Hierarchical Engineering of Sorption-Based Atmospheric Water Harvesters

Next-generation water-saving strategies for greenhouses using a nexus approach with modern technologies

The escalating food and water crisis, propelled by population growth, urbanization, and climate change, demands a reimagining of agricultural practices. Traditional water-saving irrigation methods have reached their limits, necessitating the exploration of innovative approaches. This perspective explores the potential of utilizing excess light and water in greenhouse cultivation through advanced materials and engineering technologies. We investigate the potential of four key technologies-sorption-based atmosphere water harvesting (SAWH), superabsorbent polymer water holding materials (SPWH), radiative cooling (RC), and seawater desalination. The perspective proposes suitable application methods and future development directions for greenhouse water conservation, aiming to introduce novel water-saving strategies and smarter resource management.

Cobalt-Ion Superhygroscopic Hydrogels Serve as Chip Heat Sinks Achieving a 5 °C Temperature Reduction via Evaporative Cooling

In the rapidly advancing semiconductor sector, thermal management of chips remains a pivotal concern. Inherent heat generation during their operation can lead to a range of issues such as potential thermal runaway, diminished lifespan, and current leakage. To mitigate these challenges, the study introduces a superhygroscopic hydrogel embedded with metal ions. Capitalizing on intrinsic coordination chemistry, the metallic ions in the hydrogel form robust coordination structures with non-metallic nitrogen and oxygen through empty electron orbitals and lone electron pairs. This unique structure serves as an active site for water adsorption, beginning with a primary layer of chemisorbed water molecules and subsequently facilitating multi-layer physisorption via Van der Waals forces. Remarkably, the cobalt-integrated hydrogel demonstrates the capability to harvest over 1 and 5 g g-1 atmospheric water at 60% RH and 95% RH, respectively. Furthermore, the hydrogel efficiently releases the entirety of its absorbed water at a modest 40°C, enabling its recyclability. Owing to its significant water absorption capacity and minimal dehydration temperature, the hydrogel can reduce chip temperatures by 5°C during the dehydration process, offering a sustainable solution to thermal management in electronics.

Lubricated Surface in a Vertical Double-Sided Architecture for Radiative Cooling and Atmospheric Water Harvesting

Radiative cooling significantly lowers condenser temperatures below ambient levels, enabling atmospheric water harvesting (AWH) without additional energy. However, traditional sky-facing condensers have low cooling power density, and water droplets remain pinned on surface, requiring active condensate collection. To overcome these challenges, a lubricated surface (LS) coating-consisting of highly scalable polydimethylsiloxane elastomer lubricated with silicone oil-is introduced on the condenser side in a vertical double-sided architecture. The design not only effectively doubles the local cooling power, but also eliminates contact-line pinning, enabling passive, gravity-driven collection of water. Robust AWH is demonstrated from a 30 × 30 cm2 sample in outdoor environments (of varying humidity levels and wind speeds in different months) and with no artificial flow of humidified air. In one outdoor test, the passive water collection rate of LS coating reaches 21 g m-2 h-1 double that on superhydrophobic surface, 10 g m-2 h-1. In indoor testing (20 °C and 80% relative humidity), this system achieves a condensation rate ≈87% of the theoretical limit with up to 90% of the total condensate passively collected. this approach achieves effective AWH in a decentralized approach that removes the need for piping infrastructure and external energy input.

Enhanced Hydrophilicity of DAAQ-TFP COFs via Sulfonate Modification for Air Water Harvesting in Arid Environment

The poor ability of covalent organic frameworks (COFs) based adsorbents at low relative humidity (RH) conditions limits their applications for air-water harvesting in arid environments. In the present work, the sulfonated COFs (DAAQ-TFP-SO3H@LiCl) composites are prepared through the functionalization of sulfonic acid and LiCl composite to improve its hydrophilicity. TheDAAQ-TFP-SO3H@LiCl composites exhibit a good adsorption performance, outperforming many other COF adsorbents developed so far. It can absorb 0.22 ± 0.005 g g-1 and 1.01 ± 0.027 g g-1 of water at room temperature under 20% RH and 90% RH, respectively while demonstrating good cyclic stability. Compared with the isotherm of the DAAQ-TFP, the introduction of the sulfonic acid group shifts the inflection point of the water isotherm toward low humidity, indicating that the sulfonic acid group effectively expends the working humidity range of the adsorbent and enables the effective water adsorption in an arid environment. Furthermore, the DAAQ-TFP-SO3H@LiCl composites display rapid kinetics during both the adsorption and desorption processes, reaching saturation within 60 min in the equilibrium adsorption test and completing desorption within 12 min at 50 °C. This innovative approach provides a new method for designing adsorbent materials with low energy input requirements and high daily water consumption capabilities.

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.

Biomimetic Aerogel Composite for Atmospheric Water Harvesting

Adsorption-based atmospheric water harvesting (AWH) with solar-driven photothermal desorption has become an effective means of solving freshwater scarcity in arid regions due to its low energy consumption and high efficiency. Moisture adsorption and desorption capacities are the most critical properties in AWH, and it is a challenge to improve the rate of moisture adsorption and desorption of composite adsorbents. Therefore, this paper reports a SA/carboxymethyl chitosan (CCS)/C/CaCl2-U composite aerogel adsorbents with simultaneously green, low-cost, degradable, and fast hygroscopicity and desorption kinetics. The composite adsorbent used water-soluble biomass materials sodium alginate (SA) and carboxymethyl chitosan (CCS) as the backbone of the aerogel, constructed a vertically aligned unidirectional pore structure by directional freezing, and introduced nanocarbon powder and moisture-absorbent salt calcium chloride (CaCl2) to improve the solar photothermal performance and water absorption, respectively. The results showed that the composite adsorbent had good water uptake capacity at 30-90% relative humidity (RH), the time to reach the water uptake of 1 g g-1 at 90% RH was only 2.5 h, and the final water uptake rate was up to 1.9 g g-1 within 12 h. Meanwhile, the composite sorbent can be heated and desorbed basically within 1 h at 80 °C and its evaporation efficiency is 1.3 times higher than that of the aerogel sorbent prepared by the conventional method when irradiated with 1000 W m-2 light intensity for 2 h. Therefore, the SA/CCS/C/CaCl2-U composite aerogel adsorbent of this study has a potential that can be applied in AWH due to its environmental friendliness, low cost, and faster hygroscopic desorption kinetics.

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.

An Atmospheric Water-Harvester with Ultrahigh Uptake-Release Efficiency at Low Humidity

Atmospheric water harvesting is a practical strategy that is achieved by removing materials from air moisture to relieve global water scarcity. Here we design a water-harvester (i.e., MOF-303/thiolated polymer composite (MTC)) by using a metal-organic framework (MOF-303) and thiolated chitosan (TC) skeleton. Intermolecular hydrogen bonding between TC and MOF-303 facilitates porous structures with enlarged air-polymer interfaces for long cycling life and high capacity at low relative humidity. Benefiting from synergetic effects on porosity and anchorage for accelerating the uptake-release of moisture, MTC exhibits a rapid water uptake capacity of 0.135 g/g in 60 min under 12.5 RH% and ultrafast water desorption kinetics of 0.003 g/g/min at 8.5 RH%, which is superior to the as-reported MOF-303 based adsorbents. At low heat (∼40 °C), the water desorption and collection rate, respectively, are 0.0195 and 0.0168 g/g/min within 210 min, showing ultrahigh harvesting efficiency. These results highlight the enormous potential as promising materials for solving the world's water scarcity crisis. This study offers an insight into the design of AWH materials, which can be extended into applications in some realms, e.g., freshwater development for industry in arid areas, water engineering-related devices and systems, etc.