The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

Oil spills and industrial organic pollutants have induced severe water pollution and threatened every species in the ecological system. To deal with oily water, special wettability stimulated materials have been developed over the past decade to separate oil-and-water mixtures. Basically, synergy between the surface chemical composition and surface topography are commonly known as the key factors to realize the opposite wettability to oils and water and dominate the selective wetting or absorption of oils/water. In this review, we mainly focus on the development of materials with either super-lyophobicity or super-lyophilicity properties in oil/water separation applications where they can be classified into four kinds as follows (in terms of the surface wettability of water and oils): (i) superhydrophobic and superoleophilic materials, (ii) superhydrophilic and under water superoleophobic materials, (iii) superhydrophilic and superoleophobic materials, and (iv) smart oil/water separation materials with switchable wettability. These materials have already been applied to the separation of oil-and-water mixtures: from simple oil/water layered mixtures to oil/water emulsions (including oil-in-water emulsions and water-in-oil emulsions), and from non-intelligent materials to intelligent materials. Moreover, they also exhibit high absorption capacity or separation efficiency and selectivity, simple and fast separation/absorption ability, excellent recyclability, economical efficiency and outstanding durability under harsh conditions. Then, related theories are proposed to understand the physical mechanisms that occur during the oil/water separation process. Finally, some challenges and promising breakthroughs in this field are also discussed. It is expected that special wettability stimulated oil/water separation materials can achieve industrial scale production and be put into use for oil spills and industrial oily wastewater treatment in the near future.

Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature

A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

Needleless electrospinning is expected to produce nanofibers with a large productivity. In this study, a sprocket wheel disk was used as spinneret to electrospin nanofibers. The sprocket disk shows reliable electrospinning process. In comparison with the conventional disk spinneret, which has no sprocket on the edge, the sprocket wheel produced more uniform nanofibers with smaller fiber diameter. The electric field analysis results indicated that the sprocket wheel generates higher intensity of electric field.

Needleless electrospinning using sprocket wheel disk spinneret

Most of the superamphiphilic surfaces reported so far show an oleophobic feature in underwater environments because once they are wetted with water their wettability is governed by the water layer adsorbed on the surface. In contrast, underwater oleophilic surfaces often show superhydrophobicity–oleophilicity in a dry state in air. A challenge remains in developing a superamphiphilic surface that shows both superhydrophilicity and underwater superoleophilicity (i.e. underwater superamphiphilicity). Herein, we demonstrate a novel strategy to prepare a surface that simultaneously possesses superamphiphilicity in air and underwater environments (also referred to as “amphibious superamphiphilicity” in this paper). A single-step wet-chemical coating method was employed to apply a crosslinkable polymer material, which consists of hydrophilic and oleophilic functional groups, onto a fabric substrate. The fabric after coating treatment exhibited amphibious superamphiphilicity with a contact angle of 0° for water and oils. In the dry state, it took less than 1 second for water and oil fluids of surface tension in the range of 18.4–50.8 mN m−1 to spread completely on the surface. In water, although the fabric was quickly wetted, it still allowed oils to spread completely into the wetted fabric matrix in less than 1 minute. More interestingly, the underwater superoleophilicity is self-healable against chemical damage. We further showed that such amphibious superamphiphilicity has great potential for recovery of oil from water. No matter whether the fabric was in a dry or pre-wetted state, it showed a similar oil absorption capability. The high resilience against moisture environments made oil recovery very stable. In addition, the coating is durable enough against various types of harsh damage. Such unusual superamphiphilicity may offer novel properties and applications in diverse fields.

Amphibious superamphiphilic fabrics with self-healing underwater superoleophilicity

In this study, the morphological evolution of poly(acrylonitrile) (PAN) nanofibers during electrospinning was examined via immersion precipitation of newly electrospun filaments in ethanol, and subsequently associating the fiber morphologies with the electrospinning distances (2 to 10 cm). We have observed that an uneven fiber stretching happened throughout the electrospinning process. A massive filament-thinning took place at the initial stage of whipping instability, and fiber stretching at the later whipping stage was mainly concentrated on the bead sections, leading to improved uniformity of the resultant fibers. This work has provided a new insight into the fiber formation mechanism in the electrospinning process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Evolution of fiber morphology during electrospinning

Two types of directional water transport fabrics are prepared by using cotton fabric as substrate and an electrospraying technique to apply a hydrophobic coating on one side of the fabric. The main difference between the two electrosprayed fabrics is that one of them was precoated with a hydrophilic thermoconductive resin over the fiber surface prior to electrospraying. As a result, the precoated fabric has a much higher thermoconductivity than the other, while they are similar in water transport and fibrous structure. In the wet state, the directional water-transport fabrics generate a temperature difference between the two fabric sides while drying naturally. The fabric with higher thermal conductivity shows smaller temperature difference, better thermal transfer within the fabric, stronger evaporation cooling effect, and accelerated moisture evaporation. Directional water transport fabrics with high thermal conductivity may be used to mitigate thermal burden in sportswear, summer clothing, medical fabrics, and workwear.

One-Way Water-Transport Cotton Fabrics with Enhanced Cooling Effect

Flexible energy devices with high performance and long-term stability are highly promising for applications in portable electronics, but remain challenging to develop. As an electrode material for pseudo-supercapacitors, conducting polymers typically show higher energy storage ability over carbon materials and larger conductivity than transition-metal oxides. However, conducting polymer-based supercapacitors often have poor cycling stability, attributable to the structural rupture caused by the large volume contrast between doping and de-doping states, which has been the main obstacle to their practical applications. Herein, we report a simple method to prepare a flexible, binder-free, self-supported polypyrrole (PPy) supercapacitor electrode with high cycling stability through using novel, hollow PPy nanofibers with porous capsular walls as a film-forming material. The unique fiber structure and capsular walls provide the PPy film with enough free-space to adapt to volume variation during doping/de-doping, leading to super-high cycling stability (capacitance retention > 90% after 11 000 charge–discharge cycles at a high current density of 10 A g−1) and high rate capability (capacitance retention ∼ 82.1% at a current density in the range of 0.25–10 A g−1).

A self-supported, flexible, binder-free pseudo-supercapacitor electrode material with high capacitance and cycling stability from hollow, capsular polypyrrole fibers

Haitao Niu

Papers

Needleless electrospinning using sprocket wheel disk spinneret

Amphibious superamphiphilic fabrics with self-healing underwater superoleophilicity

Evolution of fiber morphology during electrospinning

One-Way Water-Transport Cotton Fabrics with Enhanced Cooling Effect

A self-supported, flexible, binder-free pseudo-supercapacitor electrode material with high capacitance and cycling stability from hollow, capsular polypyrrole fibers