Chen Wenshuai /UT-Austin YU Guihua “AM” : Hygroscopic gel with nanostructure for rapid and large-scale passive dehumidification

2022-05-02 0 By

At different times of the year, many areas of the planet face problems with moisture.High humidity is not conducive to environmental sustainability, harms people’s physical and mental health, is conducive to the growth of harmful insects, parasites and microorganisms, causes chemical corrosion of metals, and damages building structures (Figure 1A). Therefore, dehumidification is of great significance to environmental sustainability and human health.The traditional dehumidification method consumes a lot of energy and has great impact on the environment.The core challenge is to expose the hygroscopic surface to air and properly store the captured water to avoid surface inactivation.A nanostructured hygroscopic gel (N-MAG) for passive dehumidification is composed of a hydrophilic nanocellulose network controlled by hygroscopic lithium chloride (LCL).This interconnected nanocellulose can transfer captured water to the massive n-MAg interior space, eliminating water accumulation near the surface and enabling high rates of moisture absorption.N-mag can reduce relative humidity from 96.7% to 28.7% in 6 hours, even in a space more than 2×104 times its own volume.The condensed water is completely confined in n-MAG, overcoming the environmental pollution problem.This study provides a new perspective for environmentally sustainable humidity management with no energy consumption.The work is published in the latest issue of Advanced: “A Nanostructured Moisture Absorbing Gel for Fast and large-scale Passive Dehumidification.Materials “.Figure 1. Hazards of high humidity and n-MAG based dehumidification.We designed a nano-structured hygroscopic gel (N-MAG) that integrates hygroscopic LiCl and hydrophilic biopolymer-derived nanofibrous cellulose (NFC) into a layered porous network.N-mag skeleton is supported by high aspect ratio, high crystal and entangled NFC.NFC can be extracted from renewable higher plants, has excellent mechanical properties and water retention, and has outstanding advantages in terms of sustainability.A small piece of N-MAG has strong hygroscopicity and water capture capability due to its inherent hygroscopic particle hydrophilic polymer nanofiber interlinked porous network (FIG. 1B), which can quickly remove a large amount of water in wet environments.The outer n-MAG of lithium chloride has many areas exposed to air for further moisture absorption, while the gel inside the porous NFC network acts as a storage space, thus enabling continuous, rapid, and large dehumidification (FIG. 1c).Figure 2. Preparation of N-MAg.N-mag was prepared by exchanging NFC water suspension with LiCl solution to form a composite gel, which was then lyophilized.The highly crystalline NFC consists of personalized cellulose nanofibers and nanobundles with a width of about 3.5 nm (Figure 2A). The wood was extracted by high intensity ultrasound through chemical pretreatment combined with the nanofibrils and was selected as a sustainable hydrophilic module for the construction of N-MAG.Lithium ions and chloride ions rapidly diffuse into the NFC suspension, partially replacing water molecules and storing in the NFC network.This facilitates rapid self-polymerization of NFC into independent gels (Figure 2b).During lyophilization, a large amount of LiCl self-aggregates into particles and is closely linked to the NFC network, resulting in sample shrinkage and the formation of a concave mirror aerogel named N-MAG (Figure 2C).On the n-MAG surface, a large number of micron LiCl particles uniformly fill the pores of the NFC network and are closely connected with the NFC (Figure 2D).LiCl particles were also observed in the internal region and were closely linked to the NFC network (FIG. 2E).The main internal area of n-MAG has large pores of several hundred microns in size (FIG. 2f).Figure 3. Absorption of moisture from air.N-mag is placed indoors and exposed to air to absorb moisture.It quickly absorbs water vapor from the air, gradually wetting its surface.The central region wets faster than the peripheral region (FIG. 3A), mainly because the central region is thinner, looser and has a porous structure with larger pores, while the peripheral region is more dense.A sample of 8 mm × 6 mm × 0.5 mm was taken from the n-MAG center to monitor hygroscopicity (FIG. 3B).Due to the presence of a large number of LiCl particles exposed to the air, the surrounding area of N-MAG rapidly absorbs water vapor from the air and becomes wet within 0.5 h.At constant temperature, N-MAG exhibits rapid absorption kinetics and can absorb more water from the air as ambient humidity increases (Figure 3C).In the window-opening room, the hygroscopic water absorption rate of N-MAG was further compared with that of commercial 4A molecular sieve and silica gel with the constant change of ambient temperature and humidity (Figure 3D, e).The hygroscopicity and saturated hygroscopicity of N-MAG are obviously higher than those of commercial hygroscopics.Figure 4. Air dehumidification.In a dryer with a volume of 9525 cm3(FIG. 4A), N-MAG (~1 g) can rapidly absorb water vapor and reduce the dryer RH from 95.3% to 28.4% within 1 h.The SAME quality of 4A molecular sieve and silica gel can only reduce the dryer RH from 94.4% and 93.3% to 82.3% and 59.4% in 1.5 h, respectively.For dryers with volumes up to 37100 cm3, the 1.79 cm3 N-MAG effectively reduced relative humidity from 96.7% to 28.7% within 6 h, further reduced RH to 22.6% after 24 h, and stabilized RH after 65 h (Figure 4b).Compared with LiCl blocks of the same volume, N-MAG showed higher water absorption (FIG. 4c), thanks to its porous interconnecting structure supported by hydrophilic NFC.Renewable moisture absorption, good circulation stability.Figure 5. Preparation of large size colored N-MAG.Figure 6. Fruit storage dehumidification.Summary: A class of N-MaG desiccants were developed to facilitate the formation of composite gels through solution exchange and then lyophilized for rapid and large-scale dehumidification.In the outer region of n-MAG, LiCl particles are densely packed, while the inner region has micron-sized pores.Due to the synergistic effect between hygroscopic LiCl particles and superhydrophilic nanocellulose network, N-MAG shows excellent hygroscopic performance, and its dehumidification ability is superior to that of commercial sorbents.It can rapidly reduce the relative humidity from 96.7% to 28.7% in an environment whose volume is more than 2×104 times its own.In addition, N-MAG can also be used as an effective desiccant for air dehumidification to prolong the storage life of fruits.The original link: source: polymer science frontiers