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Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40 Page 13 of 35

Figure 11. The adsorption process of water by hygroscopic polymers.

Figure 12. (A) Structure of a photothermal organogel (POG) and its synthesis process diagram. Reproduced with permission [122] .
Copyright John Wiley and Sons; (B) schematic illustration of express mode and normal mode for water release powered by solar
energy. Reproduced with permission [116] . Copyright John Wiley and Sons.

Zhang et al. synthesized CPOS (CPOS-6) containing strong organic acids (tetrakis(4-sulfophenyl) methane,
TSPM) and bases (tetrakis(4-amidinophenyl) methane, TAmPM) . The unique double-layer water
[29]
adsorption process of CPOS-6 afforded rapid water adsorption and release kinetics, leading to a high
expected water production (2.16 g g day ). In this double-layer adsorption process, the first layer of water
-1
-1
molecules adsorbed by the strong polar groups acts as a binding site for the second layer of adsorption,
which improves the second layer adsorption rate [Figure 13]. Furthermore, weak hydrogen bonds in the
second layer of adsorbed water molecules were readily desorbed during desorption. Although the
applications of CPOSs in psaAWH remain nascent, their unique advantages of highly polar pores make
them promising for psaAWH. Moreover, CPOSs can be readily synthesized at ambient temperature and
pressure, thereby simplifying industrial production. However, the water uptake should be improved.

Porous carbon
Porous carbon has been used in various fields because of its ultrahigh thermal and chemical stability and
high functionalizable porosity . Accordingly, their water adsorption performance has been widely
[139]
studied [140-143] . Depending on the pore structure, activated carbon, an archetypical classic porous carbon
material, can adsorb water via capillary action . The water vapor adsorption of porous carbon primarily
[144]
involves the following processes : nucleation, water cluster growth, and pore filling [Figure 14]. In porous
[144]
carbon, the nucleation sites can be ultramicroporous, defective, or hydrophilic groups. This process can
cause the water vapor adsorption isotherm to exhibit a steep water uptake under varying low RH. If the
affinity of the hydrophilic sites is weak, the inflection point of steep water uptake will move appropriately to
a high RH. After nucleation, it is followed by the growth and coalescence of water clusters, in which pore