Modeling and experimental validation of CO heterogeneous chemistry and electrochemistry in solid oxide fuel cells [Elektronische Ressource] / vorgelegt von Vitaliy Yurkiv

Yurk_Vt

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2010
• Nombre de lectures: 29
• Langue: Deutsch
• Poids de l'ouvrage: 2 Mo

Extrait

INAUGURAL – DISSERTATION
zur
Erlangung der Doktorwürde
der
Naturwissenschaftlich-Mathematischen
Gesamtfakultät
der
Ruprecht-Karls-Universität
Heidelberg
















Vorgelegt von
Master of Chemical Science Vitaliy Yurkiv
aus: Dolyna, Ukraine

Tag der mündlichen Prüfung: 17.12.2010







Modeling and experimental validation of CO
heterogeneous chemistry and electrochemistry in solid
oxide fuel cells













Gutachter: apl. Prof. Dr. Hans-Robert Volpp, PCI, Heidelberg
Priv.-Doz. Dr. Wolfgang G. Bessler, DLR, Stuttgart



Zusammenfassung

In der vorliegenden Arbeit wurden experimentelle und numerische Untersuchungen zur
heterogen katalysierten und elektrochemischen Oxidation von CO an Anodensystemen
(bestehend aus Nickel und yttriumdotiertem Zirkoniumdioxid, YSZ) von
Festoxidbrennstoffzellen (engl. Solid Oxide Fuel Cells, SOFCs) ausgeführt, um den
mikroskopischen Mechanismus der an der CO/CO –Gasphase/Ni-Elektrode/YSZ-Elektrolyt-2
Dreiphasen-Grenzfläche ablaufenden Ladungsübertragungsreaktion aufzuklären. Temperatur-
programmierte Desorptionsmessungen (TPD) und Temperaturprogrammierte
Reaktionsmessungen (TPR) sowie Dichtefunktionaltheorierechnungen wurden ausgeführt, um
adsorptions-, desorptions- und reaktionskinetische sowie thermodynamische Daten für die
CO/CO /Ni- und CO/CO /YSZ-Reaktionssysteme zu erhalten. Unter Verwendung dieser Daten 2 2
wurden auf Elementarreaktionen basierende mikrokinetische Modelle, die vier verschiedene
Ladungstransfermechanismen enthielten, für die elektrochemische CO-Oxidation entwickelt und
zur numerischen Simulation experimenteller elektrochemischer Literaturdaten wie
Polarisationskurven und Impedanzspektren herangezogen. Durch Vergleich zwischen Simulation
und Experiment konnte gezeigt werden, dass nur einer der vier Ladungstransfermechanismen
die vorhandenen elektrochemischen Daten über einen weiten Temperatur- und CO/CO –2
Gaszusammensetzungsbereich konsistent reproduzieren kann.




Abstract

In the present work experimental and numerical modeling studies of the heterogeneously
catalyzed and electrochemical oxidation of CO at Nickel/yttria-stabilized zirconia (YSZ) solid
oxide fuel cell (SOFC) anode systems were performed to evaluate elementary charge-transfer
reaction mechanisms taking place at the three-phase boundary of CO/CO gas-phase, Ni 2
electrode, and YSZ electrolyte. Temperature-programmed desorption and reaction experiments
along with density functional theory calculations were performed to determine
adsorption/desorption and surface diffusion kinetics as well as thermodynamic data for the
CO/CO /Ni and CO/CO /YSZ systems. Based on these data elementary reaction based models 2 2
with four different charge transfer mechanisms for the electrochemical CO oxidation were
developed and applied in numerical simulations of literature experimental electrochemical data
such as polarization curves and impedance spectra. Comparison between simulation and
experiment demonstrated that only one of the four charge transfer mechanisms can consistently
reproduce the electrochemical data over a wide range of operating temperatures and CO/CO gas 2
compositions.




Acknowledgements
There are many people who deserve credit and thanks for their part in this thesis. Without their
contributions and support, this work would not have been possible.
I wish to express my sincere gratitude to all my research advisors. First and foremost, I am most
thankful to Priv.-Doz. Dr. Wolfgang Bessler for being an outstanding supervisor and mentor.
Prof. Dr. Hans-Robert Volpp, my supervisor at PCI, for his indulgence and staying in the
institute to the late night helping me in my work.
I would like to thank the International Graduate College (IGK 710) “Complex Processes: Modeling,
Simulation and Optimization” at the University of Heidelberg for fellowships.
I have had the great pleasure of working with many talented and friendly people Marcel Vogler,
Dzmitry Starukhin, Alexandr Gorski, Christian Hellwig, Nicolas Bayer-Botero, Stefan Gewies
whose support and faithful discussion played irreplaceable role in my work. The individual
thanks are due to Dr. Marcel Vogler who has been a tremendous help in working with
calculation programs, Dzimitry Starukhin who has helped me with technical problems and made
all the tools available to me that I needed to fix or construct equipments in lab, Dr. Alexandr
Gorski for his extreme help with DFT calculations.
I would like to thank my parents, Volodymyr and Galyna who have inspired me throughout my
entire life to work hard, invest in myself through education, and to strive to be a good person.
Finally, I would like to thank my wife, Viktoriya for her unending love, encouragement, and
understanding.









Table of Contents

Acknowledgements i
List of Symbols ii

1.  Introduction and Background ..................................................................................................1 
1.1 Motivation .............................................................................................................................1 
1.2 Background...........1 
1.2.1 Fuel Cells........................................................................................................................1 
1.2.2 Types of Fuel Cells.........................................................................................................2 
1.2.3 Fuel for Fuel Cells....4 
1.3 Solid Oxide Fuel Cells...........................................................................................................5 
1.3.1 SOFC electrolyte ............................................................................................................7 
1.3.2 SOFC anode material9 
1.3.3 SOFC cathode material...................................................................................................9 
1.4 Literature Overview of CO Oxidation at SOFC Anodes.......................................................9 
2.  Quantum Chemical Calculations...........................................................................................13 
2.1 Schrödinger Equation and the Born-Oppenheimer Approximation....................................13 
2.2 Density Functional Theory ..................................................................................................14 
2.3 Pseudopotentials ..................................................................................................................16 
3.  Experimental Setup, Sample Preparation and Characterizations ..........................................18 
3.1 Experimental apparatus .......................................................................................................18 
3.2 Sample preparation..............................................................................................................20 
3.3 Characterization of the samples...........................................................................................22 
3.3.1 Auger Electron Spectroscopy .......................................................................................22 
3.3.2 Low Energy Electron Diffraction Spectroscopy ..........................................................22 
3.3.3 Sum Frequency Generation Spectroscopy....................................................................23 
3.4 TPD/TPR spectroscopy .......................................................................................................24 


3.4.1 The Basic Principles of TPD/TPR Technique..............................................................24 
3.4.2 The Analysis of TPD/TPR Spectra...............................................................................25 
4.  Electrochemical Modeling and Simulation ...........................................................................28 
4.1 Electrochemistry..................................................................................................................28 
4.2 Elementary Kinetics ............................................................................................................30 
4.2.1 Gas Phase Chemistry....................................................................................................31 
4.2.2 Surface Chemistry31 
4.2.3 Charge Transfer ............................................................................................................32 
4.2.4 Charge transfer of oxygen anion in Ni/YSZ model anode ...........................................34 
4.3 Modeling and Simulations of Ni/YSZ Model Anodes ........................................................36 
4.3.1 SOFC Model Anodes ..