Stretchable High-output Triboelectric Nanogenerators and Their Application in Energy Harvesting and Motion Sensing

Abstract

With the rapid development of Internet of Things (IoT) and wearable devices, there is an increasing demand toward more portable and intelligent electrical power source. Additionally, the need for sensors integrated into clothing or worn on the human body for health monitoring purposes has intensified. In response to these requirements, wearable energy harvesters and self-powered sensors present a promising solution. Among them, the emerging triboelectric nanogenerator (TENG) stand out due to its high output voltage and efficiency in converting low-frequency energy input, making it suitable for harvesting energy from daily activities and monitoring human body motions. Furthermore, the TENG’s simple structure, consisting of only tribolayers and electrodes, facilitates easy integration into wearable electronics.

TENG can harvest energy from various body motions, such as bending-releasing, walking, jumping, bowing, blinking, and pulse vibration, while delivering high electrical output and maintaining appropriate mechanical strength. However, although the generated voltage can reach several hundred to thousand volts, the current and transferred charge typically remain low, in the order of several microamperes (µA) and nanocoulombs (nC) for common materials.

Small tribopositive molecules, such as water and glycerol, possess abundant electron-donating groups, but cannot be used independently due to charge dissipation caused by evaporation or sputtering processes. By incorporating glycerol into biomass-derived polymers as plasticizer and tribopositive additive, sustainable and degradable TENGs with enhanced output, stretchability, and water retention have been developed. Moreover, ion doping plays a significant role in influencing crosslinking and tribopositivity. Therefore, CuCl₂, CaCl₂, NaCl and Lithium Bis(trifluoromethanesulphonyl)imide (LiTFSI) were selected for developing hydrogels, solid ionic conductors, and tribolayers. Cations are beneficial to build interconnected structures and improve mechanical strength. Furthermore, these ionic conductive structures simultaneously function as tribolayers and electrodes, preventing the conventional detachment of tribolayers and electrodes during long-term usage.

Surface modification introduces charge-donating or charge-trapping elements or groups. A green surfactant self-assembly strategy, which involves covering a layer of tribopositive surfactant on polymers, significantly enhances tribopositivity and ionic conductivity. This study found that the surfactant firmly binds on the surface of the ion-based polymer, effectively preventing charge leakage. Implementing the surfactant self-assembly strategy in stretchable ionic conductors endows polymers with enhanced tribopositivity, stretchability, and conductivity simultaneously.

Overall, this research provides a comprehensive understanding of the working mechanisms of TENGs under various conditions (relative humidity, frequency, working speed and force), and offers strategies to enhance tribopositivity and stretchability, including tuning water content, ion doping, and surfactant self-assembly. Consequently, the assembled TENGs successfully achieved energy harvesting of human body motions and monitoring of body movements.

Date of Award	16 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Walid DAOUD (Supervisor)