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Browsing by Subject "Ionic liquid"

Now showing 1 - 6 of 6
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    Activated Carbon Fibers from Cellulosic Biomass with Surface Reductive Treatment for Air Cleanup and VOCs Sensing
    (2018-12) Wang, Yu-Hsiang
    Biological volatile organic compounds (Bio-VOCs) play crucial roles in living organisms such as plants, microbes, and the animals. Sub-ppm level of Bio-VOCs could work as indicators to provide information about metabolism or hormones to facilitate different stages of growth in an organism. For example, less than 25 ppb of ethylene can reduce flowering time, increase seed weight and promote ripening of plants. Thus, there is a need for sensitive detection to provide valuable information for in situ monitoring of biological ecology, as well as for environmental controlling and managing. In situ monitoring of Bio-VOCs requires highly sensitive detections (at mostly sub-ppm concentration level) using portable and accurate sensors, which is extremely challenging for most analytical methods currently available. This work examines the feasibility of an intensified capture and detection strategy for detection of trace amounts of Bio-VOCs, with the sensor unit suitable for miniaturized design for eventually remote and unmanned vehicle sensing applications. In the first part of the study, activated carbon fibers were developed using a reductive reduction procedure, and were examined for pre-concentrating of Bio-VOCs (from ppb-level raised to ppm-level). We expect the reductive carbonization can effectively remove the oxygenated groups from the cellulosic materials, producing fine-tuned electronic properties, which promote pi-pi interaction for intensified the adsorption of nonpolar VOCs (especially for multiple pi bonds compounds). The such produced carbon fibers were examined by XPS (X-ray Photoelectron Spectroscopy), which showed that 53% of the carboxylic and hydroxy groups have been successfully removed. The performance of reductive treated carbon fibers as an adsorbent was examined. Three nonpolar VOCs, methane, ethylene and benzene, were selected as typical biological and chemical VOCs. A sixteen-times increase of benzene (has multiple pi bonds) adsorption can be observed in comparison to carbon fibers without reductive treatment. The unique network structure of the reductive treated carbon fibers also provides a fine electrical conductivity (6.36 Ωcm), that makes it possible for electrothermal desorption for material regeneration. A full regeneration of VOCs was observed in repeated adsorption-desorption cycles, indicating excellent reusability and stability of reductive carbonized carbon fibers. When applied as a pre-concentration absorbent, the carbon fibers successfully increased the concentration of typical VOCs from 500 ppb to 3.5 ppm (700% increase) within 20 min. In order to develop a miniaturized sensor with ultra-high sensitivity and stability for in situ monitoring of Bio-VOCs, the electrochemical sensing system was employed because it is able to identify and quantify various VOCs with high accuracy and sensitivity. However, most of traditional electrochemical sensors have been developed for analysis of aqueous samples, they are easily impacted by the evaporation of water (changing the concentration of electrolyte) when applied to gaseous samples as concerned in the current work. To improve the stability of electrochemical sensing, we developed a unique thin film ionic liquid (IL)-gel coated sensor employing ionic liquids in poly(acrylamide) hydrogels as a solution-free electrolyte. We assumed the stability of the analysis can be improved since the solution of the electrochemical system can be locked in a gel phase, minimizing the evaporation. The ion liquids also have been selected as an electrolyte since the acidity of IL can facilitate the detection of ethylene (one critical Bio-VOCs), preventing the oxidization of the working electrode before ethylene oxidization. A series of experiments were conducted to confirm the performance of IL-gel coating sensor. The results showed the sensor has excellent sensitivity and linearity of our sensor with low detection limit to 650 ppb and 0.99 of R2 values within 0~15 ppm. Decent stability was obtained with a relative standard deviation below 1% for 1.5 months of storage. In addition, the strategy of using reductive fabricated carbon fibers as a pre-concentrating material and a thin film IL-gel coated sensor as a detection unit was also examined. Overall, our work successfully demonstrated the capture-detection strategy is suitable for stable detecting of an extremely low (sub-ppb level) concentration of VOCs. By integrating the preconcentration and senor units, we could eventually develop sensors that are capable of detecting VOC samples in the order of ppb. This approach is promising for building up miniatured Bio-VOCs sensors for in situ monitoring in future applications.
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    Block copolymer Ion gels for CO2 Separations
    (2013-08) Gu, Yuanyan
    Block copolymer ion gel is composed of a polymer network formed by self-assembly of triblock copolymers, and an ionic liquidIn this thesis project, the target is to study the gas separation performance of ion gels for CO2 separation, and seek ways to optimize their properties in terms of the gas separation performance and mechanical strength. Ionic liquids have shown great promise as novel CO2-separation media, largely due to their highly selective gas solubility and non-volatility. It is discovered that the polymer networks not only provides the mechanical support to the ionic liquid, but help improve the gas separation performance as well.To study the CO2 separation performance of block copolymer ion gels, model ion gel systems that comprise 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]), and a triblock copolymer with a polymerized ionic liquid mid-block was prepared.. The gas separation performance was measured on a supported ion gel membrane. It was discovered that the polymerized ionic liquid gels exhibit high gas permeability due to the high liquid fraction. Moreover, the permeation selectivity is significantly increased from that of the neat ionic liquid. Comparisons with Robeson plots also indicate very promising separation performance for ion gels. Two other ion gels formed by self-assembly of poly(styrene-b-ethylene oxide-b-styrene) (SOS) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) in [EMI][TFSA] were also examined. The separation performance of ion gels was found to be strongly dependent on the polymer mid-block. It is also desirable to enhance the mechanical properties of ion gels. A novel ion gel based on poly[(styrene-r-vinylbenzyl azide)-b-ethylene oxide-b-(styrene-r-vinylbenzylazide)] (SOS-N3) was synthesized. Such a triblock copolymer ion gel can be chemically cross-linked by high temperature annealing and UV-irradiation. After cross-linking, the mechanical strength of the gel showed significant improvement, with 400% increase in the tensile strength and almost one order of magnitude increase in toughness. The mechanical stability of the supported ion gel membranes was also enhanced. More importantly, the mass transport properties are retained after the cross-linking. Overall, block copolymer ion gels represent a promising class of materials for CO2 separation applications. Through rational choice of ionic liquid and block copolymers, the properties of ion gels can be further optimized.
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    Characterization of pi-conjugated polymers for transistor and photovoltaic applications
    (2012-12) Paulsen, Bryan D.
    pi-Conjugated polymers represent a unique class of optoelectronic materials. Being polymers, they are solution processable and inherently "soft" materials. This makes them attractive candidates for the production of roll-to-roll printed electronic devices on flexible substrates. The optical and electronic properties of pi-conjugated polymers are synthetically tunable allowing material sets to be tailored to specific applications. Two of the most heavily researched applications are the thin film transistor, the building block of electronic circuits, and the bulk heterojunction solar cell, which holds great potential as a renewable energy source. Key to developing commercially feasible pi-conjugated polymer devices is a thorough understanding of the electronic structure and charge transport behavior of these materials in relationship with polymer structure. Here this structure property relationship has been investigated through electrical and electrochemical means in concert with a variety of other characterization techniques and device test beds. The tunability of polymer optical band gap and frontier molecular orbital energy level was investigated in systems of vinyl incorporating statistical copolymers. Energy levels and band gaps are crucial parameters in developing efficient photovoltaic devices, with control of these parameters being highly desirable. Additionally, charge transport and density of electronic states were investigated in pi-conjugated polymers at extremely high electrochemically induced charge density. Finally, the effects of molecular weight on pi-conjugated polymer optical properties, energy levels, charge transport, morphology, and photovoltaic device performance was examined.
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    Decoupling mechanical and ion transport properties in polymer electrolyte membranes
    (2014-08) McIntosh, Lucas
    Polymer electrolytes are mixtures of a polar polymer and salt, in which the polymer replaces small molecule solvents and provides a dielectric medium so that ions can dissociate and migrate under the influence of an external electric field. Beginning in the 1970s, research in polymer electrolytes has been primarily motivated by their promise to advance electrochemical energy storage and conversion devices, such as lithium ion batteries, flexible organic solar cells, and anhydrous fuel cells. In particular, polymer electrolyte membranes (PEMs) can improve both safety and energy density by eliminating small molecule, volatile solvents and enabling an all-solid-state design of electrochemical cells. The outstanding challenge in the field of polymer electrolytes is to maximize ionic conductivity while simultaneously addressing orthogonal mechanical properties, such as modulus, fracture toughness, or high temperature creep resistance. The crux of the challenge is that flexible, polar polymers best-suited for polymer electrolytes (e.g., poly(ethylene oxide)) offer little in the way of mechanical robustness. Similarly, polymers typically associated with superior mechanical performance (e.g., poly(methyl methacrylate)) slow ion transport due to their glassy polymer matrix. The design strategy is therefore to employ structured electrolytes that exhibit distinct conducting and mechanically robust phases on length scales of tens of nanometers.This thesis reports a remarkably simple, yet versatile synthetic strategy---termed polymerization-induced phase separation, or PIPS---to prepare PEMs exhibiting an unprecedented combination of both high conductivity and high modulus. This performance is enabled by co-continuous, isotropic networks of poly(ethylene oxide)/ionic liquid and highly crosslinked polystyrene. A suite of in situ, time-resolved experiments were performed to investigate the mechanism by which this network morphology forms, and it appears to be tied to the disordered structure observed in diblock polymer melts near the order-disorder transition. In the resulting solid PEMs, the conductivity and modulus are both high, exceeding the 1 mS/cm and approaching the 1 GPa metrics, respectively, often cited for lithium-metal batteries. In the final chapter, an alternative synthetic route to generate nanostructured PEMs is presented. This strategy relies on the formation of a thermodynamically stable network morphology exhibited by a triblock terpolymer prepared with crosslinking moieties along the backbone. Although the mechanical properties of the resulting PEM are excellent, the conductivity is found to be somewhat limited by network defects that result from the solvent-casting procedure.
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    Large enhancement of capacitance driven by electrostatic image forces.
    (2011-04) Loth, Matthew Scott
    The purpose of this thesis is to examine the role of electrostatic images in determining the capacitance and the structure of the electrostatic double layer (EDL) formed at the interface of a metal electrode and an electrolyte. Current mean field theories, and the majority of simulations, do not account for ions to form image charges in the metal electrodes and claim that the capacitance of the double layer cannot be larger than that of the Helmholtz capacitor, whose width is equal to the radius of an ion. However, in some experiments, and simulations where the images are included, the apparent width of the capacitor is substantially smaller. Monte Carlo simulations are used to examine the interface between a metal electrode and a room temperature ionic liquid (RTIL) modeled by hard spheres (the “restricted primitive model”). Image charges for each ion are included in the simulated electrode. At moderately low temperatures the capacitance of the metal/RTIL interface is so large that the effective thickness of the electrostatic double-layer is up to 3 times smaller than the ion radius. To interpret these results, an approach is used that is based on the interaction between discrete ions and their image charges, which therefore goes beyond the mean-field approximation. When a voltage is applied across the interface, the strong image attraction causes counterions to condense onto the metal surface to form compact ion-image dipoles. These dipoles repel each other to form a correlated liquid. When the surface density of these dipoles is low, the insertion of an additional dipole does not require much energy. This leads to a large capacitance C that decreases monotonically with voltage V , producing a “bell-shaped” C(V ) curve. In the case of a semi-metal electrode, the finite screening radius of the electrode shifts the reflection plane for image charges to the interior of the electrode resulting in a “camel-shaped” C(V ) curve, which is parabolic near V = 0, reaches a maximum and then decreases. These predictions are in qualitative agreement with experiment. A similarly simple model is employed to simulate the EDL of superionic crystals. In this case only small cations are mobile and other ions form an oppositely charged background. Simulations show an effective thickness of the EDL that may be 3 times smaller than the ion radius. The weak repulsion of ion-image dipoles again plays a central role in determining the capacitance in this theory, which is in reasonable agreement with experiment. Finally, the problem of a strongly charged, insulating macroion in a dilute solution of multivalent counterions is considered. While an ideal conductor does not exist in the problem, and no images are explicitly included, simulations demonstrate that adsorbed counterions form a strongly correlated liquid of at the surface of the macroion and acts as an effective metal surface. In fact, the surface screens the electric field of distant ions with a negative screening radius. The simulation results serve to confirm existing non-mean-field theories.
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    Superconductor-insulator transition induced by electrostatic charging in high temperature superconductors.
    (2011-11) Leng, Xiang
    Ultrathin YBa2Cu3O7−x films were grown on SrTiO3 substrates in a high pressure oxygen sputtering system to study the superconductor-insulator transition by electrostatic charging. While backside gating using SrTiO3 as a dielectric induces only small TC shifts, a clear transition between superconducting and insulating behavior was realized in a 7 unit cell thick film using an ionic liquid as the dielectric. Employing a finite size scaling analysis, curves of resistance versus temperature, R(T), over the temperature range from 6 K to 22 K were found to collapse onto a single function, which suggests the presence of a quantum critical point. However the scaling failed at the lowest temperatures indicating the possible presence of an additional phase between the superconducting and insulating regimes. In the presence of magnetic field, a cleaner superconductor-insulator transition was realized by electrostatic charging. A scaling analysis showed that this was a quantum phase transition. The magnetic field did not change the universality class. Further depletion of holes caused electrons to be accumulated in the film and the superconductivity to be recovered. This could be an n-type superconductor. The carriers were found to be highly localized. By changing the polarity of the gate voltage, an underdoped 7 unit cell thick film was tuned into the overdoped regime. This process proved to be reversible. Transport measurements showed a series of anomalous features compared to chemically doped bulk samples and an unexpected two-step mechanism for electrostatic doping was revealed. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level.