Synthesis Of Polyelectrolyte-based Photothermal Hydrogels For Solar Driven Seawater Desalination

Solar desalination technology is one of the most promising methods to alleviate global freshwater scarcity.The key is to develop materials with wide spectral absorption and high photothermal conversion efficiency.At present,a series of photothermal conversion materials have been developed,including metal nanoparticles,inorganic semiconductors,carbon materials,organic polymers and so on,and successfully constructed high-efficiency solar desalination devices.However,after a long time of seawater evaporation,salt gradually accumulated on the evaporation interface to form a salt layer,which seriously reduced the evaporation rate.To solve this problem,many methods have been developed to improve photothermal materials and evaporation devices.One of the most effective methods is to introduce charged polyelectrolyte into photothermal materials.The charge in polyelectrolyte repels the same charge in seawater to prevent salt analysis,which is simple and easy to operate,and attracts extensive attention.However,there are still two main problems in polyelectrolytes evaporator:(a)polyelectrolyte and photothermal materials are two independent components,resulting in uneven composition and weak water absorption capacity of evaporation materials;(b)a single polyelectrolyte based evaporation material is only suitable for low concentration seawater,and its salt resistance is not ideal.To solve these problems,two new types of polyelectrolyte hydrogel materials have been developed.The main results include the following two points:1.Development of an anionic polyelectrolyte based photothermal hydrogelIt is well known that poly(3,4-ethylenedioxythiophene:sodium p-styrene sulfonate)(PEDOT:PSS)is not only a negatively charged polyelectrolyte material,but also has wide-band light absorption performance and can carry out efficient photothermal conversion.Inspired by this,PEDOT:PSS was used as a polyelectrolyte and photothermal material,and polyvinyl alcohol(PVA)was used as the framework to prepare anionic polyelectrolyte photothermal gel(APH).The interior of APH presents a porous network structure(pore size 1-5?m),the structure is conducive to light scattering and absorption,resulting in solar absorption efficiency of?95%.The-OH and-SO^3-in APH destroy the hydrogen bond network of interfacial water,resulting in a significant decrease in the evaporation enthalpy of water from 2418.11 k J kg^-1 to 1624.14 k J kg^-1.The APH was floated on the surface of seawater.Under the irradiation of simulated sunlight(1.0 k W m^-2),the evaporation rate reaches?2.5 kg m^-2 h^-1,and the evaporation efficiency reaches 90.7%.Importantly,-SO^3-in APH repels Cl^-in seawater and avoids salt analysis during evaporation.The excellent evaporation performance and salt tolerance of APH confer broad practical prospects for solar-driven seawater desalination.2.Development of an anionic-cationic polyelectrolyte double-layer structure hydrogelTo solve the problem of poor salt resistance of single polyelectrolyte evaporation system,an anionic-cationic polyelectrolyte double-layer structure hydrogel(DLH)was designed and prepared.In this paper,sodium alginate(SA)was used as anionic polyelectrolyte and chitosan(CS)as cationic polyelectrolyte.A double-layer structure hydrogel(DLH)was prepared by simple chemical synthesis.The upper layer of DLH is an anionic hydrogel photothermal evaporation layer consisting of PVA,SA and carbon nanotubes(CNT)photothermal materials.The lower layer is a cationic hydrogel water supply layer composed of PVA and CS.The upper and lower layers of DLH show porous structure,which is conducive to the transmission of water and steam and the multiple scattering and absorption of incident light.DLH exhibits wide spectral absorption,and its sunlight absorption efficiency reaches 95%.When DLH is floating in seawater,under the irradiation of simulated sunlight(1.0 k W m^-2),the evaporation rate is 1.69 kg m^-2 h^-1 and the evaporation efficiency is 91.2%.Importantly,due to the charge repulsion effect,the cationic lower layer only allows water and Cl^-to pass through and repels Na⁺;The anionic upper layer can repel Cl^-,so only water can be transported to the material air interface and evaporate.Simulated seawater with NaCl(21 wt%)solution,DLH can be continuously evaporated for 8 h without salt on the surface,realizing continuous solar seawater evaporation with high salt resistance.In this paper,two kinds of polyelectrolyte photothermal hydrogel materials were developed,which had high evaporation and salt tolerance.It provides ideas for the design of new hydrogel evaporation materials and promotes large-scale practical application of solar desalination.