Recent advances in atmospheric water harvesting technology and its development

Abstract

Water scarcity is a pressing issue worldwide. Given the ample atmospheric water sources, water harvesting from the atmosphere presents a promising solution to this challenge. In recent years, the solar-driven atmospheric water harvesting technology utilizing an adsorption–desorption process has garnered considerable interest. This is attributed to the abundant availability of solar energy, advanced adsorbents, improved photothermal materials, sophisticated interface heating system designs, and efficient thermal management techniques, all of which collectively enhance conversion efficiency. This article provides an overview of the advancements in atmospheric water collection, specifically focusing on hygroscopic water harvesting driven by solar energy. The discussion also encompasses the roles of materials, surfaces, equipment, and systems in enhancing water collection efficiency. By outlining both the advantages and challenges of atmospheric water collection, this study aims to shed light on future research directions in this research field.

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Wider impact

We believe that this review will inspire readers’ interest in atmospheric water harvesting. In recent years, water scarcity has been a pressing issue worldwide, and the solar-driven atmospheric water harvesting technology utilizing an adsorption–desorption process has garnered considerable interest. In this review, we first introduce the status of global water scarcity, and then we propose that atmospheric water harvesting technology develops as a promising solution to address water scarcity. Following that, we introduce three developing atmospheric water harvesting technologies, namely passive water harvesting, active water harvesting, and semi-active solar-driven water harvesting (SAWH) which mainly employs hygroscopic materials. Thus, we conclude the design principles for various hygroscopic materials, and we also discuss the effects of the design of structures and systems on water collection efficiency. Moreover, based on comparing different SAWH systems, we summarize the advantages and challenges of the atmospheric water harvesting approaches. Finally, we propose future research directions based on the review, the comparison, the discussion, and the conclusions. This review comprehensively concludes recent advances in solar-powered atmospheric water harvesting technology and its development, and it offers a brand new perspective angle to understand the development and challenges.

1. Introduction

1.1. The status of global water scarcity

The imbalance between the shortage of water resources and the increase of the global population has become one of the major challenges facing society. Recent research has shown that approximately two-thirds of the world's population (about 4 billion people) face a moderate water shortage for at least one month per year. In the meantime, more than 500 million people have been confronted with extreme freshwater shortages for nearly a full year.^1–11 It is widely acknowledged that fresh water is an indispensable necessity for human beings, yet freshwater resources are severely limited. According to the US Environmental Protection Agency, freshwater resources make up only 3% of global water resources. Almost 68% of freshwater is stored in glaciers or frozen soil, while the remaining 30% is dispersed as underground water. Although there is an abundance of fresh water, only 0.3% of total freshwater resources are easily accessible for human use. This uneven distribution of freshwater resources has significant implications. According to the World Health Organization, nearly 60% of the total global freshwater supply is concentrated in just nine countries, leaving eighty countries and regions, representing 40% of the world's population, facing the lack of freshwater.^1–12 It is projected that by 2025, three billion human beings will live in water-scarce regions, with over forty countries and regions experiencing severe water scarcity.

1.2. AWH technology development opportunities and challenges

In order to address the above issue, various solutions have been proposed. With the abundance of seawater resources and the limited availability of freshwater resources, seawater desalination has become a solution to water scarcity. In coastal areas such as the Middle East, Arabia, North America, and Asia, various seawater desalination technologies such as reverse osmosis, multi-effect flash evaporation, steam compression, and electroosmosis have achieved effective results.^13–16 In addition, sewage treatment is also a conventional method of water treatment. In order to maximize water resource utilization, sewage treatment methods have been developed and refined, mainly including physical, chemical, and biological methods. However, the applicability of these methods may be limited in inland water-scarce regions. In such cases, the extraction of water from the atmosphere, known as atmospheric water harvesting (AWH), has received significant attention. As an important process in the natural water cycle, the amount of water resources in the atmosphere is deemed sufficient to meet human needs. The primary source of atmospheric water is water evaporation, with an estimated freshwater volume exceeding 12.9 × 10³ km³, which is equal to one-eighth of the total freshwater contained in rivers and lakes.¹⁷ Atmospheric water exists in three main forms: clouds, fog, and water vapor. As the captured water comes from the air, the influence of geographical factors is somewhat reduced. Besides, as a clean and renewable energy source, solar energy provides an ideal energy input source for AWH technology. Thus, solar-powered AWH systems reduce reliance on fossil fuels and contribute to the sustainable use of water resources. At the same time, with the increasing demand for clean water resources, AWH technology has broad market prospects in agricultural irrigation, drinking water supply, and industrial water usage. Although AWH technology has shown favorable performance in laboratory settings, its real-world implementation faces challenges related to stability, durability, and scale production. This technology holds the potential to revolutionize our approach to water resource management, offering a decentralized and sustainable solution to freshwater scarcity, especially in regions lacking direct access to liquid water sources. By directly obtaining usable fresh water from the natural water cycle, AWH technology minimizes adverse impacts on the local ecosystem.

2. Overview of the AWH technology

Atmospheric water exists as a liquid and gas state in the air, and it can be captured by condensation and absorption processes, respectively. For different phases of the water, various approaches have been developed, such as fog collection, dew harvesting, and absorption/desorption methods, to harvest water. The fog/dew collection techniques enhance the aggregation of small droplets, leading to the formation of larger droplets that can be further collected through the use of airflow or gravity. The primary objective of the fog/dew collection technology is to accelerate the growth of water droplets adhering to the surface of the collector. Similarly, the fundamental aim of the absorption/desorption atmospheric water harvesting method is to convert gas-phase water into liquid-phase water that is suitable for collection. The conventional method of collecting gas-phase water involves cooling the surrounding air below the dew point and then accumulating the condensate. However, given the high energy barrier of cooling water, innovative methods focus on leveraging hygroscopic materials to reduce the energy of water acquisition by facilitating interactions between water molecules and the materials.¹⁸ Therefore, numerous studies during recent years that investigated the atmosphere water harvesting have been carried out, and they are reviewed in this work.

In terms of energy demand, atmospheric water harvesting technology can be categorized into three main types.^19–22 As shown in Fig. 1, the first type is passive systems, which operate without the need for additional energy input. The second type consists of active systems that need additional energy to enable water collection processes. The third type is semi-active systems, which rely on solar energy. It is called the solar-driven hygroscopic system, which involves the utilization of adsorbent materials and photothermal components to harness solar energy for driving water evaporation and then desorption in sorbents. Each of these technologies is tailored to specific working scenarios based on their energy requirements. Passive AWH systems do not rely on external energy inputs but mainly on natural conditions such as temperature and humidity differentials to drive water collection. Examples of passive systems typically include dew collectors, fog traps, and passive radiative cooling units. On the other hand, active AWH systems require additional energy inputs, such as electricity, to power the water collection and treatment processes. These systems encompass technologies like compression refrigeration, membrane separation, as well as adsorption/absorption refrigeration methods. Semi-active systems are mainly represented by solar-powered AWH systems, which use solar energy as a sustainable and green energy source to drive the collection and release of water through photothermal conversion or photochemical processes. In this paper, the atmospheric water collection technology is classified according to energy requirements, and the performance of devices and adsorbents used are introduced. Finally, the future research direction is introduced according to the advantages and disadvantages of atmospheric water collection.

Fig. 1 The overview of the atmospheric water harvesting technology, and the classifications based on the energy input. There are main three types of atmospheric water harvesting approaches: passive water harvesting, active water harvesting, and semi-active harvesting. Each atmospheric water harvesting approach contains different specific harvesting materials and systems.^19–22 Reproduced from ref. 19 with permission from the Royal Society of Chemistry, copyright 2024. Reproduced from ref. 20 with permission from John Wiley and Sons, copyright 2024. Reproduced from ref. 21 with permission from Elsevier, copyright 2024. Reproduced from ref. 22 with permission from Advanced Materials, copyright 2024.

2.1. Performance index

For further analysis and improvement of AWH technology, some metrics are employed to quantify its performance. Four metrics are commonly used: the specific water production per day per unit collector area (SWP), the specific energy consumption per unit mass water production (SEC), the specific water production per day per unit mass adsorbent material (SMP), and the recovery ratio of the feed air (RR). SWP is often used for passive AWH, and SEC is often employed for the analysis of the energy efficiency of the active AWH.where V_w is the total water produced, A is the collector area and t is the water collection time.where E is the total energy consumed and W is the mass of water produced.where m_w is the mass of total water produced and m_abs is the mass of adsorbent material.where V_w is the volume of water produced, V_air is the volume of the feed air that passes through the system, RH represents relative humidity and Δh is the moisture-holding capacity of the air.

2.2. Passive water harvesting

Passive systems do not need extra energy input and naturally harvest water from the air under various conditions.²³ Passive atmospheric water harvesting relies on natural processes to collect water from humidity in the air. This technique typically utilizes materials that facilitate condensation, such as hygroscopic substances or specially designed surfaces that cool down at night, allowing moisture to condense from the air. The resulting water droplets are then collected, often with minimal energy input or mechanical systems. This method is particularly effective in arid regions where humidity levels can still support condensation under appropriate conditions. These systems, due to their simple mechanism, were among the first water collection systems to be used and developed.²⁴ As presented in Fig. 2, classic passive systems include rainwater collection (Fig. 2a and b),^25,26 fog capture (Fig. 2c and d),^27,28 dew collection (Fig. 2e and f),^29,30 and others. Rainwater collection, being the simplest form of water harvesting technology, has been utilized in agricultural production and daily life for thousands of years. The Maya civilization, dating back 6000 years, was already utilizing rainwater harvesting techniques. With the continuous development of industry and economy, the increasing demand for water supply has made rainwater harvesting an effective way of acquiring fresh water.³¹ Current methods of rainwater collection include the construction of large facilities such as dams, ditches, and reservoirs, followed by filtration, disinfection, and other cleaning steps before the water enters the facility. Additionally, smaller rainwater collection devices based on individual households involve the collection of rainwater through the roof drainage system. In coastal arid regions, fog capture presents a feasible solution to the shortage of freshwater resources due to the high humidity and fast wind speed in these areas. The process of fog trapping involves the capture of water droplets carried by the wind using a rectangular network device, where the accumulated droplets are eventually collected in a water collection device. The first experiments on fog harvesting were conducted in 1956 by the Catholic University of Antofagasta Norte in Northern Chile.³² Since then, similar experiments have been carried out in various arid regions around the world, such as coastal deserts in West Africa (Namibia), South America (Chile and Peru), and the Middle East (Saudi Arabia and Oman, monsoon season), leading to successful projects.

Fig. 2 Illustration of various atmospheric water harvesting systems. Passive water harvesting systems: (a) and (b) rainwater collection,^25,33 Reproduced from ref. 25 with permission from Elsevier, copyright 2024. Reproduced from ref. 33 with permission from water, copyright 2024. (c) and (d) Fog harvesting,^27,28 Reproduced from ref. 27 with permission from Oxford University Press, copyright 2024. Reproduced from ref. 28 with permission from American Chemical Society, copyright 2024. (e) and (f) Dew collection,^29,30 Reproduced from ref. 29 with permission from American Chemical Society, copyright 2024. Reproduced from ref. 30 with permission from Elsevier, copyright 2024. Active water harvesting systems: (g) and (h) compression refrigeration,^34,35 Reproduced from ref. 34 with permission from Elsevier, copyright 2024. Reproduced from ref. 35 with permission from applied sciences, copyright 2024. (i) and (j) Thermoelectric/absorption cooling,^35,36 Reproduced from ref. 35 with permission from applied sciences, copyright 2024. Reproduced from ref. 36 with permission from Elsevier, copyright 2024. (k) and (l) Membrane separation,^37,38 Reproduced from ref. 37 with permission from Elsevier, copyright 2024. Reproduced from ref. 38 with permission from Elsevier, copyright 2024. Semi-active water harvesting system using different materials: (m) adsorbent loaded with CaCl₂,³⁹ Reproduced from ref. 39 with permission from John Wiley and Sons, copyright 2024. (n) Zeolite adsorbent,⁴⁰ Reproduced from ref. 40 with permission from Elsevier, copyright 2024. (o) MOF adsorbent,⁴¹ Reproduced from ref. 41 with permission from The American Association for the Advancement of Science, copyright 2024.

As previous fog capture projects were typically situated far from residential areas, the high cost of transporting the captured water posed a significant obstacle. Wahab et al. investigated the feasibility of setting up fog traps closer to residential areas.⁴² According to Cereceda et al., a feasible and effective fog trapping system must cater to prolonged periods of fog and have a high water droplet content alongside sufficient wind speed in arid or semi-arid areas.⁴³ The main challenges faced in improving current fog capture efficiency are the fast capture of droplets, the anti-tangling of the collecting structures, and the clogging of the liquid film. Notably, the design of the surface structure of the collection device also affects the collection efficiency, with excessively large gaps reducing the capture rates and small gaps causing droplet blockage. Although superhydrophobic treatment of the surface can relieve this effect to a certain extent, the superhydrophobic treatment normally cannot be maintained for a long time. Inspired by Zibold's discovery in the first half of the 20th century, Chaptal, Goddard, and Knapen further modified the stone bowl-shaped condenser to create a water-condensing device called the air well. This device used the temperature difference between day and night and the high ocean humidity to obtain liquid water. Although the device limited the output of condensate due to the performance defects of the material, it also drew people's attention to the topic of dew condensation.^44–46 The dew condensation involves using the temperature difference at night to induce water vapor condensation on the surface through radiation heat release. In other words, the object releases heat through radiation at night; once the surface temperature of the object is lower than the dew point of the surrounding air, the dew condenses on the surface.^47,48 To improve the dew collection efficiency, materials with high emissivity and low thermal conductivity, such as white low-density polyethylene (LDPE) or specially coated metal surfaces, are usually selected. These materials are able to effectively absorb radiation from low-temperature spaces while reducing heat conduction and convection losses. The International Dew Collection and Utilization Organization recommends LDPE as the standard collection material. In summary, the condensation process remains constrained by the rate of radiant heat exchange, climatic conditions, and the surface properties of the condensing material. The application of passive water collection technologies is primarily limited by climatic conditions, altitude, and geographical locations.

2.3. Active water harvesting

In order to mitigate the constraints posed by objective factors such as climate, altitude, and geographical conditions, the efficiency of obtaining freshwater resources is improved through the utilization of additional energy sources. With the previous research on cooling, along with the development of cooling technology, active surface cooling technology with additional energy input has become the main way of atmospheric water collection. Active atmospheric water harvesting involves the use of mechanical systems to extract moisture from the air. This technique often employs cooling and dehumidification technologies, such as refrigeration units or desiccant systems, to lower air temperature and increase condensation. The collected water is then filtered and stored for use. Active systems are more efficient in capturing larger volumes of water and can operate in a wider range of humidity conditions. As illustrated by Fig. 2g–l, the method can be mainly categorized into four types: (1) compression refrigeration technology,^28,34,49 (2) electromagnetic refrigeration technology, (3) absorption refrigeration technology, and (4) thermoelectric refrigeration technology.^36,49–52 Compression refrigeration technology is the most prevalent refrigeration technology for water collection.^{35,49–51,53} Its core is the refrigeration cycle, which includes four main components: compressor, condenser, expansion valve, and evaporator. The compressor compresses the low-pressure and the low-temperature refrigerant into a high-pressure hot state and then transmits it to the condenser. Within the condenser, the hot refrigerant at high pressure releases heat and condenses into a liquid form. Subsequently, the liquid refrigerant undergoes decompression through the expansion valve and enters the evaporator. In the evaporator, the refrigerant absorbs heat from the air, condensing the water vapor in the air into water, which is then collected and stored.³¹ Although its applicability to scenarios with substantial water consumption, compression refrigeration technology incurs relatively high cost and environmental pollution problems owing to its reliance on refrigerants. The remaining three cooling methods are all considered unconventional due to equipment or material deficiencies. For example, electromagnetic and absorption cooling technologies typically feature large and expensive equipment, whereas thermoelectric cooling technology, while characterized by compact technical equipment, suffers from weak material properties, resulting in lower overall energy conversion efficiency.

In recent years, membrane separation technology, an emerging technology, has witnessed rapid development and is applicable to industries such as seawater desalination and wastewater treatment.^54–58 Membrane separation technology can be divided into electrolytic membrane separation and selective penetration based on different separation strategies.^37,38 Electrolytic membrane separation technology uses a polymer electrolyte membrane under an electric field, followed by the migration of electrolytic ions through the membrane to reunite on the opposite end as water. In contrast, selective membrane penetration exploits a membrane with superior water vapor selectivity to drive water vapor through the membrane via a pressure difference for water collection. Owing to the high selectivity of the membrane, the water generated through membrane separation is relatively clean and has minimal impact on the environment, although there exists a reliance on electric energy support.

2.4. Semi-active solar-driven hygroscopic water harvesting

Solar hygroscopic water harvesting technology is a technology that uses solar energy as an energy source, evaporating the water captured by adsorbents from the surroundings and converting it into a usable water source by condensation process. Semi-active atmospheric water harvesting combines elements of passive and active systems to optimize water collection. This approach can use energy-efficient hygroscopic materials and passive collection techniques to increase water production while minimizing energy consumption. For example, it may involve the use of solar energy rather than relying entirely on conventional energy sources. This hybrid approach offers greater flexibility in different climates and can be tailored to specific environmental needs. The technology operates on the principle of adsorbents, including conventional adsorbents, composite adsorbents, metal–organic frameworks (MOFs), hydrogels, and other original composite materials, which adsorb water from the air and then desorb it through heating in sunlight.⁵⁹ The solar hygroscopic water collection process is divided into three main processes: the absorption of water molecules from the air, the desorption of water in the adsorbent with an evaporation process, and the re-condensation process of water molecules. These three processes correspond to adsorbent materials, photothermal conversion component, and condensation component, respectively. The selection of adsorbent has an important effect on the efficiency of water collection. An ideal adsorbent should have high hydrophilicity, a large specific surface area, high porosity, rapid water adsorption/desorption rates, and good cycling stability.⁶⁰ For instance, hygroscopic salt-based materials such as calcium chloride (CaCl₂) have been widely studied due to their high adsorption capacity and good cycling stability.⁶¹ Zeolite and silica gel are favored for their porous structure and high specific surface area.^62–66 Metal–organic frameworks (MOFs) exhibit excellent adsorption properties due to their customizable pore size and surface chemistry.⁴¹ As represented in Fig. 2m–o, researchers are constantly exploring new materials, such as adsorbents with nanoporous structures, to achieve faster adsorption/desorption kinetics, thereby improving the continuous operation of solar-driven atmospheric water harvesting (SAWH) systems.⁶⁷

The photothermal conversion component affects the efficiency of converting solar energy-driven water evaporation to heat energy in the adsorbent. To be specific, in the SAWH system, solar energy is used to heat the adsorbed material during the daytime, causing it to release adsorbed water vapor. To improve energy efficiency, researchers are exploring ways to maximize the absorptivity and conversion of solar energy, such as efficient solar collectors and thermal management systems.^68,69 An ideal photothermal element should have a wide wavelength absorption range and high photothermal conversion efficiency.⁷⁰ Moreover, the condensing system is crucial in a water collection system, as it involves the efficient collection and storage of the water vapor adsorbed by the adsorbent material and its condensation into liquid water under appropriate conditions. To improve water collection efficiency, researchers are developing innovative water collection devices, like active continuous SAWH (AC-SAWH) devices with optimized adsorption bed structures.⁷¹ By configuring two adsorbent beds and using nanoporous adsorbents, the device is able to achieve water production in a continuous mode, significantly increasing water yield and thermal efficiency. Therefore, an ideal condensation system should facilitate efficient heat exchange, possess good hydrophobicity, and simplify water droplet collection (Table 1).

Table 1 The character of different AWH technologies

	AWH technology	Efficiency/size	Cost	Environmental impact	Character	Ref.
Passive water harvesting	Rain water collection	—	Low	A hydroponic green roof system (HGRS) was developed to reduce urban storm water runoff	By collecting, treating, and reusing grey- and rainwater onsite in green buildings to obtain treated water	28
		76.3% of rainwater is collected	Low	Small agricultural reservoirs make a contribution to crop production	By collecting rainwater in ponds for irrigation	40
	Fog harvesting	The fog harvesting efficiency is from 10^−1 L (m^2 day)^−1 to 10 1 L (m^2 day)^−1	Low	—	The new model uses fluid dynamics and surface wettability to predict the overall fog collection efficiency of the mesh	30
		The highest fog harvesting is 3 g (cm^2 h)^−1	Low	—	The fog harp structure is adopted instead of the traditional mesh structure	31
	Dew collection	The dew collection efficiency is 9.34 g (m^2 min)^−1	—	The influence of supercooling degree and tilt angle on dew collection efficiency is studied, which reduces the potential energy consumption for fog collection in the future	The model of Stenocara beetles was used to study the influence of subcooling and tilt angle on dew collection efficiency	32
		—	—	The effect of the Opuntia stricta structure on fog collection was investigated, reducing potential energy consumption	The dew harvesting ability of the glochids in Opuntia stricta was investigated	42
Active water harvesting	Compression refrigeration	—	High	Improve energy efficiency, reduce dependence on energy sources such as electricity, and reduce greenhouse gas emissions	Water production process of the vapor compression condense	36
		The cooling capacity is 200 W, the size of the system is 300 × 230 × 70 mm^3	High	—	A miniature vapor compression refrigeration (VCR) system for electronics cooling has been developed	72
	Thermoelectric cooling	—	—	Without using refrigerants, so it does not cause damage to the atmosphere and has no greenhouse gas emissions	A thermoelectric cooler is made by wrapping a series of N–P type semiconductors in two thin ceramic wafers fitted with additional radiators	72
	Absorption cooling	The cooling capacity is 300 W, the size of the system is 200 × 200 × 34 mm^3	—	—	First time manufacturing of a small heat-activated absorption heat pump concept for miniaturized or mobile applications	53
	Membrane separation	—	—	Low temperature dehumidification can extend the life of low temperature working devices such as refrigerators	The condensation potential of proton exchange membrane (PEM) water at low temperature and low dew points was investigated	60
		The maximum water absorption is 61.8% at 50 °C 90%RH	—	Dehumidification can address corrosion or degradation in, for example, electronics, medical, art preservation	Study on the water condensing performance of polymer electrolyte membrane (PEM) of sulfonated pentagram terpolymer	39
Semi-active water harvesting	Salt-resistant GO-based aerogel	Water production efficiency is 2.89 kg m^−2  day^−1) at 70%RH	Low	It can be heated using only sunlight, and no additional energy is required	With interface solar heating, the desorption efficiency reaches 66.9%, which is low cost and effective	11
	Zeolite	The adsorption ability of zeolites is related structure	Low	It can be used to treat industrial wastewater and domestic sewage to reduce water pollution	It is environmentally friendly and can choose from a variety of adsorbent	73
	Metal–organic frameworks (MOFs)	Water production efficiency is 2.8 L (kg  day)^−1 at 20%RH		—	In a narrow relative humidity range, MOF-801 has a high water absorption capacity and desorption requires only low grade heat	41

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3. Design principles of different hygroscopic materials

Hygroscopic materials play an important role in SAWH technology, as they have the ability to absorb water molecules from the air through various chemical or physical mechanisms. Subsequently, the adsorbed water is broken down through heat transfer facilitated by the photothermal conversion material. Key characteristics essential for effective hygroscopic materials include a large specific surface area, numerous hydrophilic functional groups, significant porosity, and a vertical pore structure. According to the shape and composition of the hygroscopic materials, they can be divided into solid hygroscopic materials, salt-based hygroscopic materials, polymer hygroscopic materials, and composite hygroscopic materials.

Solid hygroscopic materials typically have a porous structure that can provide a large number of adsorption sites, facilitating the storage and transportation of water within the material thereby enhancing water absorption efficiency. Common examples of solid hygroscopic materials include natural zeolite, silica gel, molecular sieve, activated carbon, and alumina, which belong to the class of inorganic non-metallic materials. Zeolite, known for its large internal specific surface area and diverse pore structures, possesses abundant aluminum metal sites as water adsorption centers. Similarly, silica gel, with its large specific surface area, forms hydrogen bonds with water via a hydroxyl group for adsorption. A molecular sieve, preferred for high-temperature applications due to its hygroscopic properties, excels in deep dehumidification. Activated carbon and alumina, characterized by their significant surface area and pore structure, also exhibit commendable hygroscopicity.⁷² Hygroscopic salts, on the other hand, attract water primarily through a pressure gradient-induced adsorption mechanism. During the initial stage of adsorption, water vapor adheres to the surface of the salt particles before dissolving at the solid–liquid interface. When the vapor pressure above the liquid film is equal to the pressure of the salt solution, the water adsorption reaches equilibrium.⁷⁴ Despite their high water absorption rate, hygroscopic salts experience a decline in adsorption speed time due to salt coalescence and salt leakage during the adsorption–desorption cycle, potentially reducing the service lifespan of the associated equipment.

Polymer hygroscopic materials exhibit hydrophilic properties and certain swelling characteristics, making them suitable for AWH applications. Unlike materials with fixed surface area and pore volume, the flexibility of polymer allowed for increased water storage capacity. In addition, the 3D cross-linked network of polymer hygroscopic material enables the retention of collected water. Hydrogels and organic aerogels, two types of polymer material known for their high water absorption and retention capabilities, have been thoroughly investigated and applied in recent years;^73,75–77 however, challenges such as leakage and corrosion hinder their widespread practical applications. Metal–organic frameworks represent a newer class of coordination polymers characterized by their high porosity, large specific surface area, uniform pore structure, adjustable pore size, and topological structure diversity. Despite relatively low humidity conditions, metal–organic frameworks maintain exceptional water harvesting capacity. Furthermore, composite hygroscopic materials, typically developed by embedding hygroscopic salts within a matrix with significant pore volume, showcase promising potential in water harvesting applications.

The design of the material plays a crucial role in the trapping of water molecules, which typically adhere to the surface of the material through van der Waals forces. A hydrophilic surface is more conducive to capturing water molecules, while a hydrophobic surface makes it difficult for water molecules to adhere. As such, the properties of the material surface are closely related to its ability to absorb water.⁷⁸ The trapped water molecules need to be condensed on the surface before being transported in a directional manner. Directional transport improves collection efficiency by moving water from the surface to areas with dry surface adsorption. Methods of water transport include gravity-driven mechanisms, capillary-dependent xylem vessels, Janus surfaces, and self-pumping surfaces similar to Janus surfaces. Janus materials leverage the surface energy difference among porous materials or the different wettability of two material surfaces by transferring water/droplets from one surface to the adjacent surface, accelerating the water capture and collection.⁷⁹ Besides the material surface wettability, the shape of the material structure also counts for water harvesting, cactus, for example, utilize their spines and conical shape to facilitate the transport of water to their roots.⁸⁰ As an adsorbent material for efficient water collection, it is essential for the material to have a porous structure surface with strong hygroscopic properties and a large specific surface area.⁸¹ Moreover, considering that the material will be used in a solar-driven desorption process, incorporating photothermal components that can efficiently convert solar energy is imperative.^82–84 Therefore, when designing adsorption materials,⁸⁵ it is crucial to consider the incorporation of light-absorbing materials to maximize photothermal conversion efficiency, adsorption efficiency, and desorption efficiency.^86,87

3.1. Conventional hygroscopic materials

Conventional adsorbents attract water molecules mainly through physical adsorption, with intermolecular forces playing a pivotal role. Various forms of physical adsorption include micropore filling, capillary action, and high-concentration adsorption. As shown in Fig. 3a–c, early conventional adsorbents such as silica gel and zeolite,⁴⁰ which possess polar hydroxyl groups or a porous structure, exhibit significant adsorption capacity at high relative humidity levels.⁸⁸ Unlike silica gel and zeolite, salt solutions containing salts such as LiCl and CaCl₂ capture water through hydration until the vapor pressure matches that of the surrounding air.^89–91 However, challenges such as the agglomeration of liquid and the leakage of salt particles have hampered the efficiency of water collection using conventional adsorbents. Moreover, the considerable energy consumption required to overcome intermolecular forces remains a significant obstacle in the water harvesting process. One potential solution to this issue is incorporating photothermal materials to use solar energy for interface heating. Furthermore, the development of porous adsorbents such as metal–organic frameworks and covalent organic frameworks has led to the creation of advanced materials with enhanced performance for atmospheric water collection.⁹² Conventional materials such as metals and ceramics can provide reliable structural components for systems designed to capture moisture from the air. Their strength and durability make them suitable for frameworks and supporting structures. However, the weight and rigidity of these materials can limit the design flexibility of harvesting devices. As the push for more efficient and eco-friendly water harvesting solutions grows, conventional materials will need to be integrated thoughtfully with innovative designs to enhance overall system performance. These innovative materials hold promise for further improving the efficiency and effectiveness of the AWH system.

Fig. 3 The comparison of different hygroscopic materials employed in solar-driven hygroscopic water harvesting systems. Various materials contain conventional sorbents such as solid hygroscopic materials, MOF-based hydrogels, polymeric hydrogels, and composite sorbents. (a) Crystal cell unit of zeolite,⁴⁰ Reproduced from ref. 40 with permission from Elsevier, copyright 2024. (b) COF-480-hydrazide,⁹² Reproduced from ref. 92 with permission from John Wiley and Sons, copyright 2024. (c) Crystal structures of MOF-801,⁴¹ Reproduced from ref. 41 with permission from The American Association for the Advancement of Science, copyright 2024. (d) Crystal structures of MOF-LA2-1,⁹³ Reproduced from ref. 93 with permission from The American Chemical Society and Sons, copyright 2024. (e) Crystal structures of UiO-66@MIL-101(Cr),⁹⁴ Reproduced from ref. 94 with permission from Elsevier, copyright 2024. (f) Crystal structures of CaCl₂@MOF-808,⁹⁵ Reproduced from ref. 95 with permission from Elsevier, copyright 2024. (g) Schematically illustration of the IPN gel,⁷⁸ Reproduced from ref. 78 with permission from Springer Nature, copyright 2024. (h) Design of the porous sodium polyacrylate/graphene framework (PGF),⁹⁶ Reproduced from ref. 96 with permission from John Wiley and Sons, copyright 2024. (i) Schematics of the super moisture-absorbent gel (SMAG),⁹⁷ Reproduced from ref. 97 with permission from The American Chemical Society and Sons, copyright 2024. (j) Schematic of PDA@PP-Cl hydrogels,⁹⁸ Reproduced from ref. 98 with permission from The American Chemical Society and Sons, copyright 2024. (k) Design of LiCl@MIL-101(Cr),⁹⁵ Reproduced from ref. 95 with permission from Elsevier, copyright 2024. (l) Illustration of LiCl@rGO-SA with layered GO-SA matrix and LiCl crystals,⁹⁹ Reproduced from ref. 99 with permission from The Royal Society of Chemistry, copyright 2024. (m) Fabrication of deliquescent salt-based composite sorbents using different matrix materials.¹⁰⁰ Reproduced from ref. 100 with permission from The American Chemical Society, copyright 2024.

3.2. MOF-based hygroscopic materials

Metal–organic frameworks (MOFs) are a type of porous material composed of metal ions and organic ligands. Owing to this unique feature, MOFs have the ability to adsorb water molecules through metal sites, physically adsorb water molecules through organic ligands, and adsorb water molecules through capillary action using pores.^93–95,101 Although MOFs have the advantages of high porosity and large specific surface area, their structure and composition still need to be adjusted for optimal performance in the actual AWH process. As presented in Fig. 3d–f, the first AWH device designed with MOF-801 demonstrated the effectiveness of MOF-based systems in achieving water capture at low relative humidity, producing 2.8 g_water g_MOF⁻¹ day⁻¹ at 20%RH using solar energy exclusively as a desorption energy source.⁴¹ Exploiting the diverse adsorption sites present in MOFs, the hydrophilicity of MOFs can be adjusted by modifying the metal sites and using different functional group coordinations. It is worth mentioning that although the expansion of pore size can expand the adsorption capacity, it may alter the adsorption mechanism and reduce the water production efficiency. Additionally, the high cost and instability of MOFs present challenges for practical applications.^101,102 Metal–organic frameworks (MOFs) hold great promise for atmospheric water harvesting due to their exceptionally high surface area and tunable porosity, which can significantly enhance water capture efficiency from humid air. Their ability to be engineered for specific adsorption properties allows for tailored approaches in various environmental conditions. However, challenges in scalability and cost-effectiveness hinder their widespread application in large-scale harvesting systems. As research advances in simplifying synthesis processes and improving stability, MOFs could play a pivotal role in the next generation of atmospheric water harvesters.

3.3. Polymer hygroscopic materials

Polymer hygroscopic materials present themselves as promising adsorbents because of their superabsorbent nature, adjustable structure, and interactions with water. These materials are often stimuli-responsive, showcasing exceptional moisture absorption capabilities in a variety of environmental conditions. As illustrated in Fig. 3g–j, notable examples include interpenetrating polymer networks (IPN), porous sodium polyacrylic/graphene oxide frameworks (PGFs), superabsorbent gels (SMAG), and PAAS-PNIPAAm hydrogels with core–shell structures. These polymer hygroscopic materials have excellent water absorption capacity, facilitated by their special structures, which lead to a unique adsorption mechanism. For instance, IPN hydrogels are temperature-sensitive and can change their hydrophilicity depending on temperature variations, enabling the desorption of water in liquid form with minimal temperature fluctuations.¹⁰² The PGF, developed by Yao et al., is a hydrogel composed of sodium polyacrylic acid (PAAS) and graphene oxide (rGO), featuring a porous structure that enhances water absorption under high humidity conditions. At the same time, the photothermal component of rGO promotes the release of trapped water.¹⁰³ The SMAG developed by Zhao et al. combines hygroscopic polymers with hydrophilic hydrogels with a water storage function, which can adsorb water on the surface and quickly generate water through temperature responsiveness.¹⁰⁴ The core–shell structure of the PAAS-PNIPAAm hydrogel improves the stability and water absorption capacities. Although the adsorption efficiency of polymer hygroscopic materials is superior, the long adsorption/desorption process makes them difficult to apply to continuous systems.⁹⁸ Polymers offer significant advantages due to their lightweight nature and resistance to corrosion. Their versatility allows for innovative designs in capturing moisture, such as in hydrophilic coatings and membranes that can enhance water collection efficiency. Nonetheless, their lower mechanical strength and potential environmental impact pose challenges. By improving the sustainability and performance of polymer-based systems, they can contribute effectively to addressing water scarcity issues. Nevertheless, their application and development prospects warrant attention.

3.4. Composite hygroscopic materials

Hygroscopic salts exhibit superior water absorption capabilities compared to physical adsorbents at low relative humidity. However, the bond between metal and water molecules that facilitates adsorption also presents a challenge in solar-powered AWH due to the high desorption. At the same time, issues such as easy deliquing, agglomeration, and equipment corrosion can hamper the adsorption performance of hygroscopic salts, thereby impeding their application in SAWH systems.^100,105 In order to address these challenges, composite adsorbents have been developed to confine the hygroscopic salt within a porous matrix. As shown in Fig. 3k–m, commonly used hygroscopic salts such as LiCl and CaCl₂ are relatively inexpensive and efficient choices for these composite absorbents.^{100,106–108} As a carrier of hygroscopic salts, the structure and properties of the matrix also affect the application of composite adsorbents in the SAWH system. Therefore, porous matrix materials with large pore volume and good thermal conductivity are ideal substrates, as they can accommodate more salt solution, enhance the hygroscopic performance, reduce the possibility of salt solution leakage, and facilitate desorption. For these reasons, MOFs, hollow carbon spheres, and hydrogels have been used as substrates to host hygroscopic salts.^109–112 For instance, Xu et al. successfully loaded LiCl onto the MOF substrate MIL-101(Cr), and nanoscale LiCl was contained in uniform nanoscale pores to capture water, achieving a remarkable water adsorption capacity of 0.77 g g⁻¹.¹¹³ Additionally, Li et al. packed LiCl within the cavity of hollow carbon nanoparticles, demonstrating rapid adsorption–desorption kinetics and excellent water adsorption capacity.¹¹⁴ The use of hydrogels as substrates offers advantages such as large pore volume and the ability to adjust hydrophilicity. For example, LiCl@rGO-SA, where graphene oxide (rGO) was reduced with sodium alginate (SA) as a matrix and LiCl was added as an adsorption material.^99,113 Benefiting from the vertically aligned stratified pores of the matrix and the high pore volume, the composite adsorbent achieved a high salt load of 78 wt% and exhibited rapid adsorption–desorption kinetics at as low as 15%RH. Although the composite adsorbents have made progress in improving the adsorption capacity and stability, the kinetics and water adsorption capacity at low RH still need to be further improved. The synergistic effect of the matrix and the hygroscopic salt is a promising avenue to enhance the water absorption capabilities of composite adsorbent beyond the characteristics of the hygroscopic salt alone. Composites are characterized bya high strength-to-weight ratio and customizable properties. This enables the creation of efficient, lightweight systems that can operate effectively in various environmental conditions. The integration of different materials allows for innovations in moisture collection and retention, enhancing overall system performance. However, the complexity of composite manufacturing and recycling can limit their scalability and increase costs, which may hinder widespread adoption. Continued research into more efficient production techniques and the development of recyclable composite materials will be crucial to maximizing the potential of composites in atmospheric water harvesting, providing effective solutions for sustainable water access (Table 2).

Table 2 The water uptake under different conditions and characters of different hydroscopic materials

	Types of different hygroscopic materials	Temperature (°C)	RH (%)	Water uptake	Character	Ref.
Solid hygroscopic material	Zeolite	—	—	—	Environmentally friendly and can choose from a variety of adsorbents	73
	COF-480	25	20	0.45 g g^−1	Metal-free, stable under operating conditions, and flexible structures	94
	MOF-801	25	20	More than 0.25 L kg^−1	Work under a narrow RH range, using only ambient sunlight to heat the material	69
MOF-based hygroscopic material	MOF-LA2-1	25	26	0.68 g g^−1	The increase in pore volume while retaining and reduction in regeneration heat and temperature.	78
	UiO-66@MIL-101(Cr)	25	30	0.408 g g^−1	Core–shell structure, with the outer layer's excellent water absorption under dry conditions and the inner layer's high water absorption under high humidity conditions	103
	CaCl2@MOF-808	25	30	0.56 g g^−1	The use of composite material, recycling performance and cost at low relative humidity	97
Polymer hygroscopic material	IPN	27	80	0.6 g g^−1	Interpenetrating polymer network gels in the dry state	81
	PGF	25	15	0.14 g g^−1	Can actively sorb moisture from common or even smoggy environments, efficiently grabs impurities,	115
	SMAG	20	90	1.1 g g^−1	Hygroscopic and hydrophilicity-switchable polymers in a network architecture	116
	PAAS-PNIPAAm	25	30	0.68 g g^−1	The synergistic hygroscopic action between the core layer and the shell layer	106
Composite hygroscopic material	LiCl@MIL-101(Cr)	30	30	0.77 g g^−1	Confining hygroscopic salt in a metal–organic framework matrix	97
	LiCl@rGO-SA	30	15	1.01 g g^−1	Scalable, low-cost promising strategy	117

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4. Structural design and water collection efficiency of SAWH

In order to maximize water collection efficiency, the combination of excellent equipment and high-performance adsorbents is essential. Well-designed SAWH equipment can realize the full potential of the absorbent materials. As shown in Fig. 4f, the equipment can be generally categorized into adsorbent beds, heaters, condensers, and collectors. The adsorbent bed serves as the center component of the AWH system and is responsible for capturing water molecules from the air. The selection and configuration of adsorbents directly decide the water capture efficiency and operational cost of the system. Physical adsorbents such as zeolite, silica gel, and MOFs are widely studied due to their high specific surface area and adjustable pore structure. Heaters are used in the desorption process to release water molecules trapped in the materials. Solar heaters have become mainstream in research due to their renewability and ease of access. Electric heaters, though pricier, offer a more compact design for the adsorption bed and increased water production.¹¹⁷ The design of the condenser is also critical in the AWH system, as it condenses the desorbed water vapor into liquid form. Typically, the condenser is constructed as a water-cooled or air-cooled fin heat exchanger, with cooling power density significantly affecting water condensation efficiency. A good collector can avoid the evaporation of collected water. SAWH system can be designed for intermittent water collection, continuous water collection during the daytime, and 24-hour continuous water collection. Moreover, when designing the SAWH system, factors such as energy utilization efficiency, sustainability, and cost-effectiveness should be taken into consideration. Energy utilization efficiency includes the solar energy conversion efficiency of the photothermal components and the water diffusion efficiency, affecting the adsorption/desorption rate of the adsorption bed. As illustrated in Fig. 4g and h, sustainability and economics are also crucial considerations for the commercial viability of SAWH technology.^116,118 Strategies to enhance economic feasibility include reducing material costs, improving system durability, and lowering maintenance requirements.

Fig. 4 The development of water harvesting materials and systems. (a) Images of a water-harvesting prototype with MOF-801,⁴¹ Reproduced from ref. 41 with permission from The American Association for the Advancement of Science, copyright 2024. (b) Illustrative schematic of a SAWH device undergoing adsorption (nighttime, left half) and solar-assisted water collection (daytime, right half) processes,¹¹⁹ Reproduced from ref. 119 with permission from Springer Nature, copyright 2024. (c) Design of a rotational-cylinder-based atmospheric water collector,¹¹⁴ Reproduced from ref. 114 with permission from Elsevier, copyright 2024. (d) Structure schematics of a water harvesting device with improved heat and mass transfer,¹¹³ Reproduced from ref. 113 with permission from John Wiley and Sons, copyright 2024. (e) Structural design of the TEC-driven AWH device.¹²⁰ Reproduced from ref. 120 with permission from the American Chemical Society, copyright 2024. The effect of different designs of structure, device and systems on the water collection efficiency: (f) AWH device composition, (g) diagram of the water cycle and water sources available for human beings,¹¹⁸ Reproduced from ref. 118 with permission from MDPI, copyright 2024. (h) Structural schematic of TEAD with a PMMA condenser, TCP-Li, and plant.¹¹⁶ Reproduced from ref. 116 with permission from Springer Nature, copyright 2024.

4.1. Effect of devices design on water collection efficiency

In early SAWH devices, the adsorption bed was usually used only as the basis for adsorption.¹²¹ In the subsequent practical application, advancements were made to enhance the adsorption bed. In 2017, a SAWH device based on MOF-801 was developed. This device directly heated the absorption layer at the top and then condensed the collected water through the bottom condenser. This innovation inspired the later use of the absorption layer as a solar-assisted water collection system for the photothermal components and bottom condenser, despite the challenges of low energy transfer efficiency and water yield⁴¹ (Fig. 4a). In 2018, Kim et al. proposed an air-cooled collection device based on MOF-801¹¹⁹ (Fig. 4b). By applying radiative cooling in low relative humidity environments, the thermal performance of the water collection device was improved. In addition, optically transparent thermal insulator aerogel was placed on the MOF-801 layer for insulation, and a radiative cooling mode was implemented to reduce heat loss during the absorption process.

Moving forward to 2020, Li et al. designed a rotating cylinder containing HCS-LiCl adsorbents, facilitating alternating the adsorption and desorption zones¹¹⁴ (Fig. 4c). The utilization of fast kinetic adsorbents decreased the time it took for the device to complete a cycle, emphasizing the importance of utilizing adsorbents with high dynamic performance in the device design. In 2021, Xu et al. established a continuous collection device employing four composite adsorbents with rapid adsorption/desorption kinetics¹¹³ (Fig. 4d). By matching the adsorption and desorption time among the four parts, the water production efficiency was improved. However, it is noteworthy that the device currently relies on manual switching for operation. In order to further reduce the heat loss caused by air convection, in 2023, Min et al. combined a thermoelectric unit (TEC) with a water adsorbent¹²⁰ (Fig. 4e). The TEC plays a dual role, directly heating the adsorbent after adsorption to promote the release of water and causing vapor condensation under its influence. This dual-functionality of the TEC significantly boosted the efficiency of the adsorption/desorption process. In summary, a primary challenge in current water harvesting technology revolves around the desorption that requires high temperature and the lower temperature required for condensation. Heat loss from the heat exchange at varying temperatures leads to lower thermal efficiency, serving as a pivotal factor in determining overall efficiency.

4.2. Effect of systems design on water collection efficiency

With the continuous innovation of adsorbents, various SAWH systems have been developed to facilitate the practical application of this technology. These systems can be broadly divided into intermittent collection systems, daytime continuous collection systems, and all-day continuous collection systems, each offering unique advantages. The intermittent collection system uses absorbents for adsorption at night and desorption during the day through solar energy, as shown in Fig. 5a. On the other hand, the continuous daytime collection system further improves the efficiency of the intermittent collection system, which can carry out continuous adsorption and desorption during the day, albeit with specific requirements for adsorbents and equipment as illustrated in the reference (Fig. 5b).⁹⁷ Although the daytime continuous collection system has made a breakthrough in the daytime collection, it still lacks the capacity for desorption during the night, necessitating the exploration of alternative heat sources that can replace the sun at night to facilitate continuous operation. Taking advantage of the ubiquitous sunlight, the intermittent system is the most widely used water collection system. The adsorption process usually occurs at night with high humidity and low temperature, and the high temperature during the day promotes the adsorbed water to be released. For some adsorbents, the adsorption efficiency during the day with low humidity is low, so the desorption process usually occurs during the day, and the water production efficiency is low.^122–130

Fig. 5 The development of SAWH systems. (a) An intermittent water collection system with heat exchange designs.¹²³ Reproduced from ref. 123 with permission from Springer Nature, copyright 2024. (b) A daytime continuous water collection system illustration and its performance, and the operation principles of a rapid-cycling daytime continuous water harvesting system with four sorbent units.¹¹³ Reproduced from ref. 113 with permission from John Wiley and Sons, copyright 2024. (c) A schematic of an all-day continuous water collection system with 24-hour and its productivity performance in an actual desert.¹³¹ Reproduced from ref. 131 with permission from American Chemical Society, copyright 2024.

The demand for increased water production has led to the evolution of the SAWH system from a single-cycle to a multiple-cycle operation per day.¹³² When the adsorption/desorption process can be carried out simultaneously during the day to produce water continuously, the system is called a continuous daytime collection system. This system provides a more stable water supply and achieves a larger production of water than the intermittent system, albeit at the cost of additional energy consumption and complexity in design. When the adsorption/desorption process can be carried out continuously for 24 hours, the system is called a continuous day collection system. The all-day continuous collection system can be used as a stable water source because it can produce water continuously for 24 hours. In the meantime, efficiency implies increased energy and installation costs. For example, the utilization of MOF-303 powder as an adsorbent, by Hanikel et al., in combination with fans to enhance airflow and solar panel-powered heaters to overcome the limitations of direct sunlight-driven desorption, the system and its performance are presented as in Fig. 5c.¹³¹ Likewise, Wang et al. realized a continuous 24-hour water collection by alternating two adsorption beds assisted using an electric heater.¹³³ As AWH technology continues to evolve, the focus on developing continuous working systems to optimize water yield by multiple cycles remains a key area for future work.

5. Advantages and challenges of AWH technology

5.1. Advantages of SAWH

Solar-powered atmospheric water harvesting is a method of using solar energy to capture water from the air, converting it into a viable water source. This technology shows significant advantages and potential in addressing the global shortage of freshwater resources, especially in arid and water-scarce regions. One of the primary advantages of SAWH technology lies in its sustainability. As a clean and renewable energy source, solar energy does not produce greenhouse gas emissions, aligning with international goals aimed at reducing emissions and promoting sustainable development.¹³⁴ Furthermore, SAWH technology is not constrained by geographical limitations; it can be implemented in various environments, including deserts, mountains, and islands, as long as there is ample sunshine available.^135,136 As shown in Fig. 6a–c, atmospheric water harvesting technology harnesses the natural processes of the water cycle to generate electricity. By capturing humidity from the air, systems can convert moisture variations into energy. This method provides a sustainable energy solution it offers benefits in building cooling systems by utilizing evaporative cooling methods, enhancing energy efficiency in urban environments. In agriculture, SAWH can improve crop yields by providing irrigation through captured atmospheric moisture this technology enables efficient irrigation practices that can significantly enhance agricultural productivity. Innovative systems designed to capture and distribute atmospheric water directly to crops have emerged, reducing dependency on conventional water sources. This not only supports sustainable farming but also contributes to food security by enabling farmers to cultivate crops in previously inhospitable environments, thus promoting resilience in agricultural practices. Additionally, the cost-effectiveness of SAWH technology is also very obvious. Once the system is established, operational expenses are relatively low due to the abundance of solar energy, making it economically attractive, especially in regions with higher energy costs.^137–140

Fig. 6 The multifunction of AWH technology in the future. (a) Harvesting electricity during the water cycle.¹³⁸ Reproduced from ref. 138 with permission from John Wiley and Sons, copyright 2024. (b) The application map of SAWH technology, which has potential applications in water harvesting, building cooling, agriculture, mechanical energy harvesting, and thermal management in different regions.¹³⁹ Reproduced from ref. 139 with permission from John Wiley and Sons, copyright 2024. (c) A recent application of atmosphere water harvesting in irrigating plants.¹⁴⁰ Reproduced from ref. 140 with permission from Springer Nature, copyright 2024.

Moreover, the design and material selection of SAWH technology can be optimized according to specific climatic conditions and requirements. Utilization of hygroscopic materials such as salts, zeolites, silicones, and metal–organic frameworks, alongside system improvements such as the addition of heat storage and gas drive equipment, allows for greater flexibility and control. SAWH technology also demonstrates adaptability and flexibility, enabling water during the night when the humidity level is high and optimizing system efficiency through continuous cycles. The development of this technology is expected to achieve all-weather operation, even at night; it can also achieve efficient storage and utilization of energy through solar photovoltaics, battery systems, photovoltaic/photothermal systems, and adsorption heat storage systems. Ultimately, SAWH technology holds immense potential in providing safe drinking water to billions of individuals worldwide and plays a critical role in alleviating the global water crisis. Continued research and innovation are essential to overcoming the existing limitations and unlocking the full capabilities of this transformative technology.

5.2. Challenges of SAWH

SAWH technology also faces challenges, such as the efficiency of the SAWH system being affected by climatic conditions, especially in low humidity environments, the adsorption capacity of the adsorbent will be reduced, thus affecting the water collection efficiency. In addition, while low temperatures and high humidity at night favor adsorption, high temperatures during the day may limit the desorption process.¹⁴¹ Addressing these challenges through advancements in adsorption and desorption kinetics, enhanced performance of radiative cooling materials, system compactness, and thermal management is crucial for the development of more efficient and stable atmospheric water harvesting systems. Although SAWH technology shows great potential, it still faces a number of challenges in practical application. First, one of the key challenges of SAWH technology is the selection and optimization of adsorbents. The ideal adsorbent needs to have high adsorption capacity, fast adsorption rate, low desorption temperature, and good cycling stability. Currently, many adsorbents, while performing well under laboratory conditions, may experience a decline in performance in real-world environments such as high humidity, high salinity, or extreme temperature conditions. In addition, the process of synthesizing adsorbents may involve chemicals that are not environmentally friendly, which contradicts the environmental goals of SAWH technology.^34,142 Second, the design and integration of SAWH systems also present a challenge. The system must efficiently capture, adsorb, and desorb water vapor while maintaining efficient energy conversion and transmission. This requires precise thermal management and knowledge of material science to ensure stable system operation in a variety of climatic conditions. In addition, the scalability and cost-effectiveness of the system are also factors that need to be considered in practical applications.¹⁴³

Third, the energy efficiency and sustainability of SAWH technology are another important challenge. While solar energy is a clean source of energy, the water production of the system can significantly decrease in cloudy or nighttime conditions. Therefore, current research is focused on developing SAWH systems that can operate efficiently in low light conditions or in combination with other renewable energy sources, such as wind energy, to improve the energy independence of the system. Fourth, the scale application of SAWH technology faces challenges. At the same time, SAWH systems can be finely designed at the laboratory scale; maintaining the system's efficiency and reliability while reducing costs is a complex issue when it comes to mass production and deployment. In addition, scale production involves practical issues such as supply chain management, logistics, and maintenance. Finally, the regulation and standardization of SAWH technology pose a challenge. As an emerging technology, there is currently a lack of uniform industry standards and regulatory frameworks for SAWH technology. This lack of standards can result in inconsistent product quality and affect the credibility and public acceptance of the technology. It is imperative to establish strict quality control standards and regulatory mechanisms to promote the healthy development of SAWH technology. In order to strengthen the adsorption capacity and energy efficiency, for material innovation, materials such as nanomaterials, hybrid materials and biocompatible materials could be used to improve the adsorption capacity and efficiency. On the system design side, we can optimize the reactor design, implementing modular or multi-stage reactors can improve contact time and mass transfer efficiency, leading to better adsorption performance, while designing systems that include renewable energy to reduce operating costs and improve overall energy efficiency. In conclusion, SAWH technology holds great potential to address global water scarcity. However, overcoming the aforementioned challenges is essential for its widespread application.

6. Future research direction

The core of AWH technology lies in the development of new hygroscopic materials with high water absorption capacity at low relative humidity (RH) and the excellent ability to achieve rapid water release with minimal energy input.^75,144 Materials should be designed with cost-effectiveness, synthetic sustainability, static and dynamic properties, and long-term cycling stability. Enhanced heat and mass transfer are essential to improve the performance of AWH systems. This includes optimizing adsorbent structures to enhance water vapor diffusion efficiency and designing efficient thermal management systems to minimize energy consumption. (1) Design optimization: in the realm of design optimization, the development of advanced materials and innovative structural designs is crucial for enhancing the efficiency of atmospheric water harvesting systems. By utilizing hydrophilic and hydrophobic materials, as well as nano-structured surfaces, researchers can significantly improve water collection rates while minimizing evaporation losses. The integration of solar-powered systems and modular configurations, further promotes scalability and adaptability to various environmental conditions. Collectively, these strategies not only enhance water collection capabilities but also contribute to the sustainability of AWH technologies. Future AWH devices must be carefully designed to ensure efficient water capture and release, possibly by improving the design of adsorption beds, increasing condensation efficiency, and using renewable energy sources like solar energy for the desorption process of water. The operation mode of an AWH system significantly affects its performance. (2) Operational mode transition: operational mode transition is pivotal for maximizing the performance of atmospheric water harvesting systems. Designing systems that can adapt to daily changes ensures consistent performance throughout the day, addressing the challenges posed by fluctuating humidity and temperature levels. This adaptability is essential for meeting diverse water needs in varying climates. Researchers are exploring the transition from the intermittent mode to the continuous daytime mode to the continuous 24-hour mode to achieve round-the-clock water production. This requires innovative operational strategies and potential alternative heat sources such as battery discharge or waste heat utilization. Currently, there is a lack of standardized evaluation methods for AWH technologies and materials.¹⁴⁵ Future research efforts are necessary to establish evaluation metrics that facilitate the comparison of different materials and systems and advance AWH technology. (3) Molecular engineering techniques: molecular engineering techniques offer exciting possibilities for advancing atmospheric water harvesting. The development of advanced hydrophilic polymers and metal–organic frameworks (MOFs) can significantly enhance water adsorption and retention capabilities, addressing current efficiency challenges. Additionally, biomimicry approaches inspired by nature's solutions can lead to the creation of highly effective water-collecting surfaces. Furthermore, computational modeling and molecular dynamics simulations provide valuable insights into water behavior at the molecular scale, facilitating the design of optimized materials for specific applications. Enhanced water capture and release efficiency can be achieved through molecular engineering techniques, such as designing hydrophilic controllable polymers or molecular heaters. This method allows liquid water to be released directly without a phase transition process and no need for additional energy input. AWH technology offers a geographically and climate-independent solution for water production, holding significant potential for alleviating water scarcity in remote areas and developing countries. Advances in materials science are expected to significantly reduce the cost of freshwater production in the future.²³

In conclusion, this work first illustrates the global water scarcity situation, and then various mainstream solutions to address this issue are introduced. Especially the AWH technology, a comprehensive overview was given, including the principles of AWH technology, classifications of systems, design principles of the hygroscopic materials, the designs of the structures and harvesting systems, the advantages and challenges of AWH technology, and future research focuses are introduced. In summary, despite the promise of AWH technology, a number of key challenges remain, including enhancing the water absorption capacity of the material, reducing the energy requirement for water release, improving the overall system energy efficiency, and developing stable, mass-producible materials. By consolidating this review, it can be concluded that AWH technology represents a promising area with the potential to revolutionize access and utilization, particularly in water-scarce regions. Continued advancements in materials development and system design will lead to more efficient, sustainable AWH technologies with reduced environmental impact.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is financially supported by the National Natural Science Foundation of China (NSFC) (No. 12205226) and the Hubei Provincial Department of Education (Q20221508).

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