Investigations on the Degradation of Oxide Ion Conductivity in Stabilized Zirconia Ceramic Materials
Beschreibung:
Two stabilized zirconia ceramics have been investigated for this paper. 8 mol% yttria (Y2O3) stabilized zirconia (8YSZ) is a well know and well established material used in fuel cells and other ceramic applications. It has been used as a reference material in this work. The primary material under investigation is XSZ, a stabilized zirconia that uses different dopants of the form A2O3 and BO2. This material shows a higher ionic conductivity in air and is therefore of great interest for high temperature fuel cell applications. The change of the ionic conductivity in different atmospheres; as well as, the aging of 8YSZ and XSZ in oxidizing and reducing environments is the focus of this thesis.
When 8YSZ, XSZ or any other ceramic material is used as an electrolyte in a high temperature fuel cell application, it is of major interest that the ionic conductivity is not negatively affected by the environment. The reason for that can be found in the fact that fuel cells or other electrochemical devices operate over a wide range of partial pressures of oxygen (in a SOFC: 100 - ~10-20). With different measurement techniques (Van der Pauw, 4 point measurement), it was confirmed that 8YSZ is stable in both, the reducing and oxidizing environment, respectively. On the other hand, XSZ showed a strong dependency on the partial pressure of oxygen. This goes along with a change in the optical appearance of the material (i.e. strong discoloration). The appearance of the discoloration; as well as, the analysis of the thermo-gravimetric measurement of the material proved the transition of the dopant BO2 to B2O3 in a reducing environment; and accordingly a decrease in ion mobility to be the cause of the loss of conductivity in a reducing atmosphere. In addition, the decline in ion mobility was also presented in Arrhenius plots, in which the slope suddenly changed (i.e. change in activation energy) when the material was exposed to a highly reducing atmosphere (i.e. hydrogen). In Chapter 5, the implications of the reduction of the dopant and the decrease in ionic conductivity of the material are discussed in more detail.Firstly shown in this paper is that the Electrochemical Impedance Spectroscopy (EIS) can be used as a tool to help to understand the underlying mechanisms of degradation in a solid oxide fuel cell (SOFC) electrolyte. A novel measurement setup is presented which separates the different contributors to the total ohmic resistance Rs (series resistance). Furthermore, the EIS is used to determine the contributions of the different processes taking place in an operating SOFC; as well as, the contributions made by the active layers to the overall resistance of the SOFC.With regards to the electrolyte, it was also investigated how the simultaneously applied atmospheres (oxidizing at the cathode and reducing at the anode) affect its conductivity. Interestingly, different thicknesses of the discoloration were discovered for the fuel cells, running under open circuit voltage and under current, respectively.The effect of the electrolyte thickness on the rate of degradation is also discussed in this thesis. The analysis shows that the thicker the electrolyte, the higher the degradation thereof. These findings were compared to the partial pressure of oxygen distribution within the electrolyte material under open circuit voltage as well as under applied current. With this I was able to model and correlate the dependency of ionic conductivity of the electrolyte to the partial pressure of oxygen within the material.XRD results of the pristine XSZ material and the reduced XSZ material, respectively showed no measureable change in the lattice parameters. Nevertheless, it is believed that the reduction of BO2 to B2O3 induced lattice strains because of the bigger ionic radius B3+ exhibits.Other possible reasons for the loss of ionic conductivity – such as deep donor effects, agglomeration of vacancies, and/or increased grain boundary resistance – are discussed with respect to the measurement results.There are two main reasons why the presented results are an important contribution to the field of electrolyte material in SOFC applications and consequently, highly important for future material research for electrolyte materials for the SOFC.Firstly, stabilized zirconia electrolytes are still the most favored materials for a SOFC electrolyte. Secondly, this is useful and applicable to all different fuel cell designs (i.e. tubular, planar, electrolyte supported, electrode supported etc.) and other electrochemical devices that work under a wide range of partial pressure of oxygen.
Produkt Information:
- Softcover: 164
- Sprache: Englisch
- Größe und Gewicht: DINA 5, 176g
- 1. Auflage: 2010
- ISBN-13: 978-3-9813832-1-8
