Xuemei Sun

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.

Electrochromatic carbon nanotube/ polydiacetylene nanocomposite fibres

Chromatic materials such as polydiacetylene change colour in response to a wide variety of environmental stimuli, including changes in temperature, pH and chemical or mechanical stress, and have been extensively explored as sensing devices. Here, we report the facile synthesis of carbon nanotube/polydiacetylene nanocomposite fibres that rapidly and reversibly respond to electrical current, with the resulting colour change being readily observable with the naked eye. These composite fibres also chromatically respond to a broad spectrum of other stimulations. For example, they exhibit rapid and reversible stress-induced chromatism with negligible elongation. These electrochromatic nanocomposite fibres could have various applications in sensing.

Chromatic polydiacetylene with novel sensitivity.

Conjugated polymers have been investigated for a number of applications in optoelectronics and sensing due to their important electronic and optical properties. For instance, polydiacetylene (PDA) may change color in response to external stimuli and has been extensively explored as a material for chromatic sensors. However, the practical applications of PDA materials have been largely hampered by their irreversible chromatic transitions under limited stimuli such as temperature, pH, and chemical. As a result, much effort has been paid to improve the chromatic reversibility and increase the scope of external stimuli for PDA. In this tutorial review, the recent development of PDA materials which show reversible chromatic transition and respond to new stimuli including light and electrical current has been described.

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.

Flexible, Light‐Weight, Ultrastrong, and Semiconductive Carbon Nanotube Fibers for a Highly Efficient Solar Cell

Carbon nanotubes have been widely introduced to fabricate high-efficiency organic solar cells because of their extremely high surface area (e.g., ca. 1600 mg 1 for single-walled nanotubes) and superior electrical properties. In one direction, nanotubes are used in electrode materials. For example, the incorporation of nanotubes onto titania nanoparticle films has been shown to increase the roughness factor and decrease the charge recombination of electron/hole pairs, and the replacement of platinum with nanotubes as counter electrode catalyzed the reduction of triiodide to improve the cell performance. In another direction, the distribution of nanotubes within the photoactive layer improved the short circuit current density and fill factor owing to rapid charge separation at the nanotube/electron donor interface and efficient electron transport through nanotubes. However, the degrees of improvement are far from what is expected for nanotubes, mainly because of random aggregation of nanotubes in the cells. For a random nanotube network, the electrons have to cross many more boundaries. Therefore, alignment of nanotubes will further greatly improve cell performance as charge transport is more efficient. Solar cells have typically been fabricated from rigid plates, which are unfavorable for many applications, especially in the fields of portable and highly integrated equipment. As a result, flexible devices have recently become the subject of active research as a good solution. In particular, weavable fiber solar cells are very promising and have attracted increasing attention in recent years. Fiber solar cells based on metal wires, glass fibers, or polymer fibers have been investigated. Herein, we first made a family of novel organic solar cells with excellent performance from the highly aligned nanotube fiber. Compared with traditional solar cells fabricated from rigid plates or recently explored flexible films/fibers, nanotube fiber solar cells demonstrate some unique and promising advantages. Firstly, as the building nanotubes are highly aligned, the fiber shows excellent electrical properties, which provide the novel solar cell with higher short-circuit photocurrent, better maximum incident monochromatic photon-to-electron conversion efficiency, and higher power conversion efficiency. Secondly, nanotube fibers show excellent mechanical properties, much stronger than Kevlar and comparable to the strongest commercial fibers of zylon and dyneema in tensile strength. Thirdly, these fibers are flexible, light-weight, and weavable and have tunable diameters ranging from micrometers to millimeters. The above properties provide nanotube fiber solar cells with a broad spectrum of applications, including power regeneration for space aircraft and clothing-integrated photovoltaics. To produce desired nanotube fibers, high-quality nanotube arrays were first synthesized by a typical chemical vapor deposition method. The synthetic details are reported elsewhere. To summarize, Fe/Al2O3 was used as the catalyst, ethylene served as the carbon source, and Ar with 6%H2 was used to carry the precursor to a tube furnace, where the growth took place. The reaction temperature was controlled at 750 8C and the reaction time was typically between 10 and 20 min. Fibers were directly spun from the nanotube array (see Figure S1 in the Supporting Information), and the fiber diameter was controlled from 6 to 20 mm by varying the initial ribbon, that is, a bunch of nanotubes pulled out of the array at the beginning of the spinning. The nanotube fiber can be spun with lengths of tens of meters or even longer, and the fiber is uniform in diameter. The density of the nanotube fiber was calculated to be on the order of 1 gcm , and its linear density was on the order of 10 mgm , relative to 10 mgm 1 and 20– 100 mgm 1 for cotton and wool yarns, respectively. As shown in Figure 1a, the nanotube fiber is flexible and will not break after being bent, folded, or even tied many times. Highresolution transmission electron microscopy (see Figure 1b) [*] T. Chen, S. Wang, Z. Yang, Q. Feng, X. Sun, Dr. L. Li, Prof. Z.-S. Wang, Prof. H. Peng Laboratory of Advanced Materials, Fudan University Shanghai 200438 (China) E-mail: zs.wang@fudan.edu.cn penghs@fudan.edu.cn T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Fudan University (China) T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Department of Macromolecular Science, Fudan University (China) S. Wang, Prof. Z.-S. Wang Department of Chemistry, Fudan University (China) [] These authors contributed equally to this work.

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.

Smart Electronic Textiles

This Review describes the state-of-the-art of wearable electronics (smart textiles). The unique and promising advantages of smart electronic textiles are highlighted by comparing them with the conventional planar counterparts. The main kinds of smart electronic textiles based on different functionalities, namely the generation, storage, and utilization of electricity, are then discussed with an emphasis on the use of functional materials. The remaining challenges are summarized together with important new directions to provide some useful clues for the future development of smart electronic textiles.

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.

The Alignment of Carbon Nanotubes: An Effective Route To Extend Their Excellent Properties to Macroscopic Scale

To improve the practical application of carbon nanotubes, it is critically important to extend their physical properties from the nanoscale to the macroscopic scale. Recently, chemists aligned continuous multiwalled carbon nanotube (MWCNT) sheets and fibers to produce materials with high mechanical strength and electrical conductivity. This provided an important clue to the use of MWCNTs at macroscopic scale. Researchers have made multiple efforts to optimize this aligned structure and improve the properties of MWCNT sheets and fibers. In this Account, we briefly highlight the new synthetic methods and promising applications of aligned MWCNTs for organic optoelectronic materials and devices. We describe several general methods to prepare both horizontally and perpendicularly aligned MWCNT/polymer composite films, through an easy solution or melting process. The composite films exhibit the combined properties of being flexible, transparent, and electrically conductive. These advances may pave the way to new flexible substrates for organic solar cells, sensing devices, and other related applications. Similarly, we discuss the synthesis of aligned MWCNT/polymer composite fibers with interesting mechanical and electrical properties. Through these methods, we can incorporate a wide variety of soluble or fusible polymers for such composite films and fibers. In addition, we can later introduce functional polymers with conjugated backbones or side chains to improve the properties of these composite materials. In particular, cooperative interactions between aligned MWCNTs and polymers can produce novel properties that do not occur individually. Common examples of this are two types of responsive polymers, photodeformable azobenzene-containing liquid crystalline polymer and chromatic polydiacetylene. Aligning the structure of MWCNTs induces the orientation of azobenzene-containing mesogens, and produces photodeformable polymer elastomers. This strategy also solves the long-standing problems from the traditional mechanical rubbing method, which include production of broken debris and structure damage during fabrication and building up electrostatic charge during use. Aligning MWCNTs induces a conformational change in polydiacetylene, which causes the composite fibers to be electrochromatic, a previously unknown reaction in chromatic polymers. Due to their large surface area, flexibility, electrical conductivity, and remarkable electrocatalytic activity, aligned MWCNT films can be used as counter electrodes to produce highly efficient dye-sensitized solar cells. In addition, chemists have developed new electrodes from the aligned MWCNT fibers to make a family of high-performing, wire-shaped dye-sensitized solar cells.

Photoinduced Deformation of Crosslinked Liquid-Crystalline Polymer Film Oriented by a Highly Aligned Carbon Nanotube Sheet†

Photodeformable polymeric materials have recently experienced vigorous development because of their potential applications in various fields, such as artificial muscle, photomobile soft actuator, and micro-optomechanical systems (MOMS). Among them, crosslinked liquid-crystalline polymers (CLCPs) containing photochromic moieties such as azobenzene may represent one of the most studied systems, with a bending deformation owing to a photoinduced change in the molecular orientation of mesogens triggered by the trans–cis photoisomerization in azobenzene. Various lightdriven soft actuators including plastic motors, inchwormlike walkers, flexible microrobots, high-frequency oscillators, and artificial cilia have been made from the CLCPs. For the above applications, it is critical to control the bending direction of the CLCPs which depends on the orientation format of the mesogens. For instance, a homogeneously oriented CLCP film bent towards the light source along the oriented direction of mesogens, while a homeotropically oriented film bent away from the light source. To achieve the orientation in a liquid-crystalline (LC) system, the surface of the substrate is usually modified to provide an anchoring action for LC molecules during fabrication. Of many methods, the preferred modification technique is mechanical rubbing. Typically, an aligned polyimide (PI) layer with parallel grooves is firstly generated by mechanical rubbing along one direction, and the grooves are then used to orient the LC molecules. However, there remain several challenges, such as the production of broken debris and structural damage during fabrication and the accumulation of electrostatic charge on the surface during use, which have largely limited the application of the CLCPs. On the other hand, carbon nanotubes (CNTs) have been widely investigated for their extraordinary mechanical and electrical properties as well as good absorption in the visible/ near-infrared (NIR) region. It has been reported that the dispersion of CNTs in a thermoresponsive CLCP created a light-controllable network, which enabled a contraction upon irradiation by NIR light. In this case, the CNT functions as a nanoscale heat source to absorb the NIR light and further convert it to thermal energy, which induces a thermal phase transition of the CLCP from LC to isotropy with a thermal contraction. However, both sensitivity and stability are very low for such an indirect photoactuation. In addition, it remains difficult to control and improve the orientation of mesogens in the CLCP due to the random dispersion of CNTs, and the other important properties such as mechanical strength also need to be improved in this fabrication. Herein, we report the development of a new and general method to prepare photodeformable CLCP/CNT nanocomposite films by using highly aligned CNT sheets. A photosensitive CLCP containing azobenzene has been carefully investigated as a demonstration. It was found that the aligned nanostructure of the CNT sheet could effectively orient the CLCP mesogens along the length of the CNTs without using any other aligning layer. The resulting nanocomposite film underwent bending and unbending by alternate irradiation with UV and visible light. Furthermore, the introduction of aligned CNTs remarkably increased the mechanical strength and provided electrical conductivity for the CLCP film. The chemical structures and properties of two monomers, A11AB6 and A9Bz9, and the crosslinker C9A, are shown in Scheme 1. The monomers and crosslinker were synthesized and purified according to the literature. The synthetic details, H NMR spectra, and differential scanning calorimetry (DSC) thermograms are presented in the Supporting Information. The fabrication of the CLCP/CNT composite film is shown in Figure 1. Firstly, a CNT array was grown on silicon by chemical vapor deposition. Secondly, uniform CNT sheets were pulled out of the array by dry spinning and stabilized on glass substrates. Thirdly, an LC cell was made of two CNT-sheet-covered glass slides with the CNT sheet inside. Fourthly, a molten mixture of A11AB6, A9Bz9, and C9A (molar ratio 1:1:3) containing 1 mol% photoinitiator (Irgacure 784) was injected into the LC cell at 90 8C (in an isotropic phase), and then slowly cooled to a polymerization temperature of 77 8C (in a nematic phase) at a rate of 0.1 8Cmin . The CLCP/CNT composite film was obtained after photopolymerization at a wavelength of 547 nm (2 mWcm ) under a 500 W high-pressure mercury lamp through a glass filter for 2 h. The LC cell could be further opened to produce a freestanding composite film. Figure 2a shows a typical scanning electron microscopy (SEM) image of a CNTarray on silicon. The CNTs are highly [*] W. Wang, W. Wu, Prof. Y. Yu Department of Materials Science, Fudan University 220 Handan Road, Shanghai 200433 (P. R. China) E-mail: ylyu@fudan.edu.cn X. Sun, Prof. H. Peng State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, and Laboratory of Advanced Materials, Fudan University (P. R. China) E-mail: penghs@fudan.edu.cn [] These authors contributed equally to this work.