Functional nanocomposites for advanced fuel cell...

Division of Heat and Power TechnologyDepartment of Energy Technology

School of Industrial Engineering and ManagementRoyal Institute of Technology, KTH, Sweden

Doctoral Thesis

Stockholm 2011

Rizwan Raza

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Functional nanocomposites for advanced fuel cell

technology and polygeneration

Postal address Department of Energy Technology

School of Industrial Engineering and Management

Royal Institute of Technology, KTH

SE 100 44, Stockholm, Sweden

Supervisor Dr. Bin Zhu

Email: [email protected]

Prof. Torsten H Fransson

Email: [email protected]

TRITA KRV Report 12/10 ISSN 1100-7990 ISRN KTH/KRV/11/11-SE ISBN 978-91-7501- 191-2

© Rizwan Raza, 2011

Doctoral Thesis – Rizwan Raza Page i

ABSTRACTIn recent decades, the use of fossil fuels has increased exponentially with a corresponding

sharp increase in the pollution of the environment. The need for clean and sustainable technologies for the generation of power with reduced or zero environment impact has become critical. A number of attempts have been made to address this problem; one of the most promising attempts is polygeneration. Polygeneration technology is highly efficient and produces lower emissions than conventional methods of power generation because of the simultaneous generation of useable heat and electrical power from a single source of fuel. The overall efficiency of such systems can be as high as 90%, compared to 30-35% for conventional single-product power plants.

A number of different technologies are available for polygeneration, such as micro gas turbines, sterling engines, solar systems, and fuel cells. Of these, fuel cell systems offer the most promising technology for polygeneration because of their ability to produce electricity and heat at a high efficiency (about 80%) with either low or zero emissions. Various fuel-cell technologies can be used in polygeneration systems. Of these, solid oxide fuel cells (SOFCs) are the most suitable because they offer high system efficiency for the production of electricity and heat (about 90%) coupled with low or zero emissions. Compared to other types of fuel cells, SOFCs have fuel flexibility (direct operation on hydrocarbon fuels, such as biogas, bio-ethanol, bio-methanol, etc.) and produce high-quality heat energy. The development of polygeneration systems using SOFCs has generally followed one of two approaches. The first approach involves the design of a SOFC system that operates at a temperature of 850 oC and uses natural gas as a fuel. The second approach uses low-temperature (generally 400-600 oC) SOFC (LTSOFC) systems with biomass, e.g., syngas or liquid fuels, such as bio-methanol and bio-ethanol. The latter systems have strong potential for use in polygeneration.

High-temperature SOFCs have obvious disadvantages, and challenges remain for lowering the cost to meet commercial interest. The SOFC systems need lower operating temperatures to reduce their overall costs.

This thesis focuses on the development of nanocomposites for advanced fuel-cell technology (NANOCOFC), i.e., the next generation SOFCs, which are low-temperature (400-600 oC), marketable, and affordable SOFCs. In addition, new concepts that pertain to fuel-cell science and technology—NANOCOFC (www.nanocofc.com)—are explored and developed. The content of this thesis is divided into five parts:

In the first part of this thesis (Papers 1-5), the two-phase nanocomposite electrolytes, viz.ceria-salt and ceria-oxide, were prepared and studied using different electrochemical techniques. The microstructure and morphology of the composite electrolytes were characterised using XRD, SEM and TEM, and the thermal analysis was conducted using DSC. An ionic conductivity of 0.1 S/cm was obtained at 300 ºC, which is comparable to that of conventional YSZ operating at 1000 ºC. The maximum output power density was 1000 mW/cm2 at 550 oC. A co-doped ceria-carbonate was also developed to improve the ionic conductivity, morphology, and performanceof the electrolyte.

In the second part of this thesis (Papers 7-9), composite electrodes that contained less or nonickel (Ni) were developed for a low-temperature SOFC. All of the elements were highly homogenously distributed in the composite electrode, which resulted in high catalytic activity

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and good ASOFC performance. The substitution of Ni by Zn in these electrodes could reduce their cost by a factor of approximately 25.

In the third part of this thesis (Papers 10), an advanced multi-fuelled solid-oxide fuel cell (ASOFC) with functional nanocomposites (electrolytes and electrodes) was developed. Several different types of fuel, such as gaseous (hydrogen and biogas) and liquid fuels (bio-ethanol and bio-methanol), were tested. Maximum power densities of 1000, 300, 600, and 550 mW/cm2 were achieved with hydrogen, bio-gas, bio-methanol, and bio-ethanol, respectively, in the ASOFC. Electrical and total efficiencies of 54% and 80%, respectively, were achieved when the single cell was used with hydrogen.

The fourth part of this thesis (Papers 11) concerns the design of a 5 kW ASOFC system based on the demonstrated advanced SOFC technology. A polygeneration system based on alow-temperature planar SOFC was then designed and simulated. The efficiency of the overall system was approximately 80%.

The fifth part of this thesis (Paper 12) describes a single-layer multi-fuelled electrolyte-free fuel cell that is a revolutionary innovation in renewable-energy sources. Conventional fuel cells generate electricity by ion transport through the electrolyte. However, this new device works without an electrolyte, and all of the processes occur at particle surfaces in the material. Based ona theoretical calculation, an additional 18% enhancement of the fuel cell’s efficiency will be achieved using this new technology compared to the conventional technologies.

Our developed ASOFC systems with functional nanocomposites offer significant advantages in reducing the operational and capital costs for the production of power and heat by using different fuels based on the fuel-cell technology. ASOFC systems can be used for polygeneration with renewable fuels (i.e., biomass fuels) at high efficiency as a sustainable solution to energy generation in our society. The results have been achieved for this thesis work has demonstrated an advanced fuel cell technology.

Key Words: Polygeneration, advanced fuel cell, functional materials, ceria-carbonate nanocomposites, multi-fuelled, electrolyte free fuel cell

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SAMMANFATTNINGUnder de senaste decennierna har användningen av fossila bränslen ökat exponentiellt med en

motsvarande kraftig ökning av förorening av miljön. Behovet av ren och hållbar teknik för elproduktion med minskad eller ingen miljöpåverkan har blivit kritisk. Ett antal försök har gjorts att lösa detta problem och ett av de mest lovande försöken är polygenerering. Polygenereringsteknik är mycket effektiv och ger lägre utsläpp än konventionella metoder för elproduktion på grund av samtidig produktion av användbar värme och elkraft från en enda källa till bränsle. Den totala effektiviteten i sådana system kan vara så hög som 90 %, jämfört med 30-35% för konventionella singelproduktkraftverk.

Ett antal olika tekniker finns tillgängliga för polygenerering, såsom mikrogasturbiner, stirlingmotorer, solsystem, och bränsleceller. Av dessa erbjuder bränslecellssystem den mest lovande tekniken för polygenerering på grund av deras förmåga att producera el och värme med hög verkningsgrad (ca 90%) med antingen låga eller inga utsläpp alls. Olika bränslecellstekniker kan användas i polygenererings system. Av dessa är solid-oxid bränsleceller (SOFCs) de mest lämpade eftersom de ger hög systemeffektivitet för produktion av el och värme (ca 90%) i kombination med låga eller inga utsläpp. Jämfört med andra typer av bränsleceller har SOFCs hög bränsleflexibilitet (direkt drift på kolvätebränslen, som biogas, bioetanol, bio-metanol, etc) och producerar högkvalitativ värmeenergi. Utvecklingen av polygenererings system som använder SOFCs har i allmänhet följt en av två olika vägar. Den första handlar om utformningen av ett SOFC-system som arbetar vid en drifttemperatur på 850 °C och använder naturgas som bränsle. Den andra metoden använder sig av lågtemperaturdrift (i allmänhet 400-600 oC) SOFC (LTSOFC) system med biomassa, t ex syntes gas eller flytande bränslen som till exempel bio-metanol och bioetanol. De senare system har störst potential för användning i polygenerering.

Högtemperatur SOFCs har uppenbara nackdelar och utmaningar återstår för att sänka kostnaden för att tillgodose kommersiella intressen. SOFC system behöver lägre driftstemperaturer för att minska sina totala kostnader.

Den här avhandlingen fokuserar på utveckling av nanokompositer för avancerad bränslecellsteknik (NANOCOFC), dvs nästa generation SOFCs, som är av lågtemperaturtyp (400-600 °C), kommersialiserbara och prisvärda SOFCs. Dessutom bidrar arbetet till ny forskning och utveckling av nya bränslecellskoncept map NANOCOFC teknik (www.nanocofc.com). Innehållet i denna avhandling är indelad i fem delar:

I första delen (Papers 1-5), preparerades och studerades två-fas nanokomposit elektrolyter, dvs. ceria-salt och ceria-oxid med hjälp av olika elektrokemiska metoder. Mikrostruktur och morfologi av de sammansatta elektrolyter präglades med XRD, SEM och TEM, och termisk analys utfördes med hjälp av DSC. En jonisk ledningsförmåga på 0,1 S/cm erhölls vid 300 ºC, vilket är jämförbart med konventionella YSZ i drift vid 1000 ºC. Den maximala effekttätheten var 1000 mW/cm2 vid 550 oC. Dessutom har ett co-dopat ceria-karbonat utvecklats för att förbättra jonisk ledningsförmåga, morfologi och prestanda av elektrolyten.

I den andra delen av denna avhandling (Papers 7-9), har kompositelektroder med lite eller inget nickel (Ni) utvecklats för låg temperatur SOFC. Eftersom alla elementen var mycket homogent fördelade i kompositelektroden, resulterade detta i hög katalytisk aktivitet och god

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ASOFC prestanda. Ersättning av Ni med Zn i dessa elektroder minskar kostnaden med en faktor 25.

I tredje delen (Paper 10), har en avancerad multi-bränsle fast-oxid bränslecell (ASOFC) med funktionella nanokompositer (elektrolyter och elektroder) utvecklats. Flera olika typer av bränslen, såsom gas (vätgas och biogas) samt flytande bränslen (bio-etanol och bio-metanol), har testats. En maximal effekttäthet på 1000, 300, 600 och 550 mW/cm2 uppnåddes med vätgas, biogas, bio-metanol, och bio-etanol, respektive i ASOFC. Den elektriska och totala effektiviteten låg på 54% och 80% och uppnåddes med en vätgasdriven cell.

Avhandlingens fjärde del (Paper 11) gäller utformningen av ett 5 kW ASOFC system baserat på den demonstrerade avancerade LTSOFC tekniken. Ett polygenerering system baserat på en lågtemperatur, planar SOFC designades då i samband med detta och simulerades. Effektiviteten i systemet som helhet var cirka 80%.

Den femte delen (Paper 12) beskriver en en-lagers, multi-bränsle, elektrolyt-fri bränslecell som är en revolutionerande innovation inom förnybar energi. Konventionella bränsleceller genererar el genom jontransport genom elektrolyten. Dock fungerar den här nya enheten utan elektrolyt, och alla processer sker vid partikelytor i materialet. Baserat på en teoretisk beräkning, kommer ytterligare 18% förbättring av bränslecellens övergripande effektivitet uppnås med denna nya teknik jämfört med konventionella tekniker.

Våra egenutvecklade ASOFC system med funktionella nanokompositer erbjuder betydande fördelar med minskade drifts- och kapitalkostnader för produktion av el och värme genom användning av olika bränslen baserat på bränslecellsteknik. ASOFC system kan användas för polygenerering med förnybara bränslen (dvs. biobränslen) med hög verkningsgrad som en hållbar lösning för energiproduktion i vårt samhälle. Resultaten har uppnåtts för detta examensarbete har visat en avancerad bränslecellsteknik.

Nyckelord: polygenerering, avancerad bränslecellsteknik, funktionella material, ceria-karbonat nanokompositer, multi-drivna, elektrolyt gratis bränslecell

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“The best of you are those who have the best morals"

Hazrat Muhammad (PBUH)

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Whereas the 19th Century was the century of the steam engine and the 20th Century was the century of the internal combustion engine, it is likely that the 21st Century

will be the century of the fuel cell. (Brian Cook, Canada)

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PrefaceThe work in this thesis is based on the following publications and appended at the end:1. Rizwan Raza, Xiaodi Wang, Ying Ma, Yizhong Huang, Bin Zhu, “Enhancement of

conductivity in ceria-carbonate nanocomposites for LTSOFCs”, Journal of Nano Research, 2009, 6, pp. 197-204.

2. Rizwan Raza; Xiaodi Wang; Ying Ma; X. Liu and Bin Zhu, “Improved ceria–carbonate composite electrolytes” Int. Journal of Hydrogen Energy, 2010, 35(7), pp. 2684-2688.

3. Rizwan Raza and Bin Zhu “Study on nanocomposites based on carbonate @ ceria” Int. Journal of Nanoscience and Nanotechnology, 2010, 10(2), pp. 1203-1207.

4. Rizwan Raza, Xiaodi Wang, Ying Ma, Bin Zhu, “Study on calcium and samarium co-doped ceria based nanocomposite electrolytes” Journal of Power Sources, 2010,195(19), pp. 6491-6495.

5. Rizwan Raza, Ghazanfar Abbas, Ying Ma, Xiaodi Wang, Bin Zhu “Electrochemical study of the composite electrolyte based on samaria-doped ceria and containing yttria as a second phase” Solid State Ionics, 2011, 188, pp. 58–63.

6. Rizwan Raza, H. Qin, L. Fan, Bin Zhu, “Electrochemical study on co-doped ceria-carbonate composite electrolyte” Journal of Power Sources, 2011,doi:10.1016/j.jpowsour.2011.10.124.

7. Rizwan Raza, Ying Ma, Xiaodi Wang, Bin Zhu, “Nanostructured anode for low temperature SOFC” Journal of Power Sources, 2010, 195 (24), pp. 8067-8070.

8. Rizwan Raza, Torsten Fransson, Bin Zhu, “Zn0.6 Fe0.1Cu0.3/GDC Composite anode for Low Temperature SOFC (300-600) oC” Journal of Fuel Cell Science and Technology, 2010, 8(3) 031010.

9. Rizwan Raza, Qinghua Liu, Jawad Nisar, Xiaodi Wang, Ying Ma, Bin Zhu, “ZnO/NiO nanocomposite electrode for low temperature solid oxide fuel cells” Electrochemistry Communications, 2011, 13(9), pp.917-920.

10. Rizwan Raza, Haiying Qin, Qinghua Liu, Mahrokh. Samavati, R.B. Lima, Bin Zhu,“Advanced multi-fuelled solid oxide fuel cell (ASOFC) using functional nanocomposite for polygeneration”, Advanced Energy Materials, 2011,1(6), pp.1225-1233.

11. M. Samavati, Rizwan Raza, Bin Zhu, “Design of a 5kW advanced fuel cell polygeneration system (Theoretical study)”. WIREs Energy and Environment 2011.

(accepted)12. Bin Zhu, Rizwan Raza, Ghazanfar Abbas, Manish Singh, “An electrolyte-free fuel cell

constructed with one homogenous layer with mixed conductivities”, Advanced Functional Materials, 2011, 21(13), pp. 2465-2469.

Other work not included

1. Rizwan Raza, Bin Zhu “LiAlO2–LiNaCO3 Composite electrolyte for solid oxide fuel cells” Journal of Nanoscience and Nanotechnology, 2011, 11(6), 5402-5407.

2. Rizwan Raza, Bin Zhu, “Microwave sintered nanostructured electrode” Journal of Nanoscience Nanotechnology. 2011, 11(6), pp.5402-5407.

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3. Xiaodi Wang; Ying Ma; Rizwan Raza, Mamoun Muhammed, Bin Zhu, “Novel core-shell SDC/amorphous Na2CO3 nanocomposite electrolyte for low-temperature SOFCs” Electrochemistry Communications 2008, 10 (11), pp. 1617-1620.

4. Ying Ma, Xiaodi Wang Rizwan Raza, M. Muhammad, Bin Zhu, “Thermal stability study of SDC/Na2CO3 nanocomposite electrolyte for low temperature SOFCs”, Int. Journal of Hydrogen Energy, 2010, 35(7), pp. 2880 – 2585.

5. Zhan Gao, Rizwan Raza, Zongqiang Mao, Torsten Fransson, Bin Zhu, “Development of methanol fuelled low temperature fuel cells”. Int. Journal of Energy Research, 2011,35(8), pp.690–696.

6. Zhan Gao, Rizwan Raza, Bin Zhu, Zongqiang Mao, Cheng Wang, Zhixiang Liu,“Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells”, Int. Journal of Hydrogen Energy, 36(6) 2011,pp. 3984-3988.

7. Bin Zhu, Rizwan Raza, Haiying Qin, Qinghua Liu and Liangdong Fan. “A fuel cells based on electrolyte and non-electrolyte separators”, Energy & Environmental Science,2011, 4, pp. 2986-2992.

8. Bin Zhu, Ying Ma, Xiaodi Wang, Rizwan Raza, Haiying Qin and Liangdong Fan, “A fuel cell with a single component functioning simultaneously as the electrodes and electrolyte”, Electrochemistry Communication 2011, 13(3), pp. 225-227.

9. Bin Zhu, Haiying Qin, Rizwan Raza, Qinghua Liu, Liangdong Fan, Janne Patakangas,Peter Lund, “A single-component fuel cell reactor”. Int Journal of Hydrogen Energy, 2011, 36(14), pp. 8536 -8541.

10. Bin Zhu, Rizwan Raza, Haiying Qin and Liangdong Fan, Single-component and three-component fuel cells. J Power Sources, 2011, 196(15), pp. 6362-6365.

11. Rizwan Raza, Ghazanfar Abbas, Bin Zhu, “La0.3Sr0.2Mn0.1Zn0.4 oxide -Sm0.2Ce0.8O1.9(LSMZ-SDC) nanocomposite cathode for low temperature SOFCs”, Journal ofNanoscience and Nanotechnology (2011). (accepted)

12. Haiying Qin, Z.Zhu, Qinghua Liu, Yifu Jing, Rizwan Raza, M.Singh, Ghazanfar Abbas,Bin Zhu, “Direct biofuel low-temperature solid oxide fuel cells” Energy & EnvironmentalScience, 2011, 4, pp. 1273-1276.

13. S.Khalid Imran, Rizwan Raza, Ghazanfar Abbas and Bin Zhu, “Characterization and development of bio-ethanol solid oxide fuel cell” ASME Journal of Fuel Cell Science and Engineering. 2011, 8(6), 061014.

14. Ghazanfar Abbas, Rizwan Raza, M. Ashraf Ch. and Bin Zhu, “Preparation and characterization of nanocomposite calcium doped ceria electrolyte with alkali carbonates (NKCDC) for SOFC”, ASME Journal of Fuel Cell Science and Engineering. 2011, 8(4), 041013.

15. Haiying Qin, Bin Zhu, Rizwan Raza, Manish Singh, Liangdong Fan, Peter Lund,“Integration design od membrane electrode assemblies in low temperature solid oxide fuel cell”, Int. Journal of Hydrogen Energy, 2011. (accepted)

16. Liangdong Fan, C. Wang, M. Chen, Rizwan Raza, Bin Zhu “High performance transition metal oxide composite cathode for low temperature solid oxide fuel cell”, Journal of Power Sources, 2011. (accepted)

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CONFERENCE PRESENTATIONS

Oral Presentation

1. Rizwan Raza and Bin Zhu, “Study on nanocomposites based on carbonate @ ceria”,NanoSmat08, October 2008, Barcelona, Spain.

2. Rizwan Raza, Ma Ying, Xiaodi Wang, Bin Zhu, “Nanostructured anode for low temperature SOFC” Eleventh Grove Fuel Cell Symposium, 22-24 September 2009, London, UK.

3. Rizwan Raza, and Bin Zhu, Nanocomposite anode (Zn0.65 Fe0.2Cu0.15-GDC) for Low Temperature SOFC (300-570) oC” “Third European Fuel Cell Technology and Applications Conference” 15 Dec 2009 - 18 Dec 2009, Rome, Italy

4. Zhan.Gao, Rizwan Raza, Z. Mao, Bin Zhu, "Electrochemical characterization on nanocomposite electrolyte for low temperature ceramic fuel cells" 3rd International Meeting (MPA), 21-23 July, 2009, Manchester, UK.

5. Rizwan Raza, Zhan Gao, Ma Y, X. Wang, Bin Zhu, "Microwave sintered nanostructured electrode" 4th International Conference on Surfaces Coatings and Nanostructured Materials (NanoSMat-2009), 19-22 October 2009, Rome, Italy.

6. Rizwan Raza, Ghazanfar Abbas, Xiaodi Wang, Ying Ma, Bin Zhu, "The electrochemical study of oxides coated @doped ceria" 9th ISS FIT, 1-5 June 201 0, Riga, Latvia.

7. Rizwan Raza, Ghazanfar Abbas, Bin Zhu, "GDC-Y2O3 Oxide based two phase nanocomposite electrolytes" 8th International Fuel Cell Science, Engineering & Technology Conference, ASME 2010, June 14-1 6, 2010, Brooklyn, New York, USA.

8. Rizwan Raza, Ghazanfar Abbas, Bin Zhu, "The effect of coated oxides on samarium doped ceria" 4th International Meeting (MPA), 28-30 July 2010, Braga, Portugal.

9. Rizwan Raza, Ghazanfar Abbas, and Bin Zhu, "La0.3Sr0.2Mn0.1Zn0.4 oxide -Sm0.2Ce0.8O1.9(LSMZ-SDC) Nanocomposite cathode for low temperature SOFCs" 5th International Conference on Surfaces Coatings and Nanostructured Materials (NanoSMat-2010), 19-22October 2010, Reims, France.

10. Rizwan Raza, Haying Qin, and B. Zhu, "Ceria (Zr /Sm Co-Doped) carbonate nanocomposite electrolyte for advanced solid oxide fuel cell” (MPA), 27-29 June 2011, Alvor, Portugal.

11. Rizwan Raza, B. Zhu, "Alkali-carbonates effects @ doped ceria as a composite electrolyte for low temperature (300-600 oC) solid oxide fuel cell" 6th Int. Conf. Surfaces Coatings and Nanostructured Materials (NanoSMat-2011), 17-20 October 2011, Krakow, Poland.

12. Rizwan Raza, B. Zhu, “Biogas fuel based Single Functional Layer fuel cell”, European Fuel Cell Technology & Applications, 14-16 December 2011, Rome, Italy.

(To be presented)Poster Presentation

1. Rizwan Raza, and Bin Zhu “A low temperature solid oxide fuel cell based on Zr/Sm co-doped nanocomposite electrolyte” MRS-Fall meeting, Nov.28-December 2, Boston, USA.2. Rizwan Raza, Haying Qin, Liangdong Fan, and Bin Zhu, Electrochemical characterization of co-doped ceria-carbonate composite, Int. Workshop Molten Carbonates & Related Topics", 21-22 March 2011, Chimie-ParisTech, Paris, France.

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Prizes/Awards

� Best Performance Award 2008: GETT Fuel cells International AB, Sweden� Best Poster Award- Nanosmat 2009� I2P ® Stockholm on September 22 (Idea to product) 2011, Sweden-Winner 2011� 2nd Prize Poster competition: Energy initiative day, 09th Nov. 2011, Stockholm, Sweden� Who is Who in the world-2012- selected for special biography

Author’s contribution to the papersPaper 1-10: The first author is the main author of all the papers who designed and prepared

the materials, performed the experiments, analysed the results and wrote the papers. Co-authors contributed to the data analysis and to the scientific discussions. The last author was the mainacademic supervior and acted as a reviewer.

Paper 11: The second and the last authors planned the model. The first and the second authors performed experimental and numerical work together and wrote the paper. The last author was the main academic supervior and acted as a reviewer.

Paper 12: The first author invented the one homogenous layer device while second authordesigned and prepared the materials. Other co-authors contributed to the measurements. First and second authors analyzed the data and contributed to the scientific discussions on establishing the invention and its scientific principles. The first author was the main academic supervior and acted as a reviewer.

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ACKNOWLEDGEMENTSAll praise for Allah (SWT), the most Gracious and the most Compassionate, who is the Lord of the entire universe. It will be a great honour for me if I pray to our Holy Prophet Muhammad and Aale-Muhammad (the family of Muhammad) (PBUH), who spread goodness throughout the world.

I thank Allah for gifting me a great man for my supervision, Dr. Bin Zhu (Associate Professor),who has always guided me in all aspects of academics, research, and spirituality. Without his kind support and supervision, it would have been impossible for me to complete my PhD. His innovative ideas always inspire me, and I have always accepted all challenges to perform the necessary work.

I would like to express my gratitude to my co-supervisor, Professor Torsten Fransson, for his tremendous guidance and support. I am also thankful to my internal reviewer, Dr. Catharina Erlich, for her insightful comments.

I would also like to thank our administrative staff, especially Inga (who left the department but helped me a great deal with the administrative process when I started my PhD program) and Machteld, who was always cooperative, always smiling and never said “No” to me.

I would like to thank all of my past and present research group members: Xiaodi Wang and Ying Ma for discussions and characterisations of the materials; Dr. Haiying Qin, Dr. Jiebing Li, Raquel B Lima, Dr. Qinghua Liu, Liangdong Fan, and Zhan Gao for fruitful discussions about papers and research; and Yifu Jing, Jing Wang, Dr. Xuetao Wang, Dr. Wnyi Tan, S. Khalid Imran, Manish Singh, M. Shoaib, Mahrokh Samavati, Cosimo Guerra, Xavier Fonoll Almansa, Inken Lauer and Ose Micah, who are acknowledged for their good company. I am very thankful to Mr. Jawad (Uppsala University) and Ms. Mahrokh for theoretical simulations and discussions of modelling work. I also thank our GETT-KTH joint lab engineers, Zhizang Xhu and Youquan Mi, and our departmental technician, Leif, for their technical support. I am also thankful to the GETT Fuel Cells AB, for providing the laboratory facilities.

I also acknowledge to our international collaborators, Prof. Peter Lund (Aalto University, Finland), Prof. Mamoun Muhammed (Functional Materials, KTH), Dr. Imran Patel (UK), M.Siddique (UK), Dr. Ansari (UK), Dr. Jawad (UCL, UK)

I would also like to acknowledge my departmental colleagues and friends, Dr. Waseem, Hamayun, Dr. Seksan, Dr. Rashid Ali and Justin for their good company. Some friends from Pakistan, Dr. Hafiz Ashfaq, Dr. Ghazanfar Abbas, Dr. Majid Munir, M Ajmal, Kaleemullah, Mujahid Abbas, Mazhar Abbas also acknowledge for moral support.

I would like to thanks my friends, and PhD scholars; Khurram (KTH), Dr Aamir Razaq (UU), Raza Naqvi (KTH), Ali Zaidi (KTH), Waqar (KI), Mirza Ahmed (KTH), Ali Zaidi (KTH), Ansar (KTH), Irfan (KTH), Nawaz (KTH), Qasim (KTH), Sultan(SU), Nadeem (SU), Hafiz Naeem (KI), Nadeem (KI), Jafri (UU), Shabbir Hussain, Hamad, Samar, Musarat, Aamir shah and Shaheryar.

I would like to thank some families in Sweden for arranging the social activities especially, Uncle Abbas, Riaz shah, Uncle Shamim, Uncle Baqir, Kazim Shah, Jaffar Bhai, Ali Bhai, and

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Anwar Bhai. Also thanks to Molana Zakir Hussain and Allama Zakir (Qum) for their moral and spiritual support.

I would like to thank my teacher from Pakistan, Dr. Shaukat Ali Hayat (Director, COMSATS, Lahore), who taught me the first steps of research, introduced me to the idea of working on fuel cells and always encouraged me to do research work. I am also very thankful to Dr. M. Ashraf Ch. (HOD, Physics, BZU), for supporting me and giving many kind and valuable suggestions throughout my career and research. Also, I would like to thank Dr. SM Junaid Zaidi (Rector, CIIT), who provided the opportunity and the environment to conduct research in the institute (Pakistan) before PhD.

My sponsor funding agency, the Higher Education of Pakistan (HEC), provided the funding for my PhD scholarship. Another funding organisation that supported my experiments and conferences, VINOVA (The Swedish Governmental Agency for Innovation Systems), is gratefully acknowledged.

I thank my father, Raza Haider (Late), and my uncle, M. Aslam Advocate (Late), who wanted to see me become a doctor.

My deepest thanks are extended to my mother, who is waiting for my success, for her great support, encouragement and prayers during each step. I also thank my sister and brothers, Abbas Raza, Asad Raza, Imran Raza, Sajid Raza, Abid Raza, and Baqir Raza, for their support and help throughout my life. I also thank my other family members, Aunt Narjis (UK), Aunt Kalsom, and Uncle Hussnain (UK) for their prayers.

Last but not least, I have no adequate words to thank my beloved wife, Saira Rubab, who always prayed for my success, for her love, endless support and worry during my studies and visits to conferences.

Finally, I extend all my sweet love to my lovely children, “Ali Asghar Raza and Kisaa Fatima Raza” I dedicate my work to my mother, my wife and my sweet children.

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ContentsABSTRACT...............................................................................................................................I

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

ACKNOWLEDGEMENTS...................................................................................................XI

CONTENTS.........................................................................................................................XIII

LIST OF TABLES ................................................................................................................XV

LIST OF FIGURES ............................................................................................................XVI

NOMENCLATURE.........................................................................................................XVIII

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

1.1 FUEL CELL FAMILIES..................................................................................................... 2

1.1 SOLID OXIDE FUEL CELL............................................................................................... 3

1.2 SOFC COMPONENTS/MATERIALS ................................................................................. 4

1.3 FUEL-CELL STACKING................................................................................................... 7

1.4 FUEL CELL APPLICATIONS............................................................................................. 8

1.5 THERMODYNAMIC PRINCIPLE OF SOFC......................................................................... 8

1.6 POLARISATIONS........................................................................................................... 10

1.7 POLYGENERATION THEME/CONCEPT........................................................................... 12

1.8 BIOFUELS/HYDROCARBON .......................................................................................... 14

1.9 LTSOFC OR ASOFC (300-600 OC) AND NANOCOFC THEORY................................. 15

1.10 OBJECTIVES................................................................................................................. 15

1.11 THESIS OUTLINES ........................................................................................................ 17

2. EXPERIMENTAL TECHNIQUES ....................................................................... 19

2.1 FUNCTIONAL MATERIALS-SYNTHESIS FOR ASOFC...................................................... 19

2.2 FUEL CELL FABRICATION AND ELECTROCHEMICAL CHARACTERISATION ..................... 22

2.3 MIXED CONDUCTOR SYNTHESIS FOR SINGLE COMPONENT ELECTROLYTE FREE FUELCELL............................................................................................................................ 24

2.4 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) ............................................... 25

2.5 MICROSTRUCTURE ANALYSIS TECHNIQUES ................................................................. 27

2.6 THERMAL ANALYSIS ................................................................................................... 29

2.7 DENSITY/POROSITY MEASUREMENTS........................................................................... 29

2.8 THEORETICAL MODELLING AND CALCULATIONS.......................................................... 29

3. NANOCOMPOSITE ELECTROLYTES.............................................................. 33

Page xiv Doctoral Thesis – Rizwan Raza

3.1 ENHANCEMENT OF CONDUCTIVITY IN CERIA-CARBONATE NANOCOMPOSITES ............. 33

3.2 IMPROVED CERIA CARBONATES COMPOSITE ELECTROLYTES...................................... 35

3.3 CALCIUM AND SAMARIUM CO-DOPED CERIA BASED NANOCOMPOSITE ELECTROLYTES 37

3.4 COMPOSITE ELECTROLYTE BASED ON SAMARIA-DOPED CERIA AND CONTAINING YTTRIA AS A SECOND PHASE .................................................................................................... 42

4. NANOCOMPOSITE ELECTRODES ................................................................... 47

4.1 ZNO/NIO NANOCOMPOSITE ELECTRODES.................................................................... 47

4.2 NANOSTRUCTURE ANODE (CU 0.2 ZN 0.8O) ................................................................... 50

4.3 ZN0.6 FE0.1CU0.3O/GDC COMPOSITE ANODE ................................................................ 51

4.4 ADVANCED MULTI-FUELLED SOLID OXIDE FUEL CELL (ASOFC) USING FUNCTIONAL NANOCOMPOSITE FOR POLYGENERATION .................................................................... 52

5. DESIGN OF ASOFC SYSTEM FOR 5KW .......................................................... 59

5.1 SYSTEM DESCRIPTION.................................................................................................. 59

6. ELECTROLYTE FREE FUEL CELL-A NEW ENERGY CONVERSION DEVICE .................................................................................................................... 63

7. CONCLUSIONS ...................................................................................................... 67

7.1 FUTURE WORK............................................................................................................. 68

8. REFERENCES......................................................................................................... 69

Doctoral Thesis – Rizwan Raza Page xv

List of Tables

Table 1-1 Comparison of different fuel cells............................................................................................................2

Table 1-2 Electrochemical reactions of SOFC for different fuels............................................................................3

Table 1-3 Requirements for the components of a SOFC..........................................................................................4

Table 1-4 Fuel cells with the ceria-carbonate composite electrolytes overview......................................................5

Table 1-5 The comparison of some selected anodes using different fuels................................................................6

Table 1-6 Comparison of some selected cathodes using different fuels...................................................................7

Table 2-1 Input parameters of the fuel cell stack model ........................................................................................30

Table 2-2 calculated results of the fuel cell stack ..................................................................................................31

Table 3-1 The fitted curve results and parameter ..................................................................................................42

Page xvi Doctoral Thesis – Rizwan Raza

List of Figures

Figure 1-1 Working principle of fuel cell.................................................................................................................1

Figure 1-2 Working principle of Solid oxide fuel cell..............................................................................................3

Figure 1-3 Schematic diagram of fuel-cell stack (H-TEC Wasserstoff-Energie-Systeme GmbH) ...........................8

Figure 1-4 FC polarisation curve (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006)..................11

Figure 1-5 Polygeneration systems with a multi-fuelled FC .................................................................................12

Figure 1-6 Biofuel based SOFC for Polygeneration systems ................................................................................13

Figure 1-7 Comparison of different power generation system (http://www.fuelcells.org/info/smithsonian.pdf)...14

Figure 2-1 Co-precipitation synthesis process for ceria-carbonate nanocomposite electrolyte ...........................20

Figure 2-2 Co-precipitation synthesis process for ceria-oxide nanocomposite.....................................................21

Figure 2-3 Schematic figure of asymmetrical and symmetrical cell ......................................................................23

Figure 2-4 Synthesis of mixed conductors and experimental setup for EFFC.......................................................25

Figure 3-1 (a) HRTEM image of SDC-carbonate composite, (b) diffraction pattern............................................33

Figure 3-2 A.C. conductivities of the SDC-carbonates..........................................................................................34

Figure 3-3 (a) SEM image for NSDC, (b)TEM image for NSDC ..........................................................................35

Figure 3-4 (a) FC performance with LN-SDC electrolyte (two steps), (b) FC performance with NSDC electrolyte (one step)................................................................................................................................................................36

Figure 3-5 (a) microstructure of CSDC-carbonate (SEM), (b) EDX analysis.......................................................37

Figure 3-6 Fuel cell performance with CSDC-carbonate electrolytes ..................................................................38

Figure 3-7 (a) (in Air) Oxide ion conductivity (Arrhenius plot), (b) (in H2) Proton ion conductivity ...................39

Figure 3-8 Stability of ionic and protonic conductivity @550 oC of CSDC-carbonate .........................................39

Figure 3-9 (a) EIS of CSDC-carbonate based fuel cell under OCV , (b) EIS of CSDC-carbonate based fuel cell after twice time operation ......................................................................................................................................40

Figure 3-10 Comparison of experimental conductivity data with theoretical curves in both atmosphere of air, H2...............................................................................................................................................................................41

Figure 3-11 (a) crystal structure (XRD), (b) microstructure of YSDC (SEM).......................................................42

Figure 3-12 Ion transport mechanism in two phase composite electrolytes (where core is doped ceria and shell is oxide/carbonate) ....................................................................................................................................................43

Figure 3-13 (a) FC Performance with YSDC electrolyte, (b) AC impedance at different temperature.................44

Figure 3-14 (a) YSDC electrolyte Conductivities, (b) Arrhenius plot....................................................................45

Figure 4-1 (a) XRD pattern for ZnO/NiO/SDC-Na2CO3., (b) crystal structure of Ni-doped ZnO particle, (c) SEM d) HRTEM image of ZnO/NiO material, (e) cross-section image of the cell .........................................................48

Figure 4-2 (a) Electrochemical impedance spectra, (b) conductivity of the electrode determined from impedance data at different temperature .................................................................................................................................49

Figure 4-3 (a) Cell voltage (open) and power density (solid) at different temp., (b) effect of varying the Zn : Ni ratio of electrodes on Pmax at 500 oC......................................................................................................................50

Figure 4-4 (a) conventional anode (Ni-SDC), (b) Ni free anode (CuZn-SDC composite).....................................51

Doctoral Thesis – Rizwan Raza Page xvii

Figure 4-5 (a) XRD, (b) TEM ................................................................................................................................51

Figure 4-6 (a) FC performance with H2, (b) FC performance with methanol.......................................................52

Figure 4-7 (a) X-ray diffraction before and after reduction of the composite electrode, (b) High-resolution TEM...............................................................................................................................................................................53

Figure 4-8 (a) Performance of a single cell with different bio-fuels at 550 oC, (b) Performance of a single cell with H2 fuel at 550 oC ............................................................................................................................................53

Figure 4-9 (a) Comparison of calculated theoretical and observed experimental values of open circuit voltage (OCV) for different bio-fuels at temperatures of 500 oC - 550 oC, (b) Stability of SOFC for different bio-fuels at temperatures of 500 oC - 530 oC ............................................................................................................................55

Figure 4-10 Raman spectroscopy of composite electrode after running ASOFC with different fuels at 550 oC, (a) with bio-methanol for 0, 24 and 72 hrs, (b) with bio-ethanol for 0, 24 and 72 hrs, (c) with bio-gas for 0, 24 and 48 hrs, (d) AC impedance with bio-methanol, bio-ethanol and bio-gas ................................................................56

Figure 4-11 (a) Cell efficiency, (b) Larger cell (6 × 6 cm2) performance at 550 oC .............................................58

Figure 5-1 Schematic ASOFC systems ..................................................................................................................59

Figure 5-2 (a) Electrical, heating, and combined efficiencies versus fuel utilization factor, (b) Effect of fuel cell operating temperature on electrical, heating and combined efficiencies ..............................................................60

Figure 5-3 Effect of air inlet temperature on electrical, heating, and combined efficiencies ................................61

Figure 6-1 Comparison between conventional FC and EFFC ..............................................................................63

Figure 6-2 (a) TEM micrograph, (b) Conductivity for the LiNiZn-based oxide ....................................................64

Figure 6-3 (a) Voltage versus current for the one homogenous layer at 500 oC, (b) Cell voltage and power density versus current density for one homogenous layer device at various temperatures, (c) EFFC device efficiency by comparisons with the conventional SOFC ........................................................................................65

Figure 6-4 (a) shows a schematic of a conventional three-component FC and EFFC with hydrogen and air supplies, (b) with electron release and acceptance at particle surfaces in the material, respectively. H2O is �� processes…………………………………………………………………………......…………………………….…………..66

Page xviii Doctoral Thesis – Rizwan Raza

NOMENCLATUREAC Alternating currentAFC Alkaline fuel cellASOFC Advanced solid oxide fuel cellBET Brunauer-Emmit-TellerBSCF BaSrCoFeCSDC Calcium and samarium co-doped ceriaDC Direct currentDSC Differential scanning calorimetryEFFC Electrolyte free fuel cellEIS Electrochemical impedance spectroscopyEMF Electromotive forceFC Fuel cellFWHM Full width half maximumSEM Scanning electron microscopeGDC Gadolinia doped ceriaHRS Heat recovery systemHRTEM High-resolution transmission electron microscopyHTSOFC High-temperature solid oxide fuel cellIEA International energy agencyITSOFC Intermediate-temperature solid oxide fuel cellLN-SDC Sm0.2Ce0.8O1.9 - LiNaCO3

LSCF La1-xSrxCo1-yFeyO3-�

LSGM La0.9Sr0.1Ga0.8Mg0.2O2.85

LSM La1-xSrxMnO3

LTSOFC Low-temperature solid oxide fuel cellMCFC Molten carbonate fuel cellMEA Membrane electrode assemblyNSDC Sm0.2Ce0.8O1.9 - Na2CO3

OCV Open circuit voltagePAFC Phosphoric acid Fuel CellPEMFC Proton exchange membrane fuel cellSDC Samaria doped ceriaSEM Scanning electron microscopySIC Super ionic conductorSOFC Solid oxide fuel cell

Doctoral Thesis – Rizwan Raza Page xix

SSC Sm0.5Sr0.5CoO3

SSZ Scandia-stabilized zirconiaTEC Thermal expansion coefficientTEM Transmission electron microscopyXRD X-ray diffractionYDC Yttrium doped ceriaYSDC Samaria doped ceria - YttriaYSZ Yttria-stabilized zirconia

Symbolsxa Chemical activity of substance x

C Concentration at the electrode surface Co Concentration for the bulkCp Specific heat at constant pressure, in JK�1 kg�1

DBET Particle sizeAn electron

E EMF or open circuit voltage0E EMF at standard temperature and pressure, and with pure reactants

Erev Reversible (Nernst) potential.Ecell Cell voltageF Faraday constant, the charge on one mole of electrons, 96,485 Coulombsf volume fraction of the second phase of carbonatesfs volume fraction of high-conducting interface regionG Gibbs free energy (or negative thermodynamic potential)

0G� Change in Gibbs free energy at standard temperature and pressure, and with pure reactants

g Gibbs free energy per mole

fg Gibbs free energy of formation per mole

( )f xg Gibbs free energy of formation per mole of substance X

h� Electronic holeH � Proton in electrolyte�Hcom Enthalpy included in all combustible species in the fuel gases fed to the fuel cell�H0 Enthalpy of fuel species available in the fuel cell to generate electricityI Currenti Current density, current per unit areaiF Current available for the fuel cell reactionj Complex number with value of 1��

m Mass flow rateN Avogadro’s number, 236.002 10� , also revolutions per secondn Number of moles

OO� Oxygen ion in the electrolyte

e�

Page xx Doctoral Thesis – Rizwan Raza

Pe Electrical power (mW)2HP Partial pressure of hydrogen

2OP Partial pressure of oxygen

OHP2

Partial pressure of water vapour

xP Partial pressure of gas X0P Standard pressure, 100 kPa

Qin Amount of chemical energy in the fuel fed into the fuel cell stackR Molar or ‘universal’ gas constant, 8.314 J K 1 mol 1,also electrical resistanceT Temperaturet Timetion Transport numberV VoltageVc Maximum voltage

••OV Oxygen ion vacancy

W Work done� Density (kg/m3)� Conductivity (S/cm)�th Theoratical density of the material�S Composite conductivity (S/cm)�1 Conductivity of phase 1 (S/cm)�2 Conductivity of phase 2 (S/cm)� Effieciency Polarization or overpotential (V)�

Active polarisation�

Concentration polarisation�

Ohmic polarisation Phase angle

f� Fuel utilization Z impedanceZ � Real parts of the impedanceZ �� Imaginary parts of the impedance

Doctoral Thesis – Rizwan Raza Page 1

1. INTRODUCTION

The use of fossil fuels has increased exponentially in recent decades with a corresponding increase in the pollution of the environment. The need for safe, sustainable technologies that do not pollute the environment during power generation has become critical (Hawkes, et al. 2009).A number of attempts have been made in this direction; one of the most promising attempts is polygeneration. This technology is very efficient with lower emissions than conventional methods. It simultaneously produces useable heat and electrical power from a single source of fuel (Zhu, et al. 2006b; Hawkes, et al. 2009; Margalef, et al. 2011). The overall efficiency of such a system can reach 90%, which is higher than that of conventional power plants (60-80%)(I.Knight 2005; IEA 2007). Thus, the use of polygeneration can be more efficient, less expensive and produce lower emissions than conventional methods for the generation of heat and power.

A number of different technologies are available for polygeneration, such as micro gas turbines, sterling engines, solar systems, and fuel cells. Among these technologies, FC systems offer the most promising technology because of their ability to produce electricity at a high efficiency (about 60%) with either low or zero emissions (Zhu, et al. 2006a; Zhu, et al. 2006b; Hawkes, et al. 2009; Margalef, et al. 2011). In addition to their high efficiency, FCs are outstanding with respect to their friendliness to the environment and their flexibility for various power demands. FCs are considered to be a potential solution for effective energy conversion from fossil fuels, bio-fuels and hydrogen. Among various FC technologies, SOFCs are the most suitable for polygeneration because they have the highest overall efficiency (Zhe, et al. 2010).

A FC is defined as an electrochemical device that directly converts chemical energy into electricity without the Carnot limitation of efficiency that affects all heat engines. FCs combine oxygen from air with hydrogen to produce heat, electricity, and water. A fuel cell contains two electrodes (anode and cathode) that are separated by an electrolyte. Hydrogen and oxygen are fed at the anode and the cathode, respectively. An oxidation process occurs at the anode in the presence of a catalyst such that hydrogen is split into electrons and protons. The electrons travel along an external path, through a load, and the protons transport through the electrolyte. At the cathode, the protons combine with oxygen to produce water and heat. The fuel-cell principle is shown in Figure 1-1 (Larminie, et al. 2003).

Figure 1-1 Working principle of fuel cell

Page 2 Doctoral Thesis – Rizwan Raza

1.1 Fuel Cell Families

Fuel-cell families are generally categorised by the electrolyte material. Many different electrolytes can be used in the development of fuel cells. Fuel cells can be categorised asPEMFC, AFC, PAFC, MCFC, and SOFC. A comparison of the different kinds of fuel cells is provided in Table 1-1 (Chadwick 2000; Steele et al. 2001; Barnett et al. 2003; Zhu et al. 2003).

Table 1-1 Comparison of different fuel cells

Fuel Cell Types

Type of Electrolyte

Operating temp.(°C)

Electrical Efficiency

Advantages Disadvantages

PEM polymer/solid membrane

50-100 53-60% (Mobile)and

25-35% (Stationary)

� Low operating temperature

� Absence of corrosive liquid electrolyte

� Fast start-up� Simple cell structure� Potentially very low� capital costs

� Low CO tolerance� Drying of membrane� Waste heat has low value� Low performance of the cathode

AFC aqueous alkaline

50-200 60% � Mature technology� Reliable� High efficiency� Low cost materials

� Not CO2 tolerant

PAFC phosphoric acid

200-250 40% � Mature technology� Very reliable� Fast response time� High efficiency with� partial loads

� Too expensive� Rather low efficiency� Waste heat has low

value� CO < 1%

MCFC molten carbonate

600-700 45-47% � High efficiency� CO tolerant� Internal reforming� No noble metal

catalysts� High-quality waste heat

� Material problems of cell and stack components

� CO2 circulation� Sulphur tolerance is

low (<1 ppm)

SOFC solid oxide 500-800 45-70% � Very high efficiency� High-quality waste heat� Simple system� No reformer needed� Fuel flexibility� Sulphur tolerance is

one order of magnitude higher than for MCFC

� Still far too expensive, because of the synthesized ceramic materials

� Thermal expansion� Porosity� Temperature gradients� Mixed conduction

problems


1.1 Solid Oxide Fuel CellAmong the different types of fuel cells, SOFCs have gained attention because of their high

energy conversion efficiency and fuel flexibility (Hibino, et al. 2000; Park, et al. 2000; Barnett, et al. 2003; McIntosh, et al. 2004a; Sasaki, et al. 2004a). Different kinds of fuels, such as gases (e.g., hydrogen, syngas, and bio-gas), liquids (e.g., methanol, ethanol and glycerol) and solids (e.g., carbon and lignin), can be used in SOFCs (Barnett, et al. 2003; McIntosh, et al. 2004a; Sasaki, et al. 2004a). The current SOFC technology is still expensive and requires new materials that can work more efficiently at lower temperatures. SOFCs operate at high temperatures and employ ceramics as functional elements of the cell. Each cell is composed of an anode (e.g., Ni) and a cathode (e.g., LSCF, BSCF, etc.) separated by a solid impermeable electrolyte (e.g., YSZ, SDC, GDC, etc.). During operation, the electrolyte conducts oxygen ions from the cathode to the anode, where they react chemically with the fuel. The electric charge induced by the passage of the ions may then be collected and conducted away from the cell (Steele, et al. 2001). The working principle of the SOFC is depicted in Figure 1-2.

Figure 1-2 Working principle of Solid oxide fuel cell

The electrochemical reactions of SOFCs for different fuels are shown in Table 1-2 (Chadwick 2000; Steele, et al. 2001; Barnett, et al. 2003; Zhu, et al. 2003c).

Table 1-2 Electrochemical reactions of SOFC for different fuels

Fuel Type Anode reaction Cathode reaction Overall reactionHydrogen H2 + O2-��2O + 2e- 1/2 O2 + 2e- ��2- H2 + 1/2 O2 ��2OBiogas H2 ��++2e-

CH4+2H2��4H2+CO2 +8e- O2 +4e-��2- CH4 + H2 + 5/2O2�� 2+ 3H2O

Bio-ethanol C2H5OH+3H2��2CO2+12H+ +12e-

3O2+12H++12e-

��2OC2H5OH + 3O2 �� 2+3H2O

Bio-methanol CH3OH + H2�� 2+6H+ +6e-

3/2 O2 + 6H+ +6e-

��2OCH3OH+3/2O2+ H2��CO2 +3H2O


1.2 SOFC Components/Materials

FCs are composed of three functional components: the anode, the electrolyte and the cathode(Steele, et al. 2001; Barnett, et al. 2003). This 3-layer structure is also called the MEA. The MEA structure, required to provide necessary functionalities to a FC is complex and needs to be stable. The requirements for these three components are described in Table 1-3 (Steele, et al. 2001; Barnett, et al. 2003).

Table 1-3 Requirements for the components of a SOFC

FC Components/ Requirements

Catalytic Activity /Cost

Conductivity Density/Porosity

Compatibility Stability

ElectrolyteCost should be lower

High ionic conductivity, (0.1S/cm) and negligible electronic conductivity

Fully dense/must be gas-tight

Thermally, chemically, and mechanically compatible with electrodes

Chemically, morphologically, and structurally stable in fuel and oxidant

Anode High catalytic activity and cost should be lower

Mixed conductor: High electronic conductivity to transfer electron but low ionic conductivity� ��

Porous for mass transport of gases

Thermally, chemically compatible with electrodes to prevent the segregation and crack

Chemically, morphologically, and structurally stable in fuel environments

Cathode High catalytic activity and low cost

Mixed conductor: High electronic conductivity and lower ionic

Porous for mass transport of gases

Thermally, chemically compatible with electrodes

Chemically, and structurally stable in oxidant environments

Interconnect Cost should be lower

High electronic conductivity, (0.1S/cm) withno ionic conductivity

Fully dense

Thermally compatible with electrodes to prevent the crack of cell

Stable in fuel and oxidant

1.2.1 ElectrolyteThe electrolyte is the most critical material of the fuel cell. The performance of the cell is

dependent on the conductivity of the electrolyte materials. The electrolyte has to be dense to prevent gas permeability (Steele, et al. 2001; Barnett, et al. 2003) to opposite sides and have a large area to minimise the bulk resistance. It should be an electronic insulator but a good ionic conductor. For example, for state-of-the-art SOFCs with YSZ electrolytes, temperatures above 700-800 oC are needed to ensure adequate ionic conductivity to guarantee sufficient power


outputs. An extensive range of research has been devoted to the development of new alternative electrolyte materials for SOFCs that operate at low temperatures, e.g., ion-doped ceria (GDC) (Shaorong, et al. 2000), (SDC) (Park, et al. 2005), (YDC) (HartmanovÃ�� ,perovskite-type oxides (La��SrxGa��MgyO3), (Joshi, et al. 2005) mixed-ion conductors (BaZr0.1Ce0.7Y0.2–xYbxO3–�), (Yang, et al. 2009) and complex materials such as La2Mo2O9,(Tealdi, et al. 2004) and apatite-type oxides (Islam, et al. 2003). Ceria-carbonate composite electrolytes have attracted significant attention during the last decade for their application in LTSOFCs. Such composite electrolytes are considered to be a new class of ionic conductors because of their high conductivity, which occurs via interfaces and gives these materials promising potential applications in LTSOFCs (Zhu, et al. 1994; Zhu, et al. 2000; Zhu, et al. 2001; Zhu, et al. 2003a; Zhu, et al. 2003a; Zhu, et al. 2006b; Huang, et al. 2007e). A general description of a composite electrolyte (ceria-carbonate) is given in Table 1-4.

Table 1-4 Fuel cells with the ceria-carbonate composite electrolytes overview

Ceria-carbonates

Fuel/ oxidant

Conductivity (S/cm)

Power density (mW/cm2)

Working temp.(oC) References

GDC-salt composites H2/air 0.01-1 200-800 400-660 (Zhu 2003)

GYDC-40wt.% LiKCO3

H2/air - 70-300 480-530 (Zhu 2003)

YDC-22 wt.% LiNaCO3

H2/air 0.01-0.78 200-700 400-660 (Zhu 2003)

SDC-10 wt.% LiNaCO3

H2/air 0.001-0.03 430 400-625 (Huang, et al. 2007d)

SDC-20 wt.% LiNaCO3

H2/air 0.003-0.09 940 400-625 (Huang, et al. 2007d)

SDC-30 wt.% LiNaCO3

H2/air 0.003-0.1 890 400-625 (Huang, et al. 2007d)

SDC-35 wt.% LiNaCO3

H2/air 0.1-0.15 1080 400-625 (Huang, et al. 2007d)

GDC-30 wt.% LiKCO3

H2/air 0.002-0.09 - 300-700 (Benamira, et al. 2011)

SDC-25 wt.% LiNaCO3

H2/air - 100-1100 400-600 (Hou, et al. 2008)

SDC-30 wt.% LiNaCO3

H2/air 0.01-0.2 200-1000 500-650 (Huang, et al. 2008)

SDC-30 wt.% LiNaKCO3

H2/air 0.05-0.2 200-760 500-700 (Xia, et al. 2009)

SDC-Na2CO3nanocomposite H2/air - 500-900 450-580 (Wang, et al. 2008)

Ceria-oxide nanocomposite H2/air - 200-700 450-580 (Zhu, et al. 2003a)

GYDC-LiNaCO3

H2/air 0.2 600 550 (Zhang, et al. 2010)


1.2.2 AnodeThe anode is also an important part of the fuel cell. It facilitates the fuel electro-oxidation

process and passes electrons through the circuit. The criteria for a good anode are listed below (Liu 2005; Zhang 2006; Cowin, et al. 2011)

� Mixed conductor (ionic and electronic)� Low polarisation resistance � Physical and chemical stability under a reducing atmosphere at high temperature� Thermal expansion coefficient that matches that of the electrolyte � High catalytic activity to promote reaction of the fuels (hydrogen and hydrocarbons etc)

with oxide ionsNi, Co, and Cu are common anode materials for SOFCs, and these metal oxides are mixed

with an electrolyte for anode applications. The metal oxides provide electronic conductivity, and the electrolyte component gives ionic conductivity in a mixed-anode conductor. These mixed-conductor composites give a better performance compared to pure metal oxides. Some composite anodes, as noted in the literature, are listed in Table 1-5 along with their performance in SOFCsat different temperatures.

Table 1-5 The comparison of some selected anodes using different fuels

Anode Material Fuel Temp. (oC) Power density

(mW/cm2)Life(hrs) References

BaO/Ni C3H8 750-850 600-1100 100 (Yang, et al. 2011)

BaO/Ni Gasified Carbon 750-850 800 100 (Yang, et al. 2011)

Sr2FeMoO6 CH4 850 604 20(cycles) (Wang, et al. 2011b)

Ni/YSZ H2S 750 - 300 (Cheng, et al. 2011)Ni/YSZ CH4 650 370 - (Murray, et al. 1999)

Ni/BaZrCeYYb H2 500-700 1100 100 (Selman 2009; Yang, et al. 2009)

Ni/BaZrCeYYb C3H8 500-700 500 - (Selman 2009; Yang, et al. 2009)

Ni/GDC H2 650 500 - (Liu, et al. 2004)Cu/SDC Propane 550 600 - (Xia, et al. 2002b)

YSr2Fe3O�� H2 850 35 - (Azad, et al. 2011)

LaSrTiO Humidified H2

750 500 - (Savaniu, et al. 2011)

PrSrCrMn 5%H2 900 180 - (Raj, et al. 2010)LaSrCrMn H2 700 1100 - (Tao, et al. 2003)LaSrCrMn CH4 700 700 - (Tao, et al. 2003)

Ni/YSZ Ethanol 800 600 - (Cimenti, et al. 2009)

Ni/YSZ Ethanol 650-800 300-800 - (Jiang, et al. 2001)

Ni/YSZ Methanol 650-800 600-1300 - (Cimenti, et al. 2009)


1.2.3 CathodeThe reduction of molecular oxygen, the transport of charged species to the electrolyte and the

distribution of the electrical current all occur at the cathode. The criteria for a good cathode are listed below (Steele, et al. 2001; Laboratory 2004; Liu 2005):

� Physical and chemical stability under an oxidising atmosphere � Mixed conductivity (ionic and electronic), similar to the anode � Thermal expansion that matches that of the electrolyte � High porosity

The list of the most common cathode materials using different fuels is shown in Table 1-6.

Table 1-6 Comparison of some selected cathodes using different fuels

CathodeMaterial

Fueltype

Temperature (oC)

Performance/ power density (mW/cm2) References

BSCF H2 600 1010 (Shao, et al. 2004) LSCF H2 700 500 (Simner, et al. 2006)

BZCY-SSC H2 700 900 (Yang, et al. 2008) SSC-SDC H2 650 500 (Cowin, et al. 2011) SrScSbO H2 800 500 (Aguadero, et al. 2009) BaCeFe H2 700 390 (Tao, et al. 2009) GdBaCo 5%H2 700 250 (TarancÃ³n, et al. 2007) YSrFeO H2 800 35 (Azad, et al. 2011)

SmBaSrCo H2 800 1310 (Kim, et al. 2009) LaSrMnO CH4 650 500 (Murray, et al. 1999)

SrTiCo H2 600 610 (Gao, et al. 2011)

1.2.4 Interconnects

The interconnects can be either metallic or ceramic layers that are situated between each individual cell. Their purpose is to connect each cell in series so that the electricity generated by each cell can be combined (Steele, et al. 2001; Laboratory 2004; Liu 2005).

1.3 Fuel-Cell Stacking For fuel-cell applications, single cells must be combined in a modular fashion into a cell stack

to achieve the voltage and power output level required for the application. Generally, the stacking involves connecting multiple unit cells in series via electrically conductive interconnects (Figure 1-3, http://www.h-tec.com).


Figure 1-3 Schematic diagram of fuel-cell stack (H-TEC Wasserstoff-Energie-Systeme GmbH)

1.4 Fuel Cell Applications

Four primary areas are currently being targeted for fuel-cell applications. A key point is the wide range of power requirements for fuel cells, from systems of a few watts up to megawatts. Examples of applications include mobile/transportation (cars, trucks, buses, trains, ships, and submarines), stationary (backup power, power for remote locations, stand-alone power plants for towns and cities, distributed generation for buildings, and polygeneration), portable (cell phones, radios, and portable generators), and military applications (soldier-portable power, submarines, warships, etc.) (Steele, et al. 2001; Larminie, et al. 2003; Laboratory 2004; Liu 2005).

1.5 Thermodynamic principle of SOFC

To understand the thermodynamic principles of SOFCs when hydrogen is used as a fuel and air is used as an oxidant, it is important to describe the relationship between reversible cell potentials and thermodynamic state variables. The measurement and comparison of cell efficiencies and system performance are also beneficial. If H2 is oxidised at the anode side and air is reduced at the cathode of the cell while an oxide ion conductor is used as the electrolyte, then the cell reactions are given below (Steele, et al. 2001; Larminie, et al. 2003; Laboratory 2004):

At the anode, hydrogen is split into protons and electrons, which then recombine with oxygen ions to produce water:

H2 + O2-��2O + 2e-(1.1)

At the cathode, oxygen is split into oxygen ions:


1/2 O2 + 2e- ��2-(1.2)

Thus, the overall reaction is

H2 + 1/2 O2 ��2O (1.3)

The calculation of open circuit voltage (OCV)A SOFC with an oxide-ion electrolyte under OCV conditions yields a voltage difference E

between the anode and cathode. This voltage belongs to the Gibbs free energy relation �G(Larminie, et al. 2003),

�� = �� (1.4)

� = ��

(1.5)

If the SOFC is considered to be an oxygen concentration cell, the OCV can be determined according to the Nernst equation (Larminie, et al. 2003; Laboratory 2004). The EMF (electromotive force) or reversible voltage (thermodynamic) at high temperatures is given as:

� � � � )ln(2/)/ln(2/ 21

222 OOHHo PFRTPPFRTEE �� (1.6)

According to equation 1.6, the OCV depends on the cell temperature and the concentration of hydrogen, water, and oxygen at the anode and cathode.

The calculation of performance efficiencyThe net efficiency of a fuel cell is defined as (Larminie, et al. 2003; Laboratory 2004):

in

e

QW

�� (1.7)

However, the real overall efficiency of a fuel-cell system should be written as the product of the heating efficiency, �H, and the electrochemical efficiency, �E. The electrochemical efficiency,however, is the result of three contributing efficiencies: the reversible (thermodynamic) efficiency, �R, the voltage or part-load efficiency, �V, and the current efficiency, �J:

EH �� (1.8)

where

JVRE �� (1.9)

Equation 1.8 then becomes

JVRH �� (1.10)

Heating Efficiency;

HST

HH

com

o

H ��

��

� 1� (1.11)


Thermodynamic or reversible voltage efficiency;

HG

R ��

�� (1.12)

Voltage efficiency;

rev

cellV E

E�� (1.13)

Current efficiency;

FJ i

i�� (1.14)

Calculation of the rate of water production at the anode sideThe following calculations were performed (Larminie, et al. 2003; Laboratory 2004)

smolFV

Pmc

epwater /

..2, �� (1.15)

The molecular mass of water is 18.02 × 10�3 kg mole�1, so equation (1.15) may be written as

skgVPm

c

epwater /1034.9 8

,�� (1.16)

The following equation can be used for calculating the production of heat.

WVEPP

celHeat ��

��

�� 1 (1.17)

1.6 Polarisations

The polarisation is defined as the difference between the expected reversible voltage and the operating cell voltage. It is also called the overpotential (�) (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006).

� = �� (1.18)

The polarisation of the cell is the sum of three losses, activation, concentration and ohmic.

� = ��

+ ��

+�� (1.19)

The polarisation of a fuel cell is explained in Figure 1-4.


Figure 1-4 FC polarisation curve (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006)

1.6.1 Activation Polarisation/Charge TransferThe activation polarisation arises from the kinetics of the charge-transfer reaction at the

electrode/electrolyte interface. This charge-transfer reaction/electrochemical reaction has energy barriers referred to as the activation energy. This barrier leads to the polarisation of the cell and is known as the activation polarisation. According to the Bulter–Volmer equation,

� ��

��

��

��

RTF

RTFii o

�� ./1exp..exp (1.20)

The activation polarisation follows the Tafel equation (Larminie, et al. 2003; Zhang 2006),which is given as below;

��

��

��

oiiba log� (1.21)

where “a” and “b” are constant and are related to the electrode material, the type of electrode reaction and the temperature.

1.6.2 Ohmic Polarisation/ResistanceThe ohmic polarisation arises from the resistance of the electrolyte (ionic conductor), the

electrode (electronic conductor), the current collectors and the connections between the cell components. It can be described as (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006)

iR�� (1.22)


The ohmic polarisation �� -6 S) when current flows and stops.

1.6.3 Concentration Polarisation/DiffusionThe concentration losses arise from mass transport, e.g., the diffusion of the reactant/active

species to or from the electrode surface is slower than the charging and discharging current “i”. For limited diffusion, concentration polarisation can be described as (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006)

��

��

��

oCC

nFRT ln� (1.23)

Concentration polarisation occurs in both the anode and the cathode. It can be reduced by increasing electrode porosity and by reducing reactant utilisation.

1.7 Polygeneration Theme/Concept

“Polygeneration” refers to an energy supply system that delivers more than one form of energy to the final user; for example, electricity, heating and cooling can be delivered from one polygeneration plant (Coronas, et al. 2007). The SOFCs or ceramic fuel cells, which generate power and heat at the same time during their operation, are one such system. In this case, the SOFC is also called a combined heat and power (CHP) or polygeneration system (Weber, et al. 2006; Coronas, et al. 2007). The SOFCs may use different kinds of fuels, one at a time, such asgases (e.g., hydrogen, syngas and bio-gas), liquids (e.g., methanol and ethanol and glycerol) and solids (carbon and lignin) for their operation. The operation of multi-fuelled SOFCs for polygeneration is represented in Figure 1-5.

Figure 1-5 Polygeneration systems with a multi-fuelled FC


1.7.1 EXPLORE-SOFC-based Polygeneration “Explore” is an idea from (Fransson 2008). The main objective of the EXPLORE is to

develop a polygeneration demonstration unit from different research projects at the Division of Heat and Power Technology (KHT/HPT). Fuel-cell polygeneration project is one of them. The developers of fuel-cell polygeneration systems are adopting two different commercialisation strategies. One approach is the design of a SOFC system that operates in excess of approximately850 oC and can be combined with a micro gas turbine (MGT) to provide a power plant with a total electrical efficiency of approximately 80%. The other approach involves the LTSOFCs fed by biomass fuels, such as syngas, or by liquid fuels, such as methanol and ethanol. The proposed EXPLORE fuel-cell-based polygeneration flow diagram of the ASOFC-MGT integrated process is depicted in Figure 1-6. Along with the gasifier and the ASOFC, the system uses a biogas reformer, a combustor for burning both the solid residue from the gasifier and the excess fuel from the cell, sulphur cleaner, a MGT, and the required heat exchangers.

Mix

ed

Ga

s(L

ab

Cy

lin

de

r)

Gasifier

Liquid Fuel(Ethanol/Methanol)

Evaporator

DesulphurizationUnit

Liquid fuel

Syngas/biogas

Mixed Gas

ELECTROLYTE

ANODE

CATHODE

Water distilation

Heat Pump

Refrigeration

District Heating

Other power purposes

Heat Exchange

Burner

Turbine

SOFC

500oC

Electricity

Heat

Unconventional fuel to burner

Heat

Heat

Figure 1-6 Biofuel based SOFC for Polygeneration systems

1.7.2 SOFC Polygeneration system efficienciesThe thermal efficiency of SOFCs is limited only by the thermodynamics of the cell reactions,

which is why most fuel-cell reactions at the corresponding operating temperatures are capable of achieving maximum efficiencies greater than 80%. This level of efficiency is significantly higher than that of any type of thermal cycle that involves combustion. Although fuel cells in actual operation never approach ideal efficiencies because of inevitable losses, fuel cells are still the most efficient chemical energy conversion systems. An integrated SOFC system with gas


turbines or a hybrid system has an electrical efficiency higher than that of other power sources(Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006). A comparison of the electrical efficiencies of different power generation system is presented in Figure 1-7.

Figure 1-7 Comparison of different power generation system(http://www.fuelcells.org/info/smithsonian.pdf)

1.8 Biofuels/Hydrocarbon

1.8.1 Bio-Liquid Fuels Several liquid fuels can be used in a SOFC polygeneration system. Here we only consider

bio-ethanol and bio-methanol liquid fuels for SOFC in the following sections (Larminie, et al. 2003; Laboratory 2004; Liu 2005; Zhang 2006).

1.8.2 Bio-ethanol (C2H5OH)Bio-ethanol is a renewable fuel with strong potential for future energy production. It can be

produced from the hydrolysis of plants, industrial waste or almost any raw material that containssugar or starch. Like other renewable-energy fuels, bio-ethanol has many advantages, includingreduced greenhouse gas emissions, easy transportability, and a secure supply of raw materials (Park, et al. 2000; Gorte, et al. 2003; McIntosh, et al. 2004a; Sasaki, et al. 2004a).

1.8.3 Bio-methanol (CH3OH)Among the liquid fuels, methanol may be a better fuel for SOFCs. Methanol is toxic, volatile,

colourless and flammable. It also can be used with gasoline. It can be produced from both renewable energy resources as well from fossil fuels by pyrolysis. Methanol can also be produced by gasification of different organic materials by conventional methanol synthesis(Park, et al. 2000; Gorte, et al. 2003; McIntosh, et al. 2004a; Sasaki, et al. 2004a).


1.8.4 Biogas from BiomassTwo types of processes produce gaseous fuels from solid biomasses for SOFC operation.

Pyrolysis is a mature and developed method for thermochemical conversion.

The second process is gasification to produce gaseous fuel (Park, et al. 2000; Gorte, et al. 2003; McIntosh, et al. 2004a; Sasaki, et al. 2004a). This process is used by our department of energy, KTH, for the production of biogas. The produced biogas contains CH4, CO2, CO, N2 and H2. The biogas produced in the HPT laboratory of KTH with wood pellets has a LHV value equal to 5.19 MJ/Nm³, (Carrera 2008).

1.9 LTSOFC or ASOFC (300-600 oC) and NANOCOFC theory

The development of nanocomposite materials for fuel cells (NANOCOFC) is a science and an approach for developing functional nanocomposite materials for low-temperature SOFCs. This LTSOFC is named as ASOFC due to below NANOCOFC theory. In some cases, these nanocomposite materials can be used for other solid electrochemical devices (Zhu, et al. 2002; Zhu 2003; Zhu, et al. 2003a; Zhu, et al. 2006b; Zhu 2006c; Zhu, et al. 2008a; Zhu 2009).

The development of advanced nano-architectures requires wide flexibility and feasibility. The two-phase material (TPM) nanocomposite exhibits multiple functions and unique characteristics(Zhu, et al. 2002; Zhu 2003; Zhu, et al. 2003a; Zhu, et al. 2006b; Zhu 2006c; Zhu, et al. 2008a):

i) TPM composites consist of two phases with interfaces. The material functionalities are created through the two-phase interfacial regions (typically a core–shell structure). Such interfacial functionalities can break conventional structural limits.

ii) The ionic transport phenomena differ from the conventional structure/bulk effects, which do not play a major role in two-phase composite electrolytes. Instead, the interfacial mechanisms or fast super-ionic conduction (SIC) through the interfaces determine the electrical properties and other functionalities of the composite material.

iii) The interfacial SIC and the two source ions (O2- and H+) can significantly enhance SOFC power output at 300-600 °C.

iv) The interfacial phenomena and mechanism provide a wide range of possibilities for thedevelopment of functional materials and even for the development of functional materials from non-functional materials as long as functional interfaces can be created.

v) Interfacial and surface redox reactions could provide new opportunities for the further development of functional nanocomposites for advanced fuel-cell technology.

1.10 Objectives

To commercialise SOFCs, researchers have sought to reduce their operating temperatures by developing new electrolytes and electrodes with high ionic conductivities. However, the operation of SOFCs remains costly using these methods, and a reduction in operating temperature would significantly reduce both component (including inter-connects, sealing materials, and manifold materials) and manufacturing costs.


The objective of this study was to design and develop functional nanocomposite materials (electrolytes and electrodes) with reduced operational and capital costs for the production of power and heat from ASOFCs. The nanocomposite materials were also intended for use inpolygeneration applications with respect to both theoretical and experimental investigations. Asecond objective was to develop a fuel-flexible SOFC based on these materials. More precise objectives of this research are detailed below.� Development of nanocomposite electrolytes for ASOFCs

Nanocomposite electrolytes will be synthesised through different chemical routes. The particle size will be optimised at the nano-ordered scale. The ionic conduction phenomena and ionic transport number will be measured and discussed, as they relate to the dependence on the structure, composition, and morphology of the nanoparticles in the solid and molten phases.� Design and development of compatible composite electrodes

To achieve good performance, compatible electrodes that exhibit high catalytic activity willhave to be developed.� Characterisation and analysis of the functional nanocomposite materials with advanced

electrochemical techniques

The crystalline structure, microstructure, and morphology of the synthesised nanocomposite materials will be characterised by XRD, SEM with EDX, and TEM. The main route of ionic conduction is the interface between the solid and liquid phases, which will be investigatedthrough high-temperature Raman spectroscopy. Moreover, the thermal properties and phase-transition phenomena obtained from the thermal analyses (TG-DTA, DSC) will be investigatedto determine the effect of the solid phase and optimise the operating conditions of the ASOFC.Advanced electrochemical impedance spectroscopy will be used to determine the interfaces, electrochemical mechanism, capacitance, reaction kinetics, and electrode processes.� Demonstration of the feasibility of the nanocomposite materials for an ASOFC (advanced multi-

fuelled solid oxide fuel cell, 300-600 oC)

The composition of the developed materials will be optimised based on extensive material characterisations and fuel-cell measurements. The performance of the fuel cells will be used todirect further material developments.� Fabrication and analysis of ASOFCs using functional nanocomposite materials for

polygeneration

Different-sized multi-fuelled SOFCs will be fabricated and tested with different fuels, such as gases (e.g., hydrogen, syngas, natural gas and bio-gas), liquids (e.g., methanol, ethanol andglycerol) and solids (carbon/or lignin). The experimental and theoretical efficiencies will be compared among different fed fuels. � Design and study of a 5 kW ASOFC system for polygeneration

A planar 5 kW ASOFC system will be designed and studied for polygeneration applications.The electrical efficiency of the stack and overall efficiency will be calculated.� Exploration of new ideas/applications

Based on the NANOCOFC approach, new ideas or products will be further explored.


1.11 Thesis Outlines

My thesis consists of eight chapters. The results and discussions are presented in chapters 3, 4, 5, and 6. The short description of the chapters is presented below.

Chapter 1 is based on the background of the work. An introduction and a brief literature review about the existing work are presented. The main objectives of my thesis are also presented in this chapter.

Chapter 2 deals with the synthesis of the functional nanocomposite electrolytes and electrodes materials for ASOFCs using different techniques. The different characterisation methods/techniques for the materials, e.g., XRD, SEM, TEM, AC impedance, BET, DSC, etc.,and modelling input data are discussed.

Chapter 3 describes the results and provides a discussion about nanocomposite electrolytes based on our published articles (1, 2, 3, 4, 5 and 6). The ionic transport mechanism is also described in this chapter.

Chapter 4 includes an explanation of the results and a discussion about the nanocomposite-compatible electrodes for electrolytes. The doping effect of the materials from a first-principles(quantum mechanics) standpoint is discussed. This chapter is based on papers 7, 8, 9 and 10.

Chapter 5 concerns the modelling of 5 kW advanced fuel-cell technology for polygeneration.

Chapter 6 presents an overview of our new invention, the “electrolyte-free fuel cell,” based on paper 12.

Chapter 7 includes all of the results and conclusions based on our objectives. The future work and needs are also discussed.

Chapter 8 presents all of the references included in the thesis.



2. EXPERIMENTAL TECHNIQUES

2.1 Functional materials-synthesis for ASOFC

In this chapter, the different chemical routes e.g. co-precipitation, solid state, wet chemical etc. have been discussed to develop functional nanocomposite materials for ASOFCs.

2.1.1 Nanocomposite electrolytesFirstly, different electrolytes have been synthesized (LN-SDC, NSDC, CSDC-carbonate, and

YSDC) using a simple co-precipitation method.

2.1.1.1 Synthesis of ceria-carbonate composites electrolyte (two-steps)(Raze, et al. 2009)LN-SDC composite electrolyte was synthesized by co-precipitation method. 0.5M

Ce (NO3)3.6H2O (Sigma-Aldrich, USA) solution was mixed with Sm (NO3)3.6H2O (Sigma-Aldrich, USA) using 1:4 molar ratios. An appropriate amount of ammonia (33% NH3, Sigma-Aldrich) was added in order to co-precipitate the cations, e.g., Sm3+ and Ce3+ for SDC precursor in the hydroxide state, having pH value of 10. The precipitate was sluice thrice in deionized water, followed by washing with ethanol several times in order to remove water from the particle surfaces. The resultant precipitates were dried in an oven at 100 oC overnight and then grinded in a mortar. After which the powder was sintered at 700 oC for 2 hours to obtain the SDC powder.

The composites of SDC and carbonates were then prepared. Subsequently, the SDC and LiNaCO3 powders were mixed according to suitable weight ratio. Finally, the powder was sintered at 700 ºC in the furnace for 30 minutes and two steps LN-SDC composite electrolytewas achieved.

2.1.1.2 Improved Ceria-carbonate (SDC-Na2CO3, single step)(Raza, et al. 2010a)Nanocomposite electrolyte was synthesized by a co-precipitation process (Figure 2-1). In the

synthesis of ceria carbonate composite the following raw chemicals were used for 1.0 M solutions, Ce(NO3)3.6H2O (Sigma-Aldrich, USA) and Sm (NO3)3.6H2O (Sigma-Aldrich, USA). According to desired molar ratios (4:1), the solution of Ce (NO3)3.6H2O was mixed with Sm (NO3)3.6H2O solution. According to “metal ion : carbonate ion = 1: 2” in molar ratio, a pertinent amount of Na2CO3 solution (1.0M) was gradually added at a rate of 10 ml/min to complete the ceria-carbonate composites within a wet-chemical co-precipitation process. After this process the mixture was filtered by suction filtration method. The precipitate was dried overnight in the oven at 80 oC. Finally, the dried solid powder was crushed in a mortar with pestle, and sintered at 800 oC for 2 hours and SDC-Na2CO3 (NSDC) composites were obtainedin nano-scale, a so-called nanocomposite.


.

Figure 2-1 Co-precipitation synthesis process for ceria-carbonate nanocomposite electrolyte

2.1.1.3 Co-doped SDC-based composite electrolyte (Ce0.8Sm0.2-xCaxO2-� – Na2CO3) (Raza, et al. 2010b)

A co-doped SDC-based electrolyte (CSDC – Na2CO3) was synthesized by a co-precipitation method. Initial ingredients Ce(NO3)3.6H2O (Sigma-Aldrich, USA) and Sm (NO3)3·6H2O (Sigma-Aldrich, USA) and Ca(NO3)2 · 4H2O (Sigma-Aldrich, USA) were dissolved in de-ionized water with an optimal molar ratio of Ce3+:Sm3+: Ca2+= 4:1:1 to form a 0.1 mol/L solution.

Na2CO3 solution of 0.2 mol/L was gradually introduced in droplets to this solution, which was stirred for 30 minutes, giving a white precipitate. The precipitate was washed, filtered and dried in the oven for 12 h at 80 oC and sintered at 800 oC for 4 h in the air atmosphere. Subsequently, the powder was grinded in a mortar prior to the measurements.

2.1.1.4 Composite electrolyte preparation (Ceria-Oxide)(Raza, et al. 2011b)The oxide based composite electrolytes [(Ce0.8Sm0.2O2-� – Y2O3) = YSDC)] were synthesized

using a co-precipitation method (Figure 2-2). The initial ingredients Ce (NO3)3·6H2O and Sm (NO3)3·6H2O were dissolved in de-ionized water with an optimal molar ratio of Ce3+:Sm3+= 4:1 to prepare a 0.1 mol/L solution. Oxalic acid (10% of SDC) was added to the SDC solution as the precipitation agent to prepare the precursor. The precipitate was washed three times in de-ionized water. Another solution of yttrium oxide (Y2O3) was prepared separately; 0.5 g of Y2O3 was


dissolved in 10 ml of 3M hydrochloric acid (HCl) to form a solution, heated and stirred at 80 oCfor 30 minutes.

Subsequently, Y2O3 solution was added drop by drop to the first SDC solution and stirred for 30 minutes, giving a white precipitate. After washing and filtering, the precipitate was dried in an oven overnight at 80 oC and sintered at 750 oC for 3hrs in atmosphere conditions. Ultimately, the homogenous nanocomposite electrolyte was obtained.

Figure 2-2 Co-precipitation synthesis process for ceria-oxide nanocomposite

2.1.2 Synthesis of nanocomposite electrodesDuring the experimentations, different nanocomposite electrodes were prepared. Some

electrodes were used as anode and cathode for symmetrical fuel cell configuration and some only used as anodes.

2.1.2.1 Composite electrode (Zn0.5Ni0.5)- for symmetric cell configuration(Raza, et al. 2011d)

The nanocomposite electrodes were prepared using a standard solid-state reaction method. NiCO3·2Ni (OH) 2·6H2O (Sigma Aldrich, USA) and Zn (NO3)2·6H2O (Sigma Aldrich, USA)were mixed and grinded in the appropriate stoichiometric ratios. The resulting powder was sintered for 4 hours at 800 oC in a furnace. NSDC was prepared as previously reported. NSDC and ZnO/NiO were subsequently mixed in a 50:50 (volumetric) ratio and the powder was


grinded completely in order to avoid the non-homogeneity of the composition of the final powder. This composite electrode could be used as an anode and cathode.

2.1.2.2 Composite anode (Zn0.6Fe0.1Cu0.3-GDC)-for asymmetric configuration(Raza, et al. 2011g)

The optimal composition of the Zn (NO3)2·6H2O (99.99%, Sigma Aldrich, USA), Fe (NO3)3·9H2O (99%, Merck), CuCO3·Cu (OH) 2 (99.99%, Sigma Aldrich, USA) were mixed together. The mixed powder was calcined at 500 oC for one hour, and then the grinded powder was sintered at 750 oC for 4 hours. The as-obtained anode powder (50 vol.%) was mixed with prepared nanocomposite GDC- Na2CO3 (50 vol.%). The powder was grinded again for 30 minutes in order to make it more homogeneous.

2.1.2.3 Composite electrode (Cu0.2Zn0.8)- for symmetric cell configuration(Raza, et al. 2010c)

The nanostructure anode was prepared by a solid-state reaction. A stoichiometric amount of CuCO3 (99.99%, Sigma Aldrich, USA) and ZnNO3 (99.99%, Sigma Aldrich, USA) were mixed and grinded in a mortar. The CuCO3 and ZnNO3 were pre-calcined at 700 oC for 3 h to remove the carbonates and hydroxides and then mixed with nanocomposite electrolyte (NSDC) in a 1:1 volumetric ratio. The powder was sintered for 4h at 800 oC in a furnace.

2.1.2.4 Electrode preparation (LiCuNiZn)-symmetric cell configuration(Raza, et al. 2011e)The nanocomposite electrodes were prepared using a solid state reaction method. The

appropriate stoichiometric amounts of Li2CO3 (Sigma-Aldrich, USA), CuCO3·Cu (OH)2(99.99%, Sigma Aldrich, USA), NiCO3·2Ni (OH) 2·6H2O (99.99%, Sigma Aldrich, USA), and Zn (NO3)2·6H2O ((99.99%, Sigma Aldrich, USA) ) were grinded together in a molar ratio of 2:3:5:7. The resulting powder was sintered for 4 hours at 800 oC in a furnace. The Li Cu Ni ZnO (LCNZ) and electrolyte powder were grinded thoroughly in order to avoid any non-homogeneity in the composition of the final powder. The sintered powder was again grinded for 30 minutes.

2.2 Fuel cell fabrication and electrochemical characterisation

In order to obtain performance of the SOFCs, different cells were fabricated as described below.

2.2.1 Conventional cell (three layers)Conventional fuel cells have two different configurations: (i) asymmetrical cell (where anode

and cathode was different) and (ii) symmetrical cell (where anode and cathode was similar). The difference between these two can be seen in Figure 2-3.


Figure 2-3 Schematic figure of asymmetrical and symmetrical cell

2.2.1.1 Asymmetrical cellThe asymmetric cell was fabricated in the form of small sized pellets (13 mm). The dry-

powder-pressing technique involved loading a mold with the powders of anode successively followed by the electrolyte and finally the cathode, all being pressed in one step to form a complete fuel cell assemble for measurements. The pressing load employed 250 Mpa using stainless steel die.

The cells were sintered at 650 oC in air for 30 minutes to form dense pellet. For fuel cell performance measurements, anode and cathode surfaces were painted with silver for current collectors. The active area of the fuel cell was 0.64 cm2 and tested at different temperatures.

2.2.1.2 Symmetrical cellThe electrode and electrolyte powders were pressed uniaxially with a load of 250 MPa into

pellets in a symmetrical order of Anode (LCNZ and NSDC) /Electrolyte (NSDC) /Cathode(LCNZ and NSDC). The diameter of the pellet was 13 mm. Silver paste was used on both sides of the pellet to collect the current.

The larger area cell of 6 × 6 cm2 was also constructed by hot-pressing technique at 600 °C and using �� !� �� "��#� �� collectors for the large area cell.

2.2.2 Fuel cell performanceThe electrochemical performance of the fuel cell was measured at a temperature range of 300-

600 °C under laboratory condition and under a variable resistance load, which adjusts the outputs of cell voltage and power. By collecting data of the cell voltage and current under each resistance load, I (current) - V (voltage) or P (power), a curve can be drawn from the collected data. These curves are called I-V or I-P characteristics which are displayed and discussed during the latter parts of the this thesis. Measurements were obtained using a computerized instrument (SM-102, Sanmusen Corp. China) to complete the results handling processes.


A number of different fuels were used in order to allow a comparison to be made between them. Hydrogen and bio-gas were used as gaseous fuels in the range 70–110 ml/min at atmospheric pressure on the anode side, while air was used on cathode side of the cell with a flow rate of about 100-110 ml/min. The composition of the bio-gas was H2 (25%), CH4 (25%)CO2 (25.1%), and N2 (24.9%). Different liquid bio-fuels (e.g., Bio-methanol (CH3OH) and Bio-ethanol (C2H5OH)) were fed directly into a closed cylindrical neck flask on the anode side, while air was supplied to the cathode side. Ethanol solution was prepared by mixing ethanol and de-ionized water in a mass ratio of 1:1, and the flow rate of the liquid fuel was maintained at about 3 ml/min.

The performance of the cell can be normally described by the Power density and calculated by the following;

Power density (P, mW/cm2) =Current density (I, mA/cm2) ×Voltage (V, V)

2.3 Mixed conductor synthesis for single component electrolyte free fuel cell

The electronic conducting materials, LiNiZn based oxides (LNZ), used in the one homogenous layer device were prepared by solid state reaction method. Stoichiometric amounts of Li2CO3, NiCO3, 2Ni (OH)2·6H2O, Zn (NO3)2·6H2O (all chemicals from Sigma-Aldrich, USA)were mixed, grinded and sintered at 800 °C for 2-4 hours. Some typical composition with amolar ratio among metal elements are: Li:Ni:Zn = 15: 45: 40.

The SDC was prepared by a co-precipitation process. Stoichiometric amounts of CeNO3·6H2O and SmNO3·6H2O (Sigma-Aldrich, USA) were prepared as a mixture solution in 0.5 M concentration by distilled water and stirred at 120 oC. An appropriate amount 0.5 M Na2CO3 solution in a molar ratio of Ce Sm : CO3

2- = 1.0 : 1.5 was added into the CeSm-nitrate solution with a fixed rate: 10 ml/min while stirring to form the co-precipitate following washing, filtration and drying. The resulting material was then finally sintered at 800 °C for 2-4 hours in order to prepare SDC powders.

Both sintered LNZ and SDC powders were mixed to obtain a homogenous mixture, and then it was pressed uniaxially with a load of 200-300 MPa to form one layer pellet. The end surfaces were either covered by silver or, alternatively, we used nickel foam on one side and pasted the other with silver to collect current. The pellet diameter was 13 mm, with a �#��$��&��.0mm. The detail of synthesis of mixed conductors and experimental setup for electrolyte free fuel cell (EFFC) has been described in Figure 2-4.


Figure 2-4 Synthesis of mixed conductors and experimental setup for EFFC

2.4 Electrochemical impedance spectroscopy (EIS)

Electrochemical Impedance Spectroscopy has a very important role in the applied and fundamental electrochemistry. EIS is relatively new and a very powerful technique for characterizing the electrochemical behavior of materials, and to investigate their interfaces with conducting electrodes and over a range of frequencies mHz to MHz (Zhu 1995; Ralph E. White 2002; Evgenij Barsoukov 2005). Here is some basic theory of the EIS (Zhu 1995; Sholklapper 2007; Research 2010),

In an AC impedance theory, the frequency is non-zero then Ohm’s law is

� = �� (2.1)

and the complex Impedance is defined as,

� =

�(2.2)

If the applied AC signals (10mV) to the fuel cell is shown as

� = |�|��(��) (2.3)

After applied voltage the current of the cell shown as below,

� = |�|��(��) (2.4)

Using these values in equation 2.2, we get


� =| |��(��)

|�|��(��)(2.5)

� = |�|�� (2.6)

This equation can be decomposed into real and imaginary parts,

� = |�|(�� + �!��) (2.7)

|�|(��) = ��= Real Impedance, |�|(!��) = ��= Imaginary Impedance

In this study, an Auto VersaSTAT 4 potentiostat/galvanostat (Princeton Applied Research, USA) was frequently used to analyze the interfacial behavior of the two phase nanocomposite materials. The frequency was varied from 0.01 Hz to 1 MHz at 10 mV. The as obtainedexperimental data was fitted with software (ZSimpWin, Princeton Applied Research, USA) with an equivalent circuit which contains various impedance elements representing the involved reaction steps and determined the LRQ parameters of the materials and devices used.

2.4.1 Conductivity MeasurementsThe electrical and ionic conductivity measurements were determined by using two different

techniques, as mentioned below.

2.4.1.1 AC Conductivity:The AC conductivity of the material was calculated from the open circuit impedance spectra

at high frequency arc intercept. The following formula was used for the calculations.

RAL

��(2.8)

The activation energy was calculated from the following Arrhenius equation,

)exp( RTEao �� (2.9)

2.4.1.2 DC Conductivity: The total conductivity of the nanocomposite electrode was determined by using the

conventional four- probe DC method, which employed the power supply (GW instek, PSP-2010, Taiwan) from 350-600 oC.

using the Heb-Wagner equations (Hebb 1952; Wagner, et al. 1957)

� �

��

2

2ln2 H

Hionmeasured p

pt

FRTE

(2.10)


2

2ln2 H

HNernst P

pF

RTE��

�(2.11)

� �Nernst

measuredion E

Et(2.12)

where Emeasured is obtained by current source and E Nernst was calculated from Nernst equation. The ionic conductivity was obtained with this formula

totalionion t �� (2.13)

The total conductivity is the summation of ionic and electronic conductivity,

electroniontotal �� (2.14)

Therefore, the electronic conductivity can be established using the above equation.

2.5 Microstructure analysis techniques

2.5.1 X-Ray Diffraction“XRD is a useful technique that reveals detailed information about the chemical composition

and crystallographic structure of materials. X-rays are electromagnetic radiation of wavelength about 1 Å (10-10 m), which is approximately the same size as an atom” (Whittingham 1989).They occur in the portion of the electromagnetic spectrum between gamma-rays and the ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic level. XRD is one of the most important characterization tools used in solid state chemistry and materials science” (Whittingham 1989; Zhang 2006). In this study, XRD patterns of the samples were collected using a Philips X’pert pro super d�� !��#� '�� *<�� >?��@\�^�_`��#�� {��on.

The crystallite size DXRD of as-prepared composite can be calculated by using the following Scherrer equation.

Where �

�cos9.0

BDXRD �

(2.15)

)(22 FWHMBBB sm ��


2.5.2 Scanning Electron Microscopy (SEM)SEM produces a high resolution image of a sample surface by scanning it with high energy

beam of electrons which provides the detailed information about microstructure, morphology and composition of the samples (Zhang 2006). SEM images have a characteristic three-dimensional appearance and are useful for judging the surface structure of the sample (size, shape and arrangement). In our study, the microstructure and morphology were examined using a Zeiss Ultra 55 scanning electron microscope (SEM). The composition of the sample was examined using the EDX unit attached to the SEM.

2.5.3 Tunnelling Electron Microscopy (TEM)TEM is an imaging technique which is widely used to characterize sample’s morphology

including the size, shape and arrangement of the particles. It also provides the crystallographic information such as the arrangement of atoms in the specimen and their degree of order (Zhang 2006). For the detailed morphology and microstructure including the crystal size and shape, the TEM was employed using a JEM-2100F (TEM, JEOL, Japan) with a carbon-coated copper grid.

2.5.4 BET- Surface area analysis“Brunauer-Emmit-Teller (BET) theory is a well-known rule for the physical adsorption of gas

molecules on a solid surface. The BET method or isotherm is widely used to determine the effective surface areas of solids materials with complicated shapes, such as porous powders, by physical adsorption of gas molecules” (Zhang 2006). The observation of the so-called adsorption and desorption isotherms is used to determine the amount of gas molecules adsorbed to a surface. A total surface area Stotal and a specific area SBET are evaluated by the following equations:

BETthBET S

D�

3106��

(2.16)

Surface area of the samples used in this study were determined using the BET method from the low-�� |�}�� ~&��'�� |��|��@��'��12 h.

2.5.5 Raman SpectroscopyIn this study two different Raman spectrometers were used for sample analysis. Firstly, for

ceria-carbonate electrolytes, the samples were excited by the SHG light of Nd: VYO4 solid laser at 532 nm. Raman spectra were measured by Horiba T-64000 with triple monochromator. Wavenumber was calibrated by Si and CCl4. Each spectrum was measured at the wavenumber ��|�� #��'�3

2- ion (ca. 1060-1080 cm-1) from 808-1377 /cm by CCD detector with 2048 x 512 channels. Data was collected by Jobin-Yvon Spectra Link at 2047 points of CCD elements.

Secondly; in order to analyse the deposition of carbon on the anode material under the use of bio-fuels, measurements in the 100–3000 /cm region using a backscattering geometry of a


Raman spectrometer (Waltham, MA, USA) were also carried out. All samples were recorded using five scans in the range 2400 - 2650 eV.

2.6 Thermal Analysis

2.6.1 DSC analysisThe DSC is used for measuring the direction, temperature and magnitude of the thermal

transition in material by heating and cooling rate. Thermal properties of the sample were evaluated with a (DSC) 2920 thermal analyzer (TA Instruments, DE 19720, USA) at heating and cooling rates of 20°C/min using air. There are endothermic effects in temperatures between 400°C and 450 °C.

2.6.2 Dilatometer analysisA dilatometery is a thermo-analytical techniques used for obtaining the highly precise

measurements of volume changes in solids, melt powders and pastes without any strain. This change depends upon the temperature fluctuation, chemical reactions, absorption of fluids, or physical stress such as pressure on a solid substance. To examine the phase transitions, chemical reactions (oxidation) and solid-state reaction, the dilatometery is very useful. There are many applications for the dilatometery on solids, powders, liquids in research. There are three main different types of dilatometers, 1) the flat plate dilatometer, 2) modern laser and optical dilatometers 3) the Push-rod dilatometer.

In this study, NETZSCH 402C push-rod dilatometer was used with a programmed temperature.

2.7 Density/porosity measurements

The analyzers He-pycnometry (AccuPyc 1340 Micromeritics, USA) measure the volume and density calculations on a wide variety of powders, solids, and slurries having volumes from 0.01 to 350 cm3). In this atusy “He-pycnometry” (AccuPyc 1340 Micromeritics, USA) was used for density measurements of electrolyte and porosity of electrodes.

2.8 Theoretical modelling and calculations

2.8.1 Composite effect by ab initio calculationsIn order to investigate the structure and doping effect of the composite electrode in greater

detail, computational modelling was carried out using first-principle calculations with the projector-augmented wave PAW method, as implemented in the Vienna ab initio Simulation Package (VASP) code (Blöchl 1994).


2.8.2 Design of advanced solid oxide fuel cell stack for 5kWThe planar ASOFC stack for 5kW is designed and is estimated number of cells according to

the obtained electrical power from our experiments, operating cell voltage, operating current density, and cell active area from the below equation:

AivPN��

�(2.17)

Operating cell voltage is determined regarding to the fuel cell application and the required voltage. Consequently, operating current density may be found according to the operating cell voltage from polarization curves of fuel cells from previous experiments. Finally, cell active area can be chosen according to the manufacturing methods and the materials used in the fuel cell structure.

Knowing the number of required cell for desired electrical power, it can be simply estimated by using following equations:

NvV �� (2.18)

VPI �

(2.19)

The developed stack model is based on the planar solid oxide fuel cell type with the counter-flow design. The achievable current density at the base case (operating temperature of 550 oCand fuel utilization of 86%), the current density is about 800 mA/cm2. In order to achieve the desired power output (5 kW) at cell voltage of 0.6 V, the fuel cell stack should consist of approx. 210 single cells. The input parameters of the model are listed and shown in Table 2-1 and

Table 2-2 includes calculated results for the fuel cell stack.

Table 2-1 Input parameters of the fuel cell stack model

Input parameter(single cell) Unit ValueAir and fuel temperature oC 25Air and fuel pressure bar 1.01Anode (NiZn) thickness mm 0.20Cathode (NiZn) thickness mm 0.15Electrolyte (NSDC) thickness mm 0.15Interconnect thickness mm 2Electrical power kW 5Mean current density mA/cm2 800Pressure drop in fuel cell % 2Pressure drop in heat exchangers % 5Fuel utilization % 86Cell active area cm2 50


Table 2-2 calculated results of the fuel cell stack

Parameters Unit ValueOperating temperature oC 550Cell voltage V 0.6Fuel feed ( biogas) Kg/h 4.70Oxidant feed (air) Kg/h 21.45Number of the cells --- 210

2.8.2.1 System modellingA typical lumped-parameter model is used for system modelling based on the one

dimensional steady state flow. In addition, it is assumed that there is no gas leakage in the system and the heat losses to the environment can be neglected. Furthermore, the cell operating voltage and electrical power output are kept constant (0.6 V and 5 kW). In order to assure optimum of proper system performance, the energy and mass balance equations are derived for each component in the system.

2.8.2.2 System efficiency evaluationSince fuels used in fuel cells are usually burnt to release their energy in conventional

methods, it would be reasonable to compare the produced electricity with the released heat of fuel combustion. In this way the efficiency of fuel cell can be compared with heat engines. The efficiency of the SOFC may be estimated by:

fuel

elSOFC Q

P�� (2.20)

Where, Qfuel, is the amount of chemical energy in the fuel fed into the fuel cell stack:

fuelfuelfuel LHVmQ �� (2.21)

fuel

HRSelpoly Q

PP �� (2.22)



3. NANOCOMPOSITE ELECTROLYTES

In this chapter, major results of the prepared nanocomposite electrolytes for ASOFCs from paper 1-4 are presented.

3.1 Enhancement of conductivity in ceria-carbonate nanocomposites

The purpose of this work is to study the SDC-carbonate (two steps) compositemicrostructures with emphasizing on interfacial regions between the SDC and LiNa-carbonates in order to determine the interfaces and provide proofs of the existence of the interfaces thus the interfacial mechanism for superionic conduction.

3.1.1 Interfacial regions between the SDC and carbonatesThe two-phase ranges are further observed by the high resolution TEM analysis as shown in

Figure 3.1(a), where is a bright-field image of the particle of (Ce, Sm) O2 + LiNa-carbonate. Figure 3.1(b) shows a rectangle particle which is likely (Sm,Ce)O2 and the FFT diffraction pattern (inset in Fig. 3.1b) clearly shows two sets of reflections from two phases.

It is obviously observed that the SDC-carbonate is a two-phase composite. The materials consist of grains of SDC mixed with carbonate salt. It can be seen also from Figure 3.1(a) an identify confirmation of the interfaces between the SDC and carbonate, where the carbonates seem to be coated on the SDC particle. The coated layer is ranged around 10 nm thick. From Figure 3-1, we can see nano-particle SDC ranges 10-30 nm, while the SDC-carbonate composite particles range from 50-100 nm due to two-phase composite particle where the carbonate seems to be coated on the SDC particles.

Figure 3-1 (a) HRTEM image of SDC-carbonate composite, (b) diffraction pattern

3.1.2 Conductivities Figure 3-2 shows the A.C. conductivities of the SDC-carbonates. The ionic conduction

activation energy can be deduced from the Arrhenius curve to be 0.28 eV when the conductivities experience a superionic transition at higher temperature range above 500 ºC. We

20 nm20 nm 5 nm5 nm


had taken as a case study ceria-carbonate two-phase material to discuss interfacial ion interaction, the interface electric field and the corresponding oxygen ion activation energy. By taking 10 nm of the interfacial region into calculation, we had obtained the oxygen ion activation energy less than 0.2 eV. It is well agreed between the calculated value and the result obtained from experiments to prove the interfacial superionic conduction mechanism.

Figure 3-2 A.C. conductivities of the SDC-carbonates

3.1.3 Interfacial superionic conductionUnlike the conventional superionic conduction in a single-phase material it takes place from

low conductive phase transferred to the superionic conduction one accompanying a phase structure changes. In the ceria-carbonate two-phase systems the superionic conduction occurs at the interfacial regions between the two constituent phases. It is thus determined by the interfaces, i.e., the change in the interfacial properties without involving individual phase structural changes. The conductivity is then strongly dominated by the interactions or coupling effect between the constituent phases. Therefore, the two-phase co-existence can create the interfacial effects and thus multi-functions, typically the superionic conduction.

The SDC-oxide and carbonate composite approach can offer a careful control of the carbonate amount in a level not only to form the percolative network and continuous ion conducting channels/framework, but also to maintain good mechanical strength. In our experiments 20 wt% carbonate can perform these functions. In this case, the carbonates are in stick or attached tightly with the SDC-oxide without flowing freedom causing strong mechanical strength; in the same time they can optimize the interfaces and interfacial interactions to facilitate the oxygen ion transport. Especially, when temperature near or above the carbonate melting points, the high mobility of carbonates, especially the fast rotation of

1.2 1.3 1.4 1.5 1.6 1.7 1.81E-4

1E-3

0.01

0.1

1550 500 450 400 350 300 250

Cond

uctiv

ity (S

/cm)

1000/T(K-1)

t/oC


carbonate anion groups may enhance the ceria (SDC) surface oxygen ion mobility, resulting in interfacial superionic conduction. The superionic conduction often takes place accompanying the carbonate (salt) melting. It is questioned if the conductivity enhancement is due to the molten salt behaviour, not the interfacial mechanism. As pointed out by (Schober 2005):"Conceivably, the newly formed phase between the two constituent phases could have its melting point at the transition”.

3.2 Improved Ceria carbonates Composite Electrolytes

The microstructure of sintered SDC–Na2CO3 (NSDC) was examined by scanning electron microscopy (SEM), and shown in Figure 3-3(a). The image reveals that prepared NSDC composites nanoparticles morphology is tetrahedron shaped in nano scale between 30 and 100 nm. The size and morphology of nanocomposite particles can be controlled by the preparation skills, sintering temperature and time as well. On the other hand, the conventional two-step prepared SDC–LiNaCO3 composites consist of much large particle size in micro meter level (Huang, et al. 2007d; Huang, et al. 2008).

The NSDC nanocomposite and conventional LN-SDC composite electrolytes have exhibited very different morphologies, especially, when we used the NANOCOFC approach to develop the materials. The crystallographic and more details of nanostructure of the NSDC nanocomposites were investigated by transmission electron microscopy (TEM) as shown in Figure 3-3(b).

Figure 3-3 (a) SEM image for NSDC, (b)TEM image for NSDC

The carbonate is coated on the SDC particle surface in a core shell structure as observed clearly by TEM. The carbonate shell is an amorphous. This amorphous nature can facilitate ionic conduction.


3.2.1 IV/IP Characteristics (Fuel Cell Performance)Figure 3-4 shows the I–V characteristics for comparison of two fuel cells using the

conventional LN-SDC composite and NSDC nanocomposite as electrolytes. The maximum fuel cell power density reached about 1000 m/W2 and 1150 mW/cm2 at temperatures 590 oC and 500 oC as shown in Fig.3.4(a,b) for SDC–LiNaCO3 and SDC–Na2CO3, respectively. The open circuit voltage (OCV) is 0.98 V at 590 oC for LN-SDC while 1.018 V at 500 oC for NSDC electrolytes. The improved ceria–carbonate electrolyte (one step) displays significantly higher OCV than that of the conventional one. The lower OCV in the conventional composite (two-step) electrolyte fuel cell may be due to less dense of the electrolyte which caused some gas penetration directly through the electrolyte to make electrical voltage losses. This confirms that the nanocomposite NSDC electrolyte (one step) membrane is dense. We observed that the performance of fuel cell using the NSDC nanocomposite electrolyte was enhanced significantly at lower temperatures, 300–500 oC, even functioned well between 200 and 300 oC than those using the conventional LN-SDC composite electrolytes. It may be explained due to an interfacial superionic conduction mechanism in the two-phase composites first reported by (Schober 2005), large amount of surfaces of nanoparticles ceria and interfaces in the SDC nanocomposites can significantly enhance and improve the ionic conductivity.

The best performance of about 1100 mW/cm2 was reported by Huang et al. for the conventional LN-SDC composite electrolyte fuel cell at 600 oC (Huang, et al. 2007d; Huang, et al. 2008). While the better fuel cell performance of 1150 mW/cm2 was obtained at temperature 500 oC in this work. The advantages of the nanocomposites are obvious. The excellent fuel cell performances achieved so far for the nanocomposite electrolytes show that ceria–carbonates nanocomposite electrolyte is appropriate for the low temperature (300–500 oC, even 200–300 oC) fuel cells. The stability of the fuel cell can be expected to improve due to only involving the solid carbonate and also at much lower temperatures in fuel cell operations. In the meantime it also requires developments of high catalytic electrodes we will report elsewhere.

0 400 800 1200 1600 2000 2400 28000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 400 800 1200 1600 2000 2400 2800

0

100

200

300

400

500

600

700

800

900

1000

1100

Cell v

olta

ge (V

)

Current density (mA/cm2)

(450oC) (480oC) (500oC) (520oC) (550oC) (580oC) (590oC)

(a)

Powe

r den

sity

(mW

/cm

2 )

0 400 800 1200 1600 2000 2400 28000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 400 800 1200 1600 2000 2400 2800

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Cel

l vol

tage

(V)


(350oC) (430oC) (450oC) (470oC) (490oC)

(b)

Pow

er d

ensi

ty (m

W/c

m2 )

Figure 3-4 (a) FC performance with LN-SDC electrolyte (two steps), (b) FC performance with NSDC electrolyte (one step)


3.3 Calcium and samarium co-doped ceria based nanocomposite electrolytes

3.3.1 Microstructure/morphology:Figure 3-5(a) shows the microstructure of the co-doped ceria-Na2CO3 (CSDC-carbonate)

composites, obtained by SEM. The morphology of the sample is largely homogeneous, signifying a rather dense. In the co-doped powder, homogeneity and chemical composition were determined by EDX as shown in Figure 3-5(b). The results are in good agreement with the experimental stoichiometric indexes and the nominal composition of CSDC-carbonate.

Figure 3-5 (a) microstructure of CSDC-carbonate (SEM), (b) EDX analysis

There is another way to develop ceria-host structure to form the ceria-carbonate nanocomposites, e.g. co-doping. The co-doping can stabilize the ceria properties, e.g. against the ceria reduction in H2 to cause electron conduction (Banerjee, et al. 2007). The CSDC-carbonate electrolytes have also achieved the same or even better fuel cell performance. The cell power density Pmax, with H2 as a fuel, reached a maximum of 980 mW/cm2 at 560 °C and at the very low temperature of 350 °C the power density was 200 mW/cm2 as shown in Figure 3-6. The open-circuit voltages (OCV) was 1.05 V at 560 °C and 0.99 V at 350 °C and indicates that the electrolyte membrane is sufficiently dense. The performance is very encouraging at very low temperatures with the two phase co-doped nanocomposite electrolyte. This may be attributed from improved chemical surface properties of particles prepared using the co-doping technique, which appears to be useful effective fuel cell performance at lower temperatures.

The obtained performance at lower temperature with co-doped ceria electrolyte is comparatively better with conventional electrolytes YSZ and SDC based on the previous reported results by the researchers particularly, Huang et al obtained maximum power density 131 mW/cm2

at very low temperature 350oC using YSZ electrolytes (Huang, et al. 2007c); Takashi Hibino et al was obtained 101 mW/cm2 at 350 oC using ceria based electrolytes (Hibino, et al. 2000); Zongping shao et al has succeeded 1010 mW/cm2 and 402 mW/cm2 at 600 oC and 500 oC using single phase SDC electrolyte (Shao, et al. 2004); and Toshi Suzuki et al demonstrated 1000 mW/cm2 at 600 oCusing zirconia based electrolytes (Suzuki, et al. 2009).


There is no significant electrode polarization process from Figure 3-6, because the I-Vcharacteristics are linear. The less IR drop from electrolyte ohmic behavior may be the reason for the higher performance at such low temperature. Due to the calcium co-doped two phase electrolyte, it has a higher conductivity and which also make the excellent performance. By viewing high performance SOFCs in literature (Hibino, et al. 2000; Shao, et al. 2004; Suzuki, et al. 2009), it is a common fact that the high performance corresponds to linear I-V characteristics. In our work, we used CSDC-carbonate mixed with the anode and cathode, the reaction at electrodes would be enhanced because of the high ionic conductivity of CSDC-carbonate. Then, electrode polarization resistance may be decreased for this development.

Figure 3-6 Fuel cell performance with CSDC-carbonate electrolytes

The Arrhenius plots of the ionic conductivity for different composite electrolytes have been shown in the Figure 3-7. The Figure 3-7a describes the oxide ion and Figure 3-7b for proton ion conductivity.

It can be seen that the CSDC-carbonate exhibits the best ionic conductivity as shown inFigure 3-7a. Since there are two effects: one is co-doping and the other is composite, where the interface built between ceria and carbonate. The co-doping can enhance the oxygen ion concentrations and interface between ceria and carbonate facilitates the ions oxide/proton mobilities where the super-ionic conduction pathway takes place. The conductivity of all materials is high at higher temperature due to the conductivity which largely depends on the oxygen/proton ions mobile concentration and has higher mobility at high temperature. The activation energy of the CSDC-carbonate (0.238eV) is obtained from the Arrhenius relation. It is lower than that of the co-doped ceria electrolyte as reported by (Banerjee, et al. 2007). This is attributed to the interfaces of the composite material.


0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

-1.6

-0.8

0.0

0.8

1.6

2.4

3.2

4.0

4.8

5.6

6.4

Activation energy(Ea)of CSDC-Na2CO3 = 0.238 eV

ln ��

(S*c

m-1 *K)

1000/T(K-1)

CSDC-Na2CO3

SDC - Na2CO3

SDC co-doped SDC

oxygen ion conductivity

1.1 1.2 1.3 1.4 1.5 1.6 1.7-3

-2

-1

0

1

2

3

4

5

6

ln ��(

S*cm

-1*K

)

1000/T(K-1)

CSDC-carbonate SDC-carbonate BZY-carbonate BCY BZY

Proton ion conductivity

Figure 3-7 (a) (in Air) Oxide ion conductivity (Arrhenius plot), (b) (in H2) Proton ion conductivity

Figure 3-7b shows proton ion conductivity of composite electrolyte at 500 oC. The proton conduction in bulk oxide and extensive proton exchange and transfer along the interface of the two percolating phases contributes to the enhancement of conductivity.

Another important parameter that was investigates was the stability of such nanocomposite electrolytes conductivity due to its major contribution in the performance of the cell. Therefore, we have measured the conductivity for the duration of 21hrs in 3 days (7hrs per day) at 550 oC as shown in Figure 3-8.

0 3 6 9 12 15 18 210.10

0.15

0.20

0.25

0.30

0.35

in H2

�(S.cm

-1 )

t(h)

T= 550 0C

in Air

Figure 3-8 Stability of ionic and protonic conductivity @550 oC of CSDC-carbonate

Figure 3-9 shows the EIS results for the complete cells with anode, electrolyte and cathode under OCV and after twice fuel cell operations for delivering power outputs. The AC impedance spectra of the cell under OCV are shown in Figure 3-9a.

It can be seen that the Rtot is decreasing as temperature is increasing from 450-550 oC which is a considerable increase in the polarization resistance which was observed in low-frequency


range, indicating that slow oxygen diffusion and oxygen reduction might occur in the cathode at 550 oC (Raza, et al. 2011d). The R1 resistance is assigned to Ohmic. This relatively low ohmic resistance results in higher cell performance. The R2 and R3 are assigned to polarization resistances at high frequency (HF) and low frequency (LF) arc. In nanoscale materials, the difference between bulk and grain boundaries is likely to be small and the resultant impedance spectrum may be modeled using a single semi-circle (Wang, et al. 2011a).

The interrelation of the different contributions of the anode, cathode and electrolyte are illustrated in the form of an equivalent circuit as shown inside the Figure 3-9a, b. The ohmic resistance of electrolyte is known as R1. The Q is constant phase elements (CPE). The R2Q and R3Q are in series and are due to two arcs of cell. R2Q element denotes the high frequency impedance arc and R3Q is due to low frequency impedance arc which is also related to inductance of the connecting leads/cables.

The cell was operated twice and analysed for the impedance spectra which is shown in Figure 3-9b at different temperature. It can be seen that the overall resistance of the cell decreased with the increasing temperature. However, RHF decreased significantly with the increase in temperature while RLF kept almost the same. The spikes of the arc in the figure indicate that there is electrode effect at high temperature. The experimental data was fitted carefully which shows the equivalent circuit inside the Figure 3-9b. The O2- charge transfer is associated with RHFwithin the electrode dominated the overall electrode kinetic at lower temperature; however, it decreased substantially with the increasing temperature. This impedance spectrum in a H2-containing atmosphere suggests a proton conducting mechanism, which has also been observed in LN-SDC system (Wang, et al. 2008).

Furthermore, it has been observe that the overall resistance obtained under OCV condition is larger than that under operating mode. It has been observed in other cells based on different ceria carbonate composite electrolyte due to introducing proton conductivity (Fan, et al. 2011).

Figure 3-9 (a) EIS of CSDC-carbonate based fuel cell under OCV , (b) EIS of CSDC-carbonate based fuel cell after twice time operation


3.3.2 Theoretical calculation on CSDC-carbonate conductivity using a general mixing rule (GMR)

The conductivity of the composite materials can be estimated using GMR reported by Uvarov(Uvarov, et al. 2009). Therefore, the conductivity of the ceria-carbonate was calculated using Uvarov model. The appropriate form of this rule is expressed as follow,

ffff AsssMX �� )1( (3.1)

The generalized equation from eq. 3.1 can be rewritten as for two phase composite materials;

iiiiiASiSASiiA ffffAi ��

1

21 )()(1)1( �� (3.2)

iiiiiBSiSBSiiB ffffBi ��

1

21 )()(1)1( �� (3.3)

Where “A” is air atmosphere and “B” is H2 atmosphere. The experimental conductivity and the calculated data were compared in the Figure 3-10, using eq.3.2 and eq.3.3. There is a good agreement between the qualitatively experimental and calculated data through this model. The comparative results from Figure 3-10 also agree with the effective medium and percolation theory/models (Holme, et al. 2010).

Figure 3-10 Comparison of experimental conductivity data with theoretical curves in both atmosphere of air, H2

The fitted curve results and parameter are described in Table 3-1. It can be seen that the conductivity of the composite electrolyte occurs via highly-conducting interfaces due to pure carbonate which have only a conductivity of 10-4 S/cm at 500 oC (Gao, et al. 2009; Wang, et al. 2011a). It has been observed that this enhanced conductivity is due to parallel connection between two resistances of the ceria and carbonate two phases.


Table 3-1 The fitted curve results and parameter

Atmosphere

Experimental Results(Conductivities of pure

components)Fitted data using mixing rule

CSDC(S/cm)Phase-1

Carbonate(S/cm)Phase-2

Factor-1(��1)

Factor-2(�2)

Thickness of the conductive layer �� (S/cm)

Total estimated conductivity �S

(S/cm)Air 5×10-2 1×10-4 0.3 0.99 0.2 1.3H2 18×10-2 1×10-4 0.3 0.99 0.2 1.5

3.4 Composite electrolyte based on samaria-doped ceria and containing yttria as a second phase

Figure 3-11(a) shows a comparison of the electrolyte samples with different compositions.The diffractogram is indexed and the crystalline structure of the sample is cubic fluorite. The average crystallite size of SDC and Y2O3 was calculated by Debye-Scherrer formula and values were 40 and 20 nm, respectively. In Figure 3-11(a), it can be seen that the composite electrolyte consists of doped ceria and yttria. All peaks can be indexed with two crystal reference. The small peaks show that some tiny ones are crystalline form of the Y2O3. No individual phase of samariawas found, i.e., Samaria formed a solid solution with the ceria in the SDC. In addition, there is no lattice change from SDC to Y2O3-SDC composites. This implies that yttrium did not dope the ceria thus didn’t make the SDC host structural change. The intensities of the peaks related to yttrium oxide indicate different it content in each sample. The XRD results prove the yttrium oxide-SDC electrolyte as a two-phase composite system. In addition, the diffraction patterns are also the proof of a nanocrystalline structure of the sample.

The density of the sintered electrolyte (YSDC) is 6.4939 g/cm3 and the relative density was about 91%. The highest densification is obtained at 40% concentration of yttria in the SDC electrolyte.

Figure 3-11 (a) crystal structure (XRD), (b) microstructure of YSDC (SEM)


Figure 3-11(b) shows the morphology and nanostructure of the Y2O3-SDC electrolyte of SEM. Different particle sizes are seen in the figure in the range of 30 nm-80 nm. The prepared Y2O3-SDC composite electrolyte is denser than single doped ceria materials. It has been observed that the smaller particle size, the conductivity will be higher because more and more ions can pass through the interface of the materials. The interfacial region between the yttriumoxide coating and samaria doped ceria thus plays a key role in the material conductivity and cell performance. These results also support the J. Maier theory published in the Nature journal that the interfaces are increasing with decreasing of the size of the particle (Maier 2005).

The core shell structure of the Y2O3-SDC was observed at an optimized sintering temperature of 700 oC. A nanoparticle of the synthesized electrolyte is shown in the inset of Figure 3-11(b) as a two-phase composite consisting of a coating layer (yttrium oxide) named as a surface region, an inner part named as core of the SDC particle and the region between these two parts called interface.

3.4.1 Ion transportation mechanismThe oxide/carbonate introduced in a doped ceria electrolyte to create a 2nd phase as a core-

shell structure. J.Maier, reported that there are highly conducting paths near to phase boundaries due to space charge zones and it is much higher than in bulk (Maier 2005). The ion transportation mechanism is shown in Figure 3-12. The two-phase composite may form a large interface region for ion conduction paths between the SDC and oxide/carbonate. It is so called “superionic highway” as reported before by Schober and Zhu for ceria-carbonate and ceria-oxide composite electrolytes (Shao, et al. 2004; Zhu 2009), so that to enhance greatly the material conductivity. This interface between the host SDC and yttrium oxides has no bulk structural limit for creation of high concentration of mobile ions, and might be greatly disordered. It means that the interfaces have the capability to increase the mobile ion concentration than that inside the bulk. The electric field distribution in the interfaces between two phases is a key to realize the interfacial superionic conduction, allowing ions to move on particle’s surfaces or interfaces through high conductivity pathways (Schober 2005; Zhu 2009).

Figure 3-12 Ion transport mechanism in two phase composite electrolytes (where core is doped ceria and shell is oxide/carbonate)


The performance of cell with Y2O3 40% content has the highest power density, see Figure 3-13(a). The OCV and performance of our material is relatively better than that of pure SDC and YDC ��&�� oC as reported (Hibino, et al. 2000; Shao, et al. 2004; Huang, et al. 2007c). Itmeans that the introduction of the second phase can extract the electronic conductivity of the SDC-based solid electrolyte. The electrolytes are stable in the fuel cell environment (Zhu, et al. 2006b; Zhu 2006c).

Typical EIS spectra are shown in Figure 3-13(b). These spectra were obtained in air at 400-600 oC. The impedance spectra have been modeled with an equivalent circuit shown inset. Fromthe equivalent circuit, L (inductance) is the effect of the stainless tube from the testing device. The R1 (ohm resistance), predominately includes the electrolyte resistance. The electrochemical resistance, R2, is probably associated with the electrode processes combined cathode and anode reactions. Q (constant phase element), might be due to the interfaces between the electrodes and the electrolyte. It can be seen from EIS spectra at different temperatures, Rohm decreases with the increase in temperature and, in parallel, the sample ionic conductivity increases.

It can also be seen from Figure 3-13b that there is only one arc observed which may be related to the electrode diffusion process. The arc corresponding to the bulk conductivity cannot be measured due to frequency a limit that is not sufficiently high. However, it is visible from the spectra that the intersections of the EI spectra with the Zre axis are not at the origin of the axis. A semi-circle from the origin of the impedance plot can be simulated as presented in Figure 3-13(b).

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Figure 3-13 (a) FC Performance with YSDC electrolyte, (b) AC impedance at different temperature

3.4.2 Ionic transfer number

The ionic transference number ( ion) of the best performance nanocomposite electrolyte Y2O3-SDC4 was obtained by Hebb-Wagner’s DC Polarization method (Hebb 1952) at 550 oC with an applied external field of 500 mV. The following equations were used for calculation.


�"� =��

�#= 0.035 (3.4)

�$ = 1 ��

�#= 0.965 (3.5)

where Ii and If are respectively the initial current and final current obtained from the DC polarization data. These values of lower electronic transference number and high ionic transference number (close to unity, 96%) are much better than those of the pure SDC and YDC.

The conductivity of the material was deduced from the impedance spectra. Temperature dependence of the conductivity for the different yttria-coated compositions is plotted in an Arrhenius relation as shown in Figure 3-14a. The obtained conductivity of the nanocomposite electrolytes in the air atmosphere was in the range of 0.44 - 0.92 S/cm between 300-600 oC, which is much higher (1-2 orders of magnitude) than pure SDC, GDC and YDC (Zhu, et al. 2006b; Zhu 2006c).

The conductivity of the composite electrolyte varied with increase in the yttrium oxide content. This high conductivity at lower temperature also ensures the higher performance of the fuel cells. The activation energy of optimized electrolyte (Y2O3-SDC) is calculated by Arrhenius equation. This conductivity depends on the molar percent of the coated particles of the SDC.

The activation energy (Ea) of the best sample of higher conductivity YSDC4 was derived by plotting the log (�T) and (1/T) using Arrhenius expression, Figure 3-14b. The low activation energy of 0.27 eV is obtained from linear fit curve and slope of the line. The calculated low activation energy can support the high oxide-ions (O2-) conduction/transportation.

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Figure 3-14 (a) YSDC electrolyte Conductivities, (b) Arrhenius plot



4. NANOCOMPOSITE ELECTRODES

Although there have been extensive and successful developments in the use of electrolytes at ��&��oC with ceria-based composites, a serious knowledge gap still remains in relation to high catalysis and electrode kinetics, as well as in the use of materials that function as electrodes at low temperatures. Such a shortfall in our understanding of these matters represents a considerable challenge in the development of high-performance low-temperature SOFCs (hereafter ASOFCs), especially at temperature below 500 oC. There is, accordingly, a great demand for ASOFC electrodes. In order to find the compatible electrodes for nanocomposite electrolytes, composite electrodes that contained less or no nickle (Ni) were developed which shown excellent compatibility, carbon resistance and high performance at low temperature (400-600 oC). In this work, several composite electrodes were prepared the best one are as below.

4.1 ZnO/NiO nanocomposite electrodes

The XRD pattern of ZnO/NiO/NSDC is shown in Figure 4-1. The result shows that no new phase is formed in the ZnO/NiO/NSDC materials, which implies that no chemical reaction occurs between ZnO or NiO and the electrolytes. Equally, there is no Na2CO3 phase peak, due to the amorphous structure of the NSDC, as shown in our previous study (Wang, et al. 2008). The ZnO/NiO forms a composite between the two phases at a nano-level.

Despite this advantage, a doping effect of NiO in ZnO also exists. The introduction of Ni has two functions. Firstly, ZnO is a structurally stable, n-type semiconductor. By the low-level doping of Ni (up to 3% atomic weight), the conductivity of ZnO may be increased by many orders of magnititude (Rodriguez, et al. 1997; Mandal 2006). The resulting increase in conductivity can effectively decrease the polarisation losses that occur in both the anode and the cathode. We herein describe our calculations from first principles to assess the effect of doping of Ni into the structure of ZnO. The atomic geometries were fully optimised until the forces on each atom were less than the threshold value of 10-5 eV Å�1. We used the zinc potential with 4s2

and 3d10 electrons as the valence states, the nickel potential with 4s2 and 3d8 electrons as the valence states and the oxygen potential with 2s2 and 2p4 electrons as the valence states. Figure 4-1b shows that the original unit cell was expanded to a super cell (4×2×2) that contained 64 atoms with Ni as the dopant. A K-point mesh of 3×5×3 was found to be sufficient to reach convergence for the bulk calculations. The Zn atoms were substituted with Ni atoms in the 64-atom supercell, corresponding to an effective Ni concentration of 3.12% (Zn0.969 Ni0.031 O). We carried out a careful optimisation of the geometry of the ZnO host system before and after doping, and the phase structure of ZnO was found to remain unchanged in the process. A model of the surface structure of the nanoparticles was developed in order to interpret the nanostructure and its performance regarding the compatibility of the electrodes used. The nanoparticles exhibited hexagonal curtailed morphologies that exclusively had (100) and (001) type surfaces, thereby implying that Ni-doped ZnO is able to release electrons rather effectively, resulting in a considerable increase in its conductivity.


Figure 4-1 (a) XRD pattern for ZnO/NiO/SDC-Na2CO3., (b) crystal structure of Ni-doped ZnOparticle, (c) SEM d) HRTEM image of ZnO/NiO material, (e) cross-section image of the cell

The second function of Ni relates to its composite effect in the ZnO/NiO nanocomposite, in that the physical interaction that occurs between the phases of NiO and ZnO can lead to the high dispersion of the NiO phase and can thereby improve the stability and conductivity of the ZnO phase (Mandal, et al. 2006), as well as promoting catalytic activation. The surface morphologies of the ZnO/NiO materials were inspected using a SEM. Figure 4-1c shows that the sample was porous and foam-like, and that the particles were round in shape. HRTEM microscopy images show that the particle size was in the range of 20 – 50 nm (Figure 4-1d). The cross-section image of the cell is presented in the Figure 4-1e.


Figure 4-2a shows two typical electrochemical impedance spectra (EIS) of ZnO/NiO composite, measured both in H2 and air at 500 oC under open circuit conditions. The Figure 4-2ashows an arc both for H2 and air, with small chords at around 10�3 ��2, which is a much lower value than that of the better known electrode materials, in particular of cathodic materials such as, (La, Sr) MnO3 (LSM)–Y doped ZrO2 (YSZ) composite (Song, et al. 2008), and Sr-doped LaCoO3 (Zuo, et al. 2006) at 500 oC. The findings represented by Figure 4-2a result in very small polarization losses at the electrode, and good catalytic activity in the reactions at both the anode and the cathode for symmetrical ZnO/NiO electrodes. Further analysis from the above EIS simulating work, in the equivalent circuit, L (inductance), is the effect of the stainless tube from the testing device. There exist different conductance mechanisms including electron, oxygen ion, and proton. In O2 atmosphere, R1 denotes ohmic resistance mainly caused by oxygen ion and electrons; in this case, the ohmic resistance contains both electron and ion contribution; R2Q and R3Q denote charge transfer and mass transfer, respectively. In H2 atmosphere, R1 denotes ohmic resistance for proton, oxygen ion and electron due to introducing protons from H2; R2Q and R3Q denote charge transfer and mass transfer, respectively (Kang, et al. 2011).

The ZnO/NiO electrode is mixed electronic and oxygen ion conductor (Saltsburg, et al. 1964; Carrasco, et al. 2004), therefore, semicircles in impedance as shown in Figure 4-2a are the ohmic resistance resulting from the oxygen ionic conductivity of the electrode. The conductivity of the ZnO/NiO is determined from the impedance measurements using its bulk resistance at different temperatures are shown in Figure 4-2b. The electrode exhibits high conductivity, typically of the order of 0.5 – 1.0 S/cm in air and 2.6 S/cm in H2.

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Figure 4-2 (a) Electrochemical impedance spectra, (b) conductivity of the electrode determined from impedance data at different temperature

In order to test the applicability of our findings, a fuel cell was fabricated using ZnO/NiO/NSDC composites for both electrodes and NSDC for the electrolyte. Figure 4-3ashows that maxium power densities (Pmax) of 1257, 1107 and 840 mW/cm2 were achieved at 520, 500 and 480 oC, respectively. These results show a maximum power (Pmax) 54% higher than that reported previously for SOFCs with a NiO anode and for a lithiated NiO cathode with the same


electrolyte (Raza, et al. 2010a). These performances are at least twice as good as those reported for other electrodes in this temperature range (Mandal 2006; Wang, et al. 2008; Wang, et al. 2011a). Figure 4-3b shows the effect of the composition of the ZnO/NiO electrode on the performance of the fuel cell. It can be seen that Pmax reaches a peak value at an intermediate molar ratio of 9 : 11 of ZnO and NiO. Either side of this composition, the performance of the fuel cell is less good.

Figure 4-3 (a) Cell voltage (open) and power density (solid) at different temp., (b) effect of varying the Zn : Ni ratio of electrodes on Pmax at 500 oC

4.2 Nanostructure anode (Cu 0.2 Zn 0.8O)

Our electrode preparation method produces large quantities inexpensively. In the CuZnO-SDC anode, the small Cu particle size might be important to catalytic activity (Song, et al. 2008).The function of ZnO is to alter the electronic states of these particles for improved catalytic activity.

Fuel cell performance of conventional anodes and our Ni-free anodes (NiO-SDC, CuZnO-SDC) were compared and the results are shown in Figure 4-4 a and b. Note that the fuel cells shown in Figure 4-4(b) had a higher power density, whereas the open circuit voltage (OCV) isthe same. The OCV and peak power densities shown in Figure 4-4(b) were 1000 mW/cm2 at 550 oC. The fuel cell performance was comparable to those using Ni-YSZ and Ni-SDC or other Ni-based anodes but at much lower temperatures, i.e., 500 °C in this work. The advantages of this nanostructure are obvious, as were the good interconnections between the nanoparticles and interfaces. These results show that the prepared anode is compatible and can replace the nickel-based anodes.


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Figure 4-4 (a) conventional anode (Ni-SDC), (b) Ni free anode (CuZn-SDC composite)

4.3 Zn0.6 Fe0.1Cu0.3O/GDC composite anode

The phase pattern of the ZnFeCu-GDC composite electrode gained by XRD is shown inFigure 4-5a. All peaks show the ZnO and ceria phases and no peaks were found for Fe, which indicates that the Fe was doped in to ZnO to form a solid solution. The GDC are of rhombohedra structure (space group R-3m (166)), whereas ZnO peaks are of Hexagonal structure. There are also found two peaks (111) and (113) for CuO which are face center cubic (fcc). The average crystalline size of ZnO (25-30nm) is less than the GDC-Na2CO3 crystalline size (50-60 nm)when using the Scherrer formula. These results show the nanocomposite advantages over conventional electrode materials.

The internal and detailed nanostructure analysis of composite anode was studied by TEM. The Figure 4-5b displays the distribution of the GDC and ZnO particles inside the composite. The GDC was around the surfaces of the ZnO particles which are in a crystalline. The composite size of the anode is 5 nm and was observed by TEM. The nano particles in this range enhanced the catalytically properties of the anode materials and the fuel cell performance with stability and durability.

Figure 4-5 (a) XRD, (b) TEM


The surface area of 8.91m2/g was obtained with BET of the composite anode powder and pore size was calculated to be 20-30 nm, which is agreement with the XRD, SEM and TEM results. The average diameter of the pores was found by BET to be about 10 nm. All these results from XRD, TEM and BET showed good agreement between each other, these are necessary for catalytically good anode materials for SOFC.

The (I-V/I-P) curves for a single cell are shown in Figure 4-6a using H2 as a fuel. The OCV was very good at different temperature range and even at a low temperature of 480 oC it is still higher than 1.0 V. These OCVs values were reproducible after 3 days with the optimized composite anode material. Figure 4-6a shows that the performance 1000 mW/cm2 at 570 oC, which is much better than existing electrodes for LTSOFC at this temperature(Xia, et al. 2002a; Shao, et al. 2004). Figure 4-6b shows the fuel cell performance (IV/IP characteristics) with methanol as fuel at different temperatures. The maximum power density of 540 mW/cm2 was obtained at 550 oC. Thus, the relatively high performance of the cell at lower temperatures may be due to improved electrochemical activity of the electrode material.

The enhanced performance is likely caused by catalytic reforming of the methanol. The performance results show that the anode material with nanocomposite electrolyte is capable for SOFCs fueled with hydrocarbon fuel, e.g. methanol.

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Figure 4-6 (a) FC performance with H2, (b) FC performance with methanol

4.4 Advanced multi-fuelled solid oxide fuel cell (ASOFC) using functional nanocomposite for polygeneration

The results of XRD for the electrode before and after H2 fuel was fed into the cell are shown in Figure 4-7a. The results confirm that the nanocomposite electrode used in fuel cell is chemically stable under H2 atmosphere. The substitution of NiO with CuO and ZnO improvesthe chemical stability of this anode material. The microstructure of the composite electrode is shown in Figure 4-7b as measured by a high-resolution TEM. It may be seen that the particles of the catalyst are distributed homogenously throughout the range 10-15 nm


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after operation before operation#!

$�!

ZnO#!SDC

$�!%&'"�

Figure 4-7 (a) X-ray diffraction before and after reduction of the composite electrode, (b) High-resolution TEM

The performance of the ASOFC was measured for bio-gas, bio-methanol and bio-ethanol fuel. Maximum power densities of 0.3, 0.7, and 0.6 W/cm2 were obtained for the single cell at 550 oC, using bio-gas, bio-methanol, and bio-ethanol respectively, as shown in Figure 4-8a.Performances of 970 and 860 mW/cm2 were achieved using hydrogen as a fuel, as shown in Figure 4-8b.

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Figure 4-8 (a) Performance of a single cell with different bio-fuels at 550 oC, (b) Performance of a single cell with H2 fuel at 550 oC


Bio-ethanol was mixed with 60 % water to prevent the deposition of coke or carbon. The performance of ASOFCs using bio-fuels (bio-methanol, bio-ethanol and bio-gas) at different temperatures (400 oC, 450 oC, 500 oC, 550 oC) confirmed that the internal reforming of bio-ethanol and bio-methanol was more active at the higher temperatures within the range that we tested (550 oC).

The difference in performance and OCV of the cells for different fuels at different temperatures is significant. The improvement in the catalytic reaction that results from adding ZnO to the anode materials can improve significantly the performance of the direct liquid-fuel-based SOFC, and the fuel can be reformed internally in addition to the direct oxidation of the fuel cell, which implies a degree of flexibility with respect to various different fuels (Sasaki, et al. 2004c; Kim, et al. 2007). The functional nanostructure of the electrode may provide a highdensity of power through the direct use of liquid fuels or hydrocarbons, without carbon being deposited. The use of liquid and gaseous fuels can offer several advantages, including that of internal reformation.

A number of authors have reported values of power density using hydrocarbon in a fuel-based SOFC (Jiang, et al. 2001; Barnett, et al. 2003; Gorte, et al. 2003; Jiang, et al. 2004; McIntosh, et al. 2004b; Sasaki, et al. 2004a; Sasaki, et al. 2004b; Brett, et al. 2005; Lin, et al. 2005; Huang, et al. 2006; Gong, et al. 2007; Goodenough, et al. 2007; Huang, et al. 2007a; Huang, et al. 2007b; Ji, et al. 2007; Kim, et al. 2007; Ye, et al. 2007; Hawkes, et al. 2009). Huang et al achieved 600mW/cm2 at 800 oC for ethanol (Huang et al. 2007). Jiang et al reported a power density of 800mW/cm2 at 800 oC and 300 mW/cm2 at 650 oC for ethanol and 600 mW/cm2 at 650 oC (Jiang, et al. 2001; Jiang, et al. 2004; Huang, et al. 2006; Goodenough, et al. 2007; Huang, et al. 2007a; Huang, et al. 2007b) for methanol. Weber et al and others have reported on the performance of single cells, but their experiments were conducted at temperatures greater than 600 oC and the performances were not as good (Goodenough, et al. 2007; Zhang, et al. 2010). Our device performed better than these other devices, at lower temperatures, with a variety of fuels, and without any carbon deposition or coking.

One of the major predictors of good performance of fuel cell is the compatibility between the electrode and the electrolyte (Zhu 2003; Raza, et al. 2010a). To ensure compatibility, we developed a functional nanocomposite electrolyte (Raza, et al.; Zhu, et al. 2003b; Wang, et al. 2008), then developed compatible electrodes that were also highly effective as catalysts (Raza, et al. 2010c; Raza, et al. 2011d). It may be supposed that substituting Cu and Zn with Ni in the anode would minimize the effects of coking or the deposition of carbon, while retaining the function for catalyzing oxidation in the cell. We observed that the microstructure of the electrode enables its structural integrity to be maintained without the deposition of carbon or the formation of coke.


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Figure 4-9 (a) Comparison of calculated theoretical and observed experimental values of open circuit voltage (OCV) for different bio-fuels at temperatures of 500 oC - 550 oC, (b) Stability of

SOFC for different bio-fuels at temperatures of 500 oC - 530 oC

Using the Nernst equation for calculation, the theoretical OCVs for the three different bio-fuels biogas, bio-ethanol, and bio-methanol are 1.23, 1.13, and 0.995 V, respectively, at 550 oC. Our equivalent experimental OCVs for the single cells were 1.19, 1.09 and 0.98 V. A comparison between the theoretical and experimental OCVs is shown in Figure 4-9a. The comparison confirms the catalytic activity of the anode in the oxidation of bio-fuels. There is no polarisation of the electrode, as is shown in Figure 4-8a and 4-8b. The lower reduction in IR as a result of the ohmic behaviour of the electrolyte may be responsible for the high performance at such low temperatures. In general, the high performance of our SOFCs corresponds with linear I-V characteristics (Raza, et al.; Jiang, et al. 2001; Barnett, et al. 2003; Goodenough, et al. 2007; Kim, et al. 2007).

In our study, we used composite electrodes (Raza, et al. 2010c; Raza, et al. 2011d; Raza, et al. 2011g) (anode and cathode) and electrolyte (Raza, et al. 2010; Raza, et al. 2010a; Raza, et al. 2010b). The kinetics of the reaction at the electrodes might be enhanced as a result of the high ionic conductivity of the composite NSDC (Wang, et al. 2008) electrolyte and the good electronic conductivity of the LiNiCuZn electrodes. As a result, the electrode’s resistance to polarization may be lower in this case, although it could also be related to the fact that the presence of Zn in the electrode improves the electrode’s performance as a catalyst, which reflects the electrocatalytic activity of the reactant gases (Raza, et al. 2010a). Usually, polarisation losses may be attributed to the discontinuous nature of the interaction between the electrons (in electrodes) and the ions (from the electrolyte). In our case, in which the composite materials were used, the interfacial polarisation was very low, as is shown by the electrochemical characterisation (EIS, IV and OCV) of the activation of the ASOFC electrode and its polarisation.

Figure 4-9b shows the chemical stability of the ASOFC at 550 oC, without any coke formation or carbon deposition. The OCVs of cells with different fuels were monitored for 72 hrsin 9 days (7hrs per day). It can be seen that for all fuels, after 72 hrs the OCVs were stable and did not fluctuate, which shows that the cell is stable with different fuels. It is well-known that the performance of the cell may suffer due to the pores of the electrode material becoming blocked as a result of carbon deposition. The fact that the pores did not become blocked in our case due


to highly effective catalyst functional materials, there was no deterioration in cell performance at any temperature in the range 500-530 oC (McIntosh, et al. 2004b; Zha, et al. 2004).

Figure 4-10 Raman spectroscopy of composite electrode after running ASOFC with different fuels at 550 oC, (a) with bio-methanol for 0, 24 and 72 hrs, (b) with bio-ethanol for 0, 24 and

72 hrs, (c) with bio-gas for 0, 24 and 48 hrs, (d) AC impedance with bio-methanol, bio-ethanol and bio-gas

It may be seen that the main advantage of the ASOFC is the direct operation of hydrocarbon fuels through internal reforming (i.e. anode and anode chamber), which reduces the cost and complexity of the system. Direct hydrocarbon fuel operations can only be achieved when the anode of the cell is inert and carbon deposition is not possible. Our LiNiCuZnO anode has shownthe ability to catalyze the carbon in the anode of the cell.


In order to study the effect of carbon deposition, we obtained the Raman spectra at room temperature after running the ASOFC on bio-fuels and hydrocarbon fuels to the cells operated for 0, 24 and 72 hrs at 550 oC, as shown in Figure 4-10a, b and c. It is evident from the figure that no significant peaks were observed for carbon, which implies that the amounts of carbon that were deposited during the exposure of the device to bio-fuels were not significant. The results of Raman spectroscopy confirms the belief that no carbon is formed during the operation of the cell (see Figure 4-10).

In order to provide further confirmation of the effect of carbon deposition, pure carbon was mixed in the same electrode materials, which shows up as a completely different peak from those of the different fuels. The insignificant deposition of carbon also lends encouragement to those who think it is possible to have a carbon-free fuel cell composed of composite materials. For example, electrodes composed of LiNiCuZn may be used in place of pure Ni electrodes, particularly in the case of hydrocarbon fuels, to prevent the deposition of carbon and the degradation of the cell. In cells that have such a composition, electrochemical oxidation and reforming is so fast that carbon is not deposited. In light of the foregoing, we conclude that the use of nanocomposite materials in ASOFCs is effective in the electrochemical operation of all kinds of bio-fuels and hydrocarbon fuels, and engenders a fast reaction at the surface of the anode. The findings that compatibility of the components, carbon free, and stability are important for the design and development of highly efficient low-temperature SOFC stack systems for polygeneration.

AC impedance spectroscopy was used to determine the resistance and conductivity in ASOFCs for different types of fuel. The Nyquist plots of the cell for different fuels at 550 oC are shown in Figure 4-10d. All the plots show a single semicircle at low frequencies, which exhibitsthe complex behaviour of the electrode and the diffusion in the electrolyte. These curves were modelled using an equivalent circuit, as shown in Figure 4-10d. The intercepts along the real axis are the sum of the ionic resistance of the electrolyte and the electronic resistance of the electrodes and the silver paste. The ohmic resistance of the cell at the intercept with the abscissa is about 0.5 ohm, which corresponds to a high frequency. Similar phenomena for methane and methanol were reported by Aloui, Primadahl and Cheng-Xin, but not for ethanol or bio-gas(Primdahl, et al. 2001; Aloui, et al. 2007; Li, et al. 2010).

We have calculated the theoretical electrical efficiency of our fuel cell for different fuels and compared this with other theoretical results reported elsewhere in the literature. Our experimental results showed that it is possible to achieve electrical efficiencies of 42%, 44%, 46% and 54% with biogas, bio-methanol, bio-ethanol and H2 fuels, respectively, while the heat produced from the cell was 45% , 50% , and 52% , and 51% with biogas, bio-methanol, bio-ethanol and H2 fuels, respectively. The total efficiency of the cell is 80% when used hydrogen as a fuel. These results are in good agreement with our theoretical predictions. The electrical efficiency is shown in Figure 4-11a, at different temperatures for different bio-fuels. These high efficiencies are attractive for polygeneration systems.

It may be seen that a large amount of heat can be produced. Examples of the use of such heat are in a heat engine, such as a micro gas turbine, to produce more electrical power, or in a heat exchanger to meet potential demand for heating from different sources.


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Figure 4-11 (a) Cell efficiency, (b) Larger cell (6 × 6 cm2) performance at 550 oC

The performance larger cell of a size of 6 × 6 cm2 with an active area 25 cm2 of an advanced ASOFC was measured for different kinds of fuel, namely bio-gas and hydrogen. Maximum power densities of 400 and 700 mW/cm2 were obtained for the single cell at 550 oC, using syngas and H2 respectively, as shown in Figure 4-11b.


5. DESIGN OF ASOFC SYSTEM FOR 5KW

A planar ASOFC system for 5kW electrical power generation is designed. The developed stack model is based on the planar solid oxide fuel cell type with the counter- flow design. The operating temperature of the system is assumed to be 550 oC with fuel utilization of about 86%.So, to achieve the desired power output (5 kW) at cell voltage of 0.6 V, the fuel cell stack is consist of about 210 single cells. The designed ASOFC stack is described below.

5.1 System description

Figure 5-1 shows the schematic of the designed SOFC polygeneration system. The system consists of solid oxide fuel cell stack, combustor, air heater, fuel heater, and heat recovery system (HRS). Fuel is syngas which is fed directly to the fuel cell stack at atmospheric pressure. Air is assumed to be at ambient condition and pre-heated at the air heater and then is used as an oxidant in the fuel cell stack. The electrochemical reactions occurring at the interfaces of electrolyte and electrodes of the ASOFC stack result in a flow of electrons in the external circuit, i.e. electrical power is produced. Afterward, the anode and cathode outlet gases are mixed and burned at the combustor. The main reason for using a combustor after the fuel cell is to burn non-reacted fuel and produce more heat. The exhaust gases from the combustor is further used in two heat exchangers and heat recovery system to pre-heat the fuel and air and heat up water, respectively. The exhaust gas from heat recovery system is assumed to discharge into the ambient.

Figure 5-1 Schematic ASOFC systems

The effect of fuel utilization factor on system performance was simulated at fixed operating temperature of 550 oC, and air inlet temperature of 350 oC. It is worth mentioning that higher


temperatures of air inlet temperature were impossible with this operating temperature due to the exhaust gases temperature at the air heat exchanger outlet. Figure 5.2 shows uf effects on combustor temperature, electrical efficiency, heating efficiency, combined efficiency, and heat power production at the heat recovery system. Increasing of fuel utilization factor means lower amount of fuel at the electrode is available for reaction. Therefore, at higher uf smaller portion of fuel will be remained to be burned at combustor resulting in lower combustor temperature. Consequently, heating efficiency will be decreased due to the lower amount of available heat at heat recovery system. However, since the fuel cell electrical power is assumed to be constant, the lower fuel utilization factor means lower amount of input energy to the system and therefore higher efficiency. As the descending trend of heating efficiency is smaller than raising trend of electrical efficiency, the total efficiency of the system will be enhanced. Furthermore, lower temperature at combustor results in lower temperature at heat recovery system inlet. Considering discharge temperature constant, the amount of available heat decreased and therefore the heat power production is lower.

Figure 5-2 (a) Electrical, heating, and combined efficiencies versus fuel utilization factor, (b) Effect of fuel cell operating temperature on electrical, heating and combined efficiencies

Effect of fuel cell operating temperature on system performance is also investigated. The simulation was made at constant fuel utilization factor of 0.66. From Figure 5-2, it seems that operating temperature of the fuel cell almost has no effect on electrical efficiency. However, the efficiency of the whole system will increase due to increase of heating efficiency.

Moreover, the effect of air temperature at the inlet of fuel cell stack was studied while fuel utilization factor and fuel cell operating temperature were kept constant at 0.66 and 400 oC, respectively. Increasing the air temperature at the cathode inlet means higher amount of heat extraction at the air heat exchanger and subsequently lower amount of available heat at the fuel heat exchanger and heat recovery system. So, heat power production and heat efficiency will decrease (Figure 5-3). Since air inlet temperature has no significant effect on electrical efficiency, the total efficiency will decreased as well.

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6. ELECTROLYTE FREE FUEL CELL-A NEW ENERGY CONVERSION DEVICE

In this chapter, a promising break-through for FC research and commercialization, based on new scientific principle and technology is presented. FCs are traditionally based on a three-component configuration - anode, electrolyte and cathode –leading to a complex structure and high cost as well, not least since the three components have to be chemically stable and mutually compatible. Our invention on electrolyte-free and one-component FC (EFFC), represents a radically new approach to FC R&D. Obtained results show performance as good as existing three-component FCs and prospects for further improvements are excellent. This will lead to very simple constructions and significant reduction of cost, opening up opportunity for earlier commercialisation. It can also greatly reduce the device expenditure and the complexity, helping pave the way towards more cost efficient FCs and marketing competitive and perhaps even the arrival of the hydrogen economy highlighted by Materials Views and Nature Nanotechnology(H. Gallagher 2011; Nanotechnology 2011). The difference between three components and single component FCs is described in Figure 6-1.

Figure 6-1 Comparison between conventional FC and EFFC

This field has been explored in a very late stagy of this thesis work, but with great potential for rapidly growing. This chapter makes a brief view on this new promising FC R&D field concerning material characterization, mechanism and performance.

Figure 6-2a shows a TEM micrograph for our prepared LiNiZn-oxide (LNZ) sample. Theyexhibit nano-scale particles with sizes in a range of 50-60 nm. The homogenous one layercomponent contains both LNZ and SDC thus contains both ionic (O2-) and electronic (n and p type) conductivities. We studied electrical properties and electrochemical impedance spectra (EIS) of the LNZ-SDC layer. As shown in Figure 6-2b, the material exhibits high conductivity (both electronic and ionic conductivity), typically higher than 0.25 S/cm over 600 oC. In a H2atmosphere a slight enhancement is observed compared to that in air.


Figure 6-2 (a) TEM micrograph, (b) Conductivity for the LiNiZn-based oxide

We further examined the semiconducting properties of an LNZ-SDC homogenous layer. Figure 6-3a displays the I-V measurement obtained by applying external voltage on the LNZ-SDC sample. After applying a certain bias voltage (ca. 1.1 V), the current increases significantly, indicating a semiconducting property of the material.

Figure 6-3b exhibits typical measurements of cell voltage and power versus current density for the LNZ-SDC homogeneous layer device at various temperatures. The OCV is around 1.0 V, the same as for a conventional three-component FC. The results show a FC reaction, i.e. converting H2 and O2 to H2O and electricity through an electrochemical route. At this stage our new device has delivered more than 600 mW/cm2 at 550 oC.

Figure 6-3c shows calculations on the EFFC device efficiency by comparisons with the conventional SOFC. The calculations are based on the following considerations: i) anode and cathode activations and concentrations are assumed to be the same for the EFFC and conventional SOFC, just removing the contribution of the electrolyte to the voltage/efficiency loss; ii) the same current efficiency (or fuel and oxidant utilization) and theoretical efficiency (or delta G/delta H). The overall efficiency equals to voltage efficiency/current efficiency/theoretical efficiency. In this case, up to 18% higher overall efficiency can be gained in EFC compared to that of conventional SOFC as shown in Figure 6-3c.


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Figure 6-3 (a) Voltage versus current for the one homogenous layer at 500 oC, (b) Cell voltage and power density versus current density for one homogenous layer device at various

temperatures, (c) EFFC device efficiency by comparisons with the conventional SOFC

Conventional FCs with a three components MEA realize the electricity generation via ion transportation through the electrolyte. The electrolyte acts as a critical separation barrier between the electronic and ionic conduction phases, see Figure 6-4a, besides two interfaces between electrolyte/anode and electrolyte/cathode create big polarization losses to limit serious FC performance (Steele, et al. 2001). Compared to the MEA FCs, we have shown that our single homogenous layer device as shown in Figure 6-4 a &b, can realize the FC reactions/functions directly with H+ and O�� ions by ionization, movement and reaction joint with electrons in the processes. All reactions take place in this one homogenous layer. In principle, therefore, our device may be named an “electrolyte-free fuel cell-EFFC” as regards its ability to realize the same functions without using an electrolyte separator.


Figure 6-4 (a) shows a schematic of a conventional three-component FC and EFFC with hydrogen and air supplies, (b) with electron release and acceptance at particle surfaces in the material, respectively. H2O is generated via 2H+ and O�� combination and the electricity is

generated in the processes

It is surprising how a one component layer can combine the properties of a complete three-layer/component FC and its functions for anode, cathode and electrolyte. The electrolyte is the core constituent of all traditional FCs, and acts as a critical component for transporting the ions, as well as, at the same time, as a separator to the ion and electron phases, blocking electrons passing through to prevent the device from short circuit. Conversely, in the new device a single homogenous layer mixing both electron and ion conductors can reach the same voltage and performance as normal FCs under H2/air operations.

Our results show that a single layer can perform in the same way as a complex three-layer FC, including ion transport and i-e- junction/function between the anode/electrolyte and electrolyte/cathode. At this moment there is still lack of the detailed scientific knowledge of all the processes involved, but we have this far reached an understanding that our new device has many similarities to the properties of a dye solar cell, where a mixture of electronic and ion conductors (electrolyte) is also used (O'Regan, et al. 1991). The charge and phase separations for the electronic and ionic conductions/phases can also be realized in dye solar cells. Further research is strongly needed and is currently carried out concerning both scientific principles and technical developments for applications.

Given our results to date, it can be expected to catch a glimpse of possible new directions for FC research, liberated from the constraints of electrolytes and complex multi-layered structures. In case of success, a new reality would appear concerning FC science, technology and commercialization. Further calculations show that, except demonstrating promising advantages as concern cost, simplicity and stability, the one-layer device (or the EFFC) is also able to improve energy conversion efficiency compared to conventional three-component FCs. Since our device functions can be realized through surface reactions/processes; while in conventional FC devices, interfaces between the anode/electrolyte and electrolyte/cathode cause ion and electron conduction barriers that put strong constrains for achieving high FC performances.


7. CONCLUSIONS

In this work, several types of functional nanocomposite materials for ASOFCs have beendeveloped. An advanced multi-fuelled SOFC has also been developed that uses functional nanocomposites for polygeneration applications. The following conclusions can be drawn from the results achieved from this PhD thesis research:

1. An improved homogenous nanocomposite electrolyte with an enhanced conductivity of 0.1 S/cm was prepared. The interfacial super-ionic conductor and the two source ions (O2- and H+) significantly enhanced ASOFC power output at 300-600 °C. A power density of 200 mW/cm2 was generated by the device at temperatures as low as 350 oC.The XRD, SEM and TEM results indicate a nanoparticle size range of 5-20 nm with good agreement among the results.

2. Nanocomposite electrodes have been successfully developed using a cost-effective method and used for the first time in ASOFCs at 300–600 oC. The electrodes exhibit high conductivity and dual catalytic functionalities at both the cathode and the anode for the electrochemical reduction of O2 and the oxidation of H2, respectively. Excellent fuel-cell performance, i.e., a maximum power density of 1107 mW/cm2, has been demonstratedfor a symmetrical fuel cell at 500 oC. To our knowledge, this is by far the highest power density achieved at this temperature. The performance of the fuel cell is influenced by the ratio of Zn to Ni in the electrodes, and a composite effect has been observed for the nanocomposite material reported herein.

3. We derived theoretical predictions and tested the predictions experimentally. Hydrogen, bio-gas, bio-ethanol, and bio-methanol were all tested with different nanocomposite materials at temperatures much lower than those used in conventional SOFCs (300-600 oC). The high efficiency and performance of the ASOFC at fairly low temperatures ()550oC) was demonstrated using a functional nanocomposite material. The experimental results were useful for enhancing our understanding of the proper mechanisms at play and provided insights into the mechanism of electron transfer at the anode and the cathode via electronic conduction and catalytic activity. The high power densities that were obtained for a variety of fuels and the low resistance at the interface between the electrolyte and the electrodes confirmed that the electrodes acted as good catalysts in the ASOFCs and were compatible with the nanocomposite electrolytes.

4. Our results show that ASOFC systems with functional nanocomposites offer significant advantages in reducing the operational and capital costs for the production of power and heat from bio-fuels. We conclude that ASOFCs can provide a significantly high energy density and are chemically stable. They are also environmentally friendly in that they can be used for polygeneration with renewable fuels (i.e., biomass fuels). Although aconsiderable amount of research on the use of bio-fuels in ASOFCs has now been


successfully completed, the materials-synthesis processes, device fabrications and related technologies still need to be scaled up for ASOFC commercialisation.

5. A simplified mathematical model of the 5 kW ASOFC system was developed, and the ASOFC system for polygeneration purposes was investigated. The overall efficiency of the stack was 80%, when the fuel utilisation was 86%.

6. A new invention, the “Electrolyte-free fuel cell”, has resulted from the functional nanocomposite electrolytes with further development and integration of semiconductors asnew materials with multifunctionalities. This invention represents a significantbreakthrough and will greatly stimulate the development of hydrogen-generation and FC technologies in related energy sectors.

7.1 Future work

1. The ASOFC stack, the system engineering and manufacturing for the 1 kW/5 kW case using functional nanocomposite materials and the integration of the ASOFC system with other energy components, e.g., a gas turbine, is required for the development further various hybrid polygeneration systems/units.

2. Further theoretical studies (mechanism, modelling, analysis of kinetics, diffusion, single cells, stacks, CHPs, etc.) of EFFCs will be promising research areas that require more delicate contributions in ongoing research.

3. Based on the results from the electrolyte-free fuel cells, we anticipate that this new invention will not only bring about revolutionary FC technologies, but will also allow for considerably simpler and more cost-effective constructions and system designs. The transformation of future FC markets and progress towards commercialisation in an extended range of applications will no doubt also lead to significant economic and scientific impacts. We have noted that this single-component technology, in addition to bringing about a technical revolution in the FC field, may also open new windows for other areas with potentially significant scientific and economic consequences. Many important areas in both fundamental and applied research as well as in commercialisation will be impacted by further extensive research efforts.


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