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Porous Frameworks for Effective Water Adsorption: From 3D Bulk to 2D Nano-sheets


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The generation of freshwater from ubiquitous atmospheric moisture by using appropriate water adsorbents in the atmospheric water generator, have the potential to serve as powerful strategy to effectively address the global water scarcity that threatening the lives of humankind. In this regard, preparation and selection of water adsorbents is the essential premise. In this review, we summarize the latest progress in the development of porous frameworks for water harvesting. First, we introduce the system engineering for hygroscopic salts and fabrication of nano-porous super-hygroscopic hydrogel, followed by the design of nanomaterials with controlled morphologies and structural design strategy for metal-organic frameworks (MOFs). The porous adsorbents with new forms (porous organic polymers (POPs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs) and two-dimensional (2D) materials) are then summarized in detail. Furthermore, future challenges and directions for this emerging field are outlooked.
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Cite this: Inorg. Chem. Front., 2021,
Received 16th November 2020,
Accepted 13th December 2020
DOI: 10.1039/d0qi01362e
Porous frameworks for eective water adsorption:
from 3D bulk to 2D nanosheets
Lan Zhang,Wen-Xia Fang,Cong Wang,Hui Dong, Shu-Hua Ma and
Yang-Hui Luo *
The generation of freshwater from ubiquitous atmospheric moisture via using appropriate water adsor-
bents in atmospheric water generators has the potential to serve as a powerful strategy to eectively
address global water shortages that are threatening the lives of humans. In this regard, the preparation
and selection of water adsorbents are the essential premise. In this review, we summarize the latest pro-
gress in the development of porous frameworks for water harvesting. First, we introduce systems engin-
eering for hygroscopic salts and the fabrication of nano-porous super-hygroscopic hydrogels, followed
by the design of nanomaterials with controlled morphologies and a structural design strategy for metal
organic frameworks (MOFs). Porous adsorbents with new forms ( porous organic polymers (POPs),
covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and two-dimen-
sional (2D) materials) are then summarized in detail. Finally, future challenges and directions relating to
this emerging eld are discussed.
1. Introduction
As it is the worlds most important molecule, water is the
driving force for all nature.
Water is everywhere, in the sky, on
the ground, as well as under the ground, throughout life-sus-
taining biological, chemical, and geological processes.
Hence, the manipulation of water molecules, in multifarious
dimensions, for various applications, is of fundamental impor-
tance. Among these, water adsorption by porous frameworks,
for the development of water-related industrial processes and
water-production technologies, has occupied the central posi-
tion, attributed to the enormous demand for eective removal
of trace amounts of water for industrial gas transport and
as well as the urgent need to relieve fresh-
water scarcity through facile and energy-saving methods.
should be noted that, for both of the purposes, the key aspects
lie in the design and fabrication of absorbent porous frame-
works, as well as the related device, by comprehensively balan-
cing the hydrolytic stability, water adsorption capacity and
energy-demand for water release.
To achieve an ideal water adsorption performance, such as
a high water uptake, low energy demand for water release, fast
water capture/release, high cycling stability, and low cost, sig-
nificant eorts have then been made to rationally design
materials and structures for use as water adsorbents.
Among them, the most commonly used materials are porous
frameworks such as silica gels, zeolites, zeo-type inorganic
crystalline materials, composites with hygroscopic salts in sup-
porting matrices, and metalorganic frameworks (MOFs).
Currently, investigations are centered on the following three
aspects: (i) rational functionalization of the existing adsor-
bents with enhanced water anity, the large surface area and
high porosity; (ii) precise fabrication of devices for practical
water adsorption via structural designs and systems engineer-
ing; and (iii) searching for new adsorbents for the construction
of next generation water adsorption devices with an ideal
Yang-Hui Luo
Yang-Hui Luo is an associate
professor at Southeast University
at present. He was born in 1988
in Hunan Province, P. R. China.
He received his PhD degree from
Southeast University in 2016.
His research interests are related
to the controlled preparation
and application of two-dimen-
sional (2D) metalorganic frame-
work (MOF) nanosheets and
hydrogen-bonded organic frame-
works (HOFs).
The authors are contributed equally.
School of Chemistry and Chemical Engineering, Southeast University, Nanjing,
211189, PR. China. E-mail:
898 |Inorg. Chem. Front.,2021,8,898913 This journal is © the Partner Organisations 2021
With respect to practical applications, as well as to the
development of next-generation water adsorption devices, a
comprehensive understanding of the co-relationship between
the fundamental principles of material and structural designs
and water adsorption performance is necessary. There are a lot
of review articles that have summarized the structural and
functional aspects of MOFs as water adsorbents,
no summaries for other kinds of porous adsorbents have been
published. Therefore, this review will focus on the current
strategies for the design of water adsorption materials with
promising applications. The representative strategies for struc-
tural designs and systems engineering of the commonly
involved adsorbents (hygroscopic salts, functional nano-com-
posites, hygroscopic hydrogels, as well as MOFs, Fig. 1) within
the last 3 years will be discussed in the next section. The fol-
lowing section deals with newly developed porous adsorbents
species, such as porous organic polymers (POPs), covalent
organic frameworks (COFs), hydrogen-bonded organic frame-
works (HOFs) and two-dimensional (2D) materials (Table 1). In
the final section, the outlook for water adsorbents has been
2. Functionalization of porous
A desirable adsorbent for eective water adsorption relies on
its excellent hydrolytic stability under operational conditions,
a high water capacity that can be maintained even after a mul-
titude of water uptake and release cycles, and low energy-input
for water release, as well as ideal water sorption properties.
Rational materials design and systems engineering can
provide the expected water adsorption performance for adsor- bents, by introduction of functional components that can
promote spontaneous vapor sorption.
2.1 Systems engineering for hygroscopic salts
Hygroscopic salts, such as LiCl, CaCl
or CuCl
, are classical
water adsorbents, attributed to their outstanding ability to
take up water into their bulk phase at a low relative humidity
(RH). However, the spill-over of desiccant and subsequent cor-
rosion of the device, as well as particle agglomeration attribu-
ted to the deliquesce of salt, have blocked their widespread
Furthermore, the concentrated solutions of
hygroscopic salts can also be used as adsorbents, unfortu-
nately, they suer from the drawbacks of complicated appar-
atus, high capital costs and diligent management.
To solve
these thorny aspects, one promising strategy is to integrate the
hygroscopic salts into a stable porous matrix. Wang et al. have
made anhydrous salts (copper chloride (CuCl
), copper sulfate
), and magnesium sulfate (MgSO
)) into bilayer water
collection devices,
with a silica fibrous filter substrate as the
bottom layer for salt-loading, and carbon nanotubes (CNTs) as
the top layer for photothermal assisted water release (Fig. 2).
This design strategy has enabled these devices to be capable of
capturing water from air, even at a low RH (down to 15%), and
Fig. 1 A summary of currently used water adsorbents.
Table 1 The classication of the water adsorbents listed in this review
Category Ref.
of porous
engineering of
hygroscopic salts
Fabrication of
with controlled
Nanorods 39
Nanofibers 4850
nanostructures 5357
functionalization 6163
Structural design
of metalorganic
Improve the
stability of MOFs 7175
Pursuit of S-shaped
water isotherms 7682
of MOFs 8392
Incorporation of
MOFs with
Porous adsorbents
with new forms Porous organic
polymers (POPs) 99
Covalent organic
bonded organic
(2D) materials 103
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the release of the adsorbed water can be triggered by regular
and even weakened sunlight (i.e., 0.7 kW m
Apart from a stable porous matrix, a polymer matrix can
also act as a versatile support for hygroscopic salts. Moreover,
a polymer matrix usually features excellent properties for
various applications in terms of macroscopic shaping. Wang
et al.
have further rationally fabricated a PAM-CNT-CaCl
hydrogel, by using CaCl
, acrylamide (AM) and CNTs (Fig. 3a).
Among the composite hydrogels, the AM-based platform can
maintain the solid form of CaCl
even after adsorbing a large
amount of water. As a consequence, a superior water sorption
capacity even in low humidity air, and easy release of the har-
vested water under regular sunlight via the photothermal
eect, have been achieved (Fig. 3b and c), providing a cheap
and aordable strategy for water harvesting and delivery.
Based on these results, Fröba et al. have incorporated CaCl
into a sodium alginic acid-based polymer matrix to obtain algi-
nate-based hydrogels,
the latter can further generate compo-
site beads with a size of around 2 mm via a facile ionotropic
gelation method (Fig. 4a), suggesting a cheap, non-toxic, and
easily accessible strategy for the preparation of the salt hydro-
gel. It should be noted that this kind of composite bead can
adsorb 100% of its own weight in water from air in arid
climate zones (at 10 mbar water vapor pressure and 28 °C),
and 90% of the adsorbed water can be released at 100 °C, pro-
viding a potentially solar-driven atmospheric water harvesting
More recently, Wang et al. have modified sodium alginate
by using lithium chloride (LiCl) and calcium chloride (CaCl
to give a binary hydrophilic polymeric salt for atmospheric
water harvesting for the first time,
the functionalized carbon
nanotubes (FCNTs) are embedded in the hydrogel structure to
enable the solar-driven process (Fig. 4b). As a consequence, an
adsorption capacity of about 5.6 g g
(desiccant) has been
achieved, a value that is almost three times higher than that of
the individual salts (Fig. 4c). This research team further pre-
pared a binary salt composite by using hydrophilic salts LiCl
and MgSO
, supported by an activated carbon fiber (ACF).
This obtained composite can be fabricated into a prototype for
atmospheric water harvesting, with an adsorption capacity of
0.92 g g
in an arid climate powered by solar energy.
All of the afore-mentioned strategies deal with the hygro-
scopic salts at the macro-scale, which may result in the draw-
back of local particle agglomeration. Hence, Wang et al.
incorporated LiCl into a nano carbon hollow capsule (Fig. 5a),
generating a novel nano vapor sorbent with an improved water
adsorption capacity of over 100% of its own weight. In
addition, embedding of the resulting nano sorbent to the
fibrous silica substrate created a batch-mode atmospheric
water harvesting device, which showed a water adsorption
Fig. 2 The fabrication process for bilayer water collection devices.
Reproduced with permission from ref. 25. Copyright 2018, American
Chemical Society.
Fig. 3 (a) A schematic diagram of the PAM-CNT-CaCl
hydrogel syn-
thesis process. (b) and (c) water vapor sorption curves of PAM-CaCl
and PAM-CNT-CaCl
. Reproduced with permission from ref. 26.
Copyright 2018, American Chemical Society.
Fig. 4 (a) A schematic diagram of the production of composite beads.
Reproduced with permission from ref. 27. Copyright 2018, Springer
Nature. (b) A schematic diagram of the sorption-desorption mechanism
of Bina/FCNT. (c) A comparison between the water sorption capacity
and dynamic behavior of samples at 25 °C with RH = 70%. Reproduced
with permission from ref. 28. Copyright 2020, American Chemical
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capacity of 1.6 kg
in outdoor conditions
(Fig. 5bd), hinting at a possible method of large-scale deploy-
ment of atmospheric water harvesting for practical purposes.
2.2 Fabrication of nano-porous super-hygroscopic hydrogel
It is well-known that super water-absorbent gels are capable of
absorbing ten times its own weight in liquid water, therefore
the design of special nano-porous moisture-absorbent gels
with an ultrahigh moisture-absorbing capacity and water-
storing ability, is expected. In addition, the enormous surface
area of nano-porous gels promotes fast moisture absorption
from the atmosphere, and the binding forces between water
and the material are usually dominated by physisorption
rather than hydroxylation, which thus contributes to water
release in an energy-saving manner.
Tan et al. have
designed an amorphous super-hygroscopic hydrogel, which is
a non-stoichiometric oxide of zinc derived from zinc acetate in
the presence of glycol ether, amino-alcohol and water
(Fig. 6a).
This hydrogel features nano porous and fringed
contours (Fig. 6bd), which provide an enormous surface area
for moisture absorption from the atmosphere, accompanied
by weak binding forces with water molecules. As a conse-
quence, this hydrogel is capable of absorbing water from
highly humid atmospheres (above the surface of the sea) of
over 420% of its own weight (Fig. 6e), and the desorption
process can be triggered by natural sunlight (at around 55 °C),
providing an external energy-less water harvesting approach
for the air above sea water (Fig. 6f).
Apart from the randomly synthesized amorphous super-
hygroscopic hydrogel, the precise design of a hydrogel with a
controlled hydrophilicity has also attracted attention. Yu
et al.
have constructed a super moisture-absorbent gel by
using hygroscopic poly-pyrrole chloride (PPy-Cl) penetrated
into a hydrophilicity-switchable polymeric network of poly
N-isopropylacrylamide (poly-NIPAM) (Fig. 7a and b). It was
interesting that the dierent components within the gel have
shown a clear division in functionality: the PPy-Cl was respon-
sible for moisture absorption and liquefaction, the network of
poly-NIPAM was responsible for water storage, while the hydro-
philicity switching of poly-NIPAM was responsible for rapid
water release. As a result, in situ water liquefaction, high-
density water storage and fast water release under dierent
weather conditions, have been achieved (Fig. 7c), providing a
novel design strategy to improve atmospheric water harvesting,
as well as other water management systems for environmental
cooling, surficial moisturizing and beyond.
2.3 Nanomaterials with controlled morphologies
It is well-known that nanomaterials with controlled mor-
phologies, such as nanowires, nanofibers and nanorods, often
feature interesting properties.
For water adsorption, the
application of nanomaterials with controlled morphologies
may produce unusual water adsorptiondesorption mecha-
nisms, as well as an unprecedented water adsorption
2.3.1 Nanorods. Nune and Heldebrant et al.
have pre-
pared carbon-based rods by using 1-hexanol, 1,8-diazabicy-
cloundec-7-ene and CS2, in the presence of FeCl
(Fig. 8a).
Water adsorption experiments have suggested that the confine-
Fig. 5 (a) A schematic illustration of the fabrication process for a nano-
sorbent and its related water harvesting device. (b)(d) Static RH tests of
LiCl, HCS, and HCS-LiCl under dierent RH conditions. Reproduced
with permission from ref. 30. Copyright 2019, Elsevier.
Fig. 6 (a) The simulated stable structure of Zn : O with the ratio 1 : 1.1
(blue balls represent zinc atoms and yellow balls represent oxygen
atoms). (b)(d) Scanning electron microscopy (SEM) images of the
hydrogel showing the porous network and nano-fringes on the surface.
(e) Absorption rates for dierent surface area to mass (SAW) ratio hydro-
gels. (f ) The prototype device oating on the sea surface. Reproduced
with permission from ref. 34. Copyright 2019, Wiley.
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ment-mediated solvent cavitation mechanism, the adsorption
of water at low RH beginning with the formation of a mono-
layer of water on the surfaces of rods, which is followed by the
condensation of water in the confined space between adjacent
rods, can be attributed to the interfacial forces between the
confined rod surfaces. Finally, with the increase of the RH, the
surface-induced evaporation phenomenon occurs (Fig. 8b). It
should be noted that, this process (the adsorption of water at
low RH and release at high RH) is reversible, suggesting that
this kind of rod possesses a promising potential for use in
unconventional water separations, as well as for humidity-
responsive applications.
2.3.2 Nanofibers. Among the technologies used for atmos-
pheric water capture and release, the fabrication of functional
nanofibers by using several synthetic materials with adjustable
geometries have attracted considerable attention.
corresponding nanofibers are capable of controlling their
surface wettability to change their hydrophilicity and hydro-
phobicity, which are inspired by the surface structures of
natural systems such as spider webs and cacti.
among the afore-mentioned functional nanofibers, the temp-
erature-responsive properties are of particular interest, as they
can undergo a phase transition at the lower critical solution
temperature (LCST) as a response to the external temperature,
which contributes to variation of the hydrophilichydrophobic
Choi et al. have designed a temperature-responsive hydrogel
nanofiber with a coreshell structure, by using poly(VCL-co-AA)
(VCL = N-vinylcaprolactam; AA = acrylic acid) as the shell and
PAN (polyacrylonitile) as the core (Fig. 9a).
The core part fea-
tures a high mechanical, thermal, and chemical resistance,
while the shell part acts as a temperature-sensitive system with
a self-cross-linking capacity and increased hydrophilicity,
which thus contributes to the switchable swelling capacity for
the hydrogel nanofiber. As a consequence, a higher water
uptake (234%) has been observed under high humidity air at
a low temperature (Fig. 9b and c), which can be attribute to
the diusion of water molecules in the shell layers that are
present between the polymer chains, providing a versatile
model for the design of temperature-responsive nano-struc-
tures with controllable hydrophilichydrophobic properties in
response to small temperature changes.
Apart from the functionalization of synthetic materials, the
appropriate matching of hydrophobic and hydrophilic fibers
can also enable the controllable wetting properties of nano-
fibers. Stachewicz et al.
prepared optimal meshes for harvest-
ing water from fog, by combining hydrophobic polystyrene
(PS) and hydrophilic polyamide 6 (PA6) with a two-nozzle
Fig. 7 (a) A schematic illustration of the skeleton, porous structure, and
interpenetrating network of poly-NIPAM and PPy-Cl clusters. (b) SEM
images of a dried gel at dierent magnications, showing the porous
structure. (c) Water production from 24 h of atmospheric water harvest-
ing (AWH) at dierent RH levels. Reproduced with permission from ref.
36. Copyright 2019, Wiley.
Fig. 8 (a) SEM images of a carbon rod synthesized at 230 °C. (b) A
schematic illustration of the proposed water expulsion mechanism.
Reproduced with permission from ref. 39. Copyright 2016, Springer
Fig. 9 (a) An illustration of the preparation process of thermo-respon-
sive P(VCL-co-AA)/PAN coreshell nanobers and thermo-triggered
water capture and release. (b) Water uptake kinetics of coreshell
nanobers at 25 °C and 80% RH. (c) Water sorption isotherms of the
CS-5 nanobers measured at 15, 25, 40, and 50 °C. Reproduced with
permission from ref. 48. Copyright 2019 American Chemical Society.
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electrospinning setup. Without any chemical modifications,
this kind of hierarchical composite can be used for practical
water harvesting from fog, attributed to the water condensing
ability of the hydrophobic microfibers and water delivery
ability of the hydrophilic nanofibers (Fig. 10).
In addition to the combination of dierent synthetic
materials or dierent kinds of fibers, modification of fibers
with a gradient roughness can also act as a versatile alternative
to achieve controllable hydrophilichydrophobic properties.
Zheng and Hou et al.
fabricated heterostructure rough
spindle-knot microfibers (HRSFs) via a flexible parallel-nozzle
microfluidic method (Fig. 11), which featured a roughness gra-
dient between the spindle-knots and joints. During the water
collection process, the joint part acts as the transmission
channel that coalesces the tiny droplets of moisture and trans-
ports them to the spindle-knot sections. It should be noted
that, the water collection eciency was dominated by the
surface morphology of spindle-knots, the higher the roughness
gradient, the higher the water collection eciency, suggesting
it has a high potential to be used in large-scale water
2.3.3 Bioinspired nanostructures. During the process of
the design of microstructures to facilitate water harvesting,
natural environments have provided us with abundant ideas,
especially from organisms that are native to arid environ-
ments, such as cacti spines, desert moss, lizards spider webs,
and Namib desert beetles.
The multiscale surface mor-
phology of which plays a key role in the water collection
eciency, attributed to the variation of Laplace pressures that
are determined by their conical or pointed morphologies and
the chemical composition of materials. Xue and co-workers
have proposed the idea of a cascading eect for atmospheric
water harvesting,
by controlling the Laplace gradient, the
water droplets can coalesce with the adjacent one from a
similar structure (Fig. 12a), which thus has a sucient volume
to roll downward and assimilate all the small water droplets to
be condensed on the flat surface of the harvester, eectively
clearing the PWHR ( passive water harvesting region) columns
with a considerable water harvesting ability (Fig. 12b). Inspired
by leaf veins with hierarchical micro-nanostructures for water
deposition, Sariola et al.
fabricated water-harvesting func-
tional surfaces consisting of high-density copper oxide nano-
needles (Fig. 12c), the later featured a high wettability that
could be enhanced by the hydrophobic coating, providing valu-
able insights into the design of water harvesters in arid or
semi-arid environmental conditions (Fig. 12d).
Despite the control of the Laplace gradient, the design of
nano-materials with similar structures to native organisms has
attracted significant attention. Cao and co-workers
have pre-
pared a wax-infused kirigami with an anisotropic 2D triangle,
which shows similar spines to the 3D cone of cactus kirigami
Fig. 10 An illustration of the combined mechanism for the hierarchical
composite and corresponding water harvesting performance.
Reproduced with permission from ref. 49. Copyright 2019 American
Chemical Society.
Fig. 11 (a) An illustration of the fabrication process of bioinspired
microbers via the microuidic method. (b) and (c) Optical images of
collected dehydrated microbers in large quantities. (d) and (e)
Microscopic images of the microber before (d) and after (e) dehydra-
tion. Reproduced with permission from ref. 50. Copyright 2019 Wiley.
Fig. 12 (a) A schematic representation of the mechanism of water har-
vesting that results in a cascading eect that clears the PHWR of tiny
water droplets. (b) A comparison between the water harvested per area
of dierent water harvesting arrays (WHAs). Reproduced with permission
from ref. 53. Copyright 2019 American Chemical Society. (c) The guided
transportation of water via hydrophilic milled channels in uncoated CuO
nanoneedles (left) and coated CuO nanoneedles (right). (d) Fog harvest-
ing dynamics as the volume of water collected per square meter over a
surface area of 22 500 mm
for all surfaces. Reproduced with permission
from ref. 54. Copyright 2019 Royal Society of Chemistry.
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(Fig. 13a). This kind of 2D triangle can reproduce the function
of the 3D cone, and can capture fog droplets eectively and
rapidly refresh the collecting interface through directional
droplet self-propulsion (Fig. 13b), providing a rational design
for advanced fog harvesters. More recently, inspired by the
directional transportation of water droplets to the apex on
both the beaks of shore birds and wheat awns, Zheng and co-
have constructed a topological alloy net with a
V-shaped asymmetric geometry in its mesh for fog collection
(Fig. 13c), which not only improved the water-collection rate
owing to the ecient drainage along the designated pathways,
but also resolved the issue of mesh clogging, thus providing a
useful insight into the development of novel fog-collecting
materials with excellent performances along all directions
(Fig. 13d). Inspired by the microchannels of natural woods
that serve as pathways to pump and transport water from the
ground via transpiration, Ding and co-workers
have fabri-
cated a moisture pump with multilayer wood-like cellular net-
works and interconnected open channels (Fig. 13e), by using a
facile and scalable two-step electrospinning and impregnation
method. In the pump, both a desiccant layer and a photother-
mal layer are presented, contributing to the unprecedented
moisture absorption capacity of 3.01 g g
at 90% RH triggered
by solar-irradiation (Fig. 13f and g).
2.3.4 Functionalization of the surface. Apart from the con-
struction of nanomaterials with controlled morphologies, the
functionalization of surfaces to generate a hydrophobic-hydro-
philic gradient has also served as a versatile alternative for
water adsorption.
Inspired by the lubrication eect of
Nepenthes pitcher plants, Wang and co-workers have prepared
slippery liquid-infused porous surfaces by synergistically con-
structing a regular micro-pincushion and nanoparticles, which
display the well maintained dropwise coalescence of water
from fog and an ecient water harvesting performance.
Similarly, Chen et al. have fabricated a flexible functional
surface that features a superhydrophobic region and hydro-
phobic region by using an infused lubricating oil, which has
successfully achieved directional water collection.
It should be highlighted that, although MOFs themselves
are versatile water adsorbents (which will be discussed later),
they can also be used for surface functionalization to provide
an improved water adsorption capacity or water transport per-
Song et al. performed selective hydrophobic modi-
fication on the surface of a soy protein, using the construction
of hierarchical micro-/nano-crystals of ZIF-8 (Fig. 14a). This
strategy was inspired by the Stenocara beetle that can collect
water from moist air, and have generated controllably hydro-
Fig. 13 (a) The concept of simplifying 3D into 2D for the design of
cactus kirigami. (b) The fog-collecting performance on the kirigami
spines. Reproduced with permission from ref. 55. Copyright 2020 Royal
Society of Chemistry. (c) The topological alloy net with V-shaped asym-
metric geometry. (d) Water-collection rates with dierent orientations
compared to an equivalent conventional mesh. Reproduced with per-
mission from ref. 56. Copyright 2019 American Chemical Society. (e) A
schematic diagram showing the 3D self-assembly mechanism of the
moisture pump with a multilayer wood-like cellular network structure.
(f ) and (g) The moisture absorption kinetics of PAN/MIL@LiCl NFM at
25 °C and at various humidities. Reproduced with permission from ref.
57. Copyright 2020, Springer Nature.
Fig. 14 (a) A proposed mechanism illustrating the interaction between
ZIF-8 and SPI (soy protein isolate). (b) The water collection mechanism
of SA-ZIF-8@SPI lms. Reproduced with permission from ref. 62.
Copyright 2019 Elsevier.
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philic-superhydrophobic patterns on their protein film surface
for ecient water harvesting from fog (Fig. 14b), with the
maximum water collection eciency being as high as
917.6 mg cm
2.4 Structural design for metalorganic frameworks
Among the current investigated water adsorbents, MOFs
occupy a special position. Attributed to the modular construc-
tion from molecular building blocks, MOFs feature a large
diversity of frameworks that allow for the chemical and geo-
metrical optimization necessary to achieve the desired water
sorption properties, which thus endow MOFs with the merits
of a high chemical stability to water, tailorable hydrophilicity,
as well as an adjustable pore diameter to fine tune the adsorp-
tion profile and modulate the sorption kinetics, meeting the
high requirements for next generation adsorbents.
of reviews have summarized the progress of MOFs for water
Therefore, this part will focus on the repre-
sentative strategies for MOFs-based water adsorbents within
the last three years.
2.4.1 Improving the stability of MOFs. The desired water
stability of MOFs is a precondition for their application as
water adsorbents. Generally, two strategies can be employed to
construct MOFs with a special topology and improve the water
stability, one is the incorporation of hydrophobic groups, the
other is the use of high oxidation state metals.
Li et al.
have constructed MOFs with the-a topology, by using ZrCl
with fluorescent ligand acids, 5-(4-carboxyphenyl)-2,4,6-tri-
methyl-[1,1:3,1-terphenyl]-4,4-di carboxylic acid (H3CTTA)
(Fig. 15b) and 6,6,6-(2,4,6-trimethylbenzene -1,3,5-triyl)tris(2-
naphthoic acid) (H3TTNA) (Fig. 15b). This kind of topology
endows these MOFs with an excellent stability in water, HCl
solutions and NaOH solutions. Later, guided by this topologi-
cal design approach, base-resistant,
well as stability adjustable MOFs,
have all been constructed
by Li and co-workers. The results have provided eective hints
for the development of MOFs water adsorbents.
2.4.2 Pursuit of S-shaped water isotherms. S-shaped water
isotherms, which mean that large water uptakes and releases
can be achieved through relatively small temperature or
pressure gradients, are desired for use in adsorption heat
transformation systems for water harvesting, because they are
a necessary prerequisite for energy ecient water release from
the sorbent.
MOFs usually tend to exhibit
S-shaped isotherm profiles, which can be attributed to the for-
mation of ordered, hydrogen-bonded water molecule networks
within the crystalline metal organic framework (MOF) during
the water nucleation and pore-filling process. To achieve the
desired S-shaped water isotherms, hydrophilicity, and pore
diameter are of critical importance. Ideally, the pore hydrophi-
licity must be sucient to allow for water nucleation and pore
filling below approximately 30% RH, while the pore size must
be below the critical diameter (D
) of the working fluid, to
eliminate undesirable hysteresis upon water desorption. It
should be noted that, for water at 25 °C, the D
is 20.76 Å,
demonstrating that a pore diameter of approximately 20 Å for
porous adsorbents is optimal for water adsorption with irre-
versible capillary condensation having been perfectly
hence the structural design of MOFs is of signifi-
cant importance. Dinca et al.
demonstrated a series of MOFs
with the formula M
(BTDD) (M = Mn, Co, Ni; BTDD = bis
(1H-1,2,3-triazolo [4,5-b],[4,5-i])dibenzo[1,4]dioxin) (Fig. 16a)
that are desirable materials for practical water adsorption.
These MOFs featured large mesoporous channels with a dia-
meter of 22 Å, as a consequence, S-shaped water isotherms
under simulated desert conditions (daytime 45 °C and 5% RH
and nighttime 25 °C and 35% RH) have been achieved,
accompanied by a water adsorption capacity of over 82% of its
own weight below 30% RH (Fig. 16b). Later on, Farha et al.
designed acs-MOFs by using trivalent trinuclear metal clusters
and a rigid trigonal prismatic ligand (Fig. 16c), which resulted
in a 6-c acs topology with a pore size as large as 1.4 nm
(Fig. 16d), which also featured S-shaped water isotherms with
a high water uptake of 1.09 g g
(Fig. 16e) and a considerable
cycling ability (Fig. 16f), providing a practical design strategy
for the construction of highly stable functional porous MOFs
with a targeted pore size and geometry for next-generation
porous water vapor sorbents.
As one of the smallest organic linkers, formate (HCO
) can
be used to build porous MOFs for eective water adsorption,
attributed to its various coordination modes thanks to the
smallest possible side group on the carboxylate carbon. In
addition, formate can be conveniently derived in situ from the
hydrolysis of amide-based solvents such as DMF or DEF
during the solvothermal process, which thus can contribute to
a simple, straightforward, and highly eective one-pot syn-
thesis. Lah et al.
synthesized Zr-based MOFs by using in situ
Fig. 15 Structures of (a) H3CTTA and (b) H3TTNA, as well as the crystal
structures of their related MOFs with D
8-connected Zr6 clusters.
Reproduced with permission from ref. 71. Copyright 2016 American
Chemical Society.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898913 | 905
generated formate from the hydrolysis of DMF, and produced
a macrocyclic [Zr
and super-cage-like {[Zr
building unit
(Fig. 17a). These MOFs featured a highly polar surface and flex-
ible crystal packing, which thus contributes to the high heat of
water adsorption and unrestricted uptake under high humid-
ity. In addition to the direct construction of MOFs with excel-
lent water adsorption capacities, formic acid can also be used
to regulate MOFs with improved water adsorption perform-
ances. Chakraborty et al.
have found that the addition of
formic acid can promote, to a certain extent, the increment in
water adsorption for Al Fum (aluminum fumarate) MOFs (up
to 12.5%, Fig. 17b). Additionally, a novel facile synthetic strat-
egy for MOFs could also improve the water adsorption per-
formance to some extent.
2.4.3 Functionalization of MOFs. To obtain the desired
water adsorption performance for MOFs, functionalization is
the most commonly used strategy. In most cases, the
functionalization of MOFs is focused on the metal sites, the
free anions, structural defects, or the surfaces of particles or
pores. For metal sites, grafting of active groups on coordina-
tively unsaturated metal centers is an eective strategy.
Meanwhile, the employment of high-valence or rare-earth
metals can contribute to improvement of the water adsorption
performance for MOFs,
thanks to the stronger MOFH
interactions provided by the terminal active groups bound to
the rare earth sites to give a high water anity (Fig. 18a).
Furthermore, functionalization of the metal clusters can also
precisely tune the water adsorption performances of MOFs.
For free anions, Lin et al.
calculated that the terminal anions
, Cl
, or OH
) play a key role in the water adsorption
performance for MOF MIL-100(Fe), they found that MIL-100
(Fe)_F was the best owing to the strongest interaction between
the terminal F
anion and water molecules. The conclusion
was confirmed by Dinca et al.,
who exchanged the Cl
anions in Ni
, Br
and OH
anions (Fig. 18b),
Fig. 16 (a) The crystal structure of Co
(BTDD) projected along the c
axis: Co, purple; C, gray; N, blue; O, red; and Cl, green. Hydrogen atoms
are omitted for clarity. (b) Water isotherms of Co
(BTDD) under simu-
lated desert conditions. Reproduced with permission from ref. 79.
Copyright 2017 American Chemical Society. (c) A schematic representa-
tion of the design and synthesis of the acs-a net. (d) An illustration of the
hexagonal channels inside the acs-a net. (e) A comparison of the water
sorption isotherms of the acs-a net between the rst cycle and after 20
cycles of pressure swing between 20% RH and 70% RH. (f ) The cycling
testing for the acs-a net. Reproduced with permission from ref. 80.
Copyright 2019 American Chemical Society.
Fig. 17 (a) The crystal structure of the {[Zr
building unit. Color
code: green, Zr; red, O; gray, C; white, H; and orange, Cl. Reproduced
with permission from ref. 81. Copyright 2018 American Chemical
Society. (b) The adsorptiondesorption behaviors of parent and FAM
(formic acid modulated) Al Fum MOFs at 25 °C. Reproduced with per-
mission from ref. 82. Copyright 2018 Elsevier.
Fig. 18 (a) The crystal structure of the high-valence MOF [Er
O (H
dcbp = 4,4-dicarboxy-2,2-bipyridine).
Left: The binding style of the active groups with the metal sites. Right: A
view parallel to the aaxis. Reproduced with permission from ref. 87. (b)
A view of anion-exchanged secondary building units perpendicular to
the caxis and synthetic pathways for crystal Ni
BTDD (X = Cl, F, Br,
OH). (c) Water vapor adsorption (closed symbols) and desorption (open
symbols) isotherms measured at 25 °C. Reproduced with permission
from ref. 90. Copyright 2019 American Chemical Society.
Review Inorganic Chemistry Frontiers
906 |Inorg. Chem. Front.,2021,8,898913 This journal is © the Partner Organisations 2021
and found that the F
anions can improve the water adsorp-
tion performance (Fig. 18c), attributed to the increase in the
pore size and strength of the MOF-water hydrogen bonding
Lin further cooperated with Grossman
to calculate the
eect of structural defects on the water adsorption perform-
ance of MOF-801, and they found that the high defect density
may be responsible for the hydrophilic adsorptive behaviors,
and that the water adsorption performances are dependent on
the spatial configuration of defects. In addition, the photoche-
mically induced water harvesting in MOF Ni-IRMOF74-III has
been calculated by Kim et al., (Fig. 19a)
the results suggest
that the trans-to-cis isomerization of the azopyridine molecules
can lead to a significant enhancement in the water adsorption
performance (Fig. 19b), which can be attributed to the dier-
ence in pore partitioning for trans/cis configurations, as the
trans case possesses a more available surface area than the cis
case. These results have provided a special blueprint for the
design of next generation water harvesting materials.
2.4.4 Incorporation of MOFs with functional materials.
Apart from the smart structural design, the incorporation of
MOFs with functional materials can also serve as an eective
strategy to achieve the desired water adsorption performance.
Via an in situ polymerization strategy, Zhao and Maurin et al.
have constructed MOF/polymer composite materials by using
MIL-101(Cr) and PNIPAM (poly(N-isopropylacrylamide),
(Fig. 20a).
Mediated by a LCST, the resulting polymer@por-
ous MOF composite showed an unprecedented water adsorp-
tion capacity of approximately 440 wt% at 96% RH and 25 °C
(Fig. 20b), and the adsorbed water can be released under mild
conditions (40% RH and 40 °C). It should be noted that, these
promising aspects can be attributed to the hydrophilic-to-
hydrophobic phase transition of the PNIPAM component as a
response to the thermal variation, which is expected to shed
light on the development of stimuli-responsive porous adsor-
bent materials.
Additionally, the incorporation of hygroscopic salts into
as well as the coating of MOFs on stainless-steel
meshes for the construction of underoil super-hydrophilic sur-
can significantly improve the water adsorption per-
formances of MOFs.
3. Porous adsorbents with novel
Apart from the above-mentioned popular porous frameworks,
the other novel forms of porous frameworks, such as POPs,
COFs, HOFs and 2D materials, have also shown potential to be
used in water adsorption, which has extensively extended the
scope of material classes suitable for water adsorption, as well
as bringing great benefits to water adsorption technology.
3.1 POPs
The POPs have attracted significant attention owing to their
high physicochemical stability, high surface areas, and
tunable functionalities, which thus can serve as versatile plat-
forms to provide an optimum binding energy for water mole-
cules, via balancing between the uptake capacity and regener-
ation temperature.
In particular, epoxy functional groups can
act as a sound candidate for water adsorption, owing to their
high hydrophilicity and moderate binding enthalpy for water
molecules, as a consequence, a high water uptake capacity and
low-temperature regeneration can be expected.
et al.
have prepared multi-dimensional ep-POPs via a cata-
lyst-free, one pot DielsAlder cycloaddition polymerization
(Fig. 21a), these were found to be highly microporous and
exhibit specific surface areas up to 852 m
. As a conse-
quence, high water-uptake capacities of up to 39.242.4 wt%
under a wide temperature range of 545 °C have been achieved
Fig. 19 (a) A snapshot of H
O adsorption in both trans and the cis azo-
pyridine-IRMOF74-III at 2 kPa. (b) The H
O adsorption isotherm from
azopyridine-IRMOF74-III at 25 °C. Reproduced with permission from ref.
92. Copyright 2019 American Chemical Society.
Fig. 20 (a) An illustration of the preparation of a MOF/polymer compo-
site and the temperature-triggered water capture and release process.
(b) The water sorption isotherms of various samples at 25 °C.
Reproduced with permission from ref. 93. Copyright 2020 Wiley.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898913 | 907
(Fig. 21b), providing a promising strategy for the design of
POPs materials for desiccant-driven dehumidification and
water capture.
3.2 COFs
Similar to MOFs, COFs are versatile water adsorbents owing to
their exceptional porosity, large diversity of chemical compo-
sitions and accessible topologies, furthermore, their water
sorption properties can be tuned in a large variety of ways.
However, the relatively lower crystallinity of COFs usually pre-
cludes the formation of highly ordered molecular water net-
works within the porous framework, which thus hinders the
practical applications of COFs for water adsorption. Yaghi
et al.
have synthesized a COF with a (3,4,4)-c mtf topology,
by using 1,1,2,2-tetrakis(4-aminophenyl) ethene (ETTA) and
1,3,5-triformyl benzene (TFB) (Fig. 22a). This specific topology
has endowed COFs with a high water uptake capacity that is
comparable to most of the MOFs (Fig. 22b), and accompanied
by a considerable cycling stability (Fig. 22c), which has thus
paved the way for future research on COFs as water-harvesting
3.3 HOFs
Similar to MOFs and COFs, HOFs also feature exceptional
porosity and tunable framework behaviors for a large variety of
applications. However, a high stability and permanent porosity
remain a challenge for HOFs. In our previous work,
employing the equidistance eect, HOF TCPP-1,3-DPP (TCPP
=meso-tetra(carboxy-phenyl)-porphyrin, 1,3-DPP = 1,3-di(4-
pyridyl) propane) (Fig. 23a), with a permanent porosity
(Fig. 23b), has been prepared. This shows a high anity and
selectivity to water with a maximum adsorption capacity of
about 9.8% (Fig. 23c), providing a practical strategy for the
design of HOFs-based water adsorbents.
3.4 2D materials
It is well-know that birnessite (i.e., a layered structure MnO
displays a layered structure that is intercalated by cations and
water molecules, which thus shows the potential to be used for
water harvesting.
Suib et al.
have demonstrated that the
water molecules can be quickly adsorbed into the interlayers
of birnessite at a lower RH (Fig. 24a), and the adsorbed mole-
cules can form multilayer waterwater interactions on the 2D
surfaces, via hydrogen bonding contacts. As a consequence, at
a higher RH, the water molecules can be condensed in situ.
Meanwhile, birnessite features an excellent solar absorptivity
that can convert solar to thermal energy, which thus contrib-
utes to the solar-triggered release of water. More importantly, a
prototype of a water harvesting device based on birnessite has
Fig. 21 (a) The synthesis of 2D and 3D epoxy-functionalized ep-POPs
using DielsAlder cycloaddition polymerization. (b) Volumetric water-
adsorptiondesorption isotherms of 2D and 3D ep-POPs at 298 K.
Reproduced with permission from ref. 97. Copyright 2018 Wiley.
Fig. 22 (a) The reaction of ETTA and TFB to generate (3,4,4)-c mtf
topology (shown in its augmented form). (b) Water sorption analysis
measured at dierent temperatures. (c) Water cycling stability testing for
300 adsorptiondesorption cycles conducted at a constant water vapor
pressure (1.7 kPa). Reproduced with permission from ref. 98. Copyright
2020 American Chemical Society.
Fig. 23 (a) The crystal structure of TCPP-1,3-DPP: the connecting style
of the one-dimensional (1D) porous stripe (left), and the stacking of 1D
stripes to provide 1D oval-shaped channels on the 3D framework. (b)
Low-pressure N
sorption isotherms at 77 K. (c) The sorption isotherms
of activated TCPP-1,3-DPP for H
, CH
, and H
at 25 °C; the solid
symbols represent adsorption, while the open symbols represent de-
sorption. Reproduced with permission from ref. 99. Copyright 2018
Review Inorganic Chemistry Frontiers
908 |Inorg. Chem. Front.,2021,8,898913 This journal is © the Partner Organisations 2021
been constructed, suggesting its high potential for practical
applications (Fig. 24c). One thing that should be stressed is
that the employed 2D materials are in the bulk phase, if they
were exfoliated into ultra-thin 2D nanosheets, the full
exposure of 2D surfaces could be achieved, then higher water
adsorption capacities could be expected. Considering that
there is a great variety of 2D materials, this work will inspire
further future research on 2D materials as practical water
4. Outlook for water adsorbents
Water scarcity is increasingly being perceived as a global chal-
lenge threatening the lives of humans, and has attracted sig-
nificant attention, and AWH has could be a promising solu-
tion. AWH technology can generate freshwater regardless of
the geographical and hydrologic conditions, and renewable
energy can be utilized to avoid energy consumption, which has
thus been considered to be widely used as a decentralized
water supply in the near future. For all of the AWH methods
(i.e., water harvesting, dewing, and sorption-based technology),
the key aspect lies in the design of the adsorbents to balance
the requirement of moisture concentration and water release.
As a reflection of all the aforementioned research eorts and
promising prospects, the current challenge for the develop-
ment of ideal water adsorbents can be summarized as follows:
firstly, the search for novel sorbent materials with improved
working capacities under temperature and/or pressure swing
conditions is critically needed; secondly, a comprehensive
understanding of the sorption kinetics, via both experimental
exploration and theoretical prediction, for the in-depth inspec-
tion of the migration and aggregation behaviors of water mole-
cules during the AWH process, is still lacking; and thirdly,
precise device-engineering to render sorbent materials into
freshwater delivery systems remains to be established.
Among the above-mentioned three major tasks, the search
for novel sorbent materials is the basic premise. For all of the
aforementioned materials, the pros and cons are co-existent.
Specifically, for hygroscopic salts, how to fully reach their
potential after being integrated into a stable porous matrix, is
the key issue, and requires an exact match between the salts
and matrix. For the hygroscopic hydrogels, expanding their
working RH range, especially under low RH conditions, is
necessary for their wide-spread application. For nanomaterials
with given morphologies, a clear understanding of the mor-
phology-performance relationship is highly desired. While for
the porous frameworks such as MOFs, POPs, COFs and HOFs,
the water stability, adsorption capacity, and the recyclability
are the bridges that need to be crossed prior to their practical
In our opinion, 2D materials show the greatest potential for
the development of next-generation sorbent materials; the
reasons for this are as listed as follows: (i) the interlayers of 2D
materials can serve as warehouses for the quick adsorption
and condensation of water molecules over a large range of RH
values; (ii) the bulk phases of 2D materials have already shown
high adsorption capacities, let alone exfoliated 2D nanosheets.
The latter features fully exposed 2D surfaces that can promote
the physical adsorption of water molecules via weak inter-
actions, which thus contribute to eective water release at the
expense of a small amount of energy input; and (3) the agile
size and morphology changes of 2D materials are more condu-
cive to device fabrication. Inspired by this, the application of
2D MOF nano-sheets as eective water adsorbents for practical
AWH is currently being carried out in our lab.
Conicts of interest
There are no conflicts to declare.
Fig. 24 (a) An illustration of water adsorption and condensation
between interlayers, as well as solar-triggered water release. (b) The
water vapor isotherms of dierent MnO
sorbents at 25 °C. (c) The water
sorption isotherm of the MnO
-1 sample and simulated curves; the inset
shows a schematic diagram of the water sorption mode. (d) A prototype
of a water harvesting device with images of collected water on the
surface of the condenser. Reproduced with permission from ref. 103.
Copyright 2019 American Chemical Society.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898913 | 909
This work was supported by the Natural Science Foundation of
China (Grant No. 21701023), the Natural Science Foundation
of Jiangsu Province (Grant No. BK20170660), the Zhishan
Youth Scholar Program of the SEU and PAPD of Jiangsu
Higher Education Institutions.
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... Emerging Water adsorbents involve porous organic polymers, metal-organic structures, hydrogen-bonded organic structures, covalent organic structures, bioinspired nanostructures, nano-porous water-absorbent gels, controlled morphologies nanomaterials, nanofibers, nanorods, and two-dimensional nanosheets materials. The physicochemical characteristics of merging Water adsorbents for water capture by dehumidification such as hydrophilicity, stability, binding enthalpy, surface areas, water uptake and tunable functionalities are extremely significant when designing such porous organic polymers materials 59 . AW irrigation process using solar-powered for sustainable farming has been proposed. ...
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The Arabsphere struggles with highly complicated water challenges due to climate change, desertification, coronavirus pandemic, and Russo-Ukrainian War. This paper explores how to build a robust water vision to pave the road to achieving sustainable development goals (SDGs) in the Arabsphere. A sustainable water future (SWF) necessitates an interdisciplinary and transdisciplinary research strategy. ‘Horizon scanning’ process (HSP) is one of the promising foresight methodologies. A generic process for “Horizon scanning” has been developed to cope with water crises and challenges. “DEEPEST” holistic framework has been designed to suit both the “Futurology” science and water, environment, and engineering disciplines. “DEEPEST” characterizes Demographics, Ecological, Environmental, Political, Economic, Social, and Technological features. The macro-future factors (MFF) applied in the foresight process (FP) have been presented. The results showed that Water conservation (WC), Circular Water (CW), and Emerging Water Technologies (EWTs) were the main outcomes of the ‘Horizon scanning’ process (HSP). The paper concluded that the preparing for a sustainable water future (SWF) must be right now and the opportunities range from the deepest water drop to the highest water drop on Earth. The essence of the conclusion is hydrosphere sustainability, particularly in Arabsphere, should be given extreme concentration, effort, and support.
... Recently reported composite sorbent LiCl@rGO-SA 23 with exceptional water uptake has extremely high ratio of desorption vs. sorption water uptake up to 14:1 (Fig. 3b). So far, no scientific study reported sorbent with the sorption/desorption ratio approaching 1:1 in a wide range of RH 11,24,29,30,39 . Therefore, the progress of future sorbent materials used for continuous AWH should be oriented towards equalized sorption vs. desorption time. ...
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Sorption- and radiative sky cooling-based atmospheric water harvesting (AWH) technologies hold promise to provide decentralized fresh water in highly remote and arid regions. Recent emergence of advanced energy materials such as hygroscopic metal-organic frameworks, hydrogels and nanoporous composite sorbents, in addition to radiative sky cooling materials provided the necessary means for AWH. However, truly continuously operational devices are yet to be developed and field-tested. The research focus is now facing the paradigm shift, as future AWH systems are challenged to provide water generation on a kilogram scale, finally meeting a recommended daily water intake per person. This criterion can be adeptly met with continuously operated devices in comparison to discontinuous ones, providing much needed compactness along energy and mass efficiency. Here we critically discuss drawbacks of current energy materials as well as system designs that are hindering the use of continuous AWH. Based on identified challenges we outline viable scientific and technological paths. In addition, a possible synergistic effect of sorbents and radiative sky cooling materials on material and system levels to achieve 24-hour continuous fresh water generation is discussed. Provided development paths can spur voluminous avenues into sustainable continuous AWH exploration, making the ultimate goal “to provide fresh water for all” a step closer.
With the rapid development of nuclear industry, effective management of nuclear waste and oversight of nuclear fuel cycle are critical. Radionuclides such as uranium (U), plutonium (Pu), neptunium (Np), americium (Am), curium (Cm), technetium (Tc), rhenium (Re), iodine (I), selenium (Se), thorium (Th), cesium (Cs), and strontium (Sr) transferred into environment are dangerous. It is crucial to design the corresponding materials to exhibit high adsorption capacity and selectivity among competing species in nuclear waste. Herein, this review comprehensively summarizes the application of advanced porous materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and amorphous porous organic polymers (POPs) as porous adsorbents for radionuclides removal. These porous materials feature uniform composition, large porosity, and good stability, which lay a good foundation for various applications. The tunable pore sizes, high specific surface areas, exchangeable sites, and functional groups are designed as accessible platforms for nuclides diffusion and adsorption. Specific binding mechanisms toward various radionuclides, such as complexation, electrostatic interaction, and ion exchange are presented. Beyond traditional adsorbents, the superior capacity, kinetics, selectivity, and reusability of COFs, MOFs, and POPs make them broad application prospects in radionuclides removal, providing a way for effective applications in environmental remediation.
Although metal-organic frameworks (MOFs) with intriguing physiochemical properties have recently attracted increasing attention of researchers, MOFs have not adequately achieved various applications due to their instability and unsatisfactory processability. Hence, MOF-based composites as well as structurally novel MOFs have been established to solve this challenge. Recently, some efforts have been made for the development of MOF-based hydrogels. They exhibit excellent properties compared to initial MOFs in many ways (e.g., mechanical strength, absorption capacity and total pore volume). In this review, the up-to-date research progress of MOF-based hydrogels will be briefly summarized. We mainly focus on expounding the classification of MOF-based hydrogels and their related synergistic effects. As to details, we will provide a conspectus of laconic and latest approaches for the synthesis of MOF-based hydrogels, focusing on composite systems, mechanism research, and stability. In addition, the applications of MOF-based hydrogels have been widely discussed, particularly in the environmental, energy and medical aspects. It is hoped that the detailed description of this review can help researchers working on MOF-based hydrogels to develop more novel composite materials and investigate their wider application.
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Thousands have lived without love, but none without water. The floods in Germany and China that took place in July 2021 have highlighted the need for drinkable water supply in post-disaster regions. Of all the materials that could bring potable water to those in a dire time of need, metal-organic frameworks have raised a few eyebrows because they could directly capture water from air, even under very dry conditions. But before water harvesting technologies based on metal-organic frameworks could serve their purposes by providing potable water, there are clearly more things to be done to resolve the current limitations on supporting materials, as well as the information asymmetry.
The practical application of molecular switches was highly dependent on the reversible spin-state switching under ambient conditions, which needs a delicate materials design. Here, by confining spin-crossover material into a...
Atmospheric water harvesting is a promising strategy to address the water scarcity in islands. In this work, an activated carbon fiber (ACF) templated hybrid water adsorbent ACF-cobalt(II)-ethanolamine (ACF-Co-EA) was fabricated and used to build an ecological farm (Eco-farm) for potential application on tropical coral islands. ACF-Co-EA took the advantage of both the pore structure of ACF and the superabsorbent property of the Co-EA complex, thus, exhibiting superior water harvest capacity over ACF or Co-EA. The equilibrium water adsorption of ACF-Co-EA was 763 mg·g⁻¹ at 25 °C and 70% RH (typical environment of tropical coral islands). Under 1 kW·m⁻² simulated solar irradiation, the surface temperature of ACF-Co-EA increased rapidly to 50 °C within 12 min and stabilized at 54 °C in 30 min because of the superior light absorbance of ACF, which made more than 90% of captured water released. ACF-Co-EA-based Eco-farm could harvest 6.9 g·g⁻¹·day⁻¹ of water to ensure plant growth in the tropical coral islands' environment without any additional energy or water supply. The study provided novel ideas to alleviate the problems of freshwater scarcity and food shortage in the tropical coral islands.
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Water collection from moisture in air, i.e., atmospheric water harvesting, is an urgent future need for society. It can be used for water production everywhere and anytime as an alternative water source in remote areas. However, water harvesting and collection usually relies on desalination, fog, and dewing harvesting, which are energy intensive. In this respect, metal–organic frameworks (MOFs) have broad applicability for water harvesting in water-scarce areas; therefore, the current discussion focuses on this approach. Furthermore, recent progress on MOFs for moisture harvesters is critically discussed. In addition, the design, operation, and water harvesting mechanisms of MOFs are studied. Finally, we discuss critical points for future research for the design of new MOFs as moisture harvesters for use in practical applications. Graphical Abstract MOF adsorbents offer excellent operating capacity in various temperature and pressure ranges. Rational water harvesters can thus be developed by adjusting structural properties such as the porosity, functionalities, and metal centers, thereby enabling new devices to produce water even in remote areas.
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Sorption‐based atmospheric water harvesting (SAWH) holds huge potential due to its freshwater capabilities for alleviating water scarcity stress. The two essential parts, sorbent material and system structure, dominate the water sorption–desorption performance and the total water productivity for SAWH system together. Attributed to the superiorities in aspects of sorption–desorption performance, scalability, and compatibility in practical SAWH devices, hygroscopic porous polymers (HPPs) as next‐generation sorbents are recently going through a vast surge. However, as HPPs’ sorption mechanism, performance, and applied potential lack comprehensive and accurate guidelines, SAWH's subsequent development is restricted. To address the aforementioned problems, this review introduces HPPs’ recent development related to mechanism, performance, and application. Furthermore, corresponding optimized strategies for both HPP‐based sorbent bed and coupling structural design are proposed. Finally, original research routes are directed to develop next‐generation HPP‐based SAWH systems. The presented guidelines and insights can influence and inspire the future development of SAWH technology, further achieving SAWH's practical applications.
The development of atmospheric water generator has depended critically on the preparation of hygroscopic materials. In our systematic efforts toward porous frameworks for effective humidity reduction, a super hygroscopic materials,...
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Water is essential to life. It is estimated that by 2050 nearly half of the world population will live in water stressed regions, due to either arid conditions or lack of access to clean water. This Outlook, written for the general readers, outlines the parameters of this vexing societal problem and presents a solution to the global water challenge. There is plenty of water in the air that potentially can be harvested not only from the desert atmosphere where the humidity is low but also from more humid regions of the world where clean water is needed. In principle, the materials used to harvest water from air in these climates should be applicable to deployment anywhere in the world to extract atmospheric water at any time of the year. Metal–organic frameworks (MOFs) have emerged as a unique class of porous materials capable of trapping water at relative humidity levels as low as 10%, and doing so with facile uptake and release kinetics. From laboratory testing to field trials in the driest deserts, kilogram quantities of MOFs have been tested in several generations of devices. The initial results of these experiments showed that MOFs could capture water from desert climates and deliver over one liter per kilogram of MOF per day. More than an order of magnitude increase in water productivity could be achieved with members of the MOF family when employed in an electrified device operating at many cycles per day. We show that the vision of having clean water from air anywhere in the world at any time of the year is potentially realizable with MOFs and so is the idea of giving “water independence” to the citizens of the world.
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Desiccants play vital roles in dehumidification and atmospheric water harvesting; however, current desiccants have mediocre hygroscopicity, limited recyclability, and high energy consumption. Herein, we report a wood-inspired moisture pump based on electrospun nanofibrous membrane for solar-driven continuous indoor dehumidification. The developed moisture pump with multilayer wood-like cellular networks and interconnected open channels is composed of a desiccant layer and a photothermal layer. The desiccant layer exhibits an unprecedented moisture absorption capacity of 3.01 g g−1 at 90% relative humidity (RH), fast moisture absorption and transport rates, enabling atmospheric water harvesting. The photothermal layer shows a high solar absorption of 93%, efficient solar thermal conversion, and good moisture permeability, thus promoting water evaporation. The moisture pump efficiently reduces the indoor relative humidity to a comfort level (40‒60% RH) under one-sun illumination. This work opens the way to develop new-generation, high-performance nanofibrous membrane-based desiccants for energy-efficient humidity control and atmospheric water harvesting. Desiccants are important for dehumidification, but application is hindered by limited hygroscopicity, recyclability, and energy efficiency. Here, the authors report a moisture pump comprised of an electrospun nanofibrous memebrane for solar-driven continuous indoor dehumidification.
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The water crisis is a big social problem and one of the solutions are the Fog Water Collectors (FWCs) that are placed in areas, where the use of conventional methods to collect water is impossible or inadequate. The most common fog collecting medium in FWC is Raschel mesh, which in our study is modified with electrospun polyamide 6 (PA6) nanofibers. The hydrophilic PA6 nanofibers were directly deposited on Raschel meshes to create the hierarchical structure that increases the effective surface area which enhances the ability to catch water droplets from fog. The meshes and the wetting behavior were investigated using a scanning electron microscope (SEM) and environmental SEM (ESEM). We performed the fog water collection experiments on various configurations of Raschel meshes with hydrophilic PA6 nanofibers. The addition of hydrophilic nanofibers allowed us to obtain 3 times higher water collection rate of collecting water from fog. Within this study, we show the innovative and straightforward way to modify the existing technology that improves water collection by changing the mechanisms of droplet formation on the mesh.
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The steep stepwise uptake of water vapor and easy release at low relative pressures and moderate temperatures together with high working capacities make metal–organic frameworks (MOFs) attractive, promising materials for energy efficient applications in adsorption devices for humidity control (evaporation and condensation processes) and heat reallocation (heating and cooling) by utilizing water as benign sorptive and low-grade renewable or waste heat. Emerging MOF-based process applications covered are desiccation, heat pumps/chillers, water harvesting, air conditioning, and desalination. Governing parameters of the intrinsic sorption properties and stability under humid conditions and cyclic operation are identified. Transport of mass and heat in MOF structures, at least as important, is still an underexposed topic. Essential engineering elements of operation and implementation are presented. An update on stability of MOFs in water vapor and liquid systems is provided, and a suite of 18 MOFs are identified for selective use in heat pumps and chillers, while several can be used for air conditioning, water harvesting, and desalination. Most applications with MOFs are still in an exploratory state. An outlook is given for further R&D to realize these applications, providing essential kinetic parameters, performing smart engineering in the design of systems, and conceptual process designs to benchmark them against existing technologies. A concerted effort bridging chemistry, materials science, and engineering is required.
The application scope of metal-organic frameworks (MOFs) is severely restricted by their weak chemical stability and limited pore size. A robust MOF with large mesopores is highly desired, yet poses a great synthetic challenge. Herein, two chemically stable Ni(II)-pyrazolate MOFs, BUT-32 and -33 were constructed from a conformation-matched elongated pyrazolate ligand through the isoreticular expansion. The two MOFs share the same sodalite-type net, but have different pore sizes due to the network interpenetration in BUT-32. Controlled syntheses of the two MOFs have been achieved through precisely tuning reaction conditions, where the microporous BUT-32 was demonstrated to be a thermodynami-cally stable product while the mesoporous BUT-33 is kinetically favored. To date, BUT-32 represents the first example of Ni4-pyrazolate MOF whose structure was unambiguously determined by single-crystal X-ray diffraction. Interestingly, the kinetic product BUT-33 integrates 2.6 nm large mesopores with accessible Ni(II) active sites and remarkable chemical stability even in 4 M NaOH aqueous solution and 1 M Grignard reagent. This MOF thus demonstrated an excellent catalyt-ic performance in carbon-carbon coupling reactions, superior to other Ni(II)-MOFs including BUT-32. These findings highlight the importance of kinetic control in the reticular synthesis of mesoporous MOFs, as well as their superiority in heterogeneous catalysis.
Atmospheric water harvesting (AWH) emerges as a promising means to overcome the water scarcity of arid regions, especially for inland areas lacking liquid water sources. Beyond conventional system engineering that improves the water yield, the novel moisture harvesting materials provide new aspects to fundamentally promote the AWH technology benefiting from their high tunability and processability. Innovative material and structural designs enable the moisture harvesters with desirable features such as high water uptake, facile water collection and long-term recyclability, boosting the rapid development of next-generation AWH. In this perspective, we first illustrate the sorption mechanism, including absorption and adsorption for moisture harvesting materials and summarize fundamental requirements as well as design principles of moisture harvesters. Recent progress on material and structural designs of moisture harvesters for AWH is critically discussed. We conclude with prospective directions for next-generation moisture harvesters to promote AWH from scientific research to practical application.
Air humidity, as a source of water, is more or less available everywhere. The sorption capacity of water is a significant factor for the efficiency of atmospheric water harvesting (AWH) systems, which are based on the adsorption phenomenon. Lithium chloride has a high water-uptake rate, but has a very low delinquency relative humidity (DRH). Therefore, a host is required to make a stable composite. Composite of activated carbon fiber (ACF) and lithium chloride can keep the adsorbent immobile even after the occurrence of deliquescence, which also causes the three-phase sorption. However, the amount of salt inside the composite is limited by the prevention of leakage. In this paper, a binary salt composite is produced by a new method in order to enhance the water sorption capacity in terms of volume, and mass and the prevention the leakage. The effect of adding MgSO4 to the composite has been experimentally investigated for at different levels of relative humidity. The results showed that sorption capacity per unit volume and mass can be improved by the two-stage addition of MgSO4 without leaking in the adsorbent, reaching 0.78 g water/cm³ and 2.29 gwater/gadsorbent. The prototype made by the selected composite showed that AWH energy intensity was lower at higher relative humidity. The device was tested successfully in an arid climate and produced 0.92 gwater/gadsorbent when maximum RH reached 35% during the adsorption process.
The advancement of additional methods for freshwater generation is imperative to effectively address the global water shortage crisis. In this regard, extraction of the ubiquitous atmospheric moisture is a powerful strategy allowing for decentralized access to potable water. The energy requirements as well as the temporal and spatial restrictions of this approach can be substantially reduced if an appropriate sorbent is integrated in the atmospheric water generator. Recently, metal–organic frameworks (MOFs) have been successfully employed as sorbents to harvest water from air, making atmospheric water generation viable even in desert environments. Herein, the latest progress in the development of MOFs capable of extracting water from air and the design of atmospheric water harvesters deploying such MOFs are reviewed. Furthermore, future directions for this emerging field, encompassing both material and device improvements, are outlined. The latest progress in the development of metal–organic frameworks and their integration in an apparatus for water harvesting are reviewed.
Harvesting the atmospheric water is a promising method for solving water crisis in undeveloped regions, possessing remarkable advantages as simple structure, energy independence, low cost, etc. Cactus spines with conical shape is able to achieve an efficient water harvesting, but the fabrication of such three-dimensional structure is complicated and tedious. Here we simplify the cactus-inspired fog collecting spines from 3D cone to 2D triangle by designing a cactus kirigami. The wax-infused kirigami with anisotropic shape can reproduce the function of cactus spines, i.e., the efficient capture of fog-droplets and rapid refreshing of the collecting interface through directional droplet self-propulsion. Fluid simulation suggests that the thinner spine with a small apex angle give a higher onward flow speed for the better fog capture. On the basis of the promising functions and the simplified structure, the cactus kirigami can be scaled up. Under a fog flow of ~220 cm/s, the water harvesting rate of cactus kirigami can reach ~4000 mg•cm-2•h-1, which is 1.6 and 11 times the rates of harp-like and plate collectors respectively. Furthermore, the cost of this paper-based substrate and the construction process is largely reduced to nearly 0.5 $/m2. This work provides a rational design for advanced fog harvesters, and should unlock more possibility to develop functional materials from 3D to 2D in microfluidics, condensation, liquid collection, etc.
We report an in‐situ polymerization strategy to incorporate a thermo‐responsive polymer, poly(N‐isopropylacrylamide) (PNIPAM), with controlled loadings into the cavity of a mesoporous metal‐organic framework (MOF), viz. MIL‐101(Cr). The resultant MOF/polymer composites exhibit an unprecedented temperature‐triggered water capture and release behavior originating from the thermo‐responsive phase transition of the PNIPAM component. This result sheds light on the development of stimuli‐responsive porous adsorbent materials for water capture and heat transfer applications under relatively mild operating conditions.