Title

AN EXPERIMENTAL STUDY OF FLAME-ASSISTED FUEL CELLS

Date of Award

May 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Jeongmin Ahn

Subject Categories

Engineering

Abstract

Flame-assisted fuel cells (FFCs) based on solid oxide fuel cells (SOFCs) have received more attention in recent years due to their simple configuration and capability of utilizing hydrocarbons directly to produce electricity. However, the power outputs of FFCs are relatively low compared to that of dual chamber SOFCs (DC-SOFCs) and single chamber SOFCs (SC-SOFCs). Further, the carbon coking and thermal shock issues still prohibit wide applications of FFCs. Moreover, operating conditions of FFCs such as relative positions between flames and fuel cells and fuel flow rates, are important factors that affect the FFCs power output and such data are currently lacking. The main purposes of this dissertation study are: (1) develop a composite cathode and a catalyst layer in order to improve the power output and the coking resistance of FFCs; (2) investigate the effects of operating conditions on FFCs performance; and (3) develop FFCs with high power outputs and high thermal shock resistance by adapting fuel cells with different configurations.

To improve the FFCs performance, a composite cathode Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) + Sm0.2Ce0.8O2-δ (SDC) was first investigated and optimized. Experimental results showed that the electrochemical performance of a BSCF+SDC composite cathode was greatly influenced by sintering temperatures, and the best sintering temperature for the BSCF+SDC composite cathode was determined. Later, a mesoporous SDC/Ru catalyst was developed and examined. The highly porous structure of the SDC/Ru catalyst ensured fast diffusion of gas inside of the catalyst layer and exhibited a relatively high power output and effective carbon coking resistance. Experimental results also showed that the performance of the electrolyte supported SOFC (ES-SOFC) based FFCs strongly depended on the location of the fuel cell with respect to the FFCs temperature. The ES-SOFC based FFCs also presented a low thermal shock resistance due to its thick electrolyte.

To address the thermal shock issue of the ES-SOFCs based FFCs and to further improve the FFCs performance, we have developed two different FFCs configurations based on anode-supported SOFCs (AS-SOFCs): SDC based FFCs and YSZ (Yttria stabilized Zirconia)-SDC based FFCs. The SDC based FFCs exhibited much better performance and thermal shock resistance when compared to that of ES-SOFCs. A power density of ~ 791 mW.cm-2 and a maximum current density of ~ 2300 mA.cm-2 were achieved, which are comparable to the performances of the DC-SOFCs and SC-SOFCs. The total flow rates and fuel flow rates were found to have a great effect on the FFCs temperatures and fuel concentration, thus further impacting the FFCs performance. The YSZ-SDC based FFCs were then fabricated and investigated. The sintering temperatures of the FFC electrolyte were found to determine the FFC performance, and 1350 oC is the favorable sintering temperature to achieve the dense electrolyte and the best power output. Experimental results also showed that the YSZ-SDC based FFCs exhibited the higher OCVs, but lower power densities compared to that of SDC based SOFCs. However, the performance of YSZ-SDC based FFCs dropped much slower for higher equivalent ratios compare to that of SDC based SOFCs, which could effectively expand the FFCs operating windows.

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