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Page 24 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

where D is the diffusivity of vapor molecules in air, D is the Knudsen diffusivity, and ε is the porosity of
K
Vap
the adsorbent packing layer, which is related to the size and geometry of the adsorbent. In general, a smaller
size of the adsorbent implies a smaller value of ε. Besides, the adsorbent geometry can also affect the value of
ε, but the relationship is complex. Therefore, psaAWH performance can be optimized by adjusting the size
and geometry of the adsorbent.

As discussed, adsorbent size and geometry can affect its kinetic process by affecting the intra/inter-
crystalline vapor diffusion . A rapid kinetic rate was obtained using a reasonably designed adsorbent
[164]
geometry. For example, Yilmaz et al. prepared a polymer-Au@MOF mixed-matrix material (PCA-MOF)
featuring a cone array structure, which enabled the released water to rapidly accumulate at the cone tip and
form droplets, thus accelerating intracrystalline diffusion . It also enables rapid water separation from the
[28]
adsorbent surface to accelerate the release of internal water [Figure 23A]. In addition, adsorbent films offer
an increased contact area with the atmosphere and heater, resulting in a rapid kinetic process [152,165] .
Guo et al. prepared super hygroscopic polymer films (SHPFs), which can undergo a transition from
hydrophilic to hydrophobic after being heated. During the psaAWH process, SHPFs can be attached to a
metal heating plate. Therefore, the rich porous structure and large heating area of the SHPFs enable rapid
thermoresponsive hydrophobic transformations, leading to a rapid water release rate [Figure 23B] .
[152]
In general, intracrystalline diffusivity is slower in adsorbents. The introduction of large pores in the
adsorbent can be considered as a strategy to shorten the intracrystalline diffusion path, thus accelerating the
kinetic process . One approach is to build a shell structure in adsorbent that allows water molecules to
[84]
freely diffuse inside, leading to rapid kinetics. Compared with MIL-101(Cr) powder, the hollow MIL-
101(Cr) spheres showed faster adsorption kinetics because water molecules could reach the adsorption sites
faster in the large cavity [Figure 19A] . Rapid water transport through a hierarchical pore structure is
[156]
another strategy for shortening the intracrystalline diffusion path [120,166,167] . Wang et al. prepared a
nanostructured biopolymer hygroscopic aerogel (NBHA) that exhibited rapid water transport
[Figure 24A] . Under natural light irradiation, the water adsorbed in the aerogel can be quickly
[127]
transported to the surface through the hierarchical pore structure, resulting in rapid regeneration. Xu et al.
prepared an adsorbent with a vertically arranged channel [Figure 24B] . During adsorption, water
[125]
molecules may quickly diffuse to the interior of the adsorbent through the large pores to be adsorbed by the
numerous active sites. In addition, this 1D channel minimizes the diffusion distances for water molecules
between the interior and the surface of the adsorbent. Finally, these factors led to high water production
(2.12 L kg adsorbent -1 day ).
-1
Although the aforementioned strategies have achieved significant results in accelerating the adsorption
kinetics, the intracrystalline diffusion rate, especially in micropores, has not been substantially improved.
According to Wu’s work , the smaller the pore size of the adsorbent, the lower its applied RH. Therefore,
[168]
accelerating the diffusion of vapor in the adsorbent micropores will greatly promote the development of
psaAWH. Herein, the transport behavior of water molecules in the micropores is further re-examined from
a new perspective (superfluidity).

Superfluidity, in which molecules or ions move collectively and directionally in confined channels, results in
the rapid transport of matter . The earliest report of superfluidity in 1938 [170,171] was based on observations
[169]
of this phenomenon in He below 2.17 K. When He passes through a channel with a diameter of less than
4
4
100 nm, its viscosity approaches zero, indicating that it can be transmitted in the channel at a superfast
speed without energy loss. Subsequent studies have shown that the temperature at which the superfluidity
phenomenon occurs can gradually increase with a decrease in the channel diameter . In superfluidity,
[172]