Solar-driven interfacial evaporation: materials design and device assembly

Material selection criteria

In the field of SIE, prioritizing the structural optimization of photothermal materials with advanced optical absorption properties is crucial to increase efficiency and promote sustainability. In this regard, nanostructured photothermal materials [Figure 1] have shown exceptional performance due to their ability to localize and concentrate light, resulting in enhanced photothermal conversion efficiency^[30-39]. Moreover, the incorporation of phase change materials or photonic crystals in the synthesis provides the ability to dynamically control optical properties. This allows for the customization of spectral selectivity and tunable absorption^[40-42].

Figure 1. Various types of photothermal materials used in SIE.

The thermal properties of selected materials are also significant determinants of their suitability. Notably, thermal conductivity plays a dual role. High thermal conductivity is essential for the effective spread of heat across the material, thus improving the overall thermal efficiency. On the other hand, low thermal conductivity is advantageous in specific layers of the system. This helps in reducing heat loss to the external environment, focusing the thermal energy in the crucial areas for optimized evaporation. Additionally, low thermal expansion is necessary to maintain structural integrity under stress from fluctuating temperatures, thus preventing damage and extending the operational lifespan. Moreover, a high level of thermal stability is required to withstand the temperature extremes encountered in solar-driven evaporation processes, ensuring consistent performance and durability. By meticulously considering these critical thermal properties, researchers can optimize material selection for SIE, resulting in enhanced efficiency, reliability and overall performance^[43-48].

The mechanical properties of base substrate materials, including strength, ductility, toughness and fatigue resistance, are also essential factors to consider. Ideally, materials should exhibit high tensile strength and ductility to withstand the stresses induced by fluctuating temperatures and pressures during the evaporation process. Furthermore, materials with outstanding toughness can resist fractures and deformations, while those with fatigue resistance ensure the system's durability under repetitive loading cycles. The selection of materials for SIE applications depends on finding the right balance among these mechanical properties. This collective enhancement improves the overall performance and sustainability of the technology^[49-52].

Material categories

Metal oxide and semiconductor-based materials

Metal oxide materials, notably TiO₂, ZnO, and Fe₃O₄, along with other semiconductors such as silicon, perovskite and gallium arsenide, are making significant strides in SIE and purification due to their unique photothermal properties. Both metal oxides and semiconductors possess wide band gaps that allow them to absorb sunlight efficiently. Upon exposure, they absorb photons whose energies correspond to these band gaps, causing electrons to move from the valence band to the conduction band. When these electrons eventually return to their original state, most of the energy they release is in the form of heat rather than light. Recent studies have shown that incorporating plasmonic nanoparticles or co-catalysts can further amplify the light absorption and photothermal conversion capabilities^[64-66]. Notably, Fan et al. highlighted that morphological variations of MnO nanoparticles, when tuned via pyrolysis temperatures, could optimize water transport and photothermal conversion in bifunctional solar evaporators made from upcycled waste poly(ethylene terephthalate) (PET) bottles, thereby enhancing evaporation rates and overall efficiency [Figure 2]^[67]. Semiconductor-based photothermal materials are emerging as a valuable resource in SIE due to their high photothermal conversion rates, robust chemical stability, abundance, and broad industrial application. These photothermal materials also offer antibacterial qualities, durability, and recyclability. One major benefit is their capacity to create electron-hole pairs when photons are absorbed, which helps in producing steam for clean water generation^[68]. These advantages extend to various sectors, including electronics, solar energy, LED lighting, and more^[69]. However, in SIE, these benefits face challenges such as high-temperature performance issues, doping difficulties, and manufacturing complexities, which contribute to increased costs and scalability limitations^[70,71].

Figure 2. (A) Illustration of the process for fabricating flexible MnO/C-x membrane for solar evaporation and thermoelectric power production. (B) SEM micrographs of (B-1 and B-2) Mn-MOF, (B-3) MnO/C-400, (B-4) MnO/C-500, (B-5) MnO/C-600, and (B-6) MnO/C-700. (B-7 and B-8) HRTEM micrographs of MnO/C-600. (C) IR images of cotton fabric and MnO/C-600 membrane exposed to 1 Sun radiation. (D) Rates and cumulative quantities of water production. (E) Evaporation rate of MnO/C-600 employing varying water sources. (F) Images show the salt ablation effect after adding NaCl to the MnO/C-600 evaporator's top surface. Reprinted with permission from ref.^[68]. Copyright 2023 Elsevier.

Maximizing solar absorption

To optimize solar absorption, several strategic approaches can be employed. These include the use of broadband solar absorbers and the incorporation of solar concentrators such as parabolic troughs or Fresnel lenses. In addition, researchers have investigated the design of floating structures that can follow the sun's path, altering the surface of the absorber material to reduce reflection and developing multi-layered structures that are optimized for different wavelengths. Spectral splitting techniques are employed to direct specific wavelength ranges to materials that are best suited for absorption. Effective thermal management can be achieved using thermally conductive materials. Additionally, plasmonic enhancement is realized through the incorporation of metal nanoparticles or nanostructures that support LSPR^[84,85]. Exploring various methodologies to enhance solar absorption, recent research in this field has emphasized the development of intricate structures with exceptional light-trapping properties, as illustrated in Figure 4. Jonhson et al. conducted a study on a 3D-printed porous silica mesh framework with hydrophilic-modified graphene^[86]. The modified graphene, known as MG@Silica, exhibits exceptional light absorption properties thanks to the nanopores it contains. The presence of nanopores in the material helps to lower the effective refractive index and create more surface area for light to be trapped. This results in a reduction of the angular dependence of incoming light and a decrease in light reflection. Additionally, they function as optical microcavities, trapping light via multiple light scattering and reflection processes, potentially leading to improved thermal evaporation efficiency.

Figure 4. Illustration of various solar water evaporators with different surface structures.

In a lab-scale prototype, Menon et al. introduced a selective absorber/black emitter designed to optimize absorption and minimize emission^[87]. This design enhances solar absorption, leading to improved evaporation efficiency. The addition of a reflective foil inside the water tank enhances the system performance by redirecting any unabsorbed solar radiation back to the absorber. An acrylic wall surrounds the inner pocket of a tank, acting as a barrier to reduce thermal losses. This ensures that a significant portion of the captured solar energy is used for heating the water, promoting evaporation. In addition, a copper aperture positioned above the absorber helps to eliminate unwanted light, resulting in more precise data collection and a better understanding of the factors that influence solar absorption efficiency.

Minimizing heat loss

Key strategies for achieving enhanced performance include optimizing the design and materials of the solar absorber. Ideally, the absorber should have high light absorption and low thermal conductivity to ensure efficient sunlight-to-heat conversion while minimizing heat dissipation. The use of nanofluids or other advanced heat-transfer fluids can boost thermal conductivity and heat retention during the evaporation process. In addition, ensuring proper thermal insulation for the evaporator setup helps to reduce heat loss to the surrounding environment. By integrating heat recovery systems or phase-change materials into the design, it is possible to capture and recycle the latent heat released during condensation. This further reduces overall heat loss and bolsters the performance of SIE systems^[88-90].

Enhancing vapor generation and transport

Enhanced vapor generation and transport can be achieved through the incorporation of advanced nanostructured materials, which also optimize the light absorption capacity and thermal properties of the system. The resulting improved photothermal conversion efficiency not only maximizes solar energy utilization but also creates a localized high-temperature environment for rapid vapor generation. In order to optimize vapor transport, the use of superhydrophobic and hydrophilic surfaces can be employed to promote effective water vapor release. This helps to minimize vapor entrapment and the potential for backflow^[91-93]. Xia et al. report that the device features a glass microfiber-supported Cu_2-xSe/Nb₂CT_x nanocomposite solar absorber and absorbent cotton as its water transmission pathway^[4]. The unique structure of water evaporation devices, featuring a rough surface and glass microfibers with low thermal conductivity, effectively reduces light reflections and minimizes heat loss to the surrounding environment. This enhances the photothermal conversion efficiency of devices. Moreover, the absorbent cotton with vertically aligned microchannels and intrinsically hydrophilic properties ensures effective water transfer via capillary forces. The solar vapor evaporation device demonstrates a remarkable ability to absorb light, transport water efficiently, and manage heat effectively. As a result, the device maintains a consistently high evaporation rate regardless of the intensity of the light source.

In addition, the study conducted by Feng et al. reveals intriguing discoveries that challenge the conventional understanding of SIE processes [Figure 5]^[94]. It demonstrates that the surface roughness of a solar absorber is a more decisive factor in SIE than its light absorption capacity. This is evident in the behavior of Fe₃O₄ nanoparticles, which, despite exhibiting the lowest light absorptivity, show the highest evaporation rate due to their increased surface roughness. The investigation uncovers a dynamic interplay between capillary forces and interparticle interactions influenced by surface roughness, which results in varied evaporation dynamics. It is worth noting that the study reveals the enhanced effects of a textured surface and a specific arrangement of nanoparticles on the concentration of heat and electromagnetic fields when exposed to light. This results in a boundary with high temperature and accelerated evaporation. The results of this study have significant implications for the development and production of small-scale heatable evaporators or reactors. These advancements enable a diverse array of processes with accurate temperature and kinetic control.

Figure 5. (A) Photographs, (B) SEM micrographs, and (C) contact angle images of the liquid marbles comprised of Fe₃O₄, Ni NPs, and CNTs, respectively, aligned vertically. Pictorial evidence and image analysis of evaporating liquid marbles with (D) Fe₃O₄, (E) Ni, and (F) CNT are also depicted. Tracking the surface temperature evolution is achieved via an infrared (IR) camera for (G) Fe₃O₄-LM, (H) Ni-LM, and (I) CNT-LM. (J) Demonstrates the evaporation rates. (K) Depicts morphology images of LM created through both the imprinting and rolling methods, as well as typical snapshots of a Ni NP-evaporating liquid marble. (L) elucidates the evaporation processes of LMs enveloped by either Fe₃O₄ NPs or CNTs. Reprinted with permission from ref.^[95]. Copyright 2023 Elsevier.

Device configurations

Device configurations play a significant role in minimizing heat loss, maintaining ideal water temperature, and enhancing energy conversion. Various types of floating devices such as solar stills, photothermal materials and advanced energy management systems can be employed to optimize the performance of SIE systems. Specifically, both single-layer and bilayer structures have a notable impact on SIE. Single-layer structures comprise a sole layer of photothermal material that absorbs sunlight, transferring the generated heat to the liquid-vapor interface to promote evaporation. This design offers simplicity, cost-effectiveness and ease of fabrication, making it a popular choice for small-scale applications^[95,96]. However, single-layer structures face several challenges such as low light absorption and limited heat localization which restrict their overall evaporation efficiency. On the other hand, bilayer structures comprise two distinct layers: a light-absorbing layer and a thermal-insulating layer. The light-absorbing layer efficiently captures solar energy and transfers it to the liquid-vapor interface, while the thermal-insulating layer minimizes heat loss to the bulk liquid and the environment^[97-99]. This design results in a more efficient heat transfer, leading to enhanced evaporation rates. The separation of light absorption and heat insulation functions allows for the optimization of each layer, contributing to the overall performance improvement of the SIE system. Despite the increased complexity and cost of manufacturing bilayer structures compared to single-layer ones, their exceptional efficiency and versatility make them a compelling choice for large-scale solar-driven water purification and desalination applications. In addition, there are two innovative methods for utilizing the sun's energy in different applications: solar stills and solar concentrator-based systems. Solar stills are passive devices that operate on the principles of evaporation and condensation to effectively produce purified potable water. They work by trapping solar radiation within an enclosed chamber, where it heats the contaminated water, causing it to evaporate. The water vapor then condenses on the cooler surface of the still and is collected as fresh water^[100,101]. This technology offers a sustainable and cost-effective solution, especially for remote or disaster-stricken regions where access to clean water is limited.

However, systems that utilize solar concentrators are specifically engineered to harness the power of sunlight by directing it towards a concentrated area, thereby maximizing its energy output. Using mirrors or lenses, these systems collect and concentrate sunlight onto a specific target, such as a heat-absorbing material or a photovoltaic (PV) cell^[102,103]. This concentrated solar energy can then be converted into thermal energy or electricity, depending on the application. Solar concentrator systems can be found in large-scale solar power plants and in smaller-scale applications such as solar cookers and solar water heaters.

Fabrication and multilayer integration

Several techniques are employed in the fabrication of SIE devices, each offering distinct advantages. Self-assembly^[104] is a bottom-up approach that facilitates spontaneous organization at the nanoscale, enhancing photothermal properties. Layer-by-layer (L-b-L) deposition^[105] ensures uniformity and allows for the fine-tuning of material properties. Electrospinning^[106] is notable for producing nanofibrous structures, making it ideal for high-surface-area components that enhance evaporation rates. Additionally, 3D printing^[107] stands out for its design flexibility, enabling rapid prototyping of intricate device architectures and the integration of diverse materials. Together, these methods provide precise control over material distribution and alignment within the device, promoting optimal heat management and water transport. Furthermore, integrating multiple functional layers, such as photothermal converters, hydrophilic wicking layers and hydrophobic vapor channels, within a single SIE device ensures efficient energy harvesting and water recovery^[108]. Moreover, a synergistic strategy combining innovative fabrication techniques and precise multilayer integration is required to achieve effective and scalable systems in order to fully utilize the water-energy nexus through SIE. Ongoing research in this field aims to address key challenges, such as long-term stability, cost reduction and scalability, to pave the way for widespread adoption of SIE technology for sustainable water desalination and energy generation.

Emerging trends in materials and structures

In recent studies, researchers have been investigating different materials to enhance the efficiency of SIE. These materials include but not limited to poly(vinyl alcohol) (PVA), cellulose, wood, MOFs, and covalent organic frameworks (COFs). These materials offer unique advantages including adaptability and hydrophilic properties, which enhance SIE efficiency. Research on these various materials is helping to improve the efficiency of converting solar energy into thermal energy for water treatment purposes^[112].

PVA, a low-cost, biocompatible and easy-processable polymer with a high affinity to water molecules, is often used to fabricate solar evaporators in the form of hydrogels^[113]. Various absorbers are embedded into PVA networks to form solar hydrogel evaporators. Recently, Guo et al. utilized polymethyl methacrylate (PMMA) particles as a template to create uniform and size-controlled channels in PVA-based evaporators, resulting in 40% accelerated SIE rates compared to control samples prepared by freeze-drying^[114]. Tu et al. created SIE evaporators using PVA/reduced carbon dots spherical microgels, which were inspired by natural leaves^[115]. These evaporators achieved a vapor generation rate of 1.58 kg m^-2 h^-1 and can potentially reach 2.18 kg m^-2 h^-1 according to their theoretical framework. However, PVA may have limitations in mechanical strength and long-term stability under continuous solar irradiation. Although PVA-based solar evaporators have potential, it is important to prioritize further research in order to enhance their durability and efficiency in different environmental conditions. Cellulose-based materials such as bacterial nanocellulose, plant-derived cellulose, commercial cellulose paper, and cellulose nanofiber (CNF) membranes have also been used to construct SIE systems. These materials can function as hydrophilic substrates, thermal insulation layers, or both functions can be combined in one place simultaneously^[116]. Furthermore, wood materials with vertically aligned channels and hydrophilic walls are ideal platforms. Surface-carbonized balsawood and modified basswood have been used as solar evaporators, displaying faster salt exchange and effective prevention of salt accumulation^[117]. Xie et al. innovatively applied a polypyrrole (PPy) coating to the spikes of a natural plant, Setaria viridis, creating an efficient and sizable evaporation surface^[118]. This approach proved to be both environmentally sustainable and potentially cost-effective for SIE. By adjusting the height and quantity of these coated spikes, the researchers were able to achieve a notable SIE rate of 3.72 kg m^-2 h^-1 under one sun, demonstrating the efficiency of the method in practical conditions^[118]. However, the study also acknowledges certain challenges. The scalability of using natural plant materials such as Setaria viridis on a large scale is a significant concern, especially considering the potential variations in material availability and consistency. Moreover, the long-term durability of these materials in real-world applications remains uncertain. The performance of the PPy-coated spikes might vary under different environmental conditions, such as changes in humidity, temperature, and light intensity, which could influence the consistency and effectiveness of the evaporation process. Jiang et al. incorporated GO flakes into bacterial nano cellulose (BNC) hydrogels, producing bilayer evaporators. BNC not only offers channels for water transportation but also reduces heat dissipation to the water beneath it^[119]. In the ongoing investigation of water evaporation techniques, a recent study presents a novel method for SIE by utilizing a double-layered evaporator [Figure 6]. This structure is made from a combination of bacterial cellulose and vinyltrimethoxysilane, showcasing both superhydrophilic and superhydrophobic properties. Remarkably, this evaporator achieves impressive water evaporation rates of 1.91 and 4.20 kg m^-2 h^-1 under laboratory and outdoor solar conditions (1 sun irradiation). This aerogel-based evaporator is structurally robust, exceptionally lightweight and exhibits excellent salt resistance^[120]. The innovative strategy in the design and construction of materials with tunable properties, relying on a single molecular unit, is promising for future applications in water purification and beyond.

Figure 6. Illustrating the creation and properties of wettability tunable hybrid aerogels (A) Fabrication steps for superhydrophobic VNFs (vinyltrimethoxysilane (VTMS)-based nanofibrous aerogels) and superhydrophilic PNFs (polysiloxane-based nanofibrous aerogels) using VTMS and BC nanofibers. (B) Siloxane's molecular orientation on BC nanofibers. (C) Depiction of the physical linkage between VNFs and PNFs at the evaporator's interface. (D) Procedure for incorporating an evaporator device. (E) PNFs-PPy and water movement in pure PNF representations. (F) Salt transport mechanism for self-cleaning in evaporators. (G) Photographic evidence of salt resistance in the evaporator. (H) Illustration of a VNFs insulator reducing downward heat loss. (I) Thermal images of double-layered evaporator and single-layered PNFs-PPy under sunlight for 60 min. (J) Heat dissipation chart for the evaporator. Reprinted with permission from ref.^[121]. Copyright 2023 Springer Nature.

Li et al. developed a hybrid membrane by encapsulating Cu-BTC nanorods with a polyaniline (PANI) layer, which served as both a protective layer and a solar absorber^[121]. The Cu-BTC/ polyvinylidene fluoride (PVDF) membrane with PANI coating showed enhanced stability in both water and seawater. It also achieved an impressive SIE rate and efficiency of 90.8% under 1 sun. MOFs, with their micropore size, can act as molecular sieves to separate small guest molecules, making them suitable for evaporating clean water from volatile organic compound (VOC)-contaminated sewage. Peng et al. tackled this problem by a composite membrane that involved the growth of a ZIF-8 layer on a PANI-coated polyethersulfone (PES) membrane^[122]. The aperture size of ZIF-8 allows it to effectively filter out the majority of VOCs present in the evaporated steam. Additionally, its water stability has been improved through a partial ligand-exchange reaction. The resulting hybrid membrane achieved a high VOC rejection efficiency and SIE rate under one sun and could even handle high-concentration VOCs. The critical analysis reveals that each methodology, while demonstrating innovative approaches, is accompanied by inherent challenges. The bilayer evaporators developed by Jiang et al., utilizing BNC and GO flakes, exhibit superior water evaporation rates, an attribute crucial for regions grappling with water scarcity^[119]. These evaporators, leveraging solar energy, could significantly impact water purification processes. However, the scalability and cost implications of these materials may limit their application in economically constrained regions, a factor that cannot be overlooked in global implementation strategies. In the realm of treating industrial wastewater, the introduction of the composite membrane by Peng et al. has brought about a notable breakthrough^[122]. This membrane stands out for its ability to selectively reject VOCs, making it a valuable addition to the field. Yet, its complex fabrication process and potential fouling issues raise concerns regarding its practical applicability on a larger scale. From our perspective, the technologies developed by Jiang et al. and Peng et al. hold considerable promise, particularly in the context of industrial wastewater treatment, given their high efficiency and specificity^[119,122]. However, the challenges of cost-effectiveness and scalability must be addressed to realize their full potential. A comparative assessment indicates that while Jiang et al.^[119] offers evaporators potentially more suited for large-scale industrial applications, Peng et al.^[122] demonstrates a membrane with VOC rejection capabilities, likely more beneficial in specialized industrial settings. Additionally, the membrane developed by Li et al., with its enhanced stability in varied water conditions, presents a versatile solution potentially applicable in diverse geographical regions^[121].

COFs are typically embedded in porous matrices such as PVA gels^[123], PDMS sponges^[124], or polytetrafluoroethylene (PTFE) substrates^[125]. These frameworks possess an extended π-conjugated system that enables broad light absorption, although their absorption primarily encompasses the lower wavelength portions of the visible solar spectrum. For enhanced light absorption, monomers can be used that contain specific dye molecules such as 1,4,5,8-tetrakis(phenylamino)anthr-acene-9,10-dione (TPAD), squaraine (SQ), or diketopyrrolopyrrole (DPP). TPAD-COF, SQ-COF, and DPP-COF have all shown impressive light absorption capabilities and efficient energy conversion^[125]. While COFs serve as photothermal materials, they do not perform as effectively as carbon-based materials such as graphene and CNTs. However, their adjustable porous and chemical structures make them intriguing for other functional components in solar vapor generators. By optimizing the COF ratio in dual-region hydrogels (CGHs) that comprise hydrophobic reduced GO (rGO) and hydrophilic COF-loaded rGO regions, a high SIE rate and light-utilizing efficiency can be achieved^[126]. A croconium-based molecule, CR-TPE-T, which exhibits efficient photothermal conversion due to its biradical characteristic, has been prepared. When paired with a polyurethane (PU) substrate, these evaporators achieved an SIE rate of 1.27 kg m^-2 h^-1. In addition, researchers have made advancements in solar evaporators by utilizing aggregation-induced emission (AIE) molecules in 3D fiber aerogels. This has resulted in a significant increase in the rate of solar-induced evaporation, reaching 1.43 kg m^-2 h^-1 with an impressive efficiency of 86.5%. In another approach, phosphomolybdic acid was combined with porphyrins to create 2D dendritic nanosheets with enhanced photothermal capabilities, achieving an SIE rate of 2.23 kg m^-2 h^-1 and 90.9% energy efficiency under one sun^[127,128].

Inspired by the structure of black hair, an arched solar evaporator with hydrophilic photothermal fibers was constructed for desalination application with proficient salt rejection due to its distinct surface properties and the innovative convex device architecture. The surface of black hair is replete with hydrophilic groups, which play an essential role in attracting and retaining water, forming a confined liquid film when water contacts with the hair. Subsequently, capillary forces and the Marangoni effect drive the liquid flow above the cuticle, generating a free-flowing liquid layer. This mechanism enhances the efficiency of water transport, expedites liquid flow, and maintains a constant cycle of water replenishment and circulation. The constant flow of water over the hair surface prevents the accumulation of salt crystals on the photothermal layer, enabling effective salt rejection^[129]. Zhao et al. developed a solar evaporator fabricated from hierarchically nanostructured gels (HNGs) derived from porous carbon, through the hydrothermal carbonization of chitosan^[130]. This structure exhibited significant solar absorption, leading to enhanced evaporation efficiency and reduced heat loss. The structure also facilitated rapid water transport, ensuring a continuous supply of water to the evaporation surface and, producing clean water at a rate of 3.2 kg m^-2 h^-1. Under natural sunlight, the HNG demonstrated the capability to produce fresh water at a daily yield of 18-23 liters per square meter. The integration of croconium-based molecules and HNGs, along with the unique surface properties of black hair-based evaporators, marks a significant breakthrough in the field. This development has led to high SIE rates and effective salt rejection. The creation of sophisticated materials such as croconium-based molecules or HNGs is often complex, requiring precise conditions and methodologies. This complexity can lead to challenges in reproducing these materials consistently, which is a fundamental requirement for practical application. The studies by Wang et al. and Yu et al. serve as exceptional examples of novel photothermal materials in the SIE^[131,132]. Wang et al. have achieved notable advancements in synthesizing higher-metal nitride nanosheets, including Mo5N6, W2N3, Ta3N5, and Nb4N5, using amine-functionalized transition metal dichalcogenide nanosheets as precursors^[131]. Their work highlights the extraordinary capabilities of these materials for steam generation, notably achieving a rapid temperature rise to 80 °C in just about 24 s under simulated sunlight, coupled with an impressive solar evaporation rate of 2.48 kg m^-2 h^-1 and an efficiency of 114.6%. Similarly, Yu et al. have focused their efforts on enhancing SIE for seawater desalination^[132]. They synthesized 2D MoN1.2 nanosheets integrated with rGO to form MoN1.2-rGO heterostructures, effectively lowering vaporization enthalpy. This results in a notable improvement in evaporation rates, reaching 2.6 kg m^-2 h^-1, surpassing the theoretical limit of conventional 2D evaporators and outperforming other transition metal nitride-based evaporators. Both studies are marked by meticulous scientific detail and rigor, underscoring the potential impact of these materials in advancing sustainable technologies. They present significant advancements yet also highlight the need for further research into scalability, long-term stability, and environmental impacts of these materials. Overall, these investigations contribute substantially to the advancement of efficient and sustainable technologies in SIE, marking a notable progression in the field of novel photothermal materials.

Advanced device designs

Advanced device designs for SIE stand out as transformative innovations. They combine the natural power of sunlight with cutting-edge technologies. Li et al. devised a method that involves combining CNTs with CNFs to create tunable nanocomposite inks, which are then applied to ethanol-diffused polystyrene (E-PS) films^[133]. A one-step thermal treatment induces device shrinkage and substrate foaming, creating microscale crumpled textures. These textures enhance water supply and light absorption, while mesoscale pores minimize thermal conduction and radiation. This synergistic design yields a high evaporation rate and efficiency, even with a low loading of photothermal materials. Another novel evaporator made from carbon derived from bamboo leaves was designed using light trace simulation by arranging and carbonizing the bamboo leaves and finally modified with polyacrylamide (PAM). The vertically arranged carbon structure enhances light absorption by extending the light path and increasing the light-absorbing area. The PAM hydrogel is distributed evenly between the vertical carbons, allowing for quick water delivery and reducing the distance for evaporation. This evaporator achieves an ultrahigh light absorption rate of 96.1%, a water evaporation rate of 1.75 kg m^-2 h^-1, and a solar-to-vapor efficiency of 91.9% under one sun irradiation. The device effectively performs seawater desalination, heavy metal ion removal and dye separation during water evaporation and is suitable for outdoor applications and repeated recycling^[134].

Zhao et al. combine carbonized Co-based MOF (ZIF-L) and natural wood aerogel to fabricate an efficient and stable system^[135]. Carbonized ZIF-L serves as the photothermal layer, absorbing solar radiation for photothermal conversion, while the wood aerogel transports seawater to the evaporation interface and ensures the self-floating capabilities of devices. A key feature is its resistance to salt deposition and automatic salt discharge ability, which was achieved through the macroporous channels and pits in the wood aerogel. Sodium alginate is used as a binder to improve the adhesion of the photothermal layer to the aerogel. The evaporator achieved an evaporation rate of 1.52 kg m^-2 h^-1 and an impressive evaporation efficiency of 92.42% under 1 sun illumination, making it suitable for continuous SIE. The novel evaporator design combines a PPy-impregnated nylon thread photothermal layer with an octadecane/carbonized PPy nanotube aerogel composite material. This innovative design achieves a high evaporation rate of 2.62 kg m^-2 h^-1, an excellent solar-to-vapor efficiency of 92.7%, and demonstrating the ability to suitable for all-day evaporation. The evaporator exhibits high salt resistance, maintaining efficiency above 85.3% even in a high-concentration saline solution (20%) and functioning effectively in a 3.5% brine solution for 6 h without performance degradation or salt precipitation^[136]. Natural luffa and PPy was used to design a SIE device, focusing on regulating water content by utilizing hydrophilic hierarchical channels within the luffa. These channels transform excess water from a bulk state to a film state on the porous skeleton, optimizing the performance of devices. The unique fiber arrangement and porous structure of the luffa increase the contact area between water and air, facilitating steam escape and enhancing the evaporation rate. This design emphasizes the importance of hierarchical channels in water regulation for high-efficiency SIE^[137]. A magnet-dominated photothermal device (MPD) with a spiny surface has been developed through the direct assembly of magnetic nanoparticles in the presence of a magnetic field [Figure 7]. The one-step fabrication process eliminates the need for harmful reagents and simplifies the process. The adjustable spiny morphology enhances light absorption via multiple reflections and allows for easy recovery from damage. With high solar absorbance (96.1%) and light-to-heat conversion efficiency (over 94%), the MPD accelerates water evaporation under solar irradiation to over 1.70 kg m^-2 h^-1, approximately 3.5 times higher than the natural evaporation rate^[138]. Using a magnetic field, MPD successfully created a high-efficiency evaporator, showcasing its potential as a promising device for freshwater generation.

Figure 7. (A) Depictions of the magnet in air-laid paper and the MPD with varying Fe₃O₄ nanoparticle densities, including schematics, optical views of Fe₃O₄'s spiny shape, and surface images of M-2.0 and M-3.0 post-water wetting, with a 4mm scale bar. (B) Visual transformation of Fe₃O₄'s spiny structure during wetting. (C) Water contact angles on the Fe₃O₄ layer before and after the application. (D) Optical images of prepared M-3.0 showcasing its spiny structure, the loss from finger pressure, and restoration via vibration. Reprinted with permission from ref.^[139]. Copyright 2021 John Wiley & Sons.

The cutting-edge design of the 3D evaporator includes a solar absorber layer on its top surface, which efficiently converts light into heat. Below the absorber, vertically aligned microtube bundles (MTBs) are strategically placed to connect saline water to the solar absorber. Equipped with hydrophilic microchannels, the MTBs effectively pump saline water through capillary force and facilitate the flow of excessive salt back into the bulk water. This unique structure not only ensures efficient water and salt transport but also recovers conductive heat for water evaporation, resulting in a significant enhancement of overall efficiency. Following the five-day rooftop experiment, the MTB-based evaporation system was subjected to testing in a floating configuration in the Red Sea, which has a salt content of approximately 4.3%. This test aimed to showcase its practical potential for seawater desalination^[139]. The evaporator design utilizes interconnected porous hydrogels (IPHs) synthesized via the sacrificial assembly template (SAT) method, which allows for the production of lightweight, mechanically stable hydrogels with tunable pore size and high interconnectivity. By incorporating activated carbon particles as a solar absorber, IPHs demonstrate excellent solar absorption and good heat localization. The pore size of the hydrogels significantly influences the water transport rate through the matrix, with larger pore sizes resulting in faster transport. The experiments showed that IPHs with different pore sizes had water transport rates that ranged from 0.62 to 621.9 g min^-¹. The optimized IPH attained higher solar-to-vapor conversion efficiency, approximately 90%, with an evaporation rate of 2.8 kg m^-2 h^-1. Additionally, it exhibited outstanding anti-salt-fouling properties and the capability to eliminate common heavy metal ions and organic dyes^[114].

The methodology used by Li et al. demonstrates impressive evaporation efficiency^[133]. However, the complexities associated with the nano-ink formulation and substrate interactions during thermal treatment may influence the reproducibility and scalability of the results. The bamboo leaf-derived carbon evaporator, despite its high light absorption and multifunctional capabilities, confronts issues related to the consistency of the carbon structure post-carbonization, which is crucial for maintaining performance metrics. This also raises questions about the long-term structural integrity under repeated use. The use of carbonized ZIF-L with wood aerogel by Zhao et al. introduces an eco-friendly approach with enhanced salt resistance^[135]. However, the integration of these materials at a nanoscale level poses challenges in achieving uniformity across larger surfaces, a key factor for consistent SIE performance. Moreover, the long-term stability of the wood aerogel in harsh saline environments and its ability to maintain structural integrity while preventing salt accumulation remains an area requiring further exploration. The design of the PPy-impregnated nylon thread photothermal layer provided a longer operation time and enhanced resistance to salt. However, the interfacial stability between the photothermal layer and the aerogel composite, especially under continuous solar irradiation and in high-salinity conditions, is a critical aspect that needs rigorous assessment. This is imperative to ensure sustained efficiency without degradation of photothermal properties or structural breakdown. The luffa and PPy-coated device, while innovative in optimizing water regulation, confronts the challenge of maintaining the hydrophilic-hydrophobic balance within the hierarchical channel structure. This balance is essential for consistent steam generation and escape, and any disparity could significantly affect the evaporation rate and efficiency. In the case of the MPD, the challenge lies in precisely controlling the nanoparticle assembly to achieve the desired spiny morphology. This morphology is critical for light absorption and heat conversion efficiency. Furthermore, understanding the long-term impact of magnetic field exposure on the structural and functional stability of the device is crucial, especially in outdoor environments. Efficient capillary action for continuous water transport is a crucial scientific challenge in the case of the 3D evaporator with MTBs. It is important to manage salt backflow effectively to prevent salt accumulation and maintain efficiency. This requires a delicate balance in the microtube design and the hydrophilic-hydrophobic properties of the materials used. Finally, the IPHs present a challenge in tuning the pore size for optimal water transport without compromising the structural integrity and mechanical stability of the hydrogel. The interplay between pore size, solar absorption, and heat localization directly influences the solar-to-vapor conversion efficiency and anti-salt-fouling properties, necessitating precise material engineering.

Reconfigurable graphene-wrapped Fe₃O₄ (Fe₃O₄@G) nanoparticles, known for their unique magnetic responsiveness are employed to construct a magnetically responsive conic array (CA) assembly within the solar evaporator^[140]. By leveraging the reversible assembly properties of these nanoparticles under the influence of a magnetic field, a dynamic and reconfigurable structure is formed. The strategic combination of macro magnets with CA assemblies leads to the development of a sophisticated hierarchical assembly that significantly enhances the evaporator's performance. This design approach not only improves evaporation rates but also provides excellent salt resistance, water transport and recycling capabilities. The lab-scale prototype uses a solar umbrella to improve solar evaporation efficiency through surface-based heating and radiative heat localization. Positioned above the water surface to achieve a large view factor, the solar umbrella features a commercially available selective solar absorber and a spray-on black paint emitter. This design results in an over 100% enhancement in evaporation rates compared to volumetric heating, and it achieves a 43% solar-thermal efficiency under 1 sun. The system is passive, non-contact and eliminates the risk of fouling and contamination from wastewater streams. Hence, it is ideal for long-term implementation in brine-disposal ponds to achieve zero liquid discharge^[87]. Wang et al. developed a fabrication strategy that involves a L-b-L assembly process to create an MXene/CNTs/Cotton fabric-based SIE system^[141]. The process begins with the preparation of MXene nanosheets and amino-modified CNTs which exhibit negative and positive zeta potentials, respectively. The cotton fabric is alternately dip-coated with MXene nanosheets and CNTs solutions (MC), forming a porous MC film. As the number of L-b-L cycles increases, the composite fabric exhibits a proportional increase in mass per unit area. Under one sun illumination, the evaporation rate achieves 1.35 kg m^-2h^-1 for water and greater than 1.16 kg m^-2h^-1 for textile wastewater. In another study, researchers used tannic acid (TA) and iron (III) to construct a 3D-printed interfacial solar evaporator with a conical array surface structure. This design enhances light harvesting capacity through multiple reflections and anti-reflection effects [Figure 8]. By optimizing the height of the conical arrays, the evaporator achieves an evaporation rate of 1.96 kg m^-2 h^-1 under one sun illumination and photothermal conversion efficiency of 94.4%. The evaporator exhibits excellent desalination performance, recycle stability, anti-salt properties, underwater oil resistance and adsorption capacity for organic dye contaminants, making it suitable for multipurpose water purification applications^[142].

Figure 8. (A) Graphical representation of preparation, structure, and resilience of a 3D-printed TAMA evaporator. (B) Typical models of evaporators and their related 3D-TME specimens at scale bar 5 mm. (C) COMSOL-simulated temperature distribution on 3D-TME surfaces with varying cone heights under standard sunlight. (D) Numerical models show light patterns for 3D-TMEs on flat and tall-cone surfaces. (E) Images show water angle tests on the 3D-printed sample. (F) 3D-TME images with salt, at different intervals, under standard sunlight. (G) Graphs display evaporation rates and energy efficiencies for seawater and 3D-TMEs. Reprinted with permission from ref.^[143]. Copyright 2023 Elsevier.

Zhang et al. successfully addressed the trade-off between thermal localization and salt rejection, which was a prevalent issue in previous wick structure-based evaporators [Figure 9]^[6]. They achieved this by decoupling the processes of solar-thermal conversion, thermal localization and passive water supply, thereby enhancing the efficiency of the system while maintaining optimal performance. The prototype using common materials has a solar-to-vapor conversion efficiency comparable to wick-based evaporators and salt rejection performance similar to contactless evaporators. The study emphasizes the importance of engineering passive fluidic flow and developing a mechanistic model that couples salt transport, fluidic flow and heat transport to guide evaporator design. The macrochannels in the thermal insulation were engineered to trigger natural convection due to the salinity gradient. The convective flow significantly drives salt rejection while inducing negligible additional heat losses. This model-driven, quantitative design approach can serve as general guidelines for solar evaporation devices and potentially improve biofouling resistance. Confined water layers were integrated into passive solar evaporators to enhance desalination reliability and promote zero-liquid discharge in wastewater treatment.

Figure 9. (A) (A-1) Depiction of a wick-based solar evaporator for interfacial evaporation, (A-2) heat localization, salt rejection via wick-free bounded water layer, heat loss minimization through self-floating insulation, and mechanisms for salt rejection using convection in water channels, (A-3) increased heat loss in contactless evaporation with a separated solar absorber and air gap, (A-4) Variance of Peclet numbers with flow condition (L × u) indicating convection or diffusion dominance, Use of confined water layer in normal (A-5) and contactless (A-6) evaporator modes. (B) Confined water layer prototype: (B-1) schematic, (B-2) actual image. (C) Three solar evaporation setups: the first two with basic solar-to-thermal conversion, with the second having a convection shield, and the third being contactless with an air gap. (D) Photo and IR view of the reservoir with the confined water layer. (E) Efficiency assessment across setups and salinities. Reprinted with permission from ref.^[6]. Copyright 2022 Springer Nature.

Similarly, Zhao et al. have reported on a technique for creating distillation membranes with vertically aligned channels and a gradient of hydrophilicity^[143]. This is achieved through the use of engineered defects in COF films. These functional variations enable a selective water transport pathway and precise liquid-vapor phase change interface. The COF membranes demonstrate a flux of 600 L m^-2 h^-1 with 85 °C feed at 16 kPa absolute pressure, nearly triple the performance of current state-of-the-art membranes. The study found that both COF layer thickness and etching time significantly impact desalination performance. The COF membranes effectively remove metal ions and pollutants, to obtain the purified water. Furthermore, the engineered COF membranes prevent salt crystallization and outperform commercial PTFE membranes. The results suggest that these gradient 2D COF membranes are promising candidates for water purification and desalination using waste heat sources.

Ultralong hydroxyapatite nanowires (HNs) and glass fibers (GFs) are combined to create a high-temperature-resistant paper, while black NiO nanoparticles are used as the photothermal material. The hydrophobic layer of the Janus paper is formed by modifying the HNs with sodium oleate, which prevents salt accumulation and enhances the salt-resistant properties of the evaporator. This innovative design not only exhibits high water evaporation efficiency but also demonstrates compatibility with other photothermal materials, making it a versatile and promising solution for solar-driven desalination and water purification applications. This innovative paper demonstrates a remarkable water evaporation efficiency of 83.5% under 1 kWm^-2 irradiation^[144].

The Fe₃O₄@G nanoparticle-based CA assembly offers dynamic, reconfigurable structures with enhanced evaporation and salt resistance. However, a core scientific challenge is ensuring the long-term stability and repeatability of the magnetic responsiveness under varied environmental conditions, which is crucial for the consistent performance of this dynamic system. The innovative design of solar umbrella evaporators markedly boosts evaporation rates through surface-based heating, but it faces the challenge of maintaining the structural and functional integrity of the selective solar absorber and emitter under continuous and intense solar exposure, which could lead to performance degradation over time. In the MXene/CNTs/Cotton fabric-based SIE system, while the L-b-L assembly enhances evaporation rates, the primary scientific hurdle lies in achieving a uniform and stable integration of MXene nanosheets and CNTs onto the cotton fabric. This uniformity is essential for maintaining consistent evaporation performance, and any variability can significantly affect the system efficiency. The conical array surface structure of 3D-printed interfacial solar evaporators improves light harvesting, but optimizing the geometry of these arrays for maximal light absorption and minimal reflection is a delicate and complex task, requiring precise fabrication techniques. The evaporator addressing thermal localization and salt rejection presents a breakthrough in decoupling key processes for enhanced efficiency. However, the scientific intricacy here involves engineering macrochannels in the thermal insulation to optimize salt rejection via natural convection while minimizing additional heat losses. This requires a sophisticated understanding of the interplay between fluid dynamics, heat transfer, and salt transport. The distillation membranes with vertically aligned channels in COF films, though highly effective in desalination, confront the challenge of precisely controlling the COF layer thickness and etching time. These parameters are critical for performance optimization but are challenging to manage consistently, especially in large-scale applications. Furthermore, maintaining the functional integrity of these membranes under varying operational pressures is a significant concern. Lastly, the high-temperature-resistant paper with HNs and GFs addresses the issue of high evaporation efficiency under intense irradiation. However, the main scientific challenge lies in the consistent modification of the HNs with sodium oleate to ensure uniform hydrophobicity, crucial for sustained salt resistance and evaporation performance.

Bio-inspired concepts

Researchers have been drawing inspiration from biological patterns in plants, animals, and microorganisms to develop advanced evaporation materials and structures. By emulating nature's time-tested strategies and designs, bio-inspired concepts offer innovative solutions to optimize sunlight absorption, heat localization and water transportation. These include photothermal nanocomposites, 3D water pathways, and self-healing surfaces. These bio-inspired approaches not only enhance the energy efficiency of solar evaporation but also contribute to minimizing environmental impacts. For instance, biomimetic structures have been leveraged to achieve effective light-to-heat conversion and thermal management, which, in turn, minimizes energy waste. Additionally, these bio-inspired solutions can serve as blueprints for fabricating scalable and low-cost evaporation systems.

Shi et al. introduced novel design strategies for an all-day fresh water harvesting device by combining fog collection and interfacial solar steam generation through engineering 3D micro-topologies on the surface of PVA/PPy hydrogel membranes [Figure 10]^[145]. By incorporating 3D micro-tree arrays with branched micro-cones, this system efficiently captures fog, transports droplets in a specific direction, and ensures quick drainage. It utilizes bioinspired design principles to achieve these functions. This setup achieves a fog collection rate that is 115% higher than the commercially used Raschel mesh and 61% higher than a cactus stem, emphasizing the potential of hydrogel 3D printing for environmental applications. The authors found that surface microstructures improved solar steam generation by providing a large surface area for thermal conversion and water evaporation, with conical structures being promising candidates due to increased light absorption area and reduced vapor flow resistance. The findings indicate potential strategies for enhancing interfacial solar steam systems by manipulating surface structures. This approach can be applied to other materials, particularly recently developed hydrogels with strong hydratable and light-absorbing properties. The gel membrane prototype, featuring micro-tree arrays that are 4 mm tall, 0.8 mm in diameter, and separated by 1.2 mm, displayed an impressive solar water evaporation rate of 3.64 kg m^-2 h^-1 under 1 sun. Furthermore, durability studies conducted in a lab setting revealed the remarkable stability of membranes over a period of 20 months. In a rooftop prototype, the tested bi-functional microstructured hydrogel membranes were able to produce approximately 185 mL of fresh water over the course of eight days. The daily water collection efficiencies were around ~34 L m^-2.

Figure 10. (A) PVA/PPy hydrogel membrane with micro-topologies and its schematic illustration. (B) Represents the fabricated PVA-PPy gel micro-tree array. (C) 24/7 water collection diagram of the floating device. (D) SEM images of tree topology. (E) Simulated patterns and temperature gradients for different micro-topologies and photographs of the fog collection process on a gel tree (a red circle indicates the formation of the droplet while a red arrow indicates the motion of the droplet), (F) Consistent fog collection rates of different membranes. Reprinted with permission from ref.^[146]. Copyright 2021 Springer Nature.

Following that, inspired by natural plant transpiration, a study designed a wooden cone evaporator by successively loading tannins and iron ions on the delignified wood (DW) surface [Figure 11]. Alkaline sulfite treatment of wood removes partial lignin and hemicellulose, enhancing flexibility and hydrophilicity. The unique hierarchical porous structure of wood enables swift water transportation and exceptional heat insulation. The 3D cone evaporator showcased an impressive evaporation efficiency of 1.79 kg m^-2 h^-1 when exposed to direct sunlight, surpassing the performance of conventional 2D evaporators. Furthermore, the DW-TA-Fe³⁺ device exhibited mildew resistance, making it practical and cost-effective. As a result of this feature, the device can maintain its satisfactory biostability even when submerged in water for 30 days^[146].

Figure 11. (A) Illustration of creating a photothermal conversion wood veneer. (B) A three-dimensional model of a wood cone evaporator. (C) Depictions featuring: (C-1) and (C-4) Visual representations of wood and DW. (C-2) and (C-3) SEM images showcasing wood and DW in a tangential section. (C-5) and (C-6) SEM visuals of DW and wood cross-sectioned. (D) Contact angle images of the (D-1) wood, (D-2) DW and (D-3) DE-TA-Fe³⁺. (E) Design of the device at variable underwater depths. (F) Performance analysis of the DW-TA-Fe³⁺ over multiple cycles. (G) Field test for water evaporation. Reprinted with permission from ref.^[147]. Copyright 2023 Springer Nature.

However, a few important questions need to be addressed, including the durability of the technology over extended periods of time and in different environmental conditions, the possibility of implementing it on a larger scale for desalination plants, the potential environmental effects of using chemically treated wood, and the adaptability of the technology with regards to different types of wood or other natural materials. Addressing these issues in future research could help to further improve the technology and better understand its potential for real-world applications. Furthermore, new designs are being employed in various fields. Examples include the use of the lotus leaf effect for self-cleaning and the concept of vascular networks for efficient water transport. Such creative approaches are proving to be crucial in enhancing the performance of evaporation devices. They also ensure their robustness in various environmental conditions^[147]. In essence, the confluence of these novel materials and innovative device designs is ushering in a new era for SIE technology. This amalgamation of advancements not only holds the promise to significantly enhance the efficiency and functionality of current solar evaporation systems but also represents a significant step towards a more sustainable and water-secure future. The combined efforts of material scientists, engineers, and innovators are contributing to an exciting journey that could fundamentally change how we utilize solar energy in the context of water treatment and management.

Dang et al. introduced a salt-resistant evaporator (SRE) designed using computational fluid dynamics simulations to optimize its geometry for higher hydraulic conductivity and diffusion flux, thus enabling robust convection [Figure 12]^[148]. This configuration employs well-aligned multiscale channels, facilitates quick brine transfer and enables real-time salt excretion, ensuring continuous regeneration of the evaporator. Notably, the use of carbonized rattan, a sustainable biomass material, is a significant point due to its natural composition of regular multiscale channels, making it an excellent choice for SRE construction. The study claims impressive results with the rattan-based evaporator demonstrating a high evaporation rate of 1.47 kg m^-2 h^-1 and efficiency of 91% in a highly concentrated brine (20 wt% NaCl) under 1 sun. Importantly, it maintains performance consistency over continuous weekly operations, indicating long-term system stability. This opens the door for large-scale application of solar desalination for high salinity brine. The approach by Shi et al., utilizing 3D micro-tree arrays on PVA/PPy hydrogel membranes, grapples with the intricate balance between fog capture efficiency and solar steam generation^[145]. One of the main challenges is to carefully design the micro-topologies of the hydrogel to ensure efficient water flow and ideal thermal properties for evaporation. Additionally, it is important to address the issue of maintaining the structural integrity of the hydrogel during the hydration and dehydration cycles it undergoes. The wooden cone evaporator inspired by plant transpiration, which employs DW enhanced with tannins and iron ions, confronts the challenge of maintaining the chemical and structural stability of the wood. This involves ensuring that the treatment does not compromise the porosity and hydrophilicity of wood, essential for its evaporation efficiency. In the study by Dang et al., SRE encounters the primary scientific hurdle of sustaining the hydraulic conductivity and diffusion flux in the multiscale channels^[148]. Ensuring consistent performance in high salinity conditions is of utmost importance to prevent channel clogging or compromise due to salt accumulation. Furthermore, incorporating carbonized rattan presents the task of balancing the inherent variations in its channel structure with the requirement for consistent performance throughout the evaporator. These specific scientific challenges underline the complexity of optimizing material properties and structural design to enhance the functionality and efficiency of these advanced water harvesting systems.

Figure 12. (A) Process of creating orderly, multi-level porous carbon-based photothermal material. (B) Schematic of the daytime evaporation and volatilization outpace convection and diffusion. Ionic species of salt experience enrichment and accumulation. (C) Night-time convection and diffusion exceed natural volatilization, diluting and expelling salt ions. (D) (D-1) Picture of natural rattan and inset depicts a reed diffuser, (D-2) carbon-treated rattan, (D-3) and (D-4) SEM micrographs of the top section of natural and carbonized rattan, respectively, (D-5) SEM micrograph of the side section of the carbonized rattan, (D-6) SEM micrograph of the wall of the channel C1 and (D-7) Close-up view of SEM micrograph of carbon-treated rattan. (E) Salt expulsion experiment photos using carbonized rattan. (F) Salt expulsion depicted through vertical (G). The SRE device represents the temperature distribution after 10 min of irradiation. (H) Real-time optical image of the SRE integrated device in outdoor environment. (I) Salt expulsion experiment photos using carbonized cotton column. (J) Salt expulsion depicted through a tortuous path. Reprinted with permission from ref.^[149]. Copyright 2023 Elsevier.

Hybrid systems

A hybrid system combines two or more technologies to create a more efficient and versatile setup. The advantages of these systems include greater energy efficiency, increased evaporation rates, simultaneous electricity generation and superior use of solar power which makes them a key player in advancing sustainable solutions.

PV-thermal (PV-T) systems, which ingeniously combine PV and solar thermal technologies, have emerged as a promising solution for simultaneous electricity and heat generation^[149]. This hybrid approach enables efficient utilization of solar energy while optimizing overall system performance. Simultaneously, the combination of solar-driven desalination and power generation has proven to be a successful solution for tackling water scarcity and energy challenges^[150-152]. Therefore, Duan et al. developed an innovative hybrid energy system (HES) using CNT-based prototypes^[153]. The HES effectively addresses the issue of discontinuity and instability in renewable energy generation. It features a freshwater production device that can generate freshwater at a rate of 0.84 kg m^-2h^-1, with excellent purification and desalination capabilities. Additionally, they created a solar charging system with a maximum output of 5 V, suitable for charging mobile devices and power banks during sunny conditions. Zhou et al. designed a hydrogel-based system for simultaneous water purification, electricity and hydrogen production^[154]. The researchers have utilized a unique 3D pillared PAM/CNT hydrogel with optimal hydrophobic modifications to facilitate a precise balance between water absorption and evaporation. This method leads to an impressive photothermal efficiency of 96% at the liquid/vapor interface. The holistic design of systems showcases the harmonious generation of purified water and renewable energy under natural conditions, employing thermoelectric generators as thermal insulators. These are sandwiched by PAM-based evaporators and PAM/CNT-based solar absorbers, ensuring efficient cooling and heating. The system can produce a consistent electrical output power of 4.8 W/m², which can power water splitting to generate hydrogen at a rate of 0.3 mmol/h. The studies conducted by Duan et al. and Zhou et al. have identified distinct advantages and disadvantages^[153,154]. Duan et al. introduce an innovative HES that integrates freshwater production with solar energy harvesting and includes a solar charging system, enhancing its utility^[153]. However, this system faces challenges in efficiently managing energy distribution between its dual functions and requires further investigation into its performance across varying solar intensities. Zhou et al. present a hydrogel-based system that achieves high photothermal efficiency, effectively balancing water absorption and evaporation, which is crucial for water purification and energy production^[154]. One drawback of this system is the requirement for meticulous optimization of its PAM/CNT hydrogel composition to ensure efficiency. Furthermore, incorporating thermoelectric generators adds further complexity to the overall design. When examining evaporation dynamics, comparing the rates of evaporation under different levels of sunlight can offer valuable insights. "0 Sun" evaporation, occurring without the influence of direct sunlight, relies on ambient heat and air movement and is crucial for understanding ecological and meteorological phenomena in low-light conditions. Conversely, "1 Sun" evaporation, which occurs under standard solar conditions, serves as a fundamental benchmark in scientific research, allowing for the evaluation of evaporation processes under average sunlight exposure. These contrasting conditions help in comprehensively understanding the intricacies of the water cycle and its varying responses to different environmental factors. Dao et al. developed an innovative nanogenerator inspired by the Limnobium laevigatum plant^[155]. The Limnobium laevigatum-inspired nanogenerator (LLN) is a device that incorporates a special combination of multi-walled CNTs (MWNTs) coated on cellulose paper, which is then placed on top of Polystyrene foam. Engineered to float on water, the LLN exhibits exceptional capabilities in both electricity generation and water evaporation. Under 1 sun conditions, it generates 248.57 mW/m², while in 0 sun conditions, its output is 107.38 mW/m². Additionally, it achieves a water evaporation rate of 1.48 kg/m²/h in sunlight and 0.58 kg/m²/h in the dark. The study also finds optimal performance with a 50 μm thick MWNT layer in a 0.6 M NaCl solution, indicative of its potential efficiency in seawater-like environments. In another study, Dao et al. investigate a nature-inspired hierarchical evaporator using MWNTs, focusing on its water evaporation and electricity generation capabilities^[156]. The evaporator showcases a notable water evaporation rate of 1.364 kg/m²/h under 1 sun conditions and 0.450 kg/m²/h in the absence of sunlight (0 sun). In terms of electricity generation, the device achieves power densities of 176 μW/m² under 1 sun illumination and 94 μW/m² in dark conditions when utilizing Zn wire electrodes. With gold nanowire electrodes, the power densities are 14.50 nW/m² under 1 sun and 6.48 nW/m² under 0 sun. This study highlights the importance of electrode materials and their placement in enhancing the efficiency of these nanogenerators. The studies by Dao et al. on the LLN and the hierarchical evaporator using MWNTs demonstrate commendable advancements in solar energy and water evaporation technologies^[155,156]. The high energy output and evaporation rate of LLN under optimal sunlight conditions are noteworthy. However, it presents challenges in ensuring the stability of the MWNT coating, a critical factor for sustained functionality. The adaptability of a hierarchical evaporator to different lighting conditions is a significant advantage, yet its performance is constrained by the dependency on specific electrode materials, highlighting an area for further material optimization. These insights reflect the complexities and potential areas for refinement in developing efficient and versatile energy-harvesting systems.