Multi-Component Nanofiber Composite Membrane Enabled High PM0.3 Removal Efficiency and Oil/Water Separation Performance in Complex Environment

Bio-inspired gradient poly(lactic acid) nanofibers for active capturing of PM0.3 and real-time respiratory monitoring

The concept of bio-inspired gradient hierarchies, in which the well-defined MOF nanocrystals serve as active nanodielectrics to create electroactive shell at poly(lactic acid) (PLA) nanofibers, is introduced to promote the surface activity and electroactivity of PLA nanofibrous membranes (NFMs). The strategy enabled significant refinement of PLA nanofibers during coaxial electrospinning (∼40 % decline of fiber diameter), accompanied by remarkable increase of specific surface area (nearly 1.5 m2/g), porosity (approximately 85 %) and dielectric constants for the bio-inspired gradient PLA (BG-PLA) NFMs. It largely boosted initial electret properties and electrostatic adsorption capability of BG-PLA NFMs, as well as charge regeneration by TENG mechanisms even under high-humidity environment. The BG-PLA NFMs thus featured exceptionally high PM0.3 filtration efficiencies with well-controlled air resistance (94.3 %, 163.4 Pa, 85 L/min), in contrast to the relatively low efficiency of only 80.0 % for normal PLA. During the application evaluation of outdoor air purification, excellent long-term filtering performance was demonstrated for the BG-PLA for up to 4 h (nearly 98.0 %, 53 Pa), whereas normal PLA exhibited a gradually declined filtration efficiency and an increased pressure drop. Moreover, the BG-PLA NFMs of increased electroactivity were ready to generate tribo-output currents as driven by respiratory vibrations, which enabled real-time monitoring of electrophysiological signals. This bio-inspired gradient strategy opens up promising pathways to engender biodegradable nanofibers of high surface activity and electroactivity, which has significant implications for intelligent protective membranes.

Double-drying 3D lamellar-structured aerogel membrane for efficient oil-water separation and long-lasting antibacterial activity

Conventional oil-water separation membranes are difficult to establish a trade-off between membrane flux and separation efficiency, and often result in serious secondary contamination due to their fouling issue and non-degradability. Herein, a double drying strategy was introduced through a combination of oven-drying and freeze-drying to create a super-wettable and eco-friendly oil-water separating aerogel membrane (TMAdf). Due to the regular nacre-like structures developed in the drying process and the pores formed by freeze-drying, TMAdf aerogel membrane finally develops regularly arranged porous structures. In addition, the aerogel membrane possesses excellent underwater superoleophobicity with a contact angle above 168° and antifouling properties. TMAdf aerogel membrane can effectively separate different kinds of oil-water mixtures and highly emulsified oil-water dispersions under gravity alone, achieving exceptionally high flux (3693 L·m-2·h-1) and efficiency (99 %), while being recyclable. The aerogel membrane also displays stability and universality, making it effective in removing oil droplets from water in corrosive environments such as acids, salts and alkalis. Furthermore, TMAdf aerogel membrane shows long-lasting antibacterial properties (photothermal sterilization up to 6 times) and biodegradability (completely degraded after 50 days in soil). This study presents new ideas and insights for the fabrication of multifunctional membranes for oil-water separation.

Electrospun PA6 multi-stage structured nanofiber membrane with high filtration performance for oily particles

Oily particles pollution poses a tremendous threat to people's health, so it is urgent to develop air filtration materials with the ability of removing fine oily particles effectively. In this study, a nylon 6 multi-stage structured nanofiber membrane (PA6 MSNM) for effective air filtration of fine oily particles was designed and fabricated by adding a certain amount of tetrabutylammonium hexafluorophosphate (TBAHP) via one-step electrospinning. The PA6 MSNMs were composed of coarse trunk fibres and fine branching fibres. Benefiting from the properties of small pore size and high porosity, the resulting PA6 MSNMs exhibited high average filtration efficiency of 99.80% for oily aerosol particles of 0.20-4.59 μm, a low pressure drop of 251 Pa, and the high quality factor of 0.0248 Pa-1. More importantly, its filtration efficiencies for oily aerosol particles of 0.25 and 0.30 μm were up to 99.99% and 100.00%, respectively. It is expected that the multi-stage electrospun nanofiber membranes would have wide application prospects in air filtration, particularly for filtering oily particles.

One-step fabrication of polylactic acid (PLA) nanofibrous membranes with spider-web-like structure for high-efficiency PM0.3 capture

High-efficiency air filters are in high demand to protect human health from the threat of ultrafine particulate matters (PM). However, most commercial air filters are less effective for PM0.3 capture and/or still suffer from undesirable pressure drops. They are also typically petroleum-based. Herein, a double-jet synchronous electrospinning technology was demonstrated to fabricate spider-web-like polylactic acid (PLA) nanofibrous membranes (SPNM) in one step. The properties of spinning solutions were regulated to construct favorable multi-scale nanofiber and bead structures that mimicked the structural units in spider-webs. The as-prepared SPNM exhibited excellent filtration efficiency (99.87 %) and high quality factor (0.321 Pa-1) against the PM0.3, while presenting an attractively low pressure drop (19 Pa). Additionally, the filtration performance of SPNM was almost completely preserved during 10-cycle tests and the 6-month long-term tests, showing excellent function stability and durability. Benefiting from its good hydrophobicity (WCA = 143.2°), SPNM also presented a satisfactory filtration efficiency (>99.37 %) with low pressure drop (18 Pa) at an environment with humidity at 90 % against PM0.3. Furthermore, the unique structure increased the mechanical strength of SPNM, facilitating the processability for practical applications. Overall, this work may shed light on a promising approach for developing biomass-based, highly efficient filtration materials with hierarchical structures.

Guidelines for measuring and reporting particle removal efficiency in fibrous media

Adopting standardized and reliable methodologies to accurately measure particle removal efficiency when developing fibrous materials for controlling airborne contamination is crucial. Here, the authors recommend best practices for experimental assessments and reporting to ensure a reliable evaluation of new airborne particle filtration media and technologies.

P(VDF-TrFE)/BaTiO3 Nanofibrous Membrane with Enhanced Piezoelectricity for High PM0.3 Filtration and Reusable Face Masks

In the background of air pollution and regular COVID-19 prevention, personal protective masks are necessary for our daily life. However, protective masks with high PM_0.3 filtration usually have poor air permeability and are mostly disposable, leading to a heavy burden on the environment. In this work, a reusable membrane based on piezoelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers embedded with BaTiO₃ nanoparticles (BTO NPs) was developed. The P(VDF-TrFE)/BTO composite nanofibers not only have enhanced piezoelectricity and surface polarity but also have reduced diameters that could be beneficial for electrostatic adhesion, pole-polar interactions, and mechanical sieving to increase the PM_0.3 capture capacity. Moreover, the BTO NPs also improved the charge storage capacity of the composite membrane, which could further enhance the PM_0.3 filtration efficiency after corona treatment. The piezoelectric mask based on P(VDF-TrFE)/BTO composite nanofibers has high filtration efficiencies of 96% for PM_0.3 and 98% for bacteria, while the pressure drop was only 182 Pa, which is lower than the commercial N95 standard of 343.2 Pa. Furthermore, the piezoelectric mask has a long and stable filtration performance after 5 cycles of 75% alcohol disinfection, demonstrating that the P(VDF-TrFE)/BTO composite membrane has a potential application in personal protective masks with comfortable and reusable properties.

Interfacial Engineering of Leaf-like Bimetallic MOF-Based Co@NC Nanoarrays Coupled with Ultrathin CoFe-LDH Nanosheets for Rechargeable and Flexible Zn-Air Batteries

Exploring high-efficiency, low-cost, and long-life bifunctional self-supporting electrocatalysts is of great significance for the practical application of advanced rechargeable Zn-air batteries (ZABs), especially flexible solid-state ZABs. Herein, ultrathin CoFe-layered double hydroxide (CoFe-LDH) nanosheets are strongly coupled on the surface of leaf-like bimetallic metal-organic frameworks (MOFs)-derived hybrid carbon (Co@NC) nanoflake nanoarrays supported by carbon cloth (CC) via a facile and scalable method for rechargeable and flexible ZABs. This interfacial engineering for CoFe-LDHs on Co@NC improves the electronic conductivity of CoFe-LDH nanosheets as well as achieves the balance of oxygen evolution reduction (OER) and oxygen reduction reaction (ORR) activity. The unique three-dimensional (3D) open interconnected hierarchical structure facilitates the transport of substances during the electrochemical process while ensuring adequate exposure of OER/ORR active centers. When applied as an additive-free air cathode in rechargeable liquid ZABs, CC/Co@NC/CoFe-LDH-700 demonstrates high open-circuit potential of 1.47 V, maximum power density of 129.3 mW cm^-2, and satisfactory specific capacity of 710.7 mAh g^-1_Zn. Further, the flexible all-solid-state ZAB assembled by CC/Co@NC/CoFe-LDH-700 displays gratifying mechanical flexibility and stable cycling performance over 40 h. More significantly, the series-connected flexible ZAB is further verified as a chain power supply for LED strips and performs well throughout the bending process, showing great application prospects in portable and wearable electronics. This work sheds new light on the design of high-performance self-supporting non-precious metal bifunctional electrocatalysts for OER/ORR and air cathodes for rechargeable ZABs.

High-performance multifunctional electrospun fibrous air filter for personal protection: A review

With the increasingly serious air pollution and the rampant coronavirus disease 2019 (COVID–19), preparing high–performance air filter to achieve the effective personal protection has become a research hotspot. Electrospun nanofibrous membrane has become the first choice of air filter because of its small diameter, high specific surface area and porosity. However, improving the filtration performance of the filter only cannot meet the personal needs: it should be given more functions based on high filtration performance to maximize the personal benefits, called, multifunctional, which can also be easily realized by electrospinning technology, and has attracted much attention. In this review, the filtration mechanism of high–performance electrospun air filter is innovatively summarized from the perspective of membrane. On this basis, the specific preparation process, advantages and disadvantages are analyzed in detail. Furthermore, other functions required for achieving maximum personal protection benefits are introduced specifically, and the existing high–performance electrospun air filter with multiple functions are summarized. Finally, the challenges, limitations, and development trends of manufacturing high–performance air filter with multiple functions for personal protection are presented.

Single-Side Superhydrophobicity in Si3N4-Doped and SiO2-Treated Polypropylene Nonwoven Webs with Antibacterial Activity

Meltblown (MB) nonwovens as air filter materials have played an important role in protecting people from microbe infection in the COVID-19 pandemic. As the pandemic enters the third year in this current global event, it becomes more and more beneficial to develop more functional MB nonwovens with special surface selectivity as well as antibacterial activities. In this article, an antibacterial polypropylene MB nonwoven doped with nano silicon nitride (Si₃N₄), one of ceramic materials, was developed. With the introduction of Si₃N₄, both the average diameter of the fibers and the pore diameter and porosity of the nonwovens can be tailored. Moreover, the nonwovens having a single-side moisture transportation, which would be more comfortable in use for respirators or masks, was designed by imparting a hydrophobicity gradient through the single-side superhydrophobic finishing of reactive organic/inorganic silicon coprecipitation in situ. After a nano/micro structural SiO₂ precipitation on one side of the fabric surfaces, the contact angles were up to 161.7° from 141.0° originally. The nonwovens were evaluated on antibacterial activity, the result of which indicated that they had a high antibacterial activity when the dosage of Si₃N₄ was 0.6 wt%. The bacteriostatic rate against E. coli and S. aureus was up to over 96%. Due to the nontoxicity and excellent antibacterial activity of Si₃N₄, this MB nonwovens are promising as a high-efficiency air filter material, particularly during the pandemic.

Collagen fiber membrane as multi-functional support enabled rational design of ultrahigh-flux separation membrane for the remediation of oil contamination in water

Membrane separation is a promising approach for the remediation of oil contamination in water. High-flux separation of membrane relies on the rational design of ultrathin active layer to significantly reduce mass transfer distance for achieving high separation flux, while the ultrathin active layer is usually fragile with poor mechanical strength, which has to be supported on a support. Herein, we employed collagen fiber membrane (CFM) as multi-functional support for the in-situ growth of polyacrylonitrile (PAN) layer by electrospinning to prepare the high-performance PAN/CFM composite membrane. Due to the amphiphilic nature and strong capillary effect, CFM played the role as multi-functional support to provide separation effectiveness and boosted separation flux. The PAN/CFM composite membrane enabled ultrahigh separation flux (e.g., 51751.59 L m^-2 h^-1 bar^-1) to a variety of oil-in-water emulsion, which was one order of magnitude higher than that of commercial polyethersulfone membrane and 1.86-fold to that of cellulose acetate membrane. Furthermore, the PAN/CFM composite membrane retained high separation flux (e.g., 11046.97 L m^-2 h^-1 bar^-1) during the 5th separation cycle, providing appreciable anti-fouling capability. Therefore, our findings provided a promising way to effectively resolve the problem of oil contamination in water.

Polyphenylene sulfide fabric with enhanced oxidation resistance and hydrophobicity through polybenzoxazine surface coating for emission control in harsh environment

Emissions from power generation and municipal waste incineration sources are primarily at high temperatures and contain corrosive gases, particulate pollutants and are enormously challenging on the performance of the filtration systems in use. Here, polyphenylene sulfide (PPS) nonwoven fabric, a primary material used commercially in such settings, is modified with a polybenzoxazine precursor as a coating to deliver improved thermal and oxidation resistance to the fibrous substrate. The polybenzoxazine precursor undergoes chain propagation and crosslinking upon the treatment process to provide a protective layer over the PPS fibers such that enhanced structural stability in a harsh environment was demonstrated. We have shown the improved overall tensile strength (+15%), Young's modulus (+26%), and more hydrophobic nature of the modified PPS fabric, while the superior environmental stability and better filtration performance could be achieved. Such methodology may lead to higher service temperature and extended service time of the PPS filtration bags in harsh fire exhaustion airstreams encountered in power plants or municipal garbage incineration facilities. The crosslinkable benzoxazine could also be the most cost-effective high temperature coating layer on fibers, enabling future high-performance air filtration materials.

Mechanically robust Janus nanofibrous membrane with asymmetric wettability for high efficiency emulsion separation

Water pollution caused by oil leakage or oily sewage has seriously threatened the ecological environment and human health. It remains a tough task for scientists to develop versatile materials to purify different kinds of oily wastewater. In this study, we propose a facile "carbon nanotubes (CNTs) decoration and nanofibrous membrane integration" method to prepare a mechanical robust Janus membrane (JM) composed of a superhydrophilic nanofiber composite layer and a hydrophobic nanofiber composite layer. The asymmetric wettability can be controlled by tuning the thickness of the hydrophobic layer. The nanofiber composite in both two layers possesses a core-shell structure, guaranteeing the excellent flexibility and stretchability of the JM. In addition, the strong interfacial compatibility between the two layers ensures the stability and durability of the JM even after multiple stretching. More importantly, the JM could realize on-demand separation of different kinds of oily wastewater with high separation flux and separation efficiency, including oil/water mixtures with different oil densities, oil-in-water emulsions and water-in-oil emulsions. Furthermore, the JM exhibits cycling stability and long-term serviceability for the emulsion separation. The mechanically robust and stretchable JM has promising applications in purification of various oil contaminated wastewater.

Bicomponent PLA Nanofiber Nonwovens as Highly Efficient Filtration Media for Particulate Pollutants and Pathogens

Herein, a novel form of bicomponent nanofiber membrane containing stereo-complex polylactic acid (SC-PLA) was successfully produced by the side-by-side electrospinning of Poly (L-lactic acid) (PLLA) and Poly (D-lactic acid) (PDLA). We demonstrate that through these environmentally sustainable materials, highly efficient nanofiber assemblies for filtration can be constructed at very low basis weight. The physical and morphological structure, crystalline structure, hydrophobicity, porous structure, and filtration performance of the fibrous membranes were thoroughly characterized. It was shown that the fabricated polylactic acid (PLA) side-by-side fiber membrane had the advantages of excellent hydrophobicity, small average pore size, high porosity, high filtration efficiency, low pressure drop as well as superior air permeability. At the very low basis weight of 1.1 g/m², the filtration efficiency and pressure drop of the prepared side-by-side membrane reached 96.2% and 30 Pa, respectively. Overall, this biomass-based, biodegradable filtration material has the potential to replace the fossil fuel-based polypropylene commercial meltblown materials for the design and development in filtration, separation, biomedical, personal protection and other fields.