K Wolski Directeur De Recherche Mecanique

Ingenierie S Drapier

Genie Des

Maître Procedes F Gruy

Appliquees M Mathematiques

Informatique O Batton-Hubert

Professeur Boissier

Professeur J C Pinoli

Genie Industriel

Nabil Absi

Maitre De

Professeur Ph Lalevée

Mr

Vincent Augusto

Jérémy Pourchez

Dr

Stolarz Jacques

Valdivieso François

DR Jean Paul

Wolski Krzystof

Xie Xiaolan

Yugma Gallian

Spécialités doctorales Responsables : SCIENCES ET GENIE DES MATERIAUX

Keywords: Triboelectric nanogenerators, Charge density, Energy harvesting, Smart devices, Sensors Insulator Metal Insulator-Triboelectric nanogenerator

For small autonomous objects used in IoTs, traditional techniques for harvesting energy have long been concentrated on solar, electromagnetic, and piezoelectric conversion processes. These techniques generally do not attain the energy density required at the level of the application. Recent advances in microelectronics have increased the possibility for a more effective and economically feasible energy harvesting technique that offers wide practicality and flexibility in a limited volume. First proposed by Dr. Zhong Lin Wang in 2012, the development of triboelectric nanogenerators matches these needs and has thus grown rapidly.

In this thesis, development of high-performance triboelectric nanogenerators with challenges of device optimization to increase the electrical power density in a limited volume has been investigated. Systematic materials selection investigation allows to maximize the surface charge generation and significantly increase the charge density of triboelectric nanogenerators and more generally the electrostatic conversion process. Through advanced device optimization techniques, the triboelectric nanogenerators have been engaged in a demonstration of ambient energy harvesting and allows to reach power density levels compatible to drive a fully practical commercial autonomous device. The applications demonstrated in this thesis work, which contain both scientific understanding and technology developments, exhibit triboelectric nanogenerators as a reliable power source not only for small-scale electronics but could also contribute to large scale power generation.

I owe a very important debt to Frederic Depoutot of Thales who in spite of being extraordinarily busy with his duties, took time out to hear and share his invaluable encouragements and advice on both research as well as on my career. Special thanks to Dr. Mohamed Saadaaoui, Dr. Roger Delattre, Thierry Malia, Thierry Camilloni, Paolo, Serge and Nathalie who have given me helping hand to deal with the practicalities of working in a research centre, that have always gone off their way to make me feel integrated and that have been of immense help.

I would like to acknowledge the funding agents and collaborators of the AMPEERS2 project, especially, Beatrice Dubois of Thales, who was instrumental in the project initiation and given me valuable support in the beginning of my thesis. I wish to thank all the co-authors of my publications whose assistance was a milestone in the completion of this project.

I specially thank Dr. Cyril Calmes and Marc Hamand for their stimulating discussions we shared together and enlightening me the first glance of computer aided designing in building the measurement platforms. I would also like to recognize the invaluable assistances from Id-Fab and équipe salle blanche that proved essential to the progress of this thesis work.

A big thanks to Anaïs and Michelle for their immense patience and quick support with all the administrative issues and to Veronique for her useful help in all the human resources-related issues.

I would like to thank all my PhD and Postdoc colleagues, Severine, Mohamed, Simon, Omar, Elias, Bastien, Gerwin and Joe with whom I have shared moments of deep anxiety but also of big solitaire. Thanks, guys, for always being there for me. Also, I thank my interns Alexis and Mathias for assisting me in the setting up of the measurement platforms for device characterization.

Some special words of gratitude to the doctors in Centre Hospitalier du Pays d'Aix and my cycling mates who have been a major source of support when things were completely out of control. A warm word for my colleague and great friends Isabel, Paula and Sara, that always managed to make me feel special and with whom I had one of the best discussions in my life.

Finally, my deep and sincere gratitude to my family for their continuous and unparalleled love, help and support. I am forever indebted to my parents for giving me the opportunities and experiences that have made me who I am.

Special thanks to all the people who along the way believed in me.

List of Figures

Concept of IoTs and smart applications …………………………………….…….1

Figure 2

Four basic operational modes for TENG ………………………………………….3

Introduction

The increasing demand in alternative energy sources for low-power electronics and autonomous devices has given rise to substantial research activities in the field of energy harvesting. Compared to other forms of mechanical energy harvesting technologies, triboelectric nanogenerators (TENGs) have attracted immense interests among researchers as a new energy source, having high efficiency, easy working mechanism and capable of harvesting energy from different sources under different conditions. Yet, there are still some limitations to break, which will be discussed in the further sessions. In this part of the thesis, motivation, bottlenecks, and thesis objectives will be pointed out, followed by the thesis overview.

A. Motivation

Internet of Things (IoT) is now reality. Most objects in daily life come to be "smart"they are connected to the internet via countless tiny chips with their own sensors to "feel" the world, generating exploitable data and creates value for users. Soon, large infrastructures like rail lines, bridges, roads could also be self-monitoring. At home, not just electronics, anything you can imagine from door, garage, table, curtain to air-conditioner may bear some form of wireless sensors, to make life easier (Figure 1). devices, for instance, micro-controllers with reduced power consumption [4][5] as low as 5 µW with high efficiency are as much functional than conventional bulky devices. It would be logistically expensive and environmentally challenging if each sensor nodes are powered by batteries that require frequent maintenance and replacements.

Alternatively, energy harvesting technologies provide unlimited operating lifetime for a more selfsustainable power output. A great deal of energy exists in the environment, in the form of mechanical vibrations such as human walking, water flow, wind, etc. which are getting wasted and can be efficiently converted into electrical energy to drive various practical and functional devices.

Some well-known existing energy harvesting techniques include solar [6], wind [7],

piezoelectric [8], thermoelectric [9], electromagnetic [10] and RF [11].

In recent times, TENGs have been conceived as a promising technology by harvesting ambient mechanical energy for powering such low-power electronics due to its simple, compact and inexpensive devices with a claimed energy conversion efficiency of more than 50% [12][13] [14].

Known since the 19 th century from the Kelvin and Wilmshurst machines, the triboelectricity phenomenon as an energy harvesting technique was first reported by Prof. Wang in 2012 to convert mechanical energy into electrical energy through the coupling of triboelectrification and electrostatic induction processes [15].

For TENGs, the triboelectrification or contact electrification is one of the fundamental principles, which creates static polarized charges. When two different materials come in contact, equal and opposite static charges appear on the material surfaces due to contact electrification. Following electrostatic induction principle, the charges traverse in-between the two electrodes through an external circuit and as the materials are separated, a potential difference is created. Multiple operation modes of TENGs such as vertical contact separation, linear sliding, single-electrode and free-standing mode have been proposed to efficiently convert the kinetic energy of mechanical vibrations into electrical energy as shown in Figure 2. As an universal and ubiquitous effect with abundant choice of materials, light weight, low cost, and high efficiency even at low operation frequency, triboelectric nanogenerators have been shown

to reach power density levels compatible with targeted applications [16][17] [18]. However, one of its major limitations has been its very low output in terms of average power values as the outputs are in the form of short duration pulses. Exploring the full potential of TENGs with new optimization techniques and device structures, containing both scientific understanding and technology maturation, is highly desired for accelerating the miniaturization of IoTs and selfpowered systems and may promote the understanding of TENG from a new perspective.

B. Aims of this work

The aims of this work were:

• to experimentally demonstrate and evaluate the key parameters to maximize the power generation for triboelectric energy harvesting.

• to propose and investigate demonstrators for integration of TENGs within practical applications.

C. Thesis overview

In contrast to the majority of triboelectric nanogenerator studies, this work deals with the full-scale integration of high-performance triboelectric nanogenerators, and this aspect of triboelectric materials has required a comprehensive investigation which led to the technical advances through new device architecture and optimization techniques. In Chapter 3, two original demonstrators for integration to practical application are proposed and tested: a wind actuated venturi based TENG and a sliding TENG based smart card system. All prototypes demonstrated power density levels that are more than sufficient to drive their applications. It was shown that even at low operational frequencies, the wind TENG achieved a 2.8 W.m -2 and the sliding TENG provides 750 mW.m -2 . Thanks to extensive material testing, device dimensions and performance optimizations, we have demonstrated an output power that promotes the use of TENG as a reliable and viable power source for integration into commercial future IoTs and smart applications.

A novel multi-layered architecture is presented in Chapter 4, simultaneously capable of fixing the triboelectric charge storage and accumulating the charges generated at each cycle. With this new technique, a scalable perspective to boost the output of the TENG in a limited volume can be employed for tailored application. A charge accumulation layer with various metal ratio from 5 mm to 250 µm are presented and their electrical performance is discussed. The thesis is concluded by a summary and outlook, in which future prospects of this work are proposed for further improvement.

Chapter 1 Fundamentals and literature review of triboelectric nanogenerators

Abstract

In this chapter, the development of TENGs as an alternative mechanical energy harvesting technique is reviewed with a focus on output power performance. In the first part, basic principles of contact electrification and electrostatic induction are introduced. The limitations in implementation of TENGs in practical device applications guide the reader to the second part of the review, describing different parameters influencing the output power density. The power density is shown to rely both on stored localized triboelectric material charge and device velocity.

This chapter focuses on charge optimization through triboelectrification and charge storage.

Various strategies for enhancing the stored charges and their limitations are surveyed with respect to overall output performance including device geometries and charge density. These device structures show a shift from plain surface structure to more complicated, micro patterned internal space charge regions. The development and optimization of high-performance TENGs with introduction of high charge density strategies will lead to their more promising usage in applications ranging from powering electronic devices to harvesting large-scale energy.

Introduction

Energy harvesting is of great importance for a wide range of applications ranging from sensors to smart devices, thanks to device miniaturization leading to power consumption from milliwatts in run mode to nanowatts in standby mode [1][2][3][4]. Transforming mechanical energy sources into electricity opens a lot of new applications in many remote difficult-to-access locations and also underwater where batteries and conventional power are impractical [5,6]. Ubiquitous energy sources such as solar, wind, heat and vibrations can be converted into useful power. Among all energy sources, universally available mechanical energy has become a promising energy source for powering smart systems and IoTs. Energy harvesting techniques can supply unlimited operating life for low-power applications by working alongside storage devices or even eliminate the need for batteries which cannot meet the sustainable energy requirement due to their limited lifetime, size and maintenance.

Renewables energy like solar panel can be considered as a promising source but its application adaptability operating in non-luminous environments could pose serious drawbacks in addition to production costs, material availability and toxicity [7][8][9]. Alternatively, traditional small scale energy harvesting conversion techniques include piezoelectric, thermoelectric, pyroelectric, electromagnetic, etc. can capture energy for low power applications such as smart sensors, wearables, etc [10][11][12]. The need for new sources of energy, which are not only robust and efficient but also sustainable, remains essential. Triboelectricity enables the design of simple, compact and inexpensive devices to generate electricity by converting the mechanical motions through the combination of triboelectrification and electrostatic induction [13]. The integration of triboelectric nanogenerators in our day-to-day life, presented in Figure 1.1, such as robotics, smart clothing, footwear has offered a huge advantage with better portability and adaptability [14][15][16].

Additionally, it has been showcased in various healthcare and biomedical applications such as drug delivery, pacemakers, health monitoring and nerve stimulation [17]. With TENGs proving to be a reliable energy source along with their advantages, there exists a large potential for wide integration of applications [18,19]. However, various challenges need to be addressed related to the power density which does not always meet the required standards for applications cited previously.

In the first part of this chapter, the fundamental mechanism of triboelectrification is reviewed and various factors influencing the output performance are shown and discussed. The next part describes the development of high-performance TENGs along with its optimization strategies. Soft electronics patterning techniques, utilization of micro/nano surface structure and internal-spacecharge zones are introduced and their uses for practical applications are reviewed and discussed. Improvements in charge density is associated to gain in output power, opening the field of applications. Charge density can be increased by surface modification, material functionalization, internal space regions and self-sustaining charge pump. The major applications of TENG can be broadly categorized for environmental energy harvesting, sensors, IoTs and smart systems.

Operation principle and fundamentals of TENGs

Triboelectrification or contact electrification is able to create charges when two different materials of different charge affinity come in contact with each other. A periodic contact and separation between these two materials with different electron affinities acts as a charge pump to alternately drive induced electrons to flow between the electrodes through an external load [20,21].

Working mechanism

The basic working principle for TENGs is a combination of contact electrification and electrostatic induction. Triboelectrification is driven by the material work function and electrostatic induction rely on charge movement across a load. When two different materials come into contact, a chemical bond is formed between them through surface adhesion and these charges move from one material to the other to equalize their electrochemical / electric potential [22]. Based on the 10 charge separation mechanism, TENGs can be simply divided into two main categories: contactmode based on vertical charge polarization; and sliding-mode based on in-plane charge polarization [23,24].

The electrons transfer between solid and solid is the working mechanism of triboelectrification as shown in Figure 1.2(i) a, forms the basis of all other existing TENG mode mechanisms [25][26][27].

When the two surfaces are brought into contact by applying vertical external force ( The transferred charges can be electrons or ions [28]. When separated, some of the bonded atoms tend to retain extra electrons while some tends to donate them, producing triboelectric charges on the dielectric surfaces. The presence of localized triboelectric charges generates electrostatic forces that induces charges into the electrodes. The amount of mobile charges in the electrodes is, thus, electrostatically coupled to the localized charges. The total electrical output power of TENGs is primarily dictated by the difference in electron affinities of the two materials in friction that limit the charge transfer; and the material selection that determines the charge density which could maximize the overall power density.

In the triboelectrification, a three-stage process may be initiated between the triboelectric layer and the bottom electrode: charge generation on the triboelectric surface, charge flow in the dielectric layer and charge induction on the bottom electrode [29].

The underlying fundamental model for TENG is based on Maxwell's displacement current [30].

The plane capacitor model structure presented in These dielectric layers are affixed to electrodes (Metal 1 and Metal 2) that are insulated from each other. A theoretical equation relating the three key parameters: transferred charges between the electrodes Qsc, the built-up voltage V and the relative displacement x between the triboelectric layers, termed globally as V-Q-x relationship. From the Gauss theorem, the electric field strength at each region is represented as

Inside dielectric 1: E 1 = - Q Sε 0 ε r1 (1)
Inside air medium:

E air = - Q S +σ(t) ε 0 (2)
Inside dielectric 2:

E 2 = - Q Sε 0 ε r2 (3)
12

The total voltage across the structure is the sum of voltage between the two electrodes, given by

V = E 1 d 1 + E 2 d 2 + E air x (4)
Substituting (1), ( 2) and ( 3) in (4), we get,

V = - Q Sε 0 ( d 1 ε r1 + d 2 ε r2 + x(t)) + σx(t) ε 0 (5)
Under open circuit condition, the charge transfer Q=0. The output voltage increases with increasing number of surface charges on the triboelectric layer. Then, the open-circuit voltage (Voc) is given by

V OC = σx(t) ε 0 (6)
where 𝝈, 𝒙(𝒕), and 𝜺0 are the triboelectric charge density, the gap distance between two triboelectric layers, and the vacuum permittivity, respectively. Considering the effective thickness

constant d0 = 𝑑 1 𝜀 𝑟1 + 𝑑 2 𝜀 𝑟2
, Then, during short-circuit condition, V=0. The transferred charges can be represented as

Q SC = Sσx(t) d 0 + x(t) (7)
Then, the short-circuit current (Isc) is expressed as

I SC = dQ SC dt = Sσd 0 v(t) (d 0 + x(t)) 2 (8)
The charge transfer occurs across the electrodes through an external load. With an external mechanical force, there exists a periodic motion between the triboelectric layers disrupting the charge balance, driving the free electrons to flow between the electrodes. From Ohm's law, Eq (5) becomes

V = R • I = R dQ dt = - Q Sε 0 (d 0 + x(t)) + σx(t) ε 0 (9)
Quantitative techniques developed to measure accurately the surface charge density is still under debate so as to how the material surface features such as roughness, composites and dielectric properties would affect the magnitude of charge density and increase the output power density [31].

The surface charge density can be either determined from open-circuit voltage (Voc) or shortcircuit current (ISC) obtained during triboelectrification.

The relation between (VOC), area (S) and surface charge density (σ) depends on intrinsic parameters such as dielectric constant and material thickness (d) [32].

σ = ɛ o ɛ r S V oc d (10)
The periodic contact-separation process driving the electrons back and forth across the electrodes results in an alternating current (AC) signal with the total charge density on the dielectric material (σdielectric) described by the σdielectric = σ+ + σ-, the charge density of top (σ+) and bottom (σ-) surfaces.

The mechanical force (F) applied to separate the top electrode can be expressed as

F = σ. S 2 𝐸 (11)
Where electric field E is given by V/x where V is the output voltage and x is displacement, then,

F = σ. S 2 𝑉 𝑥 (12)
The mechanical energy (W) in the system at a given position x state is given by

W = F. x = σ. S 2 𝑉 (13)
With V= (σ.S)/C= σ.x/ (13) becomes,

W = σ 2 . 𝑆 2 𝑥 ɛ (14)
From ( 14), the instantaneous mechanical power (P) can be expressed as

𝑃 𝑚𝑒𝑐ℎ = 𝑑𝑊 𝑑𝑡 = σ 2 𝑆 2ɛ 𝑑𝑥 𝑑𝑡 = σ 2 𝑆 2ɛ 𝑣 ( 15
)
Where v is the velocity of the electrode. As a consequence, the mechanical power in the device originated by a movement of the electrode depends on two major parameters:

• It is proportional to the velocity of the electrode

• It depends on the square of triboelectric charge density 

In other words, the power arises from an amount of localized charges put in movement by a macroscopic mechanical force. This mechanical power will induce some movement of charges between the electrodes through an external circuit, leading to a current through the charge and thus convert mechanical power into electricity.

In order to evaluate the amount of effective power converted to electricity, it is necessary to introduce the concept of load impedance. To calculate this, we introduce the Norton equivalent circuit of the nanogenerator. This equivalent circuit is made of a current generator whose value is the short circuit current of the generator. In parallel of this generator is connected the equivalent 14 internal impedance of the nanogenerator Zn. In open circuit configuration, the output current is zero, and the voltage is equal to Voc.

𝑍 𝑛 = 𝑉 𝑜𝑐 𝐼 𝑠𝑐 = x(t). (d 0 + x(t)) 2 ε 0 . S. d 0 . v(t) (15)
In the case of contact separation device with a moving electrode, the internal impedance is not a constant as a function of time.

Using Figure 1.4c, we can calculate the effective output current and voltage on the load Zl:

𝑉 𝑜𝑢𝑡 = 𝑍 𝑛 . 𝐼 𝑠𝑐 . 𝑍 𝑙 𝑍 𝑙 + 𝑍 𝑛 (16) 𝐼 𝑜𝑢𝑡 = 𝐼 𝑠𝑐 . 𝑍 𝑛 𝑍 𝑙 + 𝑍 𝑛 (17) The electrical output power can be calculated from the product of output current and voltage, then:

(18)
The output power is thus the product of maximum power Pm and a power factor PF depending on the impedance matching between Zn and Zl. In the case of a perfect impedance matching, PF reaches a maximum of 0.25, and one fourth of the maximum power is transferred on the load.

We can explicit the value of Pm as a function of physical parameters:

𝑃 𝑚 = 𝑉 𝑜𝑐 . 𝐼 𝑠𝑐 = σ 2 𝑆𝑣 ɛ 𝑥(𝑡) 𝑑 0 (1 + 𝑥(𝑡) 𝑑 0 )² (19)
The first term of this formula is simply the mechanical power put in the system, affected by a term depending on the ration of the electrode position to d0. The maximum value of Pm is obtained when x(t)=d0, at this point Pm=Pmech/2.

Considering this condition, we can calculate the optimum load impedance:

𝑍 𝑙𝑜𝑝𝑡 = 𝑉 𝑜𝑐 𝐼 𝑠𝑐 = 4d 0 ² ε 0 . S v (20)
And the maximum electrical output power is:

𝑃 𝑒𝑙𝑒𝑐 𝑜𝑢𝑡 𝑚𝑎𝑥 = σ 2 𝑆𝑣 ɛ. 16 = 𝑃 𝑚𝑒𝑐ℎ 8 (21)
These essential components discussed could act as the basis for effective triboelectrification and development of general power applications of TENGs for practical applications. The first approach is to increase the conversion efficiency by maximizing the charge creation through material selection and its intrinsic properties. The second approach is to transfer as much mechanical energy as possible to a TENG through external factors including environmental operating conditions.

Nevertheless, it is the charge density which forms the fundamental criterion for maximizing the output performance of TENGs. Further studies on maximizing the possible charge density of a TENG will make a significant contribution to the improvement of overall output power.

Parameters influencing charge generation

To achieve high-performance TENGs, several factors must be considered, including the contact area, contact force, contact speed, relative humidity and surface work function of the contacting materials [33][34][35].

Material selection

As said, the charge exchange between the contacting materials depends primarily on their triboelectric series position, electron affinities and work function [36]. The electrons in a material gained or lost varies is given by the triboelectric series. Almost all materials exhibit triboelectrification effect. Figure 1.5 illustrates some commercially available materials chartered according to their triboelectric series position [37]. Higher up the list gives up electron and lower down the list receives electron. To achieve higher output performance, materials further away from each other in the series are chosen due to their ability to attract electrons. Materials with -fluorine atoms such as Polytetrafluoroethylene (PTFE) and Fluorinated Ethylene Propylene (FEP) tends to be more negatively charged owing to their electron affinities [38,39].

Figure 1.5 Schematic illustrating a commonly reported triboelectric series of polymers. Reproduced with permission. [37] Copyright 2019, American Chemical Society.

Work function

The work function is the property of a material's ability to hold onto its free electrons (the electrons orbiting the outer most shell of the material). The greater the materials work function, the less likely it is to give up its free electrons during contact while lower the work function is, the more likely the material will acquire a positive charge by giving up or losing some of their free electrons [40]. Triboelectrification arises when two materials in equilibrium have built in intrinsic potentials due to difference in work function, bringing them in contact will drive the electric currents. Triboelectric effect usually involves more than one of the following mechanisms, namely, electron transfer, ion transfer and contact electrification [41].

Temperature

The temperature-dependence of contact electrification can be explained by the thermal fluctuations, the new driving force for charge transfer and the change in mechanical property of the materials in contact that can affect the contact surface area [42,43]. With increase in temperature, the performance decreases due to the electron storing ability in the material surface.

This can be understood with the reduction in material permittivity as the temperature increases and formation of defects on the surface at increasing temperatures allowing the charges to escape from the shallow traps and surface oxidation.

Humidity

The triboelectrification can be influenced by the humidity since the water can adsorb as a thin layer onto the material surface during the TENG operation resulting in lower device output performance.

This can be explained by the fact that humidity affects the material dielectric constant with increase in conductivity at higher humidity leading to higher surface discharge [44]. The relative humidity (RH) is the ratio of the partial water vapor pressure to the equilibrium vapor pressure of water at any given temperature. Conversely, the pressure takes effect through impacts on the equilibrium of the adsorbed charged species on the surface, and on the thickness of the adsorbed nanoscale water layer [45].

Despite these influencing factors of charge generation, the generated output power is inadequate to power practical applications [START_REF] Yang | Upper limits for output performance of contact-mode triboelectric nanogenerator systems[END_REF]. This results in the development of various surface charge enhancing techniques, by maximizing the surface charge density as much as possible in the contact electrification process.

Charge density

Considering the basic mechanism of TENG, the enhancement of surface charge density is mostly fundamental way to get a high performance device, since both the voltage and current density are proportional to it [START_REF] Xu | A droplet-based electricity generator with high instantaneous power density[END_REF]. The total electrical output of the TENG is primarily dictated by the difference in electron affinities across the electrodes that limits the charge transfer and the material selection that determines σ which could maximize the overall power density compatible with target applications. These are the critical parameters to play with to improve the overall triboelectric performances.

A variety of approaches has been employed in the charge density maximization for development of high-performance TENGs, namely,

• Surface texturization [START_REF] Fang | Surface texturing and dielectric property tuning toward boosting of triboelectric nanogenerator performance[END_REF] • Material functionalization [START_REF] Liu | Enhancement of Triboelectric Charge Density by Chemical Functionalization[END_REF] • External charge injection-polarization [START_REF] Sihong Wang | Maximum surface charge density for triboelectric nanogenerators achieved by ionized-air injection: Methodology and theoretical understanding[END_REF] • Space charge regions [START_REF] Cheng | A self-improving triboelectric nanogenerator with improved charge density and increased charge accumulation speed[END_REF] • Device architecture based on charge pump [START_REF] Liu | Integrated charge excitation triboelectric nanogenerator[END_REF] Only the most effective techniques (surface texturization, space charge regions, charge pump) utilized to maximize the surface charge density on triboelectric materials will be further introduced [START_REF] Shin | Triboelectric charging sequence induced by surface functionalization as a method to fabricate high performance triboelectric generators[END_REF]. b) Injecting negative ions onto the FEP surface from an air-ionization gun. The bottom electrode attached to the FEP film is grounded during this process so that the positive charges can be induced onto the electrode to screen the electrical field from the injected negative charges. Reproduced with permission. [START_REF] Sihong Wang | Maximum surface charge density for triboelectric nanogenerators achieved by ionized-air injection: Methodology and theoretical understanding[END_REF] Copyright 2014, Wiley-VCH.

Surface texturization

Surface effects like micro-nano structures and roughness of contact dielectric materials influence the amount of surface area that is in contact with the other material during triboelectrification.

Higher surface contact area between the materials results in higher charge creation. The surface charge imbalance, which is related to friction between the materials, depends on the surface adhesion on the molecular level as shown in Micro/nano structures are formed on the triboelectric material surfaces to enhance the efficient contact surface area of the TENGs leading to an enhanced output performance [START_REF] Hu | Triboelectric charging behavior of a rough surface sliding on a flat plane[END_REF]. This was first reported by Zhu et al. to achieve an ultrahigh electric output on a simplified structure [START_REF] Zhu | Toward Large-Scale Energy Harvesting by a Nanoparticle-Enhanced Triboelectric Nanogenerator[END_REF]. The contact-separation structure delivers an Voc of ∼1200 V and a maximum Isc of 2 mA with a claimed energy conversion efficiency of 14.9%. The possibility of using materials with high surface roughness enhances the contact surface area and maximizes the output performance of TENG [START_REF] Zhang | Frequency-multiplication highoutput triboelectric nanogenerator for sustainably powering biomedical microsystems[END_REF][START_REF] Chen | Enhanced stretchable graphene-based triboelectric nanogenerator via control of surface nanostructure[END_REF] fluoropolymer [38,[START_REF] Mahmud | Improving the surface charge density of a contactseparation-based triboelectric nanogenerator by modifying the surface morphology[END_REF][START_REF] Tantraviwat | Highly dispersed porous polydimethylsiloxane for boosting power-generating performance of triboelectric nanogenerators[END_REF]. PFDMA has a strong electron affinity due to presence of fluoro atoms and can be patterned directly on a PDMS molds using transfer printing process as shown in Figure 1.8. The PFDMA was used as a negatively charged material for a TENG, and both the electrical voltage and current output were measured for PFDMA with microrods of different heights (flat, 5, 10, 20 μm). The increase in the output performances of the PFDMA-TENG can be explained by the contact surface area between the PFDMA and the Al electrodes. Due to the increased surface roughness of the PFDMA, the effective area between PFDMA and electrode surface increases and produces a higher output. Reproduced with permission. [38] Copyright 2017, Elsevier.

The effective surface contact between the materials offers an opportunity for charge transfer, while the transferred charge density determines the output performance of TENGs. According to previous reports [START_REF] Min | Triboelectric Nanogenerator with Enhanced Performance via an Optimized Low Permittivity Substrate[END_REF][START_REF] Kim | Effect of the relative permittivity of oxides on the performance of triboelectric nanogenerators[END_REF][START_REF] Kwon | Triboelectric contact surface charge modulation and piezoelectric charge inducement using polarized composite thin film for performance enhancement of triboelectric generators[END_REF], the dielectric constant and thickness of the triboelectric material plays an important role in enhancing the charge density. Organic materials such as poly methyl methacrylate (PMMA), polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF) have been widely preferred owing to their advantages such as the excellent electron accepting property, easy processing, flexibility, and biocompatibility, making them potential candidates for wearable applications. Besides these pure materials, functional nanomaterials such as copper woven mesh (NP@Cu), carbon nanotube (CNT) [START_REF] Bui | Nest-inspired nanosponge-Cu woven mesh hybrid for ultrastable and high-power triboelectric nanogenerator[END_REF][START_REF] Fan | Stretchable Porous Carbon Nanotube-Elastomer Hybrid Nanocomposite for Harvesting Mechanical Energy[END_REF], gold (Au) [START_REF] Chun | Mesoporous pores impregnated with Au nanoparticles as effective dielectrics for enhancing triboelectric nanogenerator performance in harsh environments[END_REF] have also been embedded in the polymer matrix to enhance the triboelectrification and surface electrification and to obtain a scalable onestep TENG devices [START_REF] Zhu | A flexible and biocompatible triboelectric nanogenerator with tunable internal resistance for powering wearable devices[END_REF]. Fang and coworkers showed that by combining the surface texturization and nanoparticle dielectric constant and optimizing the nanoparticle weight ratio (Figure 1.9), an output voltage of 195 V, a short-circuit current density of 170mA.m -2 and a charge density of 108 mC.m -2 was achieved, which is 10-fold power improvement compared with flat surface TENGs [START_REF] Cheng | A self-improving triboelectric nanogenerator with improved charge density and increased charge accumulation speed[END_REF]. Although the surface texturization approach of micro/nanopatterning of triboelectric materials can increase the effective contact surface area and enhance the triboelectric performance, to create such patterned surfaces, plasma treatment or a template (silicon wafer with patterned structures prepared by ion etching or lithography) is needed, which increases the cost and complexity of the preparation process.

Localized charge accumulation and storage

Various parameters can alter the output triboelectric performance for realization of self-powered electronic devices. Majority of the efforts have been focused on triboelectric material pair selection and investigation of intrinsic material properties. These efforts, however, are often hindered by long-term utilization and charge retention characteristics for various smart applications [START_REF] Liu | Self-cleaning triboelectric nanogenerator based on TiO2 photocatalysis[END_REF][START_REF] Khan | A self-powered multi-broadcasting wireless sensing system realized with an all-in-one triboelectric nanogenerator[END_REF].

Any dielectric material reaches a certain saturation, over which further increase seems to be nearly impossible. Thus, it would be considered beneficial is there exists a method to both store as well as accumulate triboelectric charges. Recently, there have been few novel attempts to modify the charge dynamics through intermediate space charge regions which can store the excess charges from triboelectrification to achieve very high densities resulting in high-performance TENGs.

Intermediate space charge regions

With the triboelectric charge representing an excess or deficiency of electrons on a material, the charges are assumed to reside on the material surface. The dissipation and the retention are the key factors affecting the electron transfer during the TENG process. Triboelectric materials with better retention properties are chosen with which frictional contact causes accumulation of charges over time [START_REF] Parandeh | An eco-friendly triboelectric hybrid nanogenerators based on graphene oxide incorporated polycaprolactone fibers and cellulose paper[END_REF][START_REF] Ravichandran | Increasing surface charge density by effective charge accumulation layer inclusion for high-performance triboelectric nanogenerators[END_REF]. Another approach is to create intermediate inclusions or internal space charge regions which can store and accumulate the excess charges created by triboelectrification. The possible storage in these regions greatly influences the charge generation giving rise to the formation of internal electric field [START_REF] Shao | Quantifying the power output and structural figure-of-merits of triboelectric nanogenerators in a charging system starting from the Maxwell's displacement current[END_REF]. The triboelectric charges generated on the material surface can be 21 permanently trapped in the intermediate inclusion and cause high charge induction in the bottom electrode with higher charge transfer between the electrodes, thus resulting in an increased charge density with enhancing the output performance [START_REF] Cui | High-Performance Triboelectric Nanogenerator High-Performance Triboelectric Nanogenerator with a Rationally Designed Friction Layer Structure[END_REF][START_REF] Wang | Efficient Triboelectric Nanogenerator (TENG) Output Management for Improving Charge Density and Reducing Charge Loss[END_REF][START_REF] Du | Charge boosting and storage by tailoring rhombus allinorganic perovskite nanoarrays for robust triboelectric nanogenerators[END_REF]. This was first reported by Lai et al.

through embedding crisscrossed gold ravines and gullies within triboelectric materials [START_REF] Lai | Enhancing the Output Charge Density of TENG via Building Longitudinal Paths of Electrostatic Charges in the Contacting Layers[END_REF][START_REF] Xu | Giant Voltage Enhancement via Triboelectric Charge Supplement Channel for Self-Powered Electroadhesion[END_REF] as The oxide layer thickness influences the current through an exponential decay, further confirming the quantum mechanical tunneling process. Contact electrification causes the generated charges to tunnel through the triboelectric layer and become permanently stored in the charge traps. This behavior of dielectric charging where the charges travel through the triboelectric layer can be expressed by Frenkel-Poole relationship for insulating films [START_REF] Yi | The effect of the electric field on the output performance of triboelectric nanogenerators[END_REF][START_REF] Jonscher | Poole-Frenkel conduction in high alternating electric fields[END_REF].

J α V exp ( -q (∅ B -√ qV (π ∈) ) K B T ) ( 22
)
where J is the current density due to Frankel-Poole emissions, V is the generated voltage due to contact electrification, T is the temperature, ϕB is the barrier height, k is Boltzmann's constant and ∈ related to the material permittivity. Furthermore, the resultant electrostatic force is dependent on the charge storage sites in the triboelectric layer and the work function difference between the contacting materials resulting in maximized charge density. The electrons tunneling through the dielectric material can be expressed by Fowler-Nordheim tunneling [START_REF] Oh | Voltage shifts of Fowler-Nordheim tunneling J-V plots in thin gate oxide MOS structures due to trapped charges[END_REF] as

J FN = AE 2 e -B E ⁄ ( 23
)
where E is the electric field across the dielectric, V is the electric potential and t is the material thickness. Under these circumstances, a high electric field and low thickness results in a significant tunnel current [START_REF] Fisher | Tunneling Through Thin Insulating Layers[END_REF][START_REF] Xing | Natural triboelectric nanogenerator based on soles for harvesting lowfrequency walking energy[END_REF][START_REF] Feng | A new protocol toward high output TENG with polyimide as charge storage layer[END_REF].

Considering that 2D materials could effectively trap electrons due to the large specific surface area, Wu et al chose molybdenum disulfide: Polyimide (MoS2: PI) (material as the intermediate layer to intensify the output of TENG [START_REF] Wu | Enhanced Triboelectric Nanogenerators Based on MoS2 Monolayer Nanocomposites Acting as Electron-Acceptor Layers[END_REF]. Apart from charge trap sites, the structural design of the TENGs also makes prominent contributions to the improvement of output performance. In this section, another approach of charge density is presented. To further maximize the charge density, many researchers have been focused on materials selection, surface modification and charge accumulation, which can, to some extent, increase the charge density to few hundreds of μC.m -2 .

Recently, a novel and effective external charge pump technique working in ambient condition succeed to improve the charge density to a maximum value of 1.02 mC.m -2 , presenting a strong improvement in the output power density of TENGs, allowing the development of highperformance TENG system [START_REF] Xu | Ultrahigh charge density realized by charge pumping at ambient conditions for triboelectric nanogenerators[END_REF]. The charge pump uses capacitors as the energy-storage element in which the charges flow back and forth in a closed circuit and transferred via switching between charging and the storage elements.

In general, the flow of current can be controlled by diodes or another TENG device depending on the amount of capacitance needed [START_REF] Wang | Pumping up the charge density of a triboelectric nanogenerator by chargeshuttling[END_REF]. Le and coworkers demonstrated a simple and highly effective technique by replacing the conventional frictional layers with inner plane-parallel capacitor structures (PPCS) to accept larger quantities of charges [START_REF] Liu | Integrated charge excitation triboelectric nanogenerator[END_REF]. The self-improving TENG Xu et al. proposed an easy universal method using floating layer structure and charge pump based on an integrated self-charge-pumping TENG device. This structure adopts a floating layer to accumulate and bind charges for electrostatic induction and pump charges into the floating layer simultaneously [START_REF] Xu | Ultrahigh charge density realized by charge pumping at ambient conditions for triboelectric nanogenerators[END_REF]. Similar to capacitor charging, the TENG is used to increase the overall capacitance which accommodates more charges until the maximum breakdown is reached [START_REF] Cheng | Triboelectric nanogenerator by integrating a cam and a movable frame for ambient mechanical energy harvesting[END_REF][START_REF] Liu | A constant current triboelectric nanogenerator arising from electrostatic breakdown[END_REF]. The influence of surface morphologies is essential to increase the tribo-material contact surface area. These micro/nanopatterned morphologies on the contact surfaces aids in increasing the effective contact area of the friction layers compared to a flat surface, thereby reaching high surface charge density and improving the triboelectric performance.

The large amount of the triboelectric charges distributed on the surface as well as inside the contact materials reaches a certain saturation leading to charge perturbation and reduced retention.

Through novel techniques of charge trapping and device design investigations such as internal space charge zones, charge pump and charge excitations, the charges can either be trapped or stored so as to create a strong driving force and achieve very high output power density.

While powering small scale electronics with simple TENG device structures can be easier today, demanding applications like wireless communication, system on chips, signal processors, etc. are still a challenge. For every application, material selection, TENG operation mode and fabricating techniques will need to be chosen to provide power? compatible with target applications. A universal power enhancement technique for all applications seems unlikely. The reliability, charge retention and long-term stability are the main limitations of TENGs. From a device perspective, cheaper but higher performing TENG structure will need to be engineered to meet the electrical power density restrictions for future portable electronics.

Results summary

Chapter 2 Test procedures and measurement conditions

Context:

In order to be fully exhaustive, this chapter presents the different setups that have been used and mounted during this thesis in order to characterize the performances of the fabricated TENGs. The outcome of triboelectrification process depends on several control variables including the normal material force applied on contact, the frictional speed and the number of cycles. However, the charge generated by triboelectric effect is also related to material properties and environmental conditions such as temperature, humidity, surface roughness and contact geometry. Controlling these conditions is vital to improve the efficiency of energy harvesting. Preliminary tribocharging test was performed for several material combinations and are consistent with their position in the triboelectric series. The several factors which influence the generation of electric charge are discussed in parts 2.1-2.3. Furthermore, the comparison between the charging mechanisms in terms of surface charge generation and potential decay is presented in parts 2.4. Indeed, the surface potential decay may also be an important consideration in the selection of materials for triboelectric energy harvesters. This is why the second part of this chapter will focus on the characterization of several polymers in terms of ability to preserve a high level of charge acquired by triboelectrification process and accumulation through interfacial layers. The quantity of electric charges transferred depends on:

• The nature of the two materials (which also defines their relative sign);

• The contact surfaces.

Triboelectric series

The triboelectric series is a qualitative measure of the materials for efficient triboelectric performances. Tendency of a material to donate or receive electrons depends on its polarity and is given by the triboelectric series.

Contact voltage: Vtribo

In electrostatic, the electric field is an important parameter as it depicts the interaction between the positive and negative charges. A modified Instron traction system (Figure 2.2b) was utilized to understand the influence of electric field on the output performance respective to various materials used for TENG. The setup consists of 2 copper electrodes with an insulator material under test (PTFE in this case), chosen based on their triboelectric series. For a uniform contact between the materials, a Styrofoam placed in-between the copper electrodes and Instron holder. The distance between the active triboelectric material and electrode plays a key role in the charge interaction. Greater the distance, lower the electric field and greater the field lines interaction with the external components leading to higher perturbation and losses in the output. Lower the distance, higher the electric field leading to a higher charge density with enhanced output performance. After multiple trials, it was found that a 7 mm gap spacing produces the best result in terms of charge generation and retention characteristics with an optimum cycling speed of 10 mm.s -1 . To understand the relation between the contact voltage and the distance, the materials are put under test with a certain contact spacing. After the multiple iterations, it was found that the results during contact and separation are repeatable validating the effect of distance plays a key role in the material surface charge creation. In its initial state, contact, the material reaches a consistent contact voltage of 7.5 V. Consequently, when the material is separated, the voltage reaches -9.5 V. The VTribo or the contact voltage is different for different materials.

Test procedures and measurement conditions

Test bench setup

The test bench presented in Figure 2.6 consists of an anti-vibrational table from Thor Labs and a custom-designed mechanical part to facilitate the high-speed movements of the motor. The selfassembled mechanical parts are designed to adapt easily for contact and sliding TENG systems.

For this purpose, a high-speed linear motor (LinMot) test platform is devised to understand the charge transfer mechanism between the materials and efficiently recover the generated energy. An in-house LabVIEW program was developed to drive and communicate the motor with various measurement systems. This facilitates in the graphical programming approach visualizing all aspects of your application, including hardware configuration, measurement data, and debugging.

This visualization facilitates integration with measurement hardware representing complex logic on the diagram, developing data analysis algorithms, and enabling the design of custom engineering user interfaces.

Increasing output performance through interfacial charge layer

Charge accumulation and retention in thin film dielectric layers remain critical issues [14] in realizing TENGs as a sustainable power source. Charge injection mechanism has been studied by corona discharge [15], however little has been investigated on the accumulation phenomena from continuous movement of charges by triboelectricity to increase the charge density from the surface of the dielectric layer. The charges on the dielectric surface directly modulate the output performance. Charge storage in insulating materials still poses great scientific challenges, largely due to the lack of knowledge of the specific mechanisms related to generation, transport and trapping of charges [14]. Considering high density of triboelectric charges was transferred at the contact, parameters such as work functions of the two materials, dielectric thickness and the separation distance between the two materials are very important to achieve high surface charge density.

The present work focuses on increasing surface charge density by addition of a charge layer for triboelectric nanogenerator applications [16]. With a contact-separation TENG structure, a (iii) Long cycle life with very high dielectric strength (6800 V/mil) [21];

(iv) Thin thickness with pin-hole free conformability;

(v) Highly compatible to various friction related applications. [22] Test procedures and measurement conditions

Conclusion

In this chapter, essential information related to test bench setup and material characterization adapted to all modes of TENG was presented. The unique features and procedures employed to Further improvements on increasing the output performance with no or less charge perturbation will be also addressed in the next chapters by introducing floating charge accumulation islands to permanently store charges, thereby increasing the charge density much further. Implementation of these structures in real-world applications would answer many questions regarding compatibility, reliability, and performance.

For further information, supporting information related to this chapter are provided in Annex 1.

Chapter 3 Applications

Compact and high-performance wind actuated venturi triboelectric energy harvester

Context:

In Chapter 3, two original integrated practical application demonstrators for study of device velocity were proposed and tested: a wind actuated venturi based TENG and a sliding TENG based smart card system.

The first approach to TENG integration in practical application that we present in this chapter is a wind actuated venturi based TENG using conventional materials, for environmental wind-energy harvesting. When wind is introduced into the air channel of the venturi system, the thin-flexible polymer flag membrane repeatedly comes in contact with and separates from the side electrodes.

Thanks to extensive material testing, device dimensions and performance optimization, we have demonstrated for the first time, an output power density that promotes the use of TENG as a reliable and viable power source for integration into remote environmental applications.

Abstract

The growing need for alternative sources to power Internet of Things and autonomous devices has led to many energy harvesting solutions from ambient energy sources. Use of batteries requires complementary energy source for extending the lifetime of the device. In recent times, triboelectric nanogenerators have gained significant attention in charging applications through ambient energy harvesting field due to their simplicity, efficiency, and adaptability to many device configurations Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 50 in nature. It is deemed to sustainably address power for autonomous smart applications in various environmental conditions. In this work, a state-of-the-art triboelectric nanogenerator based on wind actuated venturi design system is demonstrated in sync with the smart system evolution for powering various sensor nodal network. Using natural wind, the 3D printed wind actuated venturi triboelectric energy harvester converts ambient mechanical energy into electricity. This simple and compact device produces an optimum average power of 1.5 mW and produces a maximum output power density of 2850 mW.m -2 (peak power output of 4.5 mW), which is much higher than the existing reports that use larger surface area at higher wind velocity. Extensive material testing and future implementation in an array of applications aids for environment friendly energy production and increase the role of triboelectric nanogenerator in autonomous applications.

Introduction

With the rapid development of autonomous devices and sensors for Internet of Things (IoTs) applications, batteries cannot meet the sustainable energy requirement due to their limited lifetime, size and maintenance. Energy harvesting often refers to photovoltaics for such applications.

However, sun illumination is sometimes scattered or restricted to locations. As an alternative, mechanical energy harvesting could be intelligently associated to diversify the energy sources.

Other ambient energies such as wind energy have been considered as an attractive energy source due to their copious, ubiquity, and feasibility in nature [1]. The wind energy turbines based on electromagnetic transduction seem to be a viable solution for our ever-demanding energy problems. Unlike the batteries, they are a clean energy source and thus do not pollute the environment. The need for compact and long-life energy sources can be achieved by the development of energy harvesting techniques that are either used as a primary energy source or work in concurrence with batteries to increase the power generation with less environmental degradation and long-term operation.

Advancement in microelectronics has led to the miniaturization of sensors and devices, for instance, micro-controllers with reduced power consumption [2][3] as low as 5 µW with high efficiency. Triboelectric nanogenerator (TENG) have been conceived as a promising technology by harvesting mechanical energy for powering such low-power electronics [4] with a potential power density of 500 W.m -2 and an energy conversion efficiency of more than 70% [5]. The triboelectric phenomenon as an energy harvesting technique was first reported by Prof. Wang in Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 51 2012 [6]. The TENGs generate electricity converting the mechanical motions through the coupling of triboelectrification and electrostatic induction [7].

With the development of the autonomous sensor systems requiring the availability of compact and efficient power sources, the wind turbines are either expensive, bulky, inefficient or not suited for these microscale applications Environmental factors such as humidity, temperature and pressure can alter the triboelectric material properties and thus the performance of the triboelectric nanogenerator [18][19] [20]. Since the triboelectric nanogenerator is deployed to stay in the environment for ambient energy harvesting for extended period of time, the long-term stability of its material property must be considered suitable for long-term operations and its easy deployment aids for environment friendly energy production for autonomous applications.

Periodic contact-separation cycles between material surfaces with equal and opposite triboelectric charges creates a potential difference across the electrodes leading to alternating electron flow through an external load [21][22]. Previous reports emphasize mostly on the application perspectives [23][24] [25], however more work has to be achieved in order to improve the device output performance [26][27] [28][29] by using application suited materials. Very few reports [30][31] acknowledge the average or RMS (Root Mean Square) output performance which is very important to access the effective available average output power. Although peak power outputs provide maximum harvested power, the key parameter is the RMS power which indicates the average power output at any condition to convert mechanical wind energy to electrical energy.

The present work is focused on the development of a Wind Actuated Triboelectric Nanogenerator (WATENG) based on flutter-driven structure that resolves the existing concerns, namely, the size, reliability and output electric performance. The wind is channelized through a venturi design system inducing periodic vibration of a flag film. To demonstrate the real-world application, the WATENG system was employed to charge a 1 mF super capacitor to 15 V in less than 12 minutes to harvest the ambient wind by mounting the WATENG on the car roof top. With an efficient charging of a storage device, we believe the WATENG is adaptable not only for powering small electronics but can contribute to ubiquitous large-scale energy harvesting.

Design and fabrication of WATENG

The wind flows into the WATENG system with inner dimensions of 2.1 cm (W) x 2.1 cm (H) inducing the periodic vibrations of the flag film in the venturi design structure, which can drive the TENG. as the side electrodes. A PC film flag (6.8 cm x 1.9 cm) with a thickness 125 µm (SABIC, Inc.)

was used as a triboelectric material in the TENG. Since the flag film flutters at various wind speeds, PC is a very good candidate owing to its easy availability, good flexibility and a very high impact strength with a tensile strength (σt) of 55-75 MPa [35]. They are both eco-friendly and has been proven to show excellent stability at temperatures as high as 140 °C [19]. A 100 μm thin PTFE film (Goodfellow Ltd.) was chosen for its high electron affinity and high dielectric strength [36] with dimensions 4.5 cm (L) x 2.1 cm (W) wedged to the adhesive copper film serving as the two electrodes. The venturi structures were 3D printed (Ultimaker 3) for precise structural control and Applications: Compact and high-performance wind actuated venturi TENG Chapter 3

53 enclosed on all sides by microscopic glass slides (7.5 cm x 2.1 cm). To create a uniform wind flow into the venturi inlet channel, custom made wind tunnel was devised. The PC flag film was fixed to the center position using the holders at the mouth of the venturi system. The venturi design creates a depression at the throat that varies the flag fluttering characteristic through the system.

The venturi effect [37] is employed to concentrate the air flow in the outlet section (divergent), which increases the velocity of fluttering motion and a low-pressure region (throat) is created behind outlet and inlet sections with an optimum gap spacing. With increase in wind velocity, the throat pressure decreases, increasing the flag fluttering frequency. The relationship between the velocity and pressure can be expressed from Bernoulli equation [38] as

𝑝1 -𝑝2 = 1 2 𝜌 (𝑣2 2 -𝑣1 2 ) (1)
where p1, p2 are the inlet and outlet pressure, ρ is the density of air and v1, v2 are the wind velocity at the converging and diverging sections. The v1 is the slower wind velocity at the converging section and v2 is the faster wind velocity at the diverging section. The above equation describes that, in the event of a pressure drop, the velocity increases and vice versa. To avoid the undue aerodynamic drag, the venturi system has an entry cone of 30 degrees and an exit cone of 5 degrees [39].

Initially, the PC flag film is at rest in middle position (Figure 3.1e.I), related to its orientation and material stiffness. The film begins to move associated to the wind flow direction thanks to viscous frictions. With increasing velocity, the film starts to collide/oscillate/flutter periodically (Figure

3.1e.II).

Therefore, the kinetic energy from the ambient source is converted into mechanical energy of oscillating motion, which can be converted into electrical energy. The incoming wind speed in the experimental section ranges from 0 to 10 m.s -1 , which includes the global average wind speed of 3.28 m.s -1 [40]. For the measurement of WATENG, the wind was propelled using a commercial air gun connected to a compressed air supply controlled by a pressure regulator with regulated 30% relative humidity

𝑄 𝑒 = 𝐶. 𝑉 (2)
The capacitance Ctotal between the electrodes determines the amount of charge the PC flag can hold. The capacitance depends on the gap distance 2w between the electrodes and the contact surface area of the electrodes A.

𝐶 𝑡𝑜𝑡𝑎𝑙 = ∈ 𝐴 2𝑤 ( 3
)
where ∈ is the vacuum dielectric constant.

When the distance between the electrodes decreases, the capacitance increases, resulting in a lower

As said, the charge transfer mechanism between the contacting materials depends on their triboelectric series position and electron affinities [42]. At first (I), there exists no electric potential difference between the electrodes. As the wind enters the system, the PC flag displaces (II) from its initial position towards one of the electrodes due to the pressure depression created in the venturi system channel. When the flag contacts the PTFE film (III), the negative charges are created on

𝐼𝑠𝑐 = |𝑄 𝑒 | . 𝑣 2𝑤 (5)
The wind velocity v can be expressed as a function of the fluttering frequency f and is given by

𝑓 = 𝑣 𝑤 (6)
A continuous alternating current is produced due to the periodic charge transfer between the top and the bottom electrode material relative to the flag fluttering movement. With increase in flag fluttering frequency, the short circuit current Isc increases due to the faster rate of charge transfer between the electrodes. The relationship between the short-circuit current Isc and the fluttering frequency f can be deduced to

𝐼 𝑠𝑐 = |𝑄 𝑒 | . 𝑓 2 (7)
Therefore, the short circuit current is proportional to fluttering frequency which increases with increase in the wind velocity.

The oscillation produced by the PC flag is non-uniform at high wind speeds. To better understand the WATENGs flag behavior between the electrodes and the flag, a high-speed camera (Phantom Miro M110) at a frame rate of 1600 frames per second at full 1280 x 800 pixels resolution was employed. The camera was modulated by a control system which adjusts the illumination and the angle of vision for synchronized high quality images. The synchronous measurement control system is setup using a trigger controlling both the oscilloscope and the camera to correlate the output electric signals to the flag movements. The fragmentation of images into six stages is proposed as seen in Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 59

Results & discussions

With an increase in the wind velocity, there exists a consequential drop in pressure at the throat resulting in a faster fluttering frequency. With the wind air density forced to pass through the reduced throat gap spacing, the minimum pressure occurs at the throat region between the inlet and outlet sections. Moreover, given an inlet pressure, an increase of pressure difference between the inlet and outlet of venturi tube makes the throat pressure drop further [43].

The gap spacing d is selected based on the fact that higher fluttering frequency leads to a faster charge transfer between the electrodes, thus increasing the output electric signals. validates the output voltage with the various gap spacing tested at 5 m.s -1 . When the gap spacing is 8 mm (wider), higher pressure at the throat leads to slower flag fluttering frequency with decrease in charge transfer while a 0.2 mm (narrow) gap spacing results in a faster fluttering frequency with faster charge transfer. The narrow gap spacing may be advantageous in terms of smaller device volume, higher contact area and increased device robustness but also for better performance of charge transfer efficiency. Gap spacing lower than 2 mm resulted lower output due to stagnation of the flag fluttering owing to high pressure created at the throat section. An optimum gap spacing of 2 mm is chosen for a better triboelectrification and device stability.

To determine the optimum electrical output performances as a function of wind speed, the measurements were performed with speeds varying from 0.5 m.s -1 to 10 m.s -1 . Figure 3.2d shows the voltage output comparison as a function of wind speed, the output voltage drifts from 20 V at 0.5 m.s -1 to 350 V at 10 m.s -1 . The average natural wind does not exceed 10 m.s -1 . The output voltage depends on the contact area and the fluttering frequency, both of which increase with higher wind speeds. It is explained by the fact that increasing the wind speed increases the flag oscillation relating to a faster charge transfer mechanism. The detailed analytical derivation is presented in the Annex 2. Furthermore, higher speed generates higher flux and higher current leading to higher charge transfer and voltage rates. According to the theory of freestanding triboelectric layer based nanogenerator [44], WATENG could be considered as a capacitor with a constant transferred charge through both electrodes. The output voltage is directly the translation of the wind velocity and the flag fluttering frequency which results in a varying output current owing to a faster charge transfer between the electrodes. As the wind speed increases, the flag oscillation becomes irregular and chaotic owing to uneven contact area and output performance resulting in saturation of the charge density.

Applications: Compact and high-performance wind actuated venturi TENG

To demonstrate the WATENGs stability for long term continuous operation to obtain the maximum performance, the output voltage was measured for more than 1000 seconds which corresponds to approximately 10 000 cycles presented in Figure 3.2e. The PC flag transfers the charges between the electrodes at a constant 5 m.s -1 wind speed evidencing no decline is observed after a long-term continuous operation.

To efficiently convert the mechanical wind energy to electrical energy by TENG, the material selection plays a key role for a better triboelectrification process. The material tendency to donate or receive electrons is based on triboelectric series as presented in Figure 3.3a [45]. When two different materials come in contact (pressed or rubbed), an electron transfer occurs between the two materials related to the difference in electronegativity. The material that gain electrons tends to have stronger affinity for negative charge of the two materials leading to a negative charge on the surface after the material separation. The material losing electrons will have the equal and opposite (positive) amount of charges. Higher up the list gives up electron and lower down the list receives electron. Further away two materials are separated apart from each other in the series, the greater the charge transferred. To achieve higher performance, materials further away from each other in the series are chosen due to their different ability to attract electrons.

The selection of two materials from the triboelectric series which are significantly separated from each other is highly important for TENGs, to induce a large charge transfer between the materials.

Measuring the RMS voltage (Vrms) or the effective voltage helps in determining the best performing flag-electrode pair. Electrode materials such as Cu, PTFE -Cu and Indium Tin Oxide (ITO) are tested against flag materials Polyimide (PI), Polyethylene terephthalate (PET) and PC.

For improved output comparison, the PTFE -Cu electrode is tested with PTFE flag material proving its lowest RMS value. Figure 3.3b shows the output RMS voltage of the electrode flag pairs. The Vrms increased from ∼4 V in the device with ITO/PET:PI pair up to ∼100 V in the device with PTFE -Cu:PC electrode flag pair; validating the PTFE -Cu electrode as the best electrode material. Due to presence of fluorine atoms, the PTFE tends to be more negatively charged owing to its faster electron creation and transfer and also due to its position in the triboelectric series, standing as the most negative triboelectric material.

To harvest electricity with a constant wind speed of 5 m.s -1 , three flag materials with high electron affinity, i.e., PET, PI, and PC were systematically studied.

Table 3.1: Comparison of WATENG output performance with previous reports. The peak output power density of the present WATENG and previous reports based on angle-shaped TENG, wind based TENGs and aeroelastic flutter TENG[24][26][32][33][34].

Considering the main advantages of TENG with its size, efficiency and cost effectiveness, we compare the peak power density of the reported single unit WATENG system (Figure 3.4a and

Applications

To demonstrate the performance of our proposed device as a dependable power source, we [33], Wind-based TENG [34][24] and Aeroelastic flutter TENG [32][26].

b) 40 series connected commercial lightemitting diodes (LEDs) at its maximum illumination powered by WATENG system without rectification circuit, respectively in both day and dark light conditions. c) 100 µF capacitor charging performance of WATENG system under different flag film materials with PTFE -Cu electrode and its equivalent circuit (Inset). d) Photographs show the large-scale WATENG mounted on car roof-top to harvest ambient wind with inset showing the 1 mF super capacitor

Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 65 charging up to 15 V. The system consumes less than 12 minutes to rapidly charge a 1 mF super capacitor. e) Measured output performance of device scaling (Large, medium and small) at a constant speed of 5 m.s -1 . f) Comparative charging profile of 100 µF up to 5 V for various device size.

To demonstrate the performance of our proposed TENG device for large-scale energy harvesting in real world condition, we developed a large size WATENG (40 x 12 x 12 cm) system to be mounted on a car rooftop and tested it for a country-side ride with varied driving speeds. This experiment provides a better vision on the possibility of WATENG in various environmental applications. The testing conditions examine various ecological and reliability challenges like varying wind speeds, humidity, pressure and mechanical force. applications in different environmental conditions and to conceive TENGs as a primary stable power source.

Conclusion

In this paper, a wind actuated venturi triboelectric energy harvester with enhanced power output has been developed to harvest ambient mechanical energy into electricity. We have performed a systematic study between the venturi system design and the charge transfer to allow large improvements in electrical performance. The output electrical performance strongly depends on the gap spacing and the incoming wind velocity, resulting in faster charge transfer owing to faster fluttering frequency. With high impact strength and excellent triboelectric property, the 125 µm polycarbonate flag film is suitable candidate for long term continuous reliable operation in complex wind environment without any significant material degradation even after 10 000 oscillation cycles. The WATENG delivers an optimum RMS power of 1.5 mW with a maximum output power density of 2850 mW.m -2 (peak power output of 4.5 mW), which is much higher than the existing reports that use larger contact surface area at higher wind velocity. The generated output power was more than adequate to glow 40 LEDs instantaneously with a very bright illumination and charge a capacitor in less than 20 secs validating the WATENG potential in various low-power autonomous applications. Device sizing not only amplifies the output performance but also addresses to various environmental applications with an efficient charging to a storage device.

For further information, supporting information related to this paper are provided in Annex 2.

Acknowledgements

This research was supported under the AMPEERS-2 project by the theme ''Networks and energy storage''. The authors thank to the facility support of the Institut des Sciences du Mouvement -Equipe Biorobotique, Marseille.

Demonstration of friction-based triboelectric nanogenerator and integration in a power-balanced fully autonomous system

Context:

Next-generation fully autonomous systems exhibits great advantages and better application adaptability. Such devices need a power source, generally batteries that must be replaced frequently, leading to increased maintenance costs. Here, we introduce a first-of-its-kind novel approach that uses TENG technology to develop battery-free electronic system for friction-based applications. This robust, ecological and cost-effective approach for harnessing mechanical motions demonstrated power density levels that are more than sufficient to drive their applications. sleep currents as low as 9 nA with power-efficient wake-up sources and run currents as low as 30 μA with wide operating ranges of 2 V to 3.6 V is required for stable operations to an application [26][27]. Considering the high voltage and low current with high matched impedance [28] of TENGs, a proper device configuration and optimization are necessary to reach the output power characteristics within a minimum operating time.

Herein, we report a sliding based TENG integration for a wide range of application systems which uses an innovative yet frugal approach to achieve an optimal power balance between the power generation and application energy requirement. A multi-layer integrated TENG (ID-TENG) structure is developed converting the human hand mechanical sliding motion into electrical output, generating sufficient power compatible to drive small-scale electronics as presented in Figure 3.5a. A prototype demonstrator is devised integrating the TENG power source to a practical commercial application, promoting the maturity of TENGs in the TRL scale. The multi-layer integrated system based on triboelectric nanogenerators consists primarily of a power generation source and a practical application integration, working autonomously. The proposed system consists of a sliding card and fixed application sections on a 3D printed fixture (Figure 3

TENG power source

The interdigitated structure based on triboelectric nanogenerators as an effective power source were fabricated.

Materials and methods

Fabrication of the TENG sliding electrode section: A rigid acrylonitrile butadiene styrene (ABS) standard card format (85 mm x 53 mm) was chosen for the sliding section substrate. The interdigitated electrodes were formed on a 75 mm × 47.5 mm copper (Cu) -Kapton material through laser ablation to form a combinational electrode pair, which represents the active area of the device. Polycarbonate (PC) material was hot laminated over Cu electrodes to form a smooth and uniform surface. The metal wires were bonded onto the electrode pads to be connected for electrical characterization.

Fabrication of the TENG fixed bottom section: A FR4 substrate material (120 mm x 50 mm) was chosen for the fixed bottom section. The commercial 100 µm polytetrafluoroethylene (PTFE) films (Goodfellow Ltd.) was affixed to the substrate. Using a board plotter (Graphtec cutting pro), PTFE strips were formed, and the excess were detached. The PTFE was chosen for its high electron affinity, high dielectric strength, and its triboelectric series position [29].

Characterization and measurement

The cross-sectional morphology of the ID-TENGs was characterized by a field-emission scanning electron microscope (Carl Zeiss Ultra 55). The robust sliding actuation of the TENG power source were driven by a mechanical linear motor (Linmot, C1100) with the samples attached to the 3D printed support structures. The output electric parameters, i.e. the short-circuit current and the output voltage were measured using a low noise current preamplifier (Model SR570), 10MΩ

impedance LeCroy WaveRunner 6100A oscilloscope and Keithley 2636 source meter, respectively, controlled by a LabVIEW program for real-time acquisition.

Working principle of the TENG power source

The working mechanism of the TENG power source is illustrated in Figure 3.6c based on sliding triboelectric mode by coupling effects of triboelectrification and electrostatic induction [30]. The driving force of triboelectrification is related to the material position in the triboelectric series and their difference in electron affinities [31]. The top section consists of interdigitated electrode pattern pairs while the bottom comprises the PTFE strip section.

𝑄 𝐴 (𝑥) = |𝑄 𝑚𝑖𝑛 -𝑄 𝑎𝑣 | 1 + ( 𝑥 -𝑥 0 𝐷 ) 2 + 𝑄 𝑎𝑣 (1)
where Qav corresponds to the charge induced in the contact without influence of the PTFE strips under a constant velocity v and an electrode position xo while Qmin is the induced charge when the strip is perfectly aligned with the contact width D of PC-PTFE interface. Introducing the time dependence of position at a constant velocity, we deduce the temporal charge dependence,

𝑄 𝐴 (𝑡) = |𝑄 𝑚𝑖𝑛 -𝑄 𝑎𝑣 | 1 + ( 𝑣. 𝑡 -𝑥 0 𝐷 ) 2 + 𝑄 𝑎𝑣 (2)
The current Iout of electrode A is expressed as

I 𝑜𝑢𝑡 (𝑡) = 𝑑𝑄 𝐴 (𝑡) 𝑑𝑡 = -2. |𝑄 𝑚𝑖𝑛 -𝑄 𝑎𝑣 |. 𝑣 𝐷 . ( 𝑣.𝑡-𝑥 0 𝐷 ) [1+( 𝑣.𝑡-𝑥 0 𝐷 ) 2 ] 2 (3)
The curve shape of the simulated current as shown in Figure 3.7d. The amplitude of current oscillation can then be derived to evaluate the performance of the device as a function of the physical parameters. The maximum positive and negative current can be calculated at the time when the derivative of the current is equal to zero. One can show that maximum positive and negative current are obtained when:

( 𝑣. 𝑡 -𝑥 0 𝐷 ) = ± 1 √3 (4) Substituting (4) in (3)
, we can obtain the short-circuit current amplitude

𝑖 𝑜𝑢𝑡 = |𝑄 𝑚𝑖𝑛 -𝑄 𝑎𝑣 |. 𝑣 𝐷 . 3. √3 4 (5)
This formula highlights the relationship between magnitude of the induced current on one finger and physical parameters. Not surprisingly, the current is proportional to the contrast of charge.

This value arises from the electrostatic map of the structure evaluated by finite elements simulation. It is also proportional to the ratio of velocity over contact width. Simulation of the short circuit current as a function of time for the overall structure have been carried out and compared to the measurements.

Results & discussions

The output performance of the TENGs strongly depend on the sliding materials in contact. For high output performance, the selection of two materials from the triboelectric series [31] which are significantly separated from each other is critical for TENGs, to achieve higher amount of triboelectrification. The charge exchange between the sliding materials depends primarily on their triboelectric series position, electron affinities and work function [32][33]. Materials with fluorine atoms such as PTFE tends to be more negatively charged owing to higher electron generation and are chosen as the contact materials for the fixed bottom section.

Applications: Friction-based TENG in a power-balanced fully autonomous system Chapter 3 77

Material sweep

Initially, the study on material selection with 12 pairs of interdigitated electrodes and 1 mm contact width for the top sliding section and a fixed bottom section with 16 PTFE individual strips was performed (Figure 3.6a-c). In order to identify the high performance triboelectric material pair for increased charge generation and withstand the various effects of mechanical sliding motions, 5 different materials (PC, Nylon, Cu, Kapton and Parylene) were methodically tested with a constant sliding speed of 1 m.s -1 and a displacement of 50 mm.

PC film thickness

To investigate the influence of sliding section PC on the output performance of TENGs, various thickness of PC film was hot laminated to obtain a smooth and uniform surface. Considering identical initial surface potentials, when the PC film thickness decreases from 200 µm to 50 μm, the output voltage increases from 30 V to 135 V giving a 5-fold enhancement. The amount of induced charges is correlated to the electrostatic coupling between the interdigitated electrodes and PTFE strips, thus, inversely proportional to the PC film thickness. As shown in Figure 3.8c, with a thickness of 50 µm, the output response achieves a maximum value of 135 V. Conversely, as the thickness of the PC film increases, the output response progressively reduces to a minimum of 30

V. PC being a highly charged positive triboelectric material, the charge transfer occurs only on the interdigitated metal electrodes. As the charge transfer is driven by the electrostatic induction caused by the movement of the frictional slider, a thinner PC film would realize a better driving force for the faster charge transfer mechanism [34].

Also, to be considered is the effect of sliding frequency as the amount of charge transfer per unit time becomes larger as the sliding velocity increases [35,36] as expressed in equation (5). To verify the influence of the sliding velocity on the output electrical response, we systematically studied the relationship between the electric output signals of the TENGs and sliding velocities. The output voltage was measured at different sliding velocities from 0.1 m.s -1 to 5 m.s -1 as shown in Figure 3.8d. Although, the total amount of charges transferred remains constant, the output signals illustrate a linear trend with increase in sliding velocity, this result is consistent with the previously established model (5). A maximum output voltage of 273 V is reached with a sliding velocity of 5 m.s -1 .

Contact force

The contact interface between the PC and PTFE strips will primarily affect the output performance of TENGs due to continuous mechanical sliding movement. The influence of contact forces on the output performance was investigated at different contact forces ranging from 1-10 N shown in force increases from 1 N to 10 N, the output voltage drifts from 8 V to 235 V. This increase in the output signal is mainly attributed to the surface roughness resulting in an increased effective contact surface area between PC and PTFE materials at higher contact forces, producing higher output performance of TENG. More information are available in Annex 3.

Contact spacing

To achieve an optimized device output performance, the influence of contact width between the interdigitated electrodes and PTFE strips should be well understood and adapted. The sliding card section slides over the fixed PTFE strip section with the electrode contact width D and strip width W with electrodes electrically connected alternatively to form an interdigitated pair and as shown in Figure 3.9a.

The electrode contact width plays a key role in the output performance by increasing the contact surface area between the sliding contact materials resulting in higher charge transfer across the electrodes.

Table 3.2. TENGs output performances summary as a function various electrode-strip spacing; electrode contact width (D) and PTFE strips width (W).

The detailed results are summarized in Table 3.2. Compared to wider electrode-strip width (5 mm/ 2 mm) and shorter width (500 µm/ 500 µm), the output increases from 28 V/ 5.7 µA/ 24 nC to 300

V/ 32 µA/ 121 nC demonstrating a 10, 5 and 5-fold enhancement respectively, in terms of output voltage, current and charge.

Stability

The stability test was performed on the TENGs for 40 000 cycles to investigate the mechanical surface wear and tear on the sliding contact triboelectric materials. With a constant sliding speed of 1 m.s -1 , Figure 3.10a shows the continuous generated output voltage of 250 V, evidencing a stable and reliable output performance without any decline even after prolonged operational usage.

Power

For practical autonomous applications, the evaluation of output power of the TENGs depends mainly on the matching with optimum load impedance. The output power of the TENG is methodically experimented using a resistance ladder configuration [37] at different load impedances from 10 MΩ to 376 MΩ with a constant 1 m.s -1 sliding velocity. that, lowering the electrode spacings, in this case, 500 µm, and leads to rapid capacitor charging in less than 15 secs. By increasing the sliding speed of the TENGs, a faster charging rate can be achieved due to higher rate of charge transfer.

Electronic paper screen application integration

The integrated system driving a commercial practical application primarily consists of a TENG power source and a low-power application in the form of a display screen. The electronic paper or electrophoretic displays are employed in a wide array of applications from e-book readers to digital signage boards to mimic appearance of ordinary ink on paper [38]. It is composed of a microencapsulated electrophoretic display formed by rearranging charged particles with an appropriate electric field.

The application based on an ultra-low-power microcontroller (STM8L152C6T6) manages the screen with text "AMPEERS" (Figure 3.11a and Annex 3). The capacitor supplies the power to the microcontroller through a Low-Dropout (LDO) linear voltage regulator. When the input voltage applied on it reaches 4 V, the LDO regulator starts-up and after 800 µs provides a constant voltage of 3.3 V needed by the MCU power supply. For the current supply, the microcontroller needs a 100 µA peak during the power-on (250 ms) and a stable current of 34 µA in low-power run mode. Applications: Friction-based TENG in a power-balanced fully autonomous system Chapter 3 86

Conclusion

In summary, a state-of-the-art multilayer integrated TENG structure in a smart card format for autonomous practical application is demonstrated. The sliding hand motion based TENG power source is integrated to drive an electronic paper display screen application controlled by an efficient power management unit. The integrated system reaches a 5.2 V threshold DC output in less than 25 secs, equivalent to 7-8 human hand sliding motions, adequate to drive the target application. The integrated system is controlled by the ultra-low-power MCU that controls the information displayed on the screen. By reducing the supply voltage, the MCU consumes a 100 µA peak during the power-on (250 ms) and a stable current of 34 µA or close to 1 mW in lowpower run mode. The power balance between the TENG power source and the application is crucial for the system's operation. The key factors of the TENG power source such as material, dimensions and operation conditions are investigated in consideration with the output electrical performances. The proposed theoretical model for output current coincides well with the experimental results and it is possible to utilize this analysis for achieving high output TENG performances. The optimized structure achieves a 17-fold enhancement in performance with a maximum output power density of 750 mW.m -2 (peak power of 2.7 mW and average power of 1.86 mW). We therefore envisage this technical and scientific development as a key progression towards truly efficient energy conversion that should help in large scale triboelectric deployment for environmentally friendly IoT applications.

For further information, supporting information related to this paper are provided in Annex 3.

Chapter 4 Metal islands structure as power booster for high performance triboelectric nanogenerators

Context:

One of the objectives of this work is to increase the charge density as much as possible to increase the overall triboelectric output performance. Charge density is one of the most important parameters of TENGs since it directly determines performance, unfortunately, it is largely restricted by the phenomenon of air breakdown. In this chapter, we present a simple multi-stack structure with improved charge density. Unlike conventional TENGs with surface remained static charges, the device architecture using thin film deposition and micro-patterning technologies, allows to accumulate and store the charges created by triboelectrification. An integrated device for LED lighting is also presented, confirming its feasibility for practical applications. This finding can be applied to all the TENG modes and it provides an important guide to the structural design for nanogenerators.

Abstract

The development of the Internet of Things requires the availability of compact and efficient power sources for the supply of autonomous systems. In recent times, electromechanical transduction devices like triboelectric nanogenerators (TENGs) have gained strong attention as they permit simple, robust, and cost-effective techniques. However, TENGs generally do not achieve the electrical power density required for the targeted applications and the challenges of optimization is therefore to increase their generated power density. In this work, a multilayer flexible composite structure is developed by employing an Insulator-Metal-Insulator island architecture in place of a single insulator material to accumulate and store the charges created by triboelectrification. This increase in the stored charges has a strong effect in inducing free charges on the electrodes resulting Metal islands structure as a power booster for higher performance TENGs Chapter 4 90 in a maximum output power density of 4.8 W.m -2 , which is ~150-fold increase compared to structure without metal inclusions. The results demonstrate a maximum charge density of 1076.56 µC.m -2 , which is to our knowledge, the best charge density value ever reported for TENGs in ambient working conditions. Therefore, this work proposes a new direction to significantly increase the electrical power density in obtaining high-performance triboelectric nanogenerators attractive to future applications for active sensors and portable electronics.

Introduction

With the rapid growth of Internet of Things (IoTs), nomadic and sustainable energy sources are of major importance. In addition to the use of batteries, harvesting energy from ambient environment can be used to power these devices [1,2] Among all the energy sources, universally available mechanical energy has become a promising solution. Traditional mechanical energy harvesting techniques have long been focused on electromagnetic, electrostatic, and piezoelectric conversion processes. [3][4][5] Alternatively, the triboelectric nanogenerators (TENGs) are used as a source of energy due to their simple, compact and inexpensive devices that allow to envisage power density levels that are compatible with the targeted applications. [6,7] Known since the 19 th century, the Kelvin and Wimshurst machines, the triboelectricity phenomenon can be used to convert mechanical energy into electrical energy. [8] The TENGs developed by Wang et al. combine triboelectrification and electrostatic induction to convert mechanical energy into electrostatic energy by a periodic mechanical movement, or a frictional movement. [9] While TENGs have been demonstrated as a practical power source for some autonomous applications, they do not yet achieve the power generation level required for the targeted applications like IoTs and sensor modules. [10,11] Challenges for device optimization is therefore to increase the electrical power density for self-powered device in a limited volume. According to previous works, enhancing output performance can be achieved by material modification, increase of contact area and surface functionalization. [12][13][14] In contact-separation mode TENGs, the triboelectrification occurs at the materials interface giving rise to a localized charge (Q) which reaches a saturation level (QSAT) after several periodic contact-separation cycles. This QSAT induces free charges periodically in the electrodes allowing the transformation of kinetic energy into electrical current. It can be shown that the energy produced during each cycle is of the form [15]: [16] , multi dielectric layer [17][18] and plane parallel capacitor [19] . b-c) Digital photograph and schematic diagram of the IMI composite film. Scale bar, 1 cm. d) Working principle of the IMI-TENG during one cycle under external force. E1 is the electric field between the top electrode and PaC while E2 is the electric field which drives the charges to conduct through the insulator later to be stored in the metal inclusions.

The metal inclusions within the insulating composite allows: (i) the triboelectric charge storage in the metal whose density of states is several orders of magnitude greater than the bare insulator and

(ii) to accumulate the charges generated at each cycle. This increase in stored charges promotes higher triboelectrification on each contact which will then induce free charges in the electrodes, thus, resulting in a maximum charge density of 1076.56 µC.m -2 , the highest charge density, to our knowledge, reported for TENGs in ambient conditions with a maximum output power density of 4.8 W.m -2 .

Metal islands structure as a power booster for higher performance TENGs

Working principle

In a conventional contact-separation TENG process, the charge builds up with cyclic contactseparation reaching a certain saturation QSAT. Herein, a method of increasing the triboelectric charge density by introducing an intermediate metal island architecture instead of a single material is proposed, and its working principle is schematically illustrated in Figure 4.1d.

In this scheme, initially the device is in a neutral and short-circuit configuration without any external force (I). When a vertical force is applied on the top electrode, it contacts the IMI composite film, creating equal and opposite charges at the interface between Cu and PaC with a coupling electric field E1 (II). During separation (III), the distance between top electrode and PaC increases leading to a decrease in E1 and increase in E2. This results in a positive driving force for the electrons towards the bottom electrode. If this electric field exceeds the onset of charge conduction through the insulator, the charges created at the insulator surface will migrate through this insulator to the metal inclusion where it will be stored permanently. The electric field can be expressed as

𝐸 1 = 𝑄 𝑆𝐴𝑇 ɛ 𝑃𝑇𝐹𝐸 . 𝑆 -𝐸 2 ɛ 𝑃𝑇𝐹𝐸 ɛ 𝑎𝑖𝑟 (2)
𝐸 2 = 𝐶 𝑒𝑞 𝐶 1 . ɛ 𝑎𝑖𝑟 . 𝑆 𝑄 𝑆𝐴𝑇 (1 + 𝑑 2 𝑑 𝑡 ) (3)
with ɛPTFE, ɛair being the permittivity of PTFE and air, respectively while d2 is the distance between the PaC layer and Cu top electrode and dt is the threshold distance. The detailed analytical derivation of the formula is presented in Annex 4. The charge conduction through the insulator can exist due to various conduction mechanisms under the influence of high electric field and reducing its thickness will increase the probability of charge conduction. [22][23][24] By reducing the PaC layer thickness, the vertical electric field E2 is enhanced which in turn impacts the charge conduction through the insulator and increases the proximity to the metal leading to a stronger charge accumulation mechanism with higher charge density. Under these circumstances, a high electric field and low thickness results in a significant conduction current through the insulator.

Further investigation will allow to identify the exact mechanism of the current conduction through the insulator.

At each contact, Q is regenerated by triboelectrification and an additional flow of charges are stored in the metal. This charge conduction process through PaC continues until the electric potential is high enough to stop further accumulation of charges, reaching a saturation.

Results and discussions

To evaluate the IMI-TENG performance, a linear motor was deployed with a 7 mm cyclic The lateral design of the metal inclusion has a significant effect on the charge storage efficiency.

A novel metal island architecture is proposed which allows for an increase in charge localization due to the confinement of permanently stored charges in the metal islands. On one hand, the density must be high enough to allow the storage of high concentration charges migrating through the insulator by increasing the electric field density, on the other hand, it should suppress any form of charge leakages. At the time scale of experiment, we do not observe any effect of leakage. By effective field engineering of the intermediate metal inclusion, the charge storage characteristics can be significantly enhanced resulting in further increase in the output performance and charge density of IMI-TENGs. For practical autonomous applications, the evaluation of output power of the IMI-TENG depends mainly on the matching with optimum load impedance. A wide-range resistance ladder method has been engaged with external load varied from 10 MΩ to 376 MΩ. [25] As illustrated in Figure Further investigations are underway to fully understand the material properties and to accurately identify the charge conduction mechanism through the insulator. Therefore, this work proposes a new direction to significantly increase the electrical power density in obtaining high-performance triboelectric nanogenerators attractive to future smart applications for active sensors and portable electronics.

Supporting information related to this paper are provided in Annex 4.

Metal islands structure as a power booster for higher performance TENGs

Acknowledgements

This research was supported under the AMPEERS-2 project by the theme "Networks and energy storage". The authors thank the technical support of ID-FAB for their suggestions on the device characterization setup.

Conclusion and outlook

The research conducted in this thesis is concentrated on the development of autonomous devices based on triboelectric nanogenerators for integration to practical applications. In contrast to the majority of triboelectric based energy harvesting studies, this work dealt with the full-scale integration of high-performance triboelectric nanogenerators, containing both scientific Finally, to study the effect of charge density on the device performance, a unique structure to boost charge density in a limited surface area through standard microfabrication process is realized. This approach offers various advantages including straight-forward, large-scale production, easy fabrication, etc. Moreover, this could be adapted to any device architecture, permitting to boost the output power density compatible with various IoTs and smart devices.

Here, we review the conclusions, and make a perspective of the main sections discussed in this work. The results and methods explored in this manuscript show large room for improvement in device fabrication and characterization. The outlook discussed here shall be used as a starting point for future works.

Chapter 1 reviews the basic principle of TENGs, including fundamentals, work mechanism, structural designs and performance enhancement techniques. Benefits and limitations of each of the enhancement techniques are discussed, as well as the challenges to increase the electrical power density in a limited volume. Challenges are to be solved in surface morphology, materials and charge generation with the perspective of harvesting energy from different sources under different conditions in an efficient manner.

In Chapter 2, a detailed overview of the measurement setup and parametric selection of triboelectric materials was presented. Relying on the fact that, when two different materials come into contact, triboelectric charges are created on its surface. These speculations have been experimentally verified methodically through a robust test platform devised to understand the charge transfer mechanism between the materials and efficiently recover the generated energy.

The results show that based on the triboelectric series, materials further away from each other in the series are chosen due to their different ability to attract electrons.

In terms of practical applications, two original demonstrators were proposed and tested in Chapter 3. At first, we presented a wind actuated Venturi based triboelectric generator with enhanced power output to harvest ambient energy into electricity. Following extensive material testing, device sizing and wing flag behavior, we demonstrated for the first time, an output power that can adapt not only for powering small electronics but also contribute to ubiquitous energy harvesting.

Furthermore, we integrated a sliding based TENG to a practical smart card system application for maturation of TENGs from research to development phase in the technology readiness level. With a maximum output power density of 750 mW.m -2 , the interdigitated TENG structure converts the human hand mechanical sliding motion to electrical energy to drive a power-balanced fully autonomous system. This work, one of its kind, promotes the use of TENG as a reliable and viable power source for integration into commercial future IoTs and smart applications.

Chapter 4 introduces an insulating metal island (IMI) architecture capable of simultaneously localizing the storage and accumulation of triboelectric charges generated at each cycle. This increase in the stored charges has a strong effect in inducing free charges on the electrodes resulting in a maximum output power density of 4.8 W.m -2 , which is ~150-fold increase compared to structure without metal inclusions. The charge density performance of the IMI structure with 20% metal ratio is 2.1 times higher than the state-of-the-art structures involving complex material functionalization and larger device size. This work presents a new scalable perspective to boost the output of the TENG in a limited volume, thus delivering a sustainable and pervasive energy solution.

Outlook

The possibility of enhancement of charge density through metal island architecture was demonstrated in this work, for the first time. Compared to the idea of complex material functionalization or hybridizing TENGs with other energy sources, our approach provides much more flexibility in use of conventional microfabrication processes. As an interesting perspective, the investigation of physical phenomenon of the charge accumulation through the insulator along with the choice of soft triboelectric polymers (elastomers like PDMS, PU, etc.) would be advantageous to further boost the charge density. This approach is currently investigated in the frame of a new PhD thesis.

Another aspect that was not addressed in this thesis is the power management system and effective energy storage for powering traditional electronics. Switched capacitance circuit (widely used for piezoelectric devices) would greatly benefit in an efficient energy transfer and conversion using capacitors. With TENGs producing pulsed output high voltage and low current as shown in this work, this circuit approach will reduce the charge leakage across the system resulting in an efficient power conversion, as close as possible to the source.

We have discussed the possibility of ambient energy harvesting (Chapter 3), another promising prospective is its integration to super capacitor which will ensure low-cost without massive volume. Additionally, the novel metal architecture presented in Chapter 4 can be combined to these structures to reach high power density levels with an efficient power management system rendering a full-fledged application system.

List of annexes Annex 1: Materials characterization

Using test bench setups described in Chapter 3, various materials were tested to understand their electrical output performance relative to their triboelectric series position. The setup as shown in 106

From numerous material testing trials, it was observed that PC-PTFE material pair produced the maximum output performance compared to other material combinations. The triboelectric charge creation on the materials is effectively utilized when the applied contact force is high enough to harvest electrical energy. When the contact force is low, only a partial region of the surface is brought into contact, so the amount of triboelectric charges is small. Furthermore, and explained in terms of kinetic energy, when the mechanical energy from motion is applied to a TENG, one part of a TENG contacts another part with a kinetic energy corresponding to the velocity, the

The capacitance Ctotal between the electrodes depends with a gap distance 2w and a contact surface area of the electrodes A.

𝐶 𝑡𝑜𝑡𝑎𝑙 = ∈ 𝐴 2𝑤 (2)
In open circuit conditions, the electrodes are not shorted resulting in an electrode potential between the electrodes known as the open circuit voltage (Voc).

The total output voltage Vtotal across the electrodes with a fixed capacitance (Ctop and Cbottom) and a displacement x is given by

𝑉 𝑡𝑜𝑡𝑎𝑙 = 𝑉 𝑡𝑜𝑝 + 𝑉 𝑏𝑜𝑡𝑡𝑜𝑚 (5)
At each half cycles when flag position x = +w and x = -w, we get from (8),

𝑉 𝑡𝑜𝑡𝑎𝑙 = 𝑄 𝑡𝑜𝑝 2𝑤 ∈ 𝐴 - |𝑄 𝑒 |(𝑤 + 𝑥) ∈ 𝐴 (9)
𝑉 𝑡𝑜𝑡𝑎𝑙 = 𝑄 𝑡𝑜𝑝 2𝑤 ∈ 𝐴 (10) For a full cycle, from ( 9) and (10), the amplitude of voltage Vpeak-to-peak is deduced to

𝑉 𝑝𝑒𝑎𝑘-𝑡𝑜-𝑝𝑒𝑎𝑘 = |𝑄 𝑒 | ∈ 𝐴 . 2𝑤 (11)
When the electrodes are shorted (Fig.

The wind velocity v is the rate of change of displacement, v=dx/dt. The flag displacement between the two electrodes is arbitrated by the fluttering frequency which inherently depends on the input wind velocity. Therefore, the frequency f as a function of wind velocity and gap distance can be expressed as

𝑓 = 𝑣 𝑤 (16)
The rate of transfer of charges primarily depends on the wind velocity and the output short current Isc and is given by

𝐼 𝑠𝑐 = |𝑄 𝑒 | . 𝑓 2 (17)

Measurement of open-circuit voltage (Voc) using Resistance ladder configuration

In order to measure the power harvested from the wind actuated venturi triboelectric energy harvester, we device a resistance ladder configuration. A resistor ladder nR is an electrical circuit made from repeating units of resistors.

The voltage VTENG across the nR and the Vosc is given by

𝑉 𝑇𝐸𝑁𝐺 = 𝑉 𝑂𝑆𝐶 (1 + 𝑛𝑅 𝑅 𝑂𝑆𝐶 ) (19)
The average voltage VRMS of a periodic waveform (sine wave or triangular waveform) is given as the average instantaneous values along time axis with a period of wave τ. The RMS value of any time varying waveform is defined as

𝑉 𝑅𝑀𝑆 = √ 1 τ ∫ 𝑉 2 (𝑡). 𝑑𝑡 τ 0 (20)
The average output voltage with a maximum voltage Vmax of a sinusoidal waveform over one fullcycle is given by

𝑉 𝑅𝑀𝑆 = 𝑉 𝑚𝑎𝑥 √2 (21)
The output waveform of the WATENG resembles a triangular output signal. Consider a full cycle triangular voltage waveform that is symmetrical about V=0 and a period of wave τ. Let the maximum voltage be +Vmax and minimum voltage -Vmax. Integrating from (0 -τ/4) to calculate the RMS output.

The average output voltage VRMS from ( 23) can be deduced as

𝑉 𝑅𝑀𝑆 = 𝑉 𝑚𝑎𝑥 √3 (24)
From ( 19) and ( 24 For thin PTFE films, a pure PTFE aqueous fluoro plastic resin (Teflon DISP40, DuPont) was deposited through spin coating technique on FR4 substrate to form the fixed section. This was followed by a methodical drying process at 120°C and finally heated above the crystalline melting point of the resin (approximately 350°C) by rapid thermal annealing (RTA) process. Similarly, for thick PTFE films (Goodfellow Inc.), the 100 µm thickness films were adhered using an adhesive spray coating to tightly stick on to the substrate. For sliding section, the interdigitated patterns were formed on the Kapton-Cu flexible films by laser patterning (LPKF Protolaser S) to remove the excess Cu. These were then sandwiched in between a rigid ABS and thin film PC (SABIC Inc.) for hot laminations. The total short-circuit current was simulated from the derivative of induced charge (see formula (1) in main text. The total induced charge was obtained by summing the contributions of a strip to the induced charge in each electrode and finally multiplying by the number of strips. The electric energy produced by ID-TENG was used to power a practical application using an efficient power conditioning circuit. The key component of the power management circuit consists of a rectifier, capacitors, Low dropout regulator and a microcontroller.

Rectifier: Ultra-low leakage diodes (BAS40-7-f) are connected to form a full wave rectifier that convert TENG AC voltage into pulsating DC voltage for use in low power applications. These diodes exhibit low forward voltage drop and fast switching.

Capacitor C: The smoothing capacitor converts the full-wave rippled output of the rectifier into a smoother DC output voltage. We have chosen a 220 µF for charging in less than 25 secs, equivalent to 7-8 human hand sliding motions, reaching a threshold of 5.2 V DC output. Note that, this capacitor acts as a voltage stabilizer and an energy buffer between the TENGs and the application.

Low dropout (LDO) regulator: A low-dropout regulator (MCP1811AT-033) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. A LDO typically consists of a reference voltage, a feedback amplifier and a series pass transistor (BJT or FET), whose voltage drop is controlled by the amplifier to maintain the output at the required value. For example, the load current decreases, causing the output to rise incrementally, the error voltage will increase, the amplifier output will rise, the voltage across the pass transistor will increase and the output will return to its original value. [1] Microcontroller: An ultra-low-power microcontroller (STM8L152C6T6) module was used for application interfacing. It operates at 1.8 V to 3.6 V with consumption of less than 1 µW under sleep mode and close to 1 mW during run mode.

Table A3.1. Output performance comparison of the reported with the existing literature reports[2][3][4]

121 propylmethacrylate present in the chamber as an adhesion promoter, finally to form the IMI composite film.

Under short-circuit condition (with an impedance R), Vtotal=0, then,

𝑉 1 = 𝑉 2 (2)
𝑉 1 = 𝑄 1 𝐶 1 (3)
In short-circuit condition,

𝑉 1 = 𝑉 2 (10)
Using ( 4) and ( 10) in (11),

𝑄
E1 is high when the materials are in contact and low when the materials are separated, similarly, E2 is low when in contact and high when materials are separated. We have plotted both the electric fields E1 and E2 in Figure A4.4. Primarily, the electric field E2 is the driving force for the charges to conduct through the insulator which will later be stored in the metal 125

  de récuperation d'énergie basés sur des nanogénérateurs triboélectriques pour des autonomes IoT et des systèmes intelligents Spécialité : Microélectrique Mots clefs : Nanogénérateurs triboélectriques, Densité de charge, Récupération d'énergie, Dispositifs intelligents, Capteur Résumé Les techniques traditionnelles de récupération de l'énergie se sont longtemps concentrées sur les processus de conversion solaire, électromagnétique et piézoélectrique. Ces techniques n'atteignent généralement pas la densité d'énergie ou la praticité requise pour des applications de l'internet des objets. Les récents progrès de la microélectronique ont permis la mise au point de techniques de collecte d'énergie plus efficace et économiquement réalisable qui offre une grande praticabilité et une grande flexibilité dans un volume limité. Proposé pour la première fois par le Dr Zhong Lin Wang en 2012, le développement des nanogénérateurs triboélectriques a connu une croissance rapide. Dans cette thèse, le développement de nanogénérateurs triboélectriques à hautes performances avec des défis d'optimisation des dispositifs pour augmenter la densité de puissance électrique dans un volume limité a été étudié. L'étude systématique de la sélection des matériaux permet de maximiser la génération de charges de surface et d'augmenter considérablement la densité d'énergie des nanogénérateurs triboélectrique; et plus généralement le processus de conversion électrostatique. Après optimisation, les nanogénérateurs triboélectriques ont été utilisés dans la démonstration de la récupération de l'énergie ambiante et ont atteint des niveaux de densité de puissance compatibles pour une application commerciale autonome. Les dispositifs développés Resumé dans ce travail de thèse, qui contient à la fois des approfondissements des connaissances scientifiques et des développements technologiques, permettent de placer les nanogénérateurs triboélectriques comme une source d'énergie fiable pour l'intégration d'un large éventail d'applications non seulement d'électronique à petite échelle mais potentiellement pour la production d'énergie à grande échelle. My supervisor Dr. Marc Ramuz has been a great resource throughout this research. He has steered me in the right direction and give me a great support throughout the work process. I could not have imagined having better mentors for my Ph.D. study. Besides my mentors, I would like to thank the rest of my thesis committee: Prof. Steven Beeby, Prof. Zhong Lin Wang, Prof. Edith Kussener and Dr. Ghislain Despesse for accepting being the reviewers and examiners of the thesis.

Narain RAVICHANDRAN Chapter 3 3 . 1

 331 Applications .......................................................................................................... Compact and high-performance wind actuated venturi triboelectric energy harvester ...................................................................................................................................... 3.1.1 Abstract .......................................................................................................................................... 3.1.2 Introduction .................................................................................................................................... 3.1.3 Design and fabrication of WATENG ............................................................................................. 3.1.4 Wind flag behavior ........................................................................................................................ 3.1.5 Results & discussions ....................................................................................................................

3. 2 Chapter 4 Annex 1 :Annex 2 :Annex 3 :Annex 4 :

 241234 Demonstration of friction-based triboelectric nanogenerator and integration in a power-balanced fully autonomous system ................................................................................3.2.1 Abstract .......................................................................................................................................... 3.2.2 Introduction .................................................................................................................................... 3.2.3 Materials and methods ................................................................................................................... 3.2.4 Working principle of the TENG power source .............................................................................. 3.2.5 Results & discussions .................................................................................................................... 3.2.6 Electronic paper screen application integration ............................................................................. 3.2.7 Conclusion ..................................................................................................................................... 3.2.8 Acknowledgements ........................................................................................................................ References ............................................................................................................................................... Metal islands structure as power booster for high performance triboelectric nanogenerators ............................................................................................................................ 4.1 Abstract ............................................................................................................................................. 4.2 Introduction ....................................................................................................................................... 4.3 Materials and methods ...................................................................................................................... 4.4 Working principle ............................................................................................................................. 4.5 Results and discussions ..................................................................................................................... 4.6 Conclusion ........................................................................................................................................ 4.7 Acknowledgements ........................................................................................................................... References ............................................................................................................................................... Conclusion and outlook ............................................................................................................ Table of Contents Scientific contributions ............................................................................................................. List of annexes ........................................................................................................................... Material characterization ............................................................................................. Compact high-performance wind actuated venturi triboelectric energy harvester ..... Demonstration of friction-based triboelectric nanogenerator and integration in a powerbalanced fully autonomous system ............................................................................................. Metal islands structure as power booster for high-performance triboelectric nanogenerators ............................................................................................................................

Figure 1 . 1 9 Figure 1 . 2 10 Figure 1 . 3 11 Figure 1 . 4 14 Figure 1 . 5 16 Figure 1 . 6 18 Figure 1 . 7 18 Figure 1 . 8 19 Figure 1 . 9 20 Figure 1 . 10 21 Figure 1 . 23 Figure 1 . 12 25 Figure 1 . 13 25 Figure 2 . 1 Figure 2 . 2 Figure 2 . 3

 11912101311141415161618171818191920110211231122511325212223 Figure 1.1 Research progress in the developmental strategies to enhance output performance of TENGs. …………………………………………….…………………………..9 Figure 1.2 Fundamental of TENGs………………………………………………...………...10 Figure 1.3 Schematic of the contact electrification ………………………………...…….….11 Figure 1.4 Norton equivalent circuit configurations ………………………………...…....... 14 Figure 1.5 Schematic illustrating triboelectric series of polymers …………………………..16 Figure 1.6 Enhancement of surface charge density through material functionalization and external charge injection and polarization………………………………............. 18 Figure 1.7 Effect of surface roughness at different scales…….………..……….…...............18 Figure 1.8 Schematic of fabricated PFDMA-TENG, SEM image of transferred PFDMA with surface-relief on the contact layer and contact angle of the transferred PFDMA surface ...……………………………..……………………..……….…...............19 Figure 1.9 Illustration of the contact-separation mode TENG with P(VDF-TrFE) and PDMShigh dielectric (PDMS-HD) particle composite film.………………..…………...20 Figure 1.10 Illustrations of electrons drift in G-TENG with the schematic diagram of electrons escape from PDMS to the gold.…………………………..……………………....21 Figure 1.11 TENG with a composite PI: MoS2 intermediate layer.………………..………....23 Figure 1.12 Schematic diagram and optical image of the self-improving triboelectric nanogenerator (SI-TENG) with its output performance …..…………….…….... 25 Figure 1.13 Device structure and output performance of the floating layer structure with a pump TENG.………………..…………………………..……………………………....25 Figure 2.1 Materials characterization based on triboelectric series……….…………………36 Figure 2.2 Influence of electric field ……………………………………………………...…38 Figure 2.3 Charge decay in contact-separation process ……………………………………..39

Figure 2 . 4 40 Figure 2 . 5

 244025 Figure 2.4 Contact voltage as a function of separation distance …………………………….40 Figure 2.5 Contact voltage of commonly used materials…………………………………….40

Figure 2 . 6 Figure 2 . 7 Figure 2 . 8 Figure 2 . 9 Figure 2 . 10 45 Figure 2 . 11 46 Figure 3 . 1 54 Figure 3 . 2 Figure 3 . 3 61 Figure 3 . 4 64 Figure 3 . 5 Figure 3 . 6 74 Figure 3 . 7 75 Figure 3 . 8 78 Figure 3 . 9 Figure 3 . 10 83 Figure 3 . 11

 26272829210452114631543233613464353674377538783931083311 Figure 2.6Test bench setup with its associated measurement system …………………….…41 Figure 2.7 Linear motor setup platform ………………………………………………..……42

Figure 4 . 1 92 Figure 4 . 2 95 Figure 4 . 3 97 Figure A1. 1 Figure A1. 2 Figure A1. 3 Figure A2. 1 Figure A2. 2 Figure A2. 3 Figure A2. 4 Figure A3. 1 Figure A3. 2 Figure A3. 3 Figure A3. 4 Figure A3. 5 Figure A3. 6 Figure A3. 7 Figure A3. 8 Figure A4. 1 Figure A4. 2 Figure A4. 3 Figure A4. 4 Figure A4. 5 Figure A4. 6 Figure A4. 7

 4192429543971231234123456781234567 Figure 4.1 Design, structure and working mechanism of the Insulator-Metal-Insulator triboelectric nanogenerator (IMI-TENG) ………………………………….….....92 Figure 4.2 Charge accumulation-storage characteristics of the IMI-TENG …………...…….95 Figure 4.3 Output performance and applications of IMI-TENG …………………………....97 Figure A1.1 Device schematic used for material characterization ….……………………….104 Figure A1.2 TENG electrical performance based on triboelectric series…………………….105 Figure A1.3 Dependence of output power on contact force and operational speed………….106 Figure A2.1 Working principle of the WATENG. ………………….…………………….….107 Figure A2.2 Sequential fluttering behavior images………………….……………………….108 Figure A2.3 Schematic showing the open and short circuit conditions.………………….. ….109 Figure A2.4 Schematic diagram of resistance ladder circuit.……………………………..….111 Figure A3.1 Detailed fabrication process of the Interdigitated TENG (ID-TENG) ….…..….113 Figure A3.2 Cross-sectional schematic of the ID-TENG. ……………………...…………….114 Figure A3.3 Microscopic images of the various electrode/ PTFE strip spacings…………….115

Figure 1 .

 1 Figure 1. The IoT concepts (a) Smart home concept [1] in IoT (b) Typical wireless sensing node system driven by an energy harvester.

Figure 2 .

 2 Figure 2. Four fundamental working modes of the triboelectric nanogenerators. (a) The vertical contactseparation mode. (b) The lateral sliding mode. (c) The single electrode mode. (d) The free-standing mode.[15]

Chapter 1

 1 systematically discusses the state-of-the-art progress of TENG devices. As a viable power source sparking interest among global research communities, investigations are being carried out to increase the device output performances. A special attention is given to the works aimed at showing how the electrostatic charge generated by triboelectric effect alters the performance at the contact interface. Various optimization techniques are compared, and their individual benefits and limitations discussed. Factors affecting the efficiency of TENG, different mechanisms used in TENG are described from the viewpoint of electrical power density.Chapter 2 provides essential information on the test platform that was designed to investigate the charge transfer mechanism between materials, and thus efficiently harvest the generated energy. It also presents the experiments carried out for evaluating the materials based on triboelectric series and effects of several factors that affect the triboelectrification process, such as triboelectric material series, electron affinities and work function. The study also compares the surface distribution and charge retention generated by triboelectric effect with the electrostatic induction.

Figure 1 . 1

 11 Figure 1.1 Research progress in the developmental strategies to enhance output performance of TENGs.Improvements in charge density is associated to gain in output power, opening the field of applications. Charge density can be increased by surface modification, material functionalization, internal space regions and self-sustaining charge pump. The major applications of TENG can be broadly categorized for environmental energy harvesting, sensors, IoTs and smart systems.

Figure 1 . 2 (

 12 ii)), equal and opposite charges are created on the material surface due to the contact-electrification effect (Figure 1.2(iii)). As the two electrodes are separated from each other, the global charge neutrality is modified leading to electrostatic forces driving the charge transfer between the electrodes as shown in Figure 1.2(iv). Further separation alters the charge distribution and results in an equilibrium state without current flow. A continuous cyclic action results in an alternating current (AC) signal.

Figure 1 . 2

 12 Figure 1.2 The fundamental working principle of a vertical contact-separation mode TENG in short-circuit conditions.

Figure 1 . 3

 13 consists of one pair of triboelectric layers (dielectric 1 and dielectric 2 that face each other creating equal and opposite triboelectric charges via physical contacts with thicknesses d1 and d2 and the relative dielectric constants εr1 and εr2, respectively.

Figure 1 . 3

 13 Figure 1.3 Schematic of the contact electrification.[30] Reproduced by permission of The Royal Society of Chemistry.

Figure 1 . 4

 14 Figure 1.4 Norton equivalent circuit configurations in open circuit (a), closed circuit (b), and load (c) In order to calculate this part of converted mechanical power, we can use the Norton equivalent circuit (Figure 1.4). From the Figure 1.4a, we can first calculate the value of Zn.

  and the obtained results discussed in the following sections. To complete the overview, the basic principles of the material functionalization and external charge injection-polarization techniques are illustrated in Figure 1.6.

Figure 1 .

 1 Figure 1.6 a) Schematic representation of the surface functionalization process on a Polyethylene terephthalate (PET) surface functionalized with poly-L-lysine solution by dip-coating, rendering positively charged surface[START_REF] Shin | Triboelectric charging sequence induced by surface functionalization as a method to fabricate high performance triboelectric generators[END_REF]. b) Injecting negative ions onto the FEP surface from an air-ionization gun. The bottom electrode attached to the FEP film is grounded during this process so that the positive charges can be induced onto the electrode to screen the electrical field from the injected negative charges. Reproduced with permission.[START_REF] Sihong Wang | Maximum surface charge density for triboelectric nanogenerators achieved by ionized-air injection: Methodology and theoretical understanding[END_REF] Copyright 2014, Wiley-VCH.

Figure 1 . 7 [

 17 54,55].

Figure 1 . 7

 17 Figure 1.7 Effect of surface roughness at different scales.[56] Copyright 2014, Woodhead Publishing Limited.

Figure 1 . 8

 18 Figure 1.8 Schematic of fabricated PFDMA-TENG, SEM image of transferred PFDMA with surface-relief on the contact layer and contact angle of the transferred PFDMA surface. Voc and Isc of PFDMA-TENG.Reproduced with permission.[38] Copyright 2017, Elsevier.

Figure 1 . 9

 19 Figure 1.9 Illustration of the contact-separation mode TENG with PVDF-TrFE and PDMS-high dielectric (PDMS-HD) particle composite films. Reproduced with permission.[48] Copyright 2018, The Royal Society of Chemistry.

presented in Figure 1 . 10 .

 110 By filling imbedding gold ravines into a PDMS negative triboelectric material to form internal space charge zones, the gold ravines act both as a charge trapper and a transporter to drift the negative charges into the internal spaces of the negative triboelectric material. Owing to the strong electric field, the triboelectric charges will flow from the PDMS surface to the gold layer which forms the internal space charges zones.

Figure 1 . 10

 110 Figure 1.10 Illustrations of electrons drift in gold-PDMS TENG(G-TENG) with the schematic diagram of 2 electrons' escape from PDMS to the gold. The transfer charge density comparison of bare PDMS and three-layer G-TENG in different working durations. Reproduced with permission.[79] Copyright 2018, American Chemical Society.By embedding superior intermediate layer into TENG provides an effective strategy to improve the output performance. In the case without an intermediate layer between the triboelectric layer and bottom electrode, electrons on the surface could drift internally under the driving force of the electric field and finally combine with the induced opposite charges on the electrode[START_REF] Nie | Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid-Solid Interface[END_REF][START_REF] Jiang | Charge-trapping-blocking layer for enhanced triboelectric nanogenerators[END_REF][START_REF] Yi | The effect of the electric field on the output performance of triboelectric nanogenerators[END_REF]. A tunneling effect could exist inside negative frictional layer and the probability of tunneling is guided by the electric field strength E as well as the thickness of the layer. The limitation of the

Figure 1 .

 1 11a demonstrates the schematic illustration of MoS2: PI-based TENG.

Figure 1 . 1 . 5 . 2 . 2

 11522 Figure 1.11 (a) TENG with a composite PI: MoS2 intermediate layer. (b) Scheme of the PI: MoS2 based metal-insulator-semiconductor (MIS) device. (c) Electron transfer process and energy band states when applying a negative voltage, (d) a larger negative voltage, (e) a positive voltage is applied, and (f) a smaller negative voltage. Reproduced with permission.[88] Copyright 2017, American Chemical Society. Similarly, to confirm the existence of electrons in single-layer MoS2 experimentally, Wu et al prepared an MIS device, as illustrated in Figure 1.11b. The energy band diagram in Figure 1.11cf could be used for a qualitative explanation of the electron transfer process. In the initial time, the device maintains a flat band state, and no electrons are trapped in the MoS2 layer. When a large negative voltage is applied to the electrode, electrons transfer from the Fermi level of the Al electrode into the lowest unoccupied molecular orbital (LUMO) of PI and then captured by MoS2, causing the accumulation of holes at the PI/p-Si interface, and increase the device capacitance.Unlike reduced graphene oxide (rGO), the interface trap states are distributed in the bandgap because the intrinsic bandgap energy of single-layer MoS2 is as high as 1.8 eV[START_REF] Lopez-Sanchez | Ultrasensitive photodetectors based on monolayer MoS2[END_REF][START_REF] Tsai | Monolayer MoS2 heterojunction solar cells[END_REF]. In the case of a positive voltage, electrons in the bottom energy state of the conduction band could be drifted to the top electrode, while electrons in the bandgap incline to be trapped by interfacial trapping sites. 1.5.2.2 Device architecture based on charge pump

(

  SI-TENG) contains two parts: part I is a vertical contact-separation mode TENG while part II is used to store the charges created by part I to form a high charge density device as shown in Figure 1.12a. The two parts are connected through a rectifier bridge to prevent the charges stored in the PPCS to flow back and neutralize the circuit. The charges stored in the PPCS generate a changing electric field which changes the potential difference between electrodes and pushes electrons flowing between the electrodes to generate AC voltage (or current) in the circuit. This charge accumulation will speed up the output performance enhancement at the start of the operation. The output performance (Figure 1.12b) of the AC current of part I converted to DC current by the rectifier bridge shows a decreasing trend of current indicating the voltage between two electrodes of PPCS increases with the charges filled into PPCS. The output of SI-TENG increased from 5.6 μA and 460 V to 57.8 μA and 922 V with the effective charge density increasing from 45 μC.m -2 to 325 μC.m -2 . A similar approach was employed by Liu et al. further increased the charge density to 1250 μC.m -2 by utilizing voltage-multiplying circuits consisting of Zener diodes and capacitors[52].

Figure 1 .

 1 Figure 1.12 a) Schematic diagram and optical image of the self-improving triboelectric nanogenerator (SI-TENG) with its output performance. b) Charge accumulating process of the SI-TENG and corresponding charge transfer direction in the circuit. The friction layers in part I are replaced by positive and negative charges, and the PET film in part II are not shown in the image. Reproduced with permission.[51] Copyright 2018, Nature Publishing Group.

Figure 1 . 2 working in ambient conditions. 1 . 6

 1216 Figure 1.13 a-b) Device structure and output performance of the floating layer structure with a pump TENG. The pumped charges and the output transferred charges for the same self-charge-pumping (SCP) TENG with a pump TENG of 4 × 4 cm 2 . c) Measured data of Q1 are flipped from negative to positive for comparison with Q2. Inlet: the enlarged view and the measurement circuit, where HI and LO mark the "input high" and "input low" terminals of the electrometer. Reproduced with permission. [94] Copyright 2018, Elsevier.The device consists of two major parts, a pump TENG and main TENG as illustrated in Figure1.13a-b. The main TENG comprises two electrodes and bound charge layer (BCL) which can retain the charges within them. The pump TENG is similar to a normal TENG with two electrodes and a dielectric which triboelectrically generates an alternating charge output to be supplied to the BCL. The pump TENG generates a small amount of energy (Ep) to pump the charges and is then added to the charge accumulation of the main TENG to create a high charge density output TENG structure with high output energy (Em). The pump TENG continuously creates charges and injects it to the floating layer which accumulates the charges in a unidirectional electron flow through the

Figure 2 .

 2 1a illustrates some commercially available materials chartered according to its position in tribo series.[3][4] Higher on the list give up electrons and lower on the list receive electrons. Further away two materials are separated apart from each other in the series, the greater the charge transferred, and vice versa. To achieve higher performance, materials further away from each other in the series are chosen due to their different ability to attract electrons. Almost all materials exhibit triboelectrification effect. The material that gain electrons tends to have stronger affinity for negative charge of the two materials leading to a negative charge on the surface after the material separation. The material losing electrons will have the equal and opposite (positive) amount of charges.The charges are generated by friction between two materials, which results in the creation of a triboelectric potential layer at the interfacial region. A mechanical compression results in a change in distance between the two electrodes, thus, varying the system capacitance resulting in flow of alternating current in the external load.

Figure 2 . 1 . 2 37 2 . 3

 21223 Figure 2.1. Materials characterization based on triboelectric series. a) Materials arranged according to triboelectric series position.[5] b) Kelvin probe characterization setup from KP Technology consisting of data acquisition system, sample holder and motorised (x, y, z) stage controller. c) Kelvin probe measurement process with the known and unknown material brought together to equalize their fermi-levels by a flow of electrons from the lower work function to the higher. Measured work function in comparison with the existing literature results.[6]

  It describes the direction and magnitude. The electric charges are balanced in nature (positive equals negative neutralizing charges). Attraction by opposite charges and repulsion by like charges help in investigating the field lines. The direction of electric field is given by the direction of force exerted on a positive charge. The electric field is radially outward from a positive charge and radially inwards to a negative charge. As shown in Figure 2.2a, the electrical field lines are imaginary lines picturing the direction and intensity of an electric field on Test procedures and measurement conditions Chapter 2 38the medium caused by the electric charge. The electric field strength in space surrounded by a charge source is directly related to the charge quantity on the source and inversely to the distance from the source. The study of effect of electric field on the triboelectric charge density and the distance between the contact surfaces is proposed. The insulator exerts a contact voltage termed as "VTribo" when the material is pressed in between two metal electrodes.

Figure 2 . 2 .

 22 Figure 2.2. a) Illustration of electric field influence on the perturbation with increasing distance. b) Modified Instron test setup deployed for contact-separation TENG mode with 7mm spacing and 10 mm.s -1 cycling speed.

Figure 2 .

 2 3a presents the triboelectric charge stored in the friction layer which decays with time. The test confirmed the charge retention on the material over a certain time during the contact separation process. For an efficient power Test procedures and measurement conditions Chapter 2 39 generation, the material needs to be permanently in contact with the active material. With continuous contact-separation cycle, the charges are accumulated on the contact triboelectric surface, driving the charges that can be induced into the electrode. A similar behavior is observed in reverse polarity, shown in Figure 2.3b. Reducing the interval between the contact and separation leads to high current, increasing the amount of accumulated surface charge density resulting in an enhanced power generation performance.

Figure 2 . 3 .

 23 Figure 2.3. a) Test performed to understand the time interval between the contact and separation process. Longer the interval, lower the charge retention leading to decreased output performance. b) Similar experiment in reverse polarity configuration.

Figure 2 .

 2 4a shows the dependance of Polycarbonate (PC) contact voltage as a function of distance. Inset shows the images at various stages of the separation process.

Figure 2 . 4 .

 24 Figure 2.4. Contact voltage as a function of separation distance at 0.2 mm.sec -1 .

Figure 2 .

 2 5a-c presents the contact voltage of commonly used triboelectric materials like PET, Kapton and PTFE. With continuous contact-separation process, the charges are accumulated on the contact triboelectric surface, driving the charges that can be induced into the electrode. An electrical field in upward direction is set up between the contact surface and the electrode. Two electron transfer modes exist: a drift process caused by the electric field and a diffusion process caused by the charge concentration[13].

Figure 2 . 5 .

 25 Figure 2.5. Contact voltage as a function of separation distance for commonly used materials. a) Kapton, b) PET and c) PTFE.

  Whenever translational movements are to be dynamically executed, with low friction and great flexibility, users choose linear systems. The heart of the LinMot system consists of control electronics, servo drive and linear motors. The 150 mm linear translation stage motor is driven by LinRS communication protocol to send/receive commands over RS 485 serial link. An outstanding maximum displacement speed of 30 m/s and actuation force of 580 N allows in the investigation of various mechanical parameters including contact interface and contact velocity.

Figure 2 . 6 .

 26 Figure 2.6. Test bench consisting of mechanical setup adapted to both vertical and sliding mode TENG systems along with their electrical measurement system measuring output voltage, current and charge capacitance.

Figure 2 .

 2 7a illustrates the schematic interface diagram of various inter-linked measurement systems with Figure 2.7b showing the Labview control pane. More information in Annex 1. The control panel serves as the user interface while the block diagram contains the graphic programming part.Figure 2.7c shows the motor movement images commanded by the Labview software. The control panel allows in setting up the essential parameters like force, speed, displacement, etc. for the mechanical TENG actuation. The system performs an automatic calibration followed by the execution of the set parameters in the control panel. The control panel allows to select the measurement to be performed, electrical measurements including open-circuit voltage, voltage under load, short-circuit current, Test procedures and measurement conditions Chapter 2 42 capacitance and transfer charge. The material under study is placed in opposite faces of the fixed and moving part of the test bench. When the command start is initiated, the mechanical displacement starts along with the electrical measurements observed directly in the control panel. The actuation continues until reaching the set cyclic movements.

Figure 2 . 7 .

 27 Figure 2.7. Linear motor setup platform. a) Schematic communication interface diagram of the test bench. b) LabVIEW control panel. c) Motor movements controlled by LinMot Talk software.

  multilayer flexible film was devised to increase the surface charge density. For this purpose, we have decided to study the output performance by using the PTFE film as a substrate and Parylene-C (PaC) as the interfacial layer structure. A thin film PaC was deposited to retain the charges from the triboelectrification process, allowing the acceptance of more charges from an electrode and thereby increasing the output performance as seen in Figure2.8. It has been used in a wide range of applications, particularly as a protective coating for biomedical devices and microelectronics[17]. Its desirable properties include chemical inertness, conformal coating and excellent barrier properties[18].

Figure 2 . 8 .

 28 Figure 2.8. Fabrication process of interfacial layer TENG structure with PaC deposited through vapor polymerization technique. The investigation of output stability through cyclic contact separation mechanism provides better understanding of the charge dissipation on the dielectric surface. The use of an organic material like PaC as a triboelectric charge layer provides many advantages: (i) Reduced fabrication time leading to a cost effective compared to the existing TENG reports[19,20], which involves micro patterning, material processing; (ii) High electrical output performance with an increased work function difference of ϕPaC= 3.27 eV, ϕPTFE= 5.6 eV and ϕCu= 4.5 eV;

Figure 2 . 9 .

 29 Figure 2.9. Design of charge layered TENG. a) Schematic diagram. b) Cross-sectional SEM image with 4 µm thickness parylene deposited on PTFE sample structure.

Figure 2 .

 2 Figure 2.9a-b shows the schematic and SEM cross-sectional image of the PTFE thin film with PaC charge layer. Material work function (ϕ) provides a better understanding towards charge transfer process. The triboelectric charge density depends on: (i) the work function difference between the two materials in contact, (ii) the dielectric constant and (iii) structural parameters (i.e. dielectric thickness and separation distance during triboelectric process).Figure 2.10a shows the

Figure 2 .

 2 10a shows the work functions of the material surfaces and have been determined by measurement of their contact differences of potential with respect to gold reference surfaces of known work function (ϕAu=4.7 eV) using Kelvin probe technique. During our experiments, enhancement of triboelectric charge density was observed when the work function difference maximized, and the dielectric thickness minimized.

Figure 2 . 10 .

 210 Figure 2.10. Triboelectric nanogenerator output performance. (a) The measured work function using kelvin technique. Electrical characterization of the 4 μm charge layered TENG. (b) Comparison of TENGs output voltage characteristics. Inset shows the 10-series connected red LEDs illuminated by TENGs. (c) Enlarged view of the output voltage comparison between a bare PTFE and charge layered TENGs. Given the same amount of charges (ΔQ) transported back and forth, a faster contact-separation process is expected to produce larger current peaks (Isc = ΔQ /Δt) than that during the slower separation, and thus increase the charge accumulation. A comparative electrical response of bare PTFE and charge layer inclusion with PaC on PTFE open circuit voltage under contact separation TENG mechanism is presented in Figure 2.10b with enlarged curves in Figure 2.10c. A bare PTFE reached 17 V but explicitly increases to reach 120 V after the inclusion of the 4 μm PaC as a charge layer, indicating an enhancement of 7 times with the charge layer. Inset photographs show the LEDs lighted up by TENG before and after the PaC charge layer, depicting an important scale improvement in the illumination.

Figure 2 .

 2 Figure 2.11a shows the influence on electrical output performance of the charge layer thickness, when varied between 0.5 μm to 10 μm. Considering identical initial surface potentials, when the thickness of PaC decreases from 10 µm to 0.5 µm, the Voc increases from 24 V to 262 V, giving a

Figure 2 .

 2 Figure 2.11. (a) Output performance as a function of PaC thickness. Influence of PaC decreases from 10 µm to 0.5 µm, the open circuit voltage (Voc) increases from 24 V to 262 V, giving a 10fold enhancement. (b) Effective charge density obtained by integrating short-circuit current increases from 0.5 μC.m -2 to 70.2 μC.m -2 with increasing thickness.

  perform triboelectrification was thoroughly described. The experiments were carried on the choice of materials based on various parameters such as work function, contact voltage and triboelectric series position. The study also included the dependance of charge decay in relation to the distance and charge generated during contact-separation process. Finally, the investigation of increasing the charge density via interfacial charge layer was presented. Addition of a charge accumulation layer, thin film Parylene-C, proved to retain the charges from the triboelectrification process, Test procedures and measurement conditions Chapter 2 47 allowing the acceptance of more charges from an electrode and thereby increasing the output performance. The relation between the material thickness and charge density are well correlated.

[ 8 ]

 8 [9]. Researchers have investigated on ambient wind energy ][11][12][13]. With the recent reports on ambient energy harvesting by triboelectricity gaining significant attention mainly due its simple fabrication, low cost and wide array of applications[14][15][16][17], challenges remain mainly on the device size, output electrical performance, efficiency and implementation in real life environment.

Figure 3 .

 3 1a shows the important sections of a venturi system consisting mainly of the inlet (convergent), outlet (divergent) and throat. When the ambient air passes through the venturi system, a low pressure exists in the throat region forcing Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 52 the flag film to flutter at high frequencies between the electrodes. This flag fluttering is translated into a faster charge transfer efficiency by displacing the charge between the electrodes. This charge movement induces a potential variation at the electrodes and an enhanced triboelectrification process. With Polycarbonate (PC) as the flag film displacing at 5 m.s-1 between Polytetrafluoroethylene (PTFE) -Copper (Cu) electrodes, the device delivers an output voltage (Vo) of 150 V and short-circuit current (Isc) of 16 µA outperforming existing reports[24][26][32][33][34] with a 1.5 mW RMS power and 4.5 mW peak power outputs. The optimized output performance of the WATENG has been accomplished by systematically evaluating the effects of the wind speeds, material screening and structural parameters. The WATENG system was used to glow 40 series connected commercial LEDs instantaneously with a very bright illumination without any rectification and charge a 100 µF capacitor to 5 V within 20 secs validating its potential to power for small scale electronics. Device sizing not only enhances the output performance but also addresses to various environmental sensor applications.

Figure 3 .

 3 1b illustrates the schematic diagram and Figure 3.1c-d of the fabricated WATENG, which consists of a venturi system design with PC as the flexible flag and PTFE -Cu

Figure 3 . 1 .

 31 Figure 3.1. Design and working mechanism of WATENG. a) Venturi system consisting the inlet (convergent), outlet (divergent) and throat sections. b) 3D schematic of the WATENG. c), d) Photographs of the WATENG consisting of venturi system design with PC as the flexible flag and PTFE -Cu as the side electrodes. e) PC flag orientation positions: (I) Initial position, (II) under oscillation. f) The working principle of the WATENG. (i) Flag at its initial position between the electrodes. (ii) The flag displaces towards one of the electrodes driving the electrons from the bottom to the top electrode. (iii) On contact the triboelectric charges (equal and opposite) are created on the PTFE and PC flag. (iv) Charges flow in reverse direction relative to the flag position.

( 3 55 3 . 1 . 4

 3314 RH) conditions. A commercial anemometer from Voltcraft PL-135 was engaged in measuring the wind velocity. The output electric parameters of WATENG were measured using a Keithley 2636 sourcemeter and LeCroy waveRunner 6100A oscilloscope with a 10 MΩ impedance by a LabVIEW program for real time data acquisition. The impedance matching was performed using a wide-range resistance ladder configuration with external load ranging from 1 kΩ to 376 MΩ. Applications: Compact and high-performance wind actuated venturi TENG Chapter Wind flag behavior The working mechanism of the WATENG consists of different processes as illustrated in Figure 3.1f. Intermittent contact separation between PTFE -Cu electrodes and PC flag film with different triboelectric polarities drive charges back and forth in short-circuit conditions due to the coupling of triboelectrification and electrostatic induction processes[7]. The triboelectrification process occurs at the mechanical contact between the flag and the electrodes. The flag is thus carrying a charge Qe and the movement of this charge either induces a voltage variation in open circuit conditions or transfers charges in the form of current in short circuit conditions. The amount of electric charge Qe created on the moving flag remains a constant and is linearly related to the change in electric potential V with capacitance C at a certain wind velocity. This relation can be seen as

  output voltage. If the two electrodes are unconnected to external load, there exists a potential difference between the electrodes creating an open-circuit voltage Voc. Assuming the gap spacing w uniform throughout the design structure, a theoretical model is conceptualized treating the electrodes and the moving flag as a parallel plate capacitor model without considering the edge effect[41]. The detailed analytical derivation is presented in the Annex 2. The amplitude of open circuit voltage can be determined by 𝑉 𝑝𝑒𝑎𝑘-𝑡𝑜-𝑝𝑒𝑎𝑘 = |𝑄 𝑒 | ∈ 𝐴 . 2𝑤

Applications: 56 the

 56 Compact and high-performance wind actuated venturi TENG Chapter 3 PTFE due to triboelectrification, leaving the PC positively charged and driving the electron flow from the bottom to the top electrode. A similar trend exists when the flag displaces to the counter electrode (IV). This continuous fluttering of the flag on the electrodes results in a cyclic alternating current (AC) output signal relative to the wind speed. The fluttering movement of the flag the electrodes results in the creation of charges on the PTFE surface. The rate of transfer of charges primarily depends on the wind velocity and the output short current Isc with a wind velocity v given by

Figure 3 .

 3 2a (I-VI) to demonstrate the contact separation mechanism between the electrodes and the flag materials.

Figure 3 .

 3 2b shows the electric output signal of the contact separation mechanism of the WATENG into the electrode channel generated for 1.317 seconds.In order to understand the wind flag behavior, one complete oscillating cycle of contact separation Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 57 with an output voltage between ±150 V was measured. When the wind enters the venturi channel system, the flag begins to move from its initial position (T0) by oscillating towards the electrode with a high mechanical force (T0+1ms). The contact area increases as the wind passes through the WATENG system resulting in triboelectrification (creation of equal and opposite charges) on the flag and the electrode (T0+2ms). The PTFE gaining negative charges on its surface while the PC flag gaining positive charges. The flag carries a fixed charge displacing it to the counter electrode (T0+3ms) resulting in the movement of mobile charges in the Cu electrode and inducing a current flow in positive direction relative to the flag fluttering displacement. The parasitic in the output signals exists due to the fast flag fluttering frequency creating a non-uniform contact with the electrodes (T0+4ms) as seen on Figure 3.2a. The wind velocity transforms the mechanical energy of flag oscillating motion, which can be converted into electrical energy. Due to the flag fluttering at high frequencies, the flag propagates along the wind flow direction with increase in contact area (T0+5ms) translating to a movement of the mobile charges in Cu electrode inducing current in opposite direction, resulting in AC current output signals in the external circuit. The resultant output voltage is the reflection of measured current output. The output current is governed primarily by the wind velocity, attributed to a faster rate of charge transfer due to higher flag fluttering frequency and higher contact surface area with the electrodes. This periodic displacement of the flag cycle continues until the wind flow is propagated in the wind channel.

Figure 3 . 2 .

 32 Figure 3.2. Relation between the voltage profile and wing flag behavior. a) Serial images of wind flag behavior of flexible PC flag captured using high speed motion camera (T0 -> T0+5ms). b) Voltage output profile of WATENG at 5 m.s -1 , inset, a single voltage output cycle. c) Output potential induced by WATENG at different gap spacing. An optimum gap spacing of 2 mm is chosen for a better charge transfer process. d) Performance of output voltage as a function of wind speed.The output voltage drifts from 20 V at 0.5 m.s -1 to 350 V at 10 m.s -1 . Increasing the wind speed increases the flag oscillation relating to a faster charge transfer mechanism. e) PC flag reliability with respect to a wind speed of 5 m.s -1 , a stable 150 V output voltage is reached without any degradation in performance.

Figure 3

 3

Figure 3 .Figure 3 .

 33 Figure 3.3d with the maximum Isc of 16 µA in the case of PTFE: PC tribo electric pair compared to 5 µA in PI and 9 µA in PET flag material. The WATENGs electric output performance measured at 5 m.s -1 outperforms the existing reports on wind harvesting TENGs[24][26][32][33] which have demonstrated lower output electrical performance even at high wind velocity with larger device size.

Figure 3 . 3 . 3 62 3 . 1 . 6 Figure 3 .

 3333163 Figure 3.3. Output performance of WATENG system. a) Materials arranged in triboelectric series reproduced from [45] . b) The generated RMS voltage (Vrms) increased from ∼4 V in the device with ITO/PET: PI pair up to ∼100 V in the device with PTFE -Cu: PC electrode flag pair. c) Output voltage for different materials with PTFE -Cu electrodes. The output voltage increases from ~80 V with PET to ~150 V in the device with flexible PC flag. Note: Identical experimental conditions were a used in all TENGs. d) Comparative short-circuit current (Isc) of the TENG with PTFE -Cu electrode with PI, PET and PC flag. e) Dependence of output power on the external load resistance 1 kΩ to 376 MΩ. The device delivers a peak power output of 4.5 mW at 198 MΩ load resistance and reaches RMS power of 1.5 mW.

  connected 40 commercial light-emitting diodes (LEDs) in series to the WATENG system as shown in Figure 3.4b. Due to the relatively high fluttering frequency of the flag film in the system at a constant wind speed of 5 m.s -1 , the generated output power was adequate to glow 40 LEDs instantaneously with a very bright illumination without any signal conditioning. The images show the series connected LEDs in OFF state and ON state in both bright and dark light conditions validating the reported WATENG system have many potential applications in instantaneous signaling and low maintenance energy applications.

Figure 3 . 64 Figure 3 . 4 .

 36434 Figure 3.4c shows the voltage profile of charging a 100 µF capacitor up to 5 V at a wind speed of 5 m.s -1 . The AC output from WATENG was converted to a DC output by a full-way rectifying bridge with equivalent circuit (Inset 3.4(c)). Materials such PET and PI are tested alongside PC flag film to compare the performance of its charging time. It is observed that PC flag film shows excellent result by charging capacitor from zero to 5 V in less than 20 seconds compared to PI and PET which consumes higher than 35 seconds validating an efficient conversion of mechanical wind energy to a stored electrical energy. Considering the rapid charging time of a 100 µF in less than 20 seconds, this rapid output from the system can efficiently power low power electronic devices and wide variety of charging applications. The faster charging time can be achieved with higher wind speeds due to fact that the current and voltage output generation increases with flag fluttering frequency in accordance with the faster electron transfer between the electrodes.

Figure 3 .

 3 4d demonstrates the test setup and the electrical performance of charging 1 mF super capacitor up to 15 V. The system takes less than 12 minutes to rapidly charge the super capacitor when commuting in countryside with speeds between 7-15 m.s -1 . Also, with long term device operation, no significant damage was observed in the flag material, especially in the flag tip due to high concentration of mechanical energy proving the stability and performance of PC flag in the WATENG.With increase in device size leading to a higher power generation due to faster charge transfer and higher contact surface area. A comparative investigation was performed on the effect of device size of the WATENG with its output electrical performances. The size of the WATENG was varied 3-dimensionally (LxWxH) adapted to various charging applications. The large (40 x 12 x 12 cm) and medium (20 x 6 x 6 cm) scale WATENGs were fabricated using laser cut plexi glass that demonstrate similar device stability characteristic as the original small-scale WATENG (7.5 x 2.1 x 2.1 cm). The dependence of the voltage Vo and current Isc of the WATENG at a constant wind speed of 5 m.s -1 is shown in Figure 3.4e. Obviously, the highest performance of the WATENG is obtained in the large-scale model (Vo = 750 V and 𝐼𝑠𝑐 = 40 µA) due to higher contact surface area and faster charge transfer rate compared to the medium-scale (Vo = 270 V and 𝐼𝑠𝑐 = 21 µA) and small-scale (Vo = 150 V and 𝐼𝑠𝑐 = 16 µA) WATENG, thus making it suitable for large scale energy harvesting for autonomous applications. Furthermore, Figure 3.4f demonstrates the charging profile of 100 µF up to 5 V for the device size comparison. The large-scale WATENG charges rapidly in less than 5 secs while the medium and the small-scale WATENG requires 14 and 20 secs respectively. Increasing the device size not only decreases the charging time and increases the output performance but also addresses to power wide array of autonomous Applications: Compact and high-performance wind actuated venturi TENG Chapter 3 66

2 (

 2 .5b). The sliding card section consists of a part of the TENG power generation along with the power conditioning circuit while the fixed application section holds the main control unit (MCU) which is composed of an ultra-low-power microcontroller (STM-8) module connected to the display screen as shown in block diagram Figure3.5c. The application, in a card sliding format for secure payment systems, consists of a power processing stage that converts the TENG AC output to a constant DC output, storing it efficiently in a capacitor and a main control unit (MCU) that controls the information displayed on the screen. The activation of low power mode in the MCU reduces the supply voltage for the system consuming less than 1 µW under sleep mode and close to 1 mW during run mode. An investigation on the physical phenomenon of current transfer between the electrodes has also been carried out and comprehensive analytical model is derived from an electrostatic model study, in excellent accordance with experimental results. Through systematic investigations on the influence of materials, device structure, operational speed, force and the effect of electrode and insulator gap spacings, a maximum output power density of 750 mW.m -average power of 1.86 mW and Peak power of 2.7 mW) is generated with stable output operations even after 40 000 operational cycles. The integrated system reaches a 5.2 V threshold DC output 72 in less than 25 secs, equivalent to 7-8 human hand sliding motions, adequate to drive the target application.

Figure 3 . 5 .

 35 Figure 3.5. Integration of TENGs in practical applications. a) 3D schematic structure of the proposed integrated autonomous system illustrating the various layers. b) Photograph of the integrated card system showing the top sliding card and fixed sections on a 3D printed fixture. Also shown is the MCU consisting of the microcontroller that controls the information displayed on the screen. c) Block diagram of the integrated autonomous card system based on sliding mode TENG.

Figure 3 .

 3 6a-b shows in detail the schematic diagram of the two main sections of the TENG power source, the fixed bottom and sliding electrode sections. Annex 3 illustrates the detailed fabrication process schematic along with the as-fabricated images and cross-sectional microscopic images of the sections.

Figure 3 . 6 .

 36 Figure 3.6. Device structure and working principle of sliding mode based inter-digitated electrodes triboelectric nanogenerators (ID-TENG). a-b) Schematic diagram of TENGs illustrating the two sections (top sliding and fixed bottom). c) Working mechanism of TENGs. (i) Triboelectrification with creation of equal and opposite charges created on PTFE and PC. (ii) Sliding section displaces, driving electrons from electrode B to A. (iii) no more electron flow reaching an equilibrium position. (iv) Electrons flow in reverse direction (A to B) relative to the sliding movement.Under short-circuit condition, the triboelectrification creates equal and opposite charges on both PC and PTFE surfaces with the PC gaining positive charges while PTFE acquiring negative charges (I). When moving from interdigitated electrode pairs A towards pairs B (II), the top section slides over the fixed section driving the electrons flow from B to A, inducing a current flow from A to B until reaching an equilibrium (III). Similarly, when the top section slides back to its initial state, the electrons flow back across the electrodes (IV), this time from A to B. This continuous sliding action results in a linear alternating current (AC) signal and by increasing the number of electrodes and strips pairs, the output current can be multiplied. In order to investigate the electrostatic coupling between the interdigitated electrodes and PTFE strips, a finite element method (FEM) by COMSOL simulation was performed to compute the induced mobile charge in the electrodes as a function of strip position as presented in Figure3.7a-b.

Figure 3 . 7 .

 37 Figure 3.7. COMSOL study of the sliding based TENG power source. a-b) Simulated images of voltage under the influence of electrode displacement with respect to the fixed PTFE strips, in short-circuit configuration (all electrodes connected to ground). c) In the absence of strip, an average negative charge Qav is induced in a given electrode by the positively charged triboelectric material. When the strip is aligned to a given electrode, a local neutrality exists at the triboelectric interface reducing the amount of mobile electrons in this electrode. d) As a result, approaching the strip to electrode A results in a flow of electrons out of electrode A and a related negative output current. d) Resulting output current calculated current from the derivative of QA.The dependence of total charges QA induced in the electrodes on the strip position can be fitted by a Lorentzian probability distribution model curve (Figure3.7c),

Figure 3 .

 3 8a-b shows the output electrical response, i.e., output voltage and short-circuit current of these materials resulting from different levels of triboelectric charge of the contact materials and is in direct relation to its position in the triboelectric series. A steady increment in the electrical response can clearly be seen when the material was changed from Parylene to PC with the maximum output performance of 150 V/19 µA shown by PC and PTFE triboelectric pairs, this output is constant over time. Enlarged image shows a single electrical output cycle of PC. More information are available in Annex 3.

Figure 3 . 8 .

 38 Figure 3.8. Electrical output characteristics of the TENGs. a-b) Output voltage and current of various sliding materials under test arranged according to triboelectric series. All the parameters are maintained constant with PTFE fixed section at 1 m.s -1 . A linear increase in output performance can clearly be seen when the material changes from Parylene to PC with the maximum output performance of 150 V/19 µA shown by PC and PTFE triboelectric pairs with its enlarged single electrical response shown in inset. c) Effect of PC thickness on the output performance from 200 µm to 50 µm increases from 30 V to 135 V. Thinner PC films aids in faster

Figure 3 .

 3 Figure 3.8e. Without any external pressure, the output voltage barely reaches about 8 V. As the

Figure 3 .

 3 9b-j presents the experimental electrical output response (voltage, current, charge quantity) under short-circuit condition of the TENGs as a function of various electrode contact width (D = 500 µm, 1 mm, 2 mm and 5 mm) with varying PTFE strips (W = 500 µm, 1 mm, 2 mm) in the fixed bottom section.

Figure 3 . 9 . 3 82

 393 Figure 3.9. Electrical output data compilation under the influence of various interdigital electrode contact width (D=5 mm to 500 μm) and varying PTFE strips (W=500 µm, 1 mm, 2 mm) at 1 m.s - 1 . a) Schematic diagram of the electrode features. b-d) Output performance of W=2 mm with varying D. e-g) Output performance of W=1 mm, output increases from 22 V/ 3 µA/ 9 nC to 273 V/ 25 µA/ 76 nC. h-j) Output performance of W=500 µm with varying D, the output increases from 28 V/ 3.5 µA/ 7 nC to 300 V/ 32 µA/ 121 nC demonstrating a 10-fold, 5-fold and 5-fold enhancement respectively, in terms of output voltage, current and charge compared to wider D/W spacings (5 mm/ 2mm).

Figure 3 . 10 .

 310 Figure 3.10. Output performance of TENG. a) The stability and durability test of ID-TENG for 40 000 cycles at 1 m.s -1 . b) Dependence of output power on external load impedance (10 MΩ to 376 MΩ). The device reaches a peak power of 2.7 mW and average power of 1.86 mW at 151 MΩ. c) 50 series-connected commercial SMD LEDs driven directly by TENG at day and night-time conditions. d) Comparative charging profile of a 100 µF capacitor charged up to 5 V for various electrode gap spacings (5 mm, 2 mm, 1 mm and 500 μm) at 1 m.s -1 . Lower electrode spacings leads to rapid capacitor charging in less than 15 secs. Inset shows the rectification circuit.

Figure 3 .

 3 10b shows the average power of 1.86 mW and an instantaneous peak power reaching 2.7 mW, corresponding to a maximum power density of 750 mW.m -2 obtained across 151 MΩ load impedance. The maximum output power density is computed directly from the maximum output power and the Applications: Friction-based TENG in a power-balanced fully autonomous system Chapter 3 84 active surface area of the sliding section. Compared to existing sliding based triboelectric nanogenerator[16][22][20], the reported ID-TENG performs 3 times higher in terms of power density.To demonstrate the capability of the TENGs as a reliable power source, it was directly connected to illuminate series connected 50 commercial LEDs to high brightness instantaneously without any charge storage unit as shown in Figure3.10c. Additionally, the capacitor charging characteristic of the TENGs were evaluated. Figure 3.10d shows the comparative charging profile of 100 μF charged to 5 V for various electrode gap spacings (5 mm, 2 mm, 1 mm and 500 μm) at 1 m.s -1 . Inset shows the schematic illustrating the TENG connected to a 100 μF commercial capacitor via a bridge rectifier circuit consisting of ultra-low leakage diodes. The results validate

Figure 3 . 11 .

 311 Figure 3.11. Integration of TENGs in practical commercial applications. a) Flow chart of the signal processing conversion from the TENG to the application (electronic paper display). b) Charging profile of a 220 µF SMD capacitor charged instantaneously in less than 25 secs reaching a threshold of 5.2 V to sufficiently power the MCU. Inset shows the rectification circuit affixed on the sliding card section and the MCU with various pins of the display. Also, shown is the display screen before (OFF) and after (ON) powering the MCU through TENGs.For the display to operate, the microcontroller controls it through three General Purpose Input/output (GPIOs), connected each to screen background, word and on independent reference terminal (ITO). The color of electronic paper screen toggles from black to white when applying potential difference across the word and background. The ITO allows to toggle the background or the word independently. The TENGs convert the human hand sliding motion into effective electrical energy and the power generated is stored in a 220 µF capacitor after rectification as shown in Figure3.11b. Inset shows the Cu-FR4 substrate laser patterned to trace the custom circuit design connecting the MCU with the display screen. By a human hand sliding motion on the top with the bottom fixed section, the capacitor is charged in less than 25 secs, equivalent to 7-8 human hand sliding motions, reaching a threshold of 5.2 V DC output. This capacitor acts as an energy buffer between the TENGs and the application. The screen status corresponding to the activated sequence of initialization (background and word in white) or activation (background in white and word in black) is shown in Figure3.11b in the supplementary information. It is important to note that the current consumption of the screen is very low (about 100 nA) compared to the microcontroller consumption, evidencing the TENGs are well adapted for low-power application.

Figure 4 . 1 .

 41 Figure 4.1. Design, structure and working mechanism of the Insulator-Metal-Insulator triboelectric nanogenerator (IMI-TENG). a) Comparison of the literature reports on techniques used to increase the effective charge density, namely, nanoparticle synthesis[16] , multi dielectric layer[17][18] and plane parallel capacitor[19] . b-c) Digital photograph and schematic diagram of the IMI composite film. Scale bar, 1 cm. d) Working principle of the IMI-TENG during one cycle under external force. E1 is the electric field between the top electrode and PaC while E2 is the electric field which drives the charges to conduct through the insulator later to be stored in the metal inclusions.

Chapter 4 93 4. 3

 933 Materials and methodsDevice fabrication: The commercial 100 µm PTFE films (Goodfellow Ltd.) were used as a substrate to fabricate the IMI composite with dimensions 40 mm x 40 mm. A 100 nm of Au was deposited onto the substrates by thermal evaporation technique followed by etching of Au layer through laser into individual charge storage islands. PaC was deposited through a vapor polymerization technique. A piece of Cu adhesive foil covered the underside side of the fabricated IMI composite and affixed to the fixed plate while a bare Cu foil was adhered to the linear motor to form the TENG. Characterization and Measurement: The TENGS were driven by a mechanical linear motor (Linmot, C1100). The electrical output signals were measured by LeCroy oscilloscope (WaveRunner 6100A @ 10MΩ), Keithley 2636 sourcemeter and Stanford low-noise current preamplifier (Model SR570), respectively controlled by a LabVIEW program for real-time acquisition. The charge density was determined from integration of the output currents. The material work functions were determined by the measurement of their contact potential differences with respect to gold reference of known work function (φAu=4.7 eV) using the Kelvin probe system (KP Technology).

  displacement and velocity of 1 m.s -1 . The contact-separation test was performed with IMI composite as the triboelectric film and Cu as top and bottom electrodes. For simplification, we represent test sample PTFE as A, PaC-PTFE as B, IMI (full metal coverage) as C and IMI (island architecture 20% coverage) as D. The comparative output voltage, current and maximum localized charge density of test samples A, B and C are presented in Figure 4.2a-c. Test sample A performed with outputs of 40 V, 2.4 µA and 6.9 μC.m -2 while the test sample B performed with increasing outputs saturated at 132 V, 6.7 µA and 71 μC.m -2 showing a slight improvement over time due to higher triboelectrification of Cu-PaC material pairs. With the introduction of an intermediate metal inclusion, the test sample C generated an output voltage, current of 105 V, 6.3 µA at the first cycle and steadily reached 330 V, 16.3 µA. This rapid surge in output performance is a clear sign of a Metal islands structure as a power booster for higher performance TENGs Chapter 4 95 build-up of localized charge through the insulator with a maximum localized charge density of 312 μC.m -2 , which is 4.3 times higher than the QSAT of structure without metal inclusion (Sample B vs Sample C). The detailed results summary are presented in Table 4.1.

Figure 4 . 2 .

 42 Figure 4.2. Charge accumulation-storage characteristics of the IMI-TENG. Comparison of (a) output voltage, (b) current, and (c) charge density of bare PTFE (Sample A), PaC-PTFE (Test sample B) and IMI triboelectric film (Test sample C) with its enlarged maximums. With the inclusion of metal (Sample C), the QSAT reaches a maximum value of 312 μC.m -2 exhibiting 4.3 times enhancement compared to TENG without metal (Sample B).

Figure 4 . 96 presented in Annex 4 .Table 4 . 1 .

 496441 3a-b shows the impact of metal island density (%metal ratio: 0 to 20%) on the output with a 3-fold enhancement (Sample C vs Sample D). More information is Metal islands structure as a power booster for higher performance TENGs Chapter 4 A maximum localized charge density of 1076.56 µC.m -2 was obtained with a 20% metal ratio island architecture, which is 2.1 times higher than the highest previously reported charge density[19].Comparison of output performances of various TENG structures (PTFE, PaC, PTFE and IMI-TENG (Full metal coverage; island architecture).To confirm the charge accumulation and storage further experimentally in the intermediate metal inclusion, Figure 4.3c illustrates the charge retention in the IMI-TENG structure (Test sample D) with effective charge accumulation during the continuous ON-OFF cycle for 1200 s. The result show that the charges are well retained in the metal during the OFF period, the output voltage grows steadily despite intermediate stops. 97

Figure 4 . 3 . 4 98

 434 Figure 4.3. Output performance and applications of IMI-TENG. a) Influence of output voltage, current as a function of increasing %metal ratio. The output performance was immensely enhanced with the island architecture with decrease in %metal due to localized charge storage region. b) Maximum charge density of IMI-TENG with 100% and 20% metal ratio after reaching a stable saturation. c) Charge retention characteristic validating the effective charge accumulation during the continuous ON-OFF cycle for 1200 s. d) Dependence of output power on the external loads delivering a peak and RMS power output of 7.7 mW and 5.3 mW at 151 MΩ. e) Comparative 10

  understanding and technology maturation. It was shown that the problem of power generation and charge retention in a limited area is critical in this case, and this aspect of triboelectric materials has required a comprehensive investigation which led to the technical advances through inclusion of charge accumulation layer and metal island device architectures. This thesis carries out extensive research on conversion of mechanical energy to electrical energy based on triboelectricity. A few first-of-its-kind novelties have been demonstrated in terms of theory, device structure and applications. The key novelties are summarized as follows: Firstly, this thesis systematically studies the several physical characteristics to understand the critical parameters in selection of materials for triboelectrification. Various underlying surface mechanisms are investigated and analysed, with the device velocity and charge density being the key parameters driving the TENG performance.Secondly, two distinct nanogenerators was fabricated to study the effect of operational velocity on device performance, both based on practical applications. The fabricated triboelectric nanogenerators demonstrated the conversion of mechanical to electrical energy through ubiquitous ambient environment and human body motions, powering full-scale commercial electronic systems.

Figure A1. 1

 1 Figure A1.1 consists of 2 metal electrodes, copper (Cu) in this case, separated by insulating material under test. The surface under contact measures 40 x 40 mm. An optimized spacing distance of 7 mm and an operational contact-separation speed of 1 m.s -1 is set between the top Cu electrode and insulator. The top and bottom electrodes are connected to electrical tester, various measuring electrical parameters.

Figure A1. 1 .

 1 Figure A1.1. Device schematic used for material characterization, arranged based on triboelectric series position.

Figure A1 .

 A1 Figure A1.2a-c shows the comparative output performance output open-circuit voltage (VOC), short circuit current (ISC) and transfer charge (QSC). The triboelectric series would help to show which pair of materials may work the best to create intentionally large static electricity by contact electrifying two materials to enhance the performance of TENGs. Materials tested are arranged in the following order based on the triboelectric series position: Copper (Cu); Aluminum (Al); Polyurethane (PU); Polycarbonate (PC); Floor mat (Mat); Sand paper (SP); Polyethylene terephthalate (PET); Polypropylene (PP); Parylene-C (PaC); Polydimethylsiloxane (PDMS); Polyvinylidene Fluoride (PVDF); Ethylene chlorotrifluoroethylene (ECTFE); Static sheet foil (Static); Teflon (PTFE).

Figure A1. 2 .

 2 Figure A1.2. TENG electrical performance based on triboelectric series. The generated comparative output response a) open-circuit voltage in V, b) short-circuit current in µA and c) effective charge density in µC.m -2 of materials from triboelectric series following identical test conditions with copper electrodes.

  electrons flowing in the external circuit reached equilibrium in a shorter time, thus, increasing the output performance linearly along with increase of operational speed. To demonstrate the influence of force and speed on the output performance of TENG, the measurements were performed on PC-PTFE triboelectric materials pair with speeds varying from 0.1 to 10 ms -1 and force from 0.1 to 20 N as shown in Figure A1.3a-b.

Figure A1. 3 .

 3 Figure A1.3. Dependence of output power on contact force from 0.1-20 N (a) and operational speed under 0 to 10 m.s -1 (b).

Annex 2 :Figure A2. 1 . 2 108Figure A2. 2 . 1 .

 21221 Figure A2.1. The working principle of the WATENG. (I) There exists no electric potential difference between the top and bottom Cu electrodes. (II) As the wind enters the system, the PC flag displaces from its initial position towards the top electrode due to the pressure depression created in the venturi system design. (III) When the flag contacts the top PTFE film the negative charges would be created due to triboelectrification, leaving the PC positively charged. The charges on the Cu electrodes flows through the external circuit due to the electrostatic induction of the PC flag. (V-VII) The flag displaces towards the bottom electrode material driving the electron flow from bottom copper layer to the top electrode until the flag contacts the top electrode material transferring the charges to the PTFE -Cu electrode due to triboelectrification process giving rise to a potential difference between the top and bottom electrodes. A continuous alternating current is produced due to the periodic charge transfer flow between the top and the bottom electrode material.

Figure A2. 3 .

 3 Figure A2.3. Schematic showing the open and short circuit conditions.

2 111Figure A2. 4 .

 24 Figure A2.4. Schematic diagram of resistance ladder circuit.

Figure A3. 1 .

 1 Figure A3.1. Detailed fabrication process of the Interdigitated TENG (ID-TENG).

Figure A3. 2 . 3 115Figure A3. 3 : 3 116Figure A3. 4 .

 23334 Figure A3.2. a) Cross-sectional schematic of the ID-TENG. b) Digital photograph of two sections (top & bottom) of the fabricated ID-TENG. c) and d) Images of fabricated various electrode/ PTFE strips spacing.

Figure A3. 5 .

 5 Figure A3.5. Enlarged curves of output electrical response of various materials under study (Parylene, Kapton, Cu, Nylon and PC).

Figure A3. 6 .

 6 Figure A3.6. AFM images of sliding materials showing their roughness: PTFE (left) and PC (right).

Figure A3. 7 .

 7 Figure A3.7. Output current for various electrode-strip spacing.

Figure A3. 8 .

 8 Figure A3.8. Detailed images of the demonstrator components; MCU, power conditioning circuit and 3D printed holder.

Figure A4. 2 .

 2 Figure A4.2. Scanning electron microscope (SEM) images of the IMI composite film. Top-view and cross-section view (inset) showing the metal island architecture with the charge accumulation layer sandwiched in-between charge retention layer and substrate.

Figure A4. 3 . 4 122

 34 Figure A4.3. Simplified device configuration of the IMI-TENG.

Figure A4. 5 . 4 126Figure A4. 6 .

 546 Figure A4.5. a-b) Digital photographs and illustrations of the setup with the IMI composite film adhered on the fixed Cu electrode. c) AFM images of Cu and Pac surfaces for the surface topography and (d) roughness images taken with a Veeco NT 1100. e) Digital photographs of the IMI composite film with various metal island density from 100 to 20% metal with respect to the PTFE substrate (40 mm x 40 mm). Scale: 5 mm. The island architecture size varies from 5 mm to 250 μm squares. (L = Length of an island = Gap spacing between the islands). Cross-sectional SEM images of the (250 μm) IMI composite film

  3.1.6 Optimum output power determination ........................................................................................... 3.1.7 Applications ................................................................................................................................... 3.1.8 Conclusion .....................................................................................................................................

3.1.9 Acknowledgements ........................................................................................................................ References ...............................................................................................................................................

Table 1 .1 Summary

 1

	Technique		TENG mode	Area	Voltage	Current	Charge density	Reference
	Buried charge reservoir	Contact-separation	2 cm 2	140 V	18 mA.m -2	11 nC.m -2	[95]
	Composite bulk MoS2 flakes	Contact-separation	1 cm 2	145 V	350 µA.cm -2	0.93 µC.cm -2	[96]
	Barium coaxial nanofiber titanate-doped	Contact-separation	2.5 cm 2	1020 V	29 µA	58 µC.m -2	[97]
	Nanopillar-array PDMS	Contact-separation	2 cm 2	568 V	25.6 µA	85 μC.cm -2	[98]
	Surface excitation	Plasmon	Contact-separation	3.5 cm 2	350 V	36.25 mA.m -2	89.7 µC.m -2	[99]
	Transition layer addition	Contact-separation	4 cm 2	1010 V	40.6 mA.m -2	105 μC.m -2	[87]
	PDMS-CCTO composite film	Contact-separation	2.5 cm 2	390 V	170 µA	108 μC.cm -2	[48]
	Ferroelectric polarization	Contact-separation	1 cm 2	145 V	35 mA.m -2	118 µC•m -2	[96]
	Phase-inversion piezoelectric membranes	Contact-separation	9 cm 2	520 V	2.7 mA.m -2	150 μC•m -2	[100]
	Self-improving accumulation	charge	Contact-separation	4 cm 2	922 V	36 mA.m -2	490 μC.m -2	[51]
	Self-charge pumping	Contact-separation	7 cm 2	250 V	489 mA.m -2	1020 μC.m -2	[91]
	External charge pump with voltage stabilization	Contact-separation	5 cm 2	630 V	187 mA.m -2	1.25 mC.m -2	[52]
	Contact based charge excitation	Contact-separation	3.2 cm 2	1000 V	360 mA.m -2	2.38 mC.m -2	[101]

of various techniques employed to maximize the output performance by increasing the triboelectric charge density. The TENGs have exhibited substantial improvements of several orders of magnitude in charge density, as high as 2.38 mC.m -2 , through rational device design which results in efficient energy conversion and enhanced power performance.

  by coupling of contact electrification and electrostatic induction processes. It is therefore a transfer of electrical charges between different materials. When two different materials come into contact, there is a charge transfer which results in two charged materials, one positive and the other negative.

	Test procedures and measurement conditions	Chapter 2
	contact-separation
	2.1 Introduction
	When two objects are rubbed against each other and separated, electrical charges appear on their
	surface, which is termed as static charges or triboelectric charges. Triboelectricity was already
	known to the ancient Greeks: the philosopher Thales de Milet discovered that by rubbing amber
	on wool, such a phenomenon was created. In recent times, triboelectric nanogenerators (TENGs)
	35

Publication Status: Part of this work has been accepted and published in MRS Communications (DOI: 10.1557/mrc.2019.64) Aravind Narain Ravichandran a , M Ramuz a , S Blayac a a Mines Saint-Etienne, Department of Flexible Electronics, France. Author contributions: A.R. designed, fabricated and characterized the devices. A.R., S.B. and M.R. analyzed the experimental data and prepared the manuscript.

have attracted exceptional attention after its first report in 2012 by Prof. Wang's group presenting its simple, robust and high efficiency for energy harvesters.

[1]

[2] It works on the principle of

Table 3 .

 3 for various storage applications for IoT and sensor networks in remote locations, especially, the asynchronous non-real time applications with limited duty cycles.

	1) with the literature reports[24][26][32][33][34], which is 8 times higher than the existing
	power density of a wind based flutter-driven triboelectric nanogenerator operating in ambient
	conditions which work at higher wind velocity and larger device size. The proposed system is

  4.3d in log scale graph format, peak and RMS power of 7.7 mW and 5.3 mW were reached at 151 MΩ load, which is ~150 times higher than the output power (54 µW/ 38 µW) of structure with no metal inclusion (Test sample B). More information is presented in Annex 4. A maximum output power density of 4.8 W.m -2 was obtained, making IMI-TENGs a sustainable autonomous electrical power source for a wide array of potential smart applications such as IoTs, sensors, or wearable electronics. The device has been used to charge 10 µF capacitor and power 50 series-connected The charge conduction through the insulator was comprehensively analyzed to understand the role of the metal island inclusions and their influence on the output performance. Compared to the TENGs without metal inclusion, the intermediate metal inclusion structure demonstrated a ~150-fold enhancement in the output power performance.

commercial LEDs directly as shown in Figure

4

.3e-f, thus, demonstrating the IMI-TENGs viability as reliable power source for use in a wide array of practical applications.

4.6 Conclusion

In summary, a state-of-the-art technique to boost the output electrical performance of triboelectric nanogenerators by intermediate metal inclusion composite has been presented. By employing an island architecture, charges are refilled during each triboelectric operation cycle, allowing localized charge accumulation and storage in the intermediate metal islands. This increase in stored charges allows for a higher rate of charge transfer resulting in a maximum charge density of 1076.56 µC.m -2 with a maximum output power density of 4.8 W.m -2 , which is 2.1 times higher than the existing reports on charge densities.

  1(b), Vtotal = 0. The charges flow freely between the two electrodes.

	Compact high-performance wind actuated venturi TENG harvester	Annex 2
	0 = 𝑄 𝑡𝑜𝑝	2𝑤 ∈ 𝐴	-	|𝑄 𝑒 |(𝑤 + 𝑥) ∈ 𝐴	(12)
		𝑄 𝑡𝑜𝑝 =	|𝑄 𝑒 |(𝑤 + 𝑥) ∈ 𝐴	. 𝐶 𝑡𝑜𝑡𝑎𝑙	(13)
	The short-circuit current (Isc) is expressed as
	𝐼 𝑠𝑐 =	𝑑𝑄 𝑡𝑜𝑝 𝑑𝑡	=	𝑑 𝑑𝑡	(|𝑄 𝑒 | . ( 1 2	+	𝑥 2𝑤	))	(14)
		𝐼𝑠𝑐 = |𝑄 𝑒 | .	𝑣 2𝑤
						110

  with ɛptfe and tptfe being the permittivity and thickness of PTFE material. During contact, the charge creation depends primarily on the work function difference Vcon between triboelectric materials in The total capacitance Ctotal is given by relation between C2 and Ccon,

	𝐶 1 =	𝜀 𝑝𝑡𝑓𝑒 . 𝑆 𝑡 𝑝𝑡𝑓𝑒	; 𝐶 2 =	𝜀 𝑎𝑖𝑟 . 𝑆 𝑑 2	(4)
	contact,
			𝑉 𝑐𝑜𝑛 =	𝑄 𝑠𝑎𝑡 𝐶 𝑐𝑜𝑛	(5)
	Representing (6) in terms of Ccon,
			𝐶 𝑐𝑜𝑛 =	𝑄 𝑠𝑎𝑡 𝑉 𝑐𝑜𝑛	(6)
	𝐶 𝑡𝑜𝑡𝑎𝑙 =	𝐶 𝑐𝑜𝑛 . 𝐶 2 𝐶 𝑐𝑜𝑛 + 𝐶 2	= 𝐶 2 ( 1 + 1 𝐶 𝑐𝑜𝑛 𝐶 2	)	(7)
			𝑉 2 =	𝑄 2 𝐶 𝑡𝑜𝑡𝑎𝑙	(8)
		=	𝑄 2 𝐶 2	. (1 +	𝐶 2 𝐶 𝑐𝑜𝑛	)

  Ceq can be expressed as 𝐶 𝑒𝑞 = 𝐶 1 .𝐶 𝑐𝑜𝑛 𝐶 1 +𝐶 𝑐𝑜𝑛 and Q2 can be shown in terms of dt and Ceq, Where dt is the threshold distance and is given by 𝑑 𝑡 =

	Metal islands structure as power booster for high performance TENGs	Annex 4
											𝑄 2 . 𝐴 + 𝑄 2 = 𝑄 𝑠𝑎𝑡	(14)
											𝑄 2 (𝐴 + 1) = 𝑄 𝑠𝑎𝑡	(15)
							𝑄 2 =	𝑄 𝑠𝑎𝑡 1 + 𝐴	=	1 +	𝐶 1 𝐶 2	𝑄 𝑠𝑎𝑡 . (1 +	𝐶 2 𝐶 𝑐𝑜𝑛	)	(16)
						=	1 𝐶 1	.	1 𝐶 1	+	𝑄 𝑠𝑎𝑡 1 𝐶 2 +	1 𝐶 𝑐𝑜𝑛	=	1 𝐶 1	.	𝑄 𝑠𝑎𝑡 𝑑 2 𝜀 𝑎𝑖𝑟 . 𝑆 +	𝐶 𝑒𝑞 1	(17)
										=	𝐶 𝑒𝑞 𝐶 1	.	𝑄 𝑠𝑎𝑡 (1 + 𝑑 2 . 𝜀 𝑎𝑖𝑟 . 𝑆 𝐶 𝑒𝑞	)	(18)
										𝑄 2 =	𝐶 𝑒𝑞 𝐶 1	.	𝑄 𝑠𝑎𝑡 (1 + 𝑑 2 𝑑 𝑡	)	(19)
	We know, E=	𝑉 𝑑	, then,
													𝐸 2 =	𝑄 2 𝜀 𝑎𝑖𝑟 . 𝑆	(20)
	Substituting (20) in (21), we get a simplified form,
										𝐸 2 =	𝐶 𝑒𝑞 𝐶 1 . 𝜀 𝑎𝑖𝑟 . 𝑆	.	𝑄 𝑠𝑎𝑡 1 + 𝑑 𝑡 𝑑 2	(21)
														𝐶 𝑒𝑞 𝜀 𝑎𝑖𝑟 .𝑆	. Furthermore, E1 can be expressed
	as,
									𝐸 1 =	𝑄 1 𝜀 𝑝𝑡𝑓𝑒 . 𝑆	=	𝑄 𝑠𝑎𝑡 -𝑄 2 𝜀 𝑝𝑡𝑓𝑒 . 𝑆	(22)
						𝐸 1 =	1 𝐶 1 ɛ 𝑃𝑇𝐹𝐸 . 𝑆 𝑄 𝑆𝐴𝑇 [1 -= 𝑄 2 𝐶 2 𝑄 1 = 𝑄 𝑠𝑎𝑡 -𝑄 2 . (1 + 𝐶 2 𝐶 𝑐𝑜𝑛 𝐶 𝑐𝑜𝑛 𝐶 1 ɛ 𝑃𝑇𝐹𝐸 2 ɛ 𝑎𝑖𝑟 2 (1 + ) 1	𝑑 𝑡 𝑑 2	)	]	(11) (12)
									𝑄 𝑠𝑎𝑡 -𝑄 2 = 𝑄 2 .	𝐶 1 𝐶 2	. (1 +	𝐶 2 𝐶 𝑐𝑜𝑛	)	(13)
	Assuming, 𝐴 =	𝐶 1 𝐶 2	. (1 +	𝐶 2 𝐶 𝑐𝑜𝑛	), we get,
														123

Acknowledgements

Acknowledgements

Firstly, I wish to express my deepest gratitude to my thesis director Prof. Sylvain Blayac for the continuous support of my Ph.D. study and related research, for his patience, motivation, and immense knowledge. His guidance helped me in all the time of research and writing of this thesis. Acknowledgements excitement. Their presence was very important in a process that is often felt as tremendously

Acknowledgements

Research was financed under the AMPEERS-2 project by the theme ''Networks and energy storage''. The authors thank the technical support of ID-FAB (funded by local authorities and European Union community), Roger Delattre and Paolo Musardo for their discussions on the device characterization and COMSOL simulations.

List of Tables

 Table 1.1

Summary of various techniques employed to maximize the output performance by increasing the triboelectric charge density…………………..………………...…27

Abstract

The deployment of IoTs and mobile objects heavily relies on batteries as power sources for integration to new applications. Alternatively, triboelectric nanogenerators (TENGs) can be seen as a promising technology by harvesting mechanical energy for powering small-scale electronics.

The integration of TENGs into practical applications within a limited power requirement, is still unexplored considering its attractive properties. In this work, a state-of-the-art multilayer integrated TENG structure with high output power density is devised compatible with practical applications to promote the technological maturity of TENGs. The proposed system consists of a sliding based TENG power source integrated to drive an e-ink™ display screen application controlled by an efficient power management unit. Through simultaneous investigation of the triboelectric power source and the application system integration focusing on power balance, an Applications: Friction-based TENG in a power-balanced fully autonomous system Chapter 3 70 optimized self-sufficient system has been demonstrated by using a well-designed hand powered sliding motion test platform, respectively. The integrated system reaches a maximum output power density of 750 mW.m -2 , adequate to drive the target application. The realization of an efficient and low cost integration with reliable power output performance showcases TENG as an attractive potential for the next generation of autonomous electronic and wireless devices.

Introduction

With the development of the Internet of Things (IoTs) and autonomous devices, their implementation and integration in day-to-day applications are increasing rapidly with the estimated number of objects to be around 26 billion currently [1,2]. These devices must be powered by batteries [3,4] exhibiting limitations in terms of space occupied and cost with minimal environmental degradation. Alternatively, energy harvesting technologies provide unlimited operating life for low power electronics and in some cases, even remove the need for batteries where it is costly, high maintenance, or unsafe. Mechanical energy available in human daily activities have received increasing attention by researchers for converting mechanical body motions into effective electricity [5][6][7][8].

In recent times, triboelectric nanogenerators (TENGs) have attracted exceptional attention after its first report in 2012 presenting its high potential for fulfilling the needs for compact and cheap power sources [9,10]. The technology readiness level (TRL) describes the technology maturity focusing on the background research, its adoption and commercialization [11,12]. For advancements of TENGs as a reliable power source eliminating the need for batteries, the TRL should mature from research (TRL 4) to development (TRL 5) phase through device demonstration, optimization, and implementation in practical applications. The lateral sliding mode has been the most adaptable device configuration of TENG to human body motion owing to high energy conversion efficiency and easy to design [13,14]. Various reports of lateral sliding TENGs have been reported for integration into smart applications such as wearables [15,16],

sensors [17,18], textiles [19,20] and so on.

Lateral sliding mode TENGs offer numerous advantages, with their wide range of applications and simple integration [21][22] [23][24] [25]. However, the much-needed power, optimization, and capability to drive applications autonomously is yet to be achieved. Typical transient low power application such as secure elements, IoTs and Wireless Sensors Network; consists of embedded microcontrollers operating in low power feature modes (Active, Idle and Sleep modes), a minimum

where

is the device capacitance with permittivity ε and surface area S, whose variation in minimum and maximum is induced by the variation of distance d. Some previous concepts have been proposed to increase the localized charge density: nanoparticles additions, multi dielectric layer and self-improving structure (Figure 4.1a) but they remain temporary phenomena as the charges decay over time. [16][17][18][19][20] In all cases, the value of QSAT is limited by the nature of the insulating material, which has an intrinsic low density of states. Wang et al. demonstrated the concept of triboelectric charge trapping leading to a higher output performance in TENGs. [21] The present work demonstrates the metallic island architecture charge accumulation layer in a composite triboelectric structure allowing the increase of localized charges, generating a strong increase in electrical output power density as shown in