Freestanding Hydrophilic/Hydrophobic Janus Covalent Organic Framework Membranes for Highly Efficient Solar Steam Generation

Solar-powered clean water production has been considered an efficient and favorable way to address the global water shortage challenge. Herein, we design and synthesize a series of robust, porous, azo-rich covalent organic frameworks (COFs) that can directly form freestanding pure membranes via one-step interface polymerization for highly efficient solar steam generation. The abundant photoresponsive groups, highly ordered porous structure, and inherent hydrophilic/hydrophobic Janus membrane structures endow COF membranes with excellent solar energy absorption, solar thermal conversion effect, water transportation, and evaporation efficiency. Without any supporting substrate or post-treatment, the steam generation rate of COF membranes reaches up to 3.02 kg/m2/h under 1 sun irradiation, and the energy conversion efficiency is up to 94%. This performance is the best among all solar thermal evaporators using COFs as solar thermal materials. The evaporation system not only works for clean and pure water generation but also for solar desalination. The good performance and reusability of COF membranes make them promising platforms for solar-thermal water production from seawater.