Houpu Li

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.

Developing polymer composite materials: carbon nanotubes or graphene?

The formation of composite materials represents an efficient route to improve the performances of polymers and expand their application scopes. Due to the unique structure and remarkable mechanical, electrical, thermal, optical and catalytic properties, carbon nanotube and graphene have been mostly studied as a second phase to produce high performance polymer composites. Although carbon nanotube and graphene share some advantages in both structure and property, they are also different in many aspects including synthesis of composite material, control in composite structure and interaction with polymer molecule. The resulting composite materials are distinguished in property to meet different applications. This review article mainly describes the preparation, structure, property and application of the two families of composite materials with an emphasis on the difference between them. Some general and effective strategies are summarized for the development of polymer composite materials based on carbon nanotube and graphene.

Aligned Carbon Nanotube Sheets for the Electrodes of Organic Solar Cells

Aligned carbon nanotube sheets are developed as a new family of electrodes to fabricate dye-sensitized solar cells. The energy conversion efficiency of the resulting cell is higher than the randomly dispersed carbon nanotube film and comparable with the platinum. Novel and flexible solar cells can be easily made from such carbon nanotube sheets with high potentials.

Stretchable, Wearable Dye‐Sensitized Solar Cells

A stretchable, wearable dye-sensitized solar-cell textile is developed from elastic, electrically conducting fiber as a counter electrode and spring-like titanium wire as the working electrode. Dyesensitized solar cells are demonstrated with energy-conversion efficiencies up to 7.13%. The high energy-conversion efficiencies can be well maintained under stretch by 30% and after stretch for 20 cycles.

Efficient Dye-Sensitized Photovoltaic Wires Based on an Organic Redox Electrolyte

An organic thiolate/disulfide redox couple with low absorption in the visible region was developed for use in fabricating novel dye-sensitized photovoltaic wires with an aligned carbon nanotube (CNT) fiber as the counter electrode. These flexible wire devices achieved a maximal energy conversion efficiency of 7.33%, much higher than the value of 5.97% for the conventional I(-)/I3(-) redox couple. In addition, the aligned CNT fiber also greatly outperforms the conventional Pt counter electrode with a maximal efficiency of 2.06% based on the thiolate/disulfide redox couple.

Wearable solar cells by stacking textile electrodes.

A new and general method to produce flexible, wearable dye-sensitized solar cell (DSC) textiles by the stacking of two textile electrodes has been developed. A metal-textile electrode that was made from micrometer-sized metal wires was used as a working electrode, while the textile counter electrode was woven from highly aligned carbon nanotube fibers with high mechanical strengths and electrical conductivities. The resulting DSC textile exhibited a high energy conversion efficiency that was well maintained under bending. Compared with the woven DSC textiles that are based on wire-shaped devices, this stacked DSC textile unexpectedly exhibited a unique deformation from a rectangle to a parallelogram, which is highly desired in portable electronics. This lightweight and wearable stacked DSC textile is superior to conventional planar DSCs because the energy conversion efficiency of the stacked DSC textile was independent of the angle of incident light.

Polymer photovoltaic wires based on aligned carbon nanotube fibers

Compared with the conventional planar structure, a wire-shaped polymer solar cell which is weavable exhibits unique and promising applications. However, it is rare to realize such a useful structure in polymer solar cells due to the difficulty in finding appropriate electrodes. Herein, we have fabricated polymer photovoltaic wires by using aligned carbon nanotube fibers as electrodes. The high flexibility, high electrical conductivity, and elaborate nanostructure of the nanotube fiber electrode enables an effective charge separation and transport. The resulting wire cell showed an open-circuit voltage, short-circuit current density and fill factor of 0.42 V, 0.98 mA cm−2 and 0.36, respectively, which produce an energy conversion efficiency of 0.15%.

Quasi-solid-state, coaxial, fiber-shaped dye-sensitized solar cells

A quasi-solid-state, coaxial, fiber-shaped dye-sensitized solar cell is developed by wrapping transparent and conducting carbon nanotube sheets on a modified Ti wire. The use of eutectic melts and design of the coaxial structure enable effective contacts between the two electrodes and active layer with a good performance, including high thermal stability and flexibility.

Flexible electroluminescent fiber fabricated from coaxially wound carbon nanotube sheets

A fiber-shaped polymer light-emitting electrochemical cell (PLEC) was developed by sandwiching an electroluminescent polymer layer between two aligned carbon nanotube (CNT) sheet electrodes. Similar to a conventional planar PLEC, the electroluminescent polymer layer and two carbon nanotube electrodes are closely and stably contacted, so that the injected charges can be rapidly and efficiently transported. Due to their one-dimensional structure, the fiber-shaped PLEC demonstrates unique and promising advantages, e.g., the luminance is almost independent on the observation angle. In addition, the fiber-shaped PLEC is thin, lightweight and flexible, which bespeaks a promising future for various electronic textiles.

Electric Current Test Paper Based on Conjugated Polymers and Aligned Carbon Nanotubes

Making sense: A novel polyacetylene composite material with incorporated aligned carbon nanotubes shows rapid changes in both fluorescent intensity and color appearance in response to applied electric currents, and these electrochromatic transitions remain reversible even after a thousand cycles. The composite material is anticipated to be suitable for various other sensing applications.