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Microfabrication Methods to Improve the Kinetics of the Yttria Stabilized Zirconia – Platinum – Oxygen Electrode

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Microfabrication Methods to Improve the Kinetics of the Yttria Stabilized Zirconia – Platinum – Oxygen Electrode Microfabrication Methods to Improve the Kinetics of the Yttria Stabilized Zirconia – Platinum – Oxygen Electrode by Joshua L. Hertz B.S. Ceramic Engineering Alfred University, 2000 SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MATERIALS SCIENCE AND ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEPTEMBER 2006 © 2006 Massachusetts Institute of Technology. All rights reserved. Authored by ______________________________________________________________________ Joshua L. Hertz Department of Materials Science and Engineering August 14, 2006 Certified by_______________________________________________________________________ Harry L. Tuller Professor of Ceramics and Electronic Materials Thesis Supervisor Accepted by ______________________________________________________________________ Samuel M. Allen POSCO Professor of Physical Metallurgy Chair, Departmental Committee on Graduate Students Microfabrication Methods to Improve the Kinetics of the Yttria Stabilized Zirconia – Platinum – Oxygen Electrode by Joshua L. Hertz Submitted to the Department of Materials Science and Engineering on August 14, 2006 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and Engineering ABSTRACT Solid oxide fuel cells are a potential electrical power source that is silent, efficient, modular, and capable of operating on a wide variety of fuels. Unfortunately, current technologies are severely limited in that they provide sufficient power output only at very high temperatures (>800°C). One reason for this is because the electrodes have very poor (and poorly understood) kinetics. The work described in this dissertation involves the microfabrication of model systems with triple phase boundary lengths that varied over an order of magnitude to systematically quantify and ultimately improve the kinetics of platinum electrodes on the surface of yttria stabilized zirconia electrolytes. Platinum electrodes with well controlled geometry were sputtered onto the surface of bulk YSZ and onto sputtered YSZ thin films. An unexpected result was found whereby YSZ films of composition Y0.09Zr0.91O2-x had an ionic conductivity remarkably enhanced by a factor of 20-30. This is attributed to the films exhibiting nanometric grain sizes and thereby stabilizing the cubic morphology at considerably lower yttrium levels than is normally needed. This metastable cubic phase is suspected of having reduced defect ordering. Grain boundary resistance, which in YSZ is normally due to impurities that segregate and block ionic transfer, was found to also be significantly reduced in YSZ films. The films had a specific grain boundary conductivity enhanced by a factor of 30-100 compared to the bulk polycrystalline sample. This was believed to be due to the very low impurity content of the film grain boundaries. Concerning the electrode polarization resistance, it was found that the electrodes placed on bulk standards and films deposited at high temperatures were on par with the best electrode conductance values from the literature. However, when the electrolyte surface was a film deposited at reduced temperature, the resistance decreased further by a factor of 300-500. The cause of this was revealed to be silicon contamination on the surfaces of the poorer-performing electrolytes. Triple phase boundary length-specific resistances as low as 3.7·104 Ω·cm at 378°C and 4.0·107 Ω·cm at 215°C were measured; these appear to be the lowest ever recorded. The measurements are possibly the first electrochemical characterization of nearly silicon-free YSZ surfaces. This study emphasizes the key role of chemical purity at the electrode-electrolyte interface. Photolithography alone is unlikely to give technologically useful triple phase boundary lengths. In an attempt to achieve the triple phase boundary lengths needed for a practical device, reactive co-sputtering was used to produce composite Pt-YSZ thin films with a bi-continuous network morphology and grain sizes on the order of 30 nm. Such intimate mixing of the electronic and ionic conducting phases created an effective mixed ionic-electronic conductor with the entire surface of the film electrochemically active to the electrode reaction. The best processing conditions resulted in electrodes with an area specific polarization resistance less than 500 Ω·cm2 at 400°C and, by extrapolation, 10 Ω·cm2 at 511°C and 1 Ω·cm2 at 608°C. These films may enable operation of a micro-solid oxide fuel cell at intermediate temperatures (400-500°C), and perhaps even lower temperatures with further microstructural optimization. Thesis Supervisor: Harry L. Tuller Title: Professor of Ceramics and Electronic Materials To Jacy CONTENTS LIST OF FIGURES.......................................................................................................................................9 LIST OF TABLES ......................................................................................................................................15 ACKNOWLEDGEMENTS ........................................................................................................................17 CHAPTER 1. INTRODUCTION 1.1 Motivation..............................................................................................................................19 1.2 Fuel Cells...............................................................................................................................20 1.2.1 Principles of Operation .......................................................................................21 1.2.2 Ohmic Polarization..............................................................................................23 1.2.3 Activation Polarization........................................................................................23 1.2.4 Concentration Polarization..................................................................................28 1.2.5 Device Performance............................................................................................29 1.2.6 Solid Oxide Fuel Cells ........................................................................................34 1.2.7 Micro Solid Oxide Fuel Cells..............................................................................35 1.3 Yttria Stabilized Zirconia.......................................................................................................42 1.3.1 Bulk Properties....................................................................................................43 1.3.2 Grain Boundary Properties..................................................................................48 1.3.3 Surface Properties...............................................................................................51 1.3.4 Sputtered and Other Thin Films..........................................................................54 1.4 Impedance Spectroscopy........................................................................................................57 1.4.1 Equivalent Circuit Modeling...............................................................................58 1.4.2 Impedance of Ceramic Electrolytes ....................................................................61 1.4.3 Microelectrodes...................................................................................................62 1.5 Platinum Electrodes for YSZ .................................................................................................65 1.6 Thin Film Metal-Ceramic Composites...................................................................................72 1.7 Conclusions of the Literature Review....................................................................................76 1.8 Objectives of the Research.....................................................................................................77 CHAPTER 2. EXPERIMENTATION 2.1 Yttria Stabilized Zirconia Deposition ....................................................................................79 2.2 Platinum – Yttria Stabilized Zirconia Composite Deposition................................................81 2.3 Deposition Calibration...........................................................................................................82 2.4 Physical Characterization.......................................................................................................82 2.4.1 Scanning Electron Microscopy ...........................................................................82 2.4.2 Atomic Force Microscopy...................................................................................83 2.4.3 X-ray Diffraction.................................................................................................83 2.4.4 X-ray Photoelectron Spectroscopy......................................................................83 2.5 Platinum Microelectrode Fabrication ....................................................................................84 2.6 Platinum-YSZ Composite Microelectrode Fabrication .........................................................87 2.7 Impedance Spectroscopy .......................................................................................................88 CHAPTER 3. RESULTS 3.1 Yttria Stabilized Zirconia Film Deposition............................................................................93 3.2 Yttria Stabilized Zirconia Physical
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