Energy harvesting based on triboelectric nanogenerators (TENGs) and applications

The rapid development of technologies and great progress of society has placed serious demands on the supply of fossil fuels and ensued consequential environmental pollution. Harvesting ambient energy from the environment is regarded as an effective way to deal with this energy crisis. Recently, triboelectric nanogenerators (TENG) based on contact electrification and electrostatic induction have been demonstrated to be a novel and efficient technique for efficiently harvesting ambient mechanical energy. TENGs have the advantages of simple structure, low cost and easy fabrication, high energy conversion efficiency at low frequencies (1-10 Hz), and are suitable for applications of wearable and implantable electronics. The key to the development of TENG is the high-performance output based on small size and application as a self-powered sensor, which can be used to monitor environmental changes such as temperature and humidity. This doctoral research aims to explore new strategies to enhance the output of TENGs and to develop TENG based self-powered sensors. The research work carried out and the results obtained are summarized as follows. Firstly, although significant work has been carried out to develop materials with high surface charge density for high-performance TENG, little attention has been paid to the roles of electrode materials that are responsible for charge collection. This work reports on a facile synthesis of laser-induced graphene (LIG) as high-efficiency electrodes for TENGs. Tribo-negative polyimide (PI) and tribo-positive cellulosic paper were converted into PI-LIG and paper-LIG, respectively, by a direct photothermal process using a conventional CO2 laser. The LIG-based TENGs showed higher electrical output characteristics with a peak-to-peak voltage of up to ~625 Vp-p, a current density of ~20 mA.m-2 and a transferred charge density of ~138 μC.m-2 with a maximum power output of ~2.25 W.m-2 , respectively, while the corresponding values for the conventional Al-tape electrode-based paper-PI TENGs were 400 Vp-p, ~10 mA.m-2 , ~85 μC.m-2 and 0.9 W.m-2 , respectively. The mechanically robust LIG electrodes show excellent stability with less than 5.0% variation in output over 12,000 contact cycles. Kelvin probe force microscopy (KPFM) measurements confirmed that the average surface potentials of the LIG triboelectric surfaces are smaller than those of the pristine ones, indicating the role of initial surface chemistry in the formation of LIGs and the performance of TENG. The performance enhancement for LIG-based TENGs is ascribed to the lowering of the charge transfer barrier energy which results in a higher surface charge of the dielectric layer, and the significantly (~ 6 orders) lower contact impedance of LIG electrodes. Thus, via the removal of the additional interface between the triboelectric surface and electrode, high-performance metal-free TENGs with excellent prospects for enabling energy harvesting applications can be realised. Secondly, a TENG based on the polarization effect of piezoelectric nanomaterials working together with the piezoelectric and triboelectric effects was proposed as a new strategy. The polarization effect of piezoelectric material can provide a higher surface charge to the friction layer. For this, a variety of ZnO materials with different nanostructures were prepared and applied to TENGs to compare the effect of the effective contact area on the output performance of TENGs. Compared with the pristine PDMS-based TENG, the outputs of the five ZnO-PDMS TENG with different nanostructures have been significantly improved. The TENG with disk-like nanostructure ZnO-PDMS shows the highest peak output of ~780Vp-p, which is 136% higher than the pristine PDMS-based TENG (~330Vp-p). Correspondingly, the short-circuit current density and charge density has been increased by 205% and 114%, respectively. The nanoflowers nanostructure showed the lowest peak output of ~470Vp-p, which was 42% higher than the pristine PDMS TENG. Correspondingly, the short-circuit current density and charge density rose by 62% and 28%, from 52 mA.m-2 to 84 mA.m-2 , and from ~80 μC.m-2 to ~102 μC.m-2 , respectively. The enhancement produced by the disk-like ZnO nanostructures arose from the increase in the surface contact area in the vertical direction to the greatest extent. Furthermore, the surface charge enhancement and distribution of ZnO with different nanostructures were demonstrated by the piezoelectric microscopy (PFM). Finally, utilizing wide absorption characteristics of a narrow bandgap (~1.8 eV) semiconductor, we report on Bismuth Oxyiodide (BiOI) based photo-enhanced TENG. The tribo-positive BiOI film deposited electrochemically on transparent Fluorine doped Indium Tin Oxide (FTO) substrate provided a way to exploit concurrently the photo-enhanced charge generation and triboelectric effects. When utilized against tribo-negative PDMS films, under illumination, the BiOI/PDMS TENGs’ outputs were significantly enhanced, wherein an increase of 21% in peak to peak output voltage (from 59Vp-p to 73Vp-p, 38% in charge density (from 40µC.m-2 to 55µC.m-2 ), and 74% in overall power density (from 0.25 W.m-2 (in dark) and 0.44 W.m-2 (under illumination)), respectively, were observed. Correspondingly, a dramatic enhancement (from ~25 mV to ~300 mV) in the average surface potential, termed as surface photovoltage (SPV), for the illuminated BiOI was observed by KPFM. For an isolated, grounded BiOI/FTO electrode, this SPV increase is slow-decaying (~3.5 h) and is attributed to the high dielectric constant, presence of deep-traps within BiOI, and slow charge-exchange with the ambient environment. The work thus not only provides an approach for the enhancement of mechanical-to-electrical efficiency of TENGs by light absorption, but also can be utilized for self-powered detection of electromagnetic radiation and photodetectors. All the high-performance TENGs produced have the potential to be used in the realisation of self-powered systems and can be of great significance as a new alternative energy harvesting source