Sustainable Atmospheric Water Harvesting Using Biomass-Derived Hydrogels: Effects of Cross-Linker Concentration and Sorption Kinetics.

Solar-driven, sorption-based atmospheric water harvesting (AWH) offers a cost-effective solution to freshwater scarcity in arid areas. Creating AWH devices capable of performing multiple adsorption-desorption cycles per day is crucial for increasing water production rates matching human water requirements. However, achieving rapid-cycling AWH in passive harvesters has been challenging due to sorbents' slow water adsorption-desorption dynamics. Here we report an MOF-derived nanoporous carbon, a sorbent endowed with fast sorption kinetics and excellent photothermal properties, for high-yield AWH. The optimized structure (40% adsorption sites and ~1.0 nm pore size) has superior sorption kinetics due to the minimized diffusion resistance. Moreover, the carbonaceous sorbent exhibits fast desorption kinetics enabled by efficient solar-thermal heating and high thermal conductivity. A rapid-cycling water harvester based on nanoporous carbon derived from metal-organic frameworks can produce 0.18 L kgcarbon-1 h-1 of water at 30% relative humidity under one-sun illumination. The proposed design strategy is helpful to develop high-yield, solar-driven AWH for advanced freshwater-generation systems.

This paper conducts a comprehensive review on the advances in atmospheric water harvesting (AWH) technology. The principles, applications, advantages, and disadvantages of conventional AWH methods, such as refrigeration condensation, fog water collection, and adsorption condensation, are introduced, and the feasibility and prospects of integrating AWH with radiative cooling technology are analyzed, especially the unique advantages of passive daytime radiative cooling-based AWH, which are discussed in detail. Compared with conventional AWH technologies, radiative cooling technology effectively lowers surface temperatures through nighttime radiative cooling, which not only increases water collection in low-humidity environments but also reduces system energy consumption, providing an environmentally friendly and sustainable solution. Finally, the review discusses the challenges faced by radiative cooling technology and its future development directions, aiming to provide theoretical support and technical guidance for the innovation and optimization of AWH technologies.

Hygroscopic salt-based composite sorbents are considered ideal candidates for solar-driven atmospheric water harvesting. The primary challenge for the sorbents lies in exposing more hygroscopically active sites to the surrounding air while preventing salt leakage. Herein, a hierarchically structured scaffold is constructed by integrating cellulose nanofiber and lithium chloride (LiCl) as building blocks through 3D printing combined with freeze-drying. The milli/micrometer multiscale pores can effectively confine LiCl and simultaneously provide a more exposed active area for water sorption and release, accelerating both water sorption and evaporation kinetics of the 3D printed structure. Compared to a conventional freeze-dried aerogel, the 3D printed scaffold exhibits a water sorption rate that is increased 1.6-fold, along with a more than 2.4-fold greater water release rate. An array of bilayer scaffolds is demonstrated, which can produce 0.63g g-1 day-1 of water outdoors under natural sunlight. This article provides a sustainable strategy for collecting freshwater from the atmosphere.

Sorption‐based atmospheric water harvesting (SAWH) provides a sustainable approach to addressing global freshwater scarcity, with significant potential for freshwater production and renewable energy utilization. However, most SAWH systems rely on hygroscopic salts, which suffer from salt leakage and agglomeration. Moreover, conventional SAWH devices operate intermittently, leading to complex processes and low energy‐time efficiency. Herein, a salt‐free, biomass‐derived bilayer aerogel, composed of chitosan, sodium alginate, and carboxymethyl cellulose, enabling continuous all‐day SAWH is presented via simultaneous adsorption‐desorption. The superhydrophilic, hierarchically porous structure with interconnected channels and a top photothermal layer achieves a high water adsorption capability of 3.97 g g−1 at relative humidity of 90% and a rapid desorption rate of 3.33 kg m−2 h−1 under one‐sun illumination. The feasibility of a simple prototype is demonstrated for automated and continuous SAWH under natural conditions, achieving a high water production of 3234 mLwater kgsorbent−1 day−1 over 1 week, and outperforming previously reported SAWH systems. Moreover, the bilayer aerogel's intrinsic antibacterial properties ensure the microbial safety of the harvested water, addressing a key challenge in practical SAWH applications. This maintenance‐free design simplifies operation and provides a sustainable, off‐grid solution for freshwater production in arid and remote regions, advancing UN Sustainable Development Goal 6.

Aerogels incorporating hygroscopic salts have been widely explored for atmospheric water harvesting (AWH). However, the scalability of these sorbents remains limited due to their reliance on energy-intensive and time-consuming drying methods such as lyophilization or supercritical drying. Here, we present a simple and scalable approach to drying hydrogels with desirable AWH properties using a freezing process followed by solvent exchange and thawing at room temperature. Our system consists of cellulose and silica nanofibers, forming hybrid xerogels with ultralow density (10.86 ± 0.32 mg cm-3), high specific surface area (104.22 m2 g-1), excellent water stability, and mechanical strength. By incorporating carbon-based photothermal materials and lithium chloride as a hygroscopic salt, the xerogels achieve exceptional water uptake capacities ranging from 0.90 to 3.21 g g-1 across a relative humidity (RH) range from 15 to 75%. Under natural sunlight, the AWH xerogel produces water at a rate of 1.17 g g-1 day-1. These results highlight a sustainable and scalable AWH strategy, leveraging ambient-dried xerogels as an energy-efficient solution to mitigate water scarcity.

Recently, atmospheric water harvesting (AWH) based on hygroscopic salt on an inorganic or organic carrier has attracted great attention because of its significant potential applications in the environment. The major technical challenges for practical applications are how to prevent the leakage of hygroscopic salt while achieving a high capacity for sorption of atmospheric water and a high sorption rate. Additionally, techniques for converting sorbed water (in the form of a lithium chloride (LiCl) solution) into clean water need to be developed. Here, a novel method for continuous atmospheric water harvesting, leveraging LiCl@PHEA hydrogels is introduced. Synthesized via one‐step UV polymerization in saturated LiCl solutions, these hydrogels exhibit remarkable air distension ability (>60 times), achieving high water sorption efficiency (11.18 gg−1 at 90% relative humidity in 30 min) with over 90 wt.% salt content and no leakage. This water collection system integrates a porous evaporator and a 3D‐printed silica substrate, ensuring an extraordinarily high evaporation rate (>11 kgm−2 h−1 under sunlight) and efficient water transmission. A prototype based on this achieves a record‐breaking collection rate of over 5 kgm−2 h−1, enabling large‐scale efficient atmospheric water harvesting. Additionally, continuous hydrogen production through electrolysis using the collected water (< 5 ppm of salts) is demonstrated.

Water scarcity and the growing demand for sustainable water production present critical challenges, particularly in arid regions where conventional desalination technologies are energy-intensive and contribute to carbon emissions. This study proposes an innovative closed-loop system for atmospheric water harvesting, leveraging natural thermal gradients between high-altitude and low-altitude locations to condense water vapor from humid air. The system integrates high-thermal-conductivity materials, energy-efficient heat exchangers, and refrigerant fluids to achieve efficient heat transfer, enabling water condensation at minimal environmental cost. Key findings highlight the system's reliance on factors such as temperature lapse rates, material properties, fluid dynamics, and heat exchanger design for optimal performance. Case studies in the Kingdom of Saudi Arabia demonstrated the feasibility of applying this technology in regions with significant elevation differences, with temperature gradients and humidity levels supporting consistent freshwater production. Although quantitative performance metrics vary due to dynamic environmental conditions, the system shows promise in achieving significant reductions in energy consumption by leveraging natural thermal gradients, thereby minimizing reliance on fossil fuels and aligning with global carbon neutrality goals. This research underscores the potential of closed-loop systems as a sustainable alternative for water generation in energy-constrained environments. By addressing both water scarcity and climate change, the study lays the groundwork for further advancements in atmospheric water harvesting technologies and their integration with renewable energy systems.

The development of porous materials exhibiting steep and stepwise adsorption of water vapor at desired humidity is crucial for implementing diverse applications such as humidity control, heat allocation, and atmospheric water harvesting. The precise molecular-level elucidation of structural characteristics and chemical components that dictate the water sorption behaviors in confined nanospaces, metal-organic frameworks (MOFs) in particular, is fundamentally important, but this has yet to be largely explored. In this work, by leveraging the isoreticular principle, we crafted two pairs of isostructural Zr-MOFs with linker backbones of benzene and pyrazine acting as hydrogen-bonding donor and acceptor, respectively. The outstanding water sorption cyclic durability of the four Zr-MOFs permits persuasive investigation of the correlation of the water sorption inflection point and steepness (the two central figures-of-merit for water sorption) with the linker functionality. The two pyrazine-carrying Zr-MOFs both show steep water uptake at lower relative pressure and slightly decreased steepness, which are quantitatively described by the Dubinin-Astakhov relation. We deciphered the privileged water clusters through single-crystal X-ray diffraction studies in which the pyrazine moiety formed stronger hydrogen-bonding interactions with guest water molecules and favored the formation of water pentamers instead of hexamers that are observed in the benzene analog. The hydrogen-bonding donor/acceptor manipulation approach presented in this work may facilitate future research endeavors focusing on molecular attribute engineering in predeterminedly ultrawater-resistant MOF platforms for efficient regulation of water sorption behaviors toward customized applications.

Simultaneous generation of atmospheric water and electricity using a hygroscopic aerogel with fast sorption kinetics

Thiamine diphosphate-dependent enzymes are involved in a wide variety of metabolic pathways. The molecular mechanism behind active site communication and substrate activation, observed in some of these enzymes, has since long been an area of debate. Here, we report the crystal structures of a phenylpyruvate decarboxylase in complex with its substrates and a covalent reaction intermediate analogue. These structures reveal the regulatory site and unveil the mechanism of allosteric substrate activation. This signal transduction relies on quaternary structure reorganizations, domain rotations, and a pathway of local conformational changes that are relayed from the regulatory site to the active site. The current findings thus uncover the molecular mechanism by which the binding of a substrate in the regulatory site is linked to the mounting of the catalytic machinery in the active site in this thiamine diphosphate-dependent enzyme.

Highly porous metal-organic framework (MOF) nanosheets have shown promising potential for efficient water sorption kinetics in atmospheric water harvesting (AWH) systems. However, the water uptake of single-component MOF absorbents remains limited due to their low water retention. To overcome this limitation, we present a strategy for fabricating vertically aligned MOF nanosheets on hydrogel membrane substrates (MOF-CT/PVA) to achieve ultrafast AWH with high water uptake. By employing directional growth of MOF nanosheets, we successfully create superhydrophilic MOF coating layer and pore channels for efficient water transportation to the crosslinked flexible hydrogel membrane. The designed composite water harvester exhibits ultrafast sorption kinetics, achieving 91.4% saturation within 15 min. Moreover, MOF-CT/PVA exhibits superior solar-driven water capture-release capacity even after 10 cycles of reuse. This construction approach significantly enhances the water vapor adsorption, offering a potential solution for the design of composite MOF-membrane harvesters to mitigate the freshwater crisis.

Insulin-degrading enzyme (IDE) exists primarily as a dimer being unique among the zinc metalloproteases in that it exhibits allosteric kinetics with small synthetic peptide substrates. In addition the IDE reaction rate is increased by small peptides that bind to a distal site within the substrate binding site. We have generated mixed dimers of IDE in which one or both subunits contain mutations that affect activity. The mutation Y609F in the distal part of the substrate binding site of the active subunit blocks allosteric activation regardless of the activity of the other subunit. This effect shows that substrate or small peptide activation occurs through a cis effect. A mixed dimer composed of one wild-type subunit and the other subunit containing a mutation that neither permits substrate binding nor catalysis (H112Q) exhibits the same turnover number per active subunit as wild-type IDE. In contrast, a mixed dimer in which one subunit contains the wild-type sequence and the other contains a mutation that permits substrate binding, but not catalysis (E111F), exhibits a decrease in turnover number. This indicates a negative trans effect of substrate binding at the active site. On the other hand, activation in trans is observed with extended substrates that occupy both the active and distal sites. Comparison of the binding of an amyloid β peptide analog to wild-type IDE and to the Y609F mutant showed no difference in affinity, indicating that Y609 does not play a significant role in substrate binding at the distal site.

Metal- and halide-free, solid-state water vapor sorbents are highly desirable for water-sorption-based applications, because most of the solid sorbents suffer from low water sorption capacity caused by their rigid porosity, while the liquid sorbents are limited by their fluidity and strong corrosivity, which is caused by the halide ions. Herein, we report a novel type of highly efficient and benign polymeric sorbent, which contains no metal or halide, and has an expandable solid state when wet. A group of sorbents are synthesized by polymerizing and crosslinking the metal-free quaternary ammonium monomers followed by an ion-exchange process to replace chloride anions with benign-anions, including acetate, oxalate, and citrate. They show significantly reduced corrosivity and improved water sorption capacity. Importantly, the water sorption capacity of the acetate paired hydrogel is among the best of the literature reported hygroscopic polymers in their pure form, even though the hydrogel is crosslinked. The hydrogel-based sorbents are further used for water-sorption-driven cooling and atmospheric water harvesting applications, which show improved coefficient of performance (COP) and high freshwater production rate, respectively. The results of this work would inspire more research interest in developing better water sorbents and potentially broaden the application horizon of water-sorption-based processes towards the water-energy nexus.

Atmospheric water harvesting is a promising technology for alleviating global water scarcity. Current water sorption materials efficiently capture water vapor from ubiquitous air; however, they are difficult to scale up due to high costs, complex device engineering, and intensive energy consumption. Fired red brick, a low-cost masonry construction material, holds the potential for developing large-scale functional architectures. Here, we utilize fired red brick for atmospheric water harvesting by integrating a microtubular coating of the conducting polymer PEDOT within its inorganic microstructure. This microtubular polymer coating affords hygroscopicity and high surface area for water nucleation, enables capillary forces to promote water transport, and enhances the water harvesting efficiency. Our brick composite achieves a maximum water vapor uptake of ∼200 wt % versus polymer mass at 95% relative humidity, decreasing to ∼15 wt % at 40% relative humidity. Facile water release is demonstrated via thermal, electrical, and illuminative heating. This proof-of-concept study demonstrates the potential of masonry construction materials for large-scale atmospheric water harvesting.

We explore the thermal oxidation of hyper-cross-linkedpolymersto enhance their hydrophilicity and efficacy in atmospheric waterharvesting. Comprehensive chemical and physical characterizationsare used to confirm the successful incorporation of polar oxygen moietiesand the preservation of porosity upon thermal treatment. Newly introducedoxygen-based functional groups significantly improve water sorptionproperties, increasing total water uptake capacities by up to 400%and shifting water uptake onsets to significantly lower relative humidity.We also investigate the regeneration of oxidized hyper-cross-linkedpolymers after water sorption to probe their potential for multiplewater harvesting cycles and reuse. Our findings outline a simple andcost-effective postsynthetic modification route for optimizing porousorganic polymers for more sustainable and efficient atmospheric waterharvesting.