Browsing by Author "Can, Hatice Merve."

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    Characterization of the effect of critical design parameters on the electrochemical performance of a lithium-sulfur battery
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Can, Hatice Merve.; Eroğlu Pala, Damla.
    Lithium-sulfur (Li-S) batteries provides high theoretical specific energy and energy density and their performance is greatly sensitive to cell design as a result of the highly complex reaction and polysulfide shuttle mechanisms within the cathode. Electrolyte-to sulfur (E/S) ratio, carbon-to-sulfur (C/S) ratio, sulfur loading and carbon type are vital design parameters with a critical influence on the battery performance. Here, an integrated research methodology coupling experimental characterization and electrochemical modeling was applied to forecast the relation between the key design parameters and the discharge capacity, cycling performance and cell- and system-level specific energy and energy density of the Li-S battery. Firstly, the effect of the E/S ratio was examined; the highest initial discharge capacity was achieved with an E/S ratio of 20 μl mg−1, whereas, the best capacity retention was observed for 13 μl mg−1. Consequently, an E/S ratio of 13 μl mg−1 presented the best performance as the impact of the E/S ratio not only on the peak discharge capacity and capacity retention but also on cell- and system-level performance were considered. Secondly, the influence of the C/ S ratio was investigated; the Li-S cell having a C/S ratio of 2 and an E/S ratio of 13 μl mg-1 has provided the highest initial capacity in addition to the best capacity retention. Model predictions suggested that increasing the C/S ratio worsens the battery metrics at the pack level at low E/S ratios. Finally, Li-S cells with different carbon type and sulfur loading were studied. The capacity retention of Li-S cells with AB (Acetylene Black) was unaffected by the S loading, but Li-S cells with Super C65 retain capacity at higher S loadings. Li-S cells with KB (Ketjen Black) were unable to attain good performance at higher S loadings, which was surprising given their significantly larger surface area. Super C65 was projected to have the best pack performance. At medium S loadings, when discharge capacities are maximized, Li-S cells with both AB and Super C65 cathodes attain the greatest system-level metrics.
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    Parametric study of catalytic CO hydrogenation for producing dimethyl ether
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Can, Hatice Merve.; Avcı, Ahmet Kerim.; Önsan, Zeynep İlsen.
    The objective of this study is to examine the catalytic performance of bi-functional catalyst systems in direct synthesis of dimethyl ether (DME). Direct synthesis method involves two consecutive steps: methanol synthesis followed by methanol dehydration. Hence, a commercial methanol synthesis catalyst (Cu-Zn based HiFuel-R120) was coupled with different methanol dehydration catalysts in a dual-bed micro-reactor. Methanol dehydration catalysts were prepared by incipient-to-wetness impregnation by varying CeO2 loading on δ-Al2O3. Syngas-to-DME performance of the bi-functional catalyst system was studied in an Autoclave Engineers' BTRS-Jr-PC high-pressure, high-temperature reaction test system with a down-flow fixed-bed reactor designed to operate up to 600oC and 100 atm. Temperature, pressure, feed composition and CeO2 loading were tested for their effects on catalyst performance expressed in terms of CO conversion; DME, methane, carbon dioxide and methanol yields, and DME selectivity. Temperatures of 250, 275 and 300oC and pressures of 25 and 34 bar were tested with CeO2 loadings of 5%, 10% and 20% CeO2 on δ-Al2O3 for methanol dehydration. Results on 5% and 10% CeO2/δ-Al2O3 catalysts indicate that increasing the temperature enhances CO conversion and increases both DME selectivity and DME yield, while CO conversion on 20% CeO2/δ-Al2O3 is not altered, which may be due to metal sintering. Increasing the pressure leads to higher catalytic activity on 5% and 10% CeO2/δ-Al2O3, due to Le Chatelier’s principle operative in methanol synthesis. Effect of H2/CO molar feed ratio on CO conversion and DME selectivity is studied at ratios of 1 and 2, and results show that a H2-rich medium increases DME selectivity. Effect of decreasing CeO2 loading is to enhance CO conversion and DME selectivity. Highest CO conversion (36.6%) and DME selectivity (74.4%) are obtained when HiFUEL R120 is coupled with 5% CeO2/δ-Al2O3 at 300 °C and 34 bar using a H2/CO molar feed ratio of 2.