Molecularly confined hydration in thermoresponsive hydrogels for efficient atmospheric water harvesting

Material-to-system tailored multilayer-cyclic strategy toward practical atmospheric water harvesting

Solar-driven atmospheric water harvesting (AWH) presents a sustainable approach for freshwater production with sunlight as the sole energy input. To address challenges posed by diurnal moisture variations and diffusive sunlight, we present a system-wide approach that synergistically enhances moisture capture and solar energy utilization in an integrated water harvester. Moisture utilization at the bulk sorbent scale is improved through the hierarchical pore structure of scalable biomass gel sheets enabling rapid regeneration and is further upscaled to system-level performance through a kinetics-matched, continuously multicyclic operation protocol in a multilayered device. Solar energy utilization is enhanced by thermoresponsive hydrogels that lower the energy threshold for water desorption and by efficient thermal and mass flow management that increases energy efficiency. Our system delivers up to 235.09 mL d-1 of water with an energy efficiency as high as 26.4%, excluding solar panel power. This work offers an insight into developing energy-, material-, and space-efficient AWH systems from a cross-scale understanding of sorbent properties, device engineering, and operation protocol tailoring.

Superhygroscopic Aerogels with Hierarchical String-Bag Structure for Effective Humidity Control

Environmental humidity regulation is crucial for diverse applications ranging from healthcare, food preservation, drug storage, to electronics protection. Herein, we employed natural cellulose as the raw material to fabricate superhygroscopic aerogels with hierarchical string-bag structure for effective humidity control. The aggregation state of cellulose chains was regulated to fabricate micronano materials, including the cellulose nanofiber network (CNFN), dendritic microfibers (CDF), and pleated microfibers (CPF), via changing the precipitation process of cellulose/ionic liquid solutions. They immobilized hygroscopic salts (LiCl, CaCl2, and MgSO4) to form uniform aerogels featuring micrometer macropores and nanometer string-bags. The molecular-level distribution of metal salts along the macropore wall and nanofibers, combined with the high hydrophilicity of cellulose, enabled rapid moisture absorption from the environment and transportation within the hierarchical string-bag structure. Moreover, the micronano hierarchical structure was conducive to the water storage. CNFN/LiCl aerogel demonstrated exceptional moisture absorption performance, achieving a water uptake of 1.36 and 3.14 g/g at 30% and 70% RH, respectively. Such superhygroscopic materials could rapidly and effectively control the environmental humidity, indicating a huge potential in food preservation, healthcare, and environmental regulation.

Novel dual-network-structured hydrogel microspheres for efficient atmospheric water collection

Atmospheric water harvesting (AWH) technology is widely regarded as a promising technology to solve the problem of fresh water shortage. Hygroscopic salt-hydrogel composites have attracted extensive attention due to their high hygroscopic salt-carrying capacity. However, their complex preparation process, salting-out and low water collection efficiency restrict their development. In this study, we prepared calcium alginate (CA) and [2-(methylpropoxy) ethyl dimethyl-(3-propyl sulfonic acid)ammonium hydroxide (PDMAPS)] double-network-structured hydrogel microspheres using a novel drip-free polymerization method. Then, a CA/PDMAPS/CNT/LiCl composite adsorbent was prepared by adding carbon nanotubes (CNTs) and LiCl. The preparation process was simple and suitable for mass production. Zwitterionic groups in the double-network structure (cationic -N+(CH3)2 - and anionic -SO3 -) could produce electrostatic effects with Li+ and Cl-, thereby binding LiCl and solving the traditional salting-out problem. A binary salt system could also be formed, which greatly enhanced water-collection capacity. At 22 °C with RH = 90%, the maximum water collection of the hydrogel microspheres was 3.586 g g-1. Compared with single-network-structured hydrogels, the reported system exhibited an enhancement of 434% in its water collection efficiency. Under natural light, it desorbed more than 80% of the adsorbed water in 3-4 h. In summary, the dual-network-structured hydrogel microspheres represent a promising material for atmospheric water collection.

Adaptive Surfaces with Stimuli-Responsive Wettability: From Tailoring to Applications

Adaptive surfaces with tunable wettability have attracted considerable attention due to their increasing importance in adapting to real applications. By incorporation of stimuli-sensitive materials that enable control over surface chemistry or topographical features, or both, a variety of adaptive surfaces are engineered to exhibit reversible tailoring in wettability. This Review provides a comprehensive review of the development of adaptive surfaces with stimuli-responsive wettability. It begins by outlining the background and significance of the adaptive surfaces. Then, this Review delves into the fundamental theories that govern surface wettability, focusing on the influence of external stimuli on wetting behavior. The discussion then shifts to highlighting various triggers, such as magnetism, photo, temperature, pH, electricity, and gas stimuli, that drive response in wettability, as well as surfaces that respond to dual or multiple stimuli. This Review further explores the primary and leading applications in droplet manipulation, oil-water separation, and water harvesting. To conclude, we encapsulate the challenges, potential solutions, and future directions for improving tunable wettability on these surfaces.

Enhancement of Water Productivity and Energy Efficiency in Sorption-based Atmospheric Water Harvesting Systems: From Material, Component to System Level

To address the increasingly serious water scarcity across the world, sorption-based atmospheric water harvesting (SAWH) continues to attract attention among various water production methods, due to it being less dependent on climatic and geographical conditions. Water productivity and energy efficiency are the two most important evaluation indicators. Therefore, this review aims to comprehensively and systematically summarize and discuss the water productivity and energy efficiency enhancement methods for SAWH systems based on three levels, from material to component to system. First, the material level covers the characteristics, categories, and mechanisms of different sorbents. Second, the component level focuses on the sorbent bed, regeneration energy, and condenser. Third, the system level encompasses the system design, operation, and synergetic effect generation with other mechanisms. Specifically, the key and promising improvement methods are: synthesizing composite sorbents with high water uptake, fast sorption kinetics, and low regeneration energy (material level); improving thermal insulation between the sorbent bed and condenser, utilizing renewable energy or electrical heating for desorption and multistage design (component level); achieving continuous system operation with a desired number of sorbent beds or rotational structure, and integrating with Peltier cooling or passive radiative cooling technologies (system level). In addition, applications and challenges of SAWH systems are explored, followed by potential outlooks and future perspectives. Overall, it is expected that this review article can provide promising directions and guidelines for the design and operation of SAWH systems with the aim of achieving high water productivity and energy efficiency.

Hydrogel-embedded vertically aligned metal-organic framework nanosheet membrane for efficient water harvesting

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.

Hydroponics for plant cultivation in space - a white paper

The microgravity conditions experienced in space prevent the proper distribution of water throughout root modules of plant growth hardware, and the lack of convective mixing and buoyancy reduces gas exchange. To overcome this problem, cultivation technologies should be designed that take advantage of the unique traits of the spaceflight environment instead of attempting to recreate Earth-like conditions. Such technologies should be adaptable to both the microgravity of spaceflight and the low gravity environments of the lunar and Martian surface. Current space plant cultivation relies on traditional terrestrial practices and uses porous substrates that are nutrient poor and difficult to regenerate, and does not consider the dominance of surface- or thermal gradient-controlled rather than gravity-controlled water flow in space as a potential beneficial property. We propose systems that control water dispensation and removal by parallel but independent means in a soil-free cultivation system that is adaptable and expandable to crops of varying sizes and shallow or deep rooting plants. Water dispensation and removal in a substrate-free hydroponic system can be achieved through the misting of nutrient solutions combined with special root module geometry and temperature gradients. The use of hydrogels as substrate, and a means of providing required nutrients and water for plant cultivation in space, can aid in the transition to low-gravity systems by eventual incorporation of on-site regolith to establish Earth-like soil.

Engineering Hygroscopic MOF-Based Silk Via Bioinspired Interfacial Assembly for Fast Moisture Manipulation

Metal-organic frameworks (MOFs) have emerged as exceptional moisture sorbents in low humidity conditions. However, their typical powdered form often results in agglomeration, impeding water diffusion kinetics and practical handling. To enhance the accessibility and diversify the integration of MOFs, a universal and scalable bionic interfacial assembly method is introduced for fabricating MOF-based silk. The resulting silk, enriched with a high content of MOF-303, demonstrates a significant water adsorption capacity of 315.1 mg g-1 at 25% relative humidity, exhibiting a three fold faster water absorption compared with that of stacked MOFs powder on a gram-scale. Furthermore, it achieves efficient water release, with a rate of 8.1 mg g-1 min-1 under sunlight after surface photothermal modification. Through one-step drawing assembly, electrothermal wires can be incorporated into MOF-based silk and demonstrate fast and reversible moisture adsorption/desorption for indoor humidity control. It is envisioned that this assembling method and integrated functional silk will yield valuable insights into the rational engineering of MOFs toward practical applications in moisture management, molecule absorption, etc.

Internally-cooled atmospheric water harvesting enabling improved productivity

Sorption-based atmospheric water harvesting holds promise for alleviating water scarcity, but current prototypes have not shown significant increases in practical yields despite efforts in the enlarged engineering scale. This is due to weakened heat and mass transfer with a packed sorbent bed. In this work, the desiccant-coated adsorbers were employed to fabricate the water harvesting device that incorporates internal fluid for cooling and heating during sorption and desorption. Featured with an internal cooling effect, practical water productivity could be improved by 1.75-9.96 times with a low desorption temperature (45-62 °C). The continuous water harvesting system could produce 0.77-3.98 Lwater/kgsorbent/day with a thermal energy consumption of 7.7-30.4 MJ/kg in wide climates from 20 % to 80 % RH, providing a reference for device design in the engineering view. The demonstration revealed that using natural cooling in the sorption stage has great benefits in improving water harvesting performance, which can be integrated into the building sectors or a wider range of scenarios.

Chemistries and materials for atmospheric water harvesting

Atmospheric water harvesting (AWH) is recognized as a crucial strategy to address the global challenge of water scarcity by tapping into the vast reserves of atmospheric moisture for potable water supply. Within this domain, sorbents lie in the core of AWH technologies as they possess broad adaptability across a wide spectrum of humidity levels, underpinned by the cyclic sorption and desorption processes of sorbents, necessitating a multi-scale viewpoint regarding the rational material and chemical selection and design. This Invited Review delves into the essential sorption mechanisms observed across various classes of sorbent systems, emphasizing the water-sorbent interactions and the progression of water networks. A special focus is placed on the insights derived from isotherm profiles, which elucidate sorbent structures and sorption dynamics. From these foundational principles, we derive material and chemical design guidelines and identify key tuning factors from a structural-functional perspective across multiple material systems, addressing their fundamental chemistries and unique attributes. The review further navigates through system-level design considerations to optimize water production efficiency. This review aims to equip researchers in the field of AWH with a thorough understanding of the water-sorbent interactions, material design principles, and system-level considerations essential for advancing this technology.

Entangled Mesh Hydrogels with Macroporous Topologies via Cryogelation for Rapid Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt-hydrogel composites (HSHCs) have emerged as attractive candidates with their high water uptake, versatile designability, and scale-up fabrication. However, achieving high-performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled-mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar-driven water desorption. Consequently, a rapid-cycling HEMHs-based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents -1 day-1 via continuous eight sorption/desorption cycles, outperforming other state-of-the-art hydrogel-based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long-standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next-generation SAWH applications.

MXene-Based Soft Humidity-Driven Actuator with High Sensitivity and Fast Response

Soft actuators possessing notable mechanical deformations, high sensitivity, and fast response speed play a crucial role in various applications, such as artificial muscles, soft robots, and intelligent devices. In this study, a smart humidity-driven actuator was successfully fabricated by utilizing MXene/cellulose nanofiber (CNF)/LiCl (MCL) through vacuum-assisted filtration with fast response speed and high sensitivity. Utilizing the excellent humidity responsiveness of MXene/CNF and the robust hygroscopicity of LiCl, the synergistic effect of these materials enhances the hygroscopic properties and response speed of the actuator. The MCL actuator demonstrates excellent actuation performance, fast deformation, and reliable cyclic stability. To illustrate the extensive potential of the soft actuator, a range of applications, from bionic devices to soft grippers and crawling actuators, are showcased. Remarkably, the crawling actuator demonstrates sustained crawling motion without necessitating a humidity switch, relying on the humidity gradient from water droplets, and exhibits spontaneous directional motions within a certain range, which makes it a promising prospect in the field of soft robotics.