted from the fuel and oxidant. Then H+ and O2- separately migrate in in the composite since SDC-carbonate composite is mixed proton and oxygen ion conductor [Zhu et al. 2006a; Wang et al. 2011; Fan et al. 2012d] and some layer transition metal oxides possess the proton intercalation capability [Jean-Claude 2001; Ip et al. 2003]. The O2- and H+ meet somewhere inside of pellet and produce water according to the following reaction: 2

2O 2H H O− ++ → Eq. 3-10 In fact, the diffusion of H2 and air inside of the pellet will form a gas pressure gradient because of the porosity and different gas diffusion kinetics of hydrogen and oxygen molecules. In our earlier opinion and understanding, the whole reaction processes may prefer to take place in rather closed nanoparticles considering the double catalytic function and short ionic migration length as shown in Figure 3-21. For instance, the hydrogen and oxygen dissociate and ionize at respective transition metal oxide, but they transport to one SDC-Na2CO3 nanoparticle, and meet to produce water. The porous structure of EFFC will


facilitate the production diffusion to the external gas stream. The reactions given in above equations repeat on the redox nanoparticles, continually produce electricity and yield H+ and O2- and finally form the steam. As a matter of fact, the overall reaction is: 2 2 2H + 0.5O H O→ Eq. 3-11 It is same as conventional anode-electrolyte-cathode TLFC reaction. Therefore, the EFFC has realized the same function as the conventional TLFCs but without the electrolyte layer. In the TLFC, the electrochemical reactions occur in the electrode/electrolyte interface, while the same reactions may take place everywhere the H+ and O2- meet in EFFC. Thus it has effectively extended the length of TPB for fuel cell reaction. In addition, the performance of the conventional TLFC is limited by the large electrolyte/electrode interfacial polarization resistances, whereas such interfaces have been removed, which is indirectly reflected in faster voltage response when the gas atmospheres are exchanged between anode and cathode [Zhu et al. 2011c]. Therefore, it is expected that the EFFC will present a higher electrochemical performance than conventional TLFCs, such as SOFC and CFC. 3.4.4 Joint fuel cell and solar cell p-n junction principle

A critical concerning with EFFC is the possible electronic short circuit problem, i.e. how can the EFFC with a simple one nanocomposite layer realize the fuel cell function as TLFC while without electronic insulated electrolyte layer? The electrolyte is indispensable in conventional FC, acting as critical component to transport ions, at the same time acting as a separator to blocking the electronic phase, and to give the fuel cell voltage, while it is never being in EFFC. At this moment, there is still a lack of the detailed scientific knowledge of all the processes involved, especially the working principle behind, but we have thus far reached an understanding that our new device performs with a joining electrochemical fuel cell and a physical p–n junction, similar to a solar cell, where semi-conductive materials are employed to build bulk p-n hetero-junction (BHJ) to separate the electron and hole pair after photon activation without internal short circuit problems [Rath et al. 2012], based on our continuous observation and extensive experiments. The fuel cell electrochemical processes have been detailed in the above section, here will present more on bulk p-n junction process, analogy to a solar cell. In fact, the materials used in EFFC are transition metal oxides, like NiOx, CuO, ZnO, FeOy and their solid-solution and composite, which are such kinds of semiconductors. NiOx with a band gap energy of 3.3-4.0 eV [Dutta et al. 2010], CuO and FeOy are typical p-type conductors and ZnO is classical n-type conductor with a band gap energy of 3.2-3.3 eV [Shin et al. 2000; Caglar et al. 2010; Joseph et al. 2011; Jung et al. 2011]. In addition, both the p-type conduction and n-type conduction can be significantly improved by lithium doping or forming the solid-solution with other chemicals [Mandal et al. 2006]. In addition, the partial reduction of p-type conductors will cause partial n-type conduction in the reduced atmosphere. Unlike the solar cells, the H2/O2 instead the photon will activate the p-n conduction of the ionic and semi-conductive nanocomposite layer (Figure 3-22a), similar to SOFC electrode [Singh et al. 2013], then the device voltage and electrical field are built up due to supply H2-air gradients as shown in Figure 3-22b. An additional charger carrier depletion layer between n-rich and p-rich region will automatically form, which will help to separate the p and n conduction. In addition, the ionic conductor, SDC or SDC-carbonate composite will


also support to separate the ionic and electronic conduction in working condition. With such kind of BHJ, the electrons and hole are not able to pass through the intern of the EFFC. Instead, they are driven by the build electrical field and move toward the corresponding counter electrode and current collector, realizing the transport of electron through an external circuit as shown in Figure 3-22b. However, the ions (proton and oxygen ion) can transfer in the nanocomposite ionic conductor to give internal current, and finally functioning as a fuel cell. Hence, the EFFC can be viewed as a new energy conversion device which combines semiconducting bulk hetero-junctions like a BHJ solar cell, nano-redox (Figure 3-21) and fuel cell processes, while the presence of ionic conduction, especially for the extremely high hybrid proton and oxygen ion conductive ceria-carbonate nanocomposite, contributes the excellent electrochemical performance, in other word, higher power output than the current BHJ solar cells. To identify the BHJ function and nano-redox process, a thin-film EFFC device is extensive under investigation.

Figure 3-22: The suggested EFFC working principle, similar to a solar cell: (A) initiated by the hydrogen and oxygen dissociation to build up the voltage and the corresponding electrical field to separate the hole and electron pair as well as the ionic and electronic phase. (B) The built up electrical field forces the electron and hole moving to the counter electrode while the ionic phase still goes through the ionic conductor to give H2O and electricity. Reproduced from [Zhu et al. 2012] with permission from The Royal Society of Chemistry.

3.4.5 Energy conversion device integration/network

Since the discovery of EFFC, attention has been given to both from the academic community [2011a; Zhu et al. 2011d; Xia et al. 2012] and from the news media, such as the “materials views” highly recommendation [2011b]. Except the much streamlined design, the potential prospective of simple one-layered nanocomposite for other possible advanced applications, such as the solar cell, electrolysis, photo-catalysis, membrane reactor or separation device, is also highly appreciated because of the mixed ionic and semi-conductive properties of lithiated or lithium-free transition metal oxide-ceria oxide composite. Here we would like to propose and develop the first integrated EFFC and solar cell and photolysis technologies for future energy conversion and storage based on one simple nanocomposite material, as shown in Figure 3-23.


Figure 3-23: A proposed integrated system combined with EFFC, solar cell and photolysis technologies built on a one-layer nanocomposite. The EFFC and solar cell are also suggested to operate at elevated temperature by integrating the solar heating technology. Reproduced from [Zhu et al. 2013a] with permission of Elsevier 2012. In this proposal, the solar energy can be captured in a water photolysis process to produce H2 and O2 with the above semi and ionic conductors, which can be subsequently as fuel for EFFC. Besides, the sunlight energy can be utilized for direct electricity generation in a solar cell. Moreover, the sunlight heat can be concentrated and stored for EFFC and solar cell further use. For example, providing heat for start-up process and accelerate the electrochemical reaction rate of EFFC. Thus this integrated system is a self-sustainable and CO2-free system with solar energy as the only energy input sources, making it an advanced system for future green energy application. In the end, EFFC, liberated from the constraints of electrolytes and complex multi-layered structures, has been demonstrated to display multi-advantages over conventional three-layer fuel cell, such as decreased materials-fabrication-operational costs, eliminated chemical and thermal compatibility, wide material choice, fast electrode kinetic response and improved fuel cell performance. The discovery of EFFC has demonstrated a considerably simpler and more cost-effective fuel cell construction and cell manufacturing for green energy conversion. Thus the EFFC presents a new solution/way to overcome the FC challenges, providing great possibilities for fuel cell commercialization, at least paving the way towards more cost efficient fuel cells as pointed out by Hilary Gallagher on the “Materials Views” [2011b]. In this sense, EFFC indicates new directions for FC research both from academic and applied aspects. An integrated multi-energy related technology platform based on one single-layer nanocomposite material is proposed for future green energy conversion. Finally, Novel EFFC, a joint multi-disciplinary principle and multi-function device, deserves delicate contributions since not only fuel cell but also other advanced energy devices/technologies, like solar cell, will join to share the benefits of ongoing research.


4 CONCLUSIONS

In this thesis, with the target of capital cost reduction and the system durability improvement, innovation two major advanced energy conversion devices, ceria-based composite based low temperature ceramic fuel cell and single-component/electrolyte-free fuel cell, have been fabricated based on the developed several functional composite materials. Conclusions drawn from this thesis are: 1. Ceria-carbonate composite with Ni-based electrode presents excellent performance

both in fuel cell mode and electrolysis mode between 500 oC and 600 oC as demonstrated by I-V characteristics and impedance spectroscopy studies, demonstrating a high ionic conductivity. The operational temperature, voltage bias, steam content and steam direction have a significant influence on the electrolysis performance. The higher of the operating temperature, voltage bias and water content, the higher of the electrolysis rate. Proton transport resistance in ceria-carbonate composite is higher than that of oxygen ion, indicating higher oxygen ionic conductivity in SDC-(Li/Na)2CO3 composite electrolyte. Electrolysis cell using ceria-carbonate composite electrolyte also provides an alternative approach to produce green hydrogen gas.

2. LiNiCuZnO oxide composite gives different crystal phases in reduced and oxidation atmospheres. Ni/Cu Alloy is observed in the reduced sample. LiNiCuZnO oxide composite presents adequate electrical conductivities and electro-catalytic activities in air and in hydrogen gas atmospheres. Respective polarization resistances of 1.63 Ω∙cm2 and 0.0941 Ω∙cm2 for oxygen reduction and hydrogen oxidation are obtained at 600 oC. The oxygen reduction reaction rate is determined by the oxygen surface diffusion, adsorption and dissociation process. The enhanced electro-catalytic of LiNiCuZnO oxide composite in fuel cell condition compared with that in air is ascribed to the hydration effect of this functional oxide composite. Proton transport in the electrolyte and the formation of water in cathode help improves the oxygen reduction process and co-contribute to the excellent fuel cell performance of LTCFC.

3. The high sintering activity of ceria-carbonate composite put the all-nanocomposite fuel cell into practice in this thesis. Ni/Fe-SDC nanocomposite anode and lithiated NiO/ZnO nanocomposite are successfully synthesized by co-precipitation and solid-state reaction methods, respectively. A promising peak power density of 550 mW∙cm-2 and a total polarization resistance of 0.65 Ω ∙cm2 are achieved at 600 oC. The short-term discharging testing and thermal-cycling stability are approved. All results make all-nanocomposite fuel cell a promising system for future highly efficient energy conversion.

4. The fundamental studies of Single-component/electrolyte-free fuel cell (EFFC) indicate that composite materials with mixed ion conducting and semiconducting properties, typically doped ceria or ceria based composite ionic conductor and transition metal (Ni, Cu, Zn, Fe) oxide composite, is the first choice to realize the fuel cell function in EFFC. The composition of ionic conductor and semi-conductor, the single pellet’s porosity and thickness, particle size, fabrication method, operating conditions, the balance of ionic conduction and semi-conduction as well as the matching of n and p-type conduction have significant influences on the EFFC performances. An initial power density of 350 mW∙cm-2 has been achieved with hydrogen as fuel, which is enhanced to 700 mW∙cm-2 at 550 oC by choosing high active catalytic and optimization of material and cell


fabrication process. EFFC based on the above functional materials also presents excellent fuel adaptability. High fuel cell performances and carbon deposition resistance have been demonstrated with liquid fuel such as methanol and ethanol, and biogas. The electrochemical performance is compared with the conventional three-layer fuel cells (TLFC). EFFC holds great promise for additional performance improvement due to the removed interface polarization resistance, while it is the major power loss for conventional TLFC.

5. Due to the high specific area and active site of ionic conductor (mixed oxygen ion and proton conduction) and semi-conductive catalyst (double catalytic activity for oxygen reduction reaction and hydrogen oxidation reaction), the fuel cell reaction is suggested to be taken place on one nanocomposite particle. The enhanced reaction active site and reduced charge migration length are expected to result higher fuel cell performance.

6. A joining fuel cell and solar cell principle is proposed for this novel EFFC. Mixed ionic and semi-conductive composite have functions for EFFC reaction. While the semi-conductive property of employed transition metal oxides/composites form p-n hetero-junction upon to hydrogen activation, similar to photon activation process in the bulk hetero-junction solar cells. The formed bulk p-n junction and the charge depleted layer force the electron and hole transport to the corresponding counter current collector, avoiding the internal short-circuit or current leakage. The research of the ion and semi-conductive nanocomposite, a new type powerful functional material, also provides a research platform for integrating different advanced energy technologies for future energy conversion and storage.

7. The simplified design and manufacturing of functional nanocomposite device will significantly lower the fuel cell technology cost and speed up its commercialization. The research results will benefit the development of advanced fuel cells and other energy and environmental related sectors, such as the solar cells.


5 FUTURE WORKS

In this work, different functional composite materials and their constructed advanced fuel cells (ceramic fuel cell, symmetrical fuel cell and single-component/electrolyte free fuel cell) or electrolysis cells have been technically realized and investigated. Their physic-chemical properties (crystal structure, morphologies, electrical conductivity, and electro-activity) and fuel cell performances (I-V, P characteristics and impedance spectroscopy) are examined. The following works are suggested for each aspect as mentioned in the abstract: 1. Considering the high oxygen ionic conductivity in ceria-carbonate composite compared

with proton conductivity, effort is needed to optimize of ceria-composite and gas environment (majorly the steam content) to maximize the hydrogen production.

2. Further fundamental studies of the catalytic activity of composite electrodes for oxygen reduction reaction under different oxygen partial pressures and water partial pressures are necessary to identify the rate-determining step and study the deeper oxygen reduction mechanism.

3. Optimization of the electrode materials composition, development of new electrode for all-nanocomposite LTCFC and long-time discharging and thermal cycling stability testing could be investigated in the near future.

4. Direct evidence of the p-n junction working principle or the detailed mechanism and science of EFFC are required. The possible safety issue, theoretical analysis of gas and charge separation, electrochemical reaction and diffusion kinetic are all crucial and challenge research directions for future work. Experimental works from electrochemical impedance studies and simulation by the equivalent circuit model and further comparative study with classic three-layer fuel cell are important to better understand this radical new science and technology. Development of next generation advanced energy and environmental technology based on one-layer nanocomposite device is highly recommended. At last, utilization of current well developed powder and advanced thin film fabrication technologies to develop large scale EFFC and stack demonstration will also be much needed to develop emerging new energy technologies.

The above functional materials and novel system developments are at the core of the NANOCOFC (www.nanocofc.com) approach: multi-phase composite with nanoscale functional interfaces brings forth the interfacial superionic conduction, hybrid ionic (O2- and H+) conducting and semi-conducting (e and h) properties, non-function to function and surface redox process. NANOCOFC has presented several strategies to exploit novel nanocomposite materials with enhanced electrical and catalytic properties or other functionalities for advanced fuel cell application, and further research dedication to NANOCOFC approach is expected to lead revolutionary technology and the new crosslink science frontier, the EFFC is, inevitably, such an exceptional example.


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