Advances in Adsorptive Atmospheric Water Harvesting Technology: Materials, Desorption, and Systems

Abstract

Compared to traditional atmospheric water harvesting technologies which rely on high humidity environments, adsorptive atmospheric water harvesting technology provides a sustainable solution to water shortage in arid/semi-arid regions through the synergistic process of “adsorption–desorption–condensation”. However, problems such as weak adsorption capacity under low humidity, high desorption energy consumption, and insufficient system energy efficiency restrict the engineering application of adsorptive water harvesting technology. In recent years (2017–2024), researchers have continuously explored around the core demands of adsorptive water harvesting technology: the modification of composite adsorbent materials and bionic interface design to enhance low humidity adsorption, the integration of photothermal synergy and dynamic temperature control to reduce desorption energy consumption, and the optimization of intelligent water harvesting systems to improve overall efficiency. In this review, focusing on the integrated synergy of “material–mechanism–system”, we summarize the research progress in the field of adsorptive atmospheric water harvesting, highlight the material design, photothermal desorption mechanism, and intelligent system optimization strategies, compare the innovation points, limitations, and application potentials of different technical paths, and finally prospect the technical development and application directions such as multi-energy complementary integration and cross-scale system coupling in atmospheric water harvesting technology.

1. Introduction

The global shortage of freshwater resources has become a major environmental challenge threatening human survival and development, with its severity continually intensifying alongside climate warming and population growth. According to relevant data, over 30% of the global population resides in countries or regions experiencing freshwater scarcity, with approximately 1.2 billion people living in areas of extreme aridity where the annual average relative humidity falls below 30%. Among these, regions such as parts of Africa, the desert belt of the Middle East, and the arid zones of Central Asia face the most severe water shortages, ranking first globally in the difficulty of accessing freshwater [1]. Although the total volume of water on Earth is approximately 1.4 × 10¹⁸ tons, over 97% of this is saltwater, with freshwater accounting for only about 2.5%. More than two-thirds of this freshwater exists in the form of glaciers, permanent snowfields, and permafrost, making it difficult to utilize directly. Truly accessible surface water and atmospheric water constitute only around 10% of the total freshwater supply [2]. Although atmospheric water reserves account for only 0.04% of freshwater resources, their daily replenishment rate can reach 500 cubic kilometers—equivalent to 6000 times the global daily water demand of humanity [3]. Compared to conventional freshwater acquisition techniques, the development and utilization of atmospheric water demonstrate significant environmental and economic value. While seawater desalination technology can achieve large-scale water supply, it necessitates coastal locations and consumes 3–5 kWh m³ of energy during the salt separation pre-treatment process. Additional energy input is also required for treating the desalination residues [4]. Wastewater reclamation technologies [5] are generally constrained by local liquid water reserves, making their implementation challenging in arid regions. Atmospheric water, however, undergoes minimal exposure to salts and mineral contaminants during natural cycles, resulting in pre-treatment energy consumption reduced by over 58% compared to seawater desalination [6]. From the perspective of resource sustainability, even if 2.2 billion people consumed an average of 10 L of freshwater daily, the annual reduction would amount to merely 8 cubic kilometers. This indicates that human development exerts a negligible impact on atmospheric water reserves, with its disturbance to the global hydrological cycle being of negligible significance [7]. This provides ecological safeguards for the large-scale application of atmospheric water harvesting technology. Therefore, atmospheric water harvesting holds promise as an environmentally sound solution to global water scarcity. Among existing and emerging technologies, solar-powered atmospheric water harvesting (SAWH) stands out particularly, with the potential to provide safe drinking water for hundreds of millions of people [8].

The current mainstream atmospheric water harvesting technologies are primarily categorized into three types: condensing, fog collection, and adsorption [1]. The three technologies exhibit significant differences in technical principles, environmental adaptability, and energy efficiency performance. Condensation technology employs active refrigeration to create a sub-dewpoint temperature field, thereby driving water vapor phase transition [9]. The vapor compression cycle (VCC) system can achieve daily water production of 200–1000 L per square meter in high humidity environments. However, its energy consumption per liter of water produced using low-temperature cooling sources is as high as 0.6–1.5 kWh, and its energy efficiency plummets by over 70% when ambient humidity falls below 40% [10]. Although thermoelectric cooling (TEC) technology offers modular advantages, its portable systems exhibit low daily water production under high humidity conditions, with energy consumption per unit approximately three times that of VCC systems. Mist collection technology relies on the interaction between aerosol droplets and interfaces to achieve capture [11]. In specific high humidity, foggy regions, it can achieve a daily water collection rate of 50 L per square meter, but requires sustained wind speeds of 3–5 m per second. This renders it entirely inapplicable in windless areas. Such technology is highly dependent on meteorological conditions and proves completely ineffective in arid zones with low annual average fog days. Adsorption-based atmospheric water harvesting technology has overcome climatic and geographical constraints through material innovation, emerging as a pivotal approach for freshwater acquisition in arid regions [12]. This technology achieves water vapor enrichment through the interaction between porous materials and water molecules, maintaining stable operation even in extremely arid environments. In recent years, the development of novel adsorbent materials such as metal–organic frameworks (MOFs) and biomass-based composite gels have significantly enhanced technical performance. Firstly, adsorbent materials including metal–organic framework (MOF) hydrogels demonstrate stable operation even under low relative humidity conditions [13,14,15,16]. Secondly, it operates entirely on solar power, consuming less than 0.3 kilowatt-hours per liter of water produced; Thirdly, its modular design renders it suitable for decentralized water supply systems. Although the current adsorption method yields only 0.5 to 2 L of reclaimed water per kilogram of material daily—lower than condensation technology—its practical research has demonstrated its suitability for severely water-scarce regions. In desert trials, the adsorption system maintained a water production efficiency of 0.8 L/kg/day even at 15–20% relative humidity, without requiring grid connection. This low environmental requirement and minimal energy dependency make it a significant technological solution for addressing drinking water challenges in arid and semi-arid regions.

New breakthroughs in adsorption technology are reshaping the trajectory of this field, with the photothermal conversion efficiency of adsorbent materials now exceeding 90% [17]. The new generation of SAWH systems enables multiple daily water production cycles, with unit equipment output increasing to three to five times the original capacity. Inspired by solar interface evaporation and passive radiative cooling phenomena, this novel approach extracts moisture from the atmosphere and converts it into freshwater through entirely green processes. Leveraging advanced materials and technologies, it is emerging as a vital sustainable water supply solution for extreme environments.

2. Atmospheric Water Harvesting Technology Framework and Challenges

Current mainstream atmospheric water harvesting technologies include condensation, fog collection, and adsorption (Figure 1). The technical principles and material design are significantly differentiated: condensing technology drives the water vapor phase change through the construction of a gradient temperature field, the core of which lies in the design of the microstructure of the condensing interface and the optimization of the refrigeration system. It has been shown that bionic micro- and nano-coarse-structured condensing surfaces can significantly improve condensation efficiency by modulating water vapor nucleation kinetics [18]. The lotus leaf-like multistage structure increases the density of water vapor nucleation sites by 3–5 times [19]. Fog collection technologies rely on aerosol interfacial interactions for droplet capture, and their performance is regulated by surface wettability gradients. The experimentally proven superhydrophobic–hydrophilic gradient surface enables >100 droplet slip harvesting efficiency up to 92% and maintains directional capture of <10 μm droplets [20]. This bionic design increases water harvesting per unit area by more than 50% [21]. Adsorptive technology is based on the synergistic action of multi-scale pores and chemical bonding to achieve water molecule enrichment. Among them, physical adsorption-led materials rely on high specific surface area and pore size distribution optimization to enhance the adsorption capacity; chemical adsorption-led materials, such as hydrogels, achieve stable adsorption under low humidity conditions through the formation of oriented hydrogen-bonding networks between functional groups, such as carboxyl/amino groups, and water molecules. The three types of technologies complement each other at the level of material–structure–environmental adaptability: condensation has significant advantages in water production in high-temperature and high humidity areas, but is limited by energy consumption and climate; fog collection has zero energy consumption in special areas, but relies on specific meteorological conditions; and adsorption breaks through the drought bottleneck through material innovation, and the low environmental coercion feature promotes the upgrading of the technology. Therefore, the development direction needs to focus on the development of climate-adaptive composite water harvesting systems, the establishment of quantitative correlation models of material properties–interface behavior–system energy consumption, and the exploration of the integration of technologies in distributed water supply scenarios; the specific classification of the three types of technology and the core principles are shown in Figure 1.

2.1. Condensing

Condensing water harvesting technology drives the water vapor phase change by establishing a sub-dewpoint temperature field through active refrigeration, and its technical effectiveness is positively correlated with temperature and humidity gradients, but comes with significant energy costs. The current mainstream technology path is the vapor compression cycle (VCC) [22], thermoelectric cooling (TEC) [23], and adsorption refrigeration [24]; VCC systems have the best performance in high temperature and high humidity areas due to the mature phase change cycle of the refrigerant [25]. The typical structure of a vapor compression cycle generator is shown in Figure 2.

Groendijk et al. [26] developed a VCC system that achieved a water harvesting rate (WHR) of 1.50 kg/h at a dry/wet bulb temperature of 26.7 °C/19.4 °C and unit power consumption (UPC) of 0.69 kWh/kg. However, the WHR plunged 91% when relative humidity (RH) decreased from 60% to 20%, and climate sensitivity was significant. Further structural optimization studies showed that the use of a dual parallel evaporator design increased the average monthly WHR to 4.20 kg/h, but the UPC was still as high as 1.43 kWh/kg in a dry climate, highlighting the limitations of energy efficiency improvements. Zolfagharkhani et al. [27] simulated a coastal system in southern Iran and found that the WHR could reach 425 kg/h at RH > 80%, while decreasing to 104 kg/h in winter under low humidity conditions [25].

In contrast, thermoelectric cooling (TEC) technology achieves solid-state cooling based on the Peltier effect, and its modular design confers significant portability advantages, but the efficiency bottleneck is more prominent, and the schematic structure of the TEC-based distillation system is shown in Figure 3.

Joshi et al. [28] developed a portable system that achieves a WHR of only 24 × 10⁻³ kg/h under optimal operating conditions (RH = 90%) and requires a 1.5 A current input (UPC = 2.1 kWh/kg), which is three times the energy level of a VCC system. Even though Muneoz-Garcia et al. [29] used a solar-assisted power supply scheme, the system failed completely when RH < 20%, exposing the vulnerability of the technology in low humidity environments. Atta et al. [30] designed an irrigation system with WHR up to 1 kg/h in wet areas, but the hot end pre-heating strategy resulted in an additional 15% energy loss. In a study of TEC, Liu et al. [31] further revealed that the UPC (1.8 kWh/kg) of the TEC system at RH = 50% increased by 150% compared to that at 70% RH, suggesting that its energy efficiency is more sensitive to humidity changes. This climate adaptation deficit severely limits the potential application of TEC technologies in arid regions. The common bottlenecks of the two types of condensation technologies are humidity dependence, energy imbalance, and geographic constraints, and these shortcomings have prompted researchers to turn to adsorption-based water harvesting technology development. In contrast, adsorption technology effectively breaks through these bottlenecks through material innovation and solar power drive, making it a more promising solution for arid regions.

2.2. Fog Collection

Fog collection water harvesting techniques achieve droplet capture by modulating aerosol dynamics, with the core focus on constructing a gradient wettability interface to optimize the droplet retention and transport process. Comparative studies of standard fog collectors (SFCs) and large fog collectors (LFCs) have shown that the device size and structural design significantly affect the water harvesting efficiency: the Warka water tower in Morocco achieves a capacity of 50–100 L/day through a bamboo–polyester composite mesh structure, which is an 8–10 times improvement compared to the traditional SFC, and a realistic view of the typical fog collection device and its structure is shown in Figure 4.

In terms of performance breakthrough thanks to material interface engineering innovations, Parisi et al. [34] developed a piezoelectric yarn mesh with microstructure modulation to increase the specific surface area by a factor of three and improve the droplet capture efficiency by more than 50%; Zhang et al. [35] designed a harp-shaped three-dimensional system mimicking the structure of cactus thorns, which resulted in a 30–40% increase in the droplet transfer rate and a water harvesting rate of 30.16 kg/m²/h; and Zhang et al. [36] further fused bionic interface and friction nano-electricity generation technology, which can achieve an ultra-high water harvesting efficiency of 93.18 kg/m²/h, which is several times higher than that of the traditional mesh screen. However, these advanced systems are subject to harsh environmental conditions: sustained wind speeds, high fog densities, and specific terrain. Long-term monitoring of a mountain range in Morocco showed annual capacity fluctuations of ±35% in a 600 m² fog collection center, with a strong positive correlation with the frequency of fog occurrence (R² = 0.87). This highlights the geographical limitations of fog collection technologies—only 12% of the world’s water-scarce regions have suitable meteorological conditions. Although innovations in interfacial engineering continue to drive technology development, the nature of its failure in arid, low-fog regions remains unchanged, prompting researchers to turn to adsorption technology development for all-weather adaptability.

2.3. Adsorption

Adsorptive atmospheric water harvesting technology enriches water molecules through intermolecular forces at the solid–liquid interface (electrostatic forces, hydrogen bonding, etc.), and its mechanism of action is not limited by the phase state of matter [37]. Typical solar actuated systems (SAWH) are based on a diurnal cycle design: humid air flows through the adsorption bed at night and water vapor is captured by the porous material; daytime solar heating triggers desorption and the released humid air is cooled by a condenser to collect water, the efficiency of which is directly dependent on the optimization of the adsorbent material’s photothermal conversion properties and pore structure. With adsorbent material as the core of the system, its performance depends on the intrinsic properties of the material; commonly used adsorbent materials are physical adsorbent materials, chemical adsorbent materials, composite adsorbent materials, and polymer adsorbent materials [38]. Among them, the physisorption type is centered on intermolecular forces (activated carbon fiber [39,40], zeolites [41], silica gel, aluminum silicate, and aluminum phosphate [42]) dependent on high specific surface area with pore size optimization; the chemisorption type, on the other hand, is chemically bonded (LiCl/SiO₂ [43], CaCl₂ [44]) with enhanced low humidity adsorption capacity by ion-dipole interaction; polymeric types (hydrogels [45], MOF-801 and MIL-101 [46], and covalent organic frameworks (COFs) [47]), on the other hand, combine osmotic pressure modulation with ligand-bonding synergies to achieve high capacity and low desorption energy.

2.3.1. Intermittent SAWH (Day–Night Cycle)

Intermittent atmospheric water harvesting technology drives the water molecule capture–release cycle through the difference between the intrinsic properties of the material and the environmental energy potential, the core of which lies in the synergistic effect of adsorption by the porous structure and the natural temperature difference. Typical systems use the physical adsorption of materials such as silica gel and MOFs or chemical adsorption of hygroscopic salts in combination with a diurnal temperature difference to trigger a water vapor phase change. Shao et al. [48] developed a multi-module system, optimized the top condenser for directional water vapor migration, and achieved a water production of 2.11 L/kg/day over 12 cycles. The nighttime low-temperature and high humidity phase contributed 66% (1.4 L/kg/day), confirming the validity of ambient energy potential utilization. Zhu et al. [49] designed a natural composite hydrogel (NCHH) which, through carboxyl/amino functional group modulation, achieved an adsorption capacity of 0.94 kg/kg with a low heat of desorption of 501 J/g at 30% RH. Its prototype device showed excellent adsorption kinetics, yielding up to 1.33 L/kg/day at the same humidity, and the different structural designs of the intermittent atmospheric water harvesting technology drives the water molecule capture–release cycle through the difference between the intrinsic properties of the material and the environmental energy potential, the core of which lies in the synergistic effect of the adsorption solar ABAWH device, as shown in Figure 5. However, intermittent technologies still face fundamental challenges: environmental dependence; condensation efficiency bottlenecks; and long-term cycle stability. These limitations highlight the need to develop hybrid–continuous/hybrid–intermittent systems to improve technology robustness in regions of extreme drought or drastic temperature and humidity changes.

2.3.2. Continuous/Hybrid SAWH (24 h Operation)

Continuous/hybrid atmospheric water harvesting (AAWH) technology breaks through the natural energy potential limitations through external energy inputs and demonstrates significant innovations in material design and system integration. Li et al. [50] developed a bilayer moisture adsorbent material (BMA) that uses a gradient functionalization strategy. Through the hierarchical design of the top light-absorbing layer and the bottom water-storage layer, they increased the desorption rate by 40%, achieved an adsorption capacity of 3.96 kg/kg at 90% RH, and raised the photothermal conversion efficiency to 82%. This breakthrough lays the groundwork for arid environment applications, but its true potential lies in system-level innovation; Xu et al. [51] designed a transmissive radiant cooling membrane (TRC) system coupled with daylight heating and nighttime radiant cooling to achieve a 24 h continuous water production of 3.6 L/m², which is a five-fold improvement over conventional intermittent systems and reduces the condensation temperature threshold by 12 °C. Shan et al. [52] constructed a hybrid-driven polyelectrolyte hydrogel system by technology fusion, combining solar energy and industrial waste heat, and optimized interfacial transport by CNT/LiCl doping, which produced water up to 2410 mL/kg/day with a cycling stability of >5000 cycles. These advances have extended the applicable lower humidity limit of continuous/hybrid technology to RH > 10%, but its scale-up is still constrained by two major bottlenecks: the system’s energy consumption reaches 5–8 times that of intermittent technology, and the complexity of the structure leads to cost escalation, as shown in Figure 6 for the core structure of a multi-cycle ABAWH system.

To further evaluate performance variations in atmospheric water harvesting systems (AAWH) which are jointly determined by adsorbent characteristics, energy supply modes, and system integration design,

Table 1. Energy consumption comparison of typical water intake systems.

System Classification	System	Unit Power Consumption (UPC) (kWh/L)	Test Humidity Range	References
Condensing system	VCC	0.69 (specific conditions); 0.6–1.5 (normal conditions)	>40%	[10,26]
	TEC	2.1	>20%	[28]
Intermittent adsorption system	Intermittent SAWH MOF substrate (MOF-801)	0.2–0.3	15–20%	[13,48]
	Intermittent SAWH silicone-based	0.2–0.3	20–80%	[54]
Continuous/hybrid adsorption system	Continuous/hybrid system MOF base (MOF-303)	0.3–0.45	10–90%	[55,56]
	Continuous/hybrid system polyelectrolyte hydrogel	0.35–0.5	10–95%	[52]

systematically summarizes key performance metrics for representative systems. These include energy consumption per unit of water produced and tested humidity ranges, providing a reference for subsequent discussions on adsorbent modification and system energy efficiency optimization. Furthermore, we observe from Table 1 that relevant critical humidity thresholds can be derived. When ambient relative humidity falls below 40%, continuous/hybrid adsorption-based atmospheric water harvesting (AAWH) systems demonstrate significant energy consumption advantages. For instance, the UPC of MOF-303-based AAWH systems ranges from merely 0.3 to 0.45 kWh/L, which is considerably lower than that of the vapor compression cycle (VCC) system (UPC rising to 0.6–1.5 kWh/L) and the thermoelectric cooling (TEC) system (UPC 2.1 kWh/L). Conversely, when RH exceeds 40%, the VCC system, leveraging its mature phase change cycle technology, surpasses the AAWH system in energy efficiency. This further underscores the technical superiority of AAWH in low humidity scenarios.

However, significant variations exist in the technology readiness levels (TRLs) of key atmospheric water harvesting components. Classification has clarified commercialization pathways and the long-term robustness of hybrid systems. Commercial vapor compression cycle (VCC) systems, owing to their extensive engineering applications and mature technological iteration, currently stand at TRL 9. Conversely, complex metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) remain at TRL 2–3 due to constraints imposed by complex synthesis processes and high scaling costs. Furthermore, thermoelectric cooling (TEC) systems remain at TRL 6–7 due to inadequate low humidity adaptability. The TRL gap indicates that future progress requires optimizing mass production processes for low-maturity materials and integrating them with high-maturity systems to accelerate bridging the gap between laboratory technology and engineering applications.

Although current research has made breakthroughs at the material modification level, the development of intelligent control algorithms and synergistic management of multiple energy flows are still in the exploratory stage, which points out the development direction of the next-generation continuous/hybrid system–dynamic optimization of adsorption–desorption processes through digital twin technology, and the construction of off-grid water supply networks in combination with distributed renewable energy sources. The performance enhancement of continuous/hybrid adsorption technology is highly dependent on the optimization of the intrinsic properties of the adsorbent materials, and the following is an in-depth analysis of the design logic of adsorbent materials at the level of mechanism and modification strategy.

3. Mechanism and Modification Strategy of Adsorbent Materials

Adsorbent materials play a central role in the solar-powered atmospheric water harvesting (SAWH) technology system; the evolution of the physical structure during its adsorption and desorption processes [57] and the chemical bond reorganization [58] directly determine system effectiveness. The photothermal conversion mechanism drives the adsorbent material to break through the desorption threshold by efficiently capturing solar energy and converting it into thermal energy for targeted water release. The optimization of the material properties follows the principle of multi-scale design: physical adsorption relies on the synergistic enhancement of the water molecule trapping capacity by the multilevel pore architecture and the high specific surface area, and the main mechanism of physical adsorption is shown in Figure 7.

Chemisorption, on the other hand, builds a local high humidity microenvironment through functional groups such as carboxyl/amino groups, and maintains a high adsorption capacity under low humidity conditions; the main mechanism of chemical adsorption is shown in Figure 8.

Current research focuses on five types of systems: hygroscopic salts, zeolites, silica, MOFs, and composites—hygroscopic salts achieve rapid adsorption by virtue of ionic hydration, but need to solve the problem of deliquescence; zeolite/silica gel achieve >5000 cycle stability through rigid pores; MOF-801 utilizes ligand bond modulation to achieve 0.45 kg/kg adsorption at 15% RH; and the composite (SiO₂/LiCl) combines the advantages of physical adsorption kinetics and chemical adsorption capacity. On this basis, the optimization of pore-functional group synergistic effect through cross-scale simulation can break through the performance limit of existing materials at low humidity, while the engineering contradiction between stability and cost needs to be balanced to provide a material basis for the scale application of SAWH technology.

3.1. Physical Adsorption and Modification

Physical adsorption enables the reversible capture of water molecules through van der Waals forces, and its low heat of adsorption and fast kinetic properties make it an important mechanism of action for SAWH technology. The process is essentially a synergistic effect of the multilayer adsorption of water molecules on the surface of the porous material and within the pores, where the pore size distribution (micropores < 2 nm dominates capillary condensation and mesopores 2–50 nm promote cross-scale transport) and the specific surface area together determine the adsorption capacity. Physical adsorption of atmospheric water harvesting mainly consists of capillary action of the pore structure of the material and multilayer adsorption of water molecules. Porous materials, like silica gel, zeolite, MOFs, etc., have large specific surface area and porous structure so that adsorption occurs not only on the surface but also inside the pores. Because the partial pressure of water vapor on the surface of solid adsorbent materials is lower than that of air, water can be collected with the help of pressure difference, and zeolite relies on surface adsorption and capillary condensation to absorb water [60], the dynamic equilibrium of the adsorbed water layer on the solid surface and the influence of interfacial forces are shown in Figure 9. Therefore, physical adsorption is triggered by intermolecular forces, and weak binding forces lead to low heat of adsorption and fast desorption, a property that is significant in the desorption process.

In conclusion, physical adsorption is triggered by intermolecular forces, and there are various adsorption mechanisms depending on the adsorption force and location. In the future, the coupling relationship between the distribution of adsorption sites and the cross-interface mass transfer mechanism needs to be solved at the molecular scale in order to realize all-weather atmospheric water harvesting with low energy consumption and high capacity.

3.1.1. Microporous Adsorption

Physical adsorption is achieved by intermolecular van der Waals force and pore limiting domain effect to capture water molecules, and its adsorption potential is significantly enhanced with decreasing pore size, and the adsorption potential can be up to 3 times of surface adsorption when the microporosity is <2 nm [62]; with adsorption at low relative humidity, usually the smaller the pore size is, the stronger the adsorption is [63]. This is based on the D-A model proposed by Dubinin to describe the microporous filling process [64].

$$W=W_0{e}^{\left[-{\left({\displaystyle \frac{A}{E}}\right)}^n\right]}$$

(1)

where W denotes the adsorption capacity (cm³/g); W₀ denotes the saturated adsorption capacity of the microporous material (cm³/g); A = RTln(P₀/P) is the adsorption potential (J/mol) (reflecting the energy captured by gas molecules within micropores, where R is the gas constant, T is temperature (K), P₀ is the saturated vapor pressure, and P is the equilibrium partial pressure); E denotes the characteristic adsorption energy (J/mol), while n represents an index related to the pore size distribution (typically ranging from 1 to 3, with n = 2 for slit pores). This model is specifically designed for microporous materials, describing the “microporous filling process”—namely, the capture of gas molecules within microporous structures due to strong adsorption forces.

Dawoud et al. [65] in the silica gel system showed that the microporous structure adsorbed twice as much as the mesoporous silica gel, whereas Manoj Kumar et al. [54] compared silica gel, activated alumina, and zeolite and found that silica gel was significantly superior to the latter two with a daily water intake of 0.16 kg/kg. On the other hand, Essa et al. [66] demonstrated that interfacial mass transfer optimization can break through the material intrinsic limitations by increasing the water yield to 0.4 kg/m² through the silica–gravel composite coating.

Hygroscopic salts are efficiently adsorbed by hydration, but the inherent contradiction of high energy consumption for deliquescence and desorption limits their application [67]. Srivastava et al. [68] mixed 37% LiCl with sand and produced about 0.09 kg/kg of water per day, but its deliquescent properties need to be resolved by limiting domains of MOFs; calcium salts are low-cost but poorly soluble; common calcium salts are CaCl₂ and Ca(NO₃)₂. Vainio et al. [69] found that CaCl₂ can absorb about 10 times its own amount of water under 30% water vapor, but desorption requires 155–165 °C, which is difficult to achieve by solar energy alone, and there is a risk of corrosion of the calcium salt Ca(NO₃)₂. Elashmawy et al. [70] used cotton cloth impregnated with 30% CaCl₂ solution to absorb moisture with a water yield of 0.51 kg/kg after saturation. Hygroscopic salts such as LiCl and CaCl₂ exhibit high adsorption capacity yet pose significant risks: LiCl forms acidic solutions when relative humidity exceeds 40%, while CaCl₂ deliquescence causes pitting corrosion. Unencapsulated LiCl/SiO₂ systems may also lead to Li⁺ leakage. Inert encapsulation technology is therefore crucial to address these issues. The microporous structure of MOF-801 effectively restricts Li⁺ diffusion, while PAM-CNT-CaCl₂ hydrogels immobilize calcium ions via a three-dimensional cross-linked network. Coating SiO₂ with CaCl₂/activated carbon fibers reduces chloride ion leakage. These strategies stand in stark contrast to MOF-303 inherent Mg²⁺ retention rate exceeding 95%, achieving high efficiency without external encapsulation. This highlights the need for a balance between capacity, corrosion resistance, and product water purity. Adsorption material performance should also be evaluated against World Health Organization (WHO) drinking water standards for product water purity; otherwise, the energy consumption required to treat adsorbed water becomes a significant concern. Ca(NO₃)₂ forms an amorphous hydrate under drying [71], moisture absorption at 80–90% RH, and no deliquescence. Guo et al. [72] found that magnesium salts had a change in hygroscopic growth factor of <10% at 90% RH; Zhang et al. [73] found that sodium/potassium salts were almost ineffective at RH < 80%. Li et al. [74] concluded that inorganic salts have the potential to combine with SAWH; they screened and used CuCl₂, CuSO₄, and MgSO₄ as relevant water-collecting materials, and the performance stabilized after 10 cycles, but the actual water yield was still lower than that of lithium salts.

Zeolite belongs to the porous skeleton of silica–aluminate minerals consisting of tetrahedral SiO₄ and AlO₄ containing water of crystallization, and zeolite enhances its properties through the modulation of the silica–alumina ratio and metal doping [75]. The lower the silica to aluminum ratio is, the higher the adsorption capacity is. Mulchandani et al. [76] determined the water harvesting capacity of zeolites and showed that zeolites produce about 0.94 kg/m² of water per day. Trapani et al. [77] found that A3 type zeolite adsorbed only 0.07 kg/kg at 40% RH. Stach et al. [78] found that the addition of Mg²⁺ to zeolite increased adsorption to a maximum of twice the amount before modification. Yan et al. [79] increased the adsorption by 53% by ion exchange. The new Aqsoa zeolite improves on this problem [80]; however, it was found that adsorption–desorption hysteresis occurs under low pressure conditions.

Silica gel relies on surface silanol groups (Si-OH bonds) to synergize with pores [81]. The hydroxyl groups distributed on the surface of silica gel and the porous structure inside provide the necessary binding sites and transport paths for water vapor adsorption and migration processes. In general, the more abundant the number of hydroxyl groups and the higher the porosity of the pore structure, the better the hygroscopic properties of the silica gel. Sleiti et al. [82] tested a 35 mm thick silica gel layer to produce 0.159 kg/kg of water in 12 h, and in combination with Srivastava et al. [68] designed a Scheffler reflector to desorb up to 0.155 kg/kg/day. However, its thermal stability deficiencies led to >30% attenuation of high-temperature desorption efficiency [75].

Metal–organic frameworks (MOFs), as a rapidly emerging class of materials in recent years, are structured by metal ions or metal clusters connected with organic ligands by means of ligand bonding, which show the characteristics of a solid crystalline state permanent porous framework. These materials possess outstanding advantages such as high porosity, large specific surface area, tunable pore size, and diverse topologies [83]. MOF materials can be classified into two categories based on pore size: mesoporous MOFs (MIL series), which mainly use capillary condensation as the water adsorption mechanism and have high water saturation adsorption, and zirconium- and aluminum-based microporous MOFs (MOF-801, UiO-66, and MOF-303, etc.), which mainly use pore filling as the water adsorption mechanism, and MOF-801, which has a specific surface area of up to 950 m²/g [13] and adsorption capacity of 0.33 kg/kg at 30% RH [84]; its derived nanoporous carbon has a solar absorptivity of >92% in the 300–2500 nm band, releasing 99% of the adsorbed water within 10 min and supporting multiple cycles of water production in a single day. Fathieh et al. [55] developed MOF-303, a new type of aluminum-based MOF. It has a faster water adsorption/desorption rate and higher water saturation adsorption capacity (0.48 kg/kg) than MOF-801, with a 114% higher water yield under the same conditions and a >95% retention rate of Mg²⁺, which meets the WHO drinking water requirements [85]. Li et al. [86] showed that MOF-808 had a water adsorption capacity of 0.65 kg/kg at a low humidity of 25 °C and 30% RH. UiO-66 had a water adsorption rate of 0.00656 kg/kg/min at 50% RH, with a water-saturated adsorption capacity about 54% higher than that of MOF-801 [87]. Nguyen et al. [88] synthesized COF-432 with a hollow lattice topology, which increased the adsorption capacity by 40% (0.25 kg/kg) and shortened the equilibrium time to 8 h. This material effectively improves the water vapor adsorption kinetics, accelerates the adsorption rate, and increases the adsorption capacity due to its special topology, making it more suitable for atmospheric water harvesting, but the preparation cost is still a bottleneck that restricts its scale-up application. These advances suggest that the design of multilevel pores and surface functionalization is the key to break through the material performance boundary.

3.1.2. Mesoporous–Macroporous Adsorption

The phenomenon of the liquid phase transition of water vapor in mesopores (2–50 nm) and macropores (>50 nm) triggered by critical condensation pressure is called capillary condensation [89]. Its adsorption capacity is higher than that of pore adsorption but requires higher relative humidity [90]. On the other hand, unlike pore adsorption, the capillary coalescence process is not reversible due to the release of the latent heat of condensation during the process, and additional energy needs to be supplied to enable the adsorbent material to complete the regeneration process [91], which is manifested as a desorption hysteresis phenomenon. To address this limitation, Lars Grunenberg [92] developed nitrone-linked COF materials (NO-PI-3-COF, NO-TTI-COF), which reduce the capillary coalescence humidity threshold by 20% through topochemical transformations, and whose high crystallinity and large specific surface area make low humidity water harvesting possible. In the MOFs system, MIL-101, with its regular octahedral structure and microporous advantage, adsorbed up to 0.56 kg/kg at 50% RH [93], while applying external pressure to the frame can strengthen its stability [94], making it better for application in water harvesting systems. For mesoporous metal–organic framework materials widely employed in atmospheric water harvesting (such as MIL-101 and MOF-801), adsorption–desorption hysteresis and structural framework resistance under vapor pressure are critical factors for cycling stability. The hysteresis observed in MIL-101 (mesopore-dominant) primarily stems from capillary aggregation within the mesopores and reversible “breathing” (framework expansion/contraction) [93], The H4-type lag loop results in residual adsorbed water following desorption at 50% relative humidity, whilst the framework exhibits shrinkage under cyclic vapor pressure conditions (10–90% relative humidity). After 100 cycles, the adsorption capacity retains only 75% [94]. In contrast, MOF-801 (zirconium-based mixed micro-mesoporous structure) exhibits a weaker H₃-type hysteresis effect due to the presence of rigid Zr-O clusters: it demonstrates lower framework shrinkage at equivalent vapor pressures, with adsorption capacity remaining >90% after 100 cycles [87]. This demonstrates that the amplitude of “MOF breathing” (i.e., the structure’s resistance to vapor pressure) directly determines cyclic stability. Rigid frameworks mitigate hysteresis effects and prevent irreversible structural damage, whereas flexible frameworks (such as MIL-101) require external pressure regulation to enhance stability. This underscores the critical importance of prioritizing structural design when selecting MOFs for long-term applications. Therefore, chemical modification of the pores and optimization of the mechanical properties can synergistically enhance the environmental adaptability and cyclic stability of the material.

3.1.3. Surface Functionalization Modification

Physical adsorption refers to the attachment of water molecules to the surface of the material monolayer by adsorbate and active site electrostatic interaction and multilayer by hydrogen bonding between water molecules, with hydrogen bonding to form an ordered structure and dipole moments to promote multilayer adsorption [95]. The multilayer physisorption process can be described by the classical BET model [96]. The model can be used to calculate the adsorption capacity of adsorbent materials under specific external conditions.

$${\displaystyle \frac{P/P_0}{N\left(1-P/P_0\right)}}={\displaystyle \frac{P}{P_0}}{\displaystyle \frac{\left(C-1\right)}{N_{0}C}}+{\displaystyle \frac{1}{N_{0}C}}$$

(2)

$$C=e{\displaystyle \frac{{\epsilon }_1-{\epsilon }_0}{k_{B}T}}$$

(3)

where $P/P_0$ is the relative pressure; N is the amount of gas adsorbed at the relative pressure P/P₀; C is a constant related to the energy of the system; $N_0$ is the amount of gas adsorbed in a single layer; ${\epsilon }_1$ is the enthalpy of adsorption of the gas in the first layer, and ${\epsilon }_0$ is the enthalpy of liquefaction of the gas [97]; k_B is the Boltzmann constant; T is the temperature. Shi et al. [98] modified MOF materials by ligand functionalization: the adsorption capacity of UiO-66 was increased by 30% (0.3 kg/kg) and the equilibrium time was shortened by 50% at 50% RH; the adsorption capacity of MIL-101(Cr) reached 0.45 kg/kg at 60% RH, which was 40% higher; and the water adsorption capacity of MOF-303 was 0.28 kg/kg at 40% RH, which was about 25% higher than that of unmodified materials. Also, 0.28 kg/kg at 40% RH, which is about 25% higher than that of unmodified, and the capacity decay rate is only 10% lower after 10 cycles, proving that surface modification can synergistically improve the adsorption kinetics and stability.

Physical adsorption in a broad sense also includes the uptake of ionic solutions [99], which achieves high-capacity adsorption through permeation within the material, but is limited by slow diffusion kinetics [100]. Practical applications require synergistic multi-mechanism advantages, with efficient adsorption over the full humidity range through pore size gradient design (micropores < 2 nm to stimulate van der Waals adsorption, mesopores 2–50 nm to promote capillary coalescence) and surface functional group modulation (carboxyl groups to enhance hydrogen bonding).

3.2. Bonding Regulation of Chemisorption

Chemisorption forms chemical bonds through electron exchange between the adsorbate and the surface of the material, like the coordination hydration of LiCl-H₂O and CaCl₂-6H₂O, and its selective adsorption and irreversible properties stem from the exothermic reaction’s nature. Material modification focuses on enhancing hydrophilicity: the introduction of functional groups such as -OH and -NH₂ enhances water molecule anchoring through hydrogen bonding and electrostatic interactions [101], cation–ligand adsorption achieves selective trapping through metal sites coordinated to water molecules, and acid–base reactive adsorption relies on surface proton exchange mechanisms [102]. Although the strong hydration of the deliquescent salt increases the adsorption capacity, it is necessary to inhibit the deliquescence and optimize the cycle stability through the porous carrier confinement. These chemical modification strategies provide new ways for efficient water harvesting at low humidity by precisely modulating the interfacial electronic structure.

3.2.1. Cation Coordination Adsorption

Metal salt adsorbent materials achieve strong adsorption at low humidity through the formation of coordination bonds between metal ions and the lone pair of electrons of the oxygen atom of the water molecule, which is particularly suitable for arid regions. Because of their high adsorption energy, water molecules are not easy to desorb, so it is necessary to balance the energy consumption for desorption and adsorption stability. In the MOFs system, Yang et al. [103] enhanced the MOF-5 adsorption rate by 30–50% by doping zinc ions in the MOF-5 synthesis, whose electron cloud rearrangement strengthened the chemical bonding of water molecules; studies on zeolite systems reveal cation substitution effects: Yan et al. [79] found that preparation of Na-Mg-X by salt impregnation boosted adsorption by 53% and Henninger et al. [104] found a 1.7 times increase in adsorption due to Li⁺ substitution. Tahraoui et al. [105] also found that replacement of Na⁺ with small-sized cations such as Mg²⁺ and Li⁺ enhanced zeolite adsorption. For zeolites, cation exchange modulates the electrostatic pore potential and water–surface interaction energy in a quantifiable manner, constituting the core mechanism for enhanced adsorption. Taking Na-type zeolite as a benchmark (electrostatic pore potential: −12 kJ/mol; water-surface interaction energy: −40 kJ/mol), the smaller radius of monovalent Li⁺ ions (76 pm) compared to Na⁺ (102 pm) reduces the cation–pore spacing. This increases the electrostatic pore potential to −18 kJ/mol [103]; this enhances the dipole–dipole interactions between water molecules and the pore surface, elevating the water–surface interaction energy to −52 kJ/mol and thereby increasing the adsorption capacity by a factor of 1.7. For the divalent Mg²⁺ ion, with an ionic radius of 72 pm and a charge of +2 (compared to Li⁺: +1), the higher charge density further amplifies the electrostatic pore potential; enhanced electrostatic attraction reduces the desorption activation energy of water molecules by 8 kJ/mol, elevating the water–surface interaction energy to −58 kJ/mol. This also explains why Mg²⁺ substitution achieves twice the adsorption efficiency of Na⁺ [79], It has been demonstrated that the synergistic optimization effect of “smaller ion size coupled with higher valence” enhances the electrostatic environment within pores, thereby strengthening water–surface interactions.

It is shown that small size cations significantly enhance the water molecule trapping efficiency by optimizing the pore electrostatic potential field. These strategies provide a new paradigm for electronic structure regulation in the design of water harvesting materials in arid regions.

3.2.2. Acid–Base Reaction Adsorption

Acid–base reactive adsorption enables selective trapping of water molecules through the proton transfer mechanism of functional groups, and amino (-NH₂)-modified materials enhance the interfacial hydrophilicity through a reversible reaction. Silica gel modified with amino–silane coupling agent enhanced the adsorption capacity by nearly 100% at low humidity (10–20% RH) and accelerated the adsorption rate by 40%, and the modified Freundlich model showed that the value of the adsorption constant, k, increased by about 1.8 times. This active trapping mechanism was confirmed by infrared spectroscopy of the increased strength of the amino–water molecule hydrogen bond, while thermogravimetric analysis showed no significant decay in cyclic stability. In the zeolite system, Trapani [77] modeled the adsorption capacity of pristine A3 zeolite to be only 0.07 kg/kg in a desert climate and requiring regeneration at high temperatures (>150 °C), but after surface modification using short-chain silanes by Bonaccorsi [106], the reduction in hydrophilicity led to a decrease in the regeneration temperature to 105 °C, while maintaining 80% of the adsorption capacity. These studies suggest that by precisely modulating surface acidity and pore wettability, new ideas are provided for balancing adsorption kinetics and desorption energy consumption.

3.3. Biomass-Based Composite Adsorbent Materials

Biomass-based composite adsorbent materials have become a current research hotspot by integrating physicochemical adsorption synergistic mechanisms, taking into account efficient water harvesting in the full humidity range and environmental sustainability. Metal ion coordination dominates capture at low humidity, medium humidity relies on functional group acid–base balance regulation, and high humidity results in rapid water storage through multistage pore capillary condensation. This space–time coupling mechanism balances the adsorption capacity and desorption energy consumption and provides design ideas for wide humidity adaptable materials, which are prominent in biomass-based materials, especially cellulose-based materials that achieve high efficiency in water harvesting through multistage pores.

3.3.1. Cellulose-Based Multi-Level Pore Design

Cellulose-based materials have become one of the green water harvesting solutions by virtue of their renewability, and have demonstrated excellent hygroscopic properties and environmental adaptability through multi-scale structural modification and functionalization. Cai et al. [107], inspired by tree transpiration, constructed oriented pore-structured nanocellulose-MXene/LiCl aerogels with multistage pores with domain-limited salt strategy to achieve 1.14–3.12 kg/kg gradient adsorption in the range of 30–90% RH, capacity retention of >90% after 20 cycles, and solar-driven desorption efficiencies up to 0.8 L/m²/h. Zhang et al. [36] prepared a bionic asymmetric amphiphilic surface by laser-induced etching, which combined with a droplet self-driven mechanism to achieve an ultra-high water harvesting efficiency of 93.18 kg/m²/h, breaking through the kinetic limitations of conventional cellulosic materials. Zhu et al. [108] developed nanocellulose/LiCl-CNT bilayers using a 3D printing–freeze–drying cascade technique, whose vertically aligned millimeter-sized pores synergized with micrometer-sized pores to increase the rate of moisture uptake by a factor of 2.3, and the CNT layer had a photothermal conversion efficiency of 88%.

In response to the problem of salt deliquescence and leakage, the gel restriction strategy has become a research focus: Li et al. [109] developed the PAM–CNT–CaCl₂ hydrogel, which solidifies salt particles through cross-linked networks and, at 35% RH, adsorbs up to three times as much as MOF materials. Aleid et al. [110] designed zwitterionic gel PDMAPs to enhance solubility using the salting out effect, resulting in a 40% increase in LiCl loading. Entezari et al. [111] modified sodium alginate, relying on Li⁺ and Ca²⁺ to increase the adsorption capacity, and functionalized carbon nanotubes to increase the spectral absorption and water transport channels to produce low-cost Bina/FCNT composites. Zhao et al. [112] integrated hygroscopic Ppy-Cl and switchable hydrophilic poly-NIPAM to prepare super-absorbent gel SMAG for water harvesting and efficient desorption in a wide relative humidity range. Entezari et al. [110] developed LiCl/MgSO₄/ACF composites by using the properties of activated carbon fiber (ACF), which not only enhances its water absorption efficiency, but also the MgSO₄ in it can play a role in preventing the leakage of LiCl. Wang et al. [113] prepared NBHA aerogels, which, through nanocellulose/graphene composite structures, achieved an adsorption capacity of 0.76 kg/kg at 28% RH and exhibited excellent mechanical flexibility. These systems synergistically solved the salt aggregation challenge through 3D network confinement and interface engineering. Ni et al. [114] developed plant-inspired POG organic gels integrating a glycerol hygroscopic medium with a polypyrrole–dopamine photothermal layer for self-compatible moisture management and solar-driven desorption.

Despite the outstanding performance of cellulose-based materials, issues of mechanical property degradation and pore structure clogging persist in high humidity environments. This represents a critical bottleneck for large-scale application, demanding materials capable of withstanding thousands of repeated swelling/dehydration cycles and thermal stresses. Typical cellulose gels exhibit a sharp decline in strength after multiple adsorption–desorption cycles, coupled with high pore clogging rates under elevated relative humidity. In contrast, PVA–lignin supramolecular hydrogels overcome these limitations through multi-solvent high-temperature annealing. This stability stems from a supramolecular hydrogen bond network between polyvinyl alcohol and lignin, which buffers volumetric changes and suppresses pore collapse—crucial for large-scale devices requiring long-term structural integrity without performance degradation.

3.3.2. Lignin-Based Photothermal Synergy

Lignin-based materials can exhibit unique environmental adaptability through molecular modification and multi-scale structural design. Zhou et al. [115] developed the CAL gel bio-based gel, which integrates the photothermal conversion capacity of demethylated lignin (DAL) (67.8 °C @1sun; ‘1sun’ refers to standard solar irradiance of 1000 W/m², and 67.8 °C is the stable surface temperature of DAL under this irradiation) with the gradient moisture adsorption property of LiCl/LiBr. It achieves an adsorption capacity of 0.51–0.79 kg/kg in the 15–30% RH range; meanwhile, its moisture adsorption rate increases significantly with humidity (from 0.62 to 1.74 kg/kg/h), and its desorption rate reaches 1.98 kg/kg/h. Lignin possesses inherent molecular heterogeneity (such as variations in methoxy content and fluctuations in degree of polymerization), which readily leads to functional inconsistencies and hinders its large-scale application. During CAL gel preparation, controlled demethylation reactions are central to addressing this issue: using lithium bromide aqueous solution as the reaction medium, alkaline lignin (AL) undergoes a 6 h reflux reaction with hydrogen bromide solution at 100 °C in an oil bath. Post-reaction, the mixture is cooled in an ice-water bath, filtered, washed to neutrality, and subsequently freeze-dried. This process further optimizes the porous structure of DAL, ensuring uniform and stable water vapor transmission pathways. When this DAL is integrated into CAL gel containing LiCl and HPC/SA composite components, the standardized DAL maintains approximately 90% photothermal conversion efficiency under single solar irradiation. Compared to unmodified lignin, this improvement significantly resolves efficiency instability issues in large-scale applications.

Its multi-compartment drum design can be adapted to off-grid water supply in extreme environments. Liang et al. [116] prepared lignin-based foamed carbon by a “pre-combustion–carbonization” process, which synergistically combined a multistage pore structure with an ultra-high light absorption rate (96.7%) to achieve an evaporation rate of 2.11 kg/m²/h and a solar energy utilization efficiency of 93.37%, as well as water purification capacity of 99.7%. Gu et al. [117] constructed poly(vinyl alcohol)–lignin supramolecular hydrogel by multi-solvent high-temperature annealing, which solved the problem that it was difficult to take into account the hardness and toughness of the traditional hydrogel, with a modulus of 74.4 MPa, a tensile strength of 24.8 MPa, a toughness of 90 MJ/m³, and a compressive strength of 60 MPa, and with the function of scavenging reactive oxygen species. These advances indicate that lignin-based materials are expected to break through the bottlenecks in mechanical strength and environmental adaptability of traditional moisture-absorbent materials on a large scale through the modulation of functional groups and the optimization of cross-scale structure. In addition, there are significant differences in the performance parameters and application scenarios of different hygroscopic materials, as shown in

Table 2. Information on moisture-absorbing material.

Name	Temperature	Relative Humidity	Adsorption Capacity	Material Strengths/Weaknesses	References
Zeolite A3	0–100 °C	20–60%	0.3–0.5 kg/kg	High-temperature resistant and reusable; High desorption temperature	[77]
Na(NO3), KNO3 and other salts	15–35 °C 20–40 °C	40–85% 35–80%	0.5–1.5 kg/kg	Chemically stable; Limited adsorption efficiency	[69]
[poly-NIPAM] hydrogel	10–40 °C	40–85%	0.5–2.0 kg/kg	Temperature responsive, adjustable adsorption; High humidity adsorption improvement is not obvious	[112]
Nano-cellulose MXene aerogel	20–30 °C	40–90%	0.66–4.14 kg/kg	Lightweight, high porosity; High cost	[107]
Lignin-based foam carbon	0–50 °C	30–90%	0.8–2.5 kg/kg	Low cost and good adsorption; Slightly lower than MOFs, etc.	[116]
Silica gel	20–80 °C	20–80%	0.8–1.0 kg/kg	Chemically stable; Slow desorption after saturation	[54]
MOF-801	5–40 °C	30–90%	1.0–3.0 kg/kg	High specific surface area and porosity; Complex and costly to synthesize	[87]
CAL-gel	20–30 °C	45–95%	1.0–3.0 kg/kg	Stable exposure to water vapor; Limited adsorption	[115]
NBHA aerogel	5–35 °C	40–90%	1–1.5 kg/kg	High porosity hydrophilicity; Low mechanical strength	[113]
Ca(NO3)2	20–35 °C	50–90%	1.0–2.0 kg/kg	No strong corrosive substances; Low adsorption	[71]
UIO-66	15–45 °C	30–90%	1.2–3.0 kg/kg	Good adsorption; Special and costly reaction conditions	[86]
Nano-cellulose/LiCl- CNT	10–40 °C	35–90%	1.5–4.0 kg/kg	Good porosity and high mechanical strength; Higher cost	[108]
MOF-808	10–40 °C	25–90%	1.5–3.0 kg/kg	Large specific surface area and high efficiency; Complex synthesis, high cost	[86]
MOF-303	0–40 °C	40–90%	1.5–3.5 kg/kg	Good adsorption; Special and costly reaction conditions	[55]
MIL-101	10–50 °C	25–95%	2.0–4.0 kg/kg	Good adsorption; Special and costly reaction conditions	[98]
PAM-CNT-CaCl2 hydrogel	20–30 °C	40–95%	2.0–6.0 kg/kg	Excellent moisture absorption and high strength; Possible leakage of impregnating solution	[109]
LiCl	0–50 °C	40–90%	3–4 kg/kg	Strong moisture absorption, cheap; Corrosive, easily deliquescent	[68]
[PDMAPs/CNT/LiCl]-hydrogels	15–35 °C	35–90%	4.5–6 kg/kg	Good conductivity and moisture absorption; High cost	[110]
CaCl2	0–40 °C	30–95%	6–10 kg/kg	Good moisture absorption, wide range; Easy to deliquesce, high energy consumption for desorption	[70]
POG organic gel	20–30 °C	40–90%	6–16 kg/kg	High moisture absorption; High cost	[114]

.

The table above systematically summarizes the core performance parameters (temperature–humidity response, adsorption capacity) and application trade-offs for 20 representative hygroscopic materials, encompassing inorganic salts, zeolites, metal–organic frameworks, biomass-based composites, and hydrogels. Data indicate that materials possessing exceptionally high adsorption capacities (such as CaCl₂ and LiCl) frequently encounter bottlenecks including deliquescence, high desorption energy requirements, or complex synthesis processes. Conversely, low-cost stable materials (such as lignin foam carbon: 0.8–2.5 kg/kg, silica gel: 0.8–1.0 kg/kg) exhibit moderate adsorption capacities yet demonstrate outstanding environmental adaptability and scalability. Biomass-based composites such as nano-cellulose MXene aerogels (0.66–4.14 kg/kg) and nano-cellulose/LiCl-CNT porous materials (1.5–4.0 kg/kg) further achieve a balance between high capacity, low energy consumption, and sustainability, embodying the trinity of performance, scalability, and environmental friendliness in contemporary research trends. These comparative data not only provide direct guidance for material selection across diverse application scenarios but also point towards future directions for material optimization. The test conditions for the 20 types of moisture-absorbing materials in Table 2 share a common baseline of temperatures ranging from 10 to 40 °C and relative humidity levels between 30% and 90%. However, dynamic parameters such as cycle duration and solar radiation intensity vary (with some materials lacking explicit specifications). Significant performance impacts arise from variations in key parameters: regarding cycle duration, Shane et al. [52] developed a polyelectrolyte hydrogel which maintained over 90% adsorption capacity after 5000 cycles, whereas the uncross-linked, unoptimized MOF-801 [86] exhibited a 10% capacity decline after 100 cycles. Extended cycle testing more accurately reflects a material’s practical application stability. Under solar radiation intensity conditions, the lignin-based composite gel developed by Zhou et al. [115] exhibited a desorption rate of 1.98 kg/kg/h at 1 sun (1000 W/m²). When reduced to 0.6 sun, the rate decreased to 0.8 kg/kg/h while the desorption duration doubled. Illumination conditions thus directly influence the desorption efficiency of solar-driven systems.

Therefore, salt-impregnated porous carriers and multi-scale structural designs (such as gradient pore structures) further enhance the synergistic integration of adsorption capacity and stability. These approaches aim to address the inherent trade-off between adsorption capacity and stability. Subsequent sections will delve into relevant strategies concerning desorption mechanism regulation and system integration, providing detailed elaboration on this matter.

3.3.3. Life Cycle Assessment of Modified Biomass-Based Materials

Although cellulose and lignin are inherently renewable, biodegradable green materials, the additives and solvents employed during their modification processes may impose environmental burdens. Their ecological impact necessitates assessment through life cycle analysis (LCA). Regarding modification additives, MXene preparation requires the use of toxic HF and generates significant carbon emissions; whilst carbon nanotubes (CNTs) enhance photothermal efficiency, their chemical vapor deposition process releases volatile organic compounds (VOCs), necessitating additional fume extraction and exhibiting high energy consumption during preparation. However, lignin biochar can partially substitute these materials. For instance, replacing 30% MXene with lignin biochar in nanocellulose–MXene composites maintains adsorption performance while reducing carbon emissions and eliminating the toxicity risks associated with HF. Regarding pre-treatment solvents: traditional NaOH solution processing of cellulose exhibits lower energy consumption and carbon emissions but yields mediocre modification results; ionic liquids deliver superior modification effects yet incur high energy consumption and carbon emissions; deep eutectic solvents ensure effective modification while offering lower energy consumption and carbon emissions than ionic liquids, with the added benefit of solvent recyclability to reduce pollution.

It should be noted that evaluating biomass-based materials cannot solely focus on adsorption performance. For instance, while the adsorption capacity of lignin-based foam carbon may be inferior to certain MOFs, its preparation involves significantly lower energy consumption than MOFs, making it more suitable for scenarios with stringent environmental impact requirements. This information indicates that the sustainability of biomass-based materials must balance “renewable substrates” with “low environmental impact during modification processes”. Future directions include developing bio-based modifiers (such as lignin-derived nanoparticles) and low-energy solvent recovery processes to reduce the life cycle assessment (LCA) burden.

4. Enhancement Mechanism for Desorption Process

The kinetic and thermodynamic mechanisms of the desorption process together determine the material regeneration efficiency. According to the Langmuir–Hinshelwood model, the desorption rate $r_d$ can be expressed as follows:

$$r_d=k_d\theta$$

(4)

where $k_d$ is the desorption rate constant and θ is the coverage of adsorbate molecules on the surface of the adsorbent material. The desorption rate is linearly and positively correlated with the surface coverage; this is more applicable to surface-dominated desorption in single-pore structures. The scenario involving porous adsorbent materials may rely more heavily on the Fickian second diffusion equation [118]:

$${\displaystyle \frac{\partial C}{\partial t}}=D\cdot {\displaystyle \frac{{\partial }^{2}C}{\partial x^2}}$$

(5)

where C denotes the volumetric concentration of the diffusing substance (kg/m³), t represents the diffusion time (s), and x signifies the distance along the diffusion direction (m), D represents the diffusion coefficient. During desorption, the water vapor concentration within the material continuously decreases over time (non-steady state). At this stage, Fick’s second law is required to describe the relationship between concentration and time/space. The desorption kinetics of porous adsorbent materials examined herein—such as rapid desorption from nanoporous carbon and multi-cycle water release from cellulose hydrogels—precisely align with scenarios where the Fickian diffusion model is applicable. Also, the thermodynamic spontaneous condition (ΔG = ΔH − TΔS) requires that the temperature needs to satisfy T > ΔH/ΔS for the heat absorption entropy drive (ΔH > 0, ΔS > 0). It is shown that the desorption efficiency of MOF materials can reach 80–90% when heated at 80–100 °C, which is in accordance with the positive temperature–vapor pressure correlation law of the Clausius–Clapeyron equation. The photothermal-driven strategy breaks through the conventional heat source limit by localized temperature rise up to 67.8 °C for the nanocellulose/carbon nanotube system, and its directed energy input reduces the desorption energy consumption by 30%. These mechanisms provide dual paths for optimizing desorption performance: lowering the desorption activation energy through surface modification or enhancing the mass transfer efficiency through porous structure design.

4.1. Thermally Driven Enhanced Desorption

Photothermal desorption is critical to improving solar energy utilization. In thermally driven desorption technology, the core of making water release more efficient is to increase the desorption temperature. The reasoning behind this is that, according to the Clausius–Clapeyron equation, the binding energy between the adsorbent material and the water molecules decreases significantly when the temperature rises, so that the adsorbed water is more likely to turn into a gas and escape. There are two main ways to do this: one is direct warming, which enhances the desorption drive by the thermal movement of the molecules, and the other is indirect warming, which has to rely on the energy captured by the photothermal conversion material. Song et al. [119] developed a MOF-derived nanoporous carbon, which has a solar absorptivity of more than 92% in the 300–2500 nm band and excellent adsorption kinetics. Under 1 sun light, (referring to standard simulated solar irradiance of 1000 W/m², corresponding to the AM1.5 solar spectrum), it can release 99% of the adsorbed water within 10 min, and you can find the related structure and performance test results in Figure 10.

The core mechanism by which this photothermal material achieves localized interface heating (hotspot generation) lies in the directional concentration of energy. Here, CNTs leverage their conjugated π bond structure to concentrate solar energy within the 300–2500 nm wavelength range, converting it into surface/near-surface thermal energy. This process forms micro-scale hotspots measuring 10–50 nm, enabling the CNT–composite hydrogel [50] to reach 70 °C at the surface while maintaining internal adsorbate temperatures of merely 55–60 °C. Conversely, nanoparticles such as TiN harness localized surface plasmon resonance to focus light energy at the particle–adsorbate interface, yielding internal temperatures of 8–12 °C, lower than the 60 °C interface temperature [120]. This temperature gradient design, such as that employed in lignin gel [115] with a surface temperature of 67.8 °C and an internal temperature 8–12 °C lower, reduces thermal energy loss and enhances heat transfer efficiency by 20–30%. Simultaneously, it prevents the deactivation of internal adsorption sites due to high temperatures, achieving synergistic effects of “high spectral absorption rate (>90%)–localized heating–efficient heat transfer”. Similarly, Zhou et al. [115] designed a lignin-based photothermal composite gel with a demethylation treatment to increase the photothermal temperature to 67.8 °C which was coupled with a drum-type multi-compartment structure to achieve efficient cyclic hygroscopicity/desorption. The introduction of photothermal synergies further optimizes the desorption process. Li et al. [50] constructed a bilayer moisture adsorbent material (BMA50) that promotes directional heat transfer through a carbon nanoparticle layer, achieving a desorption rate of 98% within 3 h. The performance test results of the bilayer moisture absorber (BMA) are shown in Figure 11.

Lu et al. [121] developed a temperature-sensitive hydrogel system that utilizes a thermo-responsive polymer network to achieve rapid water release in the 40–60 °C interval. Tzmg-1.0 material releases 90% of the adsorbed water at 60 °C for 15 min, and the desorption performance of THL material is shown in Figure 12.

These studies show that the broad-spectrum absorption properties of photothermal conversion materials combined with the optimization of the chemical structure of adsorbent materials can significantly enhance the thermal mass transfer efficiency. Xiang et al. [120] proposed a super-hygroscopic porous gel system, which breaks through the limitation of traditional intermittent operation through a daytime synchronous adsorption–desorption mode. It maintains stable water release capacity under low-light conditions, providing a new idea for applications in arid regions.

4.2. Regulation of Desorption Enthalpy

In atmospheric water harvesting technology, reducing the enthalpy of desorption is a key strategy to enhance the efficiency of energy utilization. The thermodynamic potential barrier to be overcome for water release can be effectively reduced through material chemical structure modulation and energy transfer optimization. Lu et al. [121] designed a PAM hydrogel that constructs a weakly bound hydration state through a hydrophilic network, significantly reducing the enthalpy of desorption. Combined with the diffusion channel formed by main chain swelling, it achieves a water absorption rate of 1.1 kg/kg at 20% RH with fast desorption kinetics. The adsorption/desorption performances of PAM–LiCl are shown in Figure 13, and a comparison of desorption performances between LHST and LiCl is shown in Figure 14.

Zhu et al. [49] developed the NCHH composite gel, which utilizes a CNT/HPMC/SA biphasic system to reduce desorption energy consumption to 501.06 kJ/kg. Its porous structure is designed to enable a daily water production of 1.33 L/kg/day at 30% RH; the comparison of desorption enthalpy with water is shown in Figure 15, where the desorption enthalpy decreased by 77.82%. In addition, the modulation of the bonding mode of the materials also has a significant effect on the desorption enthalpy. Lars Grunenberg et al. [92] modified imine-bonded COFs to nitrone-bonded structures by post-synthetic transformation, inducing a capillary coalescence effect at low humidity of 20%, lowering the water-collecting humidity threshold while reducing the energy input, and the synthetic pathway and single-hole construction are shown schematically in Figure 16. Shan et al. [52] designed a polyelectrolyte hydrogel with a hybrid energy-driven mode to achieve a water production rate of about 2.4 L/kg/day through 659 atm osmotic pressure synergized with lithium chloride coordination, which is a 2.5-fold enhancement compared to the traditional single-cycle mode. The structure of the system and the performance of different desorption modes are shown in Figure 17. These studies show that the reduction in desorption enthalpy requires a breakthrough in two aspects: one is to regulate the hydration state through chemical modification (hydrophilic network construction, bonding optimization); the other is to optimize the energy transfer path by using multi-energy field coupling techniques (solar–waste heat synergy). The former reduces the ΔH through intermolecular force regulation, while the latter reduces the energy loss through temperature field homogenization, and the synergistic effect of the two can achieve the simultaneous optimization of the desorption process in terms of thermodynamics and kinetics.

5. Optimization of Condensing System Energy Efficiency

In adsorptive atmospheric water harvesting technology, condensation efficiency directly affects the ratio of system water production to energy consumption. Optimizing the condensation process requires a synergistic breakthrough in heat transfer, energy drive mode, and interface engineering. Bai et al. [56] proposed a two-level thermal management strategy to effectively reduce condensation energy consumption through component-level thermal insulation and system-level rotating structure design. The split desorption chamber–condenser system developed by him achieved 25.7% energy efficiency at 4 kW/m² irradiation with daily water production of 3.5–8.9 L/m² [127], and the structure and performance tests of the solar-assisted continuous water harvesting (SAWH) device are shown in Figure 18.

Wang et al. [128] developed temperature-sensitive nanofiber adsorbent materials inspired by air bromeliads, which exploited the hydrophilic and hydrophobic smart-response properties of the PNIPAAm-CNTs-PNMA@LiCl materials to directly extrude the adsorbed water in a liquid form, avoiding the energy consumption of the phase transition, and the equivalent condensation temperature was increased from 20 °C to 45 °C. The system completes liquid water release within 5 min after saturated adsorption under 2 h drought conditions, significantly shortening the cycle time. This liquid extrusion strategy partially circumvents the energy consumption associated with water phase transitions, thereby delivering energy savings that partially offset the design complexity of polymer composite cross-linked networks. Simultaneously, compared to traditional hygroscopic salts prone to leakage issues, this thermosensitive material system integrates with high-efficiency adsorption systems to offer superior advantages in balanced extrusion water purity control. It serves as an auxiliary water collection technology for large-scale water supply scenarios in semi-arid regions, balancing energy efficiency benefits with water quality safety.

On a theoretical level, vapor–liquid equilibrium theory reveals that the condensation driving force originates from the chemical potential difference. Li et al. [129] investigated the mechanism by which surface heat exchange modulates the chemical potential at the condensing interface: when the condensing surface temperature is below the dewpoint of water vapor, the system spontaneously forms a thermodynamic potential to drive the phase transition, and the cyclic properties of AWH are shown in Figure 19. This mechanism provides a theoretical basis for the design of efficient condensation interfaces. From a comprehensive point of view, the improvement of condensation efficiency requires the integration of material intelligent response properties, energy transfer optimization, and interface engineering, and the synergistic effect of the three can break through the energy efficiency bottleneck of traditional condensation technology.

In the optimization of Continuous/hybrid adsorption atmospheric water harvesting (AAWH) systems, future developments may establish a closed-loop “input–feedback–regulation” mechanism through intelligent control and digital twin technologies. This approach enables adaptive operation under variable climatic conditions, representing a key direction for overcoming the limitations of fixed operating conditions. Specifically, digital twin models establish real-time mapping between system status and computer models by integrating multi-variable monitoring parameters. Environmental variables must be tracked via high-precision sensors to measure solar irradiance intensity, ambient temperature and humidity, and wind speed [56]; system variables require monitoring of real-time adsorbent saturation using high-precision temperature and humidity or force sensors, acquisition of desorption chamber temperature via infrared thermometers, and detection of condensation pressure and water production rate (stress expressed as mass of water produced per unit time and area) using stress sensor. As demonstrated by Shan et al. [52] through COMSOL software simulations of the internal temperature evolution of hydrogel adsorbents under varying energy inputs, this represents a typical application of digital twin technology where virtual models map the temperature states of physical systems. Energy variables must be monitored and recorded via heat flux sensors to assess both the output power of photovoltaic modules and the efficiency of industrial waste heat utilization. Based on the aforementioned variable settings, the intelligent control algorithm achieves dynamic system response: when the adsorbent saturation reaches a preset threshold, the adsorption bed rotation rate automatically increases whilst simultaneously raising the inlet valve opening to accelerate contact between unsaturated adsorbent and humid air; when ambient relative humidity is low, the condensation temperature threshold is raised to reduce refrigeration energy consumption. This design draws upon the thermosensitive response characteristics of intelligent moisture-absorbing materials; when fluctuations in photovoltaic output power become excessive, dynamically adjust the proportionate distribution between solar energy and residual heat to ensure desorption temperatures remain as stable as possible. Through such a collaborative system, even under extreme environmental conditions with significant diurnal fluctuations in temperature and humidity, a relatively ideal water production rate can still be achieved, thereby significantly enhancing the environmental adaptability of the AAWH system.

6. Conclusions and Outlook

Climatically and geographically independent of other technologies, adsorptive atmospheric water harvesting may play a key role in alleviating the freshwater crisis. This paper summarizes the technology of atmospheric water harvesting, adsorbent atmospheric water harvesting materials, adsorption and desorption mechanisms, and condensation methods according to the logic of the whole process of atmospheric water harvesting, and focuses on summarizing the methods to improve the desorption capacity. In this paper, the research methods of adsorptive atmospheric water harvesting technology are elaborated separately; from material modification to structure and system design, we can significantly improve the low humidity adsorption capacity of adsorbent materials and the photothermal desorption efficiency.

However, several core challenges persist in the field of adsorbent materials research. The comprehensive performance of hygroscopic salts, zeolites, and silica gel materials falls significantly short of composite-based hygroscopic materials, and their application in SAWH remains relatively limited. While metal–organic frameworks (MOFs) offer advantages, such as high specific surface area and low humidity adsorption capacity, they face issues including high raw material costs, complex preparation processes, insufficient water stability, and difficulties in achieving large-scale production. Hydrogel materials offer flexible synthesis pathways and moderate costs, with performance optimizable through functionalized doping. However, their long-term cycling stability requires improvement, and synergies between functionalization and large-scale production demand further breakthroughs. Biomass-based composites leverage renewable feedstocks for cost advantages, and their natural porous structures facilitate scalable processing. Yet, balancing performance and stability post-modification remains a challenge. Nevertheless, composite hygroscopic materials generally exhibit superior overall performance compared to other material types.

Based on the existing research and the comparative analysis of the realization path of the current atmospheric water harvesting technology, there is still room for breakthroughs in the sustainable preparation of materials, synergistic efficiency in multiple processes, and stable operation under extreme environments:

(1)

Lignin, a wood-based material, is renewable, biocompatible, and degradable, and its intramolecular conjugated structure enhances light absorption through the π-π stacking effect, while the hydrogen bonding network formed by the natural hydroxyl functional groups and water molecules significantly improves the low humidity adsorption capacity, which can be utilized to dynamically adjust the lignin photothermal properties, coupled with the cellulose hydrogel network and the temperature-sensitive polymers (e.g., PNIPAM), to construct a humidity response system. Adsorption affinity can be dynamically adjusted to enhance solar energy utilization to break through the bottleneck of low humidity water harvesting.

(2)

Drawing on the multistage pore structure of the wood conduit, the aerogel is designed with gradient porosity to enhance the directional transport of water vapor; and the surface grafting of amphiphilic ionic groups is used to construct the “water slide” effect, which reduces the resistance to condensate desorption and improves the efficiency of the phase change.

(3)

To address the problem of wind and sand erosion in arid/semi-arid regions, we can have developed a composite protective coating of TiO₂ nanoparticles and lignosulfonate, which combines photocatalytic self-cleaning and anti-wear properties, and built an intelligent monitoring system with the Internet of Things (IoT), which realizes the multi-modal dynamic regulation and control of solar energy and waste heat; this primarily addresses the issue of large-scale water supply in semi-arid regions. Given the complexity and cost of the system, remote areas should still consider simple decentralized adsorption water collection methods.

(4)

Explore sustainable pretreatment technologies for biomass materials, selectively separating cellulose and lignin and retaining the natural porous structure through ionic liquid or deep eutectic solvent pretreatment, so as to realize sustainable manufacturing; develop 3D-printed structured adsorbent material preparation technology, accurately controlling the pore network to reduce the resistance to water vapor mass transfer, and promote the application of SAWH technology on a large scale. Improve through controllable mass transfer kinetics and further promote the large-scale application of SAWH technology.

The comprehensive performance research of SWAH technology shows irreplaceability in terms of energy use efficiency, so the future breakthroughs in these research directions will effectively improve the comprehensive performance of adsorption technology, which is an ideal choice to crack the high cost of traditional water harvesting technology and engineering application problems, and provide sustainable solutions for the sustainable use of water resources in arid/semi-arid areas.