Chemistry & Biochemistry Theses and Dissertations

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    SOVLENT REACTIVITY AND INTERFACE EVOLUTION AT MODEL ELECTRODES FOR ENERGY APPLICATIONS
    (2016) Song, Wentao; Reutt-Robey, Janice E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Li-ion rechargeable battery (LIB) is widely used as an energy storage device, but has significant limitations in battery cycle life and safety. During initial charging, decomposition of the ethylene carbonate (EC)-based electrolytes of the LIB leads to the formation of a passivating layer on the anode known as the solid electrolyte interphase (SEI). The formation of an SEI has great impact on the cycle life and safety of LIB, yet mechanistic aspects of SEI formation are not fully understood. In this dissertation, two surface science model systems have been created under ultra-high vacuum (UHV) to probe the very initial stage of SEI formation at the model carbon anode surfaces of LIB. The first model system, Model System I, is an lithium-carbonate electrolyte/graphite C(0001) system. I have developed a temperature programmed desorption/temperature programmed reaction spectroscopy (TPD/TPRS) instrument as part of my dissertation to study Model System I in quantitative detail. The binding strengths and film growth mechanisms of key electrolyte molecules on model carbon anode surfaces with varying extents of lithiation were measured by TPD. TPRS was further used to track the gases evolved from different reduction products in the early-stage SEI formation. The branching ratio of multiple reaction pathways was quantified for the first time and determined to be 70.% organolithium products vs. 30% inorganic lithium product. The obtained branching ratio provides important information on the distribution of lithium salts that form at the very onset of SEI formation. One of the key reduction products formed from EC in early-stage SEI formation is lithium ethylene dicarbonate (LEDC). Despite intensive studies, the LEDC structure in either the bulk or thin-film (SEI) form is unknown. To enable structural study, pure LEDC was synthesized and subject to synchrotron X-ray diffraction measurements (bulk material) and STM measurements (deposited films). To enable studies of LEDC thin films, Model System II, a lithium ethylene dicarbonate (LEDC)-dimethylformamide (DMF)/Ag(111) system was created by a solution microaerosol deposition technique. Produced films were then imaged by ultra-high vacuum scanning tunneling microscopy (UHV-STM). As a control, the dimethylformamide (DMF)-Ag(111) system was first prepared and its complex 2D phase behavior was mapped out as a function of coverage. The evolution of three distinct monolayer phases of DMF was observed with increasing surface pressure — a 2D gas phase, an ordered DMF phase, and an ordered Ag(DMF)2 complex phase. The addition of LEDC to this mixture, seeded the nucleation of the ordered DMF islands at lower surface pressures (DMF coverages), and was interpreted through nucleation theory. A structural model of the nucleation seed was proposed, and the implication of ionic SEI products, such as LEDC, in early-stage SEI formation was discussed.
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    ISOTOPE EFFECTS IN THE STATE-RESOLVED COLLISION DYNAMICS OF HIGHLY EXCITED MOLECULES
    (2014) Echebiri, Geraldine Onyinyechi; Mullin, Amy S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The importance of highly excited molecules in the fields of combustion and atmospheric chemistry makes it essential to study pathways by which energy is lost from the excited molecule. One such pathway is by inelastic collisions with a bath molecule. In this dissertation, the collisional relaxation of highly excited pyrazine-h4 (Evib = 37900 cm-1) and pyrazine-d4 (Evib = 37900 cm-1) with HCl (300 K) is studied. The outcomes of the inelastic collision studies reveal quantum state-energy gaps of molecules and their intermolecular interactions affect the mechanism and dynamics of collisional energy transfer. The results from collisional relaxation of pyrazine-h4 (Evib = 37900 cm-1) with HCl were compared to those from collisional relaxation of pyrazine-h4 (Evib) with DCl in order to deduce the effects of quantum state-energy gaps on the dynamics of collisional energy transfer. The comparison shows the dynamics for collisional deactivation of pyrazine-h4 (Evib) with HCl and DCl are different, and are possibly due to their intermolecular interactions with pyrazine-h4 (Evib. The data for collisional relaxation of pyrazine-d4 (Evib = 37900 cm-1) with HCl were compared to those for pyrazine-h4 (Evib) + HCl collisions in order to determine the contributions of near-resonant vibrational energies of the collision partners on the collision dynamics. The comparison shows the energy transfer dynamics for collisional quenching of pyrazine-h4 (Evib) and pyrazine-d4 (Evib) with HCl are similar. The similarity in their energy transfer dynamics suggests near-resonance effects are not contributing significantly to the collision dynamics.