Fabrication of Electrospun Membrane for Mechanical Energy Harvesting

Abstract

As electronic devices have become miniaturized, the electricity needed is minimized. Nanogenerator, which can convert the mechanical energy to electric energy, has attracted more attention. Piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG) are two important areas for mechanical energy harvesting, which depend on different mechanisms. In this study, both piezoelectricity and triboelectricity have been investigated.

Polyvinylidene fluoride (PVDF) has been widely studied as a sensor and transducer material due to its high piezoelectric, pyroelectric and ferroelectric properties. Here, we firstly studied the piezoelectricity of PVDF electrospun membrane, and then we used the negative electricity of PVDF electrospun membrane and the positive electricity of Nylon 11 electrospun membrane to investigate the triboelectricity.

PVDF-ZnO-GNPs membrane was fabricated by electrospinning to investigate the piezoelectricity. There are many phases in PVDF structure. The β-phase in PVDF is mainly responsible for the piezoelectricity. The electric poling and mechanical stretching in the electrospinning process could convert the α-phase to β-phase. The piezoelectricity property from zinc oxide (ZnO) itself could improve the piezoelectricity of the PVDF-based electrospun membrane. The addition of graphite nanoplatelets (GNPs) could act as the bridge to connect the fibers then improve the electrical conductivity between the fibers. After adding the ZnO nanoparticles and GNPs, the piezoelectric property of PVDF-ZnO-GNPs electrospun membrane was improved.

PVDF electrospun membrane could also be used as the negative electric material to fabricate the TENG. Nylon electrospun membrane was used as the positive electric material. Upon applying the force on the nanogenerator, the two membranes with different electron attracting abilities could contact with each other and then separate. The triboelectricity is produced by the triboelectrification and electrostatic induction in the process of contact and separation. The open-circuit voltage and short-circuit current produced by PVDF-Nylon TENG could reach about 250 V and 8 μA, respectively. The transferred charge quantity could reach 85 nC. It was enough to drive LED lamps. The electricity produced by triboelectricity was generally larger than the piezoelectricity. The effects of the space between the negative electric material and the positive electric material in the device, and the post treatment for the electrospun membrane were also investigated. The space between PVDF membrane and Nylon membrane functioned for maintaining the movement of contact and separation between two kinds of materials. The post treatment could change the structure of the membrane. The suitable pressure for the post treatment could improve the triboelectricity. After 25 MPa post treatment, the maximum short-circuit current produced by the PVDF-Nylon device could approach 12 uA. The transferred charge quantity could increase to 90 nC.

Based on the PVDF-Nylon TENG, another one-dimensional piezoelectric material of zinc oxide nanowires (ZnOnws) was introduced to investigate the effects of fillers on the structure of PVDF and Nylon fibers, as well as triboelectric performance of TENG. Electrospinning, as a simple and economical fabrication procedure, not only produced triboelectric layers with high surface-to-volume ratio and surface roughness, but also favored the cooperative mutual alignment of polymer chains with ZnOnws in the fibers. That enhanced the formation of the highly polar crystalline β-phase of PVDF and δ’-phase of Nylon. The alignment of ZnOnws along the fiber direction facilitated the polar crystalline phase formation by introducing electrostatic interactions between the surface charges produced by the piezoelectric property of ZnOnws and the dipoles of polymer chains during electrospinning. The enhanced polar phase increased the triboelectric potential by improving the surface charge density on the triboelectric layer. The open-circuit voltage, short-circuit current and transferred charge quantity generated by PVDF-ZnOnws/Nylon-ZnOnws TENG device were about 320 V, 10 uA and 130 nC, respectively. The fabricated TENG with improved output performance could be used to directly power over 100 LEDs. In addition, it had great potential as a sustainable power source for driving portable electronics.

Date of Award	28 May 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Yiu Robert LI (Supervisor)