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ÖgeComposite carbon nanofiber anodes for na ion batteries(Graduate School, 2022-06-02) Abdolrazzaghian, Elham ; Yanılmaz, Meltem ; 513201009 ; Nanoscience and NanoengineeringNanofibers are one of the most important nanomaterials and have many applications in diverse fields such as biomedical engineering, drug delivery, filtration, sensors, energy storage etc. due to their unique properties like high specific surface area, small diameters, uniform interconnected 3D porous structure and low weight. There are different types of techniques to produce nanofibers like self-assembly, drawing, melt-blowing, phase separation, electrospinning and centrifugal spinning. Due to special features of centrifugal spinning such as high production yield and safe production, it is one of the most promising methods to prepare nanofibers. In this technique, nanofibers are made by applying centrifugal force on polymer solution or melt by using a high-speed rotating spinneret which lead to form nanofibers on collectors. In centrifugal spinning method, the morphology of nanofibers is dependent on diverse processing parameters such as viscosity, surface tension, molecular structure, molecular weight, solution concentration, solvent structure, additive; and operational conditions such as rotational speed, feeding rate, nozzle diameter, and nozzle-collector distance. Energy and environment head the list of top global problems facing society in the twenty-first century. Nanotechnology is responding to these challenges by designing and fabricating functional nanofibers optimized for energy and environmental applications. Nanofiber materials have been extensively studied as constituent parts of energy conversion and storage devices. Lithium ion batteries (LIBs) are rechargeable batteries and have been widely used in different applications like portable electronic devices and electric vehicles due to their high energy density, however limited lithium sources lead to find another option. Production of sodium ion batteries (SIBs) are gaining great attention due to low-cost and high abundance of sodium resources. Rechargeable batteries consist of an anode, a cathode, an electrolyte and a separator. Conventional materials used as anode suffer from large volume expansion, leakage, pulverization and low conductivity whereas carbon nanofibers have been widely used in electrochemical energy storage devices because of their excellent conductivity, extremely large surface area, high porosity, mechanical flexibility and structural stability that will improve capacity and cycling performance when they are used as electrodes in sodium-ion batteries. The performance of these materials is greatly influenced by the material type; structure; mechanical, chemical, thermal stability and physical properties. For example, if the surface area and porosity increased, the permeation of the electrolyte increases, so the electrode can provide higher sodium storage capability and the electrode will have shorter transport length for sodium ions. Heteroatom doping is one of the best ways to increase the surface area and porosity of nanofibers so enhanced sodium ion insertion and desertion at high rates can be achieved. Until now, composite carbon nanofiber using various materials such as tin, iron, antimony and molybdenum have been widely investigated as the anode materials in SIBs, since they possess high theoretical capacity, environmental benignity, safety, and low cost. In the first part of the study; it was aimed to fabricate carbon nanofibers as anodes for sodium ion batteries via centrifugal spinning. PAN was used as carbon fiber precursor because of its high carbon yield, high thermal, chemical and mechanical properties. Also, PS was blended with PAN in order to increase the porosity of carbon nanofibers. Moreover, graphene with a two-dimensional honeycomb structure was used to improve the electrochemical performance of the LIBs and SIBs, due to its high theoretical specific capacity, large specific surface area, and good electronic conductivity. In addition, the effect of Molybdenum disulfide (MoS2) on performance and electrochemical capacity of LIBs and SIBs was investigated. PAN/PS/graphene polymer blend solution was prepared in DMF and centrifugally spun in order to obtain nanofibers. Nanofibers were fabricated at the rotational speed of 4000 rpm, feeding rate of 60 ml/h, with 0.5 mm nozzle diameter and 20 cm collector distance. After obtaining nanofibers, blend nanofibers were stabilized in air atmosphere at 280 ℃ for 2.5 h with a heating rate of 5 ℃/min and then carbonized in nitrogen atmosphere at 800 ℃ for 2 h with a heating rate of 2 ℃/min. Furthermore, MoS2 decorated graphene-containing porous carbon nanofibers were fabricated via hydrothermal synthesis. The morphology and average fiber diameter distribution were analyzed with SEM. Porous structure of the carbon nanofibers was observed via TEM images. XRD and Raman spectroscopy were used for structural characterization. Porous structure enhanced the electrochemical performance of electrodes. Furthermore, MoS2 decorated graphene included porous CNF improved the electrochemical capacity up to 860 mAh/g in Li-ion cells and 455 mAh/g in Na-ion cells with excellent cycling performance. In the second part of the study; it was aimed to fabricate carbon nanofibers by using water soluble polymers such as PVA and PVP. Considering environmental concerns, it is vital to fabricate carbon nanofibers from environmentally friendly materials. PVA/PVP nanofibers were fabricated via fast and safe centrifugal spinning. The effect of PVP content on the morphology and thermal properties of PVA/PVP blend nanofibers were studied by using SEM and DSC studies. Moreover, the effect of carbonization conditions including stabilization time, stabilization temperature, carbonization time, carbonization temperature on morphology and carbon yield was investigated.
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ÖgeDevelopment of interlayer based thin-film nanofibrous composite membranes adjusted by functionalized carbon nanotubes for effectual water purification(Graduate School, 2022-01-20) Arabi, Seyedehnegar ; Gökoğlu Zeytuncu, Bihter ; 513191019 ; Nano Science and Nano Engineering ; Nano Bilim ve Nano MühendislikDrinkable water supply is one of the fundamental human prerequisites all over the world. Due to the population expansion, the changes in the global climate, and water degradation, The requirement for freshwater increases with time around the world. Based on the reported calculations, except for 2.5% of existence global water, which is classified in the potable water range for humans, 70% of the remaining freshwater (FW) is frozen. Due to recent reports, more than 700 million people worldwide have not been accessed clean water. Ascribed to the severe FW demands, which have been observed in some developing countries and sub-Saharan African countries, the water treatment technologies must be enforced in these overwhelmed countries. Nanoscience and nanotechnology are other novel solutions to water treatment technology problems. Ascribed to nanomaterials properties, including high aspect ratio, reactivity, adjustable pore volume, hydrophilic, hydrophobic, and electrostatic interactions, they have been utilized in numerous types of applications. Multiple types of batteries, optics, fuel cells, sensors, electrics, thermoelectric devices, pharmaceuticals, and cosmetics are some industries that have used nanomaterials to improve their products. Moreover, nanotechnology has been performed in economically unconventional water sources, resolving contaminant-free water for humans, and suggesting many solutions to alleviate needs with regard to reducing scarcity or removing contamination. For example, there are filters that remove pesticides from drinking water using nanochemistry. At the same time, due to the multidisciplinary feature of membrane technology and essential advantages of membrane science technology, such as being clean energy, the ability of energy-saving, high-quality products, and system versatility, it has been applied in multiple applications. The power of membrane technology to replace other purification systems, including distillation and ion exchange systems, has been distinguished as other membrane technology's benefits. Furthermore, because of the forward osmosis (FO) and nanofiltration (NF), membranes' excellent features such as energy conversion, low-cost procedure, and high water recovery ability have received much more attention in wastewater treatment, water purification, and brackish water desalination over the last decade. The electrospinning device generally consists of a high voltage power supply, a supply unit, and a grounded collector. The feed solution is sent to the feed end by a pump. An electric field is created by a high-voltage power supply connected to the supply terminal. As the applied voltage increases, the electrical forces overcome the viscoelastic forces of the solution at the feed end. After a critical voltage, a jet formation is observed at the supply end. The bubbler solution diffuses in the electrical field and accumulates randomly on the collecting plate in microscopic diameter fibers. The solvent in the solution evaporates before or after the fibers are collected in the container. Among the factors affecting the nanofiber production by electrospinning method are the type of polymer to be obtained, conductivity and dielectric properties, the solvent used, the viscosity of the feed solution, the distance between the feed unit and the collector, the feed rate (flow rate), the voltage used. More than 100 polymers can be electrospinning, and the most preferred among these polymers in nanofiber membrane construction are; polyacrylonitrile (PAN), poly(ethylene oxide) (PEO), polystyrene (PS), Nylon-6, poly(vinyl alcohol) (PVA), poly(ε-caprolactone) (PCL) and polycarbonate. PVA is a water-soluble, non-toxic, and biocompatible polyhydroxy polymer with high chemical resistance and thermal stability among these polymers. It is known that PVA easily interacts with other organic and inorganic materials. However, PVA's applications are limited due to its hydrophilic nature. Therefore, it must be modified to minimize dissolution, mainly used in aqueous applications such as filtration and adsorption. Chemical crosslinking of PVA nanofibers with dialdehydes, dicarboxylic acids, or dianhydride is advantageous in becoming insoluble in all solvents and increasing their thermal and chemical properties. Polymeric thin-film composites are essential types of compounds applied in various practical applications, including surface coatings and modifications, adsorption and immobilization, membrane technologies, and low surface energy interfaces. Also, the inherent internal concentration polarization (ICP), which causes osmotic driving force's decline, is another major problem of conventional TFC membranes which has been challenged for several years. Moreover, biological fouling is another disadvantage that limits the conventional TFC membranes' performance in multiple usages. Due to the biological fouling of TFC type membranes, microorganisms and micropollutants, which require reproduction, easily stick to the membrane's surface and cause a significant reduction of FO membranes' stability and durability. In order to break the trade-off between permeability and selectivity of TFC membranes and obtain membranes with balanced permeability and rejection performance and excellent durability, triple-layered thin film composite (TFC) forward osmosis (FO) membranes fabricated by introducing an interlayer on the porous electrospun membranes before interfacial polymerization (IP) procedure. Introducing an interlayer on the electrospun substrate overcomes the conventional TFC membranes' limitations and causes synthesizing controlled polyamide (PA) layer and improving the IP process. Carbon nanotubes (CNTs), cellulose nanocrystal, and cadmium hydroxide nano-strands are some of the nanomaterials that have been introduced as an interlayer in TFC types of membranes. The adopted interlayers develop the barrier selective layer's structure and control the IP procedure. Due to the CNT's ideal characteristics, such as large specific surface area (SSA) and excellent mechanical stability, CNTs are distinguished as superior nanomaterials that have been performed as interlayers in TFC membranes. Triple-layered TFC membranes with CNT interlayer enhance the PA layer formation with defect-free and ultrathin structure and promote the membrane's permeation ability, even rejecting monovalent and divalent ions. The membranes were utilized in this research are thin-film nanofibrous composite membranes with hydrolyzed multi-walled carbon nanotubes (MWCNTs) as an interlayer. First of all, MWCNTs had been acid-treated in the presence of sulfuric (H2SO4) and nitric (HNO3) acids. Secondly, the different amounts of hydrolyzed MWCNTs were dispersed in the distilled water using ultrasonication and then introduced as an interlayer onto the porous polyacrylonitrile (PAN) electrospun membranes by vacuum filtration procedure. Finally, TFC membranes were prepared to utilize the IP procedure. In this study, MPD and TMC solutions had been performed as aqueous and organic phases to begin the IP proceeding. The prepared membranes had been tested in dead-end filtration systems to investigate the membranes' performance in salt rejections. Also, these interlayer-based TFC membranes had been applied in the dye removal from industrial wastewaters and compared to the conventional TFC type of membranes in their filtration performance.