Fast thermo-osmotic flow through covalent organic framework multilayers for desalination

Thermo-osmosis, a liquid flow driven by temperature difference (∆T) in a solid-liquid interface, is promising to utilize low-grade heat energies for water desalination. However, the water flux of thermo-osmosis is hard to be enhanced. Herein, the potential of COF multilayers for thermo-osmotic desalination is investigated via non-equilibrium molecular dynamics (NEMD) simulations. To this end, TpMA multilayers with fine water stability and sub-nanometer pores are selected. The TpMA multilayers show excellent water flux and nearly 100 % NaCl rejections. By the analysis of interfacial and interior resistances, it is extrapolated that the TpMA nanosheet with a thickness of 200 nm has a water flux of 3096 L/(m²·h) at the ∆T of 60 K. By the molecular-level analysis, it is revealed that the coexistence of single-file and two-chains of water structure in the flow direction brings in high thermo-osmotic flows. The high NaCl rejection is due to the strong sieving effect of pores on the hydration of Cl⁻. Finally, the thermo-osmosis is compared with reverse osmosis. Based on the resistance analysis, it is found that the ∆T of 60 K is equivalent to the ∆P of 180 bar at most to reach the same water flux. These findings will inspire researchers with an alternative technology for high-efficiency desalination using low-grade heat energies.