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Al-Coated Conductive Fibrous Filters with Low Pressure Drop for Efficient Electrostatic Capture of Ultrafine Particulate Pollutants Dong Yun Choi, Soo-Ho Jung, Dong-Keun Song, Eun Jeong An, Duckshin Park, Tae-Oh Kim, Jae Hee Jung, and Hye Moon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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ACS Applied Materials & Interfaces

Al-Coated Conductive Fibrous Filters with Low Pressure Drop for Efficient Electrostatic Capture of Ultrafine Particulate Pollutants Dong Yun Choi,1 Soo-Ho Jung,1 Dong Keun Song,2 Eun Jeong An,1 Duckshin Park,3 Tae-Oh Kim,4 Jae Hee Jung,5 and Hye Moon Lee1,*

1

Powder & Ceramics Division, Korea Institute of Materials and Science, Changwondaero 797,

Seongsan-gu, Changwon 51508, Korea. 2

Environment and Energy Systems Research Division, Korea Institute of Machinery and

Materials, Gajeongbuk-ro 156, Yuseong-gu, Daejeon 34103, Korea. 3

Eco-Transport Research Division, Korea Railroad Research Institute, Cheoldobangmulgwan-ro

176, Uiwang, Gyeonggi-do 16105, Korea. 4

Department of Environmental Engineering, Kumoh National Institute of Technology, Daehak-

ro 61, Gumi, Gyeongbuk 39177, Korea. 5

Center for Environment, Health, and Welfare Research, Korea Institute of Science and

Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Korea.

ACS Paragon Plus Environment

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KEYWORDS: aluminum precursor ink, conductive fiber, air pollution, particulate matter, electrostatic effect

ABSTRACT

Here, we demonstrate a new strategy of air filtration based on an Al-coated conductive fibrous filter for high efficient nanoparticulate removals. The conductive fibrous filter was fabricated by a direct decomposition of Al precursor ink, AlH3{O(C4H9)2}, onto surfaces of a polyester air filter via a cost-effective and scalable solution-dipping process. The prepared conductive filters showed a low sheet resistance (< 1.0 Ω sq−1), robust mechanical durability and high oxidative stability. By electrostatic force between the charged fibers and particles, the ultrafine particles of 30 – 400 nm in size were captured with a removal efficiency of ~99.99%. Moreover, the conductive filters exhibited excellent performances in terms of the pressure drop (~4.9 Pa at 10 cm s−1), quality factor (~2.2 Pa−1 at 10 cm s−1), and dust holding capacity (12.5 µg mm−2). After cleaned by water, the filtration efficiency and pressure drop of the conductive filter was perfectly recovered, which indicates its good recyclability. It is expected that these promising features make the conductive fibrous filter have a great potential for use in low-cost and energy-efficient air cleaning devices as well as other relevant research areas.

INTRODUCTION

Recently, particulate matter (PM) suspended in air has emerged as one of the most serious environmental problems. Accordingly, the demand for protecting human health from air pollution is significantly increasing along with the economic growth. Many reports have


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demonstrated that airborne nanosized PM poses negative health consequences, leading, for instance, to cardiovascular and respiratory diseases, as these particles can penetrate the human bronchi and lungs by their small physical size.1-5 In addition to the serious threat to human health, PM is influencing our living environments with regard to air quality, visibility, radiative force, and climate change.6-14 Thus, in terms of the global environment and public health, the development of filter technology to remove PM effectively from the ambient surroundings is an important and urgent task. Fibrous filters are widely used in various air filtration devices because they are cost-effective, light, and easy to use. When airborne particles pass through a fibrous filter, some of them are deposited onto a fiber surface by mechanical filtration mechanisms, including inertial impaction, interception, and Brownian diffusion. Because these mechanisms function competitively depending on the particle size, fibrous filters possess minimum efficiency usually in the range of 0.1 – 0.5 µm.15,16 Moreover, there exists a trade-off relationship between the PM removal efficiency and pressure drop. As the energy consumption of a fan is directly proportional to the pressure drop across the filter,17,18 low-packing-density filters are generally installed in air purification systems in many single-family houses and large buildings.19 Considering both economic and health perspectives, it is of great significance to upgrade the low removal efficiency of these filters without increasing their pressure drop. There have been numerous efforts to solve these issues; for example, electret filters with quasi-permanent electrical charges on dielectric polymer fibers,20-25 nanofibrous filters with surface functionalities of high dipole moment and/or hydrophilicity,18,26-30, metal-organic framework based nanofibrous filters with positive surface charges,31,32 and slip-effect functional filters composed of carbon nanotubes or ultra-thin nanofibers.33-36 Each of these developments


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offers improved removal efficiency without a severe increase in the pressure drop. But, greater packing density is needed to realize removal efficiency of 99.97% which is the standard of highefficiency particulate air (HEPA) filters for 300 nm airborne particles,20 and consequently they will give pressure drops in the range of several tens to several hundreds of Pa. In addition, some concerns remain to be addressed regarding the production cost and filter life-time as well as the removal efficiency vs the pressure drop. If a fibrous filter has high electrical conductivity, it can be charged by directly applying electric potential and give a capability for effective electrostatic capture of charged particles. This will become an appealing alternative to air filtration, as the electrostatic effect can significantly augment the removal efficiency of a low-efficiency filter without deteriorating air permeability. Among existing conductive metals, aluminum (Al) is nearly ideal material in terms of the price stability and electrical conductivity per unit cost. For the high yield and cost-effective production of Al-coated fibrous filters, conductive Al features should be able to be created on filter fibers via a solution-based coating process. However, conductive Al films are hard to be realized by using an Al colloidal ink, because thin insulating oxide layers (3−10 nm) are formed on Al nanoparticles during the ink preparation by the high reactivity of Al with oxygen and moisture. To circumvent the oxidation problem, we developed an Al precursor ink, AlH3{O(C4H9)2}, enabling Al film formation by the direct decomposition of Al precursors onto a target surface. This approach was found to be very powerful for the fabrication of highly conductive Al structures with excellent electromechanical properties on glass, polymer, and paper substrates.3740

Here, for the first time, we introduce a low flow-resistive conductive fibrous filter whose excellent electrical conductivity enables effective electrostatic capture of ultrafine particulates.


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Using an Al precursor ink of AlH3{O(C4H9)2}, the conductive fibrous filters were able to be fabricated successfully via a cost-effective, simple, and scalable solution-dipping process. Charged nanoparticles ranging in size from 30 to 400 nm were captured with an efficiency exceeding >99.99% by the strong electrostatic forces toward the conductive fibers. Compared with a commercial HEPA filter, the conductive fibrous filter exhibited superior performances in terms of the removal efficiency, pressure drop (lower than ~10 times), dust holding capacity (higher than ~4 times), and filter lifetime. Moreover, the conductive filters exhibited outstanding electromechanical and oxidative stability, indicating the possibility of recycling them with a cleaning process. Their recyclability was experimentally demonstrated by comparing the performance before and after they were washed with water. This work will herald a new approach for the removal of PMs by conductive fibrous filters and will therefore be attractive in low-cost and energy-efficient air quality applications. EXPERIMENTAL METHODS Preparation of the Al Precursor Ink. The Al precursor ink, AlH3{O(C4H9)2}, was produced by an ethereal reaction of aluminum chloride (AlCl3) with lithium aluminum hydride (LiAlH4) in dibutyl ether (O(C4H9)2). In this reaction, AlCl3 acts as a precursor and LiAlH4 acts as both a precursor and a reduction agent. Dibutyl ether was used as the solvent during the reaction for both materials. All of the chemicals noted above were purchased from Sigma-Aldrich and were used as received. 15 mM of AlCl3 and 53 mM of LiAlH4 were mixed with 50 mL of dibutyl ether. This solution was then heated to ~70 °C under magnetic stirring for ~1 h. After the reaction was complete, gray slurries of any byproduct of LiCl and any unreacted precursors were filtered and carefully discarded. The entire process was carried out in a glove box under a moisture-free argon atmosphere to prevent deterioration of the electrical properties of the Al precursor ink by


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the formation of nonconductive elements such as aluminum hydroxide (Al(OH)3), aluminum oxide (Al2O3), and/or aluminum nitride (AlN). Fabrication of the Conductive Fibrous Filter. A roll of the polyester air filter, commonly used as a pre-filter in various filtration systems, was obtained from Daekyung Nonwovens Co., Ltd. and was cut into an A4 size for use in a solution-dipping (SD) process. The raw filter was cleaned successively with deionized water in an ultrasonic bath for 30 min, followed by drying overnight at 85 oC in an electric oven. As moisture adsorbed onto the fiber surfaces would lead to the generation of non-conductive Al(OH)3 and Al2O3, the raw filter should be fully dried before the SD process. The dried raw filter was immersed into a catalytic solution of Ti(O-i-Pr)4 for 30 min with mild shaking, and the catalytically treated filter was dried overnight at room temperature. After drying, the filter was dipped into the Al precursor ink for a certain time and was fully dried at room temperature. All of these coating processes were conducted in a dry glove box filled with argon gas. Filtration Test. KCl particles were generated by atomizing a KCl solution (5 g L−1 in deionized water) with an ultrasonic nebulizer (Nescosonic UN-511, Alfresa-Pharma Corp.), and they were carried by filtered dry air at a flow rate of 1.5 L min−1. The aerosol KCL particles were flowed through a diffusion dryer to remove moisture and were then mixed with particle-free dry air in a dilution chamber (with a volume ratio of 6:1) to reduce the particle number concentration within the operating range of measurement instruments. To neutralize the electrical charges on the test particles, they were passed through a soft X-ray (Aerosol Neutralizer 4530, HCT) before being introduced into an electrostatic air filtration (EAF) device where the carbon fiber ionizer and conductive filters were installed. Conductive filters are installed on both sides of a plastic separator with a thickness of 5 mm, and each of them is contacted to a metal electrode linked to


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an external high-voltage generator. The size distributions of the test particles were measured using a scanning mobility particle size (SMPS) system consisting of a differential mobility analyzer (DMA 3081, TSI) and a condensation particle counter (CPC 3771, TSI). The pressure drop across the upstream and downstream sides of the filter was measured by a differential pressure transmitter (Testo 510, Testo). The ozone generation was examined with a UV photometric ozone monitor (Model 49C, Thermo Environment Instruments Inc.) at the outlet of the EAF device. Characterization. The scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) results were obtained using a scanning electron microscope (JSM-5800, JEOL; Magellan 400, FEI). Surface characterization of the raw polyester filter and Al-coated conductive filter was analyzed by using an X-ray photoelectron spectrometer (XPS) (K-alpha, Thermo VG Scientific). For the sheet resistance measurements of the conductive fibrous filters, four contacts were drawn at each edge using silver paste to make ohmic contacts, after which the resistance was measured using the four-probe van der Pauw method (FPP-HS8, DASOLENG). In the cyclic bending tests, the conductive fibrous filters were precisely bent and flattened in a controlled manner using a motorized translation stage (SM1-0810-3S and STM-1-USB, Sciencetown) communicating with a computer. After a scheduled cyclic test, the sheet resistance measurement was conducted on the re-flattened filter. CFD Simulation. The flow field, electric field, and particle motion were computed using a commercial finite-volume solver, CFD-ACE+, developed by the ESI group. A symmetric 2D model consisting of a half fiber was constructed to reduce the computational cost and its calculation domain has a geometric size of 10df × 3df, where df is the fiber diameter. We assigned a value of 3df to take into account the packing density of the filter, with the result being


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α ≈ df/(2×3df) ≈ 0.167. After all the particle trajectories were determined, the single-fiber efficiency was obtained by dividing the number of deposited particles on the fiber surface by the number of particles generated at the inlet within the projection area of the fiber. The filter removal efficiency was then calculated by Equation (2). RESULTS AND DISCUSSION Preparation of Al-Coated Conductive Fibrous Filters. A schematic diagram of the solutiondipping (SD) process for fabricating the conductive fibrous filter is given in Figure 1a. As a template for the conductive fibrous filter, we used a commercially available polyester air filter with a low packing density. The overall process consists of two steps. The first is a catalytic treatment of the raw fibrous filter by dipping it into titanium isopropoxide (Ti(O-i-Pr)4), and the second involves immersing the catalytically treated filter into an Al precursor ink of AlH3{O(C4H9)2}. The detailed preparation procedures for the Al precursor ink and the SD process are explained in the Experimental Methods. Although results are not shown here, it has been verified that the proposed SD process was applicable to the any polymeric fibers which have high chemical stability against Al precursor ink, such as polyimide, polyurethane, polystyrene, and polypropylene etc. Figure 1b shows a sketch of the mechanism underlying the direct formation of densely structured Al features onto the surface of every fiber. The Al precursor ink permeates deeply into the filter, and the decomposition of AlH3{O(C4H9)2} into Al, H2, and O(C4H9)2 starts to occur on all fiber surfaces. Since the catalytic pretreatment significantly reduces the activation energy required for the decomposition of AlH3 into Al and 1.5H2, it is possible to form conductive Al features at room temperature; the decomposition reaction commonly occurs at ~165 °C when the catalytic pretreatment is omitted.41 The nucleation of Al takes place ubiquitously on the fiber


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surfaces, and the nucleated Al grows progressively to be large enough to cover the fiber surface. The chemical reaction of Al and Li salts for the production of Al precursor ink and its decomposition for the creation of pure Al has been elaborated in our previous work.40 As shown in the photos in Figures 1c,d, the color of the raw filter changed from white to metallic gray by the existence of Al features coated on the fiber surfaces. Also, from the SEM images, it was verified that microstructure of the raw filter remained intact without any structural damages after the SD process. As is evident from the magnified SEM images in Figure S1 in the Supporting Information, there is no clear difference between the fiber surfaces before and after the Al deposition step; accordingly, we utilized EDS to determine the characteristics of the Al features on the fibers. Figure 1e shows the EDS mapping results of the Al-coated polyester filter, indicating that both carbon (C; green dots) and aluminum (Al; red dots) are well distributed over the fiber surface. Hence, it could be confirmed that the SD process enables the direct formation of densely structured Al features onto the filter fibers. Interestingly, the junctions between adjacent fibers were well bridged by the conductive Al structures made from the infiltrated Al precursor ink (Figure 1f). Thus, electrical paths were formed not only along a single fiber but also across the fibers, resulting in a three-dimensional electrical network. This type of conformal deposition of Al thin films is scarcely achievable through vacuum sputtering methods. Figure 1g exhibits an A4-sized conductive fibrous filter fabricated by the SD process, highlighting the scalability of our process. It also reveals that the porosity of the filter is high enough to enable the viewing of the background through the filter, which implies the low pressure drop characteristic indirectly.


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Electromechanical and Chemical Stability. Figure 2a shows the electrical properties of the conductive fibrous filters according to the immersion time in the Al precursor ink. The conductive fibrous filter offers good electrical conductivity with sheet resistance (RS) of ~1.64 Ω sq−1 at an immersion time of 30 min. At longer times, RS rapidly decreased, but afterward resistance values plateaued. In addition, the color of the conductive filter gradually darkened, as shown in the insets in Figure 2a. For the reliable PM removal by electrostatic effects, the electrical property of the conductive fibrous filter should be durable against various mechanical deformations which may be introduced during typical installation, maintenance, and operation processes. Figure 2b shows the results of a cyclic bending test of the conductive filter. The filter was bent to a bending radius of 5 mm (inset of Figure 2b), and RS was measured after it was re-flattened. The initial value of the sheet resistance (RS0) increased only ~3.2% after 10000 bending cycles, indicating that the electrical conductivity did not severely deteriorate during the intensive cyclic test. The resistance changed only slightly when the filter was bent to bending radii larger than 5 mm, but it began to increase at bending radii of less than ~5 mm (Figure 2c). When the filter was nearly folded to a bending radius of 0.5 mm, ∆RS/RS0 was increased by ~16.5% due to the cracked Al features by the mechanical bending stress (the corresponding SEM image is not shown here). But, considering the low value of RS0 (~0.66 Ω sq−1), electrical property of the filter was still quite good. When it was flattened, the resistance was recovered to about 95.7% of the original value. An LED lamp attached to the filter wrapped around a highlighter pen was brightened upon the supply of DC current, which demonstrates the excellent electromechanical flexibility of our conductive fibrous filter (Inset of Figure 2c).


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We performed additional durability tests of folding, peeling, and rubbing. The resistance remained mostly unchanged when the conductive filter was folded in a zigzag fashion (Figure 2d). Scotch tape did not peel off any of the Al features, and the resistance was remained (Figure 2e). When the conductive filter was rubbed in water, the resistance was nearly unchanged (Figure 2f). These results reflect the superior mechanical and electrical endurance capabilities of the SD-processed fibrous filter. Figure 2g shows the RS of the conductive fibrous filters before and after sonication tests in ethanol, isopropyl alcohol (IPA), and deionized (DI) water. The electrical properties were well maintained without severe degradation by each solvent; the RS of the filters increased by less than 16% after sonication for 1 h. Thus, the electrical property of the SD-processed fibrous filter was scarcely damaged by oxygen and moisture because the dense Al oxide layer hinders oxygen molecules from penetrating into the layer to induce further oxidation.38 All of above results imply the possibility that the conductive fibrous filter can be reusable by a simple washing. Application to Electrostatic Air Filtration. Figure 3a presents the schematic of the EAF device composed of parallel aligned two conductive fibrous filters for capturing the PMs charged by a carbon fiber ionizer. Positive high voltage was applied to the front conductive filter, whereas the back conductive filter was grounded for all filtration tests unless noted otherwise. We applied a fixed voltage of −10 kV to the ionizer when particle charging was needed. Thereby, negative ions were generated and the particles were negatively charged before passing through the conductive filters. Any additional oxidation of the Al-coated fibers was not progressed during the particle charging process, as examined by the XPS analysis (Figure S2). In the normal fibrous filter, both particle inertia and Brownian motion make a relatively weak contribution to the collection of particles 0.1 – 0.5 µm in size, and so there exists the minimum


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removal efficiency in this size range.15 However, once electrostatic forces between the particles and fibers are present, the removal efficiency can be improved without sacrificing air permeability. There are several types of electrostatic interactions, such as Coulomb force, dielectrophoretic force, and image force.5,42 As our Al-coated fibrous filter is a good conductor, the fibers can be charged directly by an applied electric potential and strong electric fields can be created around fibers. Thus, the charged particles experience Coulomb force predominatingly and are collected onto the fibers. Compared with nonconductive electret filters made from dielectric nanofibers, the charge state and the electric field strength in the conductive filter can be controlled freely. Removal Efficiency of Ultrafine Particles. The particle removal efficiency of the conductive fibrous filter was examined using potassium chloride (KCl) nanoparticles in the size range of 30 – 400 nm (Figure S3). KCl particles have been adopted in many filtration tests as test particles.20 A full description of all necessary implementation details is given in the Experimental Section, and a schematic of the experimental setup is presented in Figure S4. The removal efficiency, η, one of the most important performance indexes for air filters, is expressed as26,36

η=

N wo − N w , N wo

(1) where Nw and Nwo are the particle number concentrations at the outlets of the EAF devices with and without filters, respectively. So as to eliminate any effects on time-dependent particle generation, efficiency measurements were conducted more than four times for each experimental condition. As is evident from Figure 3b, the filtration performance of the conductive fibrous filter was augmented greatly by the action of the electrostatic effect. The electrostatic capture mechanism


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became more pronounced as the applied voltage was increased or when the air flow rate was decreased. When 10 kV was applied to the front conductive filter, the charged particles in the range of 30 – 400 nm were captured with the removal efficiency of >99.99% at airflow velocity (u) ≤ 10 cm s−1. (The removal efficiency for the particles with the same size range was ~ 99.8% even at u = 20 cm s−1). The use of such a high electric field is beneficial for enhancing the filtration performance, but may lead to significant ozone emission of the ionizer. As shown in Figure S5, the ozone concentrations at u = 2.5 cm s−1 was increased to about 0.098 ppm when 10 kV was applied to the front conductive filter. However, considering that the application of 5 kV is sufficient to remove particles with an efficiency of > 99.99%, the ozone emissions can be reduced below 0.05 ppm which is the standard for electrostatic air cleaners (UL 867).43 Once the airflow rate was raised to 8 times (u = 20 cm s−1), the ozone concentration was highly decreased to ~0.013 ppm even for the 10 kV condition. Figure 3c shows the results of the fractional removal efficiency depending on the voltage applied to the front conductive filter at a constant u of 10 cm s−1. When the front filter was grounded, the removal efficiency for particles larger than about 100 nm was nearly constant at ~52%. Below this size, the efficiency gradually increased with decreasing the size. An onset of 1 kV gave rise to a further improvement in the removal efficiency, but the trend in the efficiency curve was unchanged. Given that diffusion charging is the major mechanism for particles smaller than ~100 nm,15 these particles could gain charges and electrical mobility sufficient to reach the fiber surfaces even under a weak electric field. When 5 kV was introduced, the removal efficiency for particles 30 – 400 nm in size was notably improved and exceeded 99.7% averagely. The effect of flow rate on the removal efficiency was also investigated at a fixed voltage of 5 kV, as shown in Figure 3d. When u < 10 cm s−1, the removal efficiency within the full size range


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was more than 90%. The inset plot indicated by the red arrow shows the efficiencies for particles in size range of 100 – 400 nm. There is minimum efficiency of ~99.6% for those with diameters close to ~200 nm when u = 10 cm s−1, whereas the efficiencies remained at a nearly constant value of ~99.99% for u of 2.5 and 5 cm s−1. The removal efficiency increased with a decrease in the air flow rate because a lower flow rate provides a longer particle charging time. The relatively low efficiency for particles smaller than ~50 nm is caused by the partial charging of the particles since smaller particles have less of a chance to collide with ions.44,45 Interestingly, the electrostatic capture process of the charged particles was almost completed within the front filter and hence particles deposited on the back filter were hardly noticeable, as shown in Figure S6. We found that the back filter merely helps to create the electric field between the two filters but does not play a major role in capturing the particles. Numerical Analysis of Electrostatic Particle Capture. To verify the electrostatic effect on the particle capture performance theoretically, we carried out numerical simulations using the commercial computational fluid dynamics (CFD) solver CFD-ACE+. The conductive fibrous filter was modeled as a filter consisting of regularly staggered fibers, which is one of the simplest models taking into account the depth of the filter.42 The particle removal efficiency (η) can be determined from the single-fiber efficiency (ηs) using the following relationship:15

 −4αηs t f  .  π d (1 − α )   f 

η = 1 − exp  (2)

Here, α is the packing density, tf is the filter thickness, and df is the fiber diameter. In order to assess ηs of our filter, we constructed a single-fiber calculation domain, as shown in Figure S7. The particle trajectories were computed by considering the inertial force, Stokes drag force, Brownian diffusion, and coulombic attraction under a steady-state laminar flow field. The


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Cunningham slip correction factor was included in the calculation of the drag force,15 and the particle charging rate was calculated based on the combined charging rate model proposed by Lawless46 which considers diffusion charging and field charging simultaneously. For simplicity, we assume that the ion concentration (N0) generated by the carbon fiber ionizer is constant regardless of the electric field strength between the ionizer and the front conductive filter. The spatial distribution of the ion concentration from the ionizer to the front filter was taken into account using the unipolar charge drift formula.47 All related equations are given in the Supporting Information, and the numerical parameter values are summarized in Table S1. Figures 4a,b show the calculated removal efficiency for particles with diameters (dp) of 300 nm at different flow rates. Fitting the calculated results to the experimental data gave N0 = 2 × 1013 ions m−3. The upward trend of the calculated efficiency according to the applied voltage was in reasonably good agreement with that of the experimental data; the removal efficiency is rapidly increased as the voltage is increased, and the minimum voltage needed to produce efficiency close to 100% is reduced with a decrease of airflow velocity (Figure 4a). As shown in Figure 4b, the simulated efficiency as a function of flow rate at a constant 5 kV was wellmatched with the expermental data. Figure 4c presents the particle trajectories around a single fiber depending on the applied electric potential at u = 10 cm s−1. When electrostatic effect is absent (Figure 4c (i)), the motion of particles follows the gas streamlines well, and none of the particles reaches the fiber surface. Since our filter has a relatively large fiber diameter of 30 µm and a low filter thickness of 250 µm, the particles are hardly captured by mechanical filtration mechanisms; from filtration theory,15 the estimated single-fiber efficiencies according to the inertial impaction, interception, and Brownian diffusion are 2.89 × 10−3, 2.66 × 10−2, and 0.24%, respectively, and the resulting removal efficiency is only 0.57%. However, when the Coulomb


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force is produced between the particles and the fiber, particle paths are significantly altered and turn towards the fiber surface, as shown in Figures 4c (ii–iv). As the applied voltage is raised, increasing numbers of charged particles are deposited onto the fiber surface. The corresponding calculated removal efficiencies at 1, 5, and 10 kV are 68.14, 99.73, and 100%, respectively. So, the simulations prove that the very high removal efficiency of our conductive filter is attributed to the Coulomb force between the charged particles and the charged fibers. Pressure Drop and Quality Factor. The characteristic of pressure drop across the filter is important along with the removal efficiency because a large flow resistance consumes considerable amounts of energy during the filtration process. As shown in Figure 5a, the pressure drop curve of the conductive fibrous filter according to the flow rate was compared with that of a commercial HEPA filter structured with dual-layers of microfibers and nanofibers (Figure S8) As the airflow velocity increased from 2.5 to 20 cm s−1, the pressure drop across two sheets of the conductive filters (the front filter and the back filter) increased from ~1.5 to ~10.3 Pa, but the pressure drop across a single sheet of the HEPA filter rose more steeply from ~13.2 to ~88.7 Pa. Considering a single conductive filter, the pressure drop of the conductive filter would be more than 10 times smaller than that of the HEPA filter. The overall performances of the air filters are evaluated with the quality factor (QF), defined as QF = −ln(1 − η)/∆P, where the ∆P is the pressure drop. A higher value of QF indicates better performance of the filter. The removal efficiency of the conductive filters as a function of the pressure drop is compared to that of the HEPA filter. These data points are plotted in conjunction with the corresponding QF curves, as shown in Figure 5b. In the filter tests, the airflow velocity was varied from 2.5 to 20 cm s−1 and the electric potential of the front conductive filter was fixed to 10 kV. The conductive filters showed better particle capture performance than that of the


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HEPA filters, while offering much higher air permeability. Consequently, the QF values of the conductive filters were more than 10 times higher than those of the HEPA filter. The performance results of the conductive filter and the HEPA filter with various parametric configurations were summarized in Table S2. Dust Holding Capacity. In order to evaluate the dust holding capacity (DHC), the quantity of particles deposited on the filter was investigated by measuring the weight of the filter before and after the filtration test. When the initial pressure drop increased by roughly threefold, we terminated the filtration test and calculated DHC by dividing the weight of the deposited particles by the filter area. To obtain the DHC of the conductive filter, a sheet of the conductive filter was used and 10 kV was applied to it. (Note that the grounded back conductive filter was not used in this case.) The DHC values of the conductive filter and the HEPA filter were 12.5 and 3.4 µg mm−2, respectively (summarized in Table 1). The conductive filter gave a higher DHC compared to that of the HEPA filter. As the filter lifetime is directly proportional to the DHC,17 this result implies that our conductive filters could operate at a higher efficiency for longer durations. Walsh et al. reported that a filter with a greater amount of electric charge is clogged upon larger mass loadings.22,23 As shown in Figures 5c,d, the superior DHC of the conductive filter seems to be attributed to the uniform deposition of particles by the electrostatic effect unlike the case of the HEPA filter. Dendrites of irregular particle chains were scatteredly deposited on the nanofibers of the HEPA filters, but hemispherical particle agglomerates were evenly built up on fibers of the conductive filter as well as along its depth direction. Thus, the uniform accumulation of particles by electrostatic forces leads to slower growth of dendrites, and which enables longer periods of operation at a higher efficiency.


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Long-Term Performance and Recyclability. Figure 6a presents the long-term performance of the conductive fibrous filter. The conditions of the voltage and the face velocity are 5 kV and 2.5 cm s−1, respectively. The pressure drop increased quasi-linearly in the early stages of filtration, and rose exponentially when the conductive filter started to become clogged.23 The removal efficiency was remained stable within the range of 99.97 – 99.99% until the pressure drop increased to ~17.6 times of the initial value. As particle loading further progressed, the efficiency began to decline noticeably due to the diminished electrostatic effect by the charge screening of the deposited particles.22 Electret filters usually exhibit a sharp drop in the removal efficiency when the particle load exceeds 2 – 3 µg mm−2.48 Compared with them, the conductive filter showed highly stable performance up to particle loading of ~39.1 µg mm−2. To check whether the filter performance can be fully recovered after cleaning the used filter, we washed the used filter in an ultrasonic water bath for 10 min and then carried out the same filtration test. As depicted by the red symbols in Figure 6a, the removal efficiency and pressure drop were restored to their initial values, manifesting the reuse possibility of the conductive filter. The recyclability was further examined by conducting multiple washing test of another sample, as shown in Figure 6b. At the end of each filtration experiment, the conductive filter was cleaned by sonication in water for 10 min. The initial removal efficiency of ~99.98% was quite stably maintained during the recyclability test and any noticeable damages or defects were not produced, verifying that the conductive fibrous filter is reusable by water cleaning. CONCLUSIONS

In summary, we proposed a new air filtration media of the Al-coated fibrous filter capable of capturing nanoparticulates effectively by the electrostatic effect. The highly conductive fibrous


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filter was successfully prepared from a polyester air filter via a cost-effective, simple, and scalable SD process based on an Al precursor ink, AlH3{O(C4H9)2}. By applying high electric potential directly to the conductive filter, the removal efficiency of the charged particles were able to be improved significantly without deteriorating air permeability. The conductive fibrous filter had a comparable filtration performance to a commercial HEPA filter, but the performance regarding the pressure drop, quality factor and DHC was significantly superior. Moreover, the conductive filter showed outstanding electromechanical and oxidative stability, which enables it to have good reusability by a simple cleaning. We expect that the conductive fibrous filter with a high removal performance and low flow resistance will be greatly helpful for reducing energy consumption by the air filtration systems in clean rooms and for the development of low-cost and energy-efficient air cleaners for home use.

ASSOCIATED CONTENT Supporting Information Magnified SEM images of the raw filter and Al-coated filter; Size distribution of the generated KCl nanoparticles; Schematic diagram of the experimental setup; Photographs of the conductive filters after filtration tests; A sketch of 2D calculation domain; Table summarizing the parameter values used in the numerical calculations; SEM image showing the microstructure of a commercial HEPA filter; Performance summary of the conductive fibrous filters and HEPA filters. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by R&D program of Ministry of Science, ICT & Future Planning/Commercializations Promotion Agency for R&D Outcomes (No. 2015K000214) and by a grant from the Railway Technology Research Project of the Ministry of Land, Infrastructure and Transport (16RTRP-B082486-03). This study was also partially supported by the KIST Institutional Program. REFERENCES (1)

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(14) Zhang, S.; Liu, H.; Zuo, F.; Yin, X.; Yu, J.; Ding, B. A Controlled Design of Ripple‐Like Polyamide‐6 Nanofiber/Nets Membrane for High‐Efficiency Air Filter. Small 2017, 1603151. (15) Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nd ed.; Wiley-Interscience: New York, 1982. (16) Podgórski, A.; Bałazy, A.; Gradoń, L. Application of Nanofibers to Improve the Filtration Efficiency of the Most Penetrating Aerosol Particles in Fibrous Filters. Chem. Eng. Sci. 2006, 61, 6804-6815. (17) Fisk, W. J.; Faulkner, D.; Palonen, J.; Seppanen, O. Performance and Costs of Particle Air Filtration Technologies. Indoor Air 2002, 12, 223-234. (18) Zhang, R.; Liu, C.; Hsu, P.-C.; Zhang, C.; Liu, N.; Zhang, J.; Lee, H. R.; Lu, Y.; Qiu, Y.; Chu, S. Nanofiber Air Filters with High Temperature Stability for Efficient Pm2. 5 Removal from the Pollution Sources. Nano Lett. 2016, 16, 3642-3649. (19) Shi, B.; Ekberg, L. Ionizer Assisted Air Filtration for Collection of Submicron and Ultrafine Particles Evaluation of Long-Term Performance and Influencing Factors. Environ. Sci. Technol. 2015, 49, 6891-6898. (20) Sim, K. M.; Park, H.-S.; Bae, G.-N.; Jung, J. H. Antimicrobial Nanoparticle-Coated Electrostatic Air Filter with High Filtration Efficiency and Low Pressure Drop. Sci. Total Environ. 2015, 533, 266-274. (21) Romay, F. J.; Liu, B. Y.; Chae, S.-J. Experimental Study of Electrostatic Capture Mechanisms in Commercial Electret Filters. Aerosol Sci. Technol. 1998, 28, 224-234. (22) Walsh, D.; Stenhouse, J. Parameters Affecting the Loading Behavior and Degradation of Electrically Active Filter Materials. Aerosol Sci. Technol. 1998, 29, 419-432.


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(23) Walsh, D.; Stenhouse, J. The Effect of Particle Size, Charge, and Composition on the Loading Characteristics of an Electrically Active Fibrous Filter Material. J. Aerosol Sci. 1997, 28, 307-321. (24) Baumgarrtner, H.; Loffler, F. The Collection Performance of Electret Filters in the Particle Size Range 10nm-10um. J. Aerosol Sci. 1986, 17, 438-445. (25) Wang, S.; Zhao, X.; Yin, X.; Yu, J.; Ding, B. Electret Polyvinylidene Fluoride Nanofibers Hybridized by Polytetrafluoroethylene Nanoparticles for High-Efficiency Air Filtration. ACS Appl. Mater. Interfaces 2016, 8, 23985-23994. (26) Liu, C.; Hsu, P.-C.; Lee, H.-W.; Ye, M.; Zheng, G.; Liu, N.; Li, W.; Cui, Y. Transparent Air Filter for High-Efficiency Pm2.5 Capture. Nat. Commun. 2015, 6, 6205. (27) Xu, J.; Liu, C.; Hsu, P.-C.; Liu, K.; Zhang, R.; Liu, Y.; Cui, Y. Roll-to-Roll Transfer of Electrospun Nanofiber Film for High-Efficiency Transparent Air Filter. Nano Lett. 2016, 16, 1270-1275. (28) Khalid, B.; Bai, X.; Wei, H.; Huang, Y.; Wu, H.; Cui, Y. Direct Blow-Spinning of Nanofibers on Window Screen for Highly Efficient Pm2. 5 Removal. Nano Lett. 2017, 17, 1140-1148. (29) Jing, L.; Shim, K.; Toe, C. Y.; Fang, T.; Zhao, C.; Amal, R.; Sun, K.-N.; Kim, J. H.; Ng, Y. H. Electrospun Polyacrylonitrile–Ionic Liquid Nanofibers for Superior Pm2. 5 Capture Capacity. ACS Appl. Mater. Interfaces 2016, 8, 7030-7036. (30) Zhao, X.; Li, Y.; Hua, T.; Jiang, P.; Yin, X.; Yu, J.; Ding, B. Cleanable Air Filter Transferring Moisture and Effectively Capturing Pm2. 5. Small 2017, 1603306.


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(39) Lee, H. M.; Choi, S.-Y.; Jung, A. Direct Deposition of Highly Conductive Aluminum Thin Film on Substrate by Solution-Dipping Process. ACS Appl. Mater. Interfaces 2013, 5, 4581-4585. (40) Lee, H. M.; Choi, S. Y.; Jung, A.; Ko, S. H. Highly Conductive Aluminum Textile and Paper for Flexible and Wearable Electronics. Angew. Chem. 2013, 125, 7872-7877. (41) Haber, J. A.; Buhro, W. E. Kinetic Instability of Nanocrystalline Aluminum Prepared by Chemical Synthesis; Facile Room-Temperature Grain Growth. J. Am. Chem. Soc. 1998, 120, 10847-10855. (42) Wang, C.-S. Electrostatic Forces in Fibrous Filters—a Review. Powder Technol. 2001, 118, 166-170. (43) Zhang, Q.; Jenkins, P. L. Evaluation of Ozone Emissions and Exposures from Consumer Products and Home Appliances. Indoor Air 2017, 27, 386-397. (44) Chen, T.-M.; Tsai, C.-J.; Yan, S.-Y.; Li, S.-N. An Efficient Wet Electrostatic Precipitator for Removing Nanoparticles, Submicron and Micron-Sized Particles. Sep. Purif. Technol. 2014, 136, 27-35. (45) Han, B.; Kim, H.-J.; Kim, Y.-J.; Sioutas, C. Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers. Aerosol Sci. Technol. 2008, 42, 793-800. (46) Lawless, P. A. Particle Charging Bounds, Symmetry Relations, and an Analytic Charging Rate Model for the Continuum Regime. J. Aerosol Sci. 1996, 27, 191-215. (47) Sigmond, R. Simple Approximate Treatment of Unipolar Space‐Charge‐Dominated Coronas: The Warburg Law and the Saturation Current. J. Appl. Phys. 1982, 53, 891-898. (48) Kim, S.; Sioutas, C.; Chang, M. Electrostatic Enhancement of the Collection Efficiency of Stainless Steel Fiber Filters. Aerosol Sci. Technol. 2000, 32, 197-213.


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Figure 1. Fabrication process of Al-coated conductive fibrous filters. (a) Scheme of the solutiondipping (SD) process for the preparation of the conductive fibrous filters. (b) Mechanism of the generation and deposition of Al during the SD process. (c) Photograph (left) and SEM image (right) of a raw polyester filter. (d) Photograph (left) and SEM image (right) of an Al-coated polyester filter. (e) EDS mapping images of the SD-processed fiber for carbon (C; green dots) and aluminum (Al; red dots). The inset shows a magnified image of the microstructure of the SD-processed fiber. (f) A tilted-view SEM image of cross-stacked fibers following the SD process. (g) Demonstration of an A4 size conductive filter with high porosity.


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Figure 2. Electromechanical and oxidative stability. (a) Sheet resistance (RS) of the conductive fibrous filters according to the immersion time in the Al precursor ink. Insets show the photographs of the conductive filters prepared at the different immersion times. (b) Plot of the relative change in the sheet resistance (∆RS/RS0) of the conductive filter under a cyclic bending test with a bending radius of 5 mm. The inset shows a flexed conductive filter mounted on a translational motion stage. (c) Plot of ∆RS/RS0 of the conductive filter bent under various bending radii. The inset shows a photograph of an illuminated LED lamp mounted onto the conductive filter wrapped around a highlighter pen. (d–f) Photographs of (d) the folding test, (e) adhesion test, and (f) rubbing test. (g) Bar graph of RS of the conductive filters before and after sonication for 60 min in various solvents.


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Figure 3. Particle removal performance of Al-coated conductive fibrous filters. (a) A sketch illustrating an electrostatic air filtration based on the conductive fibrous filters. Aerosol PMs acquire negative ions produced by the carbon fiber ionizer, and then the PMs are captured on the charged fibers by the Coulombic attraction. (b) Bar graph showing the significant improvement in the removal efficiency (η) of the conductive filters by the electrostatic effect. (c,d) Plot of the fractional removal efficiency of the conductive filter as a function of the particle diameter: (c) effect of the applied voltage to the front conductive filter at a given airflow velocity (u) of 10 cm s−1, and (d) effect of airflow velocity at a constant applied voltage of 5 kV.


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Figure 4. Numerical simulation of electrostatic capture of charged particles with a diameter (dp) of 300 nm. (a) Calculated removal efficiency curves as a function of the applied voltage at different airflow velocities. (b) Removal efficiency bar graph according to the flow rate at a constant applied voltage of 5 kV. (c) Motion trajectories of neutral particles (i) when electric fields are not formed near the single fiber and (ii–iv) when the electric potential of the single fiber is (ii) 1 kV, (iii) 5 kV, or (iv) 10 kV.


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Figure 5. (a) Pressure drop curves across the conductive fibrous filter (red line) and a commercial HEPA filter (blue line). (b) Performance comparison between the conductive filter and a commercial HEPA filter considering the removal efficiency and pressure drop simultaneously. Curves correspond to the quality factor (QF) of 6.0, 0.6, and 0.06 Pa−1, respectively. (c,d) SEM images of the filter surface on which particles were captured and the microstructure of the particle agglomerates deposited on the fibers: (c) the HEPA filter; (d) the Al-coated conductive fibrous filter.


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Figure 6. Long-term performance and recyclability. (a) Changes in the removal efficiency and pressure drop (∆P) according to the filtration time of the conductive filter. (b) Monitoring of the particle removal performance over 10 washing cycles. Inset photos show the conductive filter before filtration, after filtration, after being cleaned with DI water in an ultrasonic bath for 10 min, and after refiltration, respectively.


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Table 1. Dust holding capacity (DHC) of the Al-coated conductive fibrous filter and the HEPA filter. Sample

∆Pi (Pa)a

ηi (%)b

∆Pf (Pa)c

ηf (%)d

DHC (µg mm−2)

Al-coated fibrous filter

0.6

99.9961

1.9

99.9959

12.5

Commercial HEPA filter

13.2

99.9708

40.2

99.9882

3.4

a

Initial pressure drop across a sheet of each filter; bInitial particle removal efficiency for particles with sizes in the range of 30–400 nm; cFinal pressure drop across a sheet of each filter; dFinal particle removal efficiency for particles with sizes in the range of 30–400 nm.


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Table of Contents Graphic


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Figure 1. Fabrication process of Al-coated conductive fibrous filters. (a) Scheme of the solution-dipping (SD) process for the preparation of the conductive fibrous filters. (b) Mechanism of the generation and deposition of Al during the SD process. (c) Photograph (left) and SEM image (right) of a raw polyester filter. (d) Photograph (left) and SEM image (right) of an Al-coated polyester filter. (e) EDS mapping images of the SD-processed fiber for carbon (C; green dots) and aluminum (Al; red dots). The inset shows a magnified image of the microstructure of the SD-processed fiber. (f) A tilted-view SEM image of cross-stacked fibers following the SD process. (g) Demonstration of an A4 size conductive filter with high porosity. 105x69mm (600 x 600 DPI)


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Figure 2. Electromechanical and oxidative stability. (a) Sheet resistance (RS) of the conductive fibrous filters according to the immersion time in the Al precursor ink. Insets show the photographs of the conductive filters prepared at the different immersion times. (b) Plot of the relative change in the sheet resistance (∆RS/RS0) of the conductive filter under a cyclic bending test with a bending radius of 5 mm. The inset shows a flexed conductive filter mounted on a translational motion stage. (c) Plot of ∆RS/RS0 of the conductive filter bent under various bending radii. The inset shows a photograph of an illuminated LED lamp mounted onto the conductive filter wrapped around a highlighter pen. (d–f) Photographs of (d) the folding test, (e) adhesion test, and (f) rubbing test. (g) Bar graph of RS of the conductive filters before and after sonication for 60 min in various solvents. 86x47mm (600 x 600 DPI)



Figure 3. Particle removal performance of Al-coated conductive fibrous filters. (a) A sketch illustrating an electrostatic air filtration based on the conductive fibrous filters. Aerosol PMs acquire negative ions produced by the carbon fiber ionizer, and then the PMs are captured on the charged fibers by the Coulombic attraction. (b) Bar graph showing the significant improvement in the removal efficiency (η) of the conductive filters by the electrostatic effect. (c,d) Plot of the fractional removal efficiency of the conductive filter as a function of the particle diameter: (c) effect of the applied voltage to the front conductive filter at a given airflow velocity (u) of 10 cm s−1, and (d) effect of airflow velocity at a constant applied voltage of 5 kV. 105x79mm (600 x 600 DPI)


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Figure 4. Numerical simulation of electrostatic capture of charged particles with a diameter (dp) of 300 nm. (a) Calculated removal efficiency curves as a function of the applied voltage at different airflow velocities. (b) Removal efficiency bar graph according to the flow rate at a constant applied voltage of 5 kV. (c) Motion trajectories of neutral particles (i) when electric fields are not formed near the single fiber and (ii–iv) when the electric potential of the single fiber is (ii) 1 kV, (iii) 5 kV, or (iv) 10 kV. 131x123mm (600 x 600 DPI)



Figure 5. (a) Pressure drop curves across the conductive fibrous filter (red line) and a commercial HEPA filter (blue line). (b) Performance comparison between the conductive filter and a commercial HEPA filter considering the removal efficiency and pressure drop simultaneously. Curves correspond to the quality factor (QF) of 6.0, 0.6, and 0.06 Pa−1, respectively. (c,d) SEM images of the filter surface on which particles were captured and the microstructure of the particle agglomerates deposited on the fibers: (c) the HEPA filter; (d) the Al-coated conductive fibrous filter. 85x46mm (600 x 600 DPI)


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Figure 6. Long-term performance and recyclability. (a) Changes in the removal efficiency and pressure drop (∆P) according to the filtration time of the conductive filter. (b) Monitoring of the particle removal performance over 10 washing cycles. Inset photos show the conductive filter before filtration, after filtration, after being cleaned with DI water in an ultrasonic bath for 10 min, and after refiltration, respectively. 117x97mm (600 x 600 DPI)



Table of Contents 39x19mm (600 x 600 DPI)


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