\r\n\tA number of advanced combustion technologies have been introduced to improve performance, fuel economy and emissions levels. Research in combustion technology has highlighted the importance of new fuels in reducing the petroleum dependence and achieving high efficiency with low pollutant formation.
\r\n\tThe purpose of this book is to collect interesting and original studies on combustion methods, advanced combustion strategies and new fuels able to achieve efficiency improvements and environment compliance.
\r\n\tContributions in which experimental, theoretical and computation approaches are applied to explore how fuel properties and composition affect advanced combustion systems and how advanced combustion technology can maximize engine efficiency and be environment-friendly are invited and appreciated.
Photorefractive effect is a nonlinear optical phenomenon by which the refractive index of a medium can be modified. This effect can be used to produce rewritable, dynamic (that is, temporal) holographic images [1, 2]. Associated applications include the production of phase-conjugate beams and the amplification of optical signals. There are numerous potential uses of this effect in optical devices, such as in the fabrication of electrical transistors. As an example, the photorefractive phenomenon can result in the appearance of both electro-optic and photovoltaic properties in optically transparent materials. Numerous such substances have been reported, including organic molecular crystals, inorganic ferroelectric photoconductive crystals, amorphous organic photoconductive materials, photoconductive liquid crystals (LCs), photoconductive amphiphilic compounds, and photoconductive organic polymers [3, 4, 5]. Among these, organic compounds tend to have the shortest response times and greatest photorefractivity values, and hence these are of special interest [6]. Both electro-optic and photovoltaic mechanisms contribute to variations in refractive index that are associated with the photorefractive effect (Figure 1). The separation of photo-induced charges in a photorefractive substance and associated refractive index variations resulting from the electro-optic effect can produce a refractive index grating. The application of an electric field to such devices can also enhance the efficiency of charge generation. Photoconductive glassy polymers containing high levels of D-π-A chromophores (comprising π-conjugated systems with both donor and acceptor moieties) tend to exhibit especially high photorefractivity [3, 4, 5, 6, 7]. These materials have been suggested for use in 3D displays, real-time edge enhancement, and holographic interferometry [7]. Nevertheless, it remains necessary to reduce the electric field values of 30–50 V/μm that must be applied to activate photorefractivity, as well as to improve the sluggish response times of these materials (approximately 100 ms). LCs, which are essentially in the liquid state and thus can be driven by the application of lower electric fields, have also been investigated with regard to their photorefractive properties [7]. Nematic LCs were reported to show photorefractivity in 1994 [8], and this effect was especially pronounced when the compounds were subjected to a low electric field (several V/μm) [9]. Surface-stabilized ferroelectric liquid crystals (SS-FLCs) combined with photoconductive materials have also shown photorefractivity [10, 11]. Ferroelectric liquid crystals (FLCs) have a layered helical structure [12, 13] in conjunction with a chiral smectic C (SmC*) phase. Interestingly, these materials only exhibit ferroelectricity in the form of thin films (several μm in thickness) [12]. When such films are contained between the glass plates, the SmC* phase helix uncoils to generate a surface-stabilized (SS) state that shows spontaneous polarization (Ps) (Figure 2). Typically, a film thickness of 2 μm is employed for practical applications, such that the FLC molecules are forced into a two-dimensional SS alignment, the direction of which varies with the Ps direction (Figure 3). Applying an alternating electric field to the film results in an ongoing back-and-forth motion of the molecules on a time scale of less than 1 ms. The electric field determines the Ps direction, which in turn affects the properties of the film. An internal electric field can also modify the Ps direction in SS-FLCs, and Figure 4 presents a diagram showing the photorefractive effect mechanism in an FLC. The interference between laser beams in an FLC mixed with a photoconductive material results in an internal electric field due to charge separation between dark and light regions. This field modifies the Ps direction between the dark and light locations within the interference fringes, which in turn produces periodic variations in the FLC molecular orientation. Whereas in some organic photorefractive materials there is a bulk polarization response to the internal electric field, in the case of an FLC there is a molecular dipole response. Thus, the exceptionally rapid FLC molecular switching is a consequence of the bulk polarization response.
A diagram summarizing the photorefractive effect mechanism. (a) Dual laser beams undergo interference in a photorefractive compound; (b) charges are produced at the light regions associated with interference fringes; (c) trap sites in the light regions hold electrons, while hole migration occurs via drift or diffusion due to an external electric field, such that an internal electric field is produced between the dark and light regions; and (d) the refractive index in specific areas is modified as a result of the internal electric field.
SmC phase and SS state (SS-FLC) structures.
Electro-optical switching of an FLC in the SS state.
A diagram showing the FLC photorefractivity mechanism. (a) Dual laser beams undergo interference in a mixture of an FLC in the SS state with a photoconductive compound; (b) charge generation takes place within the light regions of the interference fringes; (c) trap sites in the light regions hold electrons, while holes move via drift or diffusion due to an external electric field, producing an internal electric field between the dark and light zones; (d) the spontaneous polarization vector orientation (that is, the mesogen orientation in the FLC) is modified by the internal electric field.
The photorefractive effect induces variations in refractive index between the dark and light points in interference fringes. As a result, the refractive index grating phase is moved away from these fringes. In the case that one photogenerated charge is not moved from the light region, the magnitude of this shift is π/2. If a substance exhibits photochemical activity but not photorefractivity, the phasing of the resulting refractive index grating is equivalent to that of the interference fringes because only a photochemical reaction takes place at the light regions (Figure 5a), and this grating diffracts the laser beams. In the case that the interfering beams have the same intensity, symmetric diffraction is obtained and so there is no change in the transmission intensities of the beams. As shown in the figure, beams 1 and 2 are diffracted toward each other. In contrast, if a photorefractive material is employed, there is a phase shift of the grating relative to the fringes that modifies both beams. Specifically, both beams undergo energetic coupling such that beams 1 and 2 become less and more intense, respectively (Figure 5b). This amplification of one beam by the second beam as a result of dual-beam coupling is referred to as asymmetric energy exchange [3]. The appearance of this phenomenon, which can be applied to numerous optical devices, is characteristic of a photorefractive material.
Diagrams of refractive index grating formations. (a) Photochromic and (b) photorefractive gratings.
Dual-beam coupling is often used to assess the appearance of photorefractivity. In this technique, two beams are obtained from a p-polarized laser beam, using a beam splitter, and these beams interfere with one another inside a film specimen to which a high-voltage electric field has been applied to enhance charge generation. Variations in the intensity of the transmitted beam are tracked, and photorefractive substances will exhibit asymmetric energy exchange. The gain coefficient is determined from changes in the beam intensity due to dual-beam coupling, and is used to communicate the magnitude of the photorefractivity. As a prerequisite to calculating this term, it is necessary to ascertain whether Bragg or Raman-Nath diffraction occurs, based on the dimensionless parameter Q, calculated as:
Specifically, a Q value greater than 1 indicates Bragg diffraction. In such cases, only a single diffraction order is obtained as multiple scattering does not take place. In contrast, a Q value less than 1 is associated with Raman-Nath diffraction involving numerous diffraction orders. Q values in excess of 10 ensure that solely Bragg diffraction occurs. The dual-beam coupling gain coefficient, Γ [cm−1], can be calculated using the equation:
Here, D = L/cos(θ) is the signal beam interaction path (where L is the sample thickness and θ is the angle of beam propagation in the sample), g is the signal beam intensity behind the sample with the pump beam ratioed to that without, and m is the pump/signal intensity ratio in front of the sample [1, 2, 3].
Research regarding the photorefractivity of commercial FLCs combined with photoconductive materials began circa 2000 [10, 11] and was continued by Sasaki et al. and Golemme et al. [7, 14]. This prior experimental work was based on dual-beam coupling trials incorporating a 488-nm Ar+ laser, employing the photoconductive materials presented in Figure 6 along with SCE8 (Clariant, SmC* at 60°C, SmA at 80°C, N* at 104°C, Ps = 4.5 nC/cm2), a combination of chiral and LC compounds, such as the FLC. In these trials, a mixture of the FLC with trinitrofluorenone (TNF, 0.1 wt.%) and carbazole diphenylhydrazone (CDH, 2 wt.%) was injected into a 10-μm gap in a glass cell having an alignment layer composed of polyimide and 1 cm2 indium tin oxide electrodes (Figure 7). The typical asymmetric energy exchange obtained upon applying a 0.1 V/μm DC electric field to an FLC(SCE8)/CDH/TNF specimen is presented in Figure 8 [15]. In these trials, one beam exhibited enhanced transmittance while the other showed reduced intensity following interference within the FLC, resulting in symmetric variations in the transmitted intensities. These results demonstrate that a refractive index grating was generated at the interference fringes in conjunction with a phase shift. This grating was associated with Bragg diffraction, without any higher-order diffractions.
Molecular structures of the photoconductive chemical CDH and the sensitizer (that is, electron trap) TNF.
The structure of an LC cell and the associated laser beam incidence.
A typical asymmetric energy exchange obtained within the FLC SCE8 when combined with 2 wt.% CDH and 0.1 wt.% TNF and having an applied electric field of +0.3 V/μm.
Figure 9a summarizes the effects of temperature on the gain coefficient of SCE8 containing 0.1 and 2 wt.% TNF and CDH, respectively. In this system, temperatures less than 46°C were required to realize asymmetric energy exchange. Figure 9b plots the effects of temperature on the spontaneous polarization of an equivalent specimen and demonstrates a loss of polarization at the same temperature. These data demonstrate that the sample had to be within the SmC* phase temperature range (that is, it had to exhibit ferroelectric properties) for the asymmetric energy exchange to appear. An FLC in the N* or SmA phase cannot undergo significant variations in molecular axis orientation in response to an internal electric field because the molecular dipole moments are perpendicular to the molecular axis and have small magnitudes. In contrast, an internal electric field can result in spontaneous polarization associated with dipole reorientation in the SmC* phase. The spontaneous polarization also causes the orientation of FLC molecules in the corresponding area to change accordingly. The data show that this specimen exhibited a maximum resolution of 0.8 μm [15].
The effect of temperature on (a) the gain coefficient and (b) the spontaneous polarization of the FLC SCE8 when combined with 2 wt.% CDH and 0.1 wt.% TNF. During these dual-beam coupling trials, a 0.1 V/μm electric field was applied.
The external applied electric field strength has a significant effect on photorefractive polymers and acts to promote photorefractivity by enhancing the efficiency of charge separation. Several tens of volts per μm are necessary, and so a typical film thickness of 100 μm requires several kilovolts. However, much weaker fields can induce photorefractivity in FLCs. As an example, an SCE8 film will exhibit its largest possible gain coefficient in conjunction with a field strength in the range of 0.2–0.4 V/μm. As such films are often 10-μm thick, a field of just several volts is sufficient. Figure 10 plots the effect of the electric field strength on the gain coefficient for an FLC(SCE8)/CDH/TNF specimen. SCE8 containing 0.5–1 wt.% CDH exhibited a coefficient that increased along with the field strength, while the addition of 2 wt.% CDH resulted in a lower coefficient at field strengths above 0.4 V/μm. Increasing the field strength to 0.2 V/μm promoted the generation of an oriented grating due to charge separation, but zigzag defects became evident in the SS state at higher field values, which would be expected to lower the gain coefficient due to light scattering. As of 2004, the gain coefficients reported for FLC-based specimens were significantly lower [15] than those obtained for certain polymers.
The effect of electric field strength on the gain coefficient of the FLC SCE8 when combined with CDH at various concentrations and 0.1 wt.% TNF at 30°C in a cell having a 10-μm gap.
Both charge separation and reorientation are associated with refractive index grating formation. Both of these processes help determine the time required to form the index grating, meaning the photorefractivity response time, and can potentially determine the rate of formation. A simple single-carrier photorefractivity model was employed to assess the time required for the formation of a refractive index grating in SCE8 [1, 2], incorporating an exponential gain transient. A single exponential function was used to fit the increases in the diffraction beam signal, written as:
Here γ(t) is the intensity of the transmitted beam at time t ratioed to the initial intensity (that is, γ(t) = I(t)/I0), while τ is the time required for grating formation. Figure 11 plots the effect of the external electric field strength on the formation time for an SCE8/CDH/TNF sample and demonstrates that raising the field strength reduces the formation time, as a consequence of more efficient charge generation. Elevated temperatures also shorten the formation time, due to the lower viscosity of the material. At 30°C, the SCE8 exhibited a response time of 20 ms, which is much shorter than the typical 100 ms time span observed for polymers [7].
The effect of electric field on the index grating formation time of the FLC SCE8 when combined with 2 wt.% CDH and 0.1 wt.% TNF in a dual-beam coupling trial. Legend: • = data acquired at 30°C (T/TSmC*-SmA = 0.95); ▪ = data acquired at 36°C (T/TSmC*-SmA = 0.97).
Prior to 2011, FLCs in the SS state tended to contain defects when combined with photoconductive materials. These defects reduced the photoreactivity of the mixtures due to scattering of the laser beam, and so new photorefractive FLC mixtures were researched, based on the synthesis of chiral photoconductive additives and subsequent mixing with SmC LCs. Figure 12 shows one such blend based on an FLC [16]. In this case, the chiral photoconductive compound 3T-2 MB was added to an SmC LC together with TNF as a sensitizer, and the resulting dual-beam coupling signal acquired at 30°C is presented in Figure 13. Pronounced coupling was obtained in these trials, such that greater than 40% of the energy of the first laser beam was absorbed by the second beam when using a 10-μm-thick FLC film. Figure 14a summarizes the effect of the electric field strength on the gain coefficients. These data demonstrate that a low field strength of 1.9 V/μm resulted in a gain coefficient in excess of 1200 cm−1 in the case of the 10 wt.% specimen. Relative to the values for FLCs in Section 2.2, this represents a 45-fold increase, and is ascribed to both the greater photoconductivity of this mixture and its improved transparency. In the case of photorefractive applications, the ability to induce photorefractivity in an FLC using only a low electric field is beneficial. Figure 14b shows that increasing the field strength also reduced the response time, by providing more efficient charge separation, with a field strength of 1.9 V/μm giving a formation time less than 1 ms. Both this rapid response and significant gain would assist in realizing applications such as distance measurement devices and real-time image amplifiers.
A photorefractive FLC sample combined with a ternary mixture of smectic LCs.
Typical data obtained at 30°C from dual-beam coupling trials using a mixture of a base LC, 3T-2 MB, and TNF.
(a) The effect of electric field on the gain coefficients of mixtures of a base LC, 3T-2 MB (2–10 wt.%), and TNF (0.1 wt.%) at 30°C. (b) The formation times for refractive index gratings (that is, response times) for these same mixtures at 30°C.
A photorefractive FLC mixture has been demonstrated to form a dynamic hologram [17], such that a spatial light modulator (SLM) could be used to display a computer-generated animation. In this prior work, the SLM was exposed to a diode-pumped solid-state (DPSS) laser beam (at 488 nm) such that the FLC received the reflected beam. This beam underwent interference with a reference beam in the FLC and a refractive index grating based on Raman-Nath diffraction was generated. In other trials, a He-Ne laser beam (at 633 nm) was applied to the FLC to produce diffraction and a moving animation was generated in the diffraction (see Figure 15). Because image retention was not observed, the holographic image (that is, the refractive index grating) in the FLC was evidently rewritten at a rate sufficient to adequately reproduce the movie.
The formation of a dynamic hologram using an FLC, displaying a computer-generated animation on the SLM. Here, the SLM modulates the 488-nm object beam that irradiates the FLC and interferes with a reference beam. A 633 -nm readout beam irradiates the FLC to generate diffraction.
The photorefractive phenomenon has an obvious application in the amplification of optical signals, and this is a vital component of various optical techniques. This effect permits selective amplification, in contrast to the more well-known effects associated with lasers and nonlinear optics. Because photorefractivity results in the generation of a hologram in a material, a particular light signal can be distinguished from other signals on the basis of variations in phase, polarization, and wavelength. As an example, a photorefractive FLC mixture has demonstrated the ability to dynamically amplify a moving optical signal [16]. In this work, a rotating image (30 fps) was shown on an SLM via irradiation with a 473-nm beam such that the FLC was irradiated by the reflected beam, followed by interference with a pump beam. In these trials, amplification of the laser beam carrying the moving image was accomplished using the incident pump beam (Figure 16) and the signal beam intensity was increased by a factor of six. These data demonstrate that the speed of the photorefractive FLC response was such that the moving image could be amplified. It should be noted that this would not have been possible based on the average response time of a photorefractive polymer (about 100 ms). In such cases, the still image would be amplified, but not the moving image intensity.
(a) A demonstration of optical image amplification, in which an SLM displays a computer-generated animation. The SLM modulates a 473 nm object beam that irradiates an FLC then interferes with a reference beam. A CCD camera monitors the image sent via the FLC sample containing 10 wt.% 3T-2MB. (b) Change in signal intensity.
There has been research regarding the robustness of photorefractive FLC mixtures with chiral terthiophene photoconductive additives [18], and Figure 17 plots the gain coefficient of a blend with 3T-2 MB against irradiation time. These data demonstrate a rapid drop in the gain coefficient, to the extent that only 20% of the original value remains after 90 min. This decay can possibly be attributed to a photochemical reaction of the 3T-2 MB, based on dimerization or polymerization of the terthiophene chromophore in response to irradiation. The resulting products would be expected to be insoluble in the FLC, thus degrading the photorefractivity. Such reactions could potentially be inhibited based on steric hindrance effects, and so a series of analogs was prepared: 3T-2 MB, 3-Me-3T-2 MB, 3′-Me-3T-2 MB, 3′′-Me-3T-2 MB and 3′,3′′-Me-3T-2 MB (Figure 18). Figure 19 summarizes variations in the gain coefficients of the resulting blends with irradiation time. Interestingly, the decay profiles were unchanged by varying the degree of steric hindrance in the dopant, despite expectations that methyl substituents would hinder access to the reaction site as well as the approach to the terthiophene moieties. Consequently, the loss of photorefractivity evidently was not due to dimerization or polymerization. Additional analyses of a 3T-2 MB specimen via 1H NMR, infrared and UV-visible spectroscopy following exposure to 488-nm light found no reaction or degradation of the terthiophene moiety. Based on these results, it appears that the loss of photorefractivity resulted from factors other than those described above.
Molecular structures of the smectic LCs 8PP8 and 8PP10, chiral photoconductive dopants and TNF, an electron trap reagent.
Variations in the gain coefficient of FLC with 6 wt.% 3T-2 MB and 0.1 wt.% TNF over time.
Variations in the gain coefficients of FLC mixed with 6 wt.% various photoconductive chiral additives and 0.1 wt.% TNF over time.
When a DC field is applied, the photorefractive gain has been shown to decay as the irradiation time is prolonged. This phenomenon was assessed by a dual-beam coupling trial employing a decayed sample stored under dark conditions. Figure 20 presents the original gain signal following laser irradiation for 60 min, after which the specimen was maintained in darkness for 12 h in the absence of an electric field. In this trial, 85% of the initial signal was recovered, demonstrating that the loss of the photorefractive effect was due to changes in the material that were reversible. The observed recoverable loss in gain can possibly be attributed to the formation of photogenerated ions that adhere to the indium tin oxide (ITO) electrode. It is known that charge transfer between the TNF and terthiophene occurs in response to 488-nm laser light to produce corresponding cations and anions, and these species may migrate to the electrode in response to the electric field. This process extracts ions from the LC phase, reducing its conductivity. Thus, the observed loss of photorefractivity could possibly be mitigated by employing a bipolar electric field, to reverse the direction of migration of the photogenerated ions such that they move away from the electrode. Of course, it would be necessary to select an optimal frequency for this bipolar field based on ion mobilities, as well as to optimize the field strength. Figure 21 presents decay profiles of the gain coefficients in conjunction with a bipolar field, and demonstrates that 80% of the original gain was retained under these conditions. These results help to confirm that the observed loss of photorefractivity in the FLC samples is due to ionic migration. The TNF anions and 3T-2 MB cations would be expected to migrate at different rates as a result of the difference in their sizes, and so the electric field strength and shape could be further tuned. In fact, the electric fields employed in present-day LC displays are somewhat complex bipolar fields rather than DC fields. This is required in order to inhibit the adsorption of ions and the LC molecules themselves so as to prevent image sticking. Thus, the use of bipolar fields is already an important aspect of photorefractive LC devices.
The dual-beam coupling signals generated by FLC doped with 3T-2 MB. The initial signal following 60 min of irradiation by laser beam (250 mW/cm2) and the signal after storage in darkness for 12 h are shown.
Variations in the gain coefficients of an FLC mixed with 6 wt.% 3 T-2 MB and 0.1 wt.% TNF over time in the presence of a bipolar electric field.
The photorefractivities of terthiophene derivatives as chiral photoconductive additives have been examined at 488 nm. The practical application of FLCs in ultrasound imaging and photoacoustic interferometry requires sensitivity at longer wavelengths. Thus, chiral compounds such as those containing quarter-thiophene chromophores and sexithiophenes, as well as more complex molecular structures, have been synthesized (Figure 22). The photorefractivity of mixtures of FLCs with these dopants at longer wavelengths has also been assessed [19]. The addition of TNF to a quarter-thiophene was found to result in absorption at longer wavelengths (Figure 23) due to the formation of a charge transfer absorption band, allowing these materials to be employed in conjunction with a 532-nm writing beam. Dual-beam trials were subsequently employed to examine the photorefractivity of the resulting FLC blends. Figure 24 shows typical data for the asymmetric energy exchange in a 2EH-4 T-2 MB specimen in conjunction with a 532-nm writing beam wavelength.
Molecular structures of the chiral photoconductive additives, smectic LCs, and TNF, an electron trap reagent.
(a) The UV-vis absorption spectra generated by chloroform solutions of the chiral photoconductive additives C8-4T-2 MB, 2EH-4T-2 MB and 3 T-2 MB; (b) the UV-vis spectra of a chloroform solution of C8-4T-2 MB and TNF; (c) the UV-vis spectrum of C8-4T-2 MB compared to that of C8-4T-2 MB and TNF (all in chloroform solutions).
A typical asymmetric energy exchange obtained from a dual-beam coupling experiment at 30°C using a combination of a base-LC, 2EH-4T-2 MB, and TNF. Pump beam incidence occurred at the 2-s mark. The beam incidence parameters are given in the figure.
Photoconductive materials having longer absorption wavelengths were additionally investigated (Figure 25). In these trials, the photoconductive compounds were mixed with a base FLC and a chiral additive. Most of the materials listed in Figure 25 could not be dissolved in the FLC, with the exception of the SM, which could be added at levels up to 1 wt.%. Thus, a blend containing 1 wt.% SM was employed for dual-beam coupling trials (Figure 26), in conjunction with continuous laser irradiation at 488, 532, and 638 nm. This sample exhibited asymmetric energy exchange even at the longest wavelength, albeit with a slow response and minimal gain. The generation of a space charge field at the interference fringes evidently involved ionic conduction, which was reflected in the slow response.
Molecular structures of the host FLC and the photoconductive compounds used in the work described herein. The chiral dopant was added at a concentration of 10 wt.% relative to the total mass of the 2:1:1 ternary mixture.
Asymmetric energy exchanges in an FLC mixture in which the photoconductive compound SM is present at less than 1 wt.%.
LCs are exceptionally birefringent and thus produce a significant photorefractive effect, although they suffer from light scattering due to heterogeneous molecular orientations. Smectic LCs show more pronounced photorefractivity, especially in the case of the bulk (or spontaneous) polarization of FLCs combined with photoconductive compounds. These mixtures exhibit photorefractivity solely in the ferroelectric phase and with reduced response times relative to those obtained from nematic LCs. The various properties of FLCs, including spontaneous polarization, phase transition temperature, viscosity and SS-state homogeneity, all greatly affect photorefractivity. Both the time required for formation of a refractive index grating (that is, the response time) and the gain coefficient are significantly affected by the last factor, and so extreme homogeneity is required in a photorefractive device. Finally, FLC blends have exhibited sensitivity to wavelengths up to 638 nm following doping with photoconductive compounds.
With the rapid development of science and technology, Internet of Things (IoT) plays an increasingly important role in the next evolution of the Internet through turning data into information, knowledge, and wisdom [1]. More recently, multiple type applications based on IoT have been developed, including health testing, safe home, intelligent transportation, logistics supply, environmental protection, infrastructure testing, and security [2]. Sensor nodes in the IoTs are widely distributed and require independent, mobile, sustainable, and maintenance-free capabilities. Under the current technologies, most sensors require an external power source to drive their operation, wherein the battery is extensively applied. However, the life cycle of the battery is limited, and replacing the battery for the massive sensors is a huge project, which consumes a lot of manpower and material resources and increases the maintenance cost. In addition, the regularly replaced battery generates a large amount of harmful substances, which seriously endangers the environment and human health. Therefore, a clean and sustainable power source should be provided to satisfy the requirement of driving these small electronic devices sustainably.
Harvesting of the ambient environment energy, as an eco-friendly and renewable collecting energy method, is regarded as a promising and effective strategy to realize continuous powering for these small electronic equipment [3]. Some possible technologies have been exploited for collecting energy from surrounding environment, such as solar cells that collect energy from sunlight [4] and thermoelectric generators that harvest energy from temperature difference [5]. However, as constrained by the intermittency nature of sunlight, the low output of thermoelectric generators, these energy harvesting technologies cannot ensure the continuous operation of electronic devices. Owing to its abundant reserves and widespread, mechanical energy are increasingly utilized to extract and convert into electricity based on different mechanisms, including electromagnetic generator (EMG) [6], piezoelectric nanogenerator (PENG) [7, 8], and triboelectric nanogenerator (TENG) [9]. Considering the large-scale power generation of EMG and low output power of PENG, TENG has been demonstrated as a promising approach for harvesting ambient mechanical energy due to the desirable features of simple structure, flexibility, low cost, light weight, high efficiency, and high power density at low frequency [10]. The operation of TENGs is depended on triboelectrification (or contact electrification) and electrostatic induction [11], and the fundamental theory is according to Maxwell’s displacement current and change in surface polarization [12]. Since the first invention of TENG in 2012, it has been extensively investigated and well confirmed that the potential of wide application is ranging from powering small electronic devices for self-powered systems, functioning as active sensors for medical, infrastructural, human-machine, environmental monitoring, and security [13, 14, 15, 16, 17, 18, 19, 20]. Various types of wasted mechanical energies in our daily life, such as human motion, vibration, wind, and flowing water can be utilized by different TENG structures. Based on these characteristics, TENG can be utilized as a small-scale energy harvester for driving mass electronic equipment continuously.
TENGs are derived from the coupling effect of contact electrification and electrostatic induction. Contact electrification, as known as static electricity and contact charging, is a common phenomenon in many manufacturing environments and has been known for thousands of years. During the process of contact electrification, the dissimilar material/surface becomes charged after contacting with each other. After contacting, the opposite’s triboelectric charge is produced on the surface of dissimilar materials with different electron affinities. Driven by external mechanical motion, the materials will be separated resulting in potential difference between the two electrodes on the back side of the materials. To maintain the electrostatic equilibrium, the free electrons in the electrodes will be driven to flow in external circuit to balance the induced potential difference, thus converting mechanical energy into electrical energy. According to the different structure designs of electrodes or moving manners of the triboelectric layer in TENGs, four different modes of TENGs have been build [9], as elaborated as follows.
The mechanism of vertical contact-separation mode can be elaborated largely by an example. As shown in Figure 1a, the simplest structure of TENG includes two metal electrodes and a dielectric film, in which two metal films work as top electrode and back electrode attached to dielectric film, respectively [21, 22]. When mechanical movement is applied in the unit, the top electrode and dielectric film will contact with each other, and thus the dielectric layer and electrode will get positively charged and negatively charged, respectively, due to the triboelectrification. Once they are separated by a short distance, the potential difference between the two electrodes will be induced, which will drive electrons to flow from the back electrode to the top electrode, resulting in a pulse current with an external circuit connected. If they are brought into contact again, the electrons will flow back and the current will be reversed.
The four fundamental modes of triboelectric nanogenerators: (a) vertical contact separation mode, (b) in-plane contact-sliding mode, (c) single-electrode mode, and (d) freestanding triboelectric-layer mode [9].
The basic structure of TENG in this model is the same as that of the vertical contact-separation mode. The difference is from the motion mode of the top electrode (Figure 1b). In the original state, the top electrode and dielectric film fully overlap and intimately contact with each other, leading to the oppositely charged surfaces. With the top electrode sliding outward, the contact surface area will decrease gradually until the complete separation of the two surfaces. The separated surface creates a potential difference across the two electrodes, generating a current flow from the top electrode to the bottom electrode. When it slides backward, then there will be a reversed current flow to balance the potential difference [23, 24].
As displayed in Figure 1c, the single-electrode mode TENG has only one bottom electrode connected to the ground [25, 26]. After contact with the top material, the two surfaces will get charged owing to the triboelectric effect. During the process of an approaching and departing of top material, the local electrical field distribution caused by charged surfaces will change. Then, there will be potential difference change between the bottom electrode and the ground, and electrons exchange between them to maintain the potential change.
As for the freestanding triboelectric-layer mode, it is the only one that the motion part is a dielectric layer [10], as shown in Figure 1d. The dielectric layer and two electrodes are in the same order, and the gap distance between the two symmetric electrodes should much smaller than the size of dielectric layer. At the original position, the state of dielectric layer and electrode is the same as what is in the lateral-sliding mode. The dielectric layer and electrode will get oppositely charged, respectively, once the motion occurs as before mentioned. When the dielectric layer is sliding forward and backward, there will be a potential difference between the two electrodes due to the change of overlapped area, which drives the electron exchanges between them.
In order to satisfy the requirement of harvesting mechanical energy from multiple type motions, various TENGs have been fabricated based on the four modes illustrated above.
Given the collection features of small scale, low frequency, and irregularity, human biomechanical motions are considered to be accessible, renewable, and the most abundant energy sources. TENG can collect this energy and convert it into electricity. Since it is first reported in 2012, TENG harvesting mechanical energy from human biomechanical movements has been fully developed.
Compared to the discrete devices, complex integrated TENGs can perform multiple functions with the merits of higher output performance, better adaptability, and sustainably. Based on the high-efficient and sustainable TENGs, various integrated TENGs have been developed for harvesting energy from human biomechanical movements. Zhu et al. introdued a packaged power-generating insole with built-in flexible multi-layered TENGs that harvested mechanical pressure during normal walking to power portable and wearable consumer electronics [27]. Bai et al. developed a flexible multilayered TENG by intergrating five layers of units on a zigzag-shaped Kapton substrate to gain pressure from normal walking [28]. Because of the unique structure and nanopore-based surface modification on the metal surface, the instantaneous short-circuit current (Isc) and the open-circuit voltage (Voc) can reach 0.66 mA and 215 V with an instantaneous maximum power density of 9.8 mW/cm2 and 10.24 mW/cm3. Triggered by press from normal walking, the TENG attached onto a shoe pad was able to instantaneously drive multiple commercial LED bulbs.
For improving the output current, Yang et al. designed an integrated rhombic gridding-based TENG to harvest vibration energy from natural human walking [29]. The newly designed TENG consists of PTFE nanowire arrays and aluminum nanopores with the hybridization of both the contact-separation mode and sliding electrification mode. Herein, Voc of the TENG could be up to 428 V, and Isc was near 1.395 mA with the peak power density of 30.7 W/m2. Moreover, based on the TENG, a self-powered backpack was developed with a considerably high vibration-to-electric energy conversion efficiency of 10.62(±1.19)%. When a person walks naturally carrying the designed backpack with a total weight of 2.0 kg, the power harvested from the body vibration is high enough to simultaneously light all the 40 LEDs.
Based on a high-output TENG, Niu et al. developed an universal self-charging system exclusively driven by random body motion for sustainable operation of mobile electronics [14]. In this system, a multilayered attached-electrode contact-mode TENG is utilized to effectively collect the energy from human walking and running (Figure 2a). The basic working principle of attached-electrode contact-mode TENGs is shown in Figure 2b. The structure of multi-unit TEMG, shown in Figure 2c, consists of 10–15 layered TENGs which used a Kapton film (a thickness of 125 μm) as the substrate and is shaped into a zigzag structure. A surface modified thin aluminum foil and fluorinated ethylene propylene (FEP) layer are utilized as the triboelectric materials. Figure 1d displays the small volume and lightweight of as-fabricated TENG (5.7 × 5.2 × 1.6 cm/29.9 g for a 10-layer TENG and 5.7 × 5.2 × 2.4 cm/43.6 g for a 15-layer TENG). As shown in Figure 2e,f, a human walking can drive this TENG to generate about 2.2 μC short-circuit transferred charge and about 700 V voltage output when embedded the TENG in the shoe insoles.
(a) System diagram of a TENG-based self-powered system, (b) working mechanism of an attached-electrode contact-mode TENG, (c) structure of the designed multilayer TENG, (d) photo of an as-fabricated TENG, (e) triboelectric charge output, and (f) Voc output of the as-fabricated TENG [14].
Shen et al. proposed a humidity resisting triboelectric nanogenerator to harvest energy from human biomechanical movements and activities for wearable electronics [30]. The obtained HR-TENG is fabricated by a nanofibrous membrane via electrospinning method. Under a relative humidity of 55%, the current and voltage output of the self-powered unit can still reach as high as 28 μA and 345 V, corresponding to a power density of 1.3 W/m2 with hand tapping. With the relative humidity raising from 30 to 90%, its electrical output still kept a relatively high level. A wide-range of electronics such as an electronic watch, a commercial calculator, a thermal meter, and a total of 400 LEDs has demonstrated to be successfully powered from human biomechanical movements under different ambient humidities.
Textile-based device is highly desirable for wearable electronics due to its low-mass, durable, flexible, and conformable [31]. As the most efficient power sources, textile substrate-based TENGs are fabricated for the features of simple structure, wide material choices, and low cost [32, 33, 34, 35, 36, 37]. Series efforts have been made to develop fabric TENGs for harvesting mechanical energy induced from body motions to sustainably drive wearable electronics [34, 38]. Lee et al. reported an electrical response of a textile substrate-based TENG including nanostructured surface provided by Al nanoparticles and polydimethylsiloxane (PDMS) [32]. The obtained TENG can power wearable electronics using low-frequency mechanical movements driven by human arm activity. Under the simple folding-releasing stage of an arm near 90o, the output voltage and current of 139 V and 39 μA are achieved, respectively.
To enhance the output performance, a highly stretchable 2D fabric was developed as a wearable TENG for harvesting footstep energy during walking to driven wearable electronic devices [39]. The fabric-structured TENG composes by Al wires and PDMS tubes with a high-aspect-ratio nanotextured surface with vertically aligned nanowires. It shows a stable high-output voltage and current of 40 V and 210 μA, corresponding to an instantaneous power output of 4 mW. The TENG also exhibits high robustness behavior even after 25% stretching, enough for use in smart clothing applications and other wearable electronics. Seung et al. reported a fully flexible, foldable nanopatterned wearable TENG with high power-generating performance and mechanical robustness [40]. Both a silver (Ag)-coated textile and PDMS nanopatterns based on ZnO nanorod arrays on a Ag-coated textile template are used as active triboelectric materials. A high voltage and current output with an average value of 170 V and 120 μA, respectively, are obtained from a four-layer-stacked wearable TNG under the compressive force of 10 kgf. Notably, there are no significant differences in the output voltages measured from the multilayer-stacked WTNG over 12,000 cycles, confirming the excellent mechanical durability of WTNGs. Without external power sources, the fabricated wearable TENG can drive the LEDs, LCD, and the keyless vehicle entry system, exihibting the potential applications in self-powered smart clothes, health care monitoring and self-powered wearable devices, and even personal electronics. Tian et al. demonstrated a high-performance double-layer-stacked triboelectric textile (DTET) for harvesting human motion energy [41]. Both the Ni-coated polyester conductive textile and the silicone rubber are adopted as effective triboelectric materials. A high output Voc of 540 V and an Isc of 140 μA can be obtained from the DTET with the size of 5 × 5 cm2, corresponding to a high peak surface power density of 0.892 mW/cm2 at a load resistance of 10 MΩ. The output peak signal of the DTET can be used as a trigger signal of a movement sensor to design movement monitoring equipment. With only the energy harvested from walking, running, or flapping, the DTET can directly light up 100 LEDs connected serially and drive portable electronics, such as competition timer, digital clock, and electronic calculator.
Owing to the high power density, stable cycle life, good safety, and potentials in integration into flexible wear, introducing supercapacitors as energy-storing devices into a fabric TENG show promising prospects. Pu et al. introduced a self-charging power textile for harvesting human motion energy. The self-charging power textile was fabricated by weaving the yarn supercapacitors together with a fabric TENG into an individual fabric [42]. Based on the integrated system, the motion-charging process is carried out by charging the yarn supercapacitors by the contact-separation motions between the TENG cloth and a common cotton cloth. The yarn supercapacitors and the fabric TENG endowed the excellent flexibility and weaveability of the self-charging power textile. Chen et al. developed a self-charging power textile, consisting of a fabric triboelectric nanogenerator and a woven supercapacitor, which can simultaneously harvest and store body motion energy to sustainably drive wearable electronics [43]. Utilizing traditional weaving craft, contact-separation mode and free-standing mode FTENG are designed and fabricated on a piece of textile by weaving the cotton, carbon, and PTFE wires. Combined with the energy-storing component, utilizing RuO2-coated carbon fiber and cotton threads, the obtained self-charging power textile can harvest energy from common daily activities such as running and walking to drive the wearable electronics, such as an electric watch.
For developing low-cost TENG, paper served as a supporting component for preparing TENG for the first time [44]. Paper-based TENGs represent an low-cost, light-weight, and environmentally friendly energy harvesting methodology. Nowadays, different types of paper-based TNEG have been designed and prepared for harvesting energy from human biomechanical movements [45]. Xia et al. proposed a X-shaped paper TENG formed from a ballpoint ink layer coated by painting with a commercial brush pen for harvesting mechanical energy from human walking [46]. In this design, paper served as both a component of the triboelectric pairs and a supporting component. When a brush pen is painted on the paper, the maximum values of current and voltage output can be achieved at 326 V, 45 μA, corresponding to a power density of 542.22 μW/cm2. The staked X-shaped paper TENG is proposed to increase the output performance and harvest the mechanical energy generated by motion of the human body, which can directly light up 101 blue high-power LEDs with a working voltage of 3.4 V.
Additionally, various efforts have been made to promote the development of TENGs for harvesting biomechanical energy based on external devices attached to the human body. In them, human skin-based TENGs are developed for converting biomechanical energy induced from human body itself into electronic energy. According to these series TENGs, human skin is used as one of the triboelectric materials with the single-electrode-mode. With the contact/separation between an area of human skin and a PDMS film, a Voc up to −1000 V, a short-circuit current density of 8 mA/m2, and the corresponding power density of 500 mW/m2 on a load of 100 MΩ were obtained from the skin-based TENG delivers, which could be used to directly drive tens of green light-emitting diodes [47]. Due to its fantastic features, skin-based TENGs are developed to transform physical parameters such as pressure, sliding, and other physiological variables into electronic signals, which exploit potential application. For realizing visual-image recognition, a self-powered brain-linked vision electronic-skin (e-skin) for mimicking retina is achieved from polypyrrole/polydimethysiloxane (ppy/PDMS) triboelectric-photodetecting pixel-addressable matrix [48]. The e-skin can directly transmit photodetecting signal into brain for participating in the vision perception and behavioral intervention. Besides visual-image recognitio, more functional sensors including sliding sensor [49], touch screen [50], pressing sensor [51], and motion sensors [52] are also deeply explored.
In order to satisfy the requirement of self-powered, highly stretchability, and transparency of triboelectric skins, different materials including silicone rubber [53], metal nanowire [54, 55], and conductive polymer [56] are widely studied. To introduce the characteristic of instilling self-healing and further enhance the performance of energy generation, stretch ability, transparency, and slime-based ionic conductors were first used as transparent current-collecting layers of TENG for harvesting mechanical [57]. The ionic-skin TENG consists of a silicone rubber layer with a thickness of 100 ± 10 μm, utilized as the triboelectrically negative material, a slime layer (a crosslinked poly(vinyl alcohol) gel) with a thickness of 1 mm that works as the ionic current collector, and a VHB tape with a thickness of 1 mm as the substrate (Figure 3a). Figure 3b shows the photograph of the real highly transparent ionic-skin TENG. As depicted in Figure 3c, the resulting ionic-skin TENG displays a transparency of 92% transmittance for visible light. The mechanism of the ionic-skin TENG is based on the single-electrode mode, wherein human skin and silicone rubber serve as frictional layer, respectively (Figure 3d). Figure 3e shows the digital photographs of the fabricated ionic-skin TENG suffering various mechanical deformations including uniaxial stretching up to 700% strain as well as folding and rolling. The produced slime exhibits high ionic conductivity due to the presence of positive (Na+) and negative ions (B(OH)4−), which is measured using electrochemical impedance spectroscopy (Figure 3f). Thanks for the series of design, the energy-harvesting performance of ionic-skin TENG is 12-fold higher than that of the silver-based electronic current collectors. Besides, fabricated ionic-skin TENG can recover its property even suffering 300 times of complete bifurcation, exhibiting an autonomously self-healing capacity.
(a) Schematic diagram of the IS-TENG. (b) Digital photo of the highly transparent IS-TENG. (c) Transmittance spectra of the slime (ionic conductor) and the IS-TENG with respect to a glass slide. (d) Schematic illustration of the working mechanism of the IS-TENG. (e) Digital photos of the IS-TENG under various mechanically deformed states such as axial strain up to 700%, rolled, and folded. (f) EIS measurement of the slime (ionic conductor) [57].
For versatile scavenging mechanical energy induced from arbitrary mechanical moving objects such as humans, a new mode of triboelectric nanogenerator is first demonstrated based on the sliding of a freestanding triboelectric-layer between two stationary electrodes on the same plane [58]. With two electrodes alternatively approached by the tribo-charges on the sliding layer, electricity is effectively generated due to electrostatic induction. To reduce the direct friction between triboelectric layers for energy loss, a linear grating-structured freestanding triboelectric-layer nanogenerator (GF-TENG), consisting of a freestanding triboelectric layer with grating segments and two interdigitated metal electrodes, was developed for high-efficiency harvesting vibration energy from human walking [59]. As shown in Figure 4a, 60 commercial LEDs (Nichia NSPG500DS) can be lighted up instantaneously with the motion of hand sliding under a slow speed and a small displacement. The GF-TENG can also havest energy from the monement of car for powering electronic components on the vehicle (Figure 4b). Four identical extension springs are used to suspend and anchore the triboelectric layer, as displayed in Figure 4c. Owing to the structure, the obtained GF-TENG can scavenge the mechanical energy from people’s walking motion when it is bonded to human legs (Figure 4d). An excellent stability and maxmiun energy conversion efficiency of 85% are realized at a matched load resistance of 88 MU under the noncontact mode (Figure 4e).
Applications of GF-TENG for harvesting a wide range of mechanical energy. (a) Harvesting energy from sliding of a human hand. (b) Harvesting energy from acceleration or deceleration of a remote control car. (c) Device structure for noncontact GF-TENG. (d) Harvesting energy from people walking by noncontact GF-TENG and the real-time measurement of Isc. (e) Total conversion efficiency of noncontact GF-TENG for harvesting slight vibration under different load resistances [59].
Vibration, as a type of common mechanical phenomena, ubiquitously exists in ambient environment in a variety of forms and wide range of scales. Therefore, vibration can be regarded as a sustainable source of power for driving small electronics if it can be effectively collected. Contributing to the distinctive working mechanism, TENG has been proposed recently and proved a promising approach for scavenging mechanical energy from vibration, especially in the low-frequency range. To date, a variety of device and machine-based TENGs have been applied to convert mechanical energy induced from vibration into electric energy.
Chen et al. presented a harmonic-resonator-based TENG as a sustainable power source and an active vibration sensor [60]. The harmonic-resonator-based TENG, held a multilayer structure consisting of aluminum with nanoporous surface as contact electrode and nanowire-modified PTFE as frictional layer, is the first TENG that can harness random and tiny ambient vibration. It can effectively respond to vibration frequencies ranging from 2 to 200 Hz with a considerably wide working bandwidth of 13.4 Hz.
The above-mentioned harmonic resonator-based TENG with a simple structure design can only scavenge vibration energy from a single direction. In practice, vibrations in living environments generally display multiple motion directions. With this in mind, a three-dimensional TENG (3D-TENG) was designed for harvesting random vibration energy from multiple directions [61]. The 3D-TENG has a multilayer structure with circular acrylic as supporting substrates, as shown schematically in Figure 5a. The cylindroid core of the 3D-TENG lies at the center of the acrylic substrate with a bottom diameter of 3 cm. On the top of the core, an iron mass is mobile and suspended by three identical springs with an included angle of 120° between each other. The designed structural symmetry ensures that the whole system has a constant resonant frequency in arbitrary in-plane directions. A layer of PTFE film as one contact surface is adhered onto the bottom side of circular iron mass with a deposited copper thin film as the back electrode. Attached to the bottom acrylic substrate, an aluminum thin film with nanopore modification plays dual roles as a contact electrode and the other contact surface. The scanning electron microscopy (SEM) images of aluminum nanopores can be observed in Figure 5b. A photograph of the real 3D-TENG device is shown in Figure 5c. Owing to the conical-shaped spring structure, the 3D-TENG can operate in a hybridization mode combining with the vertical contact-separation mode and the in-plane sliding mode, which is beneficial to harvest random vibrational energy in multiple directions over a wide bandwidth.
3D triboelectric nanogenerator: (a) schematic of a 3D-TENG, (b) SEM image of nanopores on an aluminum electrode, and (c) a photograph of the fabricated 3D-TENG [61].
For better sensitivity response to external disturbance, a suspended 3D spiral structure was integrated with a TENG for energy harvesting and sensor applications [62]. Operating in the vertical contact-separation mode, the desired TENG with unstable mechanical structure can balance itself when be oscillated, which makes it a superior choice for vibration energy harvesting and vibration detection. The newly designed TENG has a wide working bandwidth of 30 Hz in low-frequency range with a maximum output power density of 2.76 W/m2 on a load of 6 MΩ.
Beyond that, a spherical three-dimensional TENG (3D-TENG) with a single electrode, consisting of an outer transparent shell and an inner polyfluoroalkoxy (PFA) ball, was designed for scavenging ambient vibration energy in full space [63]. By working at a hybridization of both the contact-separation mode and the sliding mode, the 3D-TENG can deliver a maximal output voltage of 57 V, a maximal output current of 2.3 μA, and a corresponding output power of 128 μW on a load of 100 MΩ, which can be used to directly drive tens of green light-emitting diodes. Moreover, the TENG is utilized to design the self-powered acceleration sensor with a detection sensitivity of 15.56 V/g.
Besides multiple motion directions, ambient vibrations generally exhibit a wide spectrum of frequency distribution. To solve this problem, a TENG with a wavy-structured Cu-Kapton-Cu sandwiched between two flat nanostructured PTFE films was designed for broadband vibration energy harvesting [64]. The core of the wavy structure is composed of a set of metal rods (with a diameter of 1/4 in.), as shown in Figure 6a. PTFE films are processed with inductively coupled plasma (ICP) etching to produce the nanostructures shown in Figure 6b, which would largely enhance contact electrification. The device structure is schematically shown in Figure 6c, accompanied by a magnified schematic in Figure 6d and a picture of a real device in Figure 6e. This structure design allows the TENG to be self-restorable after impact without the use of extra springs and converts direct impact into lateral sliding. Based on the wavy structure, the TENG can harvest vibrational energy from 5 to 500 Hz, and the generator’s resonance frequency was determined to be ∼100 Hz at a broad full width at half-maximum of over 100 Hz, producing a Voc of up to 72 V, an Isc of up to 32 μA, and a peak power density of 0.4 W/m2.
(a) Schematic of the method to fabricate wavy Kapton films. (b) SEM image of the ICP-processed PTFE film surface. (c) Schematic of the device structure. (d) Magnified schematic of the device, showing that the wavy core is in periodical contact with the nanostructures on the PTFE films. (e) Photograph of an as-fabricated TENG device before packaging [64].
After that, an elastic multiunit TENG was also realized to efficiently harvest low-frequency vibration energy over a wide frequency range [65]. The obtained TENG can provide a maximum instantaneous output power density of 102 W/m3 at as low as 7 Hz and maintain its stable current outputs over a wide frequency range (from 5 to 25 Hz). Besides, it can act as an active vibration sensor to monitor the running status of equipment. Moreover, by combining the TENG with a power management unit to form a self-charging power unit, the vibration energy harvesting from ambient environment, such as an operating machine and running bicycle, can sustain power electronics such as thermometers, humidity sensors, speedometers, and a micro-meteorological instrument.
For improving the lower output current, a multi-layered stacked TENG was reported as a cost-effective, simple, and robust approach for harvesting ambient vibration energy [66]. The 3D-TENG has a multilayered structure with acrylic as supporting substrates, as schematically shown in Figure 7a. A photograph of an as-fabricated TENG and SEM image of the PTFE nanowires is shown in Figure 7b-c. With superior synchronization, the 3D-TENG produces a short-circuit current as high as 1.14 mA and an Voc up to 303 V with a remarkable peak power density of 104.6 W/m2. As a direct power source, it is capable of simultaneously lighting up 20 spot lights as well as a white G16 globe light.
Three-dimensional triboelectric nanogenerator. (a) Schematic of a 3D-TENG. (b) SEM image of nanopores on aluminum electrode. (c) A photograph of the as-fabricated 3D-TENG [66].
To reduce the direct friction between triboelectric layers, a liquid-metal-based TENG (LM-TENG) was developed for high-efficiency vibration energy harvesting [67]. Owing to an intimate contact between the liquid metal and the polymer dielectric layer, the direct friction between triboelectric layers for energy loss is effectively reduced, resulting in high effective contact, shape adaptability, and low friction coefficient with solid. Therefore, the LM-TENG exhibits an output charge density of 430 μC/m2, which is four to five times higher than that in the case if the electrode is solid film.
On the other hand, soft electrode can effectively increase the contact intimacy between the triboelectric layers [68]. Xu et al. reported a novel soft and robust TENG made of a silicone rubber-spring helical structure with nanocomposite-based elastomeric electrodes for harvesting arbitrary directional vibration energy and self-powered vibration sensing [69]. The schematic diagram and a photo of the S-TENG are shown in Figure 8a,c, respectively. As displayed, the TENG exhibits a helical structure based on the integration of elastomer and spring. A mixing well silicone rubber and carbon nanofiber, which can be stretched up to the strain of 133%, serves as the elastomeric electrode (Figure 8b). The working mechanisms of the S-TENG under vertical and horizontal vibration are shown in Figure 8d,e, respectively. Under external vertical vibration excitation, the distance between a helical structure’s adjacent surfaces changes, forming a contact-separation mode TENG. Under horizontal vibration excitation, the S-TENG’s helical structure’s adjacent surfaces can contact on one side and separate on the other side, also forming a contact-separation mode TENG. Under the resonant states of the S-TENG, its peak power density is found to be 240 and 45 mW/m2 with an external load of 10 MΩ and an acceleration amplitude of 23 m/s2. Additionally, the dependence of the S-TENG’s output signal on the ambient excitation can be used as a prime self-powered active vibration sensor that can be applied to monitor the acceleration and frequency of the ambient excitation.
(a) The device schematic of the S-TENG. Note that the gray silicone rubber layer containing a spring forms a base on which other layers can be built, and the black silicone rubber layer along with the electrode layer forms a contact-separation pair. Both top and bottom electrodes are made of carbon nanofiber-mixed silicone rubber. (b) SEM image of the carbon nanofiber for preparing the elastomeric electrode. (c) Photo of the as-prepared S-TENG. Working mechanisms of the S-TENG under (d) vertical vibration excitation and (e) horizontal vibration excitation [69].
Water energy deriving from rainwater, ocean waves, and waterfalls has been regarded as an alternative renewable energy resource source without polluting the environment. Energy harvesting from water has been further reinforced due to the abundant reserves and little dependence on environmental conditions. Through decades of exploration, a variety of wave energy converting devices and machines based on TENG has been invented to harvesting energy from water.
Liquid-solid-mode TENGs for harvesting liquid-wave energy have drawn much attention for the features of relatively stable output and durability [70, 71, 72]. For the liquid–solid-mode TENG, contact separation is the main representative strategies applied to scavenge water energy [73, 74]. A hydrophobic surface on water-solid TENGs is beneficial for inducing separation at the interface of liquid and solid [75]. Based on this, Zhu et al. reported a liquid-solid electrification-enabled TENG based on a FEP thin film for harvesting energy from a variety of water motions [76]. Owing to the modification of aligned nanowires, the thin film with a property of hydrophobicity can increase the contact area at the liquid-solid interface, leading to enhanced surface charging density and thus electric output at an efficiency of 7.7%. Due to the creation of continuous contact separation between water and the solid surface, a cylindrical water TENG was designed by using a hydrophilic surface along with the hydrophobic surface to control the water flow inside a packaged system for enhanced electrostatic induction [77].
Generally, an effective way of integrating a number of electrodes together to make them area scalable is helpful for promoting output power density. On the other hand, the electric power is highly affected by nanostructures at the solid/liquid interface. According to this, a flexible thin-film TENG was reported for harvesting kinetic wave energy [78]. Because of the integration method that use an array of surface-mounted bridge rectifiers to connect multiple parallel electrode together, the induced current between any pair of electrodes can be constructively added up, leading to a significant enhancement in output power and realizing area-scalable integration of electrode arrays. However, the thin-film TENG is only applicable to regular water waves that interact with the TENG through a linear water level. For improving the adaptive means of harvesting water energy, a networked integrated TENG was fabricated for harvesting energy from interfacing interactions with water waves of various types [79]. Additionally, interdigital electrode-based TENGs were designed in the contact-sliding mode for the harvesting of triboelectric energy from water [80], resulting in a higher output performance than those of one- and two-electrode-based TENGs.
Beside liquid-solid-mode TENGs, other structure TENGs were designed for harvesting water energy generating by flowing water, such as multi-layered disk structure [81], floating buoy structure [82], radial-arrayed rotary structure [10], and so on. Although many water-based TENGs have been fabricated, there is a lack of effort in realizing TENG harvesting water energy directly on the fabric/textile, due to the poor water resistance of the fabrics related to their intrinsic hydrophilicity that can be ascribed to their abundant hydrophilic groups, and the strong adsorption capacity because of their large specific surface area [83]. For realizing the practical wearable device harvesting energy from water flow, Xiong et al. reported a wearable fabric-based WTEG with additional self-cleaning and antifouling performance for the first time [83]. This is realized with the preparation of hydrophobic cellulose oleoyl ester nanoparticles by a nontoxic esterification method and nanoprecipitation technology based on the microcrystalline cellulose. In this study, PET fabric-based WTEG can generate the output power density of 0.14 W/m2 at a load resistance of 100 MΩ.
There are two parts to water wave energy including the electrostatic energy from the contact electrification between water and surrounding media and the mechanical impact energy. For simultaneously scavenging both the energy from water, some works have been well done. For example, Su et al. presented an all-in-one hybridized TENG based on the conjunction of liquid-solid interfacial electrification enabled TENG and impact-TENG for harvesting water wave energy and as a self-powered distress signal emitter [84]; Lin et al. designed a fully integrated TENG for harvesting water energy and as a self-powered ethanol nanosensor, which contained a water-TENG unit to collect the electrostatic energy of water and a contact-TENG unit to collect the mechanical/kinetic energy of water [85]; Cheng et al. developed a water wheel hybridized TENG, composed of a water-TENG part and a disk-TENG part, for simultaneously harvesting the two types of energies from the tap water flowing from a household faucet [86]. Based on a unique structure design, the hybridized TENGs are shown to be suitable for harvesting multiple types of energies from water.
During a working process, the acting surfaces of the above mentioned TENGs will be exposed to ambient atmosphere, which will limit their applications in some cases. The interface electrification was seriously affected by humidity, causing a quick decline of the surface charge density [87]. In order to improve the performance of TENGs under harsh conditions with the presence of water, fully enclosed or packaged TENGs should be developed for tolerating the environment. So far, different designs were developed based on packaged TENG such as wavy-shaped models [88], fully packaged contact-separation configurations [89, 90, 91], and rolling spherical structure [92]. Wang et al. designed a freestanding, fully enclosed TENG that encloses a rolling ball inside a rocking spherical shell for harvesting low-frequency water wave energy [93]. An image of the fabricated TENG floating on water is shown in Figure 9a. Figure 9b shows the schematic diagram of the freestanding structured design that consists of one rolling ball and two stationary electrodes. To enhance the electric output of the TENG, nanowire arrays are fabricated on the surface of the Kapton film (Figure 9c) that provides a large contact area to generate more triboelectric charges on the surface. Through the optimization of materials and structural parameters, a spherical TENG of 6 cm in diameter actuated by water waves can provide a peak current of 1 μA over a wide load range from a short-circuit condition to 10 GΩ, with an instantaneous output power of up to 10 mW. This rolling-structured TENG is extremely lightweight, has a simple structure, and is capable of rocking on or in water to harvest wave energy. Additionally, rolling spherical TENGs and coupled TENG networks have been demonstrated to harness the water wave energy because of the advantages of light-weight, small-resistance under the water wave motions, and easy to be integrated [94, 95].
Device structure, basic operations of the freestanding-triboelectric-layer-based nanogenerator (RF-TENG) with a rolling Nylon ball enclosed. (a) Photograph of a rocking nanogenerator floating on water. (b) Schematic diagrams of freestanding-structured design. (c) SEM image of nanorod structure on the Kapton surface [93].
For enhancing the output current and enlarging the practical applications of packaged TENG, introducing a spring structure into the TENG can store the kinetic energy from water impact and later convert into electric power via residual vibrations [96]. Combining the advantages of spring structure and integrated multilayered structure, Xiao et al. demonstrated a kind of spherical TENG with spring-assisted multilayered structure for harvesting water wave energy [97]. The introduction of spring structure enhances the output performance of the spherical TENG by transforming low-frequency water wave motions into high-frequency vibrations, while the multilayered structure increases the space utilization, leading to a higher output of a spherical unit. The structure of spherical TENG designed with spring-assisted multilayered structure floating on water surface is schematically shown in Figure 10a. Figure 10b displays a photograph of as-fabricated spherical TENG device, and the inset shows the photograph of the device in the water waves. The working principle of each TENG unit is demonstrated in Figure 10c. The periodic movement of the mass block under the triggering of water waves, which leads to the contact and separation between two surfaces of the top aluminum foil and FEP film, produces periodic electric output signals. Owing to its unique structure, the output current of one spherical TENG unit can reach 120 μA, which is two orders of magnitude larger than that of previous rolling spherical TENG, and a maximum output power up to 7.96 mW is realized as triggered by the water waves.
(a) Schematic diagram of the spherical TENG with spring-assisted multilayered structure floating on water, and schematic representation enlarged structure for the zigzag multilayered TENG with five basic units. (b) Photographs of the as-fabricated TENG device. (c) Working principle of each TENG unit of the spherical TENG [97].
Wind energy can be a renewable energy sources for energy harvesting on account of widespread and absolute abundance. The practical application of traditional wind power in our daily life is largely limited by the extra-large volume, high cost of installation, noise and geographical environment. In this regard, TENG is one of the most alternative wind energy conversion strategies on accord of its small scale, low cost, simple fabrication routes, and portability [98]. In order to harvest wind energy, flutter-driven structure [99, 100] and rotational structure [101, 102] are the two main methods for preparing wind-driven TENG.
Flutter-driven structure TENG for harvesting wind energy was realized by Yang et al. for the first time [103]. As displayed in Figure 11, the TNEG is composed of two layers of Al foils and a FEP film laying in midair of a cuboid acrylic tube. The Al foils act as both triboelectric surfaces and electrodes, respectively. The FEP film is fixed one side, leaving the other side freestanding. The FEP film will vibrate periodically to contact the two Al foils inducing from wind, resulting in an output signal in an external circuit. Output voltage and current about 100 V and 1.6 μA are achieved, and a corresponding output power of 0.16 mW is realized under a loading resistance of 100 MΩ.
(a and b) The structure and photograph of the first reported flutter-driven mode WD-TENG [103].
Although single-side fixed-based TENG exhibits good performance for scavenging wind energy, the stability of output performance is a challenge because of the arbitrary fluttering of the FEP film. For solving the problem, an elasto-aerodynamics-driven TENG, consisting of a Kapton film with two Cu electrodes fixed on two ends in an acrylic fluid channel, was reported for scavenging air-flow energy [104], where the flutter effect of Cu electrodes was induced to contact two triboelectric materials of the PTFE films and the Kapton film to realize the output performance of the device.
Based on flutter-driven structure, many other efforts have been made to enhance the performance of TENG through optimizing the structure or the morphologies of material surface design. A lightweight and freestanding flag-type woven TENG, consisting of conductive belts of Ni-coated polyester textiles and Kapton film-sandwiched Cu belts, was designed for scavenging high-altitude wind energy from arbitrary directions [105]. When wind fluttering is applied in each woven unit, wind energy converts into electrical energy induced by the interlaced interactions between the Kapton film and a conductive cloth under wind-introduced fluttering of the flag. Besides, a flutter-driven TENG, consisting of a flag and a counter plate arranged in parallel with interwoven microstructure, was fabricated for harvesting wind energy based on contact electrification caused by the self-sustained oscillation of flags [106]. As shown inFigure 12, a flexible flag and a rigid plate are arranged in face to face in order to prepare a wind-driven energy-harvesting system using fluttering behavior. Owing to the design, interaction between them can lead to a rapid periodic contact and separation, and that movement can be successfully employed for converting the kinetic energy of the wind into electrical energy.
Schematic diagrams of a wind tunnel and the structural design of a flutter-driven triboelectric generator including surface characteristics of (i) a highly flexible flag, (ii) a counter plate, and (iii) the fabrication of the counter plate [106].
For rotational structure, wind cup is a main method for scavenging wind energy. Deriving from the conventional wind cup structure, a rotary structured TENG was presented for scavenging weak wind energy in our environment [101]. As illustrated in Figure 13, the rotary structured TENG is composed of a framework, a shaft, a flexible rotor blade, and two stators. When wind flowing is utilized in the rotation of the shaft and the flexible rotor, a flexible and soft polyester (PET) rotor blade with a PTFE film adhered at the end will periodically sweep across the Al electrodes. In this process, a consecutive face-to-face contact and separation between PTFE film and Al electrodes are produced, regarding as the basic process for generating electricity.
The schematic diagram showing the structural design of the R-TENG, with the enlarged picture showing the nanowire-like structures on the surface of PTFE [101].
Aiming to improve the robustness and lifetime of wind-driven TENG, a freestanding disk-based TENG was fabricated to harvest wind energy through automatic transition between contact and noncontact working states [102]. The major structure of the disk-based TENG includes two parts: the rotational inner acrylic barrel that connects with the freestanding rotor of the disk TENG and the stationary outer barrel that connects with the stator of the TENG. Two bearings are used to link the two parts and enable the relative rotation. Benefiting from the unique structural design, the TENG can work in the noncontact state with minimum surface wear and also transit into contact state intermittently to maintain high triboelectric charge density.
Besides serving as a power source for running some electric devices, wind-driven TENG is also expected to be utilized as various self-power systems by integrating with other electric devices. Chen et al. introduced the first self-powered air cleaning system focusing on sulfur dioxide (SO2) and dust removal as driven by the electricity generated by natural wind, with the use of rotating TENG [107]. Another common wind-driven TENG-based self-power system is the wind speed sensor. Kim et al. prepared wind-driven TENG based on rolling motion of beads for harvesting wind energy as a self-power wind speed sensor [108]. Wen et al. fabricated a blow-driven TENG, acting as an active alcohol breath analyzer, which is featured as high detection gas response of ~34 under an optimized sensor working temperature, fast response time of 11 s as well as a fast recovery of 20 s [109].
Aiming to simultaneously harvesting multitypes of energies from various sources, TENG has been hybridized with various other energy harvester strategies from the environment. It is well known that solar irradiance is another clean and renewable energy sources. To develop a practical method to simultaneously scavenge solar and mechanical energies, the concept of a hybridized energy harvester integrating TENG and solar cell was presented [110, 111]. Based on lightweight and low cost, fabric-based material is served as the ideal strategy utilized to fabricate these kinds of hybrid generator [112]. Chen et al. presented a foldable and sustainable power source by fabricating an all-solid hybrid power textile with economically viable materials and scalable fabrication technologies [34]. The wearable all-solid hybrid power textile has a single-layer interlaced structure, which is a mixture of two polymer-wire-based energy harvesters, including both a fabric TENG to convert mechanical movement into electricity and a photovoltaic textile to gather power from ambient sunlight, as schematically illustrated in Figure 14a,b, respectively. An enlarged view of the interlaced structure is presented for both the fabric TENG (Figure 14c) and photovoltaic textile (Figure 14d). Under ambient sunlight with mechanical excitation, like human motion, car movement, and wind blowing, the as-woven textile was capable of generating sufficient power for various practical applications, including charging a 2 mF commercial capacitor up to 2 V in 1 min, continuously driving an electronic watch, directly charging a cell phone, and driving the water splitting reactions.
Structural design of the hybrid power textile. (a and b) Schematic illustration of the hybrid power textile, which is a mixture of two textile-based all-solid energy harvesters: fabric TENG (a) and photovoltaic textile(b). Enlarged view of the interlaced structure of both the fabric TENG (c) and the photovoltaic textile (d) [112].
Aiming to largely collect the energy from mechanical motions, an integrated TENG and an electromagnetic generator (EMG) for concurrently harvesting mechanical energy are a promising way. By integrating two kinds of mechanical energy harvesting units, the weight of the EMG can be reduced and the total output power can be increased to expand the potential applications [113, 114, 115, 116, 117]. In them, rotational structure is the typical strategy utilized to simultaneously convert mechanical energy into electrical energy from one rotating motion. By integrating an EMG and a TENG, a rotation-based hybrid generator is first fabricated to generate a high output that can sustainably drive a commercial globe light with an intensity of illumination up to 1700 lx [118]. As illustrated in Figure 15a, the main structure of the hybrid generator consists of an EMG including the top and bottom layers (1 and 5) and a TENG including the middle layers (2, 3, and 4) with the planar structures, where the rotator and the stator are composed of layers 1 and2 and layers 3–5, respectively. The corresponding photographs of each layer are displayed in Figure 15b. Based on the relative rotation between the rotator and the stator, the hybrid generator simultaneously collects biomechanical energy from human hand-induced rotating motions. In order to compare the two generators with each other systematically, Guo et al. fabricated a water-proof triboelectric-electromagnetic hybrid generator, including a fully enclosed packaging of TENG achieved by the interactions between pairs of magnets as the noncontact mechanical transmission forces [119]. Systematic study of the influences of the designed parameters, including the segment’s number of the TENG, the rotation speed, and the arrangement of the coils, on the electrical outputs of the WPHG were performed experimentally. The result demonstrated that TENG can produce a stable voltage to power commercial electronic device even under a low rotation speed compared with EMG.
(a) Schematic diagram of the designed hybridized nanogenerator. (b) Photographs of the hybridized nanogenerator [118].
Besides the above mentioned, other strategies have been applied to intergrate with TENG for collecting other types of energies. Lee et al. presented a flexible hybrid cell to simultaneously harvest thermal and mechanical energies from skin temperature and body motion [120]. For fabricating the hybrid cell, ZnO nanowires are grown on the sputtered-coated seed layer surface of a thin Al substrate. And then, a 2-μm thick poly(methyl methacrylate) (PMMA) layer is coated on the surface of the as-grown ZnO nanowires, and a thin Al substrate is stacked on the PMMA-coated layer to be used as the top electrode. Owing to the structure design, the hybrid cell can simultaneously harvest thermal and mechanical energies so that the energy resources can be effectively and complementarily utilized for power sensor network and micro/nanosystems. Addtionally, combining the TENG with piezoelectric nanogenerator (PENG) is a alternative manner for concurrently collecting mechnical energy. Guo et al. developed an all-fiber hybrid piezoelectric-enhanced TENG that fabricated by electrospinning silk fibroin and poly(vinylidene fluoride) (PVDF) nanofibers on conductive fabrics [121]. Contributing to the large specific surface area of nanofibers and the extraordinary ability of silk fibroin to donate electrons in triboelectrification, the hybrid nanogenerator exhibited an outstanding electrical performance, with a power density of 310 μW/cm2, so that it can be regarded as a self-powered wearable microsystem for falling-down detection and timely remote alarm.
In order to seek an intelligent life, trillions of electronic device for the Internet of Things are requisite with higher personal, portable, complex, multifunctional, and smart. Aiming to maintain the normal working status of these small electronic devices sustainably, an effective technology to harvest small-scale energy from renewable natural resources is highly desirable. Given the collection characteristics of simple structure, flexibility, low cost, light weight, high efficiency, high power density, and environmental friendly, the invention of TENG is served as an promising small-scale energy harvester who can convert mechanical motions into electricity, even at low frequency. Futhermore, TENGs can also be utilized to transform physical parameters such as pressure, sliding, and other physiological variables into electronic signals, which directly reflected the information of mechanical stimuli and environmental conditions without an external power source. By extensively investigating, TENG can effectively harvest mechanical energy in almost any form based on the four fundamental modes, and thus can regard as the self-powered sensors for a wide application under diffident mechanical triggerings. In the future, the continuous endeavors on TENGs will lagerly enhance their output performance. Based on deeply investigating the fundamental menchanism of triboelectrification, it is possible to realize the ultrahigh charge density of TENG via material modification, structure design, or condition optimization. Besides the output perfoermance, the durability and output stability is the other bottleneck that limited the application of TENG, especially comparing with the traditional generator. It might overcome through fabricating new materials or coupling modes of operations. Based on the above discussion and analysis, it can be anticipated that TENG will soon become an ideal small-scall energy haverter with broad application as self-powered sensors through the world wide efforts.
The authors like to thank the financial supports from the National Key R & D Project from Minister of Science and Technology (2016YFA0202704), Beijing Municipal Science & Technology Commission (Z171100000317001, Z171100002017017, Y3993113DF), and National Natural Science Foundation of China (Grant No. 61774016, 21773009, 51432005, 5151101243, 51561145021).
There is no conflict of interest.
Supporting women in scientific research and encouraging more women to pursue careers in STEM fields has been an issue on the global agenda for many years. But there is still much to be done. And IntechOpen wants to help.
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