Nanostructuring of solid oxide fuel cell for efficient energy conversion [electronic resource]

Upon the growing need for efficient and environmentally friendly energy conversion devices, solid oxide fuel cell (SOFC) is the one of the most interesting research areas due to its high efficiency and cleanliness. The high operating temperature of SOFC, however, causes practical problems such as material selection and thermal degradation, and therefore limited the wide deployment of SOFCs. SOFCs operating at low temperature below 500 °C (LT-SOFCs) are therefore attractive. The low operating temperature, on the other hand, significantly increases fuel cell losses and thus reduces fuel cell performance and efficiency. The present study therefore mainly aims to engineering and study the factors that can reduce the losses of SOFC in nano-to-atomic scale with more focusing on activation loss. Triple phase boundary (TPB) density is one of the main factors that affect the size of activation loss. TPB is the boundary where electrode, electrolyte, and gas meet, and it is where electrochemical reactions preferentially occur. For TPB engineering, two experimental approaches were examined. Fist, ultra-thin (~10 nm) platinum (Pt) cathode/catalyst layers were patterned by atomic layer deposition (ALD) and tested as catalytic electrodes (cathode). We found that 180 cycles or approximately 10nm of ALD Pt, with a Pt loading of only 0.02 mg/cm2, shows maximum TPB density, and was sufficient for the purpose of catalytic cathode. Furthermore, this ALD Pt resulted in fuel cell performance comparable to that achieved by 80nm-thick sputtered Pt. Transmission electron microscope (TEM) observations revealed the optimized number of ALD cycles of Pt for the catalytic electrode, which renders both contiguity and high TPB density. Second, the kinetic role of 2nm thin yttria-stabilized zirconia (YSZ)/Pt cermet layers on enhancing the oxygen reduction kinetics was investigated. The cermet layers were deposited between the Pt cathode and the YSZ electrolyte by ALD. For cells with a cermet interlayers, the maximum power density increased by a factor of 1.8 at 400°C, and 2.7 at 450°C. The observed enhancement in cell performance is believed to be due to the increased TPB density in the cermet interlayer. Better kinetics for the fully mixed cermet layer sample may stem from the better thermal stability of Pt islands separated by the ALD YSZ matrix, which helped to maintain high density TPBs at elevated temperature. Atomic-scale observation and quantification of oxide-ion vacancy concentration near the [Sigma]13 (510)/[001] symmetric tilt grain-boundary of a YSZ bicrystal using aberration-corrected transmission electron microscope (TEM) operated under negative spherical aberration coefficient imaging condition is presented. It shows significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width < 0.6 nm. Electron energy loss spectroscopy measurements with scanning TEM indicated increased oxide-ion vacancy concentration at the grain boundary core. Oxide-ion density distribution near a grain boundary simulated by molecular dynamics corroborated well with experimental results. Such column-by-column quantification of defect concentration in functional materials can provide new insights that may lead to engineered grain boundaries designed for specific functionalities. The author also report the successful demonstration of thin-film SOFC with maximum power density of 1.3 W/cm2 at 450 °C by 3d-nanostructuring the fuel cell membrane and interlayering a catalytic layer, i.e., doped ceria, at cathode/electrolyte interface. The high power density of our cell is attributed to the huge drop of polarization loss due to enhanced surface area of the membrane-electrode-assembly (MEA) and better catalytic activity at the cathode. This low-temperature, high performance SOFC will pave a way for wider applications of SOFCs, which may greatly impact current way of energy conversion. Lastly, the author has investigated the change of chemical composition, crystallinity, and ionic conductivity in fluorine contaminated yttrium-doped barium zirconate (BYZ) fabricated by ALD. It has been identified that fluorine contamination can significantly affect the conductivity of the ALD BYZ. The author hasalso successfully established the relationship between process temperature and contamination and the source of fluorine contamination, which was the perfluoroelastomer O-ring used for vacuum sealing. The total removal of fluorine contamination was achieved by using all-metal sealed chamber instead of O-ring seals.

Keywords: Mechanical Engineering.
Advisor: Prinz F. B. primary advisor. Thesis advisor
Kenny Thomas William advisor. Thesis advisor
Zheng Xiaolin