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WATUSO - 834134;info:eu-repo/grantAgreement/EC/H2020/834134

Abstract:

Tackling the global water scarcity crisis requires innovative solutions, and capturing water from the atmosphere is emerging as a promising and viable technology. This research explores various strategies, including established methods such as active air cooling and sorption-desorption cycles with desiccants, alongside emerging technologies like deliquescent salt reverse osmosis (DESARO) and thermo-responsive hydrophilicity switching polymers. These recently proposed technologies, characterized by a single-phase transition, offer an energetic advantage. The selection of hygroscopic materials to be applied in the aforementioned processes plays a crucial role in the effectiveness of water vapor harvesting technologies. In this study, thermodynamic equilibria and kinetics of the water vapor uptake of salts, hydrogels, and thermo-responsive polymers are investigated to build a comprehensive database currently lacking in the literature. Such a database is indispensable in an efficient material selection process for the different atmospheric water production applications. Salt are attractive materials for both standard desiccation and the new DESARO process. Although water vapor uptake by salts has been known for ages, literature provides reliable data only for a limited number of salts. This work provides the water uptake data for a large collection of 35 different salts with organic and inorganic ions. The examination of the behavior of a large number of deliquescent salts reveals systematics that have not been emphasized in the literature. This research reveals that, after deliquescence, the water uptake of salt solutions expressed in mol water per mole salt ion is consistent for all salts and correlates uniquely with the environmental relative humidity. The deliquescence point which was found to be inversely proportional to the solubility emerges as a critical parameter for salt selection, along with the chemical nature and cost. Hydrogels for water vapor absorption are an attractive alternative to deliquescent salts. Favorable absorption-desorption cycles on these materials invite for application in atmospheric water production. The compressibility and the release of liquid water upon compression are attractive features for a DESARO-inspired process. Liquid water expulsion from a sodium polyacrylate hydrogel by centrifugal forces is demonstrated. Suitable hydrogels take up water vapor mainly in the mid to high relative humidity range. The degree of crosslinking of hydrogels proves irrelevant to the water vapor uptake. The ion content emerges as the key determinant in the water vapor uptake of ionic hydrogels. Interestingly, hydrogels can also be handled as a host material for deliquescent salts, to suppress water leaching. Thermo-responsive polymers alter their behavior from hydrophilic to hydrophobic with moderate temperature changes at high energy efficiency. A minimum hydration degree is needed to obtain the hydrophilicity switch. This critical absorbed water content enabling thermally driven water expulsion of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) is determined to be 0.6 g/g. The polymer readily absorbs liquid water in sufficient quantity, but absorption of water vapor is limited. The water uptake of this polymer at 90 % relative humidity is only 0.24 g/g at 25 °C, which is insufficient to trigger liquid water expulsion by thermo-response. In literature, an interpenetrating polymer network of thermo-responsive PNIPAM and conducting polypyrrole chloride is proposed as a super moisture-absorbent gel (SMAG) able to capture sufficient water vapor to enable the thermo-response. Despite extensive efforts, the reported results of SMAG could not be confirmed experimentally. Extensive characterization of the synthesized materials and comparison with literature leads to a hypothesis on the required properties. A combination of capillary condensation and osmotic forces is proposed to be needed, a combination coined "osmocapillary". Alternative strategies are explored to enhance the water uptake such as interpenetrating polymer networks and combinations with deliquescent salts as well as the creation of mesopores suited for capillary condensation.