Niobium Pentoxide Polymorphs By Electrospinning For Energy Conversion And Storage

Abstract

Electrospinning is a cheap and scalable technique to produce composite fibers in the micro or nanometer size range. It consists in accelerating a polymeric solution with a high voltage; fibers form upon stretching and solidification in the electric field. If the composite fibers include metal ions, metal oxide fibers can be obtained by adequate annealing step. The particular 1-dimensional morphology raises interest in field such as regenerative medicine, photovoltaic, or filtration. This thesis features the synthesis of niobium metal oxide nanofibers by electrospinning. Nb2O5 is a n-type transition metal oxide semiconductor, which properties depend on the oxygen stoichiometry. Control post electrospinning sintering step allows to develop different crystal structures: pseudo-hexagonal(H), orthorhombic(O), and monoclinic(M) in the present work. The fibers are characterized by various techniques: Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), density measurement, Brunauer Emmett Teller (BET) surface area measurement, conductivity measurement.

The interesting properties of Nb2O5 allow it to find applications in various areas like gas sensors, catalysts, electrochromic system, photoelectrode for Dye-Sensitized Solar Cell (DSSC) or Lithium-Ion Battery (LIB). This thesis features an extensive study of the most common polymorphs of Nb2O5 electrospun nanostructure for application in DSSC, Solar Fabric, and LIBs.

DSSC is a silicon free photovoltaic system, using dye molecule to generate photocharges. The dye is anchored on a metal oxide network, which serves to transport electrons from the dye to the outer circuit. This specific design gives promise of cheaper photovoltaic devices with moderate efficiency compared to silicon technology. In this work, the three polymorphs of Nb2O5 are tested as photoanode in DSSC. Device performance is evaluated by current-voltage characteristic, while electron transport properties are discussed from Open Circuit Voltage Decay technique, and Electrochemical Impedance Spectroscopy (EIS). Despite the lower efficiency of M-Nb2O5 among the polymorphs because of its low surface area, it exhibits the best electron transport properties, which is reflected in the kinetic studies.

Solar Fabric is a recent photovoltaic design inspired from its DSSC counterpart, where each fiber of the fabric acts as a photovoltaic device. While still in its infancy, Solar Fabric gives promise of flexible and large area photoconversion fabric. H-Nb2O5 has been tested in such as system, providing proof of concept of Nb2O5 based solar fabric.

LIB has become an energy storage medium of choice for various applications, from portable devices to electric vehicles. In LIB, lithium ions are shuffling between the anode and cathode during charge and discharge, allowing energy storage during intercalation in the anode, and energy release during intercalation in the cathode. This thesis studies the application of Nb2O5 as cathode material in LIB, in the form of coin cell (CR2016). Cycling performance of the three polymorphs as cathode material is studied by Cyclic Voltammetry and Galvanostatic Cycling. Kinetic studies on lithium intercalation process are done by EIS and Galvanostatic Intermittent Titration Technique. Initial capacity and capacity retention are the best for the M-Nb2O5, which is reflected in the kinetic studies.