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Page 2 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

for further research in this ever-expanding field.

Keywords: Atmospheric water harvesting, porous adsorbent, kinetic process, superfluidity

INTRODUCTION
Since the beginning of the 21st century, water scarcity has become a serious global resource problem .
[1]
Humanity persists for millennia without petrochemicals but cannot without water; however, by 2025, 1.9
billion people in the world will experience a severe freshwater crisis . The world's abundant water resources
[2]
maintain a poised ecological balance and considerable biodiversity, but only 2.5 percent of them are
freshwater . Furthermore, the most abundant freshwater reserves exist as inaccessible glaciers distributed in
[3]
the northern and southern poles . The irreconcilable conflicts between limited water resources and
[4,5]
accelerating global economic and population growth foreshadow famines, the spread of diseases, and wars;
the continued existence of humanity may depend on increased freshwater production.

Currently, various technologies exist that can be used to produce fresh water to alleviate water crises, such
as wastewater treatment, seawater desalination, and rainwater collection. Geographic or weather
dependencies restrict freshwater production technologies such as seawater desalination and rainwater
collection . Wastewater treatment technology based on adsorption or membrane separation is used to
[2,6]
purify polluted liquid water [7-11] . Although it is not restricted by geography and climate, it requires a large
amount of liquid water in the local area, which indirectly restricts its application in areas of water shortage.
Therefore, existing technology cannot fundamentally solve the problem of water shortage in remote water
shortage areas. Atmospheric humidity, one of the three major water sources in the world, is abundant,
ubiquitous, and can be replenished in a timely manner through global atmospheric circulation . If
[1]
freshwater can be produced directly from atmospheric humidity, it will fundamentally solve the global water
shortage problem. Thus, atmospheric water harvesting (AWH) technology, which harvests freshwater from
air humidity, is considered an effective solution to the water shortage problem [12-15] . The three major AWH
techniques include condensation (i.e., cooling air below its dew point), fog collection, and porous sorbent
assisted AWH (psaAWH) [13,16] .

Fog collection must be performed in areas where fog often occurs, which is not suitable for arid areas . For
[2]
condensation, at the beginning of condensation, the associated sensible heat must be removed to condense
the air to the dew point, and then some latent heat is required to complete the transition of water from a
gaseous state to a liquid state. To collect a certain amount of water, the value of latent heat is usually certain
and inevitable, which is related to the characteristics of the material itself. By contrast, the sensible heat
required varies with the environment. Consequently, condensing water vapor into a liquid state at high
temperatures and in dry conditions is very energy-intensive and even impossible to achieve in several
cases [17,18] . In contrast, psaAWH has climatic flexibility for widespread applications owing to the diverse
water adsorption characteristics of different porous materials, which is considered to be the development
trend of AWH technology [19-21] . The characteristics of several freshwater production technologies are
summarized in Table 1.

The process and mechanism of action of psaAWH are shown in Figure 1A. In general, psaAWH produces
freshwater in three steps . First, water in the gas phase of the atmosphere is adsorbed by porous materials
[16]
and stored in the channel in the form of a bulk phase to realize the enrichment of atmospheric water.
Second, water is released over time through external stimuli, such as light or heat, to a confined space. With
the continuous release of water from the porous adsorbent, this space increases in humidity relative to the