Regulating solar desalination of the covalent organic framework aerogel via side group engineering and photothermal...

Covalent organic frameworks (COFs) bulk materials have attracted much interest recently in the seawater desalination field. To improve water supply and photothermal conversion efficiency is vital for designing COF-based evaporators. In this study, a bilayer COF-based aerogel with multilevel architectures and tailorable solar-driven desalination performance was developed. Four types of diamines with various groups (-H, -CH₃, -OH, -OCH₃) were used to construct COF part and it was found that the presence of oxygen-containing groups improved hydrophilicity of the aerogel and reduced water evaporation enthalpy, which guaranteed sufficient water transport with the help of the aligned pores formed via the directional freezing treatment. The mixed metal organic frameworks (MOF)/MXene particles were used as surface coating for the COF part and the photothermal conversion efficiency could be optimized by regulating the mole ratio of MOF/MXene. The enhanced surface roughness improved the broadband solar absorption (95.3 %, 250-2500 nm) of the coating and the formed channels in the coating favored reducing surface salt deposition. Thus, the as-developed COF aerogel evaporator had desirable desalination performance. Owing to the strongest hydrophilicity and the lowest evaporation enthalpy, the hydroxylated diamine-constructed aerogel coated with MOF/MXene (molar ratio 1/2) exhibited the best evaporation rate (2.67 kg m^-2h^-1) and solar-to-vapor efficiency (94 %) among all the aerogels, and the desalination performance was stable in harsh environment. Moreover, the aerogel possessed multimodal working ability, adsorbing pollutants while desalinating seawater. This work provided an attractive way to structurally architect COF-based functional materials.