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Atmospheric Water Harvesting: A Review of Material and Structural Designs
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ACS Materials Letters

Cite this: ACS Materials Lett. 2020, 2, 7, 671–684
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https://doi.org/10.1021/acsmaterialslett.0c00130
Published May 7, 2020

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Abstract

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Atmospheric water harvesting (AWH) emerges as a promising means to overcome the water scarcity of arid regions, especially for inland areas lacking liquid water sources. Beyond conventional system engineering that improves the water yield, novel moisture-harvesting materials provide new aspects to fundamentally promote the AWH technology benefiting from their high tunability and processability. Innovative material and structural designs enable the moisture harvesters with desirable features, such as high water uptake, facile water collection and long-term recyclability, boosting the rapid development of next-generation AWH. In this Perspective, we first illustrate the sorption mechanism, including absorption and adsorption for moisture-harvesting materials and summarize fundamental requirements, as well as design principles of moisture harvesters. Recent progress on material and structural designs of moisture harvesters for AWH is critically discussed. We conclude with prospective directions for next-generation moisture harvesters to promote AWH from scientific research to practical application.

Copyright © 2020 American Chemical Society

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Freshwater scarcity is a global challenge threatening the lives of humankind, especially for people in arid regions. (1,2) However, current freshwater sources, such as rivers, lakes, and groundwater, that are suffering man-made pollution and climatic water loss by environmental damage are becoming insufficient to meet the growing demand. (3) Recently, research efforts have been taken to explore new opportunities to exploit potential freshwater sources. Various water purification technologies, such as filtration, (4) reverse osmosis, (5,6) multistage flash distillation, (7) and solar water purification, (8−13) have been developed to utilize seawater or wastewater. However, because of the dependency on natural water sources, these technologies are mainly feasible in coastal areas and are generally inaccessible for landlocked regions. (14,15) Atmospheric water, which is present regardless of geographical and hydrologic conditions, is emerging as an alternative water resource. The earth’s atmosphere holds water in the form of water droplets or vapor, accounting up to ∼10% of freshwater sources and providing ∼50 000 km3 water. (16) Additionally, the natural hydrologic cycle enables a sustainable water supply. (17) Thus, atmospheric water harvesting (AWH) becomes a promising strategy for decentralized water production, overcoming the challenges of long-distance transport or delivery of potable water in rural areas.

Atmospheric water harvesting becomes a promising strategy for decentralized water production, overcoming the challenges of long-distance transport or delivery of potable water in rural areas.

The AWH discussed in this Perspective is primarily focused on moisture harvesting, different from earlier efforts on fog/dew collection to facilitate the coalescence of small water droplets, generating big droplets that can be further gathered by airflow or gravity. (18,19) The core goal of fog/dew collection technology is to accelerate the growth of water droplets adhering to the surface of collectors and decrease the critical size of water droplets falling from the surface. To this end, surface wettability engineering (20−22) and biomimic micro/nanostructures (23,24) have been investigated. However, the dependency on saturated local humidity (relative humidity, RH = 100%) significantly hinders the practical application of fog/dew collection in the parched region. (15,25) In contrast, moisture harvesting (i.e., AWH) is not restricted by geographical and climatic conditions since the moisture is gaseous water (also known as water vapor), which is widespread on the earth. (14) The key target of AWH is to liquefy moisture, generating collectible liquid water. (25,26) The conventional method to harvest moisture is cooling the ambient air below its dew point and collecting the condensate. (27,28) However, the energy consumption is high because a large amount of energy is used to power the cooler and overcome the latent heat (∼2450 kJ/kg at 20 °C) during the condensation process, which increases the cost of obtained water, weakening its practical significance. (25,29) Moisture-harvesting materials offer a new aspect to address the above issues based on various tuning knobs for tailoring the interaction between functional materials and water molecules.

Materials-enabled moisture harvesting relies on the water affinity provided by the functional components.

Instead of creating high local RH by cooling the surface, moisture-harvesting materials enable spontaneous vapor sorption to trap water molecules, extracting vapor from the air, and hence, concentrating the moisture. (15,25) They are capable of harvesting water in low RH condition as well as saturated RH condition through adsorption or/and absorption. With the release process powered by energy inputs such as heating, the harvested water can be collected efficiently, benefiting from the sufficient accumulation of moisture (Figure 1). A fundamental challenge is the high energy consumption for water release due to the strong affinity between harvesters and water. While useful strategies inherit ideas from conventional desiccants, such as silica gels, (30,31) zeolites, (32,33) and hygroscopic materials, (34,35) new concepts based on unique features of nanostructured materials including MOF-based materials (15,36,37) and hybrid gels (16,35,38−42) have also been proven promising. The ideal moisture harvesters are required to have high water uptake, low energy demand for water release, fast water capture/release, high cycling stability, and low cost, which can be simultaneously achieved by rational material and structural designs, yet to be fully demonstrated. It is essential to fundamentally understand the interaction between materials and water molecules from both experiments and theoretical modelling. Therefore, this Perspective presents the strategies to design moisture-harvesting materials for AWH. Traditional aspects of moisture capture and relevant underlying mechanisms as well as fundamental design principles are discussed first. Innovative material and structural designs, along with representative examples, are then elaborated to introduce current challenges and corresponding strategies. Instead of presenting a comprehensive review of all existing literature, we focus on the representative strategies that are promising in the near future, such as tailoring the water affinity of materials to broaden the working range of RH and enhance the water uptake, architecting micro/nanostructures to facilitate vapor diffusion and moisture capture, newly developed concepts of temperature-responsive water release and explorations on utilizing sustainable power sources with incorporation of functional materials. Finally, the potential challenges relevant to the next-generation moisture harvesters are summarized to propose the opportunities in the future development of this emerging and promising field.

Figure 1

Figure 1. Schematic of atmospheric water harvesting (AWH) based on moisture harvesters. By using moisture harvesters with high surface area, porous structure, water affinity, fast vapor diffusion, and molecular diffusion, the water vapor in atmosphere can be collected as freshwater. The moisture-harvesting materials could be MOF-based materials, hygroscopic materials, and polymeric gels.

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The ideal moisture harvesters are required to have high water uptake, low energy demand for water release, fast water capture/release, high cycling stability, and low cost, which can be simultaneously achieved by rational material and structural designs.

Understanding Moisture Sorption

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AWH based on moisture harvesters captures vapor from the air via absorption or adsorption, which refers to the attachment of water molecules to materials through physical or chemical processes (Figure 2a). (25,43) Absorption can be considered as a bulk phenomenon where gas/liquid molecules diffuse into liquid/solid materials, changing the structure and volume of absorbent materials. Water molecules can be absorbed by chemical reaction and/or physical interaction with absorbent materials. (44) The chemical absorption depends on the stoichiometry of the reaction and the concentration of the reactants, while the physical absorption is usually driven by osmotic effect. Typical moisture harvesters based on absorption are hygroscopic materials, which capture water molecules through the hydration process involving chemical absorption and physical absorption. (35,39) For example, deliquescent salts can absorb moisture when the vapor pressure of contained water is lower than the partial pressure of vapor in air, (45,46) exhibiting high water uptake. However, particle agglomeration during hydration/dehydration process of solid salts might result in performance decay due to reduced permeability of water vapor. (25) Moreover, the desorption process for hygroscopic materials is regarded as the bottleneck because of the high vaporization enthalpy. (45,46)

Figure 2

Figure 2. Moisture sorption mechanisms. (a) Schematic of absorption(left) and adsorption (right) processes. (b) Schematic of physisorption (top left, top right) and chemisorption (bottom left, bottom right).

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Adsorption is a surface phenomenon in which molecules of gas or liquid adhere to a surface through chemical (chemisorption) or physical (physisorption) interactions(Figure 2b). (25) The thermodynamics of adsorption is relevant to the intrinsic material properties. For chemical adsorption, the sorbent surface needs binding sites to adsorb molecules through strong chemical bonding, such as hydrogen bonding, coordination effect and electrostatic interactions. The energy barrier involved is relatively high (80–400 kJ/mol), which is generally irreversible without energy supply. (47,48) Langmuir isotherm (49−51) can be used to describe the chemisorption upon following assumptions: (1) all adsorption sites are identical, (2) each site of the absorbent can only accommodate one and only one molecule, and (3) all sites are energetically and sterically independent of the adsorbed quantity by eq 1
(1)
where θ is the equilibrium fraction of the adsorbate monolayer coverage, which can reflect the sorption capacity in a certain content, α is the Langmuir adsorption constant depending on the intrinsic affinity of the adsorbent toward the adsorbate and the temperature, and p is the partial pressure of adsorbate. (52) It is obvious that enhancing the interaction between materials and water molecules (larger α) and increasing local RH near the adsorption sites (larger p) will promote the moisture capture capacity. The amount of molecules adsorbed on surfaces also depends on the surface area of sorbents. Physisorption presents weak van der Waals force between the sorbent surface and adsorbed molecules. This process is readily reversible and requires energy of ∼20 kJ/mol or less. Brunauer, Emmett, and Teller (BET) isotherm (52,53) is a well-accepted model to explain the physical adsorption, which is based on following assumptions: (1) the enthalpy of adsorption for the first layer (E1) is constant and greater than the following layers and (2) molecules can adsorb on each other. In this case, the enthalpy of adsorption layers over the first layer is the same as the enthalpy of liquefaction (EL). The dependency could be described by eq 2
(2)
where p and p0 are the equilibrium and the saturation vapor pressure of the adsorbate at the temperature of adsorption, respectively, N refers to equilibrium gas adsorption amount at the relative pressure p/p0, N0 is the monolayer gas adsorption quantity, that is, the number of molecules needed to uniformly cover the substrate with one complete monolayer, and C is the BET constant related to the energetics of the system
(3)
where R is the gas constant and T is the temperature. Therefore, improving the surface area (greater N0) can help to achieve a higher sorption capacity (reflected by larger N).
Regarding the adsorption kinetics, the exposure of the absorbent to the air becomes dominative since the diffusion of moisture significantly affects the adsorption process. Linear driving force (LDF) model (54,55) assumes that the temperature of adsorbent particles is uniform (does not vary with radius) at all times and the vapor partial pressure is constant, is used to describe mass-transfer by eq 4
(4)
where μ is the sorption rate (s–1), x* and x are the equilibrium and the dynamic water uptake of the adsorbent, respectively, and k1 is the LDF adsorption rate coefficient (s–1) and is positively related to the diffusion coefficient (De) and negatively related to the adsorbent particle size (r0): (25)
(5)
It is worthy noting that the sorption behavior of current moisture harvesters is usually exhibited as the combination of physisorption and chemisorption, and may involve absorption as well. (34,35) The corresponding underlying moisture capture mechanism is rather complex compared to the theoretical models. Nevertheless, based on the understanding of the fundamental sorption mechanism, the material and structural designs could be rationally optimized to enhance the sorption properties of moisture harvesters.

Material Design Principles and Characterization

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Moisture capture capacity, the energy demand of water release, sorption and desorption kinetics, and cycling durability are essential features of moisture harvester enabled AWH technologies.

Superior moisture sorption based on materials with enhanced water affinity, large surface area and high porosity can increase the water uptake to harvest enough moisture for vapor liquefaction, delivering collectible water. Low water-release energy demand can be achieved by tailoring the sorption behavior of materials and incorporating functional materials, such as thermal responsive materials to introduce phase separation, which eliminates energy-intensive phase transition process during water collection. In addition, fast water sorption and desorption are essential for high water production within a short period of time. Moreover, since the AWH is a time-intensive process, the moisture harvesters should be stable during long-term cycling process without potential performance decay (Figure 3). Through proper material selection and rational modification, an ideal moisture harvester with high water uptake, low generation energy demand, fast sorption/desorption, and cycling stability can be obtained for efficient AWH.

Figure 3

Figure 3. Essential properties of moisture harvesters for AWH. The ideal moisture harvesters should have high sorption capacity, low regeneration energy demand, fast sorption/desorption, and long-term cycling stability.

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The sorption capacity of moisture harvesters is the most important factor for efficient water harvesting from the air, which is usually evaluated by dynamic vapor sorption. Water sorption isotherms can be obtained to present the equilibrium mass of water which can be captured by materials as a function of RH at a given temperature. At the low RH levels, water molecules are absorbed on the surface of materials and then liquefied as liquid water. When the humidity is saturated, the materials provide active sites for nucleation and growth of liquid water. The water uptake at saturated RH is the maximum capacity of materials. The shape of isotherms is important to be capable of optimizing water harvesting capability. The isotherm with stepwise water uptake (type IV and V) in the RH range between 10% and 30% is desirable for facilitated water adsorption at low RH condition in arid regions, which means that a large amount of vapor from the air can be captured as the ambient RH is higher than the position of sorption step. (15,25) However, the isotherm of traditional sorption materials, which indicates their sorption behaviors cannot satisfy this criteria, making them inappropriate for efficient AWH. For example, zeolites present fast and saturated water sorption at extremely low RH (Type I), due to their strong binding with water molecules and abundant sorption sites, however, high regeneration temperature is required to release and condense water. Silica gels can only adsorb limited amount of vapor in low RH conditions (Type II), which is not suitable for arid regions. In addition, obvious difference of sorption capacity between adsorption and desorption temperature is required for large amount of collected water, which can be evaluated by static vapor sorption techniques measuring the water uptake over time at a given RH and temperature. The water affinity and micro/nanostructures of moisture-harvesting materials should be tailored for desirable sorption behaviors and high water capacity based on the isotherms. For instance, the hydrophilicity, porosity and geometry of MOF-based materials can be modified by varying the metal ions, ligands, etc. (15) Composite gels incorporated with hygroscopic salts or polymers can be tailored by applying different polymer matrices and hygroscopic materials, changing the polymer concentration, crosslinking density, functional additives, etc.
As for traditional sorption materials for AWH, the water release (i.e. regeneration) step usually involves heating process with extensive energy consumption because of their strong interaction with water molecules, which is not suitable for practical application. By choosing materials with desired sorption behaviors, regeneration of moisture harvesters by low grade energy input can be achieved. The solar irradiation can be utilized to desorb the water molecules when the harvesters have photothermal conversion ability (16,35,39,45) or incorporating solar absorbers. In addition, functional materials, such as temperature-responsive materials, can be introduced to release water for efficient regeneration by liquid–liquid phase separation under solar irradiation. (16) Another important factor is the fast sorption and desorption of moisture harvesters, which can be evaluated by the sorption/desorption kinetics measured within different RH and temperature intervals. The high rates enable multiple water harvesting cycles for high water production in a short period of time. Micro/nanostructures with high pore density can facilitate vapor diffusion and water transport for fast sorption and desorption. Moreover, the moisture harvesters should be stable without performance decay during cycling process of sorption and regeneration. Various strategies are applied for different sorption materials. MOF-based materials are modified with hydrolytic stability through changing the strength of metal-linker bond and the energetic positioning of the frontier orbitals on the metal ions with respect to those of water. Hygroscopic salts are incorporated in supporting matrices, such as gels and MOFs, to avoid particle agglomeration for cycling stability. By designing moisture harvesters with above features, efficient AWH for practical applications can be realized.

Material and Structural Design of Moisture Harvesters for AWH

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Traditional Moisture Harvesters

For AWH technologies based on moisture harvesters, traditional desiccants, such as silica gels, zeolite, hygroscopic salts, and deliquescent liquids, are initially utilized to harvest water because of their high affinity with water. Silica gel is one of the most commonly used moisture-harvesting materials, which has polar hydroxyl groups as the chemical adsorption centers to form hydrogen bonding with water molecules (Figure 4a). (56) With the increasing number of hydroxyl groups on the surface, the adsorption ability of silica gels could be enhanced due to increased density of sorption sites. This could be reflected from the eq 1, high density of sorption sites increases the overall affinity between silica gels and water molecules, leading to a larger Langmuir adsorption constant (α) and high maximum capacity (θ). The pore diameters of common silica gels are within several nanometers and the specific surface area is about 100–1000 m2/g. (57) This porous structure and high surface area of silica gels are beneficial for water harvesting. Zeolite is another widely used solid moisture-harvesting material, which are three-dimensional networks composed of silica and alumina that contain alkali and alkaline-earth metals (Figure. 4b). (58) The aluminum metal is the sorption site which has coordination effect with water molecules, whose amount influences the water harvesting ability. The porous structure of zeolite also determines the water harvesting capacity, larger pore volume usually leads to high sorption capacity. (59) However, the water capacities of silica gels and zeolites are relatively low, which are around 0.3–0.5 kgwater kgsorbent–1. (60) Besides, the high water affinity makes the release of harvested water difficult, requiring high energy consumption. The relative low water uptake and high energy demand for desorption render them unsuitable for efficient AWH.

Figure 4

Figure 4. (a) Schematic of silica gels adsorbing water molecules. Reprinted with permission from ref (56). Copyright 2000 Elsevier Science B.V. (b) Porous structure of zeolites. Reprinted with permission from ref (58). Copyright 2008 IOS Press. (c) Deliquescent liquid sorbents for AWH assisted by an interfacial solar heating strategy. Reprinted with permission from ref (45). Copyright 2019 Wiley-VCH Verlag GmbH & Co. (d) Simultaneous sorption-desorption of liquid sorbents. Reprinted with permission from ref (46). Copyright 2019 Wiley-VCH Verlag GmbH & Co.

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Unlike silica gels and zeolite, hygroscopic materials that can harvest water molecules via absorption and/or adsorption exhibit high water uptake and are capable of taking up water at low RH. (38,61) Hygroscopic materials include hygroscopic salts and deliquescent liquid sorbents. Hygroscopic salts, such as LiCl, MgCl2, and CaCl2, can harvest moisture by hydration reaction but suffer from problems of particle agglomeration and passivation layer formation on particle surface during hydration, which will reduce the permeability of water vapor and decrease the sorption capability. Deliquescent liquid harvest water until the vapor pressure of the solution is equal to the partial pressure of vapor in air. (25) The fundamental challenge for hygroscopic materials is the high energy consumption for desorption, especially for deliquescent liquid whose whole bulk liquid is heated. Recently, photothermal materials are combined with deliquescent liquid to efficiently utilize solar energy for heating during the desorption process by interfacial evaporation. The moisture harvester is composed of an interfacial solar heating layer (GO-based aerogel) on the surface of a liquid sorbent (50 wt % CaCl2 solution), realizing a AWH performance of 2.89 kg m–2 day–1 (Figure 4c). (45) Different from bulk heating, the harvested solar energy can be concentrated on the evaporation surface, greatly reducing the energy loss to bulk water and improving energy utilization efficiency. In this system, the CaCl2 solution can capture moisture from air during night by adsorption, and the salt-resistant GO-based aerogels enable the water releasing by efficiently utilizing solar energy through interfacial heating during the daytime. Then, the moisture harvester is further designed to realize simultaneous harvesting–releasing process for continuous water production and realize a water production rate of 0.5 L m–2 h–1. (46) The system is composed of two interconnected parts, one part is filled with the liquid sorbent for water capture and another contains a photothermal layer floating on the liquid for water release (Figure 4d). Moisture in the environment can be captured by the liquid sorbent and be released continuously as vapor by interfacial solar heating, which then is condensed to liquid water to be collected. However, the dependence of water uptake capability on environmental RH hinders the practical AWH of liquid harvesters in arid regions. Besides, the difficulties in handling as well as engineering of deliquescent liquids also cause issues in practical AWH.
Although these single-component water harvesting materials work well as desiccant for dehumidification, they can barely meet the requirements for efficient AWH due to the insufficient water release. The fundamental challenge is the intrinsic high energy demand during the desorption or dehydration processes caused by their high affinity with water molecules, which is difficult to be solved. Thus, new materials with highly tailorable properties have been proposed as alternatives to meet the demand for efficient AWH.

MOF-Based Moisture Harvesters

MOFs are a kind of crystalline porous materials constructed by metal-containing inorganic building centers and polyfunctional organic ligands through coordination bonds into three dimensional (3D) interconnected networks. The highly porous structure of MOFs can provide numerous potential sorption sites for water molecules. Water sorption in MOFs can occur following three distinct mechanisms: (i) chemisorption on open metal sites, (ii) physisorption in the form of layers or clusters, and (iii) capillary condensation when the pore size is large than the critical diameter of water. (15) The desorption process requires heat input to reach the regeneration temperature, which varies from several tens to hundreds of degrees Celsius depending on the adsorption strength of the water molecules. (62) Although the water sorption/desorption mechanism of MOFs has been investigated, they draw much attention until very recently for AWH because their stability in water is a challenge and the sorption/desorption behaviors need be carefully tailored for efficient AWH. By tailoring the structures and compositions of MOFs, their properties such as hydrophilicity, and pore size/volume can be tuned for desired sorption and desorption behaviors to realize superior water harvesting performance.
One of the first MOF-based AWH devices was demonstrated based on Zr-based MOF (MOF-801) that can achieve water harvesting at a low RH environment (Figure 5a). (36) MOF-801 is chosen as the harvesting material because it can take up water within a narrow RH range and enable water release of large amount with small temperature increase, which also exhibits good stability and recycling (Figure 5b). MOF-801 shows good performance driven by aggregation of water molecules into clusters within the pores. Eventually, a water production rate of 2.8 gwater gMOF–1 day–1 at relative humidity levels as low as 20% is achieved with solar energy as the only energy input for desorption. Furthermore, a passive radiative cooling strategy is introduced into the MOF-based AWH system that can work in the arid climate (Figure 5c). (37) The passive radiative cooling can be realized by dissipating thermal radiation to the clear cold sky to increase the effective RH for adsorption. By utilizing the copper foam with high emissivity for passive radiative cooling at the top of the device, a ∼3 K temperature drop can be achieved, which corresponds to an increase in 5–7% RH experienced by the moisture harvester during the AWH process. This device can operate in an exceptionally arid climate (10–40% RH) and sub-zero dew points with a thermal efficiency (solar input to water conversion) of ∼14% (Figure 5d).

Figure 5

Figure 5. (a) Schematic of AWH based on MOF-801. (b) Water adsorption isotherms of MOF-801 at different temperatures. Reprinted with permission from ref (36). Copyright 2017 American Association for the Advancement of Science. (c) Schematic of the water harvesting device containing a radiative cooling part. (d) Water adsorption isotherms of MOF-801 as a function of RH (P Psat–1, vapour pressure over saturation pressure) at temperatures of 15, 25, 45, 65, and 85 °C. Reprinted with permission from ref (37). Copyright 2018 Nature Publishing Group.

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Since the stability, hydrophilicity, and pore diameter of MOFs determine their water harvesting performance, understanding how to rationally design MOF-based moisture harvesters for desired properties is essential to achieving efficient AWH. The sorption/desorption behaviors of MOFs can be adjusted by controlling their hydrophilicity, pore size, and geometry (Figure 6a). The hydrophilicity of MOFs can be tuned by modifying the metal nodes and organic ligands with different functional groups. As the hydrophilicity of MOF increased (Figure 6b), the adsorption isotherms shift to a lower relative pressure value, indicating that the adsorption process can occurs at a lower RH. (15,63) Therefore, higher hydrophilicity is beneficial for water capture due to the enhanced interactions between MOF and water molecules. Pore size in MOFs also plays a great role in affecting the water adsorption behaviors. Increasing the length of organic linkers can enlarge the pore size, offering a larger maximum sorption capacity (Figure 6c). (15,64) However, the increase of the pore size might change the water adsorption mechanism from pore filling to irreversible capillary condensation, which has a hysteresis loop and the captured water is extremely hard to be released. Thus, the pore size should be moderate for both high moisture capture capacity and relatively easy water release. It is worthy noting that although the modification of organic linkers can tune the hydrophilicity of the MOF for efficient water harvesting under a specific condition, it may cause a substantial decrease of porosity and thus reduce the adsorption capacity. The trade-off between the hydrophilicity and porosity should be considered when modifying the MOFs with functional groups.

Figure 6

Figure 6. (a) Tuning factors of MOF-based moisture harvesters. Water sorption isotherms of MOF-based moisture harvesters with different (b) hydrophilicity, (c) pore size, and (d) geometry. Reprinted with permission from ref (15). Copyright 2018 Wiley-VCH Verlag GmbH & Co.

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Geometry is another parameter that affects the water sorption behavior of MOFs. Water-sorption isotherms of isoreticular (M)MIL-53(R) (M = Al, Fe) with different −R groups changing from −(NH2) to −(OH) and to −(COOH)2 are shown in Figure 6d. (15) The different −R groups have effects on their geometries because of their different steric configurations. Only the (Al)MIL-53(NH2) (orange triangles) shows a reversible water adsorption/desorption behavior while the other isotherms show the obvious difference between the adsorption/desorption processes due to the reversible structural phase transition during these processes. Take (Al)MIL-53(OH) as an example, its isotherm shows a strong breathing effect at P/P0 = 0.8 and a strong hysteresis between the adsorption and desorption branch. This means that the desorption of water can only occur at lower RH due to the enlarged pore size during adsorption.
Although the MOF-based moisture harvesters can be used for AWH with rationally tailored water sorption behaviors, it has been proven difficult to synthesize most of the suitable MOFs on a commercial scale that allows for large-scale, cost-efficient water production. The exploration of more stable MOFs which possess controlled porosity and excellent water capture/release properties is the cornerstone for future industrial application. There are still several key challenges that need to be overcome. First, developing MOFs with improved water sorption capacities under different temperature/RH working conditions is necessary to optimize their inherent sorption performance for practical AWH. Second, a more detailed and fundamental understanding of the sorption/desoprtion kinetics is still lacking for further development of MOF-based moisture harvesters. Third, beyond long term stability and cycling issues, shaping/processing of MOFs is also a key aspect in the practical application in the future. (47) Therefore, new MOFs with intrinsic better water sorption/desorption properties is highly desirable for superior AWH. In addition, most of the existing MOF-based moisture harvesters can only realize one sorption/desorption cycle per day, greatly limiting their potential water production amount. Achieving multiple cycles is also a key point for better AWH performance and practical applications.

Hygroscopic Salts and Composites

Hygroscopic materials feature their interaction with water molecules in surrounding environment throughout the material, in addition to the surfaces, which enables high water uptake as many as 5–6 times of their own weight. However, the structural deterioration of these materials over wetting-drying cycles, such as particle agglomeration, drastically degrades the moisture capture rate because of insufficient exposure to the moist air. The water molecules will accumulate on the surface of materials and diffuse into the closely packed structures (e.g., crystalline structure), leading to a slow water condensation near the sorption interface. Given the augmented water affinity of this type of materials, it is not surprising to find their high energy demand of water release. Such a high energy consumption limits the practical application of hygroscopic materials in AWH. In this context, porous materials have been utilized as the supporting matrix for hygroscopic salts or polymers to overcome these challenges. Fast vapor diffusion and water release are enabled by the designed micro/nanostructures. The cycling stability of moisture harvesters is also enhanced because of the embeddedness of hygroscopic components fixing in the matrix. By incorporating photothermal materials such as solar absorbers, new energy sources can be employed for efficient AWH.
Hygroscopic salts have been demonstrated to incorporate with porous supporting matrices, including silica gels, (60,65) zeolites, (66) porous carbon, (67) MOFs, (68) and polymer hydrogels, (35,39) as composite moisture-harvesting materials, taking advantages of the stability of the porous matrix and high water capture capacity of hygroscopic salts. For example, by simply impregnating LiCl into the void core of a nanocarbon hollow capsule, a moisture harvester that can capture and release water vapor from ambient air with significantly enhanced kinetics is obtained (Figure 7a) because of the shortened vapor diffusion length inside the sorbent. (67) Attributing to the fast sorption/desorption process, a batch-mode AWH device based on this moisture harvester could conduct 3 harvesting/releasing cycles within 10 h and have water harvesting capacity of 1.6 kgwater kgsorbent–1. MOF can also be a good host for hygroscopic salts. A high-performance composite moisture harvester is prepared by encapsulating a hygroscopic salt into a MOF matrix (LiCl@MIL-101(Cr)) (Figure 7b). (68) Benefiting from the uniform porous structure and large pore volume of MOF, the moisture harvester exhibit faster water harvesting kinetic and cycling stability than bulk LiCl. A highly efficient AWH prototype based on LiCl@MIL-101(Cr) is demonstrated, enabling water harvesting of 4.5 kgwater kgsorbent–1 under outdoor ambient conditions powered by natural sunlight as the only energy input. The porous supporting matrix greatly enhances vapor diffusion and water release, improving the AWH performance.

Figure 7

Figure 7. (a) Impregnating of LiCl into a porous carbon matrix. Reprinted with permission from ref (67). Copyright 2020 Elsevier. (b) MOF as the supporting matrix for LiCl for AWH. Reprinted with permission from ref (68). Copyright 2020 Wiley-VCH Verlag GmbH & Co. (c) AWH based on PAM-CNT-CaCl2 moisture harvester. (d) Water vapor harvesting curves of PAM-CNT-CaCl2 moisture harvester under different RH. Reprinted with permission from ref (38). Copyright 2018 American Chemical Society.

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In addition to conventional porous materials, polymeric gels have been reported as a promising host matrix for hygroscopic salts. A hybrid photothermal moisture harvester (PAM–CNT–CaCl2) composed of a deliquescent salt (CaCl2), a hydrogel host (polyacrylamide, PAM) and photothermal carbon nanotubes (CNT) is developed (Figure 7c). (38) With the RH of 10%, 35%, 60%, and 80%, the water sorption amounts of PAM–CNT–CaCl2 are 5%, 69%, 110%, and 173%, respectively (Figure 7d). The CaCl2 is introduced in the hydrogel network by aqueous solution based physical impregnation and is mostly responsible for the water capture either by hydration reaction at low RH (<26%) or deliquescence at high RH (>26%). Through the introduction of CaCl2 with high water affinity, the Langmuir adsorption constant of the system can be improved and enable a higher capacity. The incorporation of photothermal materials enables the utilization of renewable solar energy. The cross-linked hydrogel network keeps the deliquescent CaCl2 solution in a solid form, and the flexible expansion behavior of the hydrogel makes the pore volume not a limiting factor and, thus, enhances the water harvesting capacity. As a result, 35 g of dry hydrogel could deliver 20 g of water in 2 h and a half under natural sunlight after capture water at 60% RH. The water capacity can be further improved by incorporating binary salts in the hydrogel network. (39) A composite hydrogel composed of alginate chains modified with binary salts and functionalized multi-walled carbon nanotube (FCNT) as an efficient solar absorber is demonstrated as the moisture harvester (Bina/FNCT). The Bina/FNCT is found to have both higher water adsorption capacity (5.6 gwater gsorbent–1) and better stability than the sorbents obtained by a single salt because the combination of the salts increases water affinity of the material and the porous structure facilitates vapor diffusion into the beads.
While much of the credit is given to the unique hygroscopic composites associated with large active area for moisture harvesting, it is unclear how much the porous structure has contributed to the vapor capture performance. The emerging studies indeed demonstrate the feasibility of porous structure architecting in hygroscopic materials for AWH. Nevertheless, the role of the porous structure should be more manifested in sorption processes involving more complex composition, structures, and functionalities, such as heat management for high energy efficiency. In addition, the fundamental process of water release still relies on the dehydration of hygroscopic components instead of reducing the intrinsic energy demand of water release, and ingenious designs for energy recycling or introduction of new energy input should be introduced.

Novel Molecular Water Reservoirs

Though the moisture capture capacity and cycling stability can be improved by directly impregnating hygroscopic salts into a porous supporting matrix and they could be enhanced by structural designs and system engineering, more versatile methods have been developed via molecular engineering involving hydrophilicity controllable polymers or molecular heaters. By combining hygroscopic materials and hydrophilicity controllable polymers at a molecular level, the water release process can be achieved by a phase separation process, where the polymer in hydrophilic state serving a molecular reservoir is controllably switched to a hydrophobic state, releasing the containing water without any energy-intensive processes such as desorption or dehydration. Such a water regulation in the molecular level, which is a key innovation synergistically inheriting the intrinsic properties of building blocks, opens new opportunities for AWH. (16,40) Taking the thermoresponsive polymer as an example of novel molecular water reservoirs, an interpenetrating polymer network (IPN) gel that can capture moisture from the air and directly extract liquid water was reported by combining a thermoresponsive component (poly(N-isopropylacrylamide), PNIPAM) with a hydrophilic sodium alginate network (Figure 8a). (40) Because of the hydrophilic property of the IPN gel at temperature below its low critical solution temperature (LCST, 32 °C), the dried IPN gel can harvest considerable moisture from air. Upon increasing temperature, the IPN gel changes from hydrophilic to hydrophobic due to the thermo-responsive property of PNIPAM, enabling water molecules to be converted from a gaseous state to a liquid state by a small temperature change above the LCST and releasing the absorbed moisture as liquid water. The water-releasing behavior of this thermoresponsive moisture harvester is significantly different from that of common harvesters, which rely on the desorption of water vapor upon high temperature. However, the overall efficiency and moisture absorption capacity of this moisture harvester are relatively low.

Figure 8

Figure 8. (a) AWH based on a thermoresponsive interpenetrating network gel with liquid water release. Reprinted with permission from ref (40). Copyright 2018 Nature Publishing Group. (b) AWH based on the SMAG and its water uptake in different RH. Reprinted with permission from ref (16). Copyright 2019 Wiley-VCH. (c) AWH based on a rGO-PAAS gel. (d) Water uptake of rGO, PAAS, and PGF (25 °C, RH = 100%). Reprinted with permission from ref (69). Copyright 2020 Wiley-VCH.

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Recently, a super moisture-absorbent gel (SMAG) composed of polypyrrole chloride (PPy-Cl) penetrating in PNIPAM network with the hydrophilicity-switchable ability (Figure 8b) is developed to enable synergistic effect of hygroscopicity and thermoresponsive water releasing to enhance water harvesting capacity. (16) This moisture harvester can simultaneously realize efficient moisture capture, in situ water liquefaction, high water storage capacity, and fast liquid water release under different weather conditions, achieving high-efficiency water production in a broad RH range. The adsorbed water molecules are first liquefied on the surface of the SMAG and then diffuse into the PNIPAM matrix to be stored and cause swelling. The molecular-level interaction between PPy-Cl and hydrophilicity-switchable PNIPAM allows internal water rearrangement when several layers of water molecules are captured from air through physisorption, reactivating the sorption points to harvest more water molecules. The liquid water can be rapidly released by liquid–liquid phase separation enabled by the thermal-responsive hydrophilicity switching of PNIPAM network when the temperature is above its LCST. After this rapid release of liquid water, the residual water can be collected via an evaporating-condensing process. Both the two water releasing processes can be powered by solar energy due to the existence of PPy-Cl with photothermal ability. This work provides insight into the synergetic effect of interpenetrated functional polymers and their potential for advanced AWH systems in the future. The controllable molecular water reservoir could be also established by the synergetic effect of a polymeric water trapper and a molecular solar heater. Porous sodium polyacrylate (PAAS)/graphene framework (PGF) is designed to capture moisture from common or smoggy environments and then release clean water with a high rejection rate of impurities under solar irradiation (Figure 8c). (69) Compared with single PAAS and PGF, the water uptake is greatly enhanced due to the synergistic effect arose from the microporous structure of PGF providing effective transport channels and an enlarged contact area for moisture (Figure 8d), leading to a high equilibrium uptake of 5.20 gwater/gPGF at 100% RH. Besides, because of the excellent solar–thermal conversion property, the harvested water in PGF can be quickly released by using natural sunlight without other energy supply, and the produced water has a high quality which is free of impurities.
Despite the delightful progress, many ideas in this direction are still conceptual. Rational selection of components and incorporated functionalities is critically needed to develop superior moisture harvesters. For instance, utilizing hydrogels with high saturated water can enhance water uptake, and using environmental responsive polymer can directly release water in the liquid form. The ideal harvesting materials should have a porous structure for quick vapor diffusion during moisture capture, appropriate interaction with water for easy release, and a stable structure for long-term cycling stability. Structural expansion resulted from insertion of water molecules in the moisture-harvesting materials is able to lower water diffusion barriers along the molecular chains in the polymeric networks, but it may potentially deteriorate the stability of the whole structure. These issues require a delicate balance over high density of active regions for moisture capture and regionally selective water trapping without sacrificing the stability of materials. A trade-off between performance and stability is usually expected in this case, and extensive optimization may be needed. In addition, a deeper understanding of the underlying mechanisms on molecular water reservoir systems by means of advanced in situ characterization and theoretical modeling is also of great importance.

Outlook and Opportunities

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Being a geographically and climatically independent water production method, AWH technology is becoming more prominent for both emergency needs, such as disaster relief and decentralized water supply.

Enabling moisture as a water resource could be considered as one of the most promising alternative strategies to overcome the challenges of long-distance potable water transport or delivery in rural areas.

We have discussed the development track of AWH technology, and it is rather clear that material innovation is emerging as a potential direction to push forward such a technology. Sorbent materials can be utilized as moisture harvesters as long as the water affinity is tuned to enable the moisture concentration under low RH condition and water release upon low energy input. To this end, the design principles of moisture harvesters should be clarified (Figure. 9). First, the chemical stability toward moisture (water) is necessary to eliminate the contamination of obtained water. Second, the interaction between materials and water molecules should be strong enough to enable the operation at low RH or high-temperature conditions. Third, the concentrated moisture can be released by relatively small energy input. Forth, the microstructure of moisture harvesters should be tailored to allow airflow passing through at high flux. Fifth, the structural strength is required to be strong enough to maintain the designed structure during the water release.

Figure 9

Figure 9. Outlook and perspective opportunities for next-generation AWH.

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Along with the achievements so far, there are several key challenges to be overcome in further promoting AWH technology. The most significant challenge is to balance the requirement of moisture concentration and water release. An expansion of the operating regime of dewing can be achieved through the use of materials with strong water affinity to increase local moisture concentration. For most of water harvesters that exhibit good performance at low RH, the energy demand of water release is significantly greater than the condensation heat since a large amount of energy has to be consumed to re-vaporize the captured water for final collection. Consequently, extra energy input is required to power a heat pump operating at the Carnot limit for the heat dissipation during the condensation process. In this context, water release should be improved, which synergistically combines sorption and dewing processes to promote the AWH in a broader range of climates. In addition, architecting porous structures will enhance the exposure of AWH materials to moisture, facilitating the moisture capture. Controllable fabrication of highly ordered porous structures allows water management in the AWH material. Moreover, the AWH system, which is generally composed of moisture-harvesting materials, materials substrate and collection devices, can be optimized for efficient moisture concentration and water delivery. (25) For example, the interfacial heat and mass transfer resistances can be reduced by coating a thin layer of moisture-harvesting materials on substrates. By constructing microstructures of the substrates, the vapor diffusion and water droplets migration could be improved, increasing the water production capacity. As such, tailoring system design corresponding to certain advanced AWH material is becoming an important strategy to explore the potential of the material. Efforts need to be taken to establish a standardized means to evaluate and select materials for AWH based on an optimized system with well-designed heat management, which can reveal the potential of new materials. Novel sorbent materials are critically needed to address such fundamental challenges. Both experimental exploration and theoretical prediction by computation simulations will be helpful in presenting a more in-depth insight into the migration and aggregation behaviors of water molecules during the AWH process.
The AWH technology is promising to be widely used for decentralized water supply in the future freshwater production market. Current water production techniques, such as multi-stage flash and multi-effect distillation, highly depend on complex infrastructure and need high energy consumption, leading to relatively high cost, and hence hindering their generalization in developing countries and remote regions. The materials enabled AWH is capable of producing water regardless of geographical and hydrologic conditions, and renewable energy can be utilized to avoid energy consumption by combusting traditional fossil fuel. Therefore, benefiting from the development of AWH materials, the cost of freshwater production will be reduced to an appealing level in the near future. In reflection of all aforementioned research efforts and promising prospects, moisture harvesters are able to expand the limits of current technologies and will significantly contribute to the alleviation of water scarcity.

Author Information

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  • Corresponding Authors
  • Authors
    • Xingyi Zhou - Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States;  Orcidhttp://orcid.org/0000-0001-7593-3137
    • Hengyi Lu - Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States;  Orcidhttp://orcid.org/0000-0002-5516-1966
  • Author Contributions

    X. Z. and H. L. contributed equally to this work.

  • Notes
    The authors declare no competing financial interest.

Biographies

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Xingyi Zhou is a graduate student at University of Texas at Austin under supervision of Professor Yu. She received her B.S. degree in Chemistry from University of Science and Technology of China.

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Hengyi Lu is a postdoctoral researcher at University of Texas at Austin with Professor Yu. He received his B. Eng. in Material Science and Engineering from Southwest Jiaotong University and Ph.D. in Polymer Chemistry and Physics from Fudan University.

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Fei Zhao is a postdoctoral researcher at University of Texas at Austin with Professor Yu. He received his B.S. in Material Science and Engineering and Ph.D. in Chemistry from Beijing Institute of Technology.

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Guihua Yu is a professor of Materials Science and Mechanical Engineering at University of Texas at Austin. He is an elected Fellow of Royal Society of Chemistry and Institute of Physics, Sloan Research Fellow, and Camile Dreyfus Teacher-Scholar. He received his B.S. degree with the highest honor from University of Science and Technology of China and Ph.D. in chemistry from Harvard University, followed by postdoctoral research at Stanford University.

Acknowledgments

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G.Y. acknowledges the financial support from Camille Dreyfus Teacher-Scholar Award, Sloan Research Fellowship, and from Lockheed Martin, Corp.

References

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    Solar vapor generation has presented great potential for wastewater treatment and seawater desalination with high energy conversion and utilization efficiency. However, technol. gaps still exist for achieving a fast evapn. rate and high quality of water combined with low-cost deployment to provide a sustainable solar-driven water purifn. system. In this study, a naturally abundant biomass, konjac glucomannan, together with simple-to-fabricate iron-based metal-org. framework-derived photothermal nanoparticles is introduced into the polyvinyl alc. networks, building hybrid hydrogel evaporators in a cost-effective fashion ($14.9 m-2 of total materials cost). With advantageous features of adequate water transport, effective water activation, and anti-salt-fouling function, the hybrid hydrogel evaporators achieve a high evapn. rate under one sun (1 kW m-2) at 3.2 kg m-2 h-1 out of wastewater with wide degrees of acidity and alky. (pH 2-14) and high-salinity seawater (up to 330 g kg-1). More notably, heavy metal ions are removed effectively by forming hydrogen and chelating bonds with excess hydroxyl groups in the hydrogel. It is anticipated that this study offers new possibilities for a deployable, cost-effective solar water purifn. system with assured water quality, esp. for economically stressed communities.
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  12. 12
    Zhao, F.; Zhou, X.; Shi, Y.; Qian, X.; Alexander, M.; Zhao, X.; Mendez, S.; Yang, R.; Qu, L.; Yu, G. Highly Efficient Solar Vapour Generation Via Hierarchically Nanostructured Gels. Nat. Nanotechnol. 2018, 13, 489– 495,  DOI: 10.1038/s41565-018-0097-z
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    12
    Highly efficient solar vapour generation via hierarchically nanostructured gels
    Zhao, Fei; Zhou, Xingyi; Shi, Ye; Qian, Xin; Alexander, Megan; Zhao, Xinpeng; Mendez, Samantha; Yang, Ronggui; Qu, Liangti; Yu, Guihua
    Nature Nanotechnology (2018), 13 (6), 489-495CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
    Solar vapor generation is an efficient way of harvesting solar energy for the purifn. of polluted or saline water. However, water evapn. suffers from either inefficient utilization of solar energy or relies on complex and expensive light-concn. accessories. Here, we demonstrate a hierarchically nanostructured gel (HNG) based on polyvinyl alc. (PVA) and polypyrrole (PPy) that serves as an independent solar vapor generator. The converted energy can be utilized in situ to power the vaporization of water contained in the mol. meshes of the PVA network, where water evapn. is facilitated by the skeleton of the hydrogel. A floating HNG sample evapd. water with a record high rate of 3.2 kg m-2 h-1 via 94% solar energy from 1 sun irradn., and 18-23 L of water per square metre of HNG was delivered daily when purifying brine water. These values were achievable due to the reduced latent heat of water evapn. in the mol. mesh under natural sunlight.
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  13. 13
    Zhou, X.; Zhao, F.; Guo, Y.; Rosenberger, B.; Yu, G. Architecting Highly Hydratable Polymer Networks to Tune the Water State for Solar Water Purification. Sci. Adv. 2019, 5, eaaw5484  DOI: 10.1126/sciadv.aaw5484
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    Tu, Y.; Wang, R.; Zhang, Y.; Wang, J. Progress and Expectation of Atmospheric Water Harvesting. Joule 2018, 2, 1452– 1475,  DOI: 10.1016/j.joule.2018.07.015
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    Progress and expectation of atmospheric water harvesting
    Tu, Yaodong; Wang, Ruzhu; Zhang, Yannan; Wang, Jiayun
    Joule (2018), 2 (8), 1452-1475CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    A review. Even if people live in an arid desert, they know that plenty of water exists in the air they breathe. However, the reality tells us the atm. water cannot help to slake the world's thirst. Thus an important question occurs: what are the fundamental limits of atm. water harvesting that can be achieved in typical arid and semi-arid areas. Here, through a thorough review on the present advances of atm. water-harvesting technologies, we identify the achievements that have been acquired and evaluate the challenges and barriers that retard their applications. Lastly, we clarify our perspectives on how to search for a simple, scalable, yet cost-effective way to produce atm. water for the community and forecast the application of atm. water harvesting in evaporative cooling, such as electronic cooling, power plant cooling, and passive building cooling.
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  15. 15
    Kalmutzki, M. J.; Diercks, C. S.; Yaghi, O. M. Metal-Organic Frameworks for Water Harvesting from Air. Adv. Mater. 2018, 30, 1704304,  DOI: 10.1002/adma.201704304
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    Zhao, F.; Zhou, X.; Liu, Y.; Shi, Y.; Dai, Y.; Yu, G. Super Moisture-Absorbent Gels for All-Weather Atmospheric Water Harvesting. Adv. Mater. 2019, 31, 1806446,  DOI: 10.1002/adma.201806446
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    Oki, T.; Kanae, S. Global Hydrological Cycles and World Water Resources. Science 2006, 313, 1068– 1072,  DOI: 10.1126/science.1128845
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    Global Hydrological Cycles and World Water Resources
    Oki, Taikan; Kanae, Shinjiro
    Science (Washington, DC, United States) (2006), 313 (5790), 1068-1072CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prep. for such anticipated changes.
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  18. 18
    Klemm, O.; Schemenauer, R. S.; Lummerich, A.; Cereceda, P.; Marzol, V.; Corell, D.; Van Heerden, J.; Reinhard, D.; Gherezghiher, T.; Olivier, J. Fog as a Fresh-Water Resource: Overview and Perspectives. Ambio 2012, 41, 221– 234,  DOI: 10.1007/s13280-012-0247-8
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    Fog as a fresh-water resource: overview and perspectives
    Klemm Otto; Schemenauer Robert S; Lummerich Anne; Cereceda Pilar; Marzol Victoria; Corell David; van Heerden Johan; Reinhard Dirk; Gherezghiher Tseggai; Olivier Jana; Osses Pablo; Sarsour Jamal; Frost Ernst; Estrela Maria J; Valiente Jose A; Fessehaye Gebregiorgis Mussie
    Ambio (2012), 41 (3), 221-34 ISSN:0044-7447.
    The collection of fog water is a simple and sustainable technology to obtain fresh water for afforestation, gardening, and as a drinking water source for human and animal consumption. In regions where fresh water is sparse and fog frequently occurs, it is feasible to set up a passive mesh system for fog water collection. The mesh is directly exposed to the atmosphere, and the foggy air is pushed through the mesh by the wind. Fog droplets are deposited on the mesh, combine to form larger droplets, and run down passing into a storage tank. Fog water collection rates vary dramatically from site to site but yearly averages from 3 to 10 l m(-2) of mesh per day are typical of operational projects. The scope of this article is to review fog collection projects worldwide, to analyze factors of success, and to evaluate the prospects of this technology.
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    Andrews, H.; Eccles, E.; Schofield, W.; Badyal, J. Three-Dimensional Hierarchical Structures for Fog Harvesting. Langmuir 2011, 27, 3798– 3802,  DOI: 10.1021/la2000014
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    Three-Dimensional Hierarchical Structures for Fog Harvesting
    Andrews, H. G.; Eccles, E. A.; Schofield, W. C. E.; Badyal, J. P. S.
    Langmuir (2011), 27 (7), 3798-3802CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Conventional fog-harvesting mechanisms are effectively pseudo-2D surface phenomena in terms of water droplet-plant interactions. In the case of the Cotula fallax plant, a unique hierarchical 3D arrangement formed by its leaves and the fine hairs covering them has been found to underpin the collection and retention of water droplets on the foliage for extended periods of time. The mechanisms of water capture and release as a function of the surface 3D structure and chem. have been identified. Of particular note is that water is retained throughout the entirety of the plant and held within the foliage itself (rather than in localized regions). Individual plant hairs form mat-like structures capable of supporting water droplets; these hairs wrap around water droplets in a 3D fashion to secure them via a fine nanoscale groove structure that prevents them from easily falling to the ground.
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    Ju, J.; Xiao, K.; Yao, X.; Bai, H.; Jiang, L. Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection. Adv. Mater. 2013, 25, 5937– 5942,  DOI: 10.1002/adma.201301876
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    20
    Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection
    Ju, Jie; Xiao, Kai; Yao, Xi; Bai, Hao; Jiang, Lei
    Advanced Materials (Weinheim, Germany) (2013), 25 (41), 5937-5942CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The author has successfully prepd. a set of conical copper wires with gradient wettability, realizing continuous and efficient fog collection by mimicking the fog-collection principle of cacti. The hydrophobic tip of the CCW ensures quick collection of water drops and the combination of the gradient of Laplace pressure arising from the conical shape and the gradient of wettability arising from the gradient chem. modification guarantees quick transportation of the drops. The investigation of this bioinspired CCW with gradient wettability may provide new ideas for future fog-collection projects and open up a new source of global water.
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  21. 21
    Yin, K.; Du, H.; Dong, X.; Wang, C.; Duan, J.-A.; He, J. A Simple Way to Achieve Bioinspired Hybrid Wettability Surface with Micro/Nanopatterns for Efficient Fog Collection. Nanoscale 2017, 9, 14620– 14626,  DOI: 10.1039/C7NR05683D
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    21
    A simple way to achieve bioinspired hybrid wettability surface with micro/nanopatterns for efficient fog collection
    Yin, Kai; Du, Haifeng; Dong, Xinran; Wang, Cong; Duan, Ji-An; He, Jun
    Nanoscale (2017), 9 (38), 14620-14626CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Fog collection is receiving increasing attention for providing water in semi-arid deserts and inland areas. Inspired by the fog harvesting ability of the hydrophobic-hydrophilic surface of Namib desert beetles, we present a simple, low-cost method to prep. a hybrid superhydrophobic-hydrophilic surface. The surface contains micro/nanopatterns, and is prepd. by incorporating femtosecond-laser fabricated polytetrafluoroethylene nanoparticles deposited on superhydrophobic copper mesh with a pristine hydrophilic copper sheet. The as-prepd. surface exhibits enhanced fog collection efficiency compared with uniform (super)hydrophobic or (super)hydrophilic surfaces. This enhancement can be tuned by controlling the mesh no., inclination angle, and fabrication structure. Moreover, the surface shows excellent anti-corrosion ability after immersing in 1 M HCl, 1 M NaOH, and 10 wt% NaCl solns. for 2 h. This work may provide insight into fabricating hybrid superhydrophobic-hydrophilic surfaces for efficient atm. water collection.
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    Söz, C. a. l. K. a.; Trosien, S.; Biesalski, M. Janus Interface Materials: A Critical Review and Comparative Study. ACS Mater. Lett. 2020, 2, 336– 357,  DOI: 10.1021/acsmaterialslett.9b00489
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    Janus Interface Materials: A Critical Review and Comparative Study
    Soz, Cagla Kosak; Trosien, Simon; Biesalski, Markus
    ACS Materials Letters (2020), 2 (4), 336-357CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
    A review. In recent years, Janus interface materials with wettability contrast have attracted remarkable attention because of their beneficial properties and versatile potential applications in materials science including transport, purifn./sepn., anal. testing, and medical applications. Regarding the wide range of highly promising possible application areas, these materials will have a major impact on the next generation of smart systems. In this Review, our aim is to highlight the current status of the research on Janus interface materials with special emphasis on wettability contrast. In the first section, a brief history of the literature on Janus-type materials and interfaces, materials possessing different chemistries or topogs. on opposing sites, is introduced. In the second section, theories behind wetting, including "wettability integration", are summarized, which can be regarded as the combination of opposing wetting properties within the same material. Afterwards, natural examples of Janus interfaces, a branch of superwettability integration, are discussed, which inspired the researchers to mimic the nature and develop artificial analogs. In the next section, the current status on artificial Janus interfaces with wettability contrast are reviewed, subcategories for which are implemented according to the (possible) application areas and also the origin of their base substrates. Then, the inorg. and org. based artificial Janus interfaces were compared in terms of advantages and disadvantages. Finally, a conclusion and outlook are given.
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    Parker, A. R.; Lawrence, C. R. Water Capture by a Desert Beetle. Nature 2001, 414, 33,  DOI: 10.1038/35102108
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    Water capture by a desert beetle
    Parker, Andrew R.; Lawrence, Chris R.
    Nature (London, United Kingdom) (2001), 414 (6859), 33-34CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    There is no expanded citation for this reference.
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    Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F.-Q.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640– 643,  DOI: 10.1038/nature08729
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    Directional water collection on wetted spider silk
    Zheng, Yongmei; Bai, Hao; Huang, Zhongbing; Tian, Xuelin; Nie, Fu-Qiang; Zhao, Yong; Zhai, Jin; Jiang, Lei
    Nature (London, United Kingdom) (2010), 463 (7281), 640-643CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Many biol. surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometer-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fiber structure that forms after wetting, with the wet-rebuilt' fibers characterized by periodic spindle-knots made of random nanofibrils and sepd. by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liq. drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibers that mimic the structural features of silk and exhibit its directional water-collecting ability.
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    LaPotin, A.; Kim, H.; Rao, S. R.; Wang, E. N. Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance. Acc. Chem. Res. 2019, 52, 1588– 1597,  DOI: 10.1021/acs.accounts.9b00062
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    25
    Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance
    LaPotin, Alina; Kim, Hyunho; Rao, Sameer R.; Wang, Evelyn N.
    Accounts of Chemical Research (2019), 52 (6), 1588-1597CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Atm. H2O harvesting (AWH) is the capture and collection of H2O that is present in the air either as vapor or small H2O droplets. AWH has been recognized as a method for decentralized H2O prodn., esp. in areas where liq. H2O is phys. scarce, or the infrastructure required to bring H2O from other locations is unreliable or infeasible. The main methods of AWH are fog harvesting, dewing, and using sorbent materials to collect vapor from the air. We 1st distinguish between the geog./climatic operating regimes of fog harvesting, dewing, and sorbent-based approaches based on temp. and relative humidity (RH). Because using sorbents has the potential to be more widely applicable to areas which are also facing H2O scarcity, we focus the discussion on this approach. We discuss sorbent materials which have been developed for AWH and the material properties which affect system-level performance. Much of the recent materials development has focused on a single material metric, equil. vapor uptake in (kg of H2O uptake per kg of dry adsorbent), as found from the adsorption isotherm. This equil. property alone, however, is not a good indicator of the actual performance of the AWH system. Understanding material properties which affect heat and mass transport are equally important in the development of materials and components for AWH, because resistances assocd. with heat and mass transport in the bulk material dramatically change the system performance. We focus the discussion on modeling a solar thermal-driven system. Performance of a solar-driven AWH system can be characterized by different metrics, including L of H2O per m2 device per day or L of H2O per kg adsorbent per day. The former metric is esp. important for systems driven by low-grade heat sources because the low power d. of these sources makes this technol. land area intensive. In either case, it is important to include rates in the performance metric to capture the effects of heat and mass transport in the system. We discuss the previously developed modeling framework which can predict the performance of a sorbent material packed into a porous matrix. This model connects mass transport across length scales, considering diffusion both inside a single crystal as well as macroscale geometric parameters, such as the thickness of a composite adsorbent layer. For a simple solar thermal-driven adsorption-based AWH system, we show how this model can be used to optimize the system. Finally, we discuss strategies which have been used to improve heat and mass transport in the design of adsorption systems and the potential for adsorption-based AWH systems for decentralized H2O supplies.
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    Fathieh, F.; Kalmutzki, M. J.; Kapustin, E. A.; Waller, P. J.; Yang, J.; Yaghi, O. M. Practical Water Production from Desert Air. Sci. Adv. 2018, 4, eaat3198  DOI: 10.1126/sciadv.aat3198
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    Jin, Y.; Zhang, L.; Wang, P. Atmospheric Water Harvesting: Role of Surface Wettability and Edge Effect. Global Challenges 2017, 1, 1700019,  DOI: 10.1002/gch2.201700019
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    Atmospheric Water Harvesting: Role of Surface Wettability and Edge Effect
    Jin Yong; Wang Peng; Zhang Lianbin
    Global challenges (Hoboken, NJ) (2017), 1 (4), 1700019 ISSN:.
    Atmospheric water is emerging as an important potable water source. The present work experimentally and theoretically investigates water condensation and collection on flat surfaces with contrasting contact angles and contact angle hysteresis (CAH) to elucidate their roles on water mass collection efficiency. The experimental results indicate that a hydrophilic surface promotes nucleation and individual droplets growth, and a surface with a low CAH tends to let a smaller droplet to slide down, but the overall water mass collection efficiency is independent of both surface contact angle and CAH. The experimental results agree well with our theoretical calculations. During water condensation, a balance has to be struck between single droplet growth and droplet density on a surface so as to maintain a constant water droplet surface coverage ratio, which renders the role of both surface wettability and hysteresis insignificant to the ultimate water mass collection. Moreover, water droplets on the edges of a surface grow much faster than those on the non-edge areas and thus dominate the contribution to the water mass collection by the entire surface, directly pointing out the very important role of edge effect on water condensation and collection.
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    Wahlgren, R. V. Atmospheric Water Vapour Processor Designs for Potable Water Production: A Review. Water Res. 2001, 35, 1– 22,  DOI: 10.1016/S0043-1354(00)00247-5
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    Atmospheric water vapour processor designs for potable water production: a review
    Wahlgren R V
    Water research (2001), 35 (1), 1-22 ISSN:0043-1354.
    Atmospheric water vapour processing (AWVP) technology is reviewed. These processors are machines which extract water molecules from the atmosphere, ultimately causing a phase change from vapour to liquid. Three classes of machines have been proposed. The machines either cool a surface below the dewpoint of the ambient air, concentrate water vapour through use of solid or liquid desiccants, or induce and control convection in a tower structure. Patented devices vary in scale and potable water output from small units suitable for one person's daily needs to structures as large as multi-story office buildings capable of supplying drinking water to an urban neighbourhood. Energy and mass cascades (flowcharts) are presented for the three types of water vapour processors. The flowcharts assist in classifying designs and discussing their strengths and limitations. Practicality and appropriateness of the various designs for contributing to water supplies are considered along with water cost estimates. Prototypes that have been tested successfully are highlighted. Absolute humidity (meteorological normals) ranges from 4.0 g of water vapour per cubic metre of surface air in the atmosphere (Las Vegas, Nevada, USA) to 21.2 g m-3 (Djibouti, Republic of Djibouti). Antofagasta, Chile has a normal absolute humidity of 10.9 g m-3. A 40% efficient machine in the vicinity of Antofagasta requires an airflow of 10 m3 s-1 to produce 3767 l of water per day. At a consumption of 50 l per person per day, 75 people could have basic water requirements for drinking, sanitation, bathing, and cooking met by a decentralized and simplified water supply infrastructure with attendant economic and societal benefits.
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    Agam, N.; Berliner, P. R. Dew Formation and Water Vapor Adsorption in Semi-Arid Environments—a Review. J. Arid Environ. 2006, 65, 572– 590,  DOI: 10.1016/j.jaridenv.2005.09.004
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    Wang, D.; Xia, Z.; Wu, J.; Wang, R.; Zhai, H.; Dou, W. Study of a Novel Silica Gel-Water Adsorption Chiller. Part I. Design and Performance Prediction. Int. J. Refrig. 2005, 28, 1073– 1083,  DOI: 10.1016/j.ijrefrig.2005.03.001
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    Study of a novel silica gel-water adsorption chiller. Part I. Design and performance prediction
    Wang, D. C.; Xia, Z. Z.; Wu, J. Y.; Wang, R. Z.; Zhai, H.; Dou, W. D.
    International Journal of Refrigeration (2005), 28 (7), 1073-1083CODEN: IJRFDI; ISSN:0140-7007. (Elsevier Ltd.)
    A novel silica gel-water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evapn./condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser, and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance esp. when the chiller is driven by a low temp. heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is > 10 kW under a typical working condition with hot water temp. of 85°, the cooling water temp. of 31°, and the chilled water inlet temp. of 15°. The COP exceeds 0.5 even under a heat source temp. of 65°.
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    Ng, K.; Chua, H.; Chung, C.; Loke, C.; Kashiwagi, T.; Akisawa, A.; Saha, B. B. Experimental Investigation of the Silica Gel-Water Adsorption Isotherm Characteristics. Appl. Therm. Eng. 2001, 21, 1631– 1642,  DOI: 10.1016/S1359-4311(01)00039-4
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    31
    Experimental investigation of the silica gel-water adsorption isotherm characteristics
    Ng, K. C.; Chua, H. T.; Chung, C. Y.; Loke, C. H.; Kashiwagi, T.; Akisawa, A.; Saha, B. B.
    Applied Thermal Engineering (2001), 21 (16), 1631-1642CODEN: ATENFT; ISSN:1359-4311. (Elsevier Science Ltd.)
    In designing adsorption chillers that employs silica gel-water as adsorbent-adsorbate pair, the overriding objective is to exploit low temp. waste-heat sources from industry. This paper describes an exptl. approach for the detn. of thermodn. characteristics of silica gel-water working pair that is essential for the sizing of adsorption chillers. The expts. incorporated the moisture balance technique, a control-vol.-variable-pressure (CVVP) app. and three types of silica gel were investigated. As evidenced by the exptl. results, the Henry-type equation is suitable for describing the isotherm characteristics of silica gel-water working pair at the conditions of adsorption chiller. The regeneration of adsorbent depends on the correct allocation of temp. as well as the amt. of regeneration time. From the expts., the isotherm characteristics of silica gel-water in the low- to high-pressure regimes and hence, its isosteric heat of adsorption is detd. Key parameters for optimizing the amt. of heat recovery such as the cycle and switching time of chiller can also be implied from the measured results.
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  32. 32
    Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.; Fuchs, A. The Effect of Local Defects on Water Adsorption in Silicalite-1 Zeolite: A Joint Experimental and Molecular Simulation Study. Langmuir 2007, 23, 10131– 10139,  DOI: 10.1021/la7011205
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    The effect of local defects on water adsorption in silicalite-1 zeolite. A joint experimental and molecular simulation study
    Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.; Fuchs, A. H.
    Langmuir (2007), 23 (20), 10131-10139CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We report a joint exptl. and mol. simulation study of water condensation in silicalite-1 zeolite. A sample was synthesized using the fluoride route and was found to contain essentially no defects. A second sample synthesized using the hydroxide route was found to contain a small amt. of silanol groups. The thermodn. of water condensation was studied in these 2 samples, as well as in a com. sample, to understand the effect of local defects on water adsorption. The mol. simulation study enabled us to qual. reproduce the exptl. obsd. condensation thermodn. features. A shift and a rounding of the condensation transition was obsd. with an increasing hydrophilicity of the local defect, but the condensation transition was still obsd. above the water satn. vapor pressure P0. Both expts. and simulations agree on the fact that a small water uptake can be obsd. at very low pressure, but that the bulk liq. does not form from the gas phase below P0. The picture that emerges from the obsd. water condensation mechanism is the existence of a heterogeneous internal surface that is overall hydrophobic, despite the existence of hydrophilic "patches". This heterogeneous surface configuration is thermodynamically stable in a wide range of reduced pressures (from P/P0 = 0.2 to a few thousands), until the condensation transition takes place.
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  33. 33
    Jänchen, J.; Ackermann, D.; Stach, H.; Brösicke, W. Studies of the Water Adsorption on Zeolites and Modified Mesoporous Materials for Seasonal Storage of Solar Heat. Sol. Energy 2004, 76, 339– 344,  DOI: 10.1016/j.solener.2003.07.036
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    Zhang, H.; Yoshino, H.; Hasegawa, K.; Liu, J.; Zhang, W.; Xuan, H. Practical Moisture Buffering Effect of Three Hygroscopic Materials in Real-World Conditions. Energy Buildings 2017, 139, 214– 223,  DOI: 10.1016/j.enbuild.2017.01.021
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    Kallenberger, P. A.; Fröba, M. Water Harvesting from Air with a Hygroscopic Salt in a Hydrogel-Derived Matrix. Commun. Chem. 2018,  DOI: 10.1038/s42004-018-0028-9
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    Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.; Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N. Water Harvesting from Air with Metal-Organic Frameworks Powered by Natural Sunlight. Science 2017, 356, 430– 434,  DOI: 10.1126/science.aam8743
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    Water harvesting from air with metal-organic frameworks powered by natural sunlight
    Kim, Hyunho; Yang, Sungwoo; Rao, Sameer R.; Narayanan, Shankar; Kapustin, Eugene A.; Furukawa, Hiroyasu; Umans, Ari S.; Yaghi, Omar M.; Wang, Evelyn N.
    Science (Washington, DC, United States) (2017), 356 (6336), 430-434CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Atm. water is a resource equiv. to -10% of all fresh water in lakes on Earth. However, an efficient process for capturing and delivering water from air, esp. at low humidity levels (down to 20%), has not been developed. We report the design and demonstration of a device based on a porous metal-org. framework {M0F-801, [Zr604(0H)4(fumarate)6]} that captures water from the atm. at ambient conditions by using low-grade heat from natural sunlight at a flux of less than 1 sun (1 kW per square meter). This device is capable of harvesting 2.8 L of water per kg of MOF daily at relative humidity levels as low as 20% and requires no addnl. input of energy.
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  37. 37
    Kim, H.; Rao, S. R.; Kapustin, E. A.; Zhao, L.; Yang, S.; Yaghi, O. M.; Wang, E. N. Adsorption-Based Atmospheric Water Harvesting Device for Arid Climates. Nat. Commun. 2018,  DOI: 10.1038/s41467-018-03162-7
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    Li, R.; Shi, Y.; Alsaedi, M.; Wu, M.; Shi, L.; Wang, P. Hybrid Hydrogel with High Water Vapor Harvesting Capacity for Deployable Solar-Driven Atmospheric Water Generator. Environ. Sci. Technol. 2018, 52, 11367– 11377,  DOI: 10.1021/acs.est.8b02852
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    38
    Hybrid Hydrogel with High Water Vapor Harvesting Capacity for Deployable Solar-Driven Atmospheric Water Generator
    Li, Renyuan; Shi, Yusuf; Alsaedi, Mossab; Wu, Mengchun; Shi, Le; Wang, Peng
    Environmental Science & Technology (2018), 52 (19), 11367-11377CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The Earth's atm. holds ∼12,900 billion tons of fresh H2O distributed all over the world with fast replenishment. Atm. H2O harvesting is emerging as a promising strategy for clean H2O prodn. in arid regions, land-locked areas, and remote communities. The H2O vapor sorbent is the key component for atm. H2O harvesting devices based on absorbing-releasing process. A flexible hybrid photothermal H2O sorbent composed of deliquescent salt and hydrogel was rationally fabricated. It possesses superior H2O sorption capacity even in low humidity air thanks to the deliquescent salt and maintains a solid form after it sorbs a large amt. of H2O owing to the hydrogel platform. The harvested H2O could be easily released under regular sunlight via the photothermal effect, and it can be directly reused without noticeable capacity fading. An easy-to-assemble-at-household prototype device with 35 g of the dry hydrogel was tested outdoors under field conditions and delivered 20 g of fresh H2O within 2.5 h under natural sunlight. It is estd. that the material cost of making such a device to supply min. daily H2O consumption for an adult (i.e., 3 kg) is only $3.20 (USD). This type of atm. H2O generator (AWG) is cheap and affordable, works perfectly with a broad range of humidity, does not need any electricity, and thus is esp. suitable for clean H2O prodn. in remote areas.
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  39. 39
    Entezari, A.; Ejeian, M.; Wang, R. Super Atmospheric Water Harvesting Hydrogel with Alginate Chains Modified with Binary Salts. ACS Mater. Lett. 2020, 2, 471– 477,  DOI: 10.1021/acsmaterialslett.9b00315
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    Super Atmospheric Water Harvesting Hydrogel with Alginate Chains Modified with Binary Salts
    Entezari, Akram; Ejeian, Mojtaba; Wang, Ruzhu
    ACS Materials Letters (2020), 2 (5), 471-477CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
    Atm. water harvesting is considered to be a promising technol. to address the global water shortage. However, researchers are still searching for optimized desiccants with all of the desired features, such as high water sorption capacity, low desorption temp., wide light spectrum absorption, ease to scale up, and low cost. Here, we modified sodium alginate by occupying both G-blocks and M-blocks with more hydrophilic cations (i.e., Li and Ca). Functionalized carbon nanotubes (FCNTs) are embedded in the hydrogel structure to increase solar spectrum absorption. In summary, these features enable binary composite to adsorb ~ 5.6 g of water per g of desiccant. For the first time, the idea of rational combination of a binary hydrophilic polymeric salt was embodied. We believe that this binary/FCNT sorbent is a promising material for application in water sorption-based technologies.
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    Matsumoto, K.; Sakikawa, N.; Miyata, T. Thermo-Responsive Gels That Absorb Moisture and Ooze Water. Nat. Commun. 2018, 9, 2315,  DOI: 10.1038/s41467-018-04810-8
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    Thermo-responsive gels that absorb moisture and ooze water
    Matsumoto Kazuya; Miyata Takashi; Sakikawa Nobuki; Miyata Takashi
    Nature communications (2018), 9 (1), 2315 ISSN:.
    The water content of thermo-responsive hydrogels can be drastically altered by small changes in temperature because their polymer chains change from hydrophilic to hydrophobic above their low critical solution temperature (LCST). In general, such smart hydrogels have been utilized in aqueous solutions or in their wet state, and no attempt has been made to determine the phase-transition behavior of the gels in their dried states. Here we demonstrate an application of the thermo-responsive behavior of an interpenetrating polymer network (IPN) gel comprising thermo-responsive poly(N-isopropylacrylamide) and hydrophilic sodium alginate networks in their dried states. The dried IPN gel absorbs considerable moisture from air at temperatures below its LCST and oozes the absorbed moisture as liquid water above its LCST. These phenomena provide energy exchange systems in which moisture from air can be condensed to liquid water using the controllable hydrophilic/hydrophobic properties of thermo-responsive gels with a small temperature change.
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    Zhao, F.; Bae, J.; Zhou, X.; Guo, Y.; Yu, G. Nanostructured Functional Hydrogels as an Emerging Platform for Advanced Energy Technologies. Adv. Mater. 2018, 30, 1801796,  DOI: 10.1002/adma.201801796
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    Zhao, F.; Shi, Y.; Pan, L.; Yu, G. Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications. Acc. Chem. Res. 2017, 50, 1734– 1743,  DOI: 10.1021/acs.accounts.7b00191
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    Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications
    Zhao, Fei; Shi, Ye; Pan, Lijia; Yu, Guihua
    Accounts of Chemical Research (2017), 50 (7), 1734-1743CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Conductive polymers have attracted significant interest over the past few decades because they synergize the advantageous features of conventional polymeric materials and org. conductors. With rationally designed nanostructures, conductive polymers can further exhibit exceptional mech., elec., and optical properties because of their confined dimensions at the nanoscale level. Among various nanostructured conductive polymers, conductive polymer gels (CPGs) with synthetically tunable hierarchical 3D network structures show great potential for a wide range of applications, such as bioelectronics, and energy storage/conversion devices owing to their structural features. CPGs retain the properties of nanosized conductive polymers during the assembly of the nanobuilding blocks into a monolithic macroscopic structure while generating structure-derived features from the highly crosslinked network. In this Account, we review our recent progress on the synthesis, properties, and novel applications of dopant crosslinked CPGs. We first describe the synthetic strategies, in which mols. with multiple functional groups are adopted as crosslinkers to crosslink conductive polymer chains into a 3D mol. network. These crosslinking mols. also act as dopants to improve the elec. cond. of the gel network. The microstructure and phys./chem. properties of CPGs can be tuned by controlling the synthetic conditions such as species of monomers and crosslinkers, reaction temp., and solvents. By incorporating other functional polymers or particles into the CPG matrix, hybrid gels have been synthesized with tailored structures. These hybrid gel materials retain the functionalities from each component, as well as enable synergic effects to improve mech. and elec. properties of CPGs. We then introduce the unique structure-derived properties of the CPGs. The network facilitates both electronic and ionic transport owing to the continuous pathways for electrons and hierarchical pores for ion diffusion. CPGs also provide high surface area and solvent compatibility, similar to natural gels. With these improved properties, CPGs have been explored to enable novel conceptual devices in diverse applications from smart electronics and ultrasensitive biosensors, to energy storage and conversion devices. CPGs have also been adopted for developing hybrid materials with multifunctionalities, such as stimuli responsiveness, self-healing properties, and super-repellency to liq. with synthetically tunable phys./chem. properties, CPGs emerge as a unique material platform to develop novel multifunctional materials that have the potential to impact electronics, energy, and environmental technologies. We hope that this Account promotes further efforts toward synthetic control, fundamental investigation, and application exploration of CPGs.
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  43. 43
    Cao, Y.; Chen, Y.; Sun, X.; Zhang, Z.; Mu, T. Water Sorption in Ionic Liquids: Kinetics, Mechanisms and Hydrophilicity. Phys. Chem. Chem. Phys. 2012, 14, 12252– 12262,  DOI: 10.1039/c2cp41798g
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    Water sorption in ionic liquids: kinetics, mechanisms and hydrophilicity
    Cao, Yuanyuan; Chen, Yu; Sun, Xiaofu; Zhang, Zhongmin; Mu, Tiancheng
    Physical Chemistry Chemical Physics (2012), 14 (35), 12252-12262CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Most of the ionic liqs. (ILs) are hygroscopic in air. The effects of structural factors of ILs (cation, anion, alkyl chain length at cation, and C2 methylation at cation) and external factors (temp., relative humidity, and impurity) on the kinetics of water sorption by 18 ILs were investigated. A modified two-step sorption mechanism was proposed to correlate the water sorption data in the ILs. Three type of parameters (sorption capacity, sorption rate and degree of difficulty to reach sorption equil.) based on the modified two-step mechanism were derived to comprehensively characterize the water sorption processes. These parameters have similar tendencies, providing an efficient way to evaluate them by one parameter that can be easily obtained. The hydrophilicity of the ILs was classified to four levels (super-high, high, medium, low) according to the water sorption capacity. The results show that cation of the ILs also plays an important role in water sorption, and the impurities affect the water sorption enormously. Acetate and halogen-based ILs have the highest hydrophilicity when combined with the imidazolium or pyridinium cation.
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    Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; John Wiley & Sons, 2013.
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    Wang, X.; Li, X.; Liu, G.; Li, J.; Hu, X.; Xu, N.; Zhao, W.; Zhu, B.; Zhu, J. An Interfacial Solar Heating Assisted Liquid Sorbent Atmospheric Water Generator. Angew. Chem., Int. Ed. 2019, 58, 12054– 12058,  DOI: 10.1002/anie.201905229
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    An Interfacial Solar Heating Assisted Liquid Sorbent Atmospheric Water Generator
    Wang, Xueyang; Li, Xiuqiang; Liu, Guoliang; Li, Jinlei; Hu, Xiaozhen; Xu, Ning; Zhao, Wei; Zhu, Bin; Zhu, Jia
    Angewandte Chemie, International Edition (2019), 58 (35), 12054-12058CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Harvesting water from air is a promising strategy for fresh-water prodn., and it is particularly desirable for areas that lack direct access to clean water. While high-concn. liq. sorbent is well-known for high sorption, it has not been widely used for atm. water collection, being primarily limited by the difficulty in desorption. Interfacial solar heating based on a salt-resistant GO-based aerogel is now shown to enable a high-concn. liq. sorbent (CaCl2 50 wt % soln.) based atm. water generator. Fresh water (2.89 kg m-2 day-1) can be produced at about 70 % relative humidity, with only solar energy input and energy efficiency of desorption as high as 66.9 %. This low-cost and effective approach provides an attractive pathway to ext. water from air, to relieve the thirst of arid, land-locked, and other areas where fresh water is scarce.
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  46. 46
    Qi, H.; Wei, T.; Zhao, W.; Zhu, B.; Liu, G.; Wang, P.; Lin, Z.; Wang, X.; Li, X.; Zhang, X. An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption. Adv. Mater. 2019, 31, 1903378,  DOI: 10.1002/adma.201903378
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    46
    An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption
    Qi, Heshan; Wei, Tianqi; Zhao, Wei; Zhu, Bin; Liu, Guoliang; Wang, Pingping; Lin, Zhenhui; Wang, Xueyang; Li, Xiuqiang; Zhang, Xiaowei; Zhu, Jia
    Advanced Materials (Weinheim, Germany) (2019), 31 (43), 1903378CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Water scarcity is one of the greatest challenges facing human society. Because of the abundant amt. of water present in the atm., there are significant efforts to harvest water from air. Particularly, solar-driven atm. water generators based on sequential adsorption-desorption processes are attracting much attention. However, incomplete daytime desorption is the limiting factor for final water prodn., 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-liq.-based sorbent, an atm. 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 atm. water generator enables a high rate of water prodn. (≈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 atm. water generator, based on the liq. sorbent with a simultaneous adsorption-desorption process opens up a promising pathway to effectively harvest water from air.
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    Canivet, J.; Fateeva, A.; Guo, Y.; Coasne, B.; Farrusseng, D. Water Adsorption in MOFs: Fundamentals and Applications. Chem. Soc. Rev. 2014, 43, 5594– 5617,  DOI: 10.1039/C4CS00078A
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    Water adsorption in MOFs: fundamentals and applications
    Canivet, Jerome; Fateeva, Alexandra; Guo, Youmin; Coasne, Benoit; Farrusseng, David
    Chemical Society Reviews (2014), 43 (16), 5594-5617CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. This review article presents the fundamental and practical aspects of water adsorption in Metal-Org. Frameworks (MOFs). The state of the art of MOF stability in water, a crucial issue to many applications in which MOFs are promising candidates, is discussed here. Stability in both gaseous (such as humid gases) and aq. media is considered. By considering a non-exhaustive yet representative set of MOFs, the different mechanisms of water adsorption in this class of materials are presented: reversible and continuous pore filling, irreversible and discontinuous pore filling through capillary condensation, and irreversibility arising from the flexibility and possible structural modifications of the host material. Water adsorption properties of more than 60 MOF samples are reported. The applications of MOFs as materials for heat-pumps and adsorbent-based chillers and proton conductors are also reviewed. Some directions for future work are suggested as concluding remarks.
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  48. 48
    Alcañiz-Monge, J.; Linares-Solano, A.; Rand, B. Water Adsorption on Activated Carbons: Study of Water Adsorption in Micro-and Mesopores. J. Phys. Chem. B 2001, 105, 7998– 8006,  DOI: 10.1021/jp010674b
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    Water adsorption on activated carbons: Study of water adsorption in micro- and mesopores
    Alcaniz-Monge, Juan; Linares-Solano, Angel; Rand, Brian
    Journal of Physical Chemistry B (2001), 105 (33), 7998-8006CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The mechanism of water adsorption is analyzed in a range of activated carbons differing widely in the relative proportions of narrow and wide micropores, mesopores, and macropores. They also display different degrees of surface functionality. By assuming that the adsorbed phase in micropores is ice-like, the whole micropore vol. is filled with water at a P/P0 of ≈0.82, at which point capillary condensation may begin. However, surface complexes control the wetting of the mesopore walls by adsorbed water and can influence the condensation process. Condensation in macropores may also occur at P/P0 values >0.95. The extent of meso-/macropore filling depends on the surface complexes. The vol. filling process is progressive and in micropores is consistent with the theory of vol. filling. The adsorption process was analyzed by interpreting the nitrogen isotherms systematically measured on samples of different pore structure with progressively increasing amts. of preadsorbed water up to satn.
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    Langmuir, I. The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids. J. Am. Chem. Soc. 1916, 38, 2221– 2295,  DOI: 10.1021/ja02268a002
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    Constitution and fundamental properties of solids and liquids. I. Solids
    Langmuir, Irving
    Journal of the American Chemical Society (1916), 38 (), 2221-95CODEN: JACSAT; ISSN:0002-7863.
    Outline of L.'s theory of the mechanism of evapn., condensation, liquefaction, adsorption and capillary phenomena, according to which solids and liquids consist of atoms held together entirely by chem. forces. The conception of the mol. is thus almost entirely eliminated except in the case of gases, and a liquid or solid is looked upon as consisting of a single large mol. The following is L.'s summary: The work of the Braggs on crystal structure is reviewed from the viewpoint of the chemist, and the relation of this work to the theories of chem. constitution, such as those of Werner, Stark, J. J. Thomson and Lewis, is discussed in detail. Substances whose structures have been studied by the X-ray spectrograph are not representative compds. in general; only polar compds. have been studied. Solid polar compds. are, in general, built up of atoms bound together by secondary or residual valences. The whole crystal must be regarded as a single mol. Solid non-polar compds. consist in general of "group mols." in which the atoms are usually held together by primary valences. These group mols. in turn are bound together by secondary valence to form a large "crystal mol." which includes the whole solid mass. There is no present justification for dividing interatomic or intermolecular forces into physical and chem. forces; it is much more profitable to consider all such forces as strictly chemical. Evapn., condensation, soln., crystn., adsorption, surface tension, etc., should all be regarded as typical chem. phenomena. From a consideration of such properties as sp. heat, compressibility and coeff. of expansion it is concluded that collisions do not take place between the atoms of solids, but that these move about equil. positions under the influence of both repulsive and attractive forces. The "time of relaxation" of solids is calcd. approx. from the heat cond., and is found to be between 10-14 and 10-7 sec. An independent calcn. based on the time of evapn. of a substance into a vacuum gave substantially the same result. Thus the time necessary for an atom of a solid to reach thermal equil. with its neighbors is very small compared to the time necessary to make a single oscillation about an equil. position. The "oscillations" are thus extremely strongly damped. Since solid substances in general are held together by secondary rather than primary valences, there are few limitations to the number of compds. which can exist in the solid state. Most of these do not show a compn. which could be predicted from the ordinary rules of valence. Metallic compds., minerals, solid solns. and glasses are discussed from this point of view. From considerations based largely on compressibility it is concluded that the attractive forces between atoms usually reach a maximum intensity when the distance between adjacent atoms in a solid is increased by about 0.6 × 10-8 cm. (10 to 30% of the normal distance between atoms). Since energy must be expended in breaking apart the atoms of a solid, the surfaces of solids must contain more potential energy than does the corresponding number of atoms in the interior. Since this potential energy is probably electromagnetic energy in the field between the atoms, the interatomic forces are more intense in the surface than in the interior. This intense surface field of force (unsatd. chem. affinity) is one of the causes of the phenomena of condensation and adsorption. Because of the small time of relaxation and because an atom approaching the surface is attracted by many, but later is repelled by few atoms, it follows that the surfaces of solids are almost wholly inelastic in regard to mols. impinging on the surface. There is exptl. evidence of this inelasticity. As a result, nearly every mol. striking the surface condenses, no matter what the temp. While condensed it is held by forces quite similar to those holding solids together (either primary or secondary valence). At high temps. evapn. may take place almost immediately after condensation, but at low temps. the condensed atom or mol. may remain indefinitely. In general the rate of evapn. (m) in a high vacuum is related to the pressure by the equation m = √M/2πRTp. Red P and some other substances are probably exceptions. When separate phases of const. compn. are present (decompn. of a solid such as CaCO3) the reaction must take place exclusively at the boundaries of these phases. This kinetic interpretation of the phase rule indicates clearly the distinction between reactions in which solid solns. are formed and those in which separate phases appear. This theory offers a ready explanation for the fact that some hydrated crystals fail to effloresce unless scratched, and for the fact that thoroughly dehydrated substances will absorb H2O only with difficulty. Adsorption is a direct consequence of the time lag between condensation and evapn. Whether the adsorbed substance is held by primary or secondary valences, it is better to regard the union as chemical. Exptl. results are given which show that adsorption is very frequently the result of the strongest kind of chem. union (primary valence) between the atoms of the adsorbed substance and the atoms of the solid. The action of a typical catalyzer poison depends on the formation of a very stable film one atom deep over the surface of the catalyzer. The chem. activity of a surface depends upon the nature, arrangement and spacing of the atoms forming the surface layer. There is a very close relation between the chem. activity of a surface and the electron emission from it (either thermionic or photo-electric emission). There is a brief development of this theory of heterogeneous reactions. A "law of surface action" analogous to, but different from, the law of mass action is proposed which is in accord with, and affords an explanation of, Reichinstein's "const. sum hypothesis." Cf. following abstract.
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  50. 50
    Langmuir, I. The Constitution and Fundamental Properties of Solids and Liquids. Ii. Liquids. J. Am. Chem. Soc. 1917, 39, 1848– 1906,  DOI: 10.1021/ja02254a006
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    Constitution and fundamental properties of solids and liquids. II. Liquids
    Langmuir, Irving
    Journal of the American Chemical Society (1917), 39 (), 1848-1906CODEN: JACSAT; ISSN:0002-7863.
    cf. C. A. 10, 3007. The ideas mentioned in part I of the paper and also by Harkins (C. A. 10, 839; 11, 731) that when a film of (say) oleic acid spreads on H2O the mols. are turned with the-COOH groups in the H2O and the hydrocarbon chain on end are developed extensively, with reference to the existing expts. of Devaux (C. A. 10, 1131) and others. L. claims priority over Harkins in the idea of orientation of mols. at a surface. New expts. are described in which a piece of paper resting on a water surface is attached to a small balance in such a way as to measure the force necessary to compress a floating film to a layer one mol. deep, and data are given for a considerable number of fatty acids and their derivs. The exptl. results are thought to afford the strongest kind of confirmation of the theory that the spreading of films on surfaces is detd. by the shape of the mols. and the relative activities of the different portions of the mols. These results are discussed at length. The expts. of Gurvich (C. A. 9, 3003) on the adsorption of liquids by solids are described in some detail. L. considers that all interatomic and intermol. forces involved in the structure of matter are chemical forces; i. e., they are of the same nature as the forces that the chemist has been studying in the past. Primary valence is specific, depends upon the degree of satn. of the atoms and acts atomistically, in definite directions and only through atomic distances. Secondary valence is less specific and depends upon the presence of neighboring atoms, it acts through distances not greatly different from those through which primary valence acts, and only on adjacent atoms, though usually not in directions radiating out from single atoms. Such phenomena as evapn., cohesion, crystn., adsorption, surface tension, viscosity, etc., apparently do not involve any atomic or inter-mol. forces other than the 2 kinds of valence already mentioned. The forces involved in the structure of liquids are not essentially different from those of solids.
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  51. 51
    Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361– 1403,  DOI: 10.1021/ja02242a004
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    The adsorption of gases on plane surfaces of glass, mica and platinum
    Langmuir, I.
    Journal of the American Chemical Society (1918), 40 (), 1361-1402CODEN: JACSAT; ISSN:0002-7863.
    According to L.'s hypothesis, gaseous mols. impinging on a liquid or solid surface do not in general rebound from it elastically, but are held or adsorbed on the surface by forces similar to those holding the atoms or group mols. of solid bodies. The adsorbed film should not exceed one mol. in thickness. Adsorption of permanent gases involves only secondary valence forces. In metals particularly, adsorption may be governed by primary valence forces. It is suggested that stoichiometric relations should govern the adsorption on a surface unless interfering effects caused by steric hindrance are involved. At room temp. the absorption by glass and mica was negligible, not over 1 % of the surface being covered by a single layer of mols. At lower temps. much larger quantities of gas were taken up. With Pt no absorption was observed at - 183° unless the Pt were first activated by proper heating. The adsorption of O2 was irreversible and corresponded to a monomolecular layer. CO likewise showed the same behavior. In the presence of one or the other gas adsorbed on the Pt the adsorbed and unadsorbed gases reacted immediately to form CO2.
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    Kolasinski, K. W. Thermodynamics and Kinetics of Adsorption and Desorption. In Surface Science; Kolasinski, K. W. Ed., pp 185– 228.  DOI: 10.1002/9781119941798.ch4 .
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    Coasne, B.; Galarneau, A.; Pellenq, R. J.; Di Renzo, F. Adsorption, Intrusion and Freezing in Porous Silica: The View from the Nanoscale. Chem. Soc. Rev. 2013, 42, 4141– 4171,  DOI: 10.1039/c2cs35384a
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    Adsorption, intrusion and freezing in porous silica: the view from the nanoscale
    Coasne, Benoit; Galarneau, Anne; Pellenq, Roland J. M.; Di Renzo, Francesco
    Chemical Society Reviews (2013), 42 (9), 4141-4171CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This review presents the state of the art of mol. simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphol. and topol. disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common mol. simulation and theor. methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evapn. mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theor. results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn-Laplace equation and Gibbs-Thomson equation to describe the thermodn. of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.
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  54. 54
    Aristov, Y. I.; Tokarev, M. M.; Freni, A.; Glaznev, I. S.; Restuccia, G. Kinetics of Water Adsorption on Silica Fuji Davison Rd. Microporous Mesoporous Mater. 2006, 96, 65– 71,  DOI: 10.1016/j.micromeso.2006.06.008
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    Kinetics of water adsorption on silica Fuji Davison RD
    Aristov, Yuri I.; Tokarev, Mikhail M.; Freni, Angelo; Glaznev, Ivan S.; Restuccia, Giovanni
    Microporous and Mesoporous Materials (2006), 96 (1-3), 65-71CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    The kinetics of water adsorption on loose grains of Fuji Davison RD silica gel was studied by a TG differential step method in the temp. range 29-64° and in the pressure range 6.5-34 mbar. Three grain sizes were selected, 0.3-0.325, 0.355-0.425, and 0.8-1.0 mm. Furthermore, adsorption isobars at PH2O = 9, 18, and 48 mbar were measured over the temp. range of 30-150° by a TG technique to det. pertinent equil. parameters which are used to calc. the coeffs. of diffusion. The equil. uptake was described as a linear function of the Dubinin-Polanyi adsorption potential. The influence of particle size, temp., and pressure on the adsorption kinetics can be well described in terms of the Fickian diffusion model. The apparent water diffusivity D ap was found to be an Arrhenius function of temp. with the apparent activation energy Ea = 41.5 kJ/mol and the pre-exponential factor Dap0 = 2.9 × 10-4 m2/s. The apparent diffusivity of water in silica pores was measured to be De = (3.7-4.7) × 10-7 m2/s and possessed a slight increase with temp. This value is close to the Knudsen diffusivity, calcd. for a cylindrical pore of radius rp = 1.0 nm. For smaller grains at T >39° the contribution of thermal effects was revealed, which decreases the rate of water sorption. In this case, application of non-isothermal kinetic model of Lee and Ruthven allowed good description of exptl. uptake curves as well as the estn. of parameters which det. simultaneous heat and mass transfer.
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    Sircar, S.; Hufton, J. Why Does the Linear Driving Force Model for Adsorption Kinetics Work?. Adsorption 2000, 6, 137– 147,  DOI: 10.1023/A:1008965317983
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    Why does the linear driving force model for adsorption kinetics work?
    Sircar, S.; Hufton, J. R.
    Adsorption (2000), 6 (2), 137-147CODEN: ADSOFO; ISSN:0929-5607. (Kluwer Academic Publishers)
    The Linear Driving Force (LDF) model for gas adsorption kinetics is frequently and successfully used for anal. of adsorption column dynamic data and for adsorptive process designs because it is simple, analytic, and phys. consistent. Yet, there is a substantial difference in the characteristics of isothermal batch uptake curves on adsorbent particles by the LDF and the more rigorous Fickian Diffusion (FD) model. It is demonstrated by using simple model systems that the characteristics of the adsorption kinetics at the single pore or the adsorbent particle level are lost in (a) evaluating overall uptake on a heterogeneous porous solid, (b) calcg. breakthrough curves from a packed adsorbent column, and (c) establishing the efficiency of sepn. by an adsorptive process due to repeated averaging of the base kinetic property. That is why the LDF model works in practice.
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    Zhuravlev, L. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf., A 2000, 173, 1– 38,  DOI: 10.1016/S0927-7757(00)00556-2
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    The surface chemistry of amorphous silica. Zhuravlev model
    Zhuravlev, L. T.
    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2000), 173 (1-3), 1-38CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
    A review, with 347 refs. on the author's own results on the properties of amorphous SiO2 surface. In any description of the surface SiO2 the hydroxylation of the surface is of crit. importance. An anal. was made of the processes of dehydration (the removal of phys. adsorbed water), dehydroxylation (the removal of silanol groups from the SiO2 surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of exptl. and theor. studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chem. of amorphous SiO2. The main advantage of this physico-chem. model lies in the possibility to det. the concn. and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the SiO2 surface as a function of the pretreatment temp. of SiO2 samples. The model makes it possible to det. the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO2 skeleton. The magnitude of the silanol no., i.e., the no. of OH groups per unit surface area, αOH, when the surface is hydroxylated to the max. degree, is considered to be a physico-chem. const. This const. has a numerical value: αOH,AVER = 4.6 (least-squares method) and αOH,AVER = 4.9 OH nm-2 (arithmetical mean) and is known in literature as the Kiselev-Zhuravlev const. Adsorption and other surface properties per unit surface area of SiO2 are identical (except for very fine pores). On the basis of data published in the literature, this model was found to be useful in solving various applied and theor. problems in the field of adsorption, catalysis, chromatog., chem. modification, etc. The Brunauer-Emmett-Teller (BET) method is the correct method and gives the opportunity to measure the real phys. magnitude of the sp. surface area, SKr (by low temp. adsorption of Kr), for SiO2s and other oxide dispersed solids.
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    Wang, L.; Wang, R.; Oliveira, R. A Review on Adsorption Working Pairs for Refrigeration. Renewable Sustainable Energy Rev. 2009, 13, 518– 534,  DOI: 10.1016/j.rser.2007.12.002
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    A review on adsorption working pairs for refrigeration
    Wang, L. W.; Wang, R. Z.; Oliveira, R. G.
    Renewable & Sustainable Energy Reviews (2009), 13 (3), 518-534CODEN: RSERFH; ISSN:1364-0321. (Elsevier Ltd.)
    A review. Solid sorption refrigeration is a type of environmental benign and energy saving technol. and the sorbents utilized can be divided into phys., chem. and composite sorbents, according to the nature of the forces involved in the adsorption process. The types, characteristics, advantages and disadvantages of different adsorbents, refrigerants and working pairs are summarized in this paper, together with the models that describe the adsorption equil. Moreover, some of the procedures to prep. composite adsorbents are presented. The application of different working pairs for different situations is related with the adsorption heat, the adaptability to the driving temp. and to the desired working pressure. The methods to measure the adsorption quantity of different working pairs are compared, and future research directions of adsorption working pairs are also analyzed.
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    Nakano, T.; Nozue, Y. Orbital Degeneracy and Magnetic Properties of Potassium Clusters Incorporated into Nanoporous Crystals of Zeolite A. J. Comput. Methods Sci. Eng. 2008, 7, 443– 462,  DOI: 10.3233/JCM-2007-75-610
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    Wei, X.; Wang, W.; Xiao, J.; Zhang, L.; Chen, H.; Ding, J. Hierarchically Porous Aluminosilicates as the Water Vapor Adsorbents for Dehumidification. Chem. Eng. J. 2013, 228, 1133– 1139,  DOI: 10.1016/j.cej.2013.05.062
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    Hierarchically porous aluminosilicates as the water vapor adsorbents for dehumidification
    Wei, Xiaolan; Wang, Weilong; Xiao, Jing; Zhang, Li; Chen, Hongyin; Ding, Jing
    Chemical Engineering Journal (Amsterdam, Netherlands) (2013), 228 (), 1133-1139CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    Hierarchically porous aluminosilicates with large surface area (930-1010 m2/g) and ordered hexagonal structures were synthesized as the water vapor adsorbents for dehumidification in this work. Compared to the com. microporous zeolite 3A (low Si/Al ratio), and mesoporous SILICA GEL (high Si/Al ratio), the synthesized aluminosilicates showed higher adsorption capacity of water vapor at both high and low humidity ranges. Different from zeolite 3A (type I) and silica gel (type III), the adsorption isotherms of aluminosilicates followed type IV isotherms. The aluminosilicate materials showed low desorption activation energy of water vapor ( < 20 kJ/mol, close to the intermol. H2O-H2O H-bonding interaction). The aluminosilicates showed a low desorption temp. of 87 °C and a relatively fast desorption rate. Moreover, regenerability test in the multiple adsorption-desorption cycles suggested that the synthesized aluminosilicates were regenerable. With the promising dehumidification characteristics, including high adsorption capacity of water vapor, fast adsorption and desorption rates, low regeneration temp., as well as good regenerability, the synthesized hierarchically porous aluminosilicates can be potential adsorbents for energy-efficient and cost-effective dehumidification process in practical applications, i.e. rotary desiccant dehumidifiers.
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    Zheng, X.; Ge, T. S.; Wang, R. Z. Recent Progress on Desiccant Materials for Solid Desiccant Cooling Systems. Energy 2014, 74, 280– 294,  DOI: 10.1016/j.energy.2014.07.027
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    William, G. E.; Mohamed, M.; Fatouh, M. Desiccant System for Water Production from Humid Air Using Solar Energy. Energy 2015, 90, 1707– 1720,  DOI: 10.1016/j.energy.2015.06.125
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    Desiccant system for water production from humid air using solar energy
    William, G. E.; Mohamed, M. H.; Fatouh, M.
    Energy (Oxford, United Kingdom) (2015), 90 (Part_2), 1707-1720CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
    Decentralized and remote areas suffer from fresh water shortage. A new sustainable energy technique called "Water Extn. from Atm. Air" is introduced as a beneficial soln. for this problem. This system involves the absorption of water vapor from ambient air during the night time. Then, simultaneous desiccant regeneration and water vapor condensation during the day time will be occurred. In this study, a trapezoidal prism solar collector with four fiberglass sides is designed and constructed. This collector has multi-shelves bed (desiccant carrier) in order to maximize bed surface area inside the collector. Consequently, the absorption and evapn. surfaces are increased. The exptl. work shows the effect of weather conditions in system operation for both day time and night time. The regeneration and absorption processes are discussed for each operation condition. Also, different host materials (cloth and sand) with calcium chloride soln. are investigated. An illustrative comparison between different bed types is demonstrated. The results revealed that the total evapd. water for cloth and sand bed can reach 2.32 and 1.23 slit/days m2 at initial satn. concn. (30%) of cacl2. However, the system efficiency is 29.3 and 17.76% for cloth and sand bed, resp.
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    Dietzel, P. D.; Johnsen, R. E.; Blom, R.; Fjellvåg, H. Structural Changes and Coordinatively Unsaturated Metal Atoms on Dehydration of Honeycomb Analogous Microporous Metal-Organic Frameworks. Chem. - Eur. J. 2008, 14, 2389– 2397,  DOI: 10.1002/chem.200701370
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    Structural changes and coordinatively unsaturated metal atoms on dehydration of honeycomb analogous microporous metal-organic frameworks
    Dietzel, Pascal D. C.; Johnsen, Rune E.; Blom, Richard; Fjellvag, Helmer
    Chemistry - A European Journal (2008), 14 (8), 2389-2397CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The dehydration process of two isostructural microporous coordination polymers, [M2(dhtp)(H2O)2]·8H2O, termed CPO-27-M (M = Co, Zn; H4dhtp = 2,5-dihydroxy-terephthalic acid) was studied by in situ variable temp. x-ray diffraction. Both compds. contain accessible metal coordination sites after removal of all solvent. However, despite the analogy of their crystal structures, they behave differently during dehydration. For CPO-27-Co, H2O desorption is a smooth topotactic process of 2nd order with no concomitant space group change and no increase in microstrain, which is beneficial for the applicability of the material. Removal of the H2O propagates from the center of the channels outwards. The coordinating H2O mol. at the metal desorbs only when almost all the bulk H2O in the pores has disappeared. In contrast, discontinuities in the powder pattern of CPO-27-Zn indicate the occurrence of 1st-order transitions. The crystal structures of four of the five individual phases could be detd. The structure of the intermediate phase occurring just before the framework is completely evacuated was elusive in respect to full structure soln. and refinement, but it is most probably related to the removal of the axis of 3-fold symmetry. The Zn-based material experiences a significant amt. of strain.
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    Damasceno Borges, D.; Maurin, G.; Galvão, D. S. Design of Porous Metal-Organic Frameworks for Adsorption Driven Thermal Batteries. MRS Adv. 2017, 2, 519– 524,  DOI: 10.1557/adv.2017.181
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    Design of Porous Metal-Organic Frameworks for Adsorption Driven Thermal Batteries
    Damasceno Borges, Daiane; Maurin, Guillaume; Galvao, Douglas S.
    MRS Advances (2017), 2 (9), 519-524CODEN: MARDCQ; ISSN:2059-8521. (Cambridge University Press)
    Thermal batteries based on a reversible adsorption/desorption of a working fluid (water, methanol, ammonia) rather than the conventional vapor compression is a promising alternative to exploit waste thermal energy for heat reallocation. In this context, there is an increasing interest to find novel porous solids able to adsorb a high energy d. of working fluid under low relative vapor pressure condition combined with an easy ability of regeneration (desorption) at low temp., which are the major requirements for adsorption driven heat pumps and chillers. The porous cryst. hybrid materials named Metal-Org. Frameworks (MOF) represent a great source of inspiration for sorption based-applications owing to their tunable chem. and topol. features assocd. with a large variability of pore sizes. Recently, we have designed a new MOF named MIL-160 (MIL stands for Materials of Institut Lavoisier), isostructural to CAU-10, built from the assembly of corner sharing aluminum chains octahedra AlO4(OH)2 with the 2,5-furandicarboxylic linker substituting the pristine org. linker, 1,4-benzenedicarboxylate. This ligand replacement strategy proved to enhance both the hydrophilicity of the MOF and its amt. of water adsorbed at low p/p0. This designed solid was synthesized and its chem. stability/adsorption performances verified. Here, we have extended this study by incorporating other polar heterocyclic linkers and a comparative computational study of the water adsorption performances of these novel structures has been performed. To that purpose, the cell and geometry optimizations of all hypothetical frameworks were first performed at the d. functional theory level and their water adsorption isotherms were further predicted by using force-field based Grand-Canonical Monte Carlo simulations. This study reveals the ease tunable water affinity of MOF for the desired application.
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    Cmarik, G. E.; Kim, M.; Cohen, S. M.; Walton, K. S. Tuning the Adsorption Properties of UiO-66 Via Ligand Functionalization. Langmuir 2012, 28, 15606– 15613,  DOI: 10.1021/la3035352
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    Tuning the Adsorption Properties of UiO-66 via Ligand Functionalization
    Cmarik, Gregory E.; Kim, Min; Cohen, Seth M.; Walton, Krista S.
    Langmuir (2012), 28 (44), 15606-15613CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    UiO-66 is one of the few known H2O-stable MOFs that are readily amenable to direct ligand substitution. UiO-66 was synthesized with amino-, nitro-, methoxy-, and naphthyl-substituted ligands to impart polar, basic, and hydrophobic characteristics. Pure-component CO2, CH4, N2, and H2O vapor adsorption isotherms were measured in the materials to study the effect of the functional group on the adsorption behavior. Heats of adsorption were calcd. for each pure gas on each material. The amino-functionalized material possesses the best adsorption properties for each pure gas due to a combination of polarity and small functional group size. The naphthyl-functionalized material exhibits a good combination of inhibited H2O vapor adsorption and high selectivity for CO2 over CH4 and N2.
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    Zheng, X.; Ge, T.; Wang, R.; Hu, L. Performance Study of Composite Silica Gels with Different Pore Sizes and Different Impregnating Hygroscopic Salts. Chem. Eng. Sci. 2014, 120, 1– 9,  DOI: 10.1016/j.ces.2014.08.047
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    Performance study of composite silica gels with different pore sizes and different impregnating hygroscopic salts
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    Chemical Engineering Science (2014), 120 (), 1-9CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
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    Water sorption on composite material "zeolite 13X modified by LiCl and CaCl2"
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    Microporous and Mesoporous Materials (2020), 299 (), 110109CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    The article presents the synthesis of composite adsorbents by impregnating zeolite 13X with binary salts (LiCl + CaCl2). The phase compn. and element content of the composite adsorbent were characterized by X-ray diffraction (XRD) and Inductively coupled plasma (ICP). And its pore structure was discussed through N2 adsorption-desorption anal. The results show that the formation of the solid soln. changes the pore structure of the composite adsorbent which affects its adsorption performance. At the temp. of 25°C and the relative humidity of 80%, CS6 exhibits the max. adsorption capacity of 1.1 g/g compared with other adsorbents. The recycling test of CS6 shows that the adsorption capacity is 91.8% of the first time after 12 cycles. Finally, the dynamic adsorption curve and sorption isotherm were fitted by the LDF model and Polanyi Potential Adsorption Theory. It is shown that the simulation curve has good consistency with the measured data.
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    Li, R.; Shi, Y.; Wu, M.; Hong, S.; Wang, P. Improving Atmospheric Water Production Yield: Enabling Multiple Water Harvesting Cycles with Nano Sorbent. Nano Energy 2020, 67, 104255,  DOI: 10.1016/j.nanoen.2019.104255
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    Improving atmospheric water production yield: Enabling multiple water harvesting cycles with nano sorbent
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    Clean water shortage has long been a challenge in remote and landlocked communities esp. for the impoverished. Atm. water is now considered as an unconventional but accessible fresh water source and sorption-based atm. water generator (AWG) has been successfully demonstrated a reliable way of harvesting atm. water. The water vapor sorbents with high water uptake capacity and esp. fast vapor sorption/desorption kinetics have become the bottleneck to a desirable clean water productivity in AWG. In this work, we developed a new nano vapor sorbent composed of a nano carbon hollow capsule with LiCl inside the void core. The sorbent can capture water vapor from ambient air as much as 100% of its own wt. under RH 60% within 3 h and quickly release the sorbed water within just half hour under 1 kW/m2 sunlight irradn. A batch-mode AWG device was able to conduct 3 sorption/desorption cycles within 10 h during one day test in the outdoor condition and produced 1.6 kgwater/kgsorbent. A prototype of continuous AWG device was designed, fabricated, and successfully demonstrated, hinting a possible way of large-scale deployment of AWG for practical purposes.
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    Xu, J.; Li, T.; Chao, J.; Wu, S.; Yan, T.; Li, W.; Cao, B.; Wang, R. Efficient Solar-Driven Water Harvesting from Arid Air with Metal-Organic Frameworks Modified by Hygroscopic Salt. Angew. Chem., Int. Ed. 2020, 59, 5202,  DOI: 10.1002/anie.201915170
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    Efficient Solar-Driven Water Harvesting from Arid Air with Metal-Organic Frameworks Modified by Hygroscopic Salt
    Xu, Jiaxing; Li, Tingxian; Chao, Jingwei; Wu, Si; Yan, Taisen; Li, Wenchen; Cao, Biye; Wang, Ruzhu
    Angewandte Chemie, International Edition (2020), 59 (13), 5202-5210CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Freshwater scarcity is a global challenge threatening human survival, esp. for people living in arid regions. Sorption-based atm. water harvesting (AWH) is an appealing way to solve this problem. However, the state-of-the-art AWH technologies have poor water harvesting performance in arid climates owing to the low water sorption capacity of common sorbents under low humidity conditions. We report a high-performance composite sorbent for efficient water harvesting from arid air by confining hygroscopic salt in a metal-org. framework matrix (LiCl@MIL-101(Cr)). The composite sorbent shows 0.77 g g-1 water sorption capacity at 1.2 kPa vapor pressure (30% relative humidity at 30°C) by integrating the multi-step sorption processes of salt chemisorption, deliquescence, and soln. absorption. A highly efficient AWH prototype is demonstrated with LiCl@MIL-101(Cr) that can enable the harvesting of 0.45-0.7 kg water per kg of material under lab. and outdoor ambient conditions powered by natural sunlight without optical concn. and addnl. energy input.
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    Yao, H.; Zhang, P.; Huang, Y.; Cheng, H.; Li, C.; Qu, L. Highly Efficient Clean Water Production from Contaminated Air with a Wide Humidity Range. Adv. Mater. 2020, 32, 1905875,  DOI: 10.1002/adma.201905875
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    Highly Efficient Clean Water Production from Contaminated Air with a Wide Humidity Range
    Yao, Houze; Zhang, Panpan; Huang, Yaxin; Cheng, Huhu; Li, Chun; Qu, Liangti
    Advanced Materials (Weinheim, Germany) (2020), 32 (6), 1905875CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The huge amt. 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 irradn., is proposed. This PGF shows a superhigh equil. uptake of 5.20 g of water per g 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 kg of PGF one day, enough to meet several people's drinking water demand. This work provides a new strategy for effective prodn. of clean water from the atm. of practical significance.
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  • Abstract

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    Figure 1

    Figure 1. Schematic of atmospheric water harvesting (AWH) based on moisture harvesters. By using moisture harvesters with high surface area, porous structure, water affinity, fast vapor diffusion, and molecular diffusion, the water vapor in atmosphere can be collected as freshwater. The moisture-harvesting materials could be MOF-based materials, hygroscopic materials, and polymeric gels.

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    Figure 2

    Figure 2. Moisture sorption mechanisms. (a) Schematic of absorption(left) and adsorption (right) processes. (b) Schematic of physisorption (top left, top right) and chemisorption (bottom left, bottom right).

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    Figure 3

    Figure 3. Essential properties of moisture harvesters for AWH. The ideal moisture harvesters should have high sorption capacity, low regeneration energy demand, fast sorption/desorption, and long-term cycling stability.

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    Figure 4

    Figure 4. (a) Schematic of silica gels adsorbing water molecules. Reprinted with permission from ref (56). Copyright 2000 Elsevier Science B.V. (b) Porous structure of zeolites. Reprinted with permission from ref (58). Copyright 2008 IOS Press. (c) Deliquescent liquid sorbents for AWH assisted by an interfacial solar heating strategy. Reprinted with permission from ref (45). Copyright 2019 Wiley-VCH Verlag GmbH & Co. (d) Simultaneous sorption-desorption of liquid sorbents. Reprinted with permission from ref (46). Copyright 2019 Wiley-VCH Verlag GmbH & Co.

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    Figure 5

    Figure 5. (a) Schematic of AWH based on MOF-801. (b) Water adsorption isotherms of MOF-801 at different temperatures. Reprinted with permission from ref (36). Copyright 2017 American Association for the Advancement of Science. (c) Schematic of the water harvesting device containing a radiative cooling part. (d) Water adsorption isotherms of MOF-801 as a function of RH (P Psat–1, vapour pressure over saturation pressure) at temperatures of 15, 25, 45, 65, and 85 °C. Reprinted with permission from ref (37). Copyright 2018 Nature Publishing Group.

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    Figure 6

    Figure 6. (a) Tuning factors of MOF-based moisture harvesters. Water sorption isotherms of MOF-based moisture harvesters with different (b) hydrophilicity, (c) pore size, and (d) geometry. Reprinted with permission from ref (15). Copyright 2018 Wiley-VCH Verlag GmbH & Co.

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    Figure 7

    Figure 7. (a) Impregnating of LiCl into a porous carbon matrix. Reprinted with permission from ref (67). Copyright 2020 Elsevier. (b) MOF as the supporting matrix for LiCl for AWH. Reprinted with permission from ref (68). Copyright 2020 Wiley-VCH Verlag GmbH & Co. (c) AWH based on PAM-CNT-CaCl2 moisture harvester. (d) Water vapor harvesting curves of PAM-CNT-CaCl2 moisture harvester under different RH. Reprinted with permission from ref (38). Copyright 2018 American Chemical Society.

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    Figure 8

    Figure 8. (a) AWH based on a thermoresponsive interpenetrating network gel with liquid water release. Reprinted with permission from ref (40). Copyright 2018 Nature Publishing Group. (b) AWH based on the SMAG and its water uptake in different RH. Reprinted with permission from ref (16). Copyright 2019 Wiley-VCH. (c) AWH based on a rGO-PAAS gel. (d) Water uptake of rGO, PAAS, and PGF (25 °C, RH = 100%). Reprinted with permission from ref (69). Copyright 2020 Wiley-VCH.

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    Figure 9

    Figure 9. Outlook and perspective opportunities for next-generation AWH.

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      Fritzmann, C.; Loewenberg, J.; Wintgens, T.; Melin, T.
      Desalination (2007), 216 (1-3), 1-76CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      A review. Throughout the world, water scarcity is being recognized as a present or future threat to human activity and as a consequence, a definite trend to develop alternative water resources such as desalination can be obsd. The most commonly used desalination technologies are reverse osmosis (RO) and thermal processes such as multi-stage flash (MSF) and multi-effect distn. (MED). In Europe, reverse osmosis, due to its lower energy consumption has gained much wider acceptance than its thermal alternatives. This review summarizes the current state-of-the art of reverse osmosis desalination, dealing not only with the reverse osmosis stage, but with the entire process from raw water intake to post treatment of product water. The discussion of process fundamentals, membranes and membrane modules and of current and future developments in membrane technol. is accompanied by an anal. of operational issues as fouling and scaling and of measures for their prevention such as adequate cleaning procedures and antiscalant use. Special focus is placed on pre-treatment of raw water and post-treatment of brine as well as of product water to meet drinking and irrigation water stds., including evaluation of current boron removal options. Energy requirements of reverse osmosis plants as well as currently applied energy recovery systems for redn. of energy consumption are described and cost and cost structure of reverse osmosis desalination are outlined. Finally, current practices of waste management and disposal as well as new trends such as the use of hybrid plants, i.e. combining reverse osmosis with thermal processes and/or power generation are addressed.
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    6. 6
      Greenlee, L. F.; Lawler, D. F.; Freeman, B. D.; Marrot, B.; Moulin, P. Reverse Osmosis Desalination: Water Sources, Technology, and Today’s Challenges. Water Res. 2009, 43, 2317– 2348,  DOI: 10.1016/j.watres.2009.03.010
      6
      Reverse osmosis desalination: Water sources, technology, and today's challenges
      Greenlee, Lauren F.; Lawler, Desmond F.; Freeman, Benny D.; Marrot, Benoit; Moulin, Philippe
      Water Research (2009), 43 (9), 2317-2348CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      A review. Reverse osmosis membrane technol. has developed over the past 40 years to a 44% share in world desalting prodn. capacity, and an 80% share in the total no. of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technol. for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (conc.) disposal options, and plant location, have created significant differences in process development, implementation, and key tech. problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technol. in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.
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    7. 7
      Alsehli, M.; Choi, J.-K.; Aljuhan, M. A Novel Design for a Solar Powered Multistage Flash Desalination. Sol. Energy 2017, 153, 348– 359,  DOI: 10.1016/j.solener.2017.05.082
      There is no corresponding record for this reference.
    8. 8
      Zhao, F.; Guo, Y.; Zhou, X.; Shi, W.; Yu, G. Materials for Solar-Powered Water Evaporation. Nat. Rev. Mater. 2020, 5, 388– 401,  DOI: 10.1038/s41578-020-0182-4
      There is no corresponding record for this reference.
    9. 9
      Guo, Y.; Zhou, X.; Zhao, F.; Bae, J.; Rosenberger, B.; Yu, G. Synergistic Energy Nanoconfinement and Water Activation in Hydrogels for Efficient Solar Water Desalination. ACS Nano 2019, 13, 7913– 7919,  DOI: 10.1021/acsnano.9b02301
      9
      Synergistic Energy Nanoconfinement and Water Activation in Hydrogels for Efficient Solar Water Desalination
      Guo, Youhong; Zhou, Xingyi; Zhao, Fei; Bae, Jiwoong; Rosenberger, Brian; Yu, Guihua
      ACS Nano (2019), 13 (7), 7913-7919CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Precisely controlled distribution of energy in solar-to-thermal energy conversion systems could allow for enhanced energy utilization. Light-absorbing hydrogels provide a means for evapg. water by using solar energy, yet targeted delivery of solar thermal energy to power the water evapn. process remains challenging. Here, we report a light-absorbing sponge-like hydrogel (LASH) that is created by in situ gelation of a light-absorbing nanoparticle-modified polymer, leading to synergistic energy nanoconfinement and water activation. By exptl. demonstration and theor. simulation, the LASH presents record high vapor generation rates up to ∼3.6 kg m-2 h-1 and stable long-term performance under 1 sun (1 kW m-2) irradn. We investigate the energy confinement at the polymer-nanoparticle interphases and the water activation enabled by polymer-water interaction to reveal the significance of such effects for high-rate solar vapor generation. The water vaporization enabled by LASHs can remove over 99.9% of salt ions in seawater through solar water desalination. The fundamental design principle, scalable fabrication route, and superior performance offer possibilities for portable solar water purifn., industrial solar-powered water treatment, and other advanced solar thermal applications.
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    10. 10
      Zhou, X.; Guo, Y.; Zhao, F.; Yu, G. Hydrogels as an Emerging Material Platform for Solar Water Purification. Acc. Chem. Res. 2019, 52, 3244– 3253,  DOI: 10.1021/acs.accounts.9b00455
      10
      Hydrogels as an Emerging Material Platform for Solar Water Purification
      Zhou, Xingyi; Guo, Youhong; Zhao, Fei; Yu, Guihua
      Accounts of Chemical Research (2019), 52 (11), 3244-3253CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Conspectus: Growing concern over water scarcity leads to increased research interest in advanced water purifn. technologies. Solar water purifn., which uses solar energy to sep. water and impurities through vaporization, enables the utilization of sustainable energy and potential freshwater resources to alleviate water scarcity. However, the essential process of solar water evapn. to remove impurities is energy intensive. Insufficient solar absorption and thermal loss limited the vapor generation rate and, thus, lowered the purified water yield. Diffuse natural sunlight cannot satisfy the intrinsic energy demand for rapid water vaporization. Therefore, developing new material platforms that can simultaneously provide high solar absorption, effective energy utilization, and low energy demand for water vaporization to achieve highly efficient solar water purifn. under natural sunlight is anticipated. In this Account, we review our recent progress on hydrogel-based evaporators for solar water purifn. in terms of material selection, mol. engineering, and structural design. First, we introduce the unique water state in hydrogels consisting of free, intermediate, and bound water, of which intermediate water has a reduced energy demand for water evapn. Then, we describe the design principles of hydrogel-based solar evaporators, where the polymeric networks are tailored to regulate the water state. The water state in hydrogels defines the vaporization behavior of water. Thus, the polymer networks of hydrogels can be architected to tune the water state and, hence, to further reduce the evapn. enthalpy of water. Armed with fundamental gelation chem., we discuss synthetic strategies of hydrogels for efficient vapor generation. By incorporating solar absorbers with hydrophilic polymer networks, solar energy is harvested and converted to heat energy, which can be in situ utilized to power the vaporization of contained water in the mol. meshes, and the solar absorbers having strong interaction with hydrogels guide the formation of microstructure to reduce the energy loss and ensure adequate water transport of evaporative water. Regulating the vaporizing fronts, engineering the surface of hydrogels has been focused to favor the evapn. of water to further enhance the solar-to-vapor efficiency. By using hydrophilic polymers as building blocks, the hydrogel-based solar evaporators have also been endowed with multiple functionalities, such as antifouling, permselectivity, and thermal responsiveness, to improve water collection and purifn. abilities. Taking advantages of these merits, hydrogels have emerged as a promising materials platform to enable efficient solar water purifn. under natural sunlight. This Account serves to promote future efforts toward practical purifn. systems using hydrogel-based solar evaporators to mitigate water scarcity by improving their performance, scalability, stability, and sustainability.
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    11. 11
      Guo, Y.; Lu, H.; Zhao, F.; Zhou, X.; Shi, W.; Yu, G. Biomass-Derived Hybrid Hydrogel Evaporators for Cost-Effective Solar Water Purification. Adv. Mater. 2020, 32, 1907061,  DOI: 10.1002/adma.201907061
      11
      Biomass-Derived Hybrid Hydrogel Evaporators for Cost-Effective Solar Water Purification
      Guo, Youhong; Lu, Hengyi; Zhao, Fei; Zhou, Xingyi; Shi, Wen; Yu, Guihua
      Advanced Materials (Weinheim, Germany) (2020), 32 (11), 1907061CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Solar vapor generation has presented great potential for wastewater treatment and seawater desalination with high energy conversion and utilization efficiency. However, technol. gaps still exist for achieving a fast evapn. rate and high quality of water combined with low-cost deployment to provide a sustainable solar-driven water purifn. system. In this study, a naturally abundant biomass, konjac glucomannan, together with simple-to-fabricate iron-based metal-org. framework-derived photothermal nanoparticles is introduced into the polyvinyl alc. networks, building hybrid hydrogel evaporators in a cost-effective fashion ($14.9 m-2 of total materials cost). With advantageous features of adequate water transport, effective water activation, and anti-salt-fouling function, the hybrid hydrogel evaporators achieve a high evapn. rate under one sun (1 kW m-2) at 3.2 kg m-2 h-1 out of wastewater with wide degrees of acidity and alky. (pH 2-14) and high-salinity seawater (up to 330 g kg-1). More notably, heavy metal ions are removed effectively by forming hydrogen and chelating bonds with excess hydroxyl groups in the hydrogel. It is anticipated that this study offers new possibilities for a deployable, cost-effective solar water purifn. system with assured water quality, esp. for economically stressed communities.
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    12. 12
      Zhao, F.; Zhou, X.; Shi, Y.; Qian, X.; Alexander, M.; Zhao, X.; Mendez, S.; Yang, R.; Qu, L.; Yu, G. Highly Efficient Solar Vapour Generation Via Hierarchically Nanostructured Gels. Nat. Nanotechnol. 2018, 13, 489– 495,  DOI: 10.1038/s41565-018-0097-z
      12
      Highly efficient solar vapour generation via hierarchically nanostructured gels
      Zhao, Fei; Zhou, Xingyi; Shi, Ye; Qian, Xin; Alexander, Megan; Zhao, Xinpeng; Mendez, Samantha; Yang, Ronggui; Qu, Liangti; Yu, Guihua
      Nature Nanotechnology (2018), 13 (6), 489-495CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
      Solar vapor generation is an efficient way of harvesting solar energy for the purifn. of polluted or saline water. However, water evapn. suffers from either inefficient utilization of solar energy or relies on complex and expensive light-concn. accessories. Here, we demonstrate a hierarchically nanostructured gel (HNG) based on polyvinyl alc. (PVA) and polypyrrole (PPy) that serves as an independent solar vapor generator. The converted energy can be utilized in situ to power the vaporization of water contained in the mol. meshes of the PVA network, where water evapn. is facilitated by the skeleton of the hydrogel. A floating HNG sample evapd. water with a record high rate of 3.2 kg m-2 h-1 via 94% solar energy from 1 sun irradn., and 18-23 L of water per square metre of HNG was delivered daily when purifying brine water. These values were achievable due to the reduced latent heat of water evapn. in the mol. mesh under natural sunlight.
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    13. 13
      Zhou, X.; Zhao, F.; Guo, Y.; Rosenberger, B.; Yu, G. Architecting Highly Hydratable Polymer Networks to Tune the Water State for Solar Water Purification. Sci. Adv. 2019, 5, eaaw5484  DOI: 10.1126/sciadv.aaw5484
      There is no corresponding record for this reference.
    14. 14
      Tu, Y.; Wang, R.; Zhang, Y.; Wang, J. Progress and Expectation of Atmospheric Water Harvesting. Joule 2018, 2, 1452– 1475,  DOI: 10.1016/j.joule.2018.07.015
      14
      Progress and expectation of atmospheric water harvesting
      Tu, Yaodong; Wang, Ruzhu; Zhang, Yannan; Wang, Jiayun
      Joule (2018), 2 (8), 1452-1475CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      A review. Even if people live in an arid desert, they know that plenty of water exists in the air they breathe. However, the reality tells us the atm. water cannot help to slake the world's thirst. Thus an important question occurs: what are the fundamental limits of atm. water harvesting that can be achieved in typical arid and semi-arid areas. Here, through a thorough review on the present advances of atm. water-harvesting technologies, we identify the achievements that have been acquired and evaluate the challenges and barriers that retard their applications. Lastly, we clarify our perspectives on how to search for a simple, scalable, yet cost-effective way to produce atm. water for the community and forecast the application of atm. water harvesting in evaporative cooling, such as electronic cooling, power plant cooling, and passive building cooling.
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    15. 15
      Kalmutzki, M. J.; Diercks, C. S.; Yaghi, O. M. Metal-Organic Frameworks for Water Harvesting from Air. Adv. Mater. 2018, 30, 1704304,  DOI: 10.1002/adma.201704304
      There is no corresponding record for this reference.
    16. 16
      Zhao, F.; Zhou, X.; Liu, Y.; Shi, Y.; Dai, Y.; Yu, G. Super Moisture-Absorbent Gels for All-Weather Atmospheric Water Harvesting. Adv. Mater. 2019, 31, 1806446,  DOI: 10.1002/adma.201806446
      There is no corresponding record for this reference.
    17. 17
      Oki, T.; Kanae, S. Global Hydrological Cycles and World Water Resources. Science 2006, 313, 1068– 1072,  DOI: 10.1126/science.1128845
      17
      Global Hydrological Cycles and World Water Resources
      Oki, Taikan; Kanae, Shinjiro
      Science (Washington, DC, United States) (2006), 313 (5790), 1068-1072CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prep. for such anticipated changes.
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    18. 18
      Klemm, O.; Schemenauer, R. S.; Lummerich, A.; Cereceda, P.; Marzol, V.; Corell, D.; Van Heerden, J.; Reinhard, D.; Gherezghiher, T.; Olivier, J. Fog as a Fresh-Water Resource: Overview and Perspectives. Ambio 2012, 41, 221– 234,  DOI: 10.1007/s13280-012-0247-8
      18
      Fog as a fresh-water resource: overview and perspectives
      Klemm Otto; Schemenauer Robert S; Lummerich Anne; Cereceda Pilar; Marzol Victoria; Corell David; van Heerden Johan; Reinhard Dirk; Gherezghiher Tseggai; Olivier Jana; Osses Pablo; Sarsour Jamal; Frost Ernst; Estrela Maria J; Valiente Jose A; Fessehaye Gebregiorgis Mussie
      Ambio (2012), 41 (3), 221-34 ISSN:0044-7447.
      The collection of fog water is a simple and sustainable technology to obtain fresh water for afforestation, gardening, and as a drinking water source for human and animal consumption. In regions where fresh water is sparse and fog frequently occurs, it is feasible to set up a passive mesh system for fog water collection. The mesh is directly exposed to the atmosphere, and the foggy air is pushed through the mesh by the wind. Fog droplets are deposited on the mesh, combine to form larger droplets, and run down passing into a storage tank. Fog water collection rates vary dramatically from site to site but yearly averages from 3 to 10 l m(-2) of mesh per day are typical of operational projects. The scope of this article is to review fog collection projects worldwide, to analyze factors of success, and to evaluate the prospects of this technology.
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    19. 19
      Andrews, H.; Eccles, E.; Schofield, W.; Badyal, J. Three-Dimensional Hierarchical Structures for Fog Harvesting. Langmuir 2011, 27, 3798– 3802,  DOI: 10.1021/la2000014
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      Three-Dimensional Hierarchical Structures for Fog Harvesting
      Andrews, H. G.; Eccles, E. A.; Schofield, W. C. E.; Badyal, J. P. S.
      Langmuir (2011), 27 (7), 3798-3802CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Conventional fog-harvesting mechanisms are effectively pseudo-2D surface phenomena in terms of water droplet-plant interactions. In the case of the Cotula fallax plant, a unique hierarchical 3D arrangement formed by its leaves and the fine hairs covering them has been found to underpin the collection and retention of water droplets on the foliage for extended periods of time. The mechanisms of water capture and release as a function of the surface 3D structure and chem. have been identified. Of particular note is that water is retained throughout the entirety of the plant and held within the foliage itself (rather than in localized regions). Individual plant hairs form mat-like structures capable of supporting water droplets; these hairs wrap around water droplets in a 3D fashion to secure them via a fine nanoscale groove structure that prevents them from easily falling to the ground.
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    20. 20
      Ju, J.; Xiao, K.; Yao, X.; Bai, H.; Jiang, L. Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection. Adv. Mater. 2013, 25, 5937– 5942,  DOI: 10.1002/adma.201301876
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      Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection
      Ju, Jie; Xiao, Kai; Yao, Xi; Bai, Hao; Jiang, Lei
      Advanced Materials (Weinheim, Germany) (2013), 25 (41), 5937-5942CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The author has successfully prepd. a set of conical copper wires with gradient wettability, realizing continuous and efficient fog collection by mimicking the fog-collection principle of cacti. The hydrophobic tip of the CCW ensures quick collection of water drops and the combination of the gradient of Laplace pressure arising from the conical shape and the gradient of wettability arising from the gradient chem. modification guarantees quick transportation of the drops. The investigation of this bioinspired CCW with gradient wettability may provide new ideas for future fog-collection projects and open up a new source of global water.
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    21. 21
      Yin, K.; Du, H.; Dong, X.; Wang, C.; Duan, J.-A.; He, J. A Simple Way to Achieve Bioinspired Hybrid Wettability Surface with Micro/Nanopatterns for Efficient Fog Collection. Nanoscale 2017, 9, 14620– 14626,  DOI: 10.1039/C7NR05683D
      21
      A simple way to achieve bioinspired hybrid wettability surface with micro/nanopatterns for efficient fog collection
      Yin, Kai; Du, Haifeng; Dong, Xinran; Wang, Cong; Duan, Ji-An; He, Jun
      Nanoscale (2017), 9 (38), 14620-14626CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Fog collection is receiving increasing attention for providing water in semi-arid deserts and inland areas. Inspired by the fog harvesting ability of the hydrophobic-hydrophilic surface of Namib desert beetles, we present a simple, low-cost method to prep. a hybrid superhydrophobic-hydrophilic surface. The surface contains micro/nanopatterns, and is prepd. by incorporating femtosecond-laser fabricated polytetrafluoroethylene nanoparticles deposited on superhydrophobic copper mesh with a pristine hydrophilic copper sheet. The as-prepd. surface exhibits enhanced fog collection efficiency compared with uniform (super)hydrophobic or (super)hydrophilic surfaces. This enhancement can be tuned by controlling the mesh no., inclination angle, and fabrication structure. Moreover, the surface shows excellent anti-corrosion ability after immersing in 1 M HCl, 1 M NaOH, and 10 wt% NaCl solns. for 2 h. This work may provide insight into fabricating hybrid superhydrophobic-hydrophilic surfaces for efficient atm. water collection.
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    22. 22
      Söz, C. a. l. K. a.; Trosien, S.; Biesalski, M. Janus Interface Materials: A Critical Review and Comparative Study. ACS Mater. Lett. 2020, 2, 336– 357,  DOI: 10.1021/acsmaterialslett.9b00489
      22
      Janus Interface Materials: A Critical Review and Comparative Study
      Soz, Cagla Kosak; Trosien, Simon; Biesalski, Markus
      ACS Materials Letters (2020), 2 (4), 336-357CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
      A review. In recent years, Janus interface materials with wettability contrast have attracted remarkable attention because of their beneficial properties and versatile potential applications in materials science including transport, purifn./sepn., anal. testing, and medical applications. Regarding the wide range of highly promising possible application areas, these materials will have a major impact on the next generation of smart systems. In this Review, our aim is to highlight the current status of the research on Janus interface materials with special emphasis on wettability contrast. In the first section, a brief history of the literature on Janus-type materials and interfaces, materials possessing different chemistries or topogs. on opposing sites, is introduced. In the second section, theories behind wetting, including "wettability integration", are summarized, which can be regarded as the combination of opposing wetting properties within the same material. Afterwards, natural examples of Janus interfaces, a branch of superwettability integration, are discussed, which inspired the researchers to mimic the nature and develop artificial analogs. In the next section, the current status on artificial Janus interfaces with wettability contrast are reviewed, subcategories for which are implemented according to the (possible) application areas and also the origin of their base substrates. Then, the inorg. and org. based artificial Janus interfaces were compared in terms of advantages and disadvantages. Finally, a conclusion and outlook are given.
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    23. 23
      Parker, A. R.; Lawrence, C. R. Water Capture by a Desert Beetle. Nature 2001, 414, 33,  DOI: 10.1038/35102108
      23
      Water capture by a desert beetle
      Parker, Andrew R.; Lawrence, Chris R.
      Nature (London, United Kingdom) (2001), 414 (6859), 33-34CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      There is no expanded citation for this reference.
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    24. 24
      Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F.-Q.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640– 643,  DOI: 10.1038/nature08729
      24
      Directional water collection on wetted spider silk
      Zheng, Yongmei; Bai, Hao; Huang, Zhongbing; Tian, Xuelin; Nie, Fu-Qiang; Zhao, Yong; Zhai, Jin; Jiang, Lei
      Nature (London, United Kingdom) (2010), 463 (7281), 640-643CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Many biol. surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometer-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fiber structure that forms after wetting, with the wet-rebuilt' fibers characterized by periodic spindle-knots made of random nanofibrils and sepd. by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liq. drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibers that mimic the structural features of silk and exhibit its directional water-collecting ability.
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    25. 25
      LaPotin, A.; Kim, H.; Rao, S. R.; Wang, E. N. Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance. Acc. Chem. Res. 2019, 52, 1588– 1597,  DOI: 10.1021/acs.accounts.9b00062
      25
      Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance
      LaPotin, Alina; Kim, Hyunho; Rao, Sameer R.; Wang, Evelyn N.
      Accounts of Chemical Research (2019), 52 (6), 1588-1597CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      Atm. H2O harvesting (AWH) is the capture and collection of H2O that is present in the air either as vapor or small H2O droplets. AWH has been recognized as a method for decentralized H2O prodn., esp. in areas where liq. H2O is phys. scarce, or the infrastructure required to bring H2O from other locations is unreliable or infeasible. The main methods of AWH are fog harvesting, dewing, and using sorbent materials to collect vapor from the air. We 1st distinguish between the geog./climatic operating regimes of fog harvesting, dewing, and sorbent-based approaches based on temp. and relative humidity (RH). Because using sorbents has the potential to be more widely applicable to areas which are also facing H2O scarcity, we focus the discussion on this approach. We discuss sorbent materials which have been developed for AWH and the material properties which affect system-level performance. Much of the recent materials development has focused on a single material metric, equil. vapor uptake in (kg of H2O uptake per kg of dry adsorbent), as found from the adsorption isotherm. This equil. property alone, however, is not a good indicator of the actual performance of the AWH system. Understanding material properties which affect heat and mass transport are equally important in the development of materials and components for AWH, because resistances assocd. with heat and mass transport in the bulk material dramatically change the system performance. We focus the discussion on modeling a solar thermal-driven system. Performance of a solar-driven AWH system can be characterized by different metrics, including L of H2O per m2 device per day or L of H2O per kg adsorbent per day. The former metric is esp. important for systems driven by low-grade heat sources because the low power d. of these sources makes this technol. land area intensive. In either case, it is important to include rates in the performance metric to capture the effects of heat and mass transport in the system. We discuss the previously developed modeling framework which can predict the performance of a sorbent material packed into a porous matrix. This model connects mass transport across length scales, considering diffusion both inside a single crystal as well as macroscale geometric parameters, such as the thickness of a composite adsorbent layer. For a simple solar thermal-driven adsorption-based AWH system, we show how this model can be used to optimize the system. Finally, we discuss strategies which have been used to improve heat and mass transport in the design of adsorption systems and the potential for adsorption-based AWH systems for decentralized H2O supplies.
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    26. 26
      Fathieh, F.; Kalmutzki, M. J.; Kapustin, E. A.; Waller, P. J.; Yang, J.; Yaghi, O. M. Practical Water Production from Desert Air. Sci. Adv. 2018, 4, eaat3198  DOI: 10.1126/sciadv.aat3198
      There is no corresponding record for this reference.
    27. 27
      Jin, Y.; Zhang, L.; Wang, P. Atmospheric Water Harvesting: Role of Surface Wettability and Edge Effect. Global Challenges 2017, 1, 1700019,  DOI: 10.1002/gch2.201700019
      27
      Atmospheric Water Harvesting: Role of Surface Wettability and Edge Effect
      Jin Yong; Wang Peng; Zhang Lianbin
      Global challenges (Hoboken, NJ) (2017), 1 (4), 1700019 ISSN:.
      Atmospheric water is emerging as an important potable water source. The present work experimentally and theoretically investigates water condensation and collection on flat surfaces with contrasting contact angles and contact angle hysteresis (CAH) to elucidate their roles on water mass collection efficiency. The experimental results indicate that a hydrophilic surface promotes nucleation and individual droplets growth, and a surface with a low CAH tends to let a smaller droplet to slide down, but the overall water mass collection efficiency is independent of both surface contact angle and CAH. The experimental results agree well with our theoretical calculations. During water condensation, a balance has to be struck between single droplet growth and droplet density on a surface so as to maintain a constant water droplet surface coverage ratio, which renders the role of both surface wettability and hysteresis insignificant to the ultimate water mass collection. Moreover, water droplets on the edges of a surface grow much faster than those on the non-edge areas and thus dominate the contribution to the water mass collection by the entire surface, directly pointing out the very important role of edge effect on water condensation and collection.
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    28. 28
      Wahlgren, R. V. Atmospheric Water Vapour Processor Designs for Potable Water Production: A Review. Water Res. 2001, 35, 1– 22,  DOI: 10.1016/S0043-1354(00)00247-5
      28
      Atmospheric water vapour processor designs for potable water production: a review
      Wahlgren R V
      Water research (2001), 35 (1), 1-22 ISSN:0043-1354.
      Atmospheric water vapour processing (AWVP) technology is reviewed. These processors are machines which extract water molecules from the atmosphere, ultimately causing a phase change from vapour to liquid. Three classes of machines have been proposed. The machines either cool a surface below the dewpoint of the ambient air, concentrate water vapour through use of solid or liquid desiccants, or induce and control convection in a tower structure. Patented devices vary in scale and potable water output from small units suitable for one person's daily needs to structures as large as multi-story office buildings capable of supplying drinking water to an urban neighbourhood. Energy and mass cascades (flowcharts) are presented for the three types of water vapour processors. The flowcharts assist in classifying designs and discussing their strengths and limitations. Practicality and appropriateness of the various designs for contributing to water supplies are considered along with water cost estimates. Prototypes that have been tested successfully are highlighted. Absolute humidity (meteorological normals) ranges from 4.0 g of water vapour per cubic metre of surface air in the atmosphere (Las Vegas, Nevada, USA) to 21.2 g m-3 (Djibouti, Republic of Djibouti). Antofagasta, Chile has a normal absolute humidity of 10.9 g m-3. A 40% efficient machine in the vicinity of Antofagasta requires an airflow of 10 m3 s-1 to produce 3767 l of water per day. At a consumption of 50 l per person per day, 75 people could have basic water requirements for drinking, sanitation, bathing, and cooking met by a decentralized and simplified water supply infrastructure with attendant economic and societal benefits.
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    29. 29
      Agam, N.; Berliner, P. R. Dew Formation and Water Vapor Adsorption in Semi-Arid Environments—a Review. J. Arid Environ. 2006, 65, 572– 590,  DOI: 10.1016/j.jaridenv.2005.09.004
      There is no corresponding record for this reference.
    30. 30
      Wang, D.; Xia, Z.; Wu, J.; Wang, R.; Zhai, H.; Dou, W. Study of a Novel Silica Gel-Water Adsorption Chiller. Part I. Design and Performance Prediction. Int. J. Refrig. 2005, 28, 1073– 1083,  DOI: 10.1016/j.ijrefrig.2005.03.001
      30
      Study of a novel silica gel-water adsorption chiller. Part I. Design and performance prediction
      Wang, D. C.; Xia, Z. Z.; Wu, J. Y.; Wang, R. Z.; Zhai, H.; Dou, W. D.
      International Journal of Refrigeration (2005), 28 (7), 1073-1083CODEN: IJRFDI; ISSN:0140-7007. (Elsevier Ltd.)
      A novel silica gel-water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evapn./condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser, and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance esp. when the chiller is driven by a low temp. heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is > 10 kW under a typical working condition with hot water temp. of 85°, the cooling water temp. of 31°, and the chilled water inlet temp. of 15°. The COP exceeds 0.5 even under a heat source temp. of 65°.
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    31. 31
      Ng, K.; Chua, H.; Chung, C.; Loke, C.; Kashiwagi, T.; Akisawa, A.; Saha, B. B. Experimental Investigation of the Silica Gel-Water Adsorption Isotherm Characteristics. Appl. Therm. Eng. 2001, 21, 1631– 1642,  DOI: 10.1016/S1359-4311(01)00039-4
      31
      Experimental investigation of the silica gel-water adsorption isotherm characteristics
      Ng, K. C.; Chua, H. T.; Chung, C. Y.; Loke, C. H.; Kashiwagi, T.; Akisawa, A.; Saha, B. B.
      Applied Thermal Engineering (2001), 21 (16), 1631-1642CODEN: ATENFT; ISSN:1359-4311. (Elsevier Science Ltd.)
      In designing adsorption chillers that employs silica gel-water as adsorbent-adsorbate pair, the overriding objective is to exploit low temp. waste-heat sources from industry. This paper describes an exptl. approach for the detn. of thermodn. characteristics of silica gel-water working pair that is essential for the sizing of adsorption chillers. The expts. incorporated the moisture balance technique, a control-vol.-variable-pressure (CVVP) app. and three types of silica gel were investigated. As evidenced by the exptl. results, the Henry-type equation is suitable for describing the isotherm characteristics of silica gel-water working pair at the conditions of adsorption chiller. The regeneration of adsorbent depends on the correct allocation of temp. as well as the amt. of regeneration time. From the expts., the isotherm characteristics of silica gel-water in the low- to high-pressure regimes and hence, its isosteric heat of adsorption is detd. Key parameters for optimizing the amt. of heat recovery such as the cycle and switching time of chiller can also be implied from the measured results.
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    32. 32
      Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.; Fuchs, A. The Effect of Local Defects on Water Adsorption in Silicalite-1 Zeolite: A Joint Experimental and Molecular Simulation Study. Langmuir 2007, 23, 10131– 10139,  DOI: 10.1021/la7011205
      32
      The effect of local defects on water adsorption in silicalite-1 zeolite. A joint experimental and molecular simulation study
      Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.; Fuchs, A. H.
      Langmuir (2007), 23 (20), 10131-10139CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      We report a joint exptl. and mol. simulation study of water condensation in silicalite-1 zeolite. A sample was synthesized using the fluoride route and was found to contain essentially no defects. A second sample synthesized using the hydroxide route was found to contain a small amt. of silanol groups. The thermodn. of water condensation was studied in these 2 samples, as well as in a com. sample, to understand the effect of local defects on water adsorption. The mol. simulation study enabled us to qual. reproduce the exptl. obsd. condensation thermodn. features. A shift and a rounding of the condensation transition was obsd. with an increasing hydrophilicity of the local defect, but the condensation transition was still obsd. above the water satn. vapor pressure P0. Both expts. and simulations agree on the fact that a small water uptake can be obsd. at very low pressure, but that the bulk liq. does not form from the gas phase below P0. The picture that emerges from the obsd. water condensation mechanism is the existence of a heterogeneous internal surface that is overall hydrophobic, despite the existence of hydrophilic "patches". This heterogeneous surface configuration is thermodynamically stable in a wide range of reduced pressures (from P/P0 = 0.2 to a few thousands), until the condensation transition takes place.
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    33. 33
      Jänchen, J.; Ackermann, D.; Stach, H.; Brösicke, W. Studies of the Water Adsorption on Zeolites and Modified Mesoporous Materials for Seasonal Storage of Solar Heat. Sol. Energy 2004, 76, 339– 344,  DOI: 10.1016/j.solener.2003.07.036
      There is no corresponding record for this reference.
    34. 34
      Zhang, H.; Yoshino, H.; Hasegawa, K.; Liu, J.; Zhang, W.; Xuan, H. Practical Moisture Buffering Effect of Three Hygroscopic Materials in Real-World Conditions. Energy Buildings 2017, 139, 214– 223,  DOI: 10.1016/j.enbuild.2017.01.021
      There is no corresponding record for this reference.
    35. 35
      Kallenberger, P. A.; Fröba, M. Water Harvesting from Air with a Hygroscopic Salt in a Hydrogel-Derived Matrix. Commun. Chem. 2018,  DOI: 10.1038/s42004-018-0028-9
      There is no corresponding record for this reference.
    36. 36
      Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.; Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N. Water Harvesting from Air with Metal-Organic Frameworks Powered by Natural Sunlight. Science 2017, 356, 430– 434,  DOI: 10.1126/science.aam8743
      36
      Water harvesting from air with metal-organic frameworks powered by natural sunlight
      Kim, Hyunho; Yang, Sungwoo; Rao, Sameer R.; Narayanan, Shankar; Kapustin, Eugene A.; Furukawa, Hiroyasu; Umans, Ari S.; Yaghi, Omar M.; Wang, Evelyn N.
      Science (Washington, DC, United States) (2017), 356 (6336), 430-434CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Atm. water is a resource equiv. to -10% of all fresh water in lakes on Earth. However, an efficient process for capturing and delivering water from air, esp. at low humidity levels (down to 20%), has not been developed. We report the design and demonstration of a device based on a porous metal-org. framework {M0F-801, [Zr604(0H)4(fumarate)6]} that captures water from the atm. at ambient conditions by using low-grade heat from natural sunlight at a flux of less than 1 sun (1 kW per square meter). This device is capable of harvesting 2.8 L of water per kg of MOF daily at relative humidity levels as low as 20% and requires no addnl. input of energy.
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    37. 37
      Kim, H.; Rao, S. R.; Kapustin, E. A.; Zhao, L.; Yang, S.; Yaghi, O. M.; Wang, E. N. Adsorption-Based Atmospheric Water Harvesting Device for Arid Climates. Nat. Commun. 2018,  DOI: 10.1038/s41467-018-03162-7
      There is no corresponding record for this reference.
    38. 38
      Li, R.; Shi, Y.; Alsaedi, M.; Wu, M.; Shi, L.; Wang, P. Hybrid Hydrogel with High Water Vapor Harvesting Capacity for Deployable Solar-Driven Atmospheric Water Generator. Environ. Sci. Technol. 2018, 52, 11367– 11377,  DOI: 10.1021/acs.est.8b02852
      38
      Hybrid Hydrogel with High Water Vapor Harvesting Capacity for Deployable Solar-Driven Atmospheric Water Generator
      Li, Renyuan; Shi, Yusuf; Alsaedi, Mossab; Wu, Mengchun; Shi, Le; Wang, Peng
      Environmental Science & Technology (2018), 52 (19), 11367-11377CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The Earth's atm. holds ∼12,900 billion tons of fresh H2O distributed all over the world with fast replenishment. Atm. H2O harvesting is emerging as a promising strategy for clean H2O prodn. in arid regions, land-locked areas, and remote communities. The H2O vapor sorbent is the key component for atm. H2O harvesting devices based on absorbing-releasing process. A flexible hybrid photothermal H2O sorbent composed of deliquescent salt and hydrogel was rationally fabricated. It possesses superior H2O sorption capacity even in low humidity air thanks to the deliquescent salt and maintains a solid form after it sorbs a large amt. of H2O owing to the hydrogel platform. The harvested H2O could be easily released under regular sunlight via the photothermal effect, and it can be directly reused without noticeable capacity fading. An easy-to-assemble-at-household prototype device with 35 g of the dry hydrogel was tested outdoors under field conditions and delivered 20 g of fresh H2O within 2.5 h under natural sunlight. It is estd. that the material cost of making such a device to supply min. daily H2O consumption for an adult (i.e., 3 kg) is only $3.20 (USD). This type of atm. H2O generator (AWG) is cheap and affordable, works perfectly with a broad range of humidity, does not need any electricity, and thus is esp. suitable for clean H2O prodn. in remote areas.
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    39. 39
      Entezari, A.; Ejeian, M.; Wang, R. Super Atmospheric Water Harvesting Hydrogel with Alginate Chains Modified with Binary Salts. ACS Mater. Lett. 2020, 2, 471– 477,  DOI: 10.1021/acsmaterialslett.9b00315
      39
      Super Atmospheric Water Harvesting Hydrogel with Alginate Chains Modified with Binary Salts
      Entezari, Akram; Ejeian, Mojtaba; Wang, Ruzhu
      ACS Materials Letters (2020), 2 (5), 471-477CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
      Atm. water harvesting is considered to be a promising technol. to address the global water shortage. However, researchers are still searching for optimized desiccants with all of the desired features, such as high water sorption capacity, low desorption temp., wide light spectrum absorption, ease to scale up, and low cost. Here, we modified sodium alginate by occupying both G-blocks and M-blocks with more hydrophilic cations (i.e., Li and Ca). Functionalized carbon nanotubes (FCNTs) are embedded in the hydrogel structure to increase solar spectrum absorption. In summary, these features enable binary composite to adsorb ~ 5.6 g of water per g of desiccant. For the first time, the idea of rational combination of a binary hydrophilic polymeric salt was embodied. We believe that this binary/FCNT sorbent is a promising material for application in water sorption-based technologies.
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    40. 40
      Matsumoto, K.; Sakikawa, N.; Miyata, T. Thermo-Responsive Gels That Absorb Moisture and Ooze Water. Nat. Commun. 2018, 9, 2315,  DOI: 10.1038/s41467-018-04810-8
      40
      Thermo-responsive gels that absorb moisture and ooze water
      Matsumoto Kazuya; Miyata Takashi; Sakikawa Nobuki; Miyata Takashi
      Nature communications (2018), 9 (1), 2315 ISSN:.
      The water content of thermo-responsive hydrogels can be drastically altered by small changes in temperature because their polymer chains change from hydrophilic to hydrophobic above their low critical solution temperature (LCST). In general, such smart hydrogels have been utilized in aqueous solutions or in their wet state, and no attempt has been made to determine the phase-transition behavior of the gels in their dried states. Here we demonstrate an application of the thermo-responsive behavior of an interpenetrating polymer network (IPN) gel comprising thermo-responsive poly(N-isopropylacrylamide) and hydrophilic sodium alginate networks in their dried states. The dried IPN gel absorbs considerable moisture from air at temperatures below its LCST and oozes the absorbed moisture as liquid water above its LCST. These phenomena provide energy exchange systems in which moisture from air can be condensed to liquid water using the controllable hydrophilic/hydrophobic properties of thermo-responsive gels with a small temperature change.
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    41. 41
      Zhao, F.; Bae, J.; Zhou, X.; Guo, Y.; Yu, G. Nanostructured Functional Hydrogels as an Emerging Platform for Advanced Energy Technologies. Adv. Mater. 2018, 30, 1801796,  DOI: 10.1002/adma.201801796
      There is no corresponding record for this reference.
    42. 42
      Zhao, F.; Shi, Y.; Pan, L.; Yu, G. Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications. Acc. Chem. Res. 2017, 50, 1734– 1743,  DOI: 10.1021/acs.accounts.7b00191
      42
      Multifunctional Nanostructured Conductive Polymer Gels: Synthesis, Properties, and Applications
      Zhao, Fei; Shi, Ye; Pan, Lijia; Yu, Guihua
      Accounts of Chemical Research (2017), 50 (7), 1734-1743CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Conductive polymers have attracted significant interest over the past few decades because they synergize the advantageous features of conventional polymeric materials and org. conductors. With rationally designed nanostructures, conductive polymers can further exhibit exceptional mech., elec., and optical properties because of their confined dimensions at the nanoscale level. Among various nanostructured conductive polymers, conductive polymer gels (CPGs) with synthetically tunable hierarchical 3D network structures show great potential for a wide range of applications, such as bioelectronics, and energy storage/conversion devices owing to their structural features. CPGs retain the properties of nanosized conductive polymers during the assembly of the nanobuilding blocks into a monolithic macroscopic structure while generating structure-derived features from the highly crosslinked network. In this Account, we review our recent progress on the synthesis, properties, and novel applications of dopant crosslinked CPGs. We first describe the synthetic strategies, in which mols. with multiple functional groups are adopted as crosslinkers to crosslink conductive polymer chains into a 3D mol. network. These crosslinking mols. also act as dopants to improve the elec. cond. of the gel network. The microstructure and phys./chem. properties of CPGs can be tuned by controlling the synthetic conditions such as species of monomers and crosslinkers, reaction temp., and solvents. By incorporating other functional polymers or particles into the CPG matrix, hybrid gels have been synthesized with tailored structures. These hybrid gel materials retain the functionalities from each component, as well as enable synergic effects to improve mech. and elec. properties of CPGs. We then introduce the unique structure-derived properties of the CPGs. The network facilitates both electronic and ionic transport owing to the continuous pathways for electrons and hierarchical pores for ion diffusion. CPGs also provide high surface area and solvent compatibility, similar to natural gels. With these improved properties, CPGs have been explored to enable novel conceptual devices in diverse applications from smart electronics and ultrasensitive biosensors, to energy storage and conversion devices. CPGs have also been adopted for developing hybrid materials with multifunctionalities, such as stimuli responsiveness, self-healing properties, and super-repellency to liq. with synthetically tunable phys./chem. properties, CPGs emerge as a unique material platform to develop novel multifunctional materials that have the potential to impact electronics, energy, and environmental technologies. We hope that this Account promotes further efforts toward synthetic control, fundamental investigation, and application exploration of CPGs.
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    43. 43
      Cao, Y.; Chen, Y.; Sun, X.; Zhang, Z.; Mu, T. Water Sorption in Ionic Liquids: Kinetics, Mechanisms and Hydrophilicity. Phys. Chem. Chem. Phys. 2012, 14, 12252– 12262,  DOI: 10.1039/c2cp41798g
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      Water sorption in ionic liquids: kinetics, mechanisms and hydrophilicity
      Cao, Yuanyuan; Chen, Yu; Sun, Xiaofu; Zhang, Zhongmin; Mu, Tiancheng
      Physical Chemistry Chemical Physics (2012), 14 (35), 12252-12262CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Most of the ionic liqs. (ILs) are hygroscopic in air. The effects of structural factors of ILs (cation, anion, alkyl chain length at cation, and C2 methylation at cation) and external factors (temp., relative humidity, and impurity) on the kinetics of water sorption by 18 ILs were investigated. A modified two-step sorption mechanism was proposed to correlate the water sorption data in the ILs. Three type of parameters (sorption capacity, sorption rate and degree of difficulty to reach sorption equil.) based on the modified two-step mechanism were derived to comprehensively characterize the water sorption processes. These parameters have similar tendencies, providing an efficient way to evaluate them by one parameter that can be easily obtained. The hydrophilicity of the ILs was classified to four levels (super-high, high, medium, low) according to the water sorption capacity. The results show that cation of the ILs also plays an important role in water sorption, and the impurities affect the water sorption enormously. Acetate and halogen-based ILs have the highest hydrophilicity when combined with the imidazolium or pyridinium cation.
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    44. 44
      Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; John Wiley & Sons, 2013.
      There is no corresponding record for this reference.
    45. 45
      Wang, X.; Li, X.; Liu, G.; Li, J.; Hu, X.; Xu, N.; Zhao, W.; Zhu, B.; Zhu, J. An Interfacial Solar Heating Assisted Liquid Sorbent Atmospheric Water Generator. Angew. Chem., Int. Ed. 2019, 58, 12054– 12058,  DOI: 10.1002/anie.201905229
      45
      An Interfacial Solar Heating Assisted Liquid Sorbent Atmospheric Water Generator
      Wang, Xueyang; Li, Xiuqiang; Liu, Guoliang; Li, Jinlei; Hu, Xiaozhen; Xu, Ning; Zhao, Wei; Zhu, Bin; Zhu, Jia
      Angewandte Chemie, International Edition (2019), 58 (35), 12054-12058CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Harvesting water from air is a promising strategy for fresh-water prodn., and it is particularly desirable for areas that lack direct access to clean water. While high-concn. liq. sorbent is well-known for high sorption, it has not been widely used for atm. water collection, being primarily limited by the difficulty in desorption. Interfacial solar heating based on a salt-resistant GO-based aerogel is now shown to enable a high-concn. liq. sorbent (CaCl2 50 wt % soln.) based atm. water generator. Fresh water (2.89 kg m-2 day-1) can be produced at about 70 % relative humidity, with only solar energy input and energy efficiency of desorption as high as 66.9 %. This low-cost and effective approach provides an attractive pathway to ext. water from air, to relieve the thirst of arid, land-locked, and other areas where fresh water is scarce.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtlOgsr3E&md5=364fc049fe842f15832a5b4240732d51
    46. 46
      Qi, H.; Wei, T.; Zhao, W.; Zhu, B.; Liu, G.; Wang, P.; Lin, Z.; Wang, X.; Li, X.; Zhang, X. An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption. Adv. Mater. 2019, 31, 1903378,  DOI: 10.1002/adma.201903378
      46
      An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption
      Qi, Heshan; Wei, Tianqi; Zhao, Wei; Zhu, Bin; Liu, Guoliang; Wang, Pingping; Lin, Zhenhui; Wang, Xueyang; Li, Xiuqiang; Zhang, Xiaowei; Zhu, Jia
      Advanced Materials (Weinheim, Germany) (2019), 31 (43), 1903378CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Water scarcity is one of the greatest challenges facing human society. Because of the abundant amt. of water present in the atm., there are significant efforts to harvest water from air. Particularly, solar-driven atm. water generators based on sequential adsorption-desorption processes are attracting much attention. However, incomplete daytime desorption is the limiting factor for final water prodn., 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-liq.-based sorbent, an atm. 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 atm. water generator enables a high rate of water prodn. (≈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 atm. water generator, based on the liq. sorbent with a simultaneous adsorption-desorption process opens up a promising pathway to effectively harvest water from air.
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    47. 47
      Canivet, J.; Fateeva, A.; Guo, Y.; Coasne, B.; Farrusseng, D. Water Adsorption in MOFs: Fundamentals and Applications. Chem. Soc. Rev. 2014, 43, 5594– 5617,  DOI: 10.1039/C4CS00078A
      47
      Water adsorption in MOFs: fundamentals and applications
      Canivet, Jerome; Fateeva, Alexandra; Guo, Youmin; Coasne, Benoit; Farrusseng, David
      Chemical Society Reviews (2014), 43 (16), 5594-5617CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. This review article presents the fundamental and practical aspects of water adsorption in Metal-Org. Frameworks (MOFs). The state of the art of MOF stability in water, a crucial issue to many applications in which MOFs are promising candidates, is discussed here. Stability in both gaseous (such as humid gases) and aq. media is considered. By considering a non-exhaustive yet representative set of MOFs, the different mechanisms of water adsorption in this class of materials are presented: reversible and continuous pore filling, irreversible and discontinuous pore filling through capillary condensation, and irreversibility arising from the flexibility and possible structural modifications of the host material. Water adsorption properties of more than 60 MOF samples are reported. The applications of MOFs as materials for heat-pumps and adsorbent-based chillers and proton conductors are also reviewed. Some directions for future work are suggested as concluding remarks.
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    48. 48
      Alcañiz-Monge, J.; Linares-Solano, A.; Rand, B. Water Adsorption on Activated Carbons: Study of Water Adsorption in Micro-and Mesopores. J. Phys. Chem. B 2001, 105, 7998– 8006,  DOI: 10.1021/jp010674b
      48
      Water adsorption on activated carbons: Study of water adsorption in micro- and mesopores
      Alcaniz-Monge, Juan; Linares-Solano, Angel; Rand, Brian
      Journal of Physical Chemistry B (2001), 105 (33), 7998-8006CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The mechanism of water adsorption is analyzed in a range of activated carbons differing widely in the relative proportions of narrow and wide micropores, mesopores, and macropores. They also display different degrees of surface functionality. By assuming that the adsorbed phase in micropores is ice-like, the whole micropore vol. is filled with water at a P/P0 of ≈0.82, at which point capillary condensation may begin. However, surface complexes control the wetting of the mesopore walls by adsorbed water and can influence the condensation process. Condensation in macropores may also occur at P/P0 values >0.95. The extent of meso-/macropore filling depends on the surface complexes. The vol. filling process is progressive and in micropores is consistent with the theory of vol. filling. The adsorption process was analyzed by interpreting the nitrogen isotherms systematically measured on samples of different pore structure with progressively increasing amts. of preadsorbed water up to satn.
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    49. 49
      Langmuir, I. The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids. J. Am. Chem. Soc. 1916, 38, 2221– 2295,  DOI: 10.1021/ja02268a002
      49
      Constitution and fundamental properties of solids and liquids. I. Solids
      Langmuir, Irving
      Journal of the American Chemical Society (1916), 38 (), 2221-95CODEN: JACSAT; ISSN:0002-7863.
      Outline of L.'s theory of the mechanism of evapn., condensation, liquefaction, adsorption and capillary phenomena, according to which solids and liquids consist of atoms held together entirely by chem. forces. The conception of the mol. is thus almost entirely eliminated except in the case of gases, and a liquid or solid is looked upon as consisting of a single large mol. The following is L.'s summary: The work of the Braggs on crystal structure is reviewed from the viewpoint of the chemist, and the relation of this work to the theories of chem. constitution, such as those of Werner, Stark, J. J. Thomson and Lewis, is discussed in detail. Substances whose structures have been studied by the X-ray spectrograph are not representative compds. in general; only polar compds. have been studied. Solid polar compds. are, in general, built up of atoms bound together by secondary or residual valences. The whole crystal must be regarded as a single mol. Solid non-polar compds. consist in general of "group mols." in which the atoms are usually held together by primary valences. These group mols. in turn are bound together by secondary valence to form a large "crystal mol." which includes the whole solid mass. There is no present justification for dividing interatomic or intermolecular forces into physical and chem. forces; it is much more profitable to consider all such forces as strictly chemical. Evapn., condensation, soln., crystn., adsorption, surface tension, etc., should all be regarded as typical chem. phenomena. From a consideration of such properties as sp. heat, compressibility and coeff. of expansion it is concluded that collisions do not take place between the atoms of solids, but that these move about equil. positions under the influence of both repulsive and attractive forces. The "time of relaxation" of solids is calcd. approx. from the heat cond., and is found to be between 10-14 and 10-7 sec. An independent calcn. based on the time of evapn. of a substance into a vacuum gave substantially the same result. Thus the time necessary for an atom of a solid to reach thermal equil. with its neighbors is very small compared to the time necessary to make a single oscillation about an equil. position. The "oscillations" are thus extremely strongly damped. Since solid substances in general are held together by secondary rather than primary valences, there are few limitations to the number of compds. which can exist in the solid state. Most of these do not show a compn. which could be predicted from the ordinary rules of valence. Metallic compds., minerals, solid solns. and glasses are discussed from this point of view. From considerations based largely on compressibility it is concluded that the attractive forces between atoms usually reach a maximum intensity when the distance between adjacent atoms in a solid is increased by about 0.6 × 10-8 cm. (10 to 30% of the normal distance between atoms). Since energy must be expended in breaking apart the atoms of a solid, the surfaces of solids must contain more potential energy than does the corresponding number of atoms in the interior. Since this potential energy is probably electromagnetic energy in the field between the atoms, the interatomic forces are more intense in the surface than in the interior. This intense surface field of force (unsatd. chem. affinity) is one of the causes of the phenomena of condensation and adsorption. Because of the small time of relaxation and because an atom approaching the surface is attracted by many, but later is repelled by few atoms, it follows that the surfaces of solids are almost wholly inelastic in regard to mols. impinging on the surface. There is exptl. evidence of this inelasticity. As a result, nearly every mol. striking the surface condenses, no matter what the temp. While condensed it is held by forces quite similar to those holding solids together (either primary or secondary valence). At high temps. evapn. may take place almost immediately after condensation, but at low temps. the condensed atom or mol. may remain indefinitely. In general the rate of evapn. (m) in a high vacuum is related to the pressure by the equation m = √M/2πRTp. Red P and some other substances are probably exceptions. When separate phases of const. compn. are present (decompn. of a solid such as CaCO3) the reaction must take place exclusively at the boundaries of these phases. This kinetic interpretation of the phase rule indicates clearly the distinction between reactions in which solid solns. are formed and those in which separate phases appear. This theory offers a ready explanation for the fact that some hydrated crystals fail to effloresce unless scratched, and for the fact that thoroughly dehydrated substances will absorb H2O only with difficulty. Adsorption is a direct consequence of the time lag between condensation and evapn. Whether the adsorbed substance is held by primary or secondary valences, it is better to regard the union as chemical. Exptl. results are given which show that adsorption is very frequently the result of the strongest kind of chem. union (primary valence) between the atoms of the adsorbed substance and the atoms of the solid. The action of a typical catalyzer poison depends on the formation of a very stable film one atom deep over the surface of the catalyzer. The chem. activity of a surface depends upon the nature, arrangement and spacing of the atoms forming the surface layer. There is a very close relation between the chem. activity of a surface and the electron emission from it (either thermionic or photo-electric emission). There is a brief development of this theory of heterogeneous reactions. A "law of surface action" analogous to, but different from, the law of mass action is proposed which is in accord with, and affords an explanation of, Reichinstein's "const. sum hypothesis." Cf. following abstract.
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    50. 50
      Langmuir, I. The Constitution and Fundamental Properties of Solids and Liquids. Ii. Liquids. J. Am. Chem. Soc. 1917, 39, 1848– 1906,  DOI: 10.1021/ja02254a006
      50
      Constitution and fundamental properties of solids and liquids. II. Liquids
      Langmuir, Irving
      Journal of the American Chemical Society (1917), 39 (), 1848-1906CODEN: JACSAT; ISSN:0002-7863.
      cf. C. A. 10, 3007. The ideas mentioned in part I of the paper and also by Harkins (C. A. 10, 839; 11, 731) that when a film of (say) oleic acid spreads on H2O the mols. are turned with the-COOH groups in the H2O and the hydrocarbon chain on end are developed extensively, with reference to the existing expts. of Devaux (C. A. 10, 1131) and others. L. claims priority over Harkins in the idea of orientation of mols. at a surface. New expts. are described in which a piece of paper resting on a water surface is attached to a small balance in such a way as to measure the force necessary to compress a floating film to a layer one mol. deep, and data are given for a considerable number of fatty acids and their derivs. The exptl. results are thought to afford the strongest kind of confirmation of the theory that the spreading of films on surfaces is detd. by the shape of the mols. and the relative activities of the different portions of the mols. These results are discussed at length. The expts. of Gurvich (C. A. 9, 3003) on the adsorption of liquids by solids are described in some detail. L. considers that all interatomic and intermol. forces involved in the structure of matter are chemical forces; i. e., they are of the same nature as the forces that the chemist has been studying in the past. Primary valence is specific, depends upon the degree of satn. of the atoms and acts atomistically, in definite directions and only through atomic distances. Secondary valence is less specific and depends upon the presence of neighboring atoms, it acts through distances not greatly different from those through which primary valence acts, and only on adjacent atoms, though usually not in directions radiating out from single atoms. Such phenomena as evapn., cohesion, crystn., adsorption, surface tension, viscosity, etc., apparently do not involve any atomic or inter-mol. forces other than the 2 kinds of valence already mentioned. The forces involved in the structure of liquids are not essentially different from those of solids.
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    51. 51
      Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361– 1403,  DOI: 10.1021/ja02242a004
      51
      The adsorption of gases on plane surfaces of glass, mica and platinum
      Langmuir, I.
      Journal of the American Chemical Society (1918), 40 (), 1361-1402CODEN: JACSAT; ISSN:0002-7863.
      According to L.'s hypothesis, gaseous mols. impinging on a liquid or solid surface do not in general rebound from it elastically, but are held or adsorbed on the surface by forces similar to those holding the atoms or group mols. of solid bodies. The adsorbed film should not exceed one mol. in thickness. Adsorption of permanent gases involves only secondary valence forces. In metals particularly, adsorption may be governed by primary valence forces. It is suggested that stoichiometric relations should govern the adsorption on a surface unless interfering effects caused by steric hindrance are involved. At room temp. the absorption by glass and mica was negligible, not over 1 % of the surface being covered by a single layer of mols. At lower temps. much larger quantities of gas were taken up. With Pt no absorption was observed at - 183° unless the Pt were first activated by proper heating. The adsorption of O2 was irreversible and corresponded to a monomolecular layer. CO likewise showed the same behavior. In the presence of one or the other gas adsorbed on the Pt the adsorbed and unadsorbed gases reacted immediately to form CO2.
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    52. 52
      Kolasinski, K. W. Thermodynamics and Kinetics of Adsorption and Desorption. In Surface Science; Kolasinski, K. W. Ed., pp 185– 228.  DOI: 10.1002/9781119941798.ch4 .
      There is no corresponding record for this reference.
    53. 53
      Coasne, B.; Galarneau, A.; Pellenq, R. J.; Di Renzo, F. Adsorption, Intrusion and Freezing in Porous Silica: The View from the Nanoscale. Chem. Soc. Rev. 2013, 42, 4141– 4171,  DOI: 10.1039/c2cs35384a
      53
      Adsorption, intrusion and freezing in porous silica: the view from the nanoscale
      Coasne, Benoit; Galarneau, Anne; Pellenq, Roland J. M.; Di Renzo, Francesco
      Chemical Society Reviews (2013), 42 (9), 4141-4171CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This review presents the state of the art of mol. simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphol. and topol. disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common mol. simulation and theor. methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evapn. mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theor. results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn-Laplace equation and Gibbs-Thomson equation to describe the thermodn. of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.
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    54. 54
      Aristov, Y. I.; Tokarev, M. M.; Freni, A.; Glaznev, I. S.; Restuccia, G. Kinetics of Water Adsorption on Silica Fuji Davison Rd. Microporous Mesoporous Mater. 2006, 96, 65– 71,  DOI: 10.1016/j.micromeso.2006.06.008
      54
      Kinetics of water adsorption on silica Fuji Davison RD
      Aristov, Yuri I.; Tokarev, Mikhail M.; Freni, Angelo; Glaznev, Ivan S.; Restuccia, Giovanni
      Microporous and Mesoporous Materials (2006), 96 (1-3), 65-71CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      The kinetics of water adsorption on loose grains of Fuji Davison RD silica gel was studied by a TG differential step method in the temp. range 29-64° and in the pressure range 6.5-34 mbar. Three grain sizes were selected, 0.3-0.325, 0.355-0.425, and 0.8-1.0 mm. Furthermore, adsorption isobars at PH2O = 9, 18, and 48 mbar were measured over the temp. range of 30-150° by a TG technique to det. pertinent equil. parameters which are used to calc. the coeffs. of diffusion. The equil. uptake was described as a linear function of the Dubinin-Polanyi adsorption potential. The influence of particle size, temp., and pressure on the adsorption kinetics can be well described in terms of the Fickian diffusion model. The apparent water diffusivity D ap was found to be an Arrhenius function of temp. with the apparent activation energy Ea = 41.5 kJ/mol and the pre-exponential factor Dap0 = 2.9 × 10-4 m2/s. The apparent diffusivity of water in silica pores was measured to be De = (3.7-4.7) × 10-7 m2/s and possessed a slight increase with temp. This value is close to the Knudsen diffusivity, calcd. for a cylindrical pore of radius rp = 1.0 nm. For smaller grains at T >39° the contribution of thermal effects was revealed, which decreases the rate of water sorption. In this case, application of non-isothermal kinetic model of Lee and Ruthven allowed good description of exptl. uptake curves as well as the estn. of parameters which det. simultaneous heat and mass transfer.
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    55. 55
      Sircar, S.; Hufton, J. Why Does the Linear Driving Force Model for Adsorption Kinetics Work?. Adsorption 2000, 6, 137– 147,  DOI: 10.1023/A:1008965317983
      55
      Why does the linear driving force model for adsorption kinetics work?
      Sircar, S.; Hufton, J. R.
      Adsorption (2000), 6 (2), 137-147CODEN: ADSOFO; ISSN:0929-5607. (Kluwer Academic Publishers)
      The Linear Driving Force (LDF) model for gas adsorption kinetics is frequently and successfully used for anal. of adsorption column dynamic data and for adsorptive process designs because it is simple, analytic, and phys. consistent. Yet, there is a substantial difference in the characteristics of isothermal batch uptake curves on adsorbent particles by the LDF and the more rigorous Fickian Diffusion (FD) model. It is demonstrated by using simple model systems that the characteristics of the adsorption kinetics at the single pore or the adsorbent particle level are lost in (a) evaluating overall uptake on a heterogeneous porous solid, (b) calcg. breakthrough curves from a packed adsorbent column, and (c) establishing the efficiency of sepn. by an adsorptive process due to repeated averaging of the base kinetic property. That is why the LDF model works in practice.
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    56. 56
      Zhuravlev, L. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf., A 2000, 173, 1– 38,  DOI: 10.1016/S0927-7757(00)00556-2
      56
      The surface chemistry of amorphous silica. Zhuravlev model
      Zhuravlev, L. T.
      Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2000), 173 (1-3), 1-38CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
      A review, with 347 refs. on the author's own results on the properties of amorphous SiO2 surface. In any description of the surface SiO2 the hydroxylation of the surface is of crit. importance. An anal. was made of the processes of dehydration (the removal of phys. adsorbed water), dehydroxylation (the removal of silanol groups from the SiO2 surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of exptl. and theor. studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chem. of amorphous SiO2. The main advantage of this physico-chem. model lies in the possibility to det. the concn. and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the SiO2 surface as a function of the pretreatment temp. of SiO2 samples. The model makes it possible to det. the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO2 skeleton. The magnitude of the silanol no., i.e., the no. of OH groups per unit surface area, αOH, when the surface is hydroxylated to the max. degree, is considered to be a physico-chem. const. This const. has a numerical value: αOH,AVER = 4.6 (least-squares method) and αOH,AVER = 4.9 OH nm-2 (arithmetical mean) and is known in literature as the Kiselev-Zhuravlev const. Adsorption and other surface properties per unit surface area of SiO2 are identical (except for very fine pores). On the basis of data published in the literature, this model was found to be useful in solving various applied and theor. problems in the field of adsorption, catalysis, chromatog., chem. modification, etc. The Brunauer-Emmett-Teller (BET) method is the correct method and gives the opportunity to measure the real phys. magnitude of the sp. surface area, SKr (by low temp. adsorption of Kr), for SiO2s and other oxide dispersed solids.
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    57. 57
      Wang, L.; Wang, R.; Oliveira, R. A Review on Adsorption Working Pairs for Refrigeration. Renewable Sustainable Energy Rev. 2009, 13, 518– 534,  DOI: 10.1016/j.rser.2007.12.002
      57
      A review on adsorption working pairs for refrigeration
      Wang, L. W.; Wang, R. Z.; Oliveira, R. G.
      Renewable & Sustainable Energy Reviews (2009), 13 (3), 518-534CODEN: RSERFH; ISSN:1364-0321. (Elsevier Ltd.)
      A review. Solid sorption refrigeration is a type of environmental benign and energy saving technol. and the sorbents utilized can be divided into phys., chem. and composite sorbents, according to the nature of the forces involved in the adsorption process. The types, characteristics, advantages and disadvantages of different adsorbents, refrigerants and working pairs are summarized in this paper, together with the models that describe the adsorption equil. Moreover, some of the procedures to prep. composite adsorbents are presented. The application of different working pairs for different situations is related with the adsorption heat, the adaptability to the driving temp. and to the desired working pressure. The methods to measure the adsorption quantity of different working pairs are compared, and future research directions of adsorption working pairs are also analyzed.
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    58. 58
      Nakano, T.; Nozue, Y. Orbital Degeneracy and Magnetic Properties of Potassium Clusters Incorporated into Nanoporous Crystals of Zeolite A. J. Comput. Methods Sci. Eng. 2008, 7, 443– 462,  DOI: 10.3233/JCM-2007-75-610
      There is no corresponding record for this reference.
    59. 59
      Wei, X.; Wang, W.; Xiao, J.; Zhang, L.; Chen, H.; Ding, J. Hierarchically Porous Aluminosilicates as the Water Vapor Adsorbents for Dehumidification. Chem. Eng. J. 2013, 228, 1133– 1139,  DOI: 10.1016/j.cej.2013.05.062
      59
      Hierarchically porous aluminosilicates as the water vapor adsorbents for dehumidification
      Wei, Xiaolan; Wang, Weilong; Xiao, Jing; Zhang, Li; Chen, Hongyin; Ding, Jing
      Chemical Engineering Journal (Amsterdam, Netherlands) (2013), 228 (), 1133-1139CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      Hierarchically porous aluminosilicates with large surface area (930-1010 m2/g) and ordered hexagonal structures were synthesized as the water vapor adsorbents for dehumidification in this work. Compared to the com. microporous zeolite 3A (low Si/Al ratio), and mesoporous SILICA GEL (high Si/Al ratio), the synthesized aluminosilicates showed higher adsorption capacity of water vapor at both high and low humidity ranges. Different from zeolite 3A (type I) and silica gel (type III), the adsorption isotherms of aluminosilicates followed type IV isotherms. The aluminosilicate materials showed low desorption activation energy of water vapor ( < 20 kJ/mol, close to the intermol. H2O-H2O H-bonding interaction). The aluminosilicates showed a low desorption temp. of 87 °C and a relatively fast desorption rate. Moreover, regenerability test in the multiple adsorption-desorption cycles suggested that the synthesized aluminosilicates were regenerable. With the promising dehumidification characteristics, including high adsorption capacity of water vapor, fast adsorption and desorption rates, low regeneration temp., as well as good regenerability, the synthesized hierarchically porous aluminosilicates can be potential adsorbents for energy-efficient and cost-effective dehumidification process in practical applications, i.e. rotary desiccant dehumidifiers.
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    60. 60
      Zheng, X.; Ge, T. S.; Wang, R. Z. Recent Progress on Desiccant Materials for Solid Desiccant Cooling Systems. Energy 2014, 74, 280– 294,  DOI: 10.1016/j.energy.2014.07.027
      There is no corresponding record for this reference.
    61. 61
      William, G. E.; Mohamed, M.; Fatouh, M. Desiccant System for Water Production from Humid Air Using Solar Energy. Energy 2015, 90, 1707– 1720,  DOI: 10.1016/j.energy.2015.06.125
      61
      Desiccant system for water production from humid air using solar energy
      William, G. E.; Mohamed, M. H.; Fatouh, M.
      Energy (Oxford, United Kingdom) (2015), 90 (Part_2), 1707-1720CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
      Decentralized and remote areas suffer from fresh water shortage. A new sustainable energy technique called "Water Extn. from Atm. Air" is introduced as a beneficial soln. for this problem. This system involves the absorption of water vapor from ambient air during the night time. Then, simultaneous desiccant regeneration and water vapor condensation during the day time will be occurred. In this study, a trapezoidal prism solar collector with four fiberglass sides is designed and constructed. This collector has multi-shelves bed (desiccant carrier) in order to maximize bed surface area inside the collector. Consequently, the absorption and evapn. surfaces are increased. The exptl. work shows the effect of weather conditions in system operation for both day time and night time. The regeneration and absorption processes are discussed for each operation condition. Also, different host materials (cloth and sand) with calcium chloride soln. are investigated. An illustrative comparison between different bed types is demonstrated. The results revealed that the total evapd. water for cloth and sand bed can reach 2.32 and 1.23 slit/days m2 at initial satn. concn. (30%) of cacl2. However, the system efficiency is 29.3 and 17.76% for cloth and sand bed, resp.
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    62. 62
      Dietzel, P. D.; Johnsen, R. E.; Blom, R.; Fjellvåg, H. Structural Changes and Coordinatively Unsaturated Metal Atoms on Dehydration of Honeycomb Analogous Microporous Metal-Organic Frameworks. Chem. - Eur. J. 2008, 14, 2389– 2397,  DOI: 10.1002/chem.200701370
      62
      Structural changes and coordinatively unsaturated metal atoms on dehydration of honeycomb analogous microporous metal-organic frameworks
      Dietzel, Pascal D. C.; Johnsen, Rune E.; Blom, Richard; Fjellvag, Helmer
      Chemistry - A European Journal (2008), 14 (8), 2389-2397CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The dehydration process of two isostructural microporous coordination polymers, [M2(dhtp)(H2O)2]·8H2O, termed CPO-27-M (M = Co, Zn; H4dhtp = 2,5-dihydroxy-terephthalic acid) was studied by in situ variable temp. x-ray diffraction. Both compds. contain accessible metal coordination sites after removal of all solvent. However, despite the analogy of their crystal structures, they behave differently during dehydration. For CPO-27-Co, H2O desorption is a smooth topotactic process of 2nd order with no concomitant space group change and no increase in microstrain, which is beneficial for the applicability of the material. Removal of the H2O propagates from the center of the channels outwards. The coordinating H2O mol. at the metal desorbs only when almost all the bulk H2O in the pores has disappeared. In contrast, discontinuities in the powder pattern of CPO-27-Zn indicate the occurrence of 1st-order transitions. The crystal structures of four of the five individual phases could be detd. The structure of the intermediate phase occurring just before the framework is completely evacuated was elusive in respect to full structure soln. and refinement, but it is most probably related to the removal of the axis of 3-fold symmetry. The Zn-based material experiences a significant amt. of strain.
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    63. 63
      Damasceno Borges, D.; Maurin, G.; Galvão, D. S. Design of Porous Metal-Organic Frameworks for Adsorption Driven Thermal Batteries. MRS Adv. 2017, 2, 519– 524,  DOI: 10.1557/adv.2017.181
      63
      Design of Porous Metal-Organic Frameworks for Adsorption Driven Thermal Batteries
      Damasceno Borges, Daiane; Maurin, Guillaume; Galvao, Douglas S.
      MRS Advances (2017), 2 (9), 519-524CODEN: MARDCQ; ISSN:2059-8521. (Cambridge University Press)
      Thermal batteries based on a reversible adsorption/desorption of a working fluid (water, methanol, ammonia) rather than the conventional vapor compression is a promising alternative to exploit waste thermal energy for heat reallocation. In this context, there is an increasing interest to find novel porous solids able to adsorb a high energy d. of working fluid under low relative vapor pressure condition combined with an easy ability of regeneration (desorption) at low temp., which are the major requirements for adsorption driven heat pumps and chillers. The porous cryst. hybrid materials named Metal-Org. Frameworks (MOF) represent a great source of inspiration for sorption based-applications owing to their tunable chem. and topol. features assocd. with a large variability of pore sizes. Recently, we have designed a new MOF named MIL-160 (MIL stands for Materials of Institut Lavoisier), isostructural to CAU-10, built from the assembly of corner sharing aluminum chains octahedra AlO4(OH)2 with the 2,5-furandicarboxylic linker substituting the pristine org. linker, 1,4-benzenedicarboxylate. This ligand replacement strategy proved to enhance both the hydrophilicity of the MOF and its amt. of water adsorbed at low p/p0. This designed solid was synthesized and its chem. stability/adsorption performances verified. Here, we have extended this study by incorporating other polar heterocyclic linkers and a comparative computational study of the water adsorption performances of these novel structures has been performed. To that purpose, the cell and geometry optimizations of all hypothetical frameworks were first performed at the d. functional theory level and their water adsorption isotherms were further predicted by using force-field based Grand-Canonical Monte Carlo simulations. This study reveals the ease tunable water affinity of MOF for the desired application.
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    64. 64
      Cmarik, G. E.; Kim, M.; Cohen, S. M.; Walton, K. S. Tuning the Adsorption Properties of UiO-66 Via Ligand Functionalization. Langmuir 2012, 28, 15606– 15613,  DOI: 10.1021/la3035352
      64
      Tuning the Adsorption Properties of UiO-66 via Ligand Functionalization
      Cmarik, Gregory E.; Kim, Min; Cohen, Seth M.; Walton, Krista S.
      Langmuir (2012), 28 (44), 15606-15613CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      UiO-66 is one of the few known H2O-stable MOFs that are readily amenable to direct ligand substitution. UiO-66 was synthesized with amino-, nitro-, methoxy-, and naphthyl-substituted ligands to impart polar, basic, and hydrophobic characteristics. Pure-component CO2, CH4, N2, and H2O vapor adsorption isotherms were measured in the materials to study the effect of the functional group on the adsorption behavior. Heats of adsorption were calcd. for each pure gas on each material. The amino-functionalized material possesses the best adsorption properties for each pure gas due to a combination of polarity and small functional group size. The naphthyl-functionalized material exhibits a good combination of inhibited H2O vapor adsorption and high selectivity for CO2 over CH4 and N2.
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    65. 65
      Zheng, X.; Ge, T.; Wang, R.; Hu, L. Performance Study of Composite Silica Gels with Different Pore Sizes and Different Impregnating Hygroscopic Salts. Chem. Eng. Sci. 2014, 120, 1– 9,  DOI: 10.1016/j.ces.2014.08.047
      65
      Performance study of composite silica gels with different pore sizes and different impregnating hygroscopic salts
      Zheng, X.; Ge, T. S.; Wang, R. Z.; Hu, L. M.
      Chemical Engineering Science (2014), 120 (), 1-9CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      Composite solid desiccant materials are fabricated by impregnating hygroscopic salts into pores of different silica gels. In this paper, sorption characteristics of composite desiccants with different pore sizes (namely, 2-3, 7-8, and 9-10 nm) of silica gels and different salts in terms of LiCl, LiBr and CaCl2 are tested and analyzed. Researches on nitrogen adsorption suggest that, due to impregnated salt particles, composite desiccants are different from those of pure silica gels and have smaller surface area and pore vol. Sorption isotherms and sorption kinetics with respect to different composite desiccant materials are tested and fitted with the Dubinin-Astakhov (D-A) equation and the Linear Driving Force (LDF) model, resp. Results showed that both the pore size of silica gel substrates and categories of impregnated salts affect the sorption characteristics of composite desiccants. Microporous silica gel with pore size of 2-3 nm is not fit for prepg. composite desiccants due to decreased water sorption quantity and sorption rate. Type B and mesoporous silica gels can be a promising host matrix of composite desiccant owing to enhanced water sorption quantity and favorable sorption rate. In addn., composite desiccants employing LiCl have the best sorption properties, while CaCl2 and LiBr follow next.
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    66. 66
      Zhao, H.; Wang, Z.; Li, Q.; Wu, T.; Zhang, M.; Shi, Q. Water Sorption on Composite Material “Zeolite 13X Modified by LiCl and CaCl2. Microporous Mesoporous Mater. 2020, 299, 110109,  DOI: 10.1016/j.micromeso.2020.110109
      66
      Water sorption on composite material "zeolite 13X modified by LiCl and CaCl2"
      Zhao, Huizhong; Wang, Zhaoyang; Li, Qianwen; Wu, Tianhao; Zhang, Min; Shi, Qiqi
      Microporous and Mesoporous Materials (2020), 299 (), 110109CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      The article presents the synthesis of composite adsorbents by impregnating zeolite 13X with binary salts (LiCl + CaCl2). The phase compn. and element content of the composite adsorbent were characterized by X-ray diffraction (XRD) and Inductively coupled plasma (ICP). And its pore structure was discussed through N2 adsorption-desorption anal. The results show that the formation of the solid soln. changes the pore structure of the composite adsorbent which affects its adsorption performance. At the temp. of 25°C and the relative humidity of 80%, CS6 exhibits the max. adsorption capacity of 1.1 g/g compared with other adsorbents. The recycling test of CS6 shows that the adsorption capacity is 91.8% of the first time after 12 cycles. Finally, the dynamic adsorption curve and sorption isotherm were fitted by the LDF model and Polanyi Potential Adsorption Theory. It is shown that the simulation curve has good consistency with the measured data.
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    67. 67
      Li, R.; Shi, Y.; Wu, M.; Hong, S.; Wang, P. Improving Atmospheric Water Production Yield: Enabling Multiple Water Harvesting Cycles with Nano Sorbent. Nano Energy 2020, 67, 104255,  DOI: 10.1016/j.nanoen.2019.104255
      67
      Improving atmospheric water production yield: Enabling multiple water harvesting cycles with nano sorbent
      Li, Renyuan; Shi, Yusuf; Wu, Mengchun; Hong, Seunghyun; Wang, Peng
      Nano Energy (2020), 67 (), 104255CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Clean water shortage has long been a challenge in remote and landlocked communities esp. for the impoverished. Atm. water is now considered as an unconventional but accessible fresh water source and sorption-based atm. water generator (AWG) has been successfully demonstrated a reliable way of harvesting atm. water. The water vapor sorbents with high water uptake capacity and esp. fast vapor sorption/desorption kinetics have become the bottleneck to a desirable clean water productivity in AWG. In this work, we developed a new nano vapor sorbent composed of a nano carbon hollow capsule with LiCl inside the void core. The sorbent can capture water vapor from ambient air as much as 100% of its own wt. under RH 60% within 3 h and quickly release the sorbed water within just half hour under 1 kW/m2 sunlight irradn. A batch-mode AWG device was able to conduct 3 sorption/desorption cycles within 10 h during one day test in the outdoor condition and produced 1.6 kgwater/kgsorbent. A prototype of continuous AWG device was designed, fabricated, and successfully demonstrated, hinting a possible way of large-scale deployment of AWG for practical purposes.
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    68. 68
      Xu, J.; Li, T.; Chao, J.; Wu, S.; Yan, T.; Li, W.; Cao, B.; Wang, R. Efficient Solar-Driven Water Harvesting from Arid Air with Metal-Organic Frameworks Modified by Hygroscopic Salt. Angew. Chem., Int. Ed. 2020, 59, 5202,  DOI: 10.1002/anie.201915170
      68
      Efficient Solar-Driven Water Harvesting from Arid Air with Metal-Organic Frameworks Modified by Hygroscopic Salt
      Xu, Jiaxing; Li, Tingxian; Chao, Jingwei; Wu, Si; Yan, Taisen; Li, Wenchen; Cao, Biye; Wang, Ruzhu
      Angewandte Chemie, International Edition (2020), 59 (13), 5202-5210CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Freshwater scarcity is a global challenge threatening human survival, esp. for people living in arid regions. Sorption-based atm. water harvesting (AWH) is an appealing way to solve this problem. However, the state-of-the-art AWH technologies have poor water harvesting performance in arid climates owing to the low water sorption capacity of common sorbents under low humidity conditions. We report a high-performance composite sorbent for efficient water harvesting from arid air by confining hygroscopic salt in a metal-org. framework matrix (LiCl@MIL-101(Cr)). The composite sorbent shows 0.77 g g-1 water sorption capacity at 1.2 kPa vapor pressure (30% relative humidity at 30°C) by integrating the multi-step sorption processes of salt chemisorption, deliquescence, and soln. absorption. A highly efficient AWH prototype is demonstrated with LiCl@MIL-101(Cr) that can enable the harvesting of 0.45-0.7 kg water per kg of material under lab. and outdoor ambient conditions powered by natural sunlight without optical concn. and addnl. energy input.
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    69. 69
      Yao, H.; Zhang, P.; Huang, Y.; Cheng, H.; Li, C.; Qu, L. Highly Efficient Clean Water Production from Contaminated Air with a Wide Humidity Range. Adv. Mater. 2020, 32, 1905875,  DOI: 10.1002/adma.201905875
      69
      Highly Efficient Clean Water Production from Contaminated Air with a Wide Humidity Range
      Yao, Houze; Zhang, Panpan; Huang, Yaxin; Cheng, Huhu; Li, Chun; Qu, Liangti
      Advanced Materials (Weinheim, Germany) (2020), 32 (6), 1905875CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The huge amt. 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 irradn., is proposed. This PGF shows a superhigh equil. uptake of 5.20 g of water per g 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 kg of PGF one day, enough to meet several people's drinking water demand. This work provides a new strategy for effective prodn. of clean water from the atm. of practical significance.
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