Development of liquid solid triboelectric nanogenerators towards low frequency mechanical harvesting

공학박사 학위논문 저주파수 기계 에너지 하베스팅을 위한 액체-고체 마찰전기 나노발전기 개발 Development of Liquid-Solid Triboelectric Nanogenerators Towards Low-Frequency Mechanical Energy Harvesting 울산대학교 대학원 기계공학부 Development of Liquid-Solid Triboelectric Nanogenerators Towards Low-Frequency Mechanical Energy Harvesting Supervisor: Professor KYOUNG KWAN AHN A Dissertation Submitted to the Graduate School of the University of Ulsan In partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mechanical Engineering by LE CHAU DUY Department of Mechanical Engineering University of Ulsan, Korea February 2023 ACKNOWLEDGEMENTS This Ph.D dissertation is completed under the guidance, the support and the assistance from a lot of esteemed people I would like to extend my sincerest gratitude to all of them First of all, I would like to express my highest respect to my advisor, Professor Kyoung Kwan Ahn, for having me the chance to join his lab as well as his precious guidance and encouragement throughout my Ph.D study I am certain that without his supports, I would never be able to complete my dissertation Besides, I also would like to give my special thanks to the professors serving on my dissertation committee who take up their valuable times to improve my dissertation It is an honor that these prestigious professors attend as my committee members I would like to derive my appreciation to my research group members and my friends in University of Ulsan, who warmly assisted and promoted me, during my Ph.D study I have received tremendous help from them, and without their kindness, I could not have gone this far Last but not least, I would like to deliver my heartfelt gratitude and appreciation to my family for their endless supports and encouragement i ABSTRACT Over the past several decades, the global demand for energy has become larger and more persistent due to the population growth in associated with human activities (i.e., residential, commercial, transportation and industrial) where the fossil fuels play a dominant role However, the fast depletion and environmental impact of fossil fuels are big challenges for our sustainable development; therefore, harvesting energy from surroundings has significantly increased for years and can be recognized as an excellent approach to replace traditional energy generation Mechanical energy is one of the most universally-existing, diversely-presenting, but usually-wasted energies in the natural environment Triboelectric nanogenerator (TENG) has been introduced recently as a novel and potent technology for this purpose, and the use of TENG for mechanical energy harvesting has been investigated to some extent The liquid-solid TENG, with its remarkable strengths, has opened an additional direction for harvesting environmental energy such as water wave, river flow or rainfall However, there is less awareness of generating power from low-frequency behaviors by using liquid-solid TENG even though they are omnipresent in human life In this dissertation, the research efforts have led to develop and analyze TENGbased energy harvesters to scavenge energy of low-frequency mechanical motion utilizing liquid-solid contact electrification principle Through rational structural design, different types of solid-liquid contact electrification TENG for low-frequency mechanical energy harvesting was proposed Initially, a rotational switched-mode water-based triboelectric nanogenerator for harvesting the rotational kinetic energy as well as road slope and wheel speed detection was carried out Then, a discontinuous-conduction based rotational triboelectric nanogenerator with radially symmetrical design to effectively improve the instantaneous power was developed Lastly, an impulsive kinetic energy regulator for harvesting mechanical energy through low frequency impulse-excited motion was fabricated and experimentally evaluated to demonstrate the functionality of harvesting mechanical energy from human and machine activities The above results ascertain the development of liquid-solid TENGs for low-frequency mechanical energy harvesting and brought a big potential of impacting people’s everyday life ii TABLE OF CONTENTS ACKNOWLEDGEMENTS i ABSTRACT ii LIST OF FIGURES vi CHAPTER I Introduction and General Background 1.1 Fundamentals of TENGs 1.1.1 Origin of triboelectrification 1.1.2 Principle theory and mathematical model of TENG 1.1.3 Working mechanism and operation modes of TENG 1.1.4 Potential applications of TENG 1.2 An early view of liquid-solid TENG 10 1.3 Motivation and Objectives 10 1.4 Organization of the Thesis 11 CHAPTER II 13 Revision of Liquid-Solid Triboelectric Nanogenerator: Fundamentals, Structures and Applications 13 2.1 Fundamentals of liquid-solid triboelectrification 13 2.2 Mechanism of liquid-solid triboelectric nanogenerator 15 2.3 Structural design of liquid-solid triboelectric nanogenerator 16 2.3.1 Droplet-based TENGs 17 2.3.2 Bulk liquid-based TENGs 19 2.3.3 Liquid-filled TENGs 21 2.4 Applications of liquid-solid TENGs 22 2.4.1 Micro/nano power sources 22 2.4.2 Active self-powered sensors 24 2.4.3 Networks of liquid-solid TENG for blue energy harvesting 26 2.5 Conclusion 27 CHAPTER III 28 iii Development and Analysis of a Rotational Switched-Mode Liquid-Solid Triboelectric Nanogenerator for Vehicle Monitoring System 28 3.1 Introduction 28 3.2 Methods 29 3.2.1 Fabrication of the RSW-TENG 29 3.2.2 Electrical measurement 30 3.3 Results and Discussion 31 3.3.1 Basic operation and working mechanism of the RSW-TENG 31 3.3.2 Output performance of the RSW-TENG 33 3.4 RSW-TENG as vehicle monitoring device 38 3.5 Conclusion 43 CHAPTER IV 44 Development and Analysis of a Liquid-Solid Triboelectric Nanogenerator based Radially Symmetrical Structure for Mechanical Energy Harvester 44 4.1 Introduction 44 4.2 Methods 45 4.2.1 Fabrication of PVDF nanoporous membrane 46 4.2.2 Fabrication of the DCR-TENG 46 4.2.3 Electrical measurement 47 4.3 Results and Discussion 47 4.3.1 Characteristics of PVDF nanoporous membrane 47 4.3.2 Working principle of the single-cell DCR-TENG 48 4.3.3 Output performance of single cell DCR-TENG 50 4.3.4 Output performance of multiple cell DCR-TENG 55 4.4 Conclusion 58 CHAPTER V 60 Development and Analysis of a Harmonic Oscillator driven Liquid-Solid Triboelectric Nanogenerator for Intermittent Excitation Input 60 5.1 Introduction 60 5.2 Methods 61 iv 5.2.1 Fabrication of the rotary TENG 61 5.2.2 Fabrication of the mechanical motion rectifier 62 5.2.3 Electrical measurement 62 5.3 Results and Discussion 62 5.3.1 Working principle of the IKER 62 5.3.2 Performance of the IKER on vertical effort 65 5.3.3 Performance of the IKER on horizontal effort 72 5.4 Demonstration of the IKER under realistic intermittent excitation 79 5.5 Conclusion 82 CHAPTER VI 83 Conclusion and Future Work 83 6.1 Summary and conclusions 83 6.2 Recommendations for Future Works 84 REFERENCES 86 v LIST OF FIGURES Figure 1.1 Global primary energy consumption 1978-2018 (Exajoules) Figure 1.2 Electron-cloud-well-potential model for explaining triboelectrification with respect to electron transfer and release between two materials Figure 1.3 Schematic showing the principle theory of displacement current for nanogenerators (including TENG) that derived from the expanded Maxwell’s equations Figure 1.4 The ideal structure and equivalent circuit (capacitive) model of the contactseparation TENG Figure 1.5 Basic operation modes of TENG The TENG operation has been categorized into four modes, including vertical contact-separation (CS) mode, relative-sliding (RS) mode, single-electrode (SE) mode, and freestanding (FT) mode with their own merits and demerits Figure 1.6 Four major applications of TENG including micro/nano direct power sources for self-powered systems (MDPS), active self-powered sensors (ASPS), basic network units for harvesting low-frequency water wave energy (LFWE), and direct power sources for high voltage instruments (HVPS) Figure 2.1 Illustration of hybrid EDL model with “two-step” process formation (a) In the first step, water molecules and ions in the solution contact with the solid surface, causing electron transfer as well as ion adsorption on the solid surface (b) In the second step, free ions in the solution are attracted by the induced electrostatic field and concentrate at the region close to the electrified surface, forming the EDL 14 Figure 2.2 Effect of surface hydrophobicity on the electron transfer and the ion transfer When the water contact angle is higher than 90º, the ratio of electron transfers to ion transfers (E/I) increases rapidly, asserting the dominance of electron transfer This can be explained by the chemical bond of hydrophilic (1) and hydrophobic (2) surface 15 Figure 2.3 Mechanism of water-PDMS based TENG (a) Initial state when no force is applied (b) PDMS layer and water contact each other (c) PDMS layer separates from water (d) Separation completes and the PDMS layer comes back to original position (e) PDMS layer come to contact with water again, starting a new cycle 16 Figure 2.4 Different structural designs of droplet-based TENG for harvesting raindrop energy (a) Schematic diagram and single-electrode mechanism of the multi-unit vi Figure 5.37 Setup of the PP-IKER for harvesting energy from boom barrier gate for powering wireless sensor Inset: Wiring circuit of the setup and data tracking by cellphone 5.5 Conclusion This chapter presented an IKER to harness the impulsive mechanical energy in the surrounding environment, in which the IKER is composed of a harmonic oscillator, a mechanical motion rectifier, and a rotary liquid-solid TENG The combination of harmonic oscillator and mechanical motion rectifier can manage the interchange of kinetic energy and potential energy, through exertion phase and resilience phase, to actuate the rotary TENG operation excessively The harmonic oscillator is adaptable to be configured either as a gravity-balancer to deal with the vertical excitation or as a planar pendulum to handle the horizontal excitation As a result, the device can produce an output energy of 25.13 nJ with a maximum current density of 2.29 nA·cm-2 concerning a single vertical incentive; whilst, the horizontal direction effort obtains an output energy of 25.43 nJ with a maximum current density of 2.41 nA·cm-2 Moreover, the device can power up a few LEDs with human footstep excitation and a wireless-based sensor during boom barrier gate operation, which shows a good possibility in waste mechanical energy harvesting 82 CHAPTER VI Conclusion and Future Work 6.1 Summary and conclusions The overall goal of this research is development of the mechanical energy harvester based on the liquid-solid contact triboelectrification, which is accomplished by the following scenario In chapter 1, the necessity and objective of this research as well as the fundamentals of triboelectric nanogenerator (such as theory, mechanisms, and applications) were presented Chapter extended the concept of liquid-solid contact for triboelectrification with “Wang’s transition” model along with the structural designs and potential applications of the liquid-solid triboelectric nanogenerator Chapter to chapter consecutively introduced three mechanical energy harvesters utilizing liquid-solid triboelectric nanogenerator, including rotational switched-mode water-based triboelectric nanogenerator (RSW-TENG), discontinuous conduction based rotational triboelectric nanogenerator (DCR-TENG), and impulsive kinetic energy regulator (IKER) The design and performance analysis of the above devices were fulfilled to reach the main goal of this research Firstly, a rotational switched water based triboelectric nanogenerator (RSWTENG) was developed to convert the rotational mechanical energy of a vehicle wheel into electricity as well as keep track of the vehicle operation The key feature of the RSWTENG is the position-dependent conduction where the charge transfer just occurs at designated locations Therefore, the electrical outputs could be altered with respect to the on-state conduction positions, by which a road slope detection device would be preferred In the case, PTFE membrane is used as the solid-phase negatively charged material and DI water is used as the liquid-phase oppositely charged material The driven rotational motion induces a slippery sliding of water over the PTFE membrane and, in accordance with the position-dependent conduction structure, the induced electrostatic charges only transfer between electrodes since they attain the maximum possible accumulation Further, the RSW-TENG directly derives unidirectional current, which is beneficial to low-power electronic devices Secondly, a discontinuous conduction based rotational triboelectric nanogenerator 83 (DCR-TENG) was developed to scavenge mechanical energy without using a rectifier circuit The key feature is the utilization of the motion-activated switches where these switches are simultaneously closed at designated positions during the operation of the TENG This design allows the induced charges accumulating on one freestanding electrode, then instantaneously discharging into another electrode when the switches are triggered, which significantly enhance the output power of the TENG PVDF nanoporous membrane is used as the solid-phase negatively charged material and DI water is used as the liquid-phase oppositely charged material The driven rotational motion induces a slippery sliding of water over the PVDF membrane With the presence of designated motion-activated switches, the induced charges only transfer between electrodes when they reach the maximum value, resulting in the improvement of electrical output Further, the TENG successfully derives unidirectional current, which can directly apply to energy storage devices Thirdly, an impulsive kinetic energy regulator (IKER) was developed to scavenge the waste mechanical energy from human and machine activities The key feature is the combination of the harmonic oscillator and the mechanical motion rectifier to drive the rotary solid-liquid triboelectric nanogenerator (R-TENG) such that it can prolong the running time after triggering by an arbitrary impulse This combination and the rational design of R-TENG for discontinuous conduction can induce a remarkable enhancement in output energy of the R-TENG The harmonic oscillator is used as mechanical energy regulator where it takes charge of the interchange of input kinetic energy and stored potential energy, through either gravity balancing or planar pendulum mechanism In addition, the mechanical motion rectifier transfers the regulated input into one-way rotational motion of R-TENG that induces a slippery sliding of water over the PTFE membrane Altogether, the R-TENG can endure for excessive cycles out of a single impulse excitation, which successfully improves the electrical output energy Further, the R-TENG directly derives unidirectional current, which is beneficial to low-power electronic devices 6.2 Recommendations for Future Works The development of mechanical energy harvesters based liquid-solid triboelectric nanogenerator were addressed in this research with comprehensive fabrication and investigation However, there are still interesting aspects for enhancement of liquid-solid 84 TENG-based systems that can be extended for further research works This section proposes the potential directions which can be targeted for further investigations on the topic of this research Firstly, the designs need to be robustly developed and optimized to better obtain the electrical output performance of the system under random excitations such as human motion, impact, and vibration New designs should have much higher stability and durability, and much smaller deterioration in performance at higher operating frequency, which is beneficial for energy harvesting Secondly, the solid triboelectric materials should be optimized to achieve the maximum surface charge density since the surface charge density is the most important factor for the output of the liquid-solid TENGs The surface modification technique, including physical modification techniques, chemical functionalization techniques and the combination of them, can be applied to improve the surface morphology Ion injection and fluorinating surfaces are very interesting directions toward the development of high performance liquid-solid TENG Thirdly, due to the high voltage pulsed output and the high internal capacitive impedance, a power management circuit (PMC) must be integrated into liquid-solid TENG to convert the harvested raw energy to a well-regulated form which is suitable for electronic devices It has a critical and indispensable role in self-powered systems with the liquid-solid TENG as the energy harvesting unit Despite various circuit topologies have been proposed, there are endless opportunities for engineers and researchers to 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Research, 2022, 46(7), 9549-9559 Duy-Linh Vu, Chau-Duy Le, Kyoung Kwan Ahn, Polyvinylidene Fluoride Surface Polarization Enhancement for Liquid-Solid Triboelectric Nanogenerator and Its Application, Polymers, 2022, 14(5), 960 Chau-Duy Le, Cong-Phat Vo, Duy-Linh Vu, Thanh-Ha Nguyen, Kyoung Kwan Ahn, Water electrification based triboelectric nanogenerator integrated harmonic oscillator for waste mechanical energy harvesting, Energy Conversion and Management, 2022, 251, 115014 Duy-Linh Vu, Chau-Duy Le, Cong-Phat Vo, Kyoung Kwan Ahn, Surface polarity tuning through epitaxial growth on polyvinylidene fluoride membranes for enhanced performance of liquid-solid triboelectric nanogenerator, Composites Part B: Engineering, 2021, 223, 109135 Duy-Linh Vu, Cong-Phat Vo, Chau-Duy Le, Kyoung Kwan Ahn, Enhancing the output performance of fluid‐based triboelectric nanogenerator by using poly (vinylidene fluoride‐co‐hexafluoropropylene)/ionic liquid nanoporous membrane, International Journal of Energy Research, 2021, 45(6), 8960-8970 Chau-Duy Le, Cong-Phat Vo, Thanh-Ha Nguyen, Duy-Linh Vu, Kyoung Kwan Ahn, Liquid-solid contact electrification based on discontinuous-conduction triboelectric nanogenerator induced by radially symmetrical structure, Nano Energy, 2021, 80, 105571 Cong-Phat Vo, M Shahriar, Chau-Duy Le, Kyoung Kwan Ahn, Mechanically active 95 transducing element based on solid–liquid triboelectric nanogenerator for selfpowered sensing, International Journal of Precision Engineering and ManufacturingGreen Technology, 2019, 6(4), 741-749 Ravi Kumar Cheedarala, Le Chau-Duy, Kyoung Kwan Ahn, Double characteristic BNO-SPI-TENGs for robust contact electrification by vertical contact separation mode through ion and electron charge transfer, Nano Energy, 2018, 44, 430-437 96