Solar‐Initiated Frontal Polymerization of Photothermic Hydrogels with High Swelling Properties for Efficient Water Evaporation

Abstract

As the population increases and environmental pollution intensifies, the scarcity of freshwater and green energy has emerged as hard nut to be urgently cracked. [1][2][3][4] Solar energy, as an abundant renewable power source, could be efficiently converted into thermal energy for seawater desalination, wastewater purification, and electricity generation to address these global issues. [5] Nevertheless, poor optical absorption, large heat losses, and high evaporation enthalpy of bulk water deliver a low solar evaporation efficiency (30%-45%). [6] Recently, many efforts have been devoted to rationalizing the design of material structures and evaporation systems to concentrate heat at the air/liquid interface, boosting evaporation efficiency to over 90%, even higher than 100%. [7][8][9][10][11][12][13] Among a variety of developed photothermal materials (e.g., semiconductors, [14,15] metallic, and carbonbased materials [16][17][18] ), hydrogel-based materials, as an emerging photothermic material possessing cross-linked interconnected three-dimensional (3D) networks, macroporous structure, and excellent hydrophilicity, have been extensively used to generate freshwater. [19][20][21][22][23] The porous structure of the hydrogels strengthens the internal light reflection and scattering, allowing enhanced light harvesting and absorption. [24,25] In addition, hydrogels with macroporous structure and hydrophilic functional groups enable them self-cleaning, anti-fouling, salt, and chemically resistant to improve the durability. [19,22] Most critically, the unique hierarchical nanostructure and tailorable surface wetting state of hydrogel evaporators enable the significant decrease of evaporation enthalpy and rapidly promote water transport, contributing to the ultrahigh evaporation rate. [25][26][27][28] However, conventional batch polymerization (BP) for hydrogel preparation is mostly achieved with assistance of pore-forming agents, complicated synthesis processes and continuous heat source or UV light that is commonly conducted in more stringent laboratory conditions. [29][30][31][32] The increasing cost and energy consumption of hydrogel fabrication and subsequent transportation in turn greatly hinder the practical application of the technology, especially for the remote area and sea platform needs.On the other hand, the swelling behavior, as the pivotal attribute of hydrogels, which influences the volume, surface area, and water storage performance of evaporators, is rarely investigated in the photothermal field. Basically, the swelling ratio of hydrogel materials could be improved by optimizing their pore structure, [33] thus enlarging the effective surface area and water

Robust and Flexible 3D Photothermal Evaporator with Heat Storage for High‐Performance Solar‐Driven Evaporation

“…In order to achieve efficient solar evaporation, the key factors lie in both high conversion of solar‐to‐thermal energy and heat localization effect. [ 48–52 ] As shown in Figure a, the top temperature of dry 3D BF/PVA/CB amounts to 58 °C within 1 min at 1 kW m –2 , while the entire side still keeps a lower temperature. The temperature distribution curve features a local sharp peak, indicating capability of efficient photothermal conversion and insulation.…”

High‐Performance Integrated Solar Steam Generator for Synergetic Freshwater Production, Salt Harvesting, and Electricity Generation

“…[4,5] Thus, there is an urgent demand to exploit emerging technologies to alleviate water-energy paradox. Solarto-thermal technique is an efficient direct route for harvesting inexhaustible solar energy to enable a broad range of applications, such as domestic heating, [6] seawater desalination, [7,8] wastewater purification, [9,10] electricity generation, [11] etc. Thereinto, the stratagem of solar-driven interfacial water vaporization is a promising implementation of solar-to-thermal technology, which can directly transfer solar energy into heat and confine it to the water/air interface for high-performance evaporation, so as to simultaneously produce freshwater and drive electrical power generation.…”

Photothermal polymerization of benzoxazines

“…53,54 In all these cases, reactions are promoted photothermally that take place at relatively moderate temperatures ( T < 100 °C), while the aid of external photothermal agents is required that eventually lie entrapped in the final material – e.g. , metal nanostructures, 37–43,53,54 carbon (nano)materials 44,46,48,51 or organic dyes, 45,47,49,50 which are responsible for light absorption and heat generation. In contrast with these precedents, in this work we aim to (a) explore the more demanding photothermal polymerization of benzoxazines, which typically takes place at much higher temperatures, and (b) avoid the use of additional photoabsorbers.…”

“…To date a narrow range of photothermally-induced polymerization processes have been described, 37,38 which mainly comprise the photothermal curing of polysiloxanes, [39][40][41] the photothermal initiation of acrylate polymerization [42][43][44][45][46][47][48][49][50][51][52] and the photothermal polymerization of polyurethanes. 53,54 In all these cases, reactions are promoted photothermally that take place at relatively moderate temperatures (T < 100 °C), while the aid of external photothermal agents is required that eventually lie entrapped in the final materiale.g., metal nanostructures, [37][38][39][40][41][42][43]53,54 carbon (nano)materials 44,46,48,51 or organic dyes, 45,47,49,50 which are responsible for light absorption and heat generation.…”

“…To date a narrow range of photothermally-induced polymerization processes have been described, 37,38 which mainly comprise the photothermal curing of polysiloxanes, [39][40][41] the photothermal initiation of acrylate polymerization [42][43][44][45][46][47][48][49][50][51][52] and the photothermal polymerization of polyurethanes. 53,54 In all these cases, reactions are promoted photothermally that take place at relatively moderate temperatures (T < 100 °C), while the aid of external photothermal agents is required that eventually lie entrapped in the final materiale.g., metal nanostructures, [37][38][39][40][41][42][43]53,54 carbon (nano)materials 44,46,48,51 or organic dyes, 45,47,49,50 which are responsible for light absorption and heat generation. In contrast with these precedents, in this work we aim to (a) explore the more demanding photothermal polymerization of benzoxazines, which typically takes place at much higher temperatures, and (b) avoid the use of additional photoabsorbers.…”