Research Progress on Material, Preparation, Properties, and Water Purification Applications of Solar Energy‐Driven Hydrogels

ABSTRACT

The global water resource crisis is continuing to intensify, and it is urgent to develop low‐energy consumption and sustainable water purification technologies. Traditional water purification technologies can address the issue of water shortage. However, these traditional water purification technologies have many limitations, including high energy consumption, secondary pollution, etc. Solar‐powered water purification technology has become a current research hotspot due to low cost, high efficiency, and environmental friendliness. Solar‐driven hydrogels (SDHs) with high hydrophilicity and porous structure, as the emerging water purification materials, can efficiently convert solar energy into thermal energy through photothermal conversion, thereby enhancing the efficiency of interface evaporation, which provides innovative solutions for seawater desalination and wastewater treatment. This article systematically reviews the advanced substrate materials and photothermal agents utilized in SDHs. Moreover, this review comprehensively introduces the preparation strategies and mechanisms of SDHs. Furthermore, this paper comprehensively explores the design principles for enhancing the efficiency of light‐to‐heat conversion and the rate of water evaporation, and improving mechanical strength, durability, and salt resistance. Finally, the review discusses the practical applications of SDHs in seawater desalination, heavy metal ion removal, organic dye degradation, etc., while exploring their innovative roles in multi‐level solar energy utilization and water resource recovery.

Solar‐driven hydrogels (SDHs) with high hydrophilicity and porous structure, as the emerging water purification materials, can efficiently convert solar energy into thermal energy through photothermal conversion, thereby enhancing the efficiency of interface evaporation, which provides innovative solutions for seawater desalination and wastewater treatment. This article reviews research progress on material, preparation, properties, and water purification applications of SDHs.

1. Introduction

The global water scarcity crisis has intensified dramatically with population growth and accelerated urbanization, which has become one of the most pressing challenges confronting humanity in the 21st century. Projections indicate that approximately 5.7 billion people will experience severe water shortages for at least one month annually by 2050 [1]. Despite concerted efforts by governments worldwide to combat water pollution through various regulatory and technological measures, water quality degradation remains a critical threat to sustainable development, particularly in densely populated urban centers and regions with intensive industrial activities. This dual challenge of water quantity and quality necessitates the urgent development of effective technologies for seawater desalination and wastewater purification.

Traditional water purification technologies, including membrane separation, adsorption, and photocatalysis, have achieved notable successes in addressing specific water quality issues. However, these conventional approaches face significant inherent limitations. Membrane separation processes require high‐pressure pumps, thus resulting in substantial energy consumption. Adsorption technologies necessitate frequent adsorbent regeneration, which may lead to operational interruptions and secondary waste generation. Photocatalysis demands continuous light sources and can produce intermediate products that pose environmental risks [2, 3, 4]. These drawbacks highlight the critical need for developing more efficient, energy‐sustainable, and environmentally benign water purification strategies.

Solar energy, recognized as a clean, abundant, and renewable resource, has emerged as a promising solution to address both energy crises and sustainable development goals [5]. Among various solar utilization pathways, including photoelectric, photothermal, and photochemical conversion, photothermal interfacial evaporation technology has attracted particular attention for water purification applications [6]. This technology utilizes photothermal materials to harness renewable solar energy and convert sunlight into localized heat, thereby driving efficient water evaporation and enabling effective separation of clean water from pollutants [7]. The technology offers significant advantages in terms of low energy consumption, high efficiency, and applicability in decentralized settings.

Solar‐driven hydrogels (SDHs, the three‐dimensional cross‐linked polymer network driven mainly by solar energy) have emerged as a revolutionary platform combining hydrophilic polymer networks with photothermal functionalities under the background of the development of photothermal interface evaporation technology. SDHs provide exceptional hydrophilicity, high specific surface area, and tunable porous structures that facilitate efficient water transport and light capture. Moreover, their abundant functional groups (‐OH, ‐NH₂, and ‐COOH) can modulate water molecular states, reduce evaporation enthalpy, and enhance vapor generation rates [8, 9]. Furthermore, the cross‐linked network structure effectively distributes mechanical stresses, thereby maintaining structural integrity and extending operational lifetime. These unique properties indicate that SDHs are highly promising materials in the fields of seawater desalination and wastewater treatment.

Recent review articles have advanced the development of SDHs from multiple perspectives. However, these contributions exhibit inherent limitations in scope and thematic focus. For instance, Zhu et al. [10] and Zou et al. [11] concentrated narrowly on chitosan‐based and conjugated polymer‐based hydrogel systems, respectively, which offered systematic discussions on the potential of single material platforms for interfacial solar evaporation and water purification. Meanwhile, Cui et al. [12] and Jing et al. [13] provided comprehensive reviews centered on photothermal conversion (the mechanism by which materials absorb the energy of photons and convert it into thermal energy) mechanisms and evaporator structural design, further delving into the physicochemical principles that govern water state regulation and energy conversion efficiency. Nevertheless, these reviews failed to provide specific discussions on the long‐term operational stability, anti‐fouling mechanism, and durability evolution process of hydrogels in real complex water environments. Similarly, Zhou et al. [14] pioneered the conceptualization of hydrogels as a material platform for solar water purification in 2019, thereby establishing a foundational theoretical framework for the field. However, this review, constrained by the developmental stage of research at that time, did not encompass recent advances in emerging photothermal materials, advanced fabrication strategies, and multifunctional integrated systems. Notably, recent studies have advanced the design of multifunctional hydrogel systems and their exploration in engineered solar evaporation applications [15, 16, 17]. Nevertheless, these efforts mainly focused on the innovative development of specific functional materials and have not yet integrated the entire chain of mechanisms linking material screening, structural regulation, performance optimization, and large‐scale application at the system level. Overall, existing reviews generally present the following critical research gaps: 1) Lack of systematic cross‐material comparative analysis; 2) Insufficient exploration of performance evolution and contamination resistance mechanisms under prolonged operation; 3) Limited in‐depth analysis of the transition from laboratory‐scale prototypes to practical large‐scale deployment. To fill this gap, this article comprehensively compares the properties of hydrogels based on natural polymers and synthetic polyelectrolytes. Additionally, this article deeply analyzes the photothermal mechanisms of various photothermal conversion components, as well as their synergistic effects with the hydrogel network structure. Furthermore, this article systematically reviews advanced manufacturing techniques and evaluates the complex regulatory mechanisms that control the photothermal efficiency, evaporation rate, mechanical strength, salt tolerance, and long‐term operational stability of the obtained hydrogels. Finally, this article summarizes recent application progress of SDHs in seawater desalination, heavy metal removal, organic pollutant degradation, and water sterilization, while also exploring the innovative application potential of solar cascade utilization and water resource recovery within this evolving field. This review aims to systematically clarify the key research directions and development paths of SDHs during their transformation from laboratory research to practical application, which provides theoretical support and engineering guidance for the construction of an efficient, stable, and scalable next‐generation material system for solar‐driven water purification.

2. The Substrate Materials of Solar‐Driven Hydrogels

2.1. Natural Polymer‐Based Hydrogel

Natural polymer‐based hydrogels, such as those derived from sodium alginate, chitosan, and cellulose (Figure 1), have garnered significant interest for solar‐driven water purification due to their excellent biocompatibility, biodegradability, and selective water absorption [18]. Natural polymer‐based hydrogels can effectively manage the internal water molecules during purification, thereby reducing energy consumption for evaporation and enhancing pollutant repulsion. Consequently, natural polymer‐based hydrogels are considered ideal materials for water purification [11]. This section provides an overview of the various natural polymer‐based hydrogel types and their distinctive properties and effects on water purification.

FIGURE 1.

The molecule structures of (A) sodium alginate, (B) chitosan, and (C) cellulose.

2.1.1. Sodium Alginate Composite Hydrogel

Sodium alginate (SA) is a natural polysaccharide composed of β‐D‐mannuronic acid and α‐L‐gururonic acid. SA, extracted from kelp or sargassum, can trigger gelation through divalent ions (Ca²⁺ and Cu²⁺) and form a stable three‐dimensional network structure, thereby enhancing the mechanical strength of SA composite hydrogels [19]. The main chain of SA contains a large number of carboxyl and hydroxyl groups [20], and its carboxyl group can remove heavy metals (with retention rates of Pb²⁺ and Cd²⁺ > 99.9%) through electrostatic adsorption. The hydroxyl group can isolate organic pollutants through hydrogen bonds [21]. In addition, SA can be used as an adsorbent to react with specific pollutants for achieving selective removal.

Previous research has indicated that SA, as the base material for photothermal hydrogels, can provide a uniform dispersion and stable fixation platform for photothermal fillers [22]. At the same time, its natural hydrophilicity and porous structure are conducive to water transport and evaporation, thereby synergistically enhancing the photothermal conversion efficiency. Compared with other synthetic polymers, the advantages of SA include its natural renewability, superior biocompatibility, lower toxicity and degradability, milder gelation conditions, and lower cost. Unlike other natural polysaccharides, the ionic cross‐linking mechanism of SA is simpler and faster, and the gel stability is higher, which makes it less soluble in water and more suitable for long‐term photothermal applications. Hence, SA is an ideal candidate for constructing efficient and stable SDHs and is widely utilized in wastewater treatment [23]. For instance, Xu et al. [24] developed highly elastic photoresponsive solar absorption hydrogels (SAG) by combining photothermal polydopamine with SA on poly (n‐isopropylacrylamide) hydrogels. The results show that SAG had a high water purification rate (7.18 kg/m²/h), strong mechanical properties, and excellent salt resistance. This was attributed to the high desalination rate of SA, which could reduce the possibility of gel fouling and enhance mechanical performance (Figure 2A). In another study, interconnected porous biomass‐derived hydrogels (SPGS) were prepared by using SA and graphite as the matrix and photothermal agent, respectively. SPG possessed an efficient evaporation rate (3.15 kWm⁻²h⁻¹) and energy conversion efficiency (91%). Besides, SPG still maintained a high evaporation rate of 2.66 kg/m²/h in high‐concentration brine (20 wt.%). SPG has shown promising prospects in the field of solar‐powered seawater desalination due to its remarkable salt resistance, mechanical properties, and long‐term stability (Figure 2B) [25]. Similarly, Li et al. [26] prepared fully bio‐based porous hydrogels using SA and pulp fibers (PF) as the matrix and reinforcing materials, respectively. The results show that the hydrogel exhibited an excellent evaporation rate (2.26 kg/m²/h). This was attributed to the fact that SA exhibited remarkable photothermal conversion efficiency and excellent water transport capacity by loading CuS nanoparticles. In addition, the hydrogel possessed excellent thermal insulation, antibacterial, and mechanical properties, further implying that the hydrogel shows great application potential in the field of wastewater purification.

FIGURE 2.

(A) Sunlight‐driven SAG technology for water production. (a) Inspired by the puffer fish, the SAG technology is based on the phase transformation of PNIPAm under natural sunlight; Reprinted with permission [24]. Copyright 2021, Wiley. (b) Preparation of SAG at room temperature; (c) The characterization diagram of SAG mechanical properties; (d) Comparison of water collection performance of SAG and the previously reported gels under one sun. (B) Sunlight‐driven SPG technology for solar desalination. (a) Fabrication process for the SPG porous hydrogel irradiation; Reprinted with permission [25]. Copyright 2024, Elsevier. (b) Characterization of SPG's evaporation performance; (c) salt resistance; (d) mechanical performance; (e) Schematic of the SPG for outdoor desalination characterization.

2.1.2. Chitosan‐Based Hydrogel

Chitosan (CS), as the only basic amino polysaccharide discovered in nature so far, originates from the deacetylation of chitin, and it is non‐toxic and environmentally friendly. CS exhibits remarkable water purification properties. This is attributed to the large number of amino groups (‐NH₂) and hydroxyl groups (‐OH) in CS molecules, which can provide electrostatic adsorption sites, thereby efficiently removing heavy metal ions (Pb^{2 +} and Cd^{2 +}) and organic dyes [27]. The presence of amino and hydroxyl groups also introduces electrostatic adsorption sites, which promote the removal of heavy metal ions (Pb²⁺ and Cd²⁺) and organic dyes [28]. In addition, CS has a remarkable sterilization effect, which is attributed to the fact that its protonated amino group (NH₂) can destroy the cell membranes of microorganisms, especially for Escherichia coli. The diameter of the inhibition zone can be as high as 12.3 mm [29]. Compared with other polysaccharide materials, the significant advantage of CS lies in its cationic properties and inherent antibacterial ability, which gives it an irreplaceable advantage in the field of environmental science. At the same time, CS's amino groups provide flexible chemical modification sites, thereby facilitating functionalization modification to further enhance the photothermal performance. Therefore, CS can be used as an ideal material for solar‐driven hydrogels.

Liu et al. [30] prepared solar‐driven evaporators based on all Marine biomass by using CS hydrogel and cuttlefish ink (CI) as the hydrated skeleton and photothermal agent, respectively. The results show that the CI/CS evaporator showed a significant evaporation rate (4.1 kg/m²/h) and self‐salt discharge capacity. This was attributed to the fact that water in the 3D network of CS hydrogel could be rapidly replenished by the interconnected porous structure and hydrophilic functional groups on the polymer chain, thereby expanding the evaporation surface area of CS hydrogel and enhancing the solar energy absorption rate and water transport capacity. In another study, a g‐C₃N₄/CS composite hydrogel (CN‐B/CS) demonstrated a remarkable ability to photocatalytically degrade antibiotics (tetracycline degradation rate of 82.5%) and an evaporation rate (3.43 kg/m²/h), further suggesting its potential in wastewater treatment and seawater desalination [31]. Similarly, high‐efficiency solar‐driven water evaporators were prepared by using C─based hydrogels as the matrix and polypyrrole (PPy) and graphene oxide (GO) as photothermal agents. This solar‐driven water evaporator showed an efficient evaporation rate (3.79 kg/m²/h) and photothermal conversion efficiency (96.16%), which was attributed to the excellent vertically arranged porous structure of CS. In addition, this solar‐driven water evaporator exhibited excellent salt resistance and antibacterial properties [32]. Unfortunately, although CS has good antibacterial properties, the microbial adhesion mechanisms in actual sea areas are complex, which is not sufficient to achieve a significant sterilization effect [33]. Therefore, it is crucial to modify CS by different methods to achieve a significant sterilization effect. For instance, high‐performance antibacterial solar steam generators were prepared by using 2D AgMXene nano composites as the photothermal agent and 3D CS‐based hydrogels as the matrix. The results show that the solar steam generator displayed high photothermal conversion efficiency (93%) and excellent antibacterial performance (100% inhibition of Escherichia coli and Staphylococcus aureus within 24 h), indicating that the solar steam generator exhibits broad application prospects in seawater desalination and wastewater antibacterial fields [34].

2.1.3. Cellulose‐Based Hydrogels

Cellulose, as a naturally occurring biopolymer, is the world's most plentiful renewable resource [35]. Cellulose exhibits biodegradability, nontoxicity, and a high specific surface area. Additionally, cellulose nanofibers (CNF) display low diameter (300–700 nm), honeycomb‐like structure, and high specific surface area, which enhances their pollutant adsorption capacity. Therefore, cellulose can serve as an ideal material for solar‐driven water purification.

Cellulose is an ideal component for photothermal hydrogels due to its nanoscale properties. The properties facilitate the formation of a three‐dimensional nanoporous network structure rich in hydrophilic groups and possess significant mechanical strength [36]. Additionally, cellulose can effectively carry photothermal additives, improve water transmission and evaporation rates, and simultaneously enhance photothermal conversion efficiency. Compared to other natural polymers, the main advantage of cellulose lies in its rigid nanofiber framework structure. This structure can provide excellent mechanical strength and form controllable nano‐porous structures, thereby ensuring the structural stability and durability of the hydrogel during long‐term utilization [37]. Therefore, cellulose can be regarded as an ideal base material for preparing high‐performance, environmentally friendly solar‐driven water purification hydrogels. For instance, Alam et al. [38] prepared banana fiber CNF filter media by using CNF extracted from banana fibers. The CNF filter media exhibited an efficient antibiotic adsorption rate of 90% due to the high adsorption capacity of CNF and the antibacterial properties of kaolin. Moreover, the abundant hydroxyl groups (‐OH) in CNF could form strong hydrogen bond networks and enhance chemical modifiability, which in turn improved the hydrophilicity and mechanical strength of the CNF filter hydrogel [39]. Carboxyl groups can significantly increase the surface negative charge of CNF, thus strengthening the CNF filter's adsorption capacity for contaminants, further suggesting that this CNF filter shows great application potential in wastewater purification. Similarly, Li et al. [40] prepared a solar‐driven interface water evaporator of rGC‐CNF composite hydrogel by introducing carbon nanotube (CNT) and graphene oxide (GO) via the extensible CNF as the matrix material. The solar‐driven interfacial evaporator exhibited high evaporation performance (2.73 kg/m²/h) and photothermal conversion efficiency (97.67%). This was attributed to the diverse functional groups of cellulose nanofibers (CNF), which enhanced and refined the water molecule state of the rGC‐CNF composite hydrogel (intermediate water: free water = 1.426). Previous review has found that cellulose‐based hydrogels could effectively adsorb heavy metal ions (the retention rate of Pb²⁺ and Cd²⁺ > 99.9%) and methylene blue (the adsorption capacity of 69.38 mg/g) [41]. A solar steam evaporator was prepared by utilizing CuS as the photothermal agent and cellulose hydrogel as the matrix. This solar steam evaporator exhibited a high‐water evaporation rate (2.2 kg/m²/h) and photothermal conversion efficiency (87%) (Figure 3A). Additionally, the solar steam evaporator showed significant photocatalytic degradation ability for MB (the degradation rate of 95%) (Figure 3B) [42]. However, the photothermal conversion efficiency of cellulose‐based hydrogels can decrease under high humidity conditions. Therefore, the combination of cellulose‐based hydrogels with functionalized materials helps to optimize their structural properties and enhance mechanical stability. Wang et al. [43] developed a cellulose‐based hydrogel evaporator (3DL Metagel) by modifying the surface of CNF with hydrophilic inorganic material Al(H₂PO₄)₃ (AP). The 3DL Metagel displayed a high‐water evaporation rate (3.61 kg/m²/h) and evaporation efficiency (94.94%). The performance was due to the integration of lotus morphology and plant transpiration in 3DL Metagel, which assisted the 3DL Metagel in capturing a large amount of additional energy from the environment. Besides, the 3DL Metagel possessed a biporous structure with Janus wettability, which promoted its self‐floating ability and unidirectional salt ion reflux channel, thus providing an innovative salt‐resistant technology for desalination. Notably, the 3DL Metagel could still achieve efficient outdoor water purification in extremely low‐humidity arid regions.

FIGURE 3.

(A) The fabrication process of rGC‐CNF hybrid hydrogel. Reprinted with permission [40]. Copyright 2024, Elsevier. (B) Mass change of water under 1 sun. (C) The diagram of rGC‐1.25CNF/EVA evaporation system. (D) The photograph and absorption spectrum of RhB and MB aqueous solution and condensed water. (E) Antibacterial measurement of Escherichia coli before and after evaporation. (F) The fabrication process of CuS@cellulose composite hydrogel. Reprinted with permission [42]. Copyright 2023, Elsevier. (G) Photocatalytic degradation of MB and RhB by CuS@cellulose composite hydrogel. Reprinted with permission [42]. Copyright 2023, Elsevier.

2.1.4. Others

In recent years, other natural polymers with abundant availability, diverse compositions, and distinctive structural features have been increasingly explored as foundational materials for SDHs evaporators, thereby broadening the spectrum of material choices in this field [44]. Typical representatives of other natural polymer materials include lignin, starch, and natural plant‐based gums.

Lignin is an amorphous 3D macromolecule, formed by different types of methoxylated phenylpropane units linked through ether bonds and C─C bonds [45]. Lignin demonstrates amphiphilic characteristics owing to its high density of functional groups (hydroxyl, carbonyl, and methoxy moieties) [46]. Lignin's rich aromatic architecture enables efficient absorption of solar radiation and subsequent conversion into thermal energy, which indicates that it is suitable for application in solar evaporation systems [47]. Moreover, lignin‐based hydrogels facilitate rapid and sustained water transport in solar‐driven water purification processes, thereby addressing critical challenges, including salt accumulation, sustainability, economic feasibility, and evaporator scalability [48]. Capitalizing on these properties, Jiang et al. [49] fabricated an evaporator by using lignin hydrogel as the base layer and polypyrrole as the upper layer. The lignin hydrogel layer exhibited superior hydrophilicity and an interconnected macroporous framework, which collectively promoted efficient water permeation and reduction in evaporation enthalpy, thereby leading to an accelerated evaporation rate. Consequently, the lignin hydrogel‐based evaporator achieved a high evaporation rate of 2.25 kg/m²/h and a solar‐to‐vapor conversion efficiency of 91.6%.

Starch is a typical natural polymer material that possesses excellent biocompatibility and safety. More importantly, starch exhibits excellent swelling capacity and elasticity in water [50, 51]. Wang et al. [52] prepared a hydrogel evaporator (SiO₂‐PILs/starch) by using corn starch and grafted polyion liquids on silica. The cumulative pore size range, porosity, and total pore area of the SiO₂‐PILs/starch hydrogel were 0–50 µm, 60.57%, and 0.195 0.195 m²/g, respectively, which were conducive to the rapid transmission of water. Moreover, the SiO₂‐PILs/starch hydrogel demonstrated a solar absorption rate of 91% and an evaporation efficiency of 91.72% in pure water.

Polysaccharide‐based natural plant gums have attracted much attention due to their excellent stability, water solubility, biodegradability, and biocompatibility [53]. In particular, guar gum (GGm) has been extensively utilized in wastewater treatment applications due to its distinctive physical properties, multifunctional behavior, and unique adsorption characteristics [54]. The mannose and galactose side chains on the GGm main chain provide numerous reactive hydroxyl groups, which can be amenable to chemical modification. Therefore, GGm is an ideal substrate for constructing functional hydrogels [55]. For instance, Chaurasiya et al. [56] prepared a nano‐composite material (XGGmHs@Fe₃O₄ NC) using the guar gum‐based hydrogel modified with xanthan acid as the base and doping Fe₃O₄ nanoparticles (NPs). The results show that the removal rate of MB dye and the photocatalytic degradation efficiency of XGGmHs@Fe₃O₄ NCs were 91.35% and 82.64%, respectively, under visible light, which provided a sustainable and effective method for the removal of dye pollutants.

2.2. Synthetic Polyelectrolyte Hydrogel

Synthetic polyelectrolyte hydrogels are three‐dimensional hydrophilic polymer networks composed of charged polymers, including polyanions, polycations, and zwitterions. Polyelectrolyte hydrogels have become a revolutionary material in the field of solar water purification. This is attributed to the osmotic pressure (up to 659 atm) generated by the synthesis of polyelectrolyte chains, which can form an internal water potential gradient and facilitate rapid water adsorption and directional migration. For instance, Shan et al. [57] prepared a polyelectrolyte hydrogel adsorbent (PAMPS‐CNT‐LiCl) by incorporating hygroscopic lithium chloride (LiCl) and carbon nanotubes (CNT) into a poly(2‐acrylamido‐2‐methylpropanesulfonic acid) (PAMPS) network. PAMPS‐CNT‐LiCl exhibited excellent water adsorption kinetics and water storage capacity, which was attributed to its large internal pressure and good swelling ratio. Moreover, the distinctive chemical composition of PAMPS‐CNT‐LiCl enabled stable salt retention via effective coordination between LiCl and the charged polymer chains. Notably, PAMPS‐CNT‐LiCl achieved a more uniform adsorption temperature and ultra‐fast water release performance through the combined solar energy and simulated residual heat hybrid desorption mode. Zhao et al. [58] constructed a Janus‐structured solar evaporator by loading PPy nanoparticles onto a polyvinyl alcohol (PVA) hydrogel substrate. These results show that the solar evaporator had an efficient water evaporation rate (2.26 kg/m²/h) and solar utilization rate (80.1%). The porous architecture of the PVA hydrogel, combined with hydrophilic PPy nanoparticles, yielded a localized photothermal conversion efficiency of 82.5%, thereby promoting rapid water transport. Moreover, the Janus structure enabled the solar evaporator to achieve significant desalination capacity (desalination rate > 99.9%) and remove 95.8% of the chemical oxygen demand (COD) from actual wastewater (Figure 4A).

FIGURE 4.

(A) PAMPS‐CNT‐LiCl hydrogel. (a) Schematic illustration of the fabrication process of PAMPS‐CNT‐LiCl hydrogel; Reprinted with permission [57]. Copyright 2023, Wiley. (b) Schematic illustration of hybrid desorption mode coupling waste heat and solar energy for water release; (c) Schematic illustration of all‐day multicyclic AWH mode driven by coupled solar energy and waste heat. (B) APH hydrogel. (a) The preparation process of APH; Reprinted with permission [59]. Copyright 2022, Elsevier. (b) Schematic diagram of solar steam generation based on the salt‐tolerant anion polyelectrolyte‐based hydrogel; (c) Evaporation rates and cycling solar evaporation tests; (C) GMS@HCDs evaporator. (a) Schematic illustration of GMS@HCDs; Reprinted with permission [60]. Copyright 2024, Wiley. (b) Salt‐rejecting function and durability in brine; (c) Schematic illustration of electricity collection.

Polyelectrolyte hydrogels (PZH) have been widely applied in interfacial solar steam generators (SSG) owing to their superior anti‐contamination and salt resistance properties. This was attributed to the charged groups of PZH that prevented salt deposition through electrostatic repulsion and coordination, thereby improving SVG performance [61]. Additionally, PZH displayed an exceptional photothermal conversion rate (4.14 kg/m²/h), and the photothermal conversion rate of PZH even increased by 20% in high‐salt water (10 wt.%) compared to pure water. This was attributed to the fact that the polymer chains of PZH contained cationic and anionic functional groups with opposite charges, which easily formed a large number of hydrated polymer networks and enhanced the ability to evaporate water [62]. A 3D reticulated porous structure of anionic polyelectrolyte photothermal hydrogel (APH) was prepared by utilizing poly(3,4‐ethylenedioxythiophene) (PEDOT) and sodium polystyrene sulfonate (PSS). The results show that APH exhibited a high evaporation rate (2.5 kg/m²/h) and an evaporation efficiency (90.7%) due to the synthesized polyelectrolytes possessing charged groups. This could disrupt the hydrogen bond network of water molecules, promote the transition of interfacial water from a “tetrahedral structure” to intermediate water, and significantly reduce the energy consumption of phase change. Additionally, the negatively charged SO₃ ⁻ in APH could separate Na⁺ and Cl⁻ through electrostatic repulsion effects, which assisted APH in achieving stable salt resistance and long‐term evaporation capability (Figure 4B) [59].

However, single synthetic polyelectrolyte hydrogels are inevitably subject to mechanical damage during operation, and their evaporation performance and service life will also decline [63]. Additionally, the evaporation rate of the hydrogel will decrease when exposed to low outdoor light. Sun et al. [64] prepared hybrid hydrogels (PD‐PAM) for solar‐driven water purification by introducing eco‐friendly β‐cyclodextrin (β‐CD) into polydopamine (PDA) and polyacrylamide (PAM). The results show that PD‐PAM exhibited a high evaporation rate (2.65 kg/m²/h), which remained largely unchanged after 150 h. This was mainly attributed to the three‐dimensional network structure of PD‐PAM, which provided abundant water transport channels. Besides, the introduction of β‐CD hydroxyl groups promoted the formation of intermediate water, enhanced the hydrophilicity of PD‐PAM, and reduced its water vaporization enthalpy, further suggesting that PD‐PAM has broad application prospects in the field of seawater desalination [65].

3. Photothermal Agent for Solar‐Driven Hydrogel

3.1. Semiconductor

Semiconductor‐based photothermal catalysts have broken through the traditional bottlenecks of low efficiency in photocatalysis and high energy consumption in thermal catalysis. Semiconductor‐based photothermal catalysts possess core advantages, such as wide spectral response, high catalytic activity, excellent stability, and environmental friendliness [66]. The photothermal conversion process primarily arises from photoinduced bandgap charge excitation and electron‐phonon interactions [67]. Common semiconductor photothermal catalysts include molybdenum disulfide (MoS₂), copper sulfide (CuS), titanium dioxide (TiO₂), bismuth sulfide (Bi₂S₃), molybdenum trioxide (MoO₃), quantum dots (QDs), and titanium suboxide (Ti₄O₇) [68].

As a representative Magnéli phase semiconductor, Ti₄O₇ has advantages, such as low cost, high chemical stability, and favorable thermal conductivity. Moreover, Ti₄O₇ has a narrow bandgap (∼0.81 eV), thus enabling efficient light absorption and photothermal conversion across the entire ultraviolet‐visible‐near‐infrared spectral range, indicating that Ti₄O₇ is an ideal candidate for photothermal materials [69, 70]. Xu et al. [71] employed the “molecular/phased dual‐locking” strategy to prepare Ti₄O₇‐PVA nanocomposite hydrogels (TPNH). This design immobilized Ti₄O₇ nanoparticles within the chemically cross‐linked PVA polymer network to ensure uniform and stable dispersion of Ti₄O₇ nanoparticles within the hydrogel. The results demonstrate that TPNH exhibited an excellent evaporation rate (4.45 kg/m²/h) and evaporation efficiency (90.69%). Recent research has further advanced the design of Ti₄O₇‐based photothermal materials. Zhang et al. [72] constructed a two‐dimensional Ti₄O₇‐HPB superlattice structure (Ti₄O₇‐HPB‐SL) through the self‐assembly of Ti₄O₇ clusters mediated by heterocyclic blue (HPB). This design significantly enhanced the photothermal performance through the synergistic effect of the Ti₄O₇ and HPB components. Meanwhile, this design utilized the long‐range ordered sub‐nanowire arrays to construct directional water transport channels. The hydrophilic HPB component acted as a hydraulic pump to drive water molecules to transport along specific paths in a directed manner, while the surface‐modified hydrophobic molecular layer effectively separated the bulk water from the photothermal active sites through a physical barrier effect, thereby significantly reducing heat conduction losses. The evaporation rate and energy efficiency of the 3D Ti₄O₇‐HPB‐SL hydrogel based on this design were 3.48 kg/m²/h and 97.39%, respectively. The 3D Ti₄O₇‐HPB‐SL hydrogel performed exceptionally well in practical seawater desalination and nuclear wastewater purification.

Black‐brown copper sulfide (CuS) is a typical semiconductor material, renowned for its broad and strong light absorption capabilities [73]. CuS shows significant potential in the realm of photocatalytic water remediation [74]. For instance, Wang et al. [75] synthesized monodisperse and uniform 3D flower‐shaped CuS superstructures via a solvothermal method. 3D flower‐shaped CuS exhibited remarkable photocatalytic performance in degrading organic pollutants in water. In another study, a solar steam evaporator was prepared using CuS as the photothermal agent within a cellulose hydrogel matrix [76]. The solar steam evaporator had a high‐water evaporation rate (2.2 kg/m²/h) and a high photothermal conversion efficiency (87%) by optimizing the CuS loading in the hydrogel. Moreover, the solar steam evaporator had significant photocatalytic degradation ability for MB (degradation rate of 95%).

3.2. Carbon‐Based Nanomaterial Composite Hydrogel

Carbon‐based nanomaterials, such as graphene, carbon nanotubes, carbon black, and MXene, are ideal for solar‐driven interface water evaporation due to their broad light absorption (>95%), high photothermal conversion efficiency (>90%), and adjustable pore structures [77, 78]. Additionally, these materials can modify the hydrogen bond network of water molecules in hydrogels through polar groups (‐OH, ‐COOH), thereby enhancing the proportion of intermediate water (> 57%) and lowering evaporation enthalpy (by over 56%) [79]. Consequently, carbon‐based nanomaterials are widely used in developing solar‐driven water purification hydrogels.

3.2.1. Graphene

Graphene, as a carbon allotrope, consists of a single layer of atoms arranged in a 2D honeycomb lattice [80]. Graphene exhibits remarkable physical and chemical properties, including high electrical conductivity (1010 S/m) and carrier mobility (20 m²/(V·s)) [81, 82], strong thermal conductivity (5300 W/(m·K)) and tensile strength (130 GPa) [83]. Graphene with high specific surface area (2620 m²/g) and high light transmittance (97.7%) is extensively used for solar‐driven water purification [84]. 3D cross‐linked honeycomb graphene foam (3DG) showed a high light absorption rate (>90%), which enabled the direct conversion of solar energy into thermal energy. Moreover, 3DG could provide capillary forces and water transport channels due to its porous structure, thereby inhibiting heat diffusion losses. Notably, 3DG displayed a high evaporation rate (2.61 kg/m²/h), an energy efficiency (80%), and an ion removal rate (99.3%) in seawater desalination [85]. However, graphene is usually limited by high emissivity in the infrared spectrum, which leads to significant heat radiation loss and a decrease in overall photothermal efficiency [86]. Carbon dots (CDs), known for their molecular‐like behavior and delocalized electronic structure [87], can be integrated with graphene to reduce its infrared emissivity while introducing numerous hydrophilic functional groups for water activation [88]. Li et al. [60] prepared a novel bi‐functional evaporator (GMS@HCDs) by combining CDs with graphene and a polyelectrolyte hydrogel‐functionalized melamine sponge (MS). The findings reveal that GMS@HCDs exhibited a high evaporation rate (3.53 kg/m²/h), energy conversion efficiency (98.6%), excellent salt resistance, and mechanical properties. Additionally, GMS@HCDs generated a continuous voltage of 0.972 V and a current density of 172.38 µA/cm², indicating significant potential for powering common electronic devices and applications in sustainable power generation and efficient water purification (Figure 4C).

Graphene oxide (GO) is a single‐atom layer of carbon with both surfaces modified by oxygen‐containing functional groups, which makes it more chemically reactive than graphene [89]. Reduced graphene oxide (rGO), which is derived from graphene nanosheets [90], can be synthesized via chemical, electrochemical, and thermal reduction methods [91]. The surface of rGO features a substantial density of sp²‐conjugated π bonds. rGO effectively transforms absorbed broad‐spectrum solar energy into thermal energy through non‐radiative relaxation mechanisms [92]. For instance, Xu et al. [93] prepared a rGO/alginic acid hydrogel sponge (RSHS) evaporator by depositing rGO/alginic acid hydrogel in situ onto a naphthalene tricarboxylic acid sponge framework. RSHS achieved excellent absorption across the entire solar spectrum by incorporating rGO into the sponge matrix and utilizing the conjugated π bonds of rGO, as well as the light‐harvesting structure of the sponge. Additionally, the abundant hydrophilic groups (‐OH, ‐COOH) in the rGO/alginic acid network significantly reduced the evaporation enthalpy of water to 1094.85 J·g⁻¹, thus enabling the evaporation rate and energy conversion efficiency of RSHS to be 3.13 kg/m²/h and 91.54%, respectively [93]. Moreover, rGO exhibits a high specific surface area and exceptional electrical conductivity, which support its broad applicability in the development of solar‐driven water purification hydrogels. Zhou et al. [94] prepared a mixed hydrogel with internal capillary channels utilizing rGO and PVA. The mixed hydrogel featured a high‐water evaporation rate (2.5 kg/m²/h) and a high continuous and stable seawater desalination capacity for 96 h, which was attributed to the fact that rGO could effectively utilize the energy in the network, and PVA significantly reduced the water evaporation enthalpy. In another study, Noureen et al. [95] prepared BiVO₄‐rGO hydrogel using BiVO₄ and rGO as the base materials. The BiVO₄‐rGO hydrogel could efficiently produce solar steam and break down organic pollutants using light. When floating on water, its water evaporation rate increased to 1.6 kg/m²/h with an energy conversion efficiency of 87%. Additionally, the BiVO₄ nanoparticles in the hydrogel could rapidly decompose the toxic organic dyes (Rhodamine B, Methyl Orange) under the activation of sunlight. The BiVO4‐rGO hydrogel has broad application prospects in wastewater treatment and efficient freshwater production.

3.2.2. Carbon Nanotube (CNT)

Carbon nanotubes (CNTs) are hollow structures made of graphite carbon that exhibit a high aspect ratio and sp² hybridization. They can be categorized into single‐walled (SWCNTs), double‐walled (DWCNTs), and multi‐walled carbon nanotubes (MWCNTs) based on the number of graphene layers. CNTs have garnered significant interest for Solar‐driven water purification due to their excellent chemical stability, remarkable mechanical and electrical properties, dense mesoporous structure, extensive specific surface area, and high adsorption capacity [96]. For example, Jiang et al. [97] developed a solar evaporation device (CSG) by incorporating CNTs into a biomass hydrogel network and assessed its photothermal conversion efficiency. The CSG demonstrated excellent hydrophilicity, a solar absorption efficiency of 94.3%, and a photothermal conversion rate of 86%, while effectively preventing salt crystallization in seawater desalination. Additionally, CSG removed heavy metal ions, like Na⁺, K⁺, Ca²⁺, and Mg²⁺, with a 99.3% removal rate and fully decolorized Methyl blue and Methyl orange. Similarly, Jiang et al. [98] created a honeycomb‐structured 3D hydrogel solar evaporator (GHCFeP) by integrating CNTs, nano‐ Fe₃O₄, and rGO into polyvinyl alcohol hydrogels. The GHCFeP exhibited a low evaporation enthalpy (1377 kJ/kg), high photothermal conversion efficiency (92.4%), and solar evaporation rate (2.304 ± 0.042 kg/m²/h in pure water and 2.045 ± 0.050 kg/m²/h in simulated seawater). These performance were attributed to the following reasons: 1) CNTs and nano‐Fe₃O₄ respectively possess excellent molecular thermal vibration effects and efficient photothermal conversion mechanisms; 2) CNTs and rGO can form a porous network structure and can be combined with PVA, which helped to enhance the hydrophilic water channels of GHCFeP, thereby reducing the evaporation enthalpy of pure water; 3) The honeycomb design helped to reduce “dead evaporation zones”, increased the effective evaporation area, and further enhanced the evaporation rate. Previous study has shown that nitrogen doping in CNTs significantly increased electron state density near the Fermi level and broadened solar spectrum absorption (with an average rate of 98.5% from 200 to 2400 nm), thus leading to effective photothermal conversion [99]. Drawing inspiration from the structure of human intestinal villi, a novel nitrogen‐doped carbon nanotube solar steam generator (N‐CNTs SSG) was prepared by in situ growing a CNT matrix on the inner surfaces of vertically aligned microchannels. This SSG featured a 3D layered CNT structure and demonstrated exceptional photothermal conversion and solar evaporation efficiency of 96.8% [100].

Traditional CNTs face issues such as hydrophobicity and a tendency to agglomerate, which can compromise the porous structure of water gels, impede water flow, and diminish evaporation efficiency. Luo et al. [101] created a robust solar evaporator with a multi‐level pore structure by integrating amino‐terminated CNTs modified with a metal phenolic network (MPN) into PVA hydrogel. The findings reveal that this evaporator achieved an impressive evaporation rate of 2.9 kg/m²/h, maintained a rate of 2.7 kg/m²/h in high‐salinity water (20 wt.%), exhibited mechanical durability over 20 cycles, and retained over 99.8% of ions. This success could be attributed to the following reasons: 1) MPN's modification of CNT surfaces through π–π interactions and covalent bonding enhanced their hydrophilicity and dispersion; 2) The d‐d transition of MPN and the broad‐spectrum absorption of CNTs worked together to enhance the light absorption rate of PVA hydrogel; 3) MPN inhibited the aggregation of CNTs and promoted the formation of a uniform microporous structure in the PVA hydrogel. At the same time, it combined with the large pores of the foamed polyurethane, thereby achieving efficient water transmission and anti‐salt crystallization properties. These offer promising strategies for designing efficient seawater desalination and water purification systems.

3.2.3. Carbon Black (CB)

CB has a high specific surface area (1000–2000 m²/g) and is rich in internal micropores, mesopores, and macropores, which enhances its ability to adsorb organic compounds (phenol and dyes), heavy metal ions, and microorganisms. Additionally, the surface of CB is abundant in multiple functional groups such as hydroxyl and carboxyl, which facilitate the removal of pollutants through chemical adsorption and electrostatic interaction. Furthermore, CB exhibits a broad absorption spectrum (300–2500 nm) with a high absorption rate (96%), which can efficiently convert solar energy into thermal energy [102]. For example, Guo et al. [103] prepared a bilayer solar‐driven interfacial evaporator using CNF‐modified CB deposited nonwoven fabric as the photothermal layer and PVA/starch composite hydrogel as the base layer through the freeze‐thaw method (Figure 5A). The interfacial evaporator demonstrated a high water evaporation rate (1.08 kg/m²/h), stable mechanical properties, and salt resistance. Besides, the interfacial evaporator could effectively separate dyes, oils, and metal ions (the ion retention rate > 99%) from contaminants. This was attributed to the excellent photothermal conversion performance of CB@NF and the superior desalination performance, high purification efficiency, high water absorption rate, and strong interfacial adhesion of PVA/starch hydrogel (Figure 5B,C).

FIGURE 5.

(A) Schematic illustration for the fabrication process of bilayer CB@NF/PVA/starch composites. Reprinted with permission [103]. Copyright 2022, Elsevier. (B) Interfacial bonding performance of the bilayer CB@NF/PVA/starch composites. (C) Water purification performance of bilayer evaporator. Reprinted with permission [103]. Copyright 2022, Elsevier. (D) Schematic illustration for preparation and solar steam generation mechanism of BDCH. Reprinted with permission [104]. Copyright 2025, Elsevier. (E) BDCHs exhibit an average light absorption of approximately 95% across the 300–2500 nm wavelength range. Reprinted with permission [104]. Copyright 2025, Elsevier. (F) Evaluation of the desalination performance with primary ions in a seawater sample before and after. (G) Salt resistance and wastewater treatment performance. Reprinted with permission [104]. Copyright 2025, Elsevier.

CB materials tend to agglomerate and are challenging to disperse uniformly in water, which impacts the stability of the gel [105]. Cellulose nanofibers (CNF) are an economical and environmentally friendly bio‐based dispersant that can enhance the dispersibility of CB particles in pure water before vacuum filtration. The dispersion mechanism of CNF mainly consists of the following two aspects: 1) The polar hydroxyl groups and non‐polar groups (‐CH) help CNF to have amphiphilic properties. CNF attaches to CB particles through the interaction between hydrophobic sites, which can improve the wettability of CB; 2) The electrostatic repulsion generated by carboxyl groups on the surface of CNF can effectively prevent the aggregation of CB particles in the system [106]. This design can significantly reduce the cost of photothermal materials for hydrogels and solve the process problem of easy aggregation of CB. Deng et al. [104] prepared a double‐layer double cross‐linked hydrogel (BDCH) solar evaporation with a hierarchical porous structure by utilizing cellulose as the supporting matrix and CB as the light absorber (Figure 5D). The results show that the BDCH evaporator displayed a high evaporation rate (2.79 kg/m²/h) and energy conversion efficiency (90%). Additionally, the BDCH evaporator demonstrated excellent salt deposition resistance (no scaling after continuous seawater desalination for 72 h) and pollutant removal performance (dye waste removal rate > 99%). This performance was attributed to CB's excellent light absorption (average of 95% in the 300–2500 nm range) and the porous structure of cellulose, which together facilitated efficient solar energy capture and localized heat conversion (Figure 5E–G). It has been reported that CB could achieve uniform dispersion within PVA/PDADMAC matrix through electrostatic interactions and π–π stacking effects, thereby addressing the issue of CB agglomeration during processing. Luo et al. [107] prepared a Janus porous sponge‐like polymer electrolyte hydrogel (PANS‐J) using PVA, PDADMAC, CB, and a foaming agent (sodium dodecyl sulfate and SDS) via a one‐step self‐delamination foaming process. PANS‐J demonstrated an impressive evaporation rate (3.35 kg/m²/h) and energy conversion efficiency (93.94%). This was attributed to the efficient light capture ability of CB and the Janus asymmetric porous structure that extended the light propagation path, thereby achieving thermal localization and reducing heat loss. Figure 6 shows the representative substrate materials and photothermal agents used for preparing hydrogels.

FIGURE 6.

Representative substrate materials and photothermal agents used for preparing hydrogels.

3.2.4. MXene

MXene is a type of two‐dimensional transition metal carbides/nitrides, with the chemical formula M_n+1X_nT_x (M represents a transition metal, and X is carbon/nitrogen, and T is a surface functional group). In recent years, it has attracted much attention in the field of solar‐driven interface evaporation [108, 109]. MXene demonstrates outstanding photothermal conversion capability owing to its metal‐level conductivity, tunable surface hydrophilicity, excellent mechanical strength, and efficient localized surface plasmon resonance effect [110]. Zhang et al. [111] prepared a 3D polyvinyl alcohol/chitosan/Nb₂C MXene composite hydrogel (PCN). The Nb₂C MXene nanosheets were uniformly dispersed in the hydrophilic polymer matrix through hydrogen bonds and electrostatic interactions, thus forming interconnected microchannel structures. This design utilized the plasmonic effect and light trapping structure of MXene to achieve solar light absorption over a wavelength range of 300–2500 nm (the absorption rate of 93%), while the weakening of hydrogen bonds within the hydrated network significantly reduced the evaporation enthalpy to 1426 J/g. The evaporation rate and photothermal conversion efficiency of the PCN hydrogel were 2.72 kg/m²/h and 93.2%, respectively.

Ionic covalent organic frameworks (iCOFs) possess inherent periodic pores and abundant hydrophilic charged functional groups, which can enhance the performance of MXene‐based SDIE evaporators [112]. Ding et al. [113] prepared functionalized hierarchical nano‐porous MXene/iCOFs hydrogels. The MXene/iCOFs hydrogels could effectively regulate the evaporation performance and salt resistance, which was attributed to the fact that iCOFs with high‐density hydrophilic functional groups and nano‐constraint effects could induce the disruption and enhancement of cluster evaporation, thereby achieving a reduction in evaporation enthalpy. Moreover, the increase in functional groups promoted ion reverse diffusion and the Donnan effect, which effectively alleviated the salt crystallization problem during long‐term seawater desalination processes. This strategy led to a 56% increase in evaporation rate (2.91 kg/m²/h).

Ti₃C₂T_x MXene has been widely used as a photothermal agent due to its metallic conductivity, strong solar absorption, abundant surface terminations (single bonds O, OH, and single bonds F), and good processing properties for membranes and porous structures [114]. For instance, Ahmad et al. [115] prepared a cellulose microfiber‐reinforced Ti₃C₂T_x/alginate hydrogel evaporator (MACMF‐8) via the freeze casting method. MACMF‐8 achieved an evaporation rate of 1.66 kg/m²/h and an evaporation efficiency of 85.41% by incorporating Ti₃C₂T_x MXene.

3.3. Conjugated Polymers

Conjugated polymers are a type of macromolecule that have a continuous conjugated structure within their molecular framework. They are typically formed by polymerizing monomers containing multiple π–π conjugated aromatic rings or alkenes. Common conjugated polymers include polypyrrole (PPy), polypyrrole‐3,4‐ethylenedioxythiophene (PEDOT), polydopamine (PDA), polyaniline (PANI), and polyacrylonitrile (PAN) [11]. These conjugated polymers exhibit extensive light absorption in the solar spectrum, covering the visible light and near‐infrared bands, thereby achieving efficient utilization of solar energy. Moreover, these polymers are ideal candidate materials for photothermal materials due to their high light absorption, efficient photothermal conversion, and tunable conjugated structures.

Polypyrrole (PPy) is regarded as a promising photothermal material due to its superior light absorption performance (>90%) [116]. Xu et al. [117] developed an Ag/PPy coating with a broadband absorption rate of up to 95.4%. Based on this, Wang et al. [116] successfully prepared a pure PPy hydrogel with excellent mechanical strength and conductivity through combining the oxidation polymerization method with the secondary growth strategy. The results show that this PPy hydrogel exhibited a solar light absorption efficiency of 98% and an evaporation rate of 3.85 kg/m²/h. Its performance only declined by 4.02% after 50 h of continuous operation, implying excellent long‐term stability. To further improve the performance, MoS₂ nanosheets were introduced to regulate sulfur vacancies, and the evaporation rate of the MoS₂/PPy composite hydrogel increased to 4.34 kg/m²/h. This was attributed to the fact that MoS₂ could optimize the microstructure of the hydrogel, inhibit salt accumulation, and promote the formation of micro‐crescent surface structures and micro‐droplets, thereby effectively activating water molecules and reducing the evaporation enthalpy. This provides a new strategy for designing next‐generation hydrogels that have high photothermal conversion efficiency, low evaporation enthalpy, and efficient water transport channels.

Poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT: PSS) is a commercially available water dispersion [118]. In this material, the conjugated PEDOT can efficiently absorb sunlight, and the hydrophilic PSS has excellent water absorption properties to achieve water permeability and has a strong flocculation ability for water treatment. This material demonstrates great application potential in the field of ideal interface photothermal materials [119]. For example, Zhao et al. [120] prepared PEDOT: PSS‐PVA hydrogel by introducing PEDOT: PSS into the PVA hydrogel matrix. This hydrogel exhibited excellent evaporation rate (about 2.84 kg/m²/h) and energy efficiency (98.0%), which was attributed to its wide spectral absorption characteristics throughout the UV, visible to near‐infrared wavelength range. The PEDOT: PSS‐PVA hydrogel shows great application prospects in seawater desalination and wastewater purification fields.

Porphyrin‐based conjugated microporous polymers (PCP) have demonstrated excellent performance as emerging photothermal materials. Wang et al. [119] constructed a biomimetic water channel protein structure by using electrospinning and in situ polymerization methods to embed PCP and amphoteric polyion liquids (PIL) together into polyacrylonitrile (PAN) nanofiber membranes. This design utilized the photothermal conversion ability of PCP and the ion selectivity of PIL to achieve rapid water transport and effective salt ion blocking. Under a solar light intensity of 1, the evaporation rate and photothermal conversion efficiency of the composite fiber membrane were 2.64 kg/m²/h and 97.6%, respectively. Notably, the composite fiber membrane could achieve a stable voltage output of 184 mV and a power density of 1.5 W m⁻² after integrating this system with a thermoelectric module, which could drive small electrical appliances and demonstrated the potential for “water‐energy” synergy utilization. Additionally, the singlet oxygen generated by PCP under light could achieve an almost 100% antibacterial efficiency to ensure the long‐term stability of the equipment operation.

Polydopamine (PDA), as a biomimetic copolymer, has attracted much attention in the field of solar‐driven water purification due to its unique mussel‐inspired chemical properties, full‐spectrum light absorption capability, and abundant surface functional groups [121]. Chen et al. [122] prepared a hydrogel by introducing PDA‐coated liquid metal nanoparticles (LM@PDA) into the amphoteric ion hydrogel matrix. The hydrogel had multiple functions, such as ultra‐high mechanical strength, self‐healing, and photothermal conversion. The isotropic stretchability of LM@PDA endowed the hydrogel with efficient energy dissipation capabilities, thereby forming a hydrogel with extremely high toughness (the tensile strength of 1.3 MPa, elongation at break of 1555%, and toughness of 7.3 MJ m⁻³). Additionally, the introduction of LM@PDA endowed the hydrogel with excellent photothermal conversion performance, such as an evaporation rate of 2.42 kg/m²/h at 1 solar light intensity and a photothermal conversion efficiency of 90.3%.

3.4. Metals

Precious metal nanomaterials (Au, Ag, and Cu) exhibited outstanding photothermal conversion performance in the field of solar‐driven interface evaporation due to their unique localized surface plasmon resonance effect [123]. For instance, Gouda et al. [124] prepared spherical gold, silver, and copper oxide nano‐fluids through an environmentally friendly liquid‐phase laser ablation technique and systematically compared the photothermal conversion properties of the three materials. The study found that under the same conditions of 1 sun's light intensity, the gold nano‐fluid exhibited the best excellent evaporation performance. The evaporation rate was 2.29 times that of pure water, and the photothermal conversion efficiency reached 80%. Compared with the base liquid, the photothermal conversion efficiency increased by 306%. The evaporation rate of silver nano‐fluid was 2.08 times that of pure water, and the photothermal conversion efficiency increased by 243%. Besides, the evaporation rate of copper oxide nano‐fluid was only 1.73 times that of pure water, and the photothermal conversion efficiency increased by 190%. This comparative study fully confirmed the performance advantages of precious metal materials in the field of photothermal conversion.

The photothermal conversion mechanism of precious metal photothermal agents is as follows: when the incident light frequency matches the collective oscillation frequency of free electrons in the noble metal nanostructures, a localized surface plasmon resonance is triggered, thereby generating thermal electrons through the Landau damping mechanism. Subsequently, the thermal electrons convert the energy into Joule heat through electron‐electron scattering and electron‐phonon coupling [125, 126, 127, 128]. This photothermal conversion process has three main characteristics: 1) The absorption cross‐section is large, and the light absorption capacity of a single nanoparticle far exceeds that of traditional photothermal materials; 2) The thermal localization effect is significant, and the heat being confined within the nanoscale region; 3) The spectral tunability is strong, and a wide spectral absorption range from visible light to near‐infrared can be achieved through the regulation of the size, morphology, and composition of the nanoparticles [129]. Fargharazi et al. [130] prepared a sponge‐like photothermal absorption material by introducing glutaraldehyde, as a cross‐linking agent, into the polyvinyl alcohol/activated carbon hydrogel matrix and loaded gold or silver nanoparticles. The results show that the introduction of gold nanoparticles increased the light absorption rate of the wet hydrogel to 74%, and its evaporation rate reached 5.58 kg/m²/h. Notably, the PVA: GA: Au hydrogel (AC) nanocomposite material still maintained a stable evaporation rate of approximately 5.08 kg/m²/h under 0.1 solar weak light conditions, which indicated that the plasmon resonance effect of gold nanoparticles could still efficiently stimulate even at low light intensities. This was attributed to the uniform distribution of gold nanoparticles in the microporous network of the hydrogel, thereby forming a multi‐level structure conducive to light capture. Silver nanoparticles, as relatively low‐cost noble metal photothermal agents, are often combined with 2D materials such as graphene and applied in hydrogels, thus achieving performance breakthroughs through multiple synergistic effects. Lin et al. [131] synthesized a polyacrylamide‐based reduced graphene oxide/silver composite hydrogel (ArG‐Ag) via a one‐step radiation polymerization and reduction self‐assembly method. ArG‐Ag integrated three functional units: 1) the hydrophilic polyacrylamide cross‐linked network provided excellent water evaporation channels; 2) reduced graphene oxide enhanced the structural strength of the composite hydrogel and provided excellent photothermal conversion performance; 3) the composite effect of silver nanoparticles and graphene enhanced photothermal conversion as well as endowed the material with excellent resistance to biological contamination. The evaporation rate and evaporation efficiency of ArG‐Ag hydrogels were 3.089 kg/m²/h and 122.7%, respectively. This efficiency value exceeded 100%, which was attributed to the significant reduction of water evaporation enthalpy by the hydrogel network. Notably, the ArG‐Ag hydrogel had a 99% antibacterial rate against Escherichia coli for 24 h and had a long‐term mold prevention performance of 210 days.

4. Preparation Strategies for Solar‐Driven Hydrogels

The fabrication methodology plays a fundamental role in determining the microstructure, physicochemical properties, and ultimately the water purification performance of SDHs [132]. Over the past decade, considerable advancements have been achieved in the development of preparation techniques that enable precise regulation of pore structure, distribution of photothermal materials, and topological structure of polymer networks. This section systematically compares and analyzes the main preparation strategies, which are classified into the following categories based on their core functions: 1) Structural template construction for pore engineering; 2) The cross‐linking effect for promoting network formation; and 3) Surface and structural design. For each category, we discuss the underlying mechanisms, their influence on hydrogel properties, and a comprehensive evaluation of their respective advantages and limitations. This structured approach is intended to enhance clarity and practical applicability, thereby facilitating its implementation in future research and development.

4.1. Self‐Assembled Templating for Hierarchical Porosity

Self‐assembled templating (SAT) is commonly used to create three‐dimensional, large‐scale, interconnected porous hydrogels [133]. This method takes advantage of the synergistic effect of sacrificing templating and interface orientation to precisely control the pore structure of the hydrogel. The SAT technology has significant application value in the field of solar‐driven water purification for the following reasons: 1) SAT can spontaneously generate ordered structures and continuous porous networks through weak intermolecular interactions, thereby optimizing pore sizes to improve water transport efficiency [134]; 2) The SAT technology enables the direct incorporation of photothermal materials into hydrogels, thereby forming uniformly dispersed nanocomposites. This structure enhances the efficiency of solar light absorption and effectively blocks the heat conduction pathways; 3) The uniform cross‐linked network bolsters the hydrogels’ resilience, thus achieving a compression recovery rate above 95% and providing self‐healing capabilities to resist brittleness [135]; 4) The dynamic network can be designed with temperature and pH responsiveness to improve the hydrogels’ ability to contract when heated, thus facilitating the removal of contaminants and ion exchange; 5) SAT is an efficient, energy‐saving method (requiring only 6–8 h of room temperature etching) that consumes 60% less energy than freeze‐drying, and it supports wet spinning for mass production over hundreds of meters while minimizing uncontrolled ice crystal growth [136].

SAT exhibits significant potential for multifunctional integration through molecular design. It can concurrently incorporate photothermal conversion elements, hydrophilic networks, and pollutant capture units, thereby creating a tri‐functional system for “water absorption‐evaporation‐pollutant interception” that enhances the purification efficiency of hydrogels. Guo et al. [133] developed an IPH evaporator through SAT. IPH demonstrated a high evaporation rate (2.8 kg/m²/h), an evaporation efficiency of 90%, and impressive mechanical durability. The results were attributed to the controlled pore size of the IPH, which were influenced by the sacrificial particle diameter in the self‐assembled template, thereby optimizing water transport capacity and thermal localization (Figure 7A). In another study, T‐ECH hydrogel was prepared via SAT technology. The hydrogel possessed tissue‐like modulus, high electrical conductivity, and significant mechanical toughness. Moreover, its elongation rate, fracture toughness, and water content were 610%, 1 MJ/m³, and 90 wt.%, respectively. Furthermore, T‐ECH exhibited excellent biocompatibility in the body due to its soft, stretchable, and elastic double‐network structure (Figure 7B) [137].

FIGURE 7.

(A) IPH evaporator. Reprinted with permission [133]. Copyright 2022, Wiley. (a) The fabrication process via SAT; (b) The evaporation rate of IPHs and corresponding energy efficiency; (c) Compression stress–strain curves of a hydrated IPH and a control sample, freeze‐dried PVA/AC hydrogel. (B) T‐ECH with high electrical conductivity and high mechanical toughness of tissue sample modulus [137]. (a) Schematic illustration of the template‐directed in situ synthesis of T‐ECH; (b) Schematic illustration and optical image of highly stretchable T‐ECH (left). (C) B‐3D sponge‐like hydrogel (B‐SPH). Reprinted with permission [138]. Copyright 2023, Wiley. (a) The preparation process of B‐SPH; (b) Water evaporation rate and energy conversion efficiency of SPHs; (c) Stable SVG performance under 7 continuous days in pure water; (d) Constant evaporation rate after 100 times of rolling, folding, and twisting.

There is a risk of contamination when using organic solvents to remove sacrificial templates in the sacrificial template‐assisted technology, which will lead to significant environmental issues. Additionally, researchers also face numerous challenges in precisely controlling the pore size, morphological structure, and biomimetic architecture to optimize the water transport performance of hydrogels. Therefore, it is urgent to develop a more sustainable and efficient cryo‐assembly template polymerization (CTP) method for preparing three‐dimensional sponge hydrogels (SPH) with controllable porous structures and enhanced mechanical properties. This method draws inspiration from the design concept of the micro‐channel structure of coniferous plants. Lei et al. [138] developed a biomimetic SPH (B‐SPH) via CTP. B‐SPH demonstrated an exceptional evaporation (3.45 kg/m²/h), an energy conversion efficiency (95%), and a high capacity for heavy metal ion sequestration. Moreover, the B‐SPH maintained outstanding mechanical flexibility and stable solar vapor generation performance over multiple deformation cycles, further indicating that CTP offers an environmentally friendly strategy for producing hydrogels with customizable porosity and efficient water purification capabilities, thereby circumventing the limitations associated with conventional traditional methods (Figure 7C).

4.2. Directional Freeze‐Casting for Anisotropic Channel Networks

The directional freezing casting technology can control the growth of ice crystals through a unidirectional temperature gradient, thereby enabling the fabrication of structured porous hydrogels. This method can precisely control the orientation of ice crystals, form a clear channel system, and significantly increase the porosity and specific surface area of the hydrogel, thereby optimizing the capillary water transport performance [139]. Moreover, this technology can effectively integrate photothermal materials to enhance solar energy absorption and thermal energy conversion efficiency, and achieve efficient water evaporation. This process has both environmental and energy‐saving advantages and can be used for large‐scale production of hydrogels with enhanced mechanical strength and anti‐fouling properties [140]. Therefore, directional freezing casting provides an innovative solution for developing new hydrogels, which can improve the efficiency of seawater desalination and accelerate the process of wastewater purification.

Shen et al. [141] prepared anisotropic porous cellulose hydrogels (APC) using this technique. APC achieved a high‐water evaporation rate (2.31 kg/m²/h) and efficiency (90.69%). The APC showed remarkable mechanical strength, photothermal conversion ability, salt tolerance, and efficient directional transmission characteristics, which were attributed to the vertically arranged channel structure formed during the freeze casting process. This structure worked in synergy with the hydrophilic property of cellulose to significantly enhance the efficiency of water transmission and the ability of salt ion migration, indicating that APC is a sustainable solution for desalination and purification (Figure 8A). Another study used Poly(N‐isopropyl acrylamide) (PNIPAM) hydrogels and MXene to prepare anisotropic PNIPAM/MXene composite hydrogels (PMD) via directional freezing [142]. Directional freezing casting technology enhanced the electrochemical and mechanical properties of PMD, whose directional channel structure and polymer aggregation degree were superior to those of isotropic hydrogels, thereby achieving a faster light response speed and remove control capability. Moreover, the directional channel structure enabled a volume reduction of up to 70% within 2 s under near‐infrared light irradiation, which demonstrated a remarkable driving strength.

FIGURE 8.

(A) Anisotropic porous cellulose (APC) hydrogels. Reprinted with permission [141]. Copyright 2025, Elsevier. (a) The preparation process of APC hydrogels; (b) The compressive stress–strain curves of cellulose hydrogels in different directions; (c) The salt resistance of the APC6‐24 evaporator in the desalination process; (d) The evaporation performance of the evaporator. (B) TA‐PVA‐CNC hydrogels [143]. (a) The preparation of TA‐PVA‐CNC hydrogel; (b) The typical tensile stress–strain curves of TA‐PVA‐CNC hydrogels. (C) The optimized new chitosan hydrogel. Reprinted with permission [144]. Copyright 2025, Elsevier. (a) The preparation of CPP‐DU hydrogel; (b) Digital photographs of the home‐made outdoor solar evaporation apparatus; (c) The water production rate, cumulative freshwater production, and surface temperature.

Conventional unidirectional freeze‐casting technology has limitations in achieving uniform microstructure and is prone to ice crystal formation defects [145]. Therefore, the pore structure can be optimized, mechanical properties can be enhanced, and the light response efficiency can be improved by introducing complementary technologies, such as the salting‐out method, physical cross‐linking, or ultrasonic assistance. This integrated technical solution significantly expands the application potential of hydrogels in seawater desalination and advanced intelligent devices. Li et al. [146] developed robust, anti‐fouling hydrogel‐based solar evaporators using polyvinyl alcohol, SA, and MXene through directional freeze‐casting combined with salting‐out processes. The evaporator exhibited impressive evaporation rates (2.53 kg/m²/h) and energy efficiency (93%), while also having excellent salt resistance and long‐term stability. The success was attributed to the well‐connected vertical channel structure formed by the directional freezing casting technology, which facilitates rapid water transmission and enables effective salt removal, while the salting‐out process improved the cross‐linking degree and mechanical properties of the polymer, further suggesting innovative strategies for hydrogels in desalination and wastewater treatment. Jin et al. [143] developed a new type of durable polyvinyl alcohol (PVA) hydrogel through directional freezing casting and physical crosslinking technology. The tensile strength of this hydrogel reached 3.7 MPa, and the toughness value was 3.27 MJ·m⁻³ per cubic meter, which were 7.7 times and 9.9 times higher than those of traditional PVA hydrogels, respectively. This performance improvement was attributed to the anisotropic pore wall structure formed by the directional freezing casting process, which could effectively disperse stress and prevent the generation of local cracks. Moreover, there were a large number of breakable and reconfigurable sacrificial hydrogen bonds in the hydrogel, which could effectively dissipate tensile energy and enhance toughness. Therefore, directional freezing casting combined with physical crosslinking technology is an effective method for preparing biomimetic PVA hydrogels with advanced mechanical properties (Figure 8B). Su et al. [144] prepared chitosan hydrogels via a novel ultrasonic‐assisted directional freezing method. The hydrogel showed high evaporation rates (3.85 kg/m²/h), excellent salt tolerance, and improved mechanical strength. The results were attributed to the directional freezing technique, which lowered liquid transport resistance and enhanced the hydrogel's anisotropic mechanical and mass transfer characteristics. Furthermore, ultrasound‐induced crystallization refined the hydrogel's pore structure to improve mechanical properties and increase the water ratio, thus boosting evaporation rates. This indicates that the ultrasound‐assisted directional freezing method is innovative and versatile (Figure 8C).

4.3. Radiation Polymerization for Network Engineering

Radiation polymerization is a highly effective technique for creating three‐dimensional polymer networks [147]. Compared with traditional techniques, this technology has significant advantages such as milder reaction conditions, no generation of by‐products, faster gelation speed, and lower production costs. Hydrogels produced via radiation polymerization demonstrate exceptional mechanical strength, thermal and electrical insulation, chemical resistance, biocompatibility, biodegradability, antibacterial properties, environmental safety, and resilience to environmental stress cracking [148]. The properties and swelling behavior of these hydrogels can be precisely controlled by adjusting the irradiation time [147]. Thus, radiation polymerization shows great promise for developing hydrogels for solar‐driven water purification [149].

For instance, Lin et al. [131] prepared GO/Ag composite hydrogels (ArG‐Ag) through a one‐step radiation reduction method. This method enabled the simultaneous cross‐linking polymerization of PAM and the reduction of rGO and Ag via high‐energy gamma rays. Gamma rays, with their strong penetrating ability, could generate hydrogen atoms, hydrogen peroxide, hydrated electrons, hydroxyl radicals, and hydrogen ions in water. Notably, these electrons and hydrogen atoms had significant reducing capabilities and could effectively reduce graphene oxide and metal ions. Additionally, the ArG‐Ag hydrogel possessed deformability and patterning properties and demonstrated excellent mechanical strength and stability in long‐term applications (Figure 9A). In another study, HEA/AMPS copolymer hydrogel was prepared through radiation‐induced copolymerization. HEA/AMPS copolymer hydrogel showed a high capacity for adsorbing toxic heavy metal ions (Pb2⁺, Cd²⁺, Cu²⁺, and Fe³⁺), thereby effectively purifying contaminated wastewater (Figure 9B) [150]. Similarly, Du et al. [151] developed a polymer quaternary ammonium salt hydrogel (PQH) via radiation cross‐linking for dye wastewater treatment. The maximum adsorption capacities of PQH for Methyl orange, Congo red, and Coomassie brilliant blue were 617.28, 427.35, and 628.93 mg/g, respectively, and also demonstrated excellent regenerative ability, fully demonstrating the advantages of radiation technology in efficient treatment and rapid gel formation. PQH also exhibited excellent regeneration capabilities, thus highlighting the efficiency and rapid gel formation advantages of radiation technology (Figure 9C) [152]. Sutradhar et al. [153] prepared a CMC/AAc hydrogel using radiation‐induced grafting technology to graft acrylic acid (AAc) onto carboxymethyl cellulose (CMC). The maximum methylene blue adsorption capacity of CMC/AAc hydrogels reached 681 mg/g, and the adsorption behavior conformed to the Langmuir isothermal model. The grafting of AAc significantly enhanced the adsorption performance and selectivity of the hydrogels (Figure 9D).

FIGURE 9.

(A) GO/Ag composite hydrogel. (a) Evaporation rate of ArG Ag hydrogel in oil‐water mixture; (b) Evaporation rate of ArG Ag hydrogel in surface active water (Span‐80); (c) Changes in ion concentration; (d) Radiation preparation process of ArG and ArG Ag. Reprinted with permission [131]. Copyright 2023, Elsevier. (B) Effect of HEA/AMPS copolymer hydrogel composition on Pb²⁺ (a), Cd²⁺ (b), Cu²⁺ (c), and Fe³⁺ (d) adsorption. Reprinted with permission [150]. Copyright 2013, Springer Nature. (C) PQH hydrogel. Reprinted with permission [152]. Copyright 2021, Elsevier. (a) Synthesis route of PQH and its dyes adsorption; (b) Langmuir isotherm model; (c) Adsorption capacity and removal efficiency of MO on PQH after five times regeneration. (D) CMC/AAc hydrogel state before adsorption of MB dye (a) and after adsorption of MB dye (b); (c) Possible mechanism of adsorption of MB dye on CMC/AAc hydrogel [153].

In a similar approach, a polyacrylamide/natural rubber (PAAM/NRL) hydrogel was successfully prepared by applying ionizing radiation to modify natural rubbe. PAAM/NRL hydrogel exhibited excellent adsorption performance for heavy metal ions (Cu²⁺, Co²⁺, and Ni²⁺), which further increased with the rise in temperature [154]. The PAAM/NRL hydrogel still maintained stable adsorption kinetics characteristics and adsorption capacity even after six consecutive adsorption–desorption cycles, indicating PAAM/NRL hydrogel's broad application prospects in environmental remediation and sustainable development fields.

4.4. Cross‐Linking Strategies for Network Stabilization

Cross‐linking, the formation of bridges between polymer chains to form three‐dimensional networks, constitutes a fundamental process in hydrogel fabrication. Cross‐linking strategies are conventionally categorized into physical and chemical approaches, each imparting distinct advantages for applications in solar‐driven water purification [155]. The selection of cross‐linking methodology critically governs key hydrogel characteristics, including mechanical properties, swelling behavior, network stability, and long‐term performance in aqueous environments.

Physical cross‐linking employs non‐covalent interactions, such as hydrogen bonding, hydrophobic association, ionic interactions, and chain entanglements, to form reversible polymer networks [156]. Although these interactions are generally weaker than covalent bonds, they offer benefits including reversibility, self‐healing capacity, and the absence of residual chemical cross‐linkers. Physically cross‐linked hydrogels can exhibit dynamic responsiveness to environmental stimuli, thus enabling adaptive behaviors within complex aqueous matrices. Zou et al. [157] developed a polyvinyl alcohol/modified needle‐like carbon (PVA/MNC) mixed hydrogel (PMH) evaporator. PMH evaporator achieved an evaporation rate of 3.18 kg/m²/h in a 3.5 wt.% NaCl solution, alongside robust mechanical properties and durability. This performance was attributed to the retention of free hydrophilic groups, which enhanced water retention capacity and reduced the enthalpy of water evaporation (Figure 10A). Similarly, Park et al. [158] prepared a polypyrrole/alginate (PPy/Alg) composite hydrogel via physical cross‐linking. PPy/Alg composite hydrogel attained an evaporation rate of 1.15 kg/m²/h and exhibited notable resistance to salt crystallization, which was attributed to repulsive interactions from multivalent cations. This hydrogel presents a promising platform for sustainable solar‐driven seawater desalination.

FIGURE 10.

(A) Preparation of hydrogels by the physical cross‐linking method. Reprinted with permission [157]. Copyright 2023, Wiley. (a) Schematic illustration of the fabrication of PMH; (b) Evaporation rates and energy efficiency of PMH2 in NaCl solution; (c) Schematic illustration of solar‐driven interfacial desalination using PMH evaporator. (B) Preparation of hydrogels by the chemical cross‐linking method. The synthetic route of the dual‐layer self‐healing PSP@PC hydrogel evaporator for solar‐driven interfacial desalination. Reprinted with permission [159]. Copyright 2025, Elsevier.

Chemical cross‐linking involves the establishment of permanent covalent bonds between polymer chains through reactions such as Schiff base formation, Michael addition, click chemistry, or radical polymerization [160]. The network structure formed by chemical cross‐linking possesses excellent mechanical strength, structural stability, and corrosion resistance, which enables it to resist dissolution under harsh conditions. Therefore, it is particularly suitable for long‐term water purification applications that require mechanical strength. Tang et al. [161] prepared a Cu‐Fe₃O₄/SA/PAM hydrogel‐based solar evaporator via a dual chemical cross‐linking strategy. The evaporator achieved high evaporation (64.7%) and thermal (95.24%) efficiencies, alongside excellent repeatability and self‐cleaning properties due to the stable network architecture and interconnected micropores. In another study, Wang et al. [159] developed a bilayer Janus polyelectrolyte hydrogel evaporator (PSP@PC) through dynamic covalent cross‐linking with borax. PSP@PC achieved an evaporation rate of 2.43 kg/m²/h and a desalination rate of 90.1% due to self‐healing dynamic bonds, an optimized pore structure enabled by precise chemical cross‐linking, and the formation of an “ion barrier” that inhibits surface salt crystallization (Figure 10B).

Dual‐network and hybrid cross‐linking strategies integrate physical and chemical methods to mitigate the limitations inherent to each individual approach [162]. The first network, typically chemically cross‐linked, imparts mechanical strength and structural integrity. Meanwhile, the second network, often physically cross‐linked, facilitates energy dissipation through reversible bond breaking and confers self‐healing capability. This hierarchical design results in significantly superior mechanical properties compared to single‐network hydrogels, while maintaining processability and functionality. Li et al. [135] prepared a composite hydrogel with a dual reinforcing network structure using polyvinyl alcohol, sodium alginate, and polyacrylate sodium (PAAS). The combination of dynamic covalent bonds formed by chemical cross‐linking and non‐covalent interactions generated by physical cross‐linking enabled this hydrogel to exhibit excellent mechanical strength (a tensile strength of up to 1.2 megapascals) and rapid self‐repairing properties. Besides, the hierarchical network structure maintained high hydrophilicity and formed a porous architecture, which was crucial for achieving efficient water transport in solar‐driven evaporation applications.

4.5. Surface Patterning for Interface Engineering

Surface patterning comprises a suite of techniques designed to fabricate specific micro‐ and nano‐scale topographic features on hydrogel surfaces, thereby engineering interfacial properties to enhance photothermal conversion, mass transport, and fouling resistance [163]. This methodology is grounded in the recognition that the evaporation interface, where solar absorption, heat localization, and vapor generation take place, is functionally distinct from the bulk hydrogel and can be independently optimized through rational surface design.

The functional benefits of surface patterning in solar‐driven hydrogels (SDHs) are multifaceted: 1) The micro‐nano structures significantly increase the specific surface area for evaporation and modulate light reflection, thereby boosting solar energy conversion and enhancing light absorption efficiency [164]; 2) Localized thermal concentration at the air–water interface intensifies capillary action and optimizes heat transfer to water molecules, leading to markedly improved evaporation rates [165]; 3) Directed salt crystallization in non‐evaporative regions prevents clogging of active evaporation sites and water transport channels, thereby enhancing anti‐fouling performance [166]; 4) Mechanical stress is more evenly distributed within the hydrogel matrix, alleviating localized deformation and improving long‐term structural durability; 5) The integration of diverse microstructures into patterned architectures enables multifunctionality, including anti‐fouling, antibacterial properties, and catalytic degradation of pollutants.

Growing research efforts are directed toward employing surface patterning strategies for developing advanced hydrogels for Solar‐driven water purification. For instance, Li et al. [167] employed directional freezing and surface patterning to fabricate an anisotropic polyvinyl alcohol/carbon nanotube (PVA/CNT) hydrogel (PCH), which achieved a high evaporation rate of 3.69 kg/m²/h and an energy conversion efficiency of 90.9%. The surface‐patterned PCH maintained stable operation for over 50 h and attained a salt collection rate of 2.38 kg m⁻² per 10 h under high‐salinity conditions (10 wt.%). This performance was attributed to the surface patterning, which enhanced gas‐liquid interfacial dynamics and energy efficiency, facilitating upward crystallization of salt from concentrated brine. This innovative strategy addresses challenges related to water scarcity and hypersaline wastewater management (Figure 11A). In another study, 3D printing combined with surface patterning was utilized to fabricate PVA/polypyrrole (PPy) hydrogel membranes featuring biomimetic tree‐like microstructures [168]. These membranes exhibited an evaporation rate of 3.64 kg/m²/h and a fog harvesting rate of 5.0 g cm⁻²h⁻¹. The PVA/PPy membranes also demonstrated robust mechanical properties, enabling continuous freshwater production in arid environments. The 3D microtopography enhanced broadband light scattering and absorption (250–2500 nm), optimized vapor dissipation and water transport, and reduced evaporation enthalpy. This approach overcomes limitations of conventional solar evaporators, such as operational discontinuity and the absence of photothermal functionality in fog collectors, through the synergistic use of 3D printing and surface patterning (Figure 11B).

FIGURE 11.

(A) The surface pattern technology preparation of hydrogels. (a) Schematic illustration of the preparation process of surface‐patterned PVA/CNT composite hydrogels (PCH); (b) Water evaporation rates of differently patterned PCHs; (c) Salt collection of PCH₃‐9u. Reprinted with permission [167]. Copyright 2024, Elsevier. (B) PVA/PPy hydrogel. (a) Schematic illustration of the fabrication of microstructured PVA/PPy gel membranes; (b) Evaporation rate and energy efficiency for different tested micro‐topologies; (c) Fog collection rates measured for different gel membrane geometries [168]. (C) rGO/SA hydrogels. (a) Schematic of the gelation, reduction, cross‐linking and laser printing for the fabrication of GAH; (b) UV–vis‐near‐infrared spectra of GAH (red curve), and rGO/alginate hydrogel without fish‐scale‐like pattern (black curve); (c) Calculated ion rejection rate of GAH‐3; (d) Regrowth test result of diluted raw lake water, control group, and GAH‐treated group [169].

Bio‐inspired surface designs have shown exceptional promise. Hao et al. [169] developed reduced graphene oxide/sodium alginate (rGO/SA) hydrogels (GAH) with densely cross‐linked internal networks and fish‐scale‐inspired surface patterns. The GAH system demonstrated superior selective water transport, salt rejection, and mechanical strength, effectively excluding volatile organic compounds (VOCs) (>99.5%), ions (>99.3%), and non‐volatile organics/bacteria (100%). Surface‐exposed rGO nanosheets conferred over 99% bacterial inactivation via dual antibacterial mechanisms. The biomimetic micro‐nano structures enabled approximately 95% broadband solar absorption, markedly improving photothermal conversion efficiency. This surface engineering approach presents a viable biomimetic strategy for designing solar water‐treatment materials that mitigate contamination issues common in traditional porous media (Figure 11C).

In addition to facilitating evaporation, surface patterning also enhances water retention capacity, which is critical for maintaining sustained operational performance. Chhetry et al. [170] investigated the influence of surface patterning on polyacrylamide (PAAm) hydrogels. Their findings demonstrated that surface patterning significantly enhanced the water retention capacity and extended the retention duration, which was attributed to the periodic grooves that reduced the local humidity available for evaporation, thereby prolonging the dehydration time by 60%. Moreover, the pore structure of the patterned PAAm hydrogels underwent a more gradual transformation, preserving a greater proportion of small pores and delaying the formation of pathways for water loss. Consequently, both water retention and swelling capacity were improved.

4.6. Biomimetic Structural Design for Multi‐Functional Integration

Biomimetic structural design derives inspiration from nature's optimized strategies for light harvesting, fluid transport, and fouling resistance, translating biological principles into engineered hydrogel architectures [171]. This approach has garnered increasing attention in the context of solar‐driven water purification, owing to the fact that biological systems have evolved over millions of years to achieve precisely the functionalities required for efficient evaporation, including solar absorption, water transport, thermal management, and self‐cleaning, across diverse environmental conditions.

The fundamental advantages of biomimetic design encompass the following aspects: 1) This methodology effectively emulates and optimizes natural mechanisms of light‐water‐energy transfer, thereby enhancing solar utilization efficiency and water evaporation performance [172]; 2) Biomimetically designed hydrogels exhibit superior capillary action and aqueous transport capabilities, thereby mitigating evaporation resistance and sustaining consistent evaporation rates [173]; 3) The inherent anti‐fouling characteristics of such structures address persistent challenges, such as salt crystallization and contaminant accumulation, in conventional evaporation systems, significantly improving the hydrogel's efficacy in treating high‐salinity and complex wastewater while enhancing operational stability [174]; 4) An optimized biomimetic design integrates functionalities such as solar capture, efficient thermal conversion, hydraulic pathways, vapor release channels, and condensation/collection mechanisms within a monolithic hydrogel structure, thereby simplifying system design and reducing manufacturing costs; 5) Hydrogels featuring biomimetic architectures demonstrate remarkable mechanical robustness, enduring repeated folding and twisting cycles without structural failure [175].

Plant‐inspired designs have demonstrated considerable potential in advancing solar‐driven evaporation technologies. In natural systems, trees transport water from roots to leaves through xylem vessels, followed by evaporation into the atmosphere, which has inspired efficient bio‐inspired solar evaporation systems [176]. Drawing on this mechanism, Zhang et al. [177] developed a cost‐effective, three‐dimensional printed bio‐inspired hydrogel evaporator (3DP‐BHE), which achieved an evaporation rate of 2.13 kg/m²/h and an evaporation efficiency of 90.5%. This study underscores the advantages of bio‐inspired hydrogel architectures for solar desalination applications (Figure 12A).

FIGURE 12.

Preparation of hydrogels by the biomimetic structure method. (A) Schematic of the fabrication and salt resistance mechanism of the 3DP‐BHE. Reprinted with permission [177]. Copyright 2024, Wiley. (B) The structure and principle of a hydrogel solar evaporator that simulates the structure of fish gills. Reprinted with permission [178]. Copyright 2025, ACS. (C) B‐SPH hydrogels. (a) Cross‐section and (b) longitudinal section of conifer trees with radially and vertically aligned channels; (c) Schematic diagram of the synthesis of bionic assembled SPHs (B‐SPH). Reprinted with permission [173]. Copyright 2023, Wiley. (D) PLBH hydrogel evaporator. (a) The picture of the as‐prepared PLBH; (b) The dye degradation rate of TB and CR and the comparison of solution colors before and after purification of TB and C. Reprinted with permission [179]. Copyright 2023, Elsevier.

The transportation of water from roots to leaves via xylem vessels and its subsequent evaporation represent an efficient natural process for continuous water supply and phase change [179]. Furthermore, microstructures inherent in plant tissues enhance light absorption and overall efficiency. Inspired by these features, Nie et al. [180] designed a biomimetic hydrogel evaporator (LCP) featuring a “coconut shell raincoat” structure. The LCP attained an exceptional evaporation rate of 3.88 kg/m²/h and a photothermal conversion efficiency of 97.16%. It also exhibited superior water permeability, salt resistance, oil repellency, and antibacterial performance, inactivating over 99.98% of Escherichia coli and Staphylococcus aureus within 1 h, indicating strong potential for safe water production in arid environments.

Another biomimetic approach derives from fish gill structures, which are evolutionarily optimized for efficient gas exchange in aquatic settings. Chen et al. [178] fabricated a hydrogel‐based solar evaporator (MHH) using ultra‐long hydroxyapatite nanowires, MXene, and hydrophilic hyaluronic acid methacrylate to replicate the architecture of fish gills. This hierarchically ordered, multiscale structure achieved a notable evaporation rate of 6.278 kg/m²/h, illustrating the efficacy of combining structural hierarchy with advanced functional materials (Figure 12B).

Coniferous plants, characterized by radial microchannels and vertical conduits, exhibit remarkable water transport capabilities under extreme conditions [181, 182]. Taking inspiration from this, Lei et al. [173] developed biomimetically assembled sponge‐like hydrogels (B‐SPH), which significantly surpassed conventional hydrogels in water transport capacity. These structures also demonstrated high evaporation rates and energy conversion efficiencies, highlighting their promise for application in water purification processes (Figure 12C).

Mushroom‐inspired configurations provide yet another avenue for multifunctional integration. Zhang et al. [179] engineered a mushroom‐shaped hydrogel evaporator (PLBH) using lotus root starch and polyvinyl alcohol. In this design, a black cap functions as an efficient photothermal layer, while a fibrous stalk optimizes water transport. This system attained an evaporation rate of 3.78 kg/m²/h and achieved a catalytic degradation rate of 91.8% for TB (Thymol Blue) and 88.9% for CR (Congo Red), offering an innovative strategy for integrated solar‐driven water purification (Figure 12D).

5. Performance of Solar‐Driven Water Purification Hydrogels

The performance of SDHs hydrogels is a key factor to determine their efficacy and practicality in water purification applications [183]. Comprehensive research and continuous optimization of SDHs are crucial for facilitating the transition of this technology from laboratory research to practical application. This section aims to summarize the core performance indicators of SDHs, including photothermal conversion efficiency, evaporation efficiency, mechanical properties, and salt tolerance, and to explore the underlying mechanisms that affect these characteristics, while proposing optimization strategies.

5.1. Photothermal Conversion Performance

The photothermal conversion performance is a core attribute of hydrogels in the field of efficient solar water purification. It directly determines the steam generation rate, energy utilization efficiency, and final water purification efficiency of hydrogels. Therefore, the excellent photothermal conversion performance of hydrogels is the foundation for achieving sustainable solar‐driven water purification. The photothermal conversion process can be described as follows: Photons transfer their energy and momentum to the molecules of the substance, and induce electronic transitions within the material when optical radiation interacts with a material surface. As the excited electrons return to their ground state, their energy is dissipated in the form of thermal energy [184, 185]. Additionally, the photothermal conversion efficiency (η) of the hydrogel evaporator can be determined by the following formula:

$$\eta =\frac{m{(\mathrm{h}}_v+Q_{1)}}{P_0}$$

$$Q_1=C(T-T_1)$$

In the formula, m represents the difference between the energy absorption rate of the hydrogel under sunlight exposure and that in a dark environment; h_v represents the enthalpy change of water between the gas phase and the liquid phase; Q ₁ represents the sensible heat per unit mass of water; P ₀ represents the power of incident light; C refers to the specific heat capacity of water (≈ 4.2 J·g^−1°C‐1); T is the temperature at which water evaporates, and T ₁ is the initial temperature of water [12].

It is reported that solar light source was an ideal light source for photothermal conversion hydrogels due to its low cost, renewability, pollution‐free nature, high safety, and ease of collection [186]. However, solar light sources face limitations during overcast conditions and at night. Therefore, the key to enhancing the photothermal conversion efficiency of hydrogels lies in the efficient capture of light energy by photothermal catalysts and the optimization and regulation of energy transfer and utilization by the hydrogel matrix [187]. These two elements can collaboratively achieve high‐efficiency photothermal conversion performance in hydrogels through molecular design, structural engineering, and functional synergy (Table 1).

TABLE 1.

Advantages and limitations of different types of Substrate materials and photothermal catalysts [138].

Materials	Type	Typical species	Advantages	Limitations
Substrate materials	• Natural polymer‐based hydrogel	Alginate	High hydrophilicity Rapid water absorption Forms gel under mild conditions Excellent for encapsulating photothermal materials	Poor long‐term stability in saline water Weak mechanical strength
		Chitosan	Natural antibacterial property Good biocompatibility Resists algae/bacteria in water channels	Soluble only in acidic media; Brittle when dry Complex processing
		Cellulose	Excellent mechanical rigidity as a skeleton Low tortuosity for water transport Abundant and low‐cost	Difficult to dissolve directly Usually requires chemical modification
		Lignin	Low‐cost industrial byproduct UV‐resistant and antioxidant Protects photothermal materials	Complex and heterogeneous structure Poor transparency
		Starch	Extremely low cost Suitable for short‐term applications	Very weak gelation Prone to degradation under high Temperature/salinity
	• synthetic polyelectrolyte hydrogel	PAM	Rich in amide groups that reduce water evaporation enthalpy Excellent for achieving high evaporation rates Good compatibility with photothermal fillers	Single‐network PAM is mechanically weak Moderate salt resistance without modification.
		PVA	Chemically stable Salt‐resistant Shape‐memory property Non‐toxic Easy film‐forming with photothermal materials	Poor elasticity recovery alone Lacks active sites for adsorption
		PAMPS	Strong anti‐fouling due to sulfonic acid groups (‐SO3 −) Maintains high evaporation rate in high‐salinity brine Acts as rigid backbone in double‐network (DN) hydrogels	Homopolymer PAMPS is brittle Synthesis requires precise control of crosslinking
Photothermal catalysts	Semiconduct photothermal	CuS TiO2 MoS2 Ti4O7	Bandgap adjustable (precise absorption of near‐infrared light) Multifunctional integration of photothermal/photodynamic/catalysis	Existence of biological toxicity Weak interface bonding
	Carbon‐based photothermal	CNT Graphene CB Biochar MXene	Ultra‐wide spectrum absorption Low cost Extremely stable Controllable biocompatibility	Shallow photothermal penetration Poor dispersibility, prone to agglomeration
	Polymer category	PPy PDA Polyaniline (PANI) Polystyrene sulfone (PSS)	Good flexibility and easy processing High biocompatibility Adjustable conductivity	Narrow absorption spectrum Poor thermal stability Low light absorption efficiency
	Metal/Metal oxide	Metal nanoparticles (Au, Ag, Cu) Metal oxide nanoparticles	Ultra‐high light absorption efficiency Easy to precisely control size/form and response wavelength Easy to functionalize the surface	Extremely high cost Easy to aggregate and deactivate in biological environment Severe scattering in deep tissues

The mechanism of action of photothermal catalysts mainly includes the following aspects: 1) Semiconductor‐based photothermal catalysts: Light‐induced bandgap charge excitation and electron‐phonon interactions [188]. Electrons in semiconductor materials are excited from the valence band (VB) to the conduction band (CB) when exposed to light of a specific wavelength, thereby forming electron‐hole pairs. Subsequently, the energy is released in the form of thermal energy through non‐radiative relaxation processes (Figure 13A); 2) Carbon‐based materials: These materials have a wide absorption range (200–2500 nm) due to their conjugated π bonds [189]. When illuminated, their π electron systems become excited, generating high‐energy electron‐hole pairs that release energy as heat via non‐radiative relaxation (Figure 13B); 3) Metal nanoparticles: These materials have a high polarization density and exhibit significant interaction with electromagnetic radiation [190]. A surface resonance phenomenon (localized surface plasmon resonance, LSPR) will be triggered when the incident light frequency matches the inherent vibration frequency of surface free electrons. This effectively enhances the local electromagnetic field energy, which then converts into heat energy through non‐radiative decay processes (Figure 13C); 4) Polymers: Photons can excite the π electrons in the conjugated structure, and they cause the electrons to transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) when the polymer surface is illuminated. Subsequently, the excited electrons transfer their energy through non‐radiative relaxation processes to the lattice, thereby triggering phonon vibrations, and ultimately converting into heat energy (Figure 13D) [12].

FIGURE 13.

The mechanism of action of photothermal catalysts. (A) Semi‐conduct photothermal. (B) Metal/Metal oxide. (C) Carbon‐based photothermal. (D) Polymer category.

The optimization and regulation of photothermal catalysts by hydrogel matrices mainly manifest in the following aspects: 1) The cross‐linked network structure of the hydrogel can effectively prevent the aggregation of photothermal materials, increase their specific surface area, and fully expose the active sites. Moreover, the physical barrier formed by the gel network in the aqueous environment significantly reduces the sedimentation, oxidation, and decomposition of photothermal materials; 2) The transparent or semi‐transparent matrix structure of hydrogels facilitates the penetration of light and internal scattering, thereby enhancing the light absorption efficiency of photothermal materials [191]; 3) The hydrogel contains abundant water molecules. The surrounding water molecules can quickly absorb heat due to the high specific heat capacity of water molecules, thereby preventing the local temperature from rising too rapidly. Hydrogel can efficiently transfer the heat to the entire gel network and the water environment when the photothermal material absorbs light energy and generates local high temperature; 4) The inherent low thermal conductivity of the gel network can effectively reduce the ineffective dissipation of heat to the external environment, thereby improving the thermal utilization efficiency of the hydrogel; 5) The components (polymer chains, water molecules and ions) in the hydrogel network can interact with the surface of the photothermal material through adsorption, hydrogen bonding and electrostatic interactions. This method can effectively control the plasmonic properties and electronic structure of the photothermal material, thereby expanding its absorption spectral range and improving the absorption efficiency [192].

5.2. High Evaporation Rate

High‐efficiency evaporation performance is a core requirement for achieving high output, efficient separation, and stable operation in hydrogel‐based water purification technologies. To realize such performance, researchers often incorporate specific structural components and functional materials during the hydrogel fabrication process. These strategies can enhance the evaporation rate and ensure long‐term operational stability and durability. Moreover, efficient evaporation helps reduce energy consumption and lower the overall cost of the water purification process, thereby facilitating large‐scale green applications in this field [193].

Previous studies have revealed that the polarity of water molecules primarily originated from the asymmetric charge distribution between oxygen and hydrogen atoms. This inherent asymmetry promoted the formation of hydrogen bonds, which enhanced the attractive forces between water molecules. Water molecules need to overcome these intermolecular forces resulting from hydrogen bonding, and undergo a phase transition from liquid to gas during the passive interface solar‐driven steam generation process. Therefore, the energy required to disrupt these interactions can be effectively controlled by regulating the number and strength of hydrogen bonds between water molecules, thereby adjusting the evaporation enthalpy [194]. Moreover, a higher evaporation enthalpy significantly limits the evaporation rate of water gels. Hence, reducing the evaporation enthalpy is crucial for increasing the evaporation rate. Essentially, the reduction of evaporation enthalpy is achieved by weakening or breaking hydrogen bonds, which lowers the energy barrier for water evaporation and accelerates the phase transition process. The state of water can be categorized into bound water, intermediate water, and free water in hydrogel systems. Intermediate water possesses a relatively weak hydrogen‐bond network, which requires the least energy for dissociation and contributes most significantly to the evaporation rate [12].

To enhance evaporation efficiency, strategies typically involve: 1) using high‐performance photothermal materials; 2) optimizing water transport pathways; 3) adding hydrophilic functional groups; 4) creating porous structures; 5) designing gradient or bilayer architectures; 6) adjusting the cross‐linking density and pore size of hydrogels.

Zheng et al. [195] prepared oxygen‐doped microporous carbon‐based hydrogels (OPCH) with high evaporation rates. The mechanism of the high evaporation performance lies in the abundant hydrophilic groups (C─O, C═O, and O─H) on the surface of the OPCH hydrogel, which could reconfigure the hydrogen bond network of water molecules through hydrogen bonding and convert 42.3% of the water into an intermediate state with weaker hydrogen bonding. This process significantly reduced the evaporation enthalpy from 2216.06 to 1107.88 J/g. Additionally, the microporous structure of the OPCH hydrogel (the specific surface area of 567.2 m²/g) generated a limiting effect that further shortened the average lifetime of hydrogen bonds between water molecules and enhanced their kinetic energy. At the same time, the vertically arranged porous channel structure of the OPCH hydrogel (with pore diameters of 1–3 µm) improved its light absorption rate (95.4%), reduced its thermal conductivity, and formed a 7.1°C interface temperature gradient, thereby jointly achieving a high evaporation rate of 2.44 kg/m²/h (Figure 14A). Similarly, Zheng et al. [196] prepared a dual‐network composite hydrogel by utilizing a polyol‐borax system as a co‐existing network. The results show that the hydrogel displayed a high evaporation rate (3.02 kg/m²/h) and an energy efficiency (119.35%), which was attributed to the ability of the hydrogel to regulate the ratio of intermediate water to free water (intermediate water/free water = 1.44) through dynamic borate ester bonds and hydrogen bonding, thereby reducing the evaporation enthalpy. Additionally, the incorporation of carbon nanotubes enabled the construction of a 3D cross‐linked fibrous network and formed a homogeneous and interconnected porous structure, which helped to possess a high photothermal conversion efficiency (91.46%).

FIGURE 14.

High‐efficiency evaporation performance. (A) Schematic diagram of the evaporation mechanism of microporous carbon‐based hydrogel. Reprinted with permission [195]. Copyright 2024, Elsevier. (B) Diagram of the gradient hydrogel. Reprinted with permission [197]. Copyright 2025, Wiley. (C) Different structures of evaporators and the evaporation mechanism at the solar interface. Reprinted with permission [198]. Copyright 2025, Elsevier. (D) Scheme for the manufacturing process and cross‐linking mechanism of PMS. Reprinted with permission [199]. Copyright 2025, Wiley.

In a similar approach, a polyelectrolyte gradient hydrogel was prepared through an electric field‐driven grafting technique [197]. This hydrogel, utilizing quaternized poly(4‐vinylpyridine) (QP4VP) as the polyelectrolyte, established a concentration gradient that was higher at the surface and decreased inwardly due to the electric field. This gradient enabled water molecules to be rapidly and directionally transported to the evaporation interface at a rate of 0.091 g/min. Additionally, the strong ion‐dipole interactions generated by QP4VP disrupted the hydrogen bond network of water molecules and reduced the evaporation enthalpy to 775 kJ/kg, which allowed the surface water molecules to be more capable of overcoming the energy barrier. At the same time, the gradient structure of the hydrogel generated high capillary pressure and enhanced its water transport capacity due to its tiny surface pore size (69 nanometers), thus achieving a high evaporation rate of 4.5 kg/m²/h and a significant freshwater production rate of 14 kg/m²/d (Figure 14B). Zhao et al. [198] prepared a double‐layered arched hydrogel (DF‐TPH5) via freeze‐drying using sodium alginate (SA) and tannic acid (TA) as the matrix. The DF‐TPH5 evaporator demonstrated a high evaporation rate (2.70 kg/m²/h), which was contributed to the following factors: 1) SA and TA provided abundant hydrophilic groups (‐OH and C═O), which formed weak hydrogen bonds with water molecules, thereby significantly reducing the evaporation enthalpy; 2) The bilayer structure enhanced radial water transport capacity within the hydrogel and improved hydraulic convection and water retention; 3) The arched design could guide the water body to rise vertically along the curved surface, form a continuous water film, and optimize the water replenishment path for evaporation interface. The development of the DF‐TPH5 hydrogel offers a new perspective for addressing challenges in the evaporation performance of hydrogel‐based systems (Figure 14C).

Wen et al. [199] prepared hydrogels (PMS‐4) through a bidirectional freeze‐casting method. PMS‐4 hydrogels exhibited an excellent evaporation rate (4.62 kg/m²/h), which was attributed to the fact that PMS‐4 had external radial channels (inhibiting longitudinal heat conduction and reducing heat loss) and internal vertical channels (providing a short path for rapid water transportation). Furthermore, the incorporated polyacrylonitrile nanofibers (aPAN) within the PMS‐4 hydrogel formed a physico‐chemical cross‐linked network with sodium alginate (SA), thereby significantly enhancing the mechanical strength and long‐term stability of the hydrogel. Simultaneously, the hydrophilic groups of SA contributed to the partial breaking of hydrogen bonds and reconstructed water molecules into an intermediate water (IW) state, thereby reducing the evaporation enthalpy by 24.8% compared to pure water. Moreover, the photothermal agent (MXene) of PMS‐4 hydrogel could be combined with the cold evaporation interface design of the composite structure to effectively capture environmental thermal energy and achieve an exceptional energy efficiency (149.57%). These results suggest that the PMS‐4 hydrogel surpasses conventional evaporation limits through synergistic structural, compositional, and energetic optimization (Figure 14D). In another study, Ge et al. [200] prepared a novel 3D hydrogel evaporator based on PVA sponge, zwitterionic polyelectrolyte hydrogel, and multi‐walled carbon nanotubes. The high evaporation performance of the hydrogel was attributed to its unique molecular structure and multi‐scale synergy mechanism, as follows: 1) The zwitterionic polyelectrolyte network maintained an extended polymer chain conformation in saline environments through the antipolyelectrolyte effect, and the 3D hierarchical porous structure facilitated rapid water transport; 2) Hydrophilic groups (─OH, ─SO₃ ⁻, and ─NH₃) reconstructed water molecules into an intermediate state through hydrogen bonding, thereby reducing the evaporation enthalpy to 1741 J/g (28.9% lower than the theoretical value); 3) Charged groups on the polymer chains released bound water molecules by disrupting the hydration shells of dissolved ions, as well as established a dynamic Donnan equilibrium and an ion‐sieving effect that inhibited salt crystallization. Ultimately, the hydrogel achieved a high salt concentration (10 wt.%) evaporation rate of 3.35 kg/m²/h, further suggesting that these mechanisms work together to break the vicious cycle of salt accumulation in traditional evaporation systems.

5.3. Mechanical Properties and Durability

The mechanical properties of hydrogels, such as elastic modulus, strength, toughness, fracture energy, and fatigue threshold, are fundamentally linked to the ability of their polymer networks to dissipate external forces. These properties are crucial for the structural integrity and longevity of hydrogels used in seawater desalination and wastewater treatment. High mechanical strength prevents hydrogels from collapsing and deforming due to water loss or external pressures during evaporation, which ensures effective water transport and photothermal conversion. Additionally, robust mechanical strength significantly improves the hydrogels’ resistance to fatigue and wear over time [201]. Therefore, the preparation of hydrogels with high mechanical strength holds great application in the field of solar‐driven water purification.

1. Nanofillers can greatly improve the mechanical properties of hydrogels through interfacial interactions. Nanofillers exhibit strong interfacial interactions with the hydrogel network and enable the transfer of external forces from the flexible matrix to the rigid fillers, thereby directly enhancing the strength and modulus of the hydrogel. Moreover, energy dissipation mechanisms, such as reversible breaking, reformation, and controlled sliding of dynamic bonds at the interface, significantly improve the toughness of the hydrogel. Furthermore, nanofillers serve as additional crosslinking points, which increase the network density of the hydrogel, inhibit crack propagation, and enhance the synergistic effects among strength, stiffness, toughness, and fatigue resistance [202]. For instance, Ma et al. [203] developed hydrogels (SPJH) by introducing nanomaterials and utilizing the strong interfacial interactions of hydrogen bonds and hydrophobic bonds of the nanomaterials. This effective nano‐confinement strategy facilitated the formation of a hierarchical microstructure templated by bubbles and ice crystals, thereby enabling SPJH to display exceptional toughness (231 kJ m⁻²) and extreme strain (310%) (Figure 15A). Similarly, Duan et al. [204] prepared a highly tough, fatigue‐resistant, and conductive hydrogel (PAM‐C‐BCN) by incorporating bamboo cellulose‐derived carbon nanomaterials (C‐BCN) into a polyacrylamide (PAM) precursor solution. The PAM‐C‐BCN composite hydrogel exhibited significantly enhanced mechanical properties, including a fracture strength of 363 kPa, a fracture elongation of 2254%, a fracture energy of 30 kJ/m², and a toughness of 3.0 MJ/m³. These improvements were attributed to the abundant active sites on the C‐BCN surface, which promoted the formation of non‐covalent interactions with the PAM matrix during in situ polymerization. Besides, the strong interfacial bonding facilitated the construction of a 3D network structure. Moreover, the synergistic effect between the rigid C‐BCN skeleton and the flexible polymer chains endows the hydrogel with outstanding mechanical toughness and efficient energy dissipation capabilities (Figure 15B–D).

2. The mechanical properties of hydrogels are enhanced by reversible bonding mechanisms through the “sacrificial bond” approach. These chemical bonds break preferentially under the influence of external forces, thereby dissipating a large amount of energy and preventing stress concentration, which helps to avoid structural failure [205]. Moreover, these bonds can rapidly and reversibly reform, thereby enabling dynamic restoration of the network structure and helping to maintain material integrity and prevent permanent damage. This cyclic “breakage‐reformation” process significantly improves the toughness, fatigue resistance, and damage tolerance of hydrogels [206]. Li et al. [135] prepared a composite hydrogel with a dual‐enhanced network through a combination of chemical and physical crosslinking. The hydrogel exhibited excellent mechanical properties and rapid self‐healing capabilities by leveraging dynamic covalent bonds and non‐covalent interactions.

3. The mechanical characteristics of hydrogels are influenced by the state of water molecules and the stability of the polymer network at extreme temperatures. Research indicated that water evaporation and freezing can make hydrogels stiff and brittle during temperature fluctuations. To combat this, Zhang et al. [207] introduced a “water‐locking” strategy, which involved fixing water molecules within the hydrogel's polymer network using H₂SO₄ and adding a sacrificial network to prevent network collapse. The sodium alginate/polyacrylamide dual‐network hydrogel effectively maintained softness and stretchability in the temperature range of ‐115°C–143°C through “water‐locking” method.

4. Double‐network (DN) or interpenetrating network (IPN) hydrogels improve mechanical properties through a hierarchical energy‐dissipation mechanism that integrates rigid and flexible components. Specifically, the rigid network (highly cross‐linked polyelectrolyte) serves as a sacrificial framework that fractures preferentially under deformation to dissipate substantial energy, while the flexible network (loosely cross‐linked neutral polymer) preserves ductility and limits crack growth. These interwoven networks are interwoven and mutually penetrating, which distributes mechanical load, provides multiple energy dissipation pathways, and synergistically achieves mechanical properties, thereby surpassing the mechanical limitations of conventional single‐network hydrogels. For example, a strong thermosensitive double‐network (T‐DN) Poly(N‐isopropyl acrylamide)‐based hydrogel was prepared by using rigid poly(sodium 2‐acrylamido‐2‐methylpropanesulfonate) (PNaAMPS) as the first network and soft poly(N‐isopropylacrylamide‐co‐acrylamide) (P(NIPAM‐co‐AAm)) as the second network (Figure 15E). The PNaAMPS/P(NIPAM‐co‐AAm) T‐DN hydrogel exhibited excellent mechanical properties (0.83–1.37 MPa) and temperature‐dependent swelling behavior (Figure 15F). Furthermore, the PNaAMPS/P (NIPAM‐co‐AAm) T‐DN hydrogel could be loaded with crystal violet (CV), thereby enabling antibacterial functionality. These finding further indicates the considerable potential of the double‐network strategy in developing high‐strength and antibacterial hydrogels [208].

5. The mechanical properties of hydrogels are improved through hierarchical energy dissipation and stress distribution mechanisms, which are made possible by multiscale porous structures and strategically designed network topologies. Gradient pores at micro/nano scales collapse or deform sequentially under deformation, thereby dissipating energy and mitigating stress concentration. Additionally, optimized topological networks, such as gradient cross‐linking and bio‐inspired hierarchical entanglements, facilitate effective stress transfer and redistribution by regulating the arrangement and mobility of polymer chains. This combined approach enhances strength, toughness, compressive resistance, and resilience, thus addressing the limitations of uniform networks [209]. Li et al. [210] introduced a nano‐confined polymerization strategy to prepare tough and nearly zero‐delay hydrogels under large deformation. These hydrogels were synthesized via in situ polymerization within the nanochannels of covalent organic frameworks or molecular sieves. The confinement of the nanochannels and the strong hydrogen bonding interactions with the polymer segments were the key to achieving rapid self‐reinforcement. The rigid nanostructures helped reduce stress concentration at crack tips and prevent crack growth, thereby achieving remarkable ultimate fracture strain (17 580 ± 308)%, toughness (87.7 ± 2.3 MJ·m⁻³), and crack propagation strain (5800%).

FIGURE 15.

Mechanical Properties and Durability. (A) Schematic diagram of assembling sponge‐like polyvinyl alcohol/Janus‐like oxygenated graphene mixed hydrogels (SPJH) through strong interfacial interactions of hydrogen bonds and hydrophobic interactions. Reprinted with permission from ref(203]. Copyright 2021, ACS. (B) Schematic diagram of the preparation of C‐BCN and PAM‐C‐BCN hydrogels [204]. (C) Characterization of the mechanical properties of PAM‐C‐BCN hydrogels [204]. (D) Schematic diagram of the internal interaction between polyacrylamide and bamboo‐based cellulose carbon nanomaterials [204]. (E) Schematic of PNaAMPS/P(NIPAM‐co‐AAm) T‐DN hydrogel formation, drug loading, and molecular structures of monomers. (F) Swelling behavior and water retention of T‐DN62.5 at different temperatures. Reprinted with permission [208]. Copyright 2021, Royal Society of Chemistry.

5.4. Salt Resistance

Salt resistance is essential for the effectiveness of hydrogel‐based water purification systems in high‐salinity conditions. High salt tolerance allows hydrogels to retain their swelling properties and improves their ability to adsorb pollutants in saline environments. Significant efforts have been made to enhance the salt resistance of hydrogels, such as employing geometric designs to position salt crystals at the hydrogel's edges [211], utilizing Janus structures to prevent salt ion accumulation on the surface, and engineering hydrogel structures to facilitate the re‐dissolution of salt crystals [212]. These innovative approaches have greatly improved hydrogels' salt resistance. Nonetheless, the following challenges still exist: 1) The complexity of the hydrogel structures increases manufacturing difficulties [213]; 2) The design of fluid exchange and longitudinal channels can lead to heat loss from the surfaces, thereby reducing steam generation rates [214]; 3) The hydration of insoluble salt ions creates a significant evaporation energy barrier that cannot be overcome. Therefore, the preparation of salt‐resistant hydrogel still faces many challenges [215].

1. Anticorrosive electrolyte design based on the combination of molecular engineering and structural optimization. By integrating molecular design to enhance chemical interactions and structural optimization to improve physical properties, Zheng et al. [216] prepared a polyelectrolyte hydrogel evaporator that achieve efficient hydration, low‐energy evaporation, and prevent salt crystallization under high salinity conditions. The hydrogel's resistance to salt was attributed to the incorporation of aromatic benzene and imidazole components (VBIPS) within its molecular framework. VBIPS significantly boosted the capacity for salt ion binding, thus strengthening the anti‐polyelectrolyte effect. Notably, the binding energy between VBIPS and Na⁺/Cl⁻ ions far exceeded that of conventional sulfobetaine methacrylate (SBMA), which allowed salt ions to effectively shield the electrostatic interactions among chains, disrupt the originally compact ionic cross‐linked network, and release more hydrated ionic groups. This resulted in a significant increase in the swelling degree of the hydrogel in the physiological saline environment (the swelling ratio in seawater reached 360%, and the swelling ratio in water containing 10 wt.% salt reached 582%). Furthermore, the structure modulated the behavior of water molecules through long‐range electrostatic interactions, which significantly enhanced the ratio of intermediate water (with a ratio of intermediate to free water of 1.58 in 10 wt.% saline, which is 1.98 times that of SBMA gel) and reduced the equivalent enthalpy of evaporation to 715.20 J·g^{− 1} (approximately one‐third of traditional gels), thus considerably lowering energy consumption during evaporation. A gradient‐distributed polypyrrole (PPy) network achieved thermal localization due to variations in longitudinal thermal conductivity. This gradient‐distributed PPy network, combined with the optimized pore structure, effectively achieved continuous water transport in a high‐salt environment. The evaporator maintained an evaporation rate of 3.17 kg/m²/h for 8 h, while effectively inhibiting salt crystallization. The evaporator also demonstrated excellent antibacterial properties due to its robust hydration layer and salt ion binding capability. The experimental data showed that the density of Escherichia coli dropped below 1.97 × 10⁴ cells per square centimeter after 7 days, thereby revealing the multi‐dimensional mechanism of its salt tolerance. (Figure 16A).

2. Ion‐specific regulation strategy based on salt solubility‐salt precipitation effect. Zhou et al. [217] prepared polyvinyl alcohol (PVA) hydro‐aerogels with cationic specificity and demonstrated their extremely high salt resistance by using the salt dissolution phenomenon and salt precipitation phenomenon in polyvinyl alcohol (PVA) gels. The salt resistance mechanism of PVA hydro‐aerogels mainly depended on their unique preparation strategy and material design. Specifically, first, borax was utilized to form borate‐ester bonds with PVA to fix the hydro‐aerogels group (hydroxyl groups) in the PVA molecular chain, thereby preventing hydroxyl groups from forming a strong hydrated layer with water and weakening the competition for hydration between cations and hydroxyl groups. This laid the foundation for subsequent cationic‐specific interactions. Second, weak hydrated anion (Cl⁻) was utilized as a counterion to prevent strong hydrated anion (SO₄ ²⁻) from masking the cation effect, which was conducive to the electrostatic repulsion of cations becoming the dominant factor. A strong electrostatic repulsion occurred between the free cations and the bound cations when high charge density cations are adsorbed onto the PVA chain and form a “compression effect”, thereby causing the PVA chain to stack directionally with the graphene oxide (GO) nanosheets, ultimately forming an interwoven layered network structure. Finally, GO nanosheets served as a rigid skeleton to support the layered network, which resisted salt‐induced shrinkage deformation. Meanwhile, salt ions (Ca²⁺, Na⁺, etc.) further enhanced the network stability by promoting the increase of PVA crystallinity and the expansion of crystal domain size. This mechanism enabled the material to maintain structural integrity for a long time in a saltwater environment, and it also had high water absorption and ion retention capabilities to achieve a unity of salt resistance, mechanical stability, and multi‐functional response (Figure 16B).

3. Develop a pure zwitterionic triple‐network (ZTN) structure for enhanced salt tolerance. Li et al. [218] prepared a ZTN hydrogel using poly (trimethylamine N‐oxide) (pTMAO) and poly (sulfobetaine) (pSB), which provided strong electrostatic interactions and network entanglements. The synthesis of the pTMAO/pSB ZTN hydrogel involved the following steps: First, a zwitterionic single‐network (ZSN) hydrogel was formed by polymerizing a precursor solution of TMAO monomer, UV initiator, and N, N'‐methylene bisacrylamide (MBAA) as a cross‐linker. Next, pSB was introduced to create a zwitterionic double‐network (ZDN) hydrogel. The ZDN hydrogel was then soaked in a sulfobetaine methacrylate (SB) monomer solution, where it absorbed significant amounts of SB due to strong electrostatic interactions. pSB formed a third‐layer network structure after photoinitiation. The ZTN hydrogel component exhibited a “locking effect”, and the increase in chain entanglement density effectively reduced the swelling degree in a high‐salt environment, thereby maintaining extremely high mechanical strength (compression stress of 18.2 MPa, modulus of 0.66 MPa) and long‐term stability (Figure 16C).

4. A dynamic complementary cross‐linked network can be established by combining two polymer segments with contrasting salt tolerances and precisely regulating their ratios to ensure that hydrogels maintain salt tolerance across a broad range of salinities. Gao et al. [219] prepared a PDEA‐co‐PDMAPS copolymer hydrogel using N, N‐diethylacrylamide (DEA) and zwitterionic 3‐Dimethyl (methylacryloyloxyethyl) ammonium propane sulfonate (DMAPS) as comonomers. The interaction between PDEA and PDMAPS enabled the crosslinking process to achieve dynamic complementarity. This characteristic could be regulated by adjusting the molar ratio of DEA to DMAPS according to the salt concentration, thereby maintaining the stability of the overall network structure. The electrostatic interaction in PDMAPS dominated the crosslinking process under low salinity conditions, while the hydrophobic interaction in PDEA took the leading role in high salinity environments. Additionally, N, N'‐methylenebis (acrylamide) (BIS) was added as a chemical cross‐linker to introduce salt‐insensitive covalent linkages, further enhancing structural integrity across a wide salinity range. Consequently, the PDEA‐co‐PDMAPS copolymer hydrogel with swelling properties, stable water retention performance, extremely strong oil repellency, and extremely low oil adhesion at different salinity conditions, demonstrated great potential for application in seawater desalination and wastewater treatment fields (Figure 16D).

5. Develop a Janus ion‐selective (Janus‐IS) hydrogel for ion‐electric migration desalination technology. He et al. [220] prepared a Janus‐IS hydrogel evaporator by designing a Janus structure. This structure was achieved by incorporating a cation‐selective hydrogel (poly‐2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid sodium) for the upper layer and an anion‐selective hydrogel (poly(3‐acrylamidopropyl) trimethylammonium chloride) for the lower layer of the evaporation surface. The salt concentrations on the photothermal surface and the waste heat surface increased simultaneously during the evaporation process, thereby prompting Na⁺ to migrate downward through the anion gel and Cl⁻ to migrate upward through the cation gel and resulting in an ion counterflow. Additionally, the internal electric field generated by the selective ionic diffusion facilitated salt removal via counter‐ion electro‐migration. The Janus‐IS hydrogel evaporator was connected to the main salt water through a narrow water channel, effectively minimizing heat loss due to water convection. An air gap was also employed as an insulating layer to further curtail heat loss. The Janus‐IS hydrogel evaporator achieved an impressive evaporation rate of 6.86 kg/m²/h in a 15 wt.% brine solution, and demonstrated stability against salt deposition over seven days (Figure 16E).

FIGURE 16.

(A) Schematic illustration of a molecularly engineered polyzwitterion to strengthen the anti‐polyelectrolyte effect. Reprinted with permission [216]. Copyright 2023, Wiley. (B) Schematic illustrations of the formation mechanism for the salt‐induced structures of gels. Reprinted with permission [217]. Copyright 2024, Wiley. (C) Schematic for preparation of the pTMAO/pSB/pSB ZTN hydrogel with strong electrostatic interaction and network entanglement. Reprinted with permission [218]. Copyright 2021, Wiley. (D) Schematic illustration of the main reactants and polymer chains with dynamic complementary crosslinks of PDEA‐co‐PDMAPS hydrogel. Reprinted with permission [219]. Copyright 2021, Wiley. (E) Schematic diagram and analysis of heat and mass transport of a typical evaporator and Janus‐IS hydrogel evaporator. Reprinted with permission [220]. Copyright 2023, Wiley.

6. Applications

The significant advancements in SDHs technology are attributed to the innovative application of new materials and the optimization of the light‐to‐heat conversion mechanism, which have substantially improved the energy conversion efficiency of hydrogels and highlighted their crucial role in renewable energy systems. Therefore, this technology offers considerable potential for diverse applications, particularly in the sustainable management of water and energy resources. Solar energy‐driven hydrogels provide a practical and feasible solution to global challenges, including water shortage and pollution. This section systematically reviews the latest research progress and engineering application cases in four core water purification fields (seawater desalination, removal of heavy metal ions, degradation of organic dyes, and water sterilization). This section will also expand the discussion to innovative applications of solar energy and the recycling and utilization of valuable resources in the water environment.

6.1. Seawater Desalination

Freshwater is a vital natural resource necessary for life and economic systems. Although water covers 71% of the Earth's surface, 96.5% of it is saline and unusable. Only 2.5% of water is freshwater, primarily existing in glaciers and permafrost [221]. This scarcity has led to seawater desalination becoming a key strategy for addressing global water shortages. Traditional desalination techniques, such as thermal processes (multi‐effect distillation, multi‐stage flash, and mechanical vapor compression), membrane methods (reverse osmosis and nanofiltration), and new technologies (freeze desalination and forward osmosis), can generally satisfy ongoing freshwater needs [222]. Unfortunately, these methods have certain limitations, including high energy consumption, high cost, and susceptibility to environmental pollution. In contrast, solar‐driven hydrogels can offer a promising alternative for desalination due to their lower cost, more eco‐friendliness, and higher efficiency and selectivity. The performance of desalination technology relies on factors such as the hydrogel's steam generation rate, salt resistance, sustainability, environmental compatibility, etc. [12].

Inspired by the durian's outer skin structure, Yang et al. [223] developed a Janus composite hydrogel photothermal evaporator (SPP@PAM) featuring a pyramid array. This evaporator achieved an impressive evaporation rate of 3.68 kg/m²/h and a photothermal conversion efficiency of 95.17%, and maintained stable performance over 18 days. The upper layer consisted of super‐hydrophobic pyramid foam coated with polypyrrole, while the lower layer was a polyvinyl alcohol/agar hydrogel with amino‐functionalized carbon nanotubes. The pyramid structure reduced light reflection loss to 8.2% and increased absorption to 96.3%. Additionally, the Janus design lowered evaporation enthalpy, enhanced water transfer rates, and minimized droplet adhesion, indicating that this evaporator shows great potential in solar‐powered seawater desalination and wastewater treatment (Figure 17A).

FIGURE 17.

(A) SPP@PAM hydrogel photothermal evaporator. Reprinted with permission [223]. Copyright 2025, Elsevier. (a) Schematic diagram of a 3D conical array solar evaporation device; (b) Simulation mapping of interface heat transfer and light capture effect on the surface of SPP@PAM pyramid array; (c) Light reflection and absorption spectra of SPP@PAM over the entire solar spectrum range. (B) PSSC hydrogel evaporation. Reprinted with permission [224]. Copyright 2025, Elsevier. (a) Process flow for the preparation of phase change composite hydrogel‐based evaporators and the corresponding reaction mechanism; (b) Evaporation rates and efficiencies of PSSC in pure water and brines. (C) LACH hydrogel evaporation. Reprinted with permission [225]. Copyright 2024, Elsevier. (a) Schematic diagram of the LACH preparation process; (b) Characterization of seawater desalination performance of LACH.

Similarly, Chen et al. [224] prepared a solar interface evaporation hydrogel (PSSC) with incorporated phase change materials by embedding sodium sulfate decahydrate within a dual‐network hydrogel made of sodium alginate and polyacrylamide. The PSSC demonstrated an evaporation rate (2.72 kg/m²/h), high efficiency (92.8%), and substantial durability with a heat storage capacity of 181.42 J·g⁻¹. Moreover, the evaporation rate of PSSC remained at 1.25 kg/m²/h without solar radiation. Furthermore, PSSC produced 16.8% more fresh water than the conventional hydrogels. This design effectively mitigates the intermittent nature of solar energy, thereby facilitating continuous water production even during nighttime and overcast conditions. This capability is particularly critical to ensure a reliable water supply in off‐grid regions (Figure 17B).

Another study introduced an ion‐engineered copper‐based hydrogel photothermal evaporator (LACH) by using oppositely charged polymers and CuO nanoparticles for light absorption [225]. LACH achieved an evaporation rate of 3.93 kg/m²/h and energy efficiency of 93.5% in a 3.5 wt.% NaCl solution, and generated 10 kg/m² of freshwater in 8 h, which was sufficient to meet the daily needs of two families, further suggesting that LACH is suitable for large‐scale seawater desalination (Figure 17C).

6.2. Removal of Heavy Metal Ions

Heavy metal ions (mercury, lead, cadmium, chromium, and arsenic) are known for their bioaccumulation and biomagnification properties. These ions resist biodegradation in wastewater and accumulate in organisms, particularly in the liver, kidneys, and bones, which lead to higher concentrations in organisms at elevated trophic levels compared to their original aquatic environments [226]. Recently, various methods, including wastewater, including activated carbon adsorption, chemical precipitation, membrane separation, ion‐exchange resins, and electrodeposition/adsorption, have been developed to eliminate heavy metal ions [227]. Unfortunately, traditional approaches often involve high energy consumption, secondary pollution, and complex operation [228]. Notably, SDHs present an innovative solution to combat severe water pollution due to low energy consumption, low costs, and environmental friendliness.

The mechanism by which solar energy promotes the removal of heavy metal ions through a hydrogel system involves a synergistic multi‐step process driven by various photophysical effects and photocatalytic effects. The core of this mechanism lies in the absorption of light energy by the functional components within the hydrogel, thereby triggering the photothermal effect and photocatalytic conversion reactions, which achieve the capture, transformation, and release of heavy metal ions. Specifically: 1) Hydrogels can induce localized photothermal effects, thus generating high‐temperature zones that enhance the concentration and subsequent removal of metal ions through adsorption or chemical reactions [95]; 2) The abundant functional groups in the hydrogel structure facilitate the sequestration of heavy metal ions via chelation [229]; 3) Semiconductor materials embedded within the hydrogel generate electron‐hole pairs under illumination and mediate redox reactions that alter the valence states of metal ions, thereby reducing their toxicity and promoting immobilization [230]; 4) The porous three‐dimensional architecture of the hydrogel promotes uniform dispersion of metal ions and helps mitigate aggregation and oxidation phenomena [231].

For instance, a TiO₂@C/PAM/SA hydrogel was prepared via the dual‐method. The results show that TiO₂@C/PAM/SA hydrogel demonstrated high light absorption (99.87%), evaporation rate (2.97 kg/m²/h), and photothermal conversion efficiency (92.13%). Moreover, TiO₂@C/PAM/SA hydrogel showed high removal efficiency (99.9%) for various heavy metal ions (Figure 18A) [231]. If this type of hydrogel is used to treat the initial lead‐containing wastewater (with a concentration of 10 mg/L), the lead concentration in the water can be reduced to below 0.01 mg/L, which is far below the limit stipulated in the “Drinking Water Hygiene Standard”. At the same time, this process also simultaneously achieves freshwater production. For every cubic meter of wastewater treated, approximately 1 ton of freshwater can be obtained (estimated based on the evaporation rate), thus achieving the recycling and reuse of water resources.

FIGURE 18.

(A) Schematic diagram of the preparation of TiO2@C/PAM/SA hydrogel. Reprinted with permission from ref [231]. Copyright 2025, Elsevier. (B) konjac glucomannan and Fe‐based metal–organic frameworks hydrogel evaporator. Reprinted with permission [232]. Copyright 2020, Wiley. (a) The water evaporation rate measured under dark conditions is greater than (1/M), and the equivalent enthalpy calculated for HHE; (b) The evaporation rate of HHE and the corresponding energy efficiency; (c) The mass change of the evaporated steam over time; (d) A schematic diagram of solar water purification using HHE. (C) Schematic diagram of the synthesis process of photothermal SA/CNC‐PPy hydrogel. Reprinted with permission [233]. Copyright 2025, Elsevier.

In another study, Guo et al. [232] fabricated a composite hydrogel evaporator (HHE) by incorporating konjac glucomannan (KGM) and Fe‐based metal–organic framework‐derived solar absorbers into a polyvinyl alcohol (PVA) network. The HHE demonstrated a high‐water evaporation rate (3.2 kg/m²/h) and an energy conversion efficiency (90%). Moreover, heavy metal ions (Cd²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Ni²⁺, Ag⁺, Zn²⁺, Pb²⁺, Se²⁻, As⁵⁺, and Hg²⁺) concentrations were reduced by 4–7 orders of magnitude when they were introduced into the aqueous medium containing the HHE for 2 h. This significant reduction was attributed to the large number of hydroxyl groups provided by KGM, which help form hydrogen bonds and chelates, thereby effectively removing heavy metal ions and organic dyes (Figure 18B). Similarly, Hamid et al. [233] prepared a cellulose nanocrystals/polypyrrole/sodium alginate hydrogel. It was found that the hydrogel achieved an evaporation rate of 1.47 kg/m²/h and a removal rate of 99% for several heavy metal ions, indicating that this hydrogel has the potential for seawater desalination and pollutant degradation (Figure 18C).

6.3. Organic Dyes Degradation

Organic dyes in wastewater pose extensive hazards to ecosystems and human health. Organic dyes can reduce light penetration in water bodies and release toxic substances due to their strong coloring ability, thereby disrupting the balance of aquatic ecosystems. Moreover, organic dyes can accumulate through the food chain, which leads to carcinogenic, mutagenic, and teratogenic effects, acute/chronic poisoning, and allergic reactions in humans. Furthermore, their persistent and recalcitrant nature increases environmental burdens and threatens food safety [234]. Hence, it is crucial to develop efficient, economical, and environmentally friendly technologies for organic dye degradation. Conventional methods include physical adsorption, chemical coagulation/flocculation, chemical oxidation, and biological treatment [235]. However, these approaches often suffer from high energy consumption, limited effectiveness in complex environments, and risks of secondary water pollution. In contrast, solar‐driven hydrogels have garnered significant attention in the field of organic dye degradation due to their high synergistic efficiency, operational simplicity, and environmental compatibility.

The mechanisms of solar‐driven hydrogel in water sterilization involve the following aspects: 1) Photocatalytic degradation: Semiconductor materials embedded within the hydrogel generate electron‐hole pairs under solar irradiation and initiate redox reactions that directly decompose dye molecules [236]; 2) Photothermal evaporation and concentration: Photothermal components can convert solar energy into thermal energy, accelerate water evaporation, and concentrate dye molecules within the gel network, which increases local concentration and enhances degradation efficiency; 3) Synergistic adsorption and catalysis: The three‐dimensional network of the hydrogel contains rich functional groups (hydroxyl, carboxyl, and amino groups), adsorbs dye molecules via electrostatic interactions or coordination bonds, and provides a high‐concentration microenvironment for subsequent catalytic reactions; 4) Reactive oxygen species (ROS) oxidation: Photosensitive components excited under visible light generate reactive oxygen species (·OH, ·O²⁻, and ¹O₂), which non‐selectively attack and break down the molecular structure of the dyes [237].

Lu et al. [238] fabricated a bifunctional hydrogel evaporator (MF/CNTs‐1) via alkaline etching of vermiculite. The results show that the MF/CNTs‐1 evaporator exhibited a high vapor generation rate of 2.22 kg/m²/h and an energy conversion efficiency of 92%. Besides, the evaporator achieved photocatalytic degradation rates of 97% for methylene blue (MB) and 94% for tetracycline. When treating the dyeing wastewater with an MB concentration of 20 mg/L, the effluent concentration dropped to 0.6 mg/L after 8 h of light exposure treatment. At the same time, this process produced fresh water, thus achieving simultaneous reduction of wastewater and resource recovery. These performance advantages are attributed to the two‐dimensional MgFe nanosheets synthesized during the alkaline etching process, which promoted the generation of highly active free radicals through the Fenton reaction, thereby effectively degrading organic pollutants (Figure 19A).

FIGURE 19.

(A) MF/CNTs‐1a hydrogel. Reprinted with permission [238]. Copyright 2024, Elsevier. (a) The preparation process of an MF/CNTs‐1a hydrogel; (b) Photocatalytic degradation rate; (c) Corresponding solar water evaporation rate and efficiency. (B) CSGOZnO hydrogel. Reprinted with permission [239]. Copyright 2025, Elsevier. (a) Schematic illustration for the formation of CSGOZnO hydrogel nanocomposites; (b) Proposed mechanism for the photocatalytic dye degradation on the surface of CSGOZnO hydrogel nanocomposite; (c) The photocatalytic degradation of MB and AR in the mixture of dye solution, as a function of time using CSGOZnO catalyst; (d) The photocatalytic dye degradation performance of the CSGOZnO hydrogel nanocomposites.

Similarly, a BV‐GH hydrogel composed of graphene oxide (GO) and bismuth vanadate (BiVO₄) demonstrated significant organic dye degradation capacity, with adsorption capacities of 258.78 mg/g for methylene blue and 217.16 mg/g for methyl orange [85]. It is estimated that 1 gram of BV‐GH hydrogel could treat approximately 2.6 liters of wastewater containing 100 mg/L methyl blue. This hydrogel could be regenerated through a photocatalytic process and reused multiple times when it reaches saturation. This functionality was attributed to the porous structure and abundant surface functional groups of the GO network, which facilitated the effective adsorption of dyes through electrostatic interactions and hydrogen bonding. Additionally, the BV‐GH hydrogel generated a large amount of reactive oxygen species (ROS), which led to the disruption of Escherichia coli cell membranes.

In another study, Singh et al. [239] prepared a nanocomposite hydrogel (CSGOZnO) through an in situ mineralization approach by incorporating zinc oxide (ZnO) nanoparticles into a chitosan graphene oxide (CS‐GO) hybrid hydrogel network. The CSGOZnO hydrogel exhibited excellent photocatalytic degradation performance for methylene blue (the degradation rate constant of 5.0 × 10⁻³min⁻¹), apigenin red (the degradation rate constant of 3.0 × 10⁻³ min⁻¹), and mixed dyes (degradation rate constant of 1.0 × 10⁻³ min⁻¹). The kinetics conformed to the pseudo‐second‐order model. The residence time in a continuous flow reactor can be predicted based on this model, thus providing practical parameters for process scaling. Moreover, the in situ mineralization method facilitated the uniform dispersion of zinc oxide nanoparticles within the polymer hydrogel matrix, thereby forming a novel three‐dimensional nanostructure with enhanced photocatalytic activity. (Figure 19B).

6.4. Sterilization of Water Bodies

Bacterial contamination in water bodies has emerged as a global issue threatening drinking water safety. The hazards associated with such contamination are manifested in two major aspects. First, pathogenic bacteria (Escherichia coli and Vibrio cholerae) can enter the human body through drinking water intake or contact and cause intestinal and various infectious diseases that pose serious risks to public health. Second, pathogenic bacteria severely disrupt the balance of aquatic ecosystems, inhibit the proliferation of beneficial species, significantly reduce the self‐purification capacity of water, ultimately leading to deterioration of water quality accompanied by odor formation [240]. Conventional sterilization methods include physical approaches (boiling, UV irradiation, and filtration) and chemical techniques (chlorination, ozonation, and disinfection with chemical agents) [241]. However, these methods face limitations such as the generation of harmful disinfection by‐products, limited efficacy, and potential ecological damage. Solar‐driven hydrogel technology for water sterilization has been developed in light of these challenges.

The mechanism of solar‐driven hydrogel in water sterilization is as follows: 1) Photothermal sterilization: Photothermal materials within the hydrogel absorb solar energy and convert it into heat, thereby generating localized high temperatures that disrupt bacterial cell membranes and denature proteins, ultimately leading to cell death [242]; 2) Photodynamic sterilization: Photosensitizers excited by light at specific wavelengths generate reactive oxygen species (ROS), which oxidize critical biological molecules in bacteria [243]; 3) ROS‐mediated oxidation: Photocatalytic materials under illumination produce free radicals (·OH and ·O²⁻) and non‐selectively oxidize cellular components of bacteria; 4) Physical inhibition: The porous structure of the hydrogel physically traps bacteria, restricts their mobility, and enhances local sterilization efficiency; 5) Chemical disinfection: The hydrogel network enables the sustained release of disinfecting agents that directly disrupt bacterial metabolism.

Hao et al. [34] prepared a three‐dimensional AgMXene@chitosan hydrogel (AM/CH) evaporator by incorporating two‐dimensional AgMXene nanocomposites into a chitosan hydrogel network. The AM/CH evaporator demonstrated exceptional antibacterial properties, which achieved complete inhibition (100%) of Escherichia coli and Staphylococcus aureus under simulated solar illumination. For instance, when dealing with wastewater with an initial bacterial concentration of 10⁷ CFU/mL, the number of viable bacteria had dropped below the detection threshold after 1 h of AM/CH treatment, which achieved a 100% sterilization rate and no observed bacterial regrowth. Moreover, the evaporator demonstrated a high evaporation rate (1.59 kg/m²/h) and high evaporation efficiency (93.0%) under a light intensity of 0.1 W·cm⁻². This system could enable simultaneous water purification and freshwater production in remote areas. Its daily water treatment capacity was approximately 15 L/m², which was sufficient to meet the drinking water needs of a household.

Similarly, Mei et al. [244] prepared a PDEAAm/PANI@QCMCS hydrogel via free radical polymerization. The hydrogel demonstrated efficient solar‐driven water purification, remarkable antibacterial performance, and real‐time monitoring capability. It achieved a bacterial inactivation rate of 100% and a dye degradation efficiency of 97%, indicating strong potential as an innovative strategy for addressing global water scarcity (Figure 20A–C).

FIGURE 20.

(A) The purification mechanism of a PDEAAm/PANI@QCMCS hydrogel. (B) The antibacterial activity of PDEAAm/PANI@QCMCS hydrogel against Escherichia coli and Staphylococcus aureus. (C) Schematic diagram of disinfection. Reprinted with permission [244]. Copyright 2025, Elsevier. (D) Schematic of the fabrication process for AHSE. (E) Representative images of viable bacteria clones on agar plates after incubating with the hydrogels. (F) Antibacterial ratio of different hydrogels. (G) SEM images of the bacterial morphologies in the presence of AHSE with different times. Reprinted with permission [245]. Copyright 2025, Elsevier.

In another study, an antibacterial hydrogel solar evaporator (AHSE) was developed through cross‐linking graphene oxide‐calcium silicate hydrate (GO‐CSH) with polyvinyl alcohol (PVA) and N‐(phosphonomethyl) iminodiacetic acid‐functionalized polyethyleneimine (PMIDA‐PEI) [245]. The results show that inhibition rates of AHSE against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and methicillin‐resistant Staphylococcus aureus were 99.48%, 99.37%, 98.33%, and 98.51%, respectively. Moreover, AHSE's high efficacy against drug‐resistant strains highlighted its unique advantages in dealing with hospital wastewater and environments contaminated with antibiotic‐resistant bacteria. Furthermore, AHSE had a high evaporation rate of 3.25 kg/m²/h, an energy conversion efficiency of 95%, and a heavy metal ion removal rate of 99.9%, indicating its broad application prospects in seawater desalination and water purification processes (Figure 20D–G).

6.5. Synergistic Utilization of Solar Energy and Water Resources

Previous sections have individually analyzed the applications of SDHs in seawater desalination, heavy metal removal, organic pollutant degradation, and water sterilization. These studies have focused predominantly on contaminant elimination and freshwater generation. However, the utilization of solar energy extends beyond merely driving the evaporation process. From a broader perspective, the Solar‐driven hydrogel‐based water purification system should be regarded as an integrated platform that facilitates the synergistic conversion and utilization of solar energy, water resources, and waste materials.

6.5.1. Multi‐Level Utilization of Solar Energy

The solar spectrum covers a wavelength range from UV to infrared, and photons of different energies can drive various physical and chemical processes. Currently, the main application of solar energy in hydrogel systems is photothermal conversion driven by visible to near‐infrared light, while high‐energy UV photons can be used to initiate photocatalytic reactions [246]. However, the integration of photothermal‐photocatalytic material systems is challenging synergistic integration of photothermal evaporation and photocatalytic degradation can be achieved on a single platform. For instance, Lin et al. [247] developed a multifunctional photothermal composite hydrogel (PG@CuSx). This material embedded reduced graphene oxide and narrow‐bandgap copper sulfide into a three‐dimensional polyacrylamide network, thus constructing a ternary coupled system that combines solar steam generation, photocatalytic degradation, and thermoelectric power generation. This hydrogel exhibited an efficient evaporation rate (8.1 kg/m²/h) and an organic pollutant degradation rate (90%). Additionally, PG@CuSx could generate a voltage output as high as 161.5 mV when coupled with a thermoelectric generator. This multi‐level utilization mode (integrating photothermal, photoelectric, and photochemical processes) enables the hydrogel evaporator to become self‐sufficient, thus allowing for the simultaneous production of fresh water, electricity, and chemical fuels.

Notably, hydrogel can convert the temperature gradient generated by the photothermal effect into electrical energy by integrating thermoelectric or piezoelectric materials into the hydrogel system, thereby providing power for water quality sensors or Internet of Things devices. Chen et al. [248] prepared an ion engineering hydrogel that could simultaneously achieve solar‐driven seawater evaporation and power generation. This system achieved molecular‐scale ion transport regulation through metal‐polymer cooperative coordination and ion preferential binding. In the evaporation experiment, the freshwater yield and peak current of the hydrogel were 2.0 kg/m²/h and 1.2 mA, respectively, which were sufficient to charge small electronic devices. Additionally, this hydrogel could recover up to 16.7 Wm⁻² of salinity gradient energy (blue energy) through reverse electrodialysis technology. This multi‐energy output mode enables the SDHs system to become a multifunctional platform with both energy and resource production capabilities.

6.5.2. Targeted Recovery and Utilization of Resources in Water Bodies

Apart from the targeted pollutants, water bodies are usually rich in various valuable elements. For instance, electroplating wastewater often contains metal elements such as copper, nickel, and chromium [249]. Dyeing wastewater has residual organic dye molecules and inorganic salts [250]. Seawater contains strategic resources like lithium, uranium, and rubidium, with reserves thousands of times greater than on land [251]. Traditional water treatment technologies often treat these components as pollutants and remove them, which leads to resource waste and poses a risk of secondary pollution. SDHs, with their three‐dimensional network structure and adsorption‐enrichment characteristics, can efficiently concentrate trace substances within the gel network, thus creating favorable conditions for subsequent resource recovery and utilization. Existing research has indicated that solar hydrogels can purify water bodies while efficiently enriching heavy metal ions, recycling dyes, and extracting strategic resources from seawater through reasonable material design, implying a “two birds with one stone” potential for resource utilization [252].

In the field of rare element extraction from seawater, Liang et al. [253] developed a solar‐driven seawater desalination system based on DNA hydrogels. Functional DNA molecules capable of specifically recognizing uranyl ions (UO₂ ²⁺) were embedded in the hydrogel matrix, and graphene oxide was used as the light‐absorbing material. Experiments conducted in the Bohai Sea of China demonstrated that the hydrogel exhibited a high evaporation rate of 3.54 kg/m²/h. Additionally, the DNA hydrogel achieved an adsorption capacity of 5.7 mg/g for uranium in natural seawater due to the rapid ion transport driven by solar interfacial evaporation and its high selectivity for uranium (with a selectivity ratio of uranium to vanadium reaching 10.4). This further indicates that the DNA hydrogel, by programming the DNA structure, can be extended to the extraction of other strategic resources, such as lithium ions.

In the field of heavy metal ion recovery, concentrated metal ions can be released into a small volume of solution through simple desorption treatments (pH adjustment, light exposure, or electrochemical methods) of the adsorption‐saturated hydrogels. The high‐concentration metal solution can be applied to the subsequent recovery of metal monomers or their compounds. For instance, a composite hydrogel evaporator based on konjac glucomannan and reduced graphene oxide exhibited a remarkable removal rate for heavy metal ions (Cr³⁺, Mn²⁺, Cu²⁺, Zn²⁺, and Pb²⁺) [232]. Moreover, the composite hydrogel evaporator could reduce the initial metal ion concentration by four orders of magnitude. Similarly, the kaolin/carbon nanotube/PAM hydrogel (KCAH) could simultaneously achieve efficient evaporation (the evaporation rate of 2.74 kg/m²/h) and heavy metal enrichment (adsorption capacity for Mn²⁺ of 94.5 mg/g), thus providing a solid foundation for the simultaneous acquisition of fresh water and metal recovery from wastewater [254]. Previous studies have shown that the application of SDHs can be extended to the field of soil heavy metal remediation. Yu et al. [252] prepared a fully natural photothermal hydrogel (CS/CNFs/C@SBE). CS/CNFs/C@SBE had a high evaporation rate of 2.17 kg/m²/h and an excellent adsorption for Cu²⁺. CS/CNFs/C@SBE could effectively reduce the Cu²⁺ concentration in contaminated soil from 500 to 45.6 mg/kg, thereby promoting the healthy growth of wheat seeds and providing a new technical platform and theoretical basis for solar‐driven water purification and environmental remediation. Similarly, Fang et al. [255] prepared the MXene/sodium alginate/MgO composite hydrogel (MMS). MMS could simultaneously extract boron during seawater desalination. The recovered boron could be used as a micro‐fertilizer in agriculture, which could increase the germination rate of mustard seeds by 13% and triple the biomass.

For organic dyes, the adsorbed saturated hydrogels can recover the complete dye molecules through solvent elution, or recover valuable intermediates by regulating the degree of photocatalytic reaction. Inspired by the composition structure of plant stems, Yang et al. [256] prepared a hydrogel skin fiber bundle photocatalytic solar evaporators (G‐RSC) in the form of willow branches. The photothermal and photocatalytic functions of G‐RSC simulated the photosynthetic characteristics of plant stems. G‐RSC demonstrated an efficient evaporation rate of 2.55 kg/m²/h due to its dual water supply system and unique evaporation structure. Moreover, this evaporator could photocatalytically degrade more than 95% of dyes such as rhodamine B and methylene blue under the dynamic fluid flow mode. Both experimental and simulation results have confirmed the synergistic enhancement effect of the photothermal process and the photocatalytic process. Furthermore, G‐RSC operated effectively without salt crystallization under high salt conditions (20 wt.% NaCl), which demonstrated excellent salt tolerance. The evaporation rate and dye degradation rate remained stable during the 7‐day cycle operation (8 h per day), thus confirming its durability for long‐term operation. Notably, the concentrated brine obtained from G‐RSC could be reused in new dyeing processes, thereby effectively improving the dye absorption rate and resource recovery rate. This further suggests that G‐RSC provides theoretical support for the solar treatment of high‐salt dyes and resource recycling.

7. Conclusions and Prospects

This review systematically compares the properties of hydrogels based on natural polymers and synthetic polyelectrolytes. Additionally, this review analyzes the photothermal action mechanisms of various photothermal conversion components and their synergistic effects with the hydrogel network structure. Furthermore, this review systematically summarizes advanced preparation techniques and evaluates the complex regulatory mechanisms for controlling photothermal efficiency, evaporation rate, mechanical strength, salt resistance, and the long‐term operational stability of the hydrogels from the dual perspectives of preparation strategies and performance optimization. Finally, this review integrates the latest application progress of SDHs in areas such as seawater desalination, heavy metal removal, organic pollutant degradation, and water body sterilization. At the same time, this review explores the innovative application potential of multi‐level utilization of solar energy and water resource recovery in this emerging field.

Although significant progress has been made, there are still many scientific and engineering obstacles that must be addressed urgently in order to transform SDHs from a laboratory setting to practical application. The main limiting factors are as follows:

1. The long‐term stability and durability are still insufficient. SDHs usually exhibit high performance in short‐term laboratory tests. However, SDHs’ operation may lead to degradation of the polymer network, leakage of photothermal fillers, and loss of mechanical integrity in complex and variable real water environments (high salinity, extreme pH values, oil contamination, and microbial activity). For instance, the progressive aggregation of hydrophobic carbon‐based nanomaterials may disrupt the porous structure of the hydrogel, thereby hindering water transmission and reducing evaporation efficiency.

2. The application scope of SDHs still has limitations. Currently, the design of SDHs has a significant effect in generating high‐purity steam, but they are almost ineffective for volatile organic compounds (VOCs), which may co‐vaporize with the distillation products and cause pollution. Moreover, although heavy metal ions are usually retained within the hydrogel matrix, the disposal and regeneration of saturated hydrogels remain a significant but underestimated environmental challenge.

3. The intermittent nature of solar energy inherently limits the continuous operation capability of SDHs. SDHs’ daily operational capacity is inevitably limited due to the dependence of the SDH system on sunlight, which makes it difficult for the standalone solar thermal pump system to meet the water production demands throughout the day. Although attempts have been made to use phase change materials (PCM) for thermal energy storage and nighttime operation, their poor heat conduction performance and high system complexity still hinder the application and promotion of this technology.

4. The scalable manufacturing of SDHs faces significant challenges. Many high‐performance SDHs rely on complex and energy‐intensive manufacturing processes, such as directional freezing casting or micro/nano patterning techniques, which are difficult to achieve economically on a large scale. Moreover, the high costs of advanced raw materials, like graphene and MXene, undermine the economic feasibility of large‐scale deployment, especially in regions with limited resources.

To overcome the aforementioned challenges and promote the practical adoption of SDHs technology, future research should focus on the following core directions based on the latest research progress in this field in recent years:

1. From “Peak Performance” to “Operational Robustness”: Future efforts should prioritize the development of intelligent hydrogels with self‐healing and environmentally responsive properties by leveraging dynamic covalent chemistry and bio‐inspired adaptive structures. Moreover, 4D printing technologies can enable the programmable fabrication of evaporators that dynamically adjust their morphology and pore architecture in response to temperature, salinity, or light intensity, thereby achieving autonomous water and salt management. These strategies address key challenges in long‐term operation, including physical damage, fouling, and photothermal agent leaching. Furthermore, robust immobilization of photothermal materials via in situ polymerization or core–shell encapsulation ensures stable photothermal conversion even under extreme pH or high‐salinity conditions, which can extend device lifetimes from hundreds to thousands of hours, thus meeting the durability requirements essential for industrial deployment.

2. From “Water‐Only Production” to “Resource‐Energy Synergy”: Deep integration of interfacial evaporation with advanced oxidation processes (AOPs) is a key direction. A dual‐functional photothermal‐photocatalytic network with customized narrow band gaps can be constructed by embedding functional materials such as metal–organic frameworks (MOFs), covalent organic frameworks (COFs), or single‐atom catalysts into the hydrogel matrix. This design can broaden the solar absorption range and utilize the interface thermal energy to promote the separation of photogenerated charges, thus achieving simultaneous freshwater production and complete mineralization treatment of organic pollutants. Additionally, it is possible to precisely recover precious metals (gold, silver, and lithium) from complex aqueous matrices by grafting ion‐imprinted polymers or selective chelating ligands onto the hydrogel. This technological innovation transforms SDH from a passive purification tool into an active resource recovery platform, significantly enhancing its economic feasibility.

3. From “Intermittent Operation” to “All‐Weather Functionality”: SDHs should be integrated with phase‐change materials (PCMs) and thermoelectric modules. Excess solar thermal energy can be stored during peak sunlight hours and released at night or on cloudy days by embedding shape‐stable composite phase change materials with high thermal conductivity, thereby achieving continuous water production throughout the day. Meanwhile, the thermoelectric module can recover the low‐grade waste heat from the evaporation process and generate micro‐scale electrical energy to power the sensors or enable real‐time self‐monitoring functions. This synergistic design of water energy significantly enhances the overall energy utilization efficiency. A hybrid multi‐source energy framework can be established to ensure stable operation in off‐grid communities or disaster relief scenarios when further integrated with photovoltaic panels or industrial waste‐heat recovery systems, even under adverse weather or high‐latitude conditions.

4. From “Lab‐Scale Fabrication” to “Industrial Manufacturing”: Transitioning from batch synthesis to continuous industrial manufacturing is critical for practical applications. Techniques, such as roll‐to‐roll coating technology, microfluidic spinning technology, and 3D printing technology, provide scalable solutions to produce large‐area hydrogel evaporators with precisely controlled microstructures. In terms of material selection, low‐cost renewable raw materials should be prioritized as the base materials, combined with solvent‐free and non‐toxic cross‐linking strategies to ensure environmental compatibility. Moreover, techno‐economic analysis and life‐cycle assessment should be integrated at the earliest research and development stages to quantify the levelized cost of water and carbon footprint, thereby guiding the design of recyclable or biodegradable hydrogel systems. Furthermore, SDHs should be encapsulated into modular, portable purification units embedded with an intelligent monitoring system (water quality sensors and machine learning algorithms) to enable adaptive and autonomous operation. This development path marks a decisive leap from technological innovation to practical application.

Author Contributions

Can Zhang: formal analysis (supporting), investigation (equal), writing – original draft (equal). Hongkun Xue, Yuchen Huang, Ziyi Ma, Lan Zhao, and Yijie Han: formal analysis (supporting), investigation (equal). Jiaqi Tan, Yanyan Zhou, and Hongfeng Li: funding acquisition (equal), project administration (equal), writing – review & editing (equal).

Funding

The authors gratefully thank the financial support provided by the National Natural Science Foundation of China (Nos. 52273263, 52203219 and 52073080), Scientific research project of the Basic Scientific Research Operating Expenses of Colleges and Universities in Heilongjiang Province (2021‐KYYWF‐0029 and 2021‐KYYWF‐0041), the Heilongjiang Provincial Natural Science Foundation Joint Guidance Projects (LH2023B022), the Natural Science Foundation of Heilongjiang (No. LH2022B014), the Heilongjiang Province key research and development plan (2022ZX07D04).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Research data are not shared.