Leveraging Hydrophilic Hierarchical Channels to Regulate Excessive Water for High-Efficiency Solar Steam Yield

High-performance, salt-resistant, and stable wood-based solar evaporator equipped with sodium alginate-based functional skin

Harnessing renewable solar energy for photothermal evaporation and seawater desalination holds significant potential for alleviating freshwater scarcity. However, evaporators reported recently often suffer from limitations in environmental compatibility, salt resistance, and stability. Herein, we report a green and straightforward approach to fabricate a dual-layer solar evaporator (STF-wood) by equipping wood-based materials with a sodium alginate-based photothermal skin. The functional skin, synthesized through the chelation of tannic acid (TA), sodium alginate (SA), and Fe3+, not only exhibits excellent photothermal conversion efficiency but also forms a dense network that restricts the movement of free water, thereby enhancing the localized heat effect. Moreover, the positively charged surface of the skin can prevent the passage of salt ions through the Donnan effect, demonstrating superior salt resistance. STF-wood is capable of operating stably in seawater with an evaporation rate of 1.92 kg m-2 h-1 and maintains a similar rate even after undergoing 5-hour ultrasonic treatment. This work presents a sustainable and efficient solution to compensate for the shortcomings of environmental compatibility and operational stability for solar evaporator.

Distribution-according-to-work: Enhancing solar vapor generation of photothermal sponge by using cellulose-based water storage platform

Interfacial solar vapor generation (ISVG) has shown extraordinary promise in achieving high-efficiency water purification. However, the rapid water supply often leads to excessive water in the solar absorber, resulting in undesired heat loss and a decrease in evaporation rate. To tackle this issue, we developed a bio-based solar evaporator comprising cellulose-based water retention resin (CRR) and straw-derived photothermal sponge. CRR serves as an effective water storage platform with a high binding capacity for water molecules, preventing water from entering the absorber and reducing the water evaporation enthalpy. The water management of CRR confines the solar-to-vapor conversion to the interface between CRR and the photothermal sponge, thereby eliminating the adverse effects of excess water. Additionally, the ISVG process operates based on the principle of Distribution-according-to-work, meaning that the quantity of generated vapor depends on the evolution of the sponge structure. Optimal sponge configuration enables evaporation rates of 2.28 and 1.53 kg/m2/h under solar irradiation of 1.0 and 0.5 kW/m2, respectively. Additionally, the obtained evaporator is capable of producing 7.1 kg/m2/day of freshwater in outdoor experiment. This report proposes a novel approach to designing an ISVG device that incorporates effective water management strategy for achieving high-efficiency water purification in real-world scenarios.

Simultaneous Solar-Thermal Desalination and Catalytic Degradation of Wastewater Containing Both Salt Ions and Organic Contaminants

Although solar steam generation is promising in generating clean water by desalinating seawater, it is powerless to totally degrade organic contaminants in the seawater. Herein, solar steam generation and catalytic degradation are integrated to generate clean water by simultaneous solar-driven desalination and catalytic degradation of wastewater containing both salt ions and organic contaminants. Stepwise decoration of three-dimensional nickel foam with polypyrrole, reduced graphene oxide (RGO), and cobalt phosphate is realized to obtain polypyrrole/RGO/cobalt phosphate/nickel foam (PGCN) hybrids for solar-driven desalination and catalytic degradation of wastewater containing antibiotics and salt ions. The oxygen-containing groups of the RGO integrated with the porous nickel foam make the porous PGCN hybrid hydrophilic and ensure the upward transport of water to the evaporation surface, and the oxygen vacancies of the cobalt phosphate allow the PGCN to generate abundant highly active singlet oxygen that could still exhibit excellent catalytic degradation performances in the high salinity and highly alkaline environment of seawater. In addition to the high solar light absorbance and satisfactory solar-thermal conversion efficiency of polypyrrole and RGO, the thermally conductive nickel foam skeleton can effectively transfer the heat generated by the solar-thermal energy conversion to the adjacent cobalt phosphate catalyst and nearby wastewater, achieving a solar-thermal-promoted catalytic degradation of organic contaminants. Therefore, a high pure water evaporation rate of 2.08 kg m-2 h-1 under 1 sun irradiation and 100% catalytic degradation of Norfloxacin and dyes are achieved. The PGCN hybrid is highly efficient in purifying seawater containing 10 ppm Norfloxacin and simultaneously achieves a high purification efficiency of 100 kg m-2 h-1.

Hydrothermally Modified 3D Porous Loofah Sponges with MoS2 Sheets and Carbon Particles for Efficient Solar Steam Generation and Seawater Desalination

Although the emerging interfacial solar steam generation technology is sustainable and eco-friendly for generating clean water by desalinating seawater and purifying wastewaters, salt deposition on the evaporation surface during solar-driven evaporation severely degrades the purification performances and adversely affect the long-term performance stability of solar steam generation devices. Herein, to construct solar steam generators for efficient solar steam generation and seawater desalination, three-dimensional (3D) natural loofah sponges with both macropores of the sponge and microchannels of the loofah fibers are hydrothermally decorated with molybdenum disulfide (MoS₂) sheets and carbon particles. Benefiting from fast upward transport of water, rapid steam extraction, and effective salt-resistant capacity, the 3D hydrothermally decorated loofah sponge with MoS₂ sheets and carbon particles (HLMC) with an exposed height of 4 cm can not only obtain heat by its top surface under the downward solar light irradiation based on the solar-thermal energy conversion but also gain environmental energy by its porous sidewall surface, achieving a competitive water evaporation rate of 3.45 kg m^-2 h^-1 under 1 sun irradiation. Additionally, the 3D HLMC evaporator exhibits long-term desalination stability during the solar-driven desalination of an aqueous salt solution with 3.5 wt % NaCl for 120 h without apparent salt deposition because of its dual type of pores and uneven structure distribution.

Designing a solar interfacial evaporator based on tree structures for great coordination of water transport and salt rejection

Solar interfacial evaporation has been receiving increasing attention but it is still a huge challenge to achieve excellent coordination between efficient water transport and salt rejection. Here, unlike the common wood-inspired evaporators with equal-diameter directional pores, we have constructed an integrated structure with highly connected gradient pores that mimic the xylem vessels and phloem sieve tubes found in trees. The bio-inspired structure can reduce the resistance of water transport and salt rejection in the same channel. The average transport speed of the 6.5 cm high (2 cm in diameter) porous structure reached 1.504 g s^-1, and water was transported 16 cm after 100 seconds. Using multilayer graphene oxide as the photothermal conversion material, the evaporators with different heights can work for more than 9 hours under the condition of 1 sun illumination and 23 wt% brine without any salt crystallization, and the evaporation rates range from 3.28 to 4.51 kg m^-2 h^-1, with the highest energy utilization efficiency of about 80%. When used in heavy metal treatment, the rejection was greater than 99.99%. This research provides a simple but innovative design idea for evaporators and is expected to further expand the application of solar interfacial evaporation.

Photothermal-Photocatalytic CSG@ZFG Evaporator for Synergistic Salt Rejection and VOC Removal during Solar-Driven Water Distillation

Sunlight-driven interfacial photothermal evaporation has been considered as a promising strategy for addressing global water crisis. Herein, we fabricated a self-floating porous triple-layer (CSG@ZFG) evaporator using porous fibrous carbon derived from Saccharum spontaneum (CS) as a photothermal material. The middle layer of the evaporator is composed of hydrophilic sodium alginate crosslinked by carboxymethyl cellulose and zinc ferrite (ZFG), whereas the top hydrophobic layer consists of fibrous (CS) integrated benzaldehyde-modified chitosan gel (CSG). Water is transported to the middle layer through the bottom elastic polyethylene foam using natural jute fiber. Such a strategically designed three-layered evaporator exhibits a broad-band light absorbance (96%), excellent hydrophobicity (120.5°), a high evaporation rate of 1.56 kg m^-2 h^-1, an energy efficiency of 86%, and outstanding salt mitigation ability under the simulated sunlight of intensity 1 sun. Adding ZnFe₂O₄ nanoparticle as a photocatalyst has been proved to be capable of restricting the evaporation of volatile organic contaminants (VOCs) like phenol, 4-nitrophenol, and nitrobenzene to ensure the purity of evaporated water. Such an innovatively designed evaporator offers a promising approach for the production of drinking water from wastewater and seawater.

Tuning the Functionalization of Graphite for Hydrovoltaic Power Generation

The recent discovery of hydrovoltaic devices for power generation has led to a rapid growth into new materials for harvesting energy specifically for this research field. Of the materials investigated, carbon materials have dominated, and graphene oxide (GO) has emerged as the leader. While graphite is conductive, it does not have functional groups to strongly interact with water, and highly functionalized GO forms strong interaction with water to generate necessary surface charges but does not typically have high conductivity. Herein, we report the fabrication and functionalization of a graphite-based structure, controlling the extent of oxidation to balance the effects of conductivity and functionalization to achieve high power outputs in hydrovoltaics. Devices prepared using the functionalized graphite achieve a power output of 53.3 μW/g. High power output and good film stability are key advances toward the practical application of hydrovoltaic devices for renewable energy.

Conjugated photothermal materials and structure design for solar steam generation

With the development of solar steam generation (SSG) for clean water production, conjugated photothermal materials (PTMs) have attracted significant interest because of their advantages over metallic and inorganic PTMs in terms of high light absorption, designable molecular structures, flexible morphology, and solution processability. We review here the recent progress in solar steam generation devices based on conjugated organic materials. Conjugated organic materials are processed into fibers, membranes, and porous structures. Therefore, nanostructure design based on the concept of nanoarchitectonics is crucial to achieve high SSG efficiency. We discuss the considerations for designing SSG absorbers and describe commonly used conjugated organic materials and structural designs.

Photothermal Aerogel Beads Based on Polysaccharides: Controlled Fabrication and Hybrid Applications in Solar-Powered Interfacial Evaporation, Water Remediation, and Soil Enrichment

Solar-powered interfacial evaporation has emerged as an innovative and sustainable technology for clean water production. However, the rapid, mass and shape-controlled fabrication of three-dimensional (3D) steam generators (SGs) for versatile hybrid applications remains challenging. Herein, composite aerogel beads with self-contained properties (i.e., hydrophilic, porous, photothermal, and durable) are developed and demonstrated for threefold hybrid applications including efficient solar-powered interfacial evaporation, water remediation, and controlled soil enrichment. The rational incorporation of selected polysaccharides enables us to fabricate bead-like aerogels with rapid gelation, continuous processing, and enhanced ion adsorption. The composite beads can attain a high water evaporation rate of 1.62 kg m-2 h-1 under 1 sun. Meanwhile, high phosphate adsorption capacity of over 120 mg g-1 is achieved in broad pH (2.5-12.4) and concentration (200-1000 mg L-1) ranges of phosphate solutions. Gratifyingly, we demonstrate the first example of recycling biomaterials from interfacial SGs for controlled nutrient release, soil enrichment, and sustainable agriculture. The phosphate-saturated beads can be gradually broken down in the soil. Macronutrients (N, P, and K) can be slowly released in 50 days, sustaining the plant germination and growth in a whole growth stage. This work shines light on the mass and controlled fabrication of aerogel beads based on double-network biopolymers, not merely scaling up solar-powered interfacial evaporation but also considering water remediation, waste material disposal, and value-added conversion.

Highly Effective Multifunctional Solar Evaporator with Scaffolding Structured Carbonized Wood and Biohydrogel

A solar evaporator that utilizes solar radiation energy can be a renewable approach to deal with energy crisis and fresh water shortage. In this study, a solar evaporator was prepared by assembling composite carbonized wood of Melaleuca Leucadendron L. and biobased hydrogel. The multilayer MXene (Ti₃C₂T_x) was embedded in the scaffolding structure of the wood to form composite carbonized wood, where the loose and ordered scaffolding structure of the carbonized wood significantly improves the efficiency of water transportation with increased capillary force. The MXene adsorbed in the carbonized wood has high binding energy with water molecules, leading to reduction of vaporization enthalpy and contact angle. Moreover, the addition of MXene can improve the light absorbance, especially for the infrared and ultraviolet light bands. The hydrogel was fabricated by crosslinking konjac glucomannan and sodium alginate polysaccharides with Ca²⁺, and it has a lower thermal conductivity than water and improves the evaporation efficiency by regulating the temperature distribution and concentrating the heat on the surface of the evaporator. This solar evaporator has an evaporation rate of 3.71 kg·m^-2·h^-1 and an evaporation efficiency of 129.64% under 2 sun illumination and is available to generate an open-circuit voltage of 1.8 mV after a 20 min hydrovoltaic, demonstrating a high performance and versatility. Also, experiments and numerical simulation were carried out to understand the mechanism and design principles of this solar evaporators.

Sustainable Self-Cleaning Evaporators for Highly Efficient Solar Desalination Using a Highly Elastic Sponge-like Hydrogel

Interfacial evaporation using light-absorbing hydrogels offers efficient solar evaporation performance under natural sunlight, ensuring an affordable clean water supply. However, achieving light-absorbing hydrogels with durable and efficient utilization is still a challenge due to inevitable salt accumulation, a difficult-to-control surface morphology, and poor mechanical properties on the surfaces of hydrogel-based evaporators. In this work, a photothermal sponge-like hydrogel with a 3D interconnected porous structure was constructed using low-cost activated carbon as a photothermal material, as well as a double-network polymer chain as the basic skeleton using a simple foaming polymerization strategy. The sponge-like hydrogel evaporator showed tailored surface topography, adequate water transport, excellent elasticity and toughness, good salt rejection, and thermal localization properties. Under the irradiation of simulated sunlight (1.0 kW/m²), a high evaporation rate of 2.33 kg·m^-2·h^-1 was achieved. Furthermore, efficient salt self-cleaning behavior was achieved due to the fast ion diffusion within the 3D interconnected porous structures. Even in highly concentrated brine of 15 wt %, continuous and efficient water evaporation was still achieved. The excellent evaporation and salt rejection properties of this photothermal sponge-like hydrogel indicated its promising long-term sustainable utilization in seawater desalination.