Membrane boost for water desalination

According to study leader Yu Han, a professor of chemistry at KAUST (King Abdullah University of Science and Technology), water desalination membranes should simultaneously exhibit high water flux and high salt rejection.

Carbon nanomaterials are expected to meet these requirements due to their unique surface chemistry and propensity to stack into channels with diameters smaller than one nanometre, but channel alignment and stacking difficulties make their large-scale use in membranes challenging.

“One way to address these limitations is through two-dimensional porous carbonaceous membranes with regular and uniformly distributed subnanometre-sized molecular transport channels,” said first author Jie Shen, a postdoc in Han’s group.

These membranes are typically synthesised in solution, which promotes the random growth of a disordered three-dimensional structure with poorly defined micropores.

Yu Han, Vincent Tung, Ingo Pinnau and former KAUST scientist Lance Li, who is now at the University of Hong Kong, have developed a method that helps control the growth of two-dimensional conjugated polymer frameworks into ultrathin carbon films using chemical vapour deposition.

The researchers deposited the monomer triethynylbenzene on atomically flat single-crystalline copper substrates in the presence of an organic base that acts as a catalyst. Triethynylbenzene bears three reactive groups that serve as anchor points for additional monomers. These groups show a 120-degree angle with respect to each other, generating organised arrays of well-defined cyclic structures that stack into subnanometre-sized rhombic hydrophobic channels.

The membrane is said to have displayed excellent water desalination performance in forward and reverse osmosis configurations, surpassing those containing advanced materials such as carbon nanotubes and graphene. It also showed strong rejection for divalent ions, plus small charged and neutral molecules.

The researchers discovered that the water molecules formed a three-dimensional network inside the membrane instead of moving through the membrane along vertical triangular channels as one-dimensional chains.

“This unexpected result revealed that the seemingly discrete vertical channels are actually interconnected by short horizontal channels that can be easily overlooked in the projected structural model,” Han said in a statement.

The team is now working on improving the antifouling property, mechanical strength and long-term chemical stability of the membrane for future practical applications. They are also fine-tuning its surface-charge properties and channel sizes.

“Our ultimate goal is to provide a versatile multifunctional platform that meets the needs of various applications, such as ion sieving, single-molecule sensing and neural interfaces,” Han said.

The team’s findings have been published in Nature Materials.