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Page 12 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

stage. When hygroscopic salt is incorporated into the hydrogel, the mechanism primarily involves water
vapor adsorption driven by the deliquescence of salt. In this case, water is captured by the formation of
crystalline water, salt dissolution, and solution adsorption, and the polymer pores generally serve as storage
space for water molecules . The water adsorption process of the hygroscopic polymer is summarized in
[112]
Figure 11, and desorption is the reverse process.

Considering their high water adsorption capacity, although hygroscopic polymers do not display S-type
water vapor adsorption isotherms, they have been used in psaAWH in recent years, and promising results
have been obtained [115-121] . Nandakumar et al. synthesized a solar-driven ZnO hydrogel for water-vapor
harvesting under high relative humidity (90% RH) . Separately, Ni et al. achieved high water production
[114]
(c.a. 4.57 kg m day ) under 90% RH and sunlight exposure by introducing photothermal segments into an
-1
-2
integrated hygroscopic photothermal organogel (POG) [Figure 12A] . Moreover, incorporating segments
[122]
with LCST in hygroscopic polymers can accelerate the release of water molecules and improve water
production [37,116,121] . For example, Zhao et al. demonstrated the release of liquid water from a hydrogel
through a photoresponse effect [Figure12B] . When the ambient temperature is lower than LCST, the
[116]
hydrogel shows hydrophilicity and can adsorb a large number of water molecules in the framework. As the
temperature rises above LCST, the poly(N-isopropylacrylamide) (poly-NIPAM) segment in the hydrogel
becomes hydrophobic, resulting in the rapid release of water molecules from the hydrogel in liquid form. To
further improve the water adsorption capacity, hygroscopic salts are commonly introduced into
polymers [123-127] . For example, Lei et al. synthesized a LiCl-doped zwitterionic hygroscopic polymer . The
[118]
water uptake of the polymer was enhanced and finally showed excellent water production (5.87 L kg day ).
-1
-1
Owing to their macroporous inner structure and rich active groups inside hygroscopic polymers, they can
adsorb and filter pollutants in the atmosphere during psaAWH application . Yao et al. synthesized a
[128]
porous sodium polyacrylate/graphene framework (PGF) that can capture atmospheric water contaminated
by harmful substances and release clean water . The abundant active groups of the hydroscopic polymers
[129]
make them tunable. After the incorporation of LiCl and MOF-101, owing to the synergistic effects of space
pressure from the rigid polymer chain and strong water affinity, active and continuous water production is
finally realized . Although hygroscopic polymers have been widely used in psaAWH, most function
[28]
optimally at higher RH because of the lack of S-type water adsorption isotherms and display volume
swelling after water absorption. Although some hydrogels offer higher water uptake at low RH by doping
with hygroscopic salts, such as CaCl and LiCl, etc, the salts may remain in the collected water, making it
2
unfit for consumption.

Crystalline porous organic salts
CPOSs are emerging crystalline porous materials composed of organic acids and organic bases formed
through hydrogen bond-assisted ion bond interactions . Compared to hydrogen-bonded organic
[130]
frameworks (HOFs), CPOSs frameworks possess both hydrogen and ionic bonds. These bonds support
channels with both high polarity and regularly distribute positive and negative charges from organic acids
and bases [130,131] . These charged and highly polar channels impart CPOSs with excellent properties compared
with other porous materials [132-138] . Owing to the strong ionic bond interactions in the framework, most
CPOSs exhibit good water stability. For example, CPOS-2 shows high proton conductivity and crystallinity
under high relative humidity (98% RH) and temperature (60 °C) . Because the pore size of CPOSs is
[138]
generally small, capillary condensation does not occur during the adsorption of water vapor; therefore, the
adsorption mechanism of CPOSs is mainly cluster adsorption. The adsorption/desorption process of the
CPOSs in psaAWH can be described in Figure 7B, and the detailed process is discussed below.