Thermally driven membrane distillation (MD) is a promising way to purify polluted water by using low-grade heat, solar energy and geothermal energy. However, membrane distillation membranes suffer from limited permeance and wetting owing to the lack of precise structural control.  

Researchers from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) and Beijing Institute of Technology proposed a strategy to fabricate covalent organic frameworks (COFs) films with a gradient structure, which accelerated the desalination from seawater. The COF membranes also showed satisfactory anti-fouling and anti-wetting capability.

This study was published in Nature Materials. 

The researchers found that the membrane on a supporting layer showed a flux nearly triple that of the state-of-the-art membrane distillation membrane for desalination.

The quantitative model of the evaporation-driven liquid flow through nanochannels previously built by USTC team, described in Physics of Fluids, could explain the ultra-high flux of the COFs films.

The enhanced evaporation near the solid-liquid interface and reduced vapor diffusion length in the engineered COF layer work in coordination to increase the water flux of the COF membrane.

Besides, the researchers also found a pure water layer gap between the water-vapor interface and the salt-vapor interface, which prevents direct contact of the ions to the pore walls or the evaporation interface. As the charged surface decreases the adjacent salt concentration and thus alleviates salt crystallization, which leads to the anti-wetting behavior of the engineered COF membranes.

COFs films feature large porosity, ease of functionalization, and periodic channels, making them promising to boost the distillation technology.