Designing Hydrogel Sorbents for Efficient Atmospheric Water Capture: A Purpose-Oriented Approach

Original Authors: Xingbang Zhao, Han Fu, Amin Mojiri, Paul Westerhoff, and Lenore L. Dai

Original paper is accessible at: https://doi.org/10.1002/smll.202507990

Atmospheric Water Harvesting (AWH) is rapidly gaining prominence as a decentralized and climate-resilient freshwater solution, particularly for arid regions where conventional water sources are declining. While sorption-based systems have historically relied on materials such as zeolites, silica gels, and metal–organic frameworks (MOFs), hydrogels have recently emerged as one of the most promising classes of sorbents for next-generation AWH. Their unique three-dimensional polymer networks, excellent hygroscopicity, structural tunability, and compatibility with solar-driven regeneration collectively position them as transformative materials for sustainable water production. The reviewed work presents a purpose-driven design paradigm for hydrogel sorbents—moving beyond maximizing uptake alone to designing materials that meet real-world operational, environmental, and application-specific needs.

1. Hydrogels as Adaptive Sorbents for Sorption-Based AWH

Hydrogels possess hydrophilic functional groups—such as hydroxyl, carboxyl, amide, and sulfonate moieties—that strongly bind atmospheric water vapor through hydrogen bonding and ion–dipole interactions. This intrinsic chemistry, combined with their swelling capacity, allows hydrogels to achieve water vapor sorption capacities ranging from 0.7–6.7 g g⁻¹, surpassing many metal-based adsorbents under comparable humidity conditions Purpose‐Driven Design and Manuf…. Unlike rigid sorbents (e.g., MOFs, zeolites), hydrogels undergo significant volumetric expansion upon hydration, permitting enhanced storage and efficient diffusion of water within their network.

A central advantage is their low regeneration temperature (≈50–100 °C), enabling passive or solar-driven water release, in contrast to the high temperatures (120–250 °C) often required by MOFs and zeolites. This low-grade heat requirement makes hydrogels suitable for off-grid deployment and mobile AWH platforms.

2. A Purpose-Driven Framework for Hydrogel Development

The authors introduce a “top-down” framework in which hydrogel design begins not with maximum capacity but with the intended purpose of the AWH system. Different applications—portable devices, agriculture, drinking water, disaster relief, photovoltaic cooling—impose distinct constraints related to mechanical durability, environmental exposure, water purity, regeneration rate, and scalability. The design strategy therefore maps application demands → required material properties → optimized synthetic routes.

This framework emphasizes:

• Climate resilience (stability under fluctuating temperature, humidity, and dust exposure)

• Controlled water release rather than uncontrolled swelling

• Salt retention and anti-leakage performance for salt-loaded hydrogels

• Mechanical integrity under repeated cycling

• Integration into modular AWH devices at scale

This approach reframes hydrogels as customizable water reservoirs whose chemistry, porosity, additives, and morphologies can be purpose-tailored for specific AWH use cases.

3. Molecular Design: Crosslinking, Structure, and Functional Chemistry

Hydrogel performance originates from molecular-level decisions in polymer architecture and crosslinking. Physical interactions—ionic bonding, hydrogen bonding, hydrophobic associations—enable reversible networks with high absorption capacity. Chemical crosslinking, typically via free-radical polymerization, produces mechanically robust gels capable of sustained cycling. Crosslink density dictates both sorption and desorption behavior: tighter networks improve durability but reduce swelling, while looser networks increase uptake but may collapse under repeated operation.

Beyond the polymer backbone, functional additives offer powerful levers for performance enhancement:

• Hygroscopic salts (CaCl₂, LiCl, NaCl) create strong osmotic gradients, enabling sorption even at low humidity

• Photothermal fillers (CNTs, graphene oxide, MXene) convert sunlight to localized heat, accelerating desorption

• Zwitterionic polymers counteract the salting-out effect, preserving chain mobility in concentrated brines

• MOF–hydrogel hybrids combine high equilibrium capacity with hydrogel flexibility and stability

These composite materials expand the operating range and reliability of hydrogel-based AWH systems, particularly under low-RH conditions.

4. Architectural Tunability: Pore Structure, Shape, and Surface Engineering

The macroscopic architecture of hydrogels is equally essential. Through freeze-casting, porogenic agents, and templating, pore size can be finely tuned from microporous to macroporous regimes. Macropores shorten diffusion pathways and enable faster cycling, while mesopores increase surface binding sites and improve uptake at low humidity.

Hydrogels can be manufactured as:

• Films for rapid thermal response and patterned designs

• Foams with interconnected macropores for enhanced vapor transport

• Beads for modular, scalable AWH cartridges

• Hybrid membranes integrating photothermal or MOF layers for high-speed capture and release

Such morphological control enables device-level optimization across diverse AWH applications.

5. Biomimetic and Sustainable Hydrogel Innovations

Nature offers powerful inspirations for AWH materials. Several recent hydrogels replicate structures found in drought-tolerant plants such as Tillandsia, which utilizes surface trichomes to absorb airborne moisture. Biomimetic hydrogels featuring graded porosity, trichome-like textures, or directional water pathways achieve improved sorption kinetics and solar-driven regeneration.

Biodegradable hydrogels—derived from alginate, cellulose, chitosan, or loofah substrates—offer sustainable alternatives, though their long-term stability currently lags behind synthetic networks. Hybrid strategies (natural polymers + nanoreinforcements) aim to balance durability with environmental responsibility.

6. Challenges and Future Directions

Despite rapid progress, several challenges must be resolved for widespread deployment:

• Salt leakage and deliquescence in salt-loaded hydrogels

• Mechanical degradation during extensive swelling–deswelling cycling

• Fouling and pollutant uptake in outdoor environments

• Scalability and techno-economic feasibility of manufacturing

• Ensuring water safety for drinking applications

• Understanding VOC sorption behavior, which remains understudied

The authors argue that field validation—including long-term outdoor testing across real climatic gradients—is essential before commercial roll-out. Likewise, lifecycle assessments and cost modeling will determine economic viability for different geographic markets.

7. Conclusion

This purpose-oriented review positions hydrogels not merely as high-capacity sorbents but as engineered water-capture systems whose molecular design, additives, structure, and shape must be tailored to the demands of specific AWH applications. By integrating smart polymer chemistry with practical engineering constraints, hydrogel-based sorption systems are poised to play a central role in decentralized, low-energy atmospheric water harvesting. Continued innovation in salt stabilization, photothermal integration, bioinspired design, and scalable fabrication will accelerate the transition of hydrogels from laboratory prototypes to real-world solutions addressing global water scarcity.