Nanoparticulate Cathode Films for Low Temperature Solid Oxide Fuel Cells Vom Fachbereich Material- und Geowissenschaften der Technischen Universität Darmstadt zur Erlangung des akademischen Titels Doktor-Ingenieur (Dr.-Ing.) genehmigte Dissertation von MSc. Azad Jaberi Darbandi aus Teheran Referent: Prof. Dr.-Ing. Horst Hahn Koreferent: Prof. Dr. Christina Roth Tag der Einreichung: 27. März 2012 Tag der mündlichen Prüfung: 29. Mai 2012 Darmstadt 2012 D17 CONTENTS 1 Introduction.................................................................................................................................1 2 Basics ..........................................................................................................................................3 2.1 Fuel Cells............................................................................................................................3 2.2 Fuel Cell Generalities.........................................................................................................5 Advantages of fuel cells..............................................................................................................5 Disadvantages of fuel cells .........................................................................................................5 Fuel Cell Types...........................................................................................................................5 Solid Oxide Fuel Cell (SOFC)....................................................................................................6 SOFC components and materials..............................................................................................13 Perovskite Structure..................................................................................................................16 Cathode material systems..........................................................................................................19 Electrochemical processes at SOFC cathodes...........................................................................21 2.3 Thin films for SOFC ........................................................................................................23 2.4 Cathode performance .......................................................................................................26 2.5 Electrochemical Impedance Spectroscopy.......................................................................27 Characteristic Parameters..........................................................................................................29 3 Experimental procedures...........................................................................................................33 3.1 Synthesis...........................................................................................................................33 3.2 Characterization methods.................................................................................................35 Inductively Coupled Plasma Optical Emission Spectrometry ..................................................35 Thermoanalysis.........................................................................................................................35 Scanning electron microscopy ..................................................................................................35 Nitrogen Adsorption .................................................................................................................35 X-ray diffraction .......................................................................................................................35 Characterization of dispersions.................................................................................................36 3.3 Preparation of symmetrical samples.................................................................................40 3.4 Electrochemical characterization......................................................................................41 4 Synthesis of nanostructured cathode materials .........................................................................45 4.1 Chemical composition......................................................................................................46 4.2 Morphology......................................................................................................................46 Electron microscopy .................................................................................................................46 I Nitrogen adsorption...................................................................................................................51 4.3 Structure ...........................................................................................................................51 5 Morphological studies and thin film cathodes ..........................................................................59 5.1 Formation of hollow spheres............................................................................................59 Effect of number density of droplets.........................................................................................61 Effect of pressure and precursor concentration on morphology ...............................................63 5.2 Morphology modification.................................................................................................65 5.3 Characterization of nanodispersions ................................................................................65 5.4 Dispersions of Nanocomposites .......................................................................................68 5.5 Nanoparticulate functional cathodes ................................................................................70 5.6 Thermal Stability..............................................................................................................72 Thermal stability of synthesized powder ..................................................................................72 Thermal stability in thin cathodes.............................................................................................75 6 Electrochemical characterization of thin film cathodes ............................................................79 6.1 La0.75Sr0.2MnO3-! cathodes................................................................................................80 LSM Nanoparticulate cathode on YSZ.....................................................................................80 LSM Cathode deposited on GDC interlayer .............................................................................83 LSM-GDC composite cathodes ................................................................................................85 6.2 Cathode polarization under oxygen pressure ...................................................................89 High frequency arcs ..................................................................................................................91 Intermediate frequency arcs......................................................................................................92 Low frequency arcs...................................................................................................................95 Mixed conductive cathodes...........................................................................................................96 7 Summary and outlook .............................................................................................................105 8 References...............................................................................................................................111 II LIST OF FIGURES Figure 2-1: General concept of hydrogen / oxygen fuel cell. ............................................................4 Figure 2-2: Schematic cross section through a SOFC element.[6]....................................................7 Figure 2-3: The dependence of the ideal fuel cell potential on temperature. The ideal fuel cell voltage decreases with increasing temperature, but usually the opposite is true for the actual fuel cell efficiency due to accelerated electrochemical reaction kinetics at higher temperatures.............9 Figure 2-4: Ideal vs. actual fuel cell IV curves. Four major losses accounts for the difference between the two curves [11]. ...........................................................................................................10 Figure 2-5: SOFC components and materials..................................................................................13 Figure 2-6 Typical Perovskite structure...........................................................................................17 Figure 2-7 Two mechanisms of cathodic oxygen reduction: If the cathode material is a pure electronic conductor (e.g. LSM), the reaction can only proceed via the surface path, where the incorporation of oxygen into the electrolyte occurs at the three phase boundaries. In case of a mixed conducting cathode material (e.g. LSCF), additional to the surface path, the bulk path is also possible. ...........................................................................................................................................21 Figure 2-8: Schematic diagram of the wire connections for a three point, four lead and two points four leads configurations for impedance measurements of SOFC electrodes. ................................27 Figure 2-9: Simulated impedance spectra