Textile-based Triboelectric Nanogenerator with Alternating Positive and Negative Freestanding Grating Structure
Despite substantial growth in the wearable and portable electronics market, most of these types of devices still rely on batteries, which require persistent recharging and replacement. An effective way to solve this problem is to introduce a wearable self-charging power system using an energy harvester to scavenge energy from the surrounding environment which would otherwise be lost. Triboelectric nanogenerators (TENGs) are one of the most promising candidates for powering these systems. They can efficiently convert kinetic energy into electricity based on contact electrification and the electrostatic induction effect. Various examples of TENGs have demonstrated flexibility, lightness, biocompatibility and good performance that are essential for wearable devices. According to these properties textile-based TENGs are determined to be highly suitable for powering wearable devices and electronic textiles (e-textiles) since textiles are universally used to clothe the human body, which is one of the most powerful kinetic energy sources for powering wearable electronics.
This paper presents a novel structure of TENG with alternate grated strips of positive and negative triboelectric material operating in freestanding triboelectric-layer mode, defined as a pnG-TENG. The proposed pnG-TENG is composed of an upper substrate with gratings of polyvinyl chloride heat transfer vinyl (PVC HTV) and nylon fabric (Fig. 1a) and a lower substrate with two screen-printed silver interdigitated electrodes (IDEs) on a PVC coated fabric (Fig. 1b). Through a sliding movement of the grated triboelectric materials across the tines of the IDEs, multiple alternating currents are generated. Nylon fabric was selected as the positive triboelectric material as it is one of the most positive fabrics in the triboelectric series. Other fabrics, such as silk and cotton, were also investigated. PVC HTV is chosen as the negative triboelectric material due to its stretchability, washability and durability. It is also widely-used in the textile industry as it can last the lifetime of the fabric with no fading or cracking. Moreover, the processes involved in the manufacture of the pnG-TENG, namely screen-printing and heat transfer, are cost-effective, straightforward and compatible with standard textile manufacturing.
The pnG-TENGs with the different grating number (N) were mounted on an acrylic substrate for testing on a belt driven linear actuator at an oscillation frequency of 2 Hz and a contact force of 5 N. The simulation and theoretical calculation reveal that the open-circuit voltage (VOC) and the short-circuit current (ISC) of the pnG-TENGs are proportional to the sum between the surface charge densities of the positive and negative triboelectric materials. Therefore, the outputs of the pnG-TENGs are equal to the sum of the outputs of the TENGs with single positive material and the TENGs with single negative material for all grating number. The experimental results for VOC and ISC show the same tendency and are in good agreement with the simulation results. The pnG-TENG (N = 10) delivers an RMS VOC of 136 V, an RMS ISC of 2.68 µA and a maximum RMS power of 125 µW at a load resistance of 50 MΩ. This corresponds to a maximum RMS power density of 38.8 mW/m2, which is 1.94 and 6.43 times greater than the power generated by the TENG with a single triboelectric material and the TENG with no gratings, respectively. To demonstrate practical applications of the pnG-TENG as energy harvesters, the pnG-TENG (N = 10) was embedded into a lab coat. The energy is generated from the relative movement between the arm and torso (Fig. 2). Its output was used to drive a digital watch, a wearable night-time warning indicator for pedestrians and a wireless transmitter. As a demonstration of a sensing device, the voltage peaks of the pnG-TENG was detected and applied for step counting by means of arm motion (pedometer).
The full paper can be found online in Nano Energy Journal at https://doi.org/10.1016/j.nanoen.2019.104148.