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.

Gradient-Janus Wires for Simultaneous Fogwater Harvesting and Electricity Generation

A Gradient-Janus wire (GJW) with a diameter of 0.3 mm has been fabricated on a large scale through liquid confined modification, enabling the opposite conical wetting phenomenon along the same orientation of the GJW, characterized by an increasing superhydrophilic region and a decreasing hydrophobic region. This property allows the GJW to exhibit controllable water hovering, transport, and pinning during fog harvesting, i.e., at a large tilting angle α of 60° (mass increased with decreased α), the GJW can hover 0.6 mg of harvested fogwater in 30 s, can transport 3 mg of fogwater along the gradient in 30 s at α = 4° (with maximal mass reaching up to 4.3 mg at α = -10°), and finally, pin the water droplet at the end of the GJW. Such ability generates an effective torque that serves as the driving force for rotation. We designed a GJWs-wheel by radially arranging 60 GJWs together, resulting in an extremely lightweight structure weighing only 1.9 g. The cumulative torque generated during fog harvesting activates the rotation of the GJWs-wheel. When loaded with a coil within a magnetic field, electricity is generated as output power peaks at around 0.25 μW while maintaining a high water harvesting efficiency averaging approximately 38 ± 2.12 mg/min. This finding is significant as it provides valuable insights into designing materials capable of efficiently harnessing both energy and water resources.

When Polymorphism in Metal-Organic Frameworks Enables Water Sorption Profile Tunability for Enhancing Heat Allocation and Water Harvesting Performance

The development of thermally driven water-sorption-based technologies relies on high-performing water vapor adsorbents. Here, polymorphism in Al-metal-organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans- or cis- µ-OH-connected corner-sharing AlO4(OH)2 octahedra. Specifically, [Al(OH)(muc)] or MIP-211, is made of trans, trans-muconate linkers, and cis-µ-OH-connected corner-sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL-53-muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL-53-muc, to P/P0 ≈ 0.3 in MIP-211. Solid-state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis-positioning in MIP-211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP-211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP-211 is among the best adsorbents for adsorption-driven air conditioning and water harvesting from the air.

Confined Water Cluster Formation in Water Harvesting by Metal-Organic Frameworks: CAU-10-H versus CAU-10-CH3

Several metal-organic frameworks (MOFs) excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10-30 % relative humidity (RH%). A precise understanding of which structural characteristics govern such behavior is lacking. Herein, CAU-10-H and CAU-10-CH3 are studied with H, CH3 corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake ≈18 RH% of interest for water harvesting, yet the subtle replacement of H by CH3 in the organic linker drastically changes the water adsorption behavior to less steep water uptake at much higher humidity values. The materials' structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations are unraveled. In CAU-10-H, an energetically favorable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework μOH groups, while for CAU-10-CH3, such a favorable cluster cannot form. By relating the findings to the features of water adsorption isotherms of a series of MOFs, it is concluded that favorable water adsorption occurs when sites of intermediate hydrophilicity are present in a hydrophobic structure, and the formation of energetically favorable water clusters is possible.

3D Printed Cellulose Nanofiber Aerogel Scaffold with Hierarchical Porous Structures for Fast Solar-Driven Atmospheric Water Harvesting

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 0.63 g g-1 day-1 of water outdoors under natural sunlight. This article provides a sustainable strategy for collecting freshwater from the atmosphere.

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.

Damage Tolerance of Superhydrophobic Coatings with Binary Cooperative Cells for Water Harvesting

Multifunction superhydrophobic coatings that facilitate water harvesting are attractive for addressing the daunting water crisis, yet, they are caught in a double bind when their durability is considered, as durable coatings will require both tough micro-textures to survive concentrated stress and high-surface-energy chemistry to form chemical bonds within the matrix. To date, a universal bulk-phase coating that combines multifunctionality, ultra-durability, and fabrication feasibility remains challenging. Here, a binary cooperative cell design is reported that can solve the contradiction between the multifunctionality and durability requirements of superhydrophobic coatings. In this strategy, mechanochemically tailored cells with releasable nanoseeds are infused in the common matrix, which serves both as a versatile chemical bridge to achieve strong bonds within the coating building blocks, and as an instantaneous self-repairing generator to improve durability. Such a strategy significantly boosted the wear resistance and outdoor stability of the coatings by over 30-100 and 18 folds, respectively, compared with conventional coatings. The coating is applied to the sustainable application, i.e., enhancing the water collection efficiency by at least 1000% even after harsh abrasion. The strategy will broaden the vision in handling the dilemma properties among functional coatings and promote the application of superhydrophobic coatings in extreme environments.

Unravelling the Water Adsorption Mechanism in Hierarchical MOFs: Insights from In Situ Positron Annihilation Lifetime Studies

Atmospheric water harvesting with metal-organic frameworks (MOFs) is a new technology providing a clean, long-term water supply in arid areas. In-situ positron annihilation lifetime spectroscopy (PALS) is proposed as a valid methodology for the mechanistic understanding of water sorption in MOFs and the selection of prospective candidates for desired applications. DUT-67-Zr and DUT-67-Hf frameworks are used as model systems for method validation because of their hierarchical pore structure, high adsorption capacity, and chemical stability. Both frameworks are characterized using complementary techniques, such as nitrogen (77 K) and water vapor (298 K) physisorption, SEM, and PXRD. DUT-67-Zr and DUT-67-Hf are investigated by PALS upon exposure to humidity for the first time, demonstrating the stepwise pore filling mechanism by water molecules for both MOFs. In addition to exploring the potential of PALS as a tool for probing MOFs during in situ water loading, this work offers perspectives on the design and use of MOFs for water harvesting.

Connectivity Analysis of Adsorption Sites in Metal-Organic Frameworks for Facilitated Water Adsorption

Metal-organic frameworks (MOFs) have recently drawn considerable attention as promising adsorbents to harvest atmospheric water. To achieve an efficient harvesting process, seeking MOFs that demonstrate sharp condensation behavior is the key. Given that the clustering of water molecules in MOFs should be driven by not only MOF-water interactions but also water-water interactions, the spatial arrangement of water adsorption sites in a MOF is therefore crucial. Specifically, this study demonstrates the critical role of continuous adsorption channels (CACs) in MOFs. Such CACs will enable water molecules to stay in proximity and in a continuous manner, thus promoting the formation of hydrogen bonds and, consequently, the clustering of water molecules. We have developed an automatic algorithm to detect CACs based on the energy grid of host-guest interactions and applied the algorithm to more than 2000 diverse structures. The results show that more than 80% of the studied MOFs displaying water condensation at 298 K and 20% relative humidity predicted by Monte Carlo simulations indeed have CACs. The developments herein are anticipated to largely facilitate the future discovery of optimal adsorbents for water harvesting or water-adsorption-related applications in general. A Python-based code for detecting CACs in porous materials is also provided along with this article to employ this approach.

Salt Confined in MIL-101(Cr)-Tailoring the Composite Sorbents for Efficient Atmospheric Water Harvesting

The adsorption method for atmospheric water harvesting (AWH) is considered as a promising heat-driven technology for potable water supply in arid regions. This research is focused on novel composite sorbents based on hygroscopic salts loaded in the pores of MIL-101(Cr) developed for AWH. The composites based on LiCl, LiBr, CaCl2 , and Ca(NO3 )2 were synthesized and comprehensively studied by SEM, XRD, N2 adsorption, and thermogravimetric methods. We evidence that the CaCl2 /MIL-101(Cr) composite demonstrates a high net water uptake of 0.52-0.59 g_(H2 O)/g_(composite) per cycle under conditions of Saudi Arabia and the Sahara desert as the reference regions with extra-dry climate, which exceeds the appropriate values for other adsorbents. It is shown that water adsorption on the composite cannot be presented as a combination of the adsorption on the components, thus indicating a synergistic effect. A detailed characterization of water coordination, mobility, and hydrogen bonding within the confined CaCl2 hydrates and salt solution using solid-state 2 H NMR spectroscopy has been performed. It is established that pore confinement promotes a prolonged transition to a dynamically melted state of the hydrated salt and a notable decrease of the melting temperature, which facilitates the molecular transport of water and causes the alteration of sorption properties of CaCl2 inside MIL-101 pores. Finally, the performance of AWH employing CaCl2 /MIL-101(Cr) was evaluated in terms of the fractions of water extracted and collected, and the specific energy consumption, demonstrating its high potential for AWH.

Hierarchically Branched Siloxane Brushes for Efficient Harvesting of Atmospheric Water

Atmospheric water harvesting is considered a viable source of freshwater to alleviate water scarcity in an arid climate. Water condensation tends to be more efficient on superhydrophobic surfaces as the spontaneous coalescence-induced droplet jumping on superhydrophobic surfaces enables faster condensate removal. However, poor water nucleation on these surfaces leads to meager water harvest. A conventional approach to the problem is to fabricate micro- and nanoscale biphilic structures. Nonetheless, the process is complex, expensive, and difficult to scale. Here, the authors present an inexpensive and scalable method based on manipulating the water-repellent coatings of superhydrophobic surfaces. Flexible siloxane can facilitate water nucleation, while a branched structure promotes efficient droplet jumping. Moreover, ToF-SIMS analysis indicated that branched siloxane provides a better water-repellent coating coverage than linear siloxane and the siloxanes comprise hydrophilic and hydrophobic molecular segments. Thus, the as-prepared superhydrophobic surface, TiO2 nanorods coated with branched siloxanes harvested eight times more water than a typical fluoroalkylsilane (FAS)-coated surface under a low 30% relative humidity and performed better than most reported biphasic materials.

Nature-Inspired Surface Engineering for Efficient Atmospheric Water Harvesting

Atmospheric water harvesting is a sustainable solution to global water shortage, which requires high efficiency, high durability, low cost, and environmentally friendly water collectors. In this paper, we report a novel water collector design based on a nature-inspired hybrid superhydrophilic/superhydrophobic aluminum surface. The surface is fabricated by combining laser and chemical treatments. We achieve a 163° contrast in contact angles between the superhydrophilic pattern and the superhydrophobic background. Such a unique superhydrophilic/superhydrophobic combination presents a self-pumped mechanism, providing the hybrid collector with highly efficient water harvesting performance. Based on simulations and experimental measurements, the water harvesting rate of the repeating units of the pattern was optimized, and the corresponding hybrid collector achieves a water harvesting rate of 0.85 kg m-2 h-1. Additionally, our hybrid collector also exhibits good stability, flexibility, as well as thermal conductivity and hence shows great potential for practical application.

Reticular Synthesis of Metal-Organic Frameworks by 8-Connected Quadrangular Prism Ligands for Water Harvesting

Utilization of rigid, highly connected organic linkers is critical for the reticular synthesis of functional metal-organic frameworks (MOFs). However, highly-stable MOFs (e.g. Al/Cr/Zr-based MOFs) based on rigid ligands with more than 6 coordinating functions have been rarely achieved thus far. Herein, we describe the construction of two bcu Zr-based MOFs (named ZrMOF-1 and ZrMOF-2) from peripherally extended pentiptycene ligands (H8PEP-1 and H8PEP-2) with rigid quadrangular prism shape possessing 8 carboxylic groups at the prism vertices. Particularly, ZrMOF-1 exhibits microporous structure with large Bruno-Emmett-Teller surface area and high water stability, endowing it a promising water harvesting material with a high water uptake capacity of 0.83 gH2O gMOF-1 at P/P0 = 0.90 and 25 °C, a steep uptake at a low P/P0 of 0.30, and excellent durability over 500 water adsorption-desorption cycles. Moreover, self-consistent charge density functional tight-binding calculations were carried out, rationalizing the water adsorbing process and amount in ZrMOF-1.

Unusual Temperature Dependence of Water Sorption in Semicrystalline Hydrogels

Water vapor sorption is a ubiquitous phenomenon in nature and plays an important role in various applications, including humidity regulation, energy storage, thermal management, and water harvesting. In particular, capturing moisture at elevated temperatures is highly desirable to prevent dehydration and to enlarge the tunability of water uptake. However, owing to the thermodynamic limit of conventional materials, sorbents inevitably tend to capture less water vapor at higher temperatures, impeding their broad applications. Here, an inverse temperature dependence of water sorption in poly(ethylene glycol) (PEG) hydrogels, where their water uptake can be doubled with increasing temperature from 25 to 50 °C, is reported. With mechanistic modeling of water-polymer interactions, this unusual water sorption is attributed to the first-order phase transformation of PEG structures, and the key parameters for a more generalized strategy in materials development are identified. This work elucidates a new regime of water sorption with an unusual temperature dependence, enabling a promising engineering space for harnessing moisture and heat.

Emissive Substoichiometric Covalent Organic Frameworks for Water Sensing and Harvesting

Emissive covalent organic frameworks (COFs) have recently emerged as next-generation porous materials with attractive properties such as tunable topology, porosity, and inherent photoluminescence. Among the different types of COFs, substoichiometric frameworks (so-called Type III COFs) are especially attractive due to the possibility of not only generating unusual topology and complex pore architectures but also facilitating the introduction of well-defined functional groups at precise locations for desired functions. Herein, the first example of a highly emissive (PLQY 6.8%) substoichiometric 2D-COF (COF-SMU-1) featuring free uncondensed aldehyde groups is reported. In particular, COF-SMU-1 features a dual-pore architecture with an overall bex net topology, tunable emission in various organic solvents, and distinct colorimetric changes in the presence of water. To gain further insights into its photoluminescence properties, the charge transfer, excimer emission, and excited state exciton dynamics of COF-SMU-1 are investigated using femtosecond transient absorption spectroscopy in different organic solvents. Additionally, highly enhanced atmospheric water-harvesting properties of COF-SMU-1 are revealed using FT-IR and water sorption studies.The findings will not only lead to in-depth understanding of structure-property relationships in emissive COFs but also open new opportunities for designing COFs for potential applications in solid-state lighting and water harvesting.

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.

Combining Polymerization and Templating toward Hyper-Cross-Linked Poly(propargyl aldehyde)s and Poly(propargyl alcohol)s for Reversible H(2)O and CO(2) Capture and Construction of Porous Chiral Networks

Two series of hyper-cross-linked microporous polyacetylene networks containing either -[CH=C(CH=O)]- or -[CH=C(CH₂OH)]- monomeric units are reported. Networks are prepared by chain-growth copolymerization of acetal-protected propargyl aldehyde and acetal-protected propargyl alcohol with a 1,3,5-triethynylbenzene cross-linker followed by hydrolytic deprotection/detemplating. Deprotection not only liberates reactive CH=O and CH₂OH groups in the networks but also modifies the texture of the networks towards higher microporosity and higher specific surface area. The final networks with CH=O and CH₂OH groups attached directly to the polyene main chains of the networks have a specific surface area from 400 to 800 m²/g and contain functional groups in a high amount, up to 9.6 mmol/g. The CH=O and CH₂OH groups in the networks serve as active centres for the reversible capture of CO₂ and water vapour. The water vapour capture capacities of the networks (up to 445 mg/g at 297 K) are among the highest values reported for porous polymers, making these materials promising for cyclic water harvesting from the air. Covalent modification of the networks with (R)-(+)-3-aminopyrrolidine and (S)-(+)-2-methylbutyric acid enables the preparation of porous chiral networks and shows networks with CH=O and CH₂OH groups as reactive supports suitable for the anchoring of various functional molecules.

Acoustic Tweezer-Modulated Biomimetic Patterned Particle-Polymer Composite for Water Vapor Harvesting

With the recent threat of climate change and global warming, ensuring access to safe drinking water is a great challenge in many areas worldwide. Designing functional materials for capturing water from natural resources like fog and mist has become one of the key research areas to maximize the production of clean water. From this aspect, nature is a great source for designing bioinspired functional materials as some of the plant leaves and animal exoskeletons can harness water and then store it to save themselves from arid, xeric conditions. Inspired by the Stenocara beetle, we have designed a composite surface structure with periodic islands made of aluminum microparticles surrounded by poly(dimethylenesiloxane) (PDMS). An acoustic tweezer-based method was used to fabricate the bioinspired composite structures, where surface acoustic waves at specific frequencies and amplitudes are applied to align the microparticles as islands in the polymer matrix. An oxygen plasma etching step was applied to expose the microparticles on the PDMS surface. The average water harvesting efficiencies for structures made with 120 and 80 kHz acoustic frequencies and 1 hour etching time were found to be 9.41 and 8.84 g cm^-2 h^-1, respectively. The acoustically patterned biomimetic composite surface showed higher water harvesting efficiency compared with completely hydrophobic PDMS and hydrophilic aluminum surfaces, demonstrating the advantages of the bioinspired composite material design and acoustic-assisted manufacturing technique. The biomimetic fog water harvesting material is a promising avenue to fulfill the demand for a cost-effective, sustainable, and energy-efficient solution to safe drinking water.

One-Step Synthesis of Dynamically Shaped Stiff Nanorods Using Soft Silicone Materials to Control Water Repulsion and Collection

One-dimensional silicone nanostructures, such as filaments, wires, and tubes, have attracted significant attention, owing to their remarkable application capabilities in a large range of material and surface science. However, the soft mechanical properties of silicone cause vulnerability and irregularity in the synthesized structures, which limits their applications. Herein, a simple, solvent-free, and efficient dynamic Droplet Assisted Growth and Shaping (d-DAGS) strategy is proposed for the one-step synthesis and in situ control of the shape of silicone nanostructures. The special designed bamboo-shaped silicone nanorods (SNRs) that are produced by the repetitive dynamic regulation of growth conditions, concomitant with the periodic purging and injection of precursors, exhibit highly-regular and tunable structure with a specific number of segments, indicating that they can be tailor-made according to the requirements of various properties. The enhanced mechanical stiffness and chemical durability strongly support their excellent performances in water-resistance under both static and dynamic wetting conditions. The SNRs significantly promote buoyancy and self-cleaning properties; and exhibit very high water-harvesting efficiency compared with existing designs. Notably, the well-structured ultra-long rods with an ultrahigh aspect ratio (≈176) can also be fabricated by the d-DAGS method, and they can remain standing straight upwards and regular, even though they consist of flexible silicone.

Hydrophilic reentrant SLIPS enabled flow separation for rapid water harvesting

Water harvesting from air has the potential to alleviate water scarcity in arid regions around the globe. To achieve efficient water harvesting, we prefer rapid vapor condensation and droplet collection simultaneously. Prior techniques are not able to separate the vapor and liquid flow, so the condensed droplets always hinder the vapor condensation. In this work, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface. The slippery reentrant channels absorb the condensed droplets, lock the liquid columns inside, and transport them to the end of each channel. As a result, the sustainable flow separation significantly increases the water harvesting rate.

Water harvesting from air is desired for decentralized water supply wherever water is needed. When water vapor is condensed as droplets on a surface the unremoved droplets act as thermal barriers. A surface that can provide continual droplet-free areas for nucleation is favorable for condensation water harvesting. Here, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface (SLIPS) that rapidly removes droplets with diameters above 50 μm. The slippery reentrant channels lock the liquid columns inside and transport them to the end of each channel. We demonstrate that the liquid columns can harvest the droplets on top of the hydrophilic reentrant SLIPS at a high droplet removal frequency of 130 Hz/mm². The sustainable flow separation without flooding increases the water harvesting rate by 110% compared to the state-of-the-art hydrophilic flat SLIPS. Such a flow-separation condensation approach paves a way for water harvesting.

Marangoni Effect Drives Salt Crystallization Away from the Distillation Zone for Large-Scale Continuous Solar Passive Desalination

Solar desalination shows great potential in dealing with global water scarcity. A multistage passive solar distiller with thermal localization is especially attractive for its high-water yield. However, achieving long-term stability in large-scale devices remains a challenge because of the easy accumulation of crystallized salt inside the distiller. Here, we reported that the Marangoni effect can drive crystallized salt away along a long distance in a capillary wick, which endow the multistage passive solar distiller with the ability of salt-rejecting. In a 36 h continuous testing, the salinity of the distillation zone is limited below 12 wt % and crystallized salt only accumulates outside the device. The water yield is about 1.7 kg m^-2 h^-1 in a three-stage device, with a solar-to-vapor conversion efficiency of 114% under one sun. This novel design proves a new principle for high efficiency and long-term stable solar desalination.

All-Day Freshwater Harvesting by Selective Solar Absorption and Radiative Cooling

Solar interfacial evaporation for freshwater harvesting has received attention recently due to its high evaporation rate and environmental friendliness. Traditional interfacial evaporation mostly uses black porous polymers to absorb solar radiation and transport water which involve high thermal radiation loss to the environment and heat conduction loss to the bulk water. In addition, the freshwater collection ratio is usually lower than the solar evaporation ratio due to the high temperature of the condensation surface under solar irradiation, and no freshwater can be harvested at night due to the absence of sunlight. Here, we design an all-day freshwater-harvesting device using a solar-selective absorber (SSA) and sky radiative cooling. The prepared SSA with a high solar absorptance of 0.92 and a mid-infrared thermal emittance of 0.11 provides a great solar-thermal conversion performance (87.1% vs 51.4% for the black porous polymer at 25 °C) by minimizing the thermal radiation loss, and a hollow structure is also used to reduce the conductive heat loss, resulting in a high solar evaporation rate (1.23 vs 0.79 kg m^-2 h^-1 for the black porous polymer). In addition, a transparent radiative cooling polymer after plasma treatment is used for freshwater collection by enhancing the solar transmittance (0.92) and mid-infrared thermal emittance (0.91 at 25 °C). A theoretical freshwater collection rate of 0.044 kg m^-2 h^-1 is achieved at night-time. Outdoor results show that the all-day water harvesting is 0.87 kg m^-2. This strategy to achieve all-day water collection by coupling with the SSA and transparent radiative cooling has potential application in the field of desalination and freshwater harvesting in tropical desert areas.

Linker Functionalization Strategy for Water Adsorption in Metal-Organic Frameworks

Water adsorption in metal-organic frameworks has gained a lot of scientific attention recently due to the potential to be used in adsorption-based water capture. Functionalization of their organic linkers can tune water adsorption properties by increasing the hydrophilicity, thus altering the shape of the water adsorption isotherms and the overall water uptake. In this work, a large set of functional groups is screened for their interaction with water using ab initio calculations. The functional groups with the highest water affinities form two hydrogen bonds with the water molecule, acting as H-bond donor and H-bond acceptor simultaneously. Notably, the highest binding energy was calculated to be -12.7 Kcal/mol for the -OSO₃H group at the RI-MP2/def2-TZVPP-level of theory, which is three times larger than the reference value. Subsequently, the effect of the functionalization strategy on the water uptake is examined on a selected set of functionalized MOF-74-III by performing Monte Carlo simulations. It was found that the specific groups can increase the hydrophilicity of the MOF and enhance the water uptake with respect to the parent MOF-74-III for relative humidity (RH) values up to 30%. The saturation water uptake exceeded 800 cm³/cm³ for all candidates, classifying them among the top performing materials for water harvesting.

Machine Learning-Assisted Computational Screening of Metal-Organic Frameworks for Atmospheric Water Harvesting

Atmospheric water harvesting by strong adsorbents is a feasible method of solving the shortage of water resources, especially for arid regions. In this study, a machine learning (ML)-assisted high-throughput computational screening is employed to calculate the capture of H2O from N2 and O2 for 6013 computation-ready, experimental metal-organic frameworks (CoRE-MOFs) and 137,953 hypothetical MOFs (hMOFs). Through the univariate analysis of MOF structure-performance relationships, Qst is shown to be a key descriptor. Moreover, three ML algorithms (random forest, gradient boosted regression trees, and neighbor component analysis (NCA)) are applied to hunt for the complicated interrelation between six descriptors and performance. After the optimizing strategy of grid search and five-fold cross-validation is performed, three ML can effectively build the predictive model for CoRE-MOFs, and the accuracy R2 of NCA can reach 0.97. In addition, based on the relative importance of the descriptors by ML, it can be quantitatively concluded that the Qst is dominant in governing the capture of H2O. Besides, the NCA model trained by 6013 CoRE-MOFs can predict the selectivity of hMOFs with a R2 of 0.86, which is more universal than other models. Finally, 10 CoRE-MOFs and 10 hMOFs with high performance are identified. The computational screening and prediction of ML could provide guidance and inspiration for the development of materials for water harvesting in the atmosphere.

Sustainable Superhydrophobic Surface with Tunable Nanoscale Hydrophilicity for Water Harvesting

Fog collection can be a sustainable solution to water scarcity in many regions around the world. Recently, great efforts have been undertaken to develop low-cost and highly efficient water collectors to address water shortages, especially in arid regions. However, the design of a scalable water harvesting surface remains elusive to the trade-off between water deposition and transport. Herein, we developed a hydrophilic/superhydrophobic surface using a "one-pot" facile approach to enable an efficient water deposition and transport process. Preferential exposure of hydrophilic cellulose nanocrystal outer surface could be used to accelerate droplet deposition, coupled with wax microspheres with distinct wetting features for the manipulation of the droplet mobility. Appropriate tuning of the wetting characteristics of the surfaces, optimizing the hydrophobicity and density of the water affinity nanodomains allowed us to enhance the water deposition rate without the sacrifice of water transport. An optimal hydrophilic/superhydrophobic topography through the control of nanoscale hydrophilic and hydrophobic domains yielded a water harvesting flux of 3.402 L/m 2 /h for a plate and 5.02 L/m 2 /h for a mesh. This strategy of decorating a superhydrophobic surface with moderately hydrophilic nanodomains allows the manipulation of droplet nucleation and removal to enhance the water collection efficiency.

Honeycomb-Inspired Robust Hygroscopic Nanofibrous Cellular Networks

Mimicking nature is a highly efficient and meaningful way for designing functional materials. However, constructing bioinspired nanofibrous 3D cellular networks with robust mechanical features is extremely challenging. Herein, a biomimetic, super-flexible, highly elastic, and tough nanofibrous membrane (NFM)-based water harvester is reported with a highly ordered honeycomb-inspired gradient network structure, self-assembled from electrospun spider-silk-like humped nanofibers. The resultant NFM exhibits super flexibility, high tensile strength (2.9 MPa), superior elasticity, and decent toughness (3.39 MJ m^-3 ), allowing it to be used as the framework of hygroscopic materials. The resulting hygroscopic NFM displays excellent moisture absorption performance, which can be used as an efficient water harvester with a superhigh equilibrium moisture absorption capacity of 4.60 g g^-1 at 95% relative humidity for 96 h, fast moisture absorption and transport rates, and long-term durability, achieving directional transport and collection of tiny water droplets. This work paves the way for the design and development of multifunctional NFMs with a honeycomb-inspired gradient network structure.

A Hydrolytically Stable Cu(II)-Based Metal-Organic Framework with Easily Accessible Ligands for Water Harvesting

Water scarcity is a critical issue in desert and arid regions, and atmospheric water harvesting is a potential solution. The challenge is lacking ideal adsorbents that can efficiently capture water from low-humidity air and be regenerated readily. Herein, we report a hydrolytically stable metal-organic framework (MOF), [Cu₂(AD)₂(SA)] (Cu-AD-SA), with excellent performance in water harvesting. More importantly, this material can be facilely prepared from two easily accessible ligands adenine (HAD) and succinic acid (H₂SA). Cu-AD-SA has a three-dimensional (3D) framework structure with the crs topology and intersecting channels of ∼5 Å in diameter. The channel surface is decorated by uncoordinated aromatic N atoms, amine groups, and alkyl moieties. Interestingly, Cu-AD-SA shows a high water adsorption capacity of 0.16 g g^-1 at low pressure of 0.2 P/P₀ and 25 °C. Furthermore, dynamic water adsorption-desorption cycling experiments demonstrated a stable working capacity of 0.13 g g^-1 for uptaking water from a low-humidity air (water partial pressure: 0.85 kPa, 20% RH at 30 °C, 5.3% RH at 55 °C) at 30 °C and desorption at 55 °C. The water adsorption mechanism was also studied by analyzing its single-crystal structure after water loading. The results indicated the existence of strong H-bonding interactions between water molecules and uncoordinated N atoms and amine groups on the framework, which should play an important role in the high adsorption at low pressure. All the above features suggest great potential of Cu-AD-SA for water harvesting in arid regions.

Domino-like water transport on Tillandsia through flexible trichome wings

Tillandsia usneoides in epiphytic bromeliads takes up water through absorptive trichomes on the shoot surface under extreme environmental conditions. Although previous studies revealed the way by which T. usneoides absorbs water and prevents water loss, its water transport remains unclear. We characterized structures of trichome wings of T. usneoides. Wing length-to-thickness ratio of 136 and trichome interval (d)-to-wing length (l) ratio (d/l) smaller than 1 caused the water film to flatten the wings sequentially, resulting in domino-like water transport. A hinge-like linkage between wing and outer ring cells and the wing size longer than the elastocapillary length (L_EC ) brought about this unique reconfiguration, which is the flattening and recovery of wings. Tillandsia usneoides transported water rapidly on the surface as the water film propagated on the exterior trichomes with flexible wings and the transport distance at the macroscopic scale grew as t^x with x = 0.68 ± 0.04, unlike the conventional scaling of t^0.5 . Empirical and theoretical investigations proved our assumption that external water transport with the domino-like effect predominated over internal vascular transport. Biomimetic trichome wings simulated the domino-like water transport, highlighting the important role of flexible wing arrays.

Microporous Hyper-Cross-Linked Polymers with High and Tuneable Content of Pyridine Units: Synthesis and Application for Reversible Sorption of Water and Carbon Dioxide

New hyper-cross-linked porous organic polymers (POPs) with a high content of pyridine segments (7.86 mmol pyridine g^-1 ), and a micro/mesoporous texture are reported. The networks are achieved by the chain-growth homopolymerization of 2,6- and 3,5-diethynylpyridines. The pyridine segments form links interconnecting the polyacetylene main chains in these networks. The content of pyridine segments in the networks can be tuned by copolymerizing diethynylpyridines with 1,3-diethynylbenzene. The pyridine rings in the networks serve as base and hydrophilic centers for the sorption of CO₂ and water. The homopolymer pyridine networks are highly efficient in the low-pressure adsorption/desorption of CO₂ . This sorption mode is promising for the postcombustion removal of CO₂ from the fuel gas. The poly(3,5-diethynylpyridine) network exhibits high efficiency in capturing and releasing water vapor (determined capacity 376 mg g^-1 at 298 K and relative humidity (RH) = 90% is one of the highest values reported for POPs) and is a promising material for the cyclic water harvesting from air. The reported networks are characterized by ¹³ C cross-polarization magic angle spinning NMR, thermogravimetric analysis, and N₂ adsorption/desorption and their efficiency in CO₂ and H₂ O capturing is discussed in relation to the content and type of incorporated pyridine segments.

Surface Potential Driven Water Harvesting from Fog

Access to clean water is a global challenge, and fog collectors are a promising solution. Polycarbonate (PC) fibers have been used in fog collectors but with limited efficiency. In this study, we show that controlling voltage polarity and humidity during the electrospinning of PC fibers improves their surface properties for water collection capability. We experimentally measured the effect of both the surface morphology and the chemistry of PC fiber on their surface potential and mechanical properties in relation to the water collection efficiency from fog. PC fibers produced at high humidity and with negative voltage polarity show a superior water collection rate combined with the highest tensile strength. We proved that electric potential on surface and morphology are crucial, as often designed by nature, for enhancing the water collection capabilities via the single-step production of fibers without any postprocessing needs.

Machine-Learning-Assisted Autonomous Humidity Management System Based on Solar-Regenerated Super Hygroscopic Complex

High levels of humidity can induce thermal discomfort and consequent health disorders. However, proper utilization of this astounding resource as a freshwater source can aid in alleviating water scarcity. Herein, a low-energy and highly efficient humidity control system is reported comprising of an in-house developed desiccant dehumidifier and hygrometer (sensor), with an autonomous operation capability that can realize simultaneous dehumidification and freshwater production. The high efficiency and energy saving mainly come from the deployed super hygroscopic complex (SHC), which exhibits high water uptake (4.64 g g-1) and facile regeneration. Machine-learning-assisted in-house developed low cost and high precision hygrometers enable the autonomous operation of the humidity management system. The dehumidifier can reduce the relative humidity (RH) of a confined room from 75% to 60% in 15 minutes with energy consumption of 0.05 kWh, saving more than 60% of energy compared with the commercial desiccant dehumidifiers, and harvest 10 L of atmospheric water in 24 h. Moreover, the reduction in RH from 80% to 60% at 32 °C results in the reduction of apparent temperature by about 7 °C, thus effectively improving the thermal comfort of the inhabitants.

A Moisture-Hungry Copper Complex Harvesting Air Moisture for Potable Water and Autonomous Urban Agriculture

The earth's atmosphere houses an enormous amount of water, which could be effectively exploited for a plethora of applications. While the development of materials for harnessing this abundant resource has gained impetus in recent years, limited efforts have been devoted to in-depth research on their agricultural applications. Herein, a novel copper(II)-ethanolamine complex (Cu-complex), which has a maximum water uptake of up to 300% and a water production rate of 2.24 g g^-1 h^-1 under natural sunlight, is reported. As a proof-of-concept application, using this material, a fully automated and self-sustainable solar-powered SmartFarm device is developed. The Cu-complex harvests atmospheric water during the night, stores the adsorbed water within, and efficiently releases the adsorbed water during the day when the device is exposed to sunlight. The water harvesting and irrigation process can be fine-tuned to suit different types of plants and local climates for an optimal cultivation. With the SmartFarm in operation, the demand for freshwater for irrigation could be greatly reduced and urban farming techniques such as large-scale rooftop farming could be promoted with a view of alleviating both water and food scarcity in the near future.

Cellulose-Based Superhydrophobic Surface Decorated with Functional Groups Showing Distinct Wetting Abilities to Manipulate Water Harvesting

Inspired by the distinct functions of desert beetles with efficient droplet nucleation and lotus leaves with excellent droplet removal, an integrated method is presented for the design of a superhydrophobic surface decorated with hydrophilic groups that can efficiently nucleate and remove water droplets. We constructed a cellulose-based superhydrophobic surface containing numerous olefin terminal groups by solvent exchange and spray coating. This surface is different from most of the reported biomimicking water harvesting surfaces that rely on complicated lithography and micropatterning techniques requiring special instruments. The obtained superhydrophobic surface was further modified using various thiol compounds via a thiol-ene reaction to manipulate the water harvesting property. The modified surfaces containing hydrophobic groups (e.g., 1-octadecanethiol and 1H,1H,2H,2H-perfluorodecanethiol) or a strong hydrophilic group (e.g., 3-mercaptopropionic acid and 6-mercapto-1-hexanol) exhibited insufficient fog collecting abilities due to poor water droplet nucleation or strong water adhesion. By contrast, the modified surface decorated with moderately hydrophilic amino groups combines the advantages of biological surfaces with distinct wetting features (such as fog-harvesting beetles and water-repellent lotus leaves), resulting in accelerated water nucleation and less compromise of the water removal efficiency. Molecular dynamic simulations revealed that the efficient droplet nucleation is attributed to the hydrophilic amino groups whereas the rapid droplet removal is due to the maintained superhydrophobicity of the amino group-modified surface. This strategy of decorating a superhydrophobic surface with moderately hydrophilic functional groups provides insight into the manipulation of droplet nucleation and removal for water collection efficiency.

Surface morphology enhances deposition efficiency in biomimetic, wind-driven fog collection

Passive interception of fog from the wind is an effective solution for accessing water in regions where fog is frequent and other sources scarce. A Namib Desert beetle is often cited as bioinspiration for further advancement, in a narrative which focuses on patterned wettability of its bumpy elytra as a means of transporting accumulated water from its back to its mouth. However, surface transport in fog collection is secondary to the role of the fluid dynamics of droplet deposition, in which inertial droplets migrate across diverging streamlines approaching an obstruction. 3D geometry of biological surface features inevitably affect this process, but its specific role in flow physics of fog collection has not previously been explored. Here, we report experimental measurements of deposition efficiency of targets with identical surface chemistry but varying surface morphology. We find a nearly threefold increase in collection upon addition of millimetric bumps to a spherical target, and provide insight into the micromechanics underlying the performance. Modifying surface morphology can be easier than overall geometry for both manufactured structures and evolved organisms and should therefore be both considered in the design of separation devices and expected in other biological systems for which extraction of particles from flow is important.

Passive Removal of Highly Wetting Liquids and Ice on Quasi-Liquid Surfaces

Surfaces with ultralow adhesion to liquids and solids have attracted broad interests in both fundamental studies and engineering applications from passive removal of highly wetting liquids and water harvesting to anti-/de-icing. The current state-of-the-art superomniphobic surfaces (rely on air lubricant) and liquid-infused surfaces (rely on liquid lubricant) suffer from severe issues for liquid repellency and ice removal: air/liquid lubricant loss or topography damage. Here, we create a durable quasi-liquid surface by tethering flexible polymer on various solid substrates. The untethered end of the polymer has mobile chains that behave like a liquid layer and greatly reduce the interfacial adhesion between the surface and foreign liquids/solids. Such a quasi-liquid surface with a 30.1 nm flexible polymer layer shows ultralow contact angle hysteresis (≤1.0°) to liquids regardless of their surface tensions. The highly wetting perfluorinated liquids like FC72 and Krytox101, as well as complex fluids like urine and crude oil, can be repelled from the surface. Moreover, wind can remove accreted ice from the surface in harsh conditions due to the negligible ice adhesion. We have demonstrated that the quasi-liquid surface shows robust performances in repelling highly wetting liquids, harvesting water, and removing ice, respectively.

Passive water harvesting by desert plants and animals: lessons from nature

Fresh water sustains human life and is vital for human health. For some of the poorest countries, 1 in 10 people do not have access to safe and easily accessible water sources. Water consumption by man continues to grow with an increasing population. The current supply of fresh water needs to be supplemented to meet future needs. Living nature provides many lessons for water harvesting. It has evolved species which can survive in the most arid regions of the world by passively collecting water from fog and condensation of water vapour in the night. Before the collected water evaporates, species have mechanisms to transport water for storage or consumption. These species possess unique chemistry and structures on or within the body for collection and transport of water. Among the high diversity of species surviving in deserts, only a handful of species have been studied. Based on lessons from nature, bioinspired water harvesters can be designed. In this paper, an overview of various desert plants and animals is given and known water harvesting mechanisms of some are presented. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 3)'.

Water Harvesting of Bioinspired Microfibers with Rough Spindle-Knots from Microfluidics

Heterostructure rough spindle-knot microfibers (HRSFs) are fabricated via a flexible parallel-nozzle microfluidic method. In this method, the bioinspired HRSF with a roughness gradient between spindle-knots and joints, can be manufactured in large-scale, and with which the size of the spindle-knots and joints can be precisely adjusted by regulating flow rates. The HRSFs, fabricated with chitosan and calcium alginate, have strong mechanical properties and corrosion resistance in acid environment (pH = 5) and alkaline environment (pH = 9), respectively. More attractively, under controlled treatment conditions, the morphology of the spindle-knots on the HRSFs can be effectively managed by changing the composite content of calcium chloride in the fluid. During the water collection process, tiny droplets of moisture can be captured on the surface of the HRSFs, subsequently, the droplets can coalesce and be transported from joint to spindle-knot sections. It is demonstrated that the surface morphology of spindle-knots directly influences the water collection efficiency, where a higher roughness gradient generates higher water collection efficiency. This parallel-nozzle microfluidic technology provides a low-cost and flexible method to manufacture high biocompatibility bioinspired rough spindle-knot microfibers, which has many potential applications in large-scale water collection, sustained drug release, and directional water collection.

Highly Efficient Clean Water Production from Contaminated Air with a Wide Humidity Range

The huge amount of moisture in the air is an unexplored and overlooked water resource in nature, which can be useful to solve the worldwide water shortage. However, direct water condensation from natural or even hazy air is always inefficient and inevitably contaminated by numerous impurities of dust, toxic gas, and microorganisms. In this regard, a drinkable and clean water harvester from complex contaminated air with a wide humidity range based on porous sodium polyacrylate/graphene framework (PGF), which can actively sorb moisture from common or even smoggy environments, efficiently grabs impurities, and then releases clean water with a high rejection rate of impurities under solar irradiation, is proposed. This PGF shows a superhigh equilibrium uptake of 5.20 g of water per gram of PGF at a relative humidity (RH) of 100% and 0.14 g g^-1 at a low RH of 15%. The rejection rate of impurities is up to 97% for the collected clean water. Moreover, a water harvesting system is established to produce over 25 L clean water per kilogram of PGF one day, enough to meet several people's drinking water demand. This work provides a new strategy for effective production of clean water from the atmosphere of practical significance.

Fog Collection on a Bio-inspired Topological Alloy Net with Micro-/Nanostructures

Because of the scarcity of freshwater resources, fog collection as one of the effective methods to solve this issue has attracted widespread concern. Inspired by several natural creatures with the capability to collect water from fog, the bio-inspired water-harvesting materials have aroused considerable attention and been widely developed. Inspired by the directional water droplets transportation to the apex on both shorebirds beaks and wheat awns, the bio-inspired topological alloy net with a V-shaped asymmetric geometry in its mesh was designed for fog collecting. Then, micro-/nano-hierarchical structures were modified on the surface of the netting wire via the cathodic electrodeposition method. Thus, the bio-inspired topological alloy net with micro/nanostructures was fabricated successfully. Through the integration of topological geometry and surface microstructure, not only the water-collection rate is improved by efficient drainage along the designated pathways, but also the issue of mesh clogging is resolved. In addition, a theoretical model was constructed to reveal the mechanism, especially the resultant force arising from the V-shaped structure. This work provides insight into the development of novel fog-collecting materials, which has potential applications in other fields, such as liquid transportation, microfluidics, and interface science.

An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption

Water scarcity is one of the greatest challenges facing human society. Because of the abundant amount of water present in the atmosphere, there are significant efforts to harvest water from air. Particularly, solar-driven atmospheric water generators based on sequential adsorption-desorption processes are attracting much attention. However, incomplete daytime desorption is the limiting factor for final water production, as the rate of water desorption typically decreases very quickly with decreased water content in the sorbents. Hereby combining tailored interfacial solar absorbers with an ionic-liquid-based sorbent, an atmospheric water generator with a simultaneous adsorption-desorption process is generated. With enhanced desorption capability and stabilized water content in the sorbent, this interfacial solar-driven atmospheric water generator enables a high rate of water production (≈0.5 L m^-2 h^-1 ) and 2.8 L m^-2 d^-1 for the outdoor environment. It is expected that this interfacial solar-driven atmospheric water generator, based on the liquid sorbent with a simultaneous adsorption-desorption process opens up a promising pathway to effectively harvest water from air.

Water Purification/Harvesting: Harnessing Solar‐Driven Photothermal Effect toward the Water–Energy Nexus (Adv. Sci. 18/2019)

In article number 1900883, Zhi‐Kang Xu, Zuankai Wang, and co‐workers discuss the latest progress and future development trend of solar‐driven low‐energy water purification/harvesting technologies, hoping to provide a novel insight into harnessing solar energy to produce clean and safe drinking water, with great implication for the exploration of advanced technologies to reconcile the water–energy nexus.

Harnessing Solar-Driven Photothermal Effect toward the Water-Energy Nexus

Producing affordable freshwater has been considered as a great societal challenge, and most conventional desalination technologies are usually accompanied with large energy consumption and thus struggle with the trade-off between water and energy, i.e., the water-energy nexus. In recent decades, the fast development of state-of-the-art photothermal materials has injected new vitality into the field of freshwater production, which can effectively harness abundant and clean solar energy via the photothermal effect to fulfill the blue dream of low-energy water purification/harvesting, so as to reconcile the water-energy nexus. Driven by the opportunities offered by photothermal materials, tremendous effort has been made to exploit diverse photothermal-assisted water purification/harvesting technologies. At this stage, it is imperative and important to review the recent progress and shed light on the future trend in this multidisciplinary field. Here, a brief introduction of the fundamental mechanism and design principle of photothermal materials is presented, and the emerging photothermal applications such as photothermal-assisted water evaporation, photothermal-assisted membrane distillation, photothermal-assisted crude oil cleanup, photothermal-enhanced photocatalysis, and photothermal-assisted water harvesting from air are summarized. Finally, the unsolved challenges and future perspectives in this field are emphasized. It is envisioned that this work will help arouse future research efforts to boost the development of solar-driven low-energy water purification/harvesting.

Solar Energy Triggered Clean Water Harvesting from Humid Air Existing above Sea Surface Enabled by a Hydrogel with Ultrahigh Hygroscopicity

Water scarcity is a ubiquitous problem with its magnitude expected to rise in the near future, and efforts to seek alternative water sources are on the rise. Harvesting water from air has intrigued enormous research interest among many groups with Scientific American listing this technology as the second most impactful technology that can bring about a massive change in people's lives. Though desalination offers a huge prospect in mitigating water crisis, its practicality is limited by exorbitant energy requirement. Alternatively, the air above sea water is moisture rich, with the quantity of vapor increasing at the rate of 0.41 kg m^-2 . Herein, a method to sustainably harvest water from this moisture rich zone is demonstrated by employing a nanoporous superhygroscopic hydrogel, which is capable of absorbing water from highly humid atmospheres by over 420% (highest) of its own weight. The desorption process from the hydrogel, occurring at 55 °C (lowest), is triggered by natural sunlight (A.M 1.5) thereby ensuing an external energy-less water harvesting approach. The hydrogel exhibits excellent stability even after 1000 absorption/desorption cycles. Through multiple absorption/desorption cycles, it is possible to harvest over 10 L water per kg of hydrogel daily.

Super Moisture-Absorbent Gels for All-Weather Atmospheric Water Harvesting

Atmospheric water harvesting (AWH)-producing fresh water via collecting moisture from air-enables sustainable water delivery without geographical and hydrologic limitations. However, the fundamental design principle to prepare materials that can convert the water vapor in the air to collectible liquid water is still mostly unknown. Here, a super moisture-absorbent gel, which is composed of hygroscopic polypyrrole chloride penetrating in hydrophilicity-switchable polymeric network of poly N-isopropylacrylamide, is shown. Based on such design, a high-efficiency water production by AWH has been achieved in a broad range of relative humidity. The synergistic effect enabled by the molecular level integration of hygroscopic and hydrophilicity-switchable polymers in a network architecture presents controllable interaction between the gel and water molecules, simultaneously realizing efficient vapor capturing, in situ water liquefaction, high-density water storage and fast water releasing under different weather conditions. Being an effective method to regulate migration of water molecules, such design represents a novel strategy to improve the AWH, and it is also fundamental to other water management systems for environmental cooling, surficial moisturizing and beyond.

Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale Topography

Water scarcity has become a global issue of severe concern. Great efforts have been undertaken to develop low-cost and highly efficient condensation strategies to relieve water shortages in arid regions. However, the rationale for design of an ideal condensing surface remains lacking due to the conflicting requirements for water nucleation and transport. In this work, we demonstrate that a biphilic nanoscale topography created by a scalable surface engineering method can achieve an ultraefficient water harvesting performance. With hydrophilic nanobumps on top of a superhydrophobic substrate, this biphilic topography combines the merits of biological surfaces with distinct wetting features (e.g., fog-basking beetles and water-repellent lotus), which enables a tunable water nucleation phenomenon, in contrast to the random condensation mode on their counterparts. By adjusting the contrasting wetting features, the characteristic water nucleation spacing can be tuned to balance the nucleation enhancement and water transport to cope with various environments. Guided by our nucleation density model, we show an optimal biphilic topography by tuning the nanoscale hydrophilic structure density, which allows an ∼349% water collection rate and ∼184% heat transfer coefficient as compared to the state-of-the-art superhydrophobic surface in a moisture-lacking atmosphere, offering a very promising strategy for improving the efficiency of water harvesting in drought areas.

Metal-Organic Frameworks for Water Harvesting from Air

Water harvesting from air in passive, adsorption-based devices holds great potential for delivering drinking water to arid regions of the world. This technology requires adsorbents that can be tailored for a maximum working capacity, temperature response, and the relative pressure range in which reversible adsorption occurs. In this respect, metal-organic frameworks (MOFs) are promising, owing to their structural diversity and the precision of their functionalization for adjusting both pore size and hydrophilicity, thereby facilitating the rational design of their water-sorption characteristics. Here, chemical and structural factors crucial for the design of hydrolytically stable MOFs for water adsorption are discussed. Prevalent water adsorption mechanisms in micro- and mesoporous MOFs alongside strategies for fine-tuning of their adsorption behavior by means of reticular chemistry are presented. Finally, an approach for the selection of promising MOFs with respect to water harvesting from air is proposed and design concepts for next-generation MOFs for application in passive adsorption-based water-harvesting devices are outlined.

Highly Efficient Water Harvesting with Optimized Solar Thermal Membrane Distillation Device

Water distillation with solar thermal technology could be one of the most promising way to address the global freshwater scarcity, with its low cost and minimum environmental impacts. However, the low liquid water productivity, which is caused by the heat loss and inadequate heat utilization in solar-thermal conversion process, hinders its practical application. Here, a compact solar-thermal membrane distillation system with three structure features: highly localized solar-thermal heating, effective cooling strategy, and recycling the latent heat, is proposed. The steam generation rate is 0.98 kg m-2 h-1 under solar illumination of 1 kW m-2 in the open system, while the liquid water productivity could be 1.02 kg m-2 h-1 with the solar efficiency up to 72% with a two-level device. The outdoor experiments show a water productivity of 3.67 kg m-2 with salt rejection over 99.75% in one cloudy day. These results demonstrate an easy and high-efficiency way for water distillation, especially suitable for household solar water purification.

Thermoresponsive Cellulose Acetate-Poly(N-isopropylacrylamide) Core-Shell Fibers for Controlled Capture and Release of Moisture

In this study, we used core-shell electrospinning to fabricate cellulose acetate-poly(N-isopropylacrylamide) (CA-PNIPAM) fibrous membranes and demonstrated the ability of these fibers to capture water from a high humid atmosphere and release it when thermally stimulated. The wettability of the fibers was controlled by using thermoresponsive PNIPAM as the shell layer. Scanning electron and fluorescence microscopes are used to investigate the microstructure of the fibers and confirm the presence of the core and shell phases within the fibers. The moisture capturing and releasing ability of these core-shell CA-PNIPAM fibers was compared with those of the neat CA and neat PNIPAM fibers at room temperature as well as at an elevated temperature. At room temperature, the CA-PNIPAM core-shell fibers are shown to have the maximum moisture uptake capacity among the three samples. The external temperature variations which trigger the moisture response behavior of these CA-PNIPAM fibers fall within the range of typical day and night cycles of deserts, demonstrating the potential use of these fibers for water harvesting applications.

Uphill Water Transport on a Wettability-Patterned Surface: Experimental and Theoretical Results

In nature, there exist many functional water-controlling surfaces, such as the water-repellent surface of lotus leaves, the superhydrophobic water-adhesive surface of rose petals, the water-harvesting surface of a beetle's back, and the water-transporting surface of the legs of Ligia exotica. These natural surfaces suggest that surface chemistry and hierarchical structures are essential for controlling the water behavior. We have reported the preparation of superhydrophobic and antireflection silicon nanospike-array structures using self-organized honeycomb-patterned films as three-dimensional dry-etching masks. Moreover, the surface wettability of the silicon nanospike-array structures can be easily transformed from superhydrophobic to superhydrophilic by changes in the surface chemistry. In this report, we show the preparation of water-controlling surfaces, such as water-harvesting and water-transporting surfaces, by the wettability patterning of silicon nanostructured surfaces. We prepared honeycomb-patterned films for dry-etching masks made from polystyrene and an amphiphilic polymer by casting a chloroform solution. After the fixation of the top layer of the honeycomb-patterned films on a single-crystal silicon substrate, reactive ion etching was performed. The as-prepared silicon nanospike-array structure showed superhydrophobicity, and the water contact angles were over 170°. After UV-O₃ treatment with photomasks, only the UV-irradiated surfaces showed superhydrophilicity, suggesting that we can obtain superhydrophobic- and superhydrophilic-patterned surfaces for which the patterns are the same as those of the photomasks. On the basis of these wettability-patterned surfaces, we demonstrated water harvesting by superhydrophilic dot-patterned surfaces and water transportation against gravity by superhydrophilic triangular-patterned surfaces. In particular, we investigated uphill water transport through the motion of droplets on tilting slopes based on the equation of motion. These results suggested that we can obtain superior microfluidic devices suitable for various applications through the use of optional wettability patterns.

Patterned Polymer Coatings Increase the Efficiency of Dew Harvesting

Micropatterned polymer surfaces, possessing both topographical and chemical characteristics, were prepared on three-dimensional copper tubes and used to capture atmospheric water. The micropatterns mimic the structure on the back of a desert beetle that condenses water from the air in a very dry environment. The patterned coatings were prepared by the dewetting of thin films of poly-4-vinylpyridine (P4VP) on top of polystyrene films (PS) films, upon solvent annealing, and consist of raised hydrophilic bumps on a hydrophobic background. The size and density distribution of the hydrophilic bumps could be tuned widely by adjusting the initial thickness of the P4VP films: the diameter of the produced bumps and their height could be varied by almost 2 orders of magnitude (1-80 μm and 40-9000 nm, respectively), and their distribution density could be varied by 5 orders of magnitude. Under low subcooling conditions (3 °C), the highest rate of water condensation was measured on the largest (80 μm diameter) hydrophilic bumps and was found to be 57% higher than that on flat hydrophobic films. These subcooling conditions are achieved spontaneously in dew formation, by passive radiative cooling of a surface exposed to the night sky. In effect, the pattern would result in a larger number of dewy nights than a flat hydrophobic surface and therefore increases water capture efficiency. Our approach is suited to fabrication on a large scale, to enable the use of the patterned coatings for water collection with no external input of energy.