Fresh water shortage is an increasingly serious challenge to social development. It is reported that 29% of the global population lacks safe drinking water, and the demand for clean water is still increasing due to population growth and environmental issues. Clean water associated with sanitation is particularly important in the context of the current outbreak of coronavirus disease (Neocoronavirus pneumonia). Solar-powered water evaporation is considered one of the most promising technologies for producing clean water from seawater and wastewater using sunlight as the only energy input. The technology relies heavily on solar absorbers, which act as evaporators to convert solar energy into heat to produce steam. To achieve high water evaporation rates and solar heat conversion efficiencies, the ideal material should have a wide range of absorbance to efficiently utilize a large portion of the solar spectrum, low thermal conductivity to suppress heat loss, and three-dimensional (3D) porous channels for efficient water transport. So far, scientists have developed various photothermal materials based on carbon (e.g., graphene or carbon nanotubes), polymers, metals and metal oxides and their hybrids, etc.
Scientists have been working hard to explore and make many breakthroughs in recent years, so let’s learn from several topical publications about the research progress in photothermal clean water generation in recent years.
- Nature Communications: Plant leaves inspire solar-powered, high-efficiency water purifier
The leaves of natural vascular plants rely on differences in osmotic pressure, transpiration and excretion to produce tons of clean water, powered by sunlight. Inspired by this, researchers in Chun Li’s group at Qinghua University have reported a sunlight-driven purifier for efficient water purification and production. This sunlight-driven purifier features a negative temperature-responsive poly(N-isopropylacrylamide) hydrogel (PN) anchored on a superhydrophilic melamine foam backbone and coated with a PNIPAm-modified graphene (PG) filter membrane on the outside. Molecular dynamics simulations and experimental results showed that the superhydrophilic nature of the relatively rigid melamine backbone significantly accelerated the swelling/de-swelling rate of the PNPG-F purifier. Under the same sunlight intensity irradiation, this rationally engineered structure provided 4.2 kg/m2/h collection and > 99% ion removal for a single PNPG-F from a brine feed through the synergistic effects of evaporation and drip addition. The author envision great potential applications for this highly efficient sunlight-driven system in various water treatment applications. The study was published as a paper entitled “Plant leaves inspired sunlight-driven purifier for high-efficiency clean water production” in the Nature subjournal Nature communications.
- Science Advances: Highly hydratable polymer networks of light-absorbing hydrogels for solar water purification
Solar water evaporation for water purification is a promising technology for freshwater production. However, solar vapor generation is energy intensive, resulting in low water production under natural daylight. Therefore, there is a great need to develop new materials that can reduce the energy demand for water evaporation and accelerate solar water purification. Professor Guihua Yu’s group at Austin, Texas, USA, reported a highly hydrated light-absorbing hydrogel (h-LAH) composed of polyvinyl alcohol and chitosan as the hydration backbone and polypyrrole as the light absorber, which can use less energy (< 50% of the volume of water) to evaporate water. It was demonstrated that increasing the hydration capacity of h-LAH can change the state of water and partially activate water, thereby facilitating water evaporation. h-LAH increased solar vapor production to a record rate of ~3.6 kg/m2/h in 1 sun. h-LAH-based solar water evaporation also exhibited long-term durability and antifouling properties against complex ionic contaminants. The research was published in a paper entitled “Plant leaves inspired sunlight-driven purifier for high-efficiency clean water production” in the Science subjournal Science Advances”.
- Angew: COF materials with superhydrophilicity and broad wavelength light absorption for photothermal water evaporation
The inherent hydrophobicity and limited light absorption of porous organic polymers, especially in the near-infrared (NIR) region, are two bottlenecks that hinder their application in solar steam generation (SSG). Researchers in Xiaoli Yan’s group at Jilin University have developed a covalent organic framework (COF) based on 1,4,5,8 tetrakis(phenylamino)anthracene-9,10-dione (TPAD) (TPAD-COF), which is characterized by superhydrophilicity and broad light absorption covering SSG from the entire UV/visible NIR region. TPAD-COF, as an efficient photothermal conversion material without any additives, exhibits excellent water evaporation of 1.42 kg/m2/h and achieves a high energy conversion efficiency of 94% under one solar intensity irradiation. A further extension of the TPAD-based COF light absorption range was achieved by post-synthetic modification of chelated BF2 molecules. Systematic control experiments and analysis confirm that the hydrophilicity of the photothermal conversion material plays a more dominant role in the current TPAD-based COFs used for photothermal water evaporation. The research work, entitled “Superhydrophilic2D Covalent Organic Frameworks as Broadband Absorbers for Efficient Solar Steam Generation,” was published in the top-tier international journal Angewise. The work was published in the top international journal Angew. Chem. Int. Ed.
- JACS: Hydrophilic-hydrophobic dual-region COF/graphene hydrogel for photothermal water evaporation
Solar water evaporation is a sustainable water treatment technology that can help alleviate global water scarcity. However, this technology faces great challenges because of the high energy consumption and low evaporation rate of water evaporation. The group of Prof. Arne Thomas at the Technical University of Berlin, Germany, reports a covalent organic framework (COF)/graphene dual-region hydrogel that contains both hydrophilic and hydrophobic regions in one material through a simple in situ growth strategy. The hydrophilic COF covers part of the hydrophobic graphene region. By precise control of the two wetting regions, the hybrid hydrogel shows effective light harvesting, tunable wettability, optimized water content, and reduced energy requirement for water evaporation. As a solar absorber, the dual-region hydrogel achieves a vapor generation rate of up to 3.69 kg/m2/h for 1 solar irradiation (1kw/m2), which is comparable to other advanced materials. In addition, this hydrogel evaporator can be used to produce potable water from seawater and sewage, demonstrating the potential of water treatment. The research work was published in the journal JACS as “A Covalent Organic Framework/Graphene Dual-Region Hydrogel for Enhanced Solar-Driven Water Generation”, JACS, a top international journal.
- Science Advances: Water lily blade structure-inspired high-efficiency photothermal water evaporator
In recent years, interfacial solar vapor generation has shown great potential in achieving high energy conversion efficiency for desalination and wastewater treatment. However, high evaporation rates cannot be maintained due to the seemingly inevitable accumulation of fouling or salt on the solar absorber. Degradation is accelerated as the solute concentration increases. To address this problem, Prof. Jia Zhu’s group at Nanjing University, inspired by the structure of water lily leaves, designed a multi-stage structure (Water-lily-inspired Hierarchical Structure (WHS) photothermal-vapor conversion device, which can achieve 80% photothermal conversion efficiency when treating 10 wt% of highly concentrated salt water and 30 wt% of wastewater. It is worth mentioning that the photothermal conversion efficiency does not decrease significantly during the treatment of highly concentrated brine and wastewater until evaporation is complete (i.e., “zero discharge”), and the surface remains clean. The work was published in Science Advance under the title “A Water-lily-inspired Hierarchical Design for Stable and Efficient Solar Evaporation of High Salinity Brine”.
- Nano energy: Molybdenum disulfide for photothermal water evaporation
Researchers in Lifeng Wan’s group at Deakin University’s Institute for Frontier Materials have developed a series of transition metal disulfide and nitrogen-rich nitride heterostructures for solar water evaporation via partial crystal transformation. In particular, these advanced 2D heterostructures exhibit enhanced spectral absorption in the infrared range and excellent chemical stability in different solutions, including water, acids and bases. More importantly, due to the enhanced spectral absorption intensity and reduced heat loss, the MoS2-Mo5N6/melamine foam (MF) solar thermal device exhibits high evaporation rate (2.31 kg m-2h-1) and conversion efficiency (106.6%), which are 1.4 and 4.7 times higher than those of MoS2/MF and MF/water solar devices, respectively. This work provides a series of advanced 2D-2D TMDs-nitrogen-rich nitride heterostructures with excellent photothermal performance for solar water generation. This work was published in Nano energy, a top international journal, as “Advanced 2D-2D heterostructures of transition metal dichalcogenides and nitrogen-rich nitrides for solar water generation”.
Solar photothermal water evaporation technology is expected to solve the global shortage of clean water, what needs to be solved now is the low-cost preparation of photothermal materials, high-efficiency operation, as well as the stability and portability of use in practical application environments, looking forward to more results in the future for the early benefit of all mankind!
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