Polyelectrolyte-based photothermal hydrogel with low evaporation enthalpy for solar-driven salt-tolerant desalination

Photothermal membranes have been widely developed for solar seawater desalination, but their practical applications are limited by high evaporation enthalpy and solid-salt crystallization. To solve this problem, we prepared an anionic polyelectrolyte-based hydrogel (APH) as an "all-in-one " evaporator, possessing both photothermal property (to evaporate seawater) and electrostatic repulsion (to avoid solid-salt crystallization). The APH was prepared by freeze-thaw method, and it was composed of poly(vinyl alcohol) as framework and poly (3,4-ethylenedioxythiophene): poly(sodium-p-styrenesulfonate) as solar absorber. The fabricated material showed porous structure with solar absorption efficiency of 95.5% in 380-2500 nm. After soaking by water, the APH exhibited an evaporation enthalpy as low as 1624.14 kJ kg(-1) with a high intermediate/free water ratio of 2.27:1, deriving from the interfacial electrostatic effect of SO3- and large amount of OH- from poly(vinyl alcohol) frame. Under the irradiation of simulated sunlight (1.0 kW m(-2)), the APH showed a high evaporation rate of 2.5 kg m(-2) h(-1) and its solar evaporation efficiency reached 90.7%. Importantly, the SO3- groups can efficiently separate anions (such as Cl-) from cations (such as Na+ and Mg2+) in the evaporator, avoiding the formation of solid-salt crystals. The excellent evaporation performance and salt tolerance of APH confer broad practical prospects for solar-driven seawater desalination.