Two-thirds of the global population faces water scarcity. While desalination and water purification are being researched, atmospheric water, estimated at over ten thousand cubic kilometers, offers a sustainable freshwater source. Atmospheric water harvesting (AWH) involves moisture capture, water release, and filtration. Earlier methods like fog capture and dew condensation require high relative humidity (>90% RH), unsuitable for regions with average annual humidity below 40% (over one-third of global terrestrial areas). Porous sorbents like zeolites and silica gels offer vapor adsorption across humidity ranges, but suffer from low water uptake and high energy desorption needs. Hygroscopic salts (LiCl, CaCl₂, MgCl₂) show higher uptake but suffer from aggregation, sluggish kinetics, and performance decay. Salt composites and metal-organic frameworks have been developed to improve kinetics and prevent leakage, showing promise. Polymeric gels also offer potential due to high water retention and tunable structures. This study introduces super hygroscopic polymer films (SHPFs) to extract water vapor from arid climates (≤30% RH) with exceptional kinetics. SHPFs consist of konjac glucomannan (KGM) and hydroxypropyl cellulose (HPC) as the polymer matrix holding uniformly dispersed LiCl solution, achieving high water uptake at low RH. The hierarchically porous structures facilitated by KGM provide enlarged interfaces and rapid water vapor transport. Thermoresponsive HPC allows controlled interactions with water molecules, enabling water release within 10 min, achieving numerous sorption-desorption cycles daily. The uniform LiCl distribution prevents particle aggregation, ensuring stable sorption performance and high daily water yield. This makes SHPFs a potential solution for sustainable AWH.

Existing atmospheric water harvesting (AWH) technologies face challenges, particularly in arid regions with low relative humidity. Traditional methods like fog harvesting and dew condensation rely on high humidity levels and are therefore unsuitable for many water-scarce areas. While materials like zeolites and silica gels have been explored as porous sorbents, they often exhibit low water uptake and high energy requirements for water desorption. Hygroscopic salts, although demonstrating high water uptake at low humidity, suffer from issues such as salt aggregation, which leads to slower kinetics and reduced cycling performance. Researchers have attempted to address these limitations through the development of salt composites and metal-organic frameworks (MOFs), showing improved water capture capability and faster kinetics. However, challenges remain in terms of cost-effectiveness, scalability, and long-term stability. Polymeric gels, with their tunable properties and high water retention capacities, have also emerged as a promising platform for AWH, offering potential advantages over other materials. This research builds upon these previous efforts by developing a novel type of polymeric material specifically designed to overcome these existing limitations.

SHPFs were synthesized using a simple casting method. A gel precursor containing KGM, HPC, and LiCl was mixed and poured into a mold, where gelation occurred within 2 minutes without chemical crosslinkers or initiators. The gelation process relies on self-agglomeration through hydrogen bonding between KGM and HPC. After freeze-drying, the SHPF film was peeled off for use. The process is scalable, allowing for the production of films with various sizes and shapes. Scanning electron microscopy (SEM) revealed a rough surface with micro-sized pores (20–50 µm), primarily due to the hydrophilic KGM, enhancing the interfacial area and promoting rapid water vapor transport. Fourier transform infrared (FTIR) spectroscopy confirmed the presence of KGM and HPC in the SHPF network, and a shift in the -OH stretching indicated hydrogen bonding between the two polymers. X-ray diffraction (XRD) analysis showed the amorphous nature of the SHPF, with no detectable crystalline LiCl due to its uniform distribution within the polymer matrix, preventing salt aggregation. Water vapor sorption was tested using a dynamic vapor sorption (DVS) system under constant humidified airflow. The sorption process involves water vapor diffusion through the pores, absorption at the liquid-gas interface, and diffusion into the polymer network. The addition of LiCl significantly increased water uptake, and the optimized SHPF exhibited high water uptake at various RH levels, outperforming many state-of-the-art sorbent materials. Water release was achieved through mild heating (60 °C), facilitated by the thermoresponsive HPC. Differential scanning calorimetry (DSC) showed that water evaporation in SHPF started at a lower temperature compared to KGM-Li films. A custom-built water collection device was used to validate the SHPF's ability to collect freshwater under arid conditions. The device included a heating plate for water release and a condenser for water collection. Cycling tests demonstrated stable performance, with high water yields per day.

The study successfully developed super hygroscopic polymer films (SHPFs) for atmospheric water harvesting. SHPFs demonstrated high water uptake (0.64 g g⁻¹ at 15% RH and 0.96 g g⁻¹ at 30% RH), surpassing many existing materials. The hierarchically porous structure, facilitated by konjac glucomannan (KGM), enabled rapid water vapor transport and increased surface area for moisture capture. The incorporation of thermoresponsive hydroxypropyl cellulose (HPC) allowed for efficient water release through a mild heating process (60 °C), leading to 14–24 cycles per day. The uniform dispersion of LiCl within the polymer matrix prevented salt aggregation and ensured stable cycling performance. In a custom-built water collection device, SHPFs achieved high water collection efficiency (87%), with daily water yields reaching 5.8 L kg⁻¹ at 15% RH and 13.3 L kg⁻¹ at 30% RH. The synthesis method is simple, scalable, and utilizes sustainable, low-cost materials. Outdoor tests confirmed the effectiveness of SHPFs under real-world arid conditions, producing approximately 5.5 L kg⁻¹ day⁻¹ at 10.6–41.6% RH.

The results demonstrate the significant potential of SHPFs for addressing water scarcity in arid regions. The combination of high water uptake, rapid sorption-desorption kinetics, and sustainable material composition makes SHPFs a promising alternative to existing AWH technologies. The scalability of the synthesis method and low cost of materials are crucial for large-scale deployment and accessibility. The high daily water yield of SHPFs, even under low relative humidity, significantly improves the practicality of AWH systems. The successful integration of renewable biomasses and the efficient water release mechanism contribute to a more sustainable and environmentally friendly water harvesting solution. The study highlights the importance of material design in optimizing AWH performance, particularly in challenging environments.

This research successfully demonstrated the feasibility of using scalable, super hygroscopic polymer films (SHPFs) for efficient atmospheric water harvesting in arid climates. The unique combination of renewable biomasses, hygroscopic salt, and thermoresponsive polymers resulted in high water uptake, fast cycling, and sustainable production. Future research could focus on further optimizing the material composition and exploring different configurations for enhanced water production capacity and scalability. The integration of SHPFs into larger-scale AWH systems could provide a viable solution to water scarcity in many parts of the world.

While the SHPFs demonstrated excellent performance in laboratory and limited outdoor settings, further research is needed to assess their long-term durability and stability under various environmental conditions. The energy consumption for heating-assisted water release should be further optimized for improved energy efficiency. Scaling up the production process to industrial levels requires additional investigation to ensure cost-effectiveness and maintain the quality and consistency of the SHPFs. The potential impact of UV radiation and other environmental factors on the material's performance should also be considered in future studies.