Hygroscopic‐Microgels‐Enabled Rapid Water Extraction from Arid Air

Abstract

Sorbent‐based atmospheric water harvesting (AWH) has emerged as a promising decentralized water‐production technology to mitigate the freshwater crisis in arid areas. Hydrogels have been regarded as attractive sorbents due to their high water retention and tailorable polymer–water interactions. Yet, the kinetics of water sorption and desorption at low relative humidity (RH) shall be improved for their practical implementation. Here, hygroscopic microgels (HMGs) composed of hydroxypropyl cellulose (HPC) and hygroscopic salt are reported, which achieve a water uptake of ca. 0.5–0.8 g g−1 at 15–30% RH. HMGs enable rapid sorption–desorption kinetics owing to the short‐distance diffusion in the microgels and hydrophilicity–hydrophobicity switching of the thermoresponsive HPC. To validate the feasibility of HMGs for moisture extraction, a potential daily water collection of up to equivalent 7.9–19.1 L kg−1 at low RH is demonstrated, enabled by 24–36 operation cycles per day based on the material‐level experiments. With renewable raw materials and superior performance, HMGs provide a sustainable approach for rapid moisture extraction in arid climates.

A General Group-Protection Synthesis Strategy to Fabricate Covalent Organic Framework Gels

“…According to methods in the literature, we measured the water content ( Q ), water contact angles, water vapor adsorption and desorption curves, and water transport rate of the four COF gels (Figure c and Figures S83–S85). − The water content and water transport rates of TpAzo, NKCOF-1, NKCOF-60, and NKCOF-61 hydrogels were 38.2, 37.4, 45.5, and 48.6 g/g and 15.2, 15.9, 22.9, and 29.3 g/min, respectively. From the results, the four COF gels have a high water content and water transfer rate, with the best performance of NKCOF-61.…”

Sorption‐Based Atmospheric Water Harvesting: Materials, Components, Systems, and Applications

“…[95] Another solution to address the energy-intensive regeneration process is developing hydrogels with thermoresponsive polymers via molecular engineering to facilitate water release. [96] Thermoresponsive hydrogels such as PNIPAm, poly (N-vinyl caprolactam) (PVCL), and hydroxypropyl cellulose (HPC) [85,97] act as water molecule reservoirs in low temperatures and store the sorbed water in their hydrophilic structure. With a small temperature variation, they transform to their hydrophobic structure and release the stored water in liquid state with minimal energy consumption.…”

“…[84] Hydrogels can sorb a considerable amount of liquid water; however, their porosity and kinetics may require further enhancement to boost the sorption of moisture from the air. [85] In the sorption kinetics of hygroscopic hydrogels, vapor transport in the hydrogel micropores and liquid transport in the polymer nanopores are two chief coupled processes. Liquid transport caused by the gradient of chemical potential in the hydrogel is a significant factor in the fast kinetics.…”

“…As opposed to MOFs with high porosity and rapid water absorption, hydrogel sorbents suffer from slow water vapor sorption kinetics (especially in low RHs) due to their dense structure and slow internal diffusivity of water molecules. [85,99] Reducing the size and increasing the surface area of the sorbent is thought to be an effective method of increasing the number of sorption sites and reducing the diffusion length of water molecules, thus enhancing the overall sorption kinetics of water vapor. [6b,88d] The incorporation of salts or MOFs with smart polymers can serve as an effective strategy to achieve the desired water sorption rate and capacity.…”

Metal–Organic-Framework-Based Polymers for Effective Water Harvesting in Arid Areas

“…4 In recent years, there has been more and more research on various atmospheric water harvesting materials based on adsorbents. Guan et al 5 reported hygroscopic microgels composed of hydroxypropyl cellulose (HPC) and hygroscopic salts. Aleid et al 1 discovered that the salting-in effect of a zwitterionic hydrogel can facilitate water vapor sorption by the hygroscopic salt under otherwise identical conditions.…”