Challenges and Opportunities for Solar Evaporation

Solar-powered water evaporation — the extraction of vapour from liquid water using solar energy — provides the basis for the development of eco-friendly and cost-effective freshwater production. Liquid water consumes and carries energy, and, thus, plays an essential role in this process. As such, extensive experimental and theoretical studies have been focused on water management to achieve efficient solar vapour generation. Many innovative materials have been proposed to enable highly controllable and efficient solar-to-thermal energy conversion to address the challenges in the energy–water nexus from the microscale to the molecular level. In this Review, we summarize the fundamental principles of materials design for efficient solar-to-thermal energy conversion and vapour generation. We discuss how to integrate photothermal materials, nanostructures/microstructures and water–material interactions to improve the performance of the evaporation system via in situ utilization of solar energy. Focusing on materials science and engineering, we overview the key challenges and opportunities for nanostructured and microstructured materials in both fundamental research and practical water-purification applications. Materials engineering enables the control of water–material interactions in solar vapour generators, which aim to efficiently utilize solar energy for the cost-effective production of clean water. This Review discusses material-design principles for solar evaporators, spanning from macrostructures to molecular configurations.

Materials for solar-powered water evaporation

Energy and water are of fundamental importance for our modern society, and advanced technologies on sustainable energy storage and conversion as well as water resource management are in the focus of intensive research worldwide. Beyond their traditional biological applications, hydrogels are emerging as an appealing materials platform for energy- and water-related applications owing to their attractive and tailorable physiochemical properties. In this review, we highlight the highly tunable synthesis of various hydrogels, involving key synthetic elements such as monomer/polymer building blocks, cross-linkers, and functional additives, and discuss how hydrogels can be employed as precursors and templates for architecting three-dimensional frameworks of electrochemically active materials. We then present an in-depth discussion of the structure-property relationships of hydrogel materials based on fundamental gelation chemistry, ultimately targeting properties such as enhanced ionic/electronic conductivities, mechanical strength, flexibility, stimuli-responsiveness, and desirable swelling behavior. The unique interconnected porous structures of hydrogels enable fast charge/mass transport while offering large surface areas, and the polymer-water interactions can be regulated to achieve desirable water retention, absorption, and evaporation within hydrogels. Such structure-derived properties are also intimately coordinated to realize multifunctionality and stability for different target devices. The plethora of stimulating examples is expounded with a focus on batteries, supercapacitors, electrocatalysts, solar water purification, and atmospheric water harvesting, which showcase the unprecedented technological potential enabled by hydrogels and hydrogel-derived materials. Finally, we study the challenges and potential ways of tackling them to reveal the underlying mechanisms and transform the current development of hydrogel materials into sustainable energy and water technologies.

Hydrogels and Hydrogel-Derived Materials for Energy and Water Sustainability.

The shortage of clean water is one of the predominant causes of human mortality, especially in remote rural areas. Currently, solar steam generation is being adopted as an efficient, sustainable, and low-cost means for water desalination to produce clean water. However, preventing salt accumulation during operation while maintaining long-term stability and a rapid evaporation rate is a critical challenge that needs to be urgently addressed to further facilitate the practical applications of solar desalination, especially for desalinating high-salinity brine. Here, we demonstrate that a bimodal porous structure (e.g., balsa wood) can serve as an efficient and stable solar vapor generator for high-salinity brine desalination. Taking advantage of the inherent bimodal porous and interconnected microstructures of balsa wood, rapid capillary transport through the microchannels and efficient transport between the micro- and macrochannels through ray cells and pits in the bimodal evaporator can lead to quick replenishment of surface vaporized brine to ensure fast and continuous clean water vapor generation. The bimodal evaporator demonstrates a rapid evaporation rate of 6.4 kg m−2 h−1 under 6 suns irradiation and outstanding long-term stability for desalination of high salinity brine. The large vessel channels play a critical role in preventing salt from accumulating, as evidenced by controlled experiments with large vessels either blocked in the bimodal evaporator (balsa evaporator) or absent in a unimodal evaporator (e.g., cedar wood) whose porous structure occurs naturally without large vessels. Both approaches demonstrate severe salt accumulation during solar desalination due to a lack of sufficient brine replenishment from the bulk solution beneath. With its unique bimodal porous and interconnected microstructure configuration obtained by a facile and scalable fabrication method, our bimodal porous structured evaporator device represents an efficient, stable, low-cost, and environmentally friendly solar vapor generator for high-salinity brine desalination.

Nature-inspired salt resistant bimodal porous solar evaporator for efficient and stable water desalination

Starch, cellulose, pectin, gum, alginate, chitin and chitosan derived (nano)materials for sustainable water treatment: A review

Solar steam generation by heat localization is considered to be a highly efficient, sustainable way to alleviate water shortage in resource-limited regions. However, most of the interfacial photothermal evaporators demonstrated so far involve non-biodegradable nanoscale materials, which can quickly pose a significant threat to the environment and ecosystems, especially marine ecosystems, after their disposal. For the first time, a flexible, scalable and, more importantly, completely biodegradable photothermal evaporator for highly efficient solar steam generation is introduced. The bilayered evaporator is comprised of bacterial nanocellulose (BNC) densely loaded with polydopamine (PDA) particles during its growth. The biodegradable foam introduced here exhibits large light absorption and photothermal conversion, heat localization, and efficient water transportation, leading to an excellent solar steam generation performance under one sun (efficiency of ∼78%). The novel material and scalable process demonstrated here can be a sustainable solution to alleviate the global water crisis.

Polydopamine-filled bacterial nanocellulose as a biodegradable interfacial photothermal evaporator for highly efficient solar steam generation

To alleviate the scarcity of clean water, solar steam generation, which utilizes the green and abundant resources of Earth, has attracted considerable attention and been recognized as a sustainable technology to purify seawater and wastewater. Within the past five to seven years, building on remarkable advances in the synthesis of nanoscale photothermal materials, significant progress has been made in the design and fabrication of solar evaporators. Here, we comprehensively review the materials and structures of the three key components of solar evaporators: solar absorbers, substrates (i.e., support layers), and water collectors towards efficient harvesting of sunlight, thermal management, water transportation, interfacial evaporation, and collection. In particular, we discuss the design principles of water collectors for solar-driven evaporation, which are in the early stages of development at present. Furthermore, bio-inspired water collectors provide exciting opportunities for the design and realization of efficient water harvesting from solar steam. Finally, we introduce photothermal-enhanced membrane distillation (PMD), a promising alternative technique involving solar-driven interfacial evaporation for producing clean water. PMD combines solar energy and membrane distillation, which facilitates efficient water purification and collection. In this review, we discuss the design guidelines and recent development of this photothermal-enhanced water purification. This review provides useful insights into the material choice and structure optimization to realize enhanced solar evaporators towards freshwater generation.

Advances in solar evaporator materials for freshwater generation

Mechanically interlocked 1T/2H phases of MoS2 nanosheets for solar thermal water purification

Solar-driven membrane distillation using photothermal membranes is of considerable interest for future water desalination systems. However, the low energy efficiency, complex synthesis, and instability of current photothermal materials hinder their further development and practicability. In this study, for the first time, we demonstrate a simple, stable, and scalable polydopamine (PDA)-coated polyvinylidene fluoride (PVDF) membrane for highly efficient solar-driven membrane distillation (MD). Our membrane shows the best energy efficiency among existing photothermal MD membranes (45%) and the highest water flux (0.49 kg m−2 h−1) using a direct contact membrane distillation (DCMD) system under 0.75 kW m−2 solar irradiation. Such a performance was facilitated by the PDA coating, whose broad light absorption and outstanding photothermal conversion properties enable higher transmembrane temperature and increased driving force for vapor transport. In addition, the excellent hydrophobicity achieved by fluoro-silanization gives the membrane great wetting resistance and high salt rejection. More importantly, the robustness of our membrane, stemming from the excellent underwater adhesion of the PDA, makes the composite membrane an outstanding candidate for real-world applications.

Localized heating with a photothermal polydopamine coating facilitates a novel membrane distillation process

Herein, the recent advances in realizing highly efficient cellulose-based solar evaporators for alleviating the global water crisis are summarized. Fresh water scarcity is one of the most threatening issues for sustainable development. Solar steam generation, which harnesses the abundant sunlight, has been recognized as a sustainable approach to harvest fresh water. In contrast to synthetic polymeric materials that can pose serious negative environmental impacts, cellulose-based materials, owing to their biocompatibility, renewability, and sustainability, are highly attractive for realizing solar steam generators. The molecular and macromolecular features of cellulose and the physicochemical properties of extracted cellulose nanoparticles (cellulose nanocrystals and cellulose nanofibrils (CNF)) and natural cellulose materials (wood and bacterial nanocellulose (BNC)) that make them attractive as supporting substrate materials in solar steam generators are briefly discussed. Recent progress in designing highly efficient cellulose-based solar evaporators, including utilizing extracted cellulose nanoparticles via bottom-up assembly CNF, natural cellulose materials with intrinsic hierarchical structure (wood and BNC), and commercial planar cellulose substrates (air-laid paper, cellulose paper, and cotton fabric) is reviewed. The outstanding challenges that need to be addressed for these materials and devices to be utilized in the real-world and in overcoming global water crisis are also briefly highlighted.

Deoukchen Ghim

Papers

Polydopamine-filled bacterial nanocellulose as a biodegradable interfacial photothermal evaporator for highly efficient solar steam generation

Advances in solar evaporator materials for freshwater generation

Mechanically interlocked 1T/2H phases of MoS2 nanosheets for solar thermal water purification

Localized heating with a photothermal polydopamine coating facilitates a novel membrane distillation process

Cellulose Nanomaterials in Interfacial Evaporators for Desalination: A "Natural" Choice.