Current Organic Chemistry - Volume 17, Issue 13, 2013

Functional nanofibrous membranes, fabricated by the electrospinning technology, can be used to remove heavy metal ions through adsorption from contaminated water. These membranes, while already being considered as superior microfiltration media as they possess higher permeation flux and lower pressure drop performance than conventional microfiltration membranes, also possess a high surface-to-volume ratio and functionalizable surface that can remove toxic metal ions with a capability comparable to typical absorbents.

An increasing demand and shortage of clean water sources have become an issue in the modern society due to the rapid development of industrialization, environmental pollution and long-term droughts. Semiconductor photocatalytic process has shown a great potential as a low-cost, environmental friendly and sustainable treatment technology to align with the “zero” waste scheme in the water/ wastewater industry. Electrospinning has been explored as a simple and scalable method to fabricate one dimensional nanomaterial with controlled morphology and components, and therefore has been recognized as an effective way to produce high performance photocatalyst nanofibers. In this review, we would like to highlight recent developments on the synthesis and characterizations of functional semiconducting nanofibers for applications as photocatalyst.

The pressing needs of global demands to generate sufficient renewable and clean energy have attracted significant attention to harvest photochemical energy. Two types of artificial photoenergy processes, including the dye-sensitized solar cells (DSSCs) and photoreduction of CO2 solar fuel cells are discussed in this review. In the artificial photoenergy application, electrospinning process has attracted wide interest in fabricating nanofibers because of low processing cost and large scale production possibilities. The fabrication of electrospun composite nanofibers is normally followed by solvothermal or annealing treatment in order to obtain highly crystallized metal oxides. Highlights: The application of electrospun composite nanofibers has shown superior performance in artificial photosynthesis process. The large surface area of nanofibers mat has facilitate the adsorption-desorption process during photocatalysis as well as has induced the growth of metal oxides on electrospun nanofibers.

The applications of electrospun carbon fiber webs to the development of energy storages devices, including both supercapacitors and lithium ion batteries (LIBs), are reviewed. Following a brief discussion of the fabrication process and characterization methods for ultrafine electrospun carbon fibers, recent advances in their performance as supercapacitors and LIB anode materials are summarized. Optimization of the overall electrochemical properties of these materials through choice of thermal treatment conditions, incorporation of additional active components (such as carbon nanotubes, metal oxides, and catalysts), and generation of novel fibrous structures (such as core-shell, multi-channel or porous fibers) is highlighted. Further challenges related to improving the conductivity, surface area, and mechanical properties of the carbon nanofiber webs, as well as the scale-up ability of the fabrication technique, are discussed.

Electrospinning is an effective technology for preparing free-standing fiber mats with ultrathin fiber diameter. In this review, the electrospinning technology for application in supercapacitors is discussed. Electrode materials assembled by electrospinning and other post-treatment and chemical modification for electrical double-layer capacitors and pseudocapacitors are summarized. Meanwhile, the electrospun nanofiber mats for separator in supercapacitor is also highlighted.

Electrospinning technique has been well known for its facile production of one-dimensional (1D) continuous fibrous structure. With the advantage of simple setup and scalable capability using this technology, electrospun carbon nanofiber has raised great interest in recent years due to its superior conductive potentiality, mechanical flexibility and well-established fabrication method. This review addresses the various applications of carbon nanofiber produced via electrospinning technique, mainly focusing on energy conversion systems, such as lithium ion battery, supercapacitor, etc. Different precursors, preparation conditions and characterization methods are also reviewed to reveal the very nature of electrospun carbon nanofiber, since its overall performance depends upon its crystallinity, molecular orientation, fiber diameter, surface area and porosity. Nanocomposites combining metal oxides with carbon nanofibers stand as another important topic in this review as the integration of both has been observed to achieve novel superiority, especially in the application of lithium ion battery and supercapacitor.

Long 1D nanofibers provide unique properties, such as a high specific surface area, a high aspect ratio and easy electron transfer. Electrospinning using a high electric voltage is the most promising and practical way to fabricate nanosized nanofibers. These nanofibers have been used in numerous fields because of their many unique properties. This review will discuss the recent application of electrospun nanofibers for energy storage. The following will provide an in-depth summary of hydrogen storage media, capacitors, lithium- ion batteries, hybrid capacitors, dye-sensitized solar cells and thermoelectric materials. The most promising ways of enhancing the efficiencies of these energy storage devices will also be discussed.

Porous carbon nanofibers (PCNFs) are excellent one dimensional (1D) carbon materials that have fascinating applications in the fields of gas separation, catalyst support, sensors, energy storage and conversion devices, due to their highly developed pore structure and large surface area. Therefore, fabrication of PCNFs has attracted much attention, and many kinds of methods have been explored. Recent developments in fabrication of PCNFs are summarized in the review. Their fundamental principles and process, applications, advantages and disadvantages are discussed in detail.With careful preparation techniques, PCNFs with high specific surface area, hierarchical and well-aligned pores can be obtained, and desired properties are also achieved. Finally, other issues regarding the technology limitations of present methods for fabricating PCNFs, research challenges, and future trends are also discussed.

Sn/carbon composite nanofibers with various compositions were prepared from Sn(II) acetate/polyacrylonitrile (PAN) precursors by a combination of electrospinning and carbonization methods, and their potential use as anode materials for rechargeable lithiumion batteries was investigated. The composite electrode derived from 20 wt% Sn(II) acetate/PAN precursor showed excellent electrochemical properties, including a large reversible capacity of 699 mAh g‾1 and a high capacity retention of 83% in 50 cycles. Sn/carbon composite nanofibers exhibited enhanced electrochemical performance ascribing to the combination of the properties of both Sn nanoparticles (large Li storage capability) and carbon matrices (long cycle life), and therefore could be potentially used in high-energy rechargeable lithium-ion batteries.

Graphene-embedded porous and smaller fibrous carbon nanofibers (CNFs) were prepared by a simple electrospinning method with the help of zinc chloride (ZnCl2), and their electrochemical properties as supercapacitor electrodes were investigated. The CNFs were characterized as having a large specific surface area of up to 520 m2g-1, mesopore volume fraction up to 35%, and high electrical conductivity (over 0.45 Scm-1) and exhibited a gravimetric capacitance of 148 Fg-1 and energy densities of 9.2-20.0 Whkg-1 over a power density range of 400-30,000 Wkg-1 in 6 M potassium hydroxide electrolyte. The introduction of ZnCl2 and graphene into the polyacrylonitrile (PAN) solution induced suitable micropores to accommodate many ions for high capacitance and mesopores for smooth ion transfer. Therefore, the cooperation of micro- and mesopores in the CNF electrode materials synergistically improves the performance of electrochemical double-layer capacitors (EDLCs) because these materials have a high rate capability, high capacitance, and long cycling life.

The novel isotropic pitch that exhibits both high solvent solubility (100% soluble in THF) and a high softening point (over 200'C) was successfully synthesized by the bromination of 2-methylnaphthalene followed by dehydrobromination/polymerization and extraction in n-hexane to eliminate lighter constituents. The molecular structure of the pitch was relatively linear with flexible methylene cross-linkages between naphthalene molecules. During bromination, a reaction temperature of 180'C was necessary to induce bromine substitution of methyl hydrogen. Methylene cross-linkages were formed after the dehydrobromination reaction. The obtained pitch is a suitable precursor for electrospinning and meltspinning to prepare carbon nanofibers and carbon fibers, respectively.