Electrospun Nanofibers for Air Filtration

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Shichao Zhang1,3, Nadir Ali Rind2, 3, Ning Tang2,3, Hui Liu2, 3, Xia Yin2, 3, Jianyong Yu3, Bin Ding3 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China1; Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China2; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China3

12.1 INTRODUCTION Air pollution has become one of the most serious environmental issues, particularly fog and haze pollution, resulting in a growing impact on public health, production efficiency, and even ecosystems. Atmospheric particulate matter (PM) pollution, including solid and liquid particles emitted into the air, is the major noxious form of pollutant, and can cause adverse effects on human health due to its ability to penetrate deep into the lungs and cardiovascular system, causing minor irritation to chronic respiratory and lung cancer and making preexisting heart and lung diseases worse (Kampa and Castanas, 2008; Rodrıguez et al., 2004; Querol et al., 2001). The major sources of PM pollution are industrial emissions (e.g., mineral powder, coal, and carbon powder), combustion from daily life, intensive road transport, secondary nitrates, and secondary sulfates. Furthermore, certain contaminants attached to PM, such as bacteria, pollen, microorganisms, and viruses, may also badly influence the environment and the aforementioned diseases (Montefusco, 2005; Chuanfang, 2012; Peukert, 1998). For example, according to the report provided by the World Health Organization in 2014, PM pollution was the main cause of death around the world for 7 million people (Wang et al., 2016a). The size of PM particles is responsible for various health hazards; for example, thoracic particles (diameter <10 mm, PM10) can be mostly blocked by our nasal cavity (Brook et al., 2010); but the fine particles (diameter <2.5 mm, PM2.5) are small enough to enter the lungs, leading to various serious health issues (Mannucci et al., 2015; Yoon et al., 2008). The larger particles (diameter >10 mm) are easier to filter by using air cleaners, such as scrubbers, cyclones, sedimentation tanks, etc.; however, the fine particles, especially the PM2.5, are technically more difficult to remove from air. Taking into account the well-known hazards of ultrafine airborne particles to ecosystems and public health, a number of strategies have been developed to prevent this problem, such as source governance, new energy development, filtration technologies, and so on (Song et al., 2006). Among them, air filtration technology has prime importance because of its various advantages, including economy with low consumption of energy, robust performance, and a broad range of applications. For instance, Huang et al. have reported that the filtration market will increase to an estimate of US$700 billion by 2020 Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00012-1 Copyright © 2019 Elsevier Inc. All rights reserved.

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(Huang et al., 2003). By virtue of their porous structures and tortuous channels, which can synchronously realize PM capture and air transmission, fiber-based filtration media have become a feasible, efficient, and promising technology for PM pollution. Conventional fiber-based filters, such as meltblown, spunbonded, and glass fibers, have been extensively used in various filtration applications (Wang and Otani, 2012); however, their filtration efficiency for fine particles is still low, which can be attributed to their structural disadvantages, like microsized fiber diameter, large pore size, and low porosity (Adiletta, 1999; Wang et al., 2016a; Park and Park, 2005; Hung and Leung, 2011). Considering that the thickness and basis weight of these air filters must be increased to reduce the pore size, and thus increase the filtration efficiency, their applications are still restricted because of high energy consumption and high pressure drop (Wang et al., 2016a). Taking into account that a decrease in fiber diameter would greatly improve the filtration performance of the fiber filters, the nanofibers emerge as a promising filtration medium because of their small diameter, open pore structure, and high porosity. Different from other techniques for the fabrication of nanofibers, electrospinning can fabricate nanofibers with a wide range of fiber diameters, from 50 to 2000 nm, by regulating solution properties and processing parameters (Park and Park, 2005; Graham et al., 2002; Kosmider and Scott, 2002; Grafe and Graham, 2003a). The resultant electrospun nanofiber membranes possess various outstanding characteristics, such as small diameter, large specific surface area, interconnected porous structures, and controllable morphologies, which differentiate them with conventional fibrous filters (Lu and Ding, 2008; Bhardwaj and Kundu, 2010; Wang et al., 2013b; Fan et al., 2016; Sridhar et al., 2015). These remarkable characteristics would endow air filters with admirable filtration performance and provide a new strategy for controllable fabrication of novel filters at low cost. This chapter will present a brief overview of recent advances in electrospun nanofiber membranes used in air filtration. After briefly introducing the technique and materials for air filtration and the structural advantages and filtration mechanisms of fibrous filtration, we highlight the types, structural characters, and application performance of the existing electrospun nanofiber filters for various filtration applications. In addition, concluding remarks are also presented in terms of current and future perspectives.

12.2 ELECTROSPUN NANOFIBER FILTERS 12.2.1 STRUCTURAL AND PERFORMANCE ADVANTAGES Nonwoven fiber-based air filters, as a kind of porous medium, have become a practicable, effectual, and promising technique for air filtration by effectively trapping PM particles while allowing air molecules to pass through the pore channels (Yang et al., 2015; Das et al., 2014). For this reason, various filtration media with nonwoven structures have been produced, such as melt-blown fibers (Lee and Wadsworth, 1990; Podgo´rski et al., 2006), spunbonded fibers, glass fibers, etc. (Sakthivel et al., 2014; Anandjiwala and Boguslavsky, 2008). However, these fiber filters usually suffer from the disadvantages of large pore size and low porosity due to their microsized diameters. Compared with these fibers, electrospun nanofibers emerge as an advanced nanomaterial, which integrates the structural features of relatively small pore size, open stacking pores, and highly adjustable porosity (Wang and Otani, 2012; Thavasi et al., 2008; Sahay et al., 2012; Zhou et al., 2011). Moreover, nanofiber filter media also exhibit enhanced service life and large dust-holding capacity in various air filtration applications, which make them promising candidates for air filtration (Grafe and Graham, 2003b).

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FIGURE 12.1 Scanning electron microscopy images of (A) a polyacrylic acid nanonet membrane and (B) a polyamide-6 nanonet membrane. (A) Modified with permission from Zhang, S., Chen, K., Yu, J., Ding, B., 2015. Model derivation and validation for 2D polymeric nanonets: origin, evolution, and regulation. Polymer 74, 182e192. © 2015 Elsevier. (B) Wang, N., Yang, Y., Al-Deyab, S. S., ElNewehy, M., Yu, J. & Ding, B. 2015. Ultra-light 3D nanofibre-nets binary structured nylon 6-polyacrylonitrile membranes for efficient filtration of fine particulate matter. Journal of Materials Chemistry A, 3, 23946-23954. © 2015 Royal Society of Chemistry.

Although the conventional electrospun nanofibers have overcome the problems related to conventional microsized fiber media, many disadvantages remain in these filters, involving restricted structure controllability and insufficient filtration capacity, due to the intrinsic limitations of thick fibers with diameters >100 nm and easily collapsed cavity structure. To further decrease the diameters of electrospun nanofibers, Ding et al. have introduced a novel electrohydrodynamic technique called electrospinning/netting, which has evoked great attention by virtue of its ability to fabricate, on a large scale, nanofiber/net materials comprising conventional electrospun nanofibers and two-dimensional (2D) spiderweb-like nanonets with diameters of w20 nm, as shown in Fig. 12.1 (Ding et al., 2006, 2011; Zhang et al., 2015; Wang et al., 2015). The resultant nanofiber/net membranes combine the general properties of conventional nanofibers with some extra outstanding characteristics, like extremely small pore size, high porosity, and enhanced interconnectivity, which allow them to emerge as a promising filtration medium to greatly promote the filtration performance of air filters (Pant et al., 2014; Zhang et al., 2015; Liu et al., 2015a).

12.2.2 FILTRATION MECHANISMS Air filtration media have been in use for a thousand years, but essential studies on filtration mechanisms were proposed only in the past century. According to the mechanism, the filtration process can be classified into two states: steady and unsteady (Qin and Wang, 2006). In the steady state, the filtration efficiency and pressure drop are fixed over time and depend merely on the intrinsic characteristics of the filtration materials, nature of the PM, and rate of the airflow. For the unsteady state, the particle capture capacity and pressure drop change over time with the particles accruing on the filters (Qin and Wang, 2006). Since the unsteady state is a very complex process it still lacks

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FIGURE 12.2 (A) The main filter mechanisms of fiber material. (B) Filtration efficiency for particles with different diameters. (C) The main filtration effect based on particle size. MPPS, most penetrating particle size. (A) Modified with permission from Chuanfang, Y., 2012. Aerosol filtration application using fibrous mediadan industrial perspective. Chinese Journal of Chemical Engineering 20, 1e9. © 2012 Elsevier. (B) and (C) Barhate, R.S., Ramakrishna, S., 2007. Nanofibrous filtering media: filtration problems and solutions from tiny materials. Journal of Membrane Science 296, 1e8. © 2007 Elsevier.

systematical theories to present an accurate prediction for the actual filtration process. The stable state of fiber filtration is further categorized by following five filtration mechanisms (Zhu et al., 2000, 2016; Qin and Wang, 2006; Bull, 2008; Chuanfang, 2012), comprising interception, inertial deposition, diffusion, electrostatic, and gravity effect, as shown in Fig. 12.2.

12.2.2.1 Interception Mechanism There are certain streamlines, for a given particle size, that move close enough to the filter fiber. Then, the particle would touch the fiber and be captured by van der Waals forces, hence being removed from the air streamline (Ramskill and Anderson, 1951). This filtration process is called “interception.” Generally, particles ranging from 0.1 to 1 mm can be captured by interception, and the capture efficiency by interception increases with increasing particle size (Chuanfang, 2012).

12.2.2.2 Inertial Impaction Mechanism With sudden changes in the streamline airflow, particles of larger size are unable to maintain the airflow direction as it changes, due to the action of inertia; they will be separated from the streamline

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and deposited on the filter fibers along their original path. This type of mechanism is most predominant when the particle size is larger than 0.3e1 mm, and it is enhanced with high airflow velocities in the dense-packing fiber filters (Chuanfang, 2012).

12.2.2.3 Diffusion Mechanism The kinetic theory of gases is important for clarifying the capture process of particles by the diffusion mechanism. According to this theory, gas is composed of a large number of small-sized molecules compared with the distances between them. And, these molecules do not travel in continuous streamlines because they are colliding with one another and moving in random paths. This random motion is called Brownian motion. The diffusion mechanism of capturing PM particles is the result of Brownian motion, which can allow the smaller sized particles (0.1 mm) to deviate from their original streamline randomly and enable collisions between particles and fibers, leading to the deposition of the particles on the filter fiber (Chuanfang, 2012). This mechanism is mostly suitable for smaller particles under low airflow velocities (Ramskill and Anderson, 1951).

12.2.2.4 Electrostatic Effect Mechanism Electrostatic interaction has the ability to change the track of the particles and attract them to the surface of the filter fiber electrets, by virtue of the electrostatic adhesion provided by the particles and/ or the fibers. This filtration mechanism is widely used to capture particles having submicrometer diameters (Sahay et al., 2012). The application of electrostatic force can enhance the efficiency of filtration media while maintaining the air resistance of the filters (Wang, 2001).

12.2.2.5 Gravity Effect Mechanism The contribution of gravity to the removal of PM particles is negligible for most of the ultrafine particle sizes; therefore, this effect has not much importance for the high-efficiency air filters (Chuanfang, 2012). According to previous studies, the particulate capturing mechanism by gravity sedimentation will be completely avoided when the particle size is smaller than 0.5 mm. Obvious conclusions can be drawn that the importance of various filtration mechanisms changes with the variation in particle size during the filtration process. And, there would be a most penetrating particle size for particles, which is usually considered to be w300 nm, according to the existing studies from industrial fields and academic circles. Considering the structural features, the separation of PM from the airflow by electrospun nanofiber filters is subject to the integrated effects of inertial deposition, interception, diffusion, electrostatic deposition, and gravity effects.

12.3 POLYMERIC NANOFIBER-BASED FILTERS Electrospun nanofiber air filters have been used as high-performance air filtration media since the late 1980s, because of their tremendous ability, thanks to their large surface area, brilliant surface adhesion, highly porous structure with uniform pore size, and light weight, to capture PM particles from the air (Lu and Ding, 2008; Selvam and Nallathambi, 2015; Thavasi et al., 2008). Electrospun nanofibers have been widely applied in various filtration devices, like vehicle cabin filters, personal respirators, indoor air cleaners, etc. A number of natural and synthetic polymers have been successfully employed to fabricate nanofiber filters with superior performance, including polyacrylonitrile (PAN), polyurethane

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(PU), polyvinyl alcohol (PVA), polyamide (PA), cellulose acetate, chitosan, polysulfone (PSU), and so on. In this portion of the chapter, some typical and advanced polymeric nanofiber filters based on single-component polymers and composite polymer membranes are discussed.

12.3.1 SINGLE-COMPONENT POLYMER MEMBRANES The unique structural features of electrospun nanofibers, such as thinner fiber diameter, smaller pore size, and higher porosity than other fibrous air filters, including melt-blown fibers, spunbonded fibers, and glass fibers, make them fascinating candidates for air filtration. Various electrospun nanofiber filters obtained from single-component polymers have been successfully designed and fabricated for PM filtration. Among them, the robust physical and chemical properties of the polymer PA cause it to be one of the most broadly applied synthetic polymers for fabrication of high-performance air filters. Gibson et al. (2001) first fabricated PA-66 electrospun nanofiber filters, and demonstrated that such filtration medium was efficient for screening ultrafine particles. Ahn et al. (2006) prepared PA-6 nanofiber filters with a fiber diameter of w200 nm, pore size of 0.24 mm, and base weight of w10.75 g/m2. These filters presented superior efficiency (99.993%) compared with the commercialized HEPA (high-efficiency particulate air) filters (99.97%, pore size of 1.7 mm, base weight of 78.2 g/m2) for removing 0.3-mm ultrafine particles, which made them excellent candidates for airborne particles, especially for ultrafine PM filtration. PAN is another commonly used polymer material in air filtration by virtue of its excellent physical and chemical properties. Barhate et al. (2006) for the first time investigated the effects of the process parameters of electrospinning, for instance, applied voltage, collection rate, and electrospinning distance, on the structural and transport properties of PA-6 nanofiber membranes. They concluded that membranes with controllable pore size and distribution could be obtained by regulating the drawing and collecting rates. Electrospun PAN nanofibers with average diameter in the range of 270e400 nm were also prepared by Yun et al. (2007). Moreover, they compared the filtration capability of PAN nanofiber filters and commercial filters for removing differently charged NaCl nanoparticles (NPs), and found that the PAN electrospun filters, with thinner and more uniform fiber diameter, could greatly reduce the penetration of NPs without any relation to particle charge state, indicating that the resultant PAN nanofibers are promising materials for the preparation of high-performance air filters. The polymer PVA, as the largest-volume water-soluble synthetic polymer, is also selected to produce air filters because of its excellent mechanical properties and chemical resistance. Qin and Wang prepared crosslinked electrospun PVA nanofiber membranes and tested their filtration performance (Qin and Wang, 2008). The results showed that the crosslinked PVA nanofibers could remarkably increase the filtration efficiency of melt-blown substrate layers (w30%) and achieve a final removal efficiency of almost 100% for large particles using the as-prepared composite membranes. Polyethylene oxide (PEO) is another typical water-soluble synthetic polymer that has been widely studied for various applications. Leung et al. (2010) prepared nanofiber filters by coating electrospun PEO nanofibers onto a nonwoven microfiber substrate and investigated the effects of material structure and face velocity on the filtration efficiency and pressure drop of the filters. Based on their detailed experiments, they found that the most penetrating particle size for this filter decreased from 140 to 90 nm along with increasing packing density of the nanofiber membranes from 3.9 to 36  10e3, and the removal efficiency decreased as the face velocity increased from 5 to 10 cm/s. In addition to the aforementioned electrospun nanofiber filters, other single-component polymeric membranes, such as

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cellulose nanofibers (Grafe and Graham, 2003b), PU nanofibers (Sambaer et al., 2011), and polyvinyl pyrrolidone nanofibers (Morozov and Mikheev, 2012), have also been fabricated via electrospinning technology and have shown good filtration performance due to their greatly reduced fiber diameters, further confirming that electrospun nanofibers are an excellent candidate material for ultrafine PM filtration.

12.3.2 COMPOSITE POLYMER MEMBRANES Single-component polymeric nanofiber membranes exhibit many remarkable features that make them suitable for air filtration, like small diameter and high porosity. However, the majority of these membranes still lack some other essential properties, such as mechanical strength, chemical resistance, antifouling properties, etc. To overcome the limitations of these materials, two or more polymers with different characteristics are combined to produce composite polymeric nanofiber membranes. In 2013, Wang et al. (2013a) fabricated a tortuously structured polyvinyl chloride (PVC)/PU composite nanofiber filter with robust mechanical properties and filtration performance. The ratio of PVC/PU in solution was optimized and the nanofibers were collected on a traditional filter paper. The authors carefully studied the stressestrain curves of the resultant filters and revealed the three-step break process for fibers upon external stress, as shown in Fig. 12.3A. Owing to the nonbonding structure of PVC nanofibers, the slippage occurred at lower stresses; therefore the single PVC nanofiber membrane exhibited low tensile strength and elongation at break. With the introduction of PU, the mechanical properties gradually increased; and it was observed that all blended samples in the first region followed

FIGURE 12.3 (A) Stressestrain curves of polyvinyl chloride/polyurethane (PU) fibrous membranes. (B) Water and oil contact angles of polyacrylonitrile/PU fibrous membranes with various fluorinated PU (FPU) concentrations. (A) Modified with permission from Wang, N., Raza, A., Si, Y., Yu, J., Sun, G., Ding, B., 2013a. Tortuously structured polyvinyl chloride/polyurethane fibrous membranes for high-efficiency fine particulate filtration. Journal of Colloid and Interface Science 398, 240e246. © 2013 Elsevier. (B) Wang, N., Zhu, Z., Sheng, J., Al-Deyab, S.S., Yu, J., Ding, B., 2014b. Superamphiphobic nanofibrous membranes for effective filtration of fine particles. Journal of Colloid and Interface Science 428, 41e48. © 2014 Elsevier.

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Hook’s law (which states that stress is directly proportional to strain up to its elastic limit), and the membranes displayed a linear curve or elastic behavior under a stress load before reaching the yield point, at which they changed from an elastic to a plastic state. After that, a further increase in stress would result in the breakage of the nanofibers. Wang et al. attributed this phenomenon to a structural transformation based on the coexistence of bonding and nonbonding structures in the composite PVC/ PU nanofiber membranes during the strain process. Significantly, benefiting from the bonding structures, the as-prepared blended nanofiber membranes presented good abrasion resistance (134 cycles) performance and robust air permeability (154.1 mm/s) due to combined 3D porous structure and high tensile strength (9.9 MPa), achieving a high removal efficiency (99.5%) with a low air resistance (114 Pa). New functional nanofiber membranes with high removal efficiency and remarkable antifouling performance, reported by Wang et al., were fabricated by combining PAN/PU composite nanofiber membranes with the new synthesized fluorinated PU (FPU) (Wang et al., 2014b). The PAN/PU composite polymer structure exhibited robust mechanical properties (tensile strength of 12.28 MPa), superior air permeability (706.84 mm/s), and good abrasion resistance. With the addition of FPU, lowsurface-energy and rough nanoscaled structures were created, which endowed the membranes with hydrophobic and oleophobic properties, with water and oil contact angles of 154 and 151 degrees, respectively (Fig. 12.3B). This superamphiphobic property of the membranes can be controlled by altering the ratio of FPU in the precursor solution. In addition, the as-prepared PAN/PU/FPU nanofiber membranes showed better filtration performance toward oil and salt aerosol particles compared with pure PAN/PU membranes, which further confirmed the importance of amphiphobicity to the air filtration application. PAN/polyacrylic acid (PAA) composite nanofiber membranes were also prepared to use as a filtration medium by Liu et al., and they carefully investigated the effects of PAN/PAA ratios of 10:0, 7:3, 6:4, 5:5, and 3:7 on the structure and filtration performance of the resultant membranes (Liu et al., 2015b). The main objective of blending PAA with PAN is to achieve membranes with sufficient tensile strength. With increasing PAA content, the tensile strength of the composite membranes significantly increased from 3.8 to 6.6 MPa, indicating a much higher mechanical property compared with pure PAN membranes. Moreover, they found that the pore size and surface area of the composite nanofiber filters were mainly dependent on the structure of the fiber and composition of the blended solution. The average pore size for different samples was in the range of 16.8e44.4 nm. The composite membrane with the smallest pore size showed high removal efficiency (99.994%) with the lowest air resistance, 160 Pa, against 0.3e0.5 mm NaCl aerosol particles at an airflow velocity of 5.3 cm/s. A novel strategy to develop anti-deformed PEO@PAN/PSU composite nanofiber membranes with binary structure for capturing PM from the air was proposed by Zhang et al. (2016a), who combined multijet electrospinning with a vacuum drying process to form bonded structures of PEO@PAN/PSU membrane filters, as exhibited in Fig. 12.4. The PAN/PSU blended nanofiber membranes with controllable packing density and small pore size were prepared and optimized by varying the jet ratios of PAN and PSU in solution. Then the PEO agent was incorporated into the optimized composite membranes, to create the bonding/nonbonding structure that endowed the membranes with stable cavity structure and anti-deformed features. The resultant PEO@PAN/PSU blended membranes exhibited small pore size, high porosity, and relatively large tensile strength (8.2 MPa) with excellent toughness and modulus (2.44 MJ/m3 and 204 MPa, respectively), and can filter ultrafine particles with a high removal efficiency of 99.992%, low air resistance of 95 Pa, and excellent quality factor of 0.1/Pa.

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FIGURE 12.4 (A) Schematic illustration of the fabrication procedure of polyethylene oxide (PEO)@polyacrylonitrile (PAN)/ polysulfone (PSU) composite membranes. (B and C) The composite membranes (B) before and (C) after heat treatment. (D) Schematic illustration of the filtration process of the anti-deformed PEO@PAN/PSU fiber-based filter. Modified with permission from Zhang, S., Liu, H., Yin, X., Yu, J., Ding, B., 2016a. Anti-deformed polyacrylonitrile/polysulfone composite membrane with binary structures for effective air filtration. ACS Applied Materials & Interfaces 8, 8086e8095. © 2016 American Chemical Society.

12.4 HYBRID NANOFIBER-BASED FILTERS 12.4.1 POLYMER/ORGANIC NANOPARTICLE MEMBRANES Electret fibrous membranes have been proven to be an efficient and promising material for adsorbing airborne particles effectively by long-range electrostatic force because of their ability to quasipermanently reserve abundant charges and create an external macroscopic electric field on the periphery of the fibers. In 2016, Wang et al. prepared novel polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) NP electret nanofiber membranes by electrospinning and investigated their application performance in air filtration (Wang et al., 2016b). They first evaluated the effect of PTFE NP concentration on the morphology and structure of the PVDF/PTFE NP membranes, as shown in Fig. 12.5. It was found that the average diameter of the fibers decreased to 380 nm by including 0.05 wt % PTFE NPs compared with the pure PVDF nanofibers (622 nm). With further increase in the concentration of PTFE NPs to 0.1 wt%, the diameter of the fibers increased to 480 nm because of the agglomeration of PTFE NPs. Porosity versus concentration of PTFE NPs revealed that fibrous membranes with 0.05 wt% PTFE NPs possessed smaller pore size and more uniform aperture

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FIGURE 12.5 (A) Scanning electron microscopy and (B) transmission electron microscopy images of polyvinylidene fluoride/ polytetrafluoroethylene (PVDF/PTFE) nanoparticle (NP) electret fibrous membranes. (C) Schematic of electric charge category existing in PVDF/PTFE NP electret fibrous membranes. (D) Filtration efficiency of PVDF/PTFE NP electret fibrous membranes with various basis weights. NF, nanofilter. Modified with permission from Wang, S., Zhao, X., Yin, X., Yu, J., Ding, B., 2016b. Electret polyvinylidene fluoride nanofibers hybridized by polytetrafluoroethylene nanoparticles for high-efficiency air filtration. ACS Applied Materials & Interfaces 8, 23985e23994. © 2016 American Chemical Society.

distribution. The filtration properties of the composite electret membranes were tested with neutralized NaCl monodispersed aerosol particles with size ranging from 0.3 to 0.5 mm. The results showed that the filtration efficiency gradually increased to a maximum value as the concentration of PTFE NPs increased to 0.05 wt%. The filtration efficiency decay test also confirmed that the composite electret membranes with 0.05 wt% PTFE NPs possessed long-term stability of filtration efficiency. To deeply study the electret mechanism of the PVDF/PTFE composite membranes, a linear temperature program was employed to measure the thermally stimulated current. It was found that more interfacial polarization charges were inspired when PTFE NP concentrations reached 0.05 wt%. To further promote the depth of the energy level, various electrospinning voltages ranging from 20 to 50 kV were

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applied to increase the quantity and reinforce the stability of the charge. The results showed that PVDF/PTFE NP membranes prepared under a voltage of 40 kV showed the highest air filtration efficiency (99.972%), which implied that the initial surface potential cloud could be enhanced through increasing voltage.

12.4.2 POLYMER/INORGANIC NANOPARTICLE MEMBRANES In 2014, Wan et al. (2014) described the fabrication of a hierarchically nanostructured and superhydrophobic nanofiber medium for air filtration by electrospinning PSU/titania NP (PSU/TiO2) hybrid nanofibers on a traditional nonwoven substrate, as shown in Fig. 12.6A. They carefully studied the effects of solvent and TiO2 content in the solution on the structure of the PSU/TiO2 hybrid nanofiber membranes. By virtue of the hierarchical roughness caused by the incorporation of TiO2 NPs (indicated by dashed circle), the resultant hybrid nanofiber membranes exhibited a robust hydrophobicity, with a water contact angle of 152 degrees, and could facilely filter ultrafine particles with a high removal efficiency of 99.997% and low air resistance of 45.3 Pa, further indicating the contribution of TiO2 NPs to the filtration performance of the nanofiber filters. Cho et al. (2013) prepared TiO2incorporated PAN nanofibers, which were deposited on a cellulose filtration medium to further increase the filtration performance of the filters. They observed that with increasing content of TiO2 NPs, the numbers of ions and charged particles on the membranes were greatly enhanced, which can be

FIGURE 12.6 (A) Field emission scanning electron microscopy (FE-SEM) images of a polysulfone/TiO2 fibrous membrane. (B) Airflow streamlines around circular and noncircular cross-sectional fibers in the no-slip flow regime. On the right are the corresponding FE-SEM images of relevant fibrous membranes. (A) Modified with permission from Wan, H., Wang, N., Yang, J., Si, Y., Chen, K., Ding, B., Sun, G., EL-Newehy, M., AL-Deyab, S.S., Yu, J., 2014. Hierarchically structured polysulfone/titania fibrous membranes with enhanced air filtration performance. Journal of Colloid and Interface Science 417, 18e26. © 2014 Elsevier. (B) Wang, N., Si, Y., Wang, N., Sun, G., El-Newehy, M., Al-Deyab, S.S.., Ding, B., 2014a. Multilevel structured polyacrylonitrile/silica nanofibrous membranes for high-performance air filtration. Separation and Purification Technology 126, 44e51. © 2014 Elsevier.

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evaluated by thermally stimulated current measurement spectra. Therefore, the filtration performance of hybrid PAN/TiO2 membranes against 100e500 nm aerosol particles showed better efficiency than pristine PAN nanofiber filters, and the air resistance of the hybrid system was much lower than that of single-component PAN filter membranes. Wang et al. (2014a) proposed a powerful yet economic approach to creating high-efficiency filtration media by combining multilevel-structured hybrid nanofiber membranes. The principal feature of this work is that these membranes were fabricated by the accumulation of SiO2 NP-incorporated bimodal-sized PAN nanofibers. The authors observed that the scaffoldlike structure of the as-prepared membranes can be regulated by regulating the jet ratio of the solutions with different PAN concentrations, which has a great influence on the filtration performance of the filters. The addition of SiO2 NPs made the fibrous surface rough, with a noncircular cross section (Fig. 12.6B), and the specific surface area was enhanced by the highly porous structure. Benefiting from the layer-by-layer stacking structures and rough fiber surface, the multilevel PAN/ SiO2 hybrid membranes possessed a high filtration efficiency of 99.989% and low air resistance of 117 Pa compared with single-layer membranes. With the design strategy of capturing PM particles from the air by electrostatic effects, the novel approach of creating electret media provided an efficient way to achieve high removal efficiency with low air resistance. Song et al. (2016) reported an electret filtration medium by incorporating magnetic Fe3O4epolyhedral oligomeric silsesquioxane (POSS) particles with PAN nanofibers. With the help of the hydrosilylation reaction, magnetic Fe3O4ePOSS particles with SiOH were first prepared, and then PAN/Fe3O4ePOSS hybrid nanofiber membranes were deposited on a conventional nonwoven substrate by the electrospinning process. Owing to the incorporation of magnetic particles, a significant increase in the stability of the surface charge was achieved and its retention capacity greatly enhanced compared with pure PAN nanofiber membranes. In 2017, Zhao et al. fabricated a low-resistance hybrid nanofiber-based air filter by electrospinning negative ion powder-doped PVDF hybrid nanofiber membranes, which were capable of releasing negative ions and capturing PM2.5 pollutants from the air effectively (Zhao et al., 2017b). Based on the observation of filters obtained from different polymers (PSU, polyvinyl butyral, and PVDF), they found that a reduction in the fiber diameter (from 1.16 to 0.41 mm) significantly decreased the pressure drop (from 9.5 to 6 Pa) of the membranes, as exhibited in Fig. 12.7. Furthermore, a slower rising rate of air resistance along with a decrease in pore size could be delivered by a thinner fiber diameter of the fibrous membranes. In addition, the PVDF/negative ion particle nanofiber membranes displayed high surface potential, due to the high electronegativity of fluorine, resulting in higher releasing amounts of negative ions, which could be varied by reducing the diameter of the fiber and the negative ion content (from 789 to 2818 ions/cm3). The as-prepared PVDF hybrid nanofiber filters showed a high filtration efficiency of 99.99% at high release amounts of negative ions and a low air resistance of 40.5 Pa. And, this novel air filter with multilayered and cavitylike structures provided a new way to fabricate high-performance filtration media for various air filtration applications. The deadly hazards of air pollution, especially PM pollution, to human health make scientists and engineers eager to develop individual protective materials with several extraordinary features. In 2017, Zhao et al. reported a gradient-structured, hybrid nanofiber-based cleanable air filter with high purification efficiency, low pressure drop, and high moisture vapor transfer rate (Zhao et al., 2017a). These membranes were fabricated by using superhydrophilic PAN/SiO2 fibers, which functioned as a medium to transfer moisture vapor, and hydrophobic PVDF nanofibers, which functioned as a repellent

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FIGURE 12.7 The pressure drops of polysulfone (PSU), polyvinyl butyral (PVB), and polyvinylidene fluoride (PVDF) fibrous membranes (A) with different fiber diameters and (B) at different airflow velocities. Modified with permission from Zhao, X., Li, Y., Hua, T., Jiang, P., Yin, X., Yu, J., Ding, B., 2017b. Low-resistance dual-purpose air filter releasing negative ions and effectively capturing PM2.5. ACS Applied Materials & Interfaces 9, 12054e12063. © 2017 American Chemical Society.

element to restrict the creation of capillary water under high humidity. This gradient structure endowed the membranes with an improved moisture vapor transfer rate of 13,612 g/m2 day, high removal efficiency of 99.99%, and low air resistance of 86 Pa, as shown in Fig. 12.8.

12.5 NANOFIBER/NET-BASED FILTERS 12.5.1 NANOFIBER/NET MEMBRANES Although the electrospun nanofiber-based filters show improved filtration efficiency and enhanced service life compared with microfiber filters, some drawbacks still remain for these filters, including inadequate filtration performance and low quality factor, due to the thick fibers (>100 nm) and relatively large pore size. In 2012, Wang et al. (2012) first presented the fabrication of PA-66 nanofiber/net membranes, which are composed of conventional nanofibers and 2D spiderweb-like nanonets through an advanced hydrodynamic technique called electrospinning/netting. These novel nanonet structures, including coverage rate and pore size, can be facilely controlled by tuning the solution properties (content of additives) and process parameters. Taking advantage of the integrated properties of small diameter, small pore size, and high porosity, the nanofiber/net membranes possessed a high purification efficiency of 99.9% and relatively low air resistance, and can be employed as filtration media in various filtration fields. In 2017, Zhang et al. fabricated novel poly(m-phenylene isophthalamide) (PMIA) nanofiber/net membranes, which were composed of conventional nanofibers and a 2D Steiner tree network (w20 nm), for high-efficiency air filtration by using the electrospinning/netting process for the first time (Zhang et al., 2017a). The bimodal structure of the as-prepared PMIA membranes can be regulated by polymer concentration optimization, dodecyl

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FIGURE 12.8 (A) Schematic illustration of the fabrication process, and (B) filtration performance of gradient composite fibrous membranes. (C) The relationship between moisture vapor transfer rate (MVTR) and pressure drop of the composite membranes and commercial samples. PAN, polyacrylonitrile; PVDF, polyvinylidene fluoride. Modified with permission from Zhao, X., Li, Y., Hua, T., Jiang, P., Yin, X., Yu, J., Ding, B., 2017a. Cleanable air filter transferring moisture and effectively capturing PM2.5. Small 13, 1603306. © 2017 Wiley-VCH.

trimethylammonium bromide additive inspiration, and ambient conditions, especially relative humidity. By virtue of structural features such as extremely small diameter, small pore size, and high porosity, PMIA nanofiber/net membranes showed superlight weight (0.365 g/m2) with ultrathin thickness (w0.5 mm) and robust mechanical property (72.8 MPa), which made them promising candidates for high-performance air filtration against 300e500 nm aerosol particles, with high a removal efficiency of 99.999%, as shown in Fig. 12.9.

12.5.2 NANOFIBER/NET MEMBRANES WITH CAVITY STRUCTURES Nanofiber/net membranes like PA-66 and PMIA have been successfully prepared, and showed relatively high filtration efficiency; however, the pressure drop of these filters was still rather high (PA-66 filter w200 Pa), due to their inadequate and unstable cavity structures and compact packing density. In

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(A)

Filtration

(B)

(C)

(D)

FIGURE 12.9 (A) In situ scanning electron microscopy (SEM) study and (B) 3D model illustration of the filtration process of poly(m-phenylene isophthalamide) (PMIA) nanonet membranes. Field emission SEM images of (C) the top surface and (D) a cross section of a PMIA nanonet after filtration. Modified with permission from Zhang, S., Liu, H., Yin, X., Li, Z., Yu, J., Ding, B., 2017a. Tailoring mechanically robust poly(mphenylene isophthalamide) nanofiber/nets for ultrathin high-efficiency air filter. Scientific Reports 7, 40550.

2015, Liu et al. (2015a) proposed a novel strategy to design bimodal-sized biobased PA-56 nanofiber/ net membranes through the electrospinning/netting technique. These membranes were made of 2D ultrathin nanonets (w20 nm) and stable cavity structures, which are indeed very important for decreasing the air resistance while maintaining high efficiency for the air filters. In this study, the coverage rate of the nanonets was controlled by tuning the solution concentration, while the stable cavity structures were optimized by regulating the HCOOH/CH3COOH weight ratio. With the combined fascinating features of extremely small diameter, high porosity, and boned scaffold, the as-prepared PA-56 membranes displayed a high removal efficiency of 99.995%, low air resistance of 111 Pa, long working life with large dust-holding capacity (49 g/m2), and dust-cleaning regeneration ability compared with commercially used fibrous filters. To construct the stable and large cavity structures, Yang et al. reported a novel strategy to fabricate composite membranes with highly porous

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FIGURE 12.10 (A) Illustration of the concept of a sandwich-structured filtration medium based on polyamide-6 (PA-6) nanonets and polyacrylonitrile (PAN) bead-on-string fiber membranes. (B) Filtration efficiency and pressure drop of PA-6/ PAN/PA-6 composite membranes with various basis weights. Modified with permission from Yang, Y., Zhang, S., Zhao, X., Yu, J., Ding, B., 2015. Sandwich structured polyamide-6/ polyacrylonitrile nanonets/bead-on-string composite membrane for effective air filtration. Separation and Purification Technology 152, 14e22. © 2015 Elsevier.

structures, by using a PA-6/PAN/PA-6 sandwich structure for effective air filtration, as shown in Fig. 12.10 (Yang et al., 2015). The PAN bead-on-string fibers and 2D PA-6 nanonet (w20 nm) in the membrane endowed this newly constructed filter with tunable porous structures, and then obtained high purification efficiency (99.999%), low air resistance (117.5 Pa), and good mechanical properties. To further enlarge the cavity structures, Zhang et al. prepared PA-6/PMIA nanonet membranes with microwave structures by using electrospinning/netting and the staple fiber intercalating process, as shown in Fig. 12.11 (Zhang et al., 2016b). This work aimed to prepare the new filters with low stacking density, small pore size, and microwave fluctuation by combining the PA-6 binary nanofiber/net structures and embedded PMIA staple fibers. The resultant PA-6/PMIA membranes exhibited high removal efficiency of 99.995%, low air resistance of 101 Pa, and the required quality factor against ultrafine airborne particles, together with a relatively high tensile strength (10.7 MPa) and high dustholding capacity (>50 g/m2), which could be attributed to the features of extremely high porosity, small pore size, and large specific surface area. Furthermore, these membranes showed long service life in real applications, due to their high mechanical properties and microwave structure, further confirming that the as-prepared membranes can be strong candidates in the field of high-performance filtration media. In 2017, Zhang et al. described a novel approach to fabricating ripplelike PA-6 nanofiber/net membranes by the combination of electrospinning/netting and receiving substrate design (Zhang et al., 2017b). As shown in Fig. 12.12, the polyethylene terephthalate (PET) framework, with optimized pleat span and pleat pitch, was first designed by regulating the diameter and interval gap of the PET filaments accordingly, to form cavity structures in the "wave-like" zone (indicated between the dashed lines) without damaging Steiner treeelike 2D nanonet assemblies. Based on this regulation, the as-prepared membranes possessed excellent features such as broadly distributed nanonets, fluffy cavity structures, and enlarged frontal surface, which made the ripplelike PA-6

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FIGURE 12.11 Schematic illustration of (A) the fabrication procedure and (B) structures of polyamide-6 (PA-6)/poly(mphenylene isophthalamide) (PMIA) nanonet-based membranes. (C) Schematic illustration of the microwave structured PA-6/PMIA filter for effective air filtration. Modified with permission from Zhang, S., Liu, H., Yu, J., Luo, W., Ding, B., 2016b. Microwave structured polyamide-6 nanofiber/ net membrane with embedded poly(m-phenylene isophthalamide) staple fibers for effective ultrafine particle filtration. Journal of Materials Chemistry 4, 6149e6157. © 2016 Royal Society of Chemistry.

membrane a promising candidate in filtration media to capture ultrafine particles, with a high removal efficiency of 99.996%, low air resistance of 95 Pa, and desired quality factor of more than 0.11/Pa. In addition, the resultant membranes showed a superlight weight (0.9 g/m2) and large dust-holding capacity of >63 g/m2.

12.5.3 NANOFIBER/NET COMPOSITE MEMBRANES Multilevel composite structures give an insight into highly integrated and efficient filtration media for various applications, including personal, industrial, and environmental protection, because of their fascinating features of controllable pore size, gradually varied pore structure, and high porosity. In view of this novel design strategy, Zhang et al. reported the fabrication of highly integrated multilevel PSU/PAN/PA-6 hybrid fibrous membranes to capture airborne particles from polluted air via the sequential electrospinning process, as shown in Fig. 12.13 (Zhang et al., 2016c). Each layer of the integrated PSU/PAN/PA-6 composite membranes possessed the particular diameters and pore sizes for removing particles of different diameters. The PSU microfiber layer, with fiber diameter of w1 mm and pore size of w2.2 mm, was employed to filter particles with diameter of >2 mm; the PAN nanofiber

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FIGURE 12.12 (A) Schematic showing (1) the fabrication of the ripplelike polyamide-6 (PA-6) nanonet membrane and (2) a cross section of the wavelike structure of the PA-6 nanonet filter. (B) Scanning electron microscopy image of the ripplelike PA-6 membrane. Inset is the corresponding image of the cross-sectional view. Scale bar in the inset represents 50 mm. (C) Filtration efficiency, pressure drop, and quality factor (QF) of the ripplelike PA-6 nanonet filters with various basis weights. NF/N, nanofiber/net; PET, polyethylene terephthalate. Modified with permission from Zhang, S., Liu, H., Zuo, F., Yin, X., Yu, J., Ding, B., 2017b. A controlled design of ripple-like polyamide-6 nanofiber/nets membrane for high-efficiency air filter. Small 13, 1603153. © 2017 Wiley-VCH.

layer, with fiber diameter of w200 nm and pore size of w0.55 mm, was used to filter >0.5-mm particles; while the PA-6 nanonet could remove w300-nm particles because of their unique 2D Steiner tree structures with extremely small pore size of 0.27 mm. These orderly assembled layers, with varied diameters, different ranges of pore size, and high porosity, enabled the composite membrane filters to work efficiently and avoid blockage of the pore structures by gradually screening particles with certain sizes. Benefiting from the gradient structure, PSU/PAN/PA-6 membranes can remove 300-nm NaCl aerosol particles with high efficiency of 99.992%, low air resistance of 118 Pa, and high quality factor value, using physical sieving. Furthermore, these composite membranes successfully got rid of the impact of electret failure and high humidity, and displayed robust mechanical (tensile strength of 5.6 MPa) and hydrophobic properties (water contact angle w130 degrees), which made them promising candidates to be used in a broad range of applications for filtration and separation devices.

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FIGURE 12.13 (A) Illustration of the concept of the three layers of the integrated filter. (B) 3D simulation of the filtration process and (C) filtration efficiency of the polysulfone (PSU)/polyacrylonitrile (PAN)/polyamide-6 (PA-6) integrated filter. Modified with permission from Zhang, S., Tang, N., Cao, L., Yin, X., Yu, J., Ding, B., 2016c. Highly integrated polysulfone/ polyacrylonitrile/polyamide-6 air filter for multilevel physical sieving airborne particles. ACS Applied Materials & Interfaces 8, 29062e29072. © 2016 American Chemical Society.

12.6 INORGANIC NANOFIBER-BASED FILTERS The studies discussed previously were focused on air filtration media working at normal temperature or mediumehigh temperature because of the limited thermal stability of those membranes. However, many industrial processes need air filters to operate under high temperatures. For example, most industrial dust-removal processes in cement plants and coal-fired boilers are performed at high temperature in the range of 150 Ce260 C (Ding and Yu, 2014); as a result, conventional polymeric membranes could not fulfill the requirements of air filtration in these fields. To resolve this bottleneck, membranes composed of inorganic electrospun nanofibers are of great interest in air filtration applications under severe conditions because of their high thermal and chemical stability (Li and Xia, 2004; Li et al., 2003, 2012; Mao et al., 2010). Mao et al. (2012) fabricated novel and flexible silica nanofiber membranes with excellent thermal stability by a combination of the solegel process and electrospinning technique. They systematically optimized the flexibility and tensile strength by tuning the composition of the precursor solution and the calcination temperature. Thermogravimetric analysis (with the temperature range of 100 Ce900 C) of various silica membranes showed that no weight loss could be observed, further indicating the outstanding thermal stability of the resultant membranes. And, the as-prepared SiO2 nanofiber membranes maintained their randomly oriented structure and flexibility at calcination temperatures from 600 C to 1000 C. By virtue of their fascinating features, like excellent thermal stability, relatively high tensile strength (5.5 MPa), and remarkable flexibility (0.0156 gf cm), the SiO2 nanofiber membranes can achieve a high removal efficiency of 99.99% and low air resistance of 163 Pa against 300e500 nm NaCl particles. This novel approach may also

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provide a new vision for the design and development of flexible inorganic nanofibrous membranes for various applications and a new strategy for creating high-temperature air filters by using inorganic nanofiber membranes. In 2014, a new approach to constructing high-temperature filtration media for effective air filtration was reported by Wang et al. (2014c). By using the electrospinning technique, they fabricated freestanding g-alumina fibrous membranes that possessed remarkable flexibility, robust tensile strength (2.98 MPa), and excellent thermal stability (w900 C). Moreover, the random arrangement of nanofibers with smaller diameter of w230 nm and high aspect ratio endowed the as-prepared membranes with uniform pore structures and high porosity. And, these features enabled their use as a hightemperature filtration medium against a fine-particulate aerosol (300-nm dioctyl phthalate), which had a high removal efficiency of 99.848% and low air resistance of 239.12 Pa. Another typical study of the fabrication of inorganic nanofibers for high-temperature air filtration was described by Mao et al. (2016). Their group fabricated a flexible and high-temperature-resistant yttria-stabilized zirconia nanofiber membrane by electrospinning followed by a thermal treatment process, as exhibited in Fig. 12.14A and B. By regulating the polymer concentration of the precursor solutions, the morphology and structure of the as-prepared membranes could be effectively controlled to achieve high flexibility and robust mechanical property. As shown in Fig. 12.14C, the resultant membranes with robust bending and heat resistance displayed a high filtration efficiency of 99.996% for 0.3e0.5 mm NaCl particles, which enabled them to be strong candidates for high-temperature filtration in various applications.

FIGURE 12.14 (A) Field emission scanning electron microscopy images of the yttria-stabilized zirconia nanofiber. (B) Optical image of the relevant soft membrane. (C) Filtration efficiency and pressure drop of the yttria-stabilized zirconia nanofiber membranes. Modified with permission from Mao, X., Bai, Y., Yu, J., Ding, B., 2016. Flexible and highly temperature resistant polynanocrystalline zirconia nanofibrous membranes designed for airfiltration. Journal of the American Ceramic Society 99, 2760e2768. © 2016 Wiley-VCH.

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12.7 CONCLUDING REMARKS AND PERSPECTIVES Making our surroundings free from air pollution, especially from fine PM, microorganisms, and volatile organic compounds, has become an important issue to ensure an ecofriendly environment for living beings. In the past few decades, electrospun nanofiber membranes have gained much attention as an effective filter medium to capture airborne particles because of their tremendous advantages of small diameter, high surface area, porosity, etc. A number of polymer materials have been produced for the fabrication of different types of electrospun nanofibers for various air filtration applications. In this chapter, we have covered the latest developments in the fabrication of electrospun nanofiber materials for air filtration through electrospinning, including their structural advantages and filtration mechanisms. Specifically, we have highlighted different types and characters of electrospun nanofiber filters for different roles in filtering PM from polluted air. Interestingly, the nanofiber-based air filters exhibit better performance, such as high removal efficiency, low air resistance, and large clogging capacity, compared with the traditional nonwoven filtration materials, due to the fascinating features discussed herein. Despite remarkable progress in the development of nanofiber filtration media, some challenges still remain unsolved, which restrict their use in large-scale applications. For instance, the mechanical properties are still not sufficiently satisfactory to make nanofiber membranes suitable for practical applications. Therefore, the nanofibers need to be collected on a nonwoven conventional substrate, otherwise, they cannot be used independently in view of their low mechanical properties. This problem, which would endanger personnel and the industrial process if the filter breaks, greatly decreases the service life of nanofiber filters and restricts their future applications. Hence, proper research on the mechanical properties along with the filtration performance of nanofiber filters needs to be carried out. In addition to the experimental research, efforts should be taken to address the issues related to the manufacturing side, such as production rate, which can be effectively increased by developing the industrial equipment. Overall, the continuous efforts of engineers and researchers are expected to deal with the current challenges and promote electrospun nanofiber filters as cost-effective and energy-saving air filtration media in the future.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51473030 and 51673037), the Military Logistics Research Project (No. AWS14L008), the Shanghai Committee of Science and Technology (No. 15JC1400500), the “111 Project” Biomedical Textile Material Science and Technology (No. B07024), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program.”

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