Abstract
Air pollution becomes more and more serious with the rapid development of the society, and the haze caused by particulate matters (PMs) has become a global problem. Thus seeking an effective technology for removing the airborne PMs or other pollutants is much desirable for alleviating the air pollution. The newly invented triboelectric nanotechnology can realize efficient air filtering with obvious advantages over traditional fibrous filtering and electrostatic precipitation. Here, a review is provided for recent progress in air filter by utilizing the triboelectric nanotechnology, starting from the choices of triboelectric materials and main features of triboelectric nanotechnology. The mechanism of triboelectric air filtering technology was presented as the coupling of triboelectric filtering and mechanical filtering. Then the approaches of air filtering were summarized as the triboelectric nanogenerator (TENG)-driven air filtering, TENG-enhanced nanofiber air filtering, and self-powered triboelectric air filtering. The device structure, working principle and filtering performance were systematically discussed. Furthermore, the industry products which have been developed based on the triboelectric filtering technology were introduced.
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1. Introduction
1.1. Air filtering technologies
Due to the rapid industrialization, urbanization, and increasing energy consumption, air pollution is becoming more and more serious [1–6]. Direct emission of industrial exhaust and automobile exhaust has resulted in the frequent occurrence of severe haze weather. Haze, mainly caused by the particulate matters (PMs) made of sulfur dioxide, nitrogen oxides, inhalable particulate dust, and so on, has a severe impact on human health, visibility, communication, and even global climate [7–10]. It can induce human cardiovascular and respiratory diseases due to the PM2.5 (aerodynamic diameter of particle is less than or equal to 2.5 μm), and the threat of traffic safety [11, 12]. Now the haze has become a global problem, especially for developing countries.
Our country has proposed several ways to harness the haze, including reducing the emission of industrial and automobile exhaust, improving the quality of fuel and coal, and artificial rainfall. The government has also used a series of measures to respond the call, such as closing the enterprises causing heavy pollution, eliminating unqualified motor vehicles, restricting motor vehicle license plates, executing an odd-and-even license plate rule, strictly monitoring fuel quality, and spraying water mist into the sky, etc. The problem of these methods is that the treatment of symptoms does not cure the root cause, and they are costly and inefficient. The closing of a number of enterprises also brings a large area of unemployment and incalculable losses to national economic growth. Therefore, fundamentally, it is more desirable to identify the pollution source for its absorption and treatment, but there are still technical difficulties and a cost problem.
Removal of PMs is one of the major objectives for air filtering technology. Nowadays, fibrous filtering and electrostatic precipitation are the two most common technologies for removing the airborne PMs [13–21]. The fibrous filter, which uses multilayered randomly or regularly organized fibers to separate the PMs from the air stream with the minimum energy consumption, as shown in figure 1(a), has been widely applied for simple structure and low material costs [13, 14]. The particles may deposit on the fibers through the actions of different mechanisms including Brownian diffusion, interception, inertial impaction, and gravitational settling [15, 16]. These mechanisms belong to the mechanical filtering. Such filter can have a high efficiency for removing larger PMs, but a lower efficiency for removing smaller PMs. As the PMs are filtered and staying on the fibers, the pressure drop increases. The removal efficiency and pressure drop have the contrary relationship, determining the performance of the fibrous filter together. The electrostatic precipitator adopting the electrostatic precipitation technology has been widely applied in industry filter [18, 19]. Compared with the fibrous filter, the electrostatic precipitator has the obvious advantages of high removal efficiency and low pressure drop [20]. However, a high voltage is applied onto sharp discharge electrodes, and the air is ionized by the high electric field in the ionization region, as shown in figure 1(b). The ions and electrons produced at the corona points make the particles charged before the collection. Due to the Coulomb force, the charged particles are absorbed onto the collecting electrodes. The ionize emission is inevitable, and the production of ozone, which may cause cancer, largely limits its applications in the indoor air filter.
Figure 1. (a) Schematic illustration of fibrous filter and particle collection mechanism in the fibrous filter. Reproduced with permission from [13]. Copyright (2017) American Chemical Society. Reprinted with permission from [15]. Copyright 2012 American Chemical Society. (b) Schematic diagram of an electrostatic precipitator. © 2000 IEEE. Reprinted, with permission, from [20]. (c) Contribution of the individual mechanism to particle filtering at various particle diameter ranges. Reprinted from [16], Copyright 2009, with permission from Elsevier.
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Standard image High-resolution imageRegarding the filtering mechanisms for removal of PMs, besides the mechanical filtering, there are also electrostatic filtering and chemisorptions. The mechanisms of mechanical filtering are usually complex and diverse, but in specific circumstances, there should be corresponding emphasis. Figure 1(c) shows the contribution of the individual mechanism to particle filtering at various particle diameter ranges [16]. In general, dust particles with a diameter of more than 20 μm are filtered mainly through direct interception and inertial impaction. The gravitational settling mainly acts on much larger particles above 100 μm. The interception is dominated for the particles on the order of submicrons to several microns. For dust particles that are smaller than 2 μm, the influence of diffusion effect should be considered. For dust particles that are smaller than 0.4 μm, the diffusion effect plays a major role. The electrostatic filtering can have a larger action range for different particles, because of the larger size span of electrostatic effect from nanometers to microns. Usually, the electrostatic precipitator can realize a removal efficiency of more than 97% for particles from tens of nanometers to ten microns [21].
1.2. Triboelectric filtering
Triboelectric nanogenerator (TENG) has emerged as a powerful technology for transforming the ambient mechanical energy into electric energy [22, 23]. Based on the principle of coupling the triboelectric effect and electrostatic induction, the TENGs have been applied to harvest many kinds of energy sources, such as human walking, mechanical vibrating, wind, and water wave energies, etc [24–34]. With the technology improvement and device optimization, the TENG can have a high output performance and energy conversion efficiency to meet the power demands of small electronics [35–38]. Its fundamental physical mechanism for TENG is Maxwell's displacement current, different from the traditional electromagnetic generator [39, 40]. The TENG possesses three main applications, as a micro-scale power source, a large-scale power source, and an active sensor, which are also the applications of Maxwell's displacement current in areas of energy and sensor [39]. The energy harvested by TENG is the energy for the new era—the era of internet of things, sensor network, big data, and blue energy [40].
The triboelectric process in the working of TENG can produce a high voltage and a high space electric field. Such a high electric field in the space makes the particulates of the air stream absorbed and removed. The technology of removing PMs through the electrostatic adsorption caused by the triboelectric charges is called as triboelectric filtering, which provides a promising approach of self-powered PM removal with obvious advantages. First, the TENG can harvest the energy from the environment, and no external power supply is required for the air filtering system. Second, because the electrostatic attraction exhibits a broad action range of particle size with high removal efficiency, the triboelectric filter can efficiently remove the particulates from nanoscale to micro-scale. Third, despite of the high voltage and electric field provided by the TENG, there is no ozone emission, because of the absence of the air ionization during the operation. When the air flows through the triboelectric filtering region, the PMs in the air collide with the filtering materials and can be charged and then removed by the electrostatic attraction. Therefore, there not only exists the triboelectrification between different filtering materials, but also exists that between the flowing air and filtering materials. Usually, the filtering materials are particle-based polymer nanomaterials, and the filtering mechanism also includes the mechanical filtering besides the electrostatic filtering. The non-close packing of filtering particles can ensure a low pressure drop.
Here, recent progress in air filters by utilizing the triboelectric nanotechnology is reviewed. The review focuses on the triboelectric materials, triboelectric nanotechnology, typical triboelectric nanotechnologies in air filter, industry products, and future perspectives. In the first segment of the review, the triboelectric material choices, surface characteristics and the main features of triboelectric nanotechnology are demonstrated. In the subsequent section, we primarily devote to elaborating on the latest progress of triboelectric nanotechnology applied for air filter, including self-powered air filter driven by the TENG, TENG-enhanced nanofiber air filter, and self-powered triboelectric air filter. Next, the commercialization of triboelectric filtering technology and developed industry products are briefly introduced. Finally, perspectives and challenges for future technology and product developments are discussed.
2. Triboelectric materials and triboelectric nanotechnology
2.1. Material choices and surface characteristics
The triboelectric effect appears universally in almost all materials, from metal, to polymer, to silk, and to wood, almost everything. The material choices for the TENG or the triboelectric filter are very huge. Figure 2 presents the triboelectric series for some common materials according to a tendency of easily losing electrons (positive) to gaining electrons (negative) [41, 42]. Two materials that are more separated in the triboelectric series can produce better triboelectric effect and greater charge transfer, providing the reference basis for the material selection in the TENG or triboelectric filter. Generally, the materials with a higher surface charge density are desirable, which can generate a higher TENG output or filtering performance.
Figure 2. Triboelectric series for different materials based on the abilities of materials for gaining/losing electrons. [42] John Wiley & Sons. © 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.
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Standard image High-resolution imageIn order to improve the triboelectric surface charge density, the material surfaces can be modified to create micro- or nanopatterns by physical techniques [23, 43, 44] or functionalized chemically using various molecules, nanotubes, nanowires or nanoparticles [45]. For example, microstructured polydimethylsiloxane (PDMS) films with rectangular lines, square cubes, and square pyramids were prepared to serve as the triboelectric layers by using the patterned Si wafer molds fabricated by traditional photolithography methods and dry/wet etching [23]. Figures 3(a), (b) shows the fabrication process of the patterned films, and the SEM images of the surface morphologies. The performances of TENGs made by PDMS films with flat surface and various patterned features were compared as shown in figure 3(c). It indicates that the surface structuring can improve the electric performance of the TENGs, and the TENG with pyramid-featured PDMS film can produce the highest output. That may be ascribed to the enhancement of the contact area and the triboelectrification. For chemical surface functionalization, gold nanoparticles were used to be adsorbed on the surface of the Au films [45]. The nanoparticle-enabled surface modification induces a large enhancement in the TENG performance, which may be due to the change in the surface contact area, surface charge transfer density, and surface friction coefficient. In addition, nanocomposites composed of polymer matrix and embedded nanoparticles, nanowires, or nanotubes can also serve as the triboelectric materials, in which the surface electrification and material permittivity for improving the electrostatic induction are both changed [46].
Figure 3. (a) Schematic illustration of the fabrication process of the patterned PDMS films. (b) SEM images of the patterned PDMS thin films with line rectangular line, square cube, and square pyramid features. (c) Comparison of the output voltage of the TENG using PDMS thin films with flat surface and various patterned features, respectively. Reproduced with permission from [23]. Copyright 2012 American Chemical Society.
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Standard image High-resolution image2.2. Main features of triboelectric nanotechnology
Triboelectric nanotechnology is a new technology for realizing the power supply, self-powered sensing, air filtering, or other functions based on the characteristics of triboelectrification for nanomaterials such as multi-shell dielectrics with high charge density. The triboelectric nanotechnology has four main features: (1) high voltage generated by triboelectric materials; (2) full particle size adsorption on PM0.01-PM10 by the triboelectric nanotechnology; (3) bactericidal and antibacterial properties of the triboelectric nanotechnology; and (4) self-powered application systems established by the triboelectric nanotechnology.
Figure 4 shows the schematic and experimental illustrations for the four main features of triboelectric nanotechnology. In our previous work, we demonstrated a freestanding-triboelectric-layer based nanogenerator and found that the dielectric-to-dielectric TENG can produce a high voltage of up to 12 kV (figure 4(a)) [47]. Such high voltage is beneficial for improving the output performance of the TENG and PM removal efficiency of triboelectric filter. Generating high voltage by the triboelectric materials is one important feature of triboelectric nanotechnology. For the second feature, the triboelectric nanotechnology can realize the excellent adsorption of the airborne PMs, from the nanoscale to the micro-scale. A triboelectric filter was fabricated to remove the PMs under severe haze with a mass concentration of about 340 μg m−3 for PM2.5, and the removal efficiencies were measured. As shown in figure 4(b), the particles at the size range from 10 nm to 1 μm can be effectively removed, and a total number removal efficiency of 97.3% can be achieved. After the filtering for 5 h, the surfaces of the filtering fabric are covered by PMs from the SEM image, and the nanostructures are barely seen [48].
Figure 4. Schematic and experimental illustrations for the four main features of triboelectric nanotechnology. (a) Output voltage of an as-fabricated dielectric-to-dielectric freestanding TENG. [47] John Wiley & Sons. © 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Particle size distributions of the air under severe haze before and after the filter and the SEM image of the filtering material surface. [48] John Wiley & Sons. © 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic figure of the synergetic mechanisms of the instant and sustained sterilization properties of the water sterilization system made of TENG and nanobrush electrodes. Reprinted from [49]. Copyright 2017 with permission from Elsevier. (d) Framework for the integrated self-powered system established by the triboelectric nanotechnology. Reprinted by permission from Macmillan Publishers Ltd: Nature Research [51], Copyright 2015. CC BY 4.0.
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Standard image High-resolution imageFigure 4(c) shows the bactericidal and antibacterial properties of the triboelectric nanotechnology. A self-powered and highly-efficient water sterilization system was developed by using a water-driven ball-in-ball TENG and nanobrush electrodes of Ag-nanoparticle integrated ZnO-nanowires [49]. The system showed both instant and sustainable sterilizations for various microbes including those in natural river water. The experiments of electroporation water sterilization by the triboelectric nanotechnology indicate that the bacteria annihilation ability of electrodes was sustained for at least 20 min after withdrawing the electric field generated by the TENG. Based on this work, Wang et al discovered the post-charging antibacterial properties in the capacitive materials made of Titania nanotubes doped with carbon (TNT-C) [50]. The dramatic electron transfer from bacteria to positively charged TNT-C surface induces the reactive oxygen species (ROS) burst in bacteria, distorting the bacteria shape and leading to death finally. These are the applications of triboelectric nanotechnology in the antibacterial and bactericidal systems. As the fourth main feature, the triboelectric nanotechnology can establish a self-powered system to power portable or functional electronics [51, 52]. Different kinds of mechanical energies can be harvested from the ambient environment by the triboelectric nanotechnology. The obtained alternating-current (AC) electric outputs can be converted into a steady direct-current (DC) output by the power management. The DC electricity can directly power the electronics or through an energy storage unit. The self-powered system will provide a complete micro-energy solution.
3. Applications of triboelectric nanotechnology in air filter
Air pollution becomes more and more serious, which is caused by the airborne PMs with different sizes, and also the pollutants such as ozone, carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2) and formaldehyde, etc [53]. The air filtering based on the triboelectric nanotechnology has been proved as an effective approach to removals of the PMs and pollutants [48, 54–59]. In this section, the recent research progress in air filter by the triboelectric nanotechnology is discussed, from the viewpoints of TENG-driven filter, TENG-enhanced nanofiber filter, and triboelectric filter.
3.1. Self-powered air filter driven by TENG
3.1.1. Self-powered removal of SO2 and PMs
The first self-powered air cleaning system was developed to remove the SO2 and PMs in the air by using a wind-driven rotating TENG [54]. The schematic illustrations of the principle of SO2 removal based on electrostatics and the experimental system are shown in figures 5(a), (b). The freestanding-mode TENG with grating electrodes was connected to a miniaturized wind cup driven by air blower. The AC electricity produced by the TENG was converted into DC electricity through a rectifying bridge. The rectified voltage was applied on the two paralleled copper meshes inside the SO2 chamber, and between the two copper meshes is the saturated NaHSO3 solution in which the SO2 is almost insoluble. At the anode, the SO2 loses two electrons and turns into sulfuric acid, while at the cathode, oxygen gains two electrons and turns into water. The SEM was used to characterize the structure of the anode copper mesh to confirm that SO2 is oxidized into sulfuric acid. The SEM structures were compared for the cases without and with the TENG, as shown in figure 5(c). It is apparent that the corrosion of the copper mesh in SO2 atmosphere accelerated when the TENG was connected. Then the effect of the concentration of remaining SO2 with respect to the wind speed was revealed (figure 5(d)). As the wind speed increases, the concentration of SO2 decreases more quickly, implying that the SO2 can be effectively oxidized and removed driven by the TENG.
Figure 5. (a), (b) Schematic illustrations of the principle of SO2 removal based on electrostatics and the experimental system. (c) SEM images of the anode copper mesh compared for the cases without and with the TENG. (d) Concentrations of SO2 as functions of the time for various wind speeds. (e), (f) Schematic illustrations of the principle of PM removal and the experimental system. (g) Weight increase of the copper meshes with respect to the time for two cases without and with TENG. (h) Weight increase of the copper meshes as a function of the electrode distance. Reprinted from [54], Copyright 2015, with permission from Elsevier.
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Standard image High-resolution imageBesides the SO2, the PMs in the air chamber can be also removed based on the electrostatic adsorption. Figures 5(e)–(f) presents the schematic principle and the experimental system for the removal of PMs. The rectified voltage produced by the wind-driven TENG was applied to the two paralleled copper meshes to generate a strong space electric field. The air chamber was filled with dust aerosol through the inlet by solid aerosol generator. Charged dust particles were absorbed onto the two copper meshes under the Coulomb force between the electrode plates and the particles. The weight increase of the copper meshes was measured to evaluate the removal efficiency of PMs. The comparison of the weight increase in the copper meshes in figure 5(g) shows the weight increase with the TENG is much larger than that without TENG, proving the capability of self-powered air cleaning system. Figure 5(h) indicates the influence of the electrode distance on the PM removal efficiency. The weight increase of the copper meshes gradually decreases with increasing the electrode distance, because the absorbing capability is closely associated with the electric field.
3.1.2. Self-powered removal of rhodamine B and formaldehyde
Recently, Feng and coworkers demonstrated a self-powered filter by combining TENG and photocatalysis technique for removing organic vapor pollutants in the indoor atmosphere [55]. They fabricated the filtering network by wearing polymer-coated stainless steel wires, whose surface was embedded on with the photocatalyst P25 or Pt/P25. Figure 6(a) shows the schematic diagrams of the self-powered adsorption from electrostatic Coulomb force and self-powered degradation based on photocatalytic effect. Two polymer layers were successively deposited on the steel wire surfaces. The metal wires in vertical direction were connected to the same strip electrodes, which were electrically connected with the electrode of a single-electrode TENG. The TENG provided voltage for the vertical metal wires, while the horizontal metal wires were connected to the ground. The surface of the filtering network was charged by the output voltage from TENG, producing a strong electric field in the region of filtering network. The charged microparticles and polar organic pollutant molecules could be absorbed on the network by the electrostatic adsorption effect. And also the degradation of indoor blastomogenic gaseous matter, like formaldehyde, into nontoxic CO2 and water under UV irradiation could be realized and enhanced by the electrostatic field created by the TENG.
Figure 6. (a) Schematic diagrams of the self-powered adsorption from electrostatic Coulomb force and self-powered degradation based on photocatalytic effect. (b), (c) Enhancement principle of the TENG on the electrostatic adsorption of RhB and the photocatalytic degradation of RhB and formaldehyde. (d) Comparison of the adsorption intensity extracted at the characteristic wavelength of RhB for the two cases without and with TENG. (e) Schematic setup for degrading the formaldehyde. (f) Comparison of the formaldehyde concentration change for the two cases without and with the TENG. Reproduced with permission from [55]. Copyright 2017 American Chemical Society.
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Standard image High-resolution imageFigures 6(b), (c) presents the enhancement principle of the TENG on the electrostatic adsorption of Rhodamine B (RhB) and the photocatalytic degradation of RhB and formaldehyde. When there is no electric field for the filtering network, only a small amount of RhB droplets are precipitated on the network surface. However, when the filtering network is connected between the TENG and the ground, the strong electric field increases the absorbed dosage of charged RhB droplets. For the photocatalytic degradation, the strong electric field generated by the TENG helps to separate the electron-hole pairs and promote the charge migration on the photocatalyst surface. Based on the reaction with water and oxygen, the separated electrons and holes generate hydroxyl group and superoxide radical, enhancing the degradation of RhB and formaldehyde. The enhancement of the electrostatic adsorption on the RhB by the TENG can be seen from the adsorption intensity extracted at the characteristic wavelength of RhB, as shown in figure 6(d). On the other hand, the formaldehyde molecules can degrade under UV irradiation. Through the designed setup (figure 6(e)), the photocatalytic degradation performance of formaldehyde was enhanced by the electrostatic field created by the TENG, which can also be viewed from the comparison of formaldehyde concentration change between the two cases without and with the TENG (figure 6(f)).
3.2. TENG-enhanced nanofiber air filters
3.2.1. Monolayered nanofiber air filters
Gu et al invented a TENG-enhanced nanofiber air filter for efficient PM removal [56]. First, they fabricated the monolayered nanofiber air filter by using the electrospinning technique to prepare the polyimide (PI) nanofibers. They applied a rotating TENG to provide voltage for the stainless steel meshes bearing the nanofibers. The nanofiber film was placed inside an acrylic tube and for the measurement of the filtering performance including the PM removal efficiency and pressure drop, as schematically shown in figure 7(a). The insert shows the FE-SEM image of the nanofiber film. The filtering mechanisms of the nanofiber air filter without and with the TENG are respectively shown in figures 7(b) and (c). When there is no TENG connected, the basic filtering mechanism is the mechanical filtering of the nanofibers, including the interception, inertial impaction, and gravitational settling. However, for the filter with TENG, the PI nanofibers are charged positively by the TENG, generating a strong electric field around the stainless steel meshes and nanofiber film. The PMs can be absorbed on the nanofibers by the electrostatic force. A larger particle size span of the electrostatic effect makes the PM removal efficiency improved, especially for smaller particles with the diameter less than 100 nm.
Figure 7. (a) Schematic illustration of the TENG-enhanced nanofiber air filter and PM removal measurement system. The insert is the FE-SEM image of the PI nanofiber film. (b), (c) Schematic filtering mechanisms of the nanofiber air filter for the two cases without and with TENG. (d) Particle size distributions of PMs in the dusty air, after the filter, after the TENG-enhanced filter in the diameter region of 15–550 nm. (e) Removal efficiencies of the filter without and with the TENG. Reprinted with permission from [56]. Copyright 2017 American Chemical Society.
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Standard image High-resolution imageIn real experiments, we generated the PMs by burning a cigarette, and measured the particle number concentration and size distribution in a 30 m3 lab by the handheld particle counter, scanning mobility particle sizer and an aerodynamic particle sizer. Figure 7(d) shows the particle number-size distribution for the particles at the size range from 15 to 550 nm. When using the nanofiber filter without the TENG, about half of the PMs can be filtered. In contrast, with the TENG the particle number is greatly decreased at different particle sizes, and about 80% of the PMs can be filtered. The removal efficiency was calculated and compared for the filter without and with the TENG, as presented in figure 7(e). Clearly, the removal efficiency of the PMs is enhanced by connected the TENG.
3.2.2. Multilayered antibacterial nanofiber air filter
Based on the monolayered nanofiber air filter, a multilayered antibacterial nanofiber air filter enhanced by a rotating TENG was designed and fabricated. Gu et al used the electrospinning technique to obtain the silver nanoparticle-doped PI (Ag-PI) nanofibers. The addition of silver nanoparticles makes the nanofibers possess the antibacterial property. A schematic structure of an as-fabricated nanofiber air filter of multilayered Ag-PI films placed in a square acrylic tube is shown in figure 8(a) [57]. A rotating TENG was connected to the stainless steel mesh of Ag-PI film to charge the nanofibers after a rectifier bridge. The particle number concentrations before and after the filter were measured by a PM particle counter, and the pressure drop between the upstream and downstream was tested by a pressure gauge. Figures 8(b), (c) shows the FE-SEM images of the Ag-PI nanofibers before and after the filter. With the adsorption of the PMs, the nanofibers are filled with large aggregated PMs, larger spherical particles at the junction, and finally a formed PM layer (figure 8(c)). Then the removal efficiency was calculated based on the particle number concentrations measured for various particle sizes. The removal efficiency was compared for the two cases without and with the TENG as well as the influence of the layer number of the Ag-PI nanofiber films on the removal efficiency. As shown in figure 8(d), with increasing the layer number, the removal efficiency of PM2.5 increases and can be enhanced by connecting the TENG.
Figure 8. (a) Schematic diagram of the TENG-enhanced multilayered nanofiber air filter. (b), (c) SEM images of the Ag-PI nanofibers (b) before the filter and (c) after the filter. (d) Comparison for the removal efficiencies of PM2.5 for filters with different layer numbers of nanofiber films obtained in 10 min without and with the TENG. (e), (f) FE-SEM images of (e) E. coli and (f) S. aureus on the Ag-PI nanofiber film. (g), (h) Photographs of E. coli colonies after co-culturing for 30 h with (g) PI film and (h) Ag-PI film. (i), (j) Photographs of S. aureus colonies after co-culturing for 30 h with (i) PI film and (j) Ag-PI film. Reprinted by permission from Springer Nature and Copyright Clearance Center: Springer Nature [57] 2018.
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Standard image High-resolution imageFor the investigation of antibacterial properties of the Ag-PI nanofibers, as the model gram-negative and gram-positive bacteria, the Escherichia coli (E. coli) and staphylococcus aureus (S. aureus) were chosen for example. The bacteria were all co-cultured for 30 h, and the number of viable E. coli and S. aureus remaining in the suspensions was estimated by a colony-forming unit (CFU) counting method. The FE-SEM images of the E. coli and S. aureus on the Ag- PI nanofiber film from figures 8(e), (f) indicate the much smaller holes of the nanofibers than the bacterium diameter, implying that the bacteria can be effectively captured by the nanofiber filter. Figures 8(g)–(j) shows the photographs of the bacteria for comparison between the PI nanofiber film and Ag-PI nanofiber film. It can be clearly found that 94.6% of E. coli and 98.5% of S. aureus are killed, demonstrating the good antibacterial property of the Ag-PI film.
3.3. Self-powered triboelectric air filters
Using the technology of triboelectric filtering to remove the PMs and pollutants of the air has obvious advantages as mentioned above. Recently, researchers have been attempting to design and fabricate kinds of self-powered triboelectric air filters, and improve their filtering performance [48, 58, 59]. As two typical examples, we introduce the recent progress of triboelectric face mask and triboelectric exhaust filter in this section. The working principle of triboelectric air filter, performance characterization, and practical application are elaborated.
3.3.1. Triboelectric face mask
A triboelectric air filtering system was established based on several layers of fabrics, whose structure is schematically shown in figure 9(a) [48]. The rubbing between the two fabrics can produce triboelectric charges on the surfaces, leading to the space electric field (figure 9(b)). The triboelectric air filter (TAF) was placed in a cylindrical acrylic tube for measuring the removal efficiency of the PMs at the gas stream, pressure drop, and flow rate. Before the rubbing between the two fabrics, there are no tribo-charges generated, and the filtering mechanism is mainly the mechanical filtering of the fabric film. The created triboelectric charges and electric field through the rubbing will induce the electrostatic attraction between the PMs and fabrics besides the mechanical filtering. The PMs with a smaller size can be effectively removed, and the filtering performance can be greatly enhanced. This is the working principle of the self-powered TAF (figure 9(c)).
Figure 9. (a) Schematic illustrations of the triboelectric air filter and measurement system of flow rate, pressure drop, and PM removal efficiency. (b) Photograph of the two fabrics charged by rubbing. (c) Schematic filtering mechanism for the charged TAF. (d) Comparison of the removal efficiencies of PMs with different particle sizes for the uncharged TAF, charged TAF, and commercial face mask. (e) Particle size distribution of the PMs in the polluted air, filtered air by the uncharged TAF and charged TAF in the range of 25.9–637.8 nm. (f) Comparison of removal efficiency between the charged TAF and a commercial face mask washed for 0–5 times. The insets show the photographs of the unwashed and washed TAF. (g) Illustration of the removal efficiency measurement for the face mask made of the TAF which has been worn for 4 h. [48] John Wiley & Sons. © 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.
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Standard image High-resolution imageThe particle number concentrations were measured by a handheld particle counter at the inlet and outlet for calculating the removal efficiency. Figure 9(d) shows the comparison of the removal efficiencies of the PMs with different sizes for the uncharged TAF, charged TAF and commercial face mask. The results indicate that the removal efficiency increases with increasing the particle size, and it can be greatly enhanced by a sufficient charging between the fabrics, which are more obvious for smaller particles. The particle size distribution of the PMs was measured and the results for the size range of 25.9–637.8 nm are shown in figure 9(e). As can be seen, the particle number has a peak at about 110 nm, and the particles are effectively removed by the TAF. Compared to the uncharged TAF, the removal efficiency of PMs for the charged TAF can be improved by over 50%. In addition, one advantage of the fabricated TAF is that it can work well after being washed. For a comparison between the TAF and commercial face mask, the removal efficiencies of PMs after being washed for 0–5 times were measured as shown in figure 9(f). The insert shows that the filter color can be changed from yellow to white by washed using water and detergent. The removal efficiency of commercial face mask decreases to about 67% after being washed five times, but that of the TAF can still maintain at about 92%, implying that the TAF is washable. Figure 9(e) illustrates the measurement results of removal efficiency for the face mask made of the TAF which has been worn for 4 h. Clearly, the number concentration of PM2.5 is greatly decreased through the TAF, and the pollution degree changes from severely polluted to good, and from heavily polluted to excellent. That proves that the fabricated TAF can be applied as a strong candidate for real face masks.
Another triboelectric face mask was designed by Liu et al based on the triboelectrification between poly(vinylidene fluoride) electrospun nanofiber film (PVDF-ESNF) and Cu film driven by the respiration [58]. The structure and working principle of the self-powered electrostatic adsorption face mask (SEA-FM) are shown in figures 10(a), (b). The respiration-driven contact-mode TENG composed of a Cu film, a silicon film, a nonwoven, a rubber mode, and a PVDF-ESNF film, is implanted into the inlet of the face mask, where the PVDF-ESNF is not only the triboelectric layer, but also the filtering layer. During the expiration process, the Cu film is pushed to contact with the PVDF-ESNF film, while during the inspiration process the Cu film is separated from the PVDF-ESNF film until the maximum separation (figure 10(b)). The produced triboelectric charges by periodic expiration and inspiration can induce the electrostatic adsorption of PMs on the nanofiber film.
Figure 10. (a) Schematic structure of the respiration-driven TENG (R-TENG) for the self-powered electrostatic adsorption face mask. (b) Working principles of the R-TENG by periodic expiration and inspiration. (c) Comparison of removal efficiency of the PVDF-ESNF and R-TENG. (d) Photograph of the face mask made of the R-TENG. (e) Durability test on the removal efficiency of the SEA-FM after 30 days. (f) Durability test on the removal efficiency of the SEA-FM in different external humidities at the particulate size of less than 1.0 μm. Reprinted with permission from [58]. Copyright 2018 American Chemical Society.
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Standard image High-resolution imageBy the same measuring methods mentioned above, the removal efficiencies of the pure PVDF-ESNF film and PVDF-ESNF-containing TENG were obtained for various particle sizes as shown in figure 10(c). It can be found that the TENG can have greatly improved the removal efficiency in contrast with the pure PVDF-ESNF film. A real SEA-FM was fabricated, whose photograph is shown in figure 10(d). When the face mask was laid aside for 30 days, the removal efficiencies were found to still remain stable during the wearing process of 240 min (figure 10(e)). Figure 10(f) shows the durability tests for the particles of smaller than 1.0 μm at various humanities of environment. As the environment humanity increases, the removal efficiency of the PMs for the SEA-FM decreases. Moreover, the removal efficiency decreases gradually until saturation during the wearing process, implying the great influence of the humidity on the filtering performance. This research of SEA-FM can provide guideline for the design and fabrication of real face mask products.
3.3.2. Triboelectric air filter for automobile exhaust
A self-powered triboelectric air filter was designed to remove the PMs from the automobile exhaust based on a vibration TENG structure [59]. The triboelectric air filter could be applied for the gasoline cars or diesel vehicles. The advantages of low cost and high efficiency over the diesel particulate filter used in heavy diesel vehicles make it have great potential to be applied in real vehicles. The triboelectric filter consists of filtering pellets and two paralleled aluminum electrodes attached on the top and bottom surfaces inside a cubic chamber, whose schematic structure is shown in figure 11(a). The collision and separation between the filtering pellets and the electrodes by an external vibration triggering on the camber can produce triboelectric charges, which are negative charges on the filtering pellet surfaces and positive charges at the aluminum electrodes. The triboelectric charges can create a time-varying space electric field for adsorbing the PMs in the automobile exhaust based on the electrostatic force. Under the short-circuit condition, the periodic motion of the pellets can produce periodic AC current, while under the open-circuit condition, a high potential difference between two electrodes can be generated (figure 11(b)). Accumulation of charges increases the electric field until the maximum value. Under the action of the space electric field, the charged particles in the exhaust from motor vehicles can be collected. The neutral particles can also be removed through the mechanism of mechanical filtering. After the triboelectric air filter, clean gas can be obtained, as schematically illustrated in figure 11(c).
Figure 11. (a), (b) Schematic structure, triboelectric charge distribution, current and voltage generation processes of the triboelectric filter based on the vibration TENG. (c) Sketch showing the triboelectric filtering of PMs from the automobile exhaust by the triboelectric filter. (d) Photograph of a car with a triboelectric filter installed on the tailpipe. (e) Particle number-size distribution of the PMs at the upstream and downstream of the filter. (f) Comparison for the collection efficiencies of the PMs for the idle speed and constant speed. Reprinted with permission from [59]. Copyright 2015 American Chemical Society.
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Standard image High-resolution imageThe laboratory test was extended to the field test of vehicle exhaust emission by installing the triboelectric air filter on a commercial vehicle, which was driven on the street. Figure 11(d) shows the photograph of the triboelectric filter integrated on the tailpipe of the car, where the emitted exhaust flow through the filter by use of a plastic tube as viewed from the enlarged figure. The natural vibration of the tailpipe provides the source of the triboelectric process and produces a space electric field to remove the PMs from the exhaust. The particle number-size distribution was measured by an electric low pressure impactor when the engine was working at a constant speed of 3000 rpm. The results at the upstream and downstream of the triboelectric filter are presented in figure 11(e). It can be found that the particles mostly have the diameters of less than 290 nm, and they can be effectively removed by the triboelectric filter. The removal efficiency of PMs can reach about 95%. Figure 11(f) shows the removal efficiencies of various PMs at the two working modes of the automobile, i.e., idle speed mode and constant speed mode. The fractions of various sizes of PMs at the two modes are shown in the inserted pie charts. Regardless of the working mode of the engine, more than 95.5% of PM2.5 can be filtered, proving the high filtering performance of the triboelectric filter. This work extends the applications of triboelectric nanotechnology in the automobile exhaust filtering, which is an important direction with potentially huge market for governing environmental pollution.
4. Commercialization and Industry Products
The triboelectric air filtering technologies invented by Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, have been recently commercialized. An industrialization company named Beijing NairTENG Technology Co. Ltd has been established to develop the related air purification products. Up to now, there are several kinds of industry products, including the triboelectric face mask, embedded triboelectric purification system, triboelectric street lamp for purification, and distributed household triboelectric air purifier, as shown in figure 12. Figure 12(a) is the photograph of the triboelectric face mask products fabricated by the company. Due to the excellent performance of the face mask, it has achieved 'The eighth batch certification of new technology and new product for Beijing'. When the airflow passes through the face mask, triboelectric charges are generated on the triboelectric nanomaterials and the fabric layer on the face mask surface. By adopting the triboelectric nanotechnology, the triboelectric face mask can realize the adsorption of ultrafine particles with the diameter of less than 0.3 μm at high filtering efficiency. The filtering efficiency meets the new national standards of N95. The face mask products have several characteristics: (1) possessing four anti-haze layers; (2) a larger breathing valve for more comfortable exercise; (3) safer fitting without the fogging of glasses; (4) highly-elastic ear hooks for keeping away from the ear pain; and (5) a hidden sealing nose clip.
Figure 12. Industry products developed based on the triboelectric air filtering technology and their unique features. (a) Photograph of the triboelectric face mask product. (b) Schematic structure of the embedded triboelectric purification system. (c) Schematic impression drawing of the triboelectric street lamp for purification. The insert shows the decomposition diagram of components. (d) Schematic diagram of distributed household triboelectric air purifier. (e) Unique features of these developed industry products.
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Another air filtering product is the embedded triboelectric purification system, whose schematic diagram is shown in figure 12(b). Such a product is a kind of embedded air filtering device developed by combing the triboelectric air filtering technology and the fan coil structure of air-conditioning. By applying the internal circulation of indoor air created by the fan coil, the product can be installed onto the return air inlet of central air-conditioning to remove the PMs and bacteria in the air through the electrostatic adsorption, finally purifying the indoor air. It can realize the three-dimensional air purification with large circulation, and will be widely applied in the office buildings, conference rooms, hotels, supermarkets, shopping malls, or hospitals, etc. The products have five main features: (1) it can efficiently remove the PMs and kill the bacteria, viruses and other microorganisms; (2) it is perfectly integrated with air-conditioning inlet and fan coil installation, and no extra installation space is required; (3) it can be removed from the inlet bottom for cleaning and maintenance; (4) an internal pneumatic switch is equipped for linkage operation with the air-conditioning; and (5) the electronic controlling system has safety protection measures including overload protection and power-off protection.
Figure 12(c) shows a triboelectric street lamp for purification developed based on the same principle. The street lamp is composed of three parts: street lamp component, purification component, and connector, as viewed from the schematic structure in the insert. Therefore, the street lamp not only provides lights for illumination, but also purifies the air to decrease the air pollution. The purification principle of the street lamp can be described as follows. When the fan is working, the dusty air can be drawn into the street lamp from the air vent of the purification component. The air flows through the PM filtering module and the gas filtering module, producing clean air flowing out from the vent of the lamp component shell. One obvious advantage for the street lamp is that the filtering materials are washable, so the cost can be largely decreased. The fourth product is the household triboelectric air purifier with a distributed structure, as shown in figure 12(d). Such an air purifier can work through the integration of three parts, but each part can also act as an individual air purifier. This distributed structure can meet the specific requirement for different rooms in a house. It can bring the convenience and diversification for a human's life.
These developed industry products have their unique features. Their core technology is the triboelectric filtering based on the triboelectric nanotechnology. They can realize the high efficiency removal of PMs, which is completely different from the traditional fibrous filter and electrostatic precipitator. As shown in figure 12(e), the products have the revolutionary advantages of zero ozone emission, no need for replacing consumable materials and no pollution. A high electric field in the triboelectric filter can be created automatically because of the triboelectric process between flowing air and nanometer materials. Thus, in view of the nonionizing process, 99.5% of the particulate matters in the range of PM0.01-PM10 can be completely filtered without ozone emissions.
5. Summary and perspective
In this review, the choices and surface characteristics of the triboelectric materials determining the triboelectric properties and the main features of triboelectric nanotechnology were briefly introduced. The mechanism of triboelectric filtering technology is essentially different from that of fibrous filter and electrostatic precipitator, making it have obvious advantages of high removal efficiency and no ozone emissions. The recent developments of the triboelectric nanotechnology in the air filter were summarized, including the self-powered air filter driven by the TENG, TENG-enhanced nanofiber air filter, and self-powered triboelectric air filter. And the technology commercialization was illustrated, and the structures and characteristics of the developed industry products were demonstrated.
The triboelectric air filtering technology and the industry products can have a large developing space in the future. Firstly, such technology should be further improved to enhance the filtering performance and the durability. The maintenance of the performance after a long-term cooperation is very important for decreasing the manpower, material resource and financial costs during the after-sale maintenance. Secondly, the structure of the products can be improved or diversified for specific requirements or applications. So far, the product structures are relatively single, limiting the market promotion greatly. Thirdly, the application areas need to be further extended, such as the air filter in a super clean room, a car chamber, and integrating with the air-conditioning. For the huge marketing prospect and expected output value of the air filtering area, the developed industry products from the triboelectric filtering technology will play a more and more important role and make a great impact on a human's daily life.
Acknowledgments
J Liu and T Jiang contributed equally to this work. Research was supported by the National Key R & D Project from Minister of Science and Technology, China (2016YFA0202703). We thank our group members and collaborators for their contributions to the work reviewed here, especially: Changbao Han, Chi Zhang, Chuan He, Zhenliang Gao, Xia Cao, Jie Wang, Guangqin Gu, Shuwen Chen, Yawei Feng, Guoxu Liu, Yu Bai, Jinhui Nie, Wangtong He, Xue Li, and Yan Chen.