Semiconductor-ionic Materials for Low Temperature Solid ...1294180/FULLTEXT01.pdf · synthesis,...

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Doctoral thesis / Muhammad Afzal Semiconductor-ionic Materials for Low Temperature Solid Oxide Fuel Cells Muhammad Afzal Doctoral Thesis 2019 Division of Heat and Power Technology Department of Energy Technology Industrial Engineering and Management KTH Royal Institute of Technology SE-100 44 Stockholm, Sweden

Transcript of Semiconductor-ionic Materials for Low Temperature Solid ...1294180/FULLTEXT01.pdf · synthesis,...

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Doctoral thesis / Muhammad Afzal

Semiconductor-ionic Materials for Low Temperature Solid Oxide Fuel Cells

Muhammad Afzal

Doctoral Thesis 2019

Division of Heat and Power Technology Department of Energy Technology

Industrial Engineering and Management KTH Royal Institute of Technology

SE-100 44 Stockholm, Sweden

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Printed in Sweden Universiteteservice US_AB Stockholm, 2019 TRITA-ITM-AVL 2019:7 ISBN: 978-91-7873-132-9 © Muhammad Afzal 2019

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Abstract

Solid oxide fuel cell (SOFC) is considered as an attractive candidate for energy conversion within the fuel cell (FC) family due to several advantages including environment friendly, use of non-noble materials and fuel flexibility. However, due to high working temperatures, conventional SOFC faces many challenges relating to high operational and capital costs besides the limited selection of the FC materials and their compatibility issues. Recent SOFC research is focused on how to reduce its operational temperature to 700 ºC or lower. Investigation of new electrolytes and electrode materials, which can perform well at low temperatures, is a comprehensive route to lowering the working temperature of SOFC. Meanwhile, semiconductor-ionic materials based on semiconductors (perovskite/composite) and ionic materials (e.g. ceria based ion conductors) have been identified as potential candidates to operate in low temperature range with adequate SOFC power outputs. This investigation focuses on the development of semiconductor-ionic materials for low temperature solid oxide fuel cell (SOFC) and electrolyte-layer free fuel cell (EFFC). The content of this work is divided into four parts: First part of the thesis consists of the work on conventional SOFC to build knowledge and bridge from conventional SOFC to the new EFFC. Novel composite electrode (semiconductor) materials are synthesized and studied using established electrochemical and analytical methods such as x-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The phase structure, morphology and microstructure of the composite electrodes are studied using XRD and SEM, and the weight loss is determined using TGA. An electrical conductivity of up to 143 S/cm of as-prepared material is measured using DC 4 probe method at 550 ºC. An electrolyte, samarium doped ceria (SDC) is synthesized to fabricate a conventional three component SOFC device. The maximum power density of 325 mW/cm2 achieved from the conventional device at 550 ºC. In the second part of the thesis, semiconductor-ionic materials based on perovskite and composite materials are prepared for low temperature SOFC and EFFC devices. Semiconductor-ionic materials are prepared via nanocomposite approach based on two-phase semiconductor electrode and ionic electrolyte. This semiconductor-ionic functional component was shown to integrate all fuel cell components anode, electrolyte and cathode functions into a single component, i.e. “three in one”, resulting in enhanced catalytic activity and improved SOFC performance. The third part of the thesis addresses the development and optimization of the EFFC technologies by studying the Schottky junction mechanism in such semiconductor-ionic type devices. Perovskite and functional nanocomposites (semiconductor-ionic materials) are developed for EFFC devices. Materials characterizations are performed using a number of standard experimental and analytical techniques. Maximum power densities from 600 mW/cm2 up to 800 mW/cm2 have been achieved at 600 ºC. Fourth part of the thesis describes the theoretical simulation of EFFCs. In this work, an updated numerical model is applied in order to study the EFFC device, which introduces some modifications to the existing relations for traditional fuel cell models. The simulated V-I and P-I curves have been compared with experimental curves, and both types of curves show a good consistency.

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Keywords: Semiconductor-ionic materials; electrolyte-layer free fuel cell; low temperature solid oxide fuel cell; fuel to electricity conversion; Schottky junction; theoretical and experimental curves

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Sammanfattning

Bränsleceller av typen fastoxid (SOFC) anses vara en attraktiv kandidat för energiomvandling bland bränsleceller (FC) beroende på flera fördelar som bl.a. miljövänlighet, användning av icke-ädla material och deras bränsleflexibilitet. På grund av hög driftstemperatur står dock konventionella SOFC inför många utmaningar. Bland dessa finns höga drifts- och kapitalkostnader samt det begränsade urvalet av FC-material och relaterade kompatibilitetsproblem. En trend inom SOFC-forskning är inriktning på hur man sänker driftstemperaturen åtminstone till 700 ºC eller lägre. Undersökning av nya elektrolyt- och elektrodmaterial som kan fungera bra vid låga temperaturer är en mödosam väg för att sänka SOFCs arbetstemperatur. Som alternativ finns halvledande-jonledande material som är baserade på perovskit/komposit- och jonledande material. Dessa är potentiella kandidater att arbeta i ett lågt temperaturområde med tillräckliga SOFC-prestanda. Denna forskning fokuserar på utveckling av halvledar-joniska material för lågtemperatur-solid oxid bränsleceller och elektrolytskikt-fria bränsleceller (EFFC). Detta arbete är indelat i fyra delar: Den första delen av avhandlingen handlar om arbetet med konventionell SOFC för att bygga kunskap och att överbrygga från konventionell SOFC till det nya EFFC. Nya halvledande kompositelektrodmaterial syntetiseras och studeras med hjälp av etablerade elektrokemiska och analytiska metoder, såsom röntgendiffraktion (XRD), scanning-elektronmikroskopi (SEM) och termogravimetrisk analys (TGA). Fasstrukturen, morfologin och mikrostrukturen hos kompositelektroderna studeras med användning av XRD och SEM, och viktminskningen bestäms med användning av TGA. En elektrisk ledningsförmåga på 143 S/cm av sådant framställt material har uppmätts med användning av DC 4-sond-metoden vid 550 ºC. En elektrolyt, Samarium-dopad Ceriumoxid (SDC) syntetiseras för att tillverka en konventionell SOFC-enhet baserad på tre komponenter. En maximal effekttäthet på 325 mW/cm2 har uppnåtts från den konventionella enheten vid 550 ºC. I andra delen av avhandlingen är halvledarjoniska material baserade på perovskit och kompositmaterial förberedda för SOFC- och EFFC-enheter med låg temperatur. Halvledar-joniska material har konstruerats genom att skapa en komposit av nano-partiklar (nanokomposit) baserat på halvledarelektrod och elektrolyt i olika kristallina faser som kombineras i en tvåfas-struktur. Denna halvledar-joniska funktionella komponent har visats integrera alla anod-, elektrolyt- och katodfunktioner i bränslecellkomponenterna i en enda komponent, dvs "tre i en", vilket resulterade i förbättrad katalytisk aktivitet och förbättrad SOFC-prestanda. Tredje delen av avhandlingen tar upp utvecklingen och optimeringen av EFFC-tekniken genom att studera Schottky-kopplingsmekanismen i sådana anordningar av halvledar-jonisk typ. Perovskit och funktionella nanokompositer (halvledar-joniska material) har utvecklats för EFFC-enheter. Materialkarakteriseringar utförs med användning av ett antal standardiserade experimentella och analytiska metoder. En maximal effekttäthet från 600 mW/cm2 upp till och 800 mW/cm2 har uppnåtts vid 600 ºC. Den fjärde delen av avhandlingen beskriver den teoretiska simuleringen av EFFCs. I detta arbete tillämpas en uppdaterad numerisk modell för att studera EFFC-enheten som introducerar vissa modifieringar av de rådande sambanden i traditionella

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bränslecellsmodeller. De simulerade V-I- och P-I-kurvorna har jämförts med experimentella kurvor, och båda typerna av kurvor visar god samstämmighet.

Nyckelord: Halvledar-joniska material; elektrolytskikt-fri bränslecell; lågtemperatur fastoxidbränslecell; bränsle till elomvandling; Schottky junction; teoretiska och experimentella kurvor

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‘‘Seek knowledge from cradle to the grave’’ Prophet Hazrat Muhammad (PBUH)

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To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.

(Albert Einstein)

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Preface

This doctoral thesis reveals the outcomes of a PhD project conducted on materials using in fuel cells, at Department of Energy Technology, KTH Royal Institute of Technology, Stockholm, Sweden. This work includes eight published journal articles. The work presented consists of investigation of semiconductor-ionic materials, their synthesis, fabrication of solid oxide fuel cells with and without electrolyte-layer, characterization of the materials and the devices, fuel cell testing at low temperatures (up to 600 oC) and theoretical methods and validation of the experimental results. Semiconductor- ionic composites based on perovskite, composite, nanocomposite and ion conducting materials are found potential candidates for energy conversion applications. Finally, conclusions have been drawn through above study and future work is also proposed to use these outcomes in fuel cell practical applications.

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Acknowledgements

All praises to Allah (SWT), the most Beneficent, and the most Merciful. He is the Lord of the whole universe. He has created everything in a very systematic and ordered manner. It is great honor for me to be associated spiritually with our beloved Holy Prophet Hazrat Muhammad (PBUH) (for everlasting support in my life), Ehl-e-bait (the family of Prophet) and Companions of Prophet; they spread goodness, peace and education throughout the world. Firstly, I would like to acknowledge the financial support provided by KTH Royal Institute of Technology and EC FP 7 TriSOFC project grant for completing my doctoral studies at KTH, Stockholm. I would also like to express my sincere gratitude to my principal supervisor Professor Andrew Martin for his well-determined supervision, guidance and invaluable support throughout my research work. His enlightened and optimistic approach has helped me for smooth completion of my PhD studies. He has significantly improved my way of thinking in research. I am extremely happy for all support from his end. I would like to express my profound gratitude to my former principal supervisor Professor Bin Zhu who provided me very active supervision and guidance during my doctoral studies and for his significant support for my active research work. He has always given me positive feedback in the research activities whenever I requested his response. He provided me a lot of confidence and respect for projects and Fuel Cell Group management. During every meeting, we had very positive and active discussion about research gaps and new research ideas to solve them. I am also grateful to my co-supervisor Professor Peter Lund from Aalto University, Finland for his support throughout my research work and for the revision of my dissertation. I would like to express my gratitude to previous co-supervisor Professor Torsten Fransson for his help and guidance during my studies. A bundle of thanks to my colleagues in Fuel Cell Group, Chen Xia and Yanyan Liu for their unforgettable cooperation and pleasant atmosphere in my office. I wish you good luck for future endeavors in your lives. Moreover, my heartfelt gratitude to Professor Björn Laumert, Peter Hagström, Hatef Madani and Mahrokh Samavati for their critical feedback and suggestions to improve this dissertation. Particularly, thanks to the advance reviewer of my PhD Thesis Professor Göran Lindbergh for his constructive criticism and positive feedback to improve the thesis. I would also like to thank Jan Wikander (Dean of ITM), Malin Selleby (Director of doctoral studies) and my other colleagues at the Department of Energy Technology: Prof. Björn Palm, Rahmatollah Khodabandeh, Jeevan Jayasuriya, Thomas Nordgreen, Justin Chiu, Miroslav Petrov, Chamindie Senaratne, Mauricio, Tobias, Nenad, Rafael, Tony Chapman, Anneli Ylitalo-Qvarfordt, Hamayun Maqbool, Shahid Siyal, Youssef Almulla, Imtisal-e-Noor, Getnet, Wujun Wang (for friendly advices), Taras, Jhonny and other friends at the department for providing me a helping and friendly workplace environment. Particularly, my special thanks to Jens Fridh, Leif Pettersson and Göran Arntyr for their technical support in my experiments throughout my research work. I would like to acknowledge the support of my previous teachers Sir Ghulam Qadir Jhamat, Professor Muhammet Toprak, Professor Yasir Jameel, Prof. Anders Malmquist

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(for Swedish revision) and Professor Per Lundqvist who always encouraged my research capabilities. They always guided me how to reach at the highest level of successes. Thanks to Syed Ijaz Husain Shah Saheb (Hujra Shah Mukeem) for spiritual guidance, prayers, and appreciating me for doing PhD. I would like to pay my regards to my research collaborators and supporters Professor Amir Rashid, Dawood Akhtar, Dr. Jung-Sik Kim, Professor Mikael Syväjärvi, Professor Lyuba Belova, Professor Weihong Yang, Rizwan Raza, Dr. Kaleem Iqbal, Ehsan Baig, Dr. Muhammad Tufail, Mohsin Saleemi, Nasar Ali, Imran Asghar, Professor Naveed Kausar Janjua, Professor Asghar Hashmi, Amjad Ali, Liangdong Fan, Baoyuan Wang, Abdul Malik Tahir, Xiaodi Wang, Ying Ma, Yunjuan He, Muhammad Usman, Muhammad Kamran and other friends for always cooperation regarding research activities. Thanks to Sushant Madaan for theoretical simulations and modelling work. Thanks to my friends and community representatives Shafqat Khatana, Rahmat Ali, Ali Lashari, Imam Muhammad Muslim, Khalid Yazdani, Asif Butt, Amer Nadeem, Dr. Ghulam Husain, Arif Kisana, Qazi Masood, Ghulam Rasool (for motivation to study in Sweden), Muhammad Sarwar, Riaz Akhtar, Adeel Anwar, Imran Cheema, Waseem Aslam, Tahir Shah, Zulfiqar Husain Shah, Kashif Bhutta, Waqar-ul-Hasan, Hamad, Sadiq, Zazy Khan, Sohail Ahmad, Amir Riaz, Aslan, Nawaz Ahmad, Ali Butt, Saud, Rajjab Ali, Ahsin, Abdullah Khan, Qaiser Waheed, Muhammad Adnan, Nasir Nawaz, Taha Ali, Hasan, Hafiz Qadeer, Musa Kazim Shah, Aqil Khan and many other friends for their moral support throughout my stay in Sweden. I would like to acknowledge my great mother Hafeezan Bibi (Late) for her endless financial support, teachings, training, guidance and uncountable prayers for my successes throughout my life. I also thank to my spiritual mother Hazrat Fatimah-Tul-Zahra (SA) for spiritual support throughout my life. Thanks to my father, Chaudhary Muhammad Yaqoob (Late) and my grandfathers Chaudhary Muhammad Nazir Lumberdar (Late) and Chaudhary Muhammad Siddique (Late) who gave me a lot of love and knowledge during my early childhood. Thanks to my father-in-law Muhammad Shafique (Late) and my nephew Chaudhary Khurram Shazad (Late) for their moral support during their lives. I would like to extend my deepest thanks to my sisters and brothers, Chaudhary Muhammad Munawar Faizi, Chaudhary Muhammad Aslam, Chaudhary Muhammad Akram Advocate and Chaudhary Muhammad Arshad for their help, financial support and affectionate love throughout my life. I also thank to my other family members, Chaudhary Muhammad Anwar, Chaudhary Intzar Hussain, Chaudhary Tariq Tabassum and my mother-in-law for their prayers and appreciations. Last but not the least, I have shortage of adequate words to thank my beloved wife, Anela Afzal for her endless support and love, who always supported me for project meetings and international conferences out of Sweden, prayed for my successes and for taking care of my lovely kids. Finally, I would like to extend my sweet love and affiliation to my lovely children, ‘‘Atal Afzal and Huda Afzal’’. I dedicate my research work to my great mother, father and spiritual mother.

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Publications included in dissertation

This doctoral thesis is based on the following eight publications. All of these publications are also appended at the end of this thesis.

1. Muhammad Afzal, Rizwan Raza, Shangfeng Du, Raquel Bohn Lima, Bin

Zhu, Synthesis of Ba0.3Ca0.7Co0.8Fe0.2O3-δ composite material as novel catalytic cathode for ceria-carbonate electrolyte fuel cells, Electrochimica Acta, Volume 178, 2015, Pages 385-391.

2. Muhammad Afzal, Chen Xia, Bin Zhu, Lanthanum-doped Calcium Manganite (La0.1Ca0.9MnO3) Cathode for Advanced Solid Oxide Fuel Cell (SOFC), Materials Today: Proceedings, Volume 3, Issue 8, 2016, Pages 2698-2706.

3. Xunying Wang, Muhammad Afzal, Hui Deng, Wenjing Dong, Baoyuan Wang, Youquan Mi, Zhaoyun Xu, Wei Zhang, Chu Feng, Zhaoqing Wang, Yan Wu, Bin Zhu, La0.1SrxCa0.9-xMnO3-δ -Sm0.2Ce0.8O1.9 composite material for novel low temperature solid oxide fuel cells. International Journal of Hydrogen Energy, Volume 42, Issue 27, 2017, Pages 17552-17558.

4. Muhammad Afzal, Mohsin Saleemi, Baoyuan Wang, Chen Xia, Yunjuan He, Jeevan Jayasuriya, Bin Zhu, Fabrication of novel electrolyte-layer free fuel cell with semi-ionic conductor (Ba0.5Sr0.5Co0.8Fe0.2O3-δ-Sm0.2Ce0.8O1.9) and Schottky barrier, Journal of Power Sources, Volume 328, 2016, Pages 136-142.

5. Bin Zhu, Peter Lund, Rizwan Raza, Ying Ma, Liangdong Fan, Muhammad Afzal, Janne Patakangas, Yunjun He, Yufeng Zhao, Wenyi Tan, Qiu-An Huang, Jun Zhang, and Hao Wang, Schottky junction effect on high performance fuel cells based on nanocomposite materials, Advanced Energy Materials, Volume 5, 2015, Pages 1-6.

6. Baoyuan Wang, Yixiao Cai, Chen Xia, Jung-Sik Kim, Yanyan Liu, Wenjing Dong, Mikael Karlsson, Hao Wang, Muhammad Afzal, Junjiao Li, Rizwan Raza, Bin Zhu, Semiconductor-ionic membrane of LaSrCoFe-oxide-doped Ceria Solid Oxide fuel cells. Electrochimica Acta, Volume 248, 2017, Pages 496-504.

7. Muhammad Afzal, Sushant Madaan, Wenjing Dong, Rizwan Raza, Chen Xia, Bin Zhu, Analysis of a perovskite-ceria functional layer-based solid oxide fuel cell. International Journal of Hydrogen Energy, Volume 42, Issue 27, 2017, Pages 17536-17543.

8. Yuzheng Lu, Muhammad Afzal, Bin Zhu, Baoyuan Wang, Jun Wang, Chen Xia, Nanotechnology Based Green Energy Conversion Devices with

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Multifunctional Materials at Low Temperatures, Recent Patents on Nanotechnology, Volume 11, Issue 2, 2017, Pages 85-92.

Publications not included in dissertation

9. Zheng Qiao, Chen Xia, Yixiao Cai, Muhammad Afzal, Hao Wang, Jinli Qiao, Bin Zhu, Electrochemical and electrical properties of doped CeO2-ZnO composite for low-temperature solid oxide fuel cell applications. Journal of Power Sources, Volume 392, 2018, Pages 33-40.

10. Youquan Mi, Chen Xia, Bin Zhu, Rizwan Raza, Muhammad Afzal, Ilan Riess, Experimental and physical approaches on a novel semiconducting-ionic membrane fuel cell, International Journal of Hydrogen Energy, in Press, 2018.

11. Amjad Ali, Farrukh Shehzad Bashir, Rizwan Raza, Asia Rafique, Muhammad Kaleem Ullah, Muhammad Afzal, Moinuddin Ghauri, Lyubov M Belova, Electrochemical study of composite materials for coal-based direct carbon fuel cell, International Journal of Hydrogen Energy, in Press, 2018.

12. Yanyan Liu, Yuanjing Meng, Wei Zhang, Baoyuan Wang, Afzal Muhammad, Chen Xia, Bin Zhu, Industrial grade rare-earth triple-doped ceria applied for advanced low-temperature electrolyte layer-free fuel cells. International Journal of Hydrogen Energy, Volume 42, Issue 34, 2017, Pages 22273-22279.

13. Yanyan Liu, Yan Wu, Wei Zhang, Jing Zhang, Baoyuan Wang, Chen Xia, Muhammad Afzal, Junjiao Li, Manish Singh and Bin Zhu, Natural CuFe2O4 Mineral for Solid Oxide Fuel Cells. International Journal of Hydrogen Energy, Volume 42, Issue 27, 2017, Pages 17514-17521.

14. Yanyan Liu, Wei Zhang, Baoyuan Wang, Muhammad Afzal, Chen Xia, Bin Zhu, Industrial Grade LaCe1.85Pr0.03Nd0.06Ox/Na2CO3 Nanocomposite for Novel Low-Temperature Semiconductor-ionic Membrane Fuel Cell. Advanced Materials Letters, Volume 8, Issue 4, 2017, Pages 346-351.

15. Chen Xia, Baoyuan Wang, Yixiao Cai, Wei Zhang, Muhammad Afzal, Bin Zhu, Electrochemical properties of LaCePr-oxide/K2WO4 composite electrolyte for low-temperature SOFCs. Electrochemistry Communications, Volume 77, 2017, Pages 44-48.

16. Chen Xia, Muhammad Afzal, Baoyuan Wang, Aslan Soltaninazarlou, Wei Zhang, Yixiao Cai, Bin Zhu. Mixed-conductive membrane composed of natural hematite and Ni0.8Co0.15Al0.05LiO2-δ for electrolyte layer-free fuel cell. Advanced Materials Letters, Volume 8, Issue 2, 2017, Pages 114-121.

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17. Chen Xia, Yixiao Cai, Baoyuan Wang, Muhammad Afzal, Wei Zhang, Aslan Soltaninazarlou, Bin Zhu. Strategy Towards Cost-Effective Low-Temperature Solid Oxide Fuel Cells: A Mixed-conductive Membrane Comprised of Natural Minerals and Perovskite Oxide. Journal of Power Sources, Volume 342, 2017, Pages 779-786.

18. Baoyuan Wang, Yixiao Cai, Chen Xia, Yanyan Liu, Afzal Muhammad, Hao Wang, Bin Zhu, CoFeZrAl-oxide based composite for advanced solid oxide fuel cells, Electrochemistry Communications, Volume 73, 2016, Pages 15-19.

19. Baoyuan Wang, Yixiao Cai, Wenjing Dong, Chen Xia, Wei Zhang, Yanyan Liu, Muhammad Afzal, Hao Wang, Bin Zhu, Photovoltaic properties of LixCo3-xO4/TiO2 heterojunction solar cells with high open-circuit voltage, Solar Energy Materials and Solar Cells, Volume 157, 2016, Pages 126-133.

20. Bin Zhu, Liangdong Fan, Hui Deng, Yunjuan He, Muhammad Afzal, Wenjing Dong, AZRA YAQUB, Naveed K. Janjua, LiNiFe-based layered structure oxide and composite for advanced single layer fuel cells, Journal of Power Sources, Volume 316, 2016, Pages 37-43.

21. Wenjing Dong, Azra Yaqub, Naveed K. Janjua, Rizwan Raza, Muhammad Afzal, Bin Zhu, All in One Multifunctional Perovskite Material for Next Generation SOFC, Electrochimica Acta, Volume 193, 2016, Pages 225-230.

22. Chen Xia, Baoyuan Wang, Ying Ma, Yixiao Cai, Muhammad Afzal, Yanyan Liu, Yunjuan He, Wei Zhang, Wenjing Dong, Junjiao Li, Bin Zhu, Industrial-grade rare-earth and perovskite oxide for high-performance electrolyte layer-free fuel cell, Journal of Power Sources, Volume 307, 2016, Pages 270-279.

23. Yunjuan He, Liangdong Fan, Muhammad Afzal, Manish Singh, Wei Zhang, Yufeng Zhao, Junjiao Li, Bin Zhu, Cobalt oxides coated commercial Ba0.5Sr0.5Co0.8Fe0.2O3−δ as high performance cathode for low-temperature SOFCs, Electrochimica Acta, Volume 191, 2016, Pages 223-229.

24. Bin Zhu, Yizhong Huang, Liangdong Fan, Ying Ma, Baoyuan Wang, Chen Xia, Muhammad Afzal, Bowei Zhang, Wenjing Dong, Hao Wang, Peter D. Lund, Novel fuel cell with nanocomposite functional layer designed by perovskite solar cell principle, Nano Energy, Volume 19, 2016, Pages 156-164.

25. Huiqing Hu, Qizhao Lin, Zhigang Zhu, Xiangrong Liu, Muhammad Afzal, Yunjuan He, Bin Zhu, Effects of Composition on the Electrochemical Property and Cell Performance of Single Layer Fuel Cell, J. Power Sources, Volume 275, 2015, Pages 476-482.

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26. Huiqing Hu, Qizhao Lin, Afzal Muhammad, Bin Zhu, Electrochemical study of lithiated transition metal oxide composite for single layer fuel cell, J. Power Sources, Volume 286, 2015, Pages 388-393.

27. Liangdong Fan, Muhammad Afzal, Chuanxin He, Bin Zhu, 12: Nanocomposites for “nano green energy” applications. Bioenergy Systems for the Future, 2017, Pages 421-449.

28. Rizwan Raza, Muhammad Kaleem Ullah, Muhammad Afzal, Asia Rafique,

Amjad Ali, Sarfraz Arshad, Bin Zhu, 15: Low-temperature solid oxide fuel

cells with bioalcohol fuels. Bioenergy Systems for the Future, 2017, Pages 521-

539.

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Table of Contents

Abstract ........................................................................................................ i

Sammanfattning ........................................................................................ iii

Preface ....................................................................................................... vii

Acknowledgements ................................................................................. viii

Publications included in dissertation ......................................................... x

INDEX OF FIGURES ........................................................................... xvii

INDEX OF TABLES .............................................................................. xix

NOMENCLATURE ............................................................................... xxi

Acronyms & Abbreviations ..................................................................................................... xxi Symbols ..................................................................................................................................... xxii

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

1.1 Solid Oxide Fuel Cell (SOFC) ........................................................................................... 2 1.2 Structures of SOFC ............................................................................................................. 3 1.3 Perovskite Materials in SOFC ............................................................................................ 7 1.4 Thermodynamics of SOFC and EFFC ............................................................................ 8 1.5 Nanocomposite Approach ................................................................................................. 9 1.6 Objectives and methodology ........................................................................................... 10 1.7 Limitations .......................................................................................................................... 11

1.8 Thesis outline ..................................................................................................................... 12

2 SUMMARY OF APPENDED PUBLICATIONS ................................ 13

3 THEORY ............................................................................................... 17

3.1 Nanotechnology Based SOFCs with Multifunctional Materials at Low Temperatures (Paper 8) ...................................................................................................................................... 18 3.2 Multi-functional Nanocomposite and Semiconductor-ionic Materials for Electrolyte-layer Free Fuel Cell .................................................................................................................... 19 3.3 Patents in Electrolyte-layer Free Fuel Cells with Nanotechnology .............................. 19 3.4 Theoretical Studies and Modeling (Paper 7) .................................................................. 20

3.4.1 From Low Temperature Solid Oxide Fuel Cell to Electrolyte-layer Free Fuel Cell 20 3.4.2 Adopted updated model for EFFC .......................................................................... 20 3.4.3 Electrode kinetics ........................................................................................................ 21 3.4.4 Reaction thermodynamics .......................................................................................... 21 3.4.5 Voltage losses ............................................................................................................... 22

4 METHODS AND MATERIALS .......................................................... 25

4.1 Electrode/Semiconductor Materials Synthesis for LTSOFC/EFFC ........................ 25 4.1.1 Synthesis of Novel Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF) Composite Material (Paper 1) 25 4.1.2 Synthesis of lanthanum-doped calcium manganite (La0.1Ca0.9MnO3) (LCM) cathode material (Paper 2) ................................................................................................... 26 4.1.3 Preparation of Perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) Material (Paper 4).... 27

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4.2 Electrolyte/ionic materials synthesis for LTSOFC/EFFC ......................................... 29 4.2.1 Preparation of samarium doped ceria Sm0.2Ce0.8O2-δ (SDC) (Paper 2) .............. 29

4.3 Semiconductor-ionic materials for EFFC ...................................................................... 30 4.4 Components and fuel cell fabrication ............................................................................. 30 4.5 Characterization Methods .................................................................................................. 32 4.6 Fuel Cell Testing Facility .................................................................................................. 32

5 RESULTS AND DISCUSSION ............................................................ 35

5.1 Conventional SOFC using BCCF as Cathode Material (Paper 1) .............................. 35 5.1.1 Structural Analysis and Morphology ...................................................................... 35 5.1.2 FTIR and TGA Analyses ......................................................................................... 37 5.1.3 Conductivity Measurement ...................................................................................... 37 5.1.4 BCCF cathode in SOFC Performance ................................................................... 38

5.2 EFFC and LTSOFC Devices using Semiconductor-ionic Materials (Papers 2 & 3) 39

5.2.1 Structural Analysis and Microstructure of LCM and SDC-LCM Composite (Paper 2) ................................................................................................................................. 39 5.2.2 ORR Activity of LCM in LTSOFC Configuration .............................................. 41 5.2.3 Performance of LCM Cathode ................................................................................ 42 5.2.4 Devices Based on Modifications in LCM Material (Paper 3) ............................. 43

5.3 EFFC Schottky Junction Effect (Papers 4, 5 & 6) ....................................................... 43 5.3.1 Crystal Structure and Microstructure of BSCF (Paper 4) .................................... 43 5.3.2 Electrochemical Properties ...................................................................................... 44 5.3.3 Electrical Conductivity Measurements ................................................................... 46 5.3.4 Fuel Cell Performance .............................................................................................. 46 5.3.5 Schottky Junction Effect in EFFC ......................................................................... 47 5.3.6 Schottky Junction Effect in EFFC (Paper 6) ........................................................ 48

5.4 Comparison of Theoretical and Experimental Results (Paper 7) ............................... 50 5.4.1 Comparison of Modelling and Experimental approaches ................................... 50

6 CONCLUSIONS AND FUTURE WORK ........................................... 53

6.1 Conclusions ........................................................................................................................ 53 6.2 Future Work ......................................................................................................................... 55

7 REFERENCES ..................................................................................... 57

8 Appendix ................................................................................................ 65

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INDEX OF FIGURES

Figure 1: Working principle of fuel cell. …………………………………………….2 Figure 2: SOFC configuration (a) 3-component device (b) 2-components device (c) Single component device. …………………………………………………………...4 Figure 3: Comparison of a conventional SOFC and single component EFFC. ……...5 Figure 4: a) A fuel cell device assembled by n-type conducting anodic regime, ionic electrolyte and p-type conducting cathode. b) Removal of the electrolyte-layer in between, the new device is an n-p junction assembly. [40] …………………………..6 Figure 5: Nano redox process at particle level in the EFFC. ………………………..6 Figure 6: Elements that can occupy sites in ABO3 perovskite structure (based on [47]).8 Figure 7: Schematic of SOFC MEA system with interconnects design. ……………17 Figure 8: Flow chart for synthesis of novel BCCF material. ……………………….26 Figure 9: Flow chart for synthesis of LCM material. ………………………………27 Figure 10: Flow chart for preparation of perovskite BSCF material. ……………....28 Figure 11: Flow chart for synthesis of SDC material. ……………………………...29 Figure 12: Fuel cell testing setup. ………………………………………………….33 Figure 13: XRD pattern of BCCF. ………………………………………………...35 Figure 14: SEM images of BCCF37 (a-c) and BCCF91 (d). (Paper 1) ……………...36 Figure 15: (a) FTIR and (b) TGA analyses. (Paper 1) ……………………….……..37 Figure 16: Conductivity vs temperature for sample Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF)…………………………………………………………………………….38 Figure 17: Fuel cell performance (3-layers) ………………………………………...39 Figure 18: XRD patterns of Perovskite LCM, 6SDC-4LCM at lower temperatures and 6SDC-4LCM at 750 oC. ……………………………………………………………40 Figure 19: SEM images of high temperature sintered LCM. ………………………..40 Figure 20: Electrochemical impedance spectra of 4LCM-6SDC. ……………………41

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Figure 21: V-I and P-I characteristics of the SOFCs and EFFC at 550 oC. …………42 Figure 22: XRD diffraction pattern of as-prepared BSCF. (Paper 4) ……………….44 Figure 23: (a & b) SEM images of BSCF. ………………………………………….44 Figure 24: EIS measurements for three layer fuel cell (a) and single layer fuel cell (b), the data points are the measured data and the solid line are the fitting results. The inset is the equivalent circuit for simulations. The curves (C) give the Bode plots for the two kinds of fuel cells. ………………………………………………………………….45 Figure 25: Temperature dependence of conductivity for lab made and commercial BSCF………………………………………………………………………………46

Figure 26: Fuel cell performance for 3-layer FC and EFFC at 550 ⁰C. (Paper 4) ……47 Figure 27: Schematic of post SEM micrograph of electrolyte-layer free fuel cell (EFFC)…………………………………………………………………………….48 Figure 28: Structure of fuel cell with Schottky junction formation. (Paper 6) ……...49 Figure 29: Diode effect measurement (a) in air (b) in FC conditions. ……………....49 Figure 30: Electrochemical impedance measurement of EFFC at different temperatures and equivalent circuit. (Paper 7) …………………………………………………....50 Figure 31: Comparison of theoretical and experimental V-I and P-I characteristics...51 Figure 32: V-I characteristics with fixing (a) m and (b) n values as constant. ……….52 Figure A-1: Old gas flow controller for hydrogen used for initial experiments. …….65 Figure A-2: Fuel cell performances with new gas flow controllers. ………………....66

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INDEX OF TABLES

Table 1 EIS (in FC conditions) parameters for equivalent circuits ………………….46

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NOMENCLATURE

Acronyms & Abbreviations

ABO3 Structural formula of perovskite oxides AC Alternating current AFC Alkaline fuel cell ASOFC Advanced solid oxide fuel cell ASR Area specific resistance BCCF BaXCa1-XCoYFe1-YO3-δ BCCF37 Ba0.3Ca0.7Co0.8Fe0.2O3-δ BCCF91 Ba0.9Ca0.1Co0.8Fe0.2O3-δ BHJ Bulk heterojunction BSCF BaxSr1-xCoyFe1-yO3-δ (e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ) CPE Constant phase element CSDC Calcium and samarium co-doped ceria DC Direct current DMFC Direct methanol fuel cell EDX Energy dispersive X-ray EDS Energy dispersive X-ray spectrometer EFFC Electrolyte-layer free fuel cell EIS Electrochemical impedance spectroscopy EMF Electromotive force FC Fuel cell FTIR Fourier transform infrared spectroscopy GDC Gadolinium doped ceria HCl Hydrochloric acid HHV Higher heating value HOR Hydrogen oxidation reaction HRTEM High-resolution transmission electron microscopy HTSOFC High-temperature solid oxide fuel cell IEA International energy agency ITSOFC Intermediate-temperature solid oxide fuel cell LCM La0.1Ca0.9MnO3 LiAlO2 Lithium Aluminum oxide LN-SDC Sm0.2Ce0.8O1.9 - LiNaCO3 LNZ LiNiZn oxide LSCF La1-xSrxCo1-yFeyO3- δ LSCM La0.1SrxCa0.9-xMnO3-δ LSM La1-xSrxMnO3 LTSOFC Low temperature solid oxide fuel cell MCFC Molten carbonate fuel cell MEA Membrane electrode assembly MFC Mass flow controllers NANOCOFC Nanocomposites for advanced fuel cell technology

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NCAL Ni0.8Co0.15Al0.05LiO2 NH4OH Ammonium hydroxide (NH4)2CO3 Ammonium carbonate Ni Nickel NiO Nickel oxide NSDC Sm0.2Ce0.8O1.9 - Na2CO3 OCV Open circuit voltage ORR Oxygen reduction reaction PAFC Phosphoric acid fuel cell PEMFC Proton exchange (or polymer electrolyte) membrane fuel cell P-I Power density vs current density R&D Research and development SCDC Sm/Ca co-doped ceria SDC Samarium doped ceria SEM Scanning electron microscopy SIC Super-ionic conduction SIFCs Semiconductor-ionic fuel cells SIM Semiconductor-ionic material SLFC Single layer fuel cell SOFC Solid oxide fuel cell SrTiO3 Strontium titanate SSC Sm0.5Sr0.5CoO3 SSZ Scandia-stabilized zirconia STP Standard temperature and pressure TEM Transmission electron microscopy TGA Thermo-gravimetric analysis TPB Triple phase boundary TPM Two phase materials UN United Nations V-I Voltage vs current density XRD X-ray diffraction YSZ Yttrium stabilized zirconia

Symbols

C Concentration at the electrode surface Ca Calcium Co Cobalt °C Degree Celsius cm Centimeter CO Carbon monoxide CO2 Carbon dioxide (CO3)2- Carbonate Cp Specific heat at constant pressure, in J K-1 kg-1

e- An electron

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e.g. For example E EMF or open circuit voltage E° EMF at STP, and with pure reactants (standard cell potential) Erev Reversible (Nernst) potential Ecell Cell voltage F Faraday’s constant, the charge on one mole of electrons, 96485 C mol-1 ∆G Change in actual free energy G Gibbs free energy (or negative thermodynamic potential) ΔGf Gibbs free energy (Gf) for overall cell reaction Gf (Products) Gibbs free energy for the products Gf (Reactants) Gibbs free energy for the reactants (Gfm)H2O Molar specific Gibbs free energy of formation in case of water g Gibbs free energy per mole h Electronic hole hf Enthalpy of formation H+ Proton H2 Hydrogen gas H2O Water I Current, current density i Current density, current per unit area io Peak exchange current density i.e. That is KOH Potassium hydroxide m Constant (with units of Volt) which depends on the inside conditions of

the FC mm Millimeter N Avogadro’s number, 6.022 x 1023 mol-1 also revolutions per second n Number of electrons n Constant (with units of the reciprocal of the current density) n Number of moles nm Nanometer O2− Oxygen ion (OH)- Hydroxyl ion O2 Oxygen gas P Electrical power (mW), also power density (mW/cm2) PH2 Partial pressure of hydrogen PO2 Partial pressure of oxygen PH2O Partial pressure of water vapour R Universal gas constant, 8.314 J mol-1 K-1, also electrical resistance Rct Charge transfer resistance Rcta Charge transfer resistivity for anode Rctc Charge transfer resistivity for cathode RH High frequency resistance Ri Internal resistance RL Low frequency resistance Rmt Mass transport resistance Rp Polarization resistance

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Scm-1 Siemens per centimeter sf Entropy of formation T Working Temperature t Time V Voltage Va Activation over-potential or activation polarization Vc Concentration loss or the over-potential loss Vcell Operating cell voltage Vohm Ohmic loss or ohmic over-potential We Electrical work z Number of the electrons transferred % Percentage & And ε Efficiency of a fuel cell α Coefficient for charge transfer, 0.2 < α < 1 (used in the updated model, α

is 0.25) αo Coefficient of transfer for oxidation reaction αr Coefficient of transfer for reduction reaction

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1 INTRODUCTION

Energy is considered central to almost all major challenges and opportunities the world is facing today. Access to clean and affordable energy is essential for all sectors of society, and is an integral part of most of the 17 Sustainable Development Goals as defined by the UN [1]. To this end, significant progress has been seen during the past several decades in harnessing solar, biomass, and wind energy resources for supplying renewables-based electricity and transportation systems. In parallel, substantial gains are apparent in energy efficiency in the built environment, industry, and transportation. Nonetheless, there is a constant requirement to focus research and development, investments, and policy efforts towards expanding the role of renewable energy sources, and to ensure that they are utilized in the most effective way. A number of scenarios for a future sustainable energy system with low climate change impact consider hydrogen as an energy carrier. This concept relies upon either water/steam electrolysis from renewables-based electricity or biomass-derived syngas to supply hydrogen, which in turn can be stored or converted further downstream. It is paramount that the involved energy conversion processes are extremely efficient in order to minimize losses from source to end use. In this setting, fuel cells for fuel-to-electricity conversion play a critical role, having inherent advantages of high efficiency, environment friendly, scalability, and good part-load performance [2-4]. As shown in Figure 1, a fuel cell (FC) is comprised of an anode, electrolyte, and cathode, utilizing the electrochemical reaction between hydrogen and oxygen to supply electrons and water. Since the invention and demonstration by Sir William Grove in 1839 [5], all of the fuel cells have been assembled with a structure based on three components. The type of electrolyte, employed as a key component [6-10], characterizes FC types. For example, the polymer electrolyte or proton exchange membrane fuel cell (PEMFC) uses solid polymer electrolyte and platinum catalyst fixed in porous carbon electrodes. Due to high power density, it is advantageous and favorable as compared to other types of fuel cells. A PEMFC operates at temperatures 50-100 °C with typical mobile application efficiencies of 53-60% and stationary application efficiencies of 25-35% [11-13]. Because of low working temperatures, PEMFCs have fast start-up times and are thus relevant for transport applications [14]. Alkaline fuel cell (AFC) utilizes several non-noble metals as catalyst for the electrode functions. Conventional AFCs operate in a temperature range of 50-200 °C whereas advanced AFCs can operate from 23-70 °C with efficiencies up to 60% [10-15]. Phosphoric acid fuel cell (PAFC) utilizes a phosphoric acid solution contained in e.g. Teflon-bonded silicon carbide matrix as the electrolyte, and porous carbon electrodes contain platinum catalyst [10]. PAFCs operate at temperatures 200-250 °C with an efficiency of about 40% [11-13]. Molten carbonate fuel cell (MCFC) is an intermediate temperature device (600-700 °C) in which molten carbonate salt mixture is used as an electrolyte by filling into porous lithium aluminum oxide (LiAlO2) matrix. However, at the electrodes, non-noble catalysts can be used. The efficiency of this fuel cell is 45-47% [10-12]. Finally, solid oxide fuel cell uses solid oxide material as an electrolyte. SOFC is a promising fuel cell type because of several advantages (e.g. non-noble materials, fuel flexibility etc.) as

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compared to other types. A more detailed overview of some key SOFC research and development issues follows.

Figure 1: Working principle of fuel cell.

1.1 Solid Oxide Fuel Cell (SOFC)

SOFCs are not bound to precious noble metal catalyst and therefore are capable for cost reduction by using ceramic materials. Due to the high working temperature, the internal reformation of the fuels is also accessible and due to this property, a variety of fuels can be used in this type of fuel cell without using an external reformer. SOFCs have a capacity to tolerate sulfur and are not poisoned due to CO. This characteristic of SOFC also allows the use of even nonrenewable synthetic gas prepared from the existing coal and can be significantly used in device operation. For a comparison, thermodynamic cycles can operate directly from fossil fuels and can deliver 30-60% efficiency while SOFC can deliver better output using different fuels [16]. However, use of clean fuel sources (e.g. hydrogen gas) is always preferred. It has an efficiency of up to 60%, increasing to as much as 85% with recovery of dissipated heat in addition to electricity [13]. In a solid oxide fuel cell, electrolyte should be sufficiently dense to block gas transport and insulate against electron conduction, while it should be a good ionic conductor. Both of the electrode materials should be porous enough for gas transport and catalytic for reduction, oxidation (redox) reactions and both should be good electrical conductors for electrons. For several decades, the relatively thick layer of electrolyte in conventional technology has introduced large power losses due to high ohmic losses. Operation at elevated temperatures above 700 oC allows the use of carbon monoxide and hydrocarbons (ethanol, methanol, etc.) in addition to hydrogen, enabling fuel flexibility [12]. Conventionally SOFC is a high temperature technology for which up to 1000 oC is required for the activation of sufficiently high ionic conductivity (0.1 S/cm) for typical electrolyte such as yttrium stabilized zirconia (YSZ). This restricts the material selection of the FC components (e.g. anode, cathode, interconnects, sealing material), leading to high operational and capital system costs [8-9, 17]. Scientists have used different approaches to minimize the SOFC power losses. First approach in this perspective was applied for the investigation of new materials that could mitigate power losses, leading to better performance. However the range of available materials is limited, and so far YSZ was found to be the most suitable

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electrolyte for conventional high temperature devices [18, 19]. Second approach was thin film technology, in which scientists attempted to make the electrolyte layer as thin as possible. Some reductions in power losses have been realized through this approach using thin films of electrolyte. However, high associated system costs need to reduce operating temperature to develop cost-effective technology [18], which can be said as the third approach. Reduction of the operating temperature can be achieved by the development of new materials by using composite approach (more details are given in the upcoming sections). According to developments through above three approaches, state of the art SOFCs operate in three ranges of temperature: conventional high temperature; 800-1000 °C; intermediate temperature; 600-800 °C; and low temperature, below 600 °C [19-24]. Today, most of the worldwide SOFC research and development is focused on the development of LTSOFCs because this solves the challenges related to high temperature, which is also the focus of this thesis. Low temperature operation of the SOFC is ultimately needed for the device stability to avoid material degradation and compatibility issues and in order to maintain the low cost of the system. Since end users are not interested to purchase high temperature technology because of its several times higher price than other available technologies, it is therefore important to focus on the low temperature range to solve the existing challenges. In a seminal article by Goodenough [25], it is proposed that the main solution to avoid drawbacks with high temperature operation is to reduce the working temperature of SOFC by focusing on new materials that should have sufficient ionic conductivity at low temperatures. It has become top research and development (R&D) goal that advanced SOFC should operate in low temperature range 300-600 °C [26-28]. However, extensive worldwide efforts have been made in recent two decades to develop LTSOFC materials (semiconductor, ionic and the composite materials), new alternative electrolyte materials to replace conventional YSZ, but have not yet been available. In this thesis work, initial focus is to develop LTSOFCs including electrolyte-layer free fuel cells (EFFCs) operating below 600 °C and investigation of the useful materials for these devices.

1.2 Structures of SOFC

Most recent advances in the technologies included in the discussion above are related to LTSOFCs, which may be divided into three sub-types according to their structures.

i) 3-component device

ii) 2-component device

iii) Single component device

In conventional 3-component device, solid electrolyte component (layer) is sandwiched between the anode and the cathode components (layers) to obtain an open circuit voltage (OCV) and both electrodes are connected through an external circuit to deliver output power across a load. Research and development work on the electrolyte has been carried out for many decades, but still challenges are not fully solved. In the latest developments, an innovative technology electrolyte-layer free fuel cell (EFFC) was invented in 2010 by Dr. Zhu’s Group at KTH, and was selected as the research highlight on Nature Nanotechnology. Without using an electrolyte-layer, EFFC

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consists of two components (layers) (composite anode and composite cathode compacted together) and single component that is a homogeneous mixture of semiconductor and ionic materials (e.g. ceria based ion conductors) forming ‘‘three in one’’ structure and similar OCV and power can be obtained as those of conventional 3-component fuel cells. In single component EFFC device, pure electrolyte-layer is not used, however, electrolyte particles are embedded into ‘three in one’ mixture of the single component.

Figure 2: SOFC configuration (a) 3-component device (b) 2-component device (c)

Single component device.

Difference among all three structures is shown in Figure 2. Technically, an EFFC is easy to fabricate and handle because of simpler technology with less layers, and it has shown promising results in the fuel cell conditions. EFFC mechanisms are responsible for the better device performance over conventional SOFC technology in the same operating conditions. As the conventional YSZ requires operation above 800 °C due to its ionic conductivity limitation, the YSZ electrolyte based SOFC is restricted by the high temperature barrier. Thin film and low temperature materials have improved the technology but still there are issues relating to compatibility and catalytic activity of the electrodes in conventional FCs, like power losses because of the physical barriers between electrodes and the electrolyte and the incompatibilities between the fuel cell components. However, if the electrolyte-layer can be replaced with some novel component, an innovative technology may emerge. Recently, such a technology has been developed that has apparently eliminated the demand of the electrolyte layer, therefore is called an electrolyte-layer free fuel cell (EFFC). A schematic comparison between working principles of conventional SOFC and the EFFC is shown in Figure 3 [28-31]. Previously, the working of EFFC has been presented based on the insufficient knowledge about this innovation during the initial years and could not address the basic questions clearly. In particular, a relevant question how short-circuiting problem was avoided without using any electrolyte-layer in EFFC was not addressed satisfactorily. However, for comprehensive understanding one should keep in mind that electrolyte material particles are embedded into the homogeneous mixture of EFFC material. As a fact, the state of the art SOFC cathode materials based on perovskite oxides themselves are semiconductor materials. The cathode material with semiconducting

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properties was often ignored or less noticed in SOFCs. The cathode component used for SOFC is usually a homogeneous mixture of the cathode perovskite and ionic electrolyte combined to form a mixed ionic and electronic conducting composite in order to reduce the electrolyte/cathode interface polarization loss [10].

Figure 3: Comparison of a conventional SOFC and single component EFFC.

Further looking at the cathode component (when we use it as composite cathode) of SOFC, it is also a type of semiconductor-ionic material that is new kind of functional materials which on one hand can work for semiconducting physical properties to build junctions to prevent the electronic passage, while on the other hand, a medium for ionic transport properties. Introduction of semiconductor-ionic approach by replacement of the electrolyte-layer barrier has led an alternative way to solve the challenges faced for current SOFCs due to poor ionic conductivity of the electrolyte materials at low temperatures. A great enhancement in the ionic conductivity (at least two orders of magnitude) has been achieved in such materials, for example, in a perovskite semiconductor strontium titanate (SrTiO3) and the ionic electrolyte material YSZ [32-34]. Similar phenomenon has also been found in case of samarium doped ceria (SDC) mixed homogeneously with SrTiO3 [34]. Even, prominent enhancement in the ionic conductivities has been achieved in these heterostructures of semiconductor-ionic materials, and SOFC device output power is good based on these reported new materials. One of the main reasons could be the strong electronic conductivity in such hybrid structured materials due to the presence of SrTiO3

[35]. According to the conventional fuel cell science, the electronic conduction through the electrolyte membrane causes serious open circuit voltage (OCV) and electrical power losses [36-39]. Recently, Singh et al. [40] pointed out that n- and p-type oxide materials can fulfill the

requirements of anode and cathode respectively, showing a new scientific view of

understanding of SOFC from the physical aspects of semiconductors, as shown in

Figure 4. This is a new pathway to understand the FC science from traditional

electrochemical cell to new semiconductor-ionic device. Here, anode region is actually

an n-type semiconductor and the cathode region, is the p-type. Then an ionic conductor

in between them is the electrolyte. Hence, a fuel cell device may be considered as an

assembly consisting of n, p and ion conducting materials/components.

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Figure 4. a) A fuel cell device assembled by n-type conducting anodic regime, ionic electrolyte and p-type conducting cathode. b) Removal of the electrolyte-layer in

between, the new device is an n-p junction assembly. [40]

It can be derived from Figure 4, if we remove the electrolyte component (layer) between

the n-anode and p-cathode, the device turns into a n-p junction assembly. Same

principle is used in solar cell, i.e. without using the electrolyte layer/component, the

device can make no electronic short-circuiting problem due to the presence of junction

blocking the electrons, and thus the electrolyte-layer free fuel cell can be one possible

structure. There is another mechanism reported for the nano-redox process that takes

place in EFFC [41] through bulk heterojunction (BHJ) built on the semiconductor (n-

and p-type) material particles in one semiconductor-ionic component of the fuel cell.

From this perspective, anode/cathode functions can be completed by n, p and

electrolyte ionic transportation through ionic particles as illustrated in Figure 5, thus the

FC redox process is occurred at nanoparticle level.

Figure 5. Nano redox process at particle level in the EFFC.

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For better understanding, we can simply consider to compact from macro SOFC redox

to EFFC nanoscale particle level. As mentioned above, by removing the electrolyte-

layer between the anode and cathode, the new device turns into an n/p junction device

that is a well-known mechanism in solar cell field. The functioning of such device using

n and p type semiconductor materials mixed with ion conducting ceria shows the

comparable EFFC performance as compared to that of conventional SOFC with

anode/electrolyte/cathode configuration. Bi-layer n-p semiconductor-ionic fuel cell is

first prototype of such n/p junction device that has been demonstrated recently [42-

43]. Following this line, further semiconductor-ionic fuel cells (SIFCs) have been

developed to construct EFFC devices with junction type designed according to energy

band alignment.

1.3 Perovskite Materials in SOFC

An important issue in reducing the working temperature of the devices is the poor catalytic activity for hydrogen oxidation reaction (HOR) at anode and oxygen reduction reaction (ORR) at cathode, therefore oxidation and reduction of the fuel and oxygen respectively becomes a significant challenge. On the other hand, lower ionic conductivity through electrolyte materials operating at lower temperatures becomes another obstacle. Perovskite oxides with the structure formula ABO3 containing high mixed electronic and ionic conductivities have attracted much attention in various SOFC concepts. Therefore, a focus on perovskite materials for LTSOFCs and EFFCs development has been considered. According to the modified periodic table shown in Figure 6, chemical elements can be doped at A or B position of the perovskite structure formula ABO3. The common anode and cathode material in SOFCs is strontium doped LaMnO3 (LSM). However, it is not fully suitable due to its poor catalytic activity and low ionic conductivity at intermediate or low temperatures [44]. According to the literature, La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) has been used as a perovskite cathode material for SOFCs at intermediate and low temperatures [45]. Another perovskite oxide, BaxSr1-xCoyFe1-yO3-δ (BSCF) as cathode material, is seen as an attractive material due to its mixed electronic and ionic conductivity and good catalytic activity especially for the composition Ba0.5Sr0.5Co0.8Fe0.2O3-δ [18, 46] meeting the requirements for perovskite structure [47]. Based on the periodic table for perovskites presented in Figure 6, several new perovskite materials can be designed for energy applications.

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Figure 6: Elements that can occupy sites in ABO3 perovskite structure (based on [47]).

1.4 Thermodynamics of SOFC and EFFC

SOFC works according to the principles of thermodynamics. When fuel (hydrogen) is fed at the anode and air (oxygen) is supplied at the cathode side, oxidation of fuel/hydrogen takes place at anode (through anode half-cell reaction) and reduction of oxygen occurs at cathode (through cathode half-cell reaction). Simultaneously the electrolyte transports oxide ions from cathode to the anode. During this process, all chemical reactions take place in the cell at certain temperature and water, heat and electricity are generated [19]. Anode half-cell reaction: anode material in case of catalytically active oxidizes hydrogen molecule to produce water molecule.

H2 + O2- → H2O + 2e- (1.1)

Cathode half-cell reaction: oxygen from air is reduced into oxide ions by getting two electrons for one atom of oxygen at cathode.

½ O2 + 2e- → O2- (1.2)

Complete cell reaction: the overall cell reaction can be given as below.

H2 + ½ O2 → H2O + heat + electricity (1.3)

Open Circuit Voltage (OCV) Open circuit voltage (OCV/E) of SOFC is developed between anode and cathode in an open circuit condition. OCV can be calculated by Gibbs free energy relation for ∆G [48].

∆G = - nFE (1.4)

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E = - ∆G/nF (1.5)

In case of SOFC as an oxygen concentration cell, OCV can be calculated by Nernst equation [48]. So, EMF (electromotive force) or the reversible voltage (thermodynamically) at the higher temperatures is given as below.

E = E° + (RT

𝐧F) (ln (

P(H2)

P(H2O)) + ln (√P(O2))) (1.6)

OCV (E) depends on operating temperature of fuel cell, concentrations of hydrogen at anode, oxygen at cathode and water at the anode. Here, ∆G is the change in actual free energy, n is number of electrons which are transferred in the cell reaction, F is Faraday’s constant, E is the electrochemical potential, E° is standard cell potential, R is universal gas constant, T is the working temperature of the cell in kelvin.

Fuel Cell Efficiency

As, fuel cells use some materials which are typically burnt (in an electrochemical way) in order to release the required energy, therefore fuel cell efficiency can be described as a ratio of an electrical energy obtained to the heat produced by burning the fuel (i.e. enthalpy of formation denoted by Δhf). Efficiency of a fuel cell can be determined from following relation [48].

ε = We/Qin (1.7) We is electrical work and given by ∆G. From equation (1.4), the quantitative value of ∆G is given by nFE. For the reaction taking place, enthalpy of formation is denoted by Qin. As two values can be calculated depending upon state of reactants therefore higher of the two values (i.e. higher heating value) is used and is denoted by HHV. Hence equation (1.7) can be written as. ε = ∆G/Qin = nFE/HHV (1.8)

1.5 Nanocomposite Approach

Nanocomposites for advanced fuel cell technology (NANOCOFC) is an advanced scientific approach to develop semiconductor and ionic functional heterostructure nanocomposite materials for constructing advanced fuel cells at low temperatures (300-600 °C). Synthesized nanocomposites can also be useful in several other electrochemical devices for energy conversion [49-57]. These nanocomposites based on semiconductor-ionic (perovskites-ionic) materials are usually two phase materials (TPMs) which give some unique characteristics as well as multiple functions which are given below [50-55].

i) TPM heterostructures generate interfaces among the particles of two

different phases. These two phase interfacial regions (often as a core-shell

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structure, e.g. NSDC) are supportive for the material functionalities.

Compared to the traditionally used single phase bulk doping material, e.g.

YSZ, the structural limits have been eliminated due to the interfacial

functionalities in semiconductor-ionic materials.

ii) Ion transport in single phase structure, so called “bulk” conduction does not

play a viable role in TPM electrolytes and is quite different from

conventional single phase bulk structures. Interfacial regions create super-

ionic conduction (SIC) paths and therefore promote the functionalities and

the electrical properties.

iii) The primary ions (O2-, H+) can rapidly move through the bulk structure and

interfaces and the SIC mechanism enhances the device performance at low

temperatures 300-600 °C.

iv) Interfacial SIC mechanisms help to develop new functional materials and

make “non-function” to function, e.g. interfaces in insulators.

v) New opportunities for development of advanced LTSOFCs and EFFCs by

introducing interfacial and surface redox processes.

1.6 Objectives and methodology

The overall aim of this research is to investigate the potential materials for advanced

SOFCs through use of perovskite nanocomposites. The first objective of this study is

to design and develop the functional nanocomposite, perovskite and composite

materials (i.e. the electrolytes and the electrodes) for fuel to electricity conversion using

LTSOFC approach. The second objective is to develop the LTSOFCs and EFFCs

based on the functional materials. Specifically, the scope of this dissertation is given

below:

i) Development of perovskite/composite electrodes and nanocomposite electrolytes

for LTSOFCs

Perovskite/composite electrodes and nanocomposite electrolytes will be prepared

through different synthesis methods. The particle size of the materials will be optimized

from micro to nano-scale to get the optimized results from the device. The electrical

conduction phenomena and ionic transport will be measured and discussed.

ii) Design and development of novel semiconductor-ionic materials for EFFCs

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To achieve improved performance through EFFCs, novel semiconductor-ionic

materials will be designed based on advanced LTSOFC cathode components, i.e. from

the cathode perovskite material mixed with the ionic electrolyte to prepare a

nanocomposite heterostructure that will exhibit high catalytic activity and will show

balanced electrical and ionic conductivity.

iii) Characterization and analysis of perovskite/composite electrodes, nanocomposite

electrolytes for LTSOFCs and semiconductor-ionic materials for EFFCs using

advanced electrochemical techniques.

The crystalline/phase structure, microstructure, particle size and morphology of the as-

prepared advanced materials will be analyzed by XRD, SEM equipped with EDX, and

TEM. The main route for electrical conductivity of semiconductor materials

measurement, DC 4 probe technique will be used at least from 300 oC to 550 oC.

Thermal and phase transition phenomena properties will be analyzed through TGA.

Electrochemical impedance spectroscopy (EIS) measurement will be employed to

investigate the electrochemical mechanism, capacitance, interfaces, electrode processes

and kinetics.

iv) Demonstration and feasibility of the prepared materials for LTSOFC and EFFC in

a temperature range of 300-600 oC

The composition and combination of the synthesized materials will be optimized based

on the extensive material characterizations and fuel cell testing. The performance (i.e.

measured voltage vs current density (V-I) and power density vs current density (P-I)

characteristics) results of the devices will be used for further material developments and

optimization/improvement of the devices.

v) Fuel cell fabrication and analysis using functional nanocomposite and

semiconductor-ionic materials

Different types of lab scale conventional LTSOFCs and EFFCs will be fabricated and

evaluated. Results of both conventional LTSOFC and EFFC technologies will be

compared at the same temperature to investigate which technology is more attractive

candidate for future development.

1.7 Limitations

This research work is centered to the certain fuel cell rig. While various operating temperatures in low range (300-600 oC) and conventional electrolyte-based LTSOFCs and electrolyte-layer free fuel cells (EFFCs) have been studied, other types of SOFC at intermediate (600-800 oC) and high (800-1000 oC) have not been studied. EFFCs are fabricated and evaluated at lab scale (cells with diameter 13 mm). Stability tests are considered to be outside the scope of this investigation. Devices have been evaluated with hydrogen alone as fuel.

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1.8 Thesis outline

This dissertation is a compilation thesis based on the work presented in eight scientifically refereed papers. In section 1, Introduction, the literature survey is given, section 1.6, Objectives and Methodology, gives brief note on the objectives and 1.7 shows the limitations of this study. In section 2, summary of appended papers, is given and a short description for each paper on the specific aspects investigated is given. Section 3, Theory, introduces the topic of this PhD study from experimental and theoretical approaches that is regarded as two separate domains of fuel-to-electricity conversion by energy technologies from conventional electrolyte-based solid oxide fuel cells (SOFCs) to electrolyte-layer free fuel cells (EFFCs). Section 4, Methods, explains the methods for materials/samples preparation and the instrumentation for the characterization of the materials, samples and devices used throughout this work. In section 5, Results and Discussion, the intention is to show highlighted results related to the papers which are included in this dissertation. Section 6, Conclusions and Recommendations, summarizes the major outcomes achieved through this work with the assessment of the objectives given below, and in the light of these conclusions, recommendations are suggested. At the end of this section, Future Work, gives the outline of those aspects in this work on which opportunities and time was limited to work, therefore these aspects should be focused in future. Section 7, References, lists the literature supporting this dissertation.

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2 SUMMARY OF APPENDED PUBLICATIONS

PAPER 1: Synthesis of Ba0.3Ca0.7Co0.8Fe0.2O3-δ composite material as novel catalytic cathode for ceria-carbonate electrolyte fuel cells

The paper investigates a novel composite material, Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF37), as an improved cathode for low temperature solid oxide fuel cells (LTSOFCs). The prepared material displays a high electrical conductivity (143 Scm-1) at a relatively low temperature (550 °C). The as-prepared material consists of composite phase structure containing perovskite as a major phase in its XRD pattern, accompanied by traces of oxide phases, e.g. calcium and cobalt oxides. Maximum power density up to 325 mWcm-2 was achieved at 550 °C using BCCF37 as a cathode in LTSOFC. Contribution in Paper 1: Discussion of research idea with supervisor, combined with background knowledge through literature survey, synthesis of materials, preparation of samples, fabrication of devices, experiments (including conductivity measurement, XRD and fuel cell testing) and data analysis, writing of manuscript and following of revision based on reviewers’ comments.

PAPER 2: Lanthanum-doped Calcium Manganite (La0.1Ca0.9MnO3) Cathode for Advanced Solid Oxide Fuel Cell (SOFC)

This work made a successful preparation of a perovskite material La0.1Ca0.9MnO3 (LCM) which was first used as a cathode for low temperature SOFC below 600 oC. A peak power density of 650 mW/cm2 was achieved using the ceria-based electrolyte LTSOFC to prove the advances of the LCM as the cathode material. Based on this, further successes have been made to prepare a new SIM where 750 mW/cm2 power density has been obtained in case of EFFC. In this paper, LCM is found as a promising candidate for both LTSOFC as cathode and EFFC as the core in the single component material. Contribution in Paper 2: Discussion of research idea with supervisor, combined with background knowledge through literature survey, preparation of samples, fabrication of devices, experiments and data analysis, writing of manuscript and following of revision based on reviewers comments.

PAPER 3: La0.1SrxCa0.9-xMnO3-δ -Sm0.2Ce0.8O1.9 composite material for novel low temperature solid oxide fuel cells

The paper describes the preparation of semiconductor-ionic LSCM-SDC composite material as a homogeneous mixture of La0.1SrxCa0.9-xMnO3-δ (LSCM) and Sm0.2Ce0.8O1.9 (SDC) and its use for construction of the cathode layer of fuel cell for low temperature applications. Open circuit voltage for new device reaches above 1.0 V, indicating no electronic short-circuiting problems through the device due to formation of the Schottky junction on H2 contact side. Compared to the conventional fuel cells using pure SDC as the electrolyte separator layer, the LSCM-SDC semiconductor-ionic material demonstrates higher ionic conductivity because of higher ions carrier concentration and the conduction through heterostructure interfaces between LSCM and SDC constituent phases. The optimal weight ratio for best device

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is found for LSCM:SDC as 40:60 and the corresponding peak power densities are reached up to 800 mW cm-2 at 550 oC, which is achieved as one of the best EFFC performances among perovskite based semiconductor-ionic materials (SIMs). Contribution in Paper 3: Discussion of research idea with the first author in detail and feedback before the project, shared recipe of LCM and designed new recipe of LSCM, help in the experiments, analyzing the results and manuscript writing and revision.

PAPER 4: Fabrication of novel electrolyte-layer free fuel cell with semi-ionic conductor (Ba0.5Sr0.5Co0.8Fe0.2O3-δ-Sm0.2Ce0.8O1.9) and Schottky barrier

This work investigates the synthesis of perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) as a cathode for LTSOFC. Prepared BSCF material exhibited electrical conductivities of more than 300 Scm-1 at 550 oC. Semiconductor-ionic material was prepared based on a homogeneous mixture of as prepared BSCF and samarium doped ceria in a ratio of SDC: BSCF (60%:40% by weight), which was actually a cathode component used for LTSOFC devices. Such semiconductor-ionic cathode component was found as a promising candidate to use in EFFCs. A peak power density of 655 mW/cm2 was achieved at 550 oC using in new EFFC while 425 mW/cm2 for the SOFC using the BSCF-SDC cathode at temperature of 550 °C. Schottky junction effect was explored in the EFFC device using NCAL (Ni0.8Co0.15Al0.05LiO2) layer towards fuel (hydrogen) side and it blocked electrons from internal short-circuiting through device keeping the standard fuel cell device voltages and constant power outputs. Contribution in Paper 4: Discussion of research idea with supervisor, combined with background knowledge through literature survey, synthesis of materials, preparation of samples, fabrication of devices, experiments (including conductivity measurement and fuel cell testing) and data analysis, writing of manuscript and following of revision based on reviewers comments.

PAPER 5: Schottky junction effect on high performance fuel cells based on nanocomposite materials

The investigation presents one of the key mechanisms explaining EFFC operation, i.e. Schottky junction that has a significant effect on EFFC performance. First, development of EFFC was completed using one layer of semiconductor-ionic component based on the mixture of LNZ-oxide and samarium doped ceria (SDC), which possesses favorable electrical and ionic conductivity, along with serving as catalyst for both H2 oxidation and O2 reduction. High power densities up to 600 mW cm−2 are demonstrated at 550 °C. The working principles that underlie the operation of new device are explored, with similarities to dye-sensitized solar cells noted. New discoveries in EFFC are described in this paper according to the Schottky junction and energy band alignments. Contribution in Paper 5: Discussion of research idea and theory with the first author, conducted all basic experiments for Schottky junction discovery, analyzing the results and participation in manuscript writing and revision.

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PAPER 6: Semiconductor-ionic membrane of LaSrCoFe-oxide-doped Ceria Solid Oxide fuel cells

This study provides evidence of a Schottky junction presented in EFFC type fuel cells. A novel semiconductor-ionic nanocomposite based on La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF)-Sm/Ca co-doped CeO2 (SCDC) mixture was developed and sandwiched between two identical layers of Ni0.8Co0.15Al0.05Li-oxide (NCAL) for EFFC fabrication. The device has presented an OCV more than 1.0 V and a maximum power density of more than 800 mW cm−2 at 550 °C. Contribution in Paper 6: Discussion of research idea with the first author, shared the knowledge and experiments about Schottky junction, participated in the experiments, analyzing the results and manuscript writing.

PAPER 7: Analysis of a perovskite-ceria functional layer-based solid oxide fuel cell

Based on extensive research on perovskite/semiconductor cathode materials and ionic ceria electrolyte materials, this work modifies first a theoretical simulation model (adopted from conventional fuel cell) for the analysis of EFFCs combined with an experimental validation. First, an EFFC was fabricated by preparing a semiconductor-ionic material based on homogeneous mixture of perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and samarium doped ceria (SDC). The device obtains a maximum power density 640.4 mWcm-2 with an OCV of 1.04 V at 560 oC using hydrogen as a fuel and air as the oxidant, respectively. Then, simulation model has also been applied in order to analyze the EFFC processes and it introduces the simplified parameters to describe the polarization and ohmic losses. This model introduces some modifications in the existing equations for conventional fuel cells used to plot the V-I and P-I characteristics of EFFCs. Theoretical curves have been compared with the experimental curves, and both results are found consistent with each other. Contribution in Paper 7: Discussion of research idea with supervisor, combined with background knowledge through literature survey, synthesis of materials, preparation of samples, fabrication of devices, experiments (including conductivity measurement and fuel cell testing) and data analysis. In parallel, assigned the task to second author for modification and applying model on EFFC and discussion on this aspect, writing of manuscript and following of revision based on reviewers’ comments.

PAPER 8: Nanotechnology Based Green Energy Conversion Devices with Multifunctional Materials at Low Temperatures

The paper presents a review of the research papers and the patents in the SOFC and EFFC field. Nanocomposites (integrating nano and composite approaches) for advanced fuel cells (NANOCOFC) show a great potential to reduce the working temperature of SOFC, especially in low temperature range 300-600 ºC. Science presented by NANOCOFC has developed the multi-functional materials which are composed of the semiconductor and ionic materials (SIMs) to meet the requirements for low temperature SOFCs and EFFCs. This paper reviews the recent developments and new patents in the relevant LTSOFC and EFFC technologies from the perspectives of nanotechnology, and also reports the advances including the fabrication methods,

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characterization techniques, material compositions and the device performance. Finally, future scope of LTSOFC and EFFC based on nanotechnology approach and the practical applications in energy sector are also discussed. Contribution in Paper 8: Discussion of research idea with supervisor, combined with background knowledge through literature survey, writing of manuscript and following of revision based on reviewers’ comments.

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3 THEORY

This chapter introduces the topic of this PhD study from experimental and theoretical approaches that is regarded as two separate domains of fuel-to-electricity conversion by energy technologies from conventional electrolyte-based solid oxide fuel cells (SOFCs) to EFFCs. Based on the review paper, progress in EFFCs is discussed and then theoretical part of the updated adopted model is presented. In conventional SOFCs, due to materials limitation and required conductivities, the working temperatures are maintained between 800-1000 °C. Due to higher ionic conductivity, YSZ is an ideal material for working as an electrolyte in this range of temperature with a conductivity of 0.1 S/cm. The ionic conductivity of the electrolyte material allows oxide ions to transfer through the electrolyte to complete the fuel cell redox reactions and make possible the electricity generation. In case of the proton conducting electrolyte, protons can alternatively be transported through the electrolyte to complete the redox reactions. Schematic of SOFC with membrane electrode assembly (MEA) with interconnects design is given in Figure 7 below.

Figure 7: Schematic of SOFC MEA system with interconnects design.

At such high temperatures, maintenance costs and cell degradation issues limit the commercialization target. Therefore, operation of SOFC in the range of 500-800 °C has attracted the attention of researchers to avoid the issues relating to high temperature operations [25]. Decrease in the working temperature minimizes the risk of materials degradation and also increases the range for material selection, minimizing the fabrication cost and time. However, a decrease in working temperature also leads to a reduction in electrode kinetics, which leads to an increase in the interfacial polarization resistance for reduction of oxygen at cathode. To decrease the polarization resistance of cathode, catalytic functions as well as sufficient ionic and electronic conductivities are highly demanded for the execution of oxygen reduction reaction (ORR). New design, electrolyte-layer free fuel cell (EFFC) is using stoichiometric compositions with the mixtures of semiconductor and ion conducting materials, giving new perspective of semiconductor-ionic material in fuel cells [58-63]. New device due to the interfacial design and multifunctionality of semiconductor-ionic materials, can work at low temperatures from 300 °C to 600 °C, therefore, can be categorized within the family of LTSOFCs [64-66].

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3.1 Nanotechnology Based SOFCs with Multifunctional Materials at Low Temperatures (Paper 8)

Extensive efforts are devoted to develop low temperature (300-600 °C) SOFCs based on ceria composite electrolytes. A key issue in the reduction of working temperature of SOFC has been poor catalytic activity of the electrodes resulting low reaction rates for oxygen reduction to oxygen ions, along with poor ionic conductivity through electrolytes at low temperatures. Most commonly found cathode materials are strontium doped LaMnO3 (LSM), La0.6Sr0.4Co0.8Fe0.2O3, BaxSr1-xCoyFe1-yO3-δ (BSCF), especially, Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskites for LTSOFCs [67]. Despite, BSCF has high electrical conductivity as well as the catalytic activity but the material cost is an issue. Nanocomposites (combining both nanomaterials and composite approaches) for advanced fuel cell has introduced the useful ways to design the multi-functional advanced materials to meet the requirements for LTSOFC based on the mechanisms including dual proton as well as oxygen ion conduction, interfacial superionic conduction, and new scientific aspects (interfacial phases, defects for oxygen and corresponding transport mechanisms, and similar topics for protons) [66]. EFFC demonstrates several advantages over conventional SOFC following different working mechanisms as described in detail in the included papers. Some special merits of EFFC can briefly be summarized here:

i) The manufacturing cost may be significantly reduced because of its simple structure ‘three in one’ design and fabrication techniques.

ii) Physical interfaces between the electrolyte and the electrodes in conventional three-layer FC contribute to its major polarization losses because of the physical barriers; such issues are not present with EFFCs. Moreover, a three-layer device requires suitable thermal and chemical compatibility among the components, while on the other hand, only one homogeneous mixture layer ‘three in one’ is used in EFFC which is also free from physical electrolyte/electrodes barriers. Therefore, losses or problems relating to thermal stress from the interfaces may be avoided in EFFC. This may propose high output power from EFFC.

iii) The EFFC constructed from multi-functional nanocomposites consist of a homogeneous mixture of semiconductor and oxygen ion conductor in a particular composition for each combination of the materials. Ionic conductivity of the device core material enhances oxygen ion transportation through ionic material and because of the extra vacancies generated by interacting particles of semiconductor and ionic materials. An enhancement effect may induce due to the electrons, holes and ions between n-type or p-type semiconductor mixed with ion conductors. This effect may make enhancements in the overall device ionic conductivity, ionic transport and device performance.

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iv) Schottky junction established on hydrogen side between the reduced metal or alloy phase and semiconductor phase, helps to prevent electronic short circuiting problems and supports the ionic conductivity of the EFFC device. Absence of physical barriers between electrodes and electrolyte in EFFC; may reduce the operational temperature of the device and provides an opportunity for wide range of materials.

3.2 Multi-functional Nanocomposite and Semiconductor-ionic Materials for Electrolyte-layer Free Fuel Cell

This section is based on the review paper 8 and work in the literature [68]. Today, most of the single component materials are of semiconductor-ionic nature based on the mixture of two different phases (i.e. semiconductors of transition metals (e.g. Mn, Fe, Ni, Cu and Zn) and ion conductors (e.g. SDC [26], GDC and other ceria composites [13]), and the designs of new composites with novel compositions of the mixing of the two phases [13]). Recent advancement in this line depicts that output of the devices depends on the individual properties of the two phases, mixing ratios, matching between the semiconductor and ionic materials, microstructure and experimental conditions. Several ionic materials, e.g. SDC, GDC, MgZn-SDC, GDC-KAlZn (KAZ), CDC, SCDC, and NSDC [13, 26-30] etc. have been investigated as potential candidates for fuel cell applications. Similarly, a number of semiconductors, e.g. NiO, CoOx, CuO, FeOx, ZnO, LSCF, BSCF, BCCF, LiNiCuZn and doped LiNiO2 [13, 26, 29, 66] etc. which are suitable for fuel cells have been investigated. Semiconductor-ionic term is derived actually from two different words, one is the ‘semiconductor’ and another one is ‘ion conductor’. So semiconductor-ionic material (SIM) presents such type of composite material where semiconductor and ion conducting materials establish an active interface among the particles of the two different material natures and create more vacancies for ion conduction. These materials are used for the fabrication of LTSOFCs especially; electrolyte-layer free fuel cells (EFFCs).

3.3 Patents in Electrolyte-layer Free Fuel Cells with Nanotechnology

Based on the achievements about EFFCs, besides a number of research articles, several patents in nanocomposites have been applied for and approved [69-73]. The number of patents related to the LTSOFC using nanocomposites increased rapidly and Paper 8 has cited some of them mainly those ones with reasonable results based on new developments. Review work for the comparative studies of conventional fuel cells and EFFCs including the recent patents in the field has also been conducted (Paper 8). In most of the literature studied [65-66], it has been found that EFFC is better device from fuel cell performance, easy fabrication and handling perspectives, however, its stability still needs more focus. A number of patents have been made with the invention of several new types of EFFC using several semiconductor-ionic materials combinations. Recently, a number of patents on single component (SC) FCs/EFFC using nanocomposite SIMs have been filed and issued [74-78]. In parallel, many research articles have been published

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in this line. These achievements in SIMs have largely promoted the actual developments in both cases of fabrications and applications.

3.4 Theoretical Studies and Modeling (Paper 7)

In this section, a theoretical simulation model is introduced for EFFC that is adopted from theoretical model for conventional fuel cells with some relevant modifications according to EFFC device.

3.4.1 From Low Temperature Solid Oxide Fuel Cell to Electrolyte-layer Free Fuel Cell

Various new functional semiconductor-ionic materials and junction devices show good fuel-to-electricity conversion performance. The new functional layer based on SIMs can integrate all the fuel cell functions of the anode, electrolyte and the cathode. The EFFC device in its new structure avoids the issues related to the compatibility, device functions and the power losses because of physical barriers during the operation. EFFC functions based on nano-redox reactions are described from the formation of the bulk heterojunction (BHJ) structures. The charge separation is formed by the energy gaps present between the n and p type semiconductors taking place at the particle sites in order to avoid short-circuiting [79]. A Schottky junction is established between the metal phase and the semiconductor. For this purpose, NCAL coated on Ni-foam is attached to the anode zone side which is reduced into metal, e.g., either Ni or its alloy by H2 oxidation. This metal or alloy having contact with p-type semiconductor thus constructs the Schottky junction [65, 79]. Schottky barrier across this junction directly prevents the passage of electrons from crossing-over and therefore avoids the short circuiting problem because of the built-in-field without using the electrolyte separator in the fuel-to-electricity device. Therefore, physics and semiconducting principles play an important role to combine with traditional fuel cell electrochemistry to fulfil the fuel cell functions. Based on the working principles, a theoretical work with modifications in the existing theoretical model used for conventional fuel cell is adopted for EFFC. Few years earlier, EFFC was being used as electrolyte free fuel cell. For comprehensive understanding, introduction of electrolyte-layer free fuel cell (instead of electrolyte free fuel cell) concept is proposed by the defendant of this thesis which is now adopted and widely used in the certain research area.

3.4.2 Adopted updated model for EFFC

This work is an attempt to model an empirical formula for EFFC operation that describes the cell voltage/potential (V) vs the current density (I) curve. A number of studies can be found in literature to elaborate the behavior of the cell voltage vs the current density in conventional fuel cell [80-81]. The significance of the updated equation is to use it in optimizing the EFFC performance with respect to the power density, heat loss generation rates and energy efficiency as a function of the current density. It can be useful to optimize the working conditions as well as microstructure of the EFFC device, particularly gas contacting sides acting as the electrodes.

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As shown below, some modifications are made to existing models of conventional fuel cell [82-88] in order to describe the EFFC processes and simulate the V-I and P-I characteristics of the EFFC device; results are validated from experimental data.

3.4.3 Electrode kinetics

An EFFC device embeds both electrodes into one layer (single component), so transportation of H+ and O2- ions takes place through the bulk and the interfaces in the SIM and the electrode reaction processes occur on the respective surfaces. Oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) take place in the corresponding zones of EFFC, therefore, for the convenience in this case of modelling, the device reactions are divided theoretically into three zones: i.e., the anode zone, the cathode zone and the intermediate zone. In case of EFFC based on SDC-BSCF, oxide-ion conduction dominates the fuel cell processes, the redox reactions in EFFC take place in the corresponding zones. Oxidation reaction in the anode zone corresponds to eq (1.1) in Introduction chapter. Whereas, reduction reaction in the cathode zone referred to eq (1.2), and the complete cell reaction corresponds to eq (1.3).

3.4.4 Reaction thermodynamics

Gibbs free energy of formation can be used to calculate an amount of the chemical energy converted through the EFFC into the electrical energy. Thus, Gibbs free energy (Gf) for overall cell reaction can be calculated from the difference between the Gibbs free energies of the reactants and the products [88].

ΔGf = Gf (Products) – Gf (Reactants) (3.1)

where Gf (Products) is the Gibbs free energy for the products, Gf (Reactants) is the Gibbs free energy for the reactants and ΔGf is the Gibbs free energy (Gf) for overall cell reaction. In order to make the comparison more easily, it can be convenient to consider the quantities in the “per mole” format. For example, (Gfm)H2O represents the molar specific Gibbs free energy of formation in case of water. Thus, Gibbs free energy for reaction (1.3) becomes:

ΔGf = (Gfm)H2O – (Gfm)H2 - ½ (Gfm)O2 (3.2)

In this equation, Gfm is the Gibbs free energy for the corresponding component regarding to the respective subscript. However, Gibbs free energy also depends upon operating temperature and can also be calculated by the following formula.

Gf = hf – T. sf (3.3)

Here, hf is the enthalpy of formation at that temperature and sf is the entropy of formation at that given temperature T. Hence, for the above reaction, we get.

ΔGf = Δhf – T. Δsf (3.4)

Here, Δhf = hf (Products) – hf (Reactants) and Δsf = sf (Products) – sf (Reactants).

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Enthalpy and the entropy of formation are taken to depend upon the operational temperature according to the equations below.

hf = h298.15 + ∫ CpdTT

298.15 (3.5)

sf = s298.15 + ∫1

TCpdT

T

298.15 (3.6)

Here, hf and h298.15 are the enthalpies of given SIM at temperature T and at the standard temperature (298.15 K i.e. almost room temperature) respectively. Similarly, the quantities sf and s298.15 are the corresponding entropies of the reactants and products (for given SIM) at T temperature and at standard room temperature (298.15 K) respectively.

3.4.5 Voltage losses

Electromotive force (EMF) also referred as reversible voltage for the hydrogen fuel cell (it is also called theoretically as open circuit voltage) and by applying for EFFC is given by the following:

E = −ΔGf

𝐧F (3.7)

This expression is well known as the Nernst voltage. Here, F is Faraday’s constant and n is number of electrons. It is the maximum value of ideal voltage that can be obtained theoretically from the cell. For an electrically open circuit, no external current is generated, and in this condition the maximum voltage observed is called the open circuit voltage (OCV). If the circuit is closed under a load, the voltage drops further due to the inherent losses including electrochemical kinetics, inability of the reactants for reaching the reaction sites, electrical and ionic resistance etc. The difference between the electrode and equilibrium potentials acts as the actual driving force behind the current generation. These factors induce the irreversibility for the voltage drop, are listed as activation, concentration and ohmic losses. The operating voltage in this case can be given as below.

Vcell = E – Va – Vohm – Vc (3.8)

Here, Vcell is an operating voltage, E is the maximum voltage, Va is the activation over-potential or activation polarization, Vohm is the ohmic loss or ohmic over-potential, Vc is the concentration loss or the concentration over-potential. 3.4.5.1 Voltage loss due to activation polarization

Rate determining reaction for the EFFC device in the electrochemical reaction works as the threshold energy barrier that needs to be overcome in the reaction to proceed further. The potential needed to overcome the activation energy is known as activation polarization. The relation between the current and the activation potential is described through the Butler-Volmer expression.

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i = i0{exp(−αzFVa

RT) − exp[(1 − α)

zFVa

RT]} (3.9)

Here, i is total current density (with units A/cm2), io is peak exchange current density (with units A/cm2), α is the coefficient for charge transfer, 0.2 < α < 1 (used in this modified model, where value of α is 0.25), z is number of the electrons transferred. F is Faraday’s constant (with a value 96485 C mol-1), R is ideal gas constant (with a value 8.314 J mol-1 K-1), T is the working temperature (with units K) and Va is activation polarization. At equilibrium, no current generation is observed even the electrochemical reaction is still taking place simultaneously in the forward and backward directions. The rate of reactions at equilibrium is known as exchange current density that can be calculated using the following formula.

i0 = RT

ZFRct (3.10)

Rct represents the charge transfer resistance during the electrode reactions taking place. The value of Rct is 0.31 ohm cm2 which is calculated from Nyquist plot of EIS results (in Paper 7 measured in low temperature range 300-600 °C). From the equation (3.10), it can be calculated for EFFC device, higher the exchange current density, lower the losses associated to the activation. It is found that low temperature FCs have higher activation losses because of smaller exchange current densities at lower temperatures and therefore reach a steady state value at the higher currents. The losses mainly appear in the anode zone and the cathode zone in the EFFC limited to the microstructure and electro-catalyst activity, determined at the triple phase boundary (TPB). Butler-Volmer equation can be depicted as:

i = i0{exp(−αrzFVa

RT) − exp[

αozFVa

RT]} (3.11)

Here, αo and αr are the coefficients of transfer for oxidation reaction and reduction reaction respectively.

And i0 = RT

ZFRcta =

RT

ZFRctc (3.12)

Rctc and Rcta are charge transfer resistivity for cathode and anode components respectively.

3.4.5.2 Ohmic losses

Ohmic loss is the resistance offered from the electrolyte and both electrodes materials in the conduction of ions and the electrons which can be calculated by applying Ohm’s law:

Vohm = i.Ri (3.13)

Here, i is current density (with units A cm-2) and Ri is internal resistance (with a value of 0.43 ohm cm2) measured using EIS data. Internal resistance includes the ionic, electronic as well as the contact resistance.

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In case of EFFC, the internal resistance measured from the single layer of device is the internal resistance of FC.

3.4.5.3 Concentration losses

The voltage loss in the region can be regarded as the exponential function of current density [84].

Vc = m.exp(n.i) (3.14)

Here, m (with units of Volt) and n (with units of the reciprocal of the current density) are the constants which depend on the inside conditions of the FC and need to be determined using non-linear regression analysis. In this model, values of m and n have been adjusted up to 5e-3 and 3e-3, respectively. By including all of the expressions in case of various losses, operating voltage for the EFFC can be expressed as below.

Voc = E –Va - i.Ri – m.exp (n.i) (3.15)

The interpretation of new parameters m and n is very important to understand the effect on the curve V vs I in the plot. Hence, a theoretical calculation is conducted by keeping the factors constant in the above equation (3.15), and the values of either m or n are only changed. Theoretical results are explained in Results section. However, the EFFC being a new technology, lacks such a study so far. An attempt is formulated to demonstrate the good fit of V vs I characteristics plotted according to the empirical equation derived for the EFFC device. To account the linearity in the plot, here two constant parameters named as m and n, respectively are introduced in case of the EFFC. These parameters have been considered for the losses regarded to the mass-transport and can also be obtained from nonlinear regression analysis. Butler-Volmer equation describes a relationship between the activation over-potential and the current density, is also modified in the last section. This work also needs more description for the physical meanings of both parameters. Therefore, further studies in this line are required to link the parameters m and n with physical factors such as tortuosity, gas diffusion depth, porosity of the functional layer etc.

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4 METHODS AND MATERIALS

This section describes the methods for the preparation of the materials/samples and instrumentation used for characterizing the materials/samples and the devices used throughout this work.

4.1 Electrode/Semiconductor Materials Synthesis for LTSOFC/EFFC

This part contains different electrode/semiconductor materials which were used for both electrolyte-based low temperature SOFCs and electrolyte-layer free fuel cell technology. Details for synthesizing these materials are provided in their corresponding following sections.

4.1.1 Synthesis of Novel Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF) Composite Material (Paper 1)

Sol gel method was used to synthesis the composite material Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF37). Stoichiometric amounts of the relevant nitrate salts of barium nitrate Ba(NO3)2.H2O and calcium nitrate tetrahydrate Ca(NO3)2.4H2O were dissolved into de-ionized water in a beaker in a molar ratio Ba:Ca = 0.3:0.7 for preparing the starting solution A. This solution was stirred on a heating stage at 70 °C so that precursors were well mixed. Similarly, cobalt (II) nitrate hexahydrate Co(NO3)2.6H2O and iron(III) nitrate nonahydrate Fe(NO3)3.9H2O were also dissolved into the de-ionized water in a separate beaker in a molar ratio Co:Fe = 0.8:0.2 for preparing the starting solution B. The solution B was also stirred on a heating stage at 70 °C. Both of the solutions A and B were mixed together to prepare resultant solution for complete material composition. Another composition of the same material, Ba0.9Ca0.1Co0.8Fe0.2O3-δ (BCCF91) with molar ratio Ba:Ca:Co:Fe = 0.9:0.1:0.8:0.2 was also prepared in parallel by the same route. All of the chemicals were used as received, and were supplied from Sigma-Aldrich. Details for the synthesis of BCCF material are shown in a flow chart in Figure 8. Resultant solution C was prepared for both of the compositions of chemical compounds of Ba:Ca:Co:Fe in molar ratios as 0.3:0.7:0.8:0.2 and 0.9:0.1:0.8:0.2, respectively. This scheme is used in order to replace strontium element with calcium at A-site of ABO3 structure formula for the perovskite materials. Solution C was consecutively stirred for half an hour at 80 °C. An amount (10% by weight) of the citric acid was added to the precursor’s solution to obtain auto-combustion of the sample, and then continued the stirring process for two more hours at 90 °C. A paste-like gel of the samples was obtained in this way, which was then either auto-combusted or fired at 250 °C in air to yield an ash-like powder material. The obtained ash was annealed at 1000 °C continuously for 9 hours to obtain the proper oxide structure of the prepared material. Heating and cooling rates of the furnace were set as 2 °C min-1. The resultant BCCF materials were ground completely in a mortar with pestle (the conventional grinding method) until fine and homogeneous powder particles were obtained.

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Figure 8: Flow chart for synthesis of novel BCCF material.

4.1.2 Synthesis of lanthanum-doped calcium manganite (La0.1Ca0.9MnO3) (LCM) cathode material (Paper 2)

La0.1Ca0.9MnO3 (LCM) was prepared through a wet chemical method. Salts of lanthanum nitrate hexahydrate (La(NO3)3 · 6H2O), calcium nitrate tetra-hydrate (Ca(NO3)2 . 4H2O) and manganese nitrate tetrahydrate (Mn(NO3)2 · 4H2O) were mixed as the raw chemicals in a molar ratio of 0.1:0.9:1.0 respectively. All chemicals were used as obtained from Sigma Aldrich-USA without any further purification. Precursors of the constituents were completely immersed into a beaker containing de-ionized water following shaking well while stirring for one hour. Powder of citric acid was added equal to 100% of total amount by mass of the original chemical mixture into the solution for auto-combustion and was again stirred at high speed for another half an hour. Consequently, a viscous solution of LCM was obtained which was heated at 120

⁰C until auto-combustion was completed, followed by the annealing of burnt ash at 1000 oC for four hours. Finally, the obtained material was ground with mortar and pestle into a homogeneous black powder. Flow chart in the Figure 9 shows the stepwise procedure for the preparation of LCM.

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Figure 9: Flow chart for synthesis of LCM material.

4.1.3 Preparation of Perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) Material (Paper 4)

Perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), an electrode/semiconductor material, was synthesized through chemical co-precipitation route. All used chemicals and reagents in this experiment are purchased from Sigma-Aldrich. The stoichiometric composition (0.5:0.5 molar) of salts of barium nitrate Ba(NO3)2.H2O and strontium nitrate Sr(NO3)2.H2O were mixed into de-ionized water for the preparation of solution A and kept at 70 oC under constant stirring for 1 hour. In parallel, another solution B was also prepared by mixing the molar ratio 0.8:0.2 of cobalt nitrate hexa-hydrate Co(NO3)2.6H2O and iron nitrate nona-hydrate Fe(NO3)3.9H2O into de-ionized water into a separate beaker. This mixture was also stirred continuously at 70 oC for 30 minutes to yield a homogeneous mixture of the starting precursors. Then, solution B

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was added dropwise to the solution A which was kept under constant vigorous stirring, and the temperature was elevated up to 80 oC. The resulting solution C was continued to further stirring for half an hour so that both solutions were mixed well. In order to complete co-precipitation reaction, an amount of 0.047 moles of the oxalic acid (as precipitating agent) dehydrated solution was prepared by stirring for 10 minutes and then added dropwise to the precursor’s solution. Two chemicals, Ammonium hydroxide (NH4OH) and hydrochloric acid (HCl) were used for pH adjust 2-4 for homogenous precipitation formation. The solution was mixed with constant stirring for two hours at 80 °C.

Figure 10: Flow chart for preparation of perovskite BSCF material.

The final solution was stirred until clear precipitates were observed. After waiting for few hours, the precipitates were filtered and washed in distilled water 3 times and collected finally on a filter paper. Obtained precipitates after washing were dried overnight in a furnace fixed at 120 °C. Then dried precipitates were ground and calcined at 1000 °C continuously for 5 hours to achieve the proper perovskite phase of the material. The detailed scheme for perovskite BSCF synthesis is shown in the flow chart in Figure 10 given above.

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4.2 Electrolyte/ionic materials synthesis for LTSOFC/EFFC

In this section, preparation of electrolyte and ionic materials for use of SOFC and EFFC is outlined.

4.2.1 Preparation of samarium doped ceria Sm0.2Ce0.8O2-δ (SDC) (Paper 2)

Samarium doped ceria Sm0.2Ce0.8O2-δ (SDC) was prepared through simple co-precipitation method using (NH4)2CO3 as the precipitant. The doping ratio of the samarium (Sm) into the cerium (Ce) was used as 20%: 80%. A 0.008 mole of cerium nitrate hexahydrate Ce(NO3)3. 6 H2O (purchased from Sigma Aldrich, USA) was taken and this chemical was mixed into 0.5 L de-ionized water and magnetic stirred (700 rev/min) for half an hour with a heating temperature at 70 oC. A 0.002 mole of samarium nitrate hexahydrate Sm(NO3)3.6H2O (purchased from Sigma Aldrich, USA) was immersed into the prepared cerium nitrate solution followed by same stirring speed for more than 30 minutes to obtain a mixture of samarium and cerium nitrate solution.

Figure 11: Flow chart for synthesis of SDC material.

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Twice the amount of (NH4)2CO3 as compared to total amount of the original chemical constituents (molar basis) was dissolved into another beaker (0.5 M) and stirred till a transparent solution obtained. Then, the samarium and cerium nitrates mixed solution was added dropwise into (NH4)2CO3 transparent solution continuing fast stirring. This process was followed for an hour until clean precipitates formed and was then kept at rest for two hours before filtration. The filtered and washed precipitates were collected on a filter paper and dried overnight at 110 oC in a heating furnace in air atmosphere. Finally, the dried precipitates were annealed at 800 oC for 4 hours to obtain well crystallized SDC phase. Complete flow chart for the preparation of SDC is given in Figure 11.

4.3 Semiconductor-ionic materials for EFFC Semiconductor-ionic materials (SIMs) are composed of a stoichiometric composition between the two types of semiconductor and ionic materials, with the objective of balancing electrical and ionic conductivities. The porosity of the SIM should be low in order to ensure that no gas transport occurs across the device to avoid the chemical short-circuiting through the device. Electronic short-circuiting is eliminated by establishing a Schottky junction. Details on how these aspects are achieved are contained in the sections below. Typically, semiconductor-ionic materials are developed by mixing semiconductor and ionic materials through a nanocomposite approach. Here, some examples are given how SIMs were prepared and then fabricated EFFCs. As presented in Paper 4, SIM with a composite 60SDC (60% of the total mixture) and 40BSCF (40% self-prepared BSCF of the total mixture) with a homogeneous mixture, were mixed together through nanocomposite method using mortar and pestle following the sintering at suitable temperatures between 600-700 oC in order to form a good particle distribution of two phases and stable heterostructure architectures between the semiconductor and ionic phases, and at the same time avoiding any chemical reactions between the two phases of the materials. Using the prepared SIM, EFFC was fabricated with a thickness between 0.8 mm to 1.0 mm. In another experiment, with the same ratio, commercially purchased BSCF and SDC were mixed homogeneously (Paper 7) through the same method described as above to prepare the SIM using the commercial BSCF semiconductor then fabricated the EFFC. Similarly, another SIM of LCM-SDC was prepared with a weight ratio 40:60 (Paper 2). The mixture of this composition was homogeneously prepared through the same method as above then fabricated the EFFC. Using another composition, perovskite La0.1SrxCa0.9-xMnO3-δ (LSCM) material was mixed with ionic conductor Sm0.2Ce0.8O1.9 (SDC) in 2:3 through the same method above to get the SIM for further use in EFFC (Paper 3). All above mentioned EFFCs were fabricated using the corresponding SIMs sandwiched between two Ni-foams coated with NCAL (Ni0.8Co0.15Al0.05LiO2-δ, a commercial product) material.

4.4 Components and fuel cell fabrication

Semiconductor materials Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), novel Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF), La0.1Ca0.9MnO3 (LCM) etc. and the ionic conductor samarium (Sm3+) doped ceria (SDC) were developed to prepare several types of the semiconductor-ionic

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materials (SIMs), i.e. BSCF-SDC, BCCF-SDC, LCM-SDC etc. which were sandwiched between two layers of semiconducting material, Ni0.8Co0.15Al0.05LiO2 (NCAL) coated on nickel foam thin layers in EFFC constructions. In this way, new concept of fuel conversion devices have been explored which operates same way to a fuel cell using a purely ion conducting electrolyte added by electron-blocking layers to prevent electronic short-circuiting. Such materials and devices have been recognized based on Schottky junction principles as new generation EFFC fuel-to-electricity conversion technologies resulting in a more simplified system with higher performances. This strategy of EFFC has a great potential improving both fundamental research and applications as well as advanced technologies to overcome the current inherent fuel cell challenges requiring more theoretical studies, and deep scientific understanding. Hence, more fundamental studies are needed on this research in future. The synthesized BSCF powder and commercially available SDC/NSDC were mixed in a weight ratio of 50:50 through solid state reaction through mortar and pestle and ground homogeneously then sintered at 700 °C for 1 hour to yield a nanocomposite cathode for SOFC use. The same cathode component was found suitable for LTSOFCs by simply adjusting its composition in a weight ratio of BSCF:SDC 40:60; it can also be used as semiconductor-ionic material for EFFCs. In case of EFFCs, the SIM of BSCF:SDC was sandwiched between the two Ni-foams coated with NCAL and compacted through simple press machine. In some devices, Ni-foam coated with NCAL was used on hydrogen side while silver paste was used on air side as current collectors. In order to investigate the oxygen reduction reaction (ORR) mechanism in LCM on SDC electrolyte, the anodes LCM-SDC (50:50 volume ratio) and Ni0.8Co0.15Al0.05Li-oxide (NCAL)-SDC (50:50 volume ratio) in two different cases, one conventional anode supported/electrolyte/ cathode fuel cells were fabricated using SDC as the electrolyte separator layer while LCM-SDC (50:50 volume ratio) in both of the cases used as the cathode component. On both sides of the fuel cell structure, two Ni-foams coated with NCAL semiconductor material were compacted as current collector parts and this material also enhanced the electrode activity. Interesting to note that all the cathode components prepared for traditional electrolyte (here SDC) based LTSOFCs can be directly used or slightly modified for the composition between the two constituent materials for EFFCs based on these cathode components’ common semiconductor-ionic properties/natures. Anode supported cells with the configurations (i) LCM-SDC/SDC/LCM-SDC (ii) NCAL-SDC/SDC/LCM-SDC were carefully fabricated. Both SOFC and EFFC devices were prepared through a dry-pressing method, i.e. fuel cell devices are fabricated by using a press machine in a stainless steel die (with diameter 13 mm) under a pressure of 200 MPa. Active area of the device was measured 0.64 cm2. Specifically for EFFCs fabrication, by removal of SDC electrolyte-layer replaced by SIM materials sandwiched between the two layers of NCAL coated on Ni-foams. Most of the EFFC fabrications, SIMs were placed between the two Ni-Foams coated with NCAL and compacted through press machine. The fabricated fuel cell pellets were sintered at 550 oC for an hour to improve the mechanical strength and material activeness of the device. During fuel cell testing, in order to measure I-V characteristics, the anode was exposed to hydrogen gas for 5 minutes to activate the fuel cells and the cathode was fed by air and fuel cell reaction

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was completed and reached an open circuit voltage (OCV) of 1.0 V for case (i) LTSOFC while up to 1.15 V for case (ii) EFFC operated below 600 oC. Results and discussion are given in the relevant sections of this thesis.

4.5 Characterization Methods

Materials characterization has been conducted in order to study the suitability and potential of prepared materials to be used in energy conversion devices. Powder X-ray diffraction (XRD) of the prepared materials was recorded through Bruker AXS D8 advanced X-ray diffractometer (Germany, Bruker Corporation), and operated with a source Cu Kα (λ= 1.54060 Å). Microstructure and the morphologies were analysed using JSM7100F field emission scanning electron microscope (FESEM, Japan) operating at 15 kV coupled with the energy dispersive X-ray spectrometer (EDS) analyzer (EDX-720, Oxford). Thermogravimetric analysis (TGA) of the sample was carried out through Mettler Toledo TGA/SDTA 851e (Greifense, Switzerland) and measurements were performed between 30 ºC to 1100 ºC with the heating rate of 10 ºC min-1 taken into a 70 µL alumina crucible and provided a continuous air flow (50 ml min-1). Fourier transform infrared spectroscopy (FTIR) analysis has been performed using Mid-infrared Perkin-Elmer Spectrum 2000 FTIR spectrometer (Waltham, MA, USA) equipped with the ATR system, Spectra MKII Golden Gate (Creekstone Ridge, GA, USA). Measurement region from 4000 to 600 cm-1 at a resolution of 4 cm-1 with 16 scans was recorded. A software with Spectrum (Molecular Spectroscopy) version 3.02.01 (Perkin Elmer Instrument) has been employed to process the spectra. The electrical conductivities of the materials were measured by DC 4 Probe method (KD2531 digital micro-ohm meter). Similarly, AC impedance analyses were performed using EIS instrument (CHI660B electrochemical workstation) and fuel cell testing for I-V and I-P data an automatic computerized measurement system (ITECH8511) was used. Pellets for conductivity measurement were compacted by uniaxial powder pressing system to get samples of typical size (with active area 0.64 cm2 and 2 mm or so thickness). The pellets used for conductivity measurement were compacted using a pressure range of 200-300 kg/cm2 with specific amounts of materials according to thickness requirements and then heat treated at 600-700 °C for 1 h. More details are given in the corresponding papers. Finally, silver paste was coated on pellets both sides as the current collector in case of electrical conductivity measurements. Different types of fuel cell devices were fabricated based on these advanced materials to test the device performance. With practical approach, devices are operated at low temperatures at ≤ 600 °C to investigate if these materials and innovative technology EFFC are useful for energy applications. Conventional SOFCs were also fabricated with most of the developed materials and both devices were operated at low temperatures.

4.6 Fuel Cell Testing Facility

Electrochemical performance of the fuel cells were tested by evaluating the cell voltage vs current density (V-I) and power density vs current density (P-I) characteristics. Values of cell voltage and current density were measured through a computerized testing tool IT7000. For this purpose, fuel cells were mounted in a stainless steel sample

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holder equipped with a pair of pipes for (inlet and outlet) air flow and with another pair of pipes for (inlet and outlet) fuel flow. The ready sample holder with fuel cell was fixed into a horizontal tubular furnace and temperature was maintained in the required range

of 500-600 ⁰C for different cells. Pure hydrogen and fresh air flows at anode and cathode were provided to the cells as fuel and oxidant (oxygen) respectively with the same flow rate. Cell voltage and current values were measured through programmable electronic load (ITECH8511, purchased from ITECH Electrical Co. Ltd.). Mass flow controllers (MFC, F-201CV) were fixed to regulate the fuel and oxidant flows. While recording the data, scanning rate was adjusted 0.05-0.1 A/s. The value of power density was determined by P = V × I. From P-I characteristics, the peak value of the parabolic curve was found as the maximum output power (measured in mW/cm2), similarly, maximum value of voltage with zero value of current was found as OCV. Schematic of complete fuel cell testing setup is demonstrated in Figure 12.

Figure 12: Fuel cell testing setup.

In addition, several factors contribute to uncertainties in the measured results. One of the uncertainties is the inaccuracy of the initially used gas flow meters (ShoRate TM 1355, shown in Appendix* Figure A-1), later upgraded with mass flow controllers (MFC, F-201CV) for monitoring hydrogen and airflow rates.

* The appendix contains a brief description of the gas measurement upgrade along with discussion showing the validity of the obtained results.

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According to supplier data, each of the F-201CV units capable of an accuracy of ±2 percentage. Second uncertainty is relating to the operational temperature measured using thermocouple and the actual temperature of the fuel cell inside the testing furnace. Practically, there is a little distance between the thermocouple and exact location of the fuel cell. Thermocouple fixed in the furnace carries an uncertainty of ±5 percentage. Another uncertainty is related to the preparation errors while using the way of material synthesis and device fabrication in case of using the manual steps which may lead to certain inconsistencies of the samples, e.g. microstructure of the materials and the density of the cell that can slightly influence the ionic conductivity in the device affecting its performance. Several fuel cells from each material fabricated and repeatedly tested to obtain the consistent and reproducible results for reducing the uncertainties.

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5 RESULTS AND DISCUSSION

This chapter highlights key results contained in the appended papers, and includes a discussion of the main findings.

5.1 Conventional SOFC using BCCF as Cathode Material (Paper 1)

Perovskite materials with ABO3 structural formula as semiconductor materials are considered as the promising potential electrode candidate for low temperature solid oxide fuel cells. However, the commercially available perovskite oxides are very expensive. Novel Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF) material was prepared with an intention to replace strontium (Sr) with calcium (Ca) from commercially available BaSrCoFe oxide in order to make it cheaper. BCCF novel material is analyzed and used as an electrode in the conventional SOFC. The phase structure, morphology and microstructure of BCCF are studied using XRD and SEM characterization techniques, and the weight loss vs temperature plot is determined using TGA. Electrical conductivity is measured by DC 4 Probe method. An electrolyte, samarium doped ceria (SDC) is also synthesized to fabricate a conventional three component SOFC device.

5.1.1 Structural Analysis and Morphology

Figure 13 shows XRD pattern of the as-synthesized BCCF material. By comparing this pattern with the perovskite material peaks, we can find that perovskite peaks dominate the pattern.

Figure 13: XRD pattern of BCCF. (Paper 1)

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In addition, some traces of cobalt oxide and calcium oxide also exist besides the perovskite phase. Before the experiment, it was expected that this material would be perovskite, but the presence of extra peaks indicated that the as-prepared BCCF was actually a composite-type material with some individual oxide peaks of the materials. Particles constituents, sizes, and structure were found as follows: barium iron oxide, particle size of 36 nm with cubic structure; calcium oxide, particle size of 42 nm with cubic structure; and cobalt oxide, particle size of 39 nm and tetragonal structure. For perovskite phase, it was assumed that calcium atoms should be doped properly at A-site in ABO3 formula. From the XRD pattern, it has been observed that perovskite peaks present as major components. Presence of some extra peaks means that calcium atoms are doped at A-sites but there could be some absorbance of other impurities at same time. However, additional peaks show presence of some carbonates as mentioned also in FTIR analysis [89]. The analysis shows as-prepared material a composite type which has influenced the original plan. Sintered BCCF material is examined by high resolution scanning electron microscopy SEM with different magnifications to investigate microstructure as shown in Figure 14 (a-c) with same composition while Figure 14 (d) for another composition of BCCF as explained in Paper 1.

Figure 14: SEM images of BCCF37 (a-c) and BCCF91 (d). (Paper 1)

Distribution of particle size in Figure 14 (a-c) varies in a certain range and agglomeration of particles has been widely observed. According to the literature, the porosity level increases with increase in Ca content therefore particles tend towards the agglomeration making the possibility of coarsening in the morphology [90] and therefore leading to larger particles. Particle size varies from 80 nm to 200 nm and are observed in the form of grains. The particles of the composite material have been observed in the shape of clusters due to the nanoparticles agglomeration. Another composition of BCCF in Figure 14 (d) shows its SEM image with bigger particle size

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up to half micron. Morphology of the material shows that particle size varies in a range of 200-500 nm, showing particles are uniformly distributed only in a certain region.

5.1.2 FTIR and TGA Analyses

The FTIR spectra of BCCF is shown in Figure 15 (a). The analysis shows that as-prepared sample is a composite type material. This result strongly suggests the presence of some carbonates in the sample due to CO2 addition. It has been mentioned in TGA analysis in Figure 15 (b) that CO2 molecules are adsorbed by perovskite oxides and are emitted by subsequent heat treatment. However, a few CO2 molecules may be trapped in the material to make CO3 based molecules. Peaks located at positions 692 cm−1 and 857 cm−1, indicate the existence of barium carbonate. Peaks at 712 cm−1 and 872 cm−1 indicate calcium carbonate, which correspond to in-plane bending modes of CO3

−2. The strong peaks at positions 1444 cm−1 are representing barium carbonate and at position 1432 cm−1 show phase of calcium carbonate are revealing the C–O stretching mode of carbonates [89, 91].

Figure 15: (a) FTIR and (b) TGA analyses. (Paper 1)

5.1.3 Conductivity Measurement

Pure Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF) powder was compacted into pellets. The thickness and effective area of BCCF pellet were measured as 2.4 mm and 0.64 cm2 respectively for conductivity measurements by DC 4 probe method. The conductivity results versus temperature (T) were plotted as shown in Figure 16. The graph represents conductivity increasing rapidly with rise in temperature. Nanocomposite characteristics of the resultant powder have shown high electronic conductivity. The conductivity was achieved up to 143 Scm-1 at 550 °C, which is much higher as compared to other compositions of the same material BCCF. As-prepared material has mixed conductivity (both electronic and ionic). Nevertheless, the measured conductivity is referred to as electronic being dominant over ionic conductivity [46]. This indicates that BCCF is very useful for SOFCs in low temperature range of 500-600 °C. It has been observed that BCCF has no material degradation during the conductivity and cell performance measurements.

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Figure 16: Conductivity vs temperature for sample Ba0.3Ca0.7Co0.8Fe0.2O3-δ (BCCF).

5.1.4 BCCF cathode in SOFC Performance

To investigate the BCCF material performance in conventional SOFC, an anode supported 3-layer device using the anode (60%), electrolyte (15%) and cathode (25%) configuration was fabricated with a thin layer of SDC electrolyte sandwiched in between the two electrodes. Hydrogen and air were supplied at the anode made of NiO:NSDC (50:50 by volume) and cathode made of BCCF:NSDC (50:50 by volume). Conventional fuel cell with as-prepared cathode material has shown very good catalytic activity for oxygen reduction, oxide ion transportation and electrical conductivity. Thin layer of SDC electrolyte created a separation between anode and cathode by blocking the conduction of electrons across the electrolyte layer. Composite anode provided catalytic activity to hydrogen oxidation, the SDC electrolyte provided conduction and transportation of oxide ions from cathode to anode side. The ionic conductivity of the SDC electrolyte material allows oxide ions to move through the electrolyte component. A true electrolyte function (i.e. a barrier separating the two electrodes) is present to ensure the redox reactions across both anode and cathode components. Performance of the fuel cell was tested at 550 °C and the results are plotted in Figure 17. It can be seen from the figure that a maximum power density above 300 mWcm-2 is obtained at 550 °C. Thickness of the fuel cell device was adjusted about 1 mm. BCCF, during the cell operation has shown a good catalytic activity which may show even better performance by controlling the size of the particles. In addition, by changing the composition of the mixed anode and cathode and choosing the more appropriate electrolyte, the fuel cell performance can still be improved.

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Figure 17: Fuel cell (3-layers) performance.

5.2 EFFC and LTSOFC Devices using Semiconductor-ionic Materials (Papers 2 & 3)

The BCCF was used as the SOFC cathode in above description, which itself is the semiconductor, so used in new concept semiconductor-ionic materials for EFFCs. The BCCF mixed with ionic SDC to form a type of the semiconductor-ionic composite material, which can be used for EFFC as functional layer material. The phase structure, morphology and microstructure of BCCF are studied using XRD and SEM characterization techniques, and conductivity is measured by electrochemical impedance spectroscopy (EIS) method. An electrolyte, samarium doped ceria (SDC) is also synthesized to fabricate and to use in a conventional three component LTSOFC and EFFC devices. At the end of this section, introduction of semiconductor-ionic materials and strategy for developing next generation fuel cell, is also described. In similar approach, other perovskite oxides La0.1Ca0.9MnO3 (LCM) and La0.1SrxCa0.9-

xMnO3-δ (LSCM) which are semiconductors, have been developed and considered as the potential electrode candidate for low temperature solid oxide fuel cells and as functional semiconductor materials for electrolyte-layer free fuel cell (EFFC) (Paper 2 & Paper 3). LCM material with good electrical conductivity, which was commonly in use for conventional fuel cell was synthesized with an intention to use also in EFFC. Similarly, LSCM with some further modification in LCM was also prepared to use in advanced solid oxide fuel cells (ASOFCs). Manganese based materials are analyzed as an electrode in the conventional SOFC (Paper 2) and as semiconductor part in EFFC (Paper 2 & Paper 3).

5.2.1 Structural Analysis and Microstructure of LCM and SDC-LCM Composite (Paper 2)

Thermal stability of the perovskite oxide LCM and its compatibility with SDC electrolyte was examined through XRD as shown in Figure 18.

0 200 400 600 800 1000

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

IV curve

IP curve

I (mA.cm-2

)

V (

volt

s)

-50

0

50

100

150

200

250

300

350

P (

mW

.cm

-2)

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Figure 18: XRD patterns of Perovskite LCM, 6SDC-4LCM at lower temperatures and

6SDC-4LCM at 750 ⁰C.

The XRD pattern of the as-prepared LCM matches with the perovskite phase whereas pattern of SDC shows ceria fluorite oxide structure. After sintering at 750 oC for 2 hours, composite LCM-SDC maintained individual phases of both LCM and SDC, it shows good chemical compatibility of SDC with LCM. The peaks from composite 6SDC-4LCM show sum of the individual phases of LCM and SDC in the composite pattern of the materials. Microstructure of the material with different magnifications is shown in Figure 19 below.

Figure 19: SEM images of high temperature sintered LCM. (Paper 2)

It is clear that the particle size is in nanometer scale (from few tens of nm to few hundreds of nm) with uniformity of morphology only up to certain region. Particles are agglomerated, and the interfaces of the particles are strongly bonded, but continuing high temperature sintering damages the microstructures as shown by severe particle aggregation and insufficient porosity (Figure 19). It leads to an insufficient LCM reduction during oxygen reduction reaction (ORR) during fuel cell performance using

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it as anode mixed with SDC electrolyte with balanced electronic and ionic conductivities and the gas diffusion capability as limited to TPB for hydrogen oxidation.

5.2.2 ORR Activity of LCM in LTSOFC Configuration

LCM was used in the SDC based LTSOFC. For this purpose, LCM and SDC were mixed in a composition (1:1 by volume) and used as the cathode in the cell for oxygen reduction reaction (ORR) whereas mixture of NCAL and SDC (1:1 by volume) was used as an anode to obtain a hydrogen oxidation reaction (HOR). Similarly, a homogeneous mixture of LCM and SDC with a composition of 60:40 was prepared as semiconductor-ionic material to construct the electrolyte-separator free fuel cell (EFFC). EIS spectra of the SDC based fuel cell and novel EFFC was measured in fuel cell conditions at 550-560 oC as shown in Figure 20. The conductivity increases gradually by increasing the temperature accompanying the EIS semicircles shrinking. So, shrinking of semicircles shows that total polarization resistance (Rp) and the area specific resistance (ASR) of the LCM in the SDC electrolyte are decreased at 560 oC as compared to the value at 550 oC. Rate processes of ORR are further assessed by observing the separate EIS arcs in order to verify the cathode performance.

Figure 20: Electrochemical impedance spectra of 4LCM-6SDC. (Paper 2)

In the past, a number of the cathode materials for conventional SOFCs have been investigated, but only a few are presented with a detailed understanding about the ORR electrode reactions. Total polarization resistance RP in case of ORR consists of the rate determining processes which are reflected from two semi-arcs in EIS plot. High frequency semi-arc, that gives a corresponding resistance value of RH, it is attributed to those processes which are responsible for the charge transfer mechanism involved in the reduction of oxygen molecules at the cathode and the ion transport mechanism within the cathode leading towards the cathode/electrolyte interface. Low frequency resistance of the semi-arc determines the resistance value denoted by RL, belongs to the

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non-charge transfer processes based on molecular gas diffusion process within the electrode pores whereas gas adsorption/dissociation takes place in the cathode region.

5.2.3 Performance of LCM Cathode

LCM has shown its potential role as cathode for low temperature SDC electrolyte based fuel cell and as semiconductor material for EFFC. In both types of devices, NCAL pasted on Ni foam was attached across the cell sides for using it as current collector and also for enhancement of the catalytic activity. LCM mixed with SDC has been tested by two ways:

i) SDC electrolyte-layer was sandwiched between the two electrodes of identical layers of composite LCM-SDC (1:1 by volume) to fabricate a symmetrical fuel cell. In parallel, Ni-foams pasted with NCAL were fixed on both sides of the cell, and compacted together. This configuration has shown a maximum power density up to 300 mW/cm2 at 550 oC.

ii) SDC was sandwiched as an electrolyte-layer between composite NCAL-SDC (as anode) and composite LCM-SDC (as cathode), and then on both sides of the cell, Ni-foams pasted with NCAL were used and pressed to construct the cell. This configuration, showed a maximum power density up to 650 mW/cm2 at 550 oC.

From the performance of above cases as shown in Figure 21, it is clear that LCM shows itself a better cathode material for ORR as compared to HOR. In a third case, EFFC based on composite LCM-SDC (40:60 by weight) core functional layer was fabricated and it reached a peak power density up to 750 mW/cm2 as shown in Figure 21. It is best device performance exhibited out of few fuel cells. By comparing above two cases using the SDC as electrolyte in SOFCs, EFFC shows better performance.

Figure 21: V-I and P-I characteristics of the SOFCs and EFFC at 550 oC.

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In case of EFFC (SLFC), total polarization loss is much less due to removal of the physical barriers between electrode/electrolyte junctions which are major factors responsible for the power losses in conventional technology [6].

5.2.4 Devices Based on Modifications in LCM Material (Paper 3)

By modifying the LCM material with Sr addition, new material LSCM was prepared and used for fuel cell fabrication. LSCM-SDC composite material was prepared and used to construct the cathode layer for low temperature SOFC applications. Such a cathode layer was used as EFFC device. The OCVs of such all devices are obtained above 1.0 V, which is indicating that there is no electronic short-circuiting of the cells internally due to the presence of Schottky junction formed on the H2 contact side due to the reduction of semiconductor material into metal or alloy. Compared to the pure SDC electrolyte-layer based device, EFFCs based on LSCM-SDC semiconductor-ionic material demonstrated higher ionic conductivity because of the high ionic conductivity and carrier concentration at the heterostructure interfaces between LSCM and SDC phases. Optimized weight ratio of LSCM:SDC mixture was 2:3 found, and the corresponding peak power density achieved up to 800 mW cm-2 at 550 oC.

5.3 EFFC Schottky Junction Effect (Papers 4, 5 & 6)

EFFC technologies are developed and optimized by evaluating the Schottky junction mechanisms and energy band alignments in the devices based on semiconductor-ionic materials. Perovskite and nanocomposites (semiconductor-ionic materials) have been designed and developed for EFFC devices. Characterizations of materials are performed using relevant experimental and analytical techniques including XRD and DC 4 probe techniques. Maximum power densities of around 600 mW/cm2 are achieved from the devices at 550 ºC. Moreover, Schottky junction as well as energy band alignments have also been discovered in these fuel cells which provide evidence how to avoid the electronic short-circuiting problem and maintain steady output power from the devices.

5.3.1 Crystal Structure and Microstructure of BSCF (Paper 4)

The crystal structure of synthesized perovskite BSCF material has been examined by X-ray diffraction (XRD) and results are shown in Figure 22. XRD pattern of the as-prepared BSCF is matching with the perovskite peaks (Paper 4). The as-prepared BSCF is a material following structure formula ABO3, where Ba and Sr atoms together make A cation while Co and Fe atoms together establish B cation in this formula. In BSCF, both cations are of almost same size and occupy the octahedral sites showing the overall cubic crystal structure. Crystallite size of the material has been calculated from its XRD pattern using Scherrer equation [92]. Here, peak broadening in XRD curves determines the nanosize and average crystallite size is calculated about 85 nm.

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Figure 22. XRD diffraction pattern of as-prepared BSCF (Paper 4)

Figure 23 (a & b) shows the microstructure and morphology of the as-prepared BSCF material. The micrographs are taken using scanning electron microscope (SEM). Figure 23 (a) was obtained with lower magnification at micron scale (X10,000) but image in Figure 23 (b) was collected with higher magnification at nanometer scale (X35,000) in order to observe the nanofeatures of synthesized sample. The particle size in a range from 50 nm to 200 nm is observed in the microscopic images presented although particles are agglomerated.

Figure 23: (a & b) SEM images of BSCF.

5.3.2 Electrochemical Properties

Figure 24 (a & b) shows spectra from electrochemical impedance spectroscopy (EIS) of conventional SOFC and EFFC under fuel cell conditions at 550 oC.

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Figure 24: EIS measurements for three layer fuel cell (a) and single layer fuel cell (b),

the data points are the measured data and the solid line are the fitting results. The

inset is the equivalent circuit for simulations. The curves (C) give the Bode plots for

the two kinds of fuel cells. (Paper 4)

Results from both fuel cell types illustrate same features, showing a semicircle followed by a tail. A simple equivalent circuit according to the model Rs/Rct(CPE1)/Rmt(CPE2) is also shown in the inset in Figure 24 (a). Fitting impedance results are presented in Table 1. Rct in high frequency region corresponds to an interfacial charge-transfer resistance and Rmt in low frequency region relates to the mass transport resistance, and CPE is a constant phase element which represents a non-ideal capacitor. In Table 1, the relatively low values of Rct (between 0.05378 Ω cm2 and 0.2157 Ω cm2) show that electrode reaction kinetics are fast, and provides evidence that the SDC-BSCF membrane shows high catalytic activity for ORR that results in good fuel cell performance. Bode plot in Figure 24 (c) indicates that main peak, referred to high frequency response, shifting from high ~30000 Hz in case of three layer SOFC to low 24000 Hz in case of single layer cell. In addition, mass transfer resistance in case of single layer fuel cell or EFFC is lower than as compared to the corresponding values in case of conventional electrolyte based SOFC, which is due to high oxygen ion migration ability in one layer device/EFFC. CPE1 of the cells are in range of 10-5 F cm-2, hence it represents that charge transfer process is a rate-determining step.

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Table 1 EIS (in FC conditions) parameters for equivalent circuits

Cell Type Rs (ohm

cm2) Rct (ohm cm2)

CPE1 (F cm-2)

Rmt (ohm cm2)

CPE2 (F cm-2)

1 0.1356 0.05378 5.651E-

5 ------- 1.294

2 0.2023 0.2157 1.593E-

5 1.109 0.4422

5.3.3 Electrical Conductivity Measurements

The as-prepared perovskite BSCF material is a mixed conductor containing ionic and electronic conductivities. DC-conductivity is measured in an air atmosphere using DC 4 probe instrument and results are plotted in Figure 25. Conductivity results are compared with conductivity of commercially purchased BSCF.

Figure 25: Temperature dependence of conductivity for lab made and commercial

BSCF.

Conductivity demonstrates a thermal activation process and increases with temperature. A maximum conductivity of as-prepared BSCF is obtained more than 300 S/cm at 550 °C whereas conductivity of commercial BSCF is much less than lab made material. High conductivity of the material confirms it a potential candidate as electrode for LTSOFC.

5.3.4 Fuel Cell Performance

A conventional SOFC using SDC as an electrolyte with cell configuration NCAL+SDC/SDC/SDC+BSCF, was tested and compared with the EFFC consisted of 60SDC-40BSCF homogeneous mixture of semiconductor-ionic material (‘‘three in one’’). Cell voltage vs current density (V-I) results and power density vs current density (P-I) characteristics of both of the devices are plotted in Figure 26 (Paper 4). A peak

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power density of 425 mW/cm2 is obtained at 550 oC for conventional SOFC with composite electrodes.

0 500 1000 1500 2000 2500

0,0

0,2

0,4

0,6

0,8

1,0

Current density (mA/cm2)

Voltage (

V)

SOFC

EFFC

0

200

400

600

Pow

er d

ensity

(mW

/cm

2)

Figure 26: Fuel cell performance for 3-layer FC and EFFC at 550 ⁰C. (Paper 4)

In case of electrolyte-layer free fuel cell (EFFC) a maximum power density up to 655 mW/cm2 is achieved at 550 oC which is also shown in Figure 26. EFFC displays higher OCV and reaches up to 1.06 V at 550 oC as compared to 0.94 V of SDC electrolyte FC.

5.3.5 Schottky Junction Effect in EFFC

A SEM cross sectional view of EFFC based on BSCF-SDC semiconductor-ionic material after fuel cell performance is shown in Figure 27. It is quite clear on air side that reduction of oxygen is limited only to NCAL and is in contact with composite BSCF-SDC zone and could not enter into BSCF-SDC core. It shows that the stability of BSCF as a cathode is not affected by ORR. On the other hand, NCAL layer can be seen acting also as an anode and on exposing to hydrogen gas it is reduced partially or completely into a metal or LiNiCo alloy during FC operation. Hydrogen is oxidized and converted into protons by emitting electrons, and NCAL is reduced into LiNiCo, forming a Schottky junction at the contact interface between the metal/alloy and semiconductor BSCF-SDC (Paper 4 & Paper 5). Across this junction, Schottky barrier is induced which resists the electron conduction from hydrogen to air side avoiding short-circuiting problem internally. However, the blocked electrons can conduct through external circuit provided giving useful power. Furthermore, on the air side, oxygen reduction takes place by gaining electrons and hence the NCAL on air side maintains its original dense shape. On the anode side, NCAL is porous due to H2 penetration and its oxidation and is therefore well merged into BSCF-SDC layer. SEM micrograph of EFFC after cell performance confirms that device is mechanically and thermally stable and is correlated to the fuel cell measurements where no material degradation is observed. It is also considered that the Schottky junction is established when alignment of energy bands of contacting metal (alloy) and semiconductor match each other to accommodate a built in potential which helps to block the electronic short-circuiting through the devices (Paper 4 & Paper 5).

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Figure 27: Schematic of post SEM micrograph of electrolyte-layer free fuel cell

(EFFC).

5.3.6 Schottky Junction Effect in EFFC (Paper 6)

In Paper 6, it is observed experimentally that Schottky junction is established. On the H2 contact side, NCAL is reduced into Ni metal or NCL alloy forming a junction with semiconductor material. A built-in potential (Vbi) is created across the junction due to difference of energy levels between metal and semiconductor phases, which blocks the electron conduction internally from hydrogen contact side to air (oxygen) contact side. A schematic diagram of Schottky junction formation is shown in Figure 28. The phenomenon shown in Figure 28 is similar to the ones also mentioned in Paper 4 and Paper 5. There was no degradation in the OCV and fuel cell performance, which confirms that no short-circuiting in the device. However, such kind of experiment was the main focus which could prove that a junction is established. Therefore, experiment for diode effect measurement was performed in two ways and the results are shown in Figure 29 (a & b). In air atmosphere, ohmic behavior is observed from the fuel cell operation under biased voltage as shown in plot (a), it confirms the absence of any diode. It means that NCAL material has no change while exposed to air.

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Figure 28: Structure of fuel cell with Schottky junction formation. (Paper 6)

Figure 29: Diode effect measurement (a) in air (b) in FC conditions. (Paper 6)

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In FC conditions, non-ohmic behavior is observed showing some rectification effect as shown in plot (b), which could confirm the presence of a diode. It shows that NCAL when exposed to hydrogen is reduced into metal or alloy, so having contact with semiconductor thus they form a diode.

5.4 Comparison of Theoretical and Experimental Results (Paper 7)

An updated numerical model is applied in order to study the EFFC device which introduces the modified parameters to describe the polarizations. This model introduces some modifications in the existing equations of the traditional fuel cell model and are used to plot cell voltage vs current density (V-I) and power density vs current density (P-I) characteristics. The simulated V-I and P-I curves have been compared with experimental curves, both types of curves show good consistency (Paper 7). The work is the first effort to integrate the theoretical and experimental results into single approach for a comprehensive understanding of novel EFFC or semiconductor-ionic fuel cell mechanisms.

5.4.1 Comparison of Modelling and Experimental approaches

Figure 30 demonstrates the results from alternating current (AC) impedance measurements for EFFC based on BSCF-SDC operated in FC conditions in certain temperature range.

Figure 30: Electrochemical impedance measurement of EFFC at different

temperatures and equivalent circuit. (Paper 7)

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Curves represent similar shapes with two ‘‘semi-circles’’ following a “tail”. Small intersections in the curves within the regions of high frequency, reflect high catalytic activity of the device for hydrogen and oxygen corresponding electrochemical reactions. The equivalent circuit model is also used to fit the measured data, and is shown in the inset of Figure 30. In the circuit, Ro represents the ohmic resistance which is contributed by oxygen ions and electrons. Similarly, RctQ and RmtQ indicate charge transfer and the mass transfer processes, respectively. Methods devised by Kellogg et al. [82] are used to determine ohmic resistance and charge transfer resistivity. Ohmic resistance (Ro) and charge transfer resistivity (Rct) of the electrode reactions, calculated from Nyquist plot which is achieved from EIS Spectra are found 0.43 ohm cm2 and 0.31 ohm cm2, respectively. And, the values of Ro and Rct are used in (Paper 7) in equations (3.12), (3.13) and (3.15) for determining the voltage losses [93-96].

Based on experimental and theoretical data (Paper 7), V-I and P-I results are plotted and shown in Figure 31 (a & b). The open circuit voltage (OCV) 1.051 V and peak power density 645.5 mWcm-2 achieved from the model reaching the current density up to 1020 mA cm-2 at 560 °C under 1 bar pressure. Corresponding results from the experiment for OCV and power density are 1.04 V and 640.4 mWcm-2, respectively, with a current density up to 1043 mAcm-2 at temperature 560 °C and 1 bar pressure, respectively. It is evident that the modelled curve is in agreement with that of experimental curve.

Figure 31: Comparison of theoretical and experimental V-I and P-I characteristics.

(Paper 7)

Figure 32 (a) demonstrates the effect when only values of n are changed in equation (3.15). Similarly, Figure 32 (b) indicates the effect when only values of m are changed in equation (3.15). Obviously, value of parameter m influences the slope of curve in linear region and high current density region at departure from linearity. Value of n shows significant effect on the slope of curve in high current density region which is observed by the sharp bend however it shows small effect on slope of the curve in linear region. It changes slope of the curve in mass-transport region mainly. Therefore, it significantly influences the limiting current density. Decreasing the value of n, value of mass-transport over-potential decreases also. Thus, in order to minimize the mass-transport

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over-potential, the factors should be optimized which mitigate value of n optimizing electrodes structure, increasing the O2 concentration or increasing the temperature. Parameters m and n influence ohmic-loss and concentration-loss regions.

Figure 32: V-I characteristics with fixing (a) m and (b) n values as constant. (Paper 7)

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6 CONCLUSIONS AND FUTURE WORK

6.1 Conclusions

In this study, many types of functional perovskite, composite, nanocomposite and semiconductor-ionic materials for LTSOFCs/ASOFCs and EFFCs are developed. Working principles and device mechanisms are established to understand the EFFC science. Following conclusions are drawn from the achievements during this PhD thesis work.

1. A novel BCCF composite material with high electrical conductivity 143 S/cm at 550 °C has been synthesized successfully through sol-gel method. BCCF material has been identified as a promising candidate being cathode for LTSOFCs and as a semiconductor material for electrolyte-layer free fuel cell (EFFC) working below 600 °C. BCCF using as cathode in SOFC, maximum power density of 325 mWcm-2 has been achieved at 550 °C. Since prepared BCCF is a semiconductor material which is mixed with the ionic SDC used for SOFC cathode; which can be, in the same time, used for the semiconductor-ionic material to replace the electrolyte in EFFC with a device performance up to 600 mWcm-2. In general, the conventional SOFC cathode component can be used as the semiconductor-ionic membrane to replace the electrolyte layer for EFFCs.

2. Perovskite oxide BSCF material with an enhanced electrical conductivity of

more than 300 S/cm at 550 °C has been prepared through co-precipitation technique. Characterization techniques XRD, SEM etc. show that the material contains nanoparticles. This material is also investigated equally important for LTSOFC and EFFC applications. Using this material as cathode component in LTSOFC, peak power density of about 400 mWcm-2 while preparing semiconductor-ionic material by mixing BSCF and SDC and using in EFFC fabrication, more than 600 mWcm-2 have been obtained below 600 °C.

3. Another perovskite material LCM was synthesized by wet chemical method.

Preliminary results demonstrate, La0.1Ca0.9MnO3 may serve as better cathode for LTSOFCs operating at low temperatures 550 °C. Additionally, peak power density up to 650 mW/cm2 is achieved from ceria-based three layers fuel cells using LCM in semiconductor-ionic material as the cathode material and reached up to 750 mW/cm2 for EFFC mixing LCM with ionic conductor in an optimum composition. This clarifies that LCM may be promising material if used effectively with SDC ionic material in SOFCs and in semiconductor-ionic material LCM-SDC as a core functional layer in EFFC and excellent power outputs can be achieved.

4. A homogenous nanocomposite electrolyte SDC with interfacial super-ionic

conduction and an enhanced fuel cell performance has been synthesized by two step co-precipitation method. SDC was used with other prepared materials in

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different combinations and achieved excellent fuel cell performances for both LTSOFC and especially for EFFC.

5. The semiconductor-ionic fuel cells (SIFCs) based on the functional

semiconductor-ionic materials (SIMs), which is often the conventional SOFC cathode component, are new type of the advanced SOFCs using the SIM membrane to replace the electrolyte-layer to build the fuel cell devices, i.e. SIFCs. Charge separation phenomena, from nanoparticle level to device level, are the key for scientific understanding of the EFFC/SIFC devices.

6. The Schottky junction effect and energy band alignment for charge separation

have been found to act from particle level to device level for blocking electron short-circuiting, and also for the promotion of the ion transport mechanism supported due to built-in field. Thus, fuel-to-electricity conversion can be realized through different ways from conventional SOFCs and EFFCs devices where semiconductor-ionic heterostrctures, junction and charge separation play key roles.

7. Electrolyte-layer free fuel cell (EFFC) has been developed by preparing a

semiconductor-ionic material based on commercial perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and samarium doped ceria (SDC). The FC device obtains peak power density 640.4 mWcm-2 and with an open circuit voltage (OCV) 1.04 V at low temperature of 560 °C providing hydrogen as a fuel and air as an oxidant, respectively. A simulation model has also been applied by making some modifications in conventional fuel cell model and updated model for EFFC adopted from existing model in order to analyze the EFFC and it introduces the simplified parameters to show the polarization and ohmic losses. This model introduces some modifications in the existing equations for conventional fuel cell used to plot the V-I and P-I characteristics. Theoretical curves are compared with the experimental curves, and are found both results consistent with each other. The work is the first effort to integrate theoretical and experimental results for comprehensive understanding of EFFC processes and therefore can help to determine the optimum power of the EFFC device.

8. The Schottky junction based EFFC device is a simple fuel cell technology, which

is constructed by one cathode component of the SOFC. The electrons are blocked on the anode surface by a metal catalyst layer to form a Schottky junction with the semiconductor (like used perovskite oxide). Thus, such devices can reach standard OCVs as three-layer SOFC technology, and outputs were significantly enhanced. This may be understood because all EFFC devices have no electrolyte-anode and cathode two interfaces which can cause major polarization loss for power output in conventional SOFC devices, but these have been removed in EFFCs.

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6.2 Future Work

Research work presented in this PhD thesis has examined the validity of perovskite, composite, nanocomposite and semiconductor-ionic materials for LTSOFC applications but meanwhile, has raised few questions. For example, influences of the impurities and microstructural imperfections on the performance of the devices, incomplete task related to the measurement of ion transport mechanism through EFFC, long-term stability measurement of the materials and the EFFC devices. In terms of future research of semiconductor-ionic materials for LTSOFCs and EFFCs, varieties of research lines need to be considered:

Super ionic conductions (SICs) through interfacial regions of semiconductor-ionic materials have been proposed to understand the enhanced ionic conductivity of the semiconductor-ionic composites. In order to investigate the exact conduction behavior, transmission electron microscopy (TEM), ion transport through EFFC single component using ion transport measurement instrument, analyses of the semiconductor-ionic composites focusing on the interfaces between the semiconductor and ion conductor particles are required for detailed understanding of the charge separation and ion conducting mechanisms in EFFCs.

Semiconductor-ionic materials (SIMs) will be used extensively in SOFCs as electrolytes. For this purpose, compatible electrodes will be investigated to match these semiconductor-ionic composites considering the chemical compatibilities and thermal expansion coefficients (TECs) among the components. Moreover, SIMs will be developed more carefully to ensure their minimal porosity to avoid chemical short-circuiting. Similarly, standard and worthy procedures will be adopted and compatible electrodes with enough porosity will be designed and developed.

Scaling-up of the materials development and fabrication of the LTSOFC and EFFC devices will be focused. In this context, large sized engineering cells (6 cm x 6 cm or 10 cm x 10 cm) will be fabricated with good mechanical strength using spark plasma sintering (SPS) or hot-press machines. Log-term durability of the devices will be oriented based on the stability tests for several hundreds to thousands hours. Finally, efforts for stack construction leading to prototype development will be made for fuel cells practical applications.

More theoretical studies will be conducted for deep understanding of the EFFC. Electrolyte based fuel cell faces major losses because of physically existence of the barriers between the electrolyte and the electrodes. Whereas, in case of EFFC, there are physically no such barriers, so in principle, I-V characteristics should indicate from this perspective which is not shown by the current modified model and updated model is adopted from the existing model for conventional fuel cell as the first effort. Therefore, it needs further work to upgrade this model in future.

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8 Appendix

Here is an overview of old gas flow controllers as shown in Figure A-1. This setup was used in initial experiments and gas flows were fixed manually. Later on, the setup was upgraded with new gas flow controllers as shown in Figure 12.

Figure A-1. Old gas flow controller for hydrogen used for initial experiments.

In repeating experiments, the following fuel cell configurations as representative samples have been fabricated and tested through installed new mass flow controllers:

i) Three layer fuel cell with (BCCF.SDC) cathode, SDC as electrolyte and

anode (NCAL.SDC). Ni-foam pasted with NCAL on anode side while Ag

pasted on cathode side.

ii) 40BSCF.60SDC-EFFC-Both sides Ni foam pasted with NCAL

Fuel cell testing results have been found in agreement with the results published in papers. Sample results are shown in Figure A-2.

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0 300 600 900 1200 15000,0

0,2

0,4

0,6

0,8

1,0

1,2

3 Layer FC with BSCF cathode-Ni foam NCAL-Ag

40BSCF-60SDC EFFC-NCAL Ni foam

Pow

er

density(m

W c

m-2)

Current density (mA cm-2)

Voltage(V

)

0

100

200

300

400

500

600

Figure A-2. Fuel cell performances with new gas flow controllers.

Three layer fuel cell was fabricated with (BCCF.SDC) cathode, SDC as electrolyte and anode (NCAL.SDC). Ni-foam pasted with NCAL on anode side and Ag-paste on cathode side were applied as current collectors. EFFC was fabricated with both sides Ni-foam pasted with NCAL, 40BSCF.60SDC (as semiconductor-ionic material) and maximum power density of 514 mW/cm2 was achieved in case of EFFC. Hydrogen flow (on new gas flow meter = 130 mln/min, while on old meter it shows = 100 mm of Hg) was provided on anode side. In publication results, hydrogen flow from old meter was showing 130 mm of Hg. Therefore, according to former set up, following calculations can give comparison of the results. 100 mm-Hg gave power density = 514 mW/cm2 1 ……………………………... = 514/100 = 5.14 mW/cm2 130 mm-Hg gives power density = 5.14 x 130 = 668 mW/cm2 This result is consistent with the result in the publication. In case of electrolyte-layer free fuel cell (EFFC) in section 5.3.4, a peak power density of 655 mW/cm2 is reported

at 550 ⁰C in Figure 26 using 130 mm of Hg as recorded by the formerly employed gas flow meter.