[Background introduction]

As we all know, fresh water is a limited natural resource, but it is vital to the survival of all ecosystems. In today‘s society, more than 3 billion people are seriously affected by the shortage of fresh water. More importantly, with population growth, pollution of fresh water resources and climate change, the challenge of providing sufficient and safe fresh water for humanity is gradually increasing. Therefore, it is urgent to develop efficient, economical and scalable technologies to purify large quantities of seawater and wastewater.

In recent decades, various water purification technologies have been well researched and developed. Among them, membrane-based technologies such as membrane filtration and reverse osmosis have the advantages of low cost and high energy efficiency, but also have problems such as low productivity, pollution and low salt suppression. However, solar water purification uses solar energy to separate water and impurities by evaporation, so using sustainable energy to purify water can help alleviate the shortage of water resources. However, insufficient solar energy absorption and heat loss limit the rate of water vapor generation, which reduces the yield of pure water. And diffused natural light cannot meet the inherent energy requirements of rapidly evaporating water. Therefore, the development of new material platforms, while providing high solar absorption, efficient energy use, and low energy demand for water evaporation, will help achieve efficient purification of solar water under natural light.

[Achievement Profile]

Recently, the team of Professor Yu Guihua (corresponding author) at the University of Texas at Austin summarized and reported on the use of hydrogels as a new material for solar water purification. In this article, the author reviews the research progress of hydrogel solar water purifiers at home and abroad in recent years from the aspects of material selection, molecular engineering and structural design. First, the unique water state in a hydrogel consisting of free water, intermediate water, and bound water is introduced, and intermediate water reduces the energy requirements for water evaporation. Then, the design principle of a hydrogel-based solar evaporator is described, in which a polymer network is customized to regulate the state of the water. The state of water in a hydrogel defines the evaporation behavior of water. Therefore, the polymer network of the hydrogel can be designed to regulate the state of the water, thereby further reducing the evaporation enthalpy of the water. Based on gelation chemistry, the authors also discuss the synthetic strategies for efficient hydrogel generation of hydrogels. By merging with the hydrophilic polymer network solar absorber, it absorbs solar energy and converts it into thermal energy, which can be used in situ to drive the evaporation of the water contained in the molecular network, and a solar absorbent with strong interaction with the hydrogel can Guide the formation of water molecules. Microstructures that reduce energy loss and ensure adequate water transport of evaporated water. The engineering design of the hydrogel surface focuses on promoting the evaporation of water and further improving the evaporation efficiency of solar energy. Hydrogel solar evaporator uses hydrophilic polymer as the basic material and has various functions such as antifouling, selectivity and thermal response, which improves the ability to collect and purify water. In short, the work hopes to improve the performance, scalability, stability and sustainability of hydrogel solar evaporators, and promote the use of hydrogel solar evaporators in practical applications in the future, in order to alleviate the shortage of water resources. The work was published in the famous journal "Acc. Chem. Res." Under the title "Hydrogels as an Emerging Material Platform for Solar Water Purification".

[Graphic interpretation]

Figure 1.Schematic diagram of hydrogel as a platform for efficient solar water purification materials

Figure 2.Unique hydrogel structure

(A) Non-covalent interactions between water molecules and functional groups in the polymer chain;

(B) Unique water state in the hydrogel.

Figure 3. Reasonably designed hydrogel-based solar evaporator

Figure 4. Polymer network engineering of hydrogel-based solar evaporator

(A) HNG consists of a layered porous structure, including internal gaps, microchannels and molecular sieves,

(B) A schematic diagram of a typical SVG system and a water restriction strategy;

(C) SVG rate and efficiency of nanostructured hydrogels;

(D) Salinity of three artificial seawater samples before and after HNG desalination.

Figure 5. Performance test of hydrogel-based solar evaporator

(A) Schematic representation of water in a h-LAH hydratable polymer network;

(B) The Raman spectrum shows the fitted peaks of intermediate water and free water.

(C-e) DSC curves of h-LAH with different water contents.

(F) Equivalent water vaporization enthalpies of bulk water and water in h-LAH1 to h-LAH5.

Figure 6.Characteristics of the surface morphology of a hydrogel-based solar evaporator

(A) Schematic diagram of SH of SVG;

(B) SEM image of SHs top layer;

(C) Enhancing the heat flux of the evaporation front through nanostructures improves SVG performance.

Figure 7.Practical functions of hydrogel-based solar evaporator

(A) Schematic diagram of hydrogel antifouling;

(B) The measured concentrations of the four primary ions accumulated in the hydrogel over time;

(C) SVG rate of HNG in seawater during long-term testing;

(D) comparing the pH value of the solution before and after purification;

(E) Compared to current technologies designed for specific ions, the amount of residual ions in the purification solution;

(F) Water purification program based on PNPG-F water purifier, which absorbs a large amount of water after immersing it in sewage

(G) Change in water quality of PNPG-F under 1 solar radiation.

Figure VIII. Summary and Outlook

【to sum up】

In summary, the performance of interfacial evaporation by a hydrogel-based evaporator is superior to many reported evaporators, and has obvious advantages. In addition, high SVG rates and efficiencies are controlled by changing the main polymer matrix and different solar absorbers, as well as sufficient water transmission. By adjusting the interaction between the polymer network and water molecules, the water state can be adjusted to reduce the energy required for water vaporization. In addition, the surface morphology of the hydrogel-based evaporator can be modified to affect the evaporation of water and thereby increase heat flux. At the same time, the hydrogel-based evaporator is continuously used in long-term testing, and has excellent stability and durability. Despite the good achievements, as shown in Figure 8, there are still some challenges in increasing the strength of hydrogel solar evaporators: (1) the need to further explore the basic evaporation mechanism in hydrogels; Improve the stability and durability of hydrogel solar evaporators under severe conditions; (3) Further research is needed to reveal the basic mechanism of water evaporation.

In addition, the application of hydrogels to actual water purification, long-term stability, and large-scale preparation requires further improvement. Hydrogel-based solar evaporators have good salt tolerance and can stably produce water for a long time. At the same time, the use of more advanced antibacterial, self-cleaning and mechanical structure design to improve the stability and durability of the hydrogel to suit different water sources. Combine some current promising strategies to replace expensive polymer backbones and absorbents with low-cost materials to achieve high yields and large-scale manufacturing of hydrogels.
Finally, the versatility of hydrogel materials should be explored and hydrogel evaporators integrated into water purification systems. In short, it is very promising to develop solar integrated evaporators with dual or even multiple functions. The thermal positioning and thermal gradient of hydrogel solar evaporator provide a new idea for cogeneration. It is believed that in the near future, hydrogel solar evaporators with high performance, scalability, stability and sustainability will play an important role in the actual water purification market.

Literature link: Hydrogels as an Emerging Material Platform for Solar Water Purification (Acc. Chem. Res., 2019, DOI: 10.1021 / acs.accounts.9b00455)
Source of information: material cattle