Development of liquid solid triboelectric nanogenerators towards low frequency mechanical harvesting

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 investigate

공 학 박 사 학 위 논 문

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

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

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 based 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

TENG-TABLE OF CONTENTS

ACKNOWLEDGEMENTS i

ABSTRACT ii

LIST OF FIGURES vi

CHAPTER I 1

Introduction and General Background 1

1.1 Fundamentals of TENGs 3

1.1.1 Origin of triboelectrification 3

1.1.2 Principle theory and mathematical model of TENG 4

1.1.3 Working mechanism and operation modes of TENG 6

1.1.4 Potential applications of TENG 8

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

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

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

LIST OF FIGURES

Figure 1.1 Global primary energy consumption 1978-2018 (Exajoules) 1 Figure 1.2 Electron-cloud-well-potential model for explaining triboelectrification with

respect to electron transfer and release between two materials 3

Figure 1.3 Schematic showing the principle theory of displacement current for

nanogenerators (including TENG) that derived from the expanded Maxwell’s equations 5

Figure 1.4 The ideal structure and equivalent circuit (capacitive) model of the

contact-separation TENG 6

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 7

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) 9

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

transparent TENG (MT-TENG) in integrating with vehicle and building (b) Schematic diagram and sliding free-standing mechanism of the self-cleaning/charging power system (SPS) (c) Schematic diagram and working mechanism of the cone-shaped interdigital electrode (IDE) based TENG (d) Schematic diagram of the integrated system consisting

of solar cell and water-drop TENG as well as the mechanism of the TENG 18

Figure 2.5 Different structural designs of bulk liquid-based TENG for harvesting water

energy (a) Schematic diagram and working principle of ww-TENG driven by flowing river (b) Schematic diagram and working principle of ww-TENG driven flowing water (c) Schematic diagram and working principle of LSEG for harvesting energy from a variety of water motions (d) Schematic diagram and working principle of Ū-shape electrode TENG for extracting power from waves’ flood and ebb processes (e) Schematic diagram and working principle of liquid-immersed TENG 20

Figure 2.6 Different structural designs of liquid-filled TENG for harvesting water energy

(a) Schematic diagram and working principle of rotational water TENG (b) Schematic diagram and working principle of water tube-based TENG (c) Schematic diagram and working principle of “SWING stick” TENG (d) Schematic diagram and working principle of MSW-TENG 21

Figure 2.7 Liquid-solid TENGs as power sources (a) Structure of networked integrated

TENG (NI-TENG) with arrayed bridge rectifiers and its ability to power a wireless transmitter (b) Diagram of the grid of WDSE-TENG for harvesting water impact energy with the potential of driving low-power electronic devices (c) Diagram of integrated TENG for raindrop energy harvesting and its demonstration of powering small electronic devices 23

Figure 2.8 Liquid-solid TENG as active self-powered sensors (a) Schematic diagram

and structure of LST-TENG for ship draft measurement The water level is detected through the peaks and valleys of voltage derivation signal (b) Photograph of oil–solid interacting TENG (O–S TENG) for monitoring the engine lubricating oil and its output performance with respect to the fraction of waste oil (c) Structural diagram of superhydrophobic liquid-solid contact TENG with current output generated by six types

of droplets and its biomedical sensor prototype 25

Figure 2.9 Networks of liquid-solid TENG for blue energy harvesting (a) Concept of

liquid–solid-contact buoy TENG with the illustration of TENG network and different types of triggering movement The outputs show linear relation with the unit number (b)

Concept and fabrication of droplet-based TENG arrays on ship model with the output performance in water tank 26

Figure 3.1 3D model design of the RSW-TENG driven by a DC motor 29

Figure 3.2 Fabrication of RSW-TENG It includes two components: a cylindrical rotating

TENG with extended electrodes and a stationary electric contact with brushes The electrodes contact the brushes twice every cycle 30

Figure 3.3 Experimental setup for evaluating the characterization of the RSW-TENG.

31

Figure 3.4 Working mechanism of the RSW-TENG and the corresponding position of

electrodes and brushes in one cycle of revolution 32

Figure 3.5 (a) Output current and (b) output voltage of RSW-TENG regarding clockwise

and counterclockwise rotation at zero-degree phase angle and rotating speed of 1.25 Hz 34

Figure 3.6 Peak output current, peak output voltage and peak output power under

different load resistances at zero-degree phase angle and rotating speed of 1.25 Hz 34

Figure 3.7 (a) Demonstration of the RSW-TENG as power source for (a) continuously

lighting up LEDs in series and (b) charging different capacitors 35

Figure 3.8 Durability and stability of the RSW-TENG over time 35 Figure 3.9 Diagram of phase angle definition 36 Figure 3.10 Current and voltage with phase angle from 0 degree to 80 degree at running

speed of 1.25 Hz 36

Figure 3.11 Charge accumulation on 10 μF capacitors at different phase angle 37 Figure 3.12 Current and voltage with rotating frequency from 0.5 Hz to 1.25 Hz at phase

angle of 0 degree 37

Figure 3.13 3D graph of output current and output voltage under effect of both phase

angle and rotating frequency 38

Figure 3.14 Experimental results for validating the dispersion of water regarding the

rotating speed of (a) 1.5 Hz, (b) 1.75 Hz and (c) 2 Hz 38

Figure 3.15 Experimental setup of vehicle wheel attached RSW-TENG where the wheel

TENG prototype is attached on a slope-adjustable framework 39

Figure 3.16 (a) Assembly of wheel-TENG prototype and (b) Installation of wheel-TENG

prototype in cooperation with motor-driven pulley system 39

Figure 3.17 The voltage signal corresponding to 10-degree slope 40 Figure 3.18 The voltage signal corresponding to 20-degree slope 40

Figure 3.19 The voltage signal corresponding to 30-degree slope 41

Figure 3.20 The voltage signal corresponding to running speed of 0.1 m/s 41

Figure 3.21 The voltage signal corresponding to running speed of 0.2 m/s 42

Figure 3 22 The voltage signal corresponding to running speed of 0.3 m/s 42

Figure 4.1 Assembly model of the DCR-TENG with radially symmetrical design 45

Figure 4.2 (a) Structural design and (b) Fabrication of the DCR-TENG Inset: mechanical switch composition 47

Figure 4.3 (a) FE-SEM image of the PVDF nanoporous membrane with the water contact angle AFM images of (b) pristine PVDF and (c) nanoporous PVDF membrane with the water contact angle of each material 48

Figure 4.4 Schematic diagram of the working principle for one completed cycle of single-cell DCR-TENG 50

Figure 4.5 Output current of single cell DCR-TENG with 20-MΩ external resistance 51 Figure 4.6 Output voltage of single cell DCR-TENG with 20-MΩ external resistance. 51

Figure 4.7 (a) The magnification of a current peak with estimated charge transferred (b) Output voltage decay curve with different resistance 52

Figure 4.8 The output current density and voltage in relation to the change of external resistance for DCR-TENG and CCR-TENG 53

Figure 4.9 The instantaneous power density in relation to the change of external resistance for DCR-TENG and CCR-TENG 53

Figure 4.10 The effect of rotating speed on the output current of single-cell DCR-TENG with load resistance of 20 MΩ 54

Figure 4.11 The correlation between current peak value and rotating speed 54

Figure 4.12 Schematic diagram of multiple cell DCR-TENG based on radially symmetrical structure 55

Figure 4.13 The output energy-per-cycle of different TENG configurations under various rotating speeds 56

Figure 4.14 The slope of change of output energy-per-cycle regarding different TENG configurations 56

Figure 4.15 Demonstration of the TENG as power source to charge a capacitor and power up LEDs in series 57

Figure 4.16 Demonstration of the multiple cell DCR-TENG powering a temperature and humidity sensor 57

Figure 4.17 Output current of the 6-cell DCR-TENG within 12000 cycles 58 Figure 5.1 Mechanical component design of the mechanical motion rectifier integrated rotary liquid-solid TENG 61

Figure 5.2 Fabrication of the mechanical motion rectifier integrated rotary liquid-solid

TENG (a) 3D assembly model (b) Mechanical motion rectifier (c) Rotary liquid-solid TENG (d) Final construction of the system 63

Figure 5.3 Working principle of the rotary liquid-solid TENG 64 Figure 5.4 The assembly of IKER with gravity-balancing mechanism (GB) as harmonic

oscillator under vertical effort 66

Figure 5.5 Diagram of spring-lever mechanism model The distance from the pivot to the

spring is called the resistance arm length The distance from the pivot to the lumped mass

is called the effort arm length State (1): Rest position State (2): Fully depressed position 66

Figure 5.6 Experimental setup for ‘vertical effort’ using gravity balancing structure

Complete setup includes (1) pneumatic valve, (2) pneumatic cylinder, (3) proposed device, and (4) PC-based valve controller 67

Figure 5.7 Output current of GB-IKER regarding the spring stiffness under hard collision.

Figure 5.15 The relationship between running time and return spring stiffness under

different input excitation of GB-IKER 71

Figure 5.16 The relationship between output energy and return spring stiffness under

different input excitation of GB-IKER 71

Figure 5.17 Energy conversion efficiency of the device under difference load conditions derived from one ‘hard collision’ trigger with k = 1.19 N/mm 72

Figure 5.18 Energy conversion efficiency of the device under difference load conditions derived from one ‘soft collision’ trigger with k = 1.19 N/mm 72

Figure 5.19 The assembly of IKER with planar pendulum mechanism as harmonic oscillator 73

Figure 5.20 Diagram of compound pendulum model The distance from the pivot to the mass is defined as pendulum length State (1): Initial (rest) position State (2): Maximum possible position after triggering State (3): Arbitrary position during swinging 73

Figure 5.21 Experimental setup for ‘horizontal effort’ using pendulum structure Complete setup includes (1) pneumatic valve, (2) pneumatic cylinder, (3) proposed device, and (4) PC-based valve controller 74

Figure 5.22 Output current of PP-IKER regarding different pendulum length 75

Figure 5.23 Output voltage of PP-IKER regarding different pendulum length 75

Figure 5.24 Transferred charge of PP-IKER regarding different pendulum length 75

Figure 5.25 Output current of PP-IKER regarding different pendulum mass 76

Figure 5.26 Output voltage of PP-IKER regarding different pendulum mass 76

Figure 5.27 Transferred charge of PP-IKER regarding different pendulum mass 76

Figure 5.28 The relationship between running time and output energy with respect to the pendulum length of PP-IKER (m = 300 g) 77

Figure 5.29 The relationship between running time and output energy with respect to the pendulum mass of PP-IKER (l = 750 mm) 77

Figure 5.30 Output power of PP-IKER at different load resistances regarding length variation and constant mass of 300 g 78

Figure 5.31 Output power of PP-IKER at different load resistances regarding mass variation and constant length of 750 mm 78

Figure 5.32 Energy conversion efficiency of the device under difference load conditions derived from one ‘horizontal effort’ trigger with m = 300 g and l = 750 mm 79

Figure 5.33 Setup of the GB-IKER for harvesting footstep energy Inset: Photo of the LEDs lighted up by footstep energy 80

Figure 5.34 Capacitor charging response of the GB-IKER under footstep excitation 80

Figure 5.35 The durability of the GB-IKER under footstep excitation 81

Figure 5.36 Conceptual design of mechanical energy harvesting using PP-IKER driven

by boom barrier gate Inset: fabrication of mechanical trigger 81

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 82

1

CHAPTER I

Introduction and General Background

Energy, from ancient times to present day, is always a crucial factor in the development and prosperity of human civilization For thousands of years, hydro power and wind power have been utilized to help humans with a vast amount of works in manufacturing and transportation By the mid-19th century, the emergence of industrial revolution marked the beginning of fossil fuel era where coal, oil and natural gas have dominated as the essential energy sources for human society Today, the coupling of economic growth and urbanization rate greatly accelerates the global energy consumption, where the fossil fuels account for about 85% of the total primary energy consumption1 However, the more fossil fuels we use, the more negative impacts we put into our planet, from air and water pollution to global warming and climate change2,3 Therefore, a significant change from current ‘fossil fuel energy sources’ to some ‘sustainable energy sources’ is inevitable for the survival of not only the present but future generations This energy transition has been commenced with a progressive replacement of fossil fuel energy by renewable electricity in certain sectors like transportation4–7

When the basic principle of electricity generation was discovered in the late nineteenth century, electrical power along with electrical machines and equipment quickly revolutionized all aspect of human life including the way of communication,

Figure 1.1 Global primary energy consumption 1978-2018 (Exajoules).

transportation, industrial production and residential life As a result of technology innovations, generating electricity from renewable energy sources has rapidly attracted attention all over the world as a prominent energy option for sustainable development goals8–10 On a large scale of generating electricity (ranging from kW to MW), the research and development activities have been thoroughly concerned with natural-free energy sources, including solar11, wind12, hydro13, biomass14 and geothermal power15 Among them, solar, wind and hydro power, which are widely exploited at the current time, are highly susceptible to the weather variability It may cause damage to expensive apparatus or lost energy production in harsh conditions, thereby mitigating the reliability and stability of the energy system On a much smaller scale (ranging from μW to mW), the deployment of renewable electricity generation has almost been neglected for quite a long time due to a lack of interesting applications16–19 However, in the coming era of Internet of Things (IoT), there is an emergent request for seeking sustainable power sources, which are able to constantly function billions of distributed IoT devices20–22 In this regard, benefiting from the miniaturization and low power consumption of up-to-date electronic devices, the self-powered energy harvesting (SEH) technology emerges as a

harness the energy in our living environment to keep up operation for a long duration without maintenance so that external power source can be exclusive

The available energy in our living environment exits in different forms, including radiant energy, thermal energy, mechanical energy and bio-chemical energy Among them, mechanical energy is the most diverse and accessible energy source which can be discovered in various systems with irregular amplitude and frequencies, such as human physical motion, structural vibration and fluid flow26,27 The conventional way to scavenge the mechanical energy is based on the electromagnetic, piezoelectric and electrostatic mechanism, which have been well-developed and utilized in many applications around the world28–30 In the past decade, a novel method for transforming mechanical energy into electricity has been introduced, which is called triboelectric nanogenerator (TENG)31 The TENG is enabled by the combination of triboelectrification effect and electrostatic induction, so that it can extract electric power from electrostatic charges generated by mechanical contact Since then, TENG has caught attention of many scholars and researchers owing to its assorted benefits, inclusive of simple fabrication process, low cost, diversity in materials selection and high efficiency at low frequency,

3

and the progression of TENG development has grown rapidly across different countries32–

35 Thus, TENG has been intended as a successive method for fulfilling the requirement

of self-sustained and independent energy source in the new era36–38 In the following, the fundamentals of TENG, including working mechanism and basic operating modes, as well as the potential applications of TENGs will be briefly introduced

1.1 Fundamentals of TENGs

1.1.1 Origin of triboelectrification

Triboelectrification is a scientific term to describe the phenomenon in which two different materials become electrically charged after they are physically brought into

Figure 1.2 Electron-cloud-well-potential model for explaining triboelectrification with

respect to electron transfer and release between two materials (modified from ref.39,

Copyright 2019 Elsevier Ltd.).

contact by rubbing one on the other (i.e., by friction) and then separated39 It has been known for 2,000 years, and it exists everywhere and at any time in our life However, the fundamental physics of this universal phenomenon is blurry until a quantitative investigation was reported in 201840 This study revealed that the electron transfer is the dominant mechanism for triboelectrification, and the authors also proposed an electron-cloud-well-potential model based on fundamental electron cloud interaction to explain

the electron transition process for all types of materials (Figure 1.2) Initially, material A

is separated from material B with a distance between electron clouds of d so that electrons

cannot transfer and are trapped in their respective potential well When two materials contact each other, their electron clouds strongly overlap and then the electron could transfer from one atom to the other As two materials are separated, most of transferred

electrons are remained in the material B by the surface potential barrier E 2, resulting in the positively charged material A and the negatively charged material B With temperature increasing, electrons can run off the potential well and likely to be thermionically emitted, showing an exponential decay of surface charges The proposed electron transition model, which is regarded as “Wang transition” model, provide a new perspective to realize the origin of triboelectrification in a general case41

1.1.2 Principle theory and mathematical model of TENG

From the fundamental physics, the theory of mechanical-to-electrical energy conversion based TENG is developed based on the expanded the Maxwell’s displacement current42,43 As previously introduced, when two materials are physically contact, triboelectrification will induce electrostatic charges on contacting surface which eventually promote time-varying surface polarization By adding this surface polarization term to the classical Maxwell’s equations, the principle theory of TENG can be arisen based on the revised Maxwell’s displacement current density as follows

charges from triboelectrification effect (Figure 1.3) Here, the first term represents the

displacement current density due to time variation of electric field, and the second term

is the displacement current density due to the time-varying surface polarization as two

materials in contact under mechanical agitation By integrating the current density over the surface plane, the total displacement current is obtained

𝐼𝐷 = ∫ 𝑱𝐷∙ 𝑑𝒔 = 𝜕𝑄

𝜕𝑡

where Q is the total free charge on the electrode In this equation, the left-hand side is the

displacement current which is the internal driving force for converting mechanical energy into electrical energy, and the right-hand side is the capacitive conduction current which

is the observed current in the external circuit

To further analysis, an ideal structure of a contact-separation TENG is presented which consists of two different dielectric materials with back electrodes coated on them

An external load is connected to the electrodes, which will absorb the electricity generated

by the cyclic contact and separation of the two dielectric materials Through the

Figure 1.3 Schematic showing the principle theory of displacement current for nanogenerators (including TENG) that derived from the expanded Maxwell’s equations (modified from ref.42, Copyright 2019 Elsevier Ltd)

mechanical impact, two dielectric surfaces are oppositely charged with a surface charge

density of ±σtribo, which is independent to the gap distance z The presence of triboelectric

charges induces an electrostatic field that prompts the electron transfer between

electrodes with transferred charge density of ±σtr, which is subject to z Therefore, the mechanical energy can be converted into electrical energy regarding the change in z

Since two oppositely charged surfaces separated by some distance can be considered as

a parallel plate capacitor with variable capacitance, the equivalent circuit of TENG may

be represented by a capacitive model, where the mathematical expression for the electrical potential difference between two electrodes is given by

𝐶(𝑧)𝑄 + 𝑉𝑜𝑐(𝑧)

From the governing equation, there are two terms on the right side: the first one, -Q/C(z), relates to the transferred charges between electrodes where C(z) is the capacitance between two electrodes, and the second one, Voc(z), is originated from the polarization of

triboelectric charges Then, the equivalent circuit of TENG can be illustrated by a serial

connection of an ideal voltage source and a variable capacitor as shown in Figure 1.4

1.1.3 Working mechanism and operation modes of TENG

The working mechanism of TENGs is a coupling of triboelectric effect and electrostatic induction, in which the former inspires the static polarized charges on two material surfaces and the latter involves in the circulation of free charges through the external circuit44 In detail, under mechanical action, two materials with different electron

Figure 1.4 The ideal structure and equivalent circuit (capacitive) model of the

Co KGaA, Weinheim)

The ideal structure and equivalent circuit (capacitive) model of the contact-separation TENG (modified from ref.36, Copyright 2018 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

7

affinities rub against each other, and then the triboelectric charges are brought on at the contact interfaces The material with higher electron affinity, deemed as electron acceptor, becomes negatively charged, while the other, esteemed as electron donor, turns into positively charged In principle, the total charge amounts distributed on the surface of both materials are the same As soon as two charged materials are separated by a distance,

an electric potential difference is established between two electrodes so that it drives free charges to flow from one electrode to the other through the external circuit When applying the force again, two materials move forward each other, which will descend the established potential difference and cause a reverse current This process occurs until two materials make contact, and then the next cycle starts Apparently, during the contact-separation cycle, the TENG produces a periodical electric output with positive and negative direction

By fusing different direction of the polarization change and electrode

configuration, the TENG operation has been categorized into four modes (Figure 1.5),

Figure 1.5Basic 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 (modified from ref.45, Copyright 2018 Elsevier Ltd)

including vertical contact-separation (CS) mode, relative-sliding (RS) mode, electrode (SE) mode, and freestanding (FT) mode45 In the CS mode, the relative motion perpendicular to the interface is applied, and the electric output is determined by the gap between material surfaces46 The RS mode is deployed by applying a relative displacement in the direction parallel to the interface, and the electric output is a result of

single-a non-fully compenssingle-ated triboelectric chsingle-arges single-at the mismsingle-atched single-aresingle-as47 The SE mode is

a specialized design to harness mechanical energy in arbitrary direction, where the ground

is now assigned as reference electrode, and the triboelectric material, without using any electric conductor, can be attached to a freely moving object48 The FT mode is an adaptation of the SE mode, where a pair of symmetric electrodes is used instead When the freely moving object dislocates from its position, an asymmetric charge distribution appears between the object and the electrodes, triggering the electric charge transfer between the two electrodes49 The theory of each operation mode has been thoroughly explored and discovered in previous studies50,51 In some specific applications, TENG may not operate in just a single mode, but could rather be a conjunction of different modes

to take benefits from each other

1.1.4 Potential applications of TENG

Based on four operation modes, tremendous amount of TENGs has been elaborated in a broad range of scenarios The applications of TENG can be divided into

four major categories (Figure 1.6): 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)52 Firstly, TENG, with the ability to harvest mechanical energy from the surrounding environment, can directly power up billions of electronic devices Abundant structures of TENG are demonstrated to generate electricity from various mechanical energy sources, including wind energy53, raindrop energy54, sonic energy55, and ocean energy56 Secondly, TENG, through its electrical signal characteristics (e.g amplitude and frequency), can reflect the strength of mechanical input without using additional signal conditioning circuit Besides, the number of transferred charges on triboelectric material surface is considered a highly sensitive probe for a variety of sensing performances Some highlighted works include angle sensors57, touch sensors58, acoustic sensors59, acceleration sensors60, and chemical sensors61 Thirdly, due to the excellent capability of harvesting low-frequency mechanical energy, TENG can effectively collect

energy in water waves Various prototypes have been designed, including the solid-liquid contact electrification TENG62, fully enclosed TENG63 and TENG network64 Thus, TENG is a perfect complement to the traditional electromagnetic generator, which normally requires a high operation frequency, to fulfill the energy production for either present macro-grid or future micro-grid Lastly, TENG is known for its ultrahigh open-circuit voltage and low short-circuit current, which makes it applicable for driving high-voltage electrical appliances without auxiliary transformer Several self-powered systems based on high-voltage TENG has been developed, such as micro/nano electromechanical systems65, capacitive sterilization systems66, electrostatic manipulation systems67, and micro-plasma control systems68 However, there are still challenges and difficulties ahead

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) (modified from ref.52, Copyright 2020 The Authors)

for the efforts to promote TENG toward industrialized markets due to the discrepancy between current TENGs’ capacity, durability and stability with real demands In order to diminish such gaps, a development roadmap of TENG is proposed with four key stages: design innovation, optimum performance, system integration and industrialization36

1.2 An early view of liquid-solid TENG

So far, TENG mechanism has dominated by solid-solid case, but its own performance can be flawed by several reasons The first reason comes from its high sensitivity to environmental conditions, especially atmospheric humidity and pressure69 The second reason is the surface abrasion of contact interface after long-term friction, which causes an adverse effect on the durability as well as the sensing stability of the TENGs70 The third reason arises from the improper contact between two solid surfaces inflicted by a large mechanical force To overcome these issues, liquid materials can be used instead in the triboelectrification process to contrive a liquid-solid mechanism According to the natural wettability property, the performance of liquid-based TENGs are not affected by environmental factors such as humidity Further, the liquid layer can act as lubricant to provide wear resistance for abrasive conditions, resulting in a better durability and stability of liquid-based TENGs Last but not least, a liquid can easily change its shape so that it makes a complete contact with the solid surface, enhancing triboelectric output signals Liquid-solid TENGs can harvest mechanical energy from diverse sources such as raindrops, tidal waves, and low-speed water streams71 Besides, the application of the liquid-solid TENGs as self-powered physical and chemical sensors has also been extensively studied72

1.3 Motivation and Objectives

The liquid-solid TENG, with its remarkable strengths, has opened an additional direction for harvesting environmental energy, alongside the prominent solid-solid TENG Nowadays, most of reported liquid-solid TENGs have been applied to harness energy offered by water wave, river flow or rainfall In contrast, there is a lack of liquid-solid TENG research regarding the power generation from low-frequency behaviors, even though they are omnipresent in human life and contain a huge amount of mechanical energy This shortage motivates an innovative approach to elevate the role of liquid-solid TENG in the awareness of low-frequency mechanical energy harvesting Thus, the

overall goal of this research is to develop and analyze different types of water-solid TENGs for scavenging energy of low-frequency mechanical excitations The major objectives of the proposed research are categorized in the following directions:

● Development and analysis of a rotational switched-mode water-based triboelectric nanogenerator (RSW-TENG) for harvesting the rotational kinetic energy The concepts and results of the proposed TENG device can be utilized to design an active sensor with the ability of road slope and wheel speed detection

● Development and analysis of a discontinuous conduction based rotational triboelectric nanogenerator (DCR-TENG) with radially symmetrical structure The radially symmetrical structure helps to synchronize the electrical output from independent TENG cells which can improve the instantaneous power effectively The results show that the TENG device can become an efficient power source for the self-powered applications

● Development and analysis of an impulsive kinetic energy regulator (IKER) for harvesting mechanical energy through low frequency impulse-excited motion The combination of harmonic oscillator and mechanical motion rectifier can manage the interchange of kinetic energy and potential energy to actuate the rotary solid-liquid TENG operation excessively It can be referred to as a decent option for harvesting mechanical energy from human and machine activities

1.4 Organization of the Thesis

As stated in previous section, this research is written up with systematic insight into three main objectives which are addressed in five chapters The presented studies from chapter 2 to chapter 6 are summarized as follows:

● In chapter 2, fundamental physics of liquid-solid triboelectrification is briefly introduced, as well as an introduction to the early researches, and structure designs of water-solid TENGs is given

● In chapter 3, design, fabrication, and experimental characterization of the rotational switched-mode water-based triboelectric nanogenerator is studied Initially, the working mechanism of the device, and then the output performance of the fabricated device under various dynamical conditions, is fully investigated The energy output capability of the proposed device is demonstrated considering different electrical loads Finally, the practical application of the device for vehicle monitoring is proved

● In chapter 4, design, fabrication, and experimental characterization of the discontinuous conduction based rotational triboelectric nanogenerator is studied The device includes six independent water-solid TENGs that are radially symmetrical alignment The performance of both single TENG and synchronized TENGs are evaluated under various structural, dynamical, and electrical conditions The presented device shows an adequate ability toward harvesting mechanical energy as well as sensing application

● In chapter 5, design, fabrication, and experimental characterization of the impulsive kinetic energy regulator is studied The device can transform the supplied kinetic energy into the relevant (elastic or gravitational) potential energy and vice versa depending on thepreset configuration; thus, it prolongs the running cycle of the device The proposed device offers a cost-effective and simple structure which is potentially useful for extracting the low frequency impulse-excited energy

● In chapter 6, the summary and conclusions of the overall research as well as the final remarks are given In addition, perspectives on future work related to this thesis are presented

2.1 Fundamentals of liquid-solid triboelectrification

Liquid-solid triboelectrification, which occurs at the liquid−solid interface, has been known for a long time, especially in the studies about electrochemistry73, catalysis74, etc Earlier, it was suggested that the ion adsorption was responsible for the contact charging between the liquid and the solid surface because of the involvement of the solution75 However, recent studies, resting on methodological experiments, pointed out that both electron transfer and ion adsorption simultaneously occur in the liquid-solid triboelectrification76 Hence, the basis of liquid-solid triboelectrification was revisited, and based on the inclusion of the electron transfer in liquid-solid triboelectrification, a

“two-step” process is proposed by Wang et al.77, where the electron transfer plays a dominant role in the formation of the electric double layer (EDL)

The EDL model with “two-step” process for the formation, on account of both

electron transfer and ion adsorption, can be seen in Figure 2.1 In the first step, water

molecules and ions in the solution contact with the solid surface, and the overlap of electron cloud between water molecules and solid atoms brings on the electron transfer from the water molecules to the solid material At the same time, some ions are also absorbed on the solid surface due to electrostatic interaction (physical adsorption) and chemical reaction (chemical adsorption) Later, the water molecules are ejected from the interface owing to the agitation of the solution such that most of the transferred electrons are preserved on the solid surface 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 Subsequently, the EDL is formed with two different regions: the Stern layer and the diffuse layer The Stern layer includes all the transferred electrons and adsorbed ions that attached on the solid material, while the diffuse layer contains free ions with a higher

concentration of counterions that decays with distance from the charged surface

There are several factors that affect the liquid-solid triboelectrification, such as the temperature and pH of the solution, solutes used in the solution, hydrophobicity of the solid surface, etc78 For instance, the hydrophilic surfaces, which have higher solid-liquid interfacial energy, are more likely to form covalent bonds with substances in solution, leading to the dominant contribution of ion adsorption In contrast, the hydrophobic surfaces, which hold smaller interfacial energy, are more likely to form intermolecular bonds, and thus, the electron transfer dominates the liquid-solid triboelectrification

(Figure 2.2) From these perspectives, different characteristics have been reported as key

contributors to the improvement of liquid-solid triboelectrification process79 With respect to solid phase, material selection, surface morphology and surface modification

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 (modified from ref.77, Copyright 2021 American Chemical Society)

are vital properties to obtain a higher hydrophobicity and surface charge density The ionic activity, polarity and volatility of the contact liquids are indispensable properties that can influence the density of the charges at the solid-liquid interface

2.2 Mechanism of liquid-solid triboelectric nanogenerator

Similar to solid-solid TENG, the electricity generated by liquid-solid TENG is based on the coupling of triboelectrification and electrostatic induction, in which triboelectrification involves in the formation of EDL, and electrostatic induction creates

a current flow regarding the potential difference developed through contact-separation

process A plain illustration for the liquid-solid TENG is shown in Figure 2.3, where a

and water contain no charges When the PDMS layer impacts the water, an EDL is formed

at the contact interface so that the PDMS becomes negatively charged, and the water becomes positively charged to maintain the charge equilibrium When two layers are separated, two electrodes attached to both PDMS and water rise the opposite charges correspondingly, yielding a potential difference in the external circuit As a consequence, electrons will flow through the external circuit, generating a positive output current The

triboelectric outputs greatly depend on the separation distance (d) between the two

electrodes, which will get maximum when the PDMS layer was taken back to its original

position (d = d3) If the PDMS moves to contact again with the water, the potential

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 (modified from ref.78, Copyright 2020, The Authors)

difference between the two electrodes will decrease, and the electrons will flow in the opposite direction until a full contact is attained In a series of contact and separation, a continuous electric output can be generated

2.3 Structural design of liquid-solid triboelectric nanogenerator

Because liquid materials appear under various shapes and dimensions, there are a vast number of possible designs for liquid-solid TENG; however, it can be classified into two groups: open systems and closed systems The open systems refer to devices in which

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 (modified from ref.80, Copyright 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

the liquid materials come from exterior sources, interacting with solid materials for triboelectrification These systems are susceptible to the change of environment so that their output may be unstable The common structures of open systems include droplet-based TENGs and bulk liquid-based TENGs Whereas, the closed systems refer to devices

in which the liquid materials are sealed inside and make use of incidents from the external environment for triboelectrification This kind of design, which is specified as liquid-filled TENGs, can minimize the environmental affection and has a more stable output

On that account, Liang et al introduced the multi-unit transparent TENG (MT-TENG) with “Top-Down” and “Bottom-Up” structure81 The MT-TENG was fabricated with hydrophobic surface so that it exhibited a self-cleaning character, and can be integrated with vehicle glasses, building glasses for harvesting clean energy from raindrop, as shown

in Figure 2.4a The MT-TENG and its units (T-TENGs) are operated in single-electrode

mode As for structure design, the MT-TENG can amplify the transferred charges in one cycle, thus enhancing its power generation efficiency as compared to T-TENG On another approach, Zhang et al reported a self-cleaning/charging power system (SPS), which compounded of a hydraulic triboelectric nanogenerator (H-TENG) and several embedded fiber supercapacitors (FSCs), for harvesting energy from the falling raindrop82 The H-TENG, with self-cleaning effect derived from the hydrophobic property of the surface layer, effectively converted power from raindrops into electricity, while the FSCs were used to store that generated energy The H-TENG is deployed by sliding free-

standing mode, as displayed in Figure 2.4b A specific effort for harvesting energy from

water droplet was proposed by Yun et al., where the authors developed interdigital electrode (IDE) based TENGs to increase the triboelectric current83 In comparison to

one-electrode and two-electrode design, the IDE-based TENG produced multiple current peaks following the continuous back-and-forth movement of the accumulated charges A cone-shaped IDE-based TENG was demonstrated to harvest energy from scattered water

droplets, as exhibited in Figure 2.4c Here, the output of this device is changed by

adjusting the volume and speed of water droplets through the folding angle To maximize the utility of droplet-based TENG, integration of D-TENGs with to other energy collectors is an appealing direction For example, Liu et al integrated D-TENGs with solar cells to scavenge energy from sunlight and raindrops at the same time84 A hydrophobic and transparent PDMS-based TENG was anchored on the top of a solar cell

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 transparent TENG (MT-TENG) in integrating with vehicle and building (modified from ref.81, Copyright 2016 Elsevier Ltd) (b) Schematic diagram and sliding free-standing mechanism of the self-cleaning/charging power system (SPS) (modified from ref.82, Copyright 2018 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim) (c) Schematic diagram and working mechanism of the cone-shaped interdigital electrode (IDE) based

the integrated system consisting of solar cell and water-drop TENG as well as the mechanism of the TENG (modified from ref.84, Copyright 2018 American Chemical Society)

to compose the hybrid device, as shown in Figure 2.4d This as-fabricated system would

not only reduce the soiling effect but benefit from the combination of high-voltage output level of the TENG and high-current output level of the solar cell

2.3.2 Bulk liquid-based TENGs

Compared to droplet, bulk liquid contains greater energy and is widely distributed

in nature such as water flows (e.g., stream, river, etc.) and water waves (e.g., sea, ocean,

based on sliding freestanding mode85 This water wheel, with 8 superhydrophobic-coated paddles on each side, was driven by low speed flowing river and an AC type signal could

be detected (Figure 2.5a) In addition, the influence of electrode size, rotating speed and

properties of water was also examined under different working conditions Another similar configuration of ww-TENG was proposed by Cheng et al., except that it operated

in single-electrode mode86 (Figure 2.5b) In both designs, a stream of flowing water hits

the surface of ww-TENG, rotating it, and giving rise of triboelectrification In the case of water wave, due to the unpredictable motion of water waves, a lightweight flexible structure, which facilitates the interaction between the installed device and the surrounding contact liquid, is mandatory for a long-term stability Besides, advanced waterproofing and antibiofouling coatings should be considered to shelter devices from

on-site aggressions Figure 2.5c shows an all-in-one liquid–solid electrification-enabled

generator (LSEG) with scalable planar structure87 The “flooding and draining” cycles of water waves causes a repetitive emerging–submerging process of the LSEG, giving rise

to alternating flows of electrons between electrodes Further, by featuring wide electrodes and nanostructured hydrophobic surface, the electric output was substantially boosted A different concept for water wave TENG was introduced by Liu et al., where Ū electrode structure (a mixture of bar electrode and U-shape electrode) performed effectively the power extraction from waves’ inflow and outflow processes88 (Figure 2.5d) The

advantage of this configuration is that it hugely minimizes the screening effect from loosely counter ions in diffuse layer, and thus enhance the outputs The bar electrode and the U-shape electrode can work separately, or synergize with each other as Ū electrode,

to collect water energy Apart from the two most common cases as already mentioned, where the liquid material plays a moving part during the operation of TENG device, there

is a less attentive case of bulk liquid-TENGs that relies on the dynamic of solid material instead, namely liquid-immersed TENG In this system, the solid material, now as moving

part, sinks down then separating from the liquid, and current signal is generated through

this process An example of liquid-immersed TENG is illustrated in Figure 2.5e, in which

Figure 2.5 Different structural designs of bulk liquid-based TENG for harvesting water

energy (a) Schematic diagram and working principle of ww-TENG driven by flowing river (modified from ref.85, Copyright 2019 Elsevier Ltd) (b) Schematic diagram and

2014 American Chemical Society) (c) Schematic diagram and working principle of LSEG for harvesting energy from a variety of water motions (modified from ref.87, Copyright 2014 American Chemical Society) (d) Schematic diagram and working principle of Ū-shape electrode TENG for extracting power from waves’ flood and ebb processes (modified from ref.88, Copyright 2019 Elsevier Ltd) (e) Schematic diagram

American Chemical Society)

21

the Cu wire wrapped by multi-dielectric layer moves up and down in a tube filled with

DI water89 It was observed that the increment of immersion depth as well as the immersion speed were beneficial to improve the electric output It also suggested that a multifunctional TENG array comprising many single-wire TENG could be effective in large-scale energy harvesting

2.3.3 Liquid-filled TENGs

Long-term experience in open environment will bring up vulnerability that significantly degrades the performance of the exposed-liquid-contact-solid TENGs, especially those operating in harsh environments Therefore, liquid-filled TENGs, where liquid material is sealed and moves inside the device, are an interesting alternative to help formulate the liquid-solid triboelectrification Since both friction materials are encased in the packaged device, liquid-filled TENG suffers negligible performance degradation with excellent mechanical durability In addition, because liquid-filled TENG carries a fixed amount of liquid, evaporation-free and contamination-free conditions are essential for

long-lasting operation Figure 2.6a shows a rotational water TENG based on the water

flow of a partially filled rotating cylinder90 During its operation, water continually slid

Figure 2.6 Different structural designs of liquid-filled TENG for harvesting water

energy (a) Schematic diagram and working principle of rotational water TENG (modified from ref.90, Copyright 2016 Elsevier Ltd) (b) Schematic diagram and working

GmbH) (c) Schematic diagram and working principle of “SWING stick” TENG (modified from ref.92, Copyright 2015 Tsinghua University Press and Springer-Verlag Berlin Heidelberg) (d) Schematic diagram and working principle of MSW-TENG (modified from ref.93, Copyright 2022 The Authors)

around the patterned electrodes on the inner surface of the TENG, and, in consequence,

an alternative current was created In addition, the dynamic behavior of water, which correlated with the water volume and the angular velocity, was observed to highly impact

on the electric output of the device Benefiting from the flexibility and easy-to-flow of water, a multi-mode water-tube-based TENG (WT-TENG) was developed with deionized (DI) water put inside small FEP tubes91 (Figure 2.6b) Four main working modes,

including rotation, swing, seesaw and horizontal linear motion, of the WT-TENG are highly desired in many energy harvesting scenarios such as ocean, wind, vibration, and biomechanical motions Besides, it indicated that design parameters (tube diameter, electrode distance, electrode length) are highly related to the electric output of the device Utilizing the tubular system, the “SWING stick” TENG, with the merit of compact size, was suggested to harvest handshaking mechanical energy92 As illustrated in Figure 2.6c,

the superhydrophobic nanostructured aluminum tube was connected to the bare aluminum tube with the two ends bounded by rubber caps, and the formed structure was filled with water The electricity can be generated through the relative motion of water inside, from the coated tube to the bare tube and vice versa Further, it was indicated that the electric output of the device varies with various types of water, where the current was negatively correlated with the concentration of the solution Based on the notion of direct contact between liquid and the conductive material, a mobile stick-type water-based TENG (MSW-TENG) was implemented, in which water inside the MSW-TENG directly contacts the electrode93 A PFA cylinder, which served as both the substrate and triboelectric material, was sealed both ends with two inner electrodes, and two other outer

electrodes were attached on the sides of the device (Figure 2.6d) One inner electrode

and one outer electrode were paired up to form a freestanding type TENG When water contacted inner electrode, the electric output was generated owing to the charge accumulation and separation induced by the charge on the PFA surface

2.4 Applications of liquid-solid TENGs

As mentioned in previous section, TENG applications could be split into four major categories: micro/nano power sources, active self-powered sensors, large-scale blue energy harvesting and high-voltage power sources Among them, several remarkable progresses for liquid-solid TENGs in first three areas will be discussed in this section

2.4.1 Micro/nano power sources

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Since water is abundant in nature under various shapes and different forms of motion, many applications regarding liquid-solid TENG have been developed for collecting those water energy that make them a good candidate as power source94–96 A networked integrated triboelectric nanogenerator (NI-TENG) was introduced by Zhao et

al for harvesting energy from water waves97 The two-dimensional electrode arrays structure helps the NI-TENG to be resistant to either regular or highly random water wave motion and achieve a high and stable electric output This NI-TENG efficiently harvests dynamic energy of random water waves to drive a wireless transmitter with a driving

voltage of 5.8 V and a power consumption of 10 mW (Figure 2.7a) It takes 67 s to charge

a 22 μF capacitor to achieve the driving voltage for the first transmission and takes 53 s

to charge for another transmission This realizes the feasibility of the NI-TENG as a power supply for sensor nodes in a wireless sensing network A grid of water-dielectric single electrode mode triboelectric nanogenerators (WDSE-TENG) was exposed by Jurado et al for harvesting breaking wave impact energy98 As shown in Figure 2.7b, the

Figure 2.7 Liquid-solid TENGs as power sources (a) Structure of networked integrated

TENG (NI-TENG) with arrayed bridge rectifiers and its ability to power a wireless transmitter (modified from ref.97, Copyright 2018 American Chemical Society) (b) Diagram of the grid of WDSE-TENG for harvesting water impact energy with the potential of driving low-power electronic devices (modified from ref.98, Copyright 2020 Elsevier Ltd) (c) Diagram of integrated TENG for raindrop energy harvesting and its demonstration of powering small electronic devices (modified from ref.99, Copyright

2019 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

grid of WDSE-TENG has the potential to drive a wireless transmitter as well as an ultrasonic range sensor The wireless transmitter was active after charging a 47 μF capacitor to a voltage of 3.19 V–9.81 V based on the transmission distance The energy stored in the capacitor and the discharging power of the capacitor were between 193.56

μJ and 2.24 mJ, and 322 μW to 3.73 mW, respectively Besides, a 470 μF capacitor was charged using the grid of energy harvesters to power up the ultrasonic range sensor In this setup, the relevant stored energy and the discharging power were 5.96 mJ and 5.18

mW, respectively This confirmed the potential to power up electronic devices of the grid

of WDSE-TENG, and hence provide a potential energy source for self-powered wireless sensing systems at water-structure interfaces An integrated TENG for harvesting the raindrop energy was successfully developed by Liu et al., which inductively coupled

shows that an umbrella with integrated TENG on the surface directly illuminates 39 LEDs

at simulated raindrop rate of 22 mL∙s−1 Under the same conditions, a 33 μF capacitor was charged for 477 s so that it reached 1.8 V The stored energy in capacitor was applied to continually power a thermometer for approximately 6 s

2.4.2 Active self-powered sensors

In the new era of IoT, we are facing challenge about how to sense the world more precisely and faster Additionally, energy crisis issue requires a more economic measure towards battery independence20,100 Liquid-solid TENG, which demonstrates merits of high sensitivity, low price, easy fabrication, and shape adaptive of liquid, are likely candidates to develop self-powered sensors A water level sensor based on liquid–solid tubular triboelectric (LST-TENG) was investigated by Zhang et al., which is made of multiple copper electrodes uniformly distributed along a PTFE tube101 When water rises/falls inside the tube, the peaks in the derivative of open-circuit voltage with respect

to time are in correspondence with electrode distribution and directly related to the water

level height (Figure 2.8a) A ship dynamic draft was successfully detected using the

LST-TENG with the accuracy of 10 mm, which shows great reliability as a robust and accurate water level sensor Furthermore, Xu et al investigated a wave sensor based on liquid-solid interfacing triboelectric nanogenerator (WS-TENG) for real-time monitoring of the wave around the simulated offshore platform102 These works show great industrial potential in liquid–solid interface monitoring for marine field For real-time monitoring the engine lubricating oil of vehicles, Zhao et al demonstrated a self-powered