Huijuan Lin

An Integrated “Energy Wire” for both Photoelectric Conversion and Energy Storage†

The use of solar energy has the potential to provide an effective solution to the energy crisis. Generally, the solar energy is converted into electric energy which is transferred through external electric wires to electrochemical devices, such as lithium ion batteries and supercapacitors, for storage. To further improve the energy conversion and storage efficiency, it is important to simultaneously realize the two functions, photoelectric conversion (PC) and energy storage (ES), in one device. Recently, attempts have been made to directly stack a photovoltaic cell and a supercapacitor into one device which can absorb and store solar energy. However, these stacked devices exhibited low overall photoelectric conversion and storage efficiencies. In addition, the planar format in such stacked devices has limited their applications, such as in electronic textiles where a wire structure is required. Herein, an integrated energy wire has been developed to simultaneously realizes photoelectric conversion and energy storage with high efficiency. The fabrication is schematically shown in the Supporting Information, Figure S1. A titanium wire was modified in sections with aligned titania nanotubes on the surface. Active materials for photoelectric conversion and energy storage were then coated onto the modified parts with titania nanotubes. Aligned carbon nanotube (CNT) fibers were finally twisted with the modified Ti wire to produce the desired device. The Ti wire and CNT fiber had been used as electrodes. Figure 1a schematically shows a wire in which one part capable of photoelectric conversion and one part capable of energy storage. This novel wire device exhibits an overall photoelectric conversion and storage efficiency of 1.5%. Aligned titania nanotubes were grown on the Ti wires by electrochemical anodization in a two-electrode system. Figures 1b and 1c show typical scanning electron microscopy (SEM) images of titania nanotubes. The diameters of titania nanotubes ranged from 50 to 100 nm with the wall thickness varying from 15 to 50 nm, and their length was about 20 mm (Figure S2). In this case, titania nanotubes were mainly used to improve the charge separation and transport in photoelectric conversion and increase the surface area in energy storage. Aligned CNT fibers were spun from spinnable CNTarrays which had been synthesized by chemical vapor deposition. They could be produced with lengths of hundreds of meters through the continuous spinning process, and were typically ranged from 10 to 30 mm in diameter. Figure 1d shows a typical SEM image of a CNT fiber which has a uniform diameter of 10 mm. Figure 1e further shows that the CNTs are highly aligned in the fiber, which enables combined remarkable properties including tensile strength of 10– 10 MPa, electrical conductivity of 10 Scm , and high electrocatalytic activity comparable to the conventional platinum. In addition, the CNT fibers were flexible and could be easily and closely twisted with each other or with the other fiber materials (Figure S3), which was critical for the success in a wire-shaped device. Photoactive materials were deposited onto the titania nanotube-modified parts on the Ti wire, for photoelectric conversion, while the desired gel electrolyte was coated onto the other sections for energy storage. Aligned CNT fibers were then twisted with both photoelectric-conversion and energy-storage parts to produce an integrated wire-shaped device. For simplicity, an “energy wire” which was composed of one photoelectric conversion section and one energy storage section had been mainly investigated in this work. Figure 2a shows a typical photograph of a wire with the left Figure 1. a) Schematic illustration of the integrated wire-shaped device for photoelectric conversion (PC) and energy storage (ES). b),c) Scanning electron microscopy (SEM) images of aligned titania nanotubes grown on a Ti wire by electrochemical anodization for 2 h at low and high magnifications, respectively. d),e) SEM images of a CNT fiber at low and high magnifications, respectively.

Novel Electric Double‐Layer Capacitor with a Coaxial Fiber Structure

A coaxial electric double-layer capacitor fiber is developed from the aligned carbon nanotube fiber and sheet, which functions as two electrodes with a polymer gel sandwiched between them. The unique coaxial structure enables a rapid transportation of ions between the two electrodes with a high electrochemical performance. These energy storage fibers are also flexible and stretchable, and can be woven into and widely used for electronic textiles.

Conducting polymer composite film incorporated with aligned carbon nanotubes for transparent, flexible and efficient supercapacitor

Polyaniline composite films incorporated with aligned multi-walled carbon nanotubes (MWCNTs) are synthesized through an easy electrodeposition process. These robust and electrically conductive films are found to function as effective electrodes to fabricate transparent and flexible supercapacitors with a maximum specific capacitance of 233 F/g at a current density of 1 A/g. It is 36 times of bare MWCNT sheet, 23 times of pure polyaniline and 3 times of randomly dispersed MWCNT/polyaniline film under the same conditions. The novel supercapacitors also show a high cyclic stability.

Twisted Aligned Carbon Nanotube/Silicon Composite Fiber Anode for Flexible Wire‐Shaped Lithium‐Ion Battery

Twisted, aligned carbon nanotube/silicon composite fibers with remarkable mechanical and electronic properties are designed to develop novel flexible lithium-ion batteries with a high cyclic stability. The core-sheath architecture and the aligned structure of the composite nanotube offer excellent combined properties.

Electrochromic Fiber‐Shaped Supercapacitors

An electrochromic fiber-shaped super-capacitor is developed by winding aligned carbon nanotube/polyaniline composite sheets on an elastic fiber. The fiber-shaped supercapacitors demonstrate rapid and reversible chromatic transitions under different working states, which can be directly observed by the naked eye. They are also stretchable and flexible, and are woven into textiles to display designed signals in addition to storing energy.

Flexible, weavable and efficient microsupercapacitor wires based on polyaniline composite fibers incorporated with aligned carbon nanotubes

A supercapacitor in a flexible wire format has potential advantages that are described in this paper. Polyaniline composite fibers incorporated with aligned multi-walled carbon nanotubes are first synthesized with high mechanical strength and electrical conductivity through an easy electrodeposition process, and two robust composite fibers have then been twisted to produce microsupercapacitor wires with a specific capacitance of 274 F g−1 or 263 mF cm−1. These energy storage wires are light-weight, flexible, strong and weavable for promising applications in various fields.

Smart, Stretchable Supercapacitors

Smart supercapacitors are developed by depositing conducting polymers onto aligned carbon-nanotube sheets. These supercapacitors rapidly and reversibly demonstrate color changes in response to a variation in the level of stored energy and the chromatic transitions can be directly observed by the naked eye.

Winding aligned carbon nanotube composite yarns into coaxial fiber full batteries with high performances.

Inspired by the fantastic and fast-growing wearable electronics such as Google Glass and Apple iWatch, matchable lightweight and weaveable energy storage systems are urgently demanded while remaining as a bottleneck in the whole technology. Fiber-shaped energy storage devices that can be woven into electronic textiles may represent a general and effective strategy to overcome the above difficulty. Here a coaxial fiber lithium-ion battery has been achieved by sequentially winding aligned carbon nanotube composite yarn cathode and anode onto a cotton fiber. Novel yarn structures are designed to enable a high performance with a linear energy density of 0.75 mWh cm(-1). A wearable energy storage textile is also produced with an areal energy density of 4.5 mWh cm(-2).

An integrated device for both photoelectric conversion and energy storage based on free-standing and aligned carbon nanotube film

An all-solid-state and integrated device in which photoelectric conversion and energy storage are simultaneously realized has been developed from free-standing and aligned carbon nanotube films or carbon nanotube–polyaniline composite films. Due to the aligned structure and excellent electronic property of the film electrode, the integrated device exhibits a high entire photoelectric conversion and storage efficiency of ∼5.12%. The novel devices can also be flexible, and show promising applications in a wide variety of fields, particularly for portable electronic equipment.

Super-stretchy lithium-ion battery based on carbon nanotube fiber

Super-stretchy, fiber-shaped lithium-ion batteries with a record strain of 600% are developed by winding two highly aligned carbon nanotube composite fibers. The fiber-shaped battery exhibits high specific capacity, energy density and power density that can be well-maintained under stretching.