Systems, methods, compositions, and designs for synthesis and use of extreme-water-uptake, leakage-free hygroscopic hydrogels

Abstract

Systems, methods, compositions, and designs for synthesis and use of hygroscopic hydrogels are provided. Hygroscopic hydrogels can be used as scalable and low-cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and / or thermal energy storage. Sorbent materials can be loaded with salt by soaking the gel in an aqueous solution to form hygroscopic composites. The swelling dynamics of hydrogels in aqueous lithium-chloride solutions, its implications on hydrogel salt loading, as well as the resulting vapor uptake of the synthesized hydrogel-salt composites can be performed. Equilibrating the swelling process of the hydrogels can result in hydrogels with high water uptake and leak-free sorbents that significantly outperform comparative hydrogels.

Description

SYSTEMS, METHODS, COMPOSITIONS, AND DESIGNS FOR SYNTHESIS AND USE OF EXTREME- WATER-UPTAKE, LEAKAGE-FREE HYGROSCOPIC HYDROGELSGOVERNMENT RIGHTS

[0001] This invention was made with government support under DE-EE0009679 awarded by the U.S. Department of Energy. The government has certain rights in the invention.CROSS REFERENCE TO RELATED APPLICATION

[0002] The present disclosure claims priority to and the benefit of U.S. Provisional Application No. 63 / 511,722, entitled “Extreme-Water-Uptake, Leakage-Free Hygroscopic Hydrogels,” filed on July 3, 2023, the content of which is incorporated by reference herein in its entirety.FIELD

[0003] The present disclosure relates to synthesis and use of extreme-water-uptake, leakage-free hygroscopic hydrogels as scalable and low-cost sorbents, and more particularly relates to synthesis and use of a high-performance lithium chloride (LiCl)-loaded polyacrylamide (PAM) hygroscopic hydrogel with extremely high salt loading and water uptake, as well as methods of designing and / or manufacturing the same.BACKGROUND

[0004] Humanity faces important challenges related to the worldwide supply of energy and water. Recently, efforts in energy and water conservation have increased due to discovered importance of keeping such efforts in line with the worldwide push towards net-zero emission strategies and adaptation to constantly evolving environmental conditions in a rapidly changing climate. Research towards innovative, functional materials, such as sorbents, can be seen as a powerful means to address these challenges. Sorption of water is commonplace in nature and widely used in a variety of technical processes that address water scarcity and enhance energy efficiency. These include freshwater production through atmospheric water harvesting, passive thermal management, thermal energy storage, and space conditioning. For sorbents to be viable candidates for these applications, low-cost, scalability, sustainability, the ability to provide high water vapor uptake (i.e., high hygroscopicity), good sorption kinetics, low desorption enthalpies, long-term cyclability, andeasy integration into respective devices can be seen as important features for their widespread adoption.

[0005] To meet these requirements, many sorbent materials have been explored, including hygroscopic salts, zeolites, silicas, and metal-organic frameworks (MOFs). To date, however, no sorbent can meet all the above criteria simultaneously. Hygroscopic hydrogels in particular, which are an emerging class of sorbent materials, have been explored to determine their potential of overcoming limitations present in other sorbents by providing extreme water uptakes, fast water capture and release kinetics, and low desorption enthalpies. Hygroscopic hydrogels may be typically synthesized by combining a hydrogel that possesses comparably low hygroscopicity with a sorbent component to substantially enhance hygroscopicity and form a composite material. The sorbent components can range from salts, metal-organic frameworks, to other hygroscopic polymers, where previous literature has found that the hygroscopicity and mass loading of these components into the hydrogel are crucial to maximizing the water uptake.

[0006] Previous approaches to optimizing the mass loading of these sorbents into hydrogels had several shortcomings. One common approach has included swelling the hydrogels in salt solutions with experimentally optimized concentrations, which has resulted in insufficient performance, partly due to limited vapor uptake of the hydrogels. For example, swelling the hydrogels in salt solutions with experimentally optimized concentrations has conventionally allowed loadings of about four (4) grams of salt per gram of polymer and a water uptake of about 0.74 g g1at 35% relative humidity (RH). Alternatively, polyzwitterionic hydrogels have been employed to enhance the salt loading via higher swelling ratios in concentrated salt solutions. Hydrogels fabricated with this approach can achieve water uptakes of up to about 0.62 g g1at 30% RH. In spite of these efforts, the hygroscopicity of these materials is not close to the fundamental limit of typical hygroscopic salts, which can be partially attributed to the limited loading of sorbents into the hydrogels during the swelling phase of the synthesis. Moreover, conventional hydrogels have traditionally resulted in solution leakage, which can reduce the hydrogel performance over time and lead to device failure. To prevent leakage, previous works have explored hydrophobic encapsulations, but with no degree of quantifiable success.

[0007] Accordingly, there is a need for improved compositions and methods for synthesizing hygroscopic hydrogels that achieve high water uptakes while preventing leakage.SUMMARY

[0008] The present disclosure is directed to synthesis and uses of hygroscopic hydrogels that can capture a significant amount of moisture from the ambient environment, even in arid conditions, without leakage problems commonly found in salt solutions. The hygroscopic hydrogels can be synthesized by loading polyacrylamide (PAM) hydrogels with salt by soaking the gel in an aqueous lithium chloride solution to form hygroscopic PAM-LiCl- composites. These composites can form a high-performance lithium chloride (LiCl)-loaded polyacrylamide (PAM) hygroscopic hydrogel with significantly higher salt loading and water uptake as compared to conventional hydrogels. Synthesis can occur by combining a highly hygroscopic sorbent with a salt loading that can exceed 20 g of LiCl per gram of polymer, to achieve record high water uptakes of hygroscopic hydrogels. Using models and specifically designed experiments, the swelling dynamics of hydrogels in aqueous LiCl solutions, its implications on hydrogel salt loading, and the resulting vapor uptake of the synthesized hydrogel-salt composites can be determined. From these values, the maximum leakage-free relative humidity (RH) as a function of hydrogel uptake and swelling ratio can be used to tune the synthesized materials to a wide variety of applications that possess different RH- conditions. In some embodiments, the material can be integrated as a sorption-based device to allow potential deployment into various water-energy nexus portfolios, including freshwater production through atmospheric water harvesting, passive thermal management, thermal energy storage, and / or space conditioning.

[0009] One exemplary method for forming a hydrogel includes exposing a sorbent material comprising a polymer matrix to an aqueous salt solution to promote swelling of the sorbent material leading to salt loading, selecting a desired salt concentration of the aqueous salt solution, and monitoring mass of the sorbent material throughout fluctuation to ensure equilibrium with the aqueous salt solution is achieved. The selection of a desired salt concentration of the aqueous salt solution is based on an equilibrium of the salt solution with a maximum allowable relative humidity in a system.

[0010] The aqueous salt solution can include one or more of LiCl, CaCh, or one or more other hygroscopic salt solutions. The water uptake can be independent of a type of polymer contained in the polymer matrix of the sorbent material. At about 30% relative humidity, the hydrogel can provide an about 15% increase in water uptake compared to conventional hydrogels.10011 1 A concentration of the aqueous salt solution can be at equilibrium with a highest relative humidity to which the hydrogel is exposed. In some embodiments, concentration of the aqueous salt solution can be approximately in a range from about 25% saturated (25%sat) to about 75% saturated (75%sat). At about 70% relative humidity, the hydrogel can retain at least 90% of the aqueous salt solution from the polymer matrix.

[0012] The method can further include tuning one or more of a concentration of the aqueous salt solution for swelling or a crosslinking of the polymer matrix. In some embodiments, an amount of aqueous salt solution within the polymer matrix of the sorbent material can exceed about 20 grams per gram of the sorbent material.

[0013] One exemplary embodiment of designing a composite includes loading a sorbent material with an aqueous salt solution, the sorbent material having a polymer matrix, and optimizing the polymer matrix. The polymer matrix includes a nanoporous network connected by one or more crosslinkers to form a gel, and receives a portion of the aqueous salt solution in it. Swelling at an interface of the sorbent material and the aqueous salt solution retains a plurality of ions of the aqueous salt solution. Optimizing the polymer matrix is accomplished by tuning one or more of: (i) type of polymer matrix; (ii) polymer matrix content; (iii) type of aqueous salt solution; (iv) degree of saturation in the aqueous salt solution; (v) crosslinking content; (vi) crosslinking architecture, or (vii) sorbent thickness.

[0014] The one or more parameters can be one of tuned individually or in combination with an other parameter of the one or more parameters. One or more crosslinkers can include A,A’-methylenebisacrylamidecrosslinker. The aqueous salt solution can include one or more of LiCl, CaCh, or one or more other hygroscopic salt solutions.

[0015] Loading the sorbent material with the aqueous salt solution can include soaking the sorbent material in hygroscopic salt. A degree of saturation of the aqueous salt solution can be one or more of about 0%sat, about 25%sat, about 50%Sat, about 75%Sat, or about 100%sat-The sorbent material can be soaked until an equilibrium swelling ratio of the composite is decoupled from its swelling kinetics.

[0016] In some embodiments, the method can further include placing the composite swollen to equilibrium in one of an about 25%Sat solution or an about 75 %sat solution into a chamber at a relative humidity of about 70% for about 48 hours. A length of time that the sorbent material is soaked can be based on a degree of saturation of the aqueous salt solution. When the degree of saturation of the aqueous salt solution is about 75 %sat, the sorbent material can be soaked for about 300 hours. In some embodiments, the method can include gelating a solution of deionized water, acrylamide monomer, N,N’- methylenebisacrylamidecrosslinker, ammonium persulfate initiator, and N,N,N',N'- tetramethylethylendiamin accelerator. The solution can be cured to form disks of the sorbent material. The disks can be dried at a relative humidity of about 0%. In some embodiments, the disks of the sorbent material can be freeze-dried.

[0017] One exemplary method of water harvesting includes absorbing water or vapor from ambient atmosphere into a sorbent material, applying energy to an energy absorber to desorb vapor, and collecting water with a condenser. One or more sorbent layers each include the sorbent material and a condenser surface that is disposed between the energy absorber and the condenser.

[0018] The sorbent material can include polyacrylamide (PAM) and the aqueous salt solution can include a hygroscopic salt selected from LiCl, CaCh, or one or more other hygroscopic salt solutions.

[0019] The method can further include a gap between the two or more sorbent layers. In some embodiments, the method can further include dissipating heat from the condenser through at least one of a heat sink or active cooling. Heat can be dissipated from a condenser surface to an adjacent sorbent layer. Applying energy can include supplying at least one of solar irradiance, biomass gasification, combustion, or electrically powered joule heating.

[0020] In some embodiments, the method can further include an enclosure containing the two or more sorbent layers and the condenser. The enclosure can be opened during dark periods for water adsorption and the enclosure is closed during light periods for water production. The two or more sorbent layers can exchange into the enclosure prior todesorbing water. The water can move from the sorbent layer to the condenser following a concentration gradient.

[0021] One exemplary embodiment of a sorbent material includes a polyacrylamide (PAM) and a hygroscopic salt selected from at least one of LiCl, CaCh, or one or more other hygroscopic salt solutions.

[0022] The hygroscopic salt can be at a concentration less than about 25% of saturation in water. The sorbent material can be recyclable. The sorbent material can be recycled by at least one of freeze drying (sublimation) or heating (evaporation).BRIEF DESCRIPTION OF THE DRAWINGS

[0023] This disclosure will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

[0024] FIG. 1 A is a perspective view of one embodiment of polyacrylamide (PAM) hydrogel disks in a wet state;

[0025] FIG. IB is a perspective view of a PAM hydrogel disk of the PAM hydrogel disks of FIG. 1 A in a dry state;

[0026] FIG. 1C is a schematic front view of a PAM hydrogel disk of the PAM hydrogel disks of FIG. 1 A in an aqueous LiCl solution illustrating swelling dynamics of the disk as a one-dimensional, transient mass transfer problem along the spatial coordinate, z, to form the hygroscopic hydrogels of present embodiments;

[0027] FIG. ID is a schematic representation of the swelling process at an interface between the hydrogel and the aqueous LiCl solution of FIG. 1C;

[0028] FIG. IE is a magnified schematic representation of a gel network formed by the swelling process of FIG. ID showing polymer chains that are connected via N,N’- methylenebisacrylamide crosslinkers to form a gel;

[0029] FIG. 2A is a graph illustrating hydrogel swelling ratio as a function of swelling time for the hygroscopic hydrogels of the present embodiments;

[0030] FIG. 2B is a graph illustrating relative sample weight as a function of swelling time, which demonstrates the PAM swelling kinetics for different LiCl concentrations in an aqueous solution of the present embodiments;

[0031] FIG. 2C is a graph illustrating salt loading at the end of a swelling process as a function of salt concentration in a swelling solution of the present embodiments measured via thermogravimetric analysis;

[0032] FIG. 3A is a graph illustrating hygroscopicity measured with a dynamic vapor sorption device comparing water uptake of pure LiCl to uptake of a PAM hydrogel after swelling in various solutions of the present embodiments;

[0033] FIG. 3B is a graph illustrating results for maximum allowable relative humidity (RH) as a function material uptake fraction (compared to the uptake of pure LiCl) and swelling ratio, while avoiding leakage;

[0034] FIG. 4A is a perspective view of one embodiment of a freeze-dried PAM hydrogel disk;

[0035] FIG. 4B is a perspective view of the freeze-dried PAM hydrogel disk of FIG. 4A after swelling to reach equilibrium with an aqueous about 25% percent saturated (25%sat) LiCl solution;

[0036] FIG. 4C is a graph illustrating water uptake defined as an amount of captured water divided by a dry weight for the half A,A’-methylenebisacrylamide (MBA) sample and the freeze-dried sample of FIG. 4B, with an inset showing the half MBA sample when being in equilibrium with an environment of about 70% RH at about 25 °C;

[0037] FIG. 5 is a schematic illustration of a dual-stage atmospheric water harvesting device;

[0038] FIG. 6A is a schematic illustration of materials characterization and modeling of efficiency and L / m2 / day (LMD);

[0039] FIG. 6B is a schematic illustration of materials characterization and modeling of efficiency and LMD;

[0040] FIG. 7 is a graph illustrating a dual-stage prototype and experimental setup;

[0041] FIG. 8A is a graphic illustration of Table 1;

[0042] FIG. 8B is a graphic illustration of Table 2;

[0043] FIG. 8C is a graphic illustration of Table 3; and

[0044] FIG. 8D is a graphic illustration of Table 4.DETAILED DESCRIPTION

[0045] Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the systems compositions, designs, and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the systems, compositions, and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure. Like-numbered components across embodiments generally have similar features unless otherwise stated or a person skilled in the art would appreciate differences based on the present disclosure and / or his / her knowledge.Accordingly, aspects and features of every embodiment may not be described with respect to each embodiment, but those aspects and features are applicable to the various embodiments unless statements or understandings are to the contrary.

[0046] Further, to the extent features, layers, sides, objects, steps, or the like are described as being “first,” “second,” third,” etc., and / or “lower,” “upper,” “middle,” etc., such numerical and / or location ordering / identification is generally arbitrary, and thus such numbering can be interchangeable unless indicated or otherwise understood by those skilled in the art to not be interchangeable. To the extent that the instant disclosure includes various terms for components and / or processes of the disclosed systems, compositions, designs, and methods, and the like, one skilled in the art, in view of the claims, present disclosure, and knowledge of the skilled person, will understand such terms are merely examples of such components and / or processes, and other components, designs, processes, and / or actions are possible. For example, to the extent that the term “salt solution” is used in the specification,it will be understood to be able to refer interchangeably to the “aqueous salt solution,” which is discussed in the specification with respect to LiCl, among others.

[0047] Commercial sorbent materials, such as silica gels and zeolites, and the state-of-the- art metal-organic frameworks have relatively low water uptakes, while high-performance hygroscopic salts typically have solution leakage (deliquescence) problems that can reduce the sorption performance over time and lead to device failure. In contrast, hygroscopic hydrogels are an emerging class of sorbent materials with the potential to overcome limitations present in these sorbents by providing high water uptakes. Hygroscopic hydrogels can be synthesized by combining a hydrogel that possesses comparably low hygroscopicity with a sorbent component to enhance hygroscopicity substantially and form a composite material. Previous approaches to optimizing the mass loading of these sorbents into hydrogels include swelling the hydrogels in salt solutions with experimentally optimized concentrations, or enhancing the salt loading via higher swelling ratios in concentrated salt solutions. In spite of these efforts, the hygroscopicity of these materials is far from the fundamental limit of typical hygroscopic salts, which can be partially attributed to the limited loading of sorbents into the hydrogels during the swelling phase of the synthesis.

[0048] At least one novel aspect of the present disclosure lies in systems, methods, compositions, and designs for synthesis of hygroscopic hydrogels that can uptake large amounts of water while being substantially leakage free as compared to commercial sorbent materials and other state-of-the-art sorbent materials. The hydrogels of the present embodiments can capture large amounts of moisture from the ambient environment, even in arid conditions, without leakage problems commonly found in salt solutions. Moreover, the instantly proposed methods can take advantage of the swelling behaviors of hydrogels by presenting an effective strategy to maximize the salt loadings of hydrogels to achieve extreme water-uptake and prevent the leakage of salts with no additional need for materials. For example, in some embodiments, the hygroscopic hydrogels can include a high-performance lithium chloride (LiCl)-loaded polyacrylamide (PAM) hygroscopic hydrogel that can be configured to exhibit dramatically improved salt loading and water uptake as compared to conventional hydrogels. The hygroscopic hydrogels can be synthesized by loading polyacrylamide (PAM) hydrogels with salt by soaking the gel in an aqueous LiCl solution to form hygroscopic PAM-LiCl-composites. These composites can achieve these extreme- water-uptake and leakage-free properties by optimizing leakage-free water uptake, whileconsidering the effect of hydrogel swelling. Optimization can include tuning of one or more parameters of the process and / or composition to synthesize hydrogels with superior performance. For example, optimization can include characterization of swelling dynamics of hydrogels in aqueous LiCl solutions, its implications on hydrogel salt loading, and / or the resulting vapor uptake of the synthesized hydrogel-salt composites.100491 In use, performance of the hygroscopic hydrogels of the present embodiments can be significantly superior to commercial sorbent materials such as silica gels and zeolites, state-of-the-art metal-organic frameworks, and other hydrogels. Additionally, due to the low- cost components used, and the optimization of the PAM hydrogels in the aqueous LiCl solution, the synthesized material, e.g., the hygroscopic PAM-LiCl-composites, can have a low-cost as compared to conventional hydrogels. In fact, the material used in preparing the hygroscopic hydrogels can be integrated as a sorption-based device to allow potential deployment into various water-energy nexus portfolios, including freshwater production through atmospheric water harvesting, passive thermal management, thermal energy storage, and / or space conditioning.

[0050] FIGS. 1A-1E illustrate an exemplary embodiment of compositions and methods that form the hygroscopic hydrogels of the present embodiments. For example, polyacrylamide (PAM) hydrogels can be synthesized based on a one-pot approach of mixing one or more of deionized water, acrylamide monomer, MAP-methylenebisacrylamide (MBA) crosslinker, ammonium persulfate initiator, and / or A,A,M,M-tetramethylethylendiamin accelerator to form a pre-gel solution. Once synthesized, the pre-gel solution can be filled into capped vials to cure. After gelation, the gel can be cut into disks 10. For example, FIG. 1A illustrates an exemplary embodiment of the disks 10 of PAM hydrogels in a wet state after gelation.

[0051] A size of the disks 10 can vary. For example, when fully swollen in deionized water (DIW), the disks 10 can have a diameter, L, of approximately 25 millimeters, while their thickness, H, can be approximately 3 millimeters. After synthesis, the disks 10 can be dried for an extended period. FIG. IB illustrates an exemplary embodiment of a dried disk 20 after drying, e.g., in a dried state. For example, in some embodiments, the disks 10 can be dried for three days at about 60 °C in an oven at an RH of about 0%, which can cause the disks 10 to lose the majority of their mass and shrink substantially to form the dried disk 20 in its dried state. For the purposes of this disclosure, the term “shrink substantially” as usedin the present disclosure can pertain to the dried disk 20 being in a range of approximately 50% to about 75% smaller than that of the disks 10.

[0052] The PAM hydrogel disks 30 of the present embodiments can be swollen in LiCl solutions to load the disks with salt and enhance their hygroscopicity. FIG. 1C illustrates swelling dynamics of the dried disk 20, e.g., the PAM hydrogel disk, in an aqueous salt solution. For example, the gel of the dried disk of FIG. IB can be soaked in a highly hygroscopic salt, e.g., LiCl solution, as a one-dimensional, transient mass transfer problem along the spatial coordinate, z- Progressive swelling of the gel in the salt solution can increase the total disk thickness, H, and its diameter, L to form a hygroscopic PAM-LiCl- composite 30, e.g., the hygroscopic hydrogel. In some embodiments, the free swelling of these initially dry hydrogel disks 20 in aqueous LiCl solutions can be observed with different degrees of saturation (%sat), such as ranging from pure DIW (about 0%sat) to Lid-solutions with about 25 %sat, about 50%sat, about 75%sat and about 100%sat, corresponding respectively to concentrations of about 0.172 g of salt per gram of solution, about 0.294 g of salt per gram of solution, about 0.384 g of salt per gram of solution, and about 0.454 g of salt per gram of solution (reference temperature: about 20 °C). While the present application discusses the use of LiCl as the salt solution to form the hygroscopic hydrogels of the present embodiments, some additional non-limiting examples of the salt solution can include calcium chloride (CaCh), lithium bromide (LiBr), sodium chloride (NaCl), and / or magnesium chloride (MgCh).

[0053] FIGS. ID- IE illustrate the swelling process of the dried disk 20 to form the hygroscopic PAM-LiCl-composite 30 in greater detail. As shown, as part of the swelling process in the salt solutions, the gels can gain substantial weight as both water 32 and solvated Li+and Cl" ions 34, 36 diffuse into the nano-porous network, e.g., matrix 40, of the hydrogel 30. For example, FIG. ID illustrates swelling at an interface 38 between the hydrogel 30 and the salt solution 32. When the ions 34, 36 migrate into the polymer matrix 40, they load the gel with hygroscopic salt. As shown, the matrix 40 can include a variety of polymers such as polyvinyl alcohol, cellulose, polyacrylamide, poly- [2- (methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (PDMAPS), polyacrylamide (PAM) and poly(N-isopropylacrylamide) (PNIPAM), polyethylene glycol (PEG), and / or polyvinyl alcohol (PVA).

[0054] This salt can rearrange the matrix, as shown in FIG. IE, to form polymer chains that are connected via AGV’-methylenebisacrylamide (MBA) crosslinkers to form the gel 30, where xrrepresents the number of repeat units in between of crosslinks. It will be appreciated that the crosslinking content of the polymer chain can impact rigidity of the resulting hygroscopic hydrogel. As shown, after swelling in salt solutions, large amounts of ions can be present inside the gel. These polymer chains can allow the hygroscopic hydrogels of the present embodiments to exhibit superior performance to commercial sorbent materials such as silica gels and zeolites, state-of-the-art metal-organic frameworks, and other hydrogels, due, at least in part, to their increased ability to capture a significant amount of moisture from the ambient environment without leakage problems commonly found in salt solutions.

[0055] FIGS. 2A-2C illustrate experimental measurements of the swelling process at room temperature, e.g., about 20 °C to about 25 °C (including about 23 °C or about 24 °C in some instances herein), as compared to modeling results. In FIG. 2A, for example, the hydrogel swelling ratio, i.e., current sample weight at a given time divided by initial, dried sample weight as measured before placing the dried PAM disk in the aqueous LiCl solution, is shown as a function of swelling time for swelling in five different aqueous LiCl solutions, e.g., curves (A)-(E) representing swelling in about 0%sat, about 25 %sat, about 50%sat, about 75%sat, and about 100%sat solution, respectively, and solid lines curves (A')-(E') representing model results considering nonlinear diffusion of salt solution into the hydrogel. For the purposes of referencing the drawings, curves (A)-(E) can be represented as circles, upward-pointing triangles, squares, downward-pointing triangles, and diamonds, respectively, each of which represent swelling in about 0%sat, about 25 %sat, about 50%sat, about 75%sat, and about 100%sat solution, respectively. The swelling time can be the time that has passed after the start of the swelling experiment, i.e., the duration that the hydrogel had time to swell in the aqueous solution. Results suggest that an amount of LiCl in the aqueous solution can affect both the equilibrium swelling ratio and the swelling kinetics. Most notably, large swelling ratios can be achievable despite high salt concentrations. In fact, all tested concentrations of LiCl increased the swelling ratio with respect to pure DIW, indicating an energetically more favorable interaction between salt solution and PAM, relative to that of water and PAM. Moreover, due to the low-cost, commercial availability, and high hygroscopicity even at low RH, hygroscopic salts such as LiCl and CaCh are promising components for hygroscopic hydrogel formation.

[0056] This result is distinct from previous works that reported the opposite trend where swelling was stopped after 48 hours before reaching equilibrium, which highlights the importance of decoupling equilibrium uptake and kinetics in the present embodiments. For example, while swelling in pure DIW (about 0%sat) can yield a swelling ratio of about 45, swelling ratios above about 70 can be obtained in an about 50%sat LiCl solution. Additionally, further increases in the amount of LiCl can reveal a non-monotonic relation between swelling ratio and LiCl concentration and reduced the equilibrium swelling ratio to about 53 for the about 100%sat LiCl solution. Given that the goal of swelling hydrogels in LiCl solutions is to achieve large contents of salt in the gel, which can then translate into higher uptake when the hydrogels are used as sorbents, this increased swelling ratio of PAM may be highly desirable and can lead to exceptional water vapor uptakes, as discussed in greater detail below. Further, beyond achieving high uptakes, the hydrogel matrices 40 can ensure containment of the salt solution, as solution leakage can reduce hydrogel performance over time, eventually leading to device failure.

[0057] Decoupling of the equilibrium swelling ratio from the swelling kinetics can show that large swelling ratios may be achievable despite high salt concentrations in the swelling solutions. For example, running the swelling experiments for extended durations can allow for full decoupling of the equilibrium swelling ratio from the swelling kinetics, which was not able to be achieved conventionally. FIG. 2B illustrates PAM swelling kinetics for different LiCl concentrations in the aqueous solution in greater detail, with the relative sample weight being measured as a function of the swelling time. Relative sample weight, for example, can be defined as the current sample weight at the moment of the measurement divided by the final, equilibrium weight of the fully swollen sample. As shown, as more LiCl is added to the water, the swelling kinetics can become slower. For example, while the sample swelling in pure water (A) reaches about 90% of its equilibrium swelling ratio within less than about three (3) days, the sample in the saturated LiCl solution (about 100%Sat) (E) can take more than thirty days (more than 720 hours) to exceed this value. Conventionally, decoupling of the equilibrium swelling ratio from the swelling kinetics by soaking the gel in aqueous concentrated LiCl solution was performed for the same duration of time as in pure water. That is, decoupling of the equilibrium swelling ratio from the swelling kinetics was considered independent of solution concentration, and was considered complete after soaking the dried disks 20 in the salt solution for 24 hours or less. A person skilled in the art would not have appreciated that swelling kinetics can be impacted by increased LiCl solutionconcentrations, much less that at increased LiCl solution concentrations it can take up to about 30 days to reach about 90% of its equilibrium swelling ratio. It will also be appreciated that increasing the swelling temperature to about 75 °C can substantially enhance kinetics, which may be important in at least some practical applications, e.g., water harvesting devices, space cooling, and so forth.100581 In some embodiments, salt loading into the hydrogel can be modeled to predict swelling dynamics by considering the free swelling of a thin hydrogel disk from its initial dry state 20 to its fully swollen state 30 in the aqueous LiCl solution. An example of such a modeled configuration is discussed above with respect to FIG. 1C. For example, due to L » H, the swelling process can be described as a one-dimensional, transient mass transfer problem along the spatial coordinate, z, as noted above. Due to the large swelling ratios observed, nonlinear swelling dynamics of a hydrogel where the concentration of solvent in the hydrogel and its mechanical stretch are coupled can be considered. In models, the salt solution can be considered as a solvent that diffuses into the hydrogel, and as such, the hydrogel stretch in the z-direction, .2, can be governed by a diffusion equation:is the stretch in the x- and y- directions, D is the effective diffusivity of the aqueous solution in the polymer network, and A is given bywhere x is the Flory-Huggins chi parameter for the interaction between the PAM network and the aqueous solution, N is the effective number of polymer chains between crosslinks per unit volume of polymer, and fl is the effective volume per mole of the solution.

[0059] The diffusivity D can be taken as a variable due to the large variations in the polymer network as a consequence of the swelling. Initially, when the solvent content in the hydrogel is small, the distance between polymer chains can be small, limiting the diffusion of the solution. In contrast, at larger swelling ratios, there is more volume available for diffusion, leading to a higher D. Specifically, the free volume theory can be followed to write:where Dois the intrinsic diffusivity of the aqueous solution, taken here as a fitting parameter, rsis the radius of the solute, rFVWis the radius of the available free volume per molecule in the aqueous solution inside the hydrogel, f is the mesh size of the polymer network, and cf)pis the volume fraction of polymer. Due to the swelling process, the hydrogel mesh size increases and the polymer volume fraction <ppdecreases. These two parameters can evolve with the stretches as is the mesh size of thehydrogel in its fully swollen, equilibrium state, and Amis the equilibrium stretch of the hydrogel, considered isotropic. The stretches A and d2are coupled by the condition of no in-plane stresses in the hydrogel:1 CH(4) = - A2dz. fl JQ

[0060] Equations 1 -4 can be solved to model the swelling dynamics of the above-discussed models. Details of the boundary conditions used and the determination of the material properties N,and x can be based on, for example, mechanical characterization and swelling experiments. From these material properties, the Flory-Huggins interaction parameters can be about 0.49458, about 0.48581, about 0.48483, about 0.49039, and about 0.49648 for concentrations of about 0%sat, about 25%sat, about 50%sat, about 75%sat, and about 100%sat, respectively. Additionally, x can be seen to have a non- monotonic dependence with the salt concentration highlighting the complex chemical interactions between water, salt ions, and polymer. For example, the interaction parameter can first decrease as the concentration increased showing a more favorable interaction between the polymer and the aqueous solution. This increased swelling in higher concentrations of LiCl in water has been previously attributed to the favorability of hydrogen bonding between the positive coordinated water molecules surrounding Li+ions 34 and the electron pairs in the dipolar amide oxygen group in polyacrylamide. When close to the saturation limit, however, the interaction parameter can increase, indicating a less energetically favorable interaction of the polymer with the concentrated salt solution, which can be a consequence of the favorability of hydration of the amide groups over cation binding to the polymers. Further, the large hydrated complexes formed around Li+34 can be entropically less favorable to mix with the polymer. These two factors can translate into the solution acting as a poorer solvent.

[0061] A thermogravimetric analysis (TGA) can be used to assess the salt content of the PAM hydrogels 30 after swelling to thermodynamic equilibrium in the different salt solutions. TGA is suitable for this task at least because the thermal decomposition of the individual components of the LiCl-loaded PAM samples can be clearly decoupled when increasing the temperature from room temperature up to about 800 °C. FIG. 2C illustrates that salt loading at the end of the swelling process as a function of salt concentration in the swelling solution can be measured via thermogravimetric analysis (open symbols) and predicted based on salt concentration (black crosses). As shown, the results can be displayed as a function of LiCl concentration in the swelling solutions, where the salt content can be defined as the mass of LiCl divided by the initial dry weight of the PAM sample. Overall, FIG. 2C illustrates that extreme salt loadings can be attainable and overlap with theoretical salt concentrations, as shown. For example, swelling the gel in the about 25 %sat, about 50%sat, about 75%sat, and about 100%sat solutions yields salt contents of about 10.9, about 20.6, about 23.6, and about 23.6, respectively, showing a plateau in sample salt content for swelling solutions with concentrations above about 75%sat. In equilibrium, given that the studied hydrogel 30 is not a polyelectrolyte, the salt concentration of the liquid inside the hydrogel can be assumed to be practically the same as the salt concentration of the surrounding swelling solution. Based on this assumption, the amount of LiCl that was loaded into the hydrogel can be computed based on the equilibrium swelling ratio. As shown in FIG. 2C, the deviation between measurement and prediction in the overlap can be less than about 2% for all concentrations. These high salt loadings can allow the hygroscopicity of the PAM- LiCLcomposites to approach the pure LiCl limit and reach up to about 96% of its uptake.

[0062] Performance of hygroscopic PAM-LiCl-composites

[0063] A function of a practical sorbent for use in the hygroscopic hydrogels of the present embodiments can be its ability to retain all, or substantially all, of the water that it captures to avoid salt leakage. For purposes of this disclosure, “substantially all” of the water being retained can refer to avoiding leakage, e.g., about 10% or less of the water leaking therefrom, about 5% or less of the water leaking therefrom, about 3% or less of the water leaking therefrom, about 2% or less of the water leaking therefrom, about 1% or less of the water leaking therefrom, about 0.5% or less of the water leaking therefrom, and / or no water leaking therefrom. Water sorption capabilities of the hygroscopic hydrogels 30 can be characterized by measuring their water uptake at different RH conditions using a dynamic vapor sorption(DVS) system. FIG. 3 A, for example, illustrates the uptake of PAM hydrogels after swelling to equilibrate in aqueous about 75%sat and about 25 %sat LiCl solutions (curves (F) and (G), respectively, and compares their performance to pure LiCl (curve H), pure PAM without salt (curve I), and PAM where about one (1) gram of LiCl per gram of acrylamide (AM) can be added before gelation via an alternative synthesis method (curve J). As shown, the PAM about 75%sat LiCl sample (F) shows an exceptional water uptake of about 1.85 g g1, 2.64 g g’ and 3.94 g g1at RH of about 30%, about 50%, and about 70%, respectively. These water uptakes can correspond to about 93% to about 96% of the respective uptakes of pure LiCl, highlighting the extreme salt loading achieved of more than about 20 grams of salt per g of polymer, which aligns with the results shown in FIG. 2C. Based on the data in FIG. 3 A, the water uptake of these hygroscopic, non-polyelectrolyte hydrogels can be predicted from the sum of the weighted water uptakes of its constituents (z.<?., pure LiCl and pure PAM). For the sample with the highest salt loading (PAM about 75%sat LiCl) (F), the deviation between measured uptake and prediction can be less than about 3% for all tested RH values. Based on these DVS measurements, sorption kinetics can be shown to be similar between pure LiCl (H) and the PAM about 75%sat LiCl (F) sample irrespective of RH.

[0064] Hydrogel swelling can present a simple solution to preventing leakage with no additional need of materials. In addition to achieving high water uptake, hygroscopic hydrogels 30 of the present embodiments can be designed to ensure that no leakage of the salt solution occurs, which is increasingly challenging for higher salt loadings of the hydrogel samples. For example, initially, dry lithium chloride and hydrogels swollen to equilibrium in about 25%sat and about 75%sat solutions can be placed in an environmental chamber at a controlled temperature of about 25 °C and a humidity of about 70% for about 48 hours. Due to the high humidity, the different hygroscopic materials can capture significant amounts of water, such as exceeding 1 kilogram of captured water per kg of material. Pure LiCl can deliquesce and spread out in the container, highlighting the potential leakage issues that hygroscopic salts can have in practical sorption applications. Similarly, the hydrogel swollen in an about 75 %sat solution can exhibit substantial leakage. In some embodiments, an amount of the leakage can be quantified by removing the hydrogel from the container and weighing the remaining solution, which, in some embodiments, may correspond to about 44% of the total mass in the sample container. The salinity of the leaked solution can be measured to be about 0. 1666 g of LiCl per gram of solution. This suggests that, along with water, there is a substantial amount of salt that may leak from the sample, highlighting the importance ofmitigating leakage. In contrast, a hydrogel prepared in an about 25%sat solution may not show any observable leakage, and the measured leaked liquid may be only about 1 % of all the salt solution present. It will be appreciated that the measured mass remaining after removing the sample from its container can be related to the retention of salt solution on the container surface because of detaching the sample from it. This analysis may suggest that leveraging the fundamental relationship between swelling (1), salt loading (2), water uptake (3), and leakage (4) in hygroscopic hydrogels can allow for enhancement of the uptake and prevention of leakage, while avoiding the need for encapsulation.

[0065] Leakage results can be assessed by comparing equilibria of LiCl with water at varying RH and salt concentrations. For example, at about 75 %sat, the aqueous solution may be at equilibrium with water vapor at about 21% RH, and a hygroscopic hydrogel with this concentration may capture more water if the ambient humidity is higher, which for hydrogels that have been swollen to their maximum volume, can cause the additional captured water to leak out of the hydrogel. In contrast, an about 25%sat solution is at equilibrium at about 75% RH, whereby a hydrogel containing aqueous solution with this concentration may experience a minor loss of water due to desorption at about a 70% RH condition and consequently show no leakage, e.g., substantially all of the aqueous salt solution can be retained.100661 The presently disclosed methods can synthesize the hydrogels 30 based on a maximum leakage-free relative humidity (RH) as a function of hydrogel uptake and swelling ratio, which allows for tuning the synthesized materials to a wide variety of applications that possess different RH-conditions. Tuning of a variety of parameters can improve water uptake and / or minimize leakage of the hygroscopic PAM-LiCl-composite 30. Some non-limiting examples of such parameters can include the type of polymer used, polymer content, type of aqueous salt solution, percentage of saturation of the salt in solution, crosslinking content, crosslinking architecture, and / or sorbent thickness. Each of the above-listed parameters can be optimized individually and / or in combination with one or more of the other parameters. In some embodiments, tuning of water uptake and / or minimizing leakage can occur independent of one or more of the above-listed parameters. For example, in some parameters, the hygroscopic hydrogels can provide superior water uptake and / or minimal leakage for any type of polymer used.

[0067] In some embodiments, the interaction between the polymer and the aqueous solution can be most favorable for intermediate salt concentrations. Moreover, equilibriumswelling ratios can be smaller at low salt concentrations and values that are close to saturation of the swelling solution. In contrast, the swelling kinetics can become increasingly slower as the salt concentration increases, due, at least in part, to a lower effective diffusivity of the increasingly viscous solution into the polymer. By optimizing one or more of salt concentration and temperature of the swelling solution, crosslinking density of the hydrogel, drying protocols, and / or environmental sorption conditions, a fundamental relationship can be established between (1) swelling, (2) salt loading, (3) water uptake, and (4) leakage in hygroscopic hydrogels 30. For example, tuning one or more of the salt concentration of the swelling solutions and / or the crosslinking properties of the gels can synthesize hygroscopic hydrogels having water uptakes relative to the mass of dry sorbent (salt and polymer) of about 1.79 g g1, about 2.58 g g1, and about 3.86 g g-1at a RH of about 30%, about 50%, and 7 about 0%, respectively. At about 30% RH, this exceeds previously reported water uptakes of metal-organic frameworks by over about 100% and of hydrogels by about 15%, bringing the uptake within about 93% of the fundamental limit achievable with the best performing hygroscopic salts, while still avoiding leakage of salt solution even at RH as high as about 70%.

[0068] Selection of appropriate salt concentrations in the aqueous solution when salt loading is important for synthesizing superior performing hydrogels of the present embodiments. For example, the salt concentration should be selected such that there is no leakage at the highest RH that the hydrogel will experience in its application, while also enabling an optimized salt loading. To simultaneously achieve these two goals, the following steps can be taken: 1) the salt concentration that is at equilibrium with the highest application RH can be chosen for the swelling of hydrogels; 2) dried hydrogels can be swollen in this aqueous salt solution; and / or 3) during swelling, the mass of the hydrogels can be monitored to ensure that the hydrogels reach equilibrium and, therefore, the highest salt loading. It should be noted that the salt concentration can be found with knowledge of the activity of the salt as a function of salt concentration and / or from an isotherm of the salt at different RH. The allowable RH during operation can be a function of the salt loading and the swelling ratio of the hydrogels, as shown in FIG. 3B, where the water uptake fraction relative to LiCl can be calculated assuming that the uptake is entirely due to the salt. Low RH applications can be able to achieve uptakes increasingly closer to those of pure LiCl as higher salt concentrations can be used without a risk of leakage. Moreover, for a given RH, the swelling ratio can be maximized to increase the water vapor uptake. That is, the uptake fraction maybe determined by an amount of salt loading of the hydrogel. For higher swelling ratios, for example, the hydrogel can soak in more salt solution, which during sorption can lead to higher uptakes. On the contrary, for lower RH, more salt can be loaded into the gel without causing leakage by using highly concentrated solutions. As a result, hydrogels with high swelling ratios can be able to capture more salt solution during swelling. In turn, the salt loading relative to the polymer can be higher and during sorption the water vapor uptake can be increasingly closer to that of LiCl. Practically, this can be achieved, for instance, by tuning polymer chemistry to favor polymer-solution interaction, as discussed above, and / or, as discussed in greater detail below, by freeze-drying or changing the hydrogel crosslinking density.

[0069] Maximizing sorbent water uptake and comparison to existing sorbents

[0070] FIG. 4A illustrates an exemplary embodiment of a freeze-dried hydrogel 50 that can be used to maximize uptake of the PAM-LiCl composite 30. For example, freeze-dried and half MBA samples can show an enhancement in equilibrium swelling ratio by about 11 % and about 37%, respectively. After an initially freeze-dried PAM disk swells with an aqueous about 25%sat LiCl solution to reach equilibrium to form a PAM-LiCl composite 60, microscopic pores 62 and large internal areas 64 can be formed in the samples, as shown in FIG. 4B, which can show a net positive effect. For example, while the pores 62 can trap bubbles during swelling, as shown, the pores can accelerate the swelling kinetics due, at least in part, to the reduced diffusion distances and / or the increased equilibrium swelling ratio as LiCl solution can be captured and retained inside the hydrophilic pores.

[0071] FIG. 4C illustrates water uptake at different relative humidities at about 25 °C of a freeze-dried (K) and half- M^’-methylenebisacrylamide (MBA) PAM samples (L) that can be swollen to reach equilibrium with an about 25%Sat LiCl solution, with both hydrogels exhibiting high water uptakes. These freeze-dried samples can exhibit high uptakes of about 1.43 g g1, about 1.77 g g1, about 2.55 g g1, about 3.84 g g1, and about 8.40 g g1at an RH of about 20%, about 30%, about 50%, about 70%, and about 90%, respectively, corresponding to about 91 to about 97% of the uptake of pure LiCl. The half MBA sample can exhibit even higher uptakes of about 1.45 g g ', about 1.79 g g ', about 2.58 g g], about 3.86 g g-1, and about 8.51 g g1at an RH of about 20%, about 30%, about 50%, about 70%, and about 90%, respectively, corresponding to about 93 to about 99% of the uptake of pure LiCl, consistent with the salt loading of about 16.6 g LiCl per gram of polymer measured via TGA. It will beappreciated that this salt loading may be about 52% higher than the loading of the oven-dried samples swollen in a solution with the same concentration as shown in FIGS. 2A-2C, highlighting the role of reduced crosslinking in enhancing the swelling ratio. Moreover, as shown in the inset (i) of FIG. 4C, up to a relative humidity of about 70%, there is negligible leakage of less than about 1 % that is resulting from the adhesion between the salt solution and the container surface during sample removal from the container.

[0072] The leakage-free equilibrium uptakes of the samples with reduced crosslinking can exceed all previous hydrogels and sorbents demonstrated in literature. At about 30% RH, the hydrogel of the present embodiments can provide an about 15% increase in uptake compared to all previously fabricated hygroscopic hydrogels, owing to the exceptional salt loading and high hygroscopicity of lithium chloride. Additionally, the uptake at about 30% RH can also exceed all previously synthesized sorbents, where the uptake can be over about two (2) times larger than the uptake of metal-organic frameworks (MOFs) at the same conditions, and over about four (4) times larger than the uptake of zeolites.

[0073] Methods

[0074] Hydrogel synthesis: Polyacrylamide (PAM) hydrogels can be synthesized based on a one-pot approach. At room temperature (about 20 °C to about 25 °C, including about 23 °C or about 24 °C in some instances herein), 100 g of deionized water can be mixed with about 8.36 g acrylamide (AM) monomer using a magnetic stirrer for about ten minutes until the monomer was fully dissolved. About 5 mg M,A’-methylenebisacrylamide (MBA) can be added as a crosslinker and about 14.2 mg ammonium persulfate (APS) as initiator while continuously stirring. Further, about 12 pL M,M-tetramethylethylendiamin (TEMED) serving as accelerator can be added and the pre-gel solution can be quickly filled into capped polypropylene vials (VWR, volume about 15 ml) to cure at room temperature. During that process, the amount of air within the vials can be minimized by filling the vials practically entirely. After about one day of curing, a PAM hydrogel can be formed that could be easily removed from the vials. Scissors can be used to cut thin disks 10 from the long, rod- like gel. The disks 10 can be stored in DIW until used for experiments. For the experiments, as shown in FIGS. 2A-2C discussed above, five disks (out of more than one hundred hydrogel disks) can be selected that possess similar swollen mass (within about ±2% deviation from the mean), which can translate into disks of practically identical geometry. Subsequently, the disks 10 can be dried for about three days at about 60 °C in an oven at an RH of about 0%.During the drying process, a spatula can be used to prevent the disks 10 from sticking to their substrate. As a result of drying, the disks 10 can lose more than about 95% of their mass with a dry weight of about 31 + about 1 mg (mean of the five disks + standard deviation) and can shrink substantially to form dried disks 20. After sample preparation, the disks 20 can be swollen at room temperature in aqueous LiCl solutions of varying concentrations, which we characterized based on percent saturation, where about 100%sat can represent a saturated aqueous LiCl solution that contains about 83.2 g of LiCl per 100 g of DIW at about 20 °C. Free swelling of the hydrogel disks 20 in pure DIW (0%sat) and in aqueous LiCl-solutions with about 25 %sat, about 50%sat, about 75%satand about 100%satcan then be performed to form the hygroscopic hydrogels 30 of the present embodiments . To track the swelling of the gels, the hygroscopic hydrogels 30 can be removed from the aqueous solutions.

[0075] An alternative approach can be used to synthesize the Lid-loaded PAM hydrogel 30. For the samples discussed with respect to FIG. 3A, about 8.36 g of LiCl can be mixed into about 100 g of deionized water leaving the solution cool down in a closed beaker.Subsequently, about 8.36 g AM, about 5 mg MBA, about 14.2 mg APS, and about 12 pL TEMED can be added. This can result in a sample that contains a moderate, well-controlled amount of about 1 gram of LiCl per gram of AM without the use of swelling in an LiCl solution, which can serve as a comparison to the regular samples. To synthesize PAM samples with reduced crosslinking, the standard recipe as outlined above can be used, with the only difference being the addition of only about half the amount of MBA (about 2.5 mg instead of about 5 mg). Further reducing the MBA content can then result in insufficient mechanical integrity of the swollen material.

[0076] Freeze-drying-. Freeze-drying can be performed using a Labconco 7934000 Lyophilizer. Before freeze-drying, the hydrogel samples can be frozen at about -80°C.

[0077] Scanning Electron Microscope Characterization: Scanning electron microcopy can be conducted with a Zeiss Merlin High-resolution SEM operated at about 1 kV. Prior to imaging, hydrogel samples can be freeze-dried and sputtered with an about 5 nm coating of gold.

[0078] Dynamic Vapor Sorption Experiments: The sorption isotherms can be characterized using the dynamic vapor sorption vacuum instrument (Surface Measurement Systems Ltd.). The samples can be initially dried at about 80 °C for about 5 hours, followed by a thermalequilibration at about 25 °C for about 3 hours. The overall dehydration process can be kept under a vacuum. Next, the mass of samples can be recorded at a series of RH steps (from about 0 to about 90%) by regulating the partial pressure of water vapor at about 25 °C. For sample equilibrium at each step of RH, an equilibrium criterion of about 0.005%SamPie min1, i.e. , a percentage change in sample mass per minute, can be used to ensure precise measurement. For measurements at about 90% RH, the measurements can be stopped at a maximum stage time of about 2000 minutes.

[0079] Thermogravimetric Analysis: A Thermal Analysis System TGA / DSC 3+ by Mettler Toledo can be used to measure the salt content of the hydrogels as well as the salinity of the leaked solution. Samples weighing about 10 mg can be placed into an inert alumina pan under nitrogen gas. At a slow rate of about 1 K min1, the samples can be heated up to about 800 °C.

[0080] Statistical Analysis: To compute the swelling ratios, the measured sample weights during swelling can be divided by the dry weight of the salt-free PAM before swelling. To compute the water uptake from the DVS measurements, the dry weight of the salt-containing PAM before sorption can be subtracted from the measured sample weights during sorption. The resulting value can be divided by the dry weight of the salt-containing PAM before sorption to obtain the water uptake in g per g of dry sorbent (PAM + LiCl).

[0081] System and Device for harvesting water

[0082] The hygroscopic hydrogels 30 of the present embodiments can be used in a variety of applications. FIG. 5 illustrates a water-harvesting system can include two or more sorbent layers comprising the hydroscopic agent such as disclosed herein and a condenser.Optionally, a second condenser can be located between sorbent layers. The sorbent layers and the condenser can be contained in a housing (not shown). An energy absorber, or other heat source, drives the desorption process in the device. Gaps can be present between the sorbent layer and adjacent condenser. A gap can also be present between the energy absorber and the first sorbent layer. The gap can be filled with air, and inert gas, or a vacuum. Gas spaces, present between sorbent layers and condensers, serve two roles during a water vapor adsorption cycle and a water vapor desorption cycle. During a water vapor adsorption cycle, the gas space contains water vapor, supplied from the ambient environment (i.e., the environment external to the system), that is adsorbed by the adsorbent layer. The gas space ineach stage also can serve to provide a necessary temperature gradient between each adsorbent layer and its condenser. The desorption process occurs when this temperature gradient is maintained. An access region can provide access to water vapor (for example, naturally- occurring water vapor in air) by allowing gas exchange between gas space and the ambient environment, such as the atmosphere. Alternatively, simply exposing an adsorbent layer to air will be sufficient for vapor adsorption from air. That is taking out sorbent layers from the housing for vapor adsorption from air and installing back for desorption. For example, the access region can be a door that opens to the housing or other opening. During a water vapor desorption cycle, the access region is close to the ambient environment. Irradiation of sorbent layers, for example, by sunlight, can cause the adsorbed water to be released from the sorbent layers and the condensers then condense the water vapor, creating liquid water that can be collected.

[0083] More than two sorbent layers can be used, particularly if heat source temperature is high enough. For example, three stages could be used.

[0084] As described herein, during the adsorption step the device can be disassembled and the sorbent layers removed to allow fresh air to flow to the adsorbent. In another embodiment, a door can open (via either human interference or actuated) to open the device to the ambient during the adsorption step. In still other embodiments, the processes can occur continuously in parallel and cyclically. On one half of the device, the solar absorber can be covered and a door can open to allow air to flow to the adsorbent for adsorption. Simultaneously, another section of the device could be in the desorption state with its solar absorber exposed to the sun and its adsorbent isolated from the ambient. The two modes can switch to allow continuous and cyclic water harvesting.

[0085] In another embodiment, two sorbent layers can be used, and the adsorption and desorption step can occur in series. The adsorption process is much slower than the desorption process. A future iteration can use several extra adsorbent layers which are waiting “charged” in the fully adsorbed state. They can be desorbed in the device and then changed out for charged layers multiple times in a daily cycle.

[0086] As described herein, solar thermal energy can be used as the heat input to the top sorbent layer. Other approaches can utilize heat from other sources including biomass gasification, combustion, electrically powered joule heating (electricity can be generatedfrom a solar panel or other sources), or other forms of waste heat. Additionally, optical concentration via mirrors or lenses can be used to increase the temperature of solar heat input. In certain circumstances, the temperature of the adsorbent layer can reach about 90 degrees C. to efficiently desorb water.

[0087] As described herein, fans can be used to circulate the air under the condenser fins and reject the heat. Natural convection can be sufficient but is less reliable especially in the unfavorable downward orientation of the condenser fins. In other embodiments, the device can be flipped over so that heat sink fins are on the top of the device which would increase the convective heat transfer coefficient and eliminate the need for the fans. If direct solar heating is used as the heat source, mirrors could be used to direct the sunlight to the solar absorber at the bottom of the device. If other heat sources are used, heat application can be more trivial.

[0088] As an alternative to fins, other cooling mechanisms can be used. For example, greater heat transfer coefficient could be achieved by immersing the fins in water (perhaps the water that was harvested) or by burying them in the dirt which would eliminate the need for the fan. Alternatively, active cooling at the condenser can be achieved via vapor compression refrigeration, evaporative cooling, or thermoelectric cooling.

[0089] In a larger scale version of this device, more practical approaches can be taken to operating on a continuous cycle with fully active systems. During the adsorption step, fans can be used to bring fresh air to the adsorbent. Then the device would be closed and isolated from the ambient surroundings. During the desorption step, a heat exchanger can exchange heat from a heat source with a heat transfer fluid which would get transported through pipes to a fin / tube heat exchanger. The adsorbent can be coated on the fins of the heat exchanger. Fans can then transport the desorbed vapor to a thermally isolated condenser region which would then reject its condensation heat to a second adsorption bed.

[0090] The housing can be made from metal, plastic, glass, or combinations thereof. Condensers or a condenser surface can be made from a conductive metal, such as aluminum, steel, or copper. A cover to reduce convective heat loss can be glass, plexiglass, plastic wrap, clear acrylic, optically transparent aerogel. When the heat source is a solar absorber, it can be a selective surface (highly absorbing in solar spectrum with low emission in thermal spectrum), black paint, graphite coating, or Pyromark paint. For example, the solar absorbercan be a conductive metal, such as aluminum, steel, or copper, that is coated with solar absorbing paint / spray (e.g., graphite coating or Pyromark paint).

[0091] The adsorbent material can have an affinity to adsorb water vapor and can also absorb a portion of sunlight. In certain circumstances, the adsorbent layer can be a porous adsorbent layer. The adsorbent material can be a zeolite, molecular sieve, metal organic framework (MOF), silica gel, activated carbon, a hygroscopic salt, a hydrogel, or mixtures thereof. The adsorbent material can be held by a binder, for example, a thermally conductive binder. Examples of thermally conductive binders can include metallic foams (aluminum, copper, nickel) or graphite.

[0092] The condenser can be a finned heat sink that dissipates heat to the ambient surrounding. The condenser surface can be sheet metal, for example, copper or aluminum. The condenser can include fins, for example, aluminum or copper extruded fins or pin fins.

[0093] The device can be insulated to control the internal temperature during the desorption cycle. The insulation can be polyisocyanurate, Styrofoam, mineral wool, polyurethane, or polystyrene.

[0094] The timing of the water vapor desorption cycle and water vapor adsorption cycle can vary. For example, the water vapor desorption cycle can be during daylight hours and the water vapor adsorption cycle can be during nighttime hours. In this circumstance, the access region can allow gas exchange between naturally-occurring water vapor in air or atmosphere surrounding the housing by opening at or near sundown for the adsorption cycle. The access region can then close for the desorption cycle at or near sunrise. As an alternative, the opening and closing of access region can change based on the amount of sunlight impinging on a surface of the device. In this circumstance access region may close during daylight hours if there is cloud cover that reduces the amount of sunlight contacting the adsorbent layer. In another embodiment, the timing of the cycle is independent of the time of day. In certain circumstances, the timing of the cycle can be irregular. In other circumstances, the timing of the cycle can be periodic on a daily or a semi daily basis. For example, the cycle can be 0.5 hour adsorption and 0.5 hour desorption — 24 cycles per day, 1 hour adsorption and 1 hour desorption — 12 cycles per day, or 2 hours adsorption and 2 hours desorption — 6 cycles per day.

[0095] Here, a dual-stage AWH device was introduced as a strategy to improve the water productivity (LIVID) [L / m2 / day, defined as liters of water produced per solar absorbing area per day] of single daily cycle devices. This device operates by using two adsorbent layers under a single solar absorber for device heat input and a single condenser for device heat rejection. While the slow kinetics of the adsorption process has limited the LIVID of solarthermal devices, the dual-stage design uses two optimized layers to achieve higher LMD. The latent heat of condensation from the top stage is recycled and used to assist in desorption of the bottom stage. While thermal efficiency can also be improved with this approach (as also seen in passive multi-stage solar distillation), the most relevant metric for single daily cycle devices is daily water productivity. LMD is more important because optimizing single daily cycle devices for efficiency does not result in maximum production due to the dependence of desorption rate on the concentration of water vapor in the adsorbent. This metric was a focus given the abundance of solar energy, the potential for its use in AWH devices, and the opportunity to increase productivity by improvements to device design.

[0096] Modeling results are presented, which indicate the dual-stage device can improve productivity provided the maximum temperature in the system (temperature of the top stage) is sufficiently high for a given material. Using the results of the model to guide the design, a dual-stage AWH device was fabricated and tested in outdoor experiments in which water was harvested from both stages and condensation occurred at ambient temperatures. By using a material with a step-wise adsorption isotherm and low desorption temperature, water was released from both stages of the device using unconcentrated sunlight by utilizing the full temperature gradient which develops between the solar absorber and the condenser of the bottom stage.

[0097] The device design was exemplified with a commercially available adsorbent material (AQSOA Z01 zeolite), indicating that performance improvements and scalability can be simultaneously achieved using the dual-stage design. Experiments showed 18% greater daily productivity over our single-stage device. The experimentally validated model was used to show that even in arid conditions of 35% RH and 25° C., the dual-stage configuration promises approximately twice the LMD of the single-stage configuration by increasing the temperature of the top stage. The work described herein shows that by using a commercially available adsorbent there is a pathway for scalable solar-thermal AWH devices. The configuration can be used with other adsorbents with more favorable properties, such asa higher uptake and lower energy required for regeneration, to further increase device performance.

[0098] The dual-stage concept (FIG. 6A) consists of two adsorbent layers separated by air gaps which create a temperature gradient between the layers. Each layer is composed of the adsorbent material packed inside a porous metal foam, creating a composite layer with enhanced thermal conductivity. The device operates on a daily cycle because it is designed to be regenerated passively by sunlight. During the night, the device is opened, and the two adsorbent layers are exposed to the ambient atmosphere. Water vapor molecules diffuse into the layers and into the pores of the adsorbent. During the day, the adsorbent layers are placed in the device and heated from the top via sunlight received at a solar absorber. A transparent convection cover protects the solar absorber to decrease convective heat loss to the ambient environment. The device is well -insulated at the side walls so thermal energy conducts from the solar absorber to the top adsorbent layer, through the device, and to the condenser. For both stages, when the temperature of the adsorbent rises, vapor desorption is driven by the temperature gradient between the adsorbent layer and its corresponding condenser due to the low thermal conductivity of air in the gap (~0.027 W / m / K). In the top stage, vapor is condensed on a copper surface in thermal contact with the bottom adsorbent layer, so the latent heat of condensation is recovered and conducted to the bottom stage. Simultaneously, vapor desorbs from the bottom layer due to the temperature gradient between the bottom layer and its corresponding condenser. Water is collected at each condenser and drained out of the device.

[0099] Water vapor diffuses into the adsorption layers, filling the pores of the adsorbent when the device is exposed to ambient air during the night. During the day, the device is closed and heated from the top by sunlight received at a solar absorber (2). The temperature and concentration difference which develops between each adsorbent layer (3) and its condenser (4) drives desorption. Desorption of the bottom layer is driven by both the latent heat of condensation from the top stage as well as heat conducted through the device. Referring to FIG. 6A, details of the dual-stage atmospheric water harvesting device prototype are shown. The device consists of two stages (two adsorbent layers separated by air gaps). The back side of the top adsorbent layer is in thermal contact with the solar absorber which is insulated by the transparent convection cover (1). The bottom adsorbent layer is in thermal contact with the condensing surface for the top stage. The condenser for the bottom stageconsists of an array of fins (5) cooled by small fans (6) (2.4 W total). The sides of the device can be well-insulated (7). The insulation can be a foam or fiber material.

[0100] Device Design and Modeling

[0101] AQSOA Z01 was selected for both modeling and prototype demonstration. AQSOA Z01 is a microporous iron aluminophosphate (framework consisting of AIO4, PO4, and FeO4 tetrahedrons) AFI-type zeolite

[0032] . AQSOA Z01 was selected because it is available commercially in large quantities, hydrothermally stable, its adsorption enthalpy ('56 kJ / mol) and desorption temperature (~60° C. at a condenser temperature of '25° C.) are relatively low compared to conventional zeolites, and it exhibits relatively fast vapor kinetics [33, 34]. Additionally, AQSOA Z01 has a favorable stepwise isotherm indicative of uniform pore sizes and leading to a sharp increase in vapor uptake over a small change in RH. This sharp, stepwise isotherm shape and the shift of the step to higher RH with temperature also lead to lower energy requirements for regeneration. The adsorption isotherm was measured in a NVvapor environment (at atmospheric pressure) using a sorption analyzer (Q5000, TA), and the equilibrium uptake as a function of RH is shown at 25° C. and 65° C. The isotherms measured in a N2 / vapor environment are representative of conditions for water harvesting from air. Isotherms were also measured in a pure vapor environment (DVS Vacuum, Surface Measurement Systems Ltd.) at a range of temperatures from 25° C. to 90° C. The higher temperature isotherms enabled predictions of the dual-stage device performance at temperatures expected by solar heating. The N 2 / vapor isotherms were used for all calculations of the kinetics and the adsorption enthalpy.

[0102] From the isotherm, the intracrystalline diffusion coefficients (diffusion of vapor inside the adsorbent crystals) were extracted at different temperatures and calculated the adsorption enthalpy. For desorption to occur, the thermal energy input needs to be high enough to both reach the desorption temperature as indicated by the adsorption isotherm and overcome the adsorption enthalpy which is endothermic during desorption. The crystal size varies, but literature values range from 5-8 pm, which matched the observed size of our sample (FIG. 6 A)

[0034] . From a device perspective, when a material with a stepwise isotherm is used, the temperature difference between each adsorbent layer and its condenser must be sufficient to bring the equilibrium uptake below the steep step in the isotherm. The greater the temperature difference that exists between the adsorbent layer and condenser, the larger the driving force there is for desorption. Based on the isotherms for our AQSOA Z01 sample(FIG. 6B), desorption can occur at ~60° C. if the condenser is at ~25° C., and at ~90° C. if the condenser is at ~60° C., although desorption rate increases with the temperature difference between the adsorbent and the condenser. The required temperature differences (~30° C.) and the temperatures achievable by solar-thermal energy without concentration ('100° C.) limit the focus of this work to a maximum of two stages.|00103| A model of the device was developed using the measured AQSOA Z01 material properties to guide design decisions and predict the performance improvements which can be achieved using the dual-stage configuration. This model framework leverages the developed and experimentally validated modeling approach we have used previously to describe heat and mass transport in composite adsorbent layers [11, 16, 35, 36]. The model equations and boundary conditions are described below. The governing mass transfer equation includes vapor diffusion in the adsorbent layers and air gaps and the linear driving force model for vapor diffusion inside the adsorbent crystals. The governing heat transfer equation captures the latent heat due to the adsorption enthalpy, transient heat conduction in the adsorbent layers, and radiative heat transfer across the air gaps. The heat flux boundary conditions include the solar energy absorbed at the solar absorber, heat loss from the top and sidewalls of the device, the heat rejection at the finned condenser, and the latent heat of condensation at each condenser surface. Calculations for the combined top heat transfer coefficient, Ut, which include convective and radiative heat loss from a selective solar absorber at the top of the device to the environment can be found below.

[0104] FIG. 7 shows the geometry that was optimized using the model — the layer thicknesses (Li and L2) and the air gap thicknesses (Lviand LV2). The adsorption process was modeled in the layers to optimize the layer thickness and packing porosity to balance the metrics of LMD and L / kg / day. Using our model, we showed that increasing the adsorbent layer thickness in a single-stage configuration to increase the daily capacity is not effective due to kinetic limitations. For daily cycle operation, there is an upper limit on layer thickness which justifies the choice of the dual-stage design over the single-stage because the device capacity is increased without influencing the time required for adsorption. The optimization was performed considering adsorption conditions in an arid environment (35% RH and 25° C.), where AWH using dewing is infeasible. Based on the optimization, LI=L2=6.4 mm and 8=0.7 were selected.

[0105] To guide the initial device design, we determined the sensitivity of water harvested to the air gap size of each stageand selected a value of LVI=LV2=20 mm, for which the maximum water productivity was ~1.2 L / m2 / day for the prototype at 35% RH and 25° C. This analysis was performed with Ut=4.5 W7m2 / K, representing a lower, but achievable

[0037] , bound on heat loss (discussed below).|00106| The model predicted both LMD and thermal efficiency (which is proportional to LMD when calculated over the same desorption period) of a dual-stage device in comparison to a single-stage device. While the LMD is the key metric for our prototype, for completeness, the thermal efficiency was also calculated. For other applications of the dualstage concept such as in cyclic systems, it is important to consider efficiency improvements. For the single-stage device, the same modeling approach described below was used but considered just one adsorbent layer and one air gap. The thermal efficiency was calculated as,where mwater is the mass of water harvested, hfgis the latent heat of condensation, and Qinis the solar energy received by the solar absorber from the time when the device is first exposed to sunlight to the time when water harvesting stops. For both the dual-stage and single-stage simulations, we assumed a constant solar flux=1000 W / m2(modified by a solar absorptance=95% and convection cover transmittance=92%), and e=0.7. We used the above optimized values of Li, L2, Lvi, and LV2. We assumed the layers were initially equilibrated at 35% RH and 25° C. prior to desorption to represent arid climate conditions. The convective boundary condition at the finned condenser assumed a fin area ratio Ar=20 and a natural convection heat transfer coefficient h=2.5 W / m2 / K. FIG. 6B shows the LMD and efficiency as a function of the maximum solar absorber temperature, TH, assuming a desorption period of 4 hours. The efficiency was calculated over this entire period which includes the sensible energy required for the adsorbent layers to reach their desorption temperature. TH was varied by parametrically changing Ut, which is the heat transfer coefficient from the solar absorber to the environment on the top side of the device. Note that Ut does not include the heat transfer from the back side of the adsorbent layer and through the device.

[0107] The LMD and efficiency of both devices have the same curve shape in which LMD and efficiency increase with TH as more water is harvested, until reaching a plateau when complete desorption of both layers is achieved in the 4 hour period. Increasing TH increasesthe desorption rate (Equation 5), which increases the water harvesting rate and is necessary to bring the adsorbent above its desorption temperature. It was shown that TH must be sufficiently high (~90° C.) before the dual-stage device can achieve greater LMD and efficiency than the single-stage device. This temperature value or even higher temperatures are achievable by engineering the solar absorber using spectrally selective surfaces or optically transparent aerogels [37, 38].

[0108] The modeling was used to guide the choice of adsorbent layer thickness (LI=L2=6.4 mm), air gap thickness (LVI=LV2=20 mm) and packing porosity (s=0.7) for the experiments. Using AQSOA Z01, the dual-stage device has the potential to achieve greater performance provided that TH is at least ~90° C. Using these design guidelines, the prototype was fabricated with details of the composite adsorbent layer. The layers were fabricated by brazing nickel foam to copper sheet metal, and we infdtrated the AQSOA Z01 into the nickel foam. Note that due to challenges in fabricating the large adsorbent layers, the two layers were slightly different (FIG. 8B), where the adsorbent mass in the top layer was 305 g ( s=0.65) and the bottom was 215 g (e=0.75). A selective solar absorber (Alanod eta plus®) with measured absorptance=93% (weighted by the AM 1.5 solar spectrum) and infrared emittance=4.2% (weighted by the blackbody spectrum at 90° C.) converted incident solar radiation to heat which was conducted to the top adsorbent layer. A glass cover (thickness '3 mm, transmittance=92%) was placed on top of the solar absorber to reduce convective heat loss.

[0109] The equilibrium adsorption isotherms of AQSOA Z01 in a N 2 / vapor environment were measured at 25° C. and 65° C. The shift of the isotherm step to higher RH with temperature reduces the temperature difference between the adsorbent and condenser required for desorption. A scanning electron microscope (SEM) image of AQSOA Z01 shows a crystal size of 5-8 pm. Referring to FIG. 6A, a schematic of the device shows the dimensions of the dual-stage device which were optimized by the model. Li and L2 are the thicknesses of the top and bottom adsorbent layers, respectively. Li and L2 are the sizes of the top and bottom air gaps, respectively. FIG. 6B depicts model predictions for LMD and efficiency of the dual-stage and single-stage devices as a function of the maximum temperature of the solar absorber, TH, and assuming four hours of exposure to sunlight. The simulation assumed overnight adsorption in arid conditions (35% RH and 25° C.). Themaximum TH shown for each device also corresponds to Ut=4.5 W / m2 / K, which we consider to be a lower bound on achievable heat transfer coefficients (see below).

[0110] The prototype device consisted of two stacked stages. Condensed water drained from tubes at the bottom of each stage into separate graduated cylinders for collection. An image of the inside of the device shows an adsorbent layer, condenser surface, and the thermocouples for temperature measurements. FIG. 7 shows a graph shows the measured optical transmittance (T) and reflectance (R) spectra characterized by UV-Vis-NIR spectrophotometer and FT-IR spectrophotometer of the solar absorber and the glass convection cover. The measured solar absorptance was 93% and the thermal emittance at 90° C. was 4.2%. The AM 1.5 solar spectrum and the blackbody spectrum at 90° C. are plotted for reference. The blackbody spectrum intensity is not to scale with respect to the solar spectrum.

[0111] Experimental Characterization of the Device

[0112] The device was tested on the rooftop of MIT (Cambridge, Mass.) for outdoor water harvesting experiments. During the adsorption period, the layers were placed outside overnight under a tarp which protected them from the environment but allowed fresh air to flow to the adsorbent. Data from a weather station adjacent to the experiment were used to determine the overnight relative humidity and temperature. Although the outdoor humidity conditions were high (average overnight 68% RH) due to the test location, the adsorbent layer thickness was selected to allow complete overnight adsorption in 35% RH conditions. For the daytime desorption period, we placed the layers in the device. Using adjustable rails, the tilt angle of the device was set with respect to the horizontal to the zenith angle of the sun)(~27° at solar noon ('12:50 pm) during the experiments, which ensured the solar flux was normal to the absorber at solar noon. This angle also allowed condensed water to flow from the copper condenser sheets to the water collection tubes. An umbrella was used to cover the device to minimize solar heating during assembly in the morning and before the beginning of data collection. Throughout the experiment, the temperature of the adsorbent layer was measured by imbedding two thermocouples (K-Type) in each layer, which were held in place by heat sink compound. Two thermocouples were attached to each condenser surface, and a single thermocouple to the solar absorber surface. The ambient temperature (shielded from the sun) was measured and a pyranometer (LP-02, Hukseflux) was used, mounted at the same angle as the solar absorber, to measure the incident solar flux. Water harvested by each stage drained through a plastic tube and was collected in a graduated cylinder which was coveredto prevent evaporation throughout the experiment. The signals from the thermocouples and pyranometer were measured by data acquisition equipment (34972A, Agilent) connected to a laptop. While modeling suggested a natural convection heat transfer coefficient h=2.5 W / m2 / K was needed to sufficiently reject the heat from the finned condenser (which had Ar~20), this could not be reliably achieved during the experiment due to the stagnation of air under the unfavorable downward orientation of the fins. Therefore, a DC power supply was used to provide power to fans under the finned condenser (2.4 W total). The adsorbent layers were placed outside in average overnight temperature and RH conditions of 20° C. and 68% RH, respectively. The mass of the layer was recorded immediately before assembling the device. The temperature measurements of the adsorbent layers, condensers, solar absorber, and ambient as well as the solar flux measured by the pyranometer throughout the experiment. The two thermocouple measurements were averaged for each layer and each condenser surface.

[0113] The desorption experiment started at 9:00 ET when the device was exposed to sunlight. The temperature of the solar absorber, absorbent layers, and condensers increased after solar exposure. Initially (9:00-10:00), the temperatures showed a linear increase with respect to time. Then, as desorption started after 10:00, the rate of temperature increase began to decrease. As a result, the solar absorber temperature stabilized above 90° C. from 12:15 to 14:30 with a maximum TH=94° C. occurring at 13: 15. However, peak solar flux (~1000 W / m2) conditions occurred between 11:30 and 12:50. The lag between absorber temperature and solar flux was due to the specific heat capacity of the layers / device and the latent heat of desorption. In general, the temperature differences between the solar absorber and the top absorbent layer (0.68° C.) as well as between the top condenser and the bottom absorbent layer (1.7° C.) were small, indicating good thermal contacts. The temperature difference between the solar absorber and the top adsorbent layer was slightly larger while desorption was occurring (due to the latent heat of desorption in the layer) than at the start and end of the experiment when desorption ceased. Likewise, there was also a larger temperature difference between the top condenser and bottom adsorbent while the bottom layer was desorbing.Besides a high solar absorber temperature, it is also important to maintain a low finned condenser temperature to establish a large temperature gradient throughout the device. In our device, the bottom condenser sufficiently rejected heat from the device, as the temperature remained less than 4° C. above the average ambient temperature. The maximum bottomadsorbent temperature was 63° C. and maximum ambient temperature was 33° C. during the experiment.

[0114] During the experiment, water collection was observed in the beakers between ~10: 15 and '14:30. Water collected from both stages throughout the experiment was measured. In total, '60 mL of water was collected with '40 mL from the top stage and '20 mL from the bottom stage. Considering the 0.078 m2area of the solar absorber, this translates to ~0.77 L / m2 / day. This water collection is a significant improvement over prior work in which ~0.34 L / m2 / day and ~0.75 mL of water was collected

[0011] . Using Equation A5, the efficiency of water collection was calculated, where Qinnow included the solar energy received by the device as well as the power consumption of the fan. The electrical power consumption of the fan (2.4 W electric) was converted back to thermal energy assuming it was powered by a solar panel at 20% conversion efficiency

[0039] . We then added this to the time-varying solar flux measured for the 5.5 hours between the start of the experiment (9:02) and the end of water collection (14:30). The calculated thermal efficiency was ~9%, which included the fan power (i.e., ~10.6% without the inclusion of fan power). Results of another experiment which was performed approximately one month earlier indicated good repeatability between experiments without degradation of uptake by the adsorbent layer. Additionally, cyclic adsorption stability studies on AQSOA Z01 showed no significant degradation in uptake after many cycles. Water quality measurements indicated that the primary contaminants in the harvested water were due to metals present in the device components, not the adsorbent itself, indicating that these issues can be resolved by selecting different component materials. To compare to the dual-stage device, experiments were also completed using a single-stage configuration which was constructed by using the top adsorbent layer (with integrated solar absorber) and the finned condenser. In this single-stage experiment, ~51 mL ('0.65 L / m2 / day) and TH=78° C. was harvested. Considering the time-varying solar flux measured over the 4 hours between the start of the experiment (9:50) and the end of water collection (13:50) as well as the fan power, the thermal efficiency was '9.5% (without including the fan power, it was ~11.6%). In the following section, reasons why these efficiencies were approximately the same and show modeling results that allow comparison between the two devices under the same conditions are discussed.

[0115] The experimental results are summarized below. The mass of the adsorbent layers was measured immediately before and after the experiment (Table 3 of FIG. 8C), and thedifference in mass was the desorbed water (Table 4 of FIG. 8D). The desorbed water was compared to the condensed water collected in the cylinders and from residual droplets in the device to determine the condensation recovery. The condensation recovery for both experiments was high (98.7-99.2%), indicating successful condensation of the desorbed water and minimal vapor leakage. The percentage of water condensed and recovered from the adsorbent layers was also calculated. Although the dual-stage device had a greater LMD, the water recovery of the single-stage was higher than the dual-stage (88% and 54.7%, respectively) due to more complete desorption by the single-stage device.

[0116] Referring to experiment results, including real-time temperatures of the device components and solar flux evolution, were measured. The solar flux reached its maximum at 12:00 while peak temperature of the device occurred at 13:00. The time delay between the two peaks is due to the heat capacity of the device and the latent heat of desorption. Time- lapsed photos recording the amount of water collected in the graduate cylinders are shown. 40 mL was collected from the top stage and ~20 mL was collected from the bottom stage (including residuals left on the interior surfaces of the device which were collected using a cloth).

[0117] Using the model, including the time-varying solar flux during the experiment as an input, we compared the measured temperature of each adsorbent layer and the bottom condenser to those predicted by the model for the dual-stage device. Based on the calculations presented below, an average top heat transfer coefficient of Ut~9 W / m2 / K (Ut varied with temperature throughout the experiment) was used. Good agreement between our experiments and model were shown, as the temperatures differed by less than 6° C. The discrepancy was attributed to more heat loss between the stages of the device by convection in the air gaps or conduction through the side walls. The water harvested during the experiment compared to the model predictions. The water harvested at different times during the experiment was estimated from time-lapse images. The rate of water harvesting (rate at which water condensed at each stage) could not be accurately measured in real time because water droplets did not immediately drain from the device due to surface tension forces on the copper condensing surfaces. Nevertheless, the time lapse photos provided an estimate for the rate of water harvesting. The model predicted a total of 66 mL of water harvested, or 0.85 L / m2 / day. The model over-predicted the water harvested by the top layer due to the over- predicted temperature difference between the top adsorbent layer and its condenser andunder-predicted the water harvested by the bottom layer due to its under-predicted temperature difference.

[0118] Although the dual-stage device produced 18% more water than the single stage, two factors contributed to the comparable efficiencies. The TH of the dual and single-stage experiments were 94° C. and 78° C., respectively. According to the general modeling in FIG. 6B, the dual-stage device was expected slightly greater efficiency and LMD for a given desorption period. However, the desorption time for the single-stage device was observed to be 4 hours and that for the dual-stage device was 5.5 hours (where these times included device sensible heating). When factoring in these time differences to the efficiency, the efficiencies became comparable. Additionally, the heat capacity of the dual-stage device was larger than the single-stage because a greater mass of materials and surrounding components were required, and sensible heating was included in the efficiency calculation. While the general model in FIG. 6B did account for the difference in heat capacity between the layers, it did not include the device-specific heat capacity which includes the surrounding components (which was determined by obtaining a fit to the slope of the adsorbent layer temperature).

[0119] Furthermore, it can be difficult to draw a direct comparison between the two device experiments for several reasons. First, the experiments were performed on different days under different solar flux conditions and overnight adsorption occurred at different RH and temperature. The total mass of adsorbent in the dual-stage experiment was consistent with the dual-stage model in FIG. 6B which assumed £=0.7 for both layers. However, as previously noted, a top layer with more mass was used in the single-stage experiment which artificially increased its performance for the purpose of direct comparison.

[0120] To better compare the two devices, both were modeled under the conditions observed in the dual-stage experiment. To draw comparisons to the experimental performance of the prototype, the modeling was performed using the device specific heat capacity, as well as the overnight adsorption RH, temperature, and time-varying solar flux measured in the dual-stage experiment. The single-stage device used a single optimized adsorbent layer with the same characteristics as an individual layer in the dual-stage device (s=0.7 and L=6.4 mm). The devices were modeled with the same convective boundary condition at the finned condenser (Ar=20 and h=2.5 W / m2 / K). A tradeoff exists between the maximum LMD and efficiency due to the greater time required to achieve complete desorption than the time at which peak efficiency is reached. This result is because the waterharvesting rate depends on the concentration in the adsorbent (Equation S5), and accordingly, the water harvested per energy input is much lower towards the end of the desorption period. Furthermore, when comparing the two devices, the dual-stage has a higher heat capacity and higher required TH than the single-stage, which leads to a greater sensible heating requirement. As noted earlier though, it is believed that LMD is the more important metric for the device discussed in this work because our solar-thermal driven device operates on a single daily cycle.

[0121] As a result, the potential to increase the LMD was investigated by design modifications to reduce the average Ut (heat loss from the solar absorber to the environment) and consequently increase TH. Comparing the device- specific model to the general model in FIG. 2D, it was seen that the two show consistent behavior so the LMD was slightly lower. For the calculated Utfor the device ('9 W / m2 / K), the LMD predicted by the model agreed well with the experiment, noting the agreement was even better at the experimental TH=94° C. which suggests small differences between the model and experiment are actually due to more heat loss from one stage to the next (radiative, convective, or conduction through the side walls) rather than from the solar absorber to the environment. By reducing Utby 50% from the experiment, the maximum water harvesting potential of the device at this humidity ("1.3 L / m2 / day) was approached and almost twice the LMD of the single-stage device can be achieved. For the same Ut, the dual-stage device can reach a higher TH because the bottom adsorbent layer acts as a more insulating boundary condition than the colder finned condenser. Thus, there is greater potential for the dual-stage device to increase LMD by increasing TH, and as the temperature difference between the stages increases. Other heat sources can be used to achieve higher TH such as combustion, gasification, or waste heat [3, 4].

[0122] Besides design modifications to reduce Ut and increase TH, reduction in the heat capacity can reduce the sensible energy required for the device to reach its desorption temperature. The dual-stage device heat capacity was greater than the single-stage which allowed the single-stage to reach steady-state conditions more-quickly. This effect negatively impacted the performance of the dual-stage device but can be modified by different materials selection. For example, thinner copper condenser surfaces and aluminum foam (which is "A the density of nickel) can be used.

[0123] The dual-stage system was demonstrated using a commercially available material which shows progress towards the scalability of solar-driven AWH devices. Greater LMD can be achieved using higher-performance materials which have a greater uptake while maintaining low energy requirements for regeneration and fast adsorption kinetics. Future studies should examine the performance in the dual- stage configuration. For the evaluation of materials for this application, the intracrystalline kinetics are needed to determine any ratedependent metric because adsorption / desorption rate and optimal layer thickness and packing porosity are all dependent on the kinetics. Two different adsorbent materials can be used with optimized properties for the temperature requirements of each stage as is typical in cascaded adsorption systems. While the experiments were performed at MIT (Cambridge, Mass.) where the humidity was high (we achieved similar performance from our two dual-stage experiments in which adsorption occurred at 57% RH and 68% RH), the isotherm characteristics of AQSOA Z01 indicate that it can perform similarly in lower humidity environments. Both the modeling in FIG. 6B as well as the optimization of the adsorbent layer, in which the adsorption kinetics were considered, were performed at 35% RH.Comparing results to the modeling at high humidity in the experiments indicates that similar results can be achievable with this material in climates with low RH.

[0124] There are various tradeoffs between performance, cost, and practicality to consider in the design of AWH systems. Different metrics (LIVID, L / kg / day, thermal efficiency, $ / L) become relevant based on cost considerations, the type (passive solar-thermal vs cyclic), and the application of the device. Because of the focus on productivity based on LIVID and scalability based on using a commercially available material, L / kg / day metric which is relevant to consider for more expensive materials or for light-weight, portable, cyclic devices was not prioritized. Consequently, the L / kg / day harvested by the dual-stage experiment was lower than the single stage (Table 4 of FIG. 8D) because the water recovery ratio was lower. However, the strategies discussed to increase the LMD herein would also result in an increase in L / kg / day. Overall, a thorough techno-economic analysis is needed to understand the value of different materials, device configurations, and the importance of different metrics

[0040] .

[0125] For the devices examined herein, efficiency varies with the period the device is exposed to sunlight and maximizing efficiency does not result in the maximum LMD. These two metrics are relevant for different classes of systems with differing applications. In the context of passive solar-thermal AWH systems that operate on a single daily cycle like thosediscussed in this work, the specific productivity metric (LIVID) is more appropriate than the efficiency metric due to its influence on land area required, the size and cost of the solar absorber, and finned condenser components. Greater LMD can reduce cost by reducing the components required for solar absorption and reduces energy required for condensation because recycling of the latent heat contributes to a reduction in the specific cooling power required at the condenser. Maximizing efficiency in these systems would only reduce the daily quantity of water harvested. However, for devices incorporating heat and mass exchangers and other active systems for cycling the adsorbent multiple times per day, the efficiency metric becomes more relevant and a dual-stage configuration can be used to improve the thermal efficiency. This configuration could be implemented in a cyclic system to reduce the electrical power consumption of the condenser.

[0126] Experimental measurements and model predictions of the temperatures of the top layer, bottom layer, and the finned condenser of the device showed good agreement within 6° C. are shown. A comparison between the amount of water harvested from both stages measured in the experiment and predicted by the model is depicted. The amount of water harvested in the experiment was estimated through a series of time-lapsed photos of the water collection cylinders.|00127| A model comparison of the single-stage and dual-stage devices is shown. Model prediction for the LMD of the dual-stage device and the maximum solar absorber temperature, TH, as the average top heat transfer coefficient Utis varied. The modeling was done at the conditions seen during the experiment including overnight adsorption at average conditions of 68% RH and 20° C. and the time-varying solar flux measured during the experiment. To compare the two, we modeled the single-stage device at identical conditions. The LMD measured in our dual-stage experiment was ~0.77 L / m2 / day and that predicted by the model was 0.85 L / m2 / day.

[0128] In conclusion, a dual-stage configuration for solar-thermal adsorption-based AWH was developed with optimized heat and mass transport to increase the daily water harvesting productivity (LMD). In outdoor experiments, a dual-stage prototype was demonstrated that harvested ~60 mL of water ('0.77 L / m2 / day), an 18% increase over the single-stage device. A heat and mass transport model of the dual-stage device was developed to optimize the design, including the adsorbent layer thickness and air gap thickness. A solar absorber temperature of ~90° C. was found to be the threshold temperature above which the dual-stage device canexhibit greater performance and showed that a reduction in heat loss of 50% from the prototype can allow the dual- stage to achieve approximately twice the LMD of the single- stage device. The device, which incorporates a commercially available adsorbent, demonstrates progress towards scaling of solar-driven AWH devices. However, other adsorbent materials with fast kinetics, higher uptake than AQSOA Z01, and lower energy requirements for regeneration can be used in this configuration to achieve greater LMD. Iterations to select device components with lower heat capacity or reduce heat loss from the solar absorber can improve the performance of this configuration for even greater LMD over single-stage devices. The latent heat recovery strategy utilized by the dual-stage system can be applied to cyclic devices to increase their thermal efficiency.

[0129] Materials and Methods

[0130] Fabrication of the Adsorbent Layers

[0011] The adsorbent layer fabrication steps are detailed below and were fabricated by infiltrating AQSOA Z01 into the metallic binder. To construct the metallic binder, nickel foam (6.4 mm thick and 28 cmx28 cm) was brazed to one side of copper sheet metal (0.8 mm thick and 30.5 cmx30.5 cm) to form binder sheets. The binder sheets were made hydrophilic by rinsing in distilled water, then acetone, sonicating in distilled water, dipping the binder in 2M HC1, and then rinsing with distilled water. The AQSOA Z01 was prepared by mixing with distilled water and sonicating until the particles were suspended and the mixture was homogenous. The AQSOA ZOl / water mixture was placed in a shallow pan and the binder sheet was placed in the mixture, after which the water could evaporate and the AQSOA Z01 was left imbedded in the foam.

[0132] The packing porosity was measured by first determining the bulk density of the layer, Player, or the mass of adsorbent per layer volume. To measure the mass, the total mass of the dry adsorbent and adsorbed water at equilibrium at a measured RH and temperature was recorded. Using the adsorption isotherm, the weight percent uptake of water was determined to find the mass of dry adsorbent in the layer. FIG. 8C shows the individual adsorbent layer components and masses. The crystal density of AQSOA Z01 is pc=1.75 g / cm3, so the packing porosity can be calculated as

[0041] ,

[0133] The mass of Z01 in layer 1 and layer 2 were 305 g and 215 g, and the packing porosity were measured to be 0.65 and 0.75, respectively.

[0134] Fabrication of the Device

[0135] The device consisted of two stages which were each composed of the adsorbent layer, air gap, and a copper condensing surface. Each adsorbent layer had an area of 0.078 m2. Nickel foam, which was used as the binder for the adsorbent, had a thermal conductivity that was high enough to maintain uniform temperature in the thickness dimension (SI). The side walls of each stage were composed of 6.4 mm thick acrylic (k=0.2 W / m / K), which allowed for high thermal resistance while maintaining structural rigidity of the device. Acrylic was selected for its low thermal conductivity and ease of cutting. Side walls with high thermal resistance are important to minimize heat loss via conduction through the walls from one stage to the next to maintain the required temperature difference between the stages. A 3.2 mm thick rubber gasket was placed at the top and bottom of each acrylic side wall to minimize vapor leakage from the device and increase the overall thermal resistance of the side wall. The entire device was insulated with 5 cm thick polyisocyanurate insulation (k=0.023 W / m / K).

[0136] For the solar absorbing surface, selective surface Bluetec-eta plus was used which we adhered to the copper back-side of the top layer using thermal grease (Arctic-MX-4, k=8.5 W / m / K). A 1.3 cm air gap above the solar absorber provided the thermal insulation properties of air without significant natural convection, while a 3.2 mm borosilicate glass convection cover protected the solar absorber from heat loss by external convection. The copper backside of the bottom layer acted as the condenser surface for the top stage. The condenser for the bottom stage consisted of a copper sheet with aluminum heat sinks bonded to the back (area ratio '20). Because forced convection from the wind was not reliable, four 0.6 W CPU fans were mounted to an aluminum plate below the heat sinks and blew upwards at the heat sinks with a velocity of '1.5 m / s. This is the approximate wind speed required to keep the fins (in the downward orientation in the prototype) within 5° C. of the ambient at the peak heat flux at the condenser expected during the experiment.

[0137] During the adsorption phase, the device had to be disassembled to remove the adsorbent layers and place them in the open. Therefore, ease of disassembly was considered in the construction. The device was clamped together between two plates and eight boltswhich ran through the insulation on the exterior of the side walls. When the bolts were tightened, the plates compressed the gaskets between the copper plate of each layer and the acrylic wall. To disassemble the device, these bolts were removed so the adsorbent layers could be removed. The bottom plate (aluminum) was mounted on two rails which could be adjusted to the zenith angle of the sun. A rubber gasket and piece of acrylic was placed below the top plate (acrylic) to provide some insulation from the solar absorber. Additional insulation was also placed over the top plate to suppress heat loss. For water collection and temperature measurements, each acrylic side wall contained two holes. One hole interfaced with a plastic tube which allowed condensed water to flow out. The other hole allowed multiple thermocouples to pass into the device for temperature measurements of the adsorbent layers and the condenser surfaces. The area around the thermocouples was sealed to mitigate vapor leakage. A hydrophobic spray (Rust-Oleum® NeverWet) was used on each copper condensing surface to aid in droplet shedding and water collection.

[0138] Adsorption isotherm, kinetics, adsorption enthalpy, and thermal conductivity of the composite adsorbent layer.

[0139] Due to limitations of the TA Q5000 device, which was used for the Nz / vapor isotherms at atmospheric pressure (FIG. 6), higher temperature measurements were challenging. Isotherms at various temperatures up to 90° C. were measured in a pure vapor environment at vacuum pressures. The N2 / vapor isotherms (FIG. 6B) were used in the kinetic and enthalpy calculations described in the remainder of this section.

[0140] An adsorption isotherm of AQSOA Z01 measured in a pure vapor environment using a sorption analyzer (DVS Vacuum, Surface Measurement Systems Ltd.) is shown in the graph.

[0141] The intracrystalline diffusion coefficient, DM, and adsorption enthalpy, had, are inputs to the model which dictate the heat transfer and kinetics of adsorption and were calculated from adsorption isotherm measurements. To calculate the intracrystalline diffusion coefficient (vapor diffusion from the exterior to interior of a micron-scale adsorbent crystal), Fick's law was used with the assumption of a spherical crystal,where mt / meq is the fractional uptake as a function of time, t, and rcis the crystal radius. Duwas determined by fitting Equation S 1 to the time-dependent adsorption isotherm for a step in the relative humidity (RH) which was measured using a sorption analyzer (Q5000, TA) in a N / vapor environment. Therefore, the calculated value of Duis a representation of kinetics in an air / vapor environment. At 25° C. and 20-25% RH the characteristic value of Du=5.7xl 0-1'’ m2 / s. For this calculation, an average rc=3 pm was assumed. A comparison between the air / vapor diffusion coefficients calculated for AQSOA Z01 as compared to MOF-801, which was characterized in prior work

[0016] , In the calculation of Dufor MOF- 801, the crystal was also assumed to be spherical and rc=0.3 pm

[0016] .

[0142] The term had was estimated using the Clausius-Clapeyron equation,where P is pressure, T is temperature, and R is the gas constant. Equation S2 can be applied to AQSOA zeolites because they do not exhibit significant hysteresis in adsorption / desorption cycles

[0033] . The thermal conductivity of the composite adsorbent layer, k, was calculated as,where y is the foam porosity, kais the thermal conductivity of the powdered adsorbent, and ksis the thermal conductivity of the metal foam. Equation S3 is the parallel model for thermal conductivity in foams, which gives an upper bound on the thermal conductivity but provides a good approximation at high foam porosity [42, 43]. In the prototype, nickel foam (ks~91 W / m / K) with y=0.96 was used. Approximating the thermal conductivity of the zeolite as ka~0.2 W / m / K, then k~4 W / m / K. This result indicates that the composite layer is approximately isothermal in the thickness direction for all thicknesses considered in the modeling (2 mm- 10 mm) based on the Biot number, Bi=hL / k, where h is the external heat transfer coefficient and L is the layer thickness. For example, for an external heat transfer coefficient h=20 W / m2 / K (upper bound on heat loss from the layer during desorption), Bi=0.05 for L=10 mm indicating even the thickest layer considered by our study is approximately isothermal.

[0143] A fitting of Fick's law in a spherical crystal to the adsorption isotherm to calculate the intracrystalline diffusion coefficient is shown. Calculated air / vapor diffusion coefficients for Z01 are compared with previously characterized MOF-801.

[0144] Modeling Framework

[0145] The model is described by four governing equations (Equation S4-S7) which were applied to the adsorbent layers and the air gaps and were solved using COMSOL Multiphysics. The dependent variables of the model are temperature, T, adsorbed concentration, Ca, and concentration in the voids between the crystals that is not yet adsorbed, C. Equation S4 describes the vapor diffusion between the adsorbent crystals and was applied to both adsorbent layers (domain 1 and 3),where t is time, Dvis the intercrystalline vapor diffusivity (diffusion of vapor between the adsorbent crystals), and E is the packing porosity of powdered adsorbent in the metal binder. Dv(~10-5m2 / s) was calculated using the method discussed in prior work by Kim et al.

[0016] . The rate of adsorption is described by the linear driving force model for adsorption,where rc(5-8 pm) is the crystal size, Du(5.7xl0-16m2 / s) is the intracrystalline diffusivity, and Ceq is the equilibrium concentration of vapor given by the adsorption isotherm and was also applied to both adsorbent layers (domain 1 and 3).

[0146] The governing heat transfer equation was applied to the adsorbent layers and the air gaps (domain 1-4),where p, cp, k, and had are the locally averaged density, heat capacity, thermal conductivity, and enthalpy of adsorption, respectively. The calculation to determine Du, had, and the composite k is discussed above. The heat source / sink term on the right-hand side of Equation S6 couples had with the adsorption / desorption rate 6ca / 6t from Equation S5. The density and heat capacity account for the binding material, dry adsorbent, adsorbed water phase, and bulk vapor.

[0147] Vapor diffusion in the air gaps (domain 2 and 4) between the adsorbent layers is described by Fick's law,where Dvapis the diffusivity of vapor in air. The model assumed the device was perfectly horizontal, so convective heat and mass transport were neglected in the air gaps. A saturated vapor concentration boundary condition was applied to each condenser surface because the air gaps quickly reached local 100% RH conditions.

[0148] The model was used to predict the performance improvements which can be obtained using the dual-stage configuration and to optimize Li, L2, Lvi, and LV2. Temperature and concentration boundary conditions were applied to solve Equations S4-S7. An incident solar flux (Q"soiar) is absorbed by the solar absorber (absorptance aa) and modified by the convection cover transmittance, t. The heat loss from the top is Q"ioss.top, where Utis the combined top heat transfer coefficient (calculated below), and Teis the temperature of the ambient environment. The heat loss from the sides of the device is Q''ioss,side, where Uside is the combined heat transfer coefficient of the insulation and external convection. The heat loss at the bottom of device, Q''ioss, condenser, is determined from the heat transfer coefficient, h, and area ratio, Ar, of the finned final condenser. At each condenser surface inside the device, a condensation heat flux is applied, Q"COnd, where J is the flux of vapor at the condenser surface, and hfgis the latent heat of condensation. The flux of vapor is determined by Fick's Law,where Dvapis the diffusion coefficient of vapor in air and C is vapor concentration. There is no flux of adsorbed concentration Caout of the adsorbent layers and there is no flux of vapor concentration C between stages. Radiation heat transfer in the air gaps is included and coupled to the other governing equations. All interior surfaces were assumed to be diffuse and gray, so the radiosity of each interior surface Ji, irradiation Hi, and net flux qi are,respectively, where Ei is the emissivity of each surface, and Fij is the view factor between surfaces.

[0149] A schematic shows domains and boundary conditions used in the dual-stage model.

[0150] Optimization of the Adsorbent Layer and Air Gap Size

[0151] Daily water harvesting capacity is set by the kinetics of adsorption, which are primarily influenced by layer thickness and packing porosity

[0036] . Using the modeling framework, heat and mass transport in the design of the adsorbent layer for a daily cycle was considered. Using Equations S4-S6 a single adsorbent layer in which adsorption occurs from one side was modeled. External boundary conditions (BC) of 35% RH and temperature of 20° C. were assumed to represent operation in an arid climate. The 35% RH BC was imposed by creating a constant concentration BC (i.e., a Dirichlet BC). Assuming water vapor is an ideal gas, temperature and RH can be related to vapor concentration for this BC,where P is the partial pressure of water vapor in air, T is the ambient temperature, Psat(T) is the saturation pressure of water vapor at T, and C is the vapor concentration.

[0152] The influence of two design parameters, layer thickness and packing porosity, was investigated on the metrics of L / m2 / day and L / kg / day. The analysis was restricted to a single daily cycle. In all packing porosity and layer thickness cases simulated, adsorption was rate limiting. Therefore, the model simulated water which can be harvested after 18 hours of adsorption (allowing for 6 hours of desorption by sunlight) to represent productivity in a single day. The external convection heat transfer coefficient was assumed to be 20 W / m2 / K for all cases (external convection due to air flow over the adsorbent layers when they are placed out of the device during the overnight adsorption process).

[0153] The L / m2 / day first increases with increasing porosity due to less restricted vapor transport, but then decreases due to lower overall adsorbent density at higher porosity. At low porosity, layer thickness does not have a large impact on L / m2 / day because it is limited by transport. As packing porosity increases, L / kg / day reaches a maximum which is its equilibrium value given by the adsorption isotherm. At low porosity, L / kg / day is restricted by transport limitations. Additionally, as layer thickness increases, L / kg / day decreases becausethicker layers saturate more slowly, especially at lower packing porosity. At higher porosity, one can see increased utilization of thicker adsorbent layers (i.e., greater L / kg / day). Maximizing the L / kg / day metric is important if the adsorbent material cost is limiting. Maximizing LMD reduces the system size and the area required for solar heating and it could increase the thermal efficiency as long as desorption time does not become significantly longer. Overall, when the adsorption conditions are at low RH (35%), a layer thickness of 6-8 mm and packing porosity of 0.6-0.7 is optimal for layers with both high LMD and high L / kg / day. Using thicker adsorbent layers produces diminishing returns on the adsorbent mass and reduces the full utilization of the adsorbent material. To balance the tradeoffs between L / kg / day and L / m2 / day, we selected a layer thickness of 6 mm and porosity of 0.7 for our prototype.

[0154] Variations in air gap size (Lviand Lvi) affect the temperatures of the adsorbent layers and condensers, and the temperature difference between a layer and its corresponding condenser surface impact the driving force for desorption based on Equation S5. This is because for a given solar heat flux, the temperature difference between a layer and its condenser increases with Lvdue to the thermal resistance across the layer of air. A parametric study of the LMD was performed as a function of Lviand LV2 (FIG. 12). This parametric study was performed under heat loss conditions in which Ut~4.5 W / m2 / K, representing a lower bound on heat loss as calculated herein. Based on the optimization, a layer thicknesses ~6 mm (6.4 mm due to standard material sizes) and s=0.7 was used. Adsorption was assumed to occur at 35% RH and 25° C. to represent arid conditions. The simulation used a constant solar flux Qsoiar= 1000 W / m2with an allowable desorption time of 6 hours, and condenser heat rejection hAr=50 W / m2 / K.

[0155] LMD of the top stage first increased with increasing Lvibefore decreasing due to slower vapor transport and increased side area for heat loss. The same trend for the LMD of the bottom stage can be seen for variations in LV2. Therefore, the results show that it is desirable to have approximately equal values of Lvi and LV2. A value of LVI=LV2=20 mm was selected, of which the maximum water productivity is ~L2 L / m2 / day if adsorption conditions are 35% RH.|00156| The amount of water harvested (LMD) varies with Lviand LV2 which influence the temperature of each adsorbent layer and condenser surface and affects the driving force for desorption.

[0157] Calculation of the Combined Heat Transfer Coefficient from the Top of the Device

[0158] The top of the device consists of a selective solar absorber, air gap, and convection cover. The total heat loss can be modeled considering convection and radiation from the solar absorber to the convection cover as well as convection and radiation from the cover to the ambient. The equations are defined in Table 1 of FIG. 8A. Solar radiation, Qsoiar, is received at the solar absorber with solar absorptivity a after passing through the convection cover with transmissivity r. In FIG. 8A, Tais the absorber temperature, Tcis the convection cover temperature, and Teis the environmental temperature. s;iis the emissivity of the absorber, « is the emissivity of the convection cover, kair is the thermal conductivity of air, and Lcis the size of the air gap between the convection cover and absorber.

[0159] hc,ce is the convection coefficient between the cover and the environment, which varies based on wind speed. The Nu correlation to describe hc,acis given by a correlation by Hollands et al. which describes a narrow horizontal cavity in which the bottom plate is heated and P is the tilt angle of the plate

[0044] . The characteristic length in the Nu correlation is the spacing between the solar absorber and the convection cover, Lc. In Equation SI 6, the plus sign indicates that the term should only be included if it is positive. That way Nu=l (pure conduction) before the critical spacing and angle for natural convection to begin. The Ra number and combined top heat transfer coefficient, Ut, are,

[0160] Using a baseline value of hc,ceof 20 W / m2 / K, Ut~4.5 W / m2 / K was obtained through the solar absorber of area 0.078 m2. This is a realistic lower bound on Utfor the device. To describe heat loss in the physical prototype, an additional resistance was introduced in parallel with the convection and radiation to the convection cover to account for heat conduction through the top cover plate (area of 394 cm2) which is in thermal contact with the edges of the solar absorber. The conduction pathway travelled through 0.8 mm thick neoprene rubber (k~0.25 W / m / K), 6.4 mm thick acrylic (k~0.25 W / m / K), and 5 mm thick Al (k~240 W / m / K) before the convection heat loss to the ambient. The thermal resistance through this pathway (including convection of hw=20 W / m2 / K) was ~2 K / W. Applying this inparallel with the heat transfer calculated using Equation S 18, gives a calculated value of Ut~9 W / m2 / K for our prototype.

[0161] Fabrication of the Adsorbent Layers

[0162] Images show fabrication of the Z01 layers involved cleaning Ni foam and brazing it to copper sheet metal, immersing the layer in a Z01 / water mixture, and evaporating the water to leave behind the Z01 infiltrated into the Ni foam.

[0163] The masses of the adsorbent layer components were measured prior to infiltrating with the zeolite (Table 2 of FIG. 8B). To determine the quantity of dry adsorbent infiltrated into the Ni foam, the layer was allowed to reach equilibrium in a room temperature environment of 50% RH and 23° C. The mass of the entire layer was measured. After subtracting the mass of the components, the weight percent uptake of water was determined using the adsorption isotherm and the dry mass of adsorbent in each layer was calculated.

[0164] Additional Dual-Stage Experiment

[0165] The temperatures and solar flux were measured during the dual-stage experiment. Adsorption occurred the night before at an average overnight RH of 57 % and average temperature of 26° C. In this experiment, we harvested '38 mL from the top stage and '21 mL from the bottom stage, for a total of ~59 mL of water harvested. Considering the 0.078 m2area of the solar absorber, this translates to ~0.76 L / m2 / day. We achieved similar results in this experiment as the experiment discussed in the main text which took place a few weeks later, indicating no degradation in performance in the device or the adsorbent layers after several experiments.

[0166] Water Quality Measurements

[0167] To characterize the quality of the harvested water, ion-coupled plasma mass spectroscopy (ICP-MS) with ppb (part per billion) resolution was used to measure the ion concentration of the water collected. Ions of interest included aluminum (Al), iron (Fe), nickel (Ni), copper (Cu), silver (Ag), and indium (In). The possible contamination sources are the adsorbent material (Al, Fe), metal foam (Ni), condenser surfaces (Cu), and brazing (Cu, Al, In). In FIG. 17, obvious differences in composition of harvested water and HPLC grade water were found in concentrations of Al, Fe, Cu, Ni, and Ag. The Al and Fe ions wereprimarily from AQSOA Z01. Ag as from the brazing between the copper plate and the adsorbent layer. The concentrations of Al, Fe, and Ag in the harvested water were greater than HPLC water, but still below the EPA secondary drinking water regulations

[0045] . Phosphorous (P) was not individually measured in the collected water due to challenges in ppb level mass spectroscopy measurements. However, Al and P exist at a 1:1 ratio in the adsorbent material, and the adsorbent should be their only source in the harvested water. Therefore, one can estimate P is present at concentrations no greater than ~10 ppb. The concentrations of Ni (~100 ppb) and Cu (~3 ppm) were not negligible. Due to the cleaning and zeolite infiltration procedure which involved cleaning the surfaces with HC1 to make them hydrophilic, the harvested water reacted heavily with the Cu and Ni. The Cu and Ni concentration can be minimized by changing the cleaning procedure of the Ni foam and Cu surfaces or selecting a different material with sufficiently high thermal conductivity. Results show In concentrations in both the collected and HPLC grade water were found to be less than 1 ppb, indicating that the In was negligible in the harvested water.

[0168] Water quality analysis of potential sources of contamination was tested by ICP-MS. The relatively high concentration of Ni and Cu can be reduced by modifying their cleaning procedure or selecting different materials.

[0169] Single-Stage Experiment

[0170] Measured temperatures and solar flux during the single-stage experiment in which ~51 mL of water, or '0.65 L / m2 / day was harvested are shown. Overnight adsorption conditions were an average of 58% RH and 20° C. The maximum temperature reached was TH=78° C. The slope of the single-stage adsorbent temperature was steeper than the dualstage because the thermal mass of the single-stage device as lower. From time-lapsed photos, water collection ended at 13:50.

[0171] Summary of Experimental Results

[0172] The uptake before and after desorption was calculated by measurements of the dry adsorbent mass (Table 3 of FIG. 8C) and measurements of the composite adsorbent layer mass taken before and after desorption. The uptake is defined as water adsorbed per mass of dry adsorbent. Calculated uptakes were consistent with the uptake expected at the average adsorption RH condition of each experiment.

[0173] Examples of the above-described embodiments can include the following:1 . A method for forming a hydrogel, comprising: exposing a sorbent material comprising a polymer matrix to an aqueous salt solution to promote swelling of the sorbent material leading to salt loading; selecting a desired salt concentration of the aqueous salt solution based on an equilibrium of the salt solution with a maximum allowable relative humidity in a system; and monitoring mass of the sorbent material throughout fluctuation to ensure equilibrium with the aqueous salt solution is achieved.2. The method of example 1 , wherein the aqueous salt solution comprises one or more of LiCl, CaCh, or one or more other hygroscopic salt solutions.3. The method of example 1 or example 2, wherein the water uptake is independent of a type of polymer contained in the polymer matrix of the sorbent material.4. The method of any of examples 1 to 3, wherein at about 30% relative humidity, the hydrogel provides an about 15% increase in water uptake compared to conventional hydrogels.5. The method of any of examples 1 to 4, wherein a concentration of the aqueous salt solution is at equilibrium with a highest relative humidity to which the hydrogel is exposed.6. The method of example 5, wherein the concentration of the aqueous salt solution is approximately in a range from about 25% saturated (25%Sat) to about 75% saturated (75%Sat)-7. The method of example 5 or example 6, wherein at about 70% relative humidity, the hydrogel retains substantially all of the aqueous salt solution from the polymer matrix.8. The method of any of examples 1 to 7, further comprising tuning one or more of a concentration of the aqueous salt solution for swelling or a crosslinking of the polymer matrix.9. The method of any of examples 1 to 8, wherein an amount of aqueous salt solution within the polymer matrix of the sorbent material exceeds about 20 grams per gram of the sorbent material.10. The method of any of examples 1 to 9, further comprising fluctuating a relative humidity of the sorbent material when exposed to the aqueous salt solution, the relative humidity being in approximately in a range from about 30% to about 70%11. A method of designing a composite, comprising: loading a sorbent material with an aqueous salt solution, the sorbent material having a polymer matrix comprised of a nanoporous network connected by one or more crosslinkers to form a gel, the polymer matrix receiving a portion of the aqueous salt solution therein and swelling at an interface of the sorbent material and the aqueous salt solution to retain a plurality of ions of the aqueous salt solution; and optimizing the polymer matrix by tuning one or more of: (i) type of polymer matrix;(ii) polymer matrix content; (iii) type of aqueous salt solution; (iv) degree of saturation in the aqueous salt solution; (v) crosslinking content; (vi)crosslinking architecture, or (vii) sorbent thickness.12. The method of example 11, wherein the one or more parameters are one of tuned individually or in combination with an other parameter of the one or more parameters.13. The method of example 11 or example 12, wherein the one or more crosslinkers comprises 7V,iV’-methylenebisacrylamidecrosslinker.14. The method of any of examples 11 to 13, wherein the aqueous salt solution comprises one or more of LiCl, CaCh, or one or more other hygroscopic salt solutions.15. The method of any of examples 11 to 14, wherein loading the sorbent material with the aqueous salt solution comprises soaking the sorbent material in hygroscopic salt.16. The method of example 15, wherein a degree of saturation of the aqueous salt solution is one or more of about 0%sat, about 25 %sat, about 50%sat, about 75%sat, or about 100%sat.17. The method of example 16, wherein the sorbent material is soaked until an equilibrium swelling ratio of the composite is decoupled from its swelling kinetics.18. The method of example 17, further comprising placing the composite swollen to equilibrium in one of an about 25 %satsolution or an about 75 %satsolution into a chamber at a relative humidity of about 70% for about 48 hours.19. The method of example 17 or example 18, wherein a length of time that the sorbent material is soaked is based on a degree of saturation of the aqueous salt solution.20. The method of example 19, wherein when the degree of saturation of the aqueous salt solution is about 75 %sat, the sorbent material is soaked for about 300 hours.21. The method of any of examples 11 to 20, further comprising gelating a solution of deionized water, acrylamide monomer, / V,.V-methylenebisacrylamidecrosslinker, ammonium persulfate initiator, and / V, / V,A',Af'-tetramethylethylendiamin accelerator.22. The method of example 21, further comprising curing the solution to form disks of the sorbent material.23. The method of example 22, further comprising drying the disks at a relative humidity of about 0%.24. The method of example 22 or example 23, wherein the disks of the sorbent material are freeze-dried.25. A method of water-harvesting comprising: a. absorbing water or vapor from ambient atmosphere into a sorbent material; b. applying energy to an energy absorber to desorb vapor, wherein one or more sorbent layers each include the sorbent material and a condenser surface that is disposed between the energy absorber and a condenser; and c. collecting water with the condenser, wherein the condenser comprises the sorbent material and an aqueous salt solution.26. The method of example 25, wherein the sorbent material comprises polyacrylamide (PAM) and the aqueous salt solution comprises a hygroscopic salt selected from LiCl, CaCh, or one or more other hygroscopic salt solutions.27. The method of example 25 or example 26, further comprising a gap between the two or more sorbent layers.28. The method of any of examples 25 to 27, further comprising dissipating heat from the condenser through at least one of a heat sink or active cooling.29. The method of any of examples 25 to 28, further comprising dissipating heat from a condenser surface to an adjacent sorbent layer.30. The method of any of examples 25 to 29, wherein applying energy includes supplying at least one of solar irradiance, biomass gasification, combustion, or electrically powered joule heating.31. The method of any of examples 25 to 30, further comprising an enclosure containing the two or more sorbent layers and the condenser.32. The method of any of examples 25 to 31, wherein the enclosure is opened during dark periods for water adsorption and the enclosure is closed during light periods for water production.33. The method of any of examples 25 to 32, wherein the two or more sorbent layers exchange into the enclosure prior to desorbing water.34. The method of any of examples 25 to 33, wherein the vapor moves from the sorbent layer to the condenser following a concentration gradient.35. A sorbent material comprising a polyacrylamide (PAM) and a hygroscopic salt selected from at least one of LiCl, CaCh, or one or more other hygroscopic salt solutions.36. The sorbent material of example 35, wherein the hygroscopic salt is at a concentration less than about 25% of saturation in water.37. The sorbent material of example 35 or example 36, wherein the sorbent material is recyclable.38. The sorbent material of any of examples 35 to 37, wherein the sorbent material is recycled by at least one of freeze drying (sublimation) or heating (evaporation).

[0174] One skilled in the art will appreciate further features and advantages of the disclosures based on the provided for descriptions and embodiments. Accordingly, the inventions are not to be limited by what has been particularly shown and described. To the extent the present disclosure includes illustrations and descriptions that include prototypes, bench models, or schematic illustrations of set-ups, a person skilled in the art will recognize how to rely upon the present disclosure to integrate the techniques, systems, composition, designs, and methods provided for into a product and / or production method. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0175] Some non-limiting claims that are supported by the contents of the present disclosure are provided below.

Claims

What is claimed is:1 . A method for forming a hydrogel, comprising: exposing a sorbent material comprising a polymer matrix to an aqueous salt solution to promote swelling of the sorbent material leading to salt loading; selecting a desired salt concentration of the aqueous salt solution based on an equilibrium of the salt solution with a maximum allowable relative humidity in a system; and monitoring mass of the sorbent material throughout fluctuation to ensure equilibrium with the aqueous salt solution is achieved.

2. The method of claim 1, wherein the aqueous salt solution comprises one or more of LiCl, CaCh, or one or more other hygroscopic salt solutions.

3. The method of claim 1, wherein the water uptake is independent of a type of polymer contained in the polymer matrix of the sorbent material.

4. The method of claim 1, wherein at about 30% relative humidity, the hydrogel provides an about 15% increase in water uptake compared to conventional hydrogels.

5. The method of claim 1, wherein a concentration of the aqueous salt solution is at equilibrium with a highest relative humidity to which the hydrogel is exposed.

6. The method of claim 1, wherein the concentration of the aqueous salt solution is approximately in a range from about 25% saturated (25%Sat) to about 75% saturated (75%sat).

7. The method of claim 5, wherein at about 70% relative humidity, the hydrogel retains substantially all of the aqueous salt solution from the polymer matrix.

8. The method of claim 1 , further comprising tuning one or more of a concentration of the aqueous salt solution for swelling or a crosslinking of the polymer matrix.

9. The method of claim 1, wherein an amount of aqueous salt solution within the polymer matrix of the sorbent material exceeds about 20 grams per gram of the sorbent material.

10. A method of designing a composite, comprising: loading a sorbent material with an aqueous salt solution, the sorbent material having a polymer matrix comprised of a nanoporous network connected by one or more crosslinkers to form a gel, the polymer matrix receiving a portion of the aqueous salt solution therein and swelling at an interface of the sorbent material and the aqueous salt solution to retain a plurality of ions of the aqueous salt solution; and optimizing the polymer matrix by tuning one or more of: (i) type of polymer matrix;(ii) polymer matrix content; (iii) type of aqueous salt solution; (iv) degree of saturation in the aqueous salt solution; (v) crosslinking content; (vi) crosslinking architecture, or (vii) sorbent thickness.

11. The method of claim 10, wherein the one or more parameters are one of tuned individually or in combination with an other parameter of the one or more parameters.

12. The method of claim 10, wherein the one or more crosslinkers comprises N,N’- methylenebisacrylamidecrosslinker.

13. The method of claim 10, wherein loading the sorbent material with the aqueous salt solution comprises soaking the sorbent material in hygroscopic salt.

14. The method of claim 13, wherein the sorbent material is soaked until an equilibrium swelling ratio of the composite is decoupled from its swelling kinetics.

15. The method of claim 14, further comprising placing the composite swollen to equilibrium in one of an about 25 %sat solution or an about 75 %sat solution into a chamber at a relative humidity of about 70% for about 48 hours.

16. The method of claim 14, wherein a length of time that the sorbent material is soaked is based on a degree of saturation of the aqueous salt solution.

17. The method of claim 10, further comprising gelating a solution of deionized water, acrylamide monomer, A,A’-methylenebisacrylamidecrosslinker, ammonium persulfate initiator, and A,A,A',A'-tetramethylethylendiamin accelerator.

18. The method of claim 17, further comprising curing the solution to form disks of the sorbent material.

19. The method of claim 18, further comprising drying the disks at a relative humidity of about 0%.

20. The method of claim 18, wherein the disks of the sorbent material are freeze-dried.

Images