Efficient Water Transport and Solar Steam Generation via Radially, Hierarchically Structured Aerogels

Controlled Vertically Aligned Structures in Polymer Composites: Natural Inspiration, Structural Processing, and Functional Application

Vertically aligned structures, which are a series of characteristic conformations with thickness-direction alignment, interconnection, or assembly of filler in polymeric composite materials that can provide remarkable structural performance and advanced anisotropic functions, have attracted considerable attention in recent years. The past two decades have witnessed extensive development with regard to universal fabrication methods, subtle control of morphological features, improvement of functional properties, and superior applications of vertically aligned structures in various fields. However, a systematic review remains to be attempted. The various configurations of vertical structures inspired from biological samples in nature, such as vertically aligned structures with honeycomb, reed, annual ring, radial, and lamellar configurations are summarized here. Additionally, relevant processing methods, which include the transformation of oriented direction, external-field inducement, template method, and 3D printing method, are discussed in detail. The diverse applications in mechanical, thermal, electric, dielectric, electromagnetic, water treatment, and energy fields are also highlighted by providing representative examples. Finally, future opportunities and prospects are listed to identify current issues and potential research directions. It is expected that perspectives on the vertically aligned structures presented here will contribute to the research on advanced multifunctional composites.

Fe3O4/PPy-Coated Superhydrophilic Polymer Porous Foam: A Double Layered Photothermal Material with a Synergistic Light-to-Thermal Conversion Effect toward Desalination

Solar steam generation has been considered as one of the most promising strategies for production of fresh water using renewable solar energy. Herein, we prepared a polymer porous foam (HPSS) by a facile hydrothermal method. The HPSS presents a superhydrophilic wettability, an interpenetrating macroporous structure, and low thermal conductivity, which can well satisfy the criteria as an ideal candidate for photothermal materials. The HPSS/Fe₃O₄/PPy (polypyrrole) evaporator, of which a Fe₃O₄/PPy binary optical system served as a light absorption layer and HPSS was used as a porous substrate, was constructed through in situ growth of Fe₃O₄ particles followed by interfacial polymerization of PPy on the surface of HPSS. HPSS/Fe₃O₄/PPy shows an excellent light absorption capacity (92%) and photothermal conversion performance, with the solar energy conversion efficiency reaching up to 94.7% under 1 sun irradiation, which is much higher than that of HPSS/PPy (84.8%) composed of a unitary PPy light absorption layer. Interestingly, the presence of Fe₃O₄ particles could make directional migration in a magnetic field possible, thus facilitating its recovery as a self-floating solar generator in an open water area. Moreover, the HPSS/Fe₃O₄/PPy evaporator displays outstanding salt resistance properties and stability in various saline solutions, thus having great potential in practical desalination.

Interfacial Engineering of Attractive Pickering Emulsion Gel-Templated Porous Materials for Enhanced Solar Vapor Generation

Solar vapor generation is emerging as one of the most important sustainable techniques for harvesting clean water using abundant and green solar energy. The rational design of solar evaporators to realize high solar evaporation performances has become a great challenge. Here, a porous solar evaporator with integrative optimization of photothermal convention, water transport and thermal management is developed using attractive Pickering emulsions gels (APEG) as templated and followed by interfacial engineering on a molecular scale. The APEG-templated porous evaporators (APEG-TPEs) are intrinsically thermal insulation materials with a thermal conductivity = 0.039 W·m−1·K−1. After hydrolysis, t-butyl groups on the inner-surface are transformed to carboxylic acid groups, making the inner-surface hydrophilic and facilitating water transport through the inter-connected pores. The introduction of polypyrrole layer endows the porous materials with a high light absorption of ~97%, which could effectively convert solar irradiation to heat. Due to the versatility of the APEG systems, the composition, compressive modulus, porosity of APEG-TPEs could be well controlled and a high solar evaporation efficiency of 69% with an evaporation rate of 1.1 kg·m−2·h−1 is achieved under simulated solar irradiation. The interface-engineered APEG-TPEs are promising in clean water harvesting and could inspire the future development of solar evaporators.

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.

Solar-Powered Sustainable Water Production: State-of-the-Art Technologies for Sunlight-Energy-Water Nexus

Alternative water resources (seawater, brackish water, atmospheric water, sewage, etc.) can be converted into clean freshwater via high-efficiency, energy-saving, and cost-effective methods to cope with the global water crisis. Herein, we provide a comprehensive and systematic overview of various solar-powered technologies for alternative water utilization (i.e., "sunlight-energy-water nexus"), including solar-thermal interface desalination (STID), solar-thermal membrane desalination (STMD), solar-driven electrochemical desalination (SED), and solar-thermal atmospheric water harvesting (ST-AWH). Three strategies have been proposed for improving the evaporation rate of STID systems above the theoretical limit and designing all-weather or all-day operating STID systems by analyzing the energy transfer of the evaporation and condensation processes caused by solar-thermal conversion. This review also introduces the fundamental principles and current research hotspots of two other solar-driven seawater or brackish water desalination technologies (STMD and SED) in detail. In addition, we also cover ST-AWH and other solar-powered technologies in terms of technology design, materials evolution, device assembly, etc. Finally, we summarize the content of this comprehensive review and discuss the challenges and future outlook of different types of solar-powered alternative water utilization technologies.

Reed Leaves Inspired Silica Nanofibrous Aerogels with Parallel-Arranged Vessels for Salt-Resistant Solar Desalination

Sufficient and clean freshwater is still out of reach for billions of people around the world. Solar desalination from brine is regarded as one of the most promising proposals to solve this severe crisis. However, most of the reported evaporators to date still suffer from the decreasing evaporation rate caused by salt crystallization accumulated on their surface. Here, inspired by the vascular tissue structure, transpiration, and antifouling function of reed leaves, we design biomimetic hierarchical nanofibrous aerogels with parallel-arranged vessels and hydrophobic surfaces for highly efficient and salt-resistant solar desalination. Foldable vessel walls and flexible silica nanofibers give the reed leaf-inspired nanofiber aerogels (R-NFAs) excellent mechanical properties and enable them to withstand repeated compression. Besides, the R-NFAs can efficiently absorb sunlight (light absorption efficiency: 94.8%) and evaporate the brine to vapor, similar to reed leaves (evaporation rate: 1.25 kg m^-2 h^-1 under 1 sun). More importantly, enabled by the hydrophobic surfaces and parallel-arranged vessels, the R-NFAs can work stably in high-concentration brine (saturated, 26.3 wt %) under high-intensity light (up to 6 sun), demonstrating potent salt resistance. It is expected that R-NFAs with combined antisalt pore and surface structures will provide a designed concept for salt-resistant solar desalination.

Natural Flammulina velutipes-Based Nerve Guidance Conduit as a Potential Biomaterial for Peripheral Nerve Regeneration: In Vitro and In Vivo Studies

The treatment and repair of serious peripheral nerve injuries remain challenging in the clinical practice, while the application of multifunctional nerve guidance conduits (NGCs) based on naturally derived polymers has attracted much attention in recent years because of their excellent physicochemical properties and biological characteristics. Flammulina velutipes (Curt. ex FV) is a popular edible mushroom characterized by hollow tubular structures, antibacterial activities, and high nutritional properties. In this study, FV is utilized to construct NGCs (labeled FVC) via a freeze-drying technique without chemical modifications. The morphology, physical properties, cellular biocompatibility, antibacterial properties, and nerve regeneration capacity of FVC were assessed both in vitro and in vivo. FVC is composed of hollow tubes and evenly irregular interconnected micropores with 73.8 ± 5.5% porosity and 476.1 ± 12.9 μm hollow tube diameter. The inner surface of the FVC presents multiple microgrooves elongated parallel to the long axis. Moreover, FVC possessed strong antibacterial activity and could inhibit Gram-positive Staphylococcus aureus growth by up to 96.0% and Gram-negative Escherichia coli growth by up to 94.8% in vitro. FVC exhibited excellent biocompatibility and effectively promoted PC-12 cell proliferation and elongation in vitro. When applied to repair critical-sized sciatic nerve defects, FVC could effectively stimulate nerve functional recovery and axonal outgrowth in a rat model. Interestingly, Western blot analysis indicated that growth-associated protein 43 (GAP-43) had increased expression levels in the FVC group compared with the autograft group. This result suggested that by activating the Janus activated kinase2 (JAK2)/Phosphorylation ofsignal transducer and activator of transcription-3 (STAT3) signaling pathway, FVC upregulated Phosphorylation of signal transducer and activator of transcription-3 (P-STAT3) in vivo, resulting in the secretion of GAP-43. Collectively, a natural NGC FVC was fabricated based on FV without chemical modifications. The morphology, physical properties, cellular biocompatibility, antibacterial properties, and nerve regeneration capacity of FVC provide new insights for its further optimization and application in the field of nerve tissue engineering.

3D Photothermal Cryogels for Solar-Driven Desalination

This paper reports the fabrication of photothermal cryogels for freshwater production via the solar-driven evaporation of seawater. Photothermal cryogels were prepared via in situ oxidative polymerization of pyrrole with ammonium persulfate on preformed poly(sodium acrylate) (PSA) cryogels. We found that the pyrrole concentration used in the fabrication process has a significant effect on the final PSA/PPy cryogels (PPCs), causing the as-formed polypyrrole (PPy) layer on the PPC to evolve from nanoparticles to lamellar sheets and to consolidated thin films. PPC fabricated using the lowest pyrrole concentration (i.e., PPC10) displays the best solar-evaporation efficiency compared to the other samples, which is further improved by switching the operative mode from floating to standing. Specifically, in the latter case, the apparent solar evaporation rate and solar-to-vapor conversion efficiency reach 1.41 kg m-2 h-1 and 96.9%, respectively, due to the contribution of evaporation from the exposed lateral surfaces. The distillate obtained from the condensed vapor, generated via solar evaporation of a synthetic seawater through PPC10, shows an at least 99.99% reduction of Na while all the other elements are reduced to a subppm level. We attribute the superior solar evaporation and desalination performance of PPC10 to its (i) higher photoabsorption efficiency, (ii) higher heat localization effect, (iii) open porous structure that facilitates vapor removal, (iv) rough pore surface that increases the surface area for light absorption and water evaporation, and (v) higher water-absorption capacity to ensure efficient water replenishment to the evaporative sites. It is anticipated that the gained know-how from this study would offer insightful guidelines to better designs of polymer-based 3D photothermal materials for solar evaporation as well as for other emerging solar-related applications.

Harvesting Solar Energy by Flowerlike Carbon Cloth Nanocomposites for Simultaneous Generation of Clean Water and Electricity

Harvesting solar energy for photothermal conversion in an efficient manner for steam-electricity cogeneration is particularly opportune in the context of comprehensive solar utilization to address the challenge of a global shortage of fresh water. However, the fragile solar thermal devices and the single-energy utilization pattern greatly hinder extensive solar energy exploitation and practical application. Herein, a flexible carbon cloth nanocomposite with a biomimetic pelargonium hortorum-petal-like surface that embraces all desirable chemical and physical properties, that is, enhanced light acquisition, excellent photothermal property, and operational durability, for high-performance solar-driven interfacial water evaporation distillation is reported. Combined with the two-dimensional water channel, the solar evaporator shows a solar-to-steam conversion efficiency of 93% under the simulated solar illumination of 1 kW m^-2. More strikingly, the solar steam generation-induced electricity based on the practical consideration toward more infusive solar thermal application is proposed. Such integrative steam-electricity generators presented here provide an attractive method to produce on-site electricity and fresh water in an individualized mode in various resource-constrained areas.

Distributed solar desalination by membrane distillation: current status and future perspectives

Membrane distillation (MD) has been proven promising in solar-driven desalination. Moreover, its unique characteristics such as simple process, module compactness, high salt rejection rate, etc. allow for a small-scale device in a distributed system. Both theoretical and experimental researches on the coupling between solar collectors and MD aiming at compact and autonomous desalination system have been devoted to enhance freshwater productivity and energy efficiency. In this paper, certain critical gaps are summarized upon a panoramic review of the current status, including limited production and energy performance compared with commercial-scale desalination, unclear relation between solar collecting area and membrane area, and few discussions on efficient condensation, etc. To tackle these challenges, perspectives on the essential future research directions are proposed. Solar direct heating and solar concentration constitute the possible resolution to enhance solar energy utilization for higher water production, which also raise the question of optimizing solar/MD areas. Meanwhile, module stacking, module internal heat recovery and external evaporation heat recovery are deemed prospective in further reducing MD energy consumption. Subsequently, an enhanced vapor condensation needs more exploration. Those aspects and a potential combination among them are the main tasks in the near future, together with more field tests on small distributed solar-driven MD systems.

Versatile Janus Composite Nonwoven Solar Absorbers with Salt Resistance for Efficient Wastewater Purification and Desalination

Solar steam generation is an efficient way of harvesting solar energy for water purification. Developing a versatile solar absorber with salt resistance and the capability to purify an oil-in-water emulsion is a grand challenge. Herein, a polypropylene (PP) nonwoven fabric-based photothermal absorber is fabricated by the combination of carbon nanotubes (CNTs), polypyrrole (PPy), and a fluorinated hydrophobic coating in a layer-by-layer approach. The specially designed architecture displays a hierarchical microstructure and Janus wetting properties, facilitating solar absorption and heat generation on the evaporation surface, and can effectively prevent salt crystallization. The water layer formed on the superhydrophilic/underwater superoleophobic bottom surface could repel oil droplets and form a channel to advect concentrated salt back into bulk water, which enabled high purity separation of an oil-in-water emulsion and continuous desalinization of seawater without the reduction of the evaporation rate. As a result, the solar absorber can achieve a remarkable evaporation rate of 1.61 kg m^-2 h^-1 and an energy efficiency of 91.2% under 1 sun irradiation and shows extraordinary performance in the purification of contaminated wastewater (over 99.8% purity). The strategy proposed provides a pathway for developing versatile high-performance solar absorbers for the sustainable treatment of saline water, wastewater, and oil-containing water.

Coating of Wood with Fe2O3-Decorated Carbon Nanotubes by One-Step Combustion for Efficient Solar Steam Generation

As the global water shortage becomes increasingly serious, it is highly imperative to develop efficient, renewable, and large-scale water purification devices. Herein, an efficient solar-driven water purification device of wood coated with Fe₂O₃ nanoparticle-decorated carbon nanotubes (Fe₂O₃/CNT) is fabricated in only a few seconds by one-step combustion of ferric acetylacetonate in an ambient environment. The thin layer of the Fe₂O₃/CNT hybrid coated on the upper surface of the wood serves as a solar-light absorber for converting solar energy to thermal energy, while the thermally insulating wood layer with vertically aligned channels endows the device with rapid water upward transport and localizes the generated heat inside the Fe₂O₃/CNT layer for solar-driven water evaporation. As a result, the wood/Fe₂O₃/CNT device achieves a high water steam generation capability of 1.42 kg m^-2 h^-1 along with an excellent evaporation efficiency of 87.2% under 1 sun irradiation, higher than most of the wood-based solar-driven water evaporation device reported. This device is also efficient in the purification of seawaters and wastewaters. This work demonstrates a rapid and facile methodology for large-scale fabrication of wood/Fe₂O₃/CNT devices for efficient solar-driven water purification.

High mass loading flower-like MnO2 on NiCo2O4 deposited graphene/nickel foam as high-performance electrodes for asymmetric supercapacitors

The implementation of high mass loading MnO₂ on electrochemical electrodes of supercapacitors is currently challenging due to the poor electrical conductivity and elongated electron/ion transport distance. In this paper, a NiCo₂O₄/MnO₂ heterostructure was built on the surface of three-dimensional graphene/nickel foam (GNF) by a hydrothermal method. The petal structured NiCo₂O₄ loaded on graphene played a wonderful role as a supporting framework, which provided more space for the growth of high mass loading MnO₂ microflowers, thereby increasing the utilization rate of the active material MnO₂. The GNF@NiCo₂O₄/MnO₂ composite was used as a positive electrode and achieved a high areal capacitance of 1630.5 mF cm^-2 at 2 mA cm^-2 in the neutral Na₂SO₄ solution. The asymmetric supercapacitor assembled with the GNF@NiCo₂O₄/MnO₂ positive electrode and activated carbon negative electrode possessed a wide voltage window (2.1 V) and splendid energy density (45.9 Wh kg^-1), which was attributed to the satisfactory electroactive area, low resistance, quick mass diffusion and ion transport caused by high mass loading MnO₂.

All-Cold Evaporation under One Sun with Zero Energy Loss by Using a Heatsink Inspired Solar Evaporator

Interfacial solar steam generation is a highly efficient and sustainable technology for clean water production and wastewater treatment. Although great progress has been achieved in improving evaporation rate and energy efficiency, it's still challenging to fully eliminate the energy loss to the surrounding environment during solar steam generation. To achieve this, a novel heatsink-like evaporator (HSE) is developed herein. During solar evaporation, the temperature on the top solar evaporation surface can be regulated by the fin structures of the HSE. For the evaporators with 5 to 7 heatsink fins, the temperature of the solar evaporation surface is decreased to be lower than the ambient temperature, which fully eliminates the radiation, convection, and conduction heat losses, leading to the absolute cold evaporation over the entire evaporator under 1.0 sun irradiation. As a result, massive energy (4.26 W), which is over 170% of the received light energy, is harvested from the environment due to the temperature deficit, significantly enhancing the energy efficiency of solar steam generation. An extremely high evaporation rate of 4.10 kg m-2 h-1 is realized with a 6-fin photothermal HSE, corresponding to an energy conversion efficiency far beyond the theoretical limit, assuming 100% light-to-vapor energy conversion.

Three-Dimensionally Structured Polypyrrole-Coated Setaria viridis Spike Composites for Efficient Solar Steam Generation

Solar-driven steam generation is a promising technology for the production of freshwater from seawater and polluted water. High water evaporation rates have been achieved via the interfacial heating scheme; however, they are still limited to meet the increasing need for freshwater due to the restricted evaporation area of two-dimensionally (2D) geometrical planar photothermal membranes. Herein, a three-dimensionally (3D) structured solar evaporator is prepared via coating photothermal polypyrrole (PPy) on the spike of Setaria viridis(S. viridis) for highly efficient evaporation. Due to the enlarged evaporation area and open structure for vapor dissipation, the PPy-coated S. viridis spike solar evaporator shows a high water evaporation rate of 3.72 kg m^-2 h^-1 under one sun illumination. The 3D solar evaporator also demonstrates good durability and anti-salt-clogging performance for real-life applications. Furthermore, we show that the 3D solar evaporator demonstrates effective decontamination of saline water, dye-contaminated water, and corrosive water. This work can inspire new paradigms toward developing high-performance solar steaming technologies for effective water purification to address the worldwide crisis of freshwater shortage.

Single Ice Crystal Growth with Controlled Orientation during Directional Freezing

Ice growth has attracted great attention for its capability of fabricating hierarchically porous microstructure. However, the formation of tilted lamellar microstructure during freezing needs to be reconsidered due to the limited control of ice orientation with respect to the thermal gradient during in situ observations, which can greatly enrich our insight into architectural control of porous biomaterials. This paper provides an in situ study of the solid/liquid interface morphology evolution of directionally solidified single crystal ice with its C-axis (optical axis) perpendicular to directions of both the thermal gradient and the incident light in poly(vinyl alcohol, PVA) solutions. Multifaceted morphology and V-shaped lamellar morphology were clearly observed in situ for the first time. Quantitative characterizations on lamellar spacing, tilt angle, and tip undercooling of lamellar ice platelets provide a clearer insight into the inherent ice growth habit in polymeric aqueous systems and are suggested to exert significant impact on future design and optimization in porous biomaterials.

Stable Self-Floating Reduced Graphene Oxide Hydrogel Membrane for High Rate of Solar Vapor Evaporation under 1 sun

Highly efficient vapor generation with considerable stability under natural solar irradiance is a promising technology for seawater desalination and wastewater purification. Here a broadband solar absorber of reduced graphene oxide hydrogel membrane (rGOHM), synthesized via an environmentally friendly one-step hydrothermal reduction process, is demonstrated, which shows a high rate of solar vapor production and superior stability. The porous rGOHM containing more than 99.5% water within its small volume floats on the surface of water, exhibiting efficient solar absorption of ≈98% across 300-2500 nm, as well as sufficient water-pumping pathways. The evaporation rate can be tuned by changing the water volume. By controlling the water volume, the self-floating rGOHM can enable efficient interfacial solar vapor generation at a high rate of ≈2.33 kg m-2 h-1 under 1 sun, which is comparable to the rate generated by the evaporator with an extra insulator. In addition, the evaporation rate of rGOHM is only slightly affected at a high saltwater concentration (at least 15 wt%), and the rGOHM shows mechanical and physical stability. The superior evaporation performance combined with efficient eradication of wastewater contaminants, cost-effectiveness, and straightforward fabrication process, makes this rGOHMs ideal for advanced high-concentration seawater desalination and wastewater treatment technologies.

Nanoenabled Photothermal Materials for Clean Water Production

Solar-powered water evaporation is a primitive technology but interest has revived in the last five years due to the use of nanoenabled photothermal absorbers. The cutting-edge nanoenabled photothermal materials can exploit a full spectrum of solar radiation with exceptionally high photothermal conversion efficiency. Additionally, photothermal design through heat management and the hierarchy of smooth water-flow channels have evolved in parallel. Indeed, the integration of all desirable functions into one photothermal layer remains an essential challenge for an effective yield of clean water in remote-sensing areas. Some nanoenabled photothermal prototypes equipped with unprecedented water evaporation rates have been reported recently for clean water production. Many barriers and difficulties remain, despite the latest scientific and practical implementation developments. This Review seeks to inspire nanoenvironmental research communities to drive onward toward real-time solar-driven clean water production.

Liquid Transport in Fibrillar Channels of Ion-Associated Cellular Nanowood Foams

A mechanical disintegration of waste wood biomass and freeze-induced assembly of colloidal nanowood were effectively deployed to explore ion-associated cellular foams (NWFs) with unidirectional channels. Under the assistance of inorganic ions, the as-fabricated foams were significantly enhanced in physical stability, compressive strength, flame retardancy, and thermal barrier, accounting for the tuning effects of pores and channels, surface charges, and microphase interaction by ion effects and freeze orientation. As a result, the vascular-like ion-doped channels benefited from quick capillary liquid transport. Under 1 sun illumination, NWF-V as a 3-D evaporator exhibited a high evaporation rate of 1.50 kg m^-2 h^-1 and a conversion efficiency of up to 88.9% for seawater desalination. Dramatically, an average of 12.5 kg m^-2 of fresh water could be generated on each sunny day by outdoor NWFs for durability beyond 15 days. Under the drive of fuel combustion, an efficient conveying of ethanol or pump oil could be at rates of 0.44 and 0.26 mL min^-1, respectively. Moreover, combustion flame with variable color was generated according to the doping cations in NWFs. Therefore, sustainable, green, facile, and multifunctional wood-based cellular foams could be tailored, scaled-up, and applied as color flame burners or desalination evaporators under combustion or solar drive in the energy and environment fields.

Janus Evaporators with Self-Recovering Hydrophobicity for Salt-Rejecting Interfacial Solar Desalination

The recent advancements in interfacial evaporation of salty water using renewable solar energy provide one of the promising pathways to solve worldwide water scarcity. Pursuing a stable evaporation rate of water has been the central focus of this field, as it is directly related to the throughput, while salt deposition on the evaporator becomes a critical issue. Although Janus-structured evaporators with an upper hydrophobic layer and a bottom hydrophilic layer have been demonstrated as an effective way to suppress the salt precipitation, the hydrophobic upper layer, achieved usually by some special organic groups, suffers from a photochemical oxidation when exposed to oxidative chemicals in water and high-energy light, resulting in a deteriorated surface hydrophobicity. Here, we report our design of an efficient salt-rejecting Janus evaporator by taking advantage of the self-recovering surface hydrophobicity of poly(dimethylsiloxane) (PDMS) against photochemical damages, which ensures a long-term surface hydrophobicity. With its upper layer partially covered with PDMS, the Janus evaporator demonstrates an excellent salt rejection capability and exhibits a stable evaporation rate of 1.38 kg·m^-2·h^-1 under 1 sun illumination for 400 min of continuous operation, or 90 d of intermittent work. By combining the advantages of high structural integration, long-term salt-rejection, and efficient evaporation, our Janus evaporator holds great promise for the stable production of clean water from seawater.

Microvessel-Assisted Environmental Thermal Energy Extraction Enabling 24-Hour Continuous Interfacial Vapor Generation

Interfacial solar evaporators have great potential for clean water production; however, their evaporation performance relies greatly on the solar illumination condition, which is restricted by daily sunshine time and climates. Here, a wood-based vapor generator in pyramid structure is fabricated to achieve efficient water evaporation under dark condition (0 kW m^-2 ) through efficient extraction of environmental thermal energy. The microvessels of wood provide fast water transportation whereas the tailored pyramid surface structure enables efficient evaporative cooling for extracting energy from the environment. The method enables fast water evaporation without the need of solar heat input. We demonstrate a vapor generation rate of up to 2.15 kg m^-2  h^-1 under dark condition (0 kW m^-2 ), which is even 1.4 times faster than the theoretical limit of conventional solar thermal evaporators working under 1 sun (1 kW m^-2 ) illumination condition. During the 24-h continuous evaporation test, the evaporator presented a daily vapor generation rate of up to 50.8 kg m^-2  day^-1 and 60.7 kg m^-2  day^-1 on cloudy and sunny day, respectively, offering a novel approach for the development of 24-h full-time water evaporators for seawater desalination and wastewater treatment.

Green fabrication of seedbed-like Flammulina velutipes polysaccharides-derived scaffolds accelerating full-thickness skin wound healing accompanied by hair follicle regeneration

A novel seedbed-like scaffold was firstly fabricated by the "frozen sectioning" processing method using Flammulina velutipes as a raw material. The Flammulina velutipes polysaccharides scaffold is composed of a natural structure imitating the "ground" (connected and aligned hollow tubes with porous walls). Meanwhile, its biologically active components include polysaccharides and proteins, mimicking the "plant nutrition" in the seedbed. To further optimize the ground and nutrition components, Flammulina velutipes polysaccharides-derived scaffolds (FPDSs) were fabricated via the treatment of original Flammulina velutipes polysaccharides scaffold (labeled FPS) by NaOH, cysteine (labeled as FPS/NaOH, FPS/Cys, respectively). FPDSs were characterized by SEM, FTIR, XRD, water absorption and retention, and mechanical evaluations. From the results, FPS/NaOH and FPS/Cys lost the characteristic big tubes of original strips and had higher water absorption capacities comparing to FPS. Simultaneously, FPS/NaOH had better ductility, FPS/Cys had showed increased stiffness. Biological activities of FPDSs were tested against different types of bacteria exhibiting excellent anti-bacterial activity, and FPS/NaOH and FPS/Cys had dramatically higher anti-bacterial activity than FPS. The cytocompatibility of FPDSs was evaluated utilizing mouse fibroblast cell line (L929), and all FPDSs showed good cytocompatibility. The FPDSs were further applied to a rat full-thickness skin wound model, and they all exhibited obviously accelerated re-epithelialization, among which FPS/NaOH showed the greatest efficiency. FPS/NaOH could shorten the wound-healing process as evidenced by dynamic alterations of the expression levels of specific stagewise markers in the healing areas. Similarly, FPS/NaOH can efficiently induce hair follicle regeneration in the healing skin tissues. In summary, FPDSs exhibit potential functions as seedbeds to promote the regeneration of the "seed" including hair follicles and injured skin, opening a new avenue for wound healing.

Biomimetic Antigravity Water Transport and Remote Harvesting Powered by Sunlight

Antigravity water transport plays important roles in various applications ranging from agriculture, industry, and environmental engineering. In natural trees, ubiquitous water-flow over 100 m high from roots through the hierarchical xylem to leaves is driven by sunlight-powered continuous evaporation and the negative pressure. Inspired by natural trees, herein an artificial trunk-leaf system is built up to structurally mimic natural trees for a continuous antigravity water delivery. The artificial tree consists of directional microchannels for antigravity water transport, and a top leaf-like hybrid hydrogel that are acts as continuous solar steam evaporator, plus a purposely engineered steam collector. It is found that continuous uniform microchannels of acetylated chitin optimize and enhance capillary rise (≈37 cm at 300 min) and reduce vertical water transport resistance. A remote water harvesting, and purification is performed with a high rate of 1.6 kg m-2 h-1 and 184 cm in height under 1 sun irradiation and the collection efficiency up to 100% by evaporative cooling technique. It is envisioned that the basic design principles underlying the artificial tree can be used to transform solar energy into potential energy.

Nanofibrous Aerogels with Vertically Aligned Microchannels for Efficient Solar Steam Generation

Utilizing solar energy to evaporate seawater or sewage to improve water quality is an environment-friendly and sustainable water treatment technology, which has been widely concerned. However, there are still many challenges for efficient solar vapor generation, such as incapable free floating, low water-transfer rates, low energy efficiency, serious salt precipitation, and short service life. Herein, photothermal conversion nanofibrous aerogels (PTCNFAs) with vertically aligned microchannels inside are fabricated. Because of the orderly framework structure and the good hydrophilicity, the PTCNFAs show excellent underwater compressive fatigue durability (nearly no plastic deformation after 50 compressive cycles) and water-transfer rate (0.5 cm s^-1 in the first second). Furthermore, the surface temperature of the PTCNFAs could rise from 28 to 94 °C in air, after being irradiated for 30 s by 1 sun. Benefiting from the excellent mechanical properties, high water-transfer rates, and outstanding photothermal properties, the PTCNFAs are more convenient in application and exhibit an efficient solar water evaporation rate (2.89 kg m^-2 h^-1), while the energy efficiency under 1 sun is about 90.3%. This work provides a new approach to design and fabricate the solar steam generation materials for water treatment.

Study of the Freeze Casting Process by Artificial Neural Networks

Freeze casting technology has experienced vast development since the early 2000s due to its versatility and simplicity for producing porous materials. A linear relationship between the final porosity and the initial solid material fraction in the suspension was reported by many researchers. However, the linear relationship cannot well describe the freeze casting for various samples. Here, we proposed an artificial neural network (ANN) to analyze the influence of critical parameters on freeze-cast porous materials. After well training the ANN model on experimental data, a porosity value can be predicted from four inputs, which describe the most influential process conditions. Based on the constructed model, two improvements are also successfully added on to infer more information. By involving big data from real experiments, this method effectively summarizes a general rule for diverse materials in one model, which gives a new insight into the freeze casting process. The good convergence and accuracy prove that our ANN model has the potential to be developed for solving more complicated issues of freeze casting. Finally, a user-friendly mini-program based on a well-trained ANN model is also provided to predict the porosity for customized freeze-casting experiments.

High Efficiency Solar Membranes Structurally Designed with 3D Core-2D Shell SiO2@Amino-Carbon Hybrid Advanced Composite for Facile Steam Generation

Steam generation through efficient utilization of solar energy is a promising technology in addressing the challenge of global freshwater shortage and water pollution. One of the biggest hurdles for traditional photothermal membranes to function continuously in a high temperature, high salt, and corrosive environment has been attributed to their rapid decline of mechanical properties. In this work, a highly efficient solar-driven interfacial water evaporation system has been developed via a polydopamine/carbon/silicon (PCS) composite membrane supported by a floating insulation foam substrate. A 3.1 fold increase in the water vaporization rate was recorded compared with the pure water system. The 2D-carbon nanolayer on the surface was successfully prepared by carbonizing low-cost linear polyethylene with a glass fiber (GF) membrane as the substrate, and then the carbon membrane was modified with dopamine to control water transport on the carbon coating and within the glass fiber. The PCS membrane has a high efficiency for solar steam generation owing to high optical absorption and has excellent solar thermal conversion capability. The evaporation rate and solar thermal conversion efficiency of the PCS membrane under simulated sunlight irradiation with 1 sun (1 kW·m^-2) are 1.39 kg·m^-2·h^-1 and 80.4% respectively, which are significantly higher compared to GF membrane, carbon/silicon (CS) membrane, and pure water without a photothermal membrane. The water evaporation system retained high efficiency after 20 cycles under simulated sunlight irradiation of 1 sun. This study provides critical insight on the design and fabrication of a highly efficient and durable evaporation system.

Flexible Salt-Rejecting Photothermal Paper Based on Reduced Graphene Oxide and Hydroxyapatite Nanowires for High-Efficiency Solar Energy-Driven Vapor Generation and Stable Desalination

Vapor generation using solar energy is emerging as an efficient technology for wastewater purification and seawater desalination to relieve global water crisis. However, salt deposition on the evaporation surface seriously impairs the long-term steady water evaporation performance. Herein, the flexible salt-rejecting photothermal paper comprising reduced graphene oxide (rGO) and ultralong hydroxyapatite nanowires (HNs) has been developed for high-performance solar energy-driven water evaporation and stable desalination of seawater. The rGO/HN photothermal paper has advantages such as the hierarchical porous structure, interconnected channels, high mechanical strength, high efficiencies of solar light absorption and photothermal conversion, fast water transportation, and good heat insulation and salt-rejecting properties. Furthermore, the hydrophilicity and hydrophobicity of the rGO/HN photothermal paper can be adjusted by regulating the thermal treatment time. The water evaporation rate and energy efficiency of the hydrophilic rGO/HN photothermal paper are 1.48 kg m^-2 h^-1 and 89.2%, respectively, under 1 sun illumination (1 kW m^-2). The hydrophobic rGO/HN photothermal paper shows a long-time stable water evaporation and salt-rejecting performance in the process of seawater desalination. The flexible salt-rejecting rGO/HN photothermal paper can produce clean water from wastewater and seawater with high rejection rates of organic dyes, metal ions, and salt ions, and it is promising for applications in water purification and seawater desalination.

Advances in Manufacturing Composite Carbon Nanofiber-Based Aerogels

This article provides an overview on manufacturing composite carbon nanofiber-based aerogels through freeze casting technology. As known, freeze casting is a relatively new manufacturing technique for generating highly porous structures. During the process, deep cooling is used first to rapidly solidify a well-dispersed slurry. Then, vacuum drying is conducted to sublimate the solvent. This allows the creation of highly porous materials. Although the freeze casting technique was initially developed for porous ceramics processing, it has found various applications, especially for making aerogels. Aerogels are highly porous materials with extremely high volume of free spaces, which contributes to the characteristics of high porosity, ultralight, large specific surface area, huge interface area, and in addition, super low thermal conductivity. Recently, carbon nanofiber aerogels have been studied to achieve exceptional properties of high stiffness, flame-retardant and thermal-insulating. The freeze casting technology has been reported for preparing carbon nanofiber composite aerogels for energy storage, energy conversion, water purification, catalysis, fire prevention etc. This review deals with freeze casting carbon nanofiber composite materials consisting of functional nanoparticles with exceptional properties. The content of this review article is organized as follows. The first part will introduce the general freeze casting manufacturing technology of aerogels with the emphasis on how to use the technology to make nanoparticle-containing composite carbon nanofiber aerogels. Then, modeling and characterization of the freeze cast particle-containing carbon nanofibers will be presented with an emphasis on modeling the thermal conductivity and electrical conductivity of the carbon nanofiber network aerogels. After that, the applications of the carbon nanofiber aerogels will be described. Examples of energy converters, supercapacitors, secondary battery electrodes, dye absorbents, sensors, and catalysts made from composite carbon nanofiber aerogels will be shown. Finally, the perspectives to future work will be presented.

Reusable and Recyclable Graphene Masks with Outstanding Superhydrophobic and Photothermal Performances

The 2019 coronavirus outbreak (COVID-19) is affecting over 210 countries and territories, and it is spreading mainly by respiratory droplets. The use of disposable surgical masks is common for patients, doctors, and even the general public in highly risky areas. However, the current surgical masks cannot self-sterilize in order to reuse or be recycled for other applications. The resulting high economic and environmental costs are further damaging societies worldwide. Herein, we reported a unique method for functionalizing commercially available surgical masks with outstanding self-cleaning and photothermal properties. A dual-mode laser-induced forward transfer method was developed for depositing few-layer graphene onto low-melting temperature nonwoven masks. Superhydrophobic states were observed on the treated masks' surfaces, which can cause the incoming aqueous droplets to bounce off. Under sunlight illumination, the surface temperature of the functional mask can quickly increase to over 80 °C, making the masks reusable after sunlight sterilization. In addition, this graphene-coated mask can be recycled directly for use in solar-driven desalination with outstanding salt-rejection performance for long-term use. These roll-to-roll production-line-compatible masks can provide us with better protection against this severe virus. The environment can also benefit from the direct recycling of these masks, which can be used for desalinating seawater.

A one-step-assembled three-dimensional network of silver/polyvinylpyrrolidone (PVP) nanowires and its application in energy storage

Creating ultralight monolithic metal foams remains an outstanding challenge despite their important applications, e.g., in electronics, sensors and energy storage. Herein, a facile methodology is developed for one-step fabrication of silver/polyvinylpyrrolidone (PVP) nanowire (AgPNW) hydrogel and high-quality robust ultralight AgPNW aerogel (AgPNWA) on a large scale. The hydrogel is directly formed by in situ assembling hydrothermally-synthesized AgPNWs. The resultant ultralight AgPNWA exhibits very high electrical conductivity. The application of this one-step fabricated AgPNWA to enhance phase change materials (PCMs) for high-efficiency thermal energy storage is investigated. The AgPNWA-paraffin composite (APC) shows ∼350% thermal-efficiency enhancement, ∼463% mechanical hardening, and strong reliability against thermal cycling due to the potentially strong AgPNW-paraffin interfacial interaction. It is also observed that the thickness of the APC shrinks significantly but there is no change in its diameter during thermal cycles. Analytical models of liquid capillary filling of deformable fiber-based 3D networks are derived for the first time and are applied to analyze the thermal-cycling-induced-shape-stabilization behavior of the APC and the vaporization-induced collapse behavior of the AgPNW network. This work provides important insights into designing a facile 3D assembly of nanomaterials, and thermal energy storage materials with high performance and reliability.

Enhanced Directional Seawater Desalination Using a Structure-Guided Wood Aerogel

Seawater desalination via solar energy has potential to alleviate freshwater scarcity. However, problems including insufficient air-water interface, large heat loss, and potential ecological impact have restricted its practical viability. Here, a novel wood-derived indirect-contact (hanging) photothermal evaporation system was designed. An evaporation rate of 1.351 kg·m^-2·h^-1 with efficiency up to 90.89% under one sun illumination (1 kW·m^-2) was achieved, which is the highest record to the best of our knowledge. More importantly, a series of simulations and numerical modeling were carried out to analyze the main factors affecting seawater collection and evaporation, and the synergetic mechanisms of oriented seawater collection, photothermal thermogenesis, and natural convection were elucidated. Taken together, this study provides a new wood-derived hanging seawater desalination system with superior mechanical strength, good repeatability, great ecological security, and excellent thermal stability. The corresponding mechanisms of the whole process are shown, and the seawater evaporation efficiency approaching the real demand is maximized.

Structure Architecting for Salt-Rejecting Solar Interfacial Desalination to Achieve High-Performance Evaporation With In Situ Energy Generation

The past few years have witnessed a rapid development of solar-driven interfacial evaporation, a promising technology for low-cost water desalination. As of today, solar-to-steam conversion efficiencies close to 100% or even beyond the limit are becoming increasingly achievable in virtue of unique photothermal materials and structures. Herein, the cutting-edge approaches are summarized, and their mechanisms for photothermal structure architecting are uncovered in order to achieve ultrahigh conversion efficiency. Design principles to enhance evaporation performance and currently available salt-rejection strategies for long-term desalination are systematically investigated. The guidelines to utilize every component in solar desalination systems for simultaneous in situ energy generation are also revealed. Finally, opportunities and challenges for future works in this field are also discussed and concluded.

A review of multifunctional nacre-mimetic materials based on bidirectional freeze casting

Nacre has achieved an excellent combination of strength and toughness through its unique brick-and-mortar structure of layered aragonite platelets bonded with biopolymers. Mimicking nacre has been considered as a practical way for the development of high-performance structural composites. Over the past years, many techniques have been developed to fabricate multifunctional nacre-mimetic materials, including freeze casting, layer-by-layer assembly, vacuum filtration, 3D printing and so on. Among them, freeze casting, especially bidirectional freeze casting, as an environmentally friendly and scalable method, has attracted extensive attention recently. In this review, we begin with the introduction and discussion of various fabrication techniques comparing their advantages and disadvantages, focusing on the most recent advances of the bidirectional freeze casting technique. Then, we summarize representative examples of applying the bidirectional freeze casting technique to assemble various building blocks into multifunctional nacre-mimetic materials and their wide applications. At the end, we discuss the future direction of using bidirectional freeze casting to make nacre-mimetic materials.

Freeze Casting: From Low-Dimensional Building Blocks to Aligned Porous Structures-A Review of Novel Materials, Methods, and Applications

Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well-controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze-casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze-casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro- and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze-cast geometries-beads, fibers, films, complex macrostructures, and nacre-mimetic composites-are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze-casting techniques and low-dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.

Graphene and Rice-Straw-Fiber-Based 3D Photothermal Aerogels for Highly Efficient Solar Evaporation

Solar-steam generation is one of the most promising technologies to mitigate the issue of clean water shortage using sustainable solar energy. Photothermal aerogels, especially the three-dimensional (3D) graphene-based aerogels, have shown unique merits for solar-steam generation, such as lightweight, high flexibility, and superior evaporation rate and energy efficiency. However, 3D aerogels require much more raw materials of graphene, which limits their large-scale applications. In this study, 3D photothermal aerogels composed of reduced graphene oxide (RGO) nanosheets, rice-straw-derived cellulose fibers, and sodium alginate (SA) are prepared for solar-steam generation. The use of rice straw fibers as skeletal support significantly reduces the need for the more expensive RGO by 43.5%, turning the rice straw biomass waste into value-added materials. The integration of rice straw fibers and RGO significantly enhances the flexibility and mechanical stability of the obtained photothermal RGO-SA-cellulose aerogel. The photothermal aerogel shows a strong broad-band light absorption of 96-97%. During solar-steam generation, the 3D photothermal aerogel effectively decreases the radiation and convection energy loss while enhancing energy harvesting from the environment, leading to an extremely high evaporation rate of 2.25 kg m^-2 h^-1, corresponding to an energy conversion efficiency of 88.9% under 1.0 sun irradiation. The salinity of clean water collected during the evaporation of real seawater is only 0.37 ppm. The materials are environmentally friendly and cost-effective, showing great potential for real-world desalination applications.

Flexible Functional Surface for Efficient Water Collection

Inspired by both the water collection strategy of desert beetles and the lubrication effect of Nepenthes pitcher plants, a new flexible functional surface for water collection is designed and can be easily fabricated. Such a functional surface consists mainly of a superhydrophobic region and a hydrophobic region with infused lubricating oil. Different functional patterns can be easily manipulated by different templates. Due to the flexibility of the surface, not only a two-dimensional surface but also a three-dimensional one can be designed. Directional water collection can be achieved. Furthermore, it is an integrative bioinspired functional surface that does not require any tailoring. Compared with existing functional surfaces, the present surface has higher water collection efficiency in fog and such a function can last 15 days. The functional degraded surfaces can also be easily reused.

Gel-Emulsion-Templated Polymeric Aerogels for Water Treatment by Organic Liquid Removal and Solar Vapor Generation

The importance of water cannot be overstated. It is the most important natural resource for our survival and development. The development of suitable materials for efficient water purification will provide a critical contribution for sustainable water use. In this context, a gel-emulsion-templated synthesis of a polymeric aerogel has been developed for water treatment. Owing to its hydrophobic nature, the aerogel shows high sorption (nearly 20 times its weight) for organic liquids, such as toluene, phenol, and nitrobenzene, and can be used to remove them from water. The aerogel shows low thermal conductivity (0.032 W m^-1  K^-1 ) and excellent light absorption efficiency (>92 %) after carbonization, which provides the possibility for the construction of an interfacial solar vapor generation system. The as-prepared materials are used to develop a two-step approach to remove both organic and inorganic contaminants (salts) from water. Importantly, the aerogel shows excellent reusability and high efficiency both for oil sorption and for solar vapor generation. Moreover, the low cost and easy scale-up of the preparation process lay a solid foundation for practical application. It is anticipated that the prepared aerogels would contribute not only to water purification but also to other related areas.

Solar-to-Steam Generation via Porous Black Membranes with Tailored Pore Structures

Solar-to-steam generation is a powerful, intense, and efficient method to harvest solar energy. Many efforts have been devoted to the development of a durable, affordable, and easy-to-manufacture solar steam device. In this study, we use a versatile polydimethylsiloxane material to fabricate an open porous black membrane with different pore structures using a simple salt water etching process and vapor deposition polymerization of pyrrole into a matrix. The porous black membrane absorbed radiation from a broad section of the light spectrum from near-infrared to ultraviolet and retained its initial pore structures and floating ability. We found that our black membrane with a tailored pore structure and surface exhibits excellent solar-to-steam generation efficiency of up to 72% at five sun irradiation. Also, a series of analyses including density functional theory calculation was used to prove the outstanding efficiency of solar-to-steam generation.

An Interfacial Solar-Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption-Desorption

Water scarcity is one of the greatest challenges facing human society. Because of the abundant amount of water present in the atmosphere, there are significant efforts to harvest water from air. Particularly, solar-driven atmospheric water generators based on sequential adsorption-desorption processes are attracting much attention. However, incomplete daytime desorption is the limiting factor for final water production, as the rate of water desorption typically decreases very quickly with decreased water content in the sorbents. Hereby combining tailored interfacial solar absorbers with an ionic-liquid-based sorbent, an atmospheric water generator with a simultaneous adsorption-desorption process is generated. With enhanced desorption capability and stabilized water content in the sorbent, this interfacial solar-driven atmospheric water generator enables a high rate of water production (≈0.5 L m^-2 h^-1 ) and 2.8 L m^-2 d^-1 for the outdoor environment. It is expected that this interfacial solar-driven atmospheric water generator, based on the liquid sorbent with a simultaneous adsorption-desorption process opens up a promising pathway to effectively harvest water from air.