Date of Graduation

2017

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Harry O Finklea

Committee Co-Chair

Suzanne Bell

Committee Member

Fabien Goulay

Committee Member

Jeffrey L Petersen

Committee Member

Nianqiang Wu

Abstract

Solid oxide fuel cells (SOFCs) are promising energy conversion devices, but their economic viability depends on long term stability of the SOFC materials at the high operating temperatures. The in-situ study of materials degradation faces a great challenge because of the unavailability of analytical methods at those high temperatures. The traditional analytical techniques for degradation measurements are widely based on post-mortem studies. The cells or cell materials are cooled down to room temperature and are then subjected to microscopy-based methods. Combining post-mortem analyses with cell modelling, several studies have reported the following microstructural degradations: (a) Lanthanum Strontium Manganite (LSM) particles, one of the most widely used cathode material, 'melt' or 'spread' at temperatures as low 850 °C without the presence of a current flow on Yttrium Stabilized Zirconia (YSZ) surface, (b) the Ni particle shapes change during oxidation of nickel to NiO and as a result of reduction of NiO to Ni during redox cycling, (c) the formation of nickel phosphide particles on the Ni/YSZ anode surface due to the presence of PH3 in coal syngas. However, these reports are limited due to the fact that analyses were done at room temperature rather at their high operating temperature. In this current study, cell materials were examined in-situ at high temperature using an Environmental Scanning Electron Microscope installed with a heating stage. High temperature images were collected for different cell materials such as silver paste, LSM particles, nickel oxide particles and secondary nickel phosphide phase formed on YSZ substrate. The images were analyzed by using ImageJ, an open source image processing software. An image analysis protocol was developed to analyze the cell materials images taken from 500 °C to 1080 °C. The image analysis provides both qualitative and quantitative insight of electrode/electrolyte interface. This novel approach of the SOFC materials study suggest that: (a) the absence of detectable microstructural changes in LSM particles as a function of temperature (no sign of 'melting'); (b) NiO particles do not shrink because of partial reduction of NiO to Ni under vacuum at high temperatures, (c) the nickel phosphide particles formed on the Ni/YSZ anode during PH3 exposure are predominantly Ni 3P and the particles melt between 900°C and 1000°C. Based on the findings this study suggests that : (a) the frequently reported increase in polarization resistance at the LSM/YSZ interface with temperature is most likely due to the cation migration rather than formation of new layers because of particle melting or migration, (b) partial reduction of nickel oxide on YSZ surface does not cause crack formation due to the volume reduction, (c) the low melting temperature of nickel phosphide particles at Ni/YSZ interface suggests that during PH3 exposure nickel phosphide particles form inside the anode first and then migrate to the surface.

Download

Share

COinS