Regulating the evaporation surface architecture of anisotropic chitosan hydrogels for high-efficiency solar desalination

Abstract

Sufficient water supply and evaporation interfaces are vital for hydrogels as solar evaporators, which require ingenious structural design, from the networks and porous structure inside the bulk to the surface morphology outside the hydrogel. A chitosan based anisotropic hydrogel with well-regulated surface architecture is reported here. The aligned channels formed inside the bulk via directional freezing ensure water supply, and the ionic crosslinking further improves the mechanical stability of the bulk and also reduces the evaporation enthalpy of the seawater inside the channels. The small vessels radially distributed over the pores of the evaporation surface, together with the large-sized pores, act as a reservoir, promoting salt exchange and water evaporation and on the other hand, as drains of concentrated seawater, favoring the accumulation of a salt shell outside the hydrogel. Thus, as-prepared solar evaporator shows desirable performance (3.86 kg m⁻² h⁻¹ evaporation rate and 96.7% conversion efficiency under 1 sun irradiation) and superior salt-collection ability. The evaporation rate can maintain well even after 10 desalination cycles (over 3.5 kg m⁻² h⁻¹, in 20% brine). This work gives an eco-friendly solution for clean water production by using chitosan-based hydrogel evaporators and proposes a method of regulating evaporation surface architecture for hydrogel-based evaporators.