Simple Design of a Porous Solar Evaporator for Salt-Free Desalination and Rapid Evaporation

Mimicked Trees for Spatial Microfluidic Solar Evaporation

The lack of freshwater is becoming an increasingly urgent problem, and clean water production with green energy is becoming a hot topic. Among these, mimicked transpiration for water treatment enabled with solar energy is attracting more and more interest. In this work, we design unique microchannels inspired by leaf veins in nature. We experimentally observe the unique microfludic phenomena exhibited during liquid transport inside the mimicked microchannels, and the underlying mechanisms of the microfluidic performance of the mimicked microchannels are also revealed. Moreover, a type of mimicked leaf integrating "veins" for fluid transport and "stomata" for solar evaporation is designed. Our mimicked leaves achieve rapid liquid transport, when the light intensity is 1 sun, the maximum evaporation rate is 1.85 kg m-2 h-1, and the photothermal conversion efficiency is higher than 92%. In addition, with the rapid liquid transport capability of our mimicked leaves, we innovatively design a mimicked tree for spatial solar evaporation, exhibiting an evaporation rate of 1.52 kg m-2 h-1 with a light intensity of 1 sun. Our mimicked trees promise the microfluidics-powered, large-scale and multidirectional transportation of water, as well as excellent spatial evaporation of water.

Polyanionic Electrolyte Ionization Desalination Empowers Continuous Solar Evaporation Performance

Solar evaporation contributes to sustainable and environmentally friendly production of fresh water from seawater and wastewater. However, poor salt resistance and high degree of corrosion of traditional evaporators in brine make their implementation in real applications scarce. To overcome such deficiency, a polyanionic electrolyte functionalization strategy empowering excellent uniform desalination performance over extended periods of time is exploited. This 3D superhydrophilic graphene oxide solar evaporator design ensures stable water supply by the enhanced self-driving liquid capillarity and absorption at the evaporation interface as well as efficient vapor diffusion. Meanwhile, the polyanionic electrolyte functionalization implemented via layer-by-layer static deposition of polystyrene sodium sulfonate effectively regulates/minimizes the flux of salt ions by exploiting the Donnan equilibrium effect, which eventually hinders local salt crystallization during long-term operation. Stable evaporation rates in line with the literature of up to 1.68 kg m-2 h-1 are achieved for up to 10 days in brine (15‰ salinity) and for up to 3 days in seawater from Hangzhou Bay in the East China Sea (9‰ salinity); while, maintaining evaporation efficiencies of ≈90%. This work demonstrates the excellent benefits of polyanionic electrolyte functionalization as salt resistance strategy for the development of high-performance solar powered seawater desalination technology and others.

Achieving "Ion Diode" Salt Resistance in Solar Interfacial Evaporation by a Tesla Valve-Like Water Transport Structure

Salt deposition is a disturbing problem that limits the development of passive solar-driven interfacial evaporation. Inspired by the passive fluid control mechanism of the Tesla valve, a novel solar evaporator is proposed with a Tesla valve-like water transport structure to prevent salt accumulation at the evaporation interface. A unique "ion diode" salt resistance of this evaporator is significantly achieved by optimizing the two asymmetric water transport structures, consisting of one Tesla valve-like side and one wide-leg side, which establish a reverse-suppressing and forward-accelerating water transport channel. In contrast to the limited ion migration of the typical symmetric solar evaporator, such a channel caused by the water/salt ions transport difference between two water supply structures, reinforces the water/salt ions supply on the wide-leg side, thus leading to an apparent unidirectional salt ions migration from the wide-leg side to bulk water through the Tesla valve-like side. Consequently, an evaporation rate of 3.25 kg m-2 h-1 and a conversion efficiency of 83.27% under 2 suns are achieved in 16 wt% NaCl solution. The development of the Tesla Valve-like evaporator provides a new perspective for solving salt deposition and realizing scalable applications of solar-driven interfacial evaporation.

Dual-Interface Solar Evaporator with Highly-Efficient Thermal Regulation via Suspended Multilayer Design

Facing the increasing global shortage of freshwater resources, this study presents a suspended multilayer evaporator (SMLE), designed to tackle the principal issues plaguing current solar-driven interfacial evaporation technologies, specifically, substantial thermal losses and limited water production. This approach, through the implementation of a multilayer structural design, enables superior thermal regulation throughout the evaporation process. This evaporator consists of a radiation damping layer, a photothermal conversion layer, and a bottom layer that leverages radiation, wherein the bottom layer exhibits a notable infrared emissivity. The distinctive feature of the design effectively reduces radiative heat loss and facilitates dual-interface evaporation by heating the water surface through mid-infrared radiation. The refined design leads to a notable evaporation rate of 2.83 kg m-2 h-1. Numerical simulations and practical performance evaluations validate the effectiveness of the multilayer evaporator in actual use scenarios. This energy-recycling and dual-interface evaporation multilayered approach propels the design of high-efficiency solar-driven interfacial evaporators forward, presenting new insights into developing effective water-energy transformation systems.

Developing Salt-Rejecting Evaporators for Solar Desalination: A Critical Review

Solar desalination, a green, low-cost, and sustainable technology, offers a promising way to get clean water from seawater without relying on electricity and complex infrastructures. However, the main challenge faced in solar desalination is salt accumulation, either on the surface of or inside the solar evaporator, which can impair solar-to-vapor efficiency and even lead to the failure of the evaporator itself. While many ideas have been tried to address this ″salt accumulation″, scientists have not had a clear system for understanding what works best for the enhancement of salt-rejecting ability. Therein, for the first time, we classified the state-of-the-art salt-rejecting designs into isolation strategy (isolating the solar evaporator from brine), dilution strategy (diluting the concentrated brine), and crystallization strategy (regulating the crystallization site into a tiny area). Through the specific equations presented, we have identified key parameters for each strategy and highlighted the corresponding improvements in the solar desalination performance. This Review provides a semiquantitative perspective on salt-rejecting designs and critical parameters for enhancing the salt-rejecting ability of dilution-based, isolation-based, and crystallization-based solar evaporators. Ultimately, this knowledge can help us create reliable solar desalination solutions to provide clean water from even the saltiest sources.

Pickering Emulsion Templated 3D Cylindrical Open Porous Aerogel for Highly Efficient Solar Steam Generation

Porous-structured evaporators have been fabricated for achieving a high clean water throughput due to their maximized surface area. However, most of the evaporation surfaces in the porous structure are not active because of the trapped vapor in pores. Herein, a three-dimensional (3D) cylindrical aerogel-based photothermal evaporator with a disordered interconnected hierarchical porous structure is developed via a Pickering emulsion-involved polymerization method. The obtained cotton cellulose/aramid nanofibers/polypyrrole (CAP) aerogel-based evaporator achieved all-cold evaporation under 1.0 sun irradiation, which not only completely eliminated energy loss via radiation, convection, and conduction, but also harvested massive extra energy from the surrounding environment and bulk water, thus significantly increasing the total energy input for vapor generation to deliver an extremely high evaporation rate of 5.368 kg m-2 h-1 . In addition, with the external convective flow, solar steam generation over the evaporator can be dramatically enhanced due to fast vapor diffusion out of its unique opened porous structure, realizing an ultrahigh evaporation rate of 18.539 kg m-2 h-1 under 1.0 sun and 4.0 m s-1 . Moreover, this evaporator can continuously operate with concentrated salt solution (20 wt.% NaCl). This work advances rational design and construction of solar evaporator to promote the application of solar evaporation technology in freshwater production.

Evolution of Morphology and Distribution of Salt Crystals on a Photothermal Layer during Solar Interfacial Evaporation

Solar interfacial evaporation (SIE) by leveraging photothermal conversion could be a clean and sustainable solution to the scarcity of fresh water, decontamination of wastewater, and steam sterilization. However, the process of salt crystallization on photothermal materials used in SIE, especially from saltwater evaporation, has not been completely understood. We report the temporal and spatial evolution of salt crystals on the photothermal layer during SIE. By using a typical oil lamp evaporator, we found that salt crystallization always initiates from the edge of the evaporation surface of the photothermal layer due to the local fast flux of the vapor to the surroundings. Interestingly, the salt crystals exhibit either compact or loose morphology, depending on the location and evaporation duration. By employing a suite of complementary analytical techniques of Raman and infrared spectroscopy and temperature mapping, we followed the evolution and spatial distribution of salt crystals, interfacial water, and surface temperature during evaporation. Our results suggested that the compact crystal structure may emerge from the recrystallization of salt in an initially porous structure, driven by continuous water evaporation from the porous and loose crystals. The holistic view provided in this study may lay the foundation for effective strategies for mitigation of the negative impact of salt crystallization in solar evaporation.

Optimized Water Supply in a Solar Evaporator for Simultaneous Freshwater Production and Salt Recycle

Solar desalination has shown great potential in alleviating global water scarcity. However, the trade-off between energy efficiency and salt rejection remains a challenge, restricting its practical applications. In this study, we report a three-dimensional nitrocellulose membrane-based evaporator featuring a high evaporation rate (1.5 kg m-2 h-1) and efficient salt precipitation at the edges. Additionally, the salt is isolated from the photothermal area of the evaporator and falls automatically with a salt recovery rate of 97 g m-2 h-1 in brine with 10 wt % salt content. The distinctive performance is attributed to the precise water supply control, which was adjusted by changing the resistance force and driven force in the evaporator. With a high evaporation rate, stable performance, and specific salt recovery ability, this solar evaporation structure holds great potential in water desalination and resource recovery.

Carbon nanofiber reinforced carbon aerogels for steam generation: Synergy of solar driven interface evaporation and side wall induced natural evaporation

Solar-based interface evaporation (SIE) is a green, efficient and cost-effective technique to harvest fresh water. 3D solar evaporators show their unique advantages in gaining energy from environment and hence possess a higher evaporation rate than 2D evaporators. However, much effort is still required to develop mechanically robust and superhydrophilic 3D evaporators with strong water transportation capability and salt-rejection performance, and at the same time reveal how they gain energy from environment via the natural evaporation. In this work, a novel carbon nanofiber reinforced carbon aerogel (CNFA) is prepared for the SIE. The CNFA has a high light absorption up to 97.2% and outstanding photothermal conversion performance. The heteroatom doping and hierarchically porous structure endow the CNFA with superhydrophilicity and thus powerful water transportation capability and salt rejection performance. Benefiting from synergy of the SIE and side wall induced natural evaporation, the CNFA evaporator exhibits a high evaporation rate and efficiency (as high as 3.82 kg m^-2h^-1 and 95.5%, respectively) with long-term stability and durability. The CNFA can also work normally in high-salinity and corrosive seawater. This study demonstrates a new method to fabricate all-carbon aerogel solar evaporators and provides insights for the effective thermal management during the interface evaporation.

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.

MXene/polypyrrole coated melamine-foam for efficient interfacial evaporation and photodegradation

The application of photothermal materials in seawater desalination, wastewater treatment have been widely studied, however, there are relatively few studies that combine photothermal effects and solar-driven photocatalysis and exhibit efficient solar-driven water evaporation performance and excellent photocatalytic ability. Form the perspective of practical application, it is of great significance to combine photothermal effect with solar-driven photocatalysis to develop environment-friendly evaporator with low cost, simple preparation process and ability of seawater desalination, wastewater treatment and photodegradation of organic dyes. In this paper, a novel multifunctional MXene/polypyrrole (PPy) coated melamine foam (MF) named as MF-MXene/PPy was successfully prepared by simple impregnation and in-situ polymerization. The MF-MXene/PPy has rich porosity (89.13 %), abundant water molecule transport channels, excellent light absorption capacity (about 94 %), low thermal conductivity (0.1047 W m^-1 K^-1), and exhibits excellent performance in solar desalination, wastewater purification and photodegradation of organic dyes. Under 1 kW m^-2 illuminate, the solar energy conversion rate and efficiency of MF-MXene/PPy reaches up to 1.5174 kg m^-2h^-1 and 91.24 %. Moreover, due to the regular pore size of MF-MXene/PPy, good salinity tolerance was shown even after continuous evaporation in 20 wt% NaCl for 8 h. After continuous evaporation in 70 mL of 20 wt% NaCl for 8 h, the amount of salt collected could reach 0.2 g. In addition, MF-MXene/PPy also possessed excellent visible light degradation ability for organic dyes, and the degradation rate of methylene blue (MB), rhodamine B (RHB) and methyl orange (MO) were 92.38 %, 88.92 % and 91.75 %, respectively. As a fundamental research, this research will open a novel way to the development of new evaporator.

Superhydrophobic lignin-based multifunctional polyurethane foam with SiO2 nanoparticles for efficient oil adsorption and separation

Superhydrophobic polyurethane foam is one of the most promising materials for oil-water separation. However, there are only limited studies prepared matrix superhydrophobic foams as adsorbents. In this paper, SiO₂ modified by 1H, 1H, 2H, 2H-perfluorododecyl trichlorosilane (F-SiO₂) was added into the lignin-based foam matrix by a one-step foaming technique. The average diameter of F-SiO₂ was about 480 nm with an water contact angle (WCA) of 160.3°. The lignin-based polyurethane foam with F-SiO₂ had a superhydrophobic water contact angle of 151.3°. There is no obvious change in contact angle after 100 cycles of compression or after cutting and abrasion. Scanning electron microscopy (SEM) analysis showed that F-SiO₂ was distributed both on the surface and inside of the foam. The efficiency for oil-water separation reached 99 %. Under the light intensity of 1 kW/m², the surface temperature of the lignin-based foam rose to 77.6 °C. In addition, the foam exhibited self-cleaning properties and degraded within 2 h in an alcoholic alkali solution. Thus, in this study, we developed a novel matrix superhydrophobic lignin-based polyurethane foam with an excellent promise to be used as oil water separation adsorbents in industrial wastewater treatment and oil spill clean-up processes.