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Page 22 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

effectively affected by regulating the hydrophilicity of the adsorbent. Wang et al. demonstrated AWH by
grafting hydrophilic [FeCl ] -doped poly(3,4-ethylenedioxythiophene) (PEDOT) microtubules onto the
-
4
surface of a red brick . These microtubules accelerate the nucleation and layer adsorption of water
[154]
molecules on the surface of red bricks at ambient humidity and transport water molecules to the adsorbent
interior through the porous network of red brick, thus accelerating the water harvesting rate [Figure 21].
Although hydrophilic modification can accelerate the adsorption kinetics, it increases the regeneration
temperature of the adsorbent; therefore, it should be used reasonably according to the actual situation.

The transport speed of molecules improves with temperature increasement. As reported by Messaoudi et al
., with the increase in temperature, the mobility of TZ molecule accelerates, which promotes the rapid
absorption of TZ by adsorbent . Therefore, increasing the desorption temperature of adsorbent can
[161]
effectively speed up the desorption kinetics [Figure 22A]. Hanikel et al. used a solar power generation
system to heat a MOF adsorbent, accelerating the water release rate, realizing multiple
adsorption/desorption cycles in one day, and obtaining high water production (1.3 L kg day ) . However,
-1
-1 [91]
direct heating relying on external factors not only consumes a significant amount of energy but also evades
adsorbent material advancements. Various advances in adsorbent kinetics based on material design have
been reported. The incorporation of photothermal materials into adsorbents to accelerate the release of
water through sunlight irradiation has been reported. Wu et al. prepared a T C (a photothermal material)-
2
3
incorporated MOF adsorbent (TUN/SA), which can increase to 100 °C in 6 min under the irradiation of 103
mW cm light, realizing the rapid release of water molecules. Compared to direct heating, the incorporation
-2
of photothermal materials can rapidly heat the adsorbent from the inside, resulting in rapid desorption
kinetics [Figure 22B] . However, the introduction of photothermal materials leads to a reduction in the
[86]
adsorbent percentage per unit mass, and may block the pores of the adsorbent, thereby sacrificing a portion
of the water adsorption capacity. In order to avoid this problem, we can reduce the amount of photothermal
materials and make them disperse as evenly as possible, so that the adsorption performance of the adsorbent
is less affected .
[162]
According to previous reports , there are three main processes for the adsorption of guest molecules by
[163]
adsorbent: external diffusion; intra-particle distribution; adsorption on the active sites. For vapor adsorption
of adsorbent, the intercrystalline diffusion also needs to be considered. External diffusion is mainly affected
by the engineering of psaAWH devices, including the design of adsorbent bed and heating module. A more
detailed overview of the engineering of psaAWH devices, including heat conduction and mass conduction,
etc. can be found in Wang’s article . For adsorbent, the intra/inter-crystalline diffusion and adsorption on
[162]
the active sites are the main factors affecting its kinetic process. The effect of active sites has been discussed
above. The intracrystalline diffusion can be expressed by Fick’s law :
[164]

where C is the vapor concentration, t is the time, r is the spherical radius of the adsorbent, and D is the
µ
c
intracrystalline diffusivity of the vapor. However, the diffusivity in Eq. 1 varies with the temperature and
vapor uptake; therefore, the characteristic intracrystalline diffusivity crystals are used to represent the
average diffusivity within a certain range of temperature and vapor uptake. The following equation can be
used to describe the average diffusion coefficient of vapor adsorption kinetics [162,164] :