Enhanced Charge Generation in Nanogenerators for Self-Powered Devices

Abstract

Widespread energy harvesting, generating self-sufficient power from the surrounding environment, such as wind, solar and geothermal, have attracted increasing attention in the past decade due to the energy crisis and global warming. Among them, many technological devices converting mechanical energy into other forms of energy or vice versa have been proposed and investigated because of an extended life time, no recharging procedures, and their scalability. Generally, mechanical energy converted into electricity, namely, by using electromagnetic, electrostatic, and piezoelectric effects. Among many technologies, energy harvesting technologies based on the piezoelectric effect, named as piezoelectric nanogenerators (PENG), have been extensively investigated because of an extended life time, no recharging procedures, and scalability. A high flexibility or stretchability is essential in generating high-power continuous electric output signals. The high flexibility can provide an opportunity applicable to a target object without any limitation of its shape and movement. Most recently, a new type of power generating device, named as triboelectric nanogenerator (TENG) based on triboelectric effects coupled with electrostatic effects have been demonstrated as powerful means of harvesting mechanical energy from living environment. In traditional TENGs, the two materials were chosen according to the difference in surface potentials, e.g., the polymer material (i.e. Teflon) terminated with the most electronegative functional group as the negative side and the low work function material (i.e. Al metal) as the positive side, apart from each other (named as airgap). When the two materials are contacted, the polymer material at the negative side tends to gain electrons from Al metal, resulting in being negative compared with Al. Thus, a key approach to improve the electrical output performance of the TENGs from the materials aspect is to increase the triboelectric surface charge density through material modification and surface functionalization. In this study, we introduce highly-ordered embossed thin films with hollow hemispheres as one of promising structures for nanogenerators and pressure sensors, as well as extremely stable and directional anisotropic power generation in the composite-type piezoelectric nanogenerators without any treatment of electrical poling. For triboelectric nanogenerators, the material modifications such as the dielectric constant, the compressibility, and the surface potential in TENGs have focused. Additionally, the increased density of surface charges primary determined by the intrinsic properties of the specific materials and the effective means to enhance the output power of TENGs have proposed, which are critically important for improving the output performance of TENGs.