Textile-based triboelectric energy harvester with alternating positive and negative freestanding structure

Abstract

Although wearable and portable electronics have been substantially developed over the past decades, most of these 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. Triboelectric energy harvesters, well-known as triboelectric nanogenerators (TENGs), are one of the most promising candidates for powering these systems. They can efficiently convert kinetic energy occurring during or after frictional contact between two dissimilar materials into electricity based on contact electrification and the electrostatic induction effect. Various examples of TENGs have demonstrated flexibility, low weight, biocompatibility and good performance that are essential for wearable devices. According to these properties, textile-based TENGs are proposed to be highly suitable for powering wearable devices and electronic textiles (e-textiles). This thesis focuses on the methods of implementing TENG into textiles to harvest energy from human motion to create a textile-based TENG (T-TENG). Two novel designs of T-TENGs have been reported. The first design is a T-TENG with alternating positive and negative freestanding grating structure, defined as pnG-TENG. The second design is a T-TENG with an alternating positive and negative freestanding woven structure for harvesting kinetic energy in all sliding directions, defined herein as woven-TENG. In each case, the definition of ‘freestanding’ in the context of TENG designs is that the triboelectric material can move freely without physical connection to the other electrode. The key novelty of these designs is the introduction of the positive and negative triboelectric materials used as the freestanding triboelectric layer in TENGs operating in the sliding freestanding triboelectric-layer mode (FT-mode). This implementation has significantly improved the electrical output performance of the devices compared to conventional TENGs with a single triboelectric material. The devices have demonstrated successful practical applications as both energy harvesters and sensors. The fabrication processes of the devices are common, low-cost and compatible with standard textile manufacturing (e.g. weaving, heat-transfer printing and screen printing). The pnG-TENG is composed of alternate grated strips of positive (nylon fabric) and negative triboelectric material (PVC heat transfer vinyl) and is operating in the sliding FT-mode. Its Ag interdigitated electrodes with matching periodicity have been fabricated using a self-developed screen-printing technique. Whereas most grating-structured TENGs operating in this mode comprise gratings of one type of triboelectric material separated by air gaps, this novel design presents a replacement of the air gaps by another triboelectric material with the opposite polarity to the existing triboelectric material. This method increases performance by increasing the percentage contact area and the amount of transferred charge of the generator. The pnG-TENG with 10 gratings of nylon fabric and PVC heat transfer vinyl delivers a root mean square (RMS) open-circuit voltage of 136 V and an RMS short-circuit current of 2.68 µA. The average power transfer reaches a maximum value of 125 µW at a load resistance of 50 MΩ, a mechanical oscillation of 2 Hz, a contact force of 5 N, a humidity of 25%RH and a temperature of 25 °C. This corresponds to an average 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. The woven-TENG operating in the sliding FT-mode has been successfully developed using a standard fabric weaving and doctor blade printing technique. Whereas most woven-structured TENGs operate in contact-separation mode or contact-separation FT-mode and consist of one type of triboelectric material, this novel woven-TENG comprises woven electrodes and woven strips of positive and negative triboelectric material, which form a checker-like pattern over the electrodes with matching periodicity. The idea of implementing an alternating positive and negative freestanding structure matches perfectly with the introduced woven electrode structure. Because of the nature of the woven structure, a symmetric and periodic arrangement of the electrodes and alternating triboelectric materials are self-formed. This implementation has also shown an improvement in the power output of the TENG by a factor of 2.2 compared to the TENG with a single triboelectric material. Furthermore, in contrast to the conventional grating-structured and woven-structured TENGs, which are designed to operate only in one moving direction, this new design also allows the woven-TENG to harvest energy from any arbitrary sliding direction. The woven-TENG with woven strips of nylon fabric and polytetrafluoroethylenevinyl (PTFE) fabric can generate an RMS open-circuit voltage of 62.9 V and an RMS short-circuit current of 1.77 µA. The average power transfer reaches a maximum value of 34.8 µW at a load resistance of 50 MΩ, a mechanical oscillation of 2 Hz, a contact force of 5 N, a humidity of 25%RH and a temperature of 25°C. This corresponds to an average power density of 5.43 mW/m2.

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ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

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