Fabrication of Energy-Boosted Triboelectric Nanogenerator Via Electric Field Assisted Thermal Nanoimprinting Process
Authors: Kim Do Wan , Choi Dongwhi , Yoo Donghyeon , Kim Dong Sung 
Published: February 2020

Energy harvesting technologies, which scavenge and capture various types of abandoned energies from an ambient environment, have grown more comprehensive and crucial as they play a role to power the emerging mobile electronic systems and micro devices. Lots of energy harvesting technologies have been successfully demonstrated and explored based on photovoltaic effect, piezoelectric effect, electromagnetic effect and so forth. Recently, the eminent concept of the triboelectric nanogenerator (TENG), where operation mechanism is coupling of triboelectricity and electrostatic induction, has been proposed. The TENG is mainly composed of 1) the contact layer, where the triboelectric effect occurs, and 2) the electrode layer inducing movement of the electrical charges to generate electric current and power. Since its first demonstration, the TENG has been proven as one of the promising energy harvesting platforms due to its high accessibility, low manufacturing cost and high energy conversion efficiency compared with the other existing energy harvesting platforms. With those advantageous aspects, so far, the TENG has been widely applied to versatile applications such as biomedical monitoring system, self-powered sensor and portable power supply to drive wearable electronics.

As above mentioned, although lots of potential applications of TENGs have been successfully demonstrated, for their reliable utilization as a practical power source, the electrical output performance of TENGs still needs to be further increased. From this point of view, enhancing the electrical output performance of the TENG has been one of the important issues in research fields from its first proposal to present. The simplest and most powerful way to enhance the electrical output performance of the TENG is based on formation of micro/nanoscale surface topographies on its contact layer inducing localized concentration of contact forces, which generates higher electrical charge amounts on its surface. As well as formation of topographies on its external surface, recently, the concept of modifying internal molecular arrangement has been suggested. For example, the synthesis of material where important derivatives are carbon increased the interface for charge storage by acting charge trapping sites. On top of that, the alignment of positive and negative charges referred to as poling process has been reported. During this poling or dipolar alignment, a large electric potential is applied across the material which induces the alignment of dipole and production of net positive charge. To be detailed, the maximized surface charge and charge difference between upper and bottom surface of material can produce the developed electrical energy compared with that of non-aligned material. In this respect, it has been considered as a potent strategy to increase the energy output of the TENG.

Previously, we reported the one-step simple method to fabricate the advanced energy output TENG by using nanoimprinting process. Through thermal nanoimprint process, there are obvious advantages in terms of increasing energy output of the TENG, and practical commercialization. Firstly, the energy output of the TENG can be enhanced due to the formation of nano topographies on the contact layer of thermoplastic fluoropolymer. Secondly, in viewpoint of commercialization, the thermal nanoimprint is one-step fabrication, with coincidently replication of nano structure and integration of contact later with the electrode layer which are essential prerequisite components of TENG. Besides these merits, low cost and mass producible nanofabrication process present the application of practical commercialization.

In this study, we first introduced the innovative strategy which is coupling process between nano imprint and poling process to maximize the electrical energy output performance of TENG based on the formation of nano-topographical surface on contact layer and modulation of the internal dipole moments of ferroelectric thermoplastic polymer. The most remarkable novelties are shown that energy output of TENG can be enhanced by achieving the alignment of dipole moments of polymer while taking advantages of fabricating the TENG by using the nanoimprint process. In aspects of the modulation of internal moments, the degree of alignment dipole moments is higher than when the conventional poling method is used. Compared with the conventional poling process, the method we suggested has distinct advantages. Given that the flexibility of thermoplastic polymer increased due to the elevated temperature, the alignment of dipole moments can be achieved more effectively. In addition, the electric field concentrated on each nano structure contributes to obtain the high portion of dipole moments alignment due to their higher electric field in comparison with when the electrode which has no nano-topograpies on surface is used to poling process. Lastly, based on these effects, the overall poling period is much shorter than that of conventional poling.

Figure 1