Atomic layer deposition for solid oxide fuel cells [electronic resource]

Solid oxide fuel cells (SOFCs) are attractive because of their high energy conversion efficiency in the usage of fuels ranging from hydrogen to hydrocarbons. Nevertheless, they have limited applications because this high efficiency requires high operating temperatures, which pose significant engineering challenges, diminishing the SOFCs practicality for applications such as portable power sources. Lowering the operating temperatures of SOFCs would improve thermal stability and offer shorter start-up times, which in turn would broaden their use as auxiliary power units for automobiles. Although many studies have aimed at lowering SOFC operating temperatures, the resulting temperature reduction has been inevitably accompanied by a decrease in fuel cell performance. A critical bottleneck in reducing the operating temperature is the sluggish oxygen incorporation kinetics at the cathode/electrolyte interface at low temperatures. To improve the oxygen incorporation kinetics at the cathode/electrolyte interface, yttria-stabilized zirconia (YSZ) electrolyte membranes were surface modified by adding a one-nanometer thin, high-yttria concentration YSZ film with the help of atomic layer deposition (ALD). The addition of the one-nanometer film led to an increase of the maximum power density of a SOFC by a factor of 1.50. The enhanced performance can be attributed to an increased oxygen incorporation rate on the surface of the modified electrolyte. To further examine the effect of surface modification with ALD YSZ, isotope exchange/depth profiling (IEDP) was used for collecting the spectrometric evidences of enhanced oxygen isotope exchange kinetics. From the IEDP result, we found that the surface modified YSZ electrolyte demonstrated superior exchange kinetics, which verified our previous observation in the electrolyte's improved electrochemical performance. Through custom tailoring the surface with ALD, we were able to enhance the oxygen incorporation kinetics of our electrolyte. In the next experiment, we deposited nanostructured platinum catalysts with high triple phase boundary (TPB) densities, which increased the oxygen incorporation rate at low temperatures. To fabricate such catalyst, we studied the nucleation behavior of ALD platinum with plan-view TEM, and identified the deposition condition which produced the nanostructure with a high TPB density. We then introduced the nanostructured platinum catalyst in fabricating the membrane electrode assembly (MEA), and characterized the electrochemical performance of the MEA. As a result, a 90% performance enhancement was observed in the MEA with the addition of the nanostructured catalyst. Finally, we fabricated a high perforamce thin-film micro-SOFC, which allowed us to reduce the SOFC operating temperature. This micro-SOFC was composed of a corrugated thin-film electrolyte membrane fabricated by nanosphere lithography (NSL) and ALD, which showed a hexagonal-pyramid array nanostructure. The resulting micro-SOFC demonstrated a reduction in polarization and ohmic losses, and achieved a power density of 1.34 W/cm2 at 500 °C. In the future, arrays of micro-SOFCs with high power density may enable a range of mobile and portable power applications.

Keywords: Mechanical Engineering
Advisor: Prinz F. B. primary advisor. Thesis advisor
Bent Stacey advisor. Thesis advisor
Cui Yi