Sorbents for Atmospheric Water Harvesting: From Design Principles to Applications

Green Synthesis of Polyurethane Sponge-Grafted Calcium Alginate with Carbon Ink Aerogel with High Water Vapor Harvesting Capacity for Solar-Driven All-Weather Atmospheric Water Harvesting

Atmospheric water harvesting (AWH) technology is a new strategy for alleviating freshwater scarcity. Adsorbent materials with high hygroscopicity and high photothermal conversion efficiency are the key to AWH technology. Hence, in this study, a simple and large-scale preparation for a hygroscopic compound of polyurethane (PU) sponge-grafted calcium alginate (CA) with carbon ink (SCAC) was developed. The PU sponge in the SCAC aerogel acts as a substrate, CA as a moisture adsorber, and carbon ink as a light adsorber. The SCAC aerogel exhibits excellent water absorption of 0.555-1.40 g·g-1 within a wide range of relative humidity (40-80%) at 25 °C. The SCAC aerogel could release adsorbed water driven by solar energy, and more than 92.17% of the adsorbed water could be rapidly released over a wide solar intensity range of 1.0-2.0 sun. In an outdoor experiment, 57.517 g of SCAC was able to collect 32.8 g of clean water in 6 h, and the water quality meets the drinking water standards set by the World Health Organization. This study suggests a new approach to design promising AWH materials and infers the potential practical application of SCAC aerogel-based adsorbents.

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.

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.

Renewable biomass-based aerogels: from structural design to functional regulation

Global population growth and industrialization have exacerbated the nonrenewable energy crises and environmental issues, thereby stimulating an enormous demand for producing environmentally friendly materials. Typically, biomass-based aerogels (BAs), which are mainly composed of biomass materials, show great application prospects in various fields because of their exceptional properties such as biocompatibility, degradability, and renewability. To improve the performance of BAs to meet the usage requirements of different scenarios, a large number of innovative works in the past few decades have emphasized the importance of micro-structural design in regulating macroscopic functions. Inspired by the ubiquitous random or regularly arranged structures of materials in nature ranging from micro to meso and macro scales, constructing different microstructures often corresponds to completely different functions even with similar biomolecular compositions. This review focuses on the preparation process, design concepts, regulation methods, and the synergistic combination of chemical compositions and microstructures of BAs with different porous structures from the perspective of gel skeleton and pore structure. It not only comprehensively introduces the effect of various microstructures on the physical properties of BAs, but also analyzes their potential applications in the corresponding fields of thermal management, water treatment, atmospheric water harvesting, CO2 absorption, energy storage and conversion, electromagnetic interference (EMI) shielding, biological applications, etc. Finally, we provide our perspectives regarding the challenges and future opportunities of BAs. Overall, our goal is to provide researchers with a thorough understanding of the relationship between the microstructures and properties of BAs, supported by a comprehensive analysis of the available data.

Structure-Property Relationships of Hydrogel-based Atmospheric Water Harvesting Systems

Atmospheric water harvesting (AWH) is considered one of the promising technologies to alleviate the uneven-distribution of water resources and water scarcity in arid regions of the world. Hydrogel-based AWH materials are currently attracting increasing attention due to their low cost, high energy efficiency and simple preparation. However, there is a knowledge gap in the screening of hydrogel-based AWH materials in terms of structure-property relationships, which may increase the cost of trial and error in research and fabrication. In this study, we synthesised a variety of hydrogel-based AWH materials, characterized their physochemcial properties visualized the electrostatic potential of polymer chains, and ultimately established the structure-property-application relationships of polymeric AWH materials. Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) hydrogel is able to achieve an excellent water adsorption capacity of 0.62 g g-1 and a high water desorption efficiency of more than 90 % in relatively low-moderate humidity environments, which is regarded as one of the polymer materials with potential for future AWH applications.

A Sulfonated Covalent Organic Framework for Atmospheric Water Harvesting

We report a sulfonated covalent organic framework (COF) capable of atmospheric water harvesting in arid conditions. The isothermal water uptake profile of the framework was studied, and the network displayed steep water sorption at low relative humidity (RH) in temperatures of up to 45 °C, reaching a water uptake of 0.12 g g-1 at 10 % RH and even 0.08 g g-1 at just 5 % RH, representing some of the most extreme conditions on the planet. We found that the inclusion of sulfonate moieties shifted uptake in the water isotherm profiles to lower RH compared to non-sulfonated equivalents, demonstrating well the benefits of including these hydrophilic sites for water uptake in hot, arid locations. Repeated uptake and desorption cycles were performed on the material without significant detriment to its adsorption performance, demonstrating the potential of the sulfonated COF for real-world implementation.

A Functionally Asymmetric Janus Hygro-Photothermal Hybrid for Atmospheric Water Harvesting in Arid Regions

Metal-organic frameworks (MOFs) are high-performance adsorbents for atmospheric water harvesting but have poor water-desorption ability, requiring excess energy input to release the trapped water. Addressing this issue, a Janus-structured adsorbent with functional asymmetry is presented. The material exhibits contrasting functionalities on either face - a hygroscopic face interfaced with a photothermal face. Hygroscopic aluminum fumarate MOF and photothermal CuxS layers are in-situ grown on opposite sides of a Cu/Al bimetallic substrate, resulting in a CuxS-Cu/Al-MOF Janus hygro-photothermal hybrid. The two faces serve as independent "factories" for photothermal conversion and water adsorption-desorption respectively, while the interfacing bimetallic layer serves as a "heat conveyor belt" between them. Due to the high porosity and hydrophilicity of the MOF, the hybrid exhibits a water-adsorption capacity of 0.161 g g-1 and a fast adsorption rate (saturation within 52 min) at 30% relative humidity. Thanks to the photothermal CuxS, the hybrid can reach 71.5 °C under 1 Sun in 20 min and desorb 97% adsorbed water in 40 min, exhibiting a high photothermal conversion efficiency of over 90%. CuxS-Cu/Al-MOF exhibits minimal fluctuations after 200 cycles, and its water-generation capacity is 3.21 times that of powdery MOF in 3 h in a self-designed prototype in one cycle.

Unraveling a Stable 16-Ring Aluminophosphate DNL-11 through Three-Dimensional Electron Diffraction for Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (AWH) is a promising solution for addressing water scarcity. Developing cost-effective and stable water adsorbents with high water uptake capacity and a low-temperature regeneration requirement is a crucially important procedure. In this Communication, we present a novel and stable aluminophosphate (AlPO) molecular sieve (MS) named DNL-11 with 16-ring channels synthesized by using an affordable and commercialized organic structure directing agent (OSDA), whose crystallographic structure is elucidated by three-dimensional electron diffraction (3D ED). DNL-11 exhibits a significant water uptake capacity (189 mg/g) at a very low vapor pressure (5% relative humidity at 30 °C). In addition, most of the adsorbed water can be effortlessly removed by purging N2 at 25 °C under ambient pressure conditions. This may expand the possibility of AWH under extreme drought conditions.

Macroporous, Highly Hygroscopic, and Leakage-Free Composites for Efficient Atmospheric Water Harvesting

Hygroscopic composites based on hygroscopic salts and hydrogels are promising for atmospheric water harvesting (AWH), but their relatively low water production and possible salt leakage hinder real applications. Here, we report highly hygroscopic and leakage-free composites from loading LiCl into emulsion-templated sodium alginate and poly(vinyl alcohol) hydrogels with macroporous structures and interpenetrating polymer networks. The resulting composites exhibited an enhanced moisture uptake (up to 3.4 g g-1) and leakage-free behavior even at an extremely high relative humidity (RH) of 90%. Moreover, the composites showed accelerated adsorption, with high adsorption (0.803 g g-1 water at 25 °C and 90% RH within 1 h) and desorption due to the emulsion-templated, highly interconnected macropores. The hygroscopic composites obtained 1.12 g g-1 water per adsorption-desorption collection cycle and showed high reusability, without obvious deterioration in adsorption, desorption, and collection after 10 cycles. With the presence of carbon nanotubes, solar-driven AWH could be realized, without the requirement of additional energy. The highly hygroscopic and leakage-free composites with enhanced and accelerated adsorption and desorption are excellent candidates for efficient AWH.

Progress and perspectives of sorption-based atmospheric water harvesting for sustainable water generation: Materials, devices, and systems

Establishing alternative methods for freshwater production is imperative to effectively alleviate global water scarcity, particularly in land-locked arid regions. In this context, extracting water from the ubiquitous atmospheric moisture is an ingenious strategy for decentralized freshwater production. Sorption-based atmospheric water harvesting (SAWH) shows strong potential for supplying liquid water in a portable and sustainable way even in desert environments. Herein, the latest progress in SAWH technology in terms of materials, devices, and systems is reviewed. Recent advances in sorbent materials with improved water uptake capacity and accelerated sorption-desorption kinetics, including physical sorbents, polymeric hydrogels, composite sorbents, and ionic solutions, are discussed. The thermal designs of SAWH devices for improving energy utilization efficiency, heat transfer, and mass transport are evaluated, and the development of representative SAWH prototypes is clarified in a chronological order. Thereafter, state-of-the-art operation patterns of SAWH systems, incorporating intermittent, daytime continuous and 24-hour continuous patterns, are examined. Furthermore, current challenges and future research goals of this cutting-edge field are outlined. This review highlights the irreplaceable role of heat and mass transfer enhancement and facile structural improvement for constructing high-yield water harvesters.

Hierarchical Engineering of Sorption-Based Atmospheric Water Harvesters

Harvesting water from air in sorption-based devices is a promising solution to decentralized water production, aiming for providing potable water anywhere, anytime. This technology involves a series of coupled processes occurring at distinct length scales, ranging from nanometer to meter and even larger, including water sorption/desorption at the nanoscale, condensation at the mesoscale, device development at the macroscale and water scarcity assessment at the global scale. Comprehensive understanding and bespoke designs at every scale are thus needed to improve the water-harvesting performance. For this purpose, a brief introduction of the global water crisis and its key characteristics is provided to clarify the impact potential and design criteria of water harvesters. Next the latest molecular-level optimizations of sorbents for efficient moisture capture and release are discussed. Then, novel microstructuring of surfaces to enhance dropwise condensation, which is favorable for atmospheric water generation, is shown. After that, system-level optimizations of sorbent-assisted water harvesters to achieve high-yield, energy-efficient, and low-cost water harvesting are highlighted. Finally, future directions toward practical sorption-based atmospheric water harvesting are outlined.

Advanced Material Design and Engineering for Water-Based Evaporative Cooling

Water-based evaporative cooling is emerging as a promising technology to provide sustainable and low-cost cold to alleviate the rising global cooling demand. Given the significant and fast progress made in recent years, this review aims to provide a timely overview on the state-of-the-art material design and engineering in water-based evaporative cooling. The fundamental mechanisms and major components of three water-based evaporative cooling processes are introduced, including direct evaporative cooling, cyclic sorption-driven liquid water evaporative cooling (CSD-LWEC), and atmospheric water harvesting-based evaporative cooling (AWH-EC). The distinctive requirements on the sorbent materials in CSD-LWEC and AWH-EC are highlighted, which helps synthesize the literature information on the advanced material design and engineering for the purpose of improving cooling performance. The challenges and future outlooks on further improving the water-based evaporative cooling performance are also provided.

Biopolymers for Hygroscopic Material Development

The effective management of atmospheric water will create huge value for mankind. Diversified and sustainable biopolymers that are derived from organisms provide rich building blocks for various hygroscopic materials. Here, a comprehensive review of recent advances in developing biopolymers for hygroscopic materials is provided. It is begun with a brief introduction of species diversity and the processes of obtaining various biopolymer materials from organisms. The fabrication of hygroscopic materials is then illustrated, with a specific focus on the use of biopolymer-derived materials as substrates to produce composites and the use of biopolymers as building blocks to fabricate composite gels. Next, the representative applications of biopolymer-derived hygroscopic materials for dehumidification, atmospheric water harvesting, and power generation are systematically presented. An outlook on future challenges and key issues worthy of attention are finally provided.

Title High Solar-Thermal Conversion Aerogel for Efficient Atmospheric Water Harvesting

The shortage of freshwater is a global problem, however, the gel that can be used for atmospheric water harvesting (AWH) in recent years studying, suffer from salt leakage, agglomeration, and slow water evaporation efficiency. Herein, a solar-driven atmospheric water harvesting (SAWH) aerogel is prepared by UV polymerization and freeze-drying technique, using poly(N-isopropylacrylamide) (PNIPAm), hydroxypropyl cellulose (HPC), ethanolamine-decorate LiCl (E-LiCl) and polyaniline (PANI) as raw materials. The PNIPAm and HPC formed aerogel networks makes the E-LiCl stably and efficiently loaded, improving the water adsorption-desorption kinetics, and PANI achieves rapid water vapor evaporation. The aerogel has low density ≈0.12-0.15 g cm-3, but can sustain a weight of 1000 times of its own weight. The synergist of elements and structure gives the aerogel has 0.46-2.95 g g-1 water uptake capability at 30-90% relative humidity, and evaporation rate reaches 1.98 kg m-2 h-1 under 1 sun illumination. In outdoor experiments, 88% of the water is harvesting under natural light irradiation, and an average water harvesting rate of 0.80 gwater gsorbent -1 day-1. Therefore, the aerogel can be used in arid and semi-arid areas to collect water for plants and animals.

Hygroscopic-Microgels-Enabled Rapid Water Extraction from Arid Air

Sorbent-based atmospheric water harvesting (AWH) has emerged as a promising decentralized water-production technology to mitigate the freshwater crisis in arid areas. Hydrogels have been regarded as attractive sorbents due to their high water retention and tailorable polymer-water interactions. Yet, the kinetics of water sorption and desorption at low relative humidity (RH) shall be improved for their practical implementation. Here, hygroscopic microgels (HMGs) composed of hydroxypropyl cellulose (HPC) and hygroscopic salt are reported, which achieve a water uptake of ca. 0.5-0.8 g g-1 at 15-30% RH. HMGs enable rapid sorption-desorption kinetics owing to the short-distance diffusion in the microgels and hydrophilicity-hydrophobicity switching of the thermoresponsive HPC. To validate the feasibility of HMGs for moisture extraction, a potential daily water collection of up to equivalent 7.9-19.1 L kg-1 at low RH is demonstrated, enabled by 24-36 operation cycles per day based on the material-level experiments. With renewable raw materials and superior performance, HMGs provide a sustainable approach for rapid moisture extraction in arid climates.

Covalent Organic Frameworks for Water Harvesting from Air

Despite approximately 70 % of the earth being covered by water, water shortage has emerged as an urgent social challenge. Sorbent-based atmospheric water harvesting stands out as a potent approach to alleviate the situation, particularly in arid regions. This method requires adsorbents with ample working capacity, rapid kinetics, low energy costs, and long-term stability under operating conditions. Covalent organic frameworks (COFs) are a novel class of crystalline porous materials and offer distinct advantages due to their high specific surface area, structural diversity, and robustness. These properties enable the rational design and customization of their water-harvesting capabilities. Herein, the basic concepts about the water sorption process within COFs, including the parameters that qualitatively or quantitatively describe their water isotherms and the mechanism are summarized. Then, the recent methods used to prepare COFs-based water harvesters are reviewed, emphasizing the structural diversity of COFs and presenting the common empirical understandings of these endeavors. Finally, challenges and research concepts are proposed to help develop next-generation COFs-based water harvesters.

Biomimetic Dual Absorption-Adsorption Networked MXene Aerogel-Pump for Integrated Water Harvesting and Power Generation System

Harvesting atmospheric water and converting it into electricity play vital roles in advancing next-generation energy conversion systems. However, the current water harvester systems suffer from a weak water capture ability and poor recyclability due to high diffusion barriers and low sorption kinetics, which significantly limit their practical application. Herein, we drew inspiration from the natural "Pump effect" observed in wood and successfully developed a dual "absorption-adsorption" networked MXene aerogel atmospheric water harvester (MAWH) through ice templating and confining LiCl processes, thereby serving multiple purposes of clean water production, passive dehumidification, and power generation. The MAWH benefits from the dual H-bond network of MXene and cellulose nanocrystals (absorption network) and the hygroscopic properties of lithium chloride (adsorption network). Furthermore, its aligned wood-like channel structure efficiently eliminates water nucleation near the 3D network, resulting in fast moisture absorption. The developed MAWH demonstrates a high moisture absorption ability of 3.12 g g-1 at 90% relative humidity (RH), featuring rapid vapor transport rates and durable cyclic performance. When compared with commercial desiccants such as the 4A molecular sieve and silica gel, the MAWH can reduce the RH from 80% to 20% within just 6 h. Most notably, our integrated MAWH-based water harvesting-power generation system achieves a high voltage of ∼0.12 V at 77% RH, showcasing its potential for practical application. These developed MAWHs are considered as high-performance atmospheric water harvesters in the water collection and power generation field.

In Situ Conversion of Ligand to a Coordination Polymer via a Core@Shell Crystal: A Multi-Step Phase-Dependent Structural Transformation

Two pseudopolymorphic 1D coordination polymers of the formulas [Cd(3,3'-pytz)(CH3OH)2(ClO4)2]n (1) and [Cd(3,3'-pytz)(CH3CN)2(ClO4)2]n (2) have been prepared using the electron-deficient 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine (3,3'-pytz) ligand and cadmium perchlorate in the chloroform/methanol and chloroform/acetonitrile solvent system, respectively. It was observed that compounds 1 and 2 experienced one-step (CPreagent → CPproduct) single-crystal-to-powder structural transformation to the pure water-coordinated compound [Cd(3,3'-pytz)(H2O)2(ClO4)2]n (3) by absorbing water vapor from air (solid-gas phase transformation). Interestingly, compounds 1, 2, and 3 undergo a different transformation path and show an in situ unique three-step (CPreagent → CPproduct → Ligandintermediate → CPproduct) single-crystal-to-single-crystal (SCSC) structural transformation process through soaking in deionized water (solid-liquid phase transformation). In this fascinating transformation, we report for the first time the direct conversion of a ligand into a coordination polymer by a rare core-shell pathway in a solid-liquid phase transformation. In this process, we obtained compound {[Cd(3,3'-pytz)(H2O)4](3,3'-pytz)2(ClO4)2(H2O)6}n (4) (single-crystal = S, crystal = C, or microcrystal = P) as mixed compounds of core-shell L@4C and 4S or core-shell L@4P and 4P for compounds (1 and 2) and 3, respectively.

Two-Dimensional Covalent Organic Frameworks as Tailor-Made Scaffolds for Water Harvesting

Developing materials to harvest water from the air is of great importance to alleviate the water shortage for people living in arid regions, where the annual average relative humidity (RH) is lower than 0.4. In this work, we report a general nitrogen atom incorporation strategy to prepare high-performance covalent organic frameworks (COFs) for water harvesting from the air in arid areas. A series of COFs, namely COF-W1, COF-W2, and COF-W3 were developed for this purpose. Different contents of nitrogen were embedded into COFs by incorporating pyridine units into the building blocks. With the increasing content of nitrogen from COF-W1 to COF-W3, the inflection points of their water isotherms shift distinctly from RH values from 0.65 to 0.25. Significantly, COF-W3 exhibits the lowest inflection point at a low RH value of 0.25 and reaches a high uptake capacity of 0.28 g g-1 at 25 °C with a low hysteresis loop. Moreover, the gram-scale COF-W3 retains its high performance, which renders it more attractive in water harvesting. This work demonstrates the feasibility of this nitrogen incorporation strategy to acquire high-performance COFs as water harvesters in the future.

Molecularly confined hydration in thermoresponsive hydrogels for efficient atmospheric water harvesting

Water scarcity is a pressing global issue, requiring innovative solutions such as atmospheric water harvesting (AWH), which captures moisture from the air to provide potable water to many water-stressed areas. Thermoresponsive hydrogels, a class of temperature-sensitive polymers, demonstrate potential for AWH as matrices for hygroscopic components like salts predominantly due to their relatively energy-efficient desorption properties compared to other sorbents. However, challenges such as limited swelling capacity due to the salting-out effect and difficulty in more complete water release hinder the effectiveness of conventional hydrogel sorbents. To overcome these limitations, we introduce molecularly confined hydration in thermoresponsive hydrogels by employing a bifunctional polymeric network composed of hygroscopic zwitterionic moieties and thermoresponsive moieties. Here, we show that this approach ensures stable water uptake, enables water release at relatively low temperatures, and exhibits rapid sorption-desorption kinetics. Furthermore, by incorporating photothermal absorbers, the sorbent can achieve solar-driven AWH with comparable water release performance. This work advances the design of AWH sorbents by introducing molecularly confined hydration in thermoresponsive hydrogels, leading to a more efficient and sustainable approach to water harvesting. Our findings offer a potential solution for advanced sorbent design with comprehensive performance to mitigate the freshwater crisis.

All-Day Multicyclic Atmospheric Water Harvesting Enabled by Polyelectrolyte Hydrogel with Hybrid Desorption Mode

Sorption-based atmospheric water harvesting (AWH) is a promising approach for mitigating worldwide water scarcity. However, reliable water supply driven by sustainable energy regardless of diurnal variation and weather remains a long-standing challenge. To address this issue, a polyelectrolyte hydrogel sorbent with an optimal hybrid-desorption multicyclic-operation strategy is proposed, achieving all-day AWH and a significant increase in daily water production. The polyelectrolyte hydrogel possesses a large interior osmotic pressure of 659 atm, which refreshes sorption sites by continuously migrating the sorbed water within its interior, and thus enhancing sorption kinetics. The charged polymeric chains coordinate with hygroscopic salt ions, anchoring the salts and preventing agglomeration and leakage, thereby enhancing cyclic stability. The hybrid desorption mode, which couples solar energy and simulated waste heat, introduces a uniform and adjustable sorbent temperature for achieving all-day ultrafast water release. With rapid sorption-desorption kinetics, an optimization model suggests that eight moisture capture-release cycles are capable of achieving high water yield of 2410 mLwater kgsorbent -1 day-1 , up to 3.5 times that of single-cyclic non-hybrid modes. The polyelectrolyte hydrogel sorbent and the coupling with sustainable energy driven desorption mode pave the way for the next-generation AWH systems, significantly bringing freshwater on a multi-kilogram scale closer.

Single-Particle Tracking in Poly(Ethylene Glycol) Diacrylate: Probe Size Effect on the Diffusion Behaviors of Nanoparticles in Unentangled Polymer Solutions

A thorough understanding of the relevant factors governing the transport of nanoparticles in poly(ethylene glycol) diacrylate (PEGDA) is crucial for many applications utilizing this polymer. Here, single-particle tracking (SPT) was used to systematically investigate the role of the probe size (3-200 nm) on the diffusion behaviors of individual fluorescent nanoparticles in semidilute and unentangled PEGDA solutions. The quantitative assessment of the SPT data via the recorded single-particle trajectories and diffusion coefficients (D) not only showed that the observed probe dynamics in PEGDA were temporally and spatially heterogeneous, but more importantly that the measured D were observed to be significantly reduced (vs in solvent) and strongly size-dependent. We explained these results based on a modified multiscale model for particle diffusion, built upon well-established hydrodynamics and obstruction theories. We furthermore showed that the presence of steric interactions and probe confinement effects in highly crowded, unentangled PEGDA microstructures can lead to deviations in the single-particle displacements from the expected Gaussian behavior, as revealed by the van Hove displacement distributions and the associated non-Gaussian parameters. This study has demonstrated the power of SPT methods in offering an advanced characterization of the transport characteristics in complex polymer structures, overcoming challenges posed by traditional characterization techniques.

Dehydration of a crystal hydrate at subglacial temperatures

Water is one of the most important substances on our planet1. It is ubiquitous in its solid, liquid and vaporous states and all known biological systems depend on its unique chemical and physical properties. Moreover, many materials exist as water adducts, chief among which are crystal hydrates (a specific class of inclusion compound), which usually retain water indefinitely at subambient temperatures2. We describe a porous organic crystal that readily and reversibly adsorbs water into 1-nm-wide channels at more than 55% relative humidity. The water uptake/release is chromogenic, thus providing a convenient visual indication of the hydration state of the crystal over a wide temperature range. The complementary techniques of X-ray diffraction, optical microscopy, differential scanning calorimetry and molecular simulations were used to establish that the nanoconfined water is in a state of flux above -70 °C, thus allowing low-temperature dehydration to occur. We were able to determine the kinetics of dehydration over a wide temperature range, including well below 0 °C which, owing to the presence of atmospheric moisture, is usually challenging to accomplish. This discovery unlocks opportunities for designing materials that capture/release water over a range of temperatures that extend well below the freezing point of bulk water.

Covalent Organic Frameworks for Atmospheric Water Harvesting

Atmospheric water harvesting using reticular materials is an innovation that has the potential to change the world. Using covalent organic frameworks (COFs) for capturing water holds great promise because COFs are metal-free, stable under working conditions, and their structure can be intentionally designed to meet the requirements for this application. To promote the chemistry and use of COFs for atmospheric water harvesting, important features for synthesizing suitable water-harvesting COFs are discussed. The achievements of using COFs as water harvesters are then highlighted, showing how the water harvesting properties are related to the structural design. Finally, perspectives and research directions for further studies in COF chemistry are provided.

2D Covalent Organic Framework for Water Harvesting with Fast Kinetics and Low Regeneration Temperature

Atmospheric water harvesting represents a promising technique to address water stress. Advanced adsorbents have been rationally designed to achieve high water uptake, yet their water sorption kinetics and regeneration temperature greatly limit water production efficiency. Herein, we demonstrated that 2D covalent organic frameworks (COFs), featuring hydrophobic skeleton, proper hydrophilic site density, and 1D open channels significantly lowered the water diffusion and desorption energy barrier. DHTA-Pa COF showed a high water uptake of 0.48 g/g at 30 % R.H. with a remarkable adsorption rate of 0.72 L/Kg/h (298 K) and a desorption rate of 2.58 L/Kg/h (333 K). Moreover, more than 90 % adsorbed water could be released within 20 min at 313 K. This kinetic performance surpassed the reported porous materials and boosted the efficiency for multiple water extraction cycles. It may shed light on the material design strategy to achieve high daily water production with low-energy input.

Hygroscopic and Photothermal All-Polymer Foams for Efficient Atmospheric Water Harvesting, Passive Humidity Management, and Protective Packaging

Environmental humidity and thermal control are of primary importance for fighting global warming, growing energy consumption, and greenhouse gas emissions. Sorption-based atmospheric water harvesting is an emerging technology with great potential in clean water production and passive cooling applications. However, sorption-based humidity management and their hybrid applications are limited due to the lack of energywise designs of hygroscopic materials and devices. Herein, all polymeric 3D foams are developed and evaluated as hygroscopic and photothermal materials. The gas-foaming method generates closed-cell structures with interconnected hydrophilic networks and wrinkled surfaces, expanding hygroscopic, photothermal, and evaporating areas of the 3D foams. These unique advantages lead to efficient water vapor sorption in a wide broad relative humidity (RH) range of 50-90% and efficient water release in a wide solar intensity (0.4-1 sun) and temperature range (27-80 °C). The reversible moisture sorption/release in 50 adsorption/desorption cycles highlights the excellent durability of the 3D foams compared to conventional inorganic desiccants. The 3D foams disclose passive and efficient apparent temperature regulation in warm and humid environments. Moreover, the use of the 3D foams as loose fill for fruit preservation and packaging is demonstrated for the first time by taking the merit of the 3D foams' moisture-absorbing, quick-drying, cushioning, and thermal-insulating properties. This work presents an integrated design of polymeric desiccants and scaffolds, not merely delivering stable water adsorption/desorption but also discovering innovative hybrid applications in humidity management and protective packaging.

Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2 N-Type Materials

Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C₂ N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C₂ N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C₂ N material condensed at 700 °C remarkably increased from 0.026 s^-1 to 0.036 s^-1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability.