Scalable super hygroscopic polymer films for sustainable moisture harvesting in arid environments

The study addresses global water scarcity by developing a decentralized method to harvest water from air, particularly under arid conditions with low relative humidity (≤30% RH). Conventional approaches like fog capture and dew condensation require high humidity (>90% RH) and are unsuitable for many regions where average humidity is below 40%. Prior sorbents such as zeolites and silica gels have broad humidity operation but suffer from low uptake and high desorption energy. Hygroscopic salts (LiCl, CaCl2, MgCl2) provide high uptake at low RH but aggregate during hydration, forming passivation layers that slow kinetics and degrade cycling. Metal-organic frameworks show promise but can be complex and costly. The research question is whether a scalable, low-cost, biomass-based polymer-salt composite can achieve high water uptake and rapid sorption-desorption cycling at low RH with stable performance. The purpose is to design super hygroscopic polymer films (SHPFs) using konjac glucomannan (KGM) and hydroxypropyl cellulose (HPC) with dispersed LiCl to enable efficient atmospheric water harvesting (AWH) in arid environments.

The paper reviews AWH strategies and materials: (1) Fog harvesting and dew condensation require high RH and are limited in arid regions. (2) Porous inorganic adsorbents like zeolites and silica gels operate across RH but have low water uptake and high desorption energy costs. (3) Hygroscopic salts (LiCl, CaCl2, MgCl2) achieve higher uptake at low RH but suffer from salt crystal aggregation, passivation, slow kinetics, and cycling decay; salt composites have been explored to immobilize salts and prevent leakage. (4) Metal-organic frameworks (MOFs) can capture moisture at low RH with relatively fast kinetics but may involve complex synthesis and cost considerations. (5) Polymeric gels offer high water retention, tunable structure, and adjustable polymer–water interactions, suggesting promise for low-RH AWH. This context motivates a polymer-based, salt-containing, scalable sorbent with fast kinetics and stable cycling.

Synthesis: SHPFs were fabricated via a simple casting and freeze-drying process. A gel precursor containing KGM, HPC, and LiCl was mixed and poured into a mold; gelation occurred within ~2 minutes via hydrogen-bond-driven self-agglomeration without chemical crosslinkers or initiators. After setting, the gel was cooled (fridge at −4 °C for 3 h), flash-frozen in liquid nitrogen (~15 min), and freeze-dried (~12 h) to yield films. Film thickness was tuned; ~100 µm films were used for AWH tests to optimize kinetics. Composition and structure: KGM provided hydrophilicity and formation of micro-sized pores (20–50 µm) increasing air–polymer interfacial area; HPC contributed sub-millimeter pores enhancing vapor transport and imparted thermo-responsiveness enabling hydrophilic-hydrophobic switching near ~45 °C. LiCl was uniformly dispersed within the KGM/HPC network (no crystalline LiCl peaks detectable by XRD at the loadings used), suppressing salt aggregation during hydration. Characterization: SEM examined porous morphology and film cross-sections; FTIR confirmed the presence of KGM and HPC and hydrogen bonding interactions (shift in –OH stretching). XRD assessed crystallinity, showing amorphous KGM, mixed phases of HPC (peaks at 2θ ≈ 9° and 20°), and good miscibility in blends with broadened peaks. TGA quantified LiCl loading. DSC evaluated evaporation behavior and thermal transitions, revealing a lowered evaporation peak (~44 °C) for SHPF relative to KGM-Li film (~53 °C), consistent with HPC phase transition. ICP-MS tracked ion residues in collected water. Sorption–desorption testing: Dynamic vapor sorption (DVS) was conducted under controlled humidified airflow (200 mL min^-1). Samples were pre-dried (90 °C, 0% RH, 60 min), stabilized at 25 °C, and subjected to RH of ~15%, 30%, and 60%. Sorption kinetics and saturation uptakes were recorded. Desorption was performed via mild heating at 60 °C under 4.5–20% RH in a closed chamber. Sorption times to reach 80% of saturation and vapor sorption rates were calculated. Additional sorption tests used a homemade RH-controlled chamber with saturated salt solutions (LiCl for ~15% RH; CH3CO2K for ~30% RH) and 250 mL min^-1 dry airflow; mass changes were tracked by microbalance. Water collection device: A centimeter-scale SHPF (25 mm × 40 mm) was mounted on a flexible heating plate with external temperature control. A tilted condenser wall (~45°) facilitated droplet runoff to a collection channel; vertical heating plates on other walls minimized undesired condensation. Thermocouples monitored temperatures; the device was sealed with a rubber ring and binder clips. Prior to desorption, the SHPF was saturated in a controlled-RH chamber. Collected water mass was measured by pipetting into pre-weighed vials. Cycling protocols: at 15% RH, 70 min capture/30 min release; at 30% RH, 30 min capture/30 min release.

• SHPFs achieved high water uptake at low RH: 0.64 g g^-1 at 15% RH, 0.96 g g^-1 at 30% RH, and 1.53 g g^-1 at 60% RH.
• Rapid sorption kinetics: time to reach 80% saturation was 67 min (15% RH), 36 min (30% RH), and 28 min (60% RH), outperforming many state-of-the-art sorbents.
• High vapor sorption rate: 1.65 L kg^-1 h^-1 at 30% RH.
• Fast water release: >70% of captured water released within 10 min at 60 °C across a wide RH range; DSC showed evaporation peak shift from 53 °C (KGM-Li) to 44 °C (SHPF), consistent with HPC thermoresponsive transition.
• Stable cycling and high daily throughput: 14 cycles per day at 15% RH (70/30 min) and 24 cycles per day at 30% RH (30/30 min), corresponding to daily water yields of 5.8 L kg^-1 (15% RH) and 13.3 L kg^-1 (30% RH).
• Water collection efficiency averaged ~87% (collected/uptake); average collected water after pre-capture: 0.56 g g^-1 (~15% RH), 0.82 g g^-1 (~30% RH), 1.31 g g^-1 (~60% RH).
• Structural advantages: hierarchically porous films (micro-pores 20–50 µm plus larger channels) enlarged air–polymer interfaces and accelerated vapor transport; uniform LiCl dispersion suppressed salt agglomeration, supporting stable performance.
• Scalability and practicality: simple casting method, wafer-scale films, low-cost, renewable biomasses (KGM, HPC); outdoor-relevant performance estimated ~5.5 L kg^-1 day^-1 at 10.6–41.6% RH.

The SHPFs directly address the need for efficient AWH in arid climates by combining high uptake at low RH with rapid and repeatable sorption–desorption cycles. KGM-induced porosity increases interfacial area and facilitates fast vapor transport, while the thermoresponsive HPC lowers the effective desorption temperature via hydrophilic–hydrophobic switching, enabling >70% water release in 10 minutes at only 60 °C. The polymer network immobilizes LiCl and prevents salt aggregation, maintaining kinetics and capacity over many cycles. The resulting daily water yields (up to 13.3 L kg^-1 at 30% RH) and high water collection efficiency indicate strong potential for practical implementation. The thin-film format suggests that multilayer sorbent beds or vertical arrays could further boost areal productivity and device compactness. Overall, the materials system provides a scalable, low-cost route to decentralized freshwater generation in water-stressed regions.

This work introduces super hygroscopic polymer films (SHPFs) comprising KGM, HPC, and LiCl that deliver high water uptake and rapid cycling under low RH conditions. The films leverage hierarchical porosity for fast sorption and HPC’s thermo-responsiveness for low-temperature desorption, achieving 14–24 cycles per day and daily water yields up to 13.3 L kg^-1. The simple, scalable casting process and sustainable, inexpensive components highlight a practical pathway toward atmospheric water harvesting in arid environments. Future directions include engineering multilayered or vertically stacked sorbent architectures to increase areal productivity, optimizing polymer composition and thickness for specific climates, integrating passive or solar-driven heating for energy-efficient desorption, and long-term outdoor durability testing across diverse environmental conditions.