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Theoretical reaction mechanistic studies on energetic nanomaterials and Li-CO2 battery via multi-scale molecular simulation

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Title
Theoretical reaction mechanistic studies on energetic nanomaterials and Li-CO2 battery via multi-scale molecular simulation
Author
Jeon, Woo Cheol
Advisor
Kwak, Sang Kyu
Keywords
Reaction mechanism; energetic nanomaterials; Li-CO2 battery; multi-scale molecular simulation
Issue Date
2020-08
Publisher
Graduate School of UNIST
Abstract
In recent years, the development of energy systems with improved efficiency has become an important issue in the industrial manufacturing field due to the depletion of traditional energy fuels and ever-increasing commercial demands. Amid this sweeping trend, nanomaterials, which can exert different novel properties to those in conventional macroscopic materials, can be utilized for various energy applications. Nanomaterials play a major role in energy release and energy source. For application in energy release and energy sources, nanomaterials need to release high thermal energy from a relatively low energy level of external shocks or need to exert a high specific capacity and power density, respectively. In other words, nanomaterials for energy applications are required to have high energy density. To enhance their energy density, several nanomaterial candidates have been considered and synthesized. However, there are several difficulties in testing all the candidates due to time constraints and economic issues. Thus, for a more efficient development of the target nanomaterial, a process to screen the candidates is necessary, and reaction mechanistic studies can be conducted as additional processes to advance real experiments. These studies facilitate maximization of energy density in nanomaterials from investigation of thermodynamically and kinetically efficient reaction pathway and comparison of energy surfaces among reaction intermediates. Moreover, theoretical methods can prove to be helpful for reaction mechanistic studies. In this doctoral dissertation, reaction mechanistic studies in energy applications of nanomaterials have been conducted via multi-scale molecular simulation technique, which can prove to be a powerful tool to understand the physico-chemical phenomena for the reaction mechanistic studies of nanomaterials in energy applications. In Chapter 2, we theoretically tracked the reaction process of Ni-Al nanoalloys. Molecular dynamics simulations had been applied to investigate the characteristics depending on molar ratio of Ni and Al, the bilayer thickness of nanolayer, and ignition temperature. It was found that the variation of stoichiometry between Ni and Al had marginal effects on the overall process of reaction coordinates, however, the reaction rate and intermixing regions were different in each system. In addition, quantitative analysis on the reaction kinetics and thermodynamics were performed under different reaction and structural conditions. In this theoretical study, the reaction characteristics of Ni-Al nanolayers were quantified with systematic calculations. Therefore, it was expected to contribute to fabricate more advanced Ni-Al nanolayer products. In Chapter 3, we investigated the explosion characteristics of a nanobomb. In a nanobomb, nitromethane is constantly protected from the external environment due to stable mechanical and thermal properties of carbon nanotube (CNT) and is confined with the built-up pressure. After injection of thermal energy into confined nitromethane (NM) at various densities, the nanobomb was completely decomposed along the bursting process. The results show that the explosion time was reduced at a higher density and initial temperature. While NM was being decomposed into intermediates, Stone-Wales (SW) defects or high-order rings were randomly constructed at both the cap and side wall of CNT. Subsequently, carbon atoms at defect sites were functionalized by the reaction intermediates, where nanoholes were generated and burst at the end of bursting phenomena. Next, physicochemical modification of CNT was considered to improve the performance of the nanobomb. Chirality, nitrogen-doping, and monovacancy defect were introduced into CNT. All types of modifications on CNT brought time reduction in bursting of nanobomb although there was similarity on overall bursting mechanism. Among modifications on CNT, monovacancy defect exhibited the most striking effects on the enhancement of bursting. This suggests that chemical reactivity increased drastically around the defect sites. To intensively study the reason for this difference, SW defect formation energy and the adsorption energies of radical products on CNT were calculated for each modification. Both the formations of SW defects and bindings of the products were more favorable on monovacancy defect and nitrogen-doping site than the sites in pristine CNT. Furthermore, two heating methods were examined (e.g. electric spark and electromagnetic induction) as the additional external shocks on nanobomb. Bursting of nanobomb with electromagnetic induction occurs much rapidly due to oscillating frequency under a continuous electric field. Additionally, synergistic effects on the bursting of nanobombs with NM-detonating molecule mixed inside CNT were investigated. Detonating molecule candidates were initially filtered by comparing detonation velocity and pressure derived from Kamlet–Jacobs (K–J) equations. When bulk mixtures which contain NM and detonating molecules were constructed and decomposed at high temperatures, HMX or RDX showed a faster decomposition rate than that of NM and supported acceleration in NM decomposition rate. Furthermore, nanobombs in which HMX or RDX is confined with NM in CNT were heated by thermal energy from CNT, and their decomposition processes were compared with pure NM nanobomb. After the confined molecules were heated, detonating molecules were decomposed prior to NM and contributed to enhanced decomposition of NM. Eventually, CNT with the detonating molecule burst by continuous functionalization of reaction intermediates in much short time than pure NM nanobomb. We believe that our theoretical explorations to improve the explosion performance of nanobomb enable much feasible manipulation of nanostructured HEMs. In Chapter 4, reaction pathways in quinary molten-salt electrolyte-based Li−CO2 battery with Ru catalyst were theoretically estimated, in which nitrate-based molten salt and Ru catalyst were introduced. This led to a significantly improved performance compared to previous Li−CO2 batteries. Additionally, the number of battery cycles that can be operated was increased, but the reasonable electrochemical reaction behind its charge and discharge process was still veiled. From DFT calculation, three plausible reaction pathways in charge processes depending on operation temperature of battery cell were derived. For the discharge process, each free energy diagram with and without Ru surface was compared to probe the catalytic role of Ru nanoparticle. Consequently, Ru surface strongly reduced the energy in thermodynamic barrier of discharge process, and this was because movement of electrons from CO2− to Ru surface energetically stabilized CO2−. We believe that mechanistic understanding of electrochemical reactions in charge and discharge processes will provide significant information for further development of Li−CO2 battery cell.
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Department of Chemical Engineering
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