Semester

Spring

Date of Graduation

2011

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Nianqiang Wu

Abstract

Fuel cells offer several advantages over conventional routines of power generation, such as substantially higher conversion efficiency, modular construction, minimal sitting restriction, and much lower production of pollutants. Solid oxide fuel cell (SOFC), in principle, can utilize all kinds of combustion fuels including coal derived syngas (CSG). The U.S. Department of Energy is currently working on coupling coal gasification and SOFC to form Integrated Gasification Fuel Cell (IGFC) systems. Such IGFC systems will enable the clean, efficient and cost-effective use of coal---the nation's most abundant fossil fuel.;However several issues need to be considered before SOFC can be really commercialized. The anode of SOFC can interact with the trace impurities in CSG such as arsenic, phosphorous, chlorine etc, which leads severe degradation of the cell performance. The operation temperature of current SOFC is also high (>800 °C), which increases the system cost. Hence, further study of both the anode and cathode is necessary in order to develop high performance, long serving time SOFC for IGFC power plant.;One of the aims of this project is to investigate the degradation mechanisms of the Ni-YSZ (yttria stabilized zirconia) anode in PH3 contained coal syngas. Materials microstructure change and electrochemical performance degradation were studied synchronously. Key factors such as the operating temperature, the impurity concentration as well as the cell operation conditions were investigated, which is essential to understanding the degradation behavior of SOFC. It has been found that Ni phosphate is the product of reaction between Ni and PH 3, leading to the loss of both the electrochemical activity and the electron conductivity of the anode. In addition, surface reconstruction is ascribed to Ni-P diffusion to the anode surface. Impedance spectra are fitted with the equivalent circuits to interpret the physical and chemical processes during degradation. The impedance analysis has shown that the mass diffusion resistance increases faster than the charge transfer resistance. The anode degradation is accelerated by increase in the operating temperature, the PH3 concentration and the electrical bias.;On the other side, novel metal oxide nanofibers have been developed as the SOFC cathode materials using the electrospinning method in order to enhance the electrochemical performance of SOFCs. A high performance cathode has been developed by infiltrating lanthanum strontium manganite (LSM) into the porous yttria-stabilized zirconia (YSZ) nanofiber backbone, which has showed superior oxygen reduction activity as compared to a conventional powder cathode. The power density of single unit of SOFC with LSCF nanofibers as the cathode can reach 1.07 W/cm2 at 750 °C. Such cathode architecture could bring the operation temperature of SOFC down to the intermediate temperature range, which will significantly reduce the cost of whole SOFC system.

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