Fundamentals of TENGs
Triboelectrification is a scientific term to describe the phenomenon in which two different materials become electrically charged after they are physically brought into
Figure 1.2 Electron-cloud-well-potential model for explaining triboelectrification with respect to electron transfer and release between two materials (modified from ref 39 , Copyright 2019 Elsevier Ltd.)
4 contact by rubbing one on the other (i.e., by friction) and then separated 39 It has been known for 2,000 years, and it exists everywhere and at any time in our life However, the fundamental physics of this universal phenomenon is blurry until a quantitative investigation was reported in 2018 40 This study revealed that the electron transfer is the dominant mechanism for triboelectrification, and the authors also proposed an electron- cloud-well-potential model based on fundamental electron cloud interaction to explain the electron transition process for all types of materials (Figure 1.2) Initially, material A is separated from material B with a distance between electron clouds of d so that electrons cannot transfer and are trapped in their respective potential well When two materials contact each other, their electron clouds strongly overlap and then the electron could transfer from one atom to the other As two materials are separated, most of transferred electrons are remained in the material B by the surface potential barrier E 2 , resulting in the positively charged material A and the negatively charged material B With temperature increasing, electrons can run off the potential well and likely to be thermionically emitted, showing an exponential decay of surface charges The proposed electron transition model, which is regarded as “Wang transition” model, provide a new perspective to realize the origin of triboelectrification in a general case 41
1.1.2 Principle theory and mathematical model of TENG
From the fundamental physics, the theory of mechanical-to-electrical energy conversion based TENG is developed based on the expanded the Maxwell’s displacement current 42,43 As previously introduced, when two materials are physically contact, triboelectrification will induce electrostatic charges on contacting surface which eventually promote time-varying surface polarization By adding this surface polarization term to the classical Maxwell’s equations, the principle theory of TENG can be arisen based on the revised Maxwell’s displacement current density as follows
𝜕𝑡 where D is the electric displacement vector, ε is the permittivity of the medium, E is the electric field and P s is the polarization contributed by the presence of surface polarization charges from triboelectrification effect (Figure 1.3) Here, the first term represents the displacement current density due to time variation of electric field, and the second term is the displacement current density due to the time-varying surface polarization as two
5 materials in contact under mechanical agitation By integrating the current density over the surface plane, the total displacement current is obtained
𝜕𝑡 where Q is the total free charge on the electrode In this equation, the left-hand side is the displacement current which is the internal driving force for converting mechanical energy into electrical energy, and the right-hand side is the capacitive conduction current which is the observed current in the external circuit
To further analysis, an ideal structure of a contact-separation TENG is presented which consists of two different dielectric materials with back electrodes coated on them
An external load is connected to the electrodes, which will absorb the electricity generated by the cyclic contact and separation of the two dielectric materials Through the
Figure 1.3 Schematic showing the principle theory of displacement current for nanogenerators (including TENG) that derived from the expanded Maxwell’s equations (modified from ref 42 , Copyright 2019 Elsevier Ltd)
6 mechanical impact, two dielectric surfaces are oppositely charged with a surface charge density of ±σ tribo, which is independent to the gap distance z The presence of triboelectric charges induces an electrostatic field that prompts the electron transfer between electrodes with transferred charge density of ±σ tr, which is subject to z Therefore, the mechanical energy can be converted into electrical energy regarding the change in z
Since two oppositely charged surfaces separated by some distance can be considered as a parallel plate capacitor with variable capacitance, the equivalent circuit of TENG may be represented by a capacitive model, where the mathematical expression for the electrical potential difference between two electrodes is given by
𝐶(𝑧)𝑄 + 𝑉 𝑜𝑐 (𝑧) From the governing equation, there are two terms on the right side: the first one, -Q/C(z), relates to the transferred charges between electrodes where C(z) is the capacitance between two electrodes, and the second one, V oc(z), is originated from the polarization of triboelectric charges Then, the equivalent circuit of TENG can be illustrated by a serial connection of an ideal voltage source and a variable capacitor as shown in Figure 1.4
1.1.3 Working mechanism and operation modes of TENG
The working mechanism of TENGs is a coupling of triboelectric effect and electrostatic induction, in which the former inspires the static polarized charges on two material surfaces and the latter involves in the circulation of free charges through the external circuit 44 In detail, under mechanical action, two materials with different electron
Figure 1.4 The ideal structure and equivalent circuit (capacitive) model of the contact- separation TENG (modified from ref 36 , Copyright 2018 WILEY-VCH Verlag GmbH &
The ideal structure and equivalent circuit (capacitive) model of the contact-separation TENG (modified from ref 36 , Copyright 2018 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)
7 affinities rub against each other, and then the triboelectric charges are brought on at the contact interfaces The material with higher electron affinity, deemed as electron acceptor, becomes negatively charged, while the other, esteemed as electron donor, turns into positively charged In principle, the total charge amounts distributed on the surface of both materials are the same As soon as two charged materials are separated by a distance, an electric potential difference is established between two electrodes so that it drives free charges to flow from one electrode to the other through the external circuit When applying the force again, two materials move forward each other, which will descend the established potential difference and cause a reverse current This process occurs until two materials make contact, and then the next cycle starts Apparently, during the contact- separation cycle, the TENG produces a periodical electric output with positive and negative direction
By fusing different direction of the polarization change and electrode configuration, the TENG operation has been categorized into four modes (Figure 1.5),
Figure 1.5Basic operation modes of TENG The TENG operation has been categorized into four modes, including vertical contact-separation (CS) mode, relative-sliding (RS) mode, single-electrode (SE) mode, and freestanding (FT) mode with their own merits and demerits (modified from ref 45 , Copyright 2018 Elsevier Ltd)
8 including vertical contact-separation (CS) mode, relative-sliding (RS) mode, single- electrode (SE) mode, and freestanding (FT) mode 45 In the CS mode, the relative motion perpendicular to the interface is applied, and the electric output is determined by the gap between material surfaces 46 The RS mode is deployed by applying a relative displacement in the direction parallel to the interface, and the electric output is a result of a non-fully compensated triboelectric charges at the mismatched areas 47 The SE mode is a specialized design to harness mechanical energy in arbitrary direction, where the ground is now assigned as reference electrode, and the triboelectric material, without using any electric conductor, can be attached to a freely moving object 48 The FT mode is an adaptation of the SE mode, where a pair of symmetric electrodes is used instead When the freely moving object dislocates from its position, an asymmetric charge distribution appears between the object and the electrodes, triggering the electric charge transfer between the two electrodes 49 The theory of each operation mode has been thoroughly explored and discovered in previous studies 50,51 In some specific applications, TENG may not operate in just a single mode, but could rather be a conjunction of different modes to take benefits from each other
Based on four operation modes, tremendous amount of TENGs has been elaborated in a broad range of scenarios The applications of TENG can be divided into four major categories (Figure 1.6): micro/nano direct power sources for self-powered systems (MDPS), active self-powered sensors (ASPS), basic network units for harvesting low-frequency water wave energy (LFWE), and direct power sources for high voltage instruments (HVPS) 52 Firstly, TENG, with the ability to harvest mechanical energy from the surrounding environment, can directly power up billions of electronic devices Abundant structures of TENG are demonstrated to generate electricity from various mechanical energy sources, including wind energy 53 , raindrop energy 54 , sonic energy 55 , and ocean energy 56 Secondly, TENG, through its electrical signal characteristics (e.g amplitude and frequency), can reflect the strength of mechanical input without using additional signal conditioning circuit Besides, the number of transferred charges on triboelectric material surface is considered a highly sensitive probe for a variety of sensing performances Some highlighted works include angle sensors 57 , touch sensors 58 , acoustic sensors 59 , acceleration sensors 60 , and chemical sensors 61 Thirdly, due to the excellent capability of harvesting low-frequency mechanical energy, TENG can effectively collect
9 energy in water waves Various prototypes have been designed, including the solid-liquid contact electrification TENG 62 , fully enclosed TENG 63 and TENG network 64 Thus, TENG is a perfect complement to the traditional electromagnetic generator, which normally requires a high operation frequency, to fulfill the energy production for either present macro-grid or future micro-grid Lastly, TENG is known for its ultrahigh open- circuit voltage and low short-circuit current, which makes it applicable for driving high- voltage electrical appliances without auxiliary transformer Several self-powered systems based on high-voltage TENG has been developed, such as micro/nano electromechanical systems 65 , capacitive sterilization systems 66 , electrostatic manipulation systems 67 , and micro-plasma control systems 68 However, there are still challenges and difficulties ahead
Figure 1.6 Four major applications of TENG including micro/nano direct power sources for self-powered systems (MDPS), active self-powered sensors (ASPS), basic network units for harvesting low-frequency water wave energy (LFWE), and direct power sources for high voltage instruments (HVPS) (modified from ref 52 , Copyright 2020 The Authors)
10 for the efforts to promote TENG toward industrialized markets due to the discrepancy between current TENGs’ capacity, durability and stability with real demands In order to diminish such gaps, a development roadmap of TENG is proposed with four key stages: design innovation, optimum performance, system integration and industrialization 36
An early view of liquid-solid TENG
So far, TENG mechanism has dominated by solid-solid case, but its own performance can be flawed by several reasons The first reason comes from its high sensitivity to environmental conditions, especially atmospheric humidity and pressure 69 The second reason is the surface abrasion of contact interface after long-term friction, which causes an adverse effect on the durability as well as the sensing stability of the TENGs 70 The third reason arises from the improper contact between two solid surfaces inflicted by a large mechanical force To overcome these issues, liquid materials can be used instead in the triboelectrification process to contrive a liquid-solid mechanism According to the natural wettability property, the performance of liquid-based TENGs are not affected by environmental factors such as humidity Further, the liquid layer can act as lubricant to provide wear resistance for abrasive conditions, resulting in a better durability and stability of liquid-based TENGs Last but not least, a liquid can easily change its shape so that it makes a complete contact with the solid surface, enhancing triboelectric output signals Liquid-solid TENGs can harvest mechanical energy from diverse sources such as raindrops, tidal waves, and low-speed water streams 71 Besides, the application of the liquid-solid TENGs as self-powered physical and chemical sensors has also been extensively studied 72
Motivation and Objectives
The liquid-solid TENG, with its remarkable strengths, has opened an additional direction for harvesting environmental energy, alongside the prominent solid-solid TENG Nowadays, most of reported liquid-solid TENGs have been applied to harness energy offered by water wave, river flow or rainfall In contrast, there is a lack of liquid-solid TENG research regarding the power generation from low-frequency behaviors, even though they are omnipresent in human life and contain a huge amount of mechanical energy This shortage motivates an innovative approach to elevate the role of liquid-solid TENG in the awareness of low-frequency mechanical energy harvesting Thus, the
11 overall goal of this research is to develop and analyze different types of water-solid TENGs for scavenging energy of low-frequency mechanical excitations The major objectives of the proposed research are categorized in the following directions:
● Development and analysis of a rotational switched-mode water-based triboelectric nanogenerator (RSW-TENG) for harvesting the rotational kinetic energy The concepts and results of the proposed TENG device can be utilized to design an active sensor with the ability of road slope and wheel speed detection
● Development and analysis of a discontinuous conduction based rotational triboelectric nanogenerator (DCR-TENG) with radially symmetrical structure The radially symmetrical structure helps to synchronize the electrical output from independent TENG cells which can improve the instantaneous power effectively The results show that the TENG device can become an efficient power source for the self-powered applications
● Development and analysis of an impulsive kinetic energy regulator (IKER) for harvesting mechanical energy through low frequency impulse-excited motion The combination of harmonic oscillator and mechanical motion rectifier can manage the interchange of kinetic energy and potential energy to actuate the rotary solid-liquid TENG operation excessively It can be referred to as a decent option for harvesting mechanical energy from human and machine activities.
Organization of the Thesis
As stated in previous section, this research is written up with systematic insight into three main objectives which are addressed in five chapters The presented studies from chapter 2 to chapter 6 are summarized as follows:
● In chapter 2, fundamental physics of liquid-solid triboelectrification is briefly introduced, as well as an introduction to the early researches, and structure designs of water-solid TENGs is given
● In chapter 3, design, fabrication, and experimental characterization of the rotational switched-mode water-based triboelectric nanogenerator is studied Initially, the working mechanism of the device, and then the output performance of the fabricated device under various dynamical conditions, is fully investigated The energy output capability of the proposed device is demonstrated considering different electrical loads Finally, the practical application of the device for vehicle monitoring is proved
● In chapter 4, design, fabrication, and experimental characterization of the discontinuous conduction based rotational triboelectric nanogenerator is studied The device includes six independent water-solid TENGs that are radially symmetrical alignment The performance of both single TENG and synchronized TENGs are evaluated under various structural, dynamical, and electrical conditions The presented device shows an adequate ability toward harvesting mechanical energy as well as sensing application
● In chapter 5, design, fabrication, and experimental characterization of the impulsive kinetic energy regulator is studied The device can transform the supplied kinetic energy into the relevant (elastic or gravitational) potential energy and vice versa depending on thepreset configuration; thus, it prolongs the running cycle of the device The proposed device offers a cost-effective and simple structure which is potentially useful for extracting the low frequency impulse-excited energy
● In chapter 6, the summary and conclusions of the overall research as well as the final remarks are given In addition, perspectives on future work related to this thesis are presented.
Fundamentals of liquid-solid triboelectrification
Liquid-solid triboelectrification, which occurs at the liquid−solid interface, has been known for a long time, especially in the studies about electrochemistry 73 , catalysis 74 , etc Earlier, it was suggested that the ion adsorption was responsible for the contact charging between the liquid and the solid surface because of the involvement of the solution 75 However, recent studies, resting on methodological experiments, pointed out that both electron transfer and ion adsorption simultaneously occur in the liquid-solid triboelectrification 76 Hence, the basis of liquid-solid triboelectrification was revisited, and based on the inclusion of the electron transfer in liquid-solid triboelectrification, a
“two-step” process is proposed by Wang et al 77 , where the electron transfer plays a dominant role in the formation of the electric double layer (EDL)
The EDL model with “two-step” process for the formation, on account of both electron transfer and ion adsorption, can be seen in Figure 2.1 In the first step, water molecules and ions in the solution contact with the solid surface, and the overlap of electron cloud between water molecules and solid atoms brings on the electron transfer from the water molecules to the solid material At the same time, some ions are also absorbed on the solid surface due to electrostatic interaction (physical adsorption) and chemical reaction (chemical adsorption) Later, the water molecules are ejected from the interface owing to the agitation of the solution such that most of the transferred electrons are preserved on the solid surface In the second step, free ions in the solution are attracted by the induced electrostatic field and concentrate at the region close to the electrified surface Subsequently, the EDL is formed with two different regions: the Stern layer and the diffuse layer The Stern layer includes all the transferred electrons and adsorbed ions that attached on the solid material, while the diffuse layer contains free ions with a higher
14 concentration of counterions that decays with distance from the charged surface
There are several factors that affect the liquid-solid triboelectrification, such as the temperature and pH of the solution, solutes used in the solution, hydrophobicity of the solid surface, etc 78 For instance, the hydrophilic surfaces, which have higher solid-liquid interfacial energy, are more likely to form covalent bonds with substances in solution, leading to the dominant contribution of ion adsorption In contrast, the hydrophobic surfaces, which hold smaller interfacial energy, are more likely to form intermolecular bonds, and thus, the electron transfer dominates the liquid-solid triboelectrification (Figure 2.2) From these perspectives, different characteristics have been reported as key contributors to the improvement of liquid-solid triboelectrification process 79 With respect to solid phase, material selection, surface morphology and surface modification
Figure 2.1 Illustration of hybrid EDL model with “two-step” process formation (a) In the first step, water molecules and ions in the solution contact with the solid surface, causing electron transfer as well as ion adsorption on the solid surface (b) In the second step, free ions in the solution are attracted by the induced electrostatic field and concentrate at the region close to the electrified surface, forming the EDL (modified from ref 77 , Copyright 2021 American Chemical Society)
15 are vital properties to obtain a higher hydrophobicity and surface charge density The ionic activity, polarity and volatility of the contact liquids are indispensable properties that can influence the density of the charges at the solid-liquid interface.
Mechanism of liquid-solid triboelectric nanogenerator
Similar to solid-solid TENG, the electricity generated by liquid-solid TENG is based on the coupling of triboelectrification and electrostatic induction, in which triboelectrification involves in the formation of EDL, and electrostatic induction creates a current flow regarding the potential difference developed through contact-separation process A plain illustration for the liquid-solid TENG is shown in Figure 2.3, where a water-PDMS based TENG is taken into consideration 80 Before the contact, both PDMS and water contain no charges When the PDMS layer impacts the water, an EDL is formed at the contact interface so that the PDMS becomes negatively charged, and the water becomes positively charged to maintain the charge equilibrium When two layers are separated, two electrodes attached to both PDMS and water rise the opposite charges correspondingly, yielding a potential difference in the external circuit As a consequence, electrons will flow through the external circuit, generating a positive output current The triboelectric outputs greatly depend on the separation distance (d) between the two electrodes, which will get maximum when the PDMS layer was taken back to its original position (d = d 3) If the PDMS moves to contact again with the water, the potential
Figure 2.2 Effect of surface hydrophobicity on the electron transfer and the ion transfer When the water contact angle is higher than 90º, the ratio of electron transfers to ion transfers (E/I) increases rapidly, asserting the dominance of electron transfer This can be explained by the chemical bond of hydrophilic (1) and hydrophobic (2) surface (modified from ref 78 , Copyright 2020, The Authors)
16 difference between the two electrodes will decrease, and the electrons will flow in the opposite direction until a full contact is attained In a series of contact and separation, a continuous electric output can be generated.
Structural design of liquid-solid triboelectric nanogenerator
Because liquid materials appear under various shapes and dimensions, there are a vast number of possible designs for liquid-solid TENG; however, it can be classified into two groups: open systems and closed systems The open systems refer to devices in which
Figure 2.3 Mechanism of water-PDMS based TENG (a) Initial state when no force is applied (b) PDMS layer and water contact each other (c) PDMS layer separates from water (d) Separation completes and the PDMS layer comes back to original position (e) PDMS layer come to contact with water again, starting a new cycle (modified from ref 80 , Copyright 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim).
17 the liquid materials come from exterior sources, interacting with solid materials for triboelectrification These systems are susceptible to the change of environment so that their output may be unstable The common structures of open systems include droplet- based TENGs and bulk liquid-based TENGs Whereas, the closed systems refer to devices in which the liquid materials are sealed inside and make use of incidents from the external environment for triboelectrification This kind of design, which is specified as liquid- filled TENGs, can minimize the environmental affection and has a more stable output
The droplet-based TENGs (D-TENGs) are designs that primarily concern the energy harvesting from rainfall, which carries a huge amount of mechanical energy due to their large quantity and continuous motions The two common designs for this task are single-electrode mode and sliding free-standing mode They are typically attached to the umbrella or outside of the building, by which they can take a full benefit of available raindrops In most cases, the interaction between raindrops and the solid layer has a crucial contribution to determine the output performance of the D-TENGs In this regard, the hydrophobicity of the contact surfaces, which results in a complete separation of the water droplet after the contact, is a key parameter to enhance the electricity generation
On that account, Liang et al introduced the multi-unit transparent TENG (MT-TENG) with “Top-Down” and “Bottom-Up” structure 81 The MT-TENG was fabricated with hydrophobic surface so that it exhibited a self-cleaning character, and can be integrated with vehicle glasses, building glasses for harvesting clean energy from raindrop, as shown in Figure 2.4a The MT-TENG and its units (T-TENGs) are operated in single-electrode mode As for structure design, the MT-TENG can amplify the transferred charges in one cycle, thus enhancing its power generation efficiency as compared to T-TENG On another approach, Zhang et al reported a self-cleaning/charging power system (SPS), which compounded of a hydraulic triboelectric nanogenerator (H-TENG) and several embedded fiber supercapacitors (FSCs), for harvesting energy from the falling raindrop 82 The H-TENG, with self-cleaning effect derived from the hydrophobic property of the surface layer, effectively converted power from raindrops into electricity, while the FSCs were used to store that generated energy The H-TENG is deployed by sliding free- standing mode, as displayed in Figure 2.4b A specific effort for harvesting energy from water droplet was proposed by Yun et al., where the authors developed interdigital electrode (IDE) based TENGs to increase the triboelectric current 83 In comparison to
18 one-electrode and two-electrode design, the IDE-based TENG produced multiple current peaks following the continuous back-and-forth movement of the accumulated charges A cone-shaped IDE-based TENG was demonstrated to harvest energy from scattered water droplets, as exhibited in Figure 2.4c Here, the output of this device is changed by adjusting the volume and speed of water droplets through the folding angle To maximize the utility of droplet-based TENG, integration of D-TENGs with to other energy collectors is an appealing direction For example, Liu et al integrated D-TENGs with solar cells to scavenge energy from sunlight and raindrops at the same time 84 A hydrophobic and transparent PDMS-based TENG was anchored on the top of a solar cell
Figure 2.4 Different structural designs of droplet-based TENG for harvesting raindrop energy (a) Schematic diagram and single-electrode mechanism of the multi-unit transparent TENG (MT-TENG) in integrating with vehicle and building (modified from ref 81 , Copyright 2016 Elsevier Ltd) (b) Schematic diagram and sliding free-standing mechanism of the self-cleaning/charging power system (SPS) (modified from ref 82 , Copyright 2018 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim) (c) Schematic diagram and working mechanism of the cone-shaped interdigital electrode (IDE) based TENG (modified from ref 83 , Copyright 2017 Elsevier Ltd) (d) Schematic diagram of the integrated system consisting of solar cell and water-drop TENG as well as the mechanism of the TENG (modified from ref 84 , Copyright 2018 American Chemical Society)
19 to compose the hybrid device, as shown in Figure 2.4d This as-fabricated system would not only reduce the soiling effect but benefit from the combination of high-voltage output level of the TENG and high-current output level of the solar cell
Compared to droplet, bulk liquid contains greater energy and is widely distributed in nature such as water flows (e.g., stream, river, etc.) and water waves (e.g., sea, ocean, etc.) In the case of water flow, Jiang et al presented a water wheel TENG (ww-TENG) based on sliding freestanding mode 85 This water wheel, with 8 superhydrophobic-coated paddles on each side, was driven by low speed flowing river and an AC type signal could be detected (Figure 2.5a) In addition, the influence of electrode size, rotating speed and properties of water was also examined under different working conditions Another similar configuration of ww-TENG was proposed by Cheng et al., except that it operated in single-electrode mode 86 (Figure 2.5b) In both designs, a stream of flowing water hits the surface of ww-TENG, rotating it, and giving rise of triboelectrification In the case of water wave, due to the unpredictable motion of water waves, a lightweight flexible structure, which facilitates the interaction between the installed device and the surrounding contact liquid, is mandatory for a long-term stability Besides, advanced waterproofing and antibiofouling coatings should be considered to shelter devices from on-site aggressions Figure 2.5c shows an all-in-one liquid–solid electrification-enabled generator (LSEG) with scalable planar structure 87 The “flooding and draining” cycles of water waves causes a repetitive emerging–submerging process of the LSEG, giving rise to alternating flows of electrons between electrodes Further, by featuring wide electrodes and nanostructured hydrophobic surface, the electric output was substantially boosted A different concept for water wave TENG was introduced by Liu et al., where Ū electrode structure (a mixture of bar electrode and U-shape electrode) performed effectively the power extraction from waves’ inflow and outflow processes 88 (Figure 2.5d) The advantage of this configuration is that it hugely minimizes the screening effect from loosely counter ions in diffuse layer, and thus enhance the outputs The bar electrode and the U-shape electrode can work separately, or synergize with each other as Ū electrode, to collect water energy Apart from the two most common cases as already mentioned, where the liquid material plays a moving part during the operation of TENG device, there is a less attentive case of bulk liquid-TENGs that relies on the dynamic of solid material instead, namely liquid-immersed TENG In this system, the solid material, now as moving
20 part, sinks down then separating from the liquid, and current signal is generated through this process An example of liquid-immersed TENG is illustrated in Figure 2.5e, in which
Figure 2.5 Different structural designs of bulk liquid-based TENG for harvesting water energy (a) Schematic diagram and working principle of ww-TENG driven by flowing river (modified from ref 85 , Copyright 2019 Elsevier Ltd) (b) Schematic diagram and working principle of ww-TENG driven flowing water (modified from ref 86 , Copyright
2014 American Chemical Society) (c) Schematic diagram and working principle of LSEG for harvesting energy from a variety of water motions (modified from ref 87 , Copyright 2014 American Chemical Society) (d) Schematic diagram and working principle of Ū-shape electrode TENG for extracting power from waves’ flood and ebb processes (modified from ref 88 , Copyright 2019 Elsevier Ltd) (e) Schematic diagram and working principle of liquid-immersed TENG (modified from ref 89 , Copyright 2021 American Chemical Society)
21 the Cu wire wrapped by multi-dielectric layer moves up and down in a tube filled with
DI water 89 It was observed that the increment of immersion depth as well as the immersion speed were beneficial to improve the electric output It also suggested that a multifunctional TENG array comprising many single-wire TENG could be effective in large-scale energy harvesting
Long-term experience in open environment will bring up vulnerability that significantly degrades the performance of the exposed-liquid-contact-solid TENGs, especially those operating in harsh environments Therefore, liquid-filled TENGs, where liquid material is sealed and moves inside the device, are an interesting alternative to help formulate the liquid-solid triboelectrification Since both friction materials are encased in the packaged device, liquid-filled TENG suffers negligible performance degradation with excellent mechanical durability In addition, because liquid-filled TENG carries a fixed amount of liquid, evaporation-free and contamination-free conditions are essential for long-lasting operation Figure 2.6a shows a rotational water TENG based on the water flow of a partially filled rotating cylinder 90 During its operation, water continually slid
Figure 2.6 Different structural designs of liquid-filled TENG for harvesting water energy (a) Schematic diagram and working principle of rotational water TENG (modified from ref 90 , Copyright 2016 Elsevier Ltd) (b) Schematic diagram and working principle of water tube-based TENG (modified from ref 91 , Copyright 2021 Wiley-VCH GmbH) (c) Schematic diagram and working principle of “SWING stick” TENG (modified from ref 92 , Copyright 2015 Tsinghua University Press and Springer-Verlag Berlin Heidelberg) (d) Schematic diagram and working principle of MSW-TENG (modified from ref 93 , Copyright 2022 The Authors)
22 around the patterned electrodes on the inner surface of the TENG, and, in consequence, an alternative current was created In addition, the dynamic behavior of water, which correlated with the water volume and the angular velocity, was observed to highly impact on the electric output of the device Benefiting from the flexibility and easy-to-flow of water, a multi-mode water-tube-based TENG (WT-TENG) was developed with deionized (DI) water put inside small FEP tubes 91 (Figure 2.6b) Four main working modes, including rotation, swing, seesaw and horizontal linear motion, of the WT-TENG are highly desired in many energy harvesting scenarios such as ocean, wind, vibration, and biomechanical motions Besides, it indicated that design parameters (tube diameter, electrode distance, electrode length) are highly related to the electric output of the device Utilizing the tubular system, the “SWING stick” TENG, with the merit of compact size, was suggested to harvest handshaking mechanical energy 92 As illustrated in Figure 2.6c, the superhydrophobic nanostructured aluminum tube was connected to the bare aluminum tube with the two ends bounded by rubber caps, and the formed structure was filled with water The electricity can be generated through the relative motion of water inside, from the coated tube to the bare tube and vice versa Further, it was indicated that the electric output of the device varies with various types of water, where the current was negatively correlated with the concentration of the solution Based on the notion of direct contact between liquid and the conductive material, a mobile stick-type water-based TENG (MSW-TENG) was implemented, in which water inside the MSW-TENG directly contacts the electrode 93 A PFA cylinder, which served as both the substrate and triboelectric material, was sealed both ends with two inner electrodes, and two other outer electrodes were attached on the sides of the device (Figure 2.6d) One inner electrode and one outer electrode were paired up to form a freestanding type TENG When water contacted inner electrode, the electric output was generated owing to the charge accumulation and separation induced by the charge on the PFA surface.
Applications of liquid-solid TENGs
As mentioned in previous section, TENG applications could be split into four major categories: micro/nano power sources, active self-powered sensors, large-scale blue energy harvesting and high-voltage power sources Among them, several remarkable progresses for liquid-solid TENGs in first three areas will be discussed in this section
Since water is abundant in nature under various shapes and different forms of motion, many applications regarding liquid-solid TENG have been developed for collecting those water energy that make them a good candidate as power source 94–96 A networked integrated triboelectric nanogenerator (NI-TENG) was introduced by Zhao et al for harvesting energy from water waves 97 The two-dimensional electrode arrays structure helps the NI-TENG to be resistant to either regular or highly random water wave motion and achieve a high and stable electric output This NI-TENG efficiently harvests dynamic energy of random water waves to drive a wireless transmitter with a driving voltage of 5.8 V and a power consumption of 10 mW (Figure 2.7a) It takes 67 s to charge a 22 μF capacitor to achieve the driving voltage for the first transmission and takes 53 s to charge for another transmission This realizes the feasibility of the NI-TENG as a power supply for sensor nodes in a wireless sensing network A grid of water-dielectric single electrode mode triboelectric nanogenerators (WDSE-TENG) was exposed by Jurado et al for harvesting breaking wave impact energy 98 As shown in Figure 2.7b, the
Figure 2.7 Liquid-solid TENGs as power sources (a) Structure of networked integrated TENG (NI-TENG) with arrayed bridge rectifiers and its ability to power a wireless transmitter (modified from ref 97 , Copyright 2018 American Chemical Society) (b) Diagram of the grid of WDSE-TENG for harvesting water impact energy with the potential of driving low-power electronic devices (modified from ref 98 , Copyright 2020 Elsevier Ltd) (c) Diagram of integrated TENG for raindrop energy harvesting and its demonstration of powering small electronic devices (modified from ref 99 , Copyright
2019 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)
24 grid of WDSE-TENG has the potential to drive a wireless transmitter as well as an ultrasonic range sensor The wireless transmitter was active after charging a 47 μF capacitor to a voltage of 3.19 V–9.81 V based on the transmission distance The energy stored in the capacitor and the discharging power of the capacitor were between 193.56 μJ and 2.24 mJ, and 322 μW to 3.73 mW, respectively Besides, a 470 μF capacitor was charged using the grid of energy harvesters to power up the ultrasonic range sensor In this setup, the relevant stored energy and the discharging power were 5.96 mJ and 5.18 mW, respectively This confirmed the potential to power up electronic devices of the grid of WDSE-TENG, and hence provide a potential energy source for self-powered wireless sensing systems at water-structure interfaces An integrated TENG for harvesting the raindrop energy was successfully developed by Liu et al., which inductively coupled plasma (ICP) method was conducted to enhance the hydrophobic surface 99 Figure 2.7c shows that an umbrella with integrated TENG on the surface directly illuminates 39 LEDs at simulated raindrop rate of 22 mL∙s −1 Under the same conditions, a 33 μF capacitor was charged for 477 s so that it reached 1.8 V The stored energy in capacitor was applied to continually power a thermometer for approximately 6 s
In the new era of IoT, we are facing challenge about how to sense the world more precisely and faster Additionally, energy crisis issue requires a more economic measure towards battery independence 20,100 Liquid-solid TENG, which demonstrates merits of high sensitivity, low price, easy fabrication, and shape adaptive of liquid, are likely candidates to develop self-powered sensors A water level sensor based on liquid–solid tubular triboelectric (LST-TENG) was investigated by Zhang et al., which is made of multiple copper electrodes uniformly distributed along a PTFE tube 101 When water rises/falls inside the tube, the peaks in the derivative of open-circuit voltage with respect to time are in correspondence with electrode distribution and directly related to the water level height (Figure 2.8a) A ship dynamic draft was successfully detected using the LST-TENG with the accuracy of 10 mm, which shows great reliability as a robust and accurate water level sensor Furthermore, Xu et al investigated a wave sensor based on liquid- solid interfacing triboelectric nanogenerator (WS-TENG) for real-time monitoring of the wave around the simulated offshore platform 102 These works show great industrial potential in liquid–solid interface monitoring for marine field For real-time monitoring the engine lubricating oil of vehicles, Zhao et al demonstrated a self-powered
25 triboelectric sensor using oil–solid interacting TENG (O–S TENG) under different conditions 103 The O–S TENG is attached to the inner wall of the oil tank and the electric signal can be obtained from the contact-separation between the oil wave and the TENG solid surface Figure 2.8b shows the output voltage of O–S TENG decreases in accordance with high fraction of waste oil, which typically comes from wear debris, deposited carbon, and age-induced oxygen-containing groups adsorbed on the TENG surfaces It has a great performance for monitoring lubricating oils for different mechanical systems in a cost-efficient way For applications in biomedical devices, Hu et al designed a superhydrophobic liquid–solid contact TENGs for accurate monitoring of clinical drainage operations and intravenous injection or blood transfusion 104 In this design, the superhydrophobic TENG with four pairs of copper electrodes was firmly attached to the inner wall of the silicone rubber tube for preparing the tubular drop counter Combining the tubular drop counter and intravenous infusion tube, it is possible to
Figure 2.8 Liquid-solid TENG as active self-powered sensors (a) Schematic diagram and structure of LST-TENG for ship draft measurement The water level is detected through the peaks and valleys of voltage derivation signal (modified from ref 101 , Copyright 2019 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim) (b) Photograph of oil–solid interacting TENG (O–S TENG) for monitoring the engine lubricating oil and its output performance with respect to the fraction of waste oil (modified from ref 103 , Copyright 2021 The Authors) (c) Structural diagram of superhydrophobic liquid-solid contact TENG with current output generated by six types of droplets and its biomedical sensor prototype (modified from ref 104 , Copyright 2020 American Chemical Society)
26 effectively and precisely count the droplet number and perceive the infusion speed in real time Other distinctive liquid-solid TENG based self-powered sensors include humidity sensor 105 , pH sensor 106 , chemical sensor 107 , liquid volume sensor 108 , tactile sensor 109 , body motion sensor 110 , air pressure sensor 111 , and micro total analysis system 112
2.4.3 Networks of liquid-solid TENG for blue energy harvesting
The energy harvested from ocean, which is regarded as blue energy, is inexhaustible and convenient as the ocean covers more than 70% of the earth's surface, and gathering blue energy is a key priority for TENG technology Using thousands of liquid-solid TENG units and effectively connecting these individual TENGs to form large-scale arrays present a huge challenge to widely harvest the blue energy In this regard, Li et al designed a liquid–solid-contact buoy TENG to harvest the blue energy 113
A small network with 18 buoy TENGs was fabricated to measure the relationship between the outputs and the unit numbers (Figure 2.9a) The maximum voltage, current and transferred charges is about 300 V, 290 àA and 16 μC, respectively; also, the current and the charge have linear relation with the unit number For further extension, a large
Figure 2.9 Networks of liquid-solid TENG for blue energy harvesting (a) Concept of liquid–solid-contact buoy TENG with the illustration of TENG network and different types of triggering movement The outputs show linear relation with the unit number (modified from ref 113 , Copyright 2018 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim) (b) Concept and fabrication of droplet-based TENG arrays on ship model with the output performance in water tank (modified from ref 114 , Copyright 2021 American Chemical Society)
27 quantity of blue energy could be harvested when thousands of such buoy TENGs are integrated On the other hand, an integrated array consisting of 24 droplet-based TENGs was fabricated to harvest wave energy from ocean 114 (Figure 2.9b) The output performance of three different solutions (rain water, sea water and DI water) was investigated where the maximum voltage, current and transferred charges is about 237 V,
746 nA and 360 nC, respectively, for the case of DI water This result show that the device has observable output performance, and it can be used for large-scale ocean energy harvesting.
Conclusion
With decent features of high contact intimacy, surface wear resistance and high softness of liquid, the liquid-solid TENG demonstrates higher output performance and energy conversion efficiency at low-frequency mechanical impact From this viewpoint, it has been used as an energy harvesting technology in various scenarios for harvesting irregular energy in the environment However, most of researches on liquid-solid TENG are still in the laboratory stage, and it is expected to see continuous improvement in many aspects toward commercialized ending
Introduction
Road vehicles have played an important role in our modern society, primarily for passengers and goods transportation, because they are a must-use for delivering passengers and goods to precise destinations; and so, driving has become a part of human’s life 5 However, there are many risks during driving activities that could induce the rise of causality; therefore, monitoring and warning of the hazardous possibility are very essential 4,115 One of the most common threats in the roadway is hill descent driving where the gravity would cause the vehicle to move faster and may go out of control Hence, road slope estimation, particularly downhill slope estimation, is necessary to prevent the quick acceleration of the vehicle, so that it can improve safety through a more accurate cruise control To perform the road slope estimation, many kinds of sensors are utilized, including GPS receivers, inertial sensors, pressure sensors, and automobile onboard sensors, which have their limitations under certain operation conditions Moreover, with the rapid technological evolution towards the Internet of Things (IoT), the size and power consumption of up-to-date sensors significantly decrease such that the self-powered sensors are headed for as a succeeding solution
With regard to the light-duty transportation safety aspect, there are many interesting works where TENGs was introduced as self-powered sensors for monitoring real-time operating conditions of vehicle Most of the reported TENGs for vehicle sensing application rely entirely on the solid-solid contact electrification principle; nevertheless, the solid-based TENGs still remains some limitations Hence, water-based TENGs progressively get more notice and are available for competing in the self-powered sensing
29 application Consequently, a rotational switched-mode water-based TENG (RSW-TENG) is presented and examined (Figure 3.1) The RSW-TENG is composed of a water-filled cylindrical TENG with PTFE as solid-phase triboelectric material, in which every single electrode is split into two portions, one portion as the rotating electrode and the other as the stationary electrode When the cylindrical component rotates as well as the rotating electrode, the RSW-TENG is typically isolated with an external circuit except at designated locations where the rotating electrode adjoins the stationary electrode every half cycle This allows the RSW-TENG to accumulate induced charges and instantly releases them following the closed-circuit state so that the output current could enhance significantly Additionally, owning to the conditional conductive of RSW-TENG, the relative position of the electrode with respect to water in the closed-circuit interval has a relevant impact on the output performance Thereby, the angular sensing has been verified with good stability and sensitivity, which represents the capability of RSW-TENG as a road slope sensor.
Methods
3.2.1 Fabrication of the RSW-TENG
The RSW-TENG is assembled from two subunits, specifically as a rotating unit and a stationary unit (Figure 3.2) The rotating unit is an enclosed cylinder with a diameter of 90 mm and a depth of 12 mm Two identical semicircle copper foil with the radius of 43 mm are secured opposite to each other on an inside base of the cylinder, regarding the gap of 2 mm; and then, two small pieces of copper are separately coupled with each copper foil through a wired connection to complete the rotary electrode element
Figure 3.13D model design of the RSW-TENG driven by a DC motor
A PTFE membrane circle of 90 mm diameter is subsequently bonded over the copper layer, followed by the filling of water inside the cylinder The PTFE is chosen as friction layer due to its high negativity in the triboelectric series which prefers to gain triboelectric charges, along with high volume and surface resistance as well as excellent water repellency which help the tribocharges remain on the surface for a long time, extending the working life of friction layer The water is deposited up to about half-filled capacity of the cylinder such that it would entirely cover a semicircle electrode if they are in parallel The stationary unit is vertical mounting support with two pieces of copper attached in line with each other as electric brushes Finally, the cylinder and its support are put together with the help of an equipped ball bearing for smooth-running work
To implement electrical measurement, a dc geared motor (GB37Y3530-12V- 90EN, China) with a driver (Motorbank DMD-400, Korea) was used to drive the TENG (Figure 3.3) The output voltage and output current of the TENG were measured using a digital multimeter (Keithley DMM7510, USA) and a low-noise current preamplifier (Stanford Research Systems SR570, USA) with the load resistance of 20 MΩ An electrometer (Keithley 6517B, USA) was used to measure net charges on the capacitor
An Arduino Mega was used in associated with the driver to control the dc motor as well
Figure 3.2 Fabrication of RSW-TENG It includes two components: a cylindrical rotating
TENG with extended electrodes and a stationary electric contact with brushes The electrodes contact the brushes twice every cycle
31 as collecting the data from the TENG device in actual wheel experiments.
Results and Discussion
3.3.1 Basic operation and working mechanism of the RSW-TENG
The structure of RSW-TENG, including a rotating cylinder and a support stand Two semicircle copper foils as electrode layer and a circle PTFE membrane as dielectric layer are sequentially secured inside the cylinder to complete the solid phase of TENG The deionized (DI) water is selected for the liquid phase of the TENG and poured into the cylinder until it totally covers an electrode surface During rotation, the water remains static due to gravitational force; meanwhile, the PTFE membrane and the electrodes move along with the cylinder in synchronous speed Then, the overlapping area between water and each brush at the closed-circuit condition certainly affects the electrical output performance which will be discussed later
The working principle of RSW-TENG derives from the sliding freestanding mode, in which PTFE is selected as the triboelectric counterpart in the water contact electrification A full running cycle of RSW-TENG is depicted in Figure 3.4 and described as follows Initially, since water is in contact with the PTFE membrane, the opposite polarity occurs at the water-PTFE interface such that the PTFE surface becomes negatively charged and the water surface becomes positively charged This appearance can be explained by means of charge distribution at solid-liquid interfaces following the
Figure 3.3 Experimental setup for evaluating the characterization of the RSW-TENG
“two-step” model or Wang’s hybrid layer As the cylinder rotates, the overlapping area between water and each electrode has a change with an increment for one electrode and a decrement for the other electrode so that the electric potential difference between the two electrodes is gradually developed (Figure 3.4(ii)) It happens until the closed-circuit condition is established, inducing a charge transfer from the former electrode to the latter one (Figure 3.4(iii)) The presence of induced current also concludes the first half cycle of RSW-TENG operation, and the new electrostatic equilibrium arises It is recognized that the closed-circuit state just occurs in a very short time; therefore, charge leakage and impedance mismatch could be minimized to enhance the electrical output of the TENG
In the second half cycle, once the TENG backs to off-state, the charge flow is terminated appropriately such that no current signal could be observed; and the functionality of each electrode relative to water is interchanged following the TENG rotation concurrently (Figure 3.4(iv)) During this stage, the electric potential difference turns up again after the charge balance breakdown and arouses a current flowing through the external circuit as soon as the electrical conduction is exposed (Figure 3.4(i)) These steps already reveal a complete operating cycle of RSW-TENG, and the next cycles could be perceived
Figure 3.4Working mechanism of the RSW-TENG and the corresponding position of electrodes and brushes in one cycle of revolution
33 likewise Furthermore, the course of transferred charges across the external load is always unchanged according to the symmetrical alignment of SE in company with static water, which literally ends up with a unidirectional current It is realized that the on-state potential difference, i.e., output voltage, depends on the relative position of dual electrodes with respect to water, which is defined as a phase angle To more extent, the output voltage of RSW-TENG could be expressed by the following relation
𝑉 0 =𝜎𝜋𝑟 2 2𝐶 where σ is the tribocharge density, r is the radius of semicircle electrode and C is the on- state capacitance of the RSW-TENG In this configuration, the on-state capacitance C is the serial connection of C 1 and C 2, where C 1 and C 2 are equivalent capacitance between each electrode and water So, at an arbitrary phase angle θ, C 1 and C 2 are given by
2𝑑 where ε 0 is the permittivity of free space, ε r is the relative permittivity of dielectric membrane and d is the thickness of dielectric membrane Finally, C is obtained as
𝐶 =𝜀 0 𝜀 𝑟 𝑟 2 2𝜋𝑑 (𝜋𝜃 − 𝜃 2 ) and thus, it is evident that the output voltage of RSW-TENG decreases corresponding to the increment of phase angle as well as on-state capacitance Accordingly, a slope angle sensing device can be adopted by taking advantage of this property
3.3.2 Output performance of the RSW-TENG
For further investigation into the empirical performance of RSW-TENG, a variable-speed DC motor is utilized to drive the rotary cylinder either clockwise or counterclockwise (Figure 3.5) In relation to basic demonstration, the phase angle is set to be zero degree together with the rotation speed of 1.25 Hz, by which the water body could fully cover an electrode at closed-circuit condition Figure 3a-b show the output voltage and output current corresponding to the relevant settings with peak voltage of 1.78 V and peak current of 0.91 μA, respectively It is visible that both electric signals
34 are unidirectional and come up with a spike waveform such that the value sharply increases from zero to maximum then decreases back to zero in a short time Additionally, there are two spikes to be recorded in one revolution of the RSW-TENG, which successfully verifies the operating mechanism Alongside, the electrical output in the case of clockwise rotation is alike those in the case of counterclockwise rotation, thanks to the rational coupling of SE and RE As depicted in Figure 3.4, at the conducting point, the electrode covered by water always connects to the left brush; meanwhile, the exposed electrode consistently joins to the right brush so that the electric charges just traverse the external circuit on one-way path regardless of rotational direction From Equation 1, the maximum output voltage indicates a linearly proportional relation to the accumulated charge density, which varies very little during operation but does not affect by external resistance Thereby, the peak output voltage shows nearly unchanged peak magnitude (approximately ranging from 1.68 V to 1.82 V) despite the change of resistor as illustrated
Figure 3.5 (a) Output current and (b) output voltage of RSW-TENG regarding clockwise and counterclockwise rotation at zero-degree phase angle and rotating speed of 1.25 Hz
Figure 3.6 Peak output current, peak output voltage and peak output power under different load resistances at zero-degree phase angle and rotating speed of 1.25 Hz
35 in Figure 3.6 In relevance, the peak output current with respect to resistance turns out an inversely proportional relationship, in which its power level drops down from 0.91 μA at
20 MΩ to 0.18 μA at 100 MΩ Likewise, the peak output power is subjected to load change such that it reduces from 1.59 μW to 0.31 μW when the resistance increases from
20 MΩ to 100 MΩ Hence, it implies that the RSW-TENG can deliver power more effectively when operating at low load resistance The output power of the RSW-TENG can use to charge capacitors without the necessity of a rectifier circuit (Figure 3.7(b)) or directly light up several LEDs in series (Figure 3.7(a)) Here, the maximum charging voltage is up to 1.41 V for every tested capacitor with the settling time of 5.63 s, 38.22 s, and 85.76 s with regard to the capacitance of 1 μF, 4.7 μF, and 10 μF, respectively Besides, as shown in Figure 3.8, the RSW-TENG ensures good durability and stability over time in which the output voltage just suffers a degradation by less than 1% after 30 days of operation Through these inspections, it turns out that the RSW-TENG possesses the necessary attributes to be developed into a self-powered device
Figure 3.7(a) Demonstration of the RSW-TENG as power source for (a) continuously lighting up LEDs in series and (b) charging different capacitors
Figure 3.8 Durability and stability of the RSW-TENG over time
As mentioned previously, the phase angle of RSW-TENG, as defined in Figure
3.9, has a significant impact on the output voltage and output current such that it directly quantifies the intrinsic capacitance as derived in Equation 3 On closer inspection, the output current and voltage are measured under various phase angles under common rotating speed of 1.25 Hz, in which the SE is predefined at a specific slope compared to the static water level As demonstrated in Figure 3.10, the electrical output has a descending tendency regarding the ascending order of slope such that the output current degrades steadily from 0.91 μA at null degree to 0.08 μA at 80 degree Simultaneously, the output voltage loses its power by roughly 88%, downgrading from 1.78 V to 0.21 V when the slope changes from zero degree to 80 degree, respectively Apparently, phase angle would determine the effective covered area of both electrodes and thus the on-state dielectric capacitance of RSW-TENG Suppose that a constant tribocharge density is held, the rise of on-state capacitance leads to the deficiency of electric field across two electrodes, resulting in a falloff in potential difference Therefore, the incremental change to phase angle, which increases the on-state capacitance correspondingly, could inflict an appropriate turn-down on output voltage and output current of the RSW-TENG Besides,
Figure 3.9 Diagram of phase angle definition.
Figure 3.10Current and voltage with phase angle from 0 degree to 80 degree at running speed of 1.25 Hz
37 the charge accumulation in a 10 μF capacitor under different phase angles is also examined and depicted in Figure 3.11, and it is obvious that the larger phase angle, the less charge accumulated on the capacitor From another perspective, the electrical output of RSW-TENG shows up a linearly proportional relationship with respect to the increase of rotating frequency, as can be seen in Figure 3.12 During observation, the phase angle is locked up such that it remains constant at zero degree Starting from 0.5 Hz, the rotating frequency ramps up step by step and lastly achieves the highest speed of 1.25 Hz which is in association with the enhancement of peak output current from 0.38 μA to 0.91 μA and peak output voltage from 0.71 V to 1.78 V This is because more opposed charges on the dielectric surface due to screen effect should be swept away as the rotating speed increases, which amplifies the potential difference along with the transferred charge between electrodes Thus, it is inferred that the output current and output voltage could
Figure 3.11 Charge accumulation on 10 μF capacitors at different phase angle
Figure 3.12Current and voltage with rotating frequency from 0.5 Hz to 1.25 Hz at phase angle of 0 degree
38 be improved by boosting the rotating speed Further, a thorough relation of output characteristics of RSW-TENG with phase angle and rotating frequency is presented in
Figure 3.13 Obviously, the issue of short on electrical output at larger phase angle can be overcome by increasing the rotating speed; however, up to certain limitations, this compensation becomes less appreciable The first one is the physical limits of the testbench assembly and the second one is to prevent the dispersion of water when running at high speeds Related to the dispersion of water, we also conducted an experiment in which the rotational speed of the driving motor adapted to a monotonically increasing function The derived results are displayed in Figure 3.14 and accordingly, the stability of the water is no longer held when the rotation speed exceeds 1.5 Hz Consequently, when surpassing this rotating speed, the electric output will become distorted where the output frequency cannot be recognized.
RSW-TENG as vehicle monitoring device
Figure 3.133D graph of output current and output voltage under effect of both phase angle and rotating frequency
Figure 3.14 Experimental results for validating the dispersion of water regarding the rotating speed of (a) 1.5 Hz, (b) 1.75 Hz and (c) 2 Hz
The RSW-TENG features a clear sensitivity to the variation of phase angle and rotating speed that fairly qualifies for road slope sensing in the vehicle monitoring system
It is pointed out in many studies that the road slope has a significant impact on the vehicle speed and the engine load, in which gravitational force decelerates the vehicle when moving upward (i.e., the engine works more) and accelerates the vehicle when moving downward (i.e., the brake is applied more) Therefore, it is necessary to determine the slope of the roadway for improving safe driving and fuel economy For further examination, a prototype of vehicle wheel attached RSW-TENG (wheel diameter of 135 mm) is constructed, as depicted in Figure 3.15-3.16, to specify the sensing attribute regarding diverse input As illustrated in Figure, through the sliding mechanic, the RSW-
Figure 3.15 Experimental setup of vehicle wheel attached RSW-TENG where the wheel TENG prototype is attached on a slope-adjustable framework.
Experimental setup of vehicle wheel attached RSW-TENG where the wheel TENG prototype is attached on a slope-adjustable framework.
Figure 3.16(a) Assembly of wheel-TENG prototype and (b) Installation of wheel-TENG prototype in cooperation with motor-driven pulley system
TENG keeps synchronously rotating along with the wheel so that it can continuously generate the electric signal During excitation, two electrodes are always aligned parallel to the road surface no matter what the slope is; and thus, the RSW-TENG can sense the change of road slope in relation to the change of phase angle when the vehicle goes uphill or downhill In particular performing analysis, the wheel prototype is manipulated at a constant speed of 0.1 m/s on a linear slide rail with the length of 1.8 m and this setup is consistently experienced in the following occasions Firstly, in the case of 10-degree road
Figure 3.17The voltage signal corresponding to 10-degree slope
Figure 3.18The voltage signal corresponding to 20-degree slope
41 slope (Figure 3.17), the quantized voltage with eight consecutive peaks is recognized with the mean peaks value of 1.15 V Subsequently, in Figure 3.18-3.19, the representational voltage for the 20-degree slope and 30-degree slope is exhibited in which the 20-degree test ends up with the mean peaks value of 1.01 V; meanwhile, the mean peaks value of 0.88 V is obtained subject to 30-degree test The mean squared error (MSE) and standard deviation (STD) as regards each measurement acquire 0.203 % and 0.048 V for 10-degree slope, 0.108 % and 0.035 V for 20-degree slope as well as 0.033 % and
Figure 3.19The voltage signal corresponding to 30-degree slope
Figure 3.20The voltage signal corresponding to running speed of 0.1 m/s
0.019 V for 30-degree slope Owning to small MSE and acceptable STD during operation, the RSW-TENG shows good reliability and high accuracy to the road slope detection task Besides, experiments about wheel speed monitoring are conducted where the rotational speed of the wheel is measured with respect to the acceleration of the prototype Figure
3.20 presents the characterized voltage of the wheel as the prototype is driven at the translational speed of 0.1 m/s, where the wheel rotational speed ω wheel can be approximated by the function ω wheel = π/ΔTp-p (ΔTp-p is time difference between two
Figure 3.21The voltage signal corresponding to running speed of 0.2 m/s
Figure 3 22 The voltage signal corresponding to running speed of 0.3 m/s.
43 consecutive peaks) Based on the approximation, the mean speed value of 1.394 rad/s is observed with the corresponding MSE of 0.319 % and STD of 0.061 rad/s When the prototype is leveled up its speed to 0.2 m/s (see Figure 3.21), the running wheel gets the mean speed value of 2.812 rad/s in relevance to the MSE of 0.621 % and STD of 0.085 rad/s Once the prototype reaches the rate of 0.3 m/s, the wheel concludes with the mean speed value of 4.182 rad/s in addition to the MSE of 2.907 % and STD of 0.184 rad/s as displayed in Figure 3.22 Obviously, the wheel speed detection has fairly good reliability and accuracy; and in association the road slope detection, they can bring about a promising approach in vehicle safety monitoring systems.
Conclusion
This chapter introduces the RSW-TENG as an active transducer for harvesting rotational kinetic energy of the wheel in addition to the ability of road slope and wheel speed detection The RSW-TENG is a rotary cylindrical TENG in combination with rotating-stationary electrode pairings where the electrical output is affected by the spatial alignment of these electrode pairings as well as the rotating speed of TENG Particularly, the electrical output demonstrates an inversely proportional relationship to the phase angle but in proportion to the rotating speed Later, based on the established relationship, the RSW-TENG is secured to a wheel model for evaluating the slope and speed detection functionality In both cases, the RSW-TENG shows up good reliability and accuracy in its performance, which provides a high potential solution for road slope sensing and wheel speed sensing in vehicle monitoring systems
Introduction
With the lightning speed of technological evolution, the size and power consumption of electronic devices have been drastically optimal, resulting in the rapid and widespread development of wireless sensor networks (WSNs) So far, the elemental components in WSNs, namely sensor nodes, have been powered by batteries, whilst the stored energy is limited, leading to cut down on sensor numbers as well as network runtime This inherent drawback of the systems is worth considering paying attention, specifically, in hard-to-access locations or uninterrupted operation, as it not only expands maintenance cost but also the overall performance loss Furthermore, the battery production and disposal process also pose negative impacts on the environment As a result, seeking for a self-renewing energy source that continuously replenishes the energy consumed by the sensor is required, hence, energy harvesting techniques have been promoted and quickly developed into an interesting research area 16–18 With various combinations of materials selection and dedicated structure, liquid-solid TENGs have been demonstrated that they can harness mechanical energy from different sources However, the liquid-solid TENGs normally come out with alternative current (AC) output where they cannot directly apply to the typical sensors, which limits them in practical applications In this case, the presence of a rectifier circuit is mandatory to convert AC power into direct current (DC) power, but it also brings up an additional energy loss to the energy conversion process Therefore, a useful method that can provide DC output without using the rectifier circuit is necessary in terms of improving the energy conversion efficiency 116 Besides, the output of liquid-solid TENGs are usually low, and
45 synergizing the outputs of multiple TENG units is a potential suggestion to enhance the instantaneous output of energy harvesting devices
According to those concerns, a discontinuous-conduction-based rotational TENG (DCR-TENG) with radially symmetrical design is developed (Figure 4.1), which takes into account the contact electrification between water and PVDF membrane, to convert mechanical energy into electrical energy without using the rectifier circuit The principal strategy is the utilization of the motion-activated switches where these switches are simultaneously closed at designated positions during the operation of the TENG The motion-activated switch is composed of two parts, i.e., a rotating part and a stationary part When the switches are on, the rotating part joins up with the stationary part to provide a pathway so that the TENG can deliver energy to the external circuit; otherwise, the connection between the TENG and external load cannot be established, and thus, no energy is released at all This design allows the induced charges accumulating on one electrode, then instantaneously discharging into another electrode when the switches are triggered, which significantly enhance the output power of the TENG Furthermore, the TENG can generate a unidirectional current so that we can directly charge a capacitor, which improves the energy conversion efficiency appropriately The radially symmetrical design helps to synergize the output of individual TENG cell so that the instantaneous energy is greatly improved With these features, the DCR-TENG can become an efficient and reliable power source for the self-powered WSNs or other applications.
Methods
Figure 4.1 Assembly model of the DCR-TENG with radially symmetrical design
4.2.1 Fabrication of PVDF nanoporous membrane
For preparing the tribomaterial, a polyvinylidene fluoride (PVDF, M.W ~ 530,000) was purchased from Sigma-Aldrich (Sigma-Aldrich, St Louis, MO, USA) and
N, N-Dimethylformamide (DMF, 99.5%) was obtained from Samchun (Samchun Chemical Co., Ltd, Korea) Besides, the copper tape and adhesive glue were purchased from a local market To process the PVDF nanoporous membrane, we have embraced a phase inversion technique In detail, the PVDF solution was prepared by dissolving PVDF in DMF with a weight ratio of 1:9 and stirring at 60°C for 6 hours to obtain a viscous homogeneous solution Then, the solution was kept at room temperature for 3 hours to remove the air bubbles before it was blade-coated onto the glass substrates with a blade- to substrate gap of 125 μm Finally, the coated film was promptly immersed into a coagulation bath for 1 hour and dried under the ambient condition for 24 hours to form the PVDF nanoporous membrane For analyzing the characteristics of fabricated PVDF membrane, its morphology was observed using a field emission scanning electron microscope (FESEM, JSM-6500F, Jeol Co., Tokyo, Japan), whereas the contact angle was analyzed to evaluate the hydrophobicity of the membrane by SmartDrop (Femtofab
4.2.2 Fabrication of the DCR-TENG
All DCR-TENG’s components were fabricated by using 3D printing method which is a simpler and more flexible process of assembly The device composition includes two parts, namely rotating disk TENG and stationary electric contact (SEC) framework (Figure 4.2) The rotating disk TENG includes six built-in TENG cells where each one consists of elements: a hollow cylinder funnel and its corresponding lid The diameter of a single TENG cell is 4 cm with an effective area of 6.28 cm 2 They are tight to each other such that no leakage is guaranteed after the water is half-filled the funnel For each TENG cell structure, a pair of copper electrodes in the style of freestanding is covered by a PVDF nanoporous membrane as the hydrophobic dielectric layer, then they are secured on the inner side of the lid Electric wires from electrodes are linked to contact rods, which are uniformly distributed and fixed on the surface of a grooved cylinder such that two contact points create a line, which is parallel to the line of separated electrodes, respectively This cylinder is then coaxially attached to the rotating shaft of the disk TENG In the SEC framework, a pair of brushes are fastened oppositely on the horizontal
47 axis as SEC’s terminal This terminal connects to an external circuit in order to extract energy from the TENG during operations
To implement electrical measurement, a DC motor was used to drive the TENG The output voltage and output current of the TENG were measured using a digital multimeter (Keithley DMM7510, USA) and a low-noise current preamplifier (Stanford Research Systems SR570, USA) with the load resistance of 20 MΩ An electrometer (Keithley 6517B, USA) was used to measure net charges on the capacitor.
Results and Discussion
4.3.1 Characteristics of PVDF nanoporous membrane
The porous structure is a compact structure with widely distributed pores that could increase the hydrophobicity of the pristine PVDF membrane In particular, the porous structure modifies the surface roughness of the pristine PVDF membrane following the fact that an increase in roughness would leads to an increase in membrane hydrophobicity The contact surface of the PVDF nanoporous membrane is characterized by FE-SEM to determine the hydrophobic property It is found that the contact angle revealed the value of 131.1 degree which proves the obvious hydrophobicity of the PVDF nanoporous membrane Using AFM analysis, it indicates the root mean square roughness (R q) values of pristine PVDF and nanoporous PVDF about 55.6 nm and 142.5 nm, corresponding to the contact angle of 99.4 º and 131.1 º , respectively (Figure 4.3)
Figure 4.2 (a) Structural design and (b) Fabrication of the DCR-TENG Inset: mechanical switch composition
Altogether, the combination of porous structure and highly negative charged PVDF can increase the TENG output performance
4.3.2 Working principle of the single-cell DCR-TENG
The working principle of single-cell implementation relies on the freestanding sliding mode of TENG which is demonstrated in Figure 4.4 When the DI water comes into contact with the PVDF layer, the contact electrification occurs so that it makes electrons moving from DI water into the PVDF surface, resulting in the negatively charged PVDF and positively charged DI water Consequently, the formation of an electric double layer (EDL) at the liquid-solid interface derives an electric potential difference between two electrodes Thereby, as illustrated in Figure 4.4(i), the DI water contacts with the PVDF layer and fully overlaps electrode E1 At this point, the switches are triggered such that the electrode E1 is coupled with the right terminal of SEC while the electrode E2 is coupled with the left terminal of the SEC This allows two electrodes to merge with the SEC to form a conductive path where the electrode E1 connects to the right terminal of SEC while the electrode E2 connects to the left terminal of the SEC Hence, an instantaneous discharging takes place to release all accumulated charges on the electrode E2 under the condition of maximum electric potential difference, resulting in a flow of charge between two electrodes, then this charge imbalance is neutralized Due to the rotation of the disk, the TENG cell turns around in which electrode E1 and electrode E2 start to interchange their position; meanwhile, DI water sticks to its position because of gravity As a consequence, the covered area of electrode E1 gradually decreases whereas the covered area of electrode E2 slowly increases, breaking the electrostatic equilibrium During this period, two electrodes and the SEC are separated, i.e., the
Figure 4.3(a) FE-SEM image of the PVDF nanoporous membrane with the water contact angle AFM images of (b) pristine PVDF and (c) nanoporous PVDF membrane with the water contact angle of each material
49 switches stay open so that the inductive charges cannot transfer between two electrodes and they remain on the electrode E1 as shown in Figure 4.4(ii) As the DI water fully overlaps electrode E2, the accumulated charge and also the electric potential difference reach their maximum values; simultaneously, two electrodes are integrated with the SEC but now the electrode E1 is paired up with the left terminal of SEC while the electrode E2 is paired up with the right terminal of SEC It activates the switches to make a closed circuit; then, all accumulated charges on the electrode E1 are instantly released which allows the maximum charge transfer between two electrodes as demonstrated in Figure
4.4(iii) The charge distribution on each electrode is now defined as
= + where σ is the surface charge density, S is the surface area of the membrane and C i is the water-electrode equivalent capacitance This capacitance can be expressed as follows
= where i stands for the corresponding electrode (i = 1 or 2), ε is the permittivity of the
PVDF membrane, S Ei is the area overlapped by the water, and d is the thickness of the membrane In this circumstance, because the electrode E2 is fully overlapped, its water- electrode capacitance is much higher than the other one (C2 >> C1); thus, Q2 gets the maximum value of σ S while Q1 gets almost zero value When the disk keeps rotating, the covered area of two electrodes changes inversely As well, two electrodes and the SEC are also separated, making the switches to be deactivated Hence, the charge transfer cannot continue, and all inductive charges should stay at the electrode E2 as can be seen in Figure 4.4(iv) Since the disk turns back to the original position, the electrode E1 is entirely covered, yielding the reverse in water-electrode capacitance relation (C1 >> C2) so that Q1 gets the maximum value and Q2 gets nearly zero These four stages represent a full cycle of the current generation process, and if we rotate the disk further, a steady current can be produced appropriately It is noticed that a pulsed current signal is obtained as a result every time the switches are prompted; and by taking advantage of the fully charged and discharged action, there is expected to be a great enhancement in output
50 energy Furthermore, as previously discussed, during the operation of the TENG, the electric potential of the SEC left terminal is always getting higher than the SEC right terminal Thus, the charges have the attraction to transfer from the left to the right terminal at all times, it implies that deriving a unidirectional current property
4.3.3 Output performance of single cell DCR-TENG
To evaluate the electrical characteristics of the single-cell DCR-TENG, it was driven by a DC motor at a constant operating speed of 18 rpm and a 20 MΩ resistor was connected to the SEC as an external load The running speed should be maintained at
Figure 4.4Schematic diagram of the working principle for one completed cycle of single- cell DCR-TENG
ABS plastic Water PVDF Copper electrode Stationary electric contact
51 certain level for avoiding the centrifugal force so that water always retain its semicircle shape, leading to the maximal area of overlapping between water and one electrode Then, the accumulated charges could reach their maximum value In Figure 4.5, the pulsed unidirectional current is recognized at all times with the maximum amplitude of 75 nA, corresponding to the current density of 11.94 nAãcm -2 , as well as the time interval between current peaks of 1.67 s The pulsed signal is observed when the electrodes and the SEC are brought into contact following the movement of DCR-TENG; otherwise, no current signal can be detected It is evident that there are exactly two current peaks within one cycle of rotation, which is appropriate to the above working principle Because of the switching function in addition to a brief conducting period, the current signal instantly raises from zero to the maximum value then gradually decays to zero again in a short time as shown in Figure 4.7(a) The corresponding charge transfer during the conducting state is estimated at 2.03 nC Simultaneously, the output voltage shows up the maximum value of about 1.5 V with the signal waveform as similar to the current waveform which is demonstrated in Figure 4.6 Subsequently, the instantaneous discharge characteristics of the TENG is examined under different resistance values of 10 MΩ, 20 MΩ, and 50 MΩ,
Figure 4.5 Output current of single cell DCR-TENG with 20-MΩ external resistance
Figure 4.6 Output voltage of single cell DCR-TENG with 20-MΩ external resistance
52 respectively, and the output voltage is illustrated in Figure 4.7(b) It can be observed that the output voltage in three cases has the same peak value but the corresponding time decays constant increases with respect to the increment of the resistance value Here, the time decay constant is calculated about 17 ms, 25 ms, and 49 ms for 10 MΩ, 20 MΩ, and
50 MΩ, respectively, which are almost linear to the external resistance The relationship between output voltage, output current, instantaneous output power and external resistance is shown in Figure 4.8-4.9 The output voltage displays independence in relationship with the resistance according to the fact that it almost derives the same output level of 1.5 V with the change of resistance value Meanwhile, the output current and the instantaneous power present a negative relationship to the resistance, where their values reduce from 11.94 nAãcm -2 and 18.48 nWãcm -2 to 1.93nAãcm -2 and 2.96 nWãcm -2 regarding the growth of resistance from 20 MΩ to 100 MΩ, respectively The maximum energy-per-cycle E max derived by the DCR-TENG can be calculated by using the following equation max max max
E = 2 Q V where Q max is the maximum charge transfer and V max is the maximum output voltage For the rotating speed of 18 rpm, the energy-per-cycle attains the value of roughly 1.7 nJ regarding the energy density of 0.27 nJãcm -2 Due to the discontinuous conduction, the induced charges always stack up to the maximum before releasing; hence, the energy-
Figure 4.7 (a) The magnification of a current peak with estimated charge transferred (b)
Output voltage decay curve with different resistance
53 per-cycle remains unchanged with regards to the change of the external loads In other to further evaluate the performance of the DCR-TENG, we fabricate a reference model namely continuous-conduction-based rotational triboelectric nanogenerator (CCR- TENG) In the reference model, instead of a stationary electric contact mechanic, we connect the electrodes to an external circuit through a slip ring, which can provide a continuous transfer of power The comparison between the two configurations is demonstrated in Figure 4.8-4.9 It realizes that, at a low resistance value, both output
Figure 4.8The output current density and voltage in relation to the change of external resistance for DCR-TENG and CCR-TENG
Figure 4.9 The instantaneous power density in relation to the change of external resistance for DCR-TENG and CCR-TENG.
54 voltage and current of DCR-TENG are higher than the CCR-TENG, leading to the more instantaneous power of DCR-TENG
In the upcoming experiment, the relationship between rotating speed and output performance of the single cell DCR-TENG will be examined Figure 4.10 shows the output current of DCR-TENG with external resistance of 20 MΩ under varying rotating speeds from 18 rpm to 126 rpm When the rotating speed gradually increases from the
Figure 4.10 The effect of rotating speed on the output current of single-cell DCR-TENG with load resistance of 20 MΩ
Figure 4.11The correlation between current peak value and rotating speed.
55 minimal value to the maximal value, the TENG has the ability to deliver more power such that the current peak raises from 75 nA (11.94 nAãcm -2 ) to 510 nA (81.21 nAãcm -2 ), i.e., approximately 637% in power level This is due to the incremental change in the sliding velocity of DI water over the PVDF surface The current peak value also exhibits a strong linear relationship with the rotating speed, as seen in Figure 4.11, where the rate of change of output current over rotating speed is roughly 3.98 nAãrpm -1 This feature could allow the DCR-TENG to be recognized as a kind of velocity transducer with high linearity and sensitivity (R 2 = 0.9862)
4.3.4 Output performance of multiple cell DCR-TENG
According to the operation principle of the single-cell TENG, it is noticed that the uncovered electrode always connects to the left terminal of the SEC and the covered
Figure 4.12 Schematic diagram of multiple cell DCR-TENG based on radially symmetrical structure
56 electrode connects to the right terminal Therefore, by means of adding a symmetrical cell in the same manner, the total charge transfer can be approximated by the summation of two individual TENG cells which increase the electrical output almost twice The schematic diagram of a symmetrical pair is shown in Figure 4.12 where two electrodes with the same electrical potential are got together and then connect to the appropriate terminal of the SEC Here, the sequential position of these paired TENGs is demonstrated
Figure 4.13 The output energy-per-cycle of different TENG configurations under various rotating speeds.
Figure 4.14The slope of change of output energy-per-cycle regarding different TENG configurations
Conclusion
Figure 4.17 Output current of the 6-cell DCR-TENG within 12000 cycles
This chapter implemented a liquid-solid based discontinuous-conducting TENG for harvesting mechanical energy With functional motion-activated switches, the TENG can hold up all the induced charges when the switch is open and instantly releases all the accumulating charges when the switch is closed, resulting in a significant improvement of electricity generation The mechanical motion-activated switches minimize the mechanical loss during the operating process by reducing the contact period; hence, the overall energy conversion could be increased To understand the operation of the TENG in-depth, the structural design and the working principle are provided along with the experimental results It is shown that the obtained results are relevant to the designed operation process which reveals compatibility between theory and practice Hereby, the influence of both TENG cell configuration and rotating speed on the output performance is systematically examined, where the output power is directly proportional to each factor The major advantage of this TENG is that it produces a unidirectional output current, which can immediately apply to some energy storage devices Without using the rectifier circuit, the energy loss can reduce significantly so that the total energy conversion efficiency of solid-liquid based TENG can improve appropriately This brings out the more effective solid-liquid-based TENG as a power source, and hence, it is expected to be a deliberate choice for self-powered applications in advance
Introduction
Waste mechanical energy, such as human walking or running, moving vehicles on the road, and structural vibration, which exists everyday and everywhere in urban life, is a promising option regarding accessible, available, affordable, and clean energy However, the mechanical stimuli that arise from daily activities are normally impulse excitation with irregular amplitude and long periods between incidents, yielding the drop- off in energy harvesting efficiency This is a critical problem that needs to be dealt with and requires continuous improvement before the TENGs can be used in practice A significant approach for increasing the energy harvesting efficiency is the integration of TENGs with a kinetic energy conversion system (KECS), which can extend the working frequency and change the directional motion of irregular mechanical inputs 6,117,118 The working mechanism of KECS is that it transforms the irregular kinetic input into modulated kinetic motion through either swing-type structure or spring-assisted structure, from which it can be exploited to run on the TENGs Nevertheless, those structures are solely based on solid-solid contact electrification which could get abrasion for long-term operation; therefore, the triboelectrification between water and solid materials is brought about as a replacement principle with great performance
From this impression, an impulsive kinetic energy regulator (IKER) for harvesting the mechanical energy through low-frequency impulse-excited motion is conducted The
61 device’s components include an adaptable harmonic oscillator, a mechanical motion rectifier, and a rotary liquid-solid TENG As regards the orientation of applying effort (vertical or horizontal), the IKER can transform the supplied kinetic energy into the relevant (elastic or gravitational) potential energy and vice versa, from which the fundamental operation of this device can be split into two sequential stages as exertion stage and resilience stage In the exertion stage, the input mechanical excitation is transferred through the harmonic oscillator to the mechanical motion rectifier, turning on the rotary TENG In the resilience stage, once the excitation is totally ceased, the harmonic oscillator is continuously engaged following the release of stored potential energy so that the rotary TENG can hold the operation Herein, the harmonic oscillator takes place as an intermediary for mechanical energy interchange such that it intentionally prolongs the rotating motion, followed by the endurance of TENG operation through time Alongside, the rotational direction is preserved due to the functionality of the mechanical motion rectifier Furthermore, inspired by the rational alignment, the designated rotary TENG can produce unidirectional output current instead of alternative output current, which is favorable to conventional TENG.
Methods
5.2.1 Fabrication of the rotary TENG
Figure 5.1Mechanical component design of the mechanical motion rectifier integrated rotary liquid-solid TENG
The structure of the solid-liquid rotary TENG is formed by two elements, the round-shaped plate (the diameter of 90 mm) that served as a lid and the shallow lidded dish (the diameter of 90 mm and the depth of 12 mm) that served as a water container Both were 3D printed using Clear Transparent Resin (Formlab Form 2, USA) First of all, two semicircle copper tapes equipped with soldered electric wires were adhered symmetrically to the plate (the gap of 3 mm) in order to make a conductive layer These electric wires were later connected to the commutator Then, a round commercial PTFE membrane (Merck Millipore, hydrophobic, 150 μm thickness and 0.22 μm pore size) was placed over and fastened strictly to the copper electrode layer so that it can make contact with water Besides, the DI water was poured into the dish until reaching half of container capacity Finally, the plate and the dish were assembled by the agency of silicone sealant as a watertight seal to consolidate the enclosure of the TENG
5.2.2 Fabrication of the mechanical motion rectifier
The mechanical motion rectifier is a mechanism that compound of two adjacent transmission gears, the spur gears and the planetary gears Here, the planetary gears adjoin the low-speed shaft and the spur gears connect to the high-speed shaft Two one-way bearings were secured to the spur gears that allows one-way rotation Both the transmission gears and the housing were 3D printed (Tough 1500 Resin for gears and PLA for framework)
The output current and output voltage are measured by a graphical sampling multimeter (Keithley DMM7510, USA) and the transferred charge is measured by a programmable electrometer (Keithley 6517B, USA) The output data is recorded and analyzed by MATLAB software.
Results and Discussion
5.3.1 Working principle of the IKER
The structure of IKER consists of an adaptable harmonic oscillator (HO) for manipulating the mechanical energy derived from the external excitation, a mechanical motion rectifier (MMR) for converting two-way rotation into one-way rotation, and a rotary solid-liquid TENG (R-TENG) The HO translates a portion of input kinetic energy
63 into potential energy and releases that stored energy afterward to keep up the movement of the MMR The MMR supplies both the unidirectional rotation and the rotational acceleration of the R-TENG during operating intervals, which sequentially comes up with the persistence of the R-TENG Therefore, by coupling the harmonic oscillator and the mechanical motion rectifier, the energy transfer and endurance of the R-TENG can be regulated to achieve the optimal result Particularly, the energy transformation of the IKER is divided into two phases, namely exertion and resilience In the exertion phase, the HO is hit by an impulsive attempt so that its steady-state condition ought to be relatively changed During the adaptation, the input kinetic energy is distributed to two primary courses, one provides for the R-TENG motion by engaging the driving shaft of MMR in counterclockwise rotation and the other is transformed into the potential energy as the reservation Consequently, the R-TENG can turn in the same direction for several cycles, whereby the electrical output can be enhanced correspondingly Up to the withdrawal of the external impact, the IKER switches to the resilience stage where the R- TENG still retains its operation and orientation because of the combination of HO and
Figure 5.2Fabrication of the mechanical motion rectifier integrated rotary liquid-solid TENG (a) 3D assembly model (b) Mechanical motion rectifier (c) Rotary liquid-solid TENG (d) Final construction of the system
MMR This is due to the potential energy stored up by the HO immediately reverses into the kinetic energy in order to keep up the MMR activity Additionally, the MMR prevents R-TENG from reverse orientation, which ends up with the unchanged counterclockwise motion of the R-TENG Herein, similar to the exertion phase, the electrical output could last for few cycles until the reservation energy is used up, then the HO turns back to the steady-state condition The detailed implementation of IKER equipped with specific HO will be presented in the next sections
The working principle of the rotary liquid-solid contact TENG is demonstrated in
Figure 5.3, which is based on the sliding freestanding triboelectrification between DI water and PTFE membrane Two identical semicircle electrodes are secured oppositely with a small gap to each other and they are entirely covered by a circular PTFE membrane Then, this composition is placed inside a round-shaped disk that has horizontal orientation, followed by the water filling In the initial state, the DI water fully overlaps electrode E2, leading to the formation of electrical double layer where the PTFE membrane adsorbs electrons at the contact interface and becomes negatively charged; meanwhile, the DI
Figure 5.3 Working principle of the rotary liquid-solid TENG
65 water becomes positively charged due to the presence of counterions in order to maintain the charge neutrality As the R-TENG rotates, the water leaves electrode E2 position and moves towards electrode E1 position, which is accompanied by the departure of counterions as shown in Figure 5.3(ii) At this step, there is an electric potential difference between two electrodes, but the R-TENG and the external circuit are not put in touch; hence, no charge transfer occurs during the movement of water Once the water finally comes up electrode E1 and entirely covers it (Figure 5.3(iii)), the electric potential difference reaches the maximum value as well; simultaneously, the R-TENG and the external circuit are brought into contact, which allows a charge flow appears between two electrodes until a new electrostatic equilibrium is settled This mechanism intentionally maximizes the energy output such that the performance of the R-TENG can be relatively improved When the R-TENG keeps moving onwards, the same procedure is implemented in a reversed manner where the water turns back to electrode E2 from electrode E1 (Figure 5.3(iv)) During this process, since the R-TENG and the external circuit are disconnected, there is no electron transfer between two electrodes even though the electric potential difference quickly develops along with the movement of the water; thus, the induced charges are gradually accumulated on the electrode E1 Up to the full overlap of water regarding electrode E2, the R-TENG again connects to the external circuit so that all accumulated charges could be released from the electrode E1 and transfer into the electrode E2 (Figure 5.3(i)); hereby it prompts the maximized energy output for the R-TENG as same as the previous step It is noticed that the charge flow is always unidirectional; therefore, the pulsating DC output current can be obtained without using any electrical rectifier circuit It is a remarkable characteristic of this rotary R- TENG in terms of enhancing the energy conversion efficiency
5.3.2 Performance of the IKER on vertical effort
To realize the IKER under vertical exertion, a gravity balancing (GB) mechanism is used as the HO for manipulating the mechanical energy derived from the incident The structure of IKER with GB as a harmonic oscillator is shown in Figure 5.4, in which it translates a portion of input kinetic energy into elastic energy by means of a tension spring Once the raised end of the lever is brought into contact (i.e., engaged), it moves downward until reaching the lowest possible position (which depends on the force amplitude) It starts the MMR, and so the R-TENG rotates appropriately for a few turns; meanwhile, the spring is stretched accordingly along with the development of the potential energy
As the lever is disengaged, the spring instantly retracts to the original shape as well as the upward movement of the lever; and throughout this step, the energy reserved by the spring is deployed to hold up the operation of the MMR together with the R-TENG For detailed description, the GB mechanism can be represented as a simple spring-lever type, which is demonstrated in Figure 5.5 The angular speed of GB is given by
𝑚𝑙 𝑒𝑓𝑓 2 (5.1) where k is the stiffness of tension spring, m is the lumped mass at the end of the effort arm, l eff is the length of the effort arm, l res is the length of resistance arm and g is the gravitational acceleration constant Then, the rotating speed of R-TENG is defined as
Figure 5.4 The assembly of IKER with gravity-balancing mechanism (GB) as harmonic oscillatorunder vertical effort
Figure 5.5 Diagram of spring-lever mechanism model The distance from the pivot to the spring is called the resistance arm length The distance from the pivot to the lumped mass is called the effort arm length State (1): Rest position State (2): Fully depressed position
67 with n is the gear transmission ratio such that the tension spring stiffness proportions to the rotating speed of R-TENG
For validating the characterization of the GB-IKER, the electrical output regarding different spring stiffness was measured under two strategies, namely, hard collision and soft collision Both stimuli are launched by an accelerating impact of 1 mãs -
2 (Figure 5.6) In the case of hard collision, the releasing happens right after the pressing is completed so that the R-TENG does not suffer deceleration during the transition Therefore, the power level of the output current could maintain momentarily before falling into decay due to the progressive decline of angular speed (Figure 5.7) Alongside,
Figure 5.6 Experimental setup for ‘vertical effort’ using gravity balancing structure Complete setup includes (1) pneumatic valve, (2) pneumatic cylinder, (3) proposed device, and (4) PC-based valve controller
Figure 5.7 Output current of GB-IKER regarding the spring stiffness under hard collision
68 the higher spring stiffness brings about the faster angular speed in accord with the increment of restoring force, which proportionally raises the current level For instance, the peak output current density at the stiffness of 0.22, 0.33, and 1.19 N/mm achieves 1.31, 1.96, and 2.29 nAãcm -2 , respectively Besides, the output voltage and charge transferred are observed to be almost unchanged with the approximate values of 1.14 V and 3.06 nC, no matter what spring stiffness is utilized (Figure 5.8-5.9) Whereas, in the case of soft collision, after being pressed, the lever is held down such that the R-TENG slowly rotates until it stops; and immediately after that, the lever is released Accordingly, with this actuation, the R-TENG is accelerated then decelerated twice within one excitation cycle, yielding the relevant attenuation in output current as shown in Figure 5.10, in which the peak output current density reaches sequentially as 1.22, 1.62, and 1.91
Figure 5.8 Output voltage of GB-IKER regarding the spring stiffness under hard collision.
Figure 5.9 Transferred charge of GB-IKER regarding the spring stiffness under hard collision
69 nAãcm -2 corresponding to the change of spring stiffness The output voltage (Figure 5.11)
Figure 5.11 Output voltage of GB-IKER regarding the spring stiffness under soft collision
Figure 5.10 Output current of GB-IKER regarding the spring stiffness under soft collision.
Figure 5.12 Transferred charge of GB-IKER regarding the spring stiffness under soft collision.
70 and charge transferred (Figure 5.12) stay consistent regardless of the spring stiffness variation, but slightly lower than the previous case (around 1.09 V and 2.61 nC) It is recognized that the GB-IKER actuated by hard collision provides more output power than the soft collision under the same configuration settings (see Figure 5.13 and Figure 5.14);
Figure 5.13 Output power of GB-IKER regarding the spring stiffness under hard collision
Figure 5.14 Output power of GB-IKER regarding the spring stiffness under soft collision
71 however, the running time of IKER prompted by hard collision is a bit shorter than the soft collision as pointed out in Figure 5.15 Consequently, the output energy of GB-IKER produced by hard collision and soft collision at a specific spring stiffness is almost the same (Figure 5.16) For a better performance evaluation of the device, the energy conversion efficiency of the GB-IKER, which is defined as the ratio of electrical output energy to the elastic energy stored in the spring, was taken into consideration and given by
Figure 5.15 The relationship between running time and return spring stiffness under different input excitation of GB-IKER
Figure 5.16The relationship between output energy and return spring stiffness under different input excitation of GB-IKER
72 where I(t) is the current delivered by the device through the resistor at the time t, R is the load resistance, k is the stiffness of the spring, and x is the elongation of the spring The calculation for the energy conversion efficiency under various load conditions in both cases, hard collision and soft collision, is presented in Figure 5.17-5.18
5.3.3 Performance of the IKER on horizontal effort
Figure 5.17 Energy conversion efficiency of the device under difference load conditions derived from one ‘hard collision’ trigger with k = 1.19 N/mm
Figure 5.18 Energy conversion efficiency of the device under difference load conditions derived from one ‘soft collision’ trigger with k = 1.19 N/mm
Demonstration of the IKER under realistic intermittent excitation
Figure 5.32 Energy conversion efficiency of the device under difference load conditions derived from one ‘horizontal effort’ trigger with m = 300 g and l = 750 mm
To evaluate the energy harvesting capability of GB-IKER and PP-IKER in practical situations, some verifications are conducted Firstly, the GB-IKER with footstep-driven excitation is implemented, where the GB is connected to a pedal via a rope so that they can move synchronously (upward and downward), as seen in Figure 5.33 The output energy from the GB-IKER is stored into a 0.1μF-capacitor (Figure 5.34), then discharging such that it can turn on 3 LEDs in series Alongside, under repetitive triggering, the GB-IKER can continuously produce a stable output voltage of around 1V within 900 s, which characterizes the durability of the GB-IKER (Figure 5.35) Another practical situation for the GB-IKER is that it can be placed underneath a speed bump to
Figure 5.33 Setup of the GB-IKER for harvesting footstep energy Inset: Photo of the LEDs lighted up by footstep energy
Figure 5.34 Capacitor charging response of the GB-IKER under footstep excitation
81 harness the energy from a moving vehicle as it passes through; alongside, the structural design for adapting the corresponding input excitation should be inspected Meanwhile, the PP-IKER actuated by the boom gate is carried out as exhibited in Figure 5.36, where the PP is sequentially triggered by the moving up and down of the boom arm once a vehicle or a person enters or leaves a place The energy derived from the PP-IKER is supplied to a wireless temperature and humidity sensor (Cypress BLE-Beacon) with data tracking obtained by cellphone (see Figure 5.37) The time interval between two data points is about 30s which is quite slow but could be adequate to a certain extent Consistently, the IKER, with diverse configurations, can be referred to as a decent option for harvesting mechanical energy from human and machine activities
Figure 5.35 The durability of the GB-IKER under footstep excitation
Figure 5.36Conceptual design of mechanical energy harvesting using PP-IKER driven by boom barrier gate Inset: fabrication of mechanical trigger
Conclusion
This chapter presented an IKER to harness the impulsive mechanical energy in the surrounding environment, in which the IKER is composed of a harmonic oscillator, a mechanical motion rectifier, and a rotary liquid-solid TENG The combination of harmonic oscillator and mechanical motion rectifier can manage the interchange of kinetic energy and potential energy, through exertion phase and resilience phase, to actuate the rotary TENG operation excessively The harmonic oscillator is adaptable to be configured either as a gravity-balancer to deal with the vertical excitation or as a planar pendulum to handle the horizontal excitation As a result, the device can produce an output energy of 25.13 nJ with a maximum current density of 2.29 nAãcm -2 concerning a single vertical incentive; whilst, the horizontal direction effort obtains an output energy of 25.43 nJ with a maximum current density of 2.41 nAãcm -2 Moreover, the device can power up a few LEDs with human footstep excitation and a wireless-based sensor during boom barrier gate operation, which shows a good possibility in waste mechanical energy harvesting
Figure 5.37 Setup of the PP-IKER for harvesting energy from boom barrier gate for powering wireless sensor Inset: Wiring circuit of the setup and data tracking by cellphone
Summary and conclusions
The overall goal of this research is development of the mechanical energy harvester based on the liquid-solid contact triboelectrification, which is accomplished by the following scenario In chapter 1, the necessity and objective of this research as well as the fundamentals of triboelectric nanogenerator (such as theory, mechanisms, and applications) were presented Chapter 2 extended the concept of liquid-solid contact for triboelectrification with “Wang’s transition” model along with the structural designs and potential applications of the liquid-solid triboelectric nanogenerator Chapter 3 to chapter
5 consecutively introduced three mechanical energy harvesters utilizing liquid-solid triboelectric nanogenerator, including rotational switched-mode water-based triboelectric nanogenerator (RSW-TENG), discontinuous conduction based rotational triboelectric nanogenerator (DCR-TENG), and impulsive kinetic energy regulator (IKER) The design and performance analysis of the above devices were fulfilled to reach the main goal of this research
Firstly, a rotational switched water based triboelectric nanogenerator (RSW- TENG) was developed to convert the rotational mechanical energy of a vehicle wheel into electricity as well as keep track of the vehicle operation The key feature of the RSW- TENG is the position-dependent conduction where the charge transfer just occurs at designated locations Therefore, the electrical outputs could be altered with respect to the on-state conduction positions, by which a road slope detection device would be preferred
In the case, PTFE membrane is used as the solid-phase negatively charged material and
DI water is used as the liquid-phase oppositely charged material The driven rotational motion induces a slippery sliding of water over the PTFE membrane and, in accordance with the position-dependent conduction structure, the induced electrostatic charges only transfer between electrodes since they attain the maximum possible accumulation Further, the RSW-TENG directly derives unidirectional current, which is beneficial to low-power electronic devices
Secondly, a discontinuous conduction based rotational triboelectric nanogenerator
(DCR-TENG) was developed to scavenge mechanical energy without using a rectifier circuit The key feature is the utilization of the motion-activated switches where these switches are simultaneously closed at designated positions during the operation of the TENG This design allows the induced charges accumulating on one freestanding electrode, then instantaneously discharging into another electrode when the switches are triggered, which significantly enhance the output power of the TENG PVDF nanoporous membrane is used as the solid-phase negatively charged material and DI water is used as the liquid-phase oppositely charged material The driven rotational motion induces a slippery sliding of water over the PVDF membrane With the presence of designated motion-activated switches, the induced charges only transfer between electrodes when they reach the maximum value, resulting in the improvement of electrical output Further, the TENG successfully derives unidirectional current, which can directly apply to energy storage devices
Thirdly, an impulsive kinetic energy regulator (IKER) was developed to scavenge the waste mechanical energy from human and machine activities The key feature is the combination of the harmonic oscillator and the mechanical motion rectifier to drive the rotary solid-liquid triboelectric nanogenerator (R-TENG) such that it can prolong the running time after triggering by an arbitrary impulse This combination and the rational design of R-TENG for discontinuous conduction can induce a remarkable enhancement in output energy of the R-TENG The harmonic oscillator is used as mechanical energy regulator where it takes charge of the interchange of input kinetic energy and stored potential energy, through either gravity balancing or planar pendulum mechanism In addition, the mechanical motion rectifier transfers the regulated input into one-way rotational motion of R-TENG that induces a slippery sliding of water over the PTFE membrane Altogether, the R-TENG can endure for excessive cycles out of a single impulse excitation, which successfully improves the electrical output energy Further, the R-TENG directly derives unidirectional current, which is beneficial to low-power electronic devices.
Recommendations for Future Works
The development of mechanical energy harvesters based liquid-solid triboelectric nanogenerator were addressed in this research with comprehensive fabrication and investigation However, there are still interesting aspects for enhancement of liquid-solid
TENG-based systems that can be extended for further research works This section proposes the potential directions which can be targeted for further investigations on the topic of this research
Firstly, the designs need to be robustly developed and optimized to better obtain the electrical output performance of the system under random excitations such as human motion, impact, and vibration New designs should have much higher stability and durability, and much smaller deterioration in performance at higher operating frequency, which is beneficial for energy harvesting
Secondly, the solid triboelectric materials should be optimized to achieve the maximum surface charge density since the surface charge density is the most important factor for the output of the liquid-solid TENGs The surface modification technique, including physical modification techniques, chemical functionalization techniques and the combination of them, can be applied to improve the surface morphology Ion injection and fluorinating surfaces are very interesting directions toward the development of high performance liquid-solid TENG
Thirdly, due to the high voltage pulsed output and the high internal capacitive impedance, a power management circuit (PMC) must be integrated into liquid-solid
TENG to convert the harvested raw energy to a well-regulated form which is suitable for electronic devices It has acritical and indispensable role in self-powered systems with the liquid-solid TENG as the energy harvesting unit Despite various circuit topologies have been proposed, there are endless opportunities for engineers and researchers to seek the most effective circuit topologies regarding maximum energy generation capability as well as its optimal energy extraction strategy
In addition, it should also be noted that from the selected material and surface properties points of view, the sensing and energy harvesting are similar meaning that for both applications the use of developed high performance materials and modified surfaces is beneficial for better performance From the structural point of view, the sensing and energy harvesting designs could be different in terms of how the mechanical energy is transferred into the device and which parameter is going to be sensed
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7 Chau-Duy Le, Cong-Phat Vo, Thanh-Ha Nguyen, Duy-Linh Vu, Kyoung Kwan Ahn, Liquid-solid contact electrification based on discontinuous-conduction triboelectric nanogenerator induced by radially symmetrical structure, Nano Energy, 2021, 80,
8 Cong-Phat Vo, M Shahriar, Chau-Duy Le, Kyoung Kwan Ahn, Mechanically active