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.