Fiber supercapacitors made of nanowire-fiber hybrid structures for wearable/flexible energy storage.

Recently, there has been great interest in flexible and wearable energy devices for applications in flexible and stretchable electronics. Even though future developments are moving toward thinner, lighter, and cheaper solutions, many existing energy-harvesting and storage devices are still too bulky and heavy for intended applications. For example, high-efficiency dye-sensitized solar cells (DSSCs) employ fluorine-doped tin oxide (FTO) glass as the substrate of working electrode. However, the use of rigid FTO glass has restricted adaptability of DSSCs during transportation, installation, and application, requiring further development of flexible cells to improve DSSC adaptability. To develop flexible and wearable electronics, not only new materials for the substrates used in energy storage devices such as batteries and supercapacitors need to be explored, but future development of higher performance energy systems still depends on the employment of new and lighter electrode materials. In recent years, electrochemical supercapacitors have attracted much attention as novel energy-storage devices because of their high power density, long life cycles, and high efficiency. Supercapacitors can deliver higher power than batteries and store more energy than conventional capacitors. Current research on supercapacitors has focused on their applications in electric vehicles, hybrid electric vehicles, and backup energy sources. Thus, conventional supercapacitors are heavy and bulky, and it is still a challenge to achieve high efficiency miniaturized energy-storage devices that are compatible with the flexible/wearable electronics. Herein, we present a prototype of a high-efficiency fiberbased electrochemical microsupercapacitor using ZnO nanowires (NWs) as electrodes. These fiber supercapacitors, which have great potential for scale-up, comprise two electrodes that employ a flexible plastic wire and a Kevlar fiber as a substrate. Both wire and fiber are covered with arrays of highquality ZnO NWs grown by the hydrothermal method, and ZnO NWs on a Kevlar fiber was coated with a thin gold film to improve the charge-collection capacity. Furthermore, employment of ZnO NWs could provide exciting solutions to the future development of wearable energy devices. Our fiber-based microsupercapacitor would be large enough to be used in self-powering nanosystems, such as a power shirt using piezoelectric ZnO NWs grown radially around textile fibers. Even though conventional research efforts on bulky supercapacitors have focused on the use of carbon-based materials, such as activated carbons, and some transition metal oxides, such as RuO2 and ZnO, could have several advantages over the conventional electrode materials of supercapacitors for the wearable electronics. First, it can be grown at low temperatures (less than 100 8C) by a chemical approach on any substrate and any shape substrate. Second, it is biocompatible and environmentally friendly material. Furthermore, ZnO NWs can provide large specific surface area, which is crucial to high-efficiency supercapacitors. Compared with other oxide materials, ZnO NWs can be grown easily on fibers or textures at low temperatures (less than 90 8C). Using this unique advantage of ZnO NW growth, we have demonstrated NW nanogenerators that can scavenge energy from the environment by using fabrics. The fiber supercapacitors presented herein could be combined with the fiber nanogenerators, possibly enabling us to achieve a wearable power system. In the future nanosystem, it will be necessary to store energy, especially when an intermittent energy source (such as ZnO-based nanogerators) is used. Fiber supercapacitors could enable us to store electrical energy that is converted from mechanical energy by simple mechanical vibration, such as light wind, footsteps, and heartbeats. Figure 1a shows a low-magnification SEM image of a poly(methyl methacrylate) (PMMA) plastic wire covered with ZnO NWs along with a close-up view (Figure 1b). The wire is highly flexible, and its diameter is about 220 mm. The as-grown ZnO NWs on the plastic wire show typical hexagonal flat-ended morphology of ZnO NWs, which grow almost vertically on the substrate of plastic wire. The NW diameters range from 500 nm to 700 nm, and the lengths are about 6 mm. The fiber-based electrochemical capacitor (Figure 1c) was assembled by entangling a plastic wire covered with NWs around a Kevlar fiber covered with gold-coated NWs. For the entangling process, the plastic wire was placed on a stage, and the Kevlar fiber was entangled carefully by using tweezers. The resistance of the device was monitored during the entangling process to ensure the two electrodes were not in contact each other. As shown in the results of electrochemical measurements (see below), the liquid or gel electrolytes seem to play a role in separating the two electrodes, and contact [*] Dr. J. Bae, M. K. Song, Prof. M. Liu, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332 (USA) Fax: (+1)404-385-3852 E-mail: [email protected]