Environmentally adaptive MOF-based device enables continuous self-optimizing atmospheric water harvesting

Rapid solar-driven atmospheric water-harvesting with MAF-4-derived nitrogen-doped nanoporous carbon

Sorption-based atmospheric water-harvesting (AWH) could help to solve global freshwater scarcity. The search for adsorbents with high water-uptake capacity at low relative humidity, rapid adsorption-desorption kinetics and high thermal conductivity is a critical challenge in AWH. Herein, we report a MAF-4 (aka ZIF-8)-derived nanoporous carbon (NPCMAF-4-800) with multiple N-doped sites, considerable micropore characteristics and inherent photothermal properties, for efficient water production in a relatively arid climate. NPCMAF-4-800 exhibited optimal water-sorption performance of 306 mg g-1 at 40% relative humidity (RH). An excellent sunlight-absorption rate was realized (97%) attributed to its high degree of graphitization. A proof-of-concept device was designed and investigated for the practical harvesting of water from the atmosphere using natural sunlight. NPCMAF-4-800 achieved an unprecedentedly high water production rate of 380 mg g-1 h-1 at 40% RH, and could produce 1.77 L kg-1 freshwater during daylight hours in an outdoor low-humidity climate of ∼25 °C and 40% RH. These findings may shed light on the potential of MOF-derived porous carbons in the AWH field, and inspire the future development of solar-driven water-generation systems.

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

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

An atmospheric water harvesting system based on the "Optimal Harvesting Window" design for worldwide water production

Atmospheric water harvesting (AWH) is a promising solution to the water shortage problem. Current sorption-based AWH (SAWH) systems seldom obtain both wide climatic adaptability and high energy efficiency due to the lack of thermodynamic optimization. To achieve the ideal harvesting circulation in SAWH systems, the "optimal harvesting window" (OHW) design based on thermodynamic analysis was first proposed and validated by our prototype. The "OHW" theory indicates the water production rate and energy efficiency could be improved by properly reducing the adsorption temperature. As the humidity increases, the optimal adsorption temperature should be closer to the dew point of the environment. Experimental results revealed that, loaded with 3 kg widely adopted silica gel, the daily water production could reach 5.76-17.64 L/d with ultrahigh energy efficiency of 0.46-1.5 L/kWh. This prototype could also achieve optimal performance in wide climatic conditions in terms of 13-35 °C and 18%-72% RH. Lastly, the performance of photovoltaic (PV)-driven SAWH was evaluated. Results showed that a 1 m2 PV panel could generate 0.66-2 L water per day in Shanghai throughout the year, the highest in opening literature. Notably, this work introduces a promising concept that can help achieve large-scale, ultra-fast, energy-efficient AWH worldwide.

Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal-Organic Framework

Porous materials, particularly metal-organic frameworks (MOFs), hold great promise for advanced applications. MIL-53(Al) is an exceptionally well-studied MOF that exhibits a phase transition upon guest capture─in this case, water─resulting in a dramatic change in the pore volume. Despite extensive studies, the structure of the water-loaded narrow-pore phase, MIL-53(Al)-np, remains controversial, particularly with respect to the positions of the adsorbed water molecules. We use terahertz spectroscopy, coupled with powder X-ray diffraction and density functional theory simulations, to unambiguously resolve this controversy. We show that the low-frequency (<100 cm-1) vibrational spectrum depends on weak long-range forces that are extremely sensitive to the orientation of the adsorbed water molecules. This enables definitively determining the correct structure of MIL-53(Al)-np while highlighting the extreme sensitivity of terahertz spectroscopy to bulk structure, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host-guest complexes.

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.

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.

Informative Training Data for Efficient Property Prediction in Metal-Organic Frameworks by Active Learning

In recent data-driven approaches to material discovery, scenarios where target quantities are expensive to compute and measure are often overlooked. In such cases, it becomes imperative to construct a training set that includes the most diverse, representative, and informative samples. Here, a novel regression tree-based active learning algorithm is employed for such a purpose. It is applied to predict the band gap and adsorption properties of metal-organic frameworks (MOFs), a novel class of materials that results from the virtually infinite combinations of their building units. Simpler and low dimensional descriptors, such as those based on stoichiometric and geometric properties, are used to compute the feature space for this model owing to their ability to better represent MOFs in the low data regime. The partitions given by a regression tree constructed on the labeled part of the data set are used to select new samples to be added to the training set, thereby limiting its size while maximizing the prediction quality. Tests on the QMOF, hMOF, and dMOF data sets reveal that our method constructs small training data sets to learn regression models that predict the target properties more efficiently than existing active learning approaches, and with lower variance. Specifically, our active learning approach is highly beneficial when labels are unevenly distributed in the descriptor space and when the label distribution is imbalanced, which is often the case for real world data. The regions defined by the tree help in revealing patterns in the data, thereby offering a unique tool to efficiently analyze complex structure-property relationships in materials and accelerate materials discovery.

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.

Metal-Organic Frameworks for Water Harvesting and Concurrent Carbon Capture: A Review for Hygroscopic Materials

As water scarcity becomes a pending global issue, hygroscopic materials prove a significant solution. Thus, there is a good cause following the structure-performance relationship to review the recent development of hygroscopic materials and provide inspirational insight into creative materials. Herein, traditional hygroscopic materials, crystalline frameworks, polymers, and composite materials are reviewed. The similarity in working conditions of water harvesting and carbon capture makes simultaneously addressing water shortages and reduction of greenhouse effects possible. Concurrent water harvesting and carbon capture is likely to become a future challenge. Therefore, an emphasis is laid on metal-organic frameworks (MOFs) for their excellent performance in water and CO2 adsorption, and representative role of micro- and mesoporous materials. Herein, the water adsorption mechanisms of MOFs are summarized, followed by a review of MOF's water stability, with a highlight on the emerging machine learning (ML) technique to predict MOF water stability and water uptake. Recent advances in the mechanistic elaboration of moisture's effects on CO2 adsorption are reviewed. This review summarizes recent advances in water-harvesting porous materials with special attention on MOFs and expects to direct researchers' attention into the topic of concurrent water harvesting and carbon capture as a future challenge.

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.

Fine Tuning the Hydrophobicity of a New Three-Dimensional Cu2+ MOF through Single Crystal Coordinating Ligand Exchange Transformations

The synthesis, characterization, and single-crystal-to-single-crystal (SCSC) exchange reactions of a new 3D Cu2+ MOF based on 5-aminoisophthalic acid (H2AIP), [Cu6(μ3-ΟΗ)3(ΑΙΡ)4(HΑΙΡ)]n·6nDMF·nH2O - UCY-16·6nDMF·nH2O, are reported. It exhibits a 3D structure based on two [Cu4(μ3-OH)2]6+ butterfly-like secondary building units, differing in their peripheral ligation, bridged through HAIP-/AIP2- ligands. This compound displays the capability to exchange the coordinating ligand(s) and/or guest solvent molecules through SCSC reactions. Interestingly, heterogeneous reactions of single crystals of UCY-16·6nDMF·nH2O with primary alcohols resulted not only in the removal of the lattice DMF molecules but also in an unprecedented structural alteration that involved the complete or partial replacement of the monoatomic bridging μ3-OH- anion(s) of the [Cu4(μ3-OH)2]6+ butterfly structural core by various alkoxy groups. Similar crystal-to-crystal exchange reactions of UCY-16·6nDMF·nH2O with long-chain aliphatic alcohols (CxH2x+1OH, x = 8-10, 12, 14, and 16) led to analogues containing fatty alcohols. Notably, the exchanged products with the bulkier alcohols UCY-16/n-CxH2x+1OH·S' (x = 6-10, 12, 14, and 16) do not mix with H2O being quite stable in this solvent, in contrast to the pristine MOF, and exhibit a hydrophobic/superhydrophobic surface as confirmed from the investigation of their water contact angles and capability to remove hydrophobic pollutants from aqueous media.

Towards a better understanding of atmospheric water harvesting (AWH) technology

Atmospheric water harvesting (AWH) technology is an emerging sustainable development strategy to deal with global water scarcity. To better understand the current state of AWH technology development, we conducted a bibliometric analysis highlighting three water harvesting technologies (fog harvesting, condensation, and sorption). By comprehensively reviewing the research progress and performing a comparative assessment of these technologies, we summarized past achievements and critically analyzed the different technologies. Traditional fog collectors are more mature, but their efficiency still needs to be improved. External field-driven fog harvesting and active condensation need to be driven by external forces, and passive condensation has high requirements for environmental humidity. Emerging bio-inspired fog harvesting and sorption technology provide new possibilities for atmospheric water collection, but they have high requirements for materials, and their commercial application is still to be further promoted. Based on the key characteristics of each technology, we presented the development prospects for the joint use of integrated/hybrid systems. Next, the water-energy relationship is used as a link to clarify the future development strategy of AWH technology in energy driving and conversion. Finally, we outlined the core ideas of AWH for both basic research and practical applications and described its limitless possibilities for drinking water supply and agricultural irrigation. This review provides an essential reference for the development and practical application of AWH technologies, which contribute to the sustainable utilization of water resources globally.

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.

Research Progress on Hygroscopic Agents for Atmospheric Water Harvesting Systems

Adsorptive atmospheric water harvesting systems (AWHs) represent an innovative approach to collecting freshwater resources from the atmosphere, with a hygroscopic agent at their core. This method has garnered significant attention due to its broad applicability, strong recycling capacity, and sustainability. It is being positioned as a key technology to address global freshwater scarcity. The core agent's hygroscopic properties play a crucial role in determining the performance of the AWHs. This article provides a comprehensive review of the latest advancements in hygroscopic agents, including their adsorption mechanisms and classifications. This study of hygroscopic agents analyzes the performance and characteristics of relevant porous material composite polymer composites and plant composites. It also evaluates the design and preparation of these materials. Aiming at the problems of low moisture adsorption and desorption difficulty of the hygroscopic agent, the factors affecting the water vapor adsorption performance and the method of enhancing the hygroscopic performance of the material are summarized and put forward. For the effect of hygroscopic agents on the volume of water catchment devices, the difference in density before and after hygroscopicity is proposed as part of the evaluation criteria. Moisture absorption per unit volume is added as a performance evaluation criterion to assess the effect of hygroscopic agents on the volume of water collection equipment. The article identifies areas that require further research and development for moisture absorbers, exploring their potential applications in other fields and anticipating the future development direction and opportunities of moisture-absorbing materials. The goal is to promote the early realization of adsorptive atmospheric water harvesting technology for large-scale industrial applications.

A humidity/thermal dual response 3D-fabric with porous poly(N-isopropyl acrylamide) hydrogel towards efficient atmospheric water harvesting

Atmospheric water harvesting is a promising approach for obtaining freshwater resources, but achieving high levels of light absorption, hygroscopic capacity, and desorption efficiency simultaneously remains a challenge. In this study, we developed an innovative atmospheric water harvester that incorporates a poly(N-isopropylacrylamide) hydrogel and a polydopamine/polypyrrole-modified 3D raised-fabric. The interlacing structure and polydopamine/polypyrrole synergistically enhance the harvester's photothermal conversion capability, while the hydrogel-modified raised-fabric with its increased pore structure and high specific surface area ensures effective contact between the internal adsorbent and external moisture, thereby improving moisture capture and storage capacity. Our results indicate that the hydrogel-modified 3D raised-fabric has excellent photothermal conversion performance, as evidenced by its rapid temperature rise to 75.9 °C under 1 sun light intensity, which effectively promotes water evaporation and harvesting. Furthermore, the 3D raised-fabric exhibits exceptional water absorption (3.1 g g-1, RH 90%) and water desorption (1.75 kg m-2h-1, 1 sun) properties. Overall, the 3D raised-fabric with its integrated photothermal, hygroscopic, and hydrophobic properties can effectively collect water under low humidity conditions, making it a promising solution for water scarcity issues.

Recent Advances in Laser-Induced Synthesis of MOF Derivatives

Metal-organic frameworks (MOFs) are crystalline materials with permanent pores constructed by the self-assembly of organic ligands and metal clusters through coordination bonds. Due to their diversity and tunability, MOFs are used as precursors to be converted into other types of functional materials by pyrolytic recrystallization. Laser-induced synthesis is proven to be a powerful pyrolytic processing technique with fast and accurate laser irradiation, low loss, high efficiency, selectivity, and programmability, which endow MOF derivatives with new features. Laser-induced MOF derivatives exhibit high versatility in multidisciplinary research fields. In this review, first, the basic principles of laser smelting and the types of materials for laser preparation of MOF derivatives are briefly introduced. Subsequently, it is focused on the peculiarity of the engineering of structural defects and their applications in catalysis, environmental protection, and energy fields. Finally, the challenges and opportunities at the current stage are highlighted with the aim of elucidating the future direction of the rapidly growing field of laser-induced synthesis of MOF derivatives.

Packed OV-SnO2-Sb bead-electrodes for enhanced electrocatalytic oxidation of micropollutants in water

Electrocatalytic oxidation is an appealing treatment option for emerging micropollutants in wastewater, however, the limited reactive surface area and short service lifetime of planar electrodes hinder their industrial applications. This study introduces an innovative electrochemical wastewater treatment technology that employs packed bead-electrodes (PBE) as a dynamic electrocatalytic filter on a dimensionally stable anode (DSA) acting as a current collector. By using PBE, the electroactive volume is expanded beyond the vicinity of the common planar anode to the thick porous media of PBE with a vast electrocatalytic surface area. This greatly enhances the efficiency of electrochemical degradation of micropollutants. The OV-SnO2-Sb PBE filter achieved a nearly 100 % degradation of moxifloxacin (MOX) in under 2 min of single-pass filtration, with a degradation rate over an order of magnitude higher than the conventional electrochemical oxidation processes. The generation of abundant radical species (•OH) and non-radical species (1O2 and O3), along with the enhanced direct oxidation, led to the outstanding performance of the charged PBE system in MOX degradation. The OV-SnO2-Sb PBE was remarkably stable, and the separation between the electroactive PBE layer and the base Ti anode allows for easy renewal of the bead-electrode materials and scaling up of the system for practical applications. Overall, our study presents a dynamic electroactive PBE that advances the electrocatalytic oxidation technology for effective control of emerging pollutants in the water environment. This technology has the potential to revolutionize electrochemical wastewater treatment and contribute to a more sustainable future environment.

Enhanced atmospheric water harvesting efficiency through green-synthesized MOF-801: a comparative study with solvothermal synthesis

Adsorption-based atmospheric water harvesting has emerged as a compelling solution in response to growing global water demand. In this context, Metal-organic frameworks (MOFs) have garnered considerable interest due to their unique structure and intrinsic porosity. Here, MOF 801 was synthesized using two different methods: solvothermal and green room temperature synthesis. Comprehensive characterization indicated the formation of MOF-801 with high phase purity, small crystallite size, and excellent thermal stability. Nitrogen adsorption-desorption analysis revealed that green-synthesized MOF-801 possessed an 89% higher specific surface area than its solvothermal-synthesized counterpart. Both adsorbents required activation at a minimum temperature of 90 °C for optimal adsorption performance. Additionally, green-synthesized MOF-801 demonstrated superior adsorption performance compared to solvothermal-synthesized MOF-801, attributed to its small crystal size (around 66 nm), more hydrophilic functional groups, greater specific surface area (691.05 m2/g), and the possibility of having a higher quantity of defects. The maximum water adsorption capacity in green-synthesized MOF-801 was observed at 25 °C and 80% relative humidity, with a value of 41.1 g/100 g, a 12% improvement over the solvothermal-synthesized MOF-801. Remarkably, even at a 30% humidity level, green-synthesized MOF-801 displayed a considerable adsorption capacity of 31.5 g/100 g. Importantly, MOF-801 exhibited long-term effectiveness in multiple adsorption cycles without substantial efficiency decline.

Enhanced removal performance of magnesium-modified biochar for cadmium in wastewaters: Role of active functional groups, processes, and mechanisms

In this study, a series of biochar products with different active functional groups were developed by one-pot coprecipitation method, including magnesium-modified biochar (MgBC) and functional group-grafted MgBC (Cys@MgBC, Try@MgBC, and Glu@MgBC), for effective adsorption of cadmium (Cd(II)) from wastewaters. These biochars exhibited excellent removal performance for Cd(II), particularly Cys@MgBC, whose maximum Cd(II) adsorption capacity reached 223.7 mg g-1. The highly active and weakly crystalline Mg could adsorb Cd(II) through precipitation and ion exchange, which was further promoted by the introduced functional groups through complexation and precipitation. After 120 d of natural process, the immobilization efficiency of Cd(II) by Cys@MgBC, Try@MgBC, and Glu@MgBC was still maintained at 98.7%, 95.2%, and 82.7% respectively. This study proposes and clarifies the complexation mechanism of functional group-grafted Mg-modified biochar for heavy metals, providing new insights into the practical application of these biochars.

Water Adsorption Dynamics on Metal-Organic Framework MOF-801: Comparative Study of Loose and Glued Grains, and Coatings

Adsorption heat transformation and storage (AHTS) is an environmentally benign and energy-saving alternative to common compression chillers and heat pumps. The low specific power (SP) of adsorption systems is a key drawback that hinders their broader dissemination. The optimization of adsorption dynamics is a prerequisite for SP enhancement. In this work, we studied the dynamics of water vapor adsorption on MOF-801-a promising adsorbent for AHTS. For the first time, two configurations of compact MOF-801 bed, namely, grains glued to the surface of a metal support and thin adsorbent coatings, are prepared, and their porous structure is characterized. The water adsorption dynamics is compared with a common loose grains configuration. The main findings are: (a) the binder can both accelerate and decelerate the water adsorption, and its chemical nature is subject to careful selection; (b) at the initial time, heat transfer between the support and adsorbent bed controls the adsorption rate, and, at a longer time, mass transfer starts to affect it; (c) polyvinylpyrrolidone, bentonite, thermal grease, and hydroxyethylcellulose increase the heat transfer coefficients and accelerate adsorption; polyvinyl alcohol and polyaniline slow it down; and (d) for the coatings prepared with polyvinylpyrrolidone, an SP of 1.6-5.1 kW/kg can be achieved, which is about twice that for the loose grains. The new configurations can be recommended for effective but expensive adsorbents such as MOFs, and their implementation will contribute to the further spread of AHTS technology.

An overview of atmospheric water harvesting methods, the inevitable path of the future in water supply

Although science has made great strides in recent years, access to fresh water remains a major challenge for humanity due to water shortage for two-thirds of the world's population. Limited access to fresh water becomes more difficult due to the lack of natural resources of water. Many of these resources are also contaminated by human activities. Many attempts have been made to harvest water from the atmosphere, and condensation systems have received much attention. One of the challenges in generation systems is the high consumption energy of the cooling feed, despite the generation of large amounts of water from the atmosphere. As other airborne contaminants condense with water vapor, the water after harvesting needs to be treated, which adds to construction and maintenance costs. Also, the need for high relative humidity in condensation systems has led scientists to find ways of atmospheric water harvesting at low relative humidity and use renewable energy sources. Sorption systems can absorb atmospheric water without the need for an energy supply and spontaneously. Desiccants such as silica gel and zeolite, due to their high affinity for water, can absorb water vapor in the air through physical or physicochemical bonding, but all of these have slow adsorption kinetics. Therefore, it takes a long time for the water harvesting cycle or they are not able to absorb water at low relative humidity, and others need a lot of energy for the water desorption phase. Metal-Organic Frameworks (MOF) are porous materials that, due to their special structure, are considered the most promising material for atmospheric water harvesting at low relative humidity. MOF-303 has been identified as the most efficient material to date and can harvest 0.7 liters of water per kilogram of MOF-303 at 10% RH and 27 °C. MOFs can harvest atmospheric water even in desert areas using only solar energy, and the water produced is drinkable and does not need to be treated. In this review, systems and methods of atmospheric water harvesting will be studied and compared and then the mechanism of adsorption and desorption in sorption systems will be discussed in detail.

MOF Linker Extension Strategy for Enhanced Atmospheric Water Harvesting

A linker extension strategy for generating metal-organic frameworks (MOFs) with superior moisture-capturing properties is presented. Applying this design approach involving experiment and computation results in MOF-LA2-1 {[Al(OH)(PZVDC)], where PZVDC^2- is (E)-5-(2-carboxylatovinyl)-1H-pyrazole-3-carboxylate}, which exhibits an approximately 50% water capacity increase compared to the state-of-the-art water-harvesting material MOF-303. The power of this approach is the increase in pore volume while retaining the ability of the MOF to harvest water in arid environments under long-term uptake and release cycling, as well as affording a reduction in regeneration heat and temperature. Density functional theory calculations and Monte Carlo simulations give detailed insight pertaining to framework structure, water interactions within its pores, and the resulting water sorption isotherm.

MOFormer: Self-Supervised Transformer Model for Metal-Organic Framework Property Prediction

Metal-organic frameworks (MOFs) are materials with a high degree of porosity that can be used for many applications. However, the chemical space of MOFs is enormous due to the large variety of possible combinations of building blocks and topology. Discovering the optimal MOFs for specific applications requires an efficient and accurate search over countless potential candidates. Previous high-throughput screening methods using computational simulations like DFT can be time-consuming. Such methods also require the 3D atomic structures of MOFs, which adds one extra step when evaluating hypothetical MOFs. In this work, we propose a structure-agnostic deep learning method based on the Transformer model, named as MOFormer, for property predictions of MOFs. MOFormer takes a text string representation of MOF (MOFid) as input, thus circumventing the need of obtaining the 3D structure of a hypothetical MOF and accelerating the screening process. By comparing to other descriptors such as Stoichiometric-120 and revised autocorrelations, we demonstrate that MOFormer can achieve state-of-the-art structure-agnostic prediction accuracy on all benchmarks. Furthermore, we introduce a self-supervised learning framework that pretrains the MOFormer via maximizing the cross-correlation between its structure-agnostic representations and structure-based representations of the crystal graph convolutional neural network (CGCNN) on >400k publicly available MOF data. Benchmarks show that pretraining improves the prediction accuracy of both models on various downstream prediction tasks. Furthermore, we revealed that MOFormer can be more data-efficient on quantum-chemical property prediction than structure-based CGCNN when training data is limited. Overall, MOFormer provides a novel perspective on efficient MOF property prediction using deep learning.

Fe-Co controlled super-hygroscopic hydrogels toward efficient atmospheric water harvesting

Extracting atmospheric moisture for freshwater production is an appealing way to mitigate the global water crisis. However, the low moisture sorption capacity and high desorption temperature are the major bottlenecks for efficient atmospheric water harvesting. Herein, we develop a transition metal super-hygroscopic hydrogel by an economical strategy, which is constructed through a facile coordination between metal salts and ethanolamine. When the empty electron orbital of the metal ion is coordinated with the lone electron pair of nitrogen or oxygen atom, the water active sorption site is formed. A single water layer is bonded on the sites by a coordination effect, followed by physical interaction with water to form multi-layer structures. The Fe and Co ions in the hydrogel function as dual sorption sites to capture moisture, which can harvest additional water by the synergistic effect of bimetals. As a result, the bimetal hydrogel contributes to a high water uptake of 5.22 g g^-1 at 95% RH, triggering the desorption process by one solar intensity due to its low desorption temperature (≤50 °C).