Nanoconfined Water-Molecule Channels for High-Yield Solar Vapor Generation under Weaker Sunlight

Recent innovations in 3D solar evaporators and their functionalities

Interfacial solar evaporation (ISE) has emerged as a promising technology to alleviate global water scarcity via energy-efficient purification of both wastewater and seawater. While ISE was originally identified and developed during studies of simple double-layered two-dimensional (2D) evaporators, observed limitations in evaporation rate and functionality soon led to the development of three-dimensional (3D) evaporators, which is now recognized as one of the most pivotal milestones in the research field. 3D evaporators significantly enhance the evaporation rates beyond the theoretical limits of 2D evaporators. Furthermore, 3D evaporators could have multifaceted functionalities originating from various functional evaporation surfaces and 3D structures. This review summarizes recent advances in 3D evaporators, focusing on rational design, fabrication and energy nexus of 3D evaporators, and the derivative functions for improving solar evaporation performance and exploring novel applications. Future research prospects are also proposed based on the in-depth understanding of the fundamental aspects of 3D evaporators and the requirements for practical applications.

Numerical Simulation Technologies in Solar-Driven Interfacial Evaporation Processes

Solar interfacial evaporation technology has the advantages of environmentally conscious and sustainable benefits. Recent research on light absorption, water transportation, and thermal management has improved the evaporation performance of solar interfacial evaporators. However, many studies on photothermal materials and structures only aim to improve performance, neglecting explanations for heat and mass transfer coupling or providing evidence for performance enhancement. Numerical simulation can simulate the diffusion paths and heat and water transfer processes to understand the thermal and mass transfer mechanism, thereby better achieving the design of efficient solar interfacial evaporators. Therefore, this review summarizes the latest exciting findings and tremendous advances in numerical simulation for solar interfacial evaporation. First, it presents a macroscopic summary of the application of simulation in temperature distribution, salt concentration distribution, and vapor flux distribution during evaporation. Second, the utilization of simulation in the microscopic is summed up, specifically focusing on the movement of water molecules and the mechanisms of light responses during evaporation. Finally, all simulation methods have the goal of validating the physical processes in solar interfacial evaporation. It is hoped that the use of numerical simulation can provide theoretical guidance and technical support for the application of solar-driven interfacial evaporation technology.

Solar-driven water evaporation using a collaborative photothermal conversion material system based on carbonized waste polyphenylene sulfide and copper sulfide

Recently, water resources have become scarce due to the growing global population and human impact on the environment, coupled with the effects of climate change. For solving the problem of global freshwater shortage and increasing the value of discarded polyphenylene sulfide (PPS) filter bags, in this study, balsa wood was used as the base of a photothermal solar evaporator, chitosan solution was used as the binder, and the main photothermal conversion materials used were polyphenylene sulfide (CP) carbide and copper sulfide. In order to create synergistic photothermal conversion materials, freeze-drying and in situ precipitation were used to deposit the photothermal conversion materials on top of the balsa wood. The prepared CP/CuS-wood evaporator has excellent water evaporation performance and light conversion capability, with a water evaporation rate of 2.68 kg m-2 h-1 and a photothermal conversion efficiency of 93.2% under simulated one solar intensity irradiation. In addition, the evaporator can effectively remove organic dyes such as methylene blue and methyl orange. The evaporator's durability and seawater desalination capability have also been confirmed through seawater desalination experiments and outdoor tests. Studies have shown that solar interface photothermal evaporators are a viable solution for desalination and wastewater treatment. This eco-friendly, economically viable and stable photothermal evaporator mentioned in this paper has pioneering features and will be a new paradigm for desalination and wastewater treatment.

Reorientation of Hydrogen Bonds Renders Unusual Enhancement in Thermal Transport of Water in Nanoconfined Environments

Liquid confined in a nanochannel or nanotube has exhibited a superfast transport phenomenon, providing an ideal heat and mass transfer platform to meet the increasingly stringent challenge of thermal management in developing high-power-density nanoelectronics and nanochips. However, understanding the thermal transport of confined liquid is currently lacking and is speculated to be fundamentally different from that of bulk counterparts due to the unprecedented thermodynamics of liquid in nanoconfined environments. Here, we report that the thermal conductivity of water confined in a silica nanotube is nearly 2-fold as that of bulk status. Further molecular dynamics simulations reveal that this unusual enhancement originates from the densification and reorientation of local hydrogen bonds close to the nanotubes. Thermal-confinement scaling law is established and quantitatively supported by comprehensive simulations with remarkable agreement. Our findings lay a theoretical foundation for designing nanofluidics-enabled cooling strategies and devices.

Magnetically driven Janus conical vertical array for all-weather freshwater collection

The engineering of multifunctional structures with special surface wettability is highly desirable for all-weather freshwater production, but relevant research is scarce. In this study, a Janus conical vertical array was designed and fabricated via a magnetically driven spray-coating method for the first time. Benefiting from the special structure and wettability enhancement of the array in terms of solar absorption, fog capture and merging, droplet movement and evaporation area, all-weather freshwater production consisting of high-quality daytime solar vapor generation (water evaporation rate approximately 2.43 kg m-2 h-1, 1 kW m-2) and nighttime fog collection (water collection rate approximately 3.536 g cm-2 h-1) can be realized concurrently. When the designed array is employed for outdoor environments (114°35'E, 30°38'N, average daily temperature 34.9 °C, average daily humidity 64.0%), reliable and efficient daily pure water yields of 19.13 kg m-2-26.09 kg m-2 are obtainable. We believe that the proposed strategy for fabricating a Janus conical vertical array is novel in the integration of solar vapor generation and fog collection, which has great significance for all-weather freshwater production.

Superhydrophobic sand evaporator with core-shell structure for long-term salt-resistant solar desalination

Solar-driven water evaporation, as an environmentally benign pathway, provides an opportunity for alleviating global clean water scarcity. However, the rapidly generated interfacial steam and localized heating could cause increased salt concentration and accumulation, deteriorating the evaporation performance and long-term stability. Herein, a novel superhydrophobic sand solar (FPPSD) evaporator with a core-shell structure was proposed through interface functionalization for continuous photothermal desalination. The collective behavior essence of the sand aggregate gave itself micron-scale self-organized pores and configurable shapes, generating desirable capillary force and supplying effective water-pumping channels. More importantly, combining the dopamine, polypyrrole (PPy), and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) through π-π conjugation and multiple hydrogen bonding effects gave the FPPSD evaporator with stable superhydrophobic property and highly efficient photothermal conversion capability. Therefore, the FPPSD evaporator showed a continuous and stable photothermal performance even after 96 h continuous evaporation under 3-sun irradiation for 10 wt% saline solution, among the best values in the reported works of literature, demonstrating its excellent salt-resistance stability. Furthermore, this novel FPPSD evaporator displayed outstanding environmental stability that kept its initial water transport capacity even after being treated under harsh conditions for 30 days. With excellent salt-resistance ability and stable environmental stability, the FPPSD evaporator will provide an attractive platform for sustainable solar-driven water management.

Interfacial Solar Evaporation: From Fundamental Research to Applications

In the last decade, interfacial solar steam generation (ISSG), powered by natural sunlight garnered significant attention due to its great potential for low-cost and environmentally friendly clean water production in alignment with the global decarbonization efforts. This review aims to share the knowledge and engage with a broader readership about the current progress of ISSG technology and the facing challenges to promote further advancements toward practical applications. The first part of this review assesses the current strategies for enhancing the energy efficiency of ISSG systems, including optimizing light absorption, reducing energy losses, harvesting additional energy, and lowering evaporation enthalpy. Subsequently, the current challenges faced by ISSG technologies, notably salt accumulation and bio-fouling issues in practical applications, are elucidated and contemporary methods are discussed to overcome these challenges. In the end, potential applications of ISSG, ranging from initial seawater desalination and industrial wastewater purification to power generation, sterilization, soil remediation, and innovative concept of solar sea farm, are introduced, highlighting the promising potential of ISSG technology in contributing to sustainable and environmentally conscious practices. Based on the review and in-depth understanding of these aspects, the future research focuses are proposed to address potential issues in both fundamental research and practical applications.

Boosting the Viable Water Harvesting in Solar Vapor Generation: From Interfacial Engineering to Devices Design

Continuously increasing demand for the life-critical water resource induces severe global water shortages. It is imperative to advance effective, economic, and environmentally sustainable strategies to augment clean water supply. The present work reviews recent reports on the interfacial engineering to devices design of solar vapor generation (SVG) system for boosting the viability of drinkable water harvesting. Particular emphasis is placed on the basic principles associated with the interfacial engineering of solar evaporators capable of efficient solar-to-thermal conversion and resulting freshwater vapor via eliminating pollutants from quality-impaired water sources. The critical configurations manufacturing of the devices for fast condensation is then highlighted to harvest potable liquid water. Fundamental and practical challenges, along with prospects for the targeted materials architecture and devices modifications of SVG system are also outlined, aiming to provide future directions and inspiring critical research efforts in this emerging and exciting field.

MXene nanosheets coated conjugated microporous polymers hollow microspheres incorporating with phase change material for continuous desalination

The inevitable intermittency of solar illumination during the interfacial evaporation process can cause a reduction in the evaporation performance of solar evaporators. Here, we report the fabrication of a new solar-driven interfacial evaporator using MXene nanosheets as the photothermal layer, modifying them with conjugated microporous polymer hollow microspheres, and then compounding them with the phase change material, in this case, cetyl alcohol, to form a composite evaporator (CE) that can perform all-weather solar interfacial evaporation. By combining interfacial evaporation photothermal conversion with energy storage, the evaporator achieves an evaporation rate of 1.57 kg⋅m-2⋅h-1 at a light intensity of 1 kW⋅m-2 and 2.79 kg⋅m-2⋅h-1 at a light intensity of 2 kW⋅m-2. In addition, the evaporator attains an excellent solar evaporation efficiency of over 91% in both cases and even in salt water. In addition, interestingly, our CE exhibits excellent continuous evaporation ability, e.g., the mass of evaporated water was increased by 0.36 kg⋅m-2 at a light intensity of 2 kW⋅m-2 compared to the cavity evaporator without the phase change material (PCM) when solar light was turned off. These results could be attributed to the fact that the energy released by the incorporated phase change material allows the evaporator to maintain stable evaporation under conditions of insufficient or intermittent solar irradiation, potentially providing a new opportunity for addressing the intermittent problem of evaporation at the solar interface due to unstable light intensity, thus showing great potential for practical continuous desalination.

Conductive Metal-Organic Framework Nanosheets Constructed Hierarchical Water Transport Biological Channel for High-Performance Interfacial Seawater Evaporation

Solar interfacial water evaporation shows great potential to address the global freshwater scarcity. Water evaporation being inherently energy intensive, Joule-heating assisted solar evaporation for addressing insufficient vapor under natural conditions is an ideal strategy. However, the simultaneous optimization of low evaporation enthalpy, high photothermal conversion, and excellent Joule-heating steam generation within a single material remain a rare achievement. Herein, inspired by the biological channel structures, a large-area film with hierarchical macro/microporous structures is elaborately designed by stacking the nanosheet of a conductive metal-organic framework (MOF), Ni3 (HITP)2 , on a paper substrate. By combining the above three features in one material, the water evaporation enthalpy reduces from 2455 J g-1 to 1676 J g-1 , and the photothermal conversion efficiency increases from 13.75% to 96.25%. Benefiting from the synergistic photothermal and Joule-heating effects, the evaporation rate achieves 2.60 kg m-2 h-1 under one sun plus input electrical power of 4 W, surpassing the thermodynamic limit and marking the highest reported value in MOF-based evaporators. Moreover, Ni3 (HITP)2 -paper exhibits excellent long-term stability in simulated seawater, where no salt crystallization and evaporation rate degradation are observed. This design strategy for nanosheet films with hierarchical macro/microporous channels provides inspiration for electronics, biological devices, and energy applications.

Macro-porous structured aerogel with enhanced ab/desorption kinetics for sorption-based atmospheric water harvesting

Sorption-based atmospheric water harvesting (SAWH) has been proven to be a promising method to alleviate the impact of the water crisis on human activities. However, the low water-sorption capacity and sluggish ab/desorption kinetics of current SAWH materials make it difficult to achieve high daily water production. In this study, a photothermal porous sodium alginate-tannic acid-5/Fe3+@lithium chloride aerogel (SA-TA-5/Fe3+@LiCl) with macroporous structure (average pore diameter ∼43.67 μm) and high solar absorbance (∼98.4 %) was fabricated via Fe3+-induced crosslinking and blackening methods. When it is employed for SAWH, moisture can enter the inner space of the aerogel and contact highly hygroscopic lithium chloride (LiCl) more easily via macroporous channels, resulting in the water uptake for the SA-TA-5/Fe3+@LiCl aerogel reaching approximately 1.229 g g-1 under dry conditions (relative humidity (RH) ∼ 45 %, 25 °C) after a short time (4 h) moisture absorption, and releasing as much as 97.7 % of the absorbed water under 1 sun irradiation within 2 h. As a proof of concept, it is estimated that the daily water yield of the fabricated SA-TA/Fe3+@LiCl aerogel can reach approximately 4.65 kg kg-1 in conditions close to the real outdoor environment (RH ∼ 45 %, 25 °C), which satisfies the daily minimum water consumption of two adults. This study demonstrates a novel strategy for developing advanced solar-driven SAWH materials with enhanced ab/desorption kinetics and efficient water sorption-desorption properties.

Water Activation in Solar-Powered Vapor Generation

Solar-powered vapor evaporation (SVG), based on the liquid-gas phase conversion concept using solar energy, has been given close attention as a promising technology to address the global water shortage. At molecular level, water molecules escaping from liquid water should overcome the attraction of the molecules on the liquid surface layer to evaporate. For this reason, it is better to reduce the energy required for evaporation by breaking a smaller number of hydrogen bonds or forming weak hydrogen bonds to ensure efficient and convenient vapor production. Many novel evaporator materials and effective water activation strategies have been proposed to stimulate rapid steam production and surpass the theoretical thermal limit. However, an in-depth understanding of the phase/enthalpy change process of water evaporation is unclear. In this review, a summary of theoretical analyses of vaporization enthalpy, general calculations, and characterization methods is provided. Various water activation mechanisms are also outlined to reduce evaporation enthalpy in evaporators. Moreover, unsolved issues associated with water activation are critically discussed to provide a direction for future research. Meanwhile, significant pioneering developments made in SVG are highlighted, hoping to provide a relatively entire chain for more scholars who are just stepping into this field.

Advances in photothermal regulation strategies: from efficient solar heating to daytime passive cooling

Photothermal regulation concerning solar harvesting and repelling has recently attracted significant interest due to the fast-growing research focus in the areas of solar heating for evaporation, photocatalysis, motion, and electricity generation, as well as passive cooling for cooling textiles and smart buildings. The parallel development of photothermal regulation strategies through both material and system designs has further improved the overall solar utilization efficiency for heating/cooling. In this review, we will review the latest progress in photothermal regulation, including solar heating and passive cooling, and their manipulating strategies. The underlying mechanisms and criteria of highly efficient photothermal regulation in terms of optical absorption/reflection, thermal conversion, transfer, and emission properties corresponding to the extensive catalog of nanostructured materials are discussed. The rational material and structural designs with spectral selectivity for improving the photothermal regulation performance are then highlighted. We finally present the recent significant developments of applications of photothermal regulation in clean energy and environmental areas and give a brief perspective on the current challenges and future development of controlled solar energy utilization.

A Nanoconfined Water-Ion Coordination Network for Flexible Energy-Dissipation Devices

Water-ion interaction in a nanoconfined environment that deeply constrains spatial freedoms of local atomistic motion with unconventional coupling mechanisms beyond that in a free, bulk state is essential to spark designs of a broad spectrum of nanofluidic devices with unique properties and functionalities. Here, it is reported that the interaction between ions and water molecules in a hydrophobic nanopore forms a coordination network with an interaction density that is nearly fourfold that of the bulk counterpart. Such strong interaction facilitates the connectivity of the water-ion network and is uncovered by corroborating the formation of ion clusters and the reduction of particle dynamics. A liquid-nanopore energy-dissipation system is designed and demonstrated in both molecular simulations and experiments that the formed coordination network controls the outflow of confined electrolytes along with a pressure reduction, capable of providing flexible protection for personnel and devices and instrumentations against external mechanical impact and attack.

Micro-Nano Water Film Enabled High-Performance Interfacial Solar Evaporation

Interfacial solar evaporation holds great promise to address the freshwater shortage. However, most interfacial solar evaporators are always filled with water throughout the evaporation process, thus bringing unavoidable heat loss. Herein, we propose a novel interfacial evaporation structure based on the micro-nano water film, which demonstrates significantly improved evaporation performance, as experimentally verified by polypyrrole- and polydopamine-coated polydimethylsiloxane sponge. The 2D evaporator based on the as-prepared sponge realizes an enhanced evaporation rate of 2.18 kg m-2 h-1 under 1 sun by fine-tuning the interfacial micro-nano water film. Then, a homemade device with an enhanced condensation function is engineered for outdoor clean water production. Throughout a continuous test for 40 days, this device demonstrates a high water production rate (WPR) of 15.9-19.4 kg kW-1 h-1 m-2. Based on the outdoor outcomes, we further establish a multi-objective model to assess the global WPR. It is predicted that a 1 m2 device can produce at most 7.8 kg of clean water per day, which could meet the daily drinking water needs of 3 people. Finally, this technology could greatly alleviate the current water and energy crisis through further large-scale applications.

Borophene Embedded Cellulose Paper for Enhanced Photothermal Water Evaporation and Prompt Bacterial Killing

Solar-driven photothermal water evaporation is considered an elegant and sustainable technology for freshwater production. The existing systems, however, often suffer from poor stability and biofouling issues, which severely hamper their prospects in practical applications. Conventionally, photothermal materials are deposited on the membrane supports via vacuum-assisted filtration or dip-coating methods. Nevertheless, the weak inherent material-membrane interactions frequently lead to poor durability, and the photothermal material layer can be easily peeled off from the hosting substrates or partially dissolved when immersed in water. In the present article, the discovery of the incorporation of borophene into cellulose nanofibers (CNF), enabling excellent environmental stability with a high light-to-heat conversion efficiency of 91.5% and water evaporation rate of 1.45 kg m^-2 h^-1 under simulated sunlight is reported. It is also demonstrated that borophene papers can be employed as an excellent active photothermal material for eliminating almost 100% of both gram-positive and gram-negative bacteria within 20 min under three sun irradiations. The result opens a new direction for the design of borophene-based papers with unique photothermal properties which can be used for the effective treatment of a wide range of wastewaters.

Transparent ultrathin SiO2 nanowire aerogel displaying novel properties when interacting with water: A promising versatile functional platform

With low density, high porosity, and outstanding physicochemical stability, ceramic nanowire aerogels and sponges exhibit various interesting properties. Herein, an ultrathin silica nanowire aerogel (SiO2 NWs-A) was achieved via a facile chemical vapor deposition route. In addition to good mechanical and thermal performances, properties resulting from active water-aerogel interactions are revealed, i.e., outstanding transparency, strong capillary effect, enhanced compressive strength (a reversible strain of ∼62%), switchable wettability and robust shape retention ability when filled with water. The physical mechanism related to these interesting properties is demonstrated basically according to its unique features (distinctly reduced nanowire diameter, enriched nanoscopic gap channels, and reinforced network). To demonstrate the superiority, an advantageous solar vapor generation system (hydrophilic NWs-A/reduced graphene oxide (rGO)/ hydrophobic NWs-A) was obtained by integrating these favorable characteristics, giving rise to remarkably promoted vapor evaporation rate and energy efficiency compared to the rGO hydrophobic NWs-A device. These results contribute to the structural design and functional exploration of nanowire aerogels.

Low-Dimensional Nanomaterial Systems Formed by IVA Group Elements Allow Energy Conversion Materials to Flourish

In response to the exhaustion of traditional energy, green and efficient energy conversion has attracted growing attention. The IVA group elements, especially carbon, are widely distributed and stable in the earth’s crust, and have received a lot of attention from scientists. The low-dimensional structures composed of IVA group elements have special energy band structure and electrical properties, which allow them to show more excellent performance in the fields of energy conversion. In recent years, the diversification of synthesis and optimization of properties of IVA group elements low-dimensional nanomaterials (IVA-LD) contributed to the flourishing development of related fields. This paper reviews the properties and synthesis methods of IVA-LD for energy conversion devices, as well as their current applications in major fields such as ion battery, moisture electricity generation, and solar-driven evaporation. Finally, the prospects and challenges faced by the IVA-LD in the field of energy conversion are discussed.

High-Porosity Lamellar Films Prepared by a Multistage Assembly Strategy for Efficient Photothermal Water Evaporation and Power Generation

The frame structure combined with water- and heat-transfer capabilities fully satisfies the requirements of photothermal conversion materials in seawater evaporation applications. Meanwhile, it must integrate the characteristics of a high photothermal conversion rate, thermal management, and water transportation. Herein, lamellar porous films were successfully designed and synthesized by a simple ultrasonic-assisted vacuum filtration method. In this process, polystyrene sulfonate@carbon nanotubes/reduced graphene oxide (PSS@CNT/rGO) lamellar films were constructed by the one-dimensional synthesis of PSS@CNT self-assembled at the molecular scale and the two-dimensional matrix material rGO. It is worth noting that the lamellar film exhibits a high specific surface area (285.5 m²·g^-1), which is reflected in its abundant nanopores. Among them, the porous network system composed of nanochannels can provide efficient water supply and steam-transfer ability and strengthen the heat insulation performance of thermal localization, which is beneficial to photothermal evaporation. The obtained PSS@CNT/rGO lamellar films achieved a condensed water yield of 1.825 kg·m^-2·h^-1 under 1 sun illumination (1 kW·m^-2), and their solar-vapor conversion efficiency was 97.1%. Simultaneously, the interaction between the water flow and the carbon material interface was also used to generate additional electric energy output. The maximum open-circuit voltage of 0.46 V was generated at both termini of the PSS@CNT/rGO lamellar film, which successfully realized the multieffect utilization of energy. These results show that the multistage assembly strategy is a facile and effective means for the development of an efficient evaporation photothermal film, which offers significant value in the field of photothermal seawater evaporation and power generation.

Ultrafast Solar-Vapor Harvesting Based on a Hierarchical Porous Hydrogel with Wettability Contrast and Tailored Water States

Optimizing the water bonding network in an evaporator is significant for efficient solar-driven vapor generation (SVG). Herein, we report a facile one-pot method to regulate the hydrated structure and wettability in a hierarchical porous hydrogel. An ovalbumin (OVA)-polyacrylamide hydrogel foam was fabricated in a cake-making fashion. Because of the enrichment of amphiphilic OVA at the interface, the hydrophobic walls of the air pores in the foam provide vaporization sites and help reduce parasitic heat loss, while the hydrophilic skeleton with the secondary pores effectively pumps capillary water. Notably, the proportion of intermediate water in the foam reaches 87.6% with the melting point as low as -10 °C. All these features contribute to an exceptional evaporation rate of 3.4-4.5 kg m^-2 h^-1 under 1 sun and robust SVG performances at high-humidity, weak sunlight, or cold weathers. The strategy of using amphiphilic molecules to optimize the hydrated structures both at the interface and in bulk promises the reasonable design of SVG materials with superior efficiency and weather adaptability.

Hierarchically Structured Black Gold Film with Ultrahigh Porosity for Solar Steam Generation

Plasmonic metals demonstrate significant potential for solar steam generation (SSG) because of their localized surface plasmon resonance effect. However, the inherently narrow absorption spectra of plasmonic metals significantly limit their applications. The fabrication of nanostructures is essential to achieve broadband solar absorption for high-efficiency vapor generation. Herein, a self-supporting black gold (Au) film with an ultrahigh porosity and a hierarchically porous structure is fabricated by formulating an extremely dilute Cu₉₉ Au₁ precursor and controlling the dealloying process. In situ and ex situ characterizations reveal the dealloying mechanism of Cu₉₉ Au₁ in a 1 m HNO₃ solution as that involving the phase transformation of Cu(Au) → Au(Cu) → Au, giant volume shrinkage (≈87%), structural evolution/coarsening of ligaments, and development of ultrahigh porosity (86.2%). The multiscale structure, consisting of ultrafine nanoporous nanowires, aligned nanogaps, and various microgaps, provide efficient broadband absorption over 300-2500 nm, excellent hydrophilicity, and continuous water transport. In particular, the nanoporous black Au film shows high SSG performance with an evaporation rate of 1.51 kg m^-2 h^-1 and a photothermal conversion efficiency of 94.5% under a light intensity of 1 kW m^-2 . These findings demonstrate that the nanoporous Au film has great potential for clean water production and seawater desalination.

Janus Fibrous Mats Based Suspended Type Evaporator for Salt Resistant Solar Desalination and Salt Recovery

Solar desalination has been recognized as an emerging strategy for solving the pressing global freshwater crisis. However, salt crystallization at the photothermal interface frequently causes evaporator failure. In addition, arbitrary discharge of concentrated brine produced during desalination results in potential ecological impacts as well as wastage of valuable minerals. In the present work, a suspended-type evaporator (STEs) constructed using Janus fibrous mats is reported. The fibrous structure wicks brine to the evaporation layer and the salt gets confined in the evaporation layer until crystallization for zero liquid discharge due to the suspended design. Enhanced evaporation is observed because STEs have an additional low-resistance vapor escape path directly from the evaporation layer to the atmosphere compared to traditional floating Janus evaporators. Moreover, owing to the drastically different wettability on both sides, the evaporator allows salt crystallization only on the hydrophilic bottom layer, thus eliminating salt accumulation at the hydrophobic photothermal interface. With this unique structural design, the proposed evaporator not only maintains a high evaporation rate of 1.94 kg m^-2 h^-1 , but also demonstrates zero liquid discharged salt resistance and ideal recovery of the mineral in brine.

A Covalent Organic Framework/Graphene Dual-Region Hydrogel for Enhanced Solar-Driven Water Generation

Solar-driven water generation is a sustainable water treatment technology, helping to relieve global water scarcity issues. However, this technology faces great challenges due to the high energy consumption of water evaporation yielding low evaporation rates. Here, a covalent organic framework (COF)/graphene dual-region hydrogel, containing hydrophilic and hydrophobic regions in one material, is developed through a facile in situ growth strategy. The hydrophilic COF is covering parts of the hydrophobic graphene regions. Through accurate control of both wetting regions, the hybrid hydrogel shows effective light-harvesting, tunable wettability, optimized water content, and lowered energy demand for water vaporization. Acting as solar absorber, the dual-region hydrogel exhibits a steam generation rate as high as 3.69 kg m^-2 h^-1 under 1 sun irradiation (1 kW m^-2), which competes well with other state-of-the-art materials. Furthermore, this hydrogel evaporator can be used to produce drinkable water from seawater and sewage, demonstrating the potential for water treatment.

Interfacial Solar Steam/Vapor Generation for Heating and Cooling

Interfacial solar steam/vapor technology uses abundant and clean solar energy and water to achieve heating and cooling, a promising technology to alleviate environmental and energy issues. To obtain higher conversion and utilization efficiency, designing and optimizing materials, structures, and devices of interfacial solar steam/vapor technologies attract the attention of the research community. Given the significant progress made in the past 5 years, it is valuable to systematically summarize and discuss recent developments and future trends in this new multidisciplinary direction. This review aims to introduce interfacial solar steam/vapor principles to realize heating and cooling and the recent progress in materials, structures, devices, and applications. Meanwhile, some unsolved scientific and technical problems with outlook will also be discussed, hoping to promote further the rapid development and application of interfacial solar steam/vapor technology in heating and cooling to alleviate energy and environmental problems.

Poly(N-phenylglycine)/MoS2 Nanohybrid with Synergistic Solar-Thermal Conversion for Efficient Water Purification and Thermoelectric Power Generation

Solar interfacial evaporation is an emerging technology in solar energy harvesting developed to remedy the global energy crisis and the lack of freshwater resources. However, developing fully enhanced thermal management to optimize solar-heat utilization efficiency and form remains a great challenge. We created a synergistic photothermal layer from a poly(N-phenylglycine) (PNPG)/MoS₂ nanohybrid via electrostatic-induced self-assembly for a broad-spectrum and efficient solar absorption. The PNPG/MoS₂ system provided effective synergistic photothermal conversion and good water transmission, enabling rapid solar steam escape. Notably, synergistic coupling of solar evaporation-thermoelectric (TE) power generation was also achieved, providing more efficient exploitation of solar heat. The system demonstrated a solar evaporation rate of up to 1.70 kg m^-2 h^-1 and achieved a maximum thermoelectric output power with 0.23 W m^-2 under one sun. The high-performance PNPG/MoS₂ synergistic photothermal system developed in this study offers potential opportunities for coupling solar water purification with thermoelectric power generation to meet the needs of resource-scarce areas.

Melamine/Silicone Hybrid Sponges with Controllable Microstructure and Wettability for Efficient Solar-Driven Interfacial Desalination

Solar-driven interfacial evaporation (SIE) has received extensive attention as a very promising desalination technique to solve the fresh water shortage crisis. However, evaporation rate decline and salt-fouling during long-term SIE seriously hinder applications of solar evaporators. Here, we report the preparation of melamine/silicone (MS) hybrid sponges with controllable microstructure and wettability for efficient SIE by further combination with carbon nanotubes (CNTs). The MS sponges are synthesized by hydrolytic condensation and phase separation of two silanes in the melamine sponge. The microstructure and wettability of the MS sponges are highly controllable by the silanes concentration. The CNTs@MS solar evaporators have a unique three-tier hierarchical macro-/micro-/nanostructure, very low thermal conductivity as well as a superhydrophilic hull and a superhydrophobic nucleus. Consequently, the CNTs@MS solar evaporators show a highly stable evaporation rate of ∼1.75 kg m^-2 h^-1 without any salt precipitation during a long-term cyclic solar desalination of 3.5 wt % NaCl solution under 1 sun illumination. Furthermore, salt precipitation is completely hindered even during SIE of 20 wt % NaCl solution under 1 sun. The CNTs@MS solar evaporators are very promising for practical SIE because of their excellent performance and simple preparation method.

Recent Progress of Sub-Nanometric Materials in Photothermal Energy Conversion

Sub-nanometric materials (SNMs) are an attractive scope in recent years due to their atomic-level size and unique properties. Among various performances of SNMs, photothermal energy conversion is one of the most important ones because it can efficiently utilize the light energy. Herein, the SNMs with photothermal energy conversion behaviors and their applications are reviewed. First, a hydrothermal/solvothermal method for the synthesis of SNMs is systematically discussed, including the LaMer pathway and the cluster-nuclei coassembly pathway. Based on this synthetic strategy, many kinds of SNMs with different morphologies are successfully prepared, such as nanorings, nanowires, nanosheets, and nanobelts. These SNMs exhibit excellent photothermal performance under the laser or solar irradiation according to their different light absorption ranges. These enhanced absorption performances of SNMs are induced by the mechanism of plasmonic localized heating or nonradiative relaxation. Finally, the applications of the photothermal SNMs are illustrated. The SNMs with photothermal behaviors can be widely applied in the fields of solar vapor generation, biomedicine, and light-responsive composites construction. It is hoped that this review can provide new viewpoints and profound understanding to the SNMs in photothermal energy conversion.

Biomimetic Hybridization of Janus-like Graphene Oxide into Hierarchical Porous Hydrogels for Improved Mechanical Properties and Efficient Solar Desalination Devices

Light-absorbing hydrogels provide a means for rapidly evaporating water by using solar energy. However, to achieve light-absorbing hydrogels with both durable mechanical properties and efficient energy utilization remains challenging due to the weak interface interactions between solar absorbers and a hydrogel matrix and difficultly controlled surface topography of swollen hydrogel-based evaporators. Herein, we demonstrate an effective nanoconfinement strategy to assemble a spongy poly(vinyl alcohol)/Janus-like graphene oxide hybrid hydrogel (SPJH) via strong interfacial interactions of hydrogen bonding and hydrophobic interaction. The resultant SPJHs with an intriguing hierarchical microstructure templated by air bubbles and ice crystals showed a high toughness (∼231 kJ m^-2) and ultimate strain (∼310%) that were more than three times as high as those of light-absorbing hydrogels and a high evaporation rate of 4.18 kg m^-2 h^-1 with an efficiency up to 95% under 1 sun irradiation (relative humidity = 20%; temperature = 25 °C), achieved by synergistic mechanical and energy nanoconfinement and tailored surface topography within the designed hybrid hydrogels. This hybrid hydrogel-based solar evaporator with an ingenious design principle provides a pathway for scalable and processable solar water purification devices.

Effect of light intensity on solar-driven interfacial steam generation

Solar-driven interfacial steam generation (SISG) has attracted much attention in recent years as a solution to freshwater scarcity and the energy crisis. Currently, research interests are mainly focused on standard conditions under "1-sun" illumination, which we believe are insufficient on their own. Gaining insight and understanding about SISG under both weak and strong irradiation have important implications for real-world use that are rarely presented in relevant discussions. In this review, we aim to discuss SISG under weak (<1 sun) and strong solar irradiation (>1 sun), both of which are often undervalued but necessary for real application. By analyzing state-of-the-art techniques and recent research progress, we provide some possible strategies, in terms of both energy and water management, for improving the performance of SISG under different irradiation powers. Finally, we also give a summary and our perspectives on the directions that the future development of this exciting field might take.

Highly Elastic Interconnected Porous Hydrogels through Self-Assembled Templating for Solar Water Purification

Interfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready-to-use hydrogel materials with durable mechanical properties. Here, self-assembled templating (SAT) is developed as a simple yet effective method to fabricate large-scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m^-2  h^-1 and ≈90 % solar-to-vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze-drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.

High-performance solar-driven interfacial evaporation through molecular design of antibacterial, biomass-derived hydrogels

Hydrogel has been regarded as one of the most promising candidates for next-generation solar evaporation technology to produce freshwater from non-potable water. However, synthesizing hydrogel absorbers that can precisely regulate water state and significantly reduce the water vaporization enthalpy remains a grand challenge. Herein, we report the rational design of a novel hydrogel hybrid solar evaporator constructed by poly(vinyl alcohol) and sodium lignosulfonate (SLS), with addition of carbon nanotube as a light absorption material. The abundant sulfonate and hydroxyl groups of SLS enhance the interplay between hydrogel and water molecule through electrostatic interaction and hydrogen bond. As such, the presence of SLS not only remarkably promotes the hydrophilicity and water transport of hydrogel, but also precisely tunes the state of water molecule and the content of intermediate water for reducing the water vaporization enthalpy. The combined advantageous features endow the as-prepared hydrogel with an evaporation rate up to 2.09 kg m^-2 h^-1 under 1 Sun illumination, along with good anti-acid/basic abilities, antibacterial property, high salt-tolerance, and self-cleaning capability in purifying different types of wastewater. Finally, an outdoor solar seawater desalination device is designed to generate drinking water from seawater. The daily drinking water production amount per square meter is ca. 13 kg, which satifies the five adults' daily water consumption (12.5 kg). The present study highlights that rationally constructing the molecular architecture of hydrogel and tuning the interplay between water and hydrogel are effective strategies to fabricate advanced hydrogel solar evaporators for addressing the global freshwater shortage.

Modified Hollow Glass Microspheres/Reduced Graphene Oxide Composite Aerogels with Low Thermal Conductivity for Highly Efficient Solar Steam Generation

Solar steam generation (SSG) as a pollution-free and sustainable way for desalination or wastewater treatment has attracted great attention in recent years. Herein, we report the fabrication of novel aerogels GAHAS and GAHAF composed of 3-aminopropyltriethoxysilane (KH550)-modified hollow glass microspheres (HGM) and reduced graphene oxide (RGO) by a sol-gel method for highly efficient SSG. The RGO can well wrap on modified HGM and form an interpenetrated porous structure with an excellent mechanical property. In addition, benefiting from the hollow structure of HGM, GAHAS obtained by supercritical CO₂ drying well maintains the original structure of the hydrogel and shows low thermal conductivity (0.0823 W m^-1 K^-1) in the wet state and self-floating ability. Combined with its superhydrophilic wettability and high light absorption (ca. 93%), the as-prepared GAHAS shows an outstanding photothermal conversion efficiency of 89.13% under 1 sun (1 kW m^-2) illumination and excellent stability. Moreover, from the simulated seawater outdoor solar desalination experiment, it was found that the concentrations of the four primary ions K⁺, Ca²⁺, Na⁺, and Mg²⁺ in purified water are 1.65, 0.09, 1.42, and 0.32 mg L^-1, respectively, and fully meet drinking water standards. Thus, our GAHAS aerogel shows great potential for practical application in SSG. This work enriches the photothermal materials and may provide a new idea for design and creation of HGM-based photothermal materials with low thermal conductivity, tunable porosity, high mechanical strength, self-floating ability, and high solar energy conversion efficiency for SSG.

Unidirectionally Driving Nanofluidic Transportation via an Asymmetric Textile Pump for Simultaneous Salt-Resistant Solar Desalination and Drenching-Induced Power Generation

Solar-driven seawater desalination provides a promising technology for sustainable water energy harvesting. Although tremendous efforts have been dedicated to developing efficient evaporators, the challenge of preventing salt accumulation while simultaneously realizing high-performance steam-electricity cogeneration remains to be addressed. In this work, inspired by the water and solute transportation in plants via the wicking mechanism, a one-way asymmetric nanofluidic photothermal evaporator fabricated by disproportionately depositing photothermal MXene nanosheets on a hydrophilic cotton textile is reported for simultaneous freshwater and power production. By unidirectionally driving dynamic saline transportation via this photothermal cotton textile pump, this evaporator not only enables self-operating salt rejection for stable steam generation but also affords continuous electric power generation induced by the formation of an asymmetric double electrode layer within MXene nanochannels under the drenching state. Specifically, the solar-driven evaporation rate and voltage generation reach 1.38 kg/m²/h (with a conversion efficiency of 83.1%) and 363 mV under 1 sun irradiation, respectively. Notably, this designed nanofluidic system suffers negligible performance depreciation after 30 h of operation and washing 15 times, which indicates its outstanding stability and reusability. This facile design of the asymmetric nanofluidic photothermal system may provide prospective opportunities for scaling up sustainable freshwater and electric power production.

Multi-Synergistic Removal of Low-Boiling-Point Contaminants with Efficient Carbon Aerogel-Based Solar Purifier

Solar steam generation is considered as an efficient way for addressing water shortage issues via seawater desalination and wastewater purification. In a solar evaporator, an absorber would convert optical energy to heat for evaporating nearby water. In this process, many low-boiling-point contaminants can also be evaporated along with water steam, which compromises the effectiveness of purification. There is, so far, no study on the removal of such low-boiling-point contaminants such as organic pesticides in wastewater. To address this problem, we demonstrate a versatile carbon hybrid aerogel (CHA) as a solar powered water purification platform. With an elaborate absorber design, the maximum solar evaporation rate of 2.1 kg m^-2 h^-1 is achieved under 1 sun illumination. More importantly, CHA can effectively suppress the evaporation of low-boiling-point contaminants including common pesticides and mercury ion via its strong adsorption and retention effect. Synergetic steaming and the adsorption of CHA will inspire more paradigms of solar steam generation technologies for applications relevant to detoxification and water remediation.

Surface Patterning of Two-Dimensional Nanostructure-Embedded Photothermal Hydrogels for High-Yield Solar Steam Generation

Improving evaporation rate is extremely important to promote the application of solar steam generation in clean water production through seawater desalination. However, the theoretical evaporation rate limit of a normal two-dimensional (2D) photothermal evaporator is only about 1.46 kg m^-2 h^-1. While 3D evaporators can break the limit, they require much more raw materials. In this work, an effective approach for achieving high-yield solar steam generation via the synergy of 2D nanostructure-embedded all-in-one hybrid hydrogel evaporator and surface patterning is reported. This improved surface-patterned evaporator is able to simultaneously lower the enthalpy of vaporization and induce the Marangoni effect near the evaporation surface, thus delivering a high evaporation rate of 3.62 kg m^-2 h^-1, which is more than twice the theoretical limit of the normal 2D photothermal evaporator. This hybrid hydrogel offers a cost-effective and energy-efficient pathway to mitigate clean water shortages.

Multifunctional Membrane for Thermal Management Applications

Space cooling and heating consume a large proportion of global energy, so passive thermal management materials (i.e., without energy input), especially dual-mode materials including cooling and heating bifunctions, are becoming more and more attractive in many areas. Herein, a function-switchable Janus membrane between cooling and heating consisting of a multilayer structure of polyvinylidene fluoride nanofiber/zinc oxide nanosheet/carbon nanotube/Ag nanowire/polydimethylsiloxane was fabricated for comprehensive thermal management applications. In the cooling mode, the high thermal radiation emissivity (89.2%) and sunlight reflectivity (90.6%) of the Janus membrane resulted in huge temperature drops of 8.2-12.6, 9.0-14.0, and 10.9 °C for a substrate, a closed space, and a semiclosed space, respectively. When switching to the heating mode, temperature rises of 3.8-4.6, 4.0-4.8, and 12.5 °C for the substrate, closed space, and semiclosed space, respectively, were achieved owing to the high thermal radiation reflectivity (89.5%) and sunlight absorptivity (74.1%) of the membrane. Besides, the Janus membrane has outstanding comprehensive properties of the membrane, including infrared camouflaging/disguising, electromagnetic shielding (53.1 dB), solvent tolerance, waterproof properties, and high flexibility, which endow the membrane with promising application prospects.

Topology-Controlled Hydration of Polymer Network in Hydrogels for Solar-Driven Wastewater Treatment

Solar-driven interfacial evaporation provides a promising method for sustainable freshwater production. However, high energy consumption of vapor generation fundamentally restricts practicality of solar-driven wastewater treatment. Here a facile strategy is reported to control the hydration of polymer network in hydrogels, where densely cross-linked polymers serving as a framework are functionalized by a highly hydratable polymeric network. The hydration of polymer chains generates a large amount of weakly bounded water molecules, facilitating the water evaporation. As a result, the hydrogel-based solar evaporator can extract water from a variety of contaminants such as salts, detergents, and heavy metal components using solar energy with long-term durability and stability. This work demonstrates an effective way to tune the interaction between water and materials at a molecular level, as well as an energy-efficient water treatment technology toward wastewater containing complex contaminants.