Patterned Thin Film Cathodes for Micro-Solid Oxide Fuel Cells
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PATTERNED THIN FILM CATHODES FOR MICRO-SOLID OXIDE FUEL CELLS Presented by: NEIL JONATHAN SIMRICK A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College The Department of Materials Imperial College London, Prince Consort Road, London SW7 2BP 2010 Abstract The miniaturisation of Solid Oxide Fuel Cells (SOFCs) to micro (u)-SOFCs in recent years has led to the implementation of metallic and ceramic thin film membranes less than one micron thick for cell components such as anodes, cathodes, electrolytes and current collectors. Electrochemical processes (particularly the oxygen reduction reaction) occurring at the SOFC cathode are regarded as the primary inhibitor to cell performance. The reduction in operating temperature to less than 600°C for portable device applications reduces the reaction kinetics further. Silver (Ag) was used as a potential low-temperature cathode material for R-SOFCs in this work due to its known oxygen permeability and high electrical conductivity. Ag films approximately 100 nm thick were thermally unstable at temperatures as low as 250°C. A dewetting process occurred via the surface self diffusion of Ag to uncover the substrate and reduce the overall energy of the system. The oxygen reduction reaction occurring at lanthanum strontium cobalt iron oxide (La0.6Sr0.4C00.2Fe0.803.8 or LSCF) SOFC cathodes was investigated using patterned LSCF thin films. LSCF was deposited via pulsed laser deposition and photolithographically patterned to produce geometrically well-defined micro- cathodes. The electrical conductivity of as-deposited and etched LSCF thin films was determined. A maximum conductivity of 5700 Snil was measured in air, however degradation in performance occurred upon the temperature cycling of LSCF films subjected to a photolithography and etching process. Electrochemical impedance spectroscopy (EIS) was employed to determine the area specific resistance (ASR) of the patterned LSCF thin film cathodes between 350°C and 500°C. A non-linear relationship between the exposed surface area of LSCF and the polarisation resistance was observed with the oxygen surface exchange reaction remaining rate- limiting down to 350°C. Declaration of Originality As the author of this thesis, I declare that the work presented within is my own and that all else is appropriately referenced. "Never regard your study as a duty, but as the enviable opportunity to learn to know the liberating influence of beauty in the realm of the spirit for your own personal joy and to the profit of the community to which your later work belongs." Albert Einstein (1879 — 1955) - iv - To My Friends and Family Throughout my life you have supported me through everything that I have attempted. No matter how large the challenge appears, I know your encouragement, nourishment, faith and love will raise me up to be all that I can. I dedicate this thesis to you all and especially to my grandmother, Margaret Lockyer, because you have been the stepping stones that kept me safely above the 'deep waters' of the PhD. For this, I am eternally grateful. To My Parents I thank you for your unconditional love, discipline, patience and most of all your belief in me. You accept me for who I am and have given me the freedom to take my own path in life, yet when I've gotten lost you've never judged me. You're never far behind to push me in the right direction again! I hope to repay you for the sacrifices you've made over the years, starting with this. Most of all, I thank God. In loving memory of my grandmother, Ivy Loveta Munroe. - v - Acknowledgements Although the PhD is ultimately a personal project, I am extremely grateful for the contribution of many people, which enabled the completion of this work. I would like to give thanks to: ■ My supervisors Professor Alan Atkinson and Professor John Kilner, for their support and guidance throughout the PhD. I am grateful for the opportunity to research at Imperial College, along with the freedom to conduct the research in my own way. Your patience throughout numerous extended meetings, fruitful discussions, insight and wisdom is appreciated. ■ Professor Nigel Brandon, in addition to my supervisors who invited me to join Imperial College. My 'Imperial experience' began four years ago in 2006 when I first contacted Professor Brandon asking "is there a lot of research on fuel cells at Imperial College?" How little did I know! ■ The entire SOFC group and the Fuel Cell Network at Imperial College for the continual exchange of ideas and interesting discussions not limited to science! ■ The entire SOFC group in the department of Nonmetallic Inorganic Materials, ETH Zurich, Switzerland, with special thanks to Dr Jennifer Rupp, Dr Anja Bieberle-Huffer, Thomas Ryll and Professor Ludwig Gauckler for their invaluable insight and use of equipment to complete a large proportion of this work. ■ The FIRST cleanroom staff at ETH Zurich for their invaluable assistance in the microfabrication techniques utilised in this work. ■ The IDEA League for the travel grant, which provided the opportunity to work at ETH, Zurich in Switzerland. ■ The Worshipful Company of Armourers and Brasiers and the Hilary Bauerman Trust for the travel grants that enabled the participation at international conferences in Boston, Massachusetts and Toronto, Canada and Lucerne, Switzerland, respectively. - vi - ■ Dr Mahmoud Ardakani of the Materials department at Imperial College for his help and advice on the scanning electron microscopy conducted during the PhD. ■ Richard Chater of the Materials department at Imperial College and Henning Galinski and Julia Martynczuk at ETH Zurich for their help in producing high quality focussed-ion beam cross-sections used in the thesis. ■ Richard Sweeney of the Materials department at Imperial College for his help and advice on the XRD work conducted during the PhD. ■ Bob Rudkin of the Materials department at Imperial College for his ability to resurrect experimental rigs from the dead and for his insight into numerous experiments. ■ Professor Eric Yeatman, Dr Munir Ahmad and Dr Werner Karl of the Electrical and Electronic Engineering department at Imperial College for their help and advice on microfabrication techniques and for the use of cleanroom facilities in preliminary studies. ■ Norma Hikel, Dagmar Durham, Darakshan Khan and Ashley Perris for making some of the more day-to-day formalities of the PhD so much easier to deal with. ■ Steve Etienne of the London Centre for Nanotechnology at University College London for his additional advice on microfabrication techniques and assistance in thin film deposition techniques. ■ The London Southbank University thin films group led by Professor Neil Alford (now at Imperial College) and in particular Dr Peter Petrov, Dr Vinoth Sarma and Dr Hsin-I Chien for their assistance and use of equipment in preliminary studies. - vii - Table of Contents List of Figures xxv List of Tables xxvii Chapter 1 1 Introduction 1 1.1 Fuel Cell Technology 1 1.2 The Solid Oxide Fuel Cell (SOFC) 4 1.3 Miniaturisation of Portable Electronics 5 1.4 Batteries vs. Micro-Fuel Cells 6 1.5 The Micro-SOFC 7 1.5.1 Miniaturisation of SOFCs 8 1.5.2 Development of Micro-SOFCs 10 1.5.2.1 Design Considerations 11 1.5.2.2 Material Requirements for 1.1-SOFCs 12 1.5.2.3 Application of MicroElectroMechanical Systems (HEMS) Fabrication Techniques and Processes 13 1.6 Electrolytes 17 1.7 Electrodes 18 1.7.1 Patterned Electrodes 20 1.8 Thesis Objectives 21 1.9 Summary 21 Chanter 2 23 Literature Survey: Thin Film SOFC Cathode Research 23 2.1 SOFC Cathodes 23 2.2 The Oxygen Reduction Reaction 25 2.2.1 Geometrically Well-Defined (GWD) Electrode Studies 31 2.2.1.1 Limitations 35 2.3 Fabrication of Geometrically Well-Defined (Patterned) Electrodes 37 2.3.1 Thin Film Deposition Processing 38 2.3.2 Patterning 41 2.3.2.1 Substrate Material 48 2.4 Material Selection For p-SOFC Cathodes Used In This Study 49 2.4.1 Thin-Film Silver Cathodes 49 2.4.2 Thin Film La,,Sri_,,CoyFel_y03±5 Cathodes 50 2.5 Summary 53 Chapter 3 54 Experimental Techniques Employed 54 3.1 Preparation of Single Crystal YSZ Substrates 54 3.2 Thermal and Electron Beam Evaporation of Silver 55 3.3 Pulsed Laser Deposition of La0.6Sr0.4Co0.2Fe0.803 56 3.3.1 Ceramic Powder Processing: PLD Target Preparation 62 3.4 X-Ray Diffraction 63 3.5 Scanning Electron Microscopy 65 3.6 Photolithography and Etching 67 3.6.1 Overview 67 3.6.2 Photoresist Pattern Transfer: Exposure and Development 69 3.6.3 Lift-Off 72 3.6.4 Etching 73 3.6.4.1 Wet Etch 73 3.6.4.2 Ar+ Sputter Etch 73 3.7 DC Four-Point Electrical Measurements 75 3.8 Electrochemical Measurements 77 3.8.1 Potential-Current Measurements 79 3.8.2 AC Impedance Spectroscopic Measurements 80 Chapter 4 84 Thermal Stability of Silver Thin Films for Low Temperature Micro-SOFC Cathodes 84 4.1 Introduction 84 4.2 Supplementary Experimental Details 85 4.3 Results and Discussion 87 4.3.1 Film Rupture Mechanisms 87 4.3.2 Evolution in Morphology of Annealed Ag Thin Films 91 4.4 Summary 106 Chapter 5 108 Characterisation of Lao.6Sro.4Coo.2Feo.803 Thin Films Produced by Pulsed Laser Deposition 108 5.1 Introduction 108 5.2 Results and Discussion 108 5.2.1 Influence of Thickness on LSCF Microstructure 108 5.2.2 Post Annealing of LSCF Thin Films 112 5.3 Summary 115 Chapter 6 117 Design and Fabrication of Patterned LSCF Cathodes with Gold Current Collectors 117 6.1 Introduction 117 6.2 Cell Design 117 6.3 Sheet Resistance of a Patterned LSCF Cathode 119 6.4 Patterned