Removal of Particulate Matter Emissions from a Vehicle Using a Self

Chang Bao Han,† Tao Jiang,† Chi Zhang,† Xiaohui Li,† Chaoying Zhang,† Xia Cao,*,†,‡ and Zhong Lin Wang*,†,§

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Removal of Particulate Matter Emissions from a Vehicle Using a Self-Powered Triboelectric Filter †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China, ‡School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China, and §School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 United States

ABSTRACT Particulate matter (PM) pollution from automobile exhaust has

become one of the main pollution sources in urban environments. Although the diesel particulate filter has been used in heavy diesel vehicles, there is no particulate filter for most gasoline cars or light-duty vehicles because of high cost. Here, we introduce a self-powered triboelectric filter for removing PMs from automobile exhaust fumes using the triboelectrification effect. The finite element simulation reveals that the collision or friction between PTFE pellets and electrodes can generate large triboelectric charges and form a space electric field as high as 12 MV/m, accompanying an open-circuit voltage of ∼6 kV between the two electrodes, which is comparable to the measured value of 3 kV. By controlling the vibration frequency and fill ratio of pellets, more than 94% PMs in aerosol can be removed using the high electric field in the triboelectric filter. In real automobile exhaust fumes, the triboelectic filter has a mass collection efficiency of ∼95.5% for PM2.5 using self-vibration of the tailpipe. KEYWORDS: triboelectrification . particulate matter . exhaustion . electrostatic effect . collection efficiency

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long with the development of an economy, urban environment pollution is attracting more and more attention.1,2 Particulate matter (PM) has become one of the air pollutants in urban environments, and the haze is mainly formed by PM2.5 (diameter of 2.5 μm or less), which results in serious health issues such as cardiovascular and respiratory diseases, inducing acute symptoms, chronic diseases, or even mortality.3
7 Especially, with the rapid growth in the number of cars, automobile exhaust fumes have gradually become one of the main sources of PM in urban areas. In order to reduce PM emissions, different diesel particulate filters (DPF), such as wall-flow ceramics filter,8,9 solid foam filter,10,11 and fiber filter,12 have been fabricated. To improve the collection efficiency, one of the effective ways is to increase the specific surface area of the filtering material. Thus, most of DPFs have a honeycomb or porous structure, but that has the drawback of increased back pressure and leads to HAN ET AL.

lower fuel efficiency and reduced driving force.13 Meanwhile, the high cost for hightemperature catalysts and low removal efficiency for low PM concentration of emission limit their applications only in powerful engines rather than in economical and enormous lightweight vehicles (such as gasoline cars), which counting a majority of traffic. Another particle filtering technology, electrostatic precipitation or adsorption, has been widely used in industry to remove suspended particles with the advantages of low pressure drop and high collection efficiency.14
19 However, supplying a high voltage requires the consumption of energy, which adds additional cost to the vehicle cleaning of PM. Recently, a triboelectric nanogenerator (TENG) has been invented to generate electricity by harvesting wind, water, or vibration energy from the environment.20
24 Based on the triboelectric and electrostaticinduction effects, an alternating charge flow can be produced in an external load to form a sustainable power source.25 A typical VOL. XXX

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* Address correspondence to [email protected], [email protected]. Received for review October 8, 2015 and accepted November 11, 2015. Published online 10.1021/acsnano.5b06327 C XXXX American Chemical Society

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RESULTS Concept and Mechanism of a Triboelectric Filter. As a particle collector, the triboelectric filter can remove the aerosol particles effectively using electrostatic effects, where the high electrostatic fields are formed by the triboelectric charges without supplying electric power. The device is mainly composed of PTFE pellets and two parallel aluminum electrode plates fixed on an insulated cube-shaped chamber, as shown in Figure 1A. When a vibration in the vertical direction is applied on the chamber, the PTFE pellets collide up and down with the two electrode plates by virtue of inertia, which forms triboelectric charges on the surface of the PTFE pellets. Because of the high electronegativity of PTFE, electrons are injected first from one aluminum electrode into PTFE pellets, resulting in negative electrostatic charges at the surface of PTFE pellets and positive charges at aluminum electrode plates (Figure 1B). Then the pellets approach and collide with another electrode plate, and the positive charges will flow from one electrode to the other electrode, forming a current when the two electrodes are electrically connected (Figure 1C). In open-circuit situation, the collision will also lead to electron transfer generating net positive charges on the two electrodes, and different distances between the pellets and electrodes will bring a HAN ET AL.

potential difference. A continuous vibration can cause a gradual accumulation of triboelectric charges on PTFE pellets and form a space electric field. The surface charge density reaches a saturation after some cycles of contacting collisions (Figure 1D). The overall process of electricity generation is shown in Figure S1 of the Supporting Information. If there is no leakage or discharge, the total number of negative charges on PTFE equals the total number of positive charges on the two electrodes. A pair of high electric fields with opposed directions will be built between PTFE pellets and two electrode plates. With the change of pellet position caused by vibration in the vertical direction as driven by the turbulance of the flowing gas or the natural vibration of the engine, the intensities of the two electric fields vary alternately if the movements of the pellets are synchronized. For the triboelectric filter, the main mechanism for particle removal includes the mechanical filtration and electrostatic filtration. When suspended particles flow through the triboelectric filter, various forces, such as van der Waals force, capillary force, and/or gravity, will attract the particles to deposit on the surface of pellets or granules and form mechanical filtration.30 The corresponding filtration theory includes interception, inertial impaction, diffusion, and gravitational settling. When flue gases with the particles on the order of submicrons to several microns flow through the pellets or granules, the particles are rebounded or retained on the surface of granular materials for interception and inertial impaction, which can remove most of the large particles.31 For submicron particles, especially nanoparticles, Brownian diffusion may significantly affect the particle trajectories and improve the collision probability with granules so that the particles can be collected effectively.32,33 For most of the granular filters with a granule size of 1
5 mm, the average collection efficiency can be as high as 90% even if the electrostatic force is neglected.30,31 So the PTFE pellets with a diameter of 2 mm in the triboelectric filter have high mechanical fitering efficiency, which is similar to that in the granular bed filter.31 Electrostatic attraction is another strong force between charged particles and charged media, which has been widely used in electrostatic precipitators.34,35 In the electric field, the electrostatic force works on the charged particles in the whole space, and the effective distance is several orders of magnitude larger than that of the van der Waals force. The collision and friction between electrodes and PTFE pellets will generate a high electric field in the triboelectric filter. When the charged particles flow through the electric field, they will be attracted and deposited on the electrodes and/ or pellets by the electrostatic force, resulting in improved collection efficiency of PMs. Generally, about 60
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characteristic of the TENG is its high output voltage of several thousand volts for self-powered devices,26
29 which is promising for cleaning aerosol particles using electrostatic effects. In this work, a triboelectric filter was first designed to realize the effective removal of PMs in automobile exhaust without applying an external power. A vibration TENG (V-TENG) was fabricated to generate a high voltage using the triboelectrification between vibratory PTFE (polytetrafluoroethylene) pellets and electrode plates. A high electric field on the order of ∼MV/m was created between the PTFE pellets and electrodes, which is effective for enhancing the PM collection. A series of particles, PM0.5, PM1.0, PM2.5, PM5.0, and PM10 (in this work, they represent that the aerodynamic diameters of the particles are less than or equal to 0.5, 1.0, 2.5, 5.0, and 10 μm, respectively), were tested at different vibration frequencies and fill ratios (the fill ratio is defined as the area ratio of close-packed pellets to the electrode area), and the collection efficiency varied from 73 to 97%, which depended on the particle size. The automobile exhaust fumes were directly tested using the triboelectric filter, and ∼95.5% of PM2.5 and ∼97.2% of PM10 were removed by relying on the vibration of the tailpipe. Durability tests for the filter under the normal driving conditions of a conventional vehicle for more than 50 h show that the collection efficiency of PM 2.5 is still 82.4%. This work opens up an entirely new approach of cleaning PMs by a self-powered triboelectric device.

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ARTICLE Figure 1. Device design and working principle. (A) Three-dimensional structure of V-TENG. (B) Triboelectric charge distribution at the initial stage. (C) Current generation process when PTFE pellets move toward the top electrode in V-TENG. (D) Voltage generation process in V-TENG. (E) Sketch showing the triboelectric collection of particulate matters in V-TENG.

be easily collected by the filter for a high electric field. Once the neutral particles from the exhaust gas are injected into the chamber, they will be removed by mechanical filtration. Therefore, the triboelectric filter can realize the removal of PMs from exhaust effectively, and the relevant sketch is shown in Figure 1E. Voltage Generation of the Triboelectric Filter. To further clarify the electric potential and electric field distributions in the triboelectric filter, a theoretical model of V-TENG was established first for two motion modes of PTFE pellets and various fill ratios. First, two vibration modes, uniform-arranging and random-arranging of PTFE pellets, were constructed by COMSOL, which correspond to synchronous and asynchronous motions of pellets, respectively. At synchronous motion, when the PTFE pellets are located at the middle of V-TENG, the electric field direction between the pellets and the two electrodes is opposite (Figure 2A). The open-circuit voltage (VOC) between two electrodes linearly decreases with the gradual separation of PTFE pellets from the bottom metal electrode, whose direction is changed at the middle position of V-TENG (Figure 2B). A large potential difference of ∼6 kV between two electrodes is generated when the pellets are close to the bottom or top electrode. We assume that the positive electric field direction is upward and the device equates to a parallel capacitor. Here, the potential difference or VOC is proportional to (σ/ε)(2x þ d0
d), where d is the distance (m) between the electrodes, d0 is the diameter (m) of the pellet, σ is saturated density (C/m2) of the surface charge on the electrode, ε is dielectric constant of air, and x is the distance (m) between the pellets and the bottom HAN ET AL.

electrode. The maximum electric field (Emz) (absolute value) was also found to have a higher value of about 12 MV/m at the position close to two electrodes, and the minimum electric field is still more than 6 MV/m. At asynchronous motion, the distribution of electric field is asymmetric (Figure 2C). Under the assigned heights of PTFE pellets, the V-TENG has a very low VOC (∼168 V), implying that the motion mode has a significant effect on the VOC. However, the influence of the motion mode on the electric field is not significant, which can be found from the minor variation of Emz (Figure 2D). In brief, regardless if the vibration mode of the pellets is uniform or random, a high electric field can always be generated. The VOC and electric field distribution are provided for various fill ratios and are shown in Figure 2E). All of the PTFE pellets are located close to the bottom electrode, and a high electric field is generated above the upper surface of the bottom electrode. When the fill ratio is 60%, VOC and Emz have reached 3288.5 V and 7.3 MV/m, respectively (Figure 2E(a)). When the fill ratio increases, the electric field distribution and VOC change are shown in Figure 2E(b
e). Note that if the fill ratio exceeds 100%, the second layer of PTFE pellets is located above the first sphere layer, and a high negative electric field will be formed above the second layer. Figure 2E(f) depicts the calculated VOC and Emz, where the Emz improves gradually with the increased fill ratio, but the VOC has the maximum at the fill ratio of 100% and then decreases at a relatively low rate. Experimentally, a mechanical vibration in the vertical direction was applied to the V-TENG by a dynamoelectric vibrator, and the photographs of the V-TENG VOL. XXX

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ARTICLE Figure 2. Finite element simulation of V-TENG. (A) Schematic electric field distribution for synchronous motion of PTFE pellets. (B) Open-circuit voltage (VOC) and maximum electric field Emz in the Z-direction as functions of the distance x between the lowest point of each pellet and the upper face of the bottom electrode. (C) Schematic electric field distribution for asynchronous motion of PTFE pellets. (D) Electric field Emz versus distance x. (E) Schematic electric field distributions for various fill ratios: (a) 60%, (b) 80%, (c) 100%, (d) 120%, (e) 140%. The values of VOC and Emz at the position of each sphere are also marked. (f) VOC and Emz for various fill ratios.

and PTFE pellets are illustrated in Figure 3A. Figure 3B shows the measured VOC and short-circuit current (ISC) when the device has the fill ratio of 100% and vibrates at the frequency of 20 Hz. The output VOC and ISC change with an alternating feature, and the maximum VOC is beyond 1750 V, which means that the Emz will be several megavolts per meter in the V-TENG. The inset in Figure 3B clearly exhibits that about 150 light-emitting diodes (LEDs) are lighted up simultaneously. When the vibration frequency varies from 5 to 60 Hz, corresponding to the vibration frequency of an engine, the peakto-peak value of VOC remains almost constant at 3000 V before 20 Hz but decreases subsequently. However, the ISC increases to the peak of 90 μA at 20 Hz and then reduces with the frequency. According to the experiments, the vibration mode of PTFE pellets changes HAN ET AL.

from synchronous to asynchronous at the frequency of 20 Hz. Therefore, the invariable VOC occurs at the complete contact between pellets and electrode plates in the synchronous vibration, while the decreased VOC results from the partial contacts in the asynchronous vibration, which is consistent with numerical simulations. In addition, the maximum ISC corresponds to the shortest time of charge flow, which appears at the highest frequency in the synchronous vibration. When the fill ratio increases from 60 to 140% (the corresponding weights of PTFE pellets are shown in Table S1), both ISC and VOC increase rapidly and then decrease slowly, exhibiting an extremum at the fill ratio of 100%, which agrees with the simulation results. Lab-Scale Test of PM Removal. The collection efficiency, η, one of the important performance indexes for VOL. XXX

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ARTICLE Figure 3. Measured electric output of V-TENG. (A) Photograph of TENG. The inset is the corresponding vibration pellets (PTFE). (B) Short-circuit current (ISC) and VOC at the vibration frequency of 20 Hz. Inset: 150 LEDs are lighted up. (C) Effect of vibration frequency on the electric output of V-TENG. (D) Electrical properties measured at different fill ratios.

electrostatic precipitator or filter, is determined by the particle concentration of the inlet and outlet, as follows:18   Co  100% (1) η ¼ 1
Cin where Co is the cumulative mass concentration (μg/m3) of particles in the outlet (downstream) and Cin is the cumulative mass concentration (μg/m3) in the inlet (upstream). The triboelectric filter was fabricated by setting two gas passages, inlet (upstream) and outlet (downstream), at two sides of the V-TENG, as shown in Figure 4A. For example, ηPM5.0 = 94.5% is the ratio between the total mass lost and the total mass on the upstream for all the particles with a size less than or equal to 5.0 μm. In order to improve the space utilization, two deflectors with gas ports were introduced to guide the gas flow along the direction of the yellow arrowhead (Figure 4B). The cross-sectional area of gas passage was ∼28 mm2, and the average velocity of air with particulate materials was 2 m/s. The test particles were generated by continuous laser ablation of acrylic materials. Figure 4C shows the vibration frequency (f)-dependent collection efficiency of the triboelectric filter for different particle size ranges at the fill ratio of 100%. The standard deviation of the data is 1.9
3.27%. When f = 0, corresponding to the situation without vibration, the triboelectric filter has a collection efficiency of ∼65% for PM2.5; only 30% of PM0.5 was HAN ET AL.

removed, which indicated that the electric field in the filter dominated the removal efficiency of submicrometer particles. According to the filtering theory, the mechanical filtration may dominate the efficiency when the PTFE pellets are uncharged. Here, the collection efficiency is lower compared with that in the granular bed filter,31 which may result from the insufficient quantities of pellets. When the f of the triboelectric filter increases, the collection efficiency rises quickly and reaches the maximum at f = 20 Hz and then decreases slowly. Here, the maximum collection efficiency (ηmax) for PM2.5 is greater than 94%. Similarly, the ηmax happens at 20 Hz for different ratios and different particle size ranges (Figure 4D). In the triboelectric filter, the alternating voltage and high electric field yielded from the vibration of V-TENG will charge PMs and make them deposit on the surfaces of PTFE pellets and electrodes, leading to a high collection efficiency for the larger PMs. When the vibration frequency and fill ratio change, the ηmax also occurs at f = 20 Hz with a fill ratio of about 100%, which matches the maximum VOC of V-TENG. Evidently, the high electric field from the triboelectric charges caused by vibration is responsible for the high collection efficiency. If the particle concentration increases (Figure 4E), the collection efficiency continues to rise at a certain range of concentrations, which means that the particles can be better collected at a high particle concentration in our device. Considering the high VOL. XXX

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ARTICLE Figure 4. Lab-scale test of PM removal. (A) Photograph of triboelectric filter. (B) Schematic diagram of triboelectric filter (top view). (C,D) Collection efficiency change with different vibration frequencies and fill ratios at certain PM concentration (PM0.5 ∼ 15 μg/m3; PM1.0 ∼ 100 μg/m3; PM2.5 ∼ 350 μg/m3; PM5.0 ∼ 600 μg/m3; PM10 ∼ 700 μg/m3). (E) Collection efficiency change with different particle concentrations at the vibration frequency of 20 Hz and fill ratio of 100%.

output voltage, the high collection efficiency should be attributed to the electric field over several megavolts per meter caused by triboelectricity during vibration, which is comparable to some electrostatic precipitators.37 Field Test of PM Removal. Based on the particle removal performance in the laboratory, the field test for vehicle exhaust emission was carried out by attaching the triboelectric filter on a commercial vehicle and driving it on the street. Here, the triboelectric filter was fixed onto the tailpipe of different automobiles to test the collection efficiency of PMs in exhaust using the natural vibration of the tailpipe. Figure 5A presents a photograph of the triboelectric filter integrated on a tailpipe of an automobile, and the exhaust emitted from gasoline burning in the engine will enter the device directly by a tube (zoomed-in view in Figure 5A). Figure 5B exhibits a scanning electron microscopy HAN ET AL.

(SEM) micrograph of PTFE pellets before and after the collection of exhaust particles obtained by continuous vibration of a tailpipe for 30 h. Before the collection, the surfaces of PTFE are smooth with low conductivity. After being covered with exhaust particles, a coarse surface with irregular particles with sizes from 1 nm to several micrometers can be found, and the conductivity increases significantly, which is also confirmed by the dark color surface from the optical images of the pellets (inset of Figure 5B). The chemical compositions of the adsorbants on the surface were analyzed by energy-dispersive spectroscopy (EDS) in the SEM (Figure 5C). The result reveals that the contents of O and C increase notably, and large amounts of impurity elements such as N, Si, Na, and S are found, which verified that the organic carbon and elemental carbon particles in exhaust38 had been effectively removed by the triboelectric filter. VOL. XXX

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ARTICLE Figure 5. Field test of PM removal. (A) Photograph of a car with a triboelectric filter on the tailpipe. Inset: Zoomed-in image of the device setting. (B) Surface SEM images of PTFE vibration pellets before and after the collection of PMs. The insets are the corresponding optical photographs. (C) Corresponding EDS spectra. (D) Particle number-size distribution upstream and downstream of the filter and cumulative number distribution of particles for different size ranges. (E) Measured electrical low pressure impactor at an engine speed of 3000 r/min. (F) Collection efficiency with different PMs for idle speed mode and constant speed mode. (G) Life testing.

The particle number size distribution was measured by ELPI (electrical low pressure impactor) when the HAN ET AL.

engine was working at a constant speed of 3000 rpm. Figure 5D reveals the average size distributions at the VOL. XXX

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(Supporting Information Figure S2 and Figure S3). When the vibration of the filter stops and the pellets become packed closely, the simulation results reveal that it still has a high electric field (several MV/m) in the space of the voids, and thus the filter still keeps high collection efficiency in the real test (Supporting Information Figure S4 and Figure S5). A spraying regeneration was confirmed to be an effective way of removing PMs deposited on the surface of pellets and electrodes when the particle collection becomes saturated (Supporting Information Figure S6).

EXPERIMENTAL SECTION

Measurement of the Device. The mechanical vibration of V-TENG was produced by a dynamoelectric vibrator. The output current and voltage signals of the V-TENG were measured by a low-noise current preamplifier (SR570, Stanford Research Systems, USA) and digital oscilloscope (Agilent InfiniiVision 2000X, Agilent Technologies, USA), respectively. The particle gas was generated by laser ablation (PLS6.75, Universal Laser Systems, USA) of acrylic materials mixed with clean air and transferred through the triboelectric filter using a vacuum pump. For exhaust tests, the triboelectric filter with the fill ratio of 100% was fixed below the tailpipe of an automobile (Camry, gasoline engine type 1AZ-FE, displacement 2.0 L, Toyota, Japan). When the engine is working or the automobile is moving, the electrical output can be generated from the triboelectric filter by the natural vibration of the tailpipe, and the exhaust acts as the source of particle gas. To compare the collection efficiency, Kia motors (K2, gasoline engine type G4FA, displacement 1.4 L, Kia, South Korea) were also tested, and the results were similar to that received from a Toyota Camry. The morphologies of PTFE pellets were observed by a SEM (Quanta FEG450, FEI Company, Czech). The mass concentration of PM particles was tested in real time by a particle counter (hand-held 3016IAQ, Lighthouse,

Fabrication of the V-TENG. The V-TENG consists of a cubic insulating chamber, two parallel Al electrode plates, and PTFE pellets. The chamber was assembled by acrylic plates with the active dimensions of 140 mm  90 mm  9 mm for supporting electrodes and sealing the pellets. The aluminum foil (140 mm  90 mm  0.1 mm) was fixed in parallel on the two inner surfaces of the acrylic chamber as electrodes. PTFE pellets with the diameter of 2 mm were used to fill the chamber, forming vibration units. The values of the electric field in Figure 2B,D represent different Emz, which were marked in Figure 2A,C. Fabrication of the Self-Powered Triboelectric Filter. First, to guide the gas flow with a long route, three parallel deflectors with tiny gas ports (1.5 mm in aperture) at the end were fixed perpendicular to electrode plates in the V-TENG. Then, two gas ports, inlet and outlet, with an aperture of 8 mm were fabricated at the two sides of the V-TENG. In order to make sure that no PTFE particles leave the filter at the strong exhaust stream, two pieces of metal grids were fixed at the entrance and exit of the filter. The gas delivery from the source to the triboelectric filter was connected by a silastic tube.

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upstream and downstream of the triboelectric filter. The error bars, in all figures, represent the standard deviation from repeated measurements. Clearly, most particles from the gasoline engine are less than 290 nm in diameter, and the majority of them have a size of ∼20 nm, which agrees with the previous report.36 Combined with the accumulation mode (Figure 5E), more than 95% of particles were removed when the exhaust flowed through the triboelectric filter. Based on Maricq's work,36 60
80% of vehicle exhaust particles are electrically charged. Therefore, these charged nanoparticles are easily removed by the electric field in the filter. Figure 5F presents the efficiency as a function of particle size at different working modes of the automobile. The pie charts represent the total amount of particulate matter at different working conditions of the engine (constant speed and idle speed). The sections represent the percentages of all kinds of PMs, such as PM1.0, PM2.5, PM5.0, and PM10. As shown from the curves, whether in idle speed conditions or in constant speed conditions, more than ∼95.5% of PM2.5 can be removed. For example, at a certain PM2.5 mass concentration of 1000 μg/m3 for the idle working condition, a high removal efficiency of ∼97.6% was achieved (Supporting Information movie 1 and movie 2). The effectiveness of the triboelectric filter was tested by continuously driving a commercial vehicle on the street for an extended period of time. After the car was driven for 50 h, the collection efficiency of PM2.5 changes from ∼95.5 to ∼82.4% during such a long time of operation of the vehicle (Figure 5G). The pressure drop is an important performance for the triboelectric filter when it was used in vehicles. The measued results show that the pressure drops are less than 1.3 kPa at different engine speeds even if the PTFE pellets are accumulated with a close-packed structure

CONCLUSION In summary, we have demonstrated a new type of triboelectric filter based on V-TENG, which can remove particulate matter from automobile exhaust fumes using the natural vibration of the tailpipe. Except for the mechanical filtration, the electrostatic attraction plays a major role in removing particles from the exhaust. The finite element method (FEM) simulation reveals that a high voltage (∼6 kV) and a strong electric field (∼12 MV/m) were generated in the triboelectric filter by the collision between PTFE pellets and aluminum electrodes without applying external power. When the device continues vibrating, a more than 3000 V difference was obtained between the two electrodes. The lab-scale test shows that the collection efficiency of the particles is related to the vibration frequency of the device, fill ratio of PTFE pellets, and the particle concentration. Working toward the real automobile exhaust fumes, more than ∼95.5% of PM2.5 can be removed by the triboelectric filter using the natural vibration of the tailpipe, and it still has the collection efficiency of ∼82.4% after continuous working for 50 h. This work creates a new area of triboelectric precipitation, which will be widely used in selfpowered haze governance technology.

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Acknowledgment. The project is supported by Beijing Natural Science Foundation, China (Grant No. 4154090), the “Thousands Talents” program for Pioneer Researchers and Innovative Teams, China, and Beijing Municipal Committee of Science and Technology (Z131100006013004, Z131100006013005). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06327. Overall process of electricity generation, weight of PTFE pellets and fill ratio, pressure drop test, collection efficiency for packed pellets, and regeneration of the triboelectric filter (PDF) Movie 1 (AVI) Movie 2 (AVI)

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USA), and ELPI (ELPIþ, Dekati, Finland) was additionally used to measure real-time size distributions. FEM Simulation via COMSOL. A 3D model was utilized to calculate the electric potential and electric field distributions under open-circuit conditions. Schematically, the V-TENG has a geometric size of 6 mm  6 mm  3.5 mm. The PTFE pellets have a regular array with the same diameter of 1.2 mm. The surface density of triboelectric charges on the spherical PTFE surfaces was assigned as 40 μC/m2, and both the top electrode and bottom electrode possess one-half of total tribo-charges based on the charge conservation. At each metal electrode, the electric potential is the same, and the potential at infinity is zero. By simulating various fill ratios, every five PTFE pellets are arranged in line, and all pellets are symmetrical. After computation, values of electric potentials on electrodes and Z-direction electric fields were picked up to obtain the VOC and maximum electric field. Conflict of Interest: The authors declare no competing financial interest.

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33. Brown, R. C.; Shi, H.; Colver, G.; Soo, S.-C. Similitude Study of a Moving Bed Granular Filter. Powder Technol. 2003, 138, 201–210. 34. Mizuno, A. Electrostatic Precipitation. IEEE Trans. Dielectr. Electr. Insul. 2000, 7, 615–624. 35. Xu, X.; Gao, X.; Yan, P.; Zhu, W.; Zheng, C.; Wang, Y.; Luo, Z.; Cen, K. Particle Migration and Collection in a HighTemperature Electrostatic Precipitator. Sep. Purif. Technol. 2015, 143, 184–191. 36. Maricq, M. M. On the Electrical Charge of Motor Vehicle Exhaust Particles. J. Aerosol Sci. 2006, 37, 858–874. 37. Dastoori, K.; Kolhe, M.; Mallard, C.; Makin, B. Electrostatic Precipitation in a Small Scale Wood Combustion Furnace. J. Electrost. 2011, 69, 466–472. 38. Karjalainen, P.; Pirjola, L.; Heikkilä, J.; Lähde, T.; Tzamkiozis, T.; Ntziachristos, L.; Keskinen, J.; Rönkkö, T. Exhaust Particles of Modern Gasoline Vehicles: A Laboratory and an OnRoad Study. Atmos. Environ. 2014, 97, 262–270.

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