A schematic illustration of the fabrication process for the ILP@MF multilayer porous air filter is shown in Fig. 1a. A solution of [C n mim][OAc] was added to the ethanol solution of the pre-dispersion polymer under stirring conditions. Then, the MF sponges were immersed in the prepared ILP solution. Vacuum pumping was employed to remove the entrapped ethanol and gas in the sponge. More details of the methods employed are included in the “Methods” section near the end of the paper. Figure 1b illustrates photographs and scanning electron microscopy (SEM) images of the bare MF sponge and ILP@MF filter. Compared with the initial sponge, the ILP composite films are generated and irregularly distributed on the multi-layered scaffold of the MF sponge. Interestingly, only a portion of the holes are coated by the ILP composite on each single layer, as seen in the schematic diagram and the magnified SEM images of the ILP@MF filter. As the illustration shows in Fig. 1c, this specific structure can allow the polluted air to flow adequately in the sponge channels, and the particles are fully in contact with the ILP composite, which may lead to low air resistance and efficient capture.

Fig. 1: Fabrication method and purification model of ILP@MF filter. a Schematic representation of the dipping-coating progress for the fabrication of the ILP@MF filter. b Photographs and SEM images of the MF sponge (red) and the ILP@MF filter (blue). c Illustration of the removal mechanism of the multilayer ILP@MF filter. Full size image

Fourier transform infrared (FTIR) spectra and powder X-ray diffraction (PXRD) patterns (Fig. 2a–d) were used to characterize the structure of the pure polymers, [C 4 mim][OAc] and ILP composites with different amounts of IL (m IL /m polymer = 0.2, 0.5 and 1). In the FTIR spectra, the strong absorption peak at 1564 cm−1 belongs to the stretching vibration of the C = N group of the [C 4 mim] cation, which enhances as the content of [C 4 mim][OAc] increases. The characteristic peaks of PAM, PVA and PVP are observed in the spectra of the ILP composites at 1061, 1723 and 1648 cm−1, respectively. FTIR spectra of different ILs and polymers are examined (Supplementary Fig. 1), and similar peaks were obtained for the ILP composites. PXRD data reveal that the diffraction peaks of [C 4 mim][OAc]–PAM are sharp and intense, indicating its highly crystalline nature (Fig. 2d). The ILPs consisting of PVA and PVP are amorphous (Supplementary Fig. 2). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results illustrate that all the ILP composites are thermodynamically stable, with decomposition temperatures higher than 205 °C (Fig. 2e, f).

Fig. 2: Basic properties characterization of ILP composites. FTIR specra of the ILP composites consisting of [C 4 mim][OAc] and a PAM, b PVA, c PVP with different amounts of ILs. d PXRD patterns of the ILP composites consisting of [C 4 mim][OAc] and PAM. e TGA and f DTA curves of the ILP composites with different polymers. g Water contact angle of the ILP composites consisting of [C 4 mim][OAc] and different polymers with different amounts of ILs. h Time dependence of the contact angles of water droplets on the different ILP films. Source data are provided as a Source Data file. Full size image

Generally, PM in air contains a large amount of atmospheric moisture, and the moisture acts as an adhesive to all metal ions and fine organic carbon, forming a complex system3,37. Therefore, good hydrophilicity is beneficial for the affinity of composites towards PM complexes38. The hydrophilicity of the ILP composites with different amounts of [C 4 mim][OAc] was estimated based on the water contact angle, and the results are presented in Fig. 2g. The water contact angle of the pure PVP film is ca. 81°, while a water contact angle of ca. 66° is obtained for [C 4 mim][OAc]–PVP (0.2). With increasing amounts of inserted [C 4 mim][OAc], the water contact angle values exhibit a downward trend, decreasing from ca. 50° ([C 4 mim][OAc]–PVP (0.5)) to ca. 41° ([C 4 mim][OAc]–PVP (1)). When more [C 4 mim][OAc] was added, we found that the ILP composites were not case hardened, resulting in the inability to form a filter. Moreover, similar decreases in the water contact angle are observed in the ILP composites of PAM and PVA. The water droplet has a time-dependent decrease and finally infiltrates the film (Fig. 2h). The surface free energy for the ILP composites above was calculated by the Owens two-zone mechanism of adhesion (Supplementary Table 1), and the results coincide with the contact angle experiments. The conclusion above indicates that the ILP composites are sufficiently hydrophilic materials, which are predicted to remove particles in air.

Due to the outstanding hydrophilicity that the ILP composites exhibit, the PM removal performance of different ILP@MF filters was tested. Figure 3a shows a schematic diagram of the designed testing device that investigates the PM filtration performance. The flow rate, pressure drop (ΔP) and concentration of PM were determined by commercial detectors. The removal efficiency (η) was calculated using the following equation:

$$\eta = \left( {1 - \frac{{C_{{\mathrm{out}}}}}{{C_{{\mathrm{in}}}}}} \right) \times 100{\mathrm{\% }}$$ (1)

where C in and C out are the mass concentrations (μg m−3) of the particles before and after removal, respectively39,40.

Fig. 3: Particulate matter purification test and performance comparison among different ILP@MF filters. a Schematic diagram of the device for the removal experiment and the measurement of air flow rate, pressure drop, and PM concentrations. b Removal efficiencies of the ILP@MF filters with different ILs, polymers, and amounts of [C 4 mim][OAc]. Error bar represents the standard deviation of three replicate measurements. c ESP-mapped molecular vdW surface of the simplified PVP, [C 4 mim][OAc]–PVP and the surface area in each range of the [C 4 mim][OAc]–PVP. Source data are provided as a Source Data file. Full size image

We first examined the removal efficiencies of ILP@MF filters with ILP composites consisting of PVP and different ILs (Fig. 3b). The average PM 2.5 removal efficiencies for the ILP@MF filters consisting of [C 4 mim][OAc], [C 6 mim][OAc] and [C 8 mim][OAc] are 98.78 ± 0.42%, 97.53 ± 0.51% and 96.21 ± 0.53%, respectively. For PM 10 , the removal efficiencies are 99.02 ± 0.48% for [C 4 mim][OAc], 98.64 ± 0.41% for [C 6 mim][OAc] and 97.47 ± 0.44% for [C 8 mim][OAc]. It is clear that the shorter the length of the alkyl group is, the better the performance in the removal test. We further investigated the filtration performance among the ILP@MF filters with three kinds of polymers. Compared with ILs, the variety of polymers has a greater impact on the removal ability of ILP@MF filters, and the removal efficiencies of the PAM, PVA and PVP composites are 92.14 ± 0.63%, 86.27 ± 0.61% and 98.81 ± 0.57% for PM 2.5 and 94.98 ± 0.59%, 88.31 ± 0.57% and 99.09 ± 0.49% for PM 10 , respectively. The results demonstrate that the PVP-based filter possesses the best removal ability of the tested polymers, which can be attributed to the differences in the morphology and hydrophilicity of the filters (Fig. 2g and Supplementary Fig. 3). Thus, the ratio of components of [C 4 mim][OAc]–PVP (0.2, 0.5 and 1) was discussed in the PM removal tests. Among them, [C 4 mim][OAc]–PVP (1) shows the highest removal efficiency of 98.77 ± 0.46% for PM 2.5 and 99.07 ± 0.54% for PM 10 .

To further study the removal mechanism of the composite, we explored the surface ESP of the PVP and [C 4 mim][OAc]–PVP composite by the density functional theory (DFT) method, where a high absolute ESP value indicates a great influence on the motion of PM36,41,42,43. Figure 3c depicts the ESP-mapped van der Waals (vdW) surface of the simplified structure of PVP and [C 4 mim][OAc]–PVP. Compared with pure PVP, [C 4 mim][OAc]–PVP exhibits a higher global maximum and minima for ESP on the surface at +48.72 and −62.95 kcal mol−1, which are electrostatic enhancements of 166.08% for the maximum and 37.24% for the minima, respectively. In addition, [C 4 mim][OAc]–PVP shows a vast vdW surface area (73.6 A2) featuring high absolute ESP values (<−30 and >+30 kcal mol−1), which are much larger than those of pure PVP (13.7 A2). The above results imply that the ILP@MF filter of [C 4 mim][OAc]–PVP has the greatest potential for air filtration applications.

The admirable properties and performance of the [C 4 mim][OAc]–PVP@MF filter inspired us to investigate its capture capacity after applying a voltage. The electrochemical stability of the [C 4 mim][OAc]–PVP composite was first researched by cyclic voltammetry (CV) (Supplementary Fig. 5). A wide electrochemical window higher than 3.1 V is observed, indicating that the [C 4 mim][OAc]–PVP composite is a stable material under an applied voltage of 3 V. Electrochemical impedance spectroscopy (EIS) was used to measure the charge transport parameters of the composite, such as the sheet resistance (R S ) and charge conductivity (σ). The Nyquist plots of the pure [C 4 mim][OAc] and [C 4 mim][OAc]–PVP composite are presented (Supplementary Fig. 6). [C 4 mim][OAc], whose initial R S value is 5.8 Ω sq−1, offers good conductivity to the composites with an R S of 13.9 Ω sq−1. Furthermore, we define the conductive ability of the [C 4 mim][OAc]–PVP composite by calculating the σ value according to the equation below:

$$R_{\mathrm{S}} = \frac{L}{{A\sigma }}$$ (2)

where A presents the active film area and L presents the thickness44. Pure PVP has poor conductivity with a σ value45 lower than 10−8 mS m−1. Compared with PVP, a conductivity value of 1.03 × 102 mS m−1 was obtained for the [C 4 mim][OAc]–PVP composite. Therefore, the [C 4 mim][OAc]–PVP composite is a desirable conductive material that can produce a high electric field to capture PM, especially NPs.

Figure 4a shows a sketch of the filtration test with an applied voltage. The variable applied voltage and electric field distribution are provided by a numerical control transformer. Two pieces of copper electrodes were inserted at the ends of the [C 4 mim][OAc]–PVP@MF filter, so that the [C 4 mim][OAc]–PVP composites distributed on the sponge can contact the electrode surface to form a closed loop. Figure 4b shows the fractional removal efficiencies under various applied voltages from 0 to 3 V. Compared to the uncharred [C 4 mim][OAc]–PVP@MF filter, the removal efficiencies of PM 2.5 increased as the applied voltage increased to 3 V (99.59 ± 0.31%). Similar phenomena were also obtained for PM 10 , whose removal efficiency increased from 99.01 ± 0.23% (0 V) to 99.75 ± 0.22% (3 V). Notably, an obvious improvement in the capture for NPs is observed, as the green points show. When the imposed voltage is 0 V, the removal efficiency of the uncharged [C 4 mim][OAc]–PVP@MF filter for NPs is 35.12 ± 0.69%, which can be attributed to the great hydrophilicity and ESP values of the [C 4 mim][OAc]–PVP composite. With the increase in the applied voltage, the removal efficiencies exhibit a significant enhancement. The removal efficiency of NPs can be up to 93.77 ± 1.05% when the imposed voltage rises to 3 V. Furthermore, the removal results of PM were confirmed by SEM and dynamic light scattering (DLS). SEM images show that the [C 4 mim][OAc]–PVP@MF filter changes significantly compared to the original, with agglomerated PM 2.5 and PM 10 particles deposited on the surface of the filter (Fig. 4c, red and blue frames). With a closer look, the nano-sized particles with nonuniform diameters ranging from dozens to hundreds of nanometres are adhered on the surface of the filter film (Fig. 4c, green frame). The DLS curves further prove the removal efficiency of the proposed filter for NPs in a wide size range (Supplementary Fig. 7). The composition of the filter after the removal test was probed by energy dispersive spectroscopy (EDS) spectra and mapping images (Supplementary Figs. 8 and 9). Compared to the original filter (C, O and N), some other metallic elements, such as Ca, Fe, K and non-metallic elements, such as Si, P, and S, are found, which means that particles derived from burned cigarettes are effectively captured. The observations above demonstrate that applying a voltage to the filter is beneficial for removing PM, particularly NPs.

Fig. 4: Performance enhancement of ILP@MF filter after applying voltage. a Schematic of the charged [C 4 mim][OAc]–PVP@MF filter. b Efficiency points of [C 4 mim][OAc]–PVP@MF filter for NPs, PM 2.5 and PM 10 under different voltage. c SEMs of the [C 4 mim][OAc]–PVP@MF filter after a 15 h removal experiment. Error bar represents the standard deviation of three replicate measurements. d 15 h test of the removal efficiency for PM 2.5 and NPs of the [C 4 mim][OAc]–PVP@MF filter. The inset images are photos of the [C 4 mim][OAc]–PVP@MF filter during the purification test. e Pressure drop of the [C 4 mim][OAc]–PVP@MF filter under various applied voltage (0, 1, 2, and 3 V) and at different air flow rate (0.3, 0.5, and 1 m s−1). Source data are provided as a Source Data file. Full size image

Then, we measured the removal performance of the [C 4 mim][OAc]–PVP@MF filter for 15 h under an applied voltage of 3 V. As shown in Fig. 4d, the removal efficiencies for PM 2.5 and NPs remained at high levels of over 97% and 92%, respectively, after the filtration test. Moreover, the removal efficiencies of the [C 4 mim][OAc]–PVP@MF filter had no significant change as the air flow rate increases from 0.3 to 1 m s−1. The photographs of the filters during the purification test are shown in the inset of Fig. 4d. After 15 h of PM removal, the colour of the [C 4 mim][OAc]–PVP@MF filter changed from white to brown, indicating uptake of the particles in polluted air. The pressure drop (ΔP) is one of most important parameters in the performance of a filter. A low ΔP value indicates a desirable air flow resistance. The pressure drop between the inlet and outlet of the [C 4 mim][OAc]–PVP@MF filter was measured under various applied voltages and flow rate conditions (Fig. 4e). The average pressure drop of the filter with a 3 V voltage is only 26 Pa at a flow rate of 0.3 m s−1, which is far lower than the fine standard of the U.S. Department of Energy (ca. 325 Pa at an air flow rate of 5 cm s−1)46.

The filter consisting of ILP composites and MF sponges can be simply regenerated. The regeneration procedure for the ILP@MF filter is illustrated (Supplementary Fig. 10). The images after regeneration and the removal efficiencies of the regenerative [C 4 mim][OAc]–PVP@MF filter are shown in Fig. 5a. A conspicuous colour variation is noticed in the inserted photographs before and after regeneration, owing to cleaning of the surface covered by dusty particle pollutants. The SEM images further demonstrate that almost all the particles are successfully removed by the regeneration process. Furthermore, the regenerated [C 4 mim][OAc]–PVP@MF filter still maintains a high level capture ability with removal efficiencies for NPs of over 90% and over 97% for PM after being recycled 10 times, indicating that the [C 4 mim][OAc]–PVP@MF filter possesses a reliable regeneration capacity.

Fig. 5: Renewability test and filtration performance comparison of ILP@MF masks and commercial face masks. a Removal efficiency of the charged [C 4 mim][OAc]–PVP@MF filter regerated for 1–10 times. The inset images are photo of the filter before and after regeration. b The photographs of wearable and self-powered face mask made from [C 4 mim][OAc]–PVP@MF filter. The button lithium cell and the silicon-based solar panel are replaceable to each other. c Comprehensive index comparison including removal efficiency, pressure drop, and quality factor between masks made of [C 4 mim][OAc]–PVP@MF filter (black lines and pillars) and commercial-PP filter (red lines and pillars) under different flow rate. Error bar represents the standard deviation of three replicate measurements. d Long-term PM removal performance. Source data are provided as a Source Data file. Full size image

A wearable and self-powered face mask consisting of a [C 4 mim][OAc]–PVP@MF filter was designed and fabricated, as depicted in Fig. 5b. Benefiting from the excellent removal properties of the [C 4 mim][OAc]–PVP@MF filter after applying a voltage, a 3 V button lithium cell was installed on the face mask as a portable power supply platform to obtain a self-powered system. The employed voltage of 3 V, which is below the secure voltage (12 V) required by the International Electrotechnical Commission47, is considered to be absolutely harmless to the human body. The quality factor (QF) is calculated according to the equation below:

$${\mathrm{QF}} = \frac{{ - \ln (1 - \eta )}}{{{\mathrm{\Delta }}P}}$$ (3)

where η represents the removal efficiency and ΔP represents the pressure drop48,49. Generally, a higher QF value indicates better behaviour of a filtration device (i.e., higher removal efficiency and lower pressure drop)36,40. The filtration performance of the face masks based on the [C 4 mim][OAc]–PVP@MF filter and the commercial PP filter were compared under high concentration particulates (above 300 μg m−3)39, whose diameters ranged from dozens of nanometres to tens of micrometres. Figure 5c shows a comparison of the comprehensive index between the [C 4 mim][OAc]–PVP@MF filter and commercial PP filter. Both the [C 4 mim][OAc]–PVP@MF filter and the commercial filter exhibit excellent removal performance for PM with the highest efficiencies being over 99%. However, the commercial PP filter suffers from a drastically increasing pressure drop with an increase in the air flow rate, leading to low QF values (0.030 Pa−1 at 0.3 m s−1 and 0.006 Pa−1 at 1 m s−1). The mask based on [C 4 mim][OAc]–PVP@MF emerges as desirable with average pressure drops (28 Pa at 0.3 m s−1 and 76 Pa at 1 m s−1), which results in admirable QF values (0.178 Pa−1 at 0.3 m s−1 and 0.063 Pa−1 at 1 m s−1). Interestingly, the button lithium cell that supplied the voltage source for the mask can be replaced by a silicon-based solar panel. The mask consisting of solar panel performs well as the mask made with the button cell (see Supplementary Fig. 11 for details), which means that the face mask can directly harness solar energy to realize PM purification without other power sources. Moreover, a switch is devised and inserted at the bottom of the mask to provide users the ability to employ the mask according to the actual atmosphere. When people are in a mild pollution environment, the switch-off mode can be chosen; when people are in a heavily polluted environment, the switch-on mode can be selected. Thus, the service life of the mask can be prolonged.