Engineering Charge Heterogeneity in COF/Graphene Hydrogels for Salt-Resistant Solar Evaporation

Solar-driven interfacial evaporation technology offers a sustainable solution for water treatment, however, its efficiency in marine environments is often hindered by salt accumulation. Here, a series of covalent organic framework (COF)/graphene hydrogels with controllable charge distributions is reported, including homo- and hetero-charged as well as zwitterionic surface functionalities. By adjusting the ratio of anionic and cationic building blocks during COF synthesis, the surface charge of the hydrogel evaporator is continuously modulated at the molecular level. The hetero-charged and zwitterionic hydrogels enable faster water evaporation in seawater than in pure water by reducing the evaporation enthalpy. In particular, the hetero-charged hydrogel achieves an evaporation rate of up to 3.23 kg m^-2 h^-1 in 3.5 wt.% saline water, 12% higher than that in pure water, and maintains strong salt resistance even in 20 wt.% brine. Beyond desalination, the hydrogel exhibits high potential for treating diverse wastewaters, providing a scalable platform for sustainable water treatment in complex environments.