Hydrogels as an Emerging Material Platform for Solar Water Purification

Abstract

Conspectus Growing concern over water scarcity leads to increased research interest in advanced water purification technologies. Solar water purification, which uses solar energy to separate water and impurities through vaporization, enables the utilization of sustainable energy and potential freshwater resources to alleviate water scarcity. However, the essential process of solar water evaporation to remove impurities is energy intensive. Insufficient solar absorption and thermal loss limited the vapor generation rate and, thus, lowered the purified water yield. Diffuse natural sunlight cannot satisfy the intrinsic energy demand for rapid water vaporization. Therefore, developing new material platforms that can simultaneously provide high solar absorption, effective energy utilization, and low energy demand for water vaporization to achieve highly efficient solar water purification under natural sunlight is anticipated. In this Account, we review our recent progress on hydrogel-based evaporators for solar water purification in terms of material selection, molecular engineering, and structural design. First, we introduce the unique water state in hydrogels consisting of free, intermediate, and bound water, of which intermediate water has a reduced energy demand for water evaporation. Then, we describe the design principles of hydrogel-based solar evaporators, where the polymeric networks are tailored to regulate the water state. The water state in hydrogels defines the vaporization behavior of water. Thus, the polymer networks of hydrogels can be architected to tune the water state and, hence, to further reduce the evaporation enthalpy of water. Armed with fundamental gelation chemistry, we discuss synthetic strategies of hydrogels for efficient vapor generation. By incorporating solar absorbers with hydrophilic polymer networks, solar energy is harvested and converted to heat energy, which can be in situ utilized to power the vaporization of contained water in the molecular meshes, and the solar absorbers having strong interaction with hydrogels guide the formation of microstructure to reduce the energy loss and ensure adequate water transport of evaporative water. Regulating the vaporizing fronts, engineering the surface of hydrogels has been focused to favor the evaporationof water to further enhance the solar-to-vapor efficiency. By using hydrophilic polymers as building blocks, the hydrogel-based solar evaporators have also been endowed with multiple functionalities, such as antifouling, permselectivity, and thermal responsiveness, to improve water collection and purification abilities. Taking advantages of these merits, hydrogels have emerged as a promising materials platform to enable efficient solar water purification under natural sunlight. This Account serves to promote future efforts toward practical purification systems using hydrogel-based solar evaporators to mitigate water scarcity by improving their performance, scalability, stability, and sustainability.

Biomass‐Derived Hybrid Hydrogel Evaporators for Cost‐Effective Solar Water Purification

“…[7,8] A feasible solardriven water purification system should not only be affordable for communities that suffer from economic water scarcity but also require scientific breakthroughs to enable efficient energy capture and conversion as well as a high rate of clean water production.One key challenge is that natural sunlight (≤1 kW m −2 ) is too diffuse to power an efficient water distillation system; therefore, expensive solar concentrators are needed, which increases the overall cost of this technology. [9,10] Approaches to circumventing this insufficient solar energy supply involve the directions of improving sunlight capture, enhancing solar-to-thermal efficiency, and reducing heat loss to the surroundings. As such, a myriad of light-absorbing materials was explored and developed to collect light over the full solar wavelength range via regulating the size and morphology of nanomaterials.…”

Functionalized MXene Enabled Sustainable Water Harvesting and Desalination

“…[ 46 ] Such a porous structure can effectively generate weak water bonds with polymeric chains, which facilitates the steam generation. [ 47,48 ] Interpenetrated Li‐MXene flakes are fully wrapped and completely blended inside the polymeric network, as depicted in the SEM image (Figures S3 and S4, Supporting Information), where no obvious naked flakes can be identified. Such a complete wrapping significantly reduced exposure of MXene flakes to water and oxygen molecules, which effectively prevents MXene degradation, and thus increased its stability to more than 6 months.…”

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

“…Unlike traditional evaporation process where water changes its phase from liquid to vapor as individual molecules, Yu's group has recently disclosed a surprising and non‐intuitive water evaporation phenomenon by using a special water‐polymer network. [ 62,122 ] As shown in Figure a, the developed polyvinyl alcohol (PVA) based hydrogel composite with a hierarchical nanostructure (HNG) confined water molecular meshes. [ 123 ] The unique water–polymer interactions induced by the functional groups in polymer chains allows for the coexistence of water in different states, which includes bound water, intermediate water and free water (Figure 9b).…”