Wenxi Guo

Triboelectric-Generator-Driven Pulse Electrodeposition for Micropatterning

By converting ambient energy into electricity, energy harvesting is capable of at least offsetting, or even replacing, the reliance of small portable electronics on traditional power supplies, such as batteries. Here we demonstrate a novel and simple generator with extremely low cost for efficiently harvesting mechanical energy that is typically present in the form of vibrations and random displacements/deformation. Owing to the coupling of contact charging and electrostatic induction, electric generation was achieved with a cycled process of contact and separation between two polymer films. A detailed theory is developed for understanding the proposed mechanism. The instantaneous electric power density reached as high as 31.2 mW/cm(3) at a maximum open circuit voltage of 110 V. Furthermore, the generator was successfully used without electric storage as a direct power source for pulse electrodeposition (PED) of micro/nanocrystalline silver structure. The cathodic current efficiency reached up to 86.6%. Not only does this work present a new type of generator that is featured by simple fabrication, large electric output, excellent robustness, and extremely low cost, but also extends the application of energy-harvesting technology to the field of electrochemistry with further utilizations including, but not limited to, pollutant degradation, corrosion protection, and water splitting.

Pyroelectric nanogenerators for harvesting thermoelectric energy.

Harvesting thermoelectric energy mainly relies on the Seebeck effect that utilizes a temperature difference between two ends of the device for driving the diffusion of charge carriers. However, in an environment that the temperature is spatially uniform without a gradient, the pyroelectric effect has to be the choice, which is based on the spontaneous polarization in certain anisotropic solids due to a time-dependent temperature variation. Using this effect, we experimentally demonstrate the first application of pyroelectric ZnO nanowire arrays for converting heat energy into electricity. The coupling of the pyroelectric and semiconducting properties in ZnO creates a polarization electric field and charge separation along the ZnO nanowire as a result of the time-dependent change in temperature. The fabricated nanogenerator has a good stability, and the characteristic coefficient of heat flow conversion into electricity is estimated to be ∼0.05-0.08 Vm(2)/W. Our study has the potential of using pyroelectric nanowires to convert wasted energy into electricity for powering nanodevices.

Rectangular bunched rutile TiO2 nanorod arrays grown on carbon fiber for dye-sensitized solar cells

Because of their special application in photovoltaics, the growth of one-dimensional single-crystalline TiO(2) nanostructures on a flexible substrate is receiving intensive attention. Here we present a study of rectangular bunched TiO(2) nanorod (NR) arrays grown on carbon fibers (CFs) from titanium by a dissolve and grow method. After a corrosion process in a strong acid solution, every single nanorod is etched into a number of small nanowires. Tube-shaped dye-sensitized solar cells are fabricated by using etched TiO(2) NRs-coated CFs as the photoanode. An absolute energy conversion efficiency of 1.28% has been demonstrated under 100 mW cm(-2) AM 1.5 illumination. This work demonstrates an innovative method for growing bunched TiO(2) NRs on flexible substrates that can be applied in flexible devices for energy harvesting and storage.

An Integrated Power Pack of Dye-Sensitized Solar Cell and Li Battery Based on Double-Sided TiO2 Nanotube Arrays

We present a new approach to fabricate an integrated power pack by hybridizing energy harvest and storage processes. This power pack incorporates a series-wound dye-sensitized solar cell (DSSC) and a lithium ion battery (LIB) on the same Ti foil that has double-sided TiO(2) nanotube (NTs) arrays. The solar cell part is made of two different cosensitized tandem solar cells based on TiO(2) nanorod arrays (NRs) and NTs, respectively, which provide an open-circuit voltage of 3.39 V and a short-circuit current density of 1.01 mA/cm(2). The power pack can be charged to about 3 V in about 8 min, and the discharge capacity is about 38.89 μAh under the discharge density of 100 μA. The total energy conversion and storage efficiency for this system is 0.82%. Such an integrated power pack could serve as a power source for mobile electronics.

Direct Growth of TiO2 Nanosheet Arrays on Carbon Fibers for Highly Efficient Photocatalytic Degradation of Methyl Orange

NSF; BES DOE; Chinese Scholars Council; National Basic Research Program of China [2012CB932900]

Hybridizing Energy Conversion and Storage in a Mechanical-to-Electrochemical Process for Self-Charging Power Cell

Energy generation and energy storage are two distinct processes that are usually accomplished using two separated units designed on the basis of different physical principles, such as piezoelectric nanogenerator and Li-ion battery; the former converts mechanical energy into electricity, and the latter stores electric energy as chemical energy. Here, we introduce a fundamental mechanism that directly hybridizes the two processes into one, in which the mechanical energy is directly converted and simultaneously stored as chemical energy without going through the intermediate step of first converting into electricity. By replacing the polyethylene (PE) separator as for conventional Li battery with a piezoelectric poly(vinylidene fluoride) (PVDF) film, the piezoelectric potential from the PVDF film as created by mechanical straining acts as a charge pump to drive Li ions to migrate from the cathode to the anode accompanying charging reactions at electrodes. This new approach can be applied to fabricating a self-charging power cell (SCPC) for sustainable driving micro/nanosystems and personal electronics.

Flexible Pyroelectric Nanogenerators using a Composite Structure of Lead-Free KNbO 3 Nanowires

Pyroelectric nanogenerators fabricated using a lead-free KNbO(3) nanowire-PDMS polymer composite are reported for the first time. The voltage/current output of the nanogenerators can be controlled by electric fields and enhanced by increasing the rate of change in temperature. The fabricated nanogenerators can be used to harvest energy from sunlight illumination and have potential applications in self-powered nanodevices and nanosystems.

Piezotronic effect on the output voltage of P3HT/ZnO micro/nanowire heterojunction solar cells.

We report the first observation of piezotronic effect on the output voltage of a flexible heterojunction solar cell. The solar cell was fabricated by contacting poly(3-hexylthiophene) (P3HT) with one end of a ZnO micro/nanowire to form a p-n heterojunction on a flexible polystyrene (PS) substrate. The open-circuit voltage V(oc) of the solar cell was characterized by tuning the strain-induced polarization charges at the interface between ZnO and P3HT. The experimental data were understood based on the modification of the band structure at the p-n junction by the piezopotential, which is referred as a result of the piezotronic effect. This study not only provides an in-depth understanding about the effect but also is useful for maximizing the output of a solar cell using wurtzite structured materials.

Fiber‐Based Hybrid Nanogenerators for/as Self‐Powered Systems in Biological Liquid

A goal of nanotechnology is to create nanosystems that are intelligent, multifunctional, super-small, extremely sensitive, and low power consuming. The search for sustainable power sources for driving such nanosystems is an emerging field in today s energy research, and harvesting energy from multiple sources available in the environment is highly desirable for creating self-powered nanosystems. For implanted nanodevices, such as a glucose sensor used to monitoring diabetes, it is rather challenging to power them since the solar energy is not available inside the body and thermal energy cannot be used because there is no temperature gradient. The only available energy in vivo is mechanical and biochemical energy. Nanogenerators (NGs) were demonstrated to convert low( Hz) and high-frequency ( 50 kHz) mechanical energy into electricity by means of piezoelectric zinc oxide (ZnO) nanowires (NWs). Following this landmark discovery, direct current (DC) and alternative current (AC) NGs, single-wire and multi-nanowire arrays-based NGs have been developed. On the other hand, biofuel cells have been demonstrated to convert biochemical energy into electricity by using active enzymes as catalyst and glucose as fuel. We have previously demonstrated that biochemical and mechanical generators can work together to harvest multiple kinds of energy in bio-liquid, however, the two units were separately arranged on plastic substrate without integration, and the output was too low and the size was too large to be used for real applications. Here we demonstrate a flexible fiberbased hybrid nanogenerater (hybrid NG) consisting of a fiber nanogenerator (FNG) and a fiber biofuel cell (FBFC), which can be used in bio-liquid (such as blood) for energy harvesting. The FNG and FBFC are totally integrated on a single carbon fiber for the first time for simultaneously or independently harvesting mechanical and biochemical energy. In addition, the hybrid NG can also serve as a self-powered pressure sensor for detecting pressure variation in bio-liquid. Our fiber-based hybrid NG is an outstanding example of selfpowered nanotechnology for applications in biological sciences, environmental monitoring, defense technology, and even personalized electronics. A hybrid nanogenerater made up of a fiber nanogenerater (FNG) and a fiber biofuel cell (FBFC) is designed onto a carbon fiber. The design of the FNG is based on the textured ZnO NW film grown on the surface of the carbon fiber. The carbon fiber serves not only as the substrate on which the ZnONW film is grown, but also as an electrode (noted as core electrode). In previous work we have fabricated a textured ZnO NW film by using physical vapor deposition. The FNG was fabricated by etching the ZnO NW film at one end of the carbon fiber, contacting the top surface using silver paste and tape, and leading out two electrodes from the surface and the core electrodes (left-hand in Figure 1a). An FBFC, which is used for converting chemical energy from bio-fluid such as glucose/blood into electricity, is fabricated at the other end of the carbon fiber (Figure 1a). A layer of soft epoxy polymer is coated on the carbon fiber as an insulator, then two gold electrodes are patterned onto it and coated with carbon nanotubes (CNTs), followed by immobilization of glucose oxidase (GOx) and laccase to form the anode and cathode, respectively. In comparison to conventional biofuel cells and miniature biofuel cells, the FBFCs described here were integrated with the NG (or nanodevices) on an individual carbon fiber, forming a self-powered nanosystem. And the size of the FBFCs shrank a lot due to eliminating the separator membrane and mediator. For easy handling and fabrication, we created our hybrid NG on individual carbon fibers, and our measurements were performed on a bundle of (ca. 1000) carbon fibers. The performance of the hybrid NG is characterized by measuring the short-circuit current Isc and the open-circuit voltage Voc. The FBFC outputs are given as VFBFC and IFBFC, the AC FNG outputs as VFNG and IFNG, and the hybrid NG outputs as VHNG and IHNG. When the hybrid NG is immersed into bio-liquid containing glucose, the FBFC generates a DC output. A typical FBFC output is shown in Figure 2a and b with IFBFC of ca. 100 nA and VFBFC of ca. 100 mV. When a pressure is periodically applied to the bio-liquid, the FNG starts to generate an AC output. The general output of VFNG is 3.0 Vat an output current of IFNG= 200 nA (Figure 2c and d) for an FNG made of ca. 1000 carbon fibers, and the corresponding current density is 0.06 mAcm . By integrating the AC FNG and DC FBFC, a hybrid NG is obtained with the output close to the sum of the FBFC and the FNG (Figure 2e and f). The shape and frequency of the AC FNG output are the same before and after the hybrid[*] Dr. C. Pan, Z. Li, W. Guo, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245 (USA) E-mail: zlwang@gatech.edu Homepage: http://www.nanoscience.gatech.edu/zlwang

Optical Fiber‐Based Core–Shell Coaxially Structured Hybrid Cells for Self‐Powered Nanosystems

An optical fiber-based 3D hybrid cell consisting of a coaxially structured dye-sensitized solar cell (DSSC) and a nanogenerator (NG) for simultaneously or independently harvesting solar and mechanical energy is demonstrated. The current output of the hybrid cell is dominated by the DSSC, and the voltage output is dominated by the NG; these can be utilized complementarily for different applications. The output of the hybrid cell is about 7.65 μA current and 3.3 V voltage, which is strong enough to power nanodevices and even commercial electronic components.