A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item DEVELOPMENT OF BILAYER ELECTROLYTE LT-SOFCs USING ALTERNATIVE BISMUTH OXIDES(2018) PESARAN, ALIREZA; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work is primarily focused on the fabrication and performance of anode supported cells based on gadolinium doped ceria (GDC) and alternative stabilized bismuth oxide bilayer electrolyte that can operate at low temperatures (500-650 °C).The well-known bismuth-based electrolyte (ESB) undergoes ordering phenomena at temperatures ≤600°C, causing rapid decay in conductivity, and in turn the power output. As alternatives, two bismuth oxide compositions: (a) Neodymium-gadolinium stabilized bismuth oxide (NGSB) in rhombohedral phase, and (b)heavily doped composition, (Bi0.75Y0.25)1.86Ce0.14O3±δ (YCSB), with Y and Ce as co-dopants in cubic phase were evaluated in this work. For the GDC/NGSB bilayer SOFCs, thickness of GDC and NGSB layers was varied between 23-70 μm and 0-25 μm, respectively. The results showed the addition of NGSB layer was effective in blocking electronic conduction which increased the OCV compared to the baseline single layer GDC cell. Further, the relative and total thickness of the two layers showed a significant impact on the OCV of the cell at different temperatures with best performance obtained with cells with lower GDC thickness and higher NGSB thickness. For the GDC/YCSB bilayer electrolyte SOFCs, a cell with a ~ 20 μm GDC layer and a ~12-13 μm YCSB layer, OCV and MPD of the cell at 650 ℃ reached 0.833 V and 760 mW/cm2 respectively. OCV stability of this bilayer was measured for 50 hours at 625 and 600 ℃ (100 hours in total) and exceptional stability of OCV and ohmic ASR was observed. In comparison, the cell with 10GDC/ESB bilayer electrolyte showed a very rapid degradation of OCV at 600 ℃ (average hourly degradation rate of 0f -0.55%/h). and the ohmic ASR of the cell with GDC/ESB bilayer electrolyte at 600 ℃ increased by 5 times over the first 50 hours of operation mainly due to the conductivity decay of ESB. Following the stable performance of GDC/YCSB bilayer electrolyte SOFCs, effect of GDC/YCSB thickness ratio on the performance of the cell was studied. It was shown that MPD of the bilayer electrolyte cells is higher than pristine GDC based cells with reduced ohmic ASR values. Specifically, a high MPD of ~1 W/cm2 at 650℃ was achieved on a GDC(20μm)/ YCSB(12μm) bilayer electrolyte based SOFC, which is 62% higher than pristine GDC based SOFC (0.64 W/cm2). Such enhancement is due to the 9.3% improvement in OCV (from 0.791 to 0.865 V) and a considerable 36% reduction in ohmic ASR values (from 0.094 to 0.069 Ω.cm2). Finally, to achieve high power density at low temperatures (≤ 600 ℃), a thin GDC (7-8 μm)/ YCSB (2 μm) bilayer electrolyte was used, and non-ohmic ASR of the cell was drastically lowered via infiltration of Ni/GDC and LSM on anode and cathode, respectively. At 600 and 550 ℃, maximum power density (MPD) of the cell reached 1.73 and 1.25 W/cm2, respectively, significantly higher than all previously reported values using non-cobalt cathode materials. The effect of infiltrating LSM on LSM-YCSB cathode was studied by varying temperatures and partial pressure of oxygen. It was revealed that by infiltrating the LSM/YCSB cathode with LSM nanoparticles, non-ohmic ASR of cathode reduced remarkably by one order of magnitude. Stability of the infiltrated symmetrical cell at 550 ℃ was measured over a period of 500 hours with no sign of decay.Item Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)(2016) Pan, Ke-Ji; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.Item HETEROGENEOUS POLYMERIZATION OF METHYL METHACRYLATE AT LOW TEMPERATURE IN DISPERSED SYSTEMS(2011) EMDADI, LALEH; CHOI, KYU YONG; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ABSTRACT Title of thesis HETEROGENEOUS POLYMERIZATION OF METHYL METHACRYLATE AT LOW TEMPERATURE IN DISPERSED SYSTEMS Laleh Emdadi, Master of Science, 2011 Directed by: Professor, Dr. Kyu Yong Choi, Chemical and Biomolecular Engineering Department Dispersion polymerization is a unique method to prepare monodisperse polymer particles of 1-10 µm in a single step process. This process is usually carried out at high temperatures that are not cost effective and suitable for special applications such as encapsulation of bio materials. Production of uniform polymer particles at low temperatures via dispersion polymerization has not been studied widely yet. In this research, dispersion polymerization of methyl methacrylate (MMA) in a nonpolar solvent, n-hexane, using N,N-dimethylaniline (DMA) and lauroyl peroxide (LPO) as redox initiators at low temperature has been studied. The evolutions of monomer conversion, polymer molecular weight distribution (MWD), and particle morphology were determined. Under specific reaction conditions, monodisperse micron-sized polymer particles were produced. The same technique was applied in the confined reaction space of a monomer droplet. Using this new process, called micro dispersive suspension polymerization, polymer particles with different internal morphologies produced with various potential applications.