358 research outputs found
Modification of Porous Materials to Improve Their Gas Storage and Catalytic Activity
Full text linkOver the past decade, researchers have shown an increased interest in the field of highly porous materials. This dissertation herein will mainly focus on design and post-synthetic modification of two types of porous materials, such as Metal-Organic Frameworks (MOFs) and Porous Coordination Cages (PCCs) for optimizing guest-host interaction.
In chapter I, an overview of the discovery of these porous materials is provided. Their properties and application were discussed. This chapter is concluded by defining specific aims of the subsequent research.
One of the drawbacks faced by MOFs is the difficulty in synthesizing frameworks that have a large pore size yet maintain framework stability under harsh conditions. To fulfill this challenge, a simple methodology for the generation of ordered mesopores in an inherently microporous MOF through Soxhlet extraction is discussed in chapter II. This innovative method demonstrates a simple and reproducible process that results in a material that possesses the benefits of mesoporous while borrowing the robustness of a micropore framework. While many mesoporous MOFs have demonstrated good methane uptake, the stability of those MOFs is an issue when unrefined natural gases are attempted to capture. In this chapter, I also report a way to overcome this problem by choosing a stable MOF and applying a post-treatment method by doping long-chain hydrocarbon in the MOF. By applying this process, we observed hydrocarbon doping improves the methane uptake performance for the material.
Based on these results, in chapter III, I discuss the investigation of the in-situ functionalization with alkyl chains and even more polar and non-polar groups during the MOF synthesis by using Ligand-Fragment Co-Assembly that will provide the additional steric and electronic interactions necessary to enhance gas binding.
In chapter IV, I discuss about another class of porous material PCCs for first row transition metal nanoparticle encapsulation. Design of a series of PCC cages and efforts towards first-row transition metal encapsulation are detailed in this chapter.
Finally, a synopsis of the research and opinions on future directions in this field are provided. Overall, my results lay the foundation for optimized post-synthetic modification techniques in porous materials for gas storage and catalysis
Vanadium Dioxide Nanocomposite Coatings as Fenestration Elements
Full text linkSmart windows have attracted interest as a method of reducing the energy consumption of buildings. These smart windows would be able to reflect and transmit infrared (IR) light in response to ambient temperature, while also retaining the ability to transmit daylight. Vanadium dioxide is a prime candidate for use in smart windows due to the fact that its transition temperature of 67��C is far closer to room temperature than is typical of materials exhibiting a reversible metal���insulator transition; the transition temperature can be further depressed by introducing dopants. Above this transition temperature, vanadium dioxide switches from an insulator to metal and is able to reflect IR light. This characteristic would efficiently cut down energy consumption, as the smart windows would be able to regulate temperatures inside of buildings with reduced aid of air-conditioning systems. Vanadium dioxide alone produces a very poor film and therefore must be interfaced with a different material for practical application. Previously, our group found that silica matrices could be utilized to form films with VO2 via a modified St��ber method. These films were formed utilizing spray coating, and though somewhat successful, the method did not provide scalable or consistent films. Later, VO2 was dispersed in thickening agents to further stabilize dispersions and casting was performed with a casting knife, which improved film consistency. In this work we are focusing on functionalizing the surface of VO2 or VO2@SiO2 nanocrystals in solution with a perfluorinated silane that will work in tandem with fluorinated surfactants to disperse these materials within acrylic acid matrices. Surface functionalization of the VO2 will protect the material from oxidation as well as allowing the use of a surfactant to reduce agglomeration, which has been found to be a problem in previous work, thereby yielding a more homogeneous distribution of particles within films. Optimization of the St��ber method by changing experimental parameters related to the shell precursor and catalyst were investigated to reduce the overall agglomeration of VO2 nanoparticles in the silica shell
Recession in the Skilled Sector and Implications for Informal Wage
Full text linkGlobal recession is likely to hit the skilled sector or the so-called white goods, white collared sector in a typical developing economy. In this paper we try to analyze the impact of such an event on informal wage as the vast majority of the workforce in the developing world is employed in the unorganized or informal sector. In particular, we demonstrate the analytical possibility that a recession in the skilled sector will actually increase real informal wage.Recession, skill, capital-labor ratio, informal wage, general equilibrium
An Assessment of Developed Thermochromic Solar Films for Energy Conservation
Full text linkEnergy consumption has become directly associated with buildings. According to the United Nations, it is estimated that buildings contribute by 30-40% of the worldwide energy consumption. The consumption of energy has increased due to urbanization in the past 20 years. Heating, cooling, in addition to external and internal heat gains are, primarily, the source of energy consumption in buildings. Solar heat gains and losses occur through several components of the buildings. For instance, the use of glass windows for indoor lighting. This research aims to assess developed thermochromic nanocomposite films that have been produced through the annealing of Vanadium Dioxide (VO2) nanocrystals within Silicon Dioxide (SiO2) shells. The nanocomposite solar films can spectrally select and dynamically tune control over the visible and infrared regions of the solar spectrum, allowing for control of the desired lighting and solar heat gain. The control of the light and heat will, consequently, lower energy consumption for heating and cooling in buildings
Analysis of Mechano-Electrochemical Coupling in Intercalation Electrodes
Full text linkLithium ion batteries (LIB), owing to their high energy and power density, have gained popularity in portable electronics and automotive markets. Diffusion induced stress (DIS), due to intercalation of lithium during lithiation/delithiation process is one of the main causes of mechanical degradation in LIB. The microcracks formed hinder the diffusion of lithium inside the active particle. Also, the microcracks linked to the surface of the particle are exposed to the electrolyte and are electrochemically active.
This study investigates the mechano-electrochemical coupling observed in intercalation electrodes. The interdependence between microcrack formation and lithium concentration distribution in the active particle and its effect on the performance of LIB has been analyzed. A microcrack prediction model has been developed that estimates microcrack formation at each time step based on the DIS calculated using the concentration gradients evaluated from the concentration profile. The microcracks affect the transport of lithium within the particle in two opposing ways. On one hand, microcracks decrease the local diffusivity of the active material thereby hindering lithium diffusion. On the other hand, microcracks emanating from the surface of the particle are electrochemically active and enhance lithium diffusion by allowing electrochemical reactions inside the active particle at the microcrack-electrolyte interface, thereby reducing the effective diffusion length. Thus, microcrack formation leads to a change in the electrochemically active surface area of the electrode. Lithium source/sink terms are introduced along the electrochemically active microcracks to simulate the electrochemical reactions. The non-uniform microcrack patterns predicted by the mechano-electrochemically coupled model closely resemble the patterns observed in SEM images of LIB electrodes.
The performance curve obtained can help identify the effect of mechanical degradation on the performance of the battery and thereby provide a guideline for optimizing the physicochemical factors to leverage mechanical degradation for better cell performance
Functionalizing Graphene Surfaces with Precise Dye Absorbed Oxygen Deposition
Full text linkThis thesis studies the properties found in graphene and combines them with the potential of increased responsivity due the addition of dyes which could allow for more efficient and effective optoelectronics. Here, we theorize the bonding between 2,4,6-trichloro-1,3,5-triazine (Cyanuric Chloride) and Graphene Oxide to allow the bonding of 1-amino-2-methylanthraquinone (Disperse Orange 11) to occur. With this bond occurrence, future studies can investigate the local bonding of such organic bonds onto graphene oxide for the use in micro and nanotechnology. The specific design is as follows. Cyanuric Chloride is covalently bonded to the oxygen of the Graphene Oxide sample. Disperse Orange 11 is then added to the mixture allowing for the covalent bonding of the dye. This sample is then tested with a series of spectroscopy instruments including a UV-Vis machine and an XPS machine as well as height testing with an AFM to determine the success of the dye bonding
Experiments and Analysis of Aqueous Electrode Processing for Energy Storage
Full text linkThere is an ever-growing demand for Lithium-Ion Batteries in a widespread series of applications, where battery life and reliability are of key importance. There exist novel materials that are helping increase battery reliability and life but there is a lack of environment friendly and cost-effective processing techniques that are used to produce such energy storage devices. Current processing techniques use N-methyl-2 pyrrolidone as a solvent for electrode slurry, which is expensive and has the potential to damage the environment, increasing the risk of cancer and reproductive toxicity. Therefore, there is a need to move towards a solvent that is environmentally friendly, cheap to produce and can serve as a potential replacement. In this work, the use of deionized water has been experimentally evaluated to create an electrode processing technique that could become an environmentally friendly and cost-effective technique to produce Lithium-Ion Batteries.
This study focuses on the concepts of Lithium-Ion Batteries and their current electrode processing techniques. The proposed Aqueous Processing technique for electrode manufacture is discussed in detail along with a discussion of challenges currently being faced in this area. A 1-D physics based drying model is also developed as part of this study that is based upon evaporation, diffusion and sedimentation.
My analysis has shown that the proposed Aqueous Processing can be implemented using low-cost preparation methods and deionized water. Drying temperature has an effect on the agglomeration of particles that could impact the electrochemical performance of the electrode. My analysis has also shown that an optimal amount of dispersant needs to be added to reduce the effect of agglomeration while maintaining good film adhesion. The results from the 1-D show that at a higher drying temperature a larger volume fraction is observed at the top surface of the electrode
Inverse Method to Estimate Heat Generation Rates in Lithium Ion Cells
Full text linkLi-ion batteries represent a pinnacle of compact energy storage. This size reduction makes them very energy dense systems, thus substantially increasing the chances of the mishap. Future applications demand a very good rate capability (i.e., fast charging), which invariably leads to higher heat generation. This heat if not dissipated properly, rapidly increases the cell temperature and eventually leads to thermal runaway. Thus, the knowledge of heat generation as a function of current is of utmost importance for the design of cooling systems. Heat generation rates are most commonly quantified using accelerating rate calorimeter.
In this study, a calorimeter-free method based on inverse heat transfer analysis is proposed. 18650 cells are electrochemically cycled at different currents (C-rate) with consecutive charge-rest-discharge-rest cycles in a constant temperature ambient. During the experiments cell temperature, ambient temperature, current and voltage data is recorded. An energy balance is carried out to model the thermal response of the 18650 cell during electrochemical cycling. The model involves volumetric heat generation rate and convective heat transfer coefficient as unknowns which are characterized by inverse heat transfer analysis. Convective heat transfer coefficient is computed from data during rest periods. It is then used to quantify heat generation rate as a function of charge/discharge capacity and C-rate. At low current operation, the contribution of reversible heat is of the similar order to irreversible heat and would lead to qualitatively different heat generation profiles during charging and discharging. On the other hand, at higher currents, irreversible heat dominates and the heat generation rates during charge and discharge are quite similar, both qualitatively and quantitatively.
The contribution of various sources towards total heat generation has been quantified. Effect of capacity fade on internal resistances and heat generation rates, while cycling at various C-rates, has been investigated. At higher C-rates, the contribution of reversible heat towards the total heat generation is found to be negligible while that of irreversible ohmic heat is found to be major and is closely related to the internal resistance. Internal resistance is found to be independent of C-rate of operation, and increasing with capacity fade in a cell
Non-Invasive Estimation of Lithium-Ion Cell Thermo-Physical Properties
Full text linkCylindrical Li-ion cells have one of the highest energy density and power density of all Li-ion cell types and typically employ a spiral electrode assembly. This spiral assembly leads to a large anisotropy leading to a drastic difference in the thermo-physical properties in the axial and the radial direction. The radial direction has multiple layers of electrodes and separators leading to a high thermal impedance in this direction, whereas in the axial direction, not many obstacles are present and hence the thermal conductivity is on the higher side. This research describes a novel experimental technique to measure the anisotropic thermal conductivity and heat capacity of Li-ion cells using thermal impedance spectroscopy (T.I.S). It is paramount to experimentally measure the radial and axial thermal conductivities of a cylindrical Li-ion cell because the assumption of isotropic thermal transport properties in Li-ion cell design would lead us to either under predict the value or over predict the value of the temperature field - both of which would lead to highly undesirable results.
The experimental measurements indicate that radial thermal conductivity is two orders of magnitude lower than axial thermal conductivity for cylindrical 18650 cells which is in sync to what we intuited. Moreover, the work presented here also establishes a trend of the change in thermos-physical properties with varying the state of charge of the cell. This is extremely helpful in order to develop an efficient cooling system for any device that needs to continuously charge and discharge over thousands of cycles. The data helps to account for the change in the thermal conductivity and heat capacity over a period of cycling of the cell and thus encouraging an update in the cooling system for the device also in order to avoid hazardous situations such as thermal runaway.
Lastly, the technique presented in this research is a non-invasive, robust, quick and extremely economical way to determine the thermos-physical properties of an 18650 lithium-ion cell. It also determines the change in thermos-physical properties with the changing state of health of the cell
Exploring Electrode Microstructural Impact on Lithium-Air Battery Performance
Full text linkWith diminishing fuel reserves, the world is facing quite a dire situation in terms of satisfying global energy demands in the near future. Electrochemical energy storage is going to be an essential part of this solution due to its inherently large efficiency (much higher than Carnot limit of heat ��� to ��� work conversion) and sufficiently good reversibility. These electrochemical storage devices have to match the present day fuel economy of gasoline engines for them to present an affordable and realistic solution. Lithium air chemistry is a strong contender to replace internal combustion engines due to their very high energy density (quite comparable to IC engines).
Here one of the reactants ��� oxygen is freely available from atmosphere and thus possess no storage needs. For Li-air cells using organic electrolyte, Li ions react with oxygen and produce insoluble lithium peroxide (Li2O2). Li2O2 being an electronic insulator, covers the electrochemically active surface of cathode and leads to cell shutdown. Alternatively, the oxygen transport from atmosphere to reaction sites could be slow enough to support desired rate of electrochemical reaction. One direction of improvement is to control morphological features of these precipitates prevent them from covering the reaction surface. On the other hand, electrode microstructure could be played with to prolong time to cell shutdown.
The electrochemical behavior of a Li-air cell is modeled using species and charge conservation. Different performance limiting modes, i.e., surface passivation and oxygen starvation, are identified. The surface passivation limits are characterized from previous experimental studies. Various cathode architectures are realized using stochastic regeneration for different mean pore size and initial porosity. They are further abstracted in terms of porous media properties and used during electrochemical simulations. The simulations explore the effects of discharge rates, microstructural properties, separator and cathode dimensions
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