Musa, Umar Gishiwa
(2025)
Design and fabrication of flexible and non-woven triboelectric nanogenerators using natural and polymeric materials.
PhD thesis, University of Nottingham.
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Abstract
Triboelectric nanogenerator (TENG) is a mechanical energy harvesting device that works by combining the effects of contact electrification and electrostatic induction between two surfaces of dissimilar materials with opposite charge polarities. TENGs serve as a micro- and nano-scale energy source for self-powered devices and sensors with minimal environmental impact compared to traditional renewable energy technologies, such as biofuels and solar cells, and exhibit higher power conversion efficiency than electromagnetic and piezoelectric energy harvesters. Since its invention in 2012, the power efficiency of TENGs has increased significantly, reaching 60% within a year and now up to 85% today. Although TENGs are often utilised as self-powered devices and sensors, they still encounter several challenges, which include the need for performance enhancement, high output impedance, lack of filler materials, and controversies regarding the charge transfer mechanism(s).
This research aimed to address the aforementioned challenges by designing and fabricating flexible, non-woven triboelectric nanogenerators using techniques such as electrospinning, spin-coating, drop casting, and thermal evaporation techniques. To achieve this, the research utilised cost-effective, abundant, and environmentally friendly materials, including polymers and natural leaves to fabricate various TENGs. The project not only focused on material processing and device fabrication but also investigated the charge transfer events and methods to enhance material performance.
Among the fabricated devices, electrospun membrane-based TENGs made from polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) fiber membranes demonstrated significant performance gains with carbon nanofiber (CNF) doping in PVDF, achieving a power density of 1.47 W/m2. Furthermore, a flexible polypropylene (PP) based TENG enriched with graphene oxide (GO) was fabricated using a spin-coating method, resulting in impressive outcomes with a maximum power density of 144.70 W/m2 at 30 % GO content, which is a five-fold increase compared to the pure PP TENG without graphene oxide.
Although performance enhancement has been achieved using different fillers, this only increased the electronegativity of the materials and voltage, not the current and electro-positivity, thereby causing high impedance in the output. Consequently, a pressing need for diverse filler materials that can improve the current density and electropositivity of the triboelectric materials emerged. To address the limitations of fillers and the issue of high output impedance, neem leaf powder was used as a new filler material to produce polydimethylsiloxane (PDMS) and neem composites as triboelectric layer materials using the drop casting technique. This material (neem leaf) effectively controls the negative charge of PDMS, allowing it to become more positive than negative. Additionally, a device utilising fresh neem leaf as the triboelectric layer for charge generation was developed and tested on various polymer fiber membranes, resulting in a series of triboelectric materials with different charge polarities. Eventually, this energy-harvesting technology will allow us to power portable electronics and sensors using naturally occurring mechanical movements.
The simultaneous observation of signals from every two interacting triboelectric materials using double oscilloscope probes in this research has revealed the actual charging phenomena that occur during contact and separation from the output signal approach, setting a foundation for determining suitable electron donor and acceptor materials for various TENG applications. Consequently, the work has proposed a new perspective on the charge transfer mechanism(s) for non-woven and flexible triboelectric materials, addressing a significant challenge in TENGs.
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