טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentElishav Oren
SubjectMultilevel Structured Electrospun Nanofibers for
Energy Applications
DepartmentDepartment of Energy
Supervisors Professor Gideon Grader
Dr. Gennady Shter
Full Thesis textFull thesis text - English Version


Abstract

One of the significant challenges in our time is to establish a sustainable and curricular economy. Doing so requires increased utilization of renewable energy sources to meet our electrical and transportation energy needs and recycling of carbon dioxide to decrease the impact on the environment. The transition to such an economy relies on the development of superior materials for catalysis and energy storage. One-dimensional nanostructured materials such as nanofibers are promising candidates for energy application due to their reduced dimension and large surface to volume ratio that provide better performance. Electrospinning is a simple method to fabricate ultrafine fibers with submicron diameter. This method can provide polymeric, ceramic, composite, and metal nanofibers with hierarchical porosity. Multilevel structures in nanofibers further enhance the surface area relative to simple nanostructured material analog, thus providing larger loading of active sites and additional contact between reactant and active sites. These secondary structures can be formed on the surface of the fiber or by having a complex inner structure, unlike solid cylindrical fibers.


This research aims to investigate the interplay between multilevel structures, properties, and performance of ceramic nanofibers in energy applications. First, the deformation of electrospun nanofibers during thermal treatment is studied. The morphology and thermal shrinkage are measured as a function of temperature, the polymer concentration, and applied voltage during electrospinning. Understanding the interplay between fibers morphology and thermal shrinkage allows optimizing the precursor composition to control the shrinkage of fiber mats. Furthermore, this study opened new research directions, where the deformation that occurs during thermal treatment is harnessed to achieve different multilevel structures of ceramic nanofibers.


Subsequently, the deformation during the thermal treatment of ceramic nanofibers was investigated to achieve surface multilevel structures. In this work, the as-spun fibers before thermal treatment consist of a polymer and metal-organic complex and have a uniform and smooth surface. The polymer content relative to the ceramic precursor dictates the properties of the fiber shell, which in turn affects the deformation process during thermal treatment. This deformation leads to different surface multilevel structured nanofibers with distinct properties. A unique lamellar surface with a large specific area was produced in different material systems consisting of single and multi-oxides, including Al2O3, Fe2O3, NiO, Fe-Al-O, Ti-Al-O, Ti-Fe-O, and Ni-Al-O. Each oxide system can be utilized as a functional material in a designated application. Fe-Al-O was investigated as a catalyst for CO2 hydrogenation to hydrocarbon, while Ni-Al-O as an anode for lithium-ion batteries.


In addition, different inner multilevel structures were realized in Fe-Al-O nanofibers by altering the mat thickness, precursor composition and heating rate. Nanobelts and hollow nanofibers formed due to the rapid oxidative decomposition of the organics in thick samples. The catalytic behavior of spinel Fe-Al-O nanobelts showed high conversion and selectivity to light olefins, while a nanopowder analog produces mainly heavy hydrocarbons. The nanobelts structure increases the available sites for promoter decoration and provides thermal stability during reaction. Thus, nanobelt geometry affects the promoter distribution during reaction leading to a more electron-rich surface, which favors light olefins production.