The adverse impact of particulate air pollution on human health1,2 has prompted the development of purification systems that filter particulates out of air3,4,5. To maintain performance, the filter units must inevitably be replaced at some point, which requires maintenance, involves costs and generates solid waste6,7. Here we show that an ion-doped conjugated polymer-coated matrix infiltrated with a selected functional liquid enables efficient, continuous and maintenance-free air purification. As the air to be purified moves through the system in the form of bubbles, the functional fluid provides interfaces for filtration and for removal of particulate matter and pollutant molecules from air. Theoretical modelling and experimental results demonstrate that the system exhibits high efficiency and robustness: its one-time air purification efficiency can reach 99.6%, and its dust-holding capacity can reach 950 g m−2. The system is durable and resistant to fouling and corrosion, and the liquid acting as filter can be reused and adjusted to also enable removal of bacteria or odours. We anticipate that our purification approach will be useful for the development of specialist air purifiers that might prove useful in a settings such as hospitals, factories and mines.
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This work was supported by the National Natural Science Foundation of China (grant nos. 52025132, 21975209, 21621091, 22021001 and 22121001), the National Key R&D Programme of China (grant no. 2018YFA0209500), the 111 Project (grant nos. B17027 and B16029) and the Science and Technology Projects of Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (grant no. RD2022070601). We thank L. Huang, L. Min, B. Chen, H. Liu, D. Wu and K. Zhan for their support and discussion of the synthesis and measurements of the materials.
The authors declare no competing interests.
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Extended data figures and tables
a. The photo of the electrochemical polymerization setup. b. Current density dependent electrochemical polymerization time. Inset: Charge density dependent electrochemical polymerization time. c. The optical images of doped PPy coated SSM at different polymerization times. d. SEM images of doped PPy coated SSM at different polymerization times.
a. Cyclic voltammetry (CV) curve of DPM in 0.1 M LiClO4 at a scan rate of 50 mV s−1. The curve shows the reversibility of the redox process with an oxidization peak at 0.12 V and a reduction peak at − 0.39 V. b. The X-ray photoelectron spectroscopy (XPS) survey scans of DPM in different states, which prove the matrix surface composition changes in the redox process. c, d. High-resolution Li 1s XPS spectra of DPM in the reduction state (c) and the oxidation state (d), which show the Li+ ions doping and dedoping during the redox processes. e, f. High-resolution N 1s XPS spectra of DPM in the reduction state (e) and the oxidation state (f). Four types of nitrogen environments are present in DPM. The higher peaks at 401.4 eV and 402.9 eV can be assigned to the positively charged –HN+– and bipolarons moieties48. The positively charged N moieties are higher in the oxidation state, which indicates that the redox reaction have occurred on the DPM.
a. The self-designed setup for in situ monitoring the contact angle of a dichloromethane droplet on the DPF during adjusting the electrochemical potentials underwater. A three-electrode system was used to control the electrochemical potential. DPF was served as the working electrode. Ag/AgCl electrode and a platinum plate were used as the reference electrode and the counter electrode, respectively. 0.1 M LiClO4 aqueous solution was used as the electrolyte. b. Reversible wettability switching of the functional liquid on DPF during the continuous cycle redox process. To obtain a wide range of wettability values with a small potential difference, we selected +0.40 V and −0.60 V as the optimal working oxidation and reduction potentials, respectively, based on the contact angle analysis at different potentials in Fig. 2b. Error bars represent the standard deviation from five independent measurements. c. Underwater wettability switching of oil on DPF during the cycle redox process. Error bars represent the standard deviation from five independent measurements. d, e. Analysis of the adhesion work between the functional liquid and the matrix in reduction state (d) and oxidation state (e): the polar part of the surface tension/energy and the corresponding surface tension/energy can be identified and the combination of them provides the optimum adhesion between DPF and functional liquid. The blue and red straight line represents the optimum adhesive force, and a range of acceptable performance can be proposed. From the results, one can conclude that the functional liquid falls in the optimum zone in the reduction state, which indicates that the affinity between the functional liquid and the matrix adheres well. f, g. 3D atomic force microscope (AFM) images show height variations on the surface of the matrix in reduction state (f) and oxidation state (g). AFM results suggest that the surface of the matrix changes from smooth to rough during the reduction process due to the influx of lithium ions49, which could also increase the hydrophilicity of the surface.
a. DPM in oxidation state without liquid-lining; b. DPM from ELBS in oxidation state with liquid-lining; c. DPM in reduction state without liquid-lining; d. DPM from ELBS in reduction state with liquid-lining; Scale bar, 100 μm. After the backwashing, we can find that few particles exist on the liquid-lined matrix while many particles exist on the matrix without liquid, which demonstrate that the liquid-lining improves the anti-fouling property of the matrix. In addition, we can use the functional liquid to regulate the wettability of the matrix, which will influence the affinity among the contaminant, functional liquid, and matrix and further prevent particle accumulation. The surface of the matrix with liquid-lining in reduction state was cleaner than that in oxidation state, because of the stronger affinity between the lined liquid and the reduction state matrix. Therefore, we can apply different potentials to change the wettability of the matrix to reduce the affinity to further prevent particles accumulation. In addition, once the regulation is completed, the redox state of the liquid-based matrix can be maintained all the time without continuous power input.
a. Optical image of the pressure measurement setup. b. Schematic of the pressure-induced bulge of the liquid-air interface. Here, L is the pore size of DPM, R is the radius of the cylindrical fiber in DPM, Rsag is the critical radius of curvature of the liquid-air interface, θ is Young’s contact angle, ψ is the texture angle, δθ is the convex angle. c. Theoretical and experimental value of critical pressure (Pc) of air passing through ELBS with different pore size matrices during the redox process. The error bars represent the standard deviation from five independent measurements. d. The theoretical maximum and minimum and experimental diameters of microbubbles generated from ELBS with different matrix pore sizes in the redox process. Error bars represent the standard deviation from five independent measurements. e. The sizes of the microbubbles generated from ELBS with the average pore size of ~9.7 μm in the repeating redox process. Error bars represent the standard deviation from ten independent bubble measurements.
a. Adhesion of the hydrophobic particles to the gas–liquid interface. b. Representation of the forces acting on the hydrophobic particle at the bubble interface. Here, Rp is the radius of the particle, Rb is the radius of the bubble, FP is the force resulting from the capillary pressure inside the gas bubble, FB is the buoyancy force, FG is the gravitational force, Fγ is the surface tension force of the liquid. c. Optical image of the in situ study customized particulate matter (PM) purification system. d. The schematic of the purification system and the capture process of the droplets generated from the bursting bubbles. e. The particle size distribution in the liquid in the inlet side and outlet side. There are two purification steps: the direct interception and removal of larger PM in the polluted air by the liquid-lined porous matrix (inlet side), and the capture of smaller PM by the functional liquid through the microbubble interface (outlet side).
Optical photographs of DPM, SSP (stainless-steel plate), and SSM after immersed in 1.0 M NaOH solutions, 0.5 M H2SO4 solutions, 1.0 M NaCl solutions, at different temperatures for 24 h, respectively. Scale bar, 1 cm.
In situ observation of PPy growing on SSM.
In situ observation of microbubble generation from ELBS. DPM is the working electrode, Au/SSM is the counter electrode, Ag/AgCl is the reference electrode and LiClO4 aqueous solution is the functional liquid.
Adhesion of different wettability particles to different gas–liquid interfaces.
Demonstration of using ELBS to absorb PM in a larger environment.
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Zhang, Y., Han, Y., Ji, X. et al. Continuous air purification by aqueous interface filtration and absorption. Nature 610, 74–80 (2022). https://doi.org/10.1038/s41586-022-05124-y
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