Fabrication of Nanostructured Electrodes and Interfaces Using Combustion CVD
Ying Liu
Georgia Institute of Technology
Abstract
Reducing fabrication and operation costs while maintaining high performance is a major consideration for the design of a new generation of solid-state ionic devices, such as fuel cells, batteries and sensors. One of the key issues is to enhance mass and charge transport throughy porous electrodes, improve electrode/electrolyte interfacial conditions, thus facilitate reaction kinetics and improve electrochemical/catalytic properties of the system. In addition to searching for new materials, devel...Moreoping new fabrication approaches and creating novel microstructures are effective methodologies to achieve this goal. The objective of this research is to fabricate nanostructured materials for energy storage and conversion applications, particularly porous electrodes with nanostructured features for solid oxide fuel cells (SOFCs) and high surface area films with desired crystallographic structures for gas sensing using combustion CVD process. The extremely large surface area combined with optimum pathway for mass and charge transport will greatly facilitate electrochemical kinetics. High fabrication temperature ensures the formation of desired crystallographic structures and improves interfacial bonding, while relatively short period of time required for combustion CVD process alleviates tendency of deleterious electrode/electrolyte boundary phases, which are normally present in systems fabricated by conventional firing approach. Four most important deposition parameters are evaluated in this study: deposition temperature, deposition time, precursor concentration, and substrate. Deposition temperature has a significant influence on electrode microstructure (grain size, porosity, and pore size). Deposition time and precursor concentration, on the other hand, have little effect on microstructure but determine the electrode thickness. The nature of the substrate has no observable effect on microstructure but dramatically influences the interfacial polarization resistance. The optimum combination of deposition parameters for Sm0.5Sr0.5CoO3 (SSC)- Sm0.1CeO2 (SDC) cathodes on Gd0.1Ce0.9O2 (GDC) substrates is found to be a deposition of 5 to 10 minutes using a 0.05 M solution and a deposition temperature of 1200 1300 C. With the optimum deposition parameters, highly porous and nano-structured electrodes for low-temperature SOFCs have been fabricated using a combustion CVD process. The electrodes fabricated consist of nano-grains of about 50 nm, exhibiting extremely high surface area and remarkably low polarization resistances. XRD patterns confirmed the formation of desired crystalline phases for as-prepared NiO-SDC anodes and SSC-SDC cathodes. It is evident that combustion CVD is a highly effective approach to fabrication of high-performance electrodes for low-temperature SOFCs, producing the lowest interfacial polarization resistances (1.09 cm2 at 500 C, and 0.17 cm2 at 600 C) ever reported for the cathode materials. Anode supported cell with a 20 m thick electrolyte demonstrated a power density of 375 mW/cm2 at 600 C. Little deterioration in either microstructure or performance was observed after 172 hours of operation. Further, nanostructured and functionally graded La0.8Sr0.2MnO2 (LSM)-La0.8SrCoO3 (LSC)-GDC composite cathodes are fabricated on 240 m thick YSZ electrolyte supports using a combustion CVD method. The fabricated cathodes were graded in both composition and structure with higher strontium-doped lanthanum manganite (LSM) content and finer primary grain size at electrolyte side while higher strontium doped lanthanum cobaltite (LSC) content and coarser primary grain size at air/oxygen side. Extremely low interfacial polarization resistances (i. e. 0.43 cm2 at 700 C) and impressively high power densities (i. e. 481 mW/cm2 at 800 C) were generated over the operating temperature range of 600 C 850 C. The original combustion CVD process is modified for fabrication of porous electrodes for solid oxide fuel cells. GDC particles suspended in an SSC ethanol solution were burned in a combustion flame, depositing a porous cathode on an anode supported GDC electrolyte. Extremely small interfacial polarization resistances were obtained, especially at low temperatures such as 450 C (1.06 cm2) and 500 C (0.45 cm2). A peak power density of 385 mW/cm2 was achieved at 600 C. Finally, all parts of the composite electrodes are introduced in the form of solid ceramic particles, which are suspended in a flammable liquid carrier, and fed to the atomizer. The high velocity flame provides the energy needed for collision and sintering of the contained solid particles to produce porous electrodes. The demonstrated fast deposition rates (i. e. 40 m in 10 min) and the elimination of post- deposition firing make this process practically valuable. The performance of the porous SOFC electrodes fabricated by the modified process is equal or better than those prepared by the conventional techniques, especially at low operating temperatures. We have demonstrated a new, simple route for preparing highly porous ceramic monoliths through the removal of metal oxide (SnO2) during high temperature sintering. Unlike the existing strategies, the new method requires neither time consuming chemical leaching nor following-up gas reduction procedures. The most critical step for our new approach is the preparation of intimately distributed 40vol.%SnO2-60vol.%CeO2 composite nanopowder using combustion CVD. This method provides a simple way to introduce additional porosity into ceramic materials, and can be directly incorporated into ceramic production routes without introducing extra procedures. Composite nanopowder (20wt%SnO2-50wt%NiO-30wt%GDC) was prepared using combustion CVD from a single precursor source. Bilayer SOFC anodes differing in porosity were fabricated by co-pressing of NiO-GDC powders and SnO2-NiO-GDC composite nanopowder. An anode supported SOFC with bilayer anode was constructed and tested for electrochemical performance. Porosity variation in the anodes was achieved by the removal of SnO2 phase during sintering and cell testing. Interfacial resistances of the bilayer anode cell were 1.20, 0.49, 0.22, and 0.1 cm2 at 500, 550, 600, and 650 C respectively. Peak output power densities measured at the corresponding temperatures were 171, 301, 441, and 544 mW/cm2, respectively. Highly porous and nanostructured SnO2 thin film gas sensors with Pt interdigitated electrodes have been fabricated via a combustion CVD process. The SnO2 films were less than 1 m thick and consisted of nanocrystallines smaller than 30 nm. The as-prepared SnO2 gas sensors have been tested for ethanol vapor sensing behavior in the temperature range of 200 500 C. At 300 C the sensitivity to 500 ppm ethanol vapor was 1075 while the corresponding response time and recovery time were 31 and 8 seconds, respectively. The corresponding low detection limit was found to be below 1 ppm. Several novel nanostrucutres, such as SnO2 nanotubes with square-shaped or rectangular cross sections, well-aligned ZnO nanorods, and two-dimensional ZnO flakes, are synthesized using combustion CVD process. Solid state gas sensors based on single piece of these nanostructures have demonstrated superior gas sensing performances. The new nanostructure has significant scientific and technological implications. The curiosity that these nanostructures grew into particular morphologies (e. g. square or rectangular shape rather than circular shape (as carbon nanotubes do)) may stimulate interesting investigation into the crystallization behavior of these materials during vapor phase deposition. These size- tunable nanostructures could be the building blocks of or a template for fabrication of functional devices. In summary, this research aims to produce significant impact on concept and fabrication technology of solid-state ionic devices, as well as on fundamental understanding of the correlation between processing conditions, microstructure, and properties of the synthesized structures.
keywords:Technology Dr; Electrodes; Using Combustion
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