Preparation and characterization of nanofibers for energy applications

Abstract

Renewable energy researches have been intensively increased to address the problems arising from massive usage of fossil fuels including serious environmental problems. Burning fossil fuels emits carbon dioxide, enhances radiative forcing and leads to global warming. By using renewable energy sources, it is possible to mitigate negative effects of fossil-based economy, reduce the dependence of fossil fuels and decrease emission of greenhouse gases. Development of energy storage systems which are capable of storing electrical energy harvested from renewable sources is important. In this regard, secondary batteries have been the center of attention due to efficient storage and delivery of electrical energy. Lithium-ion batteries (LIBs) have been used in wide range of portable electronics and electric vehicles (EVs), owing to long working life, large energy-conversion efficiency, and being environmentally-friendly. Moreover, sodium-ion batteries (NIBs) have attracted great attention because of high abundance and low-cost of sodium resources. NIBs present opportunities for potential applications in large-scale grid energy storage. Electrode materials play an important role to determine electrochemical performance of batteries, including energy density, life time, operation current densities. In LIBs, metal oxides or phosphate materials (e.g. LiCoO2, LiNiCoAlO2 and LiFePO4) and graphite-based materials are used as the cathodes and anodes, respectively. In NIBs, carbon-based anodes and phosphate-based cathodes have driven the industrialization of the NIBs. The electronic and ionic conductivity are two crucial factors presenting great influence on the electrochemical performance, that are usually altered through doping, control of materials size and introduction of conductive substrate or additives (e.g. conductive metal, polymers and carbon-based materials). Carbon nanofibers (CNFs) have been used in many applications such as sensors, energy storage and biomedical applications owing to high electronic conductivity and chemical stability of CNFs. Performance of carbon nanofibers are influenced by the physical and chemical properties of nanofibers. Furthermore, morphology, porosity and specific surface area are important properties for several applications including energy storage and sensors. There are several techniques to produce nanofibers including chemical vapor deposition, catalytic synthesis, arc discharge and electrospinning. Controlling surface area and porosity is vital considering the performance of CNFs. In this study, polyacrylonitrile (PAN) nanofibers were produced via two commonly used nanofiber production techniques; electrospinning and centrifugal spinning. The morphology of the polymeric nanofibers was studied by using SEM. Moreover, heat treatment was applied in air and the inert atmosphere to fabricate carbon nanofibers and porous carbon nanofibers. The effect of production technique on the morphology and chemical structure was investigated via SEM and XRD studies. Electrospun PAN/PS nanofibers showed beads on string morphology due to low viscosity of the spinning solution while uniform fibers without defects were obtained by using centrifugal spinning technique. Centrifugally spun PAN nanofibers have rough surface while PAN nanofibers with smooth surface was obtained by using electrospinning technique. PCNFs derived from centrifugally spun PAN/PMMA nanofibers had rough surface with porous structure and XRD analysis proved the amorphous structure of PCNFs. Germanium has been the center of interest due to its enormous conductivity and Li diffusivity. Germanium is a promising anode material for sodium ion batteries owing to high theoretical capacity as well. However, it suffers from large capacity losses during cycling because of the large volume change and loss of electronic conductivity. Herein, centrifugally spun binder free N, S doped germanium@ porous carbon nanofiber (N,S doped Ge@ PCNFs) anodes first were synthesized using a fast, safe and scalable centrifugal spinning followed by heat treatment and N, S doping. The morphology and structure of the resultant N, S doped Ge@ PCNFs were investigated by scanning electron nanoscopy, transmission electron nanoscopy, EDX mapping, Raman spectroscopy and X-ray diffraction, while electrochemical performance of N,S doped Ge@ PCNFs was studied using galvanostatic charge-discharge tests. The results demonstrate that a nanostructured Ge homogeneously distributed on highly porous carbon nanofibers. Moreover, N, S doping via thiourea treatment is beneficial for the lithium and sodium ion kinetics. While interconnected PCNFs buffer volume change and provide fast diffusion channels for Li ions and Na ions, N, S doped PCNFs further improved electronic conductivity and thus led to higher reversible capacity with better cycling performance.